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Rebar corrosion is the most commonly observed deterioration mechanism in reinforced concrete (RC) structures, severely affecting their performance and potentially resulting in premature in-service failures. Corrosion causes internal damage to RC elements, owning to the loss of steel area and the formation of associated expansive corrosion products. Corrosion damage, which may ultimately lead to inadequate performance, includes loss of steel area, loss of bond between steel and concrete, reduced concrete strength due to cracking, loss of concrete section due to spalling and reduced mechanical properties of the affected rebars, i.e. strength and ductility, mostly due to pitting formation. The severity of this damage, however, depends on the nature (i.e. uniform or/and pitting) and extent of corrosion as well as the location of its occurrence, i.e. tension, compression and/or shear reinforcements in RC beams. Accurate performance evaluation of corroding structures could allow extension of service life, where appropriate, and may contribute to a more consistent safety level across a network of structures. This would improve the efficient use of scarce resources, and minimize the impact of indirect costs through optimised inspection, maintenance and repair works. In this study, performance of corroding under-reinforced beams is examined under serviceability (i.e. deflections, cracking patterns) and ultimate (i.e. load capacity, ductility) conditions through detailed parametric and reliability analysis studies. The performance of corrosion-damaged under-reinforced beams is assessed using non-linear finite element analysis (NLFEA). Empirical models from the literature are used to consider the different effects of corrosion damage in the analysis. The NLFE models are used in parametric studies, where a number of corrosion-damage scenarios are examined. For the mid-scale beams studied within this thesis, loss of tensile steel area and concrete damage/section-loss due to corrosion in the compressive region of the beam, are found to be the main causes for loss of load capacity and bending stiffness. The numerical models predicted an increase of beam's ductility for increasing amounts of corrosion, up to approx. Qcor = 12% loss. This behaviour was found to be due to the beneficial effect of reducing tension rebar area and yield strength on beam's ductility. At higher amount of corrosion, beam's ductility started to deteriorate due to a transition in beam's behaviour at ultimate deflection from concrete crushing towards premature rupture of the corroded tension rebars. Reliability analysis, which is a widely proposed assessment tool of corroding RC structures, is used to examine the influence of uncertainties associated with different variables involved in the deterioration process. A FORM based reliability analysis methodology is developed, which uses NLFEA together with adaptive response surface method (RSM) for the approximation of the performance function. The procedure is used for the analysis of two corrosion damage scenarios on (mid-scale) under-reinforced beams. The response surface of the yield-load capacity limit state is shown to be a stationary surface, since the regression coefficients are insensitive to the amount of corrosion. The reliability index, B, was found to gradually reduce for increasing corrosion damage, Qcor. The sensitivity factors obtained for the load capacity limit state revealed that the relative importance of the random variables remained almost constant as corrosion loss increased. The approximation of the ductility limit state was found to be strongly influenced by the selection of variable ranges due to significant curvature in the response surface. The accuracy of the fitted response surface was improved by splitting the variable space into smaller ranges and evaluating the response surface for each corrosion interval. An initial increase of the reliability index, B, was observed for the two corrosion damage scenarios examined. The sensitivity factors for the ductility limit state revealed that as corrosion damage increases, a transition of the relative importance of random variables concrete compressive strength,fc and ultimate (tension) rebar strain, εu, is likely to occur.

The use of changes in vibration propelties for damage detection of civil infrastructure has been a subject of investigation for many years. However, the implementation of vibration-based techniques in reinforced concrete structures is extremely limited, due principally to the absence of suitable crack models. The aim of this research is to determine whether a flexural crack model incorporating non-linearity is able to quantitatively reproduce changes in vibration characteristics of cracked reinforced concrete beams, and to assess the sensitivity of changes in vibration properties to realistic damage scenarios. To better understand cracking mechanisms in concrete, experiments on un-notched plain concrete prisms under monotonic and cyclic four-point bending were conducted. The fracture process in the prisms was captured using a digital image correlation system. In addition, a numerical model of a plain concrete prism based on the fictitious crack approach is developed. The model adopts cyclic constitutive laws and is validated against the experimental results. To examine its suitability for vibration-based damage detection, the fictitious crack approach is used to develop a reinforced concrete beam model subject to incremental static four-point bending. The dynamic behaviour of the model, such as changes in resonant frequency, modal stiffness and level of non-linearity, is examined using different sinusoidal excitations. The beam model is validated against experimental results of beams under incremental static loading. Another set of experiments on beams under cyclic loading was carried out to assess realistically the sensitivity of changes in vibration characteristics. The results revealed that the crack model quantitatively predicts changes in vibration characteristics and that the sensitivity of these changes is influenced by the condition of SUPPOltS. The trend of changes in the level of non-linearity follows a softening behaviour and thus it may not be suitable for damage detection applications. The experimental results of the beams under cyclic loading give rise to the question of the suitability of vibration-based techniques for damage detection.

