An experimental study on reinforced concrete beams strengthened with prestressed near surface mounted CFRP strips

• 7 RC beams were tested to study the behavior of prestressed FRP-strengthened beams.
• Strengthening with prestressed NSM FRPs improved performance of beams significantly.
• Brittle failure modes included interfacial debonding and concrete cover separation.
• Interface bonding and concrete strength dominated the beams’ ultimate behavior.
• Extending FRPs bond length and using end anchorage improved beams’ performance.

Externally-bonded prestressed FRP plates and near surface mounted (NSM) FRP strips are two important techniques of strengthening concrete structures with FRP materials. However, there are considerable limitations for these two techniques. Using prestressed NSM FRP strips can combine the advantages of these two strengthening methods and it thus becomes very attractive to further promote FRP applications in retrofitting concrete structures. In this paper, a total of 7 rectangular reinforced concrete beams were constructed for static load tests, of which 1 specimen was the reference beam and the other 6 beams were strengthened with FRP strips. The behavior and failure modes of these specimens were investigated, the difference of the mechanical performance due to various strengthening techniques was compared, and the effect of parameters, such as the bond length, prestressing force, and the anchorage, on the behavior and failure modes of specimens was analyzed. The testing results showed that the load-carrying behavior of flexural members could be improved significantly by strengthening with prestressed NSM FRP strips, since the highest load-carrying capacity of the beam with prestressed NSM strips is increased by 122% and 44.2% when compared to the beam without strengthening and with non-prestressed FRP strips, respectively. While the prestressed FRP strips could be anchored by epoxy through the near surface mounting, the failure modes of beams strengthened with this technique might be the debonding of epoxy- concrete interface and delamination of the concrete cover. It was proven that these failure modes can be avoided by extending the bond length of FRP strips or using U-wraps of CFRP sheets. The stress transfer length of the prestressed-FRP strengthened beam with U-wrap anchorage is approximately 100 mm.