Deterioration Process And Carbonation Depth Prediction Model Of Concrete Under Combined Action Of Stress And Carbonation

Carbonation brings about the alkalinity drop of concrete pore solution and causes reinforcement corrosion.Moreover,in actual engineering,the external load will change the pore structure and even lead to micro-cracks in concrete.The speed and degree of carbonation damage under load are far greater than under single carbonation,so it is significant to reveal the deterioration process and deterioration mechanism of concrete under the coupling action of load and carbonation.Currently,more researchers focus on studying the carbonation of concrete.However,further breakthroughs need to be made in the carbonation of concrete under uniaxial load and in the research of changes in the pore structure.This paper studied the evolution of the carbonation depth,carbonation products,and pore structure of C30 concrete under different CO₂concentrations and the uniaxial load,and the prediction model of concrete carbonation depth under stress was established.The main tasks include:1.The change of carbonation depth（D_C）,partially carbonated width（W_P）and air permeability coefficient(k _Au)of concrete with the CO₂concentrations and the exposure period was investigated.The concrete’s k _Au increases with the CO2 concentrations and has a positive linear correlation with the D_C.The k _Au can be used to evaluate the carbonation degree of concrete to replace the D_C as a nondestructive parameter.The carbonation depth measured by the proposed new method based on the layer-by-layer p H value is 1.5～2 times that of the traditional phenolphthalein method.Therefore,the new method objectively reflects the carbonation damage and reinforcement’s corrosion risk.2.The profile of all solid phase substances（CH,C₂S,C₃S,Calcite,Vaterite,Aragonite,and C-S-H）with depth were obtained based on the XRD and TGA analysis on cement paste carbonated in 2%and 20%CO₂ for 28 days.Furthermore,it is determined that 5%～10%of CH remains in the completely carbonated zone.These results break through the hypothesis of Lothenbath’s thermodynamic model and Papadakis’s life prediction model that the CH will be completely consumed in the carbonation reaction and lay a foundation for accurately establishing the concrete carbonation depth modelling.3.The carbonation properties of OPC concrete,concrete containing 30%fly ash（FA-C）and concrete containing 50%blast furnace slag（BFS-C）with the same water/binder ratio and workability under uniaxial stress were studied,and the effects of carbonation products and stress on the pore structure of concretes were analyzed.When the compressive stress level is less than 0.45,the D_C,W_P,k _Au,porosity,and d _P of carbonated concrete under stress decrease as the stress level increase,which is due to the“densification effect”of low compressive stress and the“pore-filling effect”of carbonation products.When the compressive stress level is higher than 0.45,or under the action of tensile stress,the D_C,W_P,k _Au,porosity,and d _P of carbonated concrete all increase with the increase of stress levels,in which the“pores wall rupture effect”of high compressive stress or tensile stress and the“pore-coarsening effect”caused by C-S-H carbonation occupy the dominant role.In the completely carbonated zone,the d _Pand porosity of concrete with mineral admixture increased more obviously than that of OPC concrete,and the“pore-coarsening effect”caused by C-S-H gel carbonation was more significant.On the contrary,in the partially carbonated zone,the d _P and porosity of concrete mixed with mineral admixtures（FA-C and BFS-C）are smaller than that of OPC concrete.Moreover,the“pozzolanic reaction”of mineral admixtures and the“pore filling effect”caused by carbonation products are more obvious for concrete with mineral admixture.4.Based on the XRD,TG,MIP analysis,and k _Au test results,we optimized the 3key parameters in the existing CDep-GPL model.（1）The CO₂ consumption of carbonation reaction was optimized by adjusting the carbonated CH proportion from100%to 95%,adding the influencing factors of C₂S and C₃S carbonation;（2）The effective CO₂diffusion coefficient was optimized based on the influence of mix proportion,stress,and CO₂ concentrations on pore volume distribution;（3）The stress influence coefficient was optimized based on the k _Au of concrete with more stress levels.On this basis,the M-CDep-GPL model for predicting the carbonation depth of concrete under stress was established.As a result,the new model’s accuracy has improved by more than 30%compared with the original model.