Mechanism Analysis of RC Slabs Strengthened with Pre-stressed and Non Pre-stressed FRP

The behaviour of un-strengthened RC column-slabs and RC column-slabs strengthened with non-prestressed and prestressed FRP sheets are evaluated by use of experimental tests and numerical analyses. The results show a significant increase in the ultimate load capacity for the RC column-slab strengthened with non-prestressed FRP in comparison with the un-strengthened sample and failure mode is changed from flexural failure in the un-strengthened slab to punching failure in the slab strengthened with non-prestressed FRP. The RC specimens strengthened with prestressed FRP sheets, however, cannot reach their expected ultimate load capacity due to debonding of FRP sheets although a proper FRP-steel end plate anchoring system to avoid debonding failure is used. The finite element models show that the main reason for FRP debonding is concrete fracture near the FRP end plate which is in turn caused by the increment of the tensile stresses in the region of above the steel reinforcement and below the neutral axis. Near the endplate, prestressed FRP sheets create a local compression zone close to the concrete surface and local tension zone above the steel reinforcements. These locally stressed zones are located below the primary neutral axis of the concrete section. The superposition of the tensile stresses of the local tension zone and the tensile stresses produced by applying external load on the column stub, increase the overall tensile stress level in that area. The result of this superposition process initiates the flexural cracks which propagate due to shear stresses. The flexural-shear cracks propagation leads concrete fracture near the end plate finally and hence causes the debonding of FRP sheets.