Researches On Carbonation Behavior And Mechanism Of Shrinkage-compensated High Performance Concrete

The carbonation of covering concrete is an important cause of the corrosion of steel rebars in concrete structure, which degrades the load-bearing capacities and durability of the structure and affects the service performance. High performance concrete (HPC) is widely used in modern concrete structures, which is characterized by the use of mineral admixtures, such as fly ash and ground granulated blastfurnace slag, which can promote the mechanical properties and durability of concrete. But, researches have shown that the use of mineral admixtures affects greatly the carbonation resistance of concrete. Furthermore, modern concrete has a very low water content that increases the cracking risks. The hydration of expansive admixtures in concrete can generate expansive potential and thus, the use of expansive admixture has been an important technical solution of shrinkage prevention and crack migration. The expansive admixtures are widely used in bridge, underground structure, subway construction and railway projects. These structures are mostly designed with service life more than 100 years, which proposes high level of concrete durability.The high-performance concrete is characterized by high workability, high mechanical properties and excellent durability. The main design consideration is the low w/c ratio, low water content, the use of large amount of mineral admixture and application of high performance chemical admixture. The time-dependent microstructural changes of HPC are remarkably different from that of conventional concrete due to the low water content and reactivity difference between mineral admixture and clinker. The use of expansive admixture modifies the composition and quantity of hydration products and further changes microstructural parameters of the concrete. The carbonation behavior of high performance concrete has been intensively researches in literatures jointly with the mechanism; however, researches on the carbonation behavior and mechanism of shrinkage-compensated HPC are scarce. The hydration of expansive admixture changes the pore structure and quantity of hydration products that are subjected to carbonation. Hence, systematic analysis and testing of carbonation behavior of concrete containing expansive admixtures and necessary, which helps revealing the carbonation process and the microstructural mechanisms. Te carbonation behavior and mechanism of shrinkage-compensated HPC is investigated in this thesis. The effects of factors including the replacement levels of fly ash, expansive admixtures, curing condition, restraint conditions are on the carbonation rate are tested. The microstructural changes of concrete are analyzed with X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry, coupled thermogravimetry and differential scanning calorimetry, and Fourier transform infrared spectroscopy. The pore structure and quantities of Ca(OH)2 have been the main consideration. Carbonation of synthetic C-S-H gel is investigated, which is expected to provide direct evidence of carbonation behavior and microstructural mechanisms.The main research outputs of the thesis are concluded as:(1) The carbonation behavior and microstructural mechanisms of HPC are investigated. The effects of carbonation reaction on the pore structure and compositions of cementitious binders in HPC are explored. The results have shown that in general the carbonation reactions lowers the connectivity of pores in concrete and reduces the overall porosity thus densifies the structure. The quantity of Ca(OH)2 and pore structure are the two determinative parameters on the carbonation rate of concrete.(2) The carbonation behavior and microstructural mechanism of fly ash blend HPC are tested and analyzed. Correlation between the quantities of fly ash in concrete and the carbonation rate as well as the microstructural parameters is established. The use of large volume fly ash in HPC reduces remarkably the Ca(OH)2 content in HPC and pore structure at early ages is greatly coarsened due to the relatively low reactivity of fly ash. Therefore, the carbonation rate of large volume fly ash HPC is obviously greater than that of conventional concrete. The main mechanism that changes the carbonation rate is the modification of pore structure.(3) The effects of expansive admixture on the carbonation of HPC are experimentally studied and the microstructural mechanisms are analyzed. The use of expansive admixture in fly ash blended HPC can reduce the shrinkage of concrete and increases the amount of Ca(OH)2, but degrades the pore structure of concrete due to the expansive potential and preferential growth of Ca(OH)2 crystals in the restraint free condition. Thus, the permeation of CO2 gas in concrete is accelerated and carbonation process is enhanced. (4) The carbonation of shrinkage-compensated concrete in different curing regimes are tested and analyzed. Optimized curing regimes are established specially for the HPC containing expansive admixture or large volume of fly ash. The mechanisms are explained via microstructural analysis results.(5) The carbonation rate of shrinkage-compensated concrete in different restraint conditions is investigated, based on which the pore structure is analyzed. The results have shown that in the one-dimensional restraint condition, the hydration products of expansive admixtures fill the pores and hence densify the microstructure. The carbonation rate is consequently lowered, compared to the restraint free condition.(6) A new method for testing the carbonation depth and process of HPC is established with the microhardness tests. The method is based on the relation between the microhardness and porosity and concrete. The presence of carbonation front is proved and the carbonation profile is established. Compared to the conventional phenolphthalein indicator method, the new method is more precise and less arbitrary. The carbonation degree of concrete in different location can be quantitatively determined.(7) Carbonation of the C-S-H gel with different Ca/Si ratios are studied with microstructural analysis. The results have shown that the Ca/Si ratio is an important parameter on the carbonation rate. C-S-H gels with low Ca/Si ratios carbonates much faster than those with high Ca/Si ratios. Thus, the use of highly reactive mineral admixtures can not only reduce the amount of Ca(OH)2, but lower the Ca/Si ratio of C-S-H gel, which degrades the carbonation resistance of HPC.