Study On Law Of Bond Behavior Between Steel Bar And Surrounding Concrete With Different Air Content Under Salt-Freezing Environment

The effect of salt freeze-thaw will cause serious deterioration of the concrete performance and the interface of steel and concrete,which will affect the bond performance of the steel and concrete,and eventually lead to the decrease of the durability of the structure.Air content has great influence on both but researched little at present.In this paper,the effects of different gas content and different salt freeze-thaw cycles on the bonding properties in salt freezing and thawing environment are studied in this paper.The mechanical properties,the initial slip bond strength,ultimate bond strength,residual bond strength,and the slip corresponding to the ultimate bond strength were studied.1.3%,2.6%,3.5%,4.5%,5.6% air content of concrete were designed.The influence of air content on concrete properties and bond properties of specimens under the effect of 0 salt freeze-thaw,and the formula for calculating the ultimate bond strength containing concrete strength and air content was established.According to the performance of air content 3.5% and 4.5% of the specimens with different salt free-thaw,the change law of the concrete performance and the bond performance of the pullout specimens were studied.The effect of the air content on the concrete under the salt freezing environment was obtained.The phenomenon that the bond strength of pull out specimens first increased and then decreased with the increase of salt freeze-thaw cycles was verified and analyzed.The dry specimen of 3.5% air content were designed to be compared.After 0,25,50,75,100 and 150 salt freeze-thaw cycles,it was found that the bond strength of the specimens in the dry group was still increasing first and then decreasing,but the bond strength was lower than that in the non drying group,thus the effect of salt freeze-thaw on the interface of steel and concrete was speculated.The freeze-thaw pore water increases the confining pressure of the concrete to the steel,and makes the interface concrete slightly irreversible expand,and the confining pressure decreases due to the aggravation of the freeze-thaw damage in the later period.According to the test results,calculation formula of ultimate bond strength including concrete strength,air content and salt freeze-thaw cycles and the average bond-slip relationship of the whole process of the round bar was obtained.The effect of salt freezing on the bond behavior of ribbed bars specimens was studied.After the 0,25,50,75,100 and 150 cycles of salt freeze-thaw,through the fine pullout test of ribbed bar specimens,the steel bar strain,concrete strain and relative slip of steel bar and concrete under all loading were measured during the whole process of pullout test.The bond slip relationship was obtained,and the relationship between the bond slip and the freezing and thawing times was obtained.The bond slip relationship that can reflect the number of freeze-thaw cycles and anchorage position was obtained.