Experimental Study On Toughness And Residual Strength Of Steel Fiber Reinforced Concrete Under Bending Load

Steel fiber reinforced concrete(SFRC),which is a new kind of composite building material,has been widely used in building construction and engineering practice.In addition to strength,toughness is also an important property of SFRC,and flexural toughness is an important indicator to measure the toughness of SFRC.Some scholars and national standards have proposed different test methods and evaluation indicators to indicate the flexural toughness of SFRC.According to the "Steel fiber reinforced concrete"(JG/T 472-2015),the "Standard test methods for fiber reinforced concrete"(CECS13:2009)and the "Test method for metallic fibre concrete-Measuring the flexural tensile strength(limit of proportionality(LOP),residual)"(EN14651),flexural toughness and residual flexural tensile strength of SFRC specimens were studied and analyzed by applying a third-point load on a simply supported beam or testing a simply supported notched prism under center-point loading.The main research contents are as follows:1.Through the compressive experiment on 162 cube specimens with size of 150mm×150mm×l50mm,the third-point bending test on 72 prism specimens with size of 150mm×150mm×550mm and the center-point bending test on 216 notched beams with size of 150mm×150mm×550mm,the effects of concrete matrix strength,volume fraction and types of steel fiber on the flexural toughness and residual flexural tensile strength were investigated.The results show that the flexural toughness indicators,especially the equivalent flexural strength and residual flexural tensile strength,were increased with the increase of steel fiber volume fraction and the strength of concrete matrix under different test methods.Whereas the effect of steel fiber types is mainly reflected in the equivalent flexural strength or residual flexural tensile strength improvement in the later period of concrete failure,in which 4D,5D steel fibers have better effect in strengthening and toughening.2.Three kinds of test methods with bending toughness evaluation indicators were compared.It is concluded that the condition of the notched beam flexural strength(LOP)calculation method(deflection ? or CMOD?0.05mm)limits the flexural strength range of SFRC specimens.In contrast,the third-point bending test method is more suitable for characterizing concrete with high strength and steel fiber volume fraction.By capable of measuring the decline section of the plain concrete load-deflection/CMOD curve,the notched beam testing method is more suitable for the lower steel fiber volume fraction and plain concrete specimens.3.Based on the load-deflection/CMOD curve of three different test methods,the energy absorption value,which is the area under the curve,is analyzed.Here comes the conclusion: The load-deflection curve of the notched beam method is basically the same as the load-CMOD curve.Therefore the numerical values of the flexural strength fm and the equivalent flexural strength feq1,feq2 corresponding to the load-deflection curve are relatively close to the flexural strength(LOP)ct L ff,and the residual flexural tensile strengthR,1f,R,2f corresponding to the load-CMOD curve.The notched beam cracking energy absorption value Dcr may have situation which is not applicable to the CECS13:2009 calculation method.The use of notched beam residual flexural tensile strength to evaluate the flexural toughness can better characterize the trend of the curve.