| Early age cracking of mass concrete is a common problem in practical project.Using mineral admixtures in concrete is one of the effective methods to control the early age cracking of mass concrete.In this study,the mechanical properties of concrete containing mineral admixtures under standard curing and controlled temperature history curing were analyzed and compared.Thermal-stress testing machine(TSTM)was used to study the development of early temperature-rise,strain,restraint stress and creep of the concrete containing mineral admixtures under adiabatic temperature-rise mode and 100% restraint degree.Finally,the existing TSTM-based cracking resistance evaluation methods were applied to compare the cracking resistance of the concrete containing mineral admixtures.The cracking resistance evaluation methods were also compared and analyzed.The conclusions of this study are as follows:(1)Under standard curin,the compressive strength of concrete was reduced by the addition of fly ash and slag;both fly ash and slag reduced the early-age splitting tensile strength of concrete,but at the later stage(60d),the splitting tensile strength of concrete containing 40% fly ash and the concrete containing 30% fly ash and 20% slag were higher than that of the conference concrete;when the fly ash contents were 35% and 40%,the effect of fly ash on the flexural strength of concrete was not significant.,and the flexural strength of concrete reduced when the fly ash content was 50%,the addition of 20% slag reduced the flextural strength of fly ash concrete.(2)Under the curing condition of controlled temperature history,the compressive strength of concrete was reduced by adding fly ash,while the compressive strength of concrete was improved by adding slag;when fly ash content was 40%,the splitting tensile strength of concrete was higher than that of the reference concrete at the end of curing,but when the fly ash contents was 35% and 50%,the splitting tensile strength of concretes were lower than the conference concrete;the addition of 20% slag improved the splitting tensile strength of concrete;the addition of fly ash reduced the compressive elastic modulus of concrete,while slag improved the compressive elastic modulus.(3)According to TSTM tests,the addition of fly ash effectively alleviated the early hydration temperature rise and autogenous shrinkage of concrete.Comparatively speaking,when fly ash content was 40%,the cracking temperature was the highest.The addition of slag also alleviated the early temperature rise of concrete,but increased the autogenous shrinkage of concrete;the addition of slag reduced the cracking stress of fly ash concrete and increased the cracking temperature.(4)When the fly ash contents were 35% and 40%,the compressive creep of concrete was smaller than that of the reference concrete,and when the fly ash content was 50%,the compressive creep of concrete was larger than that of the reference concrete;the development law of tensile creep of concrete with different content of fly ash was basically the same;the addition of fly ash reduced the specific creep at the early stage of tension,and has little influence on the specific creep at the later stage of tension,and had little influence on the specific creep at the later stage of tension.The addition of slag reduces the compressive creep of fly ash concrete;the development law of tensile creep of fly ash concrete and double concrete is similar;the addition of slag reduced the specific creep of concrete in the tensile stage.(5)By applying the crack resistance evaluation method based on TSTM,it was found that the crack resistance of concrete with 40% fly ash was the best,and the addition of 20% slag reduced the crack resistance of fly ash concrete.Based on the comparison,it was concluded that the crack resistance evaluation method based on the analytic hierarchy process was a more reasonable and comprehensive method to evaluate the cracking resistance of concrete. |
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