Experimental Study On Mechanical Properties Of Recycled Concrete And Its Bond Properties With Steel Bars Under Salt-Freeze In Inner Mongolia

Inner Mongolia is located in the severe cold region of northern China,and it has a large number of saline-alkali land.Therefore,the concrete structure is suffering from corrosion that is caused by the chemical and physical effects of salt-alkali and the corrosion of freezing-thawing for a long time.Based on the coupling environment of saline-alkali and freeze-thaw in Inner Mongolia,the mechanical properties of recycled concrete in different freeze-thaw cycles was studied by changing the salt concentration and strength of concrete.At the same time,the bonding performance of steel and recycled concrete in different freeze-thaw cycles was studied by changing the anchorage length and the diameter of steel bar,strength of concrete,and salt concentration.And then the previous study was compared with ordinary concrete.Finally the damage model of recycled concrete and the bond-slip constitutive relation considering position function were established after salt freezing.(1)The recycled concrete specimens were subjected to salt-freezing cycle to test their mechanical properties under the salt-alkali and freeze-thaw coupling environment.The results showed that:the mechanical properties and bonding properties of recycled concrete are worse than ordinary concrete under the same salt-freezing cycle;And then,the resistance to salt-freezing performance of recycled concrete is gradually improved with the increase of strength grade,when the strength grade is C40,the compressive strength loss and mass loss of the recycled concrete are the minimum,which are 26.62%and 0.24%,respectively;On the one hand,the mechanical properties of the recycled concrete decreased with the increase of the composite salt concentration before the 30 time salt-freezing cycle,on the other hand,the conclusion was reversed after 30 times of salt-freezing cycle.At last,the established mechanical property damage model can better reflect the damage of recycled concrete under the salt-frozen of Inner Mongolia.(2)Through the central pull-out test was used to test the bond performance of recycled concrete in a salt-alkali and freeze-thaw coupling environment.The results showed that:the bonding performance of recycled concrete is gradually improving with the increase of the diameter of the steel bar in the same number of salt-freezing cycles,however,when the diameter of the steel bar is the largest,the ultimate bond strength of the recycled concrete is the smallest due to the decrease of the relative bonding area;When the anchoring length of the steel bar is between 3d and 5d,the anchoring length has a great influence on the improvement of the bonding performance,but when the anchoring length is between 5d and8d,the effect is slowed down,so l_a/d=5 as the recommended value for the standard length of the steel bar is given;The bonding performance of recycled concrete increases with the increase of strength grade,and the ultimate bond strength of C40 concrete is 1.82 times of C20 in the 90 freeze-thaw cycles;The composite salt concentration has a great influence on the bonding performance of steel and recycled concrete.Before 60 times of salt freezing cycle,the bonding performance of steel and recycled concrete increases with the increase of compound salt concentration,and the opposite is true after 60 salt freezing cycles;(3)The local bond performance of steel bars and recycled concrete after salt freezing cycle was tested by using strain gauges in the steel groove.The results show that:when the number of salt freezing cycles is less than 60 times,the strain of the steel bar exhibits a concave characteristic along the anchor length distribution curve,and the strain of the steel bar transmits slowly to the free end.On the contrary,after the number of salt freezes reaches90 times,the strain distribution curve exhibits a convex characteristic,and he strain transmission speed is increased;The bonding stress of steel and recycled concrete gradually shows a bimodal distribution with the increase of the number of salt-freezing cycles,because the salt damage accelerates the bonding stress from the loading end to the free end,at the same time,the peak bonding stress of the free end exceeds the loading end at 90 freeze-thaw cycles;Finally,the model of bond-slip constitutive relationship between steel bars and recycled concrete considering anchoring position under the salt-free cycle is provided,which provides a theoretical reference for the actual stress of steel and recycled concrete in complex environment.