Study On The Free Expansion Deformation Of Concrete At High Temperature

High temperature will cause thermal expansion deformation of concrete.The thermal stress produced by high temperature will cause cracks in concrete elements when it is greater than shear stress intensity in weak area,which seriously endanger the safety of concrete structures.In this paper,concrete specimens with different strength grades and different curing ages are heated to the target temperature at different heating rates.After the temperatures of the center of concrete specimens reach the target temperature,the specimens were cooled down from the target temperature to room temperature.And the influence of above factors on the free expansion deformation of concrete at high temperature was analyzed.The effect of different water content was also considered.The calculation model for high temperature deformation coefficient of concrete with different influencing factors was established based on test results.The main research findings are as follows:?1?Concrete begins to deform at 40⁶0?.The linear expansion rate and high temperature deformation coefficient of concrete increase with the increase of temperature,which reach 1.018%and 13.135×10^-6/?respectively at 800?.The linear expansion rate of concrete increases at constant temperature stage,which tends to be stable after 60⁶5min.The linear expansion rate and high temperature deformation coefficient of concrete reach 1.252%and 16.155×10^-6/?respectively at800?after the end of constant temperature stage.?2?At the initial cooling stage,the linear expansion rate of concrete at 100?and 200?is constant,while it increases slightly at 300⁸00?.After initial cooling stage,the linear expansion rate of concrete decreases with the decrease of temperature.After cooling to room temperature,the deformation of concrete with target temperature of 100³00?can be completely restored.Concrete with target temperature of 400⁸00?still has residual deformation.The higher target temperature is,the larger residual deformation is.The residual linear expansion rate of concrete can reach 0.509%after being heated at 800?.?3?At 100⁸00?,the linear expansion rates of concrete with three kinds of strength grades?C25,C35,C45?increase with the increase of temperature,which reach 1.346%at maximum.The linear expansion rate of concrete with larger strength grade is larger than that of concrete with smaller strength grade at 100³00?.When the temperature is higher than 300?,the linear expansion rate of concrete with smaller strength grade is larger than that of concrete with higher strength grade.The lower strength grades of concrete are,the larger residual linear expansion rates are.They can reach 0.572%at maximum.?4?When the heating rate of 5?/min,10?/min and 15?/min increase to 100⁸00?,the linear expansion rates of concrete increased with the increase of temperature,which reach 1.336%at maximum.The effect of heating rate on the linear expansion rate of concrete at 100⁴00?is slight.At 500⁸00?,the linear expansion rates and high temperature deformation coefficients of concrete increased with the heating rate.The residual linear expansion rates of concrete increase with the increase of heating rate,which can reach 0.558%at maximum.?5?At 100³00?,the linear expansion rates of concrete with three kinds of water content?completely dry,50%RH,fully saturated?increase with the increase of temperature,which reach 0.341%at maximum.The high temperature deformation coefficient of 50%RH concrete decreases at 100²00?,and increases at 200³00?while exceeding the other two kinds of concrete.High temperature deformation coefficients of completely dry and fully saturated concrete increase with the increase of fire temperature.?6?At 100⁸00?,the linear expansion rates of concrete with different ages?3d,14d,28d,60d,90d?increase with the increase of temperature,which reach 1.278%at800?.At 100²00?,high temperature deformation coefficients of concrete with28d age decrease,while that of concrete with other ages increase.High temperature deformation coefficients of concrete with each age increase at 200⁶00?,and tend to be stable after 600?.High temperature deformation coefficients of concrete can reach 16.49×10^-6/?at 800?.?7?The calculation models of thermal expansion coefficient of concrete at high temperature are established considering the influence of strength grade,heating rate,water content and curing age of concrete.