CONCRETE MATERIALS AND MIXTURE

1.3.3.4 Accelerators








The functions of accelerator are:


• Increase the rate of hydration of cement, and hence to

increase the rate of development of strength and greater

heat evolution




• To shorten the setting time


An increase in the rate of early strength development will

result in earlier removal of forms, reduction of curing and

earlier placement of structure.


The benefits of shorten the setting time are early finishing

of surface, reduction of pressure on forms or of period of

time during which the forms are subjected to hydraulic

pressure.



1.3.3.5 High Range Water Reducing









• Also known as superplasticizer




• Adding this material in concrete mix would



greatly increase the workability of mix



• This material can be used to facilitate the



production of flowing concrete




• Example:



– Sulphonated Melamine Formaldehyde (SMF)



– Sulphonated Naphthalene Formaldehyde (SNF)



1.3.3.5 High Range Water Reducing









• Can be used to produce concrete with



significantly superior strength, known as high


performance concrete




• Can be used to produced concrete with very



high workability




• Do not influence shrinkage, creep, modulus of



elasticity of concrete

1.3.3.6 Water proofing









• Concrete absorbs water due to surface tension



in capillary pores in the hydrated cement


paste ‘pulls in’ water by capillary suction




• This admixture prevents penetration of water



into concrete




• Their effect is mainly to make concrete



hydrophobic. Simply put, a condition whereby


the water is “pushed out” of pores



1.4 What is fresh concrete








• Basically fresh concrete is a uniform mixture of


water, cement and aggregate in wet state.




• Properties of fresh concrete:


a. Consistency or slump


- How wet or dry the concrete is



a. Uniformity


- All ingredient are evenly distributed in the mix


a. Workability



- The ease with which concrete is placed and

consolidated



1.4.1 Fresh concrete and workability




relationship





• The strength of concrete of a given proportion



is affected by the degree of its compaction



• Fresh concrete can be transported and placed


without segregation and bleeding, compacted



and finished easily



• This scenario known as “workability”



• Workability – the ease with which a concrete



mix can be handle from the mixer to its finally


compacted shape

1.4.2 Factors influencing workability




on fresh concrete












• water-cement ratio





• aggregate-cement ratio




• shape and surface texture of



aggregate





• admixture substance

Water-Cement Ratio









• Higher water-cement ratio would increase


workability consequently increase void in



concrete



• Too high workability will result in segregation and



bleeding



• Lesser water-cement ratio would lower the



workability



• Optimal water-cement ratio should be achieved



as the degree of workability in slump test

Aggregate-Cement Ratio









• For constant water-content ratio, an increase in



the aggregate-cement ratio will decrease


workability



• Lower aggregate-cement ratio will increase



workability



• If the ratio of volume of coarse to fine


aggregate is high, it will lower the workability



and cause segregation



• Lack in fine aggregate used would results in a


harsh mix that has low workability

Shape and Surface Texture of Aggregate










• Finer aggregate would reduce workability because


of high specific surface area.


• Larger size of coarse aggregate gives higher


workability because of the reduction in the total

specific surface area



• Irregular and angular aggregate reduces


workability


• Rounded aggregate increase workability because


of reduction in particle interference


• The use of elongated aggregate results in low


workability, primarily due to increase in particle

interference

Admixture










• Addition of air retaining admixture and



plasticizer will increase workability




• Addition of pozzolanic material such as slag


and Pfa would reduce workability

1.4.3 Laboratory testing workability




and strength of concrete








• Slump test


- The oldest, most widely used test for determining


workability and consistency of concrete mixture


• Compacting factor test



- To determine the workability of concrete in more


precise than slump test


• Cube test



- To determine the compressive strength of hardened

concrete







http://www.scribd.com/doc/46944936/Slump-Test

Slump test

Slump test

















http://www.youtube.com/watch?v=lwZf217v5XA



Compacting factor test
























http://www.youtube.com/watch?v=SRo2vYw_QPI

Compacting factor test

Cube test

Cube test









• The making of concrete cube




http://www.youtube.com/watch?v=80E4Z_-EgiY





• Concrete cube testing



http://www.youtube.com/watch?v=oz_q_TyNYQc



http://www.youtube.com/watch?v=gg_bIMJ7I0M

1.5 Hardened concrete








• Properties of hardened concrete




– Strength


– Deformation



– Durability



– Permeability


– Shrinkage



• Testing



– Destructive test



– Nondestructive test

1.5.1 Properties of hardened concrete









• Factor affecting strength



– Cement type




– Cement content



– Proportions of constituents



– Aggregates



– Water-cement ratio



– Time

1.5.1 Properties of hardened concrete











• Factor affecting deformation is when load applied to it.


• Two type of deformation:



– Elastic deformation

As the load increases the deflection increases. When the


load removed the concrete structures such as beams

returns to its original position.


– Creep deformation


If a load is applied to a concrete beam it will initially

undergo elastic deformation. As time progress, it will

continue to deflect even though the load does nnot


change.

1.5.1 Properties of hardened concrete









• Factor affecting durability



– Weathering




– Reaction with aggregate



– Attack by sulfates



– Efflorescence



– Reaction with organic chemicals



– Corrosion of steel



– Wear

1.5.1 Properties of hardened concrete








• Factor affecting permeability




– Water- cement ratio


– Workability



– Type of structure



– Method of compaction


– Soundness and porosity of the aggregate



– Age of concrete


– Grading of aggregate



– Curing

1.5.1 Properties of hardened concrete









• Factor affecting shrinkage depends on the



amount of drying that can take place.




• Two type of shrinkage:



– Plastic shrinkage



• Caused due to the hydration of cement


– Drying shrinkage



• Caused by withdrawal of water from concrete stored in


unsaturated air voids

1.5.2 Testing of hardened concrete













• Destructive test:



– Compressive strength test



– Tensile strength test



– Flexural strength




• Non-destructive test:



– Rebound hammer test



– Ultrasonic Pulse Velocity



– Penetration resistant test

1.6 Manufacturing of concrete









• There is more information about properties



and manufacturing of concrete. You can find


out by yourself at:




http://www.concrete.net.au/publications/pdf/concretebasics.pdf