Experimental And Numerical Investigation Of Carbonation Mechanism For Concrete With Blended Admixtures Under Load

Since the 1970s, carbon dioxide concentrations in the atmosphere increased year by year, Carbonation of concrete issues become even more serious. At the same time, modern concrete are usually added with blast-furnace slag, fly ash, and the replacement of mineral admixtures in the concrete will reduce the ability of anti-carbonation of concrete. In addition, concrete structures in service is always accompanied by complex stress field, especially tensile stress can lead to increase of porosity and the expansion of cracks, which exacerbate the transport speed of aggressive substances such as CO2 in concrete. Therefore, the impact of mineral admixture materials, load and other factors on concrete carbonation process in the real service environment should be considered further.This study focuses on the mechanism of deterioration of concrete containing admixtures under the coupling under static load and carbonization process. The effects of different types (contents) of admixtures on concrete carbonation depth, pH values, flexural strength are investigated respectively. Meanwhile, the effect of bending load with different levels and types on carbonation depth is investigated. In addition, based on Fick’s second law and mass balance law, carbonation model for concrete containing mineral admixtures under the coupling of carbonization and load is proposed. The model is solved by application of multi-physics software COMSOL Multiphysics. Finally, carbonation progress is studied from the meso-level. In this study, the key conclusions are as follows:(1) The evolution of carbonation depth and pH values for concrete with single or blended admixtures are both discussed. The results show that carbonation resistance of fly ash concrete is decreased with the increase of FA contents, while the carbonation resistance of BFS concrete is increased to the point where the value of BFS content is 0.1, then decreased with the increase of BFS content, when the value of BFS content is above 0.4, the carbonation resistance of BFS concrete decreased seriously. For the concrete with blended admixtures, carbonation resistance will be very low when the value of any single admixture content reaches to 0.4. In addition, concrete with 0.4 FA and 0.4 BFS were fully carbonated when the carbonation age is 7 days. Evolution of pH values is consistent with the evolution of carbonation depth, for all types of concrete.(2) The evolution of carbonation depth and pH values of concrete under four-point bending stress level is studied, and experimental results show that the stress level and types both have a great impact on carbonation depth and pH values. Within a certain range, bending tensile stress can significantly increase the carbonation depth, while the carbonation resistance of concrete increase due to the bending compressive stress improves the pore structure of concrete. The carbonation progress described by theoretical model are analyzed by the use of COMSOL Multiphysics.(3) Based on previous studies, carbonation reaction model where the type and content of admixtures, bending load stress levels and other factors considered is proposed.(4) Based on meso-scale level, a two-dimensional circular aggregate model and a two-dimensional structure random aggregate structure are both established. At last, the effects of aggregate volume fraction, aggregate shape and Interfacial Transition Zone(ITZ) on carbonation reaction progress are also analyzed.