Multi-scale Study On Tensile Behavior Of Engineered Cementitious Composites Incorporating Normal Sand

An Engineered Cementitious Composite(ECC)reinforced with short-cut synthetic fibers can greatly improve the brittleness of concrete due to its super-strong deformation ability.Under uniaxial tension,ECC exhibits the multiply steady-state cracking and tensile strain hardening behavior.The tensile strain can exceed 3%,which is more than300 times that of ordinary concrete.The width of the crack is usually less than 150 μm.This excellent tensile behavior of ECC results from its mix design and raw material selection.The aggregate used in the mix design is usually micro-silica sand with an average particle size of around 110 μm and a maximum particle size of around 250 μm.However,due to the small particle size and amount of micro-silica sand,the shrinkage deformation of ECC,caused by the use of more cementitious materials,is usually aggravated.In addition,due to the expensive price and the complicated preparation of micro-silica sand,ECC cost is increased and ECC application is limited.In order to improve the shrinkage,reduce the cost,and ensure the good tensile performance of ECC,this study uses normal sand to replace superfine quartz sand as aggregate to prepare ECC.Based on the resulting normal sand ECC,its economics,shrinkage deformation and mechanical properties are investigated,the multi-scale influence mechanism of normal sand on the tensile behavior of ECC is elucidated,and the design method and the prediction model of the tensile behavior for normal sand ECC is given and the prediction model.The details of the study are as follow.ECCs with different the compressive strength are prepared by using normal sand(i.e.,the maximum particle size of sand is 4750 μm and the gradation is the same as that of ordinary concrete fine aggregate)and the polyethylene fiber;the cost of normal-sand ECC and ultralfine quartz sand ECC are compared;the shrinkage deformation of normal sand ECC and ordinary superfine quartz sand ECC are compared;the compressive properties and tensile behavior of normal sand ECC and ultralfine quartz sand ECC are evaluated;the feasibility of using normal sand instead of superfine quartz sand as aggregate to prepare ECC is evaluated by considering the cost analysis,shrinkage deformation,and mechanical properties.The results showed that the use of normal sand(ordinary river sand)could reduce the cost and improve the shrinkage while keeping the mechanical properties almost unchanged,especially in the high strength grade,the tensile properties of ECC with normal sand are significantly improved.However,it is noted that normalsand ECC exhibited the different tensile behavior.Based on the differences in macroscopic tensile behavior(multiply steady-state cracking and tensile-strain hardening behavior)between normal-sand ECC and microsilica sand ECC,the mechanism of the effect of using normal sand on the tensile behavior of ECC was elucidated at the microscopic scale,that is,the mechanism of the effect of normal sand on the formation process of individual cracks in the matrix,and the types of pre-existing defects,the crack initiation,the crack propagation,and crack branching in the matrix of normal sand ECC are clarified.Based on the mechanism of the influence of normal sand on the formation process of individual cracks in the matrix,the calculation formula of the matrix cracking strength is derived,and the variation law of the matrix fracture energy of in different strength grades is explained,the influence of the normal sand content on the matrix cracking strength of ECC with different strength grades is analyzed,the mathematical expression of the steady-state cracking of normal sand ECC matrix is given.Finally,the effect of sand content on the steady-state cracking of normal sand ECC matrix is presented,that is,the mesoscopic mechanism of the effect of the use of normal sand on the tensile behavior of ECCBased on the characteristics of normal sand ECC at the microscopic and mesoscopic scales,the probability-base method design and the prediction model for the tensile behavior of normal sand ECC are given;According to the probabilistic-based design method,the potential of normal-sand ECC to achieve steady-state cracking and tensile strain hardening of multiple cracks(i.e.,that is,the macroscopic mechanism of the effect of the use of normal sand on the tensile behavior of ECC)is evaluated by the failure probability and reliability,and the recommended values of failure probability and reliability corresponding to normal-sand ECC achieving the tensile strain exceeding 8%and crack width less than 100 μm;Based on the probability,the effects of normal sand content and size on ECC tensile behavior are discussed from different scales,i.e.,the effects of normal sand content and size on the probability density of defect size,probability density of defect spacing,and probability density of matrix fracture toughness of the matrix at the microscopic scale,on the probability density of cracking strength of the matrix at the mesoscopic scale,and on the probability of failure and reliability corresponding to the realization of multiply steady-state cracking and tensile-strain hardening behavior of ECC at the macroscopic scale.