A Research On Influence On High Strength Concrete Shrinkage Reduction Performance Caused By Super Absorbent Polymer

The shrinking and cracking of concrete directly affect the structural stability and safety of a building.For high strength concrete（HSC）,in condition of the same sort of cement and water-cement ratio,most of shrinking and cracking are mainly caused by humidity decreasing,which is remarkably caused by auto-shrinkage and dry-shrinkagethe in early ages.In order to prevent the shrinking and cracking,super absorbent polymer（SAP）is used as internal curing agent to reduce the degree of shrinkage.This paper researches on the influences of hydration,structure and performance of high strength concrete with super absorbent polymer mixed into.It especially focuses on the reduction of shrinkage caused by that mixing super absorbent polymer into high strength concrete,in early ages.Experiment results prove that in condition of having the same water-cement ratio,HSC with SAP mixed into can obviously reduce the shrinkage in early ages,and reduce the total shrinkage as well.In case of mixing appropriate amount of SAP,14d autogenous shrinkage reduction rate can reach 90%,however,in dry condition the total shrinkage rate reduction can reach 75%.SAP can obviously alleviate early-age hydration and exothermic problems in HSC,and improve hydration degree in a certain time.SAP can also adjust the pore structure of the cement itself,reduce the content of the pores in medium size,and increase the content of the bigger pores（100nm¹000nm）as well as the smaller pores（100nm¹000nm）.It can decrease the rate of relative humidity reduction,especially delay the start of decreasing from the highest ratio,to increase the internal humidity of concrete in the same period of time.The existence of SAP is not conductive to the improvement of concrete strength,but this aspect of effect gradually decreases with the increasing of age.As a conclusion,to compare with simply increasing water content of HSC,mixing SAP into concrete makes better influence in the aspects such as pore structure,the degree of hydration,the shrinkage of volume.In this paper,the reduction mechanism of SAP is discussed on the basis of autogenous-drying shrinkage integrated calculation model.The results of model calculation are as follows:the model can simulate and predict the humidity change and structure change of HSC in the initial stage.At the same age,the high strength concrete with SAP has stronger contraction ability than the concrete without SAP,and the main reason is the water absorbency of SAP.In this paper,based on the calculation model,the prediction results are separated from the total deformation of concrete in the initial stage,and the early changes can be obtained to form the autogenous-expansion curve.The experimental results show that the concrete has obvious expansion in short time after the test begins.Increasing the proportion of water and ash or adding SAP can significantly increase the rate and quantity of expansion;Concrete chemical admixtures（water reducing agent and retarder,etc.）of concrete deformation has played a key role in the early ages,the effect of aspects includes hydration process and pore structure,setting time,performance change,etc..The main factors of the initial expansion and deformation of concrete depend on the type and composition of cement.The main reason for the expansion deformation is the large volume of Ca（OH）₂ and alum crystals formed during the initial hydration period,and the C-S-H gel produced by the high alkali content also plays a certain role.