Study On Mechanical Properties And Cracking Sensitivity Of High Fly Ash Concrete

With the vigorous development of the social economy,the urgent demand of the natural environment protection and the rapid production and application of various additives,the fly ash concrete have been widely used.At present,the fly ash has been used as an independent component to add to the concrete,and its proportion is larger and larger.However,the standards or regulations in many countries,including our country,have a limited amount of fly ash content.Through the experimental study of the high content of fly ash concrete,Gezhouba Group Company and Our school found that though the amount of fly ash reached 80%,the prepared concrete can still meet the basic requirements of the dam.High performance concrete can be made by adding fly ash to concrete,which can effectively improve the durability of concrete and reduce the cracking sensitivity of concrete.In this paper,based on the practical engineering application of the 35% volume of fly ash concrete as the reference concrete,80% volume of ultra high volume of fly ash concrete was prepared by using the equal volume method.The mechanical properties and crackingsensitivity of two kinds of concrete at early age were studied based on the temperature-stress testing machine(TSTM).The main contents are as follows:1.TSTM was used to test the thermal stress of the above two types of fly ash concrete in adiabatic mode and temperature matched mode under full constraint condition,and the restraining stresses in the early age of the two kinds of concrete were measured.creep coefficient and creep-shrinkage ratio of the two kinds of concrete are obtained.The results show that: the temperature history of the same kind of concrete in the early age has a larger influence on constraints stress and creep properties,which means the higher the temperature,the longer the restraint stress and creep.At the same time,faster cooling rate also increases the creep ability of the concrete members in the early time.Creep coefficient and creep-shrinkage ratio of the two concrete under the temperature matched mode can be found that the creep ability of the ultra-high fly ash concrete in the matching mode is stronger than the reference concrete.However,there is not much difference between the two in adiabatic mode.2.Based on the elastic modulus model of Kanstad and the theory of maturity,the stress relaxation of two kinds of concrete was calculated.The stress relaxation of the two kinds of concrete during the early age of cooling stage was analyzed.And the stress relaxation of the two kinds of concrete is compared.The results show that the early stress relaxationcapacity of ultra-high fly ash concrete is better than that of common fly ash concrete from stress relaxation and creep-shrink ratio.In addition,the experiment shows that the cooling rate and the development of elastic modulus have a great impact on the stress relaxation of concrete.3.Based on the results of the basic mechanical properties test,the development law of tensile strength of the two kinds of concrete was fitted to calculate the early cracking sensitivity coefficient of concrete.What's more,cracking core index and cracking composite index are used to comprehensively evaluate the cracking sensitivity of two kinds of concrete based on the data of temperature-stress test to lay the foundation for the evaluation of cracking sensitivity of the two types of fly-ash concrete in practical engineering.The results show that the cracking resistance of ultra-high fly ash concrete is stronger than that of reference concrete,indicating that the cracking resistance of ultra-high fly ash concrete increases with the increase of fly ash content.