Effects Of Aggregate Gradation On The Material Properties Of Self-compacting Concrete

The good filling capacity of Self-Compacting Concrete (SCC) is able to increase its homogeneity and reduce the labor intensity in the construction of concrete structures. Because of the higherlabour force cost and improved construction mechanization degree, in recent years, the proportion of SCC in total concrete consumption is increased and the application field is extended. For this reason, the various material properties of SCC were widely investigated by more and more research institutes and engineering units. It is well known that the workability, strength, shrinkage and durability of concrete are significantly influenced by aggregats and the volume fraction of aggregate in SCC is about60%. Therefore, it is very important to investigate the effect of aggregate property on the material performance of SCC.This study was based on YangJiang nuclear power plant,and all the raw material is provided by this project. By changing the sand ratio, and mixture proportion of crushed stone (A/B ratio, size5-10crushed stone volume/size5-16crushed stone volume; B/C ratio, size5-16crushed stone volume/size5-20crushed stone volume), we have studied the effect of sand ratio and the gradation of crushed stone on the workability of fresh SCC and the effect of the grade of crushed stone on harden SCC performance, which include compressive strength, shrinkage, creep, chloride ion diffusion coefficient and carbonation depth. We have also studied the water porosity of SCC with different stone gradation according to the vacuum saturation method.The experimental results indicate that:(1) When the cementitious material content was kept constant, a low sand ratio led to a insufficient cohesiveness of mortar in SCC and a high sand ratio will make the cohesiveness of mortar in SCC too high, both of which are bad for the workability of SCC:the former is easy to cause the SCC segregation and the latter leads toa lack of liquidity and filling capacity of SCC. In this study,when manufactured sand with a fineness modulus of2.8was used. When the content of cementitious material was480kg/m3and the gravel gradation was constant, the fresh SCC derived a good workability when the sand ratio was kept between0.51-0.53. (2) Mortar coating layer is wrapped on the stone surface, acting as a lubricating layer to reduce the friction between the stones. When the gradation of coarse aggregates was changed, the thickness of mortar coating layer changed. A thin mortar coating layer leads to a bad liquidity and filling capacity of SCC as the friction between the stones becomes larger; A thick mortar coating layer also results in a bad workability of SCC because the stones in SCC are easy to subsidence. In this study, when A/B or B/C=4:6-6:4, SCC has the best workability.(3) When the sand ratio increased, the28d compressive strength of SCC was first increased and then decreased and the highest strength was derived when the sand ratio was0.53. The highest compressive strength is also related to the cementitious material content: when the cementitious material content was450kg/m3, the highest28d compressive strength of SCC was62MPa and when the cementitious material content was460kg/m3, the highest28d compressive strength of SCC was71.7MPa.(4) The optimization of the coarse aggregates gradation can change the distribution of internal stress in SCC, thus the drying and autogenous shrinkage can be reduced. In this study, SCC prepared with the mixture, in which the small stone and big stone were mixed at a ratio60:40in volume, has a minimal drying shrinkage value. The drying shrinkage reduction by the optimization of the crushed stone gradation is equivalent to the drying shrinkage reduction by reducing20kg cementitious material in1m3SCC.(5) The gradation of crushed stone has an effect on the creep deformation of SCC. In the condition that the specific surface area of crushed stone was constant, when the void content of crush stone decreased, the supporting function of skeleton structure founded by crushed stone was enhanced. Finally the creep deformation of SCC was reduced.(6) Chlorine ion diffusion coefficient of SCC decreased with the increasing of aggregate size.When A/B=10:0, chlorine ion diffusion coefficient of SCC is4.52×10-12m2/s; when B/C=0:10, chloride ion diffusion coefficient of SCC is2.30X10-12m2/s.