The repair and strengthening of RC structures has become a major problem for civil engineers in the past few decades. To satisfy this problem, a previous method for the repair and strengthening of RC beams included bonding steel plates to the inferior structure. However, bonding steel plates to concrete presents disadvantages, including corrosion of the steel/adhesive joints and the heavy weight of the material. These problems increase installation and maintenance costs. The bonding of Fiber Reinforced Plastics (FRP) to structures provides an attractive alternative to steel plates. This material is corrosion resistant and lightweight, has a high strength-to-weight ratio and possesses nonconductive properties. The use of Fiber Reinforced Plastics (FRP) in repairing and strengthening RC beams has been researched in recent years. In particular, attaching unidirectional FRP to the tension face of RC beams has provided an increase in the stiffness and load capacity of the structure. However, due to the brittle nature of unidirectional FRP, the ductility of the beam decreases. Consequently, the safety of the structure is compromised, due to the reduction in ductility. The purpose of this research is to investigate the behavior of high strength reinforced concrete beams strengthened with FRP sheets. The major test variables included the different layouts of CFRP sheets and the tensile reinforcement ratio. More particularly, change in the strength and ductility of the beams, as the number of FRP layers and tensile reinforcement bar ratios are altered, is investigated. Eight under-reinforced concrete beams were fabricated and tested to failure. With the exception of the control beam, one or four layers of CFRP were applied to the specimens.

Limit to the tension reinforcement ratio in flexural high strength reinforced concrete (HSRC) members is based on the requirement that tension failure as sufficient rotation capacity are ensured at ultimate limit state. However, the provisions for the maximum amount of compression reinforcement ratio (r'max) in a doubly reinforced section are not associated with any rational derivation. A quantitative measure to evaluate an upper limit to the compression reinforcement ratio (r'max) of such flexural HSRC members is proposed. The quantitative criterion to r'max can be derived from i) steel congestion and ii) considerations that are related to the diagonal compression bearing capacity of the members.In this paper it is shown that, when flexural loading is dominant the shear loading, the limit to ρ' is set by the steel congestion criterion. Parameters that affect this limit such as, f'c, beams geometry (cross sectional dimension and concrete cover), and bars diameter are deeply investigated and the expressions were derived to provide an additional tool for a better design and assessment of the flexural capacity of HSRC members considering different end conditions and loading arrangement.

Limit to the tension reinforcement ratio in flexural high strength reinforced concrete (HSRC) members is based on the requirement that tension failure as sufficient rotation capacity are ensured at ultimate limit state. However, the provisions for the total amount of longitudinal reinforcement ratio ( r and r¢ ) are not associated with any rational derivation. In this paper, a quantitative measure to evaluate an upper limit to the compression reinforcement ratio r¢max of flexural HSRC members is proposed. The quantitative criterion to r¢max  can be derived from i) steel congestion and ii) considerations that are related to the diagonal compression bearing capacity of the members. In this paper it is shown that, when shear loading is dominant, the limit to r¢ is set by the diagonal compression criterion. Parameters that affect this limit are deeply investigated and the expressions were derived for different end conditions, to provide an additional tool for a better design and assessment of the flexural capacity of HSRC members.