Study On The Performance Change Of High-strength Reinforced Concrete After High Temperature

Compared with ordinary concrete,high-strength concrete has high compressive strength,low porosity,high density,high impermeability and frost resistance.These advantages of high-strength concrete can optimize the cross section,increase the building space and reduce the weight of the structure itself.In recent years,building fires have occurred frequently.After the high-strength concrete has been exposed to high temperatures,the temperature of the high-strength concrete has dropped sharply through fire extinguishing and artificial water spraying.Therefore,it is of great significance to study the natural cooling,water-cooling deterioration and mechanical properties of concrete.By using high-strength steel bars,the number of steel bars can be effectively reduced,steel materials can be saved,and the reliability of the structure can be improved.At the same time,high-strength steel bars are indispensable for the design and construction of large and heavy structural projects.At present,there are relatively few researches on the theory and application of HRB600 steel bars in my country.The bond performance between steel bar and concrete is the basis for ensuring the synergy between steel bar and concrete,so it is of great significance to study the bond performance between HRB600 steel bar and concrete after high temperature.The main contents of this article are as follows:(1)Pass 22 groups of HRB600 high-strength steel bars to test the changes in mechanical properties after high temperature.The high temperature test of steel bars adopts six temperatures of 20℃,200℃,400℃,600℃,800℃,1000℃ and two different cooling methods: natural cooling(ZL)and water cooling(SL),mainly to test its tensile properties and Reverse bending performance,tensile test piece and reverse bending test piece use HRB600 steel bar with a length of 500 mm and a diameter of 16 mm.When the heating temperature is lower than 600℃,the temperature and cooling method have no effect on the steel.After natural cooling at 800℃,the yield strength decreased by 16.63% and the ultimate strength decreased by 14.39%.After water cooling,the yield strength increased by 24.88%,and the ultimate strength increased by 29.05%.After natural cooling at 1000°C,the yield strength decreased by 19.00% and the ultimate strength decreased by 16.25%.After water cooling,the yield strength decreased by 13.40%,and the ultimate strength decreased by 12.12%.The elongation at break after water cooling is lower than that of natural cooling.After water cooling,the elongation of the steel bar is significantly reduced,showing obvious brittleness,while the elongation of the steel bar after natural cooling is increased,and the toughness is improved.The steel bar has good reverse bending performance before 1000℃,and no cracks appear.(2)Test the mechanical properties of C80 high-strength concrete after high temperature.The high temperature test of concrete adopts four temperatures of 20℃,200℃,400℃,600℃and two different cooling methods: natural cooling(ZL)and water cooling(SL).Three cube test blocks of different sizes: 70mm×70mm×70mm,100mm×100mm×100mm,150mm×150mm×150mm.According to the test results,the relationship between the concrete compressive strength,splitting tensile strength and the fire temperature and cooling method after high temperature is fitted.With the increase of temperature,the strength of high-strength concrete decreases linearly;when the temperature is the same,the strength loss of high-strength concrete under water cooling conditions is greater than that of natural cooling.In addition,compared with compressive strength,temperature and cooling method have a greater influence on splitting tensile strength.Because the high-strength concrete is relatively dense,the internal moisture is not easy to evaporate,and it is easy to burst after high temperature.The C80 concrete cube specimen(150mm×150mm×150mm)burst at about450℃.(3)Test the bonding performance of high-strength reinforced concrete after high temperature.The high-temperature test of reinforced concrete uses four temperatures of 20℃,200℃,400℃,600℃,and two different types of natural cooling(ZL)and water cooling(SL).The cooling method adopts the local bonding method.The size of the bonding test piece is designed to be 90mm×90mm×300mm,and the steel bars are 16mm HRB600 grade hot-rolled ribbed steel bars.The central pull-out test was performed on 21 reinforced concrete specimens,and the pull-out test τ-s curve was made.Through theoretical analysis,the relationship between peak bond stress,peak slip and temperature change under different cooling methods was fitted..(4)Quantitative analysis of changes in pore surface area,average pore size,porosity,pore size distribution and pore ratio of high-strength concrete after high temperature through mercury intrusion test The average pore size and porosity both increase with the increase of temperature.The pore distribution of high-strength concrete after temperature action indicates that as the temperature rises,high-strength concrete gradually deteriorates.Mesopore appear at 200℃.400℃ is a turning temperature.After high temperature,the pore size increases and the mesopore increase.After the action of 600℃,macroscopic pores appeared,and the concrete started a new deterioration process.With the increase of temperature,harmless pores and less harmful pores transform into harmful pores and more harmful pores,which is the main reason for the decline of concrete after high temperature.