ASNT NDT Handbook Volume 3 Infrared and Thermal Testing

where the laser wavelength is strongly point through the plume should be able
absorbed by the gas of interest. \'\'hen to be imaged.
achieved, the result is tl1at the normally
invisible gas becomes visible on a Figure 12 illustrates a
standard television monitor. The backscatter/absorption gas imaging system
technique has three basic constraints: viewing a gas leak occurring in a bank of
(1) there must be a topographical gas storage tanks. The television monitor
background against which the gas is in the lower right corner shows the live
imaged, (2) the system must operate in an image viewed by the operator and shows
atmospheric transmission window and the leak as a black plume. The plume is
(3) the gas of interest must absorb the black because the laser energy has been
laser radiation. Imaging equipment in the absorbed by the gas and cannot return to
infrared wavelengths fulfills these needs. the infrared thermographic imager as does
the rest of the laser energy.
Tabk 4 shows a list of detectable gases,
their maximum safe concentrations and fiGURE 12. Backscatter/absorption gas imaging system reveals
their minimum detectable gas leakage from bank of gas storage tanks.
concentrations.I 7,H;
Thermographic Leak
Infrared Absorption Test Testing Summary

Instrument Infrared thermography can be used to
detect buried and aboveground pipeline
Investigation equipment consists of a discontinuities such as leaks, cracks and
tunable infrared laser coupled to an subsurface erosion voids. Infrared
infrared imager. 1)•pically the optics of the thermography can also be used to detect
imager and laser are optically coupled to gas leaks in production processes.
let the units transmit the infrared laser
radiation to the area of interest and to Infrared thermographic testing may be
then receive the reflected laser energy. performed during day or night, depending
Typically an area consisting of a 14 to 18 on environmental conditions and the
degree field of view up to 30 m (100 ft) desired results.
from the receiver may be scanned.
Computer analysis of thermal images
The laser typically used in the gas greatly improves the accuracy and speed
imaging system is a tunable S VV carbon of test interpretations and can improve
dioxide waveguide laser. Using a luw the ability to set repair priorities for areas
power laser is possible because the optical in a state of change.
arrangement permits the laser beam and
the instantaneous field of view of an Aging chemical, oil and natural gas
infrared radiation detector to be scanned infrastructures throughout the world are
in synchronization across the area of rapidly approaching the end of their
interest. The instantaneous field of view design lives. This will necessitate more
produced by tile typically small, efficient and cost effective techniques of
0.05 x 0.05 mm (0.002 x 0.002 in.), testing pipelines under load and in place.
infrared radiation detector and a Infrared and thermal testing is a
collimating lens is scanned in a raster nondestructive, remote sensing method
pattern across the target area by two that meets these requirements.
orthogonally positioned horizontal and
vertical scan mirrors. This scan ensures
that the detector field of vic'w and the
laser beam are in perfect synchronization
and that the laser need irradiate only that
region of the target area viewed by the
detector. This keeps the laser power
requirements to a minimum and makes
the system safe for eyes.

\rVhen the infrared thermographic
investigation technique is used in the
infrared absorption mode, a tunable laser
must be coordinated with the infrared
imager. In this mode, the laser is tuned to
emit a specific frequency of diffused
infrared radiation that wiH be absorbed by
the gas being sought (see Table 4). The
laser is then scanned across the area being
investigated. \-\1hen the laser radiation is
absorbed by gas escaping from a leak, the
infrared image is lost or turns black on
the image. The entire path from the leak

586 Infrared and Thermal Testing

PART 3. Infrared Thermography of Steel
Aboveground Storage Tanks20,2 1

Historically, infrared thermography has drums. Thermal imaging has been
been used for nondestructive testing developed to locate areas of material
applications that have characteristic thickness loss within these examples
thermal fingerprints because they produce using a high resolution thermal imager
and a pulsed or stepped heat source.
their own heat. Examples include power Thermal energy from the heat ·Source is
distribution panels, motors1 generators directed toward the inspection surface
and furnaces. and begins to diffuse through the material
thickness. Areas that have been reduced
Figure 13 shows a passive in thickness will retain more heat on the
thermographic image of a low pressure inspection surface displayed by the
storage tank. Such images can be used for thermal imager.
qualitative assessment as part of a larger
strategy of condition monitoring. Test System

Aboveground storage tanks and the The heat source that is used will depend
nation's infrastructure in general do not on the thickness and the diffusivity of the
produce their own thermal fingerprints. material to be tested. For steel materials
Before 1990, infrared thermography was Jess than 2.5 mm (0.1 in.) thick and
rarely applied to steel because of its possessing a high carbon content, a high
thermal characteristics and because in intensity, short duration (4 ms) pulse is
most applications steel is relatively thick. used as the thermal stimulus. As the
initial pulse of heat decays, the decrease
Developments of the 1990s have made in the sample's surface temperature is
it possible to use pulsed infrared imaging
to evaluate these materials for material
loss due to corrosion and erosion. Steel
structures that have been evaluated
include aboveground storage tank floors,
boiler tubing, pipes and 208 L (55 gal)

FIGURE 13. Passive thermogram of low pressure propane storage tank. Cold spots indicate
possible insulation damage.

G:' 294 (21) [70]
"-. 291 (18) [65]

E 289 (16) [60]

"'1! 286 (13) [55]

il 283 (10) {50)

e•0. 280 (7) [45]

E
~ 278 (4) [40]

legend
L 287.2 K (14.0 T) [57.2 'FJ.
2. 286.6 K (13.4 °C) [56.2 °F}.
3. 274.7 K (1.6 "C) (34.8 °F].
4. 274.1 K (0.9 ~c) (33.7 °FJ.
5. 274.2 K (1.1 oc) (33.9 °F}.
6. 275.3 K (2.2 oc) [35.9 °Fj.

Chemical and Petroleum Applications of Infrared and Thermal Testing 587

monitored as a function of time. For filters {convolutions) on the raw infrared
thicker materials, >2.5 mm (>0.10 in.), a data, the resulting convoluted image looks
lower intensity, longer pulse (>3 s) of like a visual image (Fig. 14) of the
energy is introduced to the inspection corroded or bottom side of the tank floor.
surface. In this case, the increase in the
sample's surface temperature is monitored Graphing the anomalies with respect to
as the heat source steps up to its time is another function of the test
predetermined pulse length. software. An anomaly's rise time is faster
than that of an area without anomalies.
The specifications and characteristics of The program shows the time and
the infrared imager are of the utmost intensity numerically.
importance for this type of testing. The
thermal imaging technique uses a high The technique described here is
resolution thermal imaging system and a qualitative. Analytical algorithms have
specially designed heat source. The imager been used to quantify the thickness of
has 12-bit temperature accuracy and is single layer materials using these
calibrated from 273 K (0 oc; 32 °F) to techniques.
473 K (200 °C; 392 °F). The temperature
resolution is ±0.05 K (±0.05 oc; ±0.09 °F) Test Technique
and the angular subtends is 1.2 mrad. The
infrared signals of the discontinuities are There are two types of heat sources used:
characteristically very small, about 0.05 K step heating and pulsed heating. The step
(0.05 oc ~ 0.1 °F) to 0.1 K (0.1 °C ~ heating source generally uses quartz
0.2 °F), so the images must be as free of tungsten lamps, whereas pulsed heating
noise as possible to reveal any uses xenon flash lamps. The type of
discontinuities in the images. heating used depends on the material. For
soft steel, 6.0 mm (0.25 in.) thick, xenon
The thermal image processing work flash lamps or quartz heaters can be used.
station consists of the following. For material thicker than 6.0 mm (0.25
in.), quartz heaters are recommended.
1. A high resolution infrared camera has
600 optical lines in a 30 degree field of Depending on the thickness and
view. diffusivity of the steel to be inspected,
either the pulsed or the step heating
2. A spectral hood contains the stimulus is placed within a specially
illumination source and serves as the designed enclosure to provide uniform
infrared camera mount. heat delivery to the inspection surface.
\-\1ith a field of view up to 20 degrees
3. A thermal image processing computer vertical by 30 degrees horizontal, that is,
system performs multitasking. 0.36 x 0.53 m (14 x 21 in.) for 1.0 mm
(0.04 in.) resolution at about 1 m (3 ft}
The \York station includes the infrared distance. The infrared imager is
camera interface with an inline frame synchronized with the initiation of the
buffer that allnws fast infrared image heat stimulus. A programmable time
acquisition. The thermal image processing sequence of data can then be collected
computer also contains image display and
analysis, as well as the ability to archive FIGURE 14. Visible light image of corroded side of tank
data, produce hard copy printouts and to sample.
connect to a network.

The size of the area being tested, as
well as the smallest resolvable
discontinuity, is a function of the infrared
camera. The infrared camera has a
30-degree horizontal field of view. This
makes it possible to resolve millimeter
sized discontinuities while imaging a
0.174 m 2 (20 x 13.5 in.) area. This spatial
resolution is necessary because many
corrosion discontinuities are 1 to 1.0 mm
(0.04 to 0.4 in.) in diameter.

The actual testing time is a few seconds
or less. Generally, 30 frames of infrared
image data are acquired. The image data
can then be put in a continuous loop
mode and displayed in an imagE;> window.
The discontinuities show up as hot spots
on a cooler background. These data can
then be image processed to reduce noise,
correct for uneven heating and calculate
absolute temperatures.

This thermal image processing software
also provides filters with 3 x 3 to 9 x 9
sized kernels. After running one of these

588 Infrared and Thermal Testing

and stored as individual frames. ngure 15 Pulsed infrared imaging provides heat
shows a series of thermal images from the by means of a pulse and dynamically
floor of a petroleum storage tank. collects infrared images of the material
FIGURE 15. Thermography of steel plate from surface. Heat intensity and duration
petroleum storage tank floor: (a) unfiltered depend on the thermal characteristics of
thermographic image; (b) filtered image; the material. For the test to be successful
(c) early thermal inertia; (d) late thermal the heat supplied to the top surface must
inertia. penetrate and conduct through the
(a) material to the bottom surface. This
change tlT of heat to the bottom surface
(b) needs to be several degree~ for good
infrared contrast. The infrared image
(c) acquisition timing also depends on the
heat penetration time. As a consequence,
a thicker piece of steel would require a
longer heating pulse and longer infrared
acquisition times.

The stored temperature data (thermal
images) are 12~bit numbers; therefore, the
data can then be recalled from a number
of data storage devices. Each image can be
analyzed with different temperature
scales, gains and sensitivities. This
technique is sufficient to locate the areas
of material thickness los~ and to obtain a
rousll indication of severity. Because the
data are collected in near real time, a
rough estimate of material thickness can
be determined in the time it takes for the
indication to appear. There is a given time
interval during which the heat is
conducted from the warmer inspection
surface to the cooler back surface. If an
area has a reduced thickness as a result of
corrosion or erosion, it will take less time
for the heat to reach that back surface
than an adjacent full thickness area and it
will take that area longer to equilibrate
than the surrounding area. This difference
in tllermal impedance will cause the surface
directly above this affected area to have a
higl1er surface temperature, which is
detected by the infrared imaging system.

Applications

Because the loss in material thickness is

determined by a difference in surface

temperature, it becomes crucial that the

thermal stimulus be as uniform for the

inspection area as possible. Uniformity in

the heat delivery to the inspection surface

(d) will contribute to the measurement
sensitivity or discontinuity delectability. If

the temperature difference between a

good area and one that has been reduced

in thickness is only ±0.2 K (±0.2 "'C;

±0.36 °F) and if the uniformity of the heat

source is only within ±4 K (±4 oc; ±7.2 °F)

over the area, it becomes obvious that

either bad areas will be overlooked or

good <treas \Vill be falsely labeled as bad.

One of the main parameter~ that needs

to be considered when performing this

in~pection is compensating for emissivity

difference~. Surface emissivity can cause

miscalculation of material thickness based

Chemical and Petroleum Applications of Infrared and Thermal Testing 589

on temperature differences or when 4. The system works well with most tank
plotting the temperature change versus coatings.
time. Half of the inspection surface is
painted with a high emissivity coating 5. No ionizing radiation or toxic
that bisects the machined region on the chemicals are needed.
rear surface. Emissivity can reduce the
amount of heat absorbed by the surface as 6. Hard copy and digital data archiving
welJ as reduce the infrared signal are imnlediatdy available.
amplitude for the same thickness when
compared to a high emissivity surface. 7. The similarity of infrared images to
Also, when imaging a surface with low X-ray images makes indications easier
emissivity, large reflections from the heat to interpret.
source can conceal the corrosion
indications. Much of the heat source error Pulsed infrared imaging has shown
can be eliminated by using a subtraction tremendous potential for detecting
algorithm. material Joss due to corrosion in many
grades of steeL This technology has
The infrared image~ are collected in continued to find applications in
frames in a time sequence. The nondestructive testing and provide its
discontinuities show up in the time users with fast, safe and effective results.
sequence according to their depth. If
there were two areas with corrosion, one The success of this technique relies on
deeper than the other, the deeper pit (the an infrared imaging system with high
one with the most material loss) would spatial and temperature resolution, pulsed
show up in an earlier image frame than and stepped heat sources and a highly
the less corroded area. Because of this uniform thermal stimulus system to
time history relationship, it is possible to maintain the maximum discontinuity
calculate material loss. sensitivity. Corrections can be made for
differences in surface emissivity to locate
Under laboratory conditions, steel areas of material loss and to quantify the
(< 0.1 in) plates with 10 percent material actual remaining material thickness.
loss will form a surface temperature
gradient that can be detected by the
infrared imager. For plates from 2.5 to
25 mm (0.1 to 1.0 in.) thick, areas with
25 percent material loss have been
identified.

A round robin test was performed on
various tank floor configurations using a
variety of technologies. Thermographic
indications have been verified with both
magnetic flux leakage testing and B-scan
ultrasonic testing. This indication
represents more than 60 percent material
loss. Experimen.tal testing has shown that
as low as 10 percent material loss can he
located with pulsed infrared imaging.

Conclusion

Pulsed thermography has been developed
to locate areas of material loss in various
grades and thicknesses of steeL A fully
integrated infrared work station has been
developed that includes an infrared
imager and analysis software for collecting
and scmtinizing the temperature data.

Infrared thermography offers
significant advantages.

1. Thermography is an optical technique,
so no contact with the device is
necessary.

2. Large areas can be tested quickly,
including over welded joints. Testing
time is generally several seconds.

3. The test area need not be smooth or
rust free.

590 Infrared and Thermal Testing

PART 4. Radiometry of Polymer Film

Radiometric Process polypropylene (Fig. 16) and polyethylene
Control of Plastic terephtlmlate (Figs. 17 and 18).
Extrusions Polypropylene is a polymer of propylene.
The images are taken at different
Recent advances in infrared technology, conditions of mass rate (U0), drawing
such as the development of focal plane (Ut·U01), cooling and takeup distance L.
array systems, have provided better image Different material behaviors are evident.
resolution and interface friendly systems
while reducing system size and weight. At \".'ishing quantitative measurements,
the same time, interest in using infrared the semitransparent polymers behavior in
cameras as predictive maintenance and
process monitoring for plastic industry is FIGURE 16. Polypropylene film casting
continuously increasing: hat spots can be extrusions: (a) L =150 mm (6.0 in.), U0 =
easily identified and repairs can be set up1 4.5 mm·s-1 (0.27 m·min-1), UL = 392 mm.y·1
reducing equipment failure and avoiding (23.5 m·min-1); (b) L = 150 mm (6.0 in.),
costly production downtime. u,U0 =
Furthermore, the ability of recording both
thermal and visible light images of targets 392
can help to highlight problems during 4.5 mm·s-1 (0.27 m·min-1), = = mm
scheduled shutdowns. mm·s-1 (23.5 m·min-1); (c) L 50

Taking into account their short (2.0 in.), U0 = 9 mm·s-1 (0.54 m·min-1),
response time and their ability to perform UL =50 mm·s-1 (3 m·min-1). Lis takeup
noncontact temperature measurement,
the control of the temperature by means distance (mm) and U0 is mass rate.
of infrared radiometers seems to be
particularly appropriate in operations (a)
involving molten polymers in free air,
such as polymer extrusion. (b)

Extrusion is a technique ·widely used in (c)
industry to process plastics. In extrusion,
plastic is first heated above its glass
transition temperature so that it will flow.
It is then forced under high pressure
through a slit die and is slightly stretched
in the machine direction in air. Finally it
finds a path in free air in the region
between die exit to chill roll. The process
can result either in a casting or a bubble
film. The rottte in air of the material from
the die to the chill roB is essential in
determining the fmal film properties. In
fact strong variations in width, thickness
and temperature take place in a short
time, while the material solidifies. Thus,
the knowledge of temperature
development along the path in free air is
a key parameter in controlling both the
working process and the quality of the
final productP·26

Monitoring with infrared sensors for
process control of these materials is
documented in the literatureP·3>~ As an
example of qualitative process
monitoring, unprocessed raw images have
been collected for film casting (Figs. 16
and 17) and bubble extrusions (Fig. 18) of
thermoplastic polymers, namely

Chemical and Petroleum Applications of Infrared and Thermal Testing 591

the hands usually sensed by available FbuIu,G.ubU-bR1El0e1e~8x.t1rP0uo;sli(yocen)tsh:uy,(l.aeun)-e1u,t,e.~ure-1p15,h;t~ha5l;a(teb)
radiometers has to be taken into account.
Polymers exhibit a peak in the absorption (d) Ut·U-10 == 20. U0 is mass rate, UL·Uo1 is
band centered at 3.4 pm for drawing and L is tal<eup distance (mm).
carbon-to-hydrogen bonding, such as
polystyrene, and at 7.95 pm for the (a)
carbon-to-oxygen bonding, sulh as
polyester.:ts In Fig. 19, the slab

transmittance spectrum is reported for
polyethylene terephthalate and
polypropylene slabs, respectively. For such
materials and for typical film thickness,

the temperature readout cannot be
performed by automatic emissivity·
correction mode, based on the
assumption that the target behaves as a
graybody, and new ways are sought for

temperature deteelion. Temperature

Figure 17. Polyethylene terephthalate film

casting extrusions: (a) L ~ 150 mm (6.0 in.), (b)
4.5 mm·s-1 (0.27 m·min-1), U,
U0 ~ mm·s-1 (23.5 m·min-1); (b) L ~ ~
392
150
4.5 mm·s-1 (0.27
mm (6.0 in.), U0 ~ mm·s-1 (3 m·min-1); (c) L
m·min-1), U,
~50
~50 mm (2.0 in.), U0 ~ 9 mm·s-1 (0.54
m·min-1), UL ==50 mm·s-1 (3 m·min-1). Lis

takeup distance (mm) and U0 is mass rate.

