Mechanical and environmental loading of concrete beams strengthened with epoxy and polyurethane matrix carbon fiber laminates

Externally-bonded carbon fiber reinforced polymer (CFRP) sheets have become a common material used to retrofit damaged or deteriorated concrete flexural members. This study examines the effects different combinations of cyclic loading, controlled thermal cycling, uncontrolled outdoor environmental exposure, and monotonic loading until failure have on the performance and failure modes of reinforced concrete beams strengthened with externally-bonded CFRP sheets. Twelve large-scale (4.88 m in length) doubly reinforced concrete beams were strengthened with externally-bonded CFRP sheets and tested. Three different unidirectional CFRP strengthening systems were considered; two systems utilizing carbon fiber sheets and epoxy matrices, and a system utilizing pre-impregnated carbon fiber with a water activated polyurethane matrix. Epoxy matrices are commonly used, but polyurethane composite systems have not been rigorously investigated for infrastructure use. Results indicate that moderate cyclic loading and environmental conditioning do not have a significant effect on flexural capacity of strengthened members, but the quality of the field lay-up does have an influence. The performance of the polyurethane system was consistent and insensitive to all loading combinations.