CONCRETE TECHNOLOGY

CE 703: CONCRETE TECHNOLOGYCREEP, SHRINKAGE and ELASTICITYofConcrete1

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BY:

MIAAZA HUSSAIN.......10/CE/61

PRONOTI YEIN....10/CE/27

MOULI MAHASETH.10/CE/232

Three main types of deformations in hardened concrete subjected to external load and environment are:

Creep: time-dependent deformation that occurs on the prolonged application of stress

Shrinkage strains: deformations occurring either on loss of moisture from the concrete or on cooling of concrete

Elastic strains: instantaneous deformations that occur when an external stress is first applied

Deformation EffectAny one or combinations of the above types of deformations in a hardened concrete leads to cracking.3

CREEP ofConcrete

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CREEP IN CONCRETEConcrete creep is defined as deformation or increase in strain of structure under sustained mechanical loading.

long term pressure or stress on concrete can make it change shape. This deformation usually occurs in the direction the force is being applied.

This phenomenon of creep can be accounted on by the stress strain curves of concrete obtained for different ages of loading. Stress strain curve of concrete is always a curved line and the degree of curvature depends on many factors out of which the intensity of stress and time for which the load is acting has a significant effect.

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Fig: Variation in stress and strain diagram with loading time

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Creep does not necessarily cause concrete to fail or break apart. Creep is factored in when concrete structures are designed.

Fig: variation of strain with time, under constant axial compressive stress7

If the load is removed, the elastic strain is immediately recovered. However the recovered elastic strain is less than the initial elastic strain, as the elastic modulus increases with age. There is reduction of strain due to creep recovery which is less than the creep strain. There is some residual strain which cannot be recovered

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CREEP COEFFICIENTThe creep coefficient is provided for three values of age of loading.

Age of Loading Creep Coefficient, 7 days2.228 days1.61 year1.1

It can be observed that if the structure is loaded at 7 days, the creep coefficient is 2.2. This means that the creep strain is 2.2 times the elastic strain. Thus, the total strain is more than thrice the elastic strain. Hence, it is necessary to study the effect of creep in the loss of prestress and deflection of prestressed flexural members. 9

FACTOR AFFECTING CREEPAGGREGATES: Aggregate influences the creep of concrete through a restraining effect on the magnitude of creep.

MIX PROPORTIONS: The amount of paste content and its quality is one of the most important factors influencing creep. A poorer paste structure undergoes higher creep

AGE OF CONCRETE: Age at which a concrete member is loaded will have a predominant effect on the magnitude of creep. This can be easily understood from the fact that the quality of gel improves with time10

EFFECTS OF CREEP IN CONCRETEIn reinforced concrete beams, creep increases the deflection with time and maybe a critical consideration in design.

In eccentrically loaded columns, creep increases the deflection and can load to buckling.

In case of statically indeterminate structures and column and beam junctions creep may relieve the stress concentration induced by shrinkage, temperatures changes or movement of support. Creep property of concrete will be useful in all concrete structures to reduce the internal stresses due to non-uniform load or restrained shrinkage.

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SHRINKAGE ofConcrete

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SHRINKAGE IN CONCRETEConcrete is subjected to changes in volume either Autogenous or induced.

Volume change is one of the most detrimental properties of concrete, which affects the long-term strength and durability.

To the practical engineer, the aspect of volume change in concrete is important from the point of view that it causes unsightly cracks in concrete.

Shrinkage causes an increase in the tensile stress which leads to cracking and warping and external deflection before concrete is subjected to any kind of loading. It is difficult to make concrete which does not shrink and crack

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Fig: Cracks in a concrete slab due to shrinkage14

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TYPE OF SHRINKAGE IN CONCRETE(I) Plastic Shrinkage(II) Drying Shrinkage (III) Autogeneous Shrinkage(IV) Carbonation Shrinkage15

Fig: Different types of shrinkage damage to a structure

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FACTORS AFFECTING SHRINKAGEWater-cement ratio: the shrinkage increases with increase in water cement ratio. The richness of the concrete also has a significant influence on shrinkage.

Cement content: the shrinkage increases with increase in cement content, but is inter related to water-cement ratio because of the necessity to maintain workability. It is not much affected by the cement content if the water content per unit volume is constant.

Ambient Humidity: One of the most important factors that affect shrinkage is the drying condition or in other words, the relative humidity of the atmosphere at which the concrete specimen is kept.

