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

The complexity and randomness of the source of recycled aggregates are different from those of natural gravel in strength and performance,lead to the durability of recycled concretes are inferior to that of ordinary concrete.The concrete structure in the natural environment are often damaged by the erosion in the complex environment,and multiple factors restrict each other and promote each other,resulting in the destruction of concrete structures In this paper,the mechanical properties and bonding properties of rebar and regenerated concrete under the action of double salt-freeze erosion were studied.（1）The Na Cl and Na₂SO₄ salt freezing cycle tests were carried out on the recycled concrete specimens with the replacement rate of coarse aggregate being100%,normal concrete control group was set.The corrosion of chlorine and sulfate,the durability of recycled concrete with different strength after salt-freeze cycle and the variation of splitting tensile strength were studied.The experimental results show that the coarse aggregate performance of ordinary concrete is better than that of recycled coarse aggregate,as a result,the frost resistance of ordinary concrete is better than recycled concrete;The smaller the mass loss and the relative modulus loss of recycled concrete with the higher strength grade of recycled concrete.The splitting tensile strength of recycled concrete decreases with the increase of salt freezing times;The damage of ordinary concrete specimens is less than recycled group.After the first30 cycles of salt freezing,the damage of Na Cl freezing-thawing group was greater than that of Na₂SO₄,but Na₂SO₄salt damaged the recycled concrete specimen more seriously after 60 salt freezing-thawing cycles.（2）Through 11 groups of central pull-out specimens,the change rule of the bonding performance of steel rebar recycled concrete specimens under different influence factors was compared and analyzed.The results show that the bonding property of recycled concrete is worse than that of ordinary concrete.With the increase of anchorage length,the curve first increases and then flattens out,When the anchorage length is in the section of 5d～8d,the test curve tends to be flat and close to a horizontal line,it shows that the influence of anchorage length on bonding performance is reduced.The bonding performance of the specimens in the regenerated group corroded by Na Cl salt decreased more than that of Na₂SO₄ salt after the first 30 salt-freezing cycles,but in the end,the bonding performance of the regenerated Na₂SO₄ salt corroded specimens decreased to a greater extent.The relative bonding area between steel rebar and recycled concrete decreases with the increase of the diameter,which leads to the decrease of the bonding property.As the strength of recycled concrete increases,its compactness and salt-frost resistance increases,with the increase of the chemical adhesive force of the steel rebar,the bonding performance is positively correlated with the strength change.（3）The local bonding performance of steel rebar and recycled concrete after salt-frozen circulation was tested by using strain gauge in the reinforcement grooves.The test results show that the strain of reinforcement increases gradually with the increase of load.With the increase of the frequency of salt-freezing,the strain peak value of reinforcement decreases gradually,under the same grade of load,the strain of steel rebar decreases with the increase of times of salt-freezing;The bonding stress presents"double peak"in the anchoring section,the peak value of the free end finally equates with that of the loading end with the increase of freezing times.Finally,the constitutive model of bonding stress-slip is simplified,the slippage equation of steel rebar recycled concrete after salt freezing cycle is established.It provides some reliable theoretical basis for the extension application,maintenance and steel rebar and finite element simulation of the recycled concrete structure in the severe cold region of north China.