Fracture mechanics approach for flexural strengthening of reinforced concrete beams

This paper reports on the experimental testing of 9 notched reinforced concrete specimens under four point bending. The beams comprise three beam sizes and three tension reinforcing steel ratios. All beams have constant span/depth ratio of 4, initial notch/depth ratio of 0.3. Two strengthening fiber laminates were used: Glass fiber for the two lower tension reinforcing steel ratios and Carbon fiber for the higher tension reinforcing steel ratio. The strengthening laminates were designed to enhance beam moment capacity by 15% to 150% depending on the beam size and reinforcement ratio. To simulate real life strengthening situations, beams were first loaded until the notch propagated to 0.5 the beam depth. The strengthening fiber laminate was then introduced to the tension side of the beam while the load was kept applied to the other side of the beam. The fracture moment for a given crack depth was calculated through an analytical algorithm which employs Linear Elastic Fracture Mechanics. The approach takes into consideration the previous loading history of the beam prior to introducing the strengthening laminate. Test measurements of crack extension and applied load were used to compare the fracture moment recorded experimentally to that one calculated analytically. The application of the solution algorithm to different specimen sizes–cross-section dimensions, reinforcement ratio, and strengthening fiber laminate–showed that the solution algorithm is able to effectively predict the behavior of larger beam size and/or reinforcement better than that of smaller beam size and/or reinforcement. A sensitivity analysis was conducted to explore this point.