Corrosion of steel reinforcement has been identified as one of the major problems facing many existing reinforced concrete structures including bridges. The exposure to aggressive environmental conditions such as those with high concentrations of chloride ions due to the use of de-icing salt in cold regions or high concentrations of carbon dioxide due to increased greenhouse gas emissions, accelerate the initiation process of corrosion. As corrosion initiates, the structural performance in terms of load-carrying capacity, ductility, and service life deteriorate over time. Coupling the effect of reinforcement corrosion with service loads may further weaken the structural performance of reinforced concrete bridges due to the presence of transverse load-induced cracks. Accordingly, a research program was conducted to evaluate the structural performance of reinforced concrete beams subjected to coupled effects of service loads and reinforcement corrosion. The research project consisted of combined experimental and numerical investigations. The experimental phase consisted of tests of nine small-scale beams and six large-scale beams. The beams were designed, constructed, instrumented, and loaded under a four-point load test. The primary test variables were the applied corrosion current density, level of corrosion, and level of sustained loading as percentage of beam ultimate capacity (0% Pu, 40% Pu, and 60% Pu). The corrosion level of steel reinforcement was quantitatively assessed using gravimetric weight measurements and three-dimensional laser scanner technique. Test results indicated that failure of corroded RC beams was brittle due to premature rupture of corroded steel bars, which was attributed to the development of localized corrosion at the sections with flexural cracks in beams. Furthermore, it was found that beams subjected to higher levels of service loads, experienced further reductions in ultimate load capacity and ductility. In addition, tensile tests were used to evaluate the effect of corrosion on the mechanical performance of steel bars retrieved from the corroded beams. It was found that the tensile strength of corroded steel bars, based on nominal sectional area, was reduced with the increase of corrosion levels. In contrast, the tensile strength, based on minimum sectional area, was not influenced by the non-uniform distribution and localization of corrosion. In fact, there was a slight increase in strength with the increase of corrosion levels. The numerical phase consisted of finite element analyses of beams using DIANA FE analysis software. A simplified approach was implemented to introduce the damage induced by corrosion into two-dimensional nonlinear FE models, based on the experimental testing of corroded beams and corroded steel bars. The analyses were reasonably accurate in predicting cracking patterns, residual load capacity, residual ductility, and failure modes of corroded beams. Subsequently, the validated model was used to conduct a parametric study on the level of service loads, level of corrosion, strength of concrete, and tensile reinforcement ratio. It was found that the FE model of corroded beams was strongly influenced by the level of service loads, level of corrosion, and tensile reinforcement ratio.

The current research project mainly aims to investigate the corrosion resistance and bond performance of steel reinforced recycled aggregate concrete incorporating nano-silica under both normal and corrosive environmental conditions. The experimental part includes testing of 180 pull-out specimens prepared from 12 different mixtures. The main parameters studied were the amount of recycled aggregate (RCA) (i.e. 0%, 25%, 50% and 100%), nano silica (1.5% and 3%), steel embedment length as well as steel bar diameter (12 and 20mm). Different levels of corrosion were electrochemically induced by applying impressed voltage technique for 2, 5, 10 and 15 days. The experimental observations mainly focused on the corrosion level in addition to the ultimate bond, failure modes and slips occurred. Experimental results showed that the bond performance between un-corroded steel and recycled aggregate concrete slightly reduced, while a significant degradation was observed after being exposed to corrosive conditions, in comparison to normal concrete. On the other hand, the use of nano silica (NS) showed a reasonable bond enhancement with both normal and RCA concretes under normal conditions. However, much better influence in terms of bond and corrosion resistance was observed under advancing levels of corrosion exposure, reflecting the improvement in corrosion resistance. Therefore, NS was superbly effective in recovering the poor performance in bond for RCA concretes. More efficiency was reported with RCA concretes compared to the conventional concrete. The bond resistance slightly with a small amount of corrosion (almost 2% weight loss), then a significant bond degradation occurs with further corrosion. The influence of specific surface area and amount of nano silica on the performance of concrete with different water/binder (w/b) ratios has been also studied, using 63 different mixtures produced with three different types of colloidal NS having various surface areas and particle sizes. The results showed that the performance of concrete is heavily influenced by changing the surface area of nano silica. Amongst the three used types of nano silica, NS with SSA of 250 m2 /g achieved the highest enhancement rate in terms of compressive strength, water absorption and microstructure analysis, followed by NS with SSA of 500 m2/g, whilst NS with SSA of 51.4 m2 /g was less advantageous for all mixtures. The optimum nano silica ratio in concrete is affected by its particle size as well as water to binder ratio. The feasibility of the impact-echo method for identifying the corrosion was evaluated and compared to the corrosion obtained by mass loss method. The results showed that the impact echo testing can be effectively used to qualitatively detect the damage caused by corrosion in reinforced concrete structures. A significant difference in the dominant frequencies response was observed after exposure to the high and moderate levels of corrosion, whilst no clear trend was observed at the initial stage of corrosion. Artificial neural network models were also developed to predict bond strength for corroded/uncorroded steel bars in concrete using the main influencing parameters (i.e., concrete strength, concrete cover, bar diameter, embedment length and corrosion rate). The developed models were able to predict the bond strength with a high level of accuracy, which was confirmed by conducting a parametric study. / Higher Education Institute of the Libyan Government

