By Rahulreddy Chennareddy1 and Mahmoud M. Reda Taha2
Practical field applications showed that it is highly likely that reinforced concrete (RC) beams are strengthened for both flexure and shear demands. Combining the near-surface mounting-fiber-reinforced polymer (NSM-FRP) for flexural strengthening and U-wrap for shear strengthening is, therefore, a possibility. The American Concrete Institute (ACI) guidelines, however, treat these two strengthening designs separately. This paper examines the drawback of such approach. Our hypothesis is that the U-wrap FRP used for shear strengthening will confine the NSM-FRP bar and the surrounding epoxy, will improve the bond of NSM-FRP to epoxy and, thus, will change the mode of failure from a ductile debonding failure as recommended by ACI guidelines to an abrupt strength rupture of FRP bar.
Fiber-reinforced polymers (FRP) have gained wide acceptance in the construction industry for structural strengthening and rehabilitation of reinforced concrete (RC) structures. Fiber-reinforced polymer has a high specific strength, high strength-to-weight ratio, is corrosion-free and easy to install. Over the past two decades, researchers developed numerous strengthening and rehabilitation. The American Concrete Institute (ACI) has developed design guidelines for using FRP for new construction, structural strengthening, and rehabilitation.
Near-surface mounting (NSM) technique for flexural strengthening and the U-wrap for shear strengthening of RC flexural members are two well-established techniques recognized by ACI. With NSM-FRP flexural strengthening technique a saw-cut groove is made on the tension soffit of the beam, and an FRP reinforcing bar is placed in the groove and bonded to the concrete using epoxy. The performance of the NSM-FRP technique is strongly dependent on the bond between NSM-FRP bar and the surrounding adhesive. Prior research showed that failure occurs either by debonding of NSM-FRP bar (preferred mode of failure) or tension rupture of FRP bar after steel yielding. Strength rupture of FRP bar is highly unlikely irrespective of the amount of development length provided. Design guidelines developed by ACI is based on NSM-FRP bar debonding after the FRP bar experiences 60 to 70 percent of its ultimate tension capacity.
The experimental investigation showed the flexural capacity of the strengthened RC-beam to increase when NSM-GFRP for flexural strengthening was combined with U-wrap shear strengthening. The increase in flexural capacity is attributed to the confinement effect of the U-wrap that improved bond between the NSM-GFRP bar and surrounding epoxy adhesive. However, the mode of failure was changed from slow debonding to an abrupt failure caused by tension rupture of the GFRP bar. The picture below shows tension rupture of NSM-GFRP bar when combined with U-wrap GFRP shear strengthening. Nevertheless, analysis also showed an improved deformability for the beams strengthened with both NSM-GFRP for flexural strengthening and U-wrap GFRP for shear strengthening compared with the beams strengthened with NSM-GFRP for flexure only.
This study shows that combining NSM-FRP flexural strengthening with U-wrap FRP shear strengthening will improve the bond without compromising on the deformability of the beam before failure. However, combining the two techniques can lead to an abrupt and sudden failure caused by tension rupture of the FRP bar. ACI guidelines for FRP-strengthening of RC structures, do not indicate a potential issue associated with such combination. While ACI guidelines limit the FRP design strain to 70 percent of the ultimate strain capacity of the FRP bar, we recommend that limit to be reduced to 50 percent of the ultimate strain capacity of the FRP bar when NSM-FRP flexural strengthening is combined with U-wrap FRP shear strengthening. Such additional provision in ACI guidelines will be necessary to eliminate the possibility of abrupt failure due to tension rupture of FRP bars used for FRP flexural strengthening.
The research can be found in a paper titled “Effect of Combining Near-Surface-Mounted and U-Wrap Fiber-Reinforced Polymer Strengthening Techniques on Behavior of Reinforced Concrete Beams,” published by ACI Structural Journal, May-June 2017.
1Graduate Student, M.S., Dept. of Civil Engineering, MSC01 1070, 1 University of New Mexico, Albuquerque, NM
2Professor & Chair, FACI, Dept. of Civil Engineering, MSC01 1070, 1 University of New Mexico, Albuquerque, NM