Shrinkage and Creep of Concrete(Section-II)

ShrinkageThe concrete undergoes some volume changes due tosettlement of the fresh concrete, chemical combinations ofhigh alkali cements with certain reactive aggregates,changes in moisture content, changes in temperature anddue to applied loads.During the process of the hydration of the cement thereoccurs change in volume due to the reduction in volume ofthe cement paste.When the cement paste is plastic, it undergoes a volumetriccontraction.Concrete shrinks when it dries and expands when wettedagain due to moisture movement.

Plastic Shrinkage Plastic shrinkage occurs mainly due to the settlement of thecement paste and bleeding of water to the top which finallyevaporate.When the deformation is restrained by reinforcement,stresses will be developed which results in cracking ofconcrete.Higher w/c, badly proportioned concrete, rapid drying andgreater bleeding are reasons of plastic shrinkage.Cracking reduces the ability of the structure to carry thedesigned load and reduces the durability of the structure.

Plastic Shrinkage

Plastic Shrinkage Cracks

Autogenous Shrinkage

Drying Shrinkage Drying shrinkage occurs due to loss of water byevaporation from freshly hardened concrete exposed to air.Mechanism of drying like timer.It is the loss of water held in gel pores that causes inchange in volume.Finer the gel more is the shrinkage.

Drying Shrinkage Cracks

Carbonation Shrinkage

Creep in Concrete

Creep in Concrete

water

Creep

Consequences of creepCreep defined as the flow of concrete undersustained stress or load.When a concrete member is loaded, themember deforms to a certain amount assoon as the load is applied.The deformation of the specimen increaseswith time even when the load on themember is kept constant.

Consequences of creepThe creep may be due to viscous flow of thecement paste, closure of internal voids andseepage flow of colloidal water from the gelthat is formed by hydration of the cement.The greater the force, the steeper thepressure gradient with resulting increase inrate of expulsion of moisture anddeformation.

Creep recoveryWhen the load on the specimen is removed,the instantaneous strain decreasesimmediately, this instantaneous recovery isfollowed by a gradual decrease in strain,called creep recovery.The longer the loading period, the less therecovery due to the hardening of theconcrete.

Factors influencing CreepAggregate cement ratioWater cement ratioNature of aggregate and its gradingComposition and fineness of cementAge at the time of loadingIntensity and duration of stressThe humidity of the ambient air

Factors influencing CreepGreater the maximum size of an aggregatewith graded uniformly from fine to coarse,the less the creep of concrete.Variations in particle shape, surface textureof the aggregate influences the creepcharacteristics of the concrete.One of the most important factor affectingcreep is relative humidity of the airsurrounding the concrete.

Factors influencing CreepAn increase in the humidity reduces the rateof loss of moisture and this reduces theseepage. Creep is higher with lower relativehumidity.Drying shrinkage at or near the surface ofthe specimen. Concrete having highshrinkage shows high creep.C=Ci (1+ΦS), Ci is the creep under

shrinkage, S is the shrinkage under relativehumidity, Φ is the constant.

Factors influencing CreepCreep decreases with increase in the size ofthe specimen.The greater the degree of hydration of thecement at the time of load application, thelower the amount of total creep.

Creep and TimeGenerally 18 to 35% of the 20 years creepoccurs in 2 weeks.40 to 70% occurs in 3 months and 64 to 83% occurs in one year.

General effects of CreepCreep affects strains and deflections andalso stress distribution.Creep increases deflection of a reinforcedconcrete beam under sustained load.Creep affects the stability of the structuresince the deformation of the structure getsincreased.In pre-stressed concrete the loss of pre-stress due to creep in concrete is more.