Study On The Cracking Factors Of Ultra-high Volume Fly Ash Concrete Based On TSTM

While the temperature deformation and autogenous shrinkage deformation of the mass fly ash concrete are restricted by the heat of hydration in early age,it creates a greater restrained tensile stress.However,the early development of tensile strength of concrete is slow and the value of tensile strength is low,which can easily lead to the cracking of concrete.To avoid cracking is always the key and difficult point of large volume fly ash concrete,so it is very important to study the cracking factors and evaluate the crack resistance of fly ash concrete.In this paper,the 35% volume of fly ash concrete in the practical engineering is used as the reference concrete,using slurry volume method to configure the 80% volume of fly ash concrete called ultra-high volume fly ash concrete.Based on the temperature-stress testing machine(TSTM),the study on the early temperature cracking factors of ultra-high volume fly ash concrete and the evaluation of the crack resistance considering the temperature factors were carried out.The stress and strain of two kinds of concrete were measured byTSTM under the full constraint condition of temperature-matching(TMC)mode and adiabatic mode.According to the Creep of Kolver determination method,the creep of two kinds of concrete is calculated;Besides,the stress relaxation coefficients of two kinds of concrete are calculated based on the elastic modulus model of Kanstad;What's more,the coefficient of thermal expansion of concrete is determined based on the theory of maturity.Furthermore,the temperature deformation and autogenous shrinkage deformation are separated.The comprehensive evaluation method of early crack resistance was put forward.The results show that the restrained stress of ultra-high volume fly ash concrete under the condition of full constraint develops slowly,and the tensile stress appears late.The early creep performance and stress relaxation ability are better than the reference concrete.Both the thermal expansion coefficient and autogenous shrinkage strain are lower than the reference concrete.According to the crack-resistant comprehensive evaluation method based on TSTM,the crack sensitivity of ultra-high volume fly ash concrete is lower than that of reference concrete and the crack resistance is better.Measure the mechanical properties and deformation characteristics such as compressive strength,tensile strength,ultimate tensile value and elastic modulus of two kinds of concrete,and fit the testing results.Then he basic mechanical properties of concrete under the actual temperature environment can be calculated based on the equivalent age theory,so asto obtain the crack resistance indexes such as the ratio of adiabatic temperature and tensile strength,the ratio of elasticity modulus and tensile strength,the ratio of compressive strength and tensile strength,crack sensitivity and ultimate tensile value.Two kinds of concrete crack resistance were evaluated with these crack resistance indexes.The results show: the crack resistance of ultra-high volume fly ash concrete is better in the aspects of the ratio of adiabatic temperature and tensile strength,the ratio of elasticity modulus and tensile strength,the cracking sensitivity;while the crack resistance of reference concrete is better in the aspects of ultimate tensile value,the ratio of compressive strength and tensile strength.