Research On Internal Curing Mechanism And Control Technology Of High Strength Concrete

High strength concrete (HSC) has excellent mechanical performance. It could increase bearing capacity of structural member, or reduce the section area under the same bearing capacity compared to normal strength concrete. This could save raw materials or heighten aesthetic effect of construction works. So HSC is widely used in high-rise buildings and long-span bridges. But the high-strengthening measures of concrete are to lower water/binder ratio (w/b ratio) by superplasticizer and incorporate highly active mineral admixtures such as silica fume, which greatly enhance the strength of concrete but lead to serious early-age self-desiccation and autogenous shrinkage cracking due to the rapid consumption of water.This paper focuses on early-age self-desiccation and autogenous shrinkage of HSC, which includes the feature of self-desiccation and effect of w/b ratio and mineral admixtures on internal relative humidity (IRH) and autogenous shrinkage of concrete. Results indicate that lowering w/b ratio and incorporating silica fume could accelerate the decrease of IRH at early age. The relationship of IRH and autogenous shrinkage is also established, which could provide the basis for mitigating autogenous shrinkage by controlling IRH of concrete using internal curing materials.Water absorption and release of internal curing materials are discussed under the condition which is the simulation of concrete. Then internal curing materials for the next study are selected. The effect of types, dosages and incorporation methods of internal curing materials on IRH and autogenous shrinkage are studied and results indicate that water retention capacity of expanded clay and super absorbent polymer is superior to that of expanded shale. Compound of pre-saturated expanded clay and super absorbent polymer in concrete could greatly mitigate self-desiccation and reduce autogenous shrinkage at different ages and obtain a better internal curing effect compared to concrete with only one kind of internal curing material.Spacial distribution of IRH in cylindrical specimen at different ages is also discussed. Results indicate that IRH is almost the same in HSC but IRH near the surface is lower than the inner IRH. Internal curing materials could not only mitigate self-desiccation and increase IRH inside the specimen, but also take a compensation effect for evaporation of water near the surface of concrete.Effect of internal curing materials on crack resistance of HSC is studied by uniaxial restraint testing method. Based on the experimental results, a modified method for the evaluation of crack resistance of concrete with internal curing materials is put forward. The results of uniaxial restraint test indicate that internal curing materials made up of expanded clay and super absorbent polymer could reduce the shrinkage and elastic modulus as well as the tensile stress under constraint at early age. All of these effects could increase the stress reserve and enhance the crack resistance of HSC.