International Concrete Abstracts Portal

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.

This paper describes the strengthening effect of RC elements with newly developed polyacetal-fiber (PAF) sheets. Typical properties of the polyacetal fiber obtained by drawing polyoxymethylene as a material for strengthening, are high strength, high strain capacity, high resistance to shear force, light weight and easy to handle by preformability. Polyacetal fiber reinforced special epoxy-resin that is optimized for this fibre, offers an outstanding combination of properties not available from steel and other high strength fibers, such as glass, aramid and carbon fibers which are used for the seismic retrofit of concrete structures. The advantages realized were the overall cost savings and strengthening of RC elements in one day, conditionally. Tests conducted to investigate the strengthening effect of concrete elements with polyacetal FRP are introduced in this paper. The loading tests were performed on 14 RC columns. The objective of the lateral loading tests is to investigate the shear and ductility strengthening effect with polyacetal FRP, and to clarify the possibility of this FRP as a material for seismic retrofit. The research work shows the prominent validity of concrete elements post-strengthened with polyacetal fibre for shear strength and ductility.

In this research, the interaction behavior between fiber and concrete was investigated by biaxial plain loading experiments on mortar panels strengthened with various fiber sheets (carbon, aramid and glass) and analyses based on the