The current research project mainly aims to investigate the corrosion resistance and bond performance of steel reinforced recycled aggregate concrete incorporating nano-silica under both normal and corrosive environmental conditions. The experimental part includes testing of 180 pull-out specimens prepared from 12 different mixtures. The main parameters studied were the amount of recycled aggregate (RCA) (i.e. 0%, 25%, 50% and 100%), nano silica (1.5% and 3%), steel embedment length as well as steel bar diameter (12 and 20mm). Different levels of corrosion were electrochemically induced by applying impressed voltage technique for 2, 5, 10 and 15 days. The experimental observations mainly focused on the corrosion level in addition to the ultimate bond, failure modes and slips occurred. Experimental results showed that the bond performance between un-corroded steel and recycled aggregate concrete slightly reduced, while a significant degradation was observed after being exposed to corrosive conditions, in comparison to normal concrete. On the other hand, the use of nano silica (NS) showed a reasonable bond enhancement with both normal and RCA concretes under normal conditions. However, much better influence in terms of bond and corrosion resistance was observed under advancing levels of corrosion exposure, reflecting the improvement in corrosion resistance. Therefore, NS was superbly effective in recovering the poor performance in bond for RCA concretes. More efficiency was reported with RCA concretes compared to the conventional concrete. The bond resistance slightly with a small amount of corrosion (almost 2% weight loss), then a significant bond degradation occurs with further corrosion. The influence of specific surface area and amount of nano silica on the performance of concrete with different water/binder (w/b) ratios has been also studied, using 63 different mixtures produced with three different types of colloidal NS having various surface areas and particle sizes. The results showed that the performance of concrete is heavily influenced by changing the surface area of nano silica. Amongst the three used types of nano silica, NS with SSA of 250 m2 /g achieved the highest enhancement rate in terms of compressive strength, water absorption and microstructure analysis, followed by NS with SSA of 500 m2/g, whilst NS with SSA of 51.4 m2 /g was less advantageous for all mixtures. The optimum nano silica ratio in concrete is affected by its particle size as well as water to binder ratio. The feasibility of the impact-echo method for identifying the corrosion was evaluated and compared to the corrosion obtained by mass loss method. The results showed that the impact-echo testing can be effectively used to qualitatively detect the damage caused by corrosion in reinforced concrete structures. A significant difference in the dominant frequencies response was observed after exposure to the high and moderate levels of corrosion, whilst no clear trend was observed at the initial stage of corrosion. Artificial neural network models were also developed to predict bond strength for corroded/uncorroded steel bars in concrete using the main influencing parameters (i.e., concrete strength, concrete cover, bar diameter, embedment length and corrosion rate). The developed models were able to predict the bond strength with a high level of accuracy, which was confirmed by conducting a parametric study. / Higher Education Institute in the Libyan Government MONE BROS Company in Leeds (UK) for providing recycled aggregates BASF and Akzonobel Companies for providing nano silica NS, Hanson Ltd, UK, for suppling cement

