Flexural Strengthening of Concrete Slabs by a Three-stage Prestress- ing FRP System Enhanced with the Presence of GFRP Anchor Spikes

Some of the advantages that results from the structural strengthening of concrete slabs using prestressed fiber reinforced polymer (FRP) systems are as follows: the prestressed FRP delays the formation and propagation of cracks, and both the serviceability and the flexural capacity are increased. In this research program, the flexural performance of a concrete slab strengthened with a three-stage prestressing system using Carbon FRP (CFRP) sheets was investigated under a four-point static load test setup at the University of Missouri Rolla (UMR). Debonding of the CFRP sheets was prevented by installing three glass FRP (GFRP) anchor spikes at each end of the prestressed FRP sheets. An un-strengthened slab designated in this paper as the control unit was also tested under a separate research program and is described in this paper only as a source for comparison with the strengthened slab. As expected, test results indicate that both the serviceability and flexural capacity were significantly increased, and no debonding from the ends of the CFRP sheets was observed due to the presence of the GFRP anchor spikes. gated under a four-point static load test setup. Prestressing of the CFRP sheets was achieved by imposing a camber profile on the concrete slab such that upon release of the prestressing system initial stresses would develop in these CFRP sheets. In addition, debonding of the CFRP sheets was prevented by installing three glass FRP (GFRP) anchor spikes at each end of the prestressed FRP sheets. These GFRP anchor spikes were made by impregnating strands of glass fibers with resin. Strengthening of the slab by these three-stage prestressing FRP system with presence of the GFRP anchor spikes include the following steps: Step 1. Cambering of the concrete slab by stressing external steel cables with hydraulic jacks. Step 2. Bonding of CFRP sheets to the concrete surface with an epoxy-based paste and embedding of three GFRP anchor spikes into the concrete slab at both ends of the sheets. Step 3. Releasing of the steel cables stressed in step 1. An un-strengthened slab designated as the control unit was also tested under a separate research program and is described in this paper as a source for comparison with the strengthened slab. The strengthened and the un-strengthened slabs were subjected to a simply supported 4-point concentrated static load test setup. As expected, test results indicate that both the serviceability and flexural capacity were significantly increased due to the presence of the prestressed CFRP sheets. The strengthened slab failed due to rupture of the CFRP sheets at mid-span and no debonding at ends of the CFRP sheets was observed. Furthermore, bond stress profiles along the length of the CFRP sheets were computed from strain gage data and suggest that the GFRP anchor spikes were effective in preventing debonding of the CRFP sheets. This conclusion is evident as a result of an increase in the computed bond stress near the first anchor spikes. However, further researches will have to be conducted to further prove these results. 2 RESEARCH SIGNIFICANCE This study provides the experimental results on the flexural behavior of one full-scale slab strengthened with a three-stage prestressing FRP system by combining traditional steel cables with CFRP strips. Development of GFRP anchor spikes and their application in bonding CFRP strips to this slab were also investigated in this program. 3 EXPERIMENTAL PROGRAM