| As a kind of "green" concrete,recycled concrete has wide application prospects.Because the physical properties of recycled aggregates are different from those of natural aggregates,the compressive strength,tensile strength,peak strain,etc.of the prepared recycled concrete are quite different from ordinary concrete.The current research on recycled concrete is focused on improving its performance and the research on compressive strength,but the research on its flexural strength and its size effect is still insufficient.This paper systematically studies the effects of recycled aggregate replacement rate,steel fiber content,and strength grade on the flexural strength and its size effect of recycled concrete and analyzes the mechanism of size effect.The main contents are:(1)The flexural strength test was performed on prism specimens with dimensions of 70mm?70mm?310mm、100mm?100mm?400mm and 150mm?150mm?550mm under the five kinds of recycled aggregate replacement rates of 0%,25%,50%,75% and 100%,to analysis the effect of replacement rate of recycled aggregates on flexural strength and its size effect.The result show that,the flexural strength of the recycled concrete is lower than that of ordinary concrete.When the replacement rate is 25%,50%,75% and 100%,the flexural strength of the specimen with a section length of 150 mm is reduced by 5.7%,12.7%,24.2%,and 19.9%,respectively,compared to the ordinary concrete specimens.The recycled concrete has a more pronounced size effect.Within the range of 0% to 75%,the size effect tends to be obvious as the replacement rate increases;For fully-replaced recycled concrete(100% replacement rate),the size effect phenomenon is reduced.The size effect of the flexural strength of the recycled concrete is about 26.74% higher than that of ordinary concrete.(2)For fully-replaced recycled concrete specimens,steel fiber with volume content of 0.25%,0.5%,0.75% and 1.0% was used to study the effect of steel fiber content on flexural strength and its size effect.The test results show that steel fiber can significantly improves the flexural strength of recycled concrete.When the steel fiber content is 0.25%,0.5%,0.75% and 1.0%,the flexural strength of the specimen with a section length of 150 mm was increased by 8.2%,18.5%,16.1% and 14.7% compared with the specimen without steel fiber.With the increase of the steel fiber content,the size effect of the flexural strength of the recycled concrete tends to be obvious.Adding steel fiber increases the size effect of the flexural strength of recycled concrete by about 21.23%.(3)A flexural test was carried out on C20,C30 and C40 strength grades of fully-replaced recycled concrete prism specimens to study the effect of strength grades on test results.The test results show that the higher the strength grade of recycled concrete,the more obvious the size effect.The size effect of flexural strength of C40 and C30 recycled concrete is increased by about 39.36% and 19.83% compared to C20 recycled concrete.(4)The size effect theory of concrete is systematically analyzed.Based on the experimental data in this paper,the values of the parameters in the Bazant size effect law formula and Carpinteri size effect law formula are calculated,and the concepts of the critical size,critical strength and critical strength ratio of the flexural strength of recycled concrete are established.(5)Random aggregate model of recycled concrete was established by combining MATLAB and ANSYS finite element software,and the mechanical parameters of the new interface between the natural aggregate and the mortar matrix and the old interface between the recycled aggregate and the mortar matrix were obtained.Through meso-analysis of numerical specimens in a larger size range,the size effect law formula for the flexural strength of numerical specimens is obtained. |
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