NSM technique: Bond of CFRP rods and static/dynamic response of strengthened RC beams

The use of the near surface mounted (NSM) fiber reinforced polymer (FRP) technique in existing RC beams is becoming a common strengthening method in civil engineering. Nevertheless, many aspects regarding the behavior of strengthened beams have not been completely analyzed such as bond mechanisms or effects of cracking concrete and section curvature.This paper investigates the response of RC beams with NSM circular CFRP rods through static and dynamic tests, analyzing the failure mechanisms of beams due to loss of strength and/or delamination. Four RC beams were built and subjected to bending loading until failure; three of which were strengthened using NSM CFRP rods. The experimental tests regarded beams with NSM strengthening in absence of execution defects and with local lacks of adherence. Theoretical and experimental results obtained from pull-out tests of concrete elements with NSM CFRP rods, allowed obtaining shear-slip laws and estimating fracture energy value for analysis of bond capacity of the NSM CFRP rod contained in the beams. Finally, the dynamic behavior was investigated considering free–free vibrations of beams and the changes of frequency values due to damages. The experimental results obtained by static and dynamic tests were discussed and analyzed to assess the NSM strengthening.