Study On Piezoresistive Effect And Mechanism Of Ultra High Performance Concrete Under Tension

Ultra High Performance Concrete(UHPC)is a new type of fiber reinforced concrete.In comparison with ordinary fiber reinforced concrete UHPC exhibits tensile strain hardening behavior,higher load bearing capabilities and energy absorbing capabilities.Based on structural performances of UHPC,the self-sensing behavior is studied in this thesis so as to develop multi-functional UHPC.In this thesis,effects of the aspect ratios,shapes and contents of copper-coated fine steel fibers on the tensile mechanical properties,electrical conductivity and piezoresistive effects of UHPC were investigated experimentally.The mechanical properties and contact resistance changes of fiber-matrix interface were studied by pulling out single steel fiber and multiple fibers from the matrix to analyze the mechanisms of piezoresistive effects of UHPC under direct tension.The mechanical model and electrical conduction model were proposed to establish relationship between the fractional variation of resistance and the applied tensile strain.The theoretical results were compared with experimental results.The main conclusions drawn up are as follows:(1)With the increase of the aspect ratio and the volume content of steel fiber,tensile strength,ability of deformation,electrical conductivity and strain self-sensing ability of UHPC are improved.The resistivity of UHPC decreases first and then rises slowly as the tensile strain increases.Straight fibers are more conducive to improve the tensile properties,electrical conductivity and strain self-sensing ability of UHPC than the end-hook fibers and the corrugated fibers.UHPC exhibits tensile strain hardening phenomenon accompanied by the generation of multiple cracks.(2)During the pull-out test,the interface bond strength between steel fiber and the cement matrix decreases with an increase of the embedded length of fiber into the matrix,and increases with an increase of the fiber volume content added into the matrix.In aspect of the shape of steel fibers,the bond strength of the end-hook type fiber with the matrix is the highest,and that of the corrugated type fiber is higher than that of the straight fiber.The interface contact resistance between steel fiber and the matrix increases during the slip of fiber,while it remains unchanged during the elastic phase,and the resistance is reduced till the applied ultimate load indicating that the debonding of fiber from the matrix starts,which is due to that as the fiber is debonded the conductive length of the matrix is shortened,so the electrical resistivity of the sample is lowered.(3)According to the bond-slip mechanical model of steel fiber and matrix interface,the slip-debonding length relationship is derived theoretically and the fitting formula is obtained.Then,the number of cracks and the crack width are substituted into the fitting formula to get the debonding length of fiber around every crack.Using the fraction variation of resistance-strain relationship of UHPC,the calculated results are in consistent with the experimental results.