(a)

(c)
(b)

(d)
(c)

592 Infrared and Thermal Testing

readout for plastic film, unlike that for surrounding for which the intensities
opaque materials where radiation is a incident on both faces are known.
superficial phenomenon, must take into A radiometer views the slab at a fixed
account that a portion of the radiation angle to the surface.
comes from the bulk and is related to the
inner temperature distribution. Radiation Field inside Slab

To perform significant temperature According to the local radiant energy
readout, the present work deals with a balance equation in the direction (f), the
simple model setup for the radiation field radiation behavior of the slab material in
in a plane plastic filmi the model
establishes key parameters influencing the terms of spectral intensity h.w is described
temperature readout for different
materials and geometrical configurations, by two intensive pammeters: the spectral
depending on the instrument in use. absorption coefficient K1. and the
refractive index 111. both weakly
Radiation Field dependent on temperature. Here they are
assumed to be temperature independent.
A slab with a known temperature
distribution, T = T(x), is considered; the The formal solution to the local energy
slab is immersed in air and confined by a balance can be conveniently obtained
with respect to the forward and backward
FIGURE 19. Typical transmittance curves: intensity components:]6 it is expressed in
(a) for ester band; (b) for carbon hydrogen terms of the local temperature
band. L is takeup distance. distribution, the media refractlve index,

(a) slab thickness L and optical thickness Sv

) I I ~ L~lJ-lm where S1. = L·Kf1•

Loo lOpm (,~, Radiation Boundary Conditions

rA ~~~~\ Proper radiation boundary conditions will
make it possible to determine the
''I interface forward and backward
intensities. For a transparent interface, the
0 10 boundary condition has to take into
0 account both the radiation transmitted
Wavelength A(1-Jm) through the interface inside the medium
(b) and the radiation reflected internally from
f~\L~l"m the medium.

~ wI For the problems under study, that is,
the process and the wavelength of
L=lOpm I interest, the surfaces of the slab can be
assumed to be optica1ly smooth. For such
~ L/ a problem, the radiation interface
phenomena are described by the Snell and
' I)' Fresnel relations in terms of the ratio of
the complex refractive indices of the two
\ 10 media. The two media can be considered
dielectric ones and the interface relations
l~ are simplified, depending only on the
ratio of the two refractive indices.
0
0 The enerm' interface phenomena can
be expressed in terms of the spectral
Wavelength 'A (1-lm) directional transmissivity t 1.1,) or
reflectivity Pi.<•J = 1 - t 1.w defined with
reference to the radiation energy balance
at the interface. Under the above
assumptions, a simple relationship stands
between the spectral reflectivity and the
ratio of the refraction index of the two
media (the slab material and the
surrounding air).

Hot Slab and Cold Slab

Once the radiation boundary conditions
<lfe known, the radiation field in the slab
and the radiation leaving the slab can be
obtained.

The problem being linear, the radiation
field can be sought as resulting from the
sum of two simpler problems: (1) the llol

Chemical and Petroleum Applications of Infrared and Thermal Testing 593

slab, in which the radiation is due only to The curves presented in I~ig. 21 reveal
emission, and (2) the cold slab, in which that, for optically thick limit ~L----7c-o, the
the radiation is due to only external radiation phenomenon turns out to he a
forward aud backward radiation. The surface one and absorptance decreases as
radiation leaving the hot slab is expressed the incidence angle increases:
in terms of the spectral directional
transmissivity, the ratio of the refractive (6) 1 - Pt.(•)
index, the optical thickness and the local
temperature distribution. For the optically thin limit, the behavior
is the opposite because the beam
The intensity leaving the cold slab is transmittance effect is greater than the
due to the transmitted and backward reflection interface effect.
reflected contribution; it can he expressed
in terms of the spectral directional slab Isothermal Slab
transmittance lAw and spectral directional
slab reflectance r,,w An overall energy Consider a slab at known uniform
balance relates the spectral directional temperature Tslab' immersed in air and
absorptance a1.w to the transmittance and confined by a stirrounding ·with a knmvn
reflectance. radiation field. ln this case the hot slab
solution simplifies and the radiation field
As an example, in Figs. 20 and 21, the can be easily obtained in closed form. The
spectral slab absorptance and spectral slab intensity leaving the slab because of
transmittance are reported versus optical emission can be obtained in terms of the
thickness, for n,:ni~air = 1.72, typical value spectral directional slab emittance c1.w As
for plastic corresponding to PH = 0.07. expected, the spectral directional
The plots of Fig. 20 show that, for emittance is not revealed to be an extra
increasing value of the optical thickness, radiation bulk parameter to specify
the absorptance increases to constant because it is equal to the spectral
directional absorptance.
value, au = 1 - Pu. and the transmittance
decreases to zero; for an optically thin Basic Spectral Functions
limit, ~t---70, the absorptance vanishes and
the transmittance goes to: As observed before, for the isothermal slab
the spectral directional radiation response
(5) tu 1 - PH is completely described by two functions:
+ P;u

FIGURE 20. Spectral normal absorptance and FIGURE 21. Spectral directional absorptance
transmittance for cold slab. for cold slab.

g0' 1 ~ra - - - - - - - - - - 0'

\c:! 6·;o ~ 1 - r u - - - - - - - - -

Be- ~ euc~ e oo- 60 degree
= 0 degrees
0
8
~ .r0o e = 80 degree.

.0 .-r;0;o;

ro "'c
.r0o
-;;; •0t

"§' ~

c0 '0
~
~
t t
5 0 5
a~_ a~_ 0

~0 ~
0

Spectral optical thickness (K).L) Spectral optical thickness (K1.L)

legend Legend

ou = spectral normal absorptance l = slab thickness (mm)
L = slab thickness (mm) K). = spectra! absorptivity coefficient
Pu =spectral normal reflectivity
Kt. = spectral absorptivity coefficient
Pi.l = spectral normal reflectivity 8 = angle of incidence
th = spectral normal transmiltance

594 Infrared and Thermal Testing

slab emittance and slab transmittance, maximum spectral intensity of the
·which turn out to depend on the spectral blackbody at Tslah· The plots reveal that, at
radiation parameters of the slab material low thickness, the emitted intensity
and the slab thickness- that is, spectral markedly resembles the fingertip of the
relative refractive index 11).·1f).1,a1r and spectral absorption coefficient; this effect
tends to vanish for increasing thickness,
spectral optical thickness St· that is, approaching the opaque slab limit.

It can be shown that, for high values Nonisothermal Slab
of the thickness measured with respect to
the absorption length, L·KJ. > 1, the Once the temperature distribution across
emittance attains constant value, the slab is known, the spectral radiation
recovering the limit behavior of optically field due to emission can be calculated
thick slab; whereas for L·K,. << 1, the but not in the close form as for the
optically thin approximation is recovered: isothermal slab. This limitation can be
the slab absorbs and emits little so the removed if the blackbody planck function
emittance tends to vanish. For increasing is approximated by a power series.
values of absorption coefficient, the
curves are shifted on the left, the optically Below, the technique is applied to a
thick limit being recovered for lower slab supposed to exhibit a parabolic
values of thickness.
FIGURE 22. Radiosity (exitance): (a) for
Semitransparent Slab kmax·k-;i.rn = 100; (b) for kmax·k~rn = 10.

The absorption spectrum for (a)
polypropylene is typical for most polymer
characterized by the carbon-hydrogen: its 1 - Pu = 0.93 - - - -/-~~"-'
main feature is a peak around 3.43 pm. To
simplify the description that follows and L=10mm(0.40in.)
to evidence the relevant aspect to which
the semitransparent slab behavior is ~
related, the absorption spectrum is
characterized with a single absorption ~
band, described by a gaussian curve, %> l = 1.0 mm 0.04 in.)
centered at I'D with wavelength
amplitude "'A.: -~

"~!i_y_ -· c

(7) Kmin ~

where Kmin is the absorption coefficient -"c l = 0.5 mm (0.02 in.)
far from the absorption band and Kmax is
the maximum value attained inside the 0
band. The foJJowing values are assumed: -~
Kmin = 1.0 mm-1, 1'0 = 3.5 pm, ~!.A = 0.2 ij
IJm. This choice makes it possible to ro
consider a variable peak intensity: the
absorption coefficient ratio Kmax·Kn\1n "'
remains the only free parameter to L=O.l mm(0.004in.)
describe the radiation behavior of the slab
material. For the polymer refraction 0 7
index, its variation in the region around 1
the peak is limited, so a constant value is Wavelength), (J.lm)
assumed: n}:llJ.!alr = 1.72, ·which leads to (b)
PH= 0.07.
L= 10 mm (0.40 in.)
For the above assumptions the leaving l = 1.0 mm (O.Q4 in.)
intensity turns out to depend on the slab
and ambient temperature, the absorption L= 0.5 mm (0.02 in.)
coefficient ratio Kmax·Kj"1~1n and the slab
thickness; this dependence is. shown in L = 0.1 mm (0.004 in.)
Fig. 22, for different thickness and two
slab materials, Kmax·Kn~1n = 100 and 7
Kmax·Kn!1n = 10, respectively. For sake of
simplicity, a slab at high temperature, Wavelength ), (J.lm)
such that 1~mb·Tsl~b---70, is considered,
'iNith Tslah = 473 K (200 °C = 392 °F); the
spectral intensities are reported in
dimensionless form with respect to the

Chemical and Petroleum Applications of Infrared and Thermal Testing 595

temperature distribution: region: the radiation impinging on the
radiometer is the same as that leaving the
* l -(8) 'f(J target. The radiation stimulates the
Tmax ~ To detector according to the radiometer
spectral transfer function, that is, the
with T0 ~ 373 K (100 oc ~ 212 Of') and spectral transmittance of the optical
system and the spectral detector
Tmax::: 413 K (140 °C::: 284 °F); the sensitivity. In the following, for
corresponding planck function convenience and without loss of
distribution across the slab results in a generality, an ideal radiometer will be
known function, whose shape depends on considered, that is, one for which the
the wavelength and on the ratio transfer function assumes a constant value
fbt.,max·fbt~,o (the maximum to minimum in the radiometer sensitivity band. The
blackbody intensity inside the slab), To fix response of two different radiometers will
the shape, the reference wavelength is be considered: a short wave infrared
chosen as')..~= 5 )..1111. Then, the planck radiometer with sensitivity band between
function can be approximated by a 2 and 5 pm and a long wave infrared
second order power series. radiometer with sensitivity hand between
8 and 12 pm.
It is easy to calculate the radiation field
and hence tlle radiation leaving the slab. Isothermal Semitransparent Slab

For any value of the slab's optical For an isothermal slab embedded in air
thickness, the radiation emitted from the and confined by a surrounding at known
slab is the same as that from an temperature Tamb the slab radiosity
isothermal slab whose temperature is measured by a radiometer in its sensitivity
contained within the minimum wall and band ht.,w is a combination of radiation
maximum temperature. \.Yithin the limit emitted from, reflected from and
of a thick slab the emission approaches transmitted through the slab. The
that of an isothermal slab maintained at intensity (u,m turns out to be a known
wall temperature- that is, the slab function of ambient and slab temperature
behaves as an opaque body. In the and two parameters: the radiometer/slab
optically thin limit, the equivalent emittance e,u,w and the radiometer/slab
isothermal temperature is about 395 K absorptance a~.,t,)· Both parameters are
related not only to the material but also
(122 oc ~ 252 °F). to the radiometer sensitivity window.

For radiation leaving the slab versus The radiometer slab emittance and
the wavelength for a given absorptance are both evaluated within the
semitransparent slab, where sensitivity window of the infrared
radiometer. As expected, the radiometer
Kmax·K!1l!n = 100 and L = 0.50 mm slab emittance turns out to be not equal
to the radiometer slab absorptance; these
(0.020 in.), the spectrum shows that: for parameters, both related to the spectral
wavelengths far from the absorption slab emittance, depend on the slab and
coefficient peak, the slab behaves 1ike an ambient temperature, respectively.
optically thin slab and emits as an
In the limiting case of gray transparent
ocequivalent isothermal one at 395 K
slab, Pt.w and Kt. do not depend on
(122 = 252 °}:); in the region around wavelength. The radiometer slab
the peak, tile curve approaches the one emittance and absorptance are the same
related to the wall temperature. as the corresponding spectral functions:

Infrared Radiometer = =e,v.,m e/.(r) and a,\J.,w a/.(r} = Ct.w: as a result
Measurements
they are independent of temperature. The
Formulas have been presented to radiometer infers the slab temperature by
determine the radiation field in an slab assuming the target to be a gray opaque
with a given temperature profile across body, so a single object radiation
the thickness, once the radiation behavior parameter is needed: the radiometer
of slab and the external radiation emissivity €, reflectivity p being its
impinging are known. complement to unity. Thus, the
radiometer slab emit!ance can he used to
The following discussion focuses on set the radiometer emissivity r = r,,1..(J) and
the fundamental infrared radiometric the online temperature readout can be
problem with plastic's, that is, how to performed by the radiomett"r's built-in
infer the temperature of an isothermal software. Note, however, the meaning of
semitransparent slab and how to the term related to the ambient
characterize the case of nonisothermal contribution: for an opaque body that
semitransparent slabs. term accotmts only for the reflected
radiation whereas for the transparent slab
The air attenuation along the path ·with foreground temperature equal to
between the target and the radiometer
can be neglected because of the low
values of the optical path in the infrared

