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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. [ABSTRACT FROM AUTHOR]

Cracks influence the durability of reinforced concrete (RC) structures in aggressive environments by accelerating the ingress of corrosive agents to the embedded steel. This study investigated the influence of crack width (0·7 mm, 0·4 mm and incipient cracks), binder type (100% CEM I (ordinary Portland cement - OPC) and 50/50 OPC/Corex slag blend), water-to-binder (w/b) ratio (0·40 and 0·55) and crack reopening on chloride-induced corrosion in RC specimens with constant cover of 40 mm. Over the study period (31 weeks), corrosion rates varied from passive values of 0·01 A/cm to active values of 1·50 A/cm. Results indicate that corrosion rate is sensitive to crack width, concrete quality (i.e. binder type and w/b ratio) and crack reopening. For a given binder type and w/b ratio, corrosion rate increased with increasing crack width; however, this increase was smaller in Corex slag specimens (increases of 40% or more) than in OPC specimens (increases 210% or more). Corex slag specimens generally had lower corrosion rates compared to OPC specimens. For both OPC and Corex slag specimens, corrosion rate decreased with decreasing w/b ratio. Crack reopening increased the corrosion rate in the cracked specimens. This increase in corrosion rate was higher if the specimen was actively corroding prior to the crack reopening. [ABSTRACT FROM AUTHOR]

The bond between substrate and overlay is often the main factor concerning the serviceability of a composite member consisting of concretes of different ages. In the current paper, existing bond strength test methods are discussed and fundamental influences on bond strength development and bond serviceability and durability are investigated. Two new experimental methods for the evaluation of interface shear strength are introduced, which have proved to be practical and indicative of fundamental bond mechanisms and factors influencing bond durability. Shear bond strength between substrate and overlay was found to be governed largely by mechanical and physical material properties of the overlay close to the interface. Bond strength development could be linked to the development of the compressive strength of the overlay. Interface texture was found to have a significant influence on the durability of bond strength. 'Interface bond failure' could, in most instances, be identified visually to have occurred in the overlay close to the interface. The term 'interface failure' commonly used for the discussion of bond characteristics therefore appears to be ambiguous. [ABSTRACT FROM AUTHOR]