The assessment and behaviour of crack bridging and crack accommodating protective coatings on reinforced concrete.

The ingress of carbon dioxide and chloride ions into reinforced concrete can cause corrosion of the steel reinforcement. Cracks and poor quality cover can accelerate this process. Coating the surface of the concrete can increase the life of the structure. A coating must have the ability to bridge cracks that form in the substrate concrete and to accommodate any subsequent movement of that crack without failure. If the coating itself is damaged by substrate cracking then the coating becomes less effective as it no longer provides a barrier to the deleterious agents. There are currently few methods to assess the performance of a coating on a reinforced concrete surface subject to crack formation and subsequent movement. In this thesis a test specimen, machine and methodology are developed to evaluate the crack bridging and crack accommodation performance of these coatings. The specimen is 40mm x 40mm x 160mm mortal prism, coated on one face and axially reinforced with an 8mm steel bar. A crack in the specimen is opened by the application of a tensile load to the ends of the reinforcing bar by a pneumatically powered testing machine. Initial investigations confirmed that the testing machine was reliable and gave reproducible results. A testing program was then carried out to investigate the effects of temperature, coating thickness, artificial weathering and crack width on the crack bridging and crack accommodation behaviour of the coatings. It was found that increasing the thickness of a coating allowed wider cracks to be bridged. Crack accommodating behaviour was found to fall into three regions dependent upon the test temperature. At the lowest temperatures both crack bridging and crack accommodation did not occur. As the temperature was increased crack bridging and crack accommodation occurred but the crack accommodation behaviour was highly variable. As the temperature was increased further a region is encountered where reliable crack bridging and crack accommodation occurred and the variability in crack accommodation behaviour was reduced. Artificial weathering was found to have a detrimental effect on both crack bridging and accommodation performance