A mechanics solution for predicting the durability and bond interface deterioration of environmentally loaded FRP-strengthened RC flexural members

Adhesively bonding fibre-reinforced polymer (FRP) composites to reinforced concrete (RC) members is a simple and highly effective method to restore strength, increase loading capacities and prolong the service life of structural elements. Widespread application of FRP retrofits have been hampered by the uncertainty surrounding the long-term durability of the FRP strengthening system, particularly in relation to the critical bond between FRP and concrete and its implications on member flexural behaviour. RC members strengthened by adhesively bonding FRP to tension faces are susceptible to brittle debonding failures that can severely limit the effectiveness of the strengthening system. These debonding failures are attributed to the stress concentrations that occur at the plate-end, at flexure-shear cracks, and through the intermediate-crack (IC) debonding mechanism associated with the widening of flexural cracks. While plate-end debonding can be avoided via sound detailing practices, IC debonding is unavoidable since the failure mechanism is associated to the displacement, or slip, that occurs at the bond interface between FRP and concrete as flexure and flexure-shear cracks widen under load. Hence it is this form of failure which is investigated in this thesis. To isolate the conditions that lead to IC debonding, researchers have devised shear bond tests that allow the bond behaviour between FRP and concrete, central to IC debonding, to be studied. Over the past 20 years there has been widespread laboratory testing of environmentally loaded FRP-to-concrete joints via these shear bond tests, therefore there is a need for a global assessment of all data to determine the driving mechanisms influencing bond deterioration. First, through a regression analysis of a comprehensive shear bond test database, factors contributing to the instances of joint strength gain and loss across a range of studies and various environmental conditions are identified. It is found that the durability of the joint is generally not attributable to the type of fibre, strengthening scheme or surface preparation method, rather the strong influence is the relationship between the strength development of the concrete substrate and deterioration of the adhesive over the duration of environmental conditioning. With limited reporting of the full-range load-displacement behaviour, the analysis is limited to quantifying bond deterioration in terms of joint strength only. Lower bound statistical models are developed for predicting the change in bond strength under moisture conditions, elevated temperatures and freeze-thaw cycling. Having established that there is limited reporting of the load-displacement behaviour of bonded joints, the deterioration of the bond is further investigated through an experimental study. Samples are exposed by continuous immersion in water, saltwater and for the first time, a sulphuric acid solution simulating the pooling of acid rain in highly industrialised cities. Then, a numerical model, based on the mechanics of partial-interaction, is applied to extract bond characteristics in the form of the bond-slip relationship from the present experimental results as well as those in published literature. Significantly, it is shown that the majority of test results gathered for the analysis can only be accurately assessed with the proposed model as samples are often outside the bounds of existing predictive strength and bond-slip models. Extracted bond properties are statistically analysed to propose a set of factors that quantify bond characteristic deterioration and the resulting extension of the effective bond length that occurs with environmental loading. Finally, a displacement-based moment-rotation approach for IC debonding in FRP-strengthened flexural members is adapted into a numerical solution to predict the implications of bond deterioration on flexural performance. The approach uses partial-interaction theory to simulate the load-slip response of the FRP plate relative to the concrete, from the formation of the first flexural crack to IC debonding, which is significant as any changes at any bond interface due to environmental loading can be accommodated. At the initiation of IC debonding, the FRP laminate is considered completely detached from the RC member over a certain length, no longer acting as external reinforcement, but rather as unbonded prestressing tendon that exerts a force equivalent to the IC debonding resistance, allowing for concrete softening should it occur prior to the complete debonding. The numerical model is shown to be capable of quantifying local bond characteristics by matching the predicted load-deflection response to the experimental load-deflection response. Hence, a new methodology for extracting bond characteristics from flexural tests is developed. Application of the methodology to deteriorated FRP-strengthened flexural members reveals the change in bond characteristics that occur at the member level. Through the extraction of material bond characteristics in this thesis, it is shown that bond deterioration at the member level can be more substantial than that indicated by shear bond tests. This change in bond characteristics, due to environmental loading, is found to reduce the ability of the member to accommodate the deformation of the FRP laminate over its length, thereby limiting the load above IC debonding that the beam can support. This result is subject to three limitations: (1) due to the absence of common test methodologies and guidelines, the test conditions of flexural tests gathered for the study are outside the range of equivalent shear bond tests. (2) Changes to material behaviour or steel-to-concrete bond interface behaviour are lumped into the FRP-to-concrete interface. (3) In shear bond tests the failure mode is isolated, whereas debonding failure in flexural members can be difficult to identify due to sudden and catastrophic failure of specimens. This research has shown that bond deterioration at the FRP-to-concrete interface can promote unstable IC debonding along the plated length of FRP-strengthened RC flexural members, compromising both strength and ductility