Study On Cracking Resistances Of Layered Hybrid Steel Fiber Recycled Aggregate Concrete Beams

In order to reduce the project cost and construction difficulty,the mixing mode of steel fiber in the integral steel fiber recycled aggregate concrete has been changed,the optimal steel fiber hybrid proportion has been selected,and the structural form of the layered hybrid steel fiber recycled aggregate concrete(LHSFRAC)has been adopted.Through the compressive strength test,splitting tensile strength test and static compression elastic modulus test of 10 groups of recycled aggregate concrete(RAC)test blocks,the effects of the layers and contents of hybrid steel fiber on the mechanical properties of LHSFRAC were analyzed.The influence of the number of steel fiber layers,the contents of steel fiber and the ratio of longitudinal reinforcement on the cracking resistances,ductility and energy dissipation capacity of LHSFRAC beams were analyzed through the bending test of 12 groups of beams.The main research results are as follows:(1)With the increase of the layers of hybrid steel fiber,the compressive strength of LHSFRAC has little change as a whole,the splitting tensile strength is gradually increasing(the maximum increase is 36.1%),the elastic modulus is lower(the maximum decrease is 10.1%),the tension compression ratio is higher(the maximum increase is 32.1%),and the elastic strength ratio increases or decreases(the maximum decrease is 6.3%).With the increase of the contents of hybrid steel fiber,the compressive strength increases first and then decreases,the splitting tensile strength increases 22.2%,the elastic modulus increases 3.8%,the tensile compression ratio and the elastic strength ratio decrease first and then increase.(2)When the layers of steel fiber are 4 and the contents of steel fiber are 2.0 kg/m2,although the splitting tensile strength of concrete is improved and the brittleness is reduced,considering the elastic strength ratio,the cracking resistance is poor.The failure of RAC changed from brittleness to plasticity after the hybrid steel fiber layers were distributed.(3)With the change of the number of steel fiber spreading layers,the load value of LHSFRAC beam increases by 3.3%～91.7%when the crack width is 0.5 mm compared with that of ordinary RAC beam,the load value increases by 4.2%-33.7%when the crack width is 3 mm,the maximum crack width is reduced by 46.7%when the beam is damaged,the number of cracks increases by 5 at most,the average crack spacing decreases by 0.2%-23.7%,and the strain of longitudinal bars of the same load level is generally increased.(4)With the increase of steel fiber content,the load values of 0.5 mm and 3 mm cracks increase by 32.5%and 11.2%respectively,the crack width decreases gradually when the beam is damaged,the number of cracks increases by 2-3 compared with group B0-S0-R16,the average crack spacing first decreases by 7.8%and then slightly increases,the strain of longitudinal reinforcement decreases gradually under the same load,the crack resistance of the beam increases gradually.(5)With the increase of reinforcement ratio,the load values of 0.5 mm and 3 mm cracks increase by 37.5%and 22.4%respectively,the crack width increases gradually when the beam is damaged,the number of cracks decreases gradually,the average crack spacing decreases first and then increases,the strain of longitudinal reinforcement decreases under the same load.(6)The effect of steel fiber on cracking load is obvious,and the effect of reinforcement ratio on yield and ultimate load is more obvious.With the increase of the number of layers,the amount of steel fiber and the reinforcement ratio,the maximum displacement ductility coefficient increases by 8.8%,4.1%and 36.0%respectively,and the maximum energy dissipation capacity increases by 41.8%,4.8%and 85.1%respectively.(7)By modifying the influence coefficient of steel fiber on tensile strength at and the influence coefficient of steel fiber on crack width the calculation formulas of cracking resistances(crack moment and maximum crack width under short-term load)of LHSFRAC beams are put forward based on the fitting analysis of the measured data and the formulas in current codes,regulations and literatures.(8)The loading experience of LHSFRAC beam is basically similar to that of ordinary RAC beam,which has four characteristics of elasticity,cracking,yielding and failure.The crack development of the finite element simulation is similar to that observed in the test process,and the simulation values of deflection,cracking loads and yield loads are in good agreement with the test results.