Evaluation of externally bonded CFRP systems for the strengthening of RC slabs
The use of carbon fiber-reinforced polymer (CFRP) as externally bonded reinforcement (EBR) for the repair and strengthening of deficient structures have been taking place since the late 1980’s. Continuous efforts in material development and research activities, with strong links to engineering practice, give this application more and more interest worldwide. This paper presents an experimental study on flexural strengthening of reinforced concrete (RC) slabs with different CFRP systems, using different EBR techniques. All the slabs were tested to failure under simply supported conditions. CFRP EBR increased the flexural strength and reduced the deflections and crack widths of the strengthened slabs. Two modes of failure were observed, delamination and rupture of the CFRP reinforcement.
Flexural Strengthening of Concrete Slabs by a Three-stage Prestressing 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
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.