Research On Internal Humidity And Shrinkage And Their Effect On Plastic Shrinkage Cracking Of Self-compacting Concrete At Early Age

In the resent years, Self-Compacting Concrete(SCC) is applied more and moreextensively in China in large-scale projects, because SCC has excellent mobility,self-compacting and stability, while it can greatly simplify the constructing techniquesof the concrete structure, and shorten the cycle of construction. In addition, comparedwith ordinary concrete, SCC which includes high content of cementitious materials,high sand ratio, and low cement-binder, results in the increase of shrinkage and areaof shrinkage cracking. It has an urgent need to address to improve durability ofconcrete structure.This thesis which is with the background of nuclear power engineering, based oncharacteristics of local materials and environment, focus on the property of resistanceto cracks of early new mixing concrete, the effects and law of internal humidity andshrinkage of SCC. The study might validate the actual quality of SCC in practicalproject.First of all, the effects of water-binder ratio, sand ratio and the dosage of fly ashon plastic shrinkage cracking of concrete at early age were studied. It is found that,while the water-binder ratio was of0.30-0.55, the evaporation rate of fresh concreteincreased with the rise of the water-binder ratio; the evaporation rate increased withthe increase of sand ratio in the sand ratio of51%-55%; in the content of15%-40%for fly ash, the evaporation rate reached the minimum for the content of fly ash at30%. In the water-binder ratio of0.30-0.55, cracking area at the water-binder ratio0.45reached the maximum; cracking area decreased with the increase of sand ratio of51%-55%; cracking area increased as the dosage of fly ash increasing in the range of15%-40%.Moreover, the influencing factors on internal humidity of SCC were investigated.The results showed that, the nearer the internal humidity of SCC was decreased moreseriously; with the sand ratio of51%-55%, the extent of internal humidity of concretedecreased with the sand ratio increasing; in the range of0.35-0.40of water-binder,internal humidity of concrete reached the minimum at the water-binder0.40; if difference of paste content of concrete was because of different water-binder theinternal humidity of concrete dropped seriously with paste content increasing; pastecontent which was depended on the binder-aggregate increased when internalhumidity of concrete decreased; the dosage of fly ash which resulted in change ofpaste content of concrete raised when internal humidity of concrete decreased slightly.Furthermore, the efforts on internal shrinkage of SCC were researched. Fordifferent mixing proportion of SCC, the closer to surface of concrete,the degree ofinternal contraction of concrete was greater; when sand ratio with the range of51%-55%increased, the shrinkage deformation of concrete increased; in the range of0.35-0.40of water-binder ratio, internal contraction of concrete reached the maximumat water-binder0.38; in the range of33%-37%of paste content, in general, pastecontent of concrete increased when internal contraction of concrete increased,especially, when water-binder was relative smaller and paste content of concrete waslarger, internal contraction of concrete was relative larger.Lastly, internal shrinkage of concrete was related with the depth of the surface ofconcrete in single-side drying conditions. Internal humidity of concrete decreasedwith the depth of concrete increasing. Besides, the change of internal humidity ofSCC was later than internal contraction of SCC. And it was found that the internalshrinkage strain of concrete had a linear relationship with values of internal humidityand ln(RH).