Experimental Study Of Crack Resistance Of High-Volume Fly Ash Concrete

As a byproduct of the industry, the annual emission of the fly ash is huge. A large number of fly ash doesn't get enough rational utilization. This is not only a waste of resources, but will pollute the environment. The use of fly ash in concrete, can improve the nature of cement matrix while reducing the amount of fly ash storage. Compared to ordinary concrete, high-volume fly ash can maximize the use of fly ash while reducing the amount of cement. It has better social, economic and environmental benefits. But the high-volume fly ash concrete has not been widely used at present. The reason is, first, because of the research of high-volume fly ash concrete's own properties, especially the durability is not enough, and second, the concept of engineering and technical personnel also limits the application of high volume fly ash concrete.This paper mainly study on crack resistance of the high-volume fly ash concrete combining a project of Ningxia water resource department. In this paper the ultimate tensile values, shrinkage, and carbonation of different dosage of fly ash are experimental studied, and the ultimate tensile values experiment is studied by the numerical methods. To study the mechanism of the high-volume fly ash concrete's crack resistance from the microscopic point of view, this paper microanalysis the cement paste and interfacial transition zone of the concrete with 0%, 50%, 60%, 70%, and 80% of fly ash. This paper concluded that:(1)Fly ash can significantly increase the ultimate tensile value of concrete under the same strength . It is beneficial to crack resistance of concrete. However, with the growth of curing period, the growth rate of axis tensile strength of concrete and elastic modulus will exceed the growth rate of the value of ultimate tensile.(2)The shrinkage rate of fly ash concrete is significantly lower than the reference concrete that is not mixed with fly ash. And the shrinkage rate of concrete will reduce with the increasing of the dosage of fly ash. However, when the dosage of fly ash is larger, due to the reduction of cement content, the effect of reducing concrete shrinkage is not obvious. The shrinkage high- volume fly ash concrete will no longer develop after 60d age.(3)When the dosage of fly ash is less than 60%, The admixture of fly ash can improve the early crack resistance of concrete. As the age increases, the development of compressive strength of concrete is faster than the tensile strength, and the ratio of tension and compression of fly ash concrete is lower than the early age, but still higher than the reference concrete.(4)The regression equation of exponential model shows good agreement with the experimental results. It can be used for the prediction of the ultimate tensile results, and the crack resistance of fly ash concrete.(5)The results of the FEM analysis and simulation of ultimate tensile test show that increasing the cross-section of the specimen can improve the anti-crack toughness of specimen. When the fly ash content is less than 60%, we can reduce the amount of cement and meet the ultimate tensile properties in the same time. It will significantly weaken the ultimate tensile properties of concrete, when the flyash content is 80%.(6)When the dosage of fly ash is higher, its secondary hydration can significantly reduce the content and growth-oriented of Ca(OH)2 in cement paste and transition zone, so that the Ca(OH)2 becomes amorphous. But the fly ash can not fully play its active effect when the content of fly ash is higher. Too much fly ash that is not involved with the hydration will gather in the around of aggregate, and because there is not enough gel, the structure will become loose and the structure of cement paste and transition zone will become weak.