The Design And Performance Research On High Temperature Resistance Ultra High Performance Concrete

Unlike ordinary concrete,the interior of ultra-high performance concrete is denser,which is a characteristic of high-strength materials.However,the dense structure inside can expose fatal defects of ultra-high performance concrete in some special working environments.When it comes to high temperatures and fires,the increasing temperature can lead to temperature gradients at different positions of concrete components.The varying degrees of expansion and contraction of the material can be caused by temperature difference,leading to internal cracks in the components.The generation of cracks will lead to a rapid decrease in material strength,and even affect the overall safety of the structure.In addition to the temperature gradient,under high temperature conditions,the free water inside the material,the water decomposed from C-S-H gel,etc.,will evaporate,and the water vapor inside the dense material can not escape through the internal pores easily,thus the high pressure inside will be generated,leading to the generation of internal cracks and material damage.Based on this,under the guidance of the maximum density theory,combined with the orthogonal test method,Calcium Aluminate Cement（CAC）and Ordinary Portland Cement（OPC）were used as the cementing materials for different mix proportions this paper.The aggregate was divided into corundum（alumina）and quartz sand,and a total of four groups of mix proportions were designed.In addition,S105 slag,fly ash and silica fume were added to inhibit the strength degradation of CAC in the later period.In order to minimize the impact of temperature gradient on the material,3 vol.%of thin and straight copper plated Steel Fiber（SF）was added,and was used to transfer heat.The dissipation of water vapor was increased and internal pressure was reduced through the pores left by the addition of polypropylene fiber（PP）melting after the temperature rose to 180-190℃.After the curing period of the specimens,in order to simulate the high temperature,fire and other environments,the high temperature furnace was used to heat the specimen at high temperature.The normal temperature group（20℃）was set as the control group,and the other test groups were 250℃,500℃,750℃and 1000℃,respectively.Statics tests were carried out subsequently,cube compressive strength test,axial tensile test,and bending test were included,and dynamic impact resistance tests were conducted by Split Hopkinson Pressure Bar（SHPB）.The internal microstructure of the specimen was observed by using scanning electron microscope,and XRD test was used to investigate the phase and product changes of different mixtures under different temperature conditions.CT scanning was used to explore and characterize the internal porosity of materials after high temperatures accurately.After the analysis of experimental data,the following conclusions can be drawn:（1）With the temperature gradually rose from 20℃to 1000℃,the CAC specimen could maintain the integrity of the test block,and there was no crack on the surface.In addition to the warping of the tensile specimen and a small number of nanocracks on the surface were deserved,there were also a few small cracks on the surface of the dynamic impact resistance specimen and the prism bending test block,and no crack on the cube compression specimen was found,which indicated that the mix proportion of this experiment was equpped with excellent high-temperature peeling resistance capacity.（2）According to the analysis of the mass data at different heating stages,the mass loss rate of the mix ratio M₁was largeer between 20-500℃.When the temperature exceeded 500℃,the average mass loss rate dropped from 6.49%to 2.49%,indicated that after the temperature reached 500℃,most of the free water and bound water inside the test block were evaporated.The average mass loss rates of M₂-M₄ during the temperature increased from 20℃to 500℃were 7.53%,6.10%,and 7.33%,respectively.（3）The tests of compressive performance on 10cm cube specimens were conducted and stress-strain curves were obtained.After heating at 250℃,the compressive strength of all specimens were increased by compareing to 20℃.The average compressive strength of the mix proporations M₁,M₂,M₃,and M₄ increased from 152.94MPa,149.21MPa,141.12MPa,and 135.02MPa to 172.99MPa,164.39MPa,153.56MPa,and 149.05MPa,respectively.The compressive strength of the CAC experimental group were 161.89MPa and 149.94MPa after heaating at 500℃,while the mean compressive strength of the OPC group M₃ and M₄ were 129.01MPa and121.74MPa after the temperature reached 500℃.The compressive strength of the M₁decreased from 152.94MPa at 20℃to 54.97MPa at 1000℃,with a decrease of 64.06%.（4）The tensile test results show that at 250℃,the axial tensile strength of the mixture proporation M₁-M₄ increased from 9.62MPa,9.34MPa,8.71MPa,and 8.54MPa to 10.68MPa,10.11MPa,10.17MPa,and 9.31MPa,respectively.After heating at 500℃,the axial tensile strength of M₁-M₄ were 8.45MPa,7.56MPa,6.95MPa,and 6.54MPa,with a certain degree of decrease compared to the strength at room temperature,however,the strength loss of the OPC group was greater.Moreover,the failure of the M₁ is complete fracture failure model,and the internal steel fibers were completely oxidized,losing adhesion when the temperature reached 1000℃.The tensile strength decreases from 9.6MPa at 20℃to 3.48MPa at1000℃,with a decrease of 63.75%.（5）The flexural strength of the mixture proporation from M₁ to M₄ increased from44.75MPa,41.51MPa,39.23MPa,and 35.64MPa at room temperature to 49.71MPa,47.46MPa,44.14MPa,and 40.49MPa at 250℃,respectively.When the temperature continues to rose to 500℃,due to the appearance of cracks on the surface of the specimen and the failure of the bonding force between the steel fiber and the UHPC matrix,the flexural strength of the M₁-M₄ mixture were 36.59 MPa,33.57MPa,28.96MPa,and25.47MPa,respectively.Besides,the flexural strength was only 13.64 MPa after heating at 1000℃,.（6）The SHPB device was used to conducted the dynamic impact resistance test,and a significant strain rate effect was showed through the analysis of the data.The impact strength was increasd with the increasing of strain rate,and the degree of failure of the test block was also higher.When the temperature reached 1000℃,due to the failure of steel fibers,the bridging effect of UHPC matrix is lost,resulted in higher degree of damage to the specimens.（7）The crystal phase changes of the internal products of UHPC made with M₁was tested by XRD,and the formation and decomposition of internal materials under different temperature conditions was analysed,it was found that a large amount of anorthite（CAS₂）was generated at 1000℃.The SEM observation results indicated that a large number of pores and cracks were generated inside the UHPC,and the 3D visualizatione model established by CT scanning showed the variation of the porosity of UHPC with the change of temperature more intuitively.