Guidelines for Embedment Length of Carbon Fiber Reinforced Polymer (CFRP) Strips in Near Surface Mount (NSM) Retrofitted Concret

Download Final Report

CAIT project no.: CAIT-UTC-036

Fiscal Year: 2013/2014

Status: Final

Rutgers-CAIT Author(s): Nakul Ramanna, Ph.D., Patrick Szary, Ph.D.

External Author(s): Jason Arndt

Sponsor(s): Delaware DOT, FHWA - RITA



The NSM-CFRP method for retrofitting is becoming an effective way of strengthening reinforced concrete structures compared to the conventional external bonded retrofitting (EBR) technique. This can be attributed to increased stiffness and strength resulting from the significantly more effective bonding in the NSM method. In this method grooves are cut on the concrete surface, epoxy is filled in and then CFRP reinforcement is inserted in the slits. The commonly used CFRP reinforcements for strengthening in the NSM method are round and square/rectangular bars. Recent studies have shown narrow rectangular CFRP strips to be highly effective and more economical than round or rectangular bars in resisting static, fatigue and impact loads. However, research highlights the need to provide larger development length for these strips than what is recommended by ACI 440.2R-08 [1] to avoid premature failure. There is also clear lack in the literature for the recommended embedment length of CFRP strips in concrete owing to limited experimental data. It is also noteworthy that the development length formula for rectangular reinforcements given in ACI 440.2R-08 is only applicable to normal strength concrete (i.e. up to 5000 psi) and it does not distinguish between flexural and compression strengthening. However, in-situ concrete strengths are often found to be well above this limit and CFRP strips are known to perform differently in tension and compression. As a result, the use of same formulation for both cases cannot be justified. Finally, although rectangular in nature, CFRP strips should be treated differently than conventional square/rectangular reinforcements due to higher stress concentrations at the edges. All these factors provide impetus for this research project.