Study On The Mechanism Of Internal Curing With SAP Based On Water-Cement Ratio Correction Method

Super Absorbent Polymer(SAP)has become a common internal-curing admixture in concrete.The existing researches show that SAP internal-curing reduces the early shrinkage of concrete,reduces the cracking,improves the strength and increases the mobility.SAP releases water inside the concrete,which effectively improves the hardening process of cement paste and the internal humidity of the service process.All the effects achieve the result of curing concrete from the inside.However,SAP can not only release the curing water in the concrete curing period,but also exchange water with the slurry during mixing time.The process will cause the change of cement paste water-cement ratio,which will lead to the change of the mechanism of the influences of SAP on the shrinkage,strength and fluidity of concrete,and having difficulty clarifying the actual effect of internal curing.In this thesis,in response to the problem,it is necessary to measure the water absorption of SAP in different water-cement ratio of paste through the fluidity balance test accurately;then the actual water-cement ratio of the cement paste under different SAP dosage is unified.Armed with the material strength,moisture content,self-shrinkage and hydration heat test,the effects of SAP on the mechanical properties,shrinkage and hydration process of mortar with different water-cement ratio were discussed on the basis of eliminating the effect of SAP on the ratio of water to cement.The main work and conclusions are as follows:(1)The flowability of paste and the water absorption of SAP in paste with different water cement ratio was measured.The influence of water cement ratio,mixing method and SAP particle size on the water absorption of SAP was analyzed.The results showed that SAP reaches the water absorption stability in 5min in the cement paste.Lower water to cement ratio leads to smaller water absorption of SAP.The water absorption of dry SAP was greater than that of pre wetting.The particle size had little effect on the water absorption of SAP.(2)The autogenous shrinkage and moisture content tests of mortar under four water to cement ratio were carried out.The effects of water-cement ratio,SAP content,SAP particle size on the autogenous shrinkage and moisture content were analyzed.The results showed that lower water-cement ratio and moisture content lead to larger autogenous shrinkage.SAP effectively reduces the autogenous shrinkage of mortar with different water-cement ratio,especially surpassing 80%of the reduction effect under the optimum dosage.For the mortar specimens with water-cement ratio of 0.21 and 0.26,higher SAP content lead to better the reduction of shrinkage.For the mortar specimens with water cement ratio of 0.31 and 0.36,when the SAP content exceeds 0.2%,the reduction of shrinkage will not increase anymore.SAP particle size has little effect on the shrinkage performance.When the pastes have same SAP content,lower water-cement ratio leads to smaller reduction of shrinkage,which indicates that the mortar with low water cement ratio is more dependent on internal curing.(3)The strength tests and hydration heat tests under four water-cement ratios of mortar were carried out.The influence rule of water-cement ratio,SAP content and SAP particle size on strength and hydration process was analyzed.The results showed that the mechanical properties of concrete can be reduced by adding SAP,the higher the dosage of SAP was,the more significant the decrease of compressive and flexural strength was;but for cement paste with low water-cement ratio,the right amount of SAP may increase the strength of the material;after excluding the influence of water cement ratio on the hydration process,it was found that the effect of SAP on the hydration process of gelling material was not very obvious;when the content of SAP was the same,the lower the water-cement ratio was,the more obvious the effect on the hydration process was and the smaller the degree of strength reduction was;the effect of large particle size SAP on hydration of cement is slightly better than that of small particle size SAP,so the effect of large particle size SAP on the decrease of strength is slightly smaller than that of small particle size SAP.