Study On The Mechanical Performance Degradation And Spalling Mechanism Of High Strength Concrete Exposed To Fire

In this paper, by using the orthogonal design method, the degradation characteristic,mechanical properties, fracture properties and the spalling characteristics of high-strengthconcrete exposed to fire with the fire temperature, different strength grad, pore water saturationwere studied, and compared the feasibility and applicability of different nondestructive testingmethod for detection of concrete strength after high temperature, in the comprehensive testresults were studied and the spalling mechanism on the basis of the existing, the hightemperature concrete high temperature “vapor explosion" model was put forward, the mainresearch findings are as follows:The “double mixing” technology was successfully used on the preparation of C80high-strength concrete, and the experience of concrete preparation was summarized. For highstrength concrete cube block after high temperature mechanical properties were studied. Theeffect of fire temperature, concrete strength grade, pore water saturation on the relative resistpressure strength after high temperature and high temperature curing strength after waterrecovery was analyzed, the relative residual compressive strength of concrete changes with thetemperature, strength, pore water saturation were got. By the analysis of variance showed that,the influence factors on the relative residual compressive strength of concrete after hightemperature, the fire temperature highly significant influence, the fire temperature is higher, themechanical properties of concrete deterioration is more serious; effect of pore water saturation,strength grade is not significantly affected.Using the rebound method, ultrasonic method, synthetic method and infrared thermalimage method for the different fire temperature, strength grade, pore water saturation of thecompressive strength of high strength concrete after high temperature nondestructive testing,established he test indexes and the compressive strength and the fire temperature regressioncurve. Through the error analysis results show that, the non-destructive testing methods thispaper used meet the requirements of related technical regulations accuracy, were suitable for thequality detection of concrete after high temperature. Among them, the highest precision ofconcrete strength testing is ultrasonic-rebound combined method，and infrared thermal imagemethod has the highest precision for temperature determination of concrete. When thetemperature is above600℃, the infrared thermal image method’s comprehensive applicabilityis better.Through the three point incision bending beam test of concrete after high temperaturefracture performance were studied, analyzed the influence of fire temperature, strength grade,pore water saturation for relative residual fracture toughness and fracture energy of concrete after high temperature, through the finite element software for stress intensity factor of threepoint bending incision beam was verified. The results of variance analysis showed that, amongthe factors affecting the fracture properties deterioration of concrete after high temperature, thefire temperature is highly significant, pore water saturation had effect, strength grade is notsignificantly affected. The residual fracture toughness of concrete and compressive strengthafter high temperature was linear positive correlation, fracture energy changes with thecompressive strength is not obvious.Studied the different fire temperature, strength grades, pore water saturation effects ofconcrete block crack width, ignition loss, porosity degradation characteristics and spallingcharacteristics after high temperature, the internal temperature field test and finite elementnumerical simulation on the high temperature were carried out. Finally, on the basis of thecomprehensive test results and the existing concrete spalling mechanism at high temperature,the “vapor explosion" model was put forward, using the model of concrete spalling, sometypical high temperature spalling phenomenon and representative research results are analyzedin more reasonable explanation.