Study On Mechanical Properties And Toughness Of Basalt Fiber Recycled Aggregate Concrete

Researches and applications of recycled aggregate concrete,an important component of the construction waste reutilization and the sustainable developmental needs of building materials,have become hot issues in global academic and engineering fields in recent years.At present,it is generally believed that recycled aggregate concrete has original defects including the weakness of interface transition zone,which is more complex than normal concrete.Therefore,recycled aggregate concrete is weaker than normal concrete in mechanical properties,crack resistance and deformation capacity.The dubiousness of recycled aggregate concrete in the engineering performance and long-term performance limits its application.To expand the application range and improve the utilization rate of recycled aggregate concrete,improving the engineering performance of recycled aggregate concrete is a key focus.According to the New Material Planning in 14^th Five-Year Plan（2021-2025）by Shanxi Provincial People’s Government,the accelerating application of basalt fibers in concrete is stated explicitly.On the basic of low construction waste reutilization in Shanxi Province and local basalt fiber application planning,this paper is designed to combine basalt fiber with recycled aggregate concrete which improving the concrete performance with the help of basalt fiber toughening perfomance.Based on the related project supported by the National Natural Science Foundation of China and previous researches in this group,this paper concentrates on the researches of the mechanical properties,crack resistance,and toughness of basalt fiber recycled aggregate concrete,which aims to reveal the improvement mechanism of basalt fiber characteristic parameters on the engineering performance of recycled aggregate concrete and provides references for further engineering application of basalt fiber recycled aggregate concrete.The main findings and conclusions are as follows:（1）Experimental research on the effect of mechanical properties of basalt fiber recycled concrete was carried out.The effects of volume fraction and length-diameter ratio of basalt fiber on the compressive strength,tensile strength and flexural strength of recycled concrete were analyzed by cubic compressive strength test,splitting tensile strength test and flexural strength test,and compared with those of ordinary concrete.The results showed that the volume fraction and length-diameter ratio of basalt fibers had a significant effect on the basic mechanical properties of recycled concrete.When the volume fraction and length-diameter ratio of basalt fibers were within a reasonable range,the maximum effect on the tensile strength was 32.84%.Compared with ordinary concrete,the enhancement of mechanical properties of recycled concrete by basalt fibers is more significant.As the recycled coarse aggregate caused certain strength loss to the mechanical properties of recycled concrete,when the replacement ratio of recycled coarse aggregate was 100%,basalt fiber could make up 56.00%of the cubic compressive strength loss and 62.10%of the flexural strength loss,respectively.When the replacement rate of recycled coarse aggregate is 50%,it can compensate 94.40%of the splitting tensile strength loss.（2）Experimental study on uniaxial compressive/tensile principal structure relationship of basalt fiber recycled concrete was conducted.The effects of basalt fiber volume fraction and length-diameter ratio on the uniaxial compressive/tensile stress-strain curves and load-deformation properties of the recycled concrete were analyzed by uniaxial compressive and uniaxial tensile tests.The results showed that both the volume fraction and length-diameter ratio of basalt fibers affected the stress-strain curve.When the fiber volume fraction and length-diameter ratio are within reasonable range,the maximum uniaxial compressive peak stress elevation is 23.15%and 26.94%,and the maximum peak strain elevation is 29.03%and 29.15%for the basalt fiber recycled concrete with 50%and 100%recycled coarse aggregate replacement ratio,respectively,compared with the recycled concrete without fiber addition.Its uniaxial tensile peak stress maximum elevation rates were 29.41%and 33.45%,and the peak strain maximum elevation rates were both 16.70%.From the engineering design,the uniaxial compressive/tensile stress-strain full curve equation of concrete in the Code for Design of Concrete Structures（GB50010）is used as the basis,and the uniaxial compressive/tensile stress-strain principal structure model of basalt fiber recycled concrete is proposed with consideration of fiber reinforcement factor and high applicability.（3）The experimental study of early cracking performance of basalt fiber recycled aggregate concrete was carried out to analyze the influence law of different volume fraction and length-diameter ratio of basalt fiber on early cracking evaluation index of recycled aggregate concrete and reveal the mechanism of early cracking resistance enhancement of basalt fiber.The results show that basalt fiber can significantly improve the early cracking performance of recycled aggregate concrete,and when the volume fraction and length-diameter ratio of fiber are within reasonable range,the maximum improvement of early cracking performance of recycled aggregate concrete with 50%and 100%of recycled coarse aggregate substitution is 94.10%and 94.60%,respectively,compared with that of recycled aggregate concrete without fiber;the improvement of early cracking performance of recycled aggregate concrete by basalt fiber is higher than that of normal concrete.The improvement effect of basalt fiber on the early crack resistance of recycled aggregate concrete is more significant than that of normal concrete,and the improvement effect is more obvious as the replacement rate of recycled coarse aggregate increases;basalt fiber not only makes up for the adverse effects brought by recycled coarse aggregate,but also makes the early cracks of recycled aggregate concrete disappear basically.（4）Experimental research on fracture performance of basalt fiber recycled concrete was carried out.The effect of fiber volume fraction and length-diameter ratio on the fracture performance index of recycled concrete was analyzed by three-point bending test of precast cutout beam.The results showed that the larger the replacement ratio of recycled coarse aggregate,the more obvious the effect of fiber on the fracture performance of recycled concrete.The volume fraction and length-diameter ratio of basalt fibers exist in a reasonable range.When the replacement rate of recycled coarse aggregate was 50%and 100%,the fiber could improve the fracture energy of recycled concrete by 9.34%and 35.44%,respectively.As the recycled coarse aggregate causes some fracture energy loss to the recycled concrete,the fiber can make up 29.58%and 51.27%of the fracture energy loss,respectively.A comprehensive analysis of the relationship between fracture performance index and fiber reinforcement factor established a significant second-order nonlinear relationship between cohesive toughness and basalt fiber reinforcement factor for the first time.（5）The analysis of the toughening effect of basalt fiber on recycled aggregate concrete was carried out,and the toughening mechanism and microstructure of basalt fiber on recycled aggregate concrete were explored based on the fiber-cementitious interface bonding performance test and microscopic performance test.The results show that:the critical bond length of fiber is 13.4mm,and when the bond length is less than 13.4mm,the fiber is pulled out in the cement matrix;strengthening the bond performance at the fiber-cement matrix interface can increase the energy dissipation capacity of the matrix and improve the toughness of recycled aggregate concrete;the weak zone at the interface between fiber and cement matrix is about60μm from the fiber surface,and the saturated pre-wetting of recycled coarse aggregate is conducive to giving full play to recycled coarse aggregate"Internal curing"effect,when the recycled coarse aggregate soaking time is greater than 0.25h can give full play to the"Internal curing",effectively reduce the thickness of the interface transition zone,promote the hydration of the interface transition zone between the fiber and cementitious materials and recycled coarse aggregate.The interfacial weak zone for 50%replacement rate is 50μm～70μm,while the interfacial weak zone for 100%replacement rate of recycled coarse aggregate is less than 50μm.