596 Infrared and Thermal Testing

background temperature, it accounts for distribution. huthennore, the spectral
hoth reflected and transmitted energy. slab emittance looses its meaning: there is
not a unique temperature related to the
The situation is quite different for a slab.Js,36
semitransparent slab, because the
radiometer slab emittance and At this point, the procedure outlined
absorptance turn out to be different. for the isothermal case can be applied
Hence the automatic emissivity correction again with reference to the first two steps
mode cannot be used and suitable to calculate the emitted radiation. The
software must used to accomplish the third step seems to f<Jil because the slab is
following tasks. not isothermal. To overcome this
difficulty, the investigator must refer tl1e
1. The software must read the target known value of the emitted radiation to
radiosity measured by the radiometer. an equivalent problem characterized by a
single temperature and strictly related to
2. The software must calculate the the problem under examination. It seems
radiometer slab absorptance for a reasonable to refer to the same
known ambient temperature and semitransparent slab, held at uniform
subtract the ambient contribution to temperature to emit the same radiation as
the target radiosity to obtain the the nonisothermal one. Therefore, the
emitted radiation. third step can still be applied to obtain an
apparent temperature, depending on the
3. The software must solve the known slab temperature distdbution.
emitted radiation with respect to the
slab temperature. The radiometer slab The apparent temperature is expected
emittance, the slab blackbody to coincide with the surface temperature
intensity and the slab temperature are for slab thickness increasing, that is,
related. approaching the opaque body limit, and
is expected to attain an opportune average
Thus, to determine the slab temperature of the slab temperature distribution for
by radiometric measurements in thickness decreasing, that is, approaching
semitransparent isothermal slabs, a single the optically thin slab limit.
basic radiometer slab function is needed:
this function ·will give the radiometer/slab The apparent temperature as measured
emittance or absorptance depending on by short wave and long wave radiometers
the selected temperature. can be drawn slab thickness for fixed
value of Kmax·Kmin-I, assuming a parabolic
The basic radiometer slab function temperature distribution inside the slab.
versus slab thickness can be drawn for J;or both radiometers, the apparent
different slab materials and temperatures. temperature recovers the surface
If the temperature is sought as the slab temperature for optically thick slabs
temperature, the radiometer slab function whereas for decreasing thickness the
meaning is related to the emitted apparent temperature increases to a value
radiation. The radiation detected by the very close to the average temperature
radiometers increases with thickness up to across the slab.
the opaque graybody limit. For the short
wave radiometer, the radiation increases For a fixed thickness, the radiation
as the absorption band peak increases, sensed by long wave radiometer comes
weakly depending on temperature. On the more from the bulk than from the surface,
contrary the long wave radiometer is and vice versa for the short wave
insensitive to the absorption band because radiometer. Thus the apparent
it looks at the slah as a gray transparent temperature readout results are higher for
body. The radiometer slab emittance turns long wave than for short wave radiometer.
out to be independent of slab This suggests that two radiometers,
temperature. The short wave radiometer sensitive in regions ·where the spectral
detects more radiation energy than the absorption coefficients markedly differ,
long wave radiometer detects. make it possible to have two distinct
temperature values related to slab
Nonisothermal Semitransparent temperature distribution; this happens if
Slab the s1ab radiation behavior is far from the
thin and thick limits.
A further problem arises when infrared
radiometry interrogates a nonisothermal Finally, in the optically thin limit, the
semitransparent slab with a temperature radiation leaving the slab and hitting the
distribution across the thickness, T = T(x). radiometer is so slight that it is difficult to
The intensity leaving the slab is still due measure radiometrically.
to ambient and slab contributions.
Conclusions
The radiation due to the ambient
assumes the same form presented for the Quantitative infrar~d thermography can
isothermal slab, because this contribution be applied to temperature measurement
is not affected by slab temperature. This is in plastic film technology but c;:ue must
not the case for the emitted radiation,
related to the inner temperature

Chemical and Petroleum Applications of Infrared and Thermal Testing 597

be taken when the materials cind
thickness are such that the film exhibits a
semitransparent radiation behavior. In
this case the radiometer temperature
readout cannot use automatic emissivHy
correction but must use suitable software
off line. The program is mainly based on
the knowledge of the slab radiation field.
The output from this field will give a
reference temperature that coincides with
the slab temperature for an isothermal
slab whereas it is an apparent temperature
for a nonisothermal slab. The apparent
temperature is a sort of average value
between the surface and bulk
temperatures/ shifted toward the surface
value as the slab behavior tends to the
opaque limit. Radiometers sensible in
diffetent spectral regions will give
different temperature readouts.

\-\'ith reference to a nonisothermal
transparent slab, the radiometer seems to
be limited, giving a single temperature.
Further information can be obtained both
by working with different sensitivity
bands (for example, with different
radiometers or spectral filters).

Looking at the target from different
directions does not provide additional
information. As shown by plots on the
interface reflectivity of plastics, the
internal cutoff angle is about 35 degrees
and two regions can be distinguished. In
the first region, the reflectivity assumes a
constant value close to the one related to
the normal direction. The emitted
radiation is affected mainly by the value
of the beam length, which in turn does
not vary appreciably with respect to the
normal one. In the second region, a
strong variation can be observed: the
surface effect predominates but the
amount of energy leaving the slab is
vanishing.

598 Infrared and Thermal Testing

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27. Acierno, D.1 L. DiMaio and Proceedi11gs Poly1ner Processi11g Sociel)'
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Orlando, FL: Academic Press (1975):

p 317-387.

600 Infrared and Thermal Testing

CHAPTER

Infrastructure and
Conservation Applications of
Infrared and Thermal Testing

Ermanno Grinzato, Consiglio Nazionale delle
Richerche, lstituto per Ia Tecnica del Freddo, Padua,
Italy (Part 5)
Hollis E. Humphries, American Society for
Nondestructive Testing, Columbus, Ohio (Part 6)
Minh Phong Luong, Ecole Polytechnique, Palaiseau,
France (Part 3)
Phillip C. McMullan, TSI Thermo Scan Inspections,
Carmel, Indiana (Part 4)
Yoshizo Okamoto, East Asia University, Shemonoseki,
japan (Part 2)
Elisabetta Rosina, Politecnico di Milano, Milan, Italy
(Part 5)
Gary). Weil, EnTech Engineering, Saint Louis, Missouri
(Part 1)

PART 1. Techniques of Infrared Therrnographic
leak Testingl

Infrared thermographic leak testing general thermal anomalies, either hotter
techniques can be accurate and cost or colder than the surrounding
effective processes for ·water, sewer, steam, background surfaces, that could indicate
petroleum, chemical and gas pipeline subsurface leaks. The infrared emission
rehabilitation programs and for locating technique is based on the measurement of
leak discontinuities in storage facilities an emission pattern of the leaking fluid
and manufacturing programs.1·5 These different from that of the background
techniques have been used to test surface using a thermal infrared imager.
petroleum transmission pipelines, The near infrared and infrared
chemical plants, water supply systems, radiometers can detect the leaked location
steam power plants, natural gas pipelines of the fluid by measuring the difference in
and sewer systems. radiation properties between the wall
surface and leaking fluid. Pressurized
Thermographic technology makes it leaking fluid exhausted from the
possible to inspect large areas, from penetrated discontinuity of the waH flows
remote distances, \Vith 100 percent over through the opening and exchange.~>
coverage. In addition, certain infrared the heat to the wall surface. The infrared
thermographic techniques can locate radiometer detects an invisible leaking
voids and erosion areas surrounding location by visualizing a small
buried pipelines, making their testing temperature difference around the leakage
capabilities unique and highly desirable. opening of a wall. This technique can be
used with portable imagers, tmck
Infrared thermographic leak testing mounted imagers or helicopter and fixed
techniques can be divided into three main wing mounted infrared imagers. The
categories: (1) infrared emission pattern decision as to whether to look for
techniques, (2) infrared absorption anomalies hotter or colder than
techniques and (3) infrared photoacoustic background is determined ·with
techniques. The first two techniques rely knowledge of the type of leak being
on using an Infrared thermographic sought, the ambient conditions and the
imager to image either the infrared energy time of day. This technique has been used
emitted by a leak and the effect it has on to investigate up to 800 km (500 mi) of
its surroundings or to absorb a specWc pipeline daily for leaks.
frequency of infrared energy. Both
techniques have the following aspects in The second category is based on the
conunon. measurement of a pattern of energy
absorption of the leaking fluid using an
1. They are accurate. infrared emitter "\\'ith specific spectral
2. They are noncontacting and bands such as those emitted from a
halogen lamp or tunable laser. The
nondestructive. absorption of specific infrared frequencies
3. They are used to inspect large areas as of the absorbed energy by the leaking
fluid is transformed into the
well as localized areas. concentration intensity pattern. The
4. They are efficient in terms of both infrared imager presents images of small
and medium sized areas to the operator,
labor and equipment. who looks for areas where the image is
5. They are economical. black or missing because of eneq,Jy
6. They are not obtrusive to the absorption. Imagers can be hand carried
or ran he carried by inspectors or
surrounding environment. mounted on trucks. This technique is
7. They do not inconvenience the specifically designed to locate leaks in a
variety of situations, such as locating
pipeline's users or the production fugitive emission leaks in chemical plants
process. or small gas leaks in manufacturing and
assembly operations.
The third technique is based on using a
laser with a specific frequency in the The third category is based on using a
infrared spectrum to cause leaking gas to tuned laser lo excite a specific leak testing
emit an acoustic signal. gas in a repetitive manufacturing process,-
such as air conditioning heat exchanger
Their differences come into play on the
types of leaks they are used for and the
auxiliary equipment used with the basic
infrared thermographic imagers.

The first category, based on infrared
emission pattern techniques, uses an
infrared imager to view large ground
surface areas and lets the operator look for

602 Infrared and Thermal Testing

testing. The excitation of the gas by. the For pipelines carrying fluids at
tuned laser causes the tracer gas to emit a temperatures above or below the ambient
specific acoustic signature that can be ground temperatures (steam, oil, liquified
picked up by nearby microphones. From gases or chemicals), an alternative is to
the information gathered, the exact use the heat sinking ability of the earth to
location of the leakage can be accurately draw heat from the pipeline under test.
determined. The crucial point to remember is that the
energy must be flowing through the
Factors Influencing ground. It doesn't matter what direction it
Radiometric Signals is going.

Effects of Subsurface Conditions Effects of Ground Cover on
on Temperature Measurement Temperature Measurement

An infrared thermographic imaging The ground cover is a second important
system measures the energy emitted from factor to consider for apparent
a ground surface only. But the temperature variations in the condition of
temperatures that are measured on the the test surfaces, caused by changes in
surface of the ground above a buried emissivity, shadowing and heat
pipeline depend on the subsurface insulation.
conditions.
The ability of a material to radiate
The subsurface configuration effects are energy is expressed as the emissivity of
based on the theory that energy cannot the material. This is defined as the ability
be stopped from flowing from warmer to of the material to release energy as
cooler areas and that it can only be compared to a perfect blackbody radiator.
slowed down by the insulating effects of This is strictly a surface property.
the material through which it flows. Emissivity normally exhibits itself in
Various types of construction materials higher values for rough surfaces and lower
have different insulating abilities. In values for smooth surfaces. For example,
addition, differing types of pipeline rough concrete may have an emissivity of
discontinuities have different insulating 0.95 whereas a shiny piece of tin foil may
values. have an emissivity of only 0.05. \\'hen
looking at large areas of ground cover, the
There are three ways of transferring engineer in charge of testing must be
energy: (1) conduction, (2) convection aware of differing surface textures caused
and (3) radiation. Good solid backfill by such things as broom roughed spots,
should have the least resistance to tire rubber tracks, oil spots, loose sand
conduction of energy and the convection and dirt on the surface and even the
effects should be negligible. The various height of grassy areas and trees.
types of problems associated '"ith soil
erosion and poor backfill surrounding Effects of Environment on
buried pipelines increase the insulating Temperature Measurement
ability of the soil by reducing the energy
conduction properties without The final system that affects the
substantially increasing the convection temperature measurement of a ground
effects. This is because dead air spaces or cover surface is the environment
voids do not allow the formation of surrounding the surface to be measured.
substantial convection currents. Some of the various parameters that affect
the surface temperature measurements are
An energy flow must start with an sunlight, clouds, ambient te-mperature,
energy source. Because buried pipeline wind and moisture on the ground.
testing caninvolve large areas, the heat
source should be both low cost and able Solar Radiation. Testing should be
to distribute heat evenly in the ground performed during times of the day or
surface above the pipeline. The sun fulfills night when the solar radiation or lack of
both of these requirements. Allowing the solar radiation would produce the most
sun to warm the ground surface above the rapid heating or cooling of the ground
pipeline areas under test will normally cover surface.
supply all the energy needed. At night,
the process may be reversed with the Cloud Cover. Clouds will absorb and
ground as the heat source and the night scatter infrared radiation. This has the
sky as the heat sink. This theory and effect of slowing the heat transft>r process
methodology works best with pipelines between sky and ground. Therefore
carrying fluids at the same ambient testing should he performed during times
temperature as_ the ground -natural gas, of little or no cloud cover to allow the
water or sewage. most efficient transfer of ene-rgy out of or
into the ground.

Ambient Temperatures. Atmospheric
temperature should have a negligible

Infrastructure and Conservation Applications of Infrared and Thermal Testing 603

effect 011 the accuracy of the testing emitted or reflected by objects is absorbed
because the important consideration is by the moisture in the atmosphere.
the rapid heating or cooling of the ground
surface. This paraineter wil1 affect the In addition, the imager's sensor is
length of time (that is, the window) normally cooled to reduce the effects of
during which high contrast temperature background heating of the infrared sensor.
measurements can be made. Normally the infrared scanner's highly
sensitive detector is cooled by liquid
Wind Speed. Wind has a definite cooling nitrogen or a mechanical stirling cooler,
effect on surface temperatures.
Measurements should be taken at wind to a temperature of 77 K (-196 oc =
speeds of Jess than 6.67 m·s-1 (IS mi·h-1).
-321 °F) and can detect temperature
Moisture on Ground. Moisture tends to variations as s1ight as 0.05 K
disperse the surface heat and mask the
temperature differences and thus the (0.05 oc = 0.1 °F).
subsurface anomalies. Tests should not he
performed while the ground has standing Alternate techniques of cooling the
water. infrared radiation detectors use
compressed gases or electric cooling.
Selection of Test Area These two coo1ing techniques may not
give the same resolution because they
Once the proper conditions are cannot bring the detector temperatures as
established for imaging, a relatively large low as liquid nitrogen or the stirling
area should be selected for calibration cooler. In addition, compressed gas
purposes. This area should encompass cylinders .may present safety problems
both good and bad pipeline areas (areas while storing or handling.
with voids, delaminations, cracks or
leaks). Each type of anomaly will display The second major component of the
a unique signature depending on the infrared imaging system is a real time
conditions present. microprocessor coupled to a display
monitor. 'A,Iith this component, cooler
.,,__ .- items being scanned are normally
represented by darker gray tones whereas
Test Equipment warmer areas are represented by lighter
gray tones. A color monitor may also be
To test ground cover for subsurface voids, installed in the monitoring system to
pipeline leaks and other types of make the images easier to understand for
anomalies, all that is really needed is a those unfamiliar \Vith interpreting
sensitive contact thermometer. In even monochrome images. The color monitor
the smallest test area, however, thousands will quantize the continuous graytone
of readings would have to be made energy images into tvw, three or more
simultaneously to outline the anomaly energy levels and assign them contrasting
precisely. This means that to inspect large visual colors representing relative thermal
areas of ground cover efficiently a high energy levels.
resolution infrared thermographic imager
is recommended. This type of equipment The third major component of the
allows entire areas to be imaged and the infrared imaging system is data
resulting data to be displayed as pictures acquisition and analysis equipment. It
with areas of differing temperatures comprises an analog-to-digital converter, a
designated by differing gray tones on a digital computer with high resolution
black and ·white image or by various color monitor and storage and analysis
colors on a color image. A wide variety of software. The computer allows the
auxiliary equipment can be used to transfer of moving instrumentation video
facilitate the data recording, referencing tape or Jive images of infrared scenes to
and interpretation. single frame computer images. The images
can then be stored individually and later
The actual imaging and analysis system retrieved for enhancement and individual
can be divided into four main subsystems. analysis. The computer allows the
The first is the infrared sensor and optics engineer in charge of testing to set
head that normally can be used ·with specific analysis standards based on
interchangeable lens. It is similar in destructive sam pie tests, such as corings,
appearance to a portable video camera. and to apply them uniformly to every
The scanner's optical system, however, is square centimeter of ground cover.
transparent only to short wave infrared Standard off-the-shelf image analysis
radiation in the spectrum field of 3.0 to programs may be used or custom written
5.6 pm or the medium wave infrared software may be developed.
spectrum field of 8 to 12 pm. These two
spectrum bands are selected because The fourth major component system
outside these ranges the thermal radiation consists of various image recording and
retrieving devices. These should be used
to record both visual and thermal images.
They may be composed of video tape
recorders, still frame film cameras with
either instant and 35· mm or larger
formats or computer printed images.

