| Author: | Das, Chandra Sekhar |
| Title: | Steel corrosion in coastal RC structures under coupled effects of chloride ingress and elevated environmental temperatures |
| Advisors: | Dai, Jian-guo (CEE) Zhao, Xiao-lin (CEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxi, 231 pages : color illustrations |
| Language: | English |
| Abstract: | Reinforcement corrosion is a primary contributor to the early-age deterioration of coastal reinforced concrete (RC) structures, leading to significant economic costs on a global scale. As a key climatic parameter, temperature affects the corrosion risk of structures. Based on the geographic locations and the influence of urban heat islands, structures experience daily, seasonal, and annual fluctuations in temperatures. The global temperatures are also forecasted to rise by 4.4°C compared to current average temperatures by 2100 due to climate change, influencing service temperatures. Thus, accurately assessing corrosion initiation under various service conditions needs to consider these time-dependent temperature variations and their effect on the physical, chemical and electrochemical properties of corrosion. However, the temperature has traditionally been considered to influence the physical nature of chloride transport through concrete pores based on the Arrhenius equation, with less focus directed towards other interactions. This simplistic assumption has been the basis for service life prediction models considering chloride-induced corrosion initiation in RC structures, which may lead to an inaccurate assessment of the service life of RC structures against corrosion. This thesis involves experimental works to elucidate the interactions during chloride ingress through the concrete pore network up to the steel surface, leading to the breakdown of the passive film and corrosion initiation for different service temperatures. Based on the findings from these studies, an improved numerical framework has been developed that incorporates temperature-dependent chloride transport rate, chloride binding, and chloride threshold value in the modelling of steel corrosion initiation in coastal RC structures. The passivation and depassivation (i.e., corrosion initiation) of steel bars were first analysed in a simulated concrete environment maintained at three different temperatures of 25 °C, 35 °C and 45 °C through combined electrochemical and steel surface characterisation techniques. Findings reveal that exposure of the reinforcing steel to pore solutions at elevated temperatures of 35 °C and 45 °C accelerated the formation of Fe(III) oxides compared to 25 °C, resulting in a higher Fe(III)/Fe(II) ratio and nobler stable potential values. However, the overall metal oxide content was lower, reducing the passive film's protective ability. Additionally, the passive film formed on the steel surface had more defects. Consequently, corrosion initiated at a significantly lower chloride concentration when chlorides were introduced into the pore solution. Interestingly, the passivity breakdown was triggered for nearly the same pore solution resistivity value, even when exposed to the three different temperatures. To understand the contents of free chlorides in cement pore solution that pose a corrosion risk to steel reinforcement in uncarbonated conditions, chloride binding by cement hydration products at 25 °C and 45 °C was subsequently explored at different chloride concentrations. The main purpose was to (a) quantify the physically and chemically bound chlorides, (b) relate the physically bound chlorides to the structure and composition of C-(A)-S-H, and (c) assess the implications of elevated temperatures on steel corrosion risk. Results reveal that when exposed to low chloride concentration (0.5M), akin to seawater conditions, total chloride binding remained unchanged for OPC and increased for lower content of fly ash addition with increased temperature. However, it decreased significantly for binders with a high fly ash content and silica fume addition despite increased chemical chloride binding. At higher chloride concentrations (3M), total chloride binding increased for all the mixes with increasing temperatures. A hypothesis was then proposed to explain these trends in chloride binding with exposure temperatures. The long-term corrosion performance (20 months) of steel reinforcement was also monitored in different mortars prepared with fly ash and silica fume when exposed to chloride solutions at two environmental temperatures of 25 °C and 45 °C. The aim was (a) to quantify temperature-dependent chloride threshold values, (b) to assess any benefits of adding siliceous SCM contents in these extreme conditions, and (c) to characterize the evolution of mortar properties. It was observed that incorporating SCMs marginally offset the adverse effect of elevated temperatures on corrosion initiation time due to enhanced pore refinement in mortars. Analysis of mortars around the steel-mortar interface revealed that the chloride threshold value for corrosion initiation was significantly reduced when the samples were exposed to chlorides at elevated temperatures, supporting the findings from the simulated concrete pore solution study. The chloride threshold value increased with fly ash but reduced drastically when silica fume was added to the mix. Elevated temperature also modified the corrosion morphology and the products formed on the steel surface due to chloride attack. Finally, considering future climate projections, a multi-physics modelling framework was developed that integrates mass transport, electrochemical reactions, and material damage to simulate the corrosion processes in RC structures. The experimentally validated parameters, such as temperature-dependent chloride transport rates, binding capacities, and threshold values, were included to enhance its fidelity. The model was used to predict the corrosion initiation time for reinforced mortar mix, and the framework was then applied to assess the corrosion risk of representative RC structures in Hong Kong under various projected climate scenarios. The results showed that while higher temperatures accelerated chloride transport, increased chloride binding partially suppressed this effect, resulting in only a slight reduction in corrosion initiation time for a temperature rise of 5 °C. Thus, the different climate projections had minimal impact on tidal exposure zones, as corrosion was primarily due to chlorides. The effect was more complicated for atmospheric zones due to the interaction between the deeper carbonation front and chlorides. An improved understanding of corrosion in RC structures under different service conditions will enable the development of accurate life cycle models to forecast maintenance needs for coastal infrastructures and ensure efficient carbon management during their service life. |
| Rights: | All rights reserved |
| Access: | open access |
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