Behaviour of FRP-to-concrete bonded joints under cyclic and thermal loading

Abstract

Flexural strengthening of reinforced concrete (RC) structures with externally bonded (EB) carbon fibre reinforced polymers (CFRP) plate has become increasingly popular over the last several decades, primarily due to the many advantages like a high strength-to-weight ratio, excellent corrosion resistance, and ease of installation. In EB CFRP strengthened RC structures, the effectiveness of the strengthening system largely depends on the interfacial shear stress transfer mechanism of the bonded interface. Extensive research has revealed that knowledge of the local bond shear stress-slip (bond-slip) relation is vital in understanding and modelling the behaviour of CFRP-to-concrete bonded joints under mode II loading (i.e. interfacial shear loading). Theoretical models have been developed to predict both the local bond-slip relationship and bond strength, and hence the full-range behaviour of CFRP-to-concrete bonded joints under mode II loading. Bond-slip models for modelling the constitutive behaviour of the bonded interface assume that the interfacial shear slip will reduce to zero when unloaded to zero shear stress (i.e. damage elasticity).While extensive research has been carried out on the behaviour of CFRP-to-concrete bonded joints under monotonic loading, the behaviour under different loading conditions, such as quasi-static cyclic loading, fatigue loading and thermal loading, has received much less attention. These loading conditions are quite common in structures such as bridges. Therefore, a sound understanding of CFRP-to-concrete bonded joints under cyclic and thermal loading conditions is necessary.Limited existing experimental studies on CFRP-to-concrete bonded joints subjected to cyclic loading have revealed that the damage elasticity assumption commonly made in bond-slip models is inaccurate, and may yield erroneous results when used to model the behaviour under cyclic loading. Theoretical models currently available for modelling the constitutive behaviour of CFRP-to-concrete bonded joints under cyclic loading lack sufficient experimental evidence for verifying the underlying assumptions. Limited studies available on the behaviour of CFRP-to-concrete bonded joints at elevated temperatures have shown that the failure mode of the bonded joints may change from cohesion failure within the concrete to cohesion failure within the adhesive. However, no in-depth study has been carried out in investigating how the change of adhesive mechanical properties may affect the behaviour of CFRP-to-concrete bonded joints.Against this background, the work presented in this thesis investigates the behaviour of CFRP-to-concrete bonded joints under quasi-static cyclic loading, fatigue cyclic loading and elevated temperature conditions.The behaviour of CFRP-to-concrete bonded joints under both quasi-static cyclic and fatigue cyclic loadings was investigated experimentally through a carefully designed series of single-shear pull-off tests. Experimental results showed that a) the damage elasticity assumption is incorrect, b) the failure mode of bonded joints could change from cohesion failure within the concrete to FRP delamination or cohesion failure within the adhesive layer as a result of fatigue cyclic loading, and c) the failure mode is sensitive to the loading amplitude, concrete strength and as well as the CFRP laminate used in the bonded joints. A damage plasticity model was proposed, where damage was defined as a function of the ratio between the energy dissipated in the loading process and the total fracture energy. Data from samples failed in concrete under fatigue loading was used to develop a damage accumulation relation with respect to the load amplitude and the damage parameter.Analytical and experimental investigations were carried out to study the behaviour of CFRP-to-concrete and CFRP-to-steel bonded joints under elevated temperatures. CFRP-to-steel bonded joints were selected to isolate the failure mode to cohesion failure within the adhesive, so that the effect of temperature on adhesive mechanical properties, and thus the bond behaviour could be investigated. Analytical and experimental investigations revealed that a) the bond strength of bonded joints between two intermediate cracks increased due to the initial shear stress induced by the different thermal expansion coefficients of CFRP and the substrates, b) the bond strength of CFRP-to-steel bonded joints increased with temperature until the heat deflection temperature of the adhesive was reached, c) for bonded joints with sufficiently long bond length, the ultimate load depends only on the fracture energy of the final temperature, and d) the maximum load of the bonded joints depends on the ratio between the loading and heating rates.The present work provides the following key contributions to the knowledge of the bond behaviour of CFRP-to-concrete or steel joints under different loading conditions: a) an accurate bond-slip model for CFRP-to-concrete bonded joints under quasi-static cyclic loading, b) an accurate damage accumulation model for CFRP-to-concrete bonded joints under mode II fatigue cyclic loading, c) insight into the effects of adhesive mechanical properties at elevated temperatures on the behaviour of bonded joints, and d) insight into the effect of key parameters such as temperature dependent interfacial fracture energy, bond length and loading/heating rates on the behaviour of CFRP-to-steel bonded joints at elevated temperatures. These findings significantly advance the knowledge on the long-term performance of FRP strengthened structures.