604 Infrared and Thermal Testing

All of the above equipment may be Software. Software is used for data
carried into the field or parts of it may be analysis and presentation.
left in the laboratory or office for
additional use. If all of the equipment is Data Synchronization between Data Sets.
transported to the field to al1ow The data sets include infrared
simultaneous data acquisition and thermographic data, normal image data,
analysis, it is prudent to use an reference data such as global positioning
automotive van to set up and transport system (GPS) information, meter distance
the equipment. The van should also counters and other data sources (Fig. 1).
include a technique to elevate the scanner Data synchronizati_on is critical because at
head and accompanying video camera to normal video rates of data collection,
allow scanning of the widest area possible, 60 fields per second, if separate video and
depending on the system optics used. The infrared thermographic recorders are used
equipment may also be transported by a·nd are 1 s off in synchronization, then
fixed wing aircraft or helicopters, the images can be looking at areas
depending on the length of pipeline to be hundreds of meters (a thousand feet or
inspected. more) out of synchronization, making the
data worthless.
Several manufacturers produce infrared
thermographic equipment. Each "!·.
manufacturer's equipment has .its own
strengths and weaknesses. These Pipeline Leak Testing Case
variations are in a constant state of Histories
change as each manufacturer alters and
improves equipment. Therefore, Buried Water Pipeline
equipment comparisons should be made
before purchase. In 1983, an infrared thermographic leak
and erosion void investigation was
Equipment Considerations performed on Duncan street in midtown
St. Louis. Before the inspection, crews
Items of major importance when selecting from the Metropolitan St. Louis Sewer
equipment include the following. District had observed street pavement
sinking up to 150 mm (6 in.) along a
Thermal Resolution. The smaller the 180m (600ft) long section of Duncan
better. Street. Visual inspections using both
Spatial Resolution. The smaller the better. television cameras and crawl crews had
located only three dime sized water
Field of View. Appropriate to requirements infiltration points in the 1.5 m (5 ft)
of the job. diameter se·wer located about 4.0 m (13 ft)
below the surface. Running alongside the
Data Format. Data may be collected in sewer was a pressurized water line.
analog or digital format. Analog Jets more
data be collected and stored at less cost During the thermographic
but detail information may be lost in the investigation a cool area was located
storage process. perpendicular to the buried water pipe. It
began at the \Vater line and spread
FIGURE 1. Screen data collection system developed for outward to the sewer line. It was
thermographic inspection. determined that the cooler surface area
was caused by the heat sinking ability of
Date Time the water plume as it spread from the
water line leak and flowed down the
Reference outside of the nearby sewer pipeline.
footage Some of the fresh water "\Vas entering the
counter sewer line through the three dime sized
oc holes that the crawl crew had located.
global
positioning Jn addition to the water leak, the
system infrared thermographic investigation also
located an erosion area above the water
Text box line. Evidently the water flowing from the
water pipeline to the sewer pipeline was
carrying soil that was washing away down
the sewer line. This void area had caused
some of the pavement sinking and further
street collapse was inevitable. The void
above the water line was evidenced !Jy a
warmer signature in the thermographic
image (Fig. 2).

Infrastructure a.nd Conservation Applications of Infrared and Thermal Testing 605

Buried Drain Pipeline possible voids with 100 percent coverage,
without interrupting airport traffic.
In May 1990, at an airport in New
England, the landing gear of an airplane The inspection of. over 190 000 m2
(2 000000 ft2) of pavement was
carrying a run load of passengers fell conducted by using infrared
thermographic techniques at night, nftcr
through the taxiway pavement while 23:00 when air traffic was at a minimum.
approaching its unloading gate (Fig. 3). The entire investigation took three nights
Damage to the airplane cost $500 000 and and uncovered twelve subsurface voids of
included areas of the landing gear, varying sizes, some of which could have
fuselage and fuel system leaks. caused major damage to airplanes if they
had collapsed (Fig. 4).
Upon removal of the passengers and
containment of the leaking fueC airport FIGURE 3. Airplane landing gear collapsing
authorities removed the airplane. During into taxiway void caused by drain pipe
the removal process, it was determined infiltration leakage void.
that a 1.8 m (6ft) by 1.8 m (6ft) by 2.4 m
(8 ft) deep void had formed underneath
the pavement because of leaks and
infiltration of the soil into a 40 year old
buried storm water drainage pipe. \'\'hen it
was determined that the drainage system
was located throughout the entire airport
pavement system, airport authorities and
their consultants concluded that more
drainage system leaks and erosion areas
probably existed. Airport authorities then
requested that the consultants determine
a technique of locating the leaks and

FiGURE 2. Surface images showing water FIGURE 4. leaking drain pipe at airport:
pipeline, water leakage, leakage plume and (a) visible light photograph; (b) thermogram.
void area forming above pipeline: (a) visible
light photographs; (b) infrared (a)
thermograms.

(a)

~~" ·' (b)

(b)

606 Infrared and Thermal Testing

Buried Hot Water Pipeline 3.6 m (12ft) belOw the pavement surface
in most locations. It was the beginning of
In 1986, the State of Utah used infrared winter and several large industrial
thermography to inspect the hot ·water, customers downstream of ·where the line
radiant heat system used to heat steps, crossed Seventh Street along \".'ashington
driveways and roads near the state capital Avenue complained of a laCk of capacity.
buildings in Salt Lake City, Utah. These Union Electric personnel were able to
pavements were heated during the winter localize the leak to an area between hvo
months to melt ice and snow before it manholes 78 m (256 ft) apart (Fig. 6).
could become dangerous to pedestrians
and automobile traffic. Infrared thermographic techniques
were used to locate the leak without
The 20 year old system normally digging or halting traffic on the major
worked properly but was beginning to downtown street·. The inspection was
show its age by higher than normal water pefformed from a nearby parking garage
usage, higher than normal boiler fuel bills rooftop and occurred at about 5:00p.m. It
and higher than normal quantities of took less than 10 min to locate and mark
boiler chemical additives used to reduce the pavement above what turned out to
pipe fouling. be a major leak on the bottom of a 0.30 m
(12 in.) insulated pipeline buried.3.6m
Several water leaks were detected as (12ft) below the surface. The major
warm spots on the thermographic images. signatme of the thermographic images .
Most leaks, however, were more difficult was a central hot spot and gradual coolmg
to locate because they did not start with a along the pipeline length.
hot spot and radiate in a circular pattern
from the leaks. Instead, these leaks started Buried Oil Cooled Electric Cable
with a smaller warm spot and spread out
along the pipeline for just a short In 1989, infrared thermography was used
distance. It was determined that to locate leaks in a buried 400 kV-A
significantly smaller leaks had not cracked electric cable that carried power for
the pipeline concrete encasement but 25 percent of the city of ~~ome, Italy. Due
rather had exited the water pipe and to the importance and h1gh current
traveled along the outside of the pipeline carrying capacity of this cable, it was
until they found an exfiltration point designed with a circulating oil filled
somewhere downstream in the pipe cooling system. \".'henever leaks occurred
casement. The heat dissipated quickly
because the line acted as a heat sink and ftGURE 6. Buried steam pipeline leakage,
brought the outside water to the line Saint Louis, Missouri: (a) visible light
temperature very quickly (Fig. S). photograph; (b) thermogram.

Buried Steam Pipeline (a)

In 1981, Union Electric Company, the
steam generating and distributio~1 utility
company in Saint Louis, fvfissoun, used
thermographic techniques to locate
buried steam pipeline leaks. The steam
distribution loop in downtown Saint
Louis was about 29 km (18 mi) long and

fiGURE 5. Thermogram of buried hot water (b)
pipeline grid used to melt snow and ice on
roadway pavement.

Infrastructure and Conservation Applications of Infrared and Thermal Testing 607

in the system, controls automatically shut Range of Applications
off electricity to the line, effectively
shutting down 25 percent of the power to Infrared thermography can be used to
the city of Rome. Leaks normally took up nondestructively detect buried and
to 48 h to locate and repair. aboveground discontinuities such as leaks,
cracks and subsurface erosion voids.
During the test, which was performed Infrared thermographic testing may be
at night because of high traffic volume performed during day or night, depending
during daylight hours, one leak was on environmental conditions, the
detected as evidenced by a temperature investigated problem and the desired
higher than the average ground cover results. Computer analysis of thermal
temperature (Fig. 7). This area was images greatly improves the accuracy and
brought to the attention of the speed of test interpretations.
authorities. It was confirmed that the area
located was the site of a previous oil leak. Aging chemical, oil, natural gas, water,
It was certain that the recorded images steam and sewage pipeline infrastructures
were caused by the small poo1s of leaked throughout the world are rapidly
oil. This site was determined to be the site approaching the end of their design lives.
of the active leak and contained about This will necessitate more efficient and
1 dm' (0.25 gal) of oil. cost effective techniques of testing
pipelines under load and in place.
The inspection process, including Infrared thermography is a
equipment setup, calibration and nondestructive, remote sensing technique
scanning, took about 30 min for 180 m that can meet these requirements.
(600ft) of pipeline inspected.
Additional applications not discussed
fiGURE 7. Oil leakage in buried, oil cooled here include inspection of hazardous
electrical cable, Rome, Italy: (a) visible light \Vaste containment sites7 and condition
photograph; (b) thermogram. monitoring of shotcretcd slopes. Slopes
bordering on railroad tracks are reinforced
(a) by shotcrctc in various countries. The
maintenance of such slopes is an
important infrastructure concern.s

(b)

608 Infrared and Thermal Testing

PART 2. Thermographic Modeling of leakage
through Walls

From the viewpoint of predictive controlled uniformly by regulating a heat
maintenance and safety management of exchanger and an air conditioner.
structures and plants, it is quite important
to detect the presence and location of The size of the mortar test plate is
external and/or internal leaked fluids and 300 X 180 X 50111111 (12 X 7 X 2 in.). The
their flow rate from the wall surface of
tank, pipe line, dam, canal, river bank and leakage hole of 1 mm (0.04 in.) in
so on. In particular, it is urgently diameter is drilled in the plate. The water
necessary to diagnose the leaked location is supplied from an upper tank and is
of the wall with remote sensing heated to a desired temperature by a
nondestructive testing devices and to regulation heater. The flow rate of the
repair it before the rupture takes place. leaking water is measured by a rotameter.
The measured radiation temperature is
Several application techniques using equal to the real temperature, because the
the near infrared and infrared radiometer emissivity of the surface becomes nearly
have been used to detect water leaked
from a waH: thermal tuft and wire mesh FIGURE 8. Experimental setups: (a) vertical test piece;
grid, thermal tracer, gas absorption, cell (b) horizontal test piece.
convection, bypass and permeation flow
of the internal insulation structure of (a)
high temperature and high pressure tanks
and pipelines. The technique can forecast DCathodeturbaye D Personal Water
the splay and eventual collapse of a computer tank
crevice through water's leakage and flow
undercutting a mortar layer. j@Central ~~~~="'1/"""""::''\ t

Several techniques can be applied to processing unit
detect and evaluate the leakage location
of components containing fluid.1,6,9,JO r,:=:::;:~==~---,
Test
Among these are (1) evaluation of leaking
water flow on vertical and horizontal piece Heater
wa1ls, (2) tracer gas flow on a thermal tuft
and ·wire net grid, (3) thermographic
detection of gas flow and {4) absorption
of leaking gas.

Water Leakage on Vertical (b)
and Horizontal Walls
DCathode D Personal Water
A thermal technique has been used to computer tank
evaluate the flow mechanism of the ray tube
leaking water from the wall of the banks,
tunnels and canals. Figure 8 shows an Central ~~;~;;==~f~=~=~>~~
experimental modeling apparatus for processing unit
measuring temperature and flow
distributions of the le<tking 'i\'ater flow on Heater t
a mortar ·wall.
Infrared sensor
Figure 8a illustrates a vertical mort<tr
wall. The leaking water flows downward Water
according to the balance of gravity and gage
viscous forces. Figure Sb illustrates the
horizontal nonmetal and sand walls
submerged O.OS to 0.20 m (2 to 8 in.)
deep in the water, which simulates spring
·water on the submerged mortar wall.

A polystyrene ·wall, insulated to keep
the environment temperature constant,
encloses the test plate. Temperatmes of
the test plate and ambient air are

Infrastructure and Conservation Applications of Infrared and Thermal Testing 609

unity. An infrared radiometer can be used can help evaluate the leaking flm\•
to detect leakage by changing the water behavior of water supply, pipelines and
temperature and flow ratP of the leakine- sewer systems.
·water.
Water Leakage on Horizontal Wall
Water Leakage on Vertical Wall
A horizontal wall was submerged in water
Figure 9 shows a thermography of the to simulate model tests for detecting small
leakage flow on the vertical waH. The leakage of water under the water layer.
leaked water flows downward. The Thermography of the water surface was
horizontal '\\'idth of the surface flow and used to detect the leakage. The wall was
its temperature Tare increasing with submerged in the water to a depth of 50
increase in the distance from leakage mm (2.0 in.). Hot water was injected from
hole X. the small hole on the horizontal wall.

Normalized correlation is useful to Figure 10 shows the thermography of
obtain the location of the leak point and the leaking water on the horizontal wall.
flow rate of the leakage water? Results on The temperature of the submerged water
the modeling experiment and its analysis is 298 K (2S 'C ~ 77 'F) and that of the
leaking water is 293 K (20 'C ~ 68 'F). The
FIGURE 9. Thermography of volumetric leakage of 0.5 cm3.s-1 flow rate of the water is 0.5 m L·s-1 (30
(30 cm'·min·1 ~ 0.0636 ft'·h· 1) on vertical wall: (a) leaked cm]·min-1). The thermograms are
water temperature T0 ~ 293 K(20 'C ~ 68 'F) and test plate recorded after 30 s.
temperature Tw ~ 303 K(30 'C ~ 86 'F); (b) leaked water
temperature To ~ 293 K(20 'C ~ 68 'F) and test plate It is clear from Fig. 10 that the leaking
temperature Tw ~ 308 K(35 'C ~ 95 'F). water on the horizontal wall diffuses in
nearly a square shape. The detection of
(a) the leaking flow is limited in case that the
temperature difference between the wall
and leaking water is smaller than 2 K (< 2
oc; < 4 °F). The diffusion becomes
extended with decrease in the depth of
the leakage water. tvteasuring the axial
temperature distribution by means of
infrared thermography, it is possible to
estimate the leakage flow rate and the
location of the leakage point, as in case of
the vertical wall.

The modeling technique can be applied
to locate the invisible leaking flow or
sprung puddle before rupture in the river
bank or shotcrete slope protection.

FIGURE 10. Thermogram showing water leaked from
horizontal test piece.

(b)

610 Infrared and Thermal Testing

Thermal Indication of Air leaking gap. Leakage occurs commonly in
leakage places such as (1) registers for the floor
heating, (2) clearance gaps between
In cases where the temperature of the wall sliding doors and their frames and
surface differs from that of the leaking gas (3) convection leaks at entrance door gaps
flow, heat transfer between the wall and eaves. The infrared radiometer detects
surface and leaking gas creates thermal the leak by locating the thermal
indications on the surface near the indication of gas leaking from the house.JO

FIGURE 11. Thermogram and gap flow pattern of wooden The infrared radiometer detects
door. thermal indication of the hot spot caused
by the bypass hot streak and the cell
(a) convection heat leakage in the high
pressure high temperature vessel and
Hot streaks piping with the inner insulation layer.

I Figure 11 shows a thermogram with
thermal streak indication caused by
Inside room II leakage through the wooden door gap in
winter. The thermography of the inside of
I'I'"!'"" the door represents the cold streak mark
caused by leakage at left and bottom
II! corner, through the parallel gap of 2.0 to
It J /Wooden door 3.0 mm (0.08 to 0.12 in.) between the
door and side wall. The cold streaks are
,IJ,,I thermal indications, caused by the flow of
gas through the gap from the cold
ill outside environment past the door to the
inside entrance room (Fig. lla).
\\\ Outside of room
The thermogram in Fig. 12 shows the
ill gap flow pattern at the outside of the
door. The thermography of the outside of
111 the door represents the hot gap zone
caused by gas convection leakage, visible
111 at the right corner gap of the door. The
streak mark is caused by leakage at the
111 door frame past the door from indoors to
I1 the cold outside environment.

I --<-- Figure 13 shows the thermography and
gap flow pattern of a steel sliding door by
========:'J....,._--<-- bypass airflow in winter. The radiation
Cold streaks temperature of the door increases with

FIGURE 12. Thermogram shows pattern of flow through gap
(b) as seen from outside of door.

Cold streaks

Infrastructure and Conservation Applications of Infrared and Thermal Testing 611

height, because air trapped between the two-dimensional and three-dimensional
glass window and door panels is heated temperature distributions of the invisible
and flows tlpward (Fig. 13a). gas flow from the leak can be seen by
using a portable thermal template of the
Wire Grids for Leak wire lattice and wire net.11
Location
The square wire lattice is a simple
Several fundamental visualization detecting device, a grid of parallel plmtic
techniques are used to measure wires less than 0.5 mm (0.02 in.) in
temperature distribution and thermal diameter with a constant pitch of 2.0 mm
fluid flov..• velocity in components, (0.08 in.). Because vinyl and nylon wires
including schlieren interferometry and have low thermal conductivity they are
liquid crystal thermal testing. A tuft wire used as the thermal indicator to eliminate
technique of flow visualization can be the lateral heat conduction along the
applied with gross leaking testing wire. The wire lattice marker aligned with
methods such as bubble testing. In cases the thermal leaking gas flow from the
where the temperature of the leaking gas opening of the wall can be used with an
differs from that of the ambient gas, infrared radiometer to visualize a hot spot
of two-dimensional temperature
FIGURE 13. Steel sliding door: (a) gap flow pattern; distribution.
(b) temperature gradient between panels of door;
(c) thermogram. The wire net mark is also composed of
a net or grid of parallel plastic wires,
(a) (b) similar to the square wire lattice. The

square wire lattice is placed on the testing
wall to detect the leaking point of the gas

flow. The temperature distribution of the
leaking gas gives information about the
presence and location of leaking fluid on
the ·wall.

