Externally Bonded GFRP and NSM Steel Bars for Improved Strengthening of Rectangular Concrete Beam

Abstract

The technology of fiber-reinforced polymer (FRP) strengthening has matured to a great extent. However, there is always room for performance improvements. In this study, external bonding of glass-fiber reinforced polymer (GFRP) and near surface mounting of regular steel bars is combined to improve the behavior, delay the failure, and enhance the economy of the strengthening. E-Glass FRP is selected due to its inexpensive cost and non-conductive properties to shield the NSM steel bars from corrosion. On the other hand, the use of NSM bars gives redundancy against vandalism and environmental deterioration of the GFRP. An experimental program is conducted in which five rectangular cross-section beams are designed and built. The first beam is tested as a control beam failing at about 12 kips (53.4 kN). The second beam is strengthened using 5 layers of carbon-fiber reinforced polymer (CFRP), which failed at 27.1 kips (120.5 kN). CFRP U-wraps were used to anchor this external reinforcement. The third beam is strengthened using two #5 steel NSM bars and 1 layer of GFRP, both extending to the support. GFRP U-wraps were applied to anchor this external reinforcement. This beam failed at 31.5 kips (140 kN). The fourth beam is strengthened with the same system used for the third beam. However, the NSM steel bars were cut short covering 26% of the shear-span only while the GFRP was extended to the support. This beam failed at 30.7 kips (136.5 kN) due to the lack of sufficient development of the NSM steel bars and the shear stress concentration at the steel bar cut off point. Nevertheless, the failure load developed was higher than that of 5 layers of CFRP used for beam 2. The fifth beam was strengthened exactly as the fourth beam, but once strengthened, was loaded five times to cracking load and then submerged in a highly concentrated saline solution for six months. The beam was then tested to failure with a failure load of 29.8 kips (132.6 kN), showing that the GFRP wrapping provided good corrosion resistance.