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By: chandani chandra neupanest115587SHIRNKAGE of concreteOUTLINEIntroductionTypesFactors affecting shrinkageEffectsPrevention ConclusionIntroductionShrinkage is a reduction in volume, and in concrete, it is mainly caused by the loss of water. In most cases, shrinkage is measured by monitoring longitudinal strain.

When tensile stresses due to restrained volume contraction exceed the tensile strength of concrete, the shrinkage leads to cracking, which is called shrinkage cracking.

IntroductionConcrete 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. All concrete, even shrinkage-compensating concrete, shrinks. Because of a loss in volume, concrete shrinkage can lead to cracking when base friction or other restraint occurs. Shrinkage also causes curling/warping which can lead to a variety of slab issues including decreased load-carrying capacity (structural cracking) and joint stability problem.Types of Shrinkage in Concrete

Shrinkage can beclassifiedin the following way:

(a) Plastic Shrinkage(b) Drying Shrinkage(c) Autogeneous Shrinkage(d) Carbonation Shrinkage

Plastic ShrinkageVolumetric contraction ,which takes place while the concrete is still in the plastic state (Fresh Concrete) is known as Plastic Shrinkage(also called Early Volume Change)Loss of water byEvaporation from surfaceof concrete Absorption by aggregate or subbase

Plastic ShrinkageRate of evaporation is accelerated by ofHigh wind speedLow relative humidityHigh ambient temperature

Other causes of Plastic ShrinkageUnintended vibration or yielding of formwork support High water/cement ratio Badly proportioned concrete Rapid drying Greater bleedingPlastic ShrinkageReported if rate of evaporation exceeds about 0.5 kg/m2/h.IN ACI 305R[2] ,precautions should be taken if rate expected to approach 1.0 kg/m2/h.Evaporation rate can be estimated by using nomograph.

Plastic ShrinkagePlastic shrinkage can be reduced by

Preventing the rapid loss of water from surface Covering the surface with polyethylene sheeting immediately on finishing operationMonomolecular coatings by fog spray that keeps the surface moist Working at night

Revibrate the concrete in a controlled mannerUse of small quantity of aluminium power is also suggested to offset the effect of plastic shrinkagePlastic ShrinkageReducing the temperature of concrete mixture and increasing its rate of setting (accelerating admixture)Use of monosynthetic fibersTemporary windbreaks and sunshadesExpansive cement or shrinkage compensating cement also can be used Further, use of unneeded high slump concrete, over sanded mix, higher air entraining should be discouraged

Drying shrinkageAfter hardening, concrete begins to shrink as water not consumed by cement hydration leaves the system. This is known as drying shrinkage.

It is inevitable unless the concrete is completely submerged in water or in an 100% RH environmentIn general, the higher water content, the higher the shrinkage potential.

Drying shrinkageIt is the loss of water held in gel pores that causes the change in the volume. Under drying conditions, the gel water is lost progressively over a long time, as long as the concrete is kept in drying conditions.The drying shrinkage of concrete can be determined in laboratory.Expressed as a percentage or in millionths(*10^-6) Physically, concrete that experiences a drying shrinkage of about 0.05%.(50mm for every 100m)Drying shrinkageCement paste shrinks more than mortar and mortar shrinks more than concrete. Concrete made with smaller size aggregate shrinks more than concrete made with bigger size aggregate. The magnitude of drying shrinkage is also a function of the fineness of gel. The finer the gel the more is the shrinkage. It has been pointed out earlier that the high pressure steam cured concrete with low specific surface of gel, shrinks much less than that of normally cured cement gel.

Drying shrinkageDrying shrinkage can be reduced by Reducing water content as low as possible By Using a high content of hard and rigid aggregate By Using Water reducing admixturesShrinkage reducing admixture Reduce Drying shrinkage and rate of drying shrinkageImprove cracking resistanceReduce curling height and rate of curlingReduce joint opening and rate of joint opening

Autogeneous Shrinkage No moisture movement to or from the paste is permitted and when temperature is constant , some shrinkage may occur. The shrinkage is known as autogeneous shrinkage. If no extra water is available for cement hydration, all pore water is progressively consumed. This process empties the pores in the concrete and produces a drop of the internal relative humidity (self-desiccation). Emptying of the pores and self-desiccation leads to capillary stresses in the pore water, which induce an external volume contraction, called autogenous shrinkage. Autogeneous ShrinkageContrary to drying shrinkage, autogenous shrinkage occurs without any loss of moisture from the concrete. Autogenous shrinkage becomes important with w/c