International Concrete Abstracts Portal

Strength and stiffness properties of Glass Fiber Reinforced Plastic (GFRP) bars under various conditioning schemes with and without the application of sustained loads are described in this paper. Alkaline conditioning was more detrimental to the strength of GFRP bars as compared to salt conditioning. Increasing temperatures and stress resulted in a corresponding decrease in the strength of GFRP bars. Based on accelerated test results calibrated with respect to naturally aged results it is safe to conclude that the service life of the FRP bars with durable low viscosity urethane modified vinylester resin is about 60 years as a minimum with 20% sustained stress on the bar. Concrete cover protection on the GFRP bars enhanced the service life up to an additional 60 years.

New repair and maintenance technique for existing reinforced concrete structures has been developed as a result of a marriage of corrosion prevention and carbon fiber reinforced polymer (CFRP) installation techniques. Rebar corrosion prevention is basically provided by penetrating liquid and/or mortar, while mechanical degradation is compensated by CFRP. CFRP can also provide an effective protective layer for subsequent corrosive agent penetration. The technique can overcome various drawbacks of the current repair techniques by minimizing repair time and unnecessary chipping of damaged concrete, and protect degrading concrete structures. Such excellent performances have been proven by a series of experiments. Steel rebar embedded in a concrete block brushed with lithium nitrite-based penetrating corrosion inhibitor has been proven rust-free for years. The tensile strength of CFRP sheet have been proven unchanged after the exposure of 10,000 hours of accelerated weathering conditions. The CFRP sheet has also been proven impermeable to salt ion and water by experiments. Both shear and tensile strengths of concrete columns, damaged by salt penetration and then repaired with this new technique, have been proven equal or greater than the original strengths of control column. The performance of this technique has also been granted an official “Examination and Certification of Building Preservation & Maintenance Techniques” by the authorized organizations of the Ministry of Construction, Japan in July, 1998.

Experimental behavior of prestressed concrete beams strengthened with carbon sheets and the analytical predictions are reported. Two sets of beams made with normal strength (35 MPa, 5 ksi) and high strength (70 MPa, 10 ksi) concrete were strengthened with two and three layers of carbon sheet reinforcement. The beams were instrumented to measure deflection, and strains in composite, concrete, prestressed steel, and nonprestressed steel, and tested to failure using third point loading over a simply supported span of 3 m(10 ft). The results indicate that prestressed concrete beams can be effectively strengthened using high strength composites. As compared to reinforced concrete beams strengthened with carbon sheets, the loss of ductility was minimal for prestressed concrete beams. A strength increase of 86% was obtained for 3 layers, and the decrease in failure deflection was less than 10%. The authors believe that the stress-strain behavior of a prestressing strand, which has a continuous strain hardening with no yield plateau, is the main reason for this different behavior. The strength capacities can be predicted with good accuracy.

The anchorage of externally bonded reinforcement is extremely important. Due to high shear stress concentrations, premature failure is often initiated in the end zones. A non linear model to describe the phenomena at the end of the externally bonded reinforcement has been set up to allow a safe design of the anchoring capacity and the anchorage length. Appropriate safety factors have to be used to take into account the brittle behaviour of the system. A series of 24 direct shear tests were performed to verify the assumptions and to check the validity of the model. These test specimens consist of two concrete prisms bonded together with one, two or three layers of CFRP using different bonded lengths and widths.

The use of fiber reinforced polymers (FRP) to strengthen sagging and hogging moment regions of continuous beams is discussed in this paper. Five two-span reinforced concrete beams with “T” cross sections were tested. Four different strengthening systems were examined. Two beams were strengthened with two different types of carbon fiber reinforced polymer (CFRP) sheets. The first beam was strengthened for flexure only while the second beam was strengthened for both flexure and shear. The third beam was strengthened with glass fiber reinforced polymer (GFRP) sheets, while CFRP plates were used in strengthening the fourth beam. The fifth beam was a control. Each beam was loaded and unloaded for at least one cycle of loading before failure. The effects of FRP strengthening on failure modes, load capacity, cracking pattern and propagation, and deflections are presented. It was concluded that the use of FRP laminates to strengthen continuous beams is effective in reducing deflections and increasing their load carrying capacity. Furthermore, beams strengthened with FRP laminates exhibit smaller and better-distributed cracks.