Research On Performance And Damage Detection Of Reactive Powder Concrete After Elevated Temperatures

Reactive powder concrete(RPC)is an advanced cement-based composite material with ultra-high strength,high toughness,high volumetric stability and high durability,which has broad application prospects in bridges,tunnels,nuclear power plants and other projects.However,so far,a large number of investigations have mostly focused on the mechanical properties and preparation technology of RPC at ordinary temperature,limited research has been reported the high-temperature performance of RPC.Consequently,to bridge this gap,the mechanical properties,physical properties,and microstructure of RPC after high temperature were systematically studied in this paper,and the corresponding computational models of RPC were established.Besides,the performance of RPC after high temperature was evaluated by ultrasonic pulse velocity(UPV)and resonance frequency(RF)tests by analogy with ordinary concrete nondestructive test(NDT),and the corresponding strength-measuring curves were proposed.The main work and conclusions of this paper are as follows:(1)The phenomenon of RPC high-temperature test,the apparent change and mass loss of RPC specimens after high temperature were studied through high-temperature test.With the increase of temperature,the appearance changes and mass loss of RPC specimens with different fiber content show a similar variation law.The apparent color of RPC specimens gradually changes from dark to light,and the mass loss rate gradually increases with the rising temperature.The apparent color of RPC specimens varies with temperature as follows: greenish gray-light gray-grayish brown-brown-grayish white-light red accompanied by the development of cracks and pores in the process.After heating to 800℃,the specimens exhibit varying degrees of peeling and missing corners,and the knocking sound is low.Besides,the specimens become loose and porous,with a mass loss rate of up to 10%.Moreover,the influence of polypropylene(PP)fiber content on the mass loss of RPC is not remarkable.Based on the experimental results,the quantitative relationship between mass loss rate and temperature as well as fiber content is established.(2)The cubic compressive test,axial compressive test and splitting tensile test were carried out on RPC after high temperature.The failure mode of the specimen was studied,and the effects of fiber content and temperature on the residual strength of RPC were analyzed.The results show that RPC with different fiber content exhibits similar compression and tensile failure patterns at various target temperatures.At 20℃～600℃,the cubic compressed specimen shows brittle damage,the tensile specimen exhibits ductile damage,and the prismatic specimen shows shear damage.While at 600℃～800℃,the cubic compressed specimen shows plastic damage,the tensile specimen exhibits brittle damage,and the prismatic specimen shows splitting damage.The residual strength of RPC tends to increase first and then decrease as the enhancing temperature,but the critical temperature is different,the critical temperature of cubic compressive strength and splitting tensile strength is 300℃,while the critical temperature of axial compressive strength is 200℃.The splitting tensile strength loss rate of RPC after high temperature is higher than the cubic compressive loss rate.Below 300℃,the variation amplitude of axial compressive strength and cubic compressive strength is basically the same.After 300℃,the decline rate of axial compressive strength is faster than that of cubic compressive strength.Through regression analysis,the formula for calculating the ratio of axial compressive strength to cubic compressive strength with temperature is developed.In addition,based on the experimental results,the calculation models of cubic compressive strength,axial compressive strength and splitting tensile strength after high temperature of RPC are established,respectively,considering the effects of temperature and PP fiber content.(3)The physical properties and microstructure changes of RPC after high temperature were investigated,and the performance degradation mechanism of RPC at high temperature was revealed.The water absorption and water sorptivity of RPC increase as the temperature increases,and the maximum values are observed at 700℃.Steel fiber and PP fiber exhibit positive hybrid effects,which can effectively improve the physical properties of RPC after high temperature.Below 300℃,the microscopic morphology of RPC matrix is dense,intact and continuous,mainly composed of continuous bulk C-S-H gels.After 300℃,the micromorphology of the RPC matrix deteriorates with increasing temperature,and the C-S-H gel changes from continuous block to dispersed phase with small size.The interface between steel fiber,PP fiber and matrix gradually cracks and finally debonding.The melting of PP fiber provides microchannels for the matrix,which alleviates the high temperature vapor pressure to a certain extent,but also increases the internal defects of the matrix.The deterioration of the RPC microstructure is the root cause of the micro-mechanical property recession.(4)UPV and RF tests were carried out,and the variation of NDTs evaluation parameters with temperature was studied.It can be found an enhancement in damage degree is observed as the temperature raises,while relative UPV and relative RF as well as relative dynamic elastic modulus demonstrate a declining trend.Besides,the effect of PP fiber on NDTs evaluation parameters is not significant at the same temperature.It is reasonable and feasible to assess the performance of RPC after elevated temperature by using relative UPV,relative RF,damage degree and relative dynamic elastic modulus as damage evaluation parameters.High fitting accuracy and large correlation coefficient between damage evaluation parameters and temperature regression equation.There is a good correlation between damage evaluation parameters and RPC strength.Based on the experimental results,a prediction model between the macroscopic mechanical properties and the damage evaluation parameters is proposed.