Study On Frost Resistance And Bonding Performance Of Recycled Concrete Under Salt Freezing

The process of the expansion and the technological iterations of the infrastructure industry has produced a large amount of construction solid waste whose main reason is the demolishing of massive old concrete structures and buildings.The use of construction solid waste in the production of industrial recycled concrete meets the needs of national economic strategy and environmental protection.Inner Mongolia is a cold area,and the soil contains a large number of SO₄^2-ions and Cl^-ions,SO₄^2-ions will produce expansive products,and Cl^-will corrode steel bars,both of which will affect the durability of the structure.At present,there are relatively few studies on the frost resistance and bond performance of recycled concrete after salt freezing cycle.Therefore,this paper uses contrast method to study the freezing resistance and bond performance of recycled concrete after freeze-thaw cycles with different solutions（water,sodium chloride,sodium sulfate）.（1）Failure morphology,strength,mass and relative dynamic elastic modulus of recycled concrete specimens were studied after different freeze-thaw cycles（0,30,60,90,120）in different solutions（water,sodium chloride,sodium sulfate）.The results showed that the quality loss rate of the sodium sulfate group specimens was reduced after 30freeze-thaw cycles.After 120 freeze-thaw cycles,the specimens in the sodium sulfate solution had the most serious damage.With a large amount of mortar peeling off at the edges,corners and middle parts of the specimens,the mass loss rate of the sodium sulfate group specimens was the largest,which was 2.12%and 0.92%higher than that in the water group and the sodium chloride group,respectively.when the number of freeze-thaw cycles increased,the strength loss rate and the relative dynamic elastic modulus loss rate of each group of specimens were constantly increasing.After 120 freeze-thaw cycles,the strength loss rate of the sodium sulfate group specimens were 11.7%and 10.62%higher than that of the water group and the sodium chloride group,respectively.（2）Uniaxial compression test was carried out on recycled concrete specimens in each group to obtain stress-strain curves,and then two-dimensional finite element simulation was carried out on the specimens to observe their failure and summarize the reasons for their failure.It is found that with the increase of the number of freezing-thawing cycles,the peak strain of prismatic specimens in each group increases while the peak stress of specimens in each group decreases.When the number of freezing-thawing cycles is 90,the peak stress of sodium sulfate group and sodium chloride group decreases by 28.12%and26.5%compared with the water freezing group,respectively.Based on Weibull distribution function,the damage constitutive model of each specimen can reflect the stress-strain relationship of recycled concrete in different salt freeze groups well,and the correlation coefficient of each specimen is above 0.9.For groups of two-dimensional elastic compression simulation for prism specimens found that most of the recycled coarse aggregate concrete macroscopic damage from mesoscopic tensile damage,but because of natural coarse aggregate tensile strength is much higher than the new and old mortar and the new and old interface area,so the recycled coarse aggregate concrete damage area is generally mortar and interface transition zone tensile damage.The large stiffness of natural aggregate leads to the accumulation of stress,which controls the development of crack,and its distribution affects the damage process and failure path of renewable coarse aggregate concrete.（3）the freeze-thaw cycle（0,30,60,90）after the frozen water,sodium chloride and sodium sulfate（slot）group center drawing specimen to load,get each specimen load sliding steel strain curve and sodium chloride group found that as the increase of freeze-thaw cycles,the groups of recycled concrete specimens of limit load are decreasing;When the number of freezing-thawing cycles reaches 90 times,the peak load of sodium chloride group,sodium sulfate group and water frozen specimen loses 46.6%,43.7%and 27.26%,respectively.As the number of freezing-thawing cycles increases,the shape of the curve of reinforcing bar with anchorage length gradually develops from"single peak"to"double peak",which is because the salt-freezing cycle accelerates the transfer speed of drawing load from loading end to free end.When the freezing-thawing cycle is 0,the shape of the curve of the bond stress with anchorage length is obviously"single peak",and the single peak appears about 20mm from the loading end.（4）Two-dimensional mesoscopic finite element simulation and three-dimensional finite element simulation were carried out for each group of central-drawing specimens.It was found that the failure of the concrete part of the central-drawing specimen had a certain local effect,and the failure occurred around the edge of the reinforcement rib,while the part far from the reinforcement did not have damage failure.The simulated values of the drawing specimens are in good agreement with the experimental values,indicating that the bond slip constitutive relation between recycled concrete and steel bar obtained by the research group has good applicability.