The use of externally bonded fiber-reinforced polymer (FRP) sheets and strips has recently been established as an effective tool for rehabilitating and strengthening reinforced concrete structures. Several experimental investigations have been reported on the behavior of concrete beams strengthened for flexure using externally bonded FRP plates, sheets, or fabrics. One of the drawbacks experienced by most of these strengthened beams was a considerable loss in beam ductility. An examination of the load-deflection behavior of the beams, however, showed that the majority of the gained increase in load was experienced without much increase in yield load. Hence, the significant increase in service level loads could hardly be gained.
Apart form the condition of the concrete element before strengthening, the steel reinforcement contributes significantly to the flexural response of
the strengthened beam. Unfortunately, available FRP strengthening materials have a behavior that is different than steel. Although FRP materials have high strengths, most of them stretch to relatively high strain values before providing their full strength. Because steel has a relatively low yield strain value when compared with the ultimate strains of most of the FRP materials, the contribution of both the steel and the strengthening FRP materials differ with the deformation of the strengthened element. As a result, steel reinforcement may yield before the strengthened element gains any measurable load increase. Some designers place a greater FRP cross section, which generally increases the cost of the strengthening, to provide a measurable contribution, even when deformations are limited (before the yield of steel). Debonding of the strengthening material from the surface of the concrete, however, is more likely to happen in these cases due to higher stress concentrations. Debonding is one of the non-desired brittle failures involved with this technique of strengthening. Although using some special low-strain fibers such as ultra-high modulus carbon fibers may appear to be a solution, it would result in brittle failures due to the failure of fibers. The objective of this is to introduce a new pseudo-ductile FRP fabric that has a low strain at yield so that it has the potential to yield simultaneously with the steel reinforcement, yet provide the desired strengthening level.
RESEARCH SIGNIFICANCE
FRPs have been increasingly used as materials for rehabilitating and strengthening reinforced concrete structures. Currently available FRP materials, however, lack the ductility and have dissimilar behaviors to steel reinforcement. As a result, the strengthened beams may exhibit a reduced ductility, lack the desired strengthening level, or both. This study presents an innovative pseudo-ductile FRP strengthening fabric. The fabric provides measurably higher yield loads for the strengthened beams and helps to avoid the loss of ductility that is common with the use of currently available FRP.
Stress Test 1 (video)
Triax Braid (video)
Fire Chamber Test PDF