Glass door Steel door

~ /

Outside air

Inside room

Buoyancy flow Temperature

~/

(c)

612 Infrared and Thermal Testing

PART 3. Vibrothermography of Earthquake
Resistant Structures

Current technological developments tend nondestructively and to illustrate the
toward increased exploitation of materials onset of damage process, stress
strengths and toward tackling extreme concentration and heat dissipation
loads and environmental actions such as localization in loaded zones.24 In addition,
offshore structures subject to wind and this method can be used as a
·wave Joading,IZ,B or buildings in seismic nondestructive means for evaluating the
areas.l 4 Concrete is widely used as a fatigue limit of concrete structure subject
construction material because of its high to repeated loading. This approach
strength-to-cost ratio in many represents a departure from the traditional
applications. Earthquakes and laboratory empirical approach to fatigue analysis and
tests have shown that well designed and offers promise for improved estimation of
detailed reinforced concretelS is suitable fatigue performance in complex
for earthquake resistant structures. The structures. The development of
most severe of likely earthquakes can be satisfactory design procedures not only
survived if the members are sufficiently enables a rational selection of allowable
stresses but also permits tradeoff studies
ducti1e to absorb and dissipate seismic between allowable stresses, makes possible
energy by inelastic deformations.l6 This alternative materials fabrication
procedures and provides guidance in the
requires a .designer to realistically assess selection of surveillance/maintenance
the acceptable levels of strength and to policies.
ensure adequate dissipation.17
Characteristics of Concrete
Fatigue of plain and reinforced Materials
concrete structural members has been
studied for many years to design safe Plain concrete is the most popular
stmcturesi8,I9 such as bridges, pmver engineering material, consisting of coarse
aggregates embedded in a continuous
plants, high rise buildings and other matrix of mortar- a mixture of hydraulic
engineering structures. The phenomenon binding materials, additives and
of fatigue damage20 in brittle concrete admixtures distributed in a suitable
must be critically examined, in view of homogeneous suspension. Under applied
the general perception that brittle loading, the concrete as a whole deforms
materials are not perceptible to despite significant incompatibilities
fatigue.2I, 22 Mechanisms established for between the aggregates and the matrix
fatigue crack growth in ductile metals are that promote further breakdown. At the
based on dislocation activities in the crack macroscopic level, breakdm\'n is
tip region, leading to a view that brittle accompanied by both losses in stiffness
materials are insensitive to fatigue and accumulation of irrecoverable
damage. Empirical techniques using deformations. At the structural level,
wOhler cmves (curves expressing the ratio breakdm\'Il appears as microcracking and
of stress to the number of load cycles)23 possibly as slippage at the
have traditionally been considered with aggregate-to-cement paste interfacesT'
statistical treatment of data.
The formation and propagation of
Infrared thermography offers three microcracks have been detected using well
advantages as a nondestructive, known measuring methods such as the
noncontact and real time technique: following.
(1) observation of the physical
manifestation of damage and the
mechanism of failure of concrete,
(2) detection of the occurrence of intrinsic
dissipation localization and (3) relatively
fast evaluation of the fatigue strength
through dissipative phenomena. In
addition, infrared thermography readily
describes the damage location and the
evolution of structural failure. The
investigated parameter is heat generation
because of intrinsic dissipation of concrete
subject to various and complex loadings.

Infrared thermography can be used to
study damage mechanisms

Infrastructure and Conservation Applications of Infrared and Thermal Testing 613

1. The ultrasonic pulse velocity dissipation to be a highly accurate
technique26·27 involves measurement indicator of damage manifestation and
of the transit time of an ultrasonic assumes that intrinsic dissipation and
pulse through a path of known length damage present the same evolution under
in a specimen. The velocity of the fatigue loading up to failure.
ultrasnnic pulse in a solid material will
depend on the density and the Infrared Scanner
modulus of elasticity. Ultrasound
propagation will be affected by the A scanning camera analogous to a
presence of more or less unstable television camera uses an infrared detector
in a sophisticated electronics system to
cracksP~ detect radiated energy and to convert it
into a detailed real time thermal image in
2. The acoustic emission method29 is a color and monochrome video system.
based on the principle that the Response limes are shorter than 1 ps.
formation and propagation of Temperature differences in the heat
microcracks are associated with the patterns are discernible instantly and
release of energy. \Vhen a crack forms represented by several distinct hues. The
or spreads, part of the original strain quantity lV of energy (\.Y·m-2·pm-1)
energy is dissipated in the form of emitted as infrared radiation is a function
heat, mechanical vibrations and in the of the temperature and emissivity of the
creation of new surfaces. The specimen. The higher the temperature,
mechanical vibration component can the more important the emitted energy.
be detected by acoustic techniques Differences of radiated energy correspond
and recorded. Hence microcracking to differences of temperature. Calibration
may be readily detected by studying and correction procedures have to he
sounds emitted from the materials. applied because the received radiation has
a nonlinear relationship to the object's
Stress concentrations occur and result temperature, because it can be affected by
in localized forces that are sufficient to atmosphere damping and because it
promote plasticity, elasticity or both. includes reflected radiation from object's
Damage and failure may thus be viewed surroundings. Knmving the temperature
as a microstructural process through the of the reference, the object's temperature
activation and growth of one preexisting can then be calculated with a sensitivity
discontinuity or of a site of weakness or of ±0.1 K (±0.1 'C; ±0.2 'F) at 293 K (20 'C
through the coalescence of a system of == 68 oF). The infrared scanner converts
interacting smaH discontinuities and electromagnetic thermal enerb~r radiated
growing microcracks. ~vfacroscopically it from the tested specimen into electronic
occurs as localization of intrinsic video signals. These signals are amplified
dissipation before a visible failure. The and transmitted via an interconnecting
stress level, corresponding to the cable to a display monitor where the
activation of the discontinuities, is related signals are further amplified. The resultant
to the discontinuity size and connected image is displayed on the monitor screen.
with the encompassing microstructure.
Vibrothermography of Plain
Nondestructive and noncontact tests Concrete Specimens
are thus needed to define concrete
properties (1) to establish strength taking Concrete materials present a luw
into account of a threshold of acceptable thermomechanical conversion under
damage, {2) to optimize design values and monotonic loading hut plastic
(3) to ensure quality control. deformation, whereby microcracking and
slips occur, creating permanent changes
Vibrothermography of globally or locally, is one of the most
Concrete efficient heat production m~chanisms.
!vfost of the energy required to cause such
Infrared thermography is a convenient plastic deformations is dissipated as heat.
technique for producing thermal images Such heat generation is more easily
from the invisible radiant energy emitted observed when it is produced in a fixed
from stationary or moving objects at any location by reversed applied loads. These
distance and without surface contact or in considerations dcfiJH:' vihrothermography
any way influencing the actual surface as a nondestructive and noncontact
temperature of the objects viewed. It is technique for observing the damage
successfully used as an experimental process of concrete materials.3-l In the
technique for detection of plastic laboratory, the high frequency
deformation during crack propagation of servohydraulic test machine provide~ a
steel plate under monotonous loading or means of vibration and dymunk testing
as a laboratory technique for investigating of engineering materials. A vibratory
damage or failure mechanisms occurring loading at 100Hz, applied on a specimen
in engineering materials.Z4·30·3J The work
reported here considers intrinsic

614 Infrared and Thermal Testing

(Fig. 14) subjected to increasing static Infrared thermography readily depicts
compression levels, exhibits in a intrinsic dissipation localization
nondestructive manner the irreversible announcing quite different mechanisms
plastic strain concentrations around gaps of damage preceding concrete failme. The
or cracks. The contribution of the different phases of heat dissipation,
plasticity term is revealed by the rapid operating during an unstable failure, are
evolution of heat dissipation once the readily described by heat patterns. \'\7hen
stable reversible stress domain is discontinuities or weak zones are present
exceeded, demonstrating the occurrence on the specimen, infrared observations
of an unstable crack propagation or evidence the progressive mechanism of
coalescence of discontinuities existing in discontinuity coalescence (Fig. IS). These
the concrete specimen. results have been readily extended to rock
materials,31 metals35 and various
Equation 1 describes the uniaxial composites.J3
compression aN:
Short Term Evaluation of Fatigue
(1) F Limit of Plain Concrete
So
In accordance with the coupled
Experimental results have already thermomechanical equation, the analysis
shown the following. of thermal images consists in isolating the
intrinsic dissipation from thermal noises
1. Under a vibratory excitation between by simply subtracting the thermal image
25 and 50 percent of the nominal at reference time from the thermal image
uniaxial compression aN, the heat at 1000 load cycles. Computer aid
dissipation detected for 2000 load thermography software allowed the data
cycles is smaH, even at the hottest reduction of the thermal images using the
location. function subtraction of images. The
resulting image is a subtracted image
2. V\'hen 0.50 s a·aN1 s 0.75, stress shmving the tempNature cl1ange between
concentrations around cracks or two compared images, obtained under
discontinuities are readily detected at nearly identical test conditions. This
the 1OOOth load cycle.
FIGURE 15. Infrared thermography of
3. For 0.63 s; cr·crNl s 0.88, cracking concrete specimen under vibratory
compressive excitation (0.63 to 0.88 percent
occurs increasingly in the reduced of nominal uniaxial compression). Weakness
section of the specimen. zones are readily detected by heat patterns
after 7000 load cycles, where scale
FIGURE 14. Concrete specimen under represents 0.2 K(0.2 'C = 0.36 'F) per hue.
vibratory compressive excitation at 100 Hz.

t±M 100Hz

1.6 1'1

legend oA ' If

F = frequency (Hz) 0

,v = frequency change (Hz) legend
AT"' temperature change (kelvin, where
50 = area (m 1)
o- = stress or compression 1 K = 1 <-c = 1.8 "F)

oN= nominal uniaxial compression (at maximum) W = whHe
Y =yellow
R =red
G =green
B = blue

Infrastructure and Conservation Applications of Infrared and Thermal Testing 615

image processing provided quantitative develop. The most severe of likely
values of intrinsic dissipation. earthquakes can be survived if the
members are sufficiently ductile to absorb
This procedure is applied for ('<lch load and dissipate seismic energy by inelastic
step. The manifestation of the fatigue deformations with little decrease in
damage mechanism is revealed by a break strength.
of the intrinsic dissipation regime. The
starting load level must be chosen below Under quasiseismic loading, simulated
the fatigue limit. It significantly depends by a rotating mass exciter placed on the
on concrete characteristics. For example, top of the building, plastic hinges form
the test is started at a stress level of a!Jout progressively at the column bases where
20 percent of failure nominal stress, then heat dissipation can be observed using
is increased to 30 percent, 40 percent and infrared thermography as a function of
higher until the temperature rises to a the number of load cycles.
anticipated threshold. For each load step,
an averaging treatment (among 4, 8, 16 or Computer aided thermographic
32 thermograms) provides more stable sofhvare allows the data reduction of the
thermal images. thermal images by using the function
subtraction of images and shows the
Experimental results confirm that the progressive evolution of heat dissipation
fatigue limit can be expressed by a at a column base before crack line
graphical procedure. The threshold of becomes visible. The resulting image is a
critical thermal dissipation is roughly the subtracted image showing the
same for different chosen number of load temperature change between the reference
cycles. It roughly corresponds to the value time and after N load cycles. This image
deduced from standard procedures. These processing provided quantitative values of
experiments have shown that the infrared intrinsic dissipation corresponding to
thermographic technique can provide the different numbers of load cycle~. Results
fatigue limit of concrete within a few obtained after 270 load cycles and after
hours instead of several months when 360 load cycles lead to an intrinsic
using for instance the standard staircase dissipation plot that permits evaluation of
technique. These results are consistent fatigue lifetime of a column base of the
with those obtained on concrete prisms experimental reinforced concrete structure
subject to compressive fatigue testingY• subject to simulated seismic loading.
The limit so determined could also be
understood as a threshold of acceptable Energy Dissipation in Concrete
damage for concrete. Structures Subjected to Shaking
Table Loading
Thermography of Earthquake
Resistant Concrete Structure Load bearing walls in reinforced concrete
structures are of common usc in France.
The damaged areas are located and i\-\1 thin the framework of the ECOEST2
highlighted by heat patterns. These results (European Consortium of Earthquake
support and validate the assumptions to Shaking Tables) and ICONS (Innovative
he taken into consideration in numerical Seismic Design New and Existing
procedures for stability assessment of Structures, Topic 5 Shear wall structures)
concrete structures. The research project supported by the
phenomenological behavior in European Commission, t\vo large
consideration is therefore the standard of specimens of 36 t (79 000 Ibm) each are
reference, permitting techniques and used to simulate one-third scaled
results of continuum mechanics for five-story buildings. Referred to by their
analyzing and modeling their engineering program name, the model buildings are
performance. Information about the called Conception et Analyse de Murs
location and significance of structural sous Seismes Ill and IV (CAMUS II I and
discontinuities as a basis for maintenance CA1vfUS IV) and have been tested under
decisions, including the extreme case of dynamic seismic-like loading on the
removal from service, can be obtained major shaking table Azalee at the
through inspection and nondestructive
evaluation. Commissariat aI'Energie Atomique Saclay

The proposed infrared thermographic (French Atomic Energy Agency, Saclay,
procedure involves careful examination of France). The loading input signal is an
areas where discontinuities are most likely artificial accelerogram (far field
to occur. Analyzing the structure and the earthquake) characterized by its peak
service histories of similar structures in ground acceleration {PGA) values. The~e
similar environments can identify the performed mockup tests aim to
critical areas. The application of infrared demonstrate the major influence of
scanning to inspection of concrete boundary conditions at the base of the
structure relies on the fact that the energy model and the feasibility of optimizing
is dissipated during accumulative damage low ratio and adequate distribution of
when internal cracks or discontinuities reinforcements to obtain multicracking
zones {multifuse concept) in opposition

616 Infrared and Thermal Testing

with the traditional pseudoplastic hinge internal parameter. Two supplementary
localized at the base of a steel reinforced terms can represent the cross coupling
wall (monofuse concept). effects;18 the former is caused by the
dependence of the stress tensor on
Test of Reinforcement Ratios temperature (reversilJie) whereas the latter
is induced by the same dependence of the
The CAMUS III specimen, composed of generalized force conjugates to the
two lightly reinforced walls anchored to internal state vector (irreversible). This
the shaking table, is designed according to phenomenon appeared on the concrete
a European code that allows a plastic surface with a delay depending on the
hinge at the base. Special attention has depth of reinforcements. Thus infrared
been paid on the influence of different thermography readily evidenced and
reinforcement ratios and boundary localized, on the scanned wall surface, the
conditions.37 During the performed tests plasticity of steel reinforcements with a
the CAMUS Ill mockup suffered from delay because of heat conduction
high damage levels. Its behavior was characteristics of concrete (Fig. 17).
mostly conditioned by its flexural
bending. Examination after tests Test of Boundary Conditions
evidenced failure of steel reinforcements
(Fig. 16). The CAMUS IV specimen, composed of
two lightly reinforced walls, was designed
In this case, the dissipation mechanism according to recommendations39 and
caused by plasticity of steel simply rested on a 400 mm (16 in.) thick
reinforcements can be considered as an

fiGURE 16. Conception et Analyse de Murs FIGURE 17. Infrared thermographic detection
of dissipation caused by plasticity of steel
sous Seismes (CAMUS) Ill mockup_on reinforcements: (a) heat image of
Conception et Analyse de Murs sous S€ismes
shaking table of Commissariat a I'Energie (CAMUS) Ill recorded 3 min after testing at
peak ground acceleration = 7.85 m·s-2
Atomique, Saclay, France. (0.8 G); (b) CAMUS Ill recorded 3 min after
testing at peak ground acceleration =
8.34 m·s-2 (0.85 G).

(a)

(b)

Infrastructure and Conservation Applications of Infrared and Thermal Testing 617

sand layer -(Fig. 18). This test aimed to it is expected that soft boundary
reproduce the phenomenon of uplift and conditions \Viii determine the seismic
the fact that such a nonlinear behavior of structural walls.
phenomenon could isolate the structure
from ground borne excitation. In this case As in the above case, infrared
thermography evidenced friction between
FIGURE 18. Conception et Analyse de Murs steel reinforcements and concrete matrix
sous Seismes (CAMUS) IV mockup resting with a delay necessitated by heat
on fine sand layer subjected to far field conduction through the concrete layer
earthquake (artificial accelerogram). (Fig. 19).

