Influence of substrate moisture preparation on concrete overlay bond strength

Master Thesis


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University of Cape Town

A considerable amount of progress has been made over the years in understanding the fundamentals of concrete composition and performance in both safe and harsh environments. Nevertheless, premature concrete deterioration remains a concern and is often experienced either due to poor workmanship or design. The bonded concrete overlay technique has become a popular method in repairing these deteriorated concrete structures, as it has a simplistic application procedure and can be applied to a number of different scenarios. This technique involves the removal of the distressed layer of concrete (substrate), followed by the application of a fresh, new layer of concrete (overlay). One of the fundamentals behind bonded concrete overlays is the bond between the existing and new concrete layer. Sufficient bond strength is a prerequisite for the durability and serviceability of a repaired concrete structure. Factors which have been considered most important for achieving a good bond between the substrate and overlay are cleanliness and preparation of the substrate, together with overlay compaction and composition. However, the impact of substrate moisture condition on the bond strength of a repaired member has not been fully investigated and still raises many debates amongst engineers. Current best practice suggests that a concrete substrate which is preconditioned to a saturated surface dry state prior to overlay application will achieve higher bond strengths. This investigation provides insight into the aforementioned through both literature research and practical experiments performed. In this investigation the effects of moisture preparation on bond strength were tested on three different substrate concretes, together with the application of four overlays. The substrates varied according to strength grade (50, 30 and 25 MPa), whereas the overlays varied in both strength (40 and 25 MPa) and workability (30 and 120 mm slump). The accompanied bond strength was established through interface shear testing. All substrates of the repaired specimens were subjected to a constant surface roughness and equally aged to prevent any differential shrinkage which may falsify bond strength readings. The bond strength results were plotted, statistically evaluated and compared with existing literature.

Includes bibliographical references.