European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Use of NSM FRP for the strengthening of thin wall tubular RC structures

Chandan Gowda

The University of Minho



Thin-walled reinforced concrete (RC) beams (e.g. box girders) have been used extensively as main structural elements in bridge construction since the 1960s and a large number of bridges across Europe and worldwide are now approaching their end-of-life. This ageing infrastructure requires extensive maintenance and, owing to the ever-increasing traffic load on the road networks and the enforcement of more stringent regulations, a large number of bridges are in urgent need of repair and strengthening. However, efficient means of increasing the torsional resistance of existing thin-walled elements, which is of paramount importance in bridges and other critical infrastructure, has not yet been developed.

Hence the project has developed an innovative torsional strengthening solution using advanced composite materials. The near surface mounted (NSM) technique involved application of carbon fibre reinforced polymer (FRP) laminates into the grooves extracted on the strengthened element. Near surface mounted technique has many advantages like protection against fire and vandalism, ease of application, effective utilization of the strengthening FRP material.

The research was split into three work packages; literature review, finite element analysis and experimental work. The literature review gave an insight into the different kinds of available strengthening solutions using traditional (steel and concrete) and innovative methods (FRP) along with different parameters to be analysed during research. The finite element analysis was used to perform numerical analysis to finalise different strengthening configurations and the test setup necessary for the laboratory testing’s. Finally, the experimental work was carried out based on the knowledge gained from the first two parts of the project.

In the laboratory work, the FRP laminates were applied in different volumes, both in horizontal and vertical directions of the beam. The application was also tested on strengthening three faces and four faces of the beams, giving an idea on practical feasibility since three sides are usually available for strengthening. In addition, special “L-laminates” were manufactured for exploring the strengthening efficiency in corners, as corners are the weak link in torsional deficient structures. All the tested elements showed improved ultimate torsional capacity and mitigation of crack development. The increase in ultimate torsional capacity varied between 17.5% to 44.4% with respect to the reference beam without any strengthening.

Further research needs to be performed to form the design guidelines for torsional strengthening, predicting the limitation for the FRP material. Finite element analysis should also be carried out to validate the numerical model and to predict the efficiency of a new strengthening proposals. The proposed NSM technique guaranteed the optimal and cost-effective use of the strengthening materials and provides a sustainable solution, requiring minimum time of installation and successfully extending the service life of the aging infrastructure.


Experimental specimen failure (plan and side view)


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