Dissipative Mechanisms and Their
Range of Temperature Changes

Experimental results showed that the
discrimination of the involved dissipative
mechanisms is very delicate. This work,
originally intended to validate diverse
different dissipative mechanisms,
provided the interesting discriminative
characteristics of temperature changes
(Table 1).

Concluding Remarks

This work has demonstrated that the
dissipalivity of the tested materials under
loading is a highly sensitive and accurate
mmzi(estation ufdamage.

Owing to the thermom~chanical
coupling, infrared thermography provides
a nondestructive, noncontact and real
time test to observe the physical process
of concrete degradation and to detect the
occurrence of its intrinsic dissipation.
Thus it readily provides a measure of the
material damage and makes it possible- to
define a limit of acceptable damage or
fatigue limit of concrete under load

FIGURE 19. Infrared thermographic detection of dissipation caused by slippage of steel
reinforcements embedded in concrete matrix: (a) before seismic test; (b) after peak ground
acceleration of 10.79 m·s-2 (1.1 G) during 30 s on shaking table.

(a) (b)

618 Infrared and Thermal Testing

TABlE 1. Magnitude order of temperatu~e change. Temperature Range I'!T Time Delay
(s)
Dissipative Mechanisms K or <rC eF)
in realtime
Plasticity of concrete under compression 10 (20) some minutes of delay
Plasticity of steel reinforcements tens of minutes of delay
Slippage between steel reinforcements and concrete matrix 1to10 (2to20)

0.1 to 1.0 (0.2 to 2.0)

beyond which the material is susceptible
to failure.

The technique not only permits
qualitative work such as finding
discontinuities or \Veakness zones but also
permits quantitative analysis of the effects
of discontinuities on strength and
durability of concrete structural
components. This useful and promising
technique offers an accurate illu.stration of
crack initiation and readily detects the
onset of its unstable propagation through
the material and/or discontinuity
coalescence when cyclic loading generates
increasing irreversible microcracking.

The main interest of this energy
approach is to unify microscopic and
macroscopic test data. The parameter
intrinsic dissipation under consideration is
a scalar quantity, easy to evaluate with
accuracy. Subsequently it may suggest
multiaxial design criteria, highly relevant
for full scale testing on engineering
structures.

Mechanical test data generated under
noncyclic conditions are insufficient to
provide a comprehensive insight into the
damage development in brittle concrete
under cyclic loading. Design procedures
ignoring fatigue phenomena may be
seriously dysfunctional if the concrete
structures are loaded cyclically.

Infrastructure and Conservation Applications of Infrared and Thermal Testing 619

PART 4. Inspection of Thermal Envelopes of New
Buildings40,41

An extremely valuable inspection tool in easily control the timing of the
the inspection of buildings for thermal inspection, thereby allowing for more
performance is the infrared scanner. ideal conditions during which to conduct
Assessing the heat loss and gain the inspection. However, if the building is
characteristics of buildings in a field occupied (as is most often the case) the
application can be difficult because of the occupants must be accommodated. A visit
wide range of varying construction before inspection is often helpful to
practices. An infrared scanner can assist in determine the nature of obstacles such as
this assessment by providing a means for furniture, equipment or inventory that
a whole building approach to ·building may prohibit full access to the building
inspections. envelope for inspection.

Depending on the type of building \".'ith both new and existing
being inspected, both the thermal and, inspections, the thermographer should
where applicable, the structural have access to the plans and specifications
performance of the building envelope for the building and have freedom to
materials can be quantified. However, for move throughout the building and
the procedure to be useful, the assessment construction site. Before conducting the
process must be relatively inexpensive. infrared inspection, the thermographer
Infrared scanners find application in should familiarize himself with the plans
building envelope inspections and explore and specifications as they relate to the
different building configurations where building envelope. Careful documentation
thermography provides an inexpensive of areas being inspected is necessary.
assessment procedure for identifying
thermal and structural characteristics of Following are the basic criteria to
the buildings.42 satisfy before a building envelope is ready
for inspection.
Inspection Technique
Complete Building Envelope
The primary purpose of any construction
inspection is to ensure that the intent of All building envelope components that
the designer and owner has been fulfilled separate the conditioned space from the
in the construction process. Inspections unconditioned space should be in place,
can be performed as quality control including doors, windows and roof
evaluations on new construction or as components. Insulation should be
problem solving tools for existing completed along with all required
buildings. To be effective in conducting caulking and glazing.
building envelope inspections for new
construction, the thermographer must Heating, Ventilating and Air
become an integral part of the Conditioning System Functional
construction team.
The heating, ventilating and air
As part of the construction team the conditioning system and all associated
thermographer may want to discuss with ductwork should be complete:B The
the contractor the nature of the infrared heating, ventilating and air conditioning
inspection and results that generally can system should be operational to create a
be expected from a thermal building normal operating positive pressure within
envelope inspection. This simple step will the building. If the system is not
help avoid unnecessary confusion and operational, an induced positive or
assist everyone in performing in the most negative pressure can be achieved using a
advantageous and profitable manner. At blower door system;H·46 However, this
the same time, a discussion of inspection technique requires that temporary heating
timing is valuable. If possible the and cooling systems be in place. A single
thermographer should gain the assistance blower door may not be sufficient for a
of the site manager in notifying that large or high rise building.
manager of the exact time when the
building can be inspected. Conductive Heat Loss

\.Yhen an existing building is being There should be a minimum of 10 K
inspected, the thermographer can more
(1 0 oc: = 18 °F) between the inside and the

outside surface temperatures of the

620 Infrared and Thermal Testing

building for at least 3.0 h before the the thermographer in identifying thermal
survey. \.Vhenever possible, the infrared anomalies and their pt'obable causes.
inspection should be conducted when
there is little or no solar loading on the Building Envelope Thermal
building envelope. However, some Inspection
building envelopes can be inspected by
using solar load as the source of \\'hen a building envelope inspection is
temperature difference. being performed for the purpose of
identifying the location of conductive and
Typically, four different thermal convective losses, pressurization or
patterns arc identified with the infrared depressurization of the structure can
scanner. A fully insulated wall will appear greatly enhance the thermal vis.tmlization
light colored from the inside of a heated of leakage. Therefore a series of tests
building. The framing members will look performed in the following manner can
darker in the image, because of their document both the conductive and
lower insulating value. A partially convective losses of the building
insulated or uninsulated wall w1H have envelopeP
partial or whole cavities where the surface
temperature is cooler and the image 1. Thermographically inspect the
darker than the adjacent framing building under normal pressure, that
members. The temperature difference is, settings maintained during
between insulated areas and insulated operation of the heating, ventilating
voids is sharp and consistent. and air conditioning system. In
multistory buildings, determine which
A fully insulated wall with retained floor represents the neutral plane of
moisture is the third pattern sometimes the building. \<\'here possible,
found during an infrared heat loss document operating pressures at each
evaluation. The moisture within the wall floor above and belo-w the neutral
components can usually be traced to plane floor.
severe condensation problems or flashing
failures at the roof, window or door areas. 2. Pressurize or depressurize the
The thermal pattern ·will show dark structure. Sustain the highest even
streaks along the framing members where interior pressure, not to exceed 50 Pa
they are wet and uneven dark splotches (7.25 x 10·:< lbrin.·1), through each
and streaks where saturated insulation has floor of the structure. Document
lost its thermal resistance. sustained pressure at each floor of the
structure.
Finally, a fourth pattern normally
found in construction is caused by 3. ThermographicalJy inspect the
convective air currents or infiltration. In building envelope during sustained
some cases, cold air will enter a wa11 positive operating pressure. Following
cavity through a top plate penetration same inspection pattern as conducted
such as a \\'ire pull. The air will follow the in the first infrared inspection.
wire to a point where it may appear to be Document surface temperatures for
uninsulated. There are many examples of comparison ·with first inspection.
these convective currents in building wall
assemblies. Because air leakage accounts for a
significant portion of the thermal space
Interior Finish conditioning load and greatly affects the
thermographic images of a building
Ideally, in the case of a new building, the envelope, it is important to discuss the
building envelope should be complete. nature of building envelope air leakage. It
However, when structural inspections of affects occupant comfort and indoor air
masonry construction are undertaken on quality. Jn most buildings indoor outdoor
certain types of construction where air exchange is attributable primarily tO
multiple layers of a building envelope are air leakage through cracks, penetrations
involved, the survey can only be and construction joints and inefficient
accomplished from the inside of the makeup air designs.
building envelope and is most detailed
when there is no interior finish at the Air leakage under natural conditions is
time of the survey. This is further very difficult to calculate because it
discussed later in this report. depends not only on wind speed and
indoor-versus-cJUtdoor temperature
VVith existing buildings, the building differences but also the quality of
envelope must be dealt with as a single workmanship and other building
unit regardless of the envelope makeup. elements. Although there are standard
Therefore the thermal conductance of the formulas to estimate air leakage, they are
envelope will include all components of only approximations.
the building envelope. Detailed
investigation at this point when
compared to as-built drawings can assist

Infrastructure and Conservation Applications of Infrared and Thermal Testing 621

Building Envelope conducling building envelope inspection~,
Structural Inspection the thermographer must become an
integral part of the construction team.
To assist design professionals in making Given that masonry construction is a
masonry specification more consistent labor intensive craft passed from
and reliable, design and construction generation to generation and primarily
specifications and standards have been dependent on site prepared nwterials, it is
developed. These procedures are written important that the thermographer be
to improve the inspection technique for professional and use tact when dealing
and hence the quality of m<Jsonry with the contractor.
structures:H1.49 The intent of the standards
is to assist contractors in bidding by As part of the construction team, the
reducing" unknowns, to help architects in thermographer may want to discuss with
preparing uniform specifications and to the contractor the nature of the infrared
assure owners of more uniform quality in inspection and what results can generally
construction. be expected from a thermal building
envelope inspection. This simple step will
Of special interest in the standards are help avoid unnecessary confusion and
the requirements dealing with quality assist everyone in performing his work in
control and quality assurance. Quality the most advantageous and profitable
assurance deals with the action to be manner. At the same time, a discussion of
taken by the owner or his representatives inspection timing is valuable. Again, if
to ensure that the construction is in possible the thermographer should gain
accordance with the written the assistance of the site manager by
specifications. Quality control refers to announcing when the building can be
the requirements placed on the contractor inspected.
in the form of construction testing and
inspection.50·52 During the process of conducting the
infrared inspection, it is imperative that
Infrared thermography has proven to accurate detailed field data be gathered for
be an effective tool to assist in the inclusion in the final report. The building
required inspections. These inspections envelope inspection includes both visible
focus on evaluating masonry for light photographs and infrared
compliance with the design specifications thermograms for the most complete
with regard to material, structural permanent record of conditions at the
strength and thermal performance.s3,S4 time of the inspection.

Video infrared thermography provides The inspection should be conducted
a thorough, systematic technique for concisely and systematically. A written
inspection of structural solids and record of conditions at the time of the
ensuring the thermal integrity of masonry survey includes but is not limited to
structures. In conducting masonry weather, building, site and equipment.
inspections, the creation of a permanent, The direction of the survey and the exact
well documented record is valuable in location of any significant thermal
avoiding potential controversy over the anomaly should he marked on the
inspection findings. construction drawings. All air and surface
temperatures should be recorded along
To understand the vast information with the emissivity values of the surface
from an infrared building envelope and a defined reference component to
inspection, nonthermographers - such as provide quantitative information.
the architect, engineer, contractor and
owner- need to have the data 1n nondestructive thermal
documented and presented in a report investigation, all thermal anomalies
that they can understand. Often the should be investigated as thoroughly as
project architect wi11 specify that an possible. The component should be
infrared inspection be performed on his visually tested and compared with the
building for a specific purpose, such as construction documents. All findings
the verification of the insulation in the should be noted on the ·written report
wall cavities. In performing the infrared and/or verbally stated on a permanent
inspection for this particular item, the video record.
entire building envelope is being
inspected and documented. These data After all components of the building
can then be used to address not only the envelope have been inspected and video
specified component hut also other taped, after the written record is
potential problems and provide assistance completed and after the direction of the
to solve them. inspection along with therm<ll anomalies
is marked on the building prints, the field
The primary purpose of any data are complete.
construction inspection is to ensure that
the intent of the designer and owner has Masonry Bloc!< Walls
been fulfilled. As discussed earlier, for the
thermographer to be effective in There are several types of reinforced
masonry construction including

622 Infrared and Thermal Testing

reinforced grOuted cavity, reinforced solid, heating sources and different room
reinforced diaphragm and reinforced fin temperatures. Additionally the outside
walls. The most common type of masonry surface temperatures can be greatly
wall used in new one·story and two-story affected by weather conditions such as
commercial construction is hollow, 'i\'ind, precipitation and solar loading.
partially reinforced, load bearing concrete
block. In a typical wall section, with Summary
vertical reinforcement consisting of
reinforcement bars set in grout, these The thermal pattern of buildings is a
grouted vertical supports along with the function of the construction style, which
bond beams become visible under the is influenced by the architectural style,
proper conditions during an ihfrared climate, building codes and prevailing
survey. quality of construction. An infrared
scanner can quantify the thermal
In performing an infrared survey of performance of the lmilding envelope: the
this type construction, the exterior finish materials' resistive value, its conductive
proves to be the determining factor in loss and the convective air leakage
whether the survey should be conducted (infiltration and exfiltration) through the
from the interior or exterior of the envelope.
building. If the block forms both the
interior and exterior surfaces, the survey Once the thermal performance of the
can be performed from either surface. building has been quantified, assessment
However, should exterior be brick veneer, for identifying corrective structural
only interior infrared surveys prove to be requirements and energy savings
effective. opportunities can be identified. After
required building envelope improvements
Code requirements for partially have been implemented, the building
reinforced masonry wall have limited the envelope can be inspected again.
maximum spacing of vertical
reinforcement to 2.4 m (8 ft). To successfully accomplish this type of
Reinforcement must also be provided at detailed infrared building envelope
both sides of openings and at each wall inspection, the thennographer must know
corner. Certain building codes require that construction techniques, materials and
horizontal reinforcement of at least documents and how to interpret building
130 mm2 (0.2 in.2) in area be provided in materials thermal patterns. Finally, to
bond beams at the top of footings, at the achieve the best results from infrared
bottom and top of wall openings, at roof building envelope inspections, the
and floor levels and at the top of parapet thennogwpher should work to overcome
walls. Other code standards require the confusion and possible resentment
reinforcement at these same locations but that can be created by such an inspection.
do not stipulate a minimmn bar diameter. A good program must educate the parties
The thermographer should look for this involved and work to build confidence so
type of wall pattern to know if the that the thermographer can become an
conditions are present for the inspection. integral part of the construction team.

Concrete Tilt-Up Precast Panels

Concrete precast modular waH panels are
extruded using hoHow core machinery.
Infrared thermography can easily detect
the thermal mass of structural solid cores.
Given the variation of thermal mass
present in this type of concrete panel,
·with a surface temperature variation as
little as 2 K (2 'C = 4 'F) between the
exterior and interior operating
temperatures, structural components
become visiblei huwever, 10 K
(10 'C = 18 'F) is the preferred
temperature spread.

The infrared survey can be performed
from either the interior or exterior sides of
the precast wall panels. Should interior
finish work prevent inspection of bare
concrete panels, the survey can be
conducted from the outside of the
building. Care should be taken to
compensate for interior conditions that
may present thermal anomalies on the
outside of the building such as interior

Infrastructure and Conservation Applications of Infrared and Thermal Testing 623

PART 5. Infrared and Thermid Testing for
Conservation of Historic Buildings

Purpose of Inspection Planning

Thermal scanning of buildings makes it The examination of all the documents
possible to gather information regarding available regarding the project and the
building technology and elements, their components of the structure should be a
shape, their material characteristics and mandatory prerequisite. Nevertheless,
their state of decay. Different kinds of historical buildings sometimes have
discontinuities affecting building hundreds of years of history and little
structures are detected by thermal analysis preliminary information is available. Even
of the surface temperature, submitted at the plan, the design and the project have
particular boundary conditions. often been lost with the time.

