The influence of concrete substrate preparation on overlay bond strength

Repair and strengthening of concrete structures often include the application of a concrete overlay to the existing substrate. Sufficient bond strength between substrate and overlay is a prerequisite for the durability and serviceability of the repaired or strengthened structure. Factors considered most important for good bond strength between substrate and overlay are cleanliness and texture of the substrate concrete surface, as well as overlay placement and curing techniques. In terms of substrate moisture condition prior to overlay application, a saturated, surface-dry substrate is generally considered best practice to achieve high bond strength. In some cases, bonding agents are specified, which may consist of commercial products or site-made cement slurries. Despite the extensive application of bonded concrete overlays, the usefulness of bonding agents is still the subject of much debate among researchers, specifiers and practitioners. Conflicting opinions also still exist as to whether pre-wetting the substrate concrete can be expected to have a positive influence on bond strength. The laboratory-based research described in this paper identified that a saturated, surface-dry substrate concrete has generally no beneficial influence on overlay bond strength. In many instances, the use of substrate surfaces prepared to what is commonly considered the optimum moisture condition resulted in significantly lower bond strength, compared with non-preconditioned substrates. This phenomenon was ascribed to better mechanical interlock, which exists when the fresh overlay material can flow into the unsaturated cavities and pores of the substrate. Bonding agents were found to enhance bond strength only when an overlay of low workability was applied, both commercial and self-made bonding agents performing equally well. For overlays with workability characteristics of conventional concretes (slump values between 70 and 110 mm), bonding agents were found to have no noticeable influence on bond strength. This relates to the circumstance that overlays of sufficient fluidity can fill the pores and cavities of the substrate without the help of a bonding agent.