Localised strain and stress in bonded concrete overlays subjected to differential shrinkage

Bonded concrete overlays are widely used for repair and strengthening of existing structures as well as for precast elements which receive an in-situ topping. The performance of such overlays relates mainly to their resistance to cracking and debonding. Associated failure mechanisms are a result largely of differential volume changes between substrate and overlay. The objective of this paper is to provide an analytical tool to facilitate the design of bonded overlays for crack-resistance when subjected to shrinkage restraint.Fundamental strain characteristics of composite members are identified and existing analytical models for the prediction of strains and stresses in bonded overlays are evaluated. Results from experimental work indicate that existing models, which are based on simple beam theory, are deficient in modelling overlay strains realistically. The degree of overlay restraint was found to depend far less on relative section dimensions of substrate and overlay than is commonly assumed. Based on fundamental aspects concerning strain characteristics of bonded overlays, an analytical prediction model is introduced, based on localised strain conditions at the interface.