Effect Of Interfacial Properties On Bonding Behaviors Of UHPC-NC

Ultra-high performance concrete(UHPC)that can be used as a repair material has excellent mechanical properties and durability.However,in the field of repair and strengthening of concrete,there are few experimental researches on UHPC as a repair material.This paper focuses on the bonding properties between UHPC and normal concrete(NC).This paper mainly studied the effect of interfical roughness,bonding agents and steel fiber contents on mechanical properties and durability properties of bonding interface of UHPC-NC by the splitting tensile test and the freeze-thaw test.The main work and conclusions are as follows:(1)The effect of the roughness(type ?,type ?,type ?,and type ?)and steel fiber contents(2%,3%and 4%)on bonding properties of UHPC-NC without bonding agents were investigated by the splitting tensile test of UHPC-NC.The results showed that the bond splitting tensile strength of UHPC-NC increased with the increase of roughness,and the failure mode was transformed from failure happened at interface to failure happened at NC;The bond splitting tensile strength had little change with the increase of the steel fiber content,which indicated that the steel fiber content had little effect on the mechanical properties of bonding interface of UHPC-NC.(2)Four kinds of typical bonding agents were selected.Based on the rheological and mechanical properties of bonding agents,the effect of bonding agents and steel fiber contents on bonding properties of UHPC-NC was studied by the splitting tensile test.The results showed that bonding properties of UHPC-NC can be improved to different degree by the cement paste,the cement paste with silica fume and the cement paste with expansive agent.With the increase of the steel fiber content,the bonding strength of UHPC-NC is significantly improved,when the cement paste and the cement paste with expansive agent are used.The maximum splitting tensile strength of UHPC-NC can reach over 90%of the splitting tensile strength of NC.(3)The influence of the roughness and steel fiber contents on the freeze resistance of UHPC-NC without bonding agents was studied by the freeze-thaw test of UHPC-NC.The results indicated that the type IV with the highest roughness showed the best frost resistance;the higher the steel fiber content,the better the freeze resistance of the UHPC-NC bonding interface;and the roughness and the steel fiber content have a mutually reinforcing effect on the enhancement of the freeze resistance of the interface,under 150 freez-thaw cycles,the bonding strength of the bonding specimen with 4%volume fraction of steel fibers and the highest roughness only decreased by 8%.(4)The effect of bonding agents and steel fiber contents on the freeze resistance of UHPC-NC was studied by the freeze-thaw test of UHPC-NC.The results showed that bonding effect of the cement paste was the best after freeze-thaw cycles,and the cement paste with silica fume was the second.The bonding strength of UHPC-NC with 3%and 4%volume fraction of steel fibers were close,but both were better than 2%when using the cement paste or the cement paste with silica fume.Under 100 freez-thaw cycles,The failure mode of UHPC-NC wih the cement paste was transformed from failure happened at the bonding agent to failure happened at the interface between the bonding agent and NC,which indicated that the freeze-thaw cycle had more damage to the interface between bonding agents and NC than to the bonding agent.