The influence of the yield strength of the longitudinal reinforcement on the bending capacity of beams reinforced with CFRP strips

Effective application of FRP laminates to concrete is possible only with proper understanding of physical phenomena accompanying of failure mechanisms. The most probable failure mode of RC beams strengthened with externally bonded FRP systems is an intermediate crack-induced debonding. Better knowleddge of this failure mode will lead to more accurate designing that have important influence on safety and cost. This paper describes laboratory test on flexural strengthening of reinforced concrete elements with CFRP strips attached to the tensile soffit of the beams. Four RC beams were made of concrete with low strength of about 20 MPa with longitudinal reinforcement made with different types of steel and tested to failure in four-point flexure. Theoretical yield strength of steel reinforcement varied from 350 MPa to 1860MPa but the reinforcement ratio remained almost constant at about 0.75%. All the beams had shear ratio about 3 and the same transverse reinforcement. All the beam failed due the intermediate crack-induced debonding or concrete crushing. At the failure, different strains and tensile stress were reached depending on the steel properties used as the longitudinal reinforcement. It was concluded that the important influence on the level of the FRP strips' strain at collapse have the yield strength of applied reinforcement. With the increase of the yield strength of steel raised ultimate force and the strain in the tape at the delamination. For high strength steels damage occurred as a result of crushing concrete and thus delamination of the strip.