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No description available.

One of the main causes of deterioration of reinforced concrete structures is the corrosion of the reinforcement. This may be as a result of carbonation or chloride diffusion into the concrete. During the lifetime of any reinforced concrete structure it is likely to require maintenance and repair. Repair materials are used as a form of "corrosion prevention" on deteriorated areas of a structure. The properties of the repair material offer maximum protection to the steel at the repair. Combining two cementitious materials with different properties can create a difference in oxygen or chloride concentration between two materials, creating a galvanic cell. Matching the physical properties of the repair and substrate materials may mitigate the corrosion of the reinforcement. The theoretical and experimental work to support the idea of matching properties is currently limited. This project has examined the effect of combining repair and substrate materials with different physical properties on the corrosion of reinforcement embedded in these materials. The initial part of the experimental programme examined the physical properties of typical repair and substrate materials to quantify the range of the properties. This concentrated on mass-transport properties such as, the density, porosity, permeability, chloride diffusion and resistivity of each material. The electrochemical compatibility of the materials was measured using potentiodynamic polarisation measurements. This showed that steel embedded in materials with lower permeability coefficients had lower corrosion potentials. The permeability and chloride concentration in the materials were used to produce test specimens with range of differences in cell potentials between the steel embedded in the repair and substrate materials. The specimens of the different repair substrate combinations were exposed to a saline solution for twelve months. Measurements of the resistivity, half-cell potentials and corrosion currents of the specimens monitored with time assess the corrosion rate of the specimens. Two methods were used to measure corrosion currents, impedance spectroscopy and linear polarisation resistance. The half-cell potential measurements indicated that a low permeability material would be anodic when combined with a high permeability material, which would be cathodic. The corrosion current of the material identified as anodic by half-cell potentials was higher for larger mismatch in permeability. This indicated a higher corrosion rate in the anodic material for substrate repair combinations that result in large differences in halfcell potentials. This would suggest that a galvanic cell was formed due to oxygen concentration differences between the repair and substrate materials. The results from the experimental work were used to model the distribution of current between the anodic and cathodic sites. The model shows that it is the difference in potential that has the greatest influence on the current flowing in the cell and the resistivity of the material controls the distribution of the current within the cell. This indicted that corrosion would be concentrated at the interface between the repair and substrate material with the low permeability material being anodic. The study shows that the corrosion resulting from a disparity in properties between repair and substrate materials is likely to be small. However higher corrosion rates may occur at the interface between repair and substrate that may require additional corrosion protection systems to be used. Matching the permeabilities of materials would not be practical, as permeability has been found to change with time altering the match between repair and substrate.

This thesis investigates gaps in the current understanding of some key aspects of Fibre Reinforced Polymer (FRP) strengthened reinforced concrete (RC) members. There are four important issues have been investigated. Firstly, a novel pullout test was developed to investigate the stress transfer mechanics and failure modes of near surface mounted (NSM) FRP strengthened RC blocks at a fundamental level. Secondly, the (FEA) was used to gain a detailed understanding of stress distribution both along the bond line and through the thickness of the adhesive layer for the RC members strengthened either by NSM or externally bonded plate (EBP) FRP technique.

The finite element (FE) method is a powerful technique that can provide numerical solutions to the response of reinforced concrete (RC) structures. However, results obtained from FE models are often not objective in the sense that the numerical solutions of FE models depend on aspects such as the selection of mesh size, load step size etc. FE model objectivity aims at the development of FE models for which the predicted results converge with refinement. To date, many research studies have been carried out on the objectivity of FE solutions for RC structures. However, considerable uncertainty still exists because of the many parameters which are involved in the analysis. The parameters affecting FE analysis of RC structures may be divided into two groups: material parameters and procedural parameters. The main parameters related to the material behaviour are tension softening and interaction between steel reinforcement and concrete. On the other hand, the procedural parameters which affect directly the results of the analysis are the load step, mesh size, iterative scheme, and number of cracks allowed per load step, numerical integration rule, and the use of static vs. dynamic analysis. In an effort to investigate these parameters, the current research is primarily aimed towards developing finite element formulations and solution procedures that facilitate the objective modelling of RC structures. The present study focuses on a subset of the above parameters that appear to be most relevant to objective modelling. Two new formulations have been developed in this work which allows the objective modelling of RC beam-column members, including geometric and material nonlinearity as well as bond slip. Particular emphasis is placed on predicting crack localisation in the concrete and stress concentrations in the steel reinforcement across such cracks, as this is particularly relevant to the modelling of RC structures under extreme loading. Several verification and validation studies are presented in the thesis to illustrate the key features of the proposed formulations and their applicability to the objective modelling of RC framed structures.