Experimental Study On Vapor Pressure Of Beams And Slabs In C80 High Performance Concrete Exposed To Elevated Temperatures

High-Strength Concrete and High-Performance Concrete?HSC/HPC?has many characteristics such as high durability,high workability,high strength and high volume stability because of its dense structure,then it has been used for much large construction projects as bridges,high-rise buildings and seaport buildings,etc.However,because HSC/HPC present the brittleness and are poor in permeability,it may cause the spalling of HSC/HPC and other phenomena when exposed to elevated temperature.Then the bearing capacity of the building structure drops sharply,producing great security risks and disastrous consequences even.According to the acient research on the spalling of HSC/HPC mainly includes three types when exposed to elevated temperature:vapor pressure spalling,thermal stress spalling and thermal crack spalling.There are many reasons for the spalling of HSC/HPC,among which vapor pressure is recognized as one of the main causes for spalling of HSC/HPC at high temperature.Because there is little research on the vapor pressure test of HSC/HPC at high temperature,and the combination of engineering practice,the study about the effect of vapor pressure on the spalling of structural members in HSC/HPC at high temperature are almost none.Therefore,further study on the spalling of structural members in HSC/HPC at high temperature is carried out.It has far-reaching significance for expanding its engineering application field.Based on the National Natural Science Foundation of China?51478290?,took C80HPC as the research object,and conducted a vapor pressure test at high temperature to investigate the effect of vapor pressure on the high temperature performance of beams and slabs in C80HPC at high temperature and the effect of PP fiber and steel mesh on vapor pressure of beams and slabs in C80HPC at high temperature.The main research contents of this paper are as follows:1.The test study on strength of C80HPC specimens after high temperature.According to the water content test method,the measured equilibrium moisture content of the concrete specimens in the same environment is 4.13%;The spalling temperature of the P0 was between 200?and 400?,and no spalling was occured in the P1 and P2;20²00?,the compressive strength and splitting tensile strength of P1 and P2 were less than that of P0;200³00?,the compressive strength and splitting tensile strength of P1 and P2 were larger than that of P0;400⁷00?,the strength of P0,P1,P2 decreased rapidly,and the high temperature damage was intensified.2.The test study on vapor pressure in C80HPC slabs at high temperature.Combined with the vapor pressure study of SP0,RSP0,S8P2 and RS8P2 in the previous period,except for S15P2,the vapor pressure peaks of the other types of concrete slabs at the depth of 50mm were larger than the vapor pressure peaks at depths of 25mm and 75mm;The vapor pressure peak at the depths of 25mm,50mm and 75mm from the heated surface of 10 types of concrete slabs?including the previous S8P2,RS8P2?appeared successively,which showed a tendency of continuously migrate from the hested surface to the rear surface;The peak value of vapor pressure at the depth of 25mm from the heated surface of 10 kinds of concrete slabs:P_RSP0SP0 is maximum,P_S15P2 is the smallest;The peak value of the vapor pressure at 50mm deep from the heated surface:P_RSP0 is maximum,P_S15P2 is the smallest;The maximum of the vapor pressure at a 75mm deep from the heated surface:P_RSP0 is maximum,P_S15P1 is the smallest.At the same PP fiber content,the concrete slabs with the reinforced mesh with the flaking surface of the heated surface was smaller was less damaged than the concrete slabs without the reinforced mesh.3.The test study on water pressure of C80HPC specimens.Statistics on the temperature at which the vapor pressure peaks are reached.It is designed to compare and analyze the high temperature test results of water pressure strength,splitting tensile strength of standard cube specimens and vapor pressure peak of slabs.From the comparison of water pressure,splitting tensile strength and vapor pressure peak of various PP fiber specimens,the splitting tensile strength are all highest,while the vapor pressure peaks of slabs are the smallest a 25mm deep peak temperature;The water pressure are all highest in the specimens,but the maximum vapor pressure of slabs was the smallest at the 50mm and 75mm deep peak temperature.In this test,the maximum vapor pressure of the slabs is0.558 MPa at the high temperature,which is less than the water pressure and splitting tensile strength.4.The test study on vapor pressure of C80HPC beams in furnace heated.The time of reaching the vapor pressure peak at different depths of BP0 and BP2was counted.The time of reaching vapor pressure peak appeared at each position of BP0:T _{50mm from beams bottom} is maximum,T _{25mm from beams side} is the smallest;The time of reaching vapor pressure peak at each position of BP2:T_{50mm from beams side} is maximum,T _{25mm from beams side} is the smallest;BP0 did not spall at high temperature;The vapor pressure of BP0 and BP2 increased first and then decreased with the heating time;The vapor pressure peak at different positions of BP0 and BP2:P _{50mm from beams bottom} is maximum,P _{25mm from beams side} is the smallest.5.The test study on vapor pressure of C80HPC beams in open flame-load coupling.The time of reaching the vapor pressure peak at different depths of FBP0 and FBP2 was counted.The time of vapor pressure peak at each position of FBP0 and FBP2:T_{50mm from beams bottom}0mm from beams bottom is maximum,T_{25mm from beams side} is the smallest;The time reached the vapor pressure peak of FBP0 and FBP2 smaller than that of BP0 and BP2;The vapor pressure of FBP0 and FBP2 increased first and then decreased with the heating time.The vapor pressure peak at different positions:P _{50mm from beams bottom} is maximum,P _{25mm from beam side} is the smallest.6.The temperature variation at different depths of the beams and slabs with the heating time is analyzed.At the same heating rate,the heating rate of the slab?single-sided fire?is lower than that of the beam?three sides exposed to fire?.Maximum vapor pressure in RSP0 and BP0,RS8P2 and BP2 at high temperature?resistance furnace heating?,and SFSP0 and FBP0,FRS8P2 and FBP2 at high temperature?open flame-load coupling?were compared and analyzed.During the different heating times,the temperature at different depths in beams is greater than that of the corresponding depth in the slabs.The maximum vapor pressure at a 50 mm deep in the beams and slabs is:P_RSP0>P_BP0,P_FRSP0>P_FBP0,indicating that the damage of slabs is more serious than that of the beams at the two kind of heating method.