Bonded concrete overlays are frequently used as a repair method. Three special issues are connected to bonded overlays: (i) bond between substrate concrete and overlay, (ii) stresses due to differential shrinkage, and (iii) durability of the repaired structure. Good bond has an uppermost importance for obtaining monolithic behaviour and, consequently, maintaining the strength and stiffness of the original structure. A good bond also prevents salt and contaminated water to flow through the interface having access to both the substrate and the overlay. Absence of micro cracking, absence of laitance layer, cleanliness, compaction, and curing are the main factors affecting bond. Numerous filed tests show that these factors are more important than other factors, e.g., surface roughness, surface preparation, traffic vibrations, stress state, and environmental conditions. However, there are still questions on how some of these factors affect the bond and on the understanding of the phenomena behind. Differential shrinkage, i.e., shrinkage difference between the new-cast concrete overlay and the previously shrunk concrete substrate leads theoretically to the development of shrinkage stresses of such a magnitude that the overlay would crack. Field and laboratory tests show, however, that this cracking must not be inevitable. Two beneficial and crack-reducing factors are restraint less than complete and concrete creep. Numerous investigators have tried to develop models for determination of the stresses, but the “final” theory accepted by the entire concrete society is still missing. The durability of the repaired concrete structure is dependent on the durability of the substrate concrete, the durability of the overlay, and the durability of the bond. The two first aspects have been investigated thoroughly during the latest decades. Here, the focus is on the third aspect, i.e., the bond development. Whereas Swedish investigations show an increased bond with time, some North American studies show the opposite. These differences are analysed in the paper.
2006. 193-206 p.
2nd International RILEM Symposium on Advances in Concrete through Science and Engineering, Sept. 11-16, Quebec City, Canada