Studies And Controls On Shrinkage Of Modern Concrete

Concrete is known as the most widely used construction material so far, butconcrete is easy to crack due to early-age shrinkage, which can induce many severedamages to concrete structures. So, present research is aimed to reduce and controlearly-age shrinkage and interior humidity reduction of modern high-, moderate-andlow-strength concrete through both experimental and theoretical approaches. Researchfocus is placed on the three aspects of common coarse aggregate, internal curing bypre-saturated lightweight aggregate(PSLWA) and external cover by permanentformwork.To investigate the effect of coarse aggregate volume ratio on early-age shrinkageof concrete, experimental measurements on development of shrinkage and internalhumidity of early-age concrete were carried out under both sealing and dryingconditions. Increasing coarse aggregate volume ratio, both early-age drying andautogenous shrinkages of high-, moderate-and low-strength concrete are reduced. Andthe controlling effect of coarse aggregate on drying shrinkage is more outstanding thanthat on autogenous shrinkage. With the increase in volume ratio of coarse aggregate,elastic modulus of concrete is increased monotonously, and the mechanical propertiesare also improved. The effects of coarse aggregate volume ratio on interior humidity,moisture diffusion coefficient and degree of cement hydration as well in early-ageconcrete are neglectable. The developments of both early-age drying and autogenousshrinkages of high-, moderate-and low-strength concrete with different volume ratios ofcoarse aggregate, can well be predicted using the improved two-stage integrativeshrinkage model.In order to study the internal curing effect of PSLWA on early-age concrete, alarge number of measurements on development of early-age shrinkage and internalhumidity of concrete internal cured with PSLWA were carried out under both sealingand drying conditions. Furthermore, measurements on early-age internal humiditydistribution and development inside of concrete slabs with a larger scale and withPSLWA curing were also performed under one-dimensional drying condition. Theexperimental results show that early-age shrinkage development and interior humidityreduction in high-strength concrete are mitigated by internal curing with PSLWA. The higher the internal curing level, the better its controlling effect on early-age shrinkage ofhigh-strength concrete. Uner drying condition, the internal curing effect of PSLWA ondrying shrinkage of high-, moderate-and low-strength concrete at eary age is decreasedas compared with that under the sealing case. What’s more, the higher thewater-to-binder ratio, the weaker the internal curing effect of PSLWA on dryingshrinkage and internal humidity of concrete. The improved two-stage shrinkage modelcan well predicts the development of shrinkage of concrete internal cured with PSLWAat early age. In the study of the distribution and development of interior humidity insideof concrete slabs, as PSLWA was used to introduce internal curing water, the reductionof interior humidity at early age in high and moderate strength concrete slabs isobviously mitigated, and the gradients of interior humidity are decreased as well. But,internal curing with PSLWA can not completely eliminate near-surface dryingphenomenon in high-, moderate-and low-strength concrete slabs. By developing anone-dimensional internal humidity field model, the distribution and development ofinternal humidity inside of high-, moderate-, low-strength concrete slabs with andwithout PSLWA are well simulated. Afterwards, the effect of PSLWA on thedistribution and development of early-age drying shrinkage in the concrete slabs isanalyzed. Finally, the results of the steel ring tests indicate that the cracking resistanceof high-, moderate-and low-strength concrete is improved by internal curing usingPSLWA, under both one-face drying and fully plastic sealing conditions.At the end, experimental study indicates that the drying shrinkages of high-,moderate-and low-strength concrete at early age are effectively reduced when externalcover by permanent formwork and internal curing by PSLWA were used simulta-neously. The lower the water-to-binder ratio, the bigger the mitigating effect of thecomposite shrinkage-reducing method on drying shrinkage of concrete. However,early-age drying shrinkage of concrete can not completely be eliminated even thecomposite shrinkage-reducing method is used.