O presente estudo trata do problema da corrosão de armaduras em estruturas de concreto armado submetidas à penetração de cloretos. É amplamente reconhecido nos meios técnico e científico que, dentre os diversos processos causadores de patologias nas estruturas de concreto armado, destacam-se aqueles que desencadeiam a corrosão das armaduras. O processo de corrosão é iniciado quando a concentração de cloretos na interface armadura/concreto atinge um valor limite, despassivando a camada química protetora ao redor da armadura. A utilização de uma abordagem probabilística é capaz de tratar o problema de forma mais consistente, uma vez que as variáveis envolvidas no fenômeno possuem um alto grau de aleatoriedade associado. A formulação utilizada para a análise não linear mecânica emprega o método dos elementos finitos, utilizando o modelo de dano de Mazars para descrever o comportamento do concreto e o modelo elastoplástico para descrever o comportamento do aço. A lei de Fick é empregada para descrever o mecanismo de difusão dos íons cloretos no interior dos elementos de concreto armado e leis empíricas são utilizadas para representar a penalização das áreas de aço das armaduras bem como a redução de sua tensão de escoamento. A formulação utilizada para a análise probabilística emprega o FORM (First Order Reliability Method), o método de superfície de resposta e a simulação de Monte Carlo. Enfoque especial é dado na análise de estruturas hiperestáticas, com o objetivo de determinar a probabilidade de falha do sistema, sendo esta uma das contribuições desse estudo. Os resultados obtidos indicam de que forma os efeitos corrosivos considerados contribuem para a redução da durabilidade estrutural. / This work deals with the problem of reinforcement corrosion of concrete structures subjected to chloride penetration. It is widely recognized in the technical and scientific communities that, among the different processes which cause pathologies in reinforced concrete structures, reinforcement corrosion is one of the most relevant. The corrosion process starts when chloride concentration at the reinforcement/concrete interface reaches a threshold value, causing depassivation of the protecting chemical layer surrounding the armor. A probabilistic approach is capable of dealing with the problem in a more consistent manner, since the variables involved in the phenomenon have a high degree of randomness. The formulation used in the mechanical non-linear analysis utilizes the finite element method, employing Mazars\' damage model to describe the concrete behavior and the elastoplastic model to describe the steel behavior. Fick\'s law is used to describe the diffusion mechanism of chloride ions inside the concrete elements and empirical laws are used to represent the steel area reduction as well as the reduction of steel yield stress. The formulation used for the probabilistic analysis employs the FORM (First Order Reliability Method), the response surface method and the Monte Carlo simulation. A contribution of this study concerns the analysis of hyper-static structures, in order to evaluate the failure probability of the system, giving special attention to this type of problem. The obtained results show how the considered corrosive effects influences the structural durability reduction.

O presente estudo trata do problema da corrosão de armaduras em estruturas de concreto armado submetidas à penetração de cloretos. É amplamente reconhecido nos meios técnico e científico que, dentre os diversos processos causadores de patologias nas estruturas de concreto armado, destacam-se aqueles que desencadeiam a corrosão das armaduras. O processo de corrosão é iniciado quando a concentração de cloretos na interface armadura/concreto atinge um valor limite, despassivando a camada química protetora ao redor da armadura. A utilização de uma abordagem probabilística é capaz de tratar o problema de forma mais consistente, uma vez que as variáveis envolvidas no fenômeno possuem um alto grau de aleatoriedade associado. A formulação utilizada para a análise não linear mecânica emprega o método dos elementos finitos, utilizando o modelo de dano de Mazars para descrever o comportamento do concreto e o modelo elastoplástico para descrever o comportamento do aço. A lei de Fick é empregada para descrever o mecanismo de difusão dos íons cloretos no interior dos elementos de concreto armado e leis empíricas são utilizadas para representar a penalização das áreas de aço das armaduras bem como a redução de sua tensão de escoamento. A formulação utilizada para a análise probabilística emprega o FORM (First Order Reliability Method), o método de superfície de resposta e a simulação de Monte Carlo. Enfoque especial é dado na análise de estruturas hiperestáticas, com o objetivo de determinar a probabilidade de falha do sistema, sendo esta uma das contribuições desse estudo. Os resultados obtidos indicam de que forma os efeitos corrosivos considerados contribuem para a redução da durabilidade estrutural. / This work deals with the problem of reinforcement corrosion of concrete structures subjected to chloride penetration. It is widely recognized in the technical and scientific communities that, among the different processes which cause pathologies in reinforced concrete structures, reinforcement corrosion is one of the most relevant. The corrosion process starts when chloride concentration at the reinforcement/concrete interface reaches a threshold value, causing depassivation of the protecting chemical layer surrounding the armor. A probabilistic approach is capable of dealing with the problem in a more consistent manner, since the variables involved in the phenomenon have a high degree of randomness. The formulation used in the mechanical non-linear analysis utilizes the finite element method, employing Mazars\' damage model to describe the concrete behavior and the elastoplastic model to describe the steel behavior. Fick\'s law is used to describe the diffusion mechanism of chloride ions inside the concrete elements and empirical laws are used to represent the steel area reduction as well as the reduction of steel yield stress. The formulation used for the probabilistic analysis employs the FORM (First Order Reliability Method), the response surface method and the Monte Carlo simulation. A contribution of this study concerns the analysis of hyper-static structures, in order to evaluate the failure probability of the system, giving special attention to this type of problem. The obtained results show how the considered corrosive effects influences the structural durability reduction.

This dissertation deals with the study of durability and degradation repair mortars for reinforced concrete structures. In its theoretical part, there are the research findings on the behavior of repair mortars exposed to corrosive environments with different exposure time and selected high temperatures. Processes occurring in repair mortars during their loading at high temperatures or when exposed to chemically aggressive environments are described, We can find recipes designed for cementitious binder based with a specific replacement using slag or fly ash in its practical part. The object of the research was to determine the durability of the proposed repair mortars, and determine their suitability for use on concrete structures, which may be exposed to a synergistic effect of chemically aggressive environments with high temperatures simulating fire.