Shear Strength of Reinforced Concrete Beam Strengthened by Transverse External Post-tension

Abstract

Problem statement: Shear failure of concrete beam is brittle manner without warning so inadequate design for shear of beam and/or material deterioration lead to the possibility of sudden failure of beam. The change of functional use and future increased load of structure lead to the need for strengthening of concrete structure. Approach: This research focuses on behaviors under static loading of reinforced concrete beam, with shear strengthening by transverse external prestressing force. Post-tension high strength steel is vertically applied in shear span. Total eight beam specimens are divided into two groups each having shear span to depth ratios 2 and 1.5. Each beam, possessing the same reinforcing steels, is intentionally designed to be failing in shear. One of the beams from each group is used as reference, without shear strengthening. The other three specimens from each group are applied different amounts of external prestressing force. Results: The experimental result shows that ultimate load carrying capacity of all shear strengthened specimens significantly improves over the reference specimen. The higher the amount of applied strengthening force, the greater the ability to carry ultimate loading. Failure mode shifts from brittle shear failure closer to ductile flexural failure, with higher ductility and stiffness. External prestressing force in transverse direction of shear span of beam enhances ultimate shear capacity by improving aggregate interlocking, preventing splitting cracks caused by debonding of longitudinal reinforcing steels due to dowel action. Furthermore, load at first diagonal tension cracks are increased as a result of pre-compressed prestressing force leading to the higher ultimate load carrying capacity. Moreover, concrete in compression zone remains uncrushed at ultimate state. Strut-and-tie model can be used to predict ultimate loading capacity of beam specimen and failure mechanism of both specimens with or without strengthening. Conclusion: Shear strength of reinforced concrete beams strengthened by transverse external post tension at shear span is effectively improved over reference beam specimen. Strut and tie model can be conservatively predicted the ultimate shear capacity of both reference and strengthened beam specimen.