It is inevitable that the concrete structure in the natural atmospheric environment is carbonated by CO2 in air. One of the causes of reinforced concrete structure deterioration is the carbonation of concrete. During the practical application of the concrete structure, the deterioration of its performance often occurs under the load and environment coupling effects. The damage of micro-structure in concrete is often caused by load, such as increasing the micro-cracks in concrete, changing the connectivity of pores, increasing the CO2 diffusion channel and leading to the intensification of carbonation reaction. The deterioration of concrete structure caused by load influences the carbonation resistance ability of concrete to a great extent. Therefore, it is significant to study the durability of cement based materials under coupling effect of carbonation and load.
In this paper, phenolphthalein spraying method, thermal gravimetric and differential scanning calorimetric analysis, X-ray computed tomography method, nanoindentation, backscattered electron image analysis technology and mercury intrusion porosimetry method are carried out to analyze the microstructure and components of cement-based materials qualitative and quantitative. The law of the phase composition and microstructure of cement-based materials under different carbonation conditions was decided, and the phase composition and pore structure evolution of the interface transition zone before and after carbonation were quantitatively characterized, and at the same time the influence of load on the micro structure of cement based materials was also carried out. The carbon dioxide distribution model which based on the microstructure evolution law and considered the load, aggregate and interfacial transition zone was set up. The following conclusions can be drawn from the results:
Compared with that in the natural carbonation condition, the effect of 20% carbon dioxide concentration on the size of the partially carbonated zone was significant. However, no effect was seen for 3% carbon dioxide concentration. MIP showed that, in the same carbonation zone, low carbon dioxide concentration led to low total porosity; there was a clear refinement in the pore size and there was a high degree of carbonation.The specimens with different water cement ratio were nondestructive tested by XCT. The results showed that the specimens with low water cement ratio had the clear carbonation front shape, and the size of partial carbonation zone was small, while the high water cement ratio samples had the opposite law.For the supplementary cementing materials (SCM) specimens with 0.35 water binder ratio, the addition of 30% fly ash and 50% slag had little effect on the carbonated zone size and had significant influence on the partial carbonation zone.For the specimens with 0.53 water binder ratio, the carbonation depth of the SCM specimen is higher than that of the pure cement paste. Under the condition of the same SCM, the smaller water binder ratio is, the smaller size of the carbonated zone and partial carbonation zone. The carbonation results of different humidity showed that the effect of humidity on the carbonation zone was significant. There was almost no carbonation occur when the humidity was 90%; when the humidity was 70%, the complete carbonation zone size and partial carbonation zone size was almost the same, when the humidity was 50%, the partial carbonation zone size was larger than the full carbonation zone size.
Through researching on the influence of aggregate on the carbonation of cement based materials, found that the dilution and tortuosity effect of aggregate improves the carbonation resistance ability of concrete and mortar, while the interfacial percolation effect reduces the carbonation resistance ability. Which effect is in the dominant position depends on the aggregate content. The paste thickness had a good correlation with the carbonation depth, so the paste thickness can be used to characterize the effect of aggregate on carbonation. The ITZ effect amplification experiment results show that the diffusion rate of the ITZ is several times the matrix diffusion rate; the BSE test results show that before the carbonation the porosity of ITZ is higher than the matrix, and after carbonation the porosity of ITZ is reduced but still greater than the matrix porosity, after carbonation the ITZ is still the fast channel for CO2 transmission. The nanoindentation test results show that the size of ITZ decrease with the decrease of water cement ratio gradually, after carbonation ,the thickness of ITZ decreases, but its size still remains at 20μm ~30μm. It is shown that the ITZ is still the weakest part of concrete after carbonation. Compared with the pure cement sample, the microstructure of the specimen with fly ash is improved, and the porosity of the SCM specimen is smaller than that of the pure cement sample, and the hydration degree of the cement is more sufficient.
The effect of loading on the carbonation is tested by thermal gravimetric analysis method and mercury intrusion porosimetry method, the results show that carbonation depth increased under the tensile stress and carbonation depth reduced under the compressive stress; in the same carbonation depth, the residual CH in the compressive side is higher than that in the tensile side, it’s shown that the sample carbonated more sufficient under the tensile stress, the tension stress can accelerate the carbonation process. The pore structure parameters show that the compressive stress can refine the pores of the cement based materials.
The effect of load on the interface effect zone is tested by XCT, the results show that the interface effect zone in the tensile region is higher than that of the compression zone. The BSE results show that the ITZ and aggregate bond very close in the compression zone, and there is no obvious difference between the ITZ and the matrix, while in the tensile zone, there are obvious cracks between the interface transition zone and the aggregate, the obvious cracks also appear in the matrix. The gray scale analysis results show that the different load has significant influence on the porosity of cement-based materials: the porosity of ITZ and matrix under compressive stress is smaller than that of the specimens without loading, while the porosity of ITZ and matrix under tensile stress is greater than that of the specimens without loading. The appropriate compressive stress can improve the strength of ITZ in some extent, and strengthen the ITZ, while the tensile stress has the deterioration effect on the cement-based materials.
Finally, based on the quality conservation law and Fick diffusion law the CO2 distribution model established. In the model a variety of factors (load, aggregate, interface, porosity, pore distribution, water cement ratio) are considered, and determine the effective diffusion coefficient of CO2 and the diffusion coefficient of interfacial transition zone in the concrete. The relation curve between CO2 concentration and pH and the relation curve between CH and pH were obtained by ion balance equation. By solving the CO2 distribution model, the distribution of CO2 under different stress levels is calculated. According to the CO2 concentration corresponding to the pore solution pH=9, the depth of the complete carbonation zone is determined and according to the CO2 concentration corresponding to the pH=9-11.5, the depth of the partial carbonation zone is determined, and compared with the measured value, the simulation results agree well with the measured values.