Various developments in the history of Because thermography is mostly used
architecture prevent any attempt to for preliminary investigation, surveys and
determine a common threshold defming lab tests on materials are usually still in
what a historical building is. Probably the progress during the planning phase.
simplest definition is its registration with Therefore, a direct survey of the materials
the public authority. It is the practice to and their damage is required to know the
classify buildings according to building real state of the test surface. These data
technology. Most common types of make it possible for the inspector to target
historical buildings can be classed as areas of interest and to specify test
follo·ws. techniques and even the integration with
other testing methods. Finally, the
1. Structural masonry buildings are made planning of the scanning has a range of
of bricks, stones and cast iron approximation. For instance the heating
structures (iron or cast iron supporting time may vary, depending on unexpected
structure ~ frames, girders, columns changes in the structure.
and buttresses).
Testing Procedure
2. Balloon frame structures include
timber frames and other supporting Very often walls of ancient buildings are
structures. not regular~ their thickness, structure and
number of layers may change
3. Adobe buildings are made of mud and unpredictably. J:or this reason procedures
unfired bricks. Different moulds cause reported must be considered flexible. TlJe
varying thickness of the walls even in following set of examples helps in
the same settlement. Regardless of the planning the actual procedure tailored on
thickness, the adobe structures are the specific case, by specifically trained
usually not high. personnel. Because of the large thickness
of walls, some thermograpHic
Even if it is uneasy to pinpoint the most nondestructive testing techniques
widespread types of building structures, developed for industrial purposes are not
often the bonding results from different useful here. UsuaiJy for ancient building
walling phases. In those cases several only nondestructive testing can be
types of structures can lie together, faced applied. As a consequence, the 'mtegration
·with a homogeneous coating (because of with destructive methods is exceptional
the last finishing phase). lvfany detailed and strict limitations are also imposed to
studies have been carried out about all the temperature maximum value.
these technologies and materials. Jt is
helpful to bear in mind their results Lab Postprocessing
during investigations:~5 ·-~7 Indeed, a basic
Old surfaces often )lave heterogeneous
knowledge of the building structural colors; materials and their state of
pattern is required for a correct evaluation conservation are variable m1d thickness of
of thermograms. structures is high. As a consequence,
many false alarms may be found. In these
Infrared thermOgraphr is applied with cases, the processing of ra\'\' thermogram~
similar procedures for both modern and and usage of software filters b't~sed on
ancient buildings. The following visual analysis, are effective tools to
differences exist for historic buildings. reduce undesired information.

624 Infrared and Thermal Testing

The temperature of the surface is a composed of different materials. The more
function of heat flmN crossing the wall common are described in the following.
and local bqundary conditions.
External Layer (Roughness and
The surface temperature may give
information regarding the inside Colors)
structure. The heat is tranSferred more
quickly throughout the most cohesive Bricks, concrete, stones, adobe and cast
materials or materials with high thermal iron masonry are often faced with a
diffusivity. protective slab that prevents weather
damage to the supporting structure. The
Differences of surface temperature optical and thermal properties of this first
because of different thermal properties of layer affects any information regarding
elements such as timber, bricks, stone, the inner layers. Most common materials
mortar can be visualized at proper time as are plasters and parget, spread with at
a footpriuf of their shapes projected on the least two layers and having a variable
overlapping plaster. Any thin thickness of 10 to 30 mm (0.4 to 1.2 in.)
delamination of the coating strongly -even more in cases of refurbishment.
reduces the heat transfer and adds its
signal to that given by the structure. Ancient coatings, where present,
consist of parget of lime; only newer
As mentioned, thermal properties of plaster recipes contain cement. Their state
ancient building materials are not easy to of conservation is often an object of
fincl. The range in the values reported by investigation by thermography. The
literature is very large. In addition these surface of the finishing materials nearly
properties change with time and always shows different colors, different
environmental conditions. Thermal porosity and damages. For instance,
properties of building materials submitted frescoes, stains and black crusts are often
to significant aging were not measured present on ancient buildings' surfaces.
systematically in the 1900s. The change of color of the surface caused
by environmental factors and pollution is
Objectives of Investigation an important topic in diagnosis of ancient
monuments. Optical characteristics could
The objectives of investigation are the be important to evaluate the cleaning and
detection and evaluation of thermal restoration treatments of the surface.
anomalies corresponding both to Thermal properties of such materials are
discontinuities because of the decay and roughly known and hardly determinable,
to the hidden elements of the building. In especially in situ.
particular, for ancient buildings, infrared
thermography is used to determine the Thermography can be applied also to
existence, position, shape and dimensions investigate the optical signal related to
of structural elements, textures of bricks discontinuities of the surface. The thin
and stones beneath the plaster. Elements colored layer affects the solar radiation
buried inside the wall as voids or chimney absorbed by the wall, in the visible and
tubes, anchorage chains, pillars, direct near infrared bands, according to the
arches, relieving arches, beams spacing, optical spectral characteristics of
plugging, plumbing systems, cables and reflectance and absorption of the surface.
others are detectable by the their thermal This causes a major flux of heat into those
footprint on the surface. zones where the reflectance of the surface
is lower. rvfeanwhile, the surface cools by
Furthermore, thermography irradiation, more intensively where the
characterizes different materials having emissivity is higher, including the infrared
the same visual state. Another important bands captured by thermography. In
application is the monitoring of thermal addition, a part of the impinging
and hygrometrical conditions of surfaces. radiation is reflected on the surface.
Hence a measurement's total flux must be
11Iermogi'aphic rmdout refers to the subdivided into its components-
surface temperature, integrating signals reflected and emitted. The same occurs in
due to the whole masonry. Therefore, the the case of alteration of materials because
knowledge of the wall is crucial for data of salt crystallization or mechanical
interpretation. Vertical and horizontal damage (freezing, bumps, coring, lack of
structures are described below, starting finishing, porosity, roughness and others).
from the external layer and proceeding
through the thickness. The object of the test is seldom the
alteration itself; nevertheless these
Facing Materials over changes are the main filter to any other
Vertical Supporting signal coming from inside the structure.
Structures These anomalies could be considered as
noises investigating discontinuities
In many historical buildings, an external behind the surface. The comparison with
facing covers the masonry inner part the visual state could be useful to improve
diagnosis when a surface alteration exists.

Infrastructure and Conservation Applications of Infrared and Thermal Testing 625

The application of waterproof products Examples of Chromatic Alterations
on the surface of porous materials {lime
mortars, cement, bricks, sandstone and A typical case in which the influence of
others) changes the absorption coefficient the color was observed is the fa\-·ade of
as well. The choice of the product to San Omobono church, Cremona, Italy
apply may be supported by (Figs. 20 and 21). The building consists of
thermographic tests before and after the structural masonry. The dark area at the
preliminary application of different base of the fa\ade is due to the chrornatic
products. The reduction of porosity gives alteration of the external layer of the
the reduction of the radiation absorption. bricks, similar to a very thin black crust.
Up to now, a clear mathematical Capillary rising affected the wall in the
correlation between porosity and
absorption has not been found but the FIGURE 20. San Omobono church, Cremona,
phenomenon is often observed in field Italy, 22 May 1994: (a) visible light
tests. photograph; (b) composite of thermograms,
showing chromatic alteration on lower part.
Test Procedure
Air temperature, 296 K(23 •c = 73 "F);
A standard testing procedure to calculate
the ratio of energy due to surface color relative humidity, 45 percent.
has not been available in the 1990s (a)
although studies and research have been
carried out.58 (b)

Nevertheless, radiance measurements
of the surface under investigation in
different spectral bands indirectly allows
rating at about the absorptivity of painted
surfaces (the table of the values is valid
only case by case). This test may be
applied to a portion of the surface selected
in advance by the operator.

1. Select a reference area within the field
of view as close as possible to the
investigated spot and possibly to be
viewed in each thermogram.

2. Set up of lamps having a known
emission spectrum disposed
perpendicularly to the surface to
achieve the most homogeneous
irradiation. The heating from the
lamps is inversely proportional to the
distance of the investigated surface.
For example, for air temperature 281
to 288 K (8 to 15 "C; 46 to 59 "F),
place 2 x 500 W lamps at 0.60 m
(2.0 ft) from the surface for a heated
area about 1 m 2 (11 ft2). The heating
required must increase the surface
temperature about 3 to 5 K (3 to 5 oC;
about 5 to 9 "F) and keep it stable.

3. Recording time depends on the
diffusivity of the material and the
source inertia. The recording has to
begin before the heating and be
repeated at regular intervals. Time and
power depend on the materials.

4. For data processing, the analysis is
based on the ratio of the temperature
of any spot investigated and the
reference spot. The emissivity
correction could be a crucial point.
The temperature has to be analyzed
before and after the heating. Air
temperature and radiation emitted
from the surrounding must be
monitored.

626 Infrared and Thermal Testing

past years and caused the alteration of the crystallization cycles of soluble salts. Tht>
surface. The infrared images were shot parget/plaster layers may embody an air
after solar irradiation: the dark color at gap inside or between the wall and the
the bottom of the wall corresponds to a plaster. The detection of the discontinuity
major thermal difference at the surface is more reliable in transient condition
(Fig. 21). even if a periodic or quasistationary
approach may give results as well.59
Finishing Detachment
Natural sources include air streams and
The detachments or lack of adhesion solar radiation, both giving a passive
between the finishing surface and the heating. Solar radiation may he also
substrate are very common properly shielded to enhance the transient
discontinuities. The discontinuities are behavior.no
due to bad setting of the plaster or the
This kind of discontinuity appears as a
FIGURE 21. San Omobono church, Cremona, warmer area when the net heat flux enters
Italy: (a) visible light photograph of right the building- that is, usually during the
side of fat;ade with demarcation showing first phase of the heating or cooling,
damp area; (b) thermogram of darker zone. following the thermal excitation of the
surface. The heat remains in the area
Air temperature 296 K(23 oc ~ 73 °F); insulated by the air layer instead of
flowing inside the structure. In fact,
relative humidity 45 percent. thermal signals may appear also during
(a) the heating phase. A signal lasts for a
particular time depending on the depth
(b) and thickness of the discontinuity. The
actual procedure must be tailored to the
optimum frequency of recording; the
amount, starting and duration of the
heating according to material and of the
discontinuity depth.

The localization of discontinuities can
be obtained comparing the response of
the anomalous areas with a reference one
at the optimum time. A quantitative
evaluation requires the time and the space
analysis of the 'whole sequence.
Irradiating the surfaces increases the
differences of absorption. Convective
heating is better for avoiding effects due
to local optical characteristics. The sizing
of the discontinuity is normally accurate
and easy.

\-\'hen the surface is painted ·with
precious frescoes, the most sensitive
techniques such as pulse phase
thermography may be applied."'

Procedure for Transient Tests

Heating. Heating must be homogeneous
to avoid false indications. In case of
natural heating (solar irradiation) the
scanning has to be planned as a function
of surface lighting. According to the
season and the latitude of the site, several
minutes (about 300 s) of radiation can be
enough to heat a parget to a depth of 20
to 30 mm (about 1 in.). The temperature
over about 2.0 m2 (21 ft2) can he

increased by 10 K (10 oc ~ 18 °F) by

means of artificial sources. For instance a
set of 4 halogen lamps of 1 k\,V each for
2 min is suitable for 25 mm (l in.) thick
plaster.

Recording. Time depends mainly on the
diffusivity of the material and the depth
of the discontinuity (one infrared image
any 10 to 15 s for IO to 15 min could be
enough). The recording has to begin

Infrastructure and Conservation Applications of Infrared and Thermal Testing 627

FIGURE 22. Plaster detachment on Palazzo della Ragione, Milano, Italy: (a) visible light

photograph, May 1994; (b) thermogram during night of 31 May 1994, air temperature
295 K(22 oc ~ 72 of), relative humidity 59.9 percent; (c) thermogram during foggy day,
16 November 1994, air temperature 281.55 K(8.4 oc ~ 47.1 °f), relative humidity

91.9 percent; (d) thermogram during early cooling phase, 30 March 1995, air temperature

280.65 K(7.5 oc ~ 45.5 of), relative humidity 34.5 percent; (e) thermogram at beginning of
heating, 22:00 12 October 1995, air temperature 296 K(22.7 oc ~ 72.9 of), relative humidity
60.5 percent; (f) thermogram after 2 h of heating, 12:00 noon 12 October 1995, air
temperature 297.8 K(24.6 °C ~ 76.3 of), relative humidity 43.2 percent; (g) thermogram

during cooling phase, 17:00 12 October 1995, air temperature 280.7 K (7.5 oc = 45.5 of),

relative humidity 34.5 percent.
(a) (e)

(b)
(f)

(c)

(g)

(d)

Legend
1. Anomalous region.
2. Region without anomaly.

628 Infrared and Thermal Testing

before the heating and go on for the This anomalous appearance wa~
whole thermal process. Generally, the probably due to the decreased air
temperature history of any point is temperature. In Hg. 22c the infrared
compared with a reference one, chosen on images were taken at the opposite weatheJ
a sound zone. If a reference area is condition (air temperature 281.6 K =
needed, a preliminary visual analysis and
knocking test is useful. 8.4 oc = 47.1 °F, relative humidity

Data Processing. Discontinuity location 91.9 percent, foggy weather, no direct
and thermal characterization are normally radiation all day long). In such
performed off line. Several algorithms are
available in the literaturef>2 Among them, 1
thermal tomography uses the thermal
contrast as informative parameter. The conditions it is quite impossible to detect
contrast is defined as the temperature anomalous areas. The thermogram shown
difference between the analyzed points in Fig. 22d was recorded 1 h after the
and reference one, normalized by the direct solar irradiation shadowing. The
reference temperature. Several variants of detached area is wanner than the
the contrast exist, for instance the remaining plaster, even if the zones les~
maximum temperature reached by any protruding are now at the same
point is used for the normalization. In temperature of the sound coating (ail
such a way, the influence of different heat temperature 280.7 K = 7.5 "'C = 4:1.5 'F;
absorption on the surface is reduced. The relative humidity 34.5 percent),
analysis of the whole set of data gives two
synthetic images, where the maximum of A further set of measures for a dynamic
the contrast is mapped, concurrently with test were performed on 12 October 1995.
the time when it occurs. This procedure The sampling interval was every S min
characterizes the discontinuity, using from 10:00 to 17:00. \Veather conditions
proper calibration functions. According to were monitored some days before and
this, a window in time evolution during the test. Figures 22e to 22g show
corresponds to a layer in depth. This the different temperatures of the detached
allows separating the searched area (2) and the reference (1) achieved at
discontinuity from the whole structure. the beginning of the heating (Fig. 22e),
after 2 h (Fig. 22f) and during the cooling
Scanning. A preliminary scanning on the
\Vhole surface is useful to set device (Fig. 22g).
parameters and to localize the anomalies. Slabs of marble or stone and mtificial
A further scanning on these areas
pinpoints the discontinuities. \Vhen a stone (cement} have varying thickness,
complete scanning of the surface is 20 to 30 mm (0.8 to 1.2 in.). Metallic
needed attention must be paid to anchorage and mortar fixes back the slabs
geographical aspects and topology of the
building. to the wall.
An application of infrared
Examples of Plaster Detachments
and Debonding thermography is the detection of the
detached elements and the location of
The investigation at Palazzo della Ragione, metallic anchorage. For example, the
rvman, Italy,63 was finalized to localize the method could be used to inspect extern<JI
delamination of the pargets on the South weather tile cladding1 very thin 5 to 12
facade. Four scannings were performed mm (0.2 to 0.5 in.) tile that simulates
during 1993 to 1995 to cover the first terracotta and is usually embedded in
floor. mortar. ·wall tiling is another, earlier use
of mosaic. The vertical application of roof
The scans were shot before, during and tiling makes more waterproof the external
after direct solar irradiation. Here, three
image sets are reported taken at different layer of masonry.
environmental conditions using a Moreover withstanding weather tiles
quasistationary approach and anotheJ for
the transient one. The restricted area also protect walls against evaporation of
shown in Fig. 22a is used as example rising damp. Concrete and mortar can fill
indicating (1) the reference area and their hollow section. The porosity of the
(2) a large delamination. In Fig. 22h the surface changes under pollution and frost.
detachment appears colder than the These objects in most cases withstand
parget adhesive to the substrate. weather; nevertheless, because of wate1
Thermograms have been taken 2 h after infiltration, the metal bracketing and the
the shading of the solar radiation (air mortar may suffer damage. Thermography
is applied to find detachments, cracks,
temperature 295 K = 22 oc = 72 °1~ relative chromatic alteration and decay or the
surface in the same manner as for modern
humidity 59.9 percent, clear sky). buildings.

Terracotta works are used too, as bas
relief, cornices, pilasters, friezes, swags,
boasts and dressing of every kind, having
a variable thickness (in any case much
thicker th<ln tiles), The backs of such
facings are fixed to metallic anchorage-
copper, protected steel and bronze.

Figure 23 57 shows an example of the

executive project of a tcrracotta cornice in

Infrastructure and Conservation Applications of Infrared and Thermal Testing 629

a late nineteenth century building. The heating of the surface lets the edges of the
metallic brackets can be seen in the cracking be detected better because of the
section through the cornice. faster heating.

In Santa tvfaria in Cantuello, Ricengo, A crack represents a higher thermal
Italy, a set of measures was performed to resistance perpendicular to the surface,
detect the metallic anchorage behind a detectable when a heat flux parallel to the
marble slab at the base of the main surface exists. Linear motion of the heat
elevation. source at constant speed makes it possible
to stimulate the surface uniformly and to
In Fig. 24 a thermogram shows metallic increase productivity. Of course, the
anchorage as warmer (demarcated in the maximum temperature at each point is
pictures). Colder areas at the base of the reached at a different time.
slab indicate water infiltration into the
mortar in the slab. The image ·was Therefore, a dedicated algorithm is
recorded after 2 h of direct solar applied for the reconstruction of imagesf>-I
irradiation (21 March 1997, air The equipment is placed on a contro!l('d
temperature 294 K = 21 °C = 70 oF, relative motorized rail. The focused linear lamp
humidity 42 percent). heats the surface on a moving strip while
a sequence of thermograms is stored at
Cracking suitable frequency. The emitted radiation
may be shifted toward the infrared hand,
Usually the solutions of continuity of the reducing differences in the energy
coating are visible. The most important absorption.
information is about their
depth -whether the cracking is only In Santa Maria in Cantuello church,
across the coating, across the thickness of Ricengo, Italy, a survey of the cracking
the brick or across the whole thickness of representative of the structural decay was
the masonry. Under transient condition it performed. The cracking of the edge of
is possible to survey the main occurrences the diagonal arch in the vault is shown in
of cracking thermographically. Fig. 25a. The infrared image was shot in
transient conditions, after 8 h of sunning
Surveys of crossing cracking are best on the roof. It is a cracking across the
performed when a modulated thermal thickness of the solid masonry (21 t._,farch
gradient between the two sides occurred.
The heat due to mass transfer flows across 1997, air temperature 294 K = 21 oc =
the cracking and it is possible to survey its
shape. In case of surface cracking the 70 oF, relative humidity 32 percent).
In Fig. 25b is shown cracking of the
FIGURE 23. Example of executive project of terracotta cornice
in late nineteenth century building. Metallic brackets are parget in the triumphal arch (zone l) in
detailed in section of cornice. 57 Santa Jvfargherita church, Cremona, Italy.
The scanning was recorded in transient
conditions whereas the solar irradiation
has been heating the roof for 3 h
(21 lvlarch 1997, air tempemture

FIGURE 24. Infrared image with metallic
anchorages of slab (demarcated), Santa
Maria in Cantuello, Ricengo, Italy, 21 March
1997, air temperature 294 K(21 oc ~
70 °F), relative humidity 42 percent.

630 Infrared and Thermal Testing

288 K = 15 "C = 59 "1;, relative humidity (28 in.) long, moving 30 mm {1.2 in.)
58 percent). from the surface at a constant speed of
0.1 m-s-! (19.7 ft·min- 1). In this image
Tests run inside the Palazzo della two crackS-and a large detachme11t Wt'n'
Ragione hall, Padua, Italy, where the found and confirmed by conservationists.
whole fresco is more than 1000 m2
(11 000 ft 2) rising up to a height of 10 m Hidden Structures
(33 ft), used the lateral heating. The visual
image of frescoes and the equipment is In brick and stone buildings the masonry
shown in Fig. 26a. Figure 26b shmvs is massive.
processed thermograms covering an area
of about 4 m2 (44 ft2), taken after the Usually, the thickness depends on thl'
complete scanning by the heat source, disposition of the clements. The slimmest
3000 VV of electrical power, 700- mm wall consists of a single leaf of.half brick,
which me-asures about 60 mm (2.4 in.).
fiGURE 25. Santa Maria in Cantuello church,
Ricengo, Italy, 21 March 1997: (a) cracks in FIGURE 26. Active thermography of plaster wall of Palazzo del
vault after 8 h of solar irradiation, air Podesta, Padua, Italy, 10 july 1998: (a) setup, showing
temperature 294 K (21 oc = 70 oF), relative scaffolding needed to view wide surface 4 m (13 ft) above
humidity 32 percent; (b) thermogram of floor, horizontal rail supporting lamps and telescopic frame
cracks in parget in triumphal arch, transient for lifting of thermocamera (on left); (b) in tomographic map
condition after 3 h of solar irradiation on of plaster discontinuities, differences of gray levels from
roof. Thermographer superimposed background indicate cracks or discontinuities at different
numbers on Fig. 25b to identify regions of depths. Two kinds of discontinuities are detected-
interest. Air temperature 288 K (15 oc = detachments at different depths and continuous crack with
59 oF), relative humidity 58 percent. inverted W shape.

(a) (a)

(b)
(b)

2 20 ,,

c d'•·'''

c0 40 .~

g ,{''-'

:e 60

"""U 80

·~

"'v 100

~
120

50 100 !50 200 250
Horizontal grid (arbitrary unit)

Infrastructure and Conservation Applications of Infrared and Thermal Testing 631

The brick noggin walls (or infill in timber has a structural function like the brick\ or
frames, used as the inner partitions of the stones. The thickness of the whole wall is
buildings until the nineteenth century) more than 0.5 to 0.6 m (20 to 24 in.); the
have similar thickness. Greater thickness u~ual thickness is 0.80 to I .00 m \3 1 to
depends on the bond (texture of the 39 in.). Infrared thermography can detect
masonry), where two or more stretchers a lack of adhesion between the leaves of
and headers elements are laid across the bricks and the filling if the thickness is
wall. not prohibitive.

Most common types of structural Example of Discontinuity inside
masonry are composed of ashIars of bricks Solid Masonry
or stone and joined by lime mortar. The
bond is characteristic of the age and the In Palazzo Ducale,C.S Urbino, Italy, the
place and depends on the size of the investig<Jtion was carried out on the wall
elements, as the thickness. A widespread between the southern aisle and the throne
typology consists of rubble walls: stones hall to detect voids and drains 150 to
of different shapes, dimensions and 300 mm {6 to 12 in.) deep inside the wall
provenience are embedded in lime mortar, (see Hg. 27). Infrared lamps supplied 12 h
in regular courses. Stretched and heating, enough to increase the surface
herringbone bonding arc the typical
disposals. Thickness is not less than 0.4 m temperature about 4 to 5 K (4 to 5 oc; 7 to
(16 in.). Best results of thermography are
achieved in case of stone wa!Hng, coated 9 T). In such a wny, the inner layers of
by lime pargcts. A suitable choice of the wall {total thickness more than 1 m)
heating, time and boundary conditions has been thermally stimulated. The
permits detection of the bond beneath the scanning was shot 1 h after the end of the
coating. heating on the same side.

Often relieving arches are disposed in The infrared image (Fig. 28) shows a
the masonry. Builders insert them in the warmer upper zone, clearly separated from
walls to change the vertical distribution of the colder area at the bottom. Here, the
the loads, so to prevent the cracking of external layer of brick is completely
the weaker part of the masonry (for detached from the substrate (28 june
example, near the openings or the
reduced thickness of the wall). Their 1994, air temperature 297 K = 24 oc =
pattern is almost different from the bond
of the remaining wall. The survey of the 75 og relative humidity 42 percent).
structural pattern of the building provides
important design information for its Inspection with a fiber optic borescope
historical restoration. Also in these cases later confirmed the anomaly.
infrared thermography can he usefully
applied to detect such structural elements, Voids, Inclusions and Thickness
thanks to the different thermal properties Variations in Walls
of materials constituting ashlars, bricks
and mortar. Thermal imaging is mainly used for
investigation of shallow discontinuities,
Cavity walls are composed of two from 0 to 30 mm (0 to 1.2 in.) long.
leaves of external bricks, with stretched Nevertheless it can also help to detect
bond, mortar and rubble (or pieces of voids such as cavities, weak pockets,
bricks, tiles and other material) filling the
cavity inside, without a regular FIGURE 28. Cavity wall debonding shown in
disposition. In these kinds of walls mortar thermogram of whitewashed surface in
Palazzo Oucale, Urbina, Italy, 28 June 1994
FiGURE 27. Visible light photograph of third after 12 h of heating. Air temperature 297 K
aisle in Palazzo Ducale, Urbina, Italy,
28 june 1994. oc(24 ~ 75 °F), relative humidity

42 percent.

legend

1. Wilrmer upper zone.
2. Colder area at bottom.

632 Infrared and Thermal Testing

chimney stacks, smoke tubes, dead spaces properties of the investigated materials
or thickness variations. In these cases, and the environmental conditions during
active techniques are preferable and the tlw tests. Unfortunately such data are
thermal excitation of the wall must be often not available.
properly designed.
In Carboni ~vfansion, an adobe building
~v1athematical models are very useful in Assemini, ltaly, thermography shows
for this purpose. The models must take the flue over the oven in the kitchen (Fig.
into account the thermophysical 29). The composite of infrared images
were shot after 15 min of heating by
FIGURE 29. Thermogram of flue over oven, in convection (1.1 k\Al). The heater was put
kitchen in Carboni Mansion, Assemini, Italy, at the base of the ftue inside the m:ren.
detected by artificial heating, 3 September The thickness of the wall crossed is about
1996. Air temperature 300 K 100 mm (4.0 in.). The heat flows up
(27 oc ~ 81 of), relative humidity following the shape of the cavity and at
42 percent. the top is a timber lath closing the flue.
At the bottom, the two iron doors of the
oven are visible. Electric wires are seen too
(3 September 1996, air temperature 300 K

= 27 oc = 81 °F, relative humidity 42

percent).

Examples of Hidden Structures

In Malpaga Castle, a thin wall of 0.15 m
(6 in.) thickness was investigated. Noggin
bricks fill it and it is timber framed. Both
surfaces are frescoed and whitewashed. It
is a partition of an eastern loggia at the
first floor. Thermograms have been
recorded after convective heating
(4000 \·V). The volume of air to heat was

FIGURE 30. Internal structures detection on
rear side of wall in Malpaga Castle,
Bergamo, Italy, 10 january 1997. After 6 h
of heating whole timber frame is delectable
as warmer areas. Air temperature 277 K
(4 oc ~ 39 of), relative humidity 72 percent.

1

Infrastructure and Conservation Applications of Infrared and Thermal Testing 633

about 30 m·1 (39 yd'). Objects in the as colder areas in Fig. 32 (demarcated
image grow warm at different rates, and area) beneath the frescoed parget. Under
this variety can be seen in thermograms the encountered favorable boundary
over a 6 h period. A timber frame appears conditions it was not necessary to heat
colder at first and adhesive disband the surface artificially (12 September
discontinuities appear warmer. The bond 1997, air temperature 290 K ==
of the wall (bricks and mortar) begins to 17 oc = 63 oF, relative humidity
appear too. After 6 h of heating, the 65 percent, clear sky).
whole timber frame is detectable on the
rear side as warmer areas (Fig. 30) Closed Openings
(10 january 1997, air temperature
277 K = 4 "C = 39 "F, relative humidity An earlier appearance of a historical
72 percent). building is a very important topic for
nondestructive testing and evaluation.
The structural masonry was identified The presence of closed openings gives a
by infrared thermography inside the aisle documentation of the usage of rooms and
at the first floor of Villa Arconati, infrared thermography is well suited to
Castellazzo di Bollate, Italy, at steady state this task. The fenestration is often the
conditions. An unusual pattern of the result of refurbishment during the life of
wall was discovered, as reported in Fig. 31, the building. Therefore, the position of
where colder and warmer strips are seen. the openings may depend on the changes
of the internal arrangement of rooms and
This unusual pattern of the wall can be their furniture.
explained by the particular structure. The
presence of timber buttresses is recognized According to the new plan of the
in the infrared image. In fact the elevations, the ancient doors and
investigated wall is just a secondary windows may he walled up while new
partition of a larger room, corresponding openings are made. Moreover a new
to a vault at the lower level. For this coating covers the facades and hides the
reason, its structure was lightened by irregularity of the masonry caused by
timber buttresses, which are continued by those alterations. Infrared thermography
the vault centerings (11 june 1997, air makes it possible to locate openings fil1ed
temperature 287 K = 14 oc:::: 57 °F, relative and no longer visible. In fact the
humidity 51 percent). structural elements of the opening
(lintels, edges, abutments, thresholds,
In Malpaga Castle, the wall under doorsteps and others) can be recognized
investigation is located over the secondary by their thermal properties. Moreover, the
entrance door, inside the castle. Infrared ne'i\' openings constitute weaknesses in
thermography made it possible to locate the masonry, because their presence
the metallic tie cotter of the lifting bridge changes the distribution of the vertical
loads. Therefore cracking of the masonry
FIGURE 31. Presence of timber buttresses is may occur.
evident in infrared image as colder and
warmer strips. Vault centerings are In the following cases infrared
demarcated in square. Villa Arconati aisle, thermography makes it possible to
Castellazzo di Bollate, Italy, 11 june 1997. localize the cracking as described above.
Air temperature 287 K(14 "C =57 "F);
relative humidity 51 percent. 1. The structural masonry (bricks and
clay mortar) of Sant'Abbondio cloister,
Cremona, Italy, is documented in
infrared images not shown here
(7 june 1995). An ancient opening was
walled up by a thinner filling of wall
bricks. The thermography was shot

FIGURE 32. Metallic tie cotter detection
(colder areas in highlighted zones) of lifting
bridge beneath frescoed parget, northern
tower, Malpaga Castle, Bergamo, Italy,
12 September 1997. Air temperature 290 K
(17 oc == 63 °F), relative humidity 65 percent.

634 Infrared and Thermal Testing

from inside the building, during direct humidity (relative humidity below
solar irradiation of the exterior. The 60 percent) is required.
reduced thickness of the panel allowed
a higher heat flux than in the In case of discontinuities~ because of
remaining wall to be transferred. the lack of finishing, damage to the
Therefore, the panel filling appears in finishing or colored decorations-
the thermogram as a ·warmer area. convective heating is more homogeneous.
2. At Malpaga Castle, the structural In addition, convective heating can be
masonry at the Ooor level was more effective reducing the volume of the
completely frescoed. Nevertheless, the air near the test surface. For this purpose,
filling of a previous door can be easily it is possible to endose the reduced
detected by active thermography using volume to heat by means of frames,
convective heating. The-thermogram supporting plastic sheets. In this way, less
(not shown here) shows the filling as a expensive and more effective heating can
darker area, where black spots are be performed in a shorter time.
rubble. The arch shape is clearly
delimitated too. 'fhe vertical structures The wall under investigation in this
correspond to the edge of the wall and case is the inward side of the tower of
the corner with the perpendicular wall Malpaga Castle (Pig. 33). Under favorable
(3 Pebruary 1997). weather conditions it was possible to
3. Another example was found also in detect the bond of the masonry. The
Sant' Abbondio cloister where the thermograms of Fig. 34 show squared
structural masonry is coated by Jime
and clay mortar and does not show FIGURE 33. Visual state of inward west side of
any traces of past openings. On the tower in Malpaga Castle, Bergamo, Italy,
contrary, passive thermography (20 1 October 1997.
June 1995) reveals the filling of an
ancient window. Rising damp is also ltt~~,,i····
detected at the base of the wall as a
darker area (see also Fig. 40, belmv). \

Bonds of Walls FIGURE 34. Mosaic of thermogram: bond of
masonry appears in infrared images of tower
A direct analysis of the building of Malpaga Castle, Bergamo, Italy,
(geometrical and material surveys) makes 1 October 1997. Air temperature 295 K
it possible to think about many steps of (22 oc = 72 of), relative humidity
the historical evolution up to the final 52 percent.
asset of the masonry, increasing the
knowledge about the building.

Nevertheless, if the parget covers the
structures, it is impossible to get
information about the bond. Of course, it
is not advisable to scrap the parget in
order to dig out the traces of alterations
occurring over centuries. Infrared and
thermal testing is a very effective tool for
obtaining the required information about
the state of the elementary components
of the wall.

Also in this case, infrared scanning has
to he applied after an adequate heating of
the surface. Best results are achieved
where materials with different thermal
characteristics are juxtaposed, for
example, stone-to-lime mortar masonry.
Active approach is more reliable because
environmental conditions may seldom
allow a good survey of the bond beneath
the parget. The cases presented were
obtained using hath active and passive
heating. In the latter case, a direct
irradiation or a natural warm draught
licking the surfaces for few hours may be
exploited. Hence, a high daily thermal
excursion (temperature increasing more
than +10 K [+10 oc = +18 °F]), high
average air temperature !higher than
288 K = 15 °C =59 °1:] and low relative

Infrastructure and Conservation Applications of Infrared and Thermal Testing 635