Experimental Study On Mechanical Properties Of Concrete Under The Interaction Of Load And Carburization

Carbonation is one of the main durability properties of concrete.In recent years,due to the increase of acid gases such as carbon dioxide in the atmosphere,the reaction of the alkaline substances in concrete with carbon dioxide results in a decrease in alkalinity of concrete,which leads to corrosion of steel bars and the loss of the bearing capacity of concrete structure.In practical engineering,most concrete members are also subjected to constant loads for a long time.Therefore,the study on mechanical and carbonation properties of concrete under the combined action of load and carbonization is of great significance to the engineering practice.In present study,the rapid carbonation test of four-point bending beams of concrete with a constant preload was carried out to study the influence of different load levels and carbonization time on the carbonization resistance of concrete.The results showed that,with the same carbonation time,the carbonation depth and rate increased with the increase of stress level in the tensile stress zone,and decreased with the increase of stress level in the compressive stress zone.When the stress level was the same,the carbonation depths of the tensile stress zone and the compressive stress zone increased with the increase of carbonation time.Based on the experimental results,the prediction models of carbonation depth under the action of bending tensile stress and compressive stress were established respectively.The model was in good agreement with the experimental data.The compressive and flexural tests of concrete with different load levels and carbonation time were carried out.It was found that when the stress level was the same,the compressive strength increased with the increase of carbonation time,and the flexural strength decreased at first and then increased.When the carbonation time was the same,the flexural strength decreased with the increase of stress level.X-ray diffraction analysis and differential thermal gravimetric analysis showed that the content of Ca?OH?₂ in the uncarbonized concrete was higher than that in the carbonized concrete,and the content of CaCO₃ had the opposite result.In the initial stage of carbonation?7 days?,the carbonation reaction was more adequate.The evolution of concrete pore structure under loading and carbonization was studied by mercury compression method.It was found that the total pore volume,total pore surface area,porosity,less harmful pore,and harmless pore decreased with the increase of stress level and carbonation time.At the same stress level,the pore structure at the initial stage of carbonation turned out to be better than that at the later stage in the compressive zone of concrete.When the carbonation time was the same,the internal defects and pores in the tensile zone increase with the increase of stress level.Digital image correlation technique?DIC?was used to analyze the variation of strain field on the surface of bend specimen.The horizontal strain statistical index was defined to represent the damage degree of concrete.The results showed that when the carbonation time was the same,the higher the stress level was,the earlier the turning point of the statistical index of horizontal strain in the tensile zone of concrete appeared which lead to the earlier damage,while the earlier the damage accumulation occurred,and the earlier it exited the stage of damage accumulation.When the stress level was the same,with the increase of carbonation time,the turning point of the statistical index of horizontal strain in the tensile zone appeared later,indicating the damage appeared later,while the damage formation and accumulation occurred earlier and the failure occurred later in the compression zone.