Abstract:
The performance of bonded concrete overlays relates mainly to the resistance against cracking and debonding. The associated failure mechanisms are largely a result of differential volume changes between substrate and overlay. The objective of this research was to develop analytical tools to facilitate the design of bonded overlays subjected to differential shrinkage. The experimental programme included the identification of fundamental strain characteristics and bond strength development of composite members in relation to different interface textures and overlay materials. Existing analytical models for the prediction of strains and stresses in bonded overlays were evaluated. Results from the experimental work indicated that existing models, which are based on simple beam theory, are deficient in modelling overlay strains in a realistic manner. The degree of overlay restraint was found to depend far less on relative sectional dimensions of substrate and overlay as commonly assumed. Based on fundamental aspects concerning strain characteristics of bonded overlays identified through experimental tests and numerical simulations, an analytical prediction model was developed based on localised strain conditions at the interface. The most important material parameters in respect to overlay crack resistance are overlay shrinkage, relaxation properties, elasticity, and tensile strength. Tests on composite specimens revealed that substrate creep is a major component in composite members subjected to differential shrinkage. These parameters were combined in the analytical model to allow prediction of time-dependent overlay stresses. In fully bonded members, shear bond strength development was found to relate closely to mechanical overlay strength at the interface. Failure patterns and long-term bond strength development indicate that an interface transition zone exists, similarly to that between aggregates and cement paste. Long-term bond strength could be related to the combined influences of macro-mechanical interaction at the interface and overlay shrinkage. Changing environmental conditions were found to have no detrimental influence on interface shear strength. Bond development depends on a large range of influences that are difficult to control in practice. It therefore appears appropriate to rely on prescriptive design recommendations for interface bond strength, considering substrate surface preparation, overlay materials and application methods. However, in terms of crack resistance, analytical modelling of shrinkage-induced stresses, taking into account the combined influences of various material parameters and the characteristics of the structural system, can greatly facilitate the design for long-term performance.