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Fig: Effect of water cement-ratio and cement content on drying shrinkage

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Fig: Variation in Shrinkage with decreasing relative humidity of surrounding air19

Aggregate: The quantum of an aggregate, its size, and its modulus of elasticity influence the magnitude of drying shrinkage. The aggregate which exhibit moisture movement themselves and have low elastic modulus causes large shrinkage.Harder aggregate with higher modulus of elasticity like quartz shrinks much less than softer aggregates such as sandstone.An increase in maximum size decreases the shrinkage. The grading and shape has little effect on shrinkage.

Fig: Effect of type of aggregate on shrinkage of concrete20

Size and shape of specimen: both the rate and ultimate magnitude of shrinkage decreases with surface/volume ratio of the specimen

Type of cement: The rapid hardening cement shrinks somewhat more than the others

Admixtures: The shrinkage increases with the addition of calcium chloride and reduces with lime replacement

Effect of time: Shrinkage takes place over long periods. However, large fraction of the ultimate shrinkage (which is mainly the drying shrinkage) takes place at early times and the small fraction of the ultimate shrinkage (which is mainly the carbonation shrinkage) takes place over long periods.

Other factors: The steam curing has effect on shrinkage unless applied at high pressure. 21

ELASTICITY ofConcrete

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ELASTICITY OF CONCRETEElastic Strains: These are the instantaneous deformations that occur when an external stress is first applied.

Elastic strain in concrete, as defined above, depends on the externally applied stress and the modulus of elasticity of concreteElastic strain = externally applied stress Modulus of elasticity of concrete

Modulus of elasticity is the property of concrete where the material is treated as elastic. Concrete is not a truly elastic material, as evident from the nonlinear stress-strain curve for concrete.

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fig: stress-strain curve for concrete

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Stress-Strain Plot of Concrete

At stress below 30% of ultimate strength, the transition zone cracks remain stable. The stress-strain plot remains linear.

At stress between 30% and 50% of ultimate strength, the transition zone microcracks begin to increase in length, width and numbers. The stress-strain plot becomes non-linear.

At 50 to 60% of the ultimate stress, cracks begin to form in the matrix. With further increase to about 75% of the ultimate stress, the cracks in the transition become unstable, and crack propagation in the matrix will increase. The stress-strain curve bends towards the horizontal.

At 75 to 80% of the ultimate stress, the stress reaches a critical stress level for spontaneous crack growth under a sustained stress. Cracks propagate rapidly in both the matrix and the transition zone. Failure occurs when the cracks join together and become continuous.

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DETERMINATION OF MODULUS OF ELASTICITY OF CONCRETEThe modulus of elasticity of concrete can also be determined by subjecting a cube or cylinder specimen to uniaxial compression and measuring the deformations by means of dial gauge fixed between certain gauge lengths. Dial gauge reading divided by gauge length will give the strain and load applied divided by the area of cross section will give the stress. Strain =Dial gauge reading/gauge length = dl/L Stress = Load/Cross sectional area= P/A

A series of readings are taken and the stress-strain relationship is established.

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Deflection method: E can be determined from testing of beam also. For central point load, Max. Deflection, = WL3/48EIxx27

DIFFERENT MODULUS OF ELASTICITY

1. Initial tangent modulus

2. Tangent modulus

3. Secant modulus

4.Chord modulus28

FACTORS AFFECTING MODULUS OF ELASTICITYElasticity of concrete is effected by:Cement and aggregate factors : Since concrete is a composite material, consisting of cement paste and aggregate, its modulus of elasticity depends on the moduli of elasticity and the volume fractions of cement paste and aggregate.

Moisture condition factor: The moisture condition of the specimen is a factor: a wet specimen has a modulus of elasticity higher than by 3 to 4 GPa than a dry one.

Condition of curing: Another factor affecting the modulus of elasticity of concrete is the manner in which the test cylinders were cured. In general, concrete specimens that were cured in moist conditions resulted in a modulus value higher than those cured in dry conditions.

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Age of concrete: As age increases, E increases

Mix proportion (C + A + W): All ingredients will have its own effect. For a given mix, the effect of one variable should be considered keeping all other variables constant.

Strength of concrete: As strength increases, E increases as shown in Table below

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Fig: variation of modulus of elasticity with compressive strength

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Rate of loading: As the rate of loading increases, E also increases as the creep effect is lessSize and shape of specimen : Cube vs. cylinder, small vs. large

Effect of transition zone: The void spaces and the micro cracks in the transition zone play a major role in affecting the stress-strain behavior of concrete. The transition zone characteristics affect the elastic modulus more than it affects the compressive strength of concrete. Silica fume, metakaolin, RHA in concrete has significant effect on E

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THANK YOU

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