Study On Crack Resistance Of High Strength Fly Ash Concrete Under The Condition Of Equal Strength

Fly ash has been used to improve the crack resistance of concrete,but the research on crack resistance is often not carried out at a strength level.Therefore,by changing the water-cement ratio and fly ash content,the compressive strength of fly ash concrete and pure cement concrete at the age of 28d is basically the same.Tests on mechanical properties,adiabatic temperature rise and shrinkage performance were carried out under the condition of equal strength(compressive strength)to evaluate the crack resistance,and the crack resistance mechanism was analyzed by combining SEM and MIP.The specific contents are as follows:In this paper,the mixture ratio design of C60,C80 and other strong groups was first completed.The water-binder ratio of C60 group was 0.41,0.31 and 0.33,the fly ash content was 0%,50%and 50%,the fly ash content of C80 group was 0.24,0.20 and 0.16,and the fly ash content was 0%,30%and 50%.On the premise that the compressive strength is basically the same,the differences between the mixture ratio of fly ash and the mixture ratio of pure cement in the splitting tensile strength,uniaxial tensile strength and tensile elastic modulus in the equivalent strength are compared.Then,the adiabatic temperature rise and shrinkage performance of fly ash concrete under the condition of equivalent strength were studied,and the temperature difference tensile stress and temperature shrinkage stress under self-constraint and external constraint conditions were calculated to evaluate the crack resistance performance.Finally,combined with SEM and MIP microscopic test methods,the crack resistance mechanism was analyzed.The test results show that:within the range of 0.16-0.41,30%fly ash is added at the same time to 0.02 water-cement ratio is reduced or 50%fly ash is added at the same time to 0.1 water-cement ratio is reduced or 50%fly ash is added,and 1%Na2SO4 is added at the same time to reduce the water-cement ratio,so that the compressive strength of fly ash concrete and pure cement concrete can reach same at the age of 28d..Sample mixed with 50%fly ash and activator in the strong group C60 showed the best performance in mechanical properties,adiabatic temperature rise and shrinkage performance,and no crack risk appeared in crack resistance calculation.Sample mixed with 50%fly ash and activator in the strong group C80 had better crack resistance performance than pure cement concrete.SEM and MIP analysis showed that the micropores and cracks in the internal structure of concrete,as well as the large holes larger than 100000nm,were the important reasons for the internal cracks.