| Concrete durability research has been a hot topic in the field of civil engineering research.Sulfate attack is one of the main factors affecting the durability of concrete,and the influence of load and carbonation cannot be ignored.In November 2019,our research group was invited to join the RILEM TC 281-CCC WG4 Technical Committee of the International Federation of Experimental Materials and Structures,and participated in an experiment on the durability of concrete under the coupling effect of load and carbonation,which was carried out by China Building Materials Academy.The test was carried out by domestic and international universities and organizations at the same time,and regular international conferences were held to exchange test progress and test results.The purpose is to prepare an international specification for the study of concrete durability under load and carbonization.On this basis,considering that many coastal and salt lake concrete buildings are not only subjected to the coupling effect of load and carbonization,but also inevitably subjected to dry-wet circulation sulfate erosion.Therefore,this paper carried out the experimental study on the damage characteristics of 252 concrete cube specimens under the combined action of axial compression load-carbonization-dry-wet cycle sulfate,and carried out the damage degradation analysis of concrete from multiple perspectives and deep levels by means of macro and micro combination,so as to provide reference for concrete degradation research and engineering practice.The main results are as follows :（1）Innovation of axial compression loading device : an axial compression loading device was designed to facilitate the coupling between axial compression load and carbonation.The main body of the device is assembled by four pull rods,and the roof is fixed by the support cylinder and the gasket.The force is transmitted by the ball hinge and the energy is stored by the disc spring,so as to impose a specified size of axial compression load on the test block.When the device is pressurized,the loading plate is connected with the lower device by loading the ball hinge.The force applied by the press is evenly transmitted to the gasket through the ball,and then transmitted to the disc spring by the gasket to store the force.Disc spring is selected because it can bear large load and small deformation,small space required,easy to combine and replace.On the other hand,the innovation of the device lies in the sensor designed on four support tubes.When the load is applied,the four sensors can show the magnitude of the force they are subjected to,which not only solves the problem of whether there is axial eccentricity during loading,but also monitors the loss of stress at any time in the test process.It is convenient to adjust in time to reduce the test error and provide more reliable data.（2）The damage performance of concrete under the coupling effect of axial compression load and dry-wet cycle sulfate was studied.Under no-load condition,dry-wet circulation sulfate plays an ’ optimization ’ role in the internal structure of concrete,and the greater the sulfate concentration,the more obvious the ’ optimization ’ effect;compared with the no-load group,under the same sulfate concentration,the existence of axial compression stress inhibits the deterioration of concrete,and the inhibition effect of0.3 stress ratio is higher than 0.15.With the development of erosion age,concrete specimens will be compacted first and then damaged.The greater the axial compression load,the higher the concentration of sulfate solution,the more obvious the effect.（3）The damage performance of concrete under the coupling effect of axial compression load and carbonation was studied.The damage mechanism of concrete was analyzed by testing the change of ultrasonic velocity,uniaxial compressive strength and carbonization depth of cube specimens before and after test.The results show that the relative dynamic elastic modulus and compressive strength of concrete increase with the increase of carbonization age.Different axial compression stress ratios have different effects on improving the carbonation resistance of concrete specimens.Under no-load condition,the carbonation depth of cube specimen is the largest,followed by that of specimen with stress ratio of 0.3,and that of specimen with stress ratio of 0.15 is the smallest.The carbonation depth of concrete increased rapidly in the first 7 days,and then increased slowly.（4）The damage performance of concrete under axial compression load-carbonization-dry-wet cycling sulfate attack was studied.The results show that the axial compression stress level and sulfate solution concentration have obvious effects on the damage performance of concrete in the three-factor composite test.With the development of erosion age,the concrete specimens with axial compression stress ratio of0.3 under different sulfate concentrations continue to deteriorate and develop,and the strength decreases continuously.The specimens with no load show a trend of first increasing and then decreasing,while the specimens with axial compression stress ratio of0.15 show different concentrations : first increasing and then decreasing at low concentrations,and then decreasing at high concentrations;in addition,different concentrations of sulfate solution will also affect the degree of carbonization of the specimen.When the sulfate concentration is constant,the change of carbonation depth is also affected by the axial compression stress ratio.Under the action of 0.3 axial compression stress ratio,the growth rate is the fastest and the carbonation depth is the largest.（5）At the micro level,the evolution law of the internal microstructure of concrete with the erosion age under the combined action of different axial compression stress ratios,different sulfate concentrations and carbonation was analyzed.The initial state of concrete surface structure is relatively close and smooth,there are a small number of tiny pores,there are aggregation of lamellar structure around the pores of calcium hydroxide;c F2S2 concrete shows obvious deterioration tendency in T4 period.The connection between cracks and pores can be observed in the concrete,and a large number of hexagonal rhombic crystal calcium carbonate exists in the pores.0.3Axial compression stress ratio,sulfate solution concentration on the microstructure of concrete damage degradation has a significant impact.With the increase of axial compressive stress ratio and the concentration of sodium sulfate solution,the microstructure of concrete shows that the sulfate attack product ettringite deposits continuously around the pores,the development of original cracks and the generation of new cracks accelerate,and cracks penetrate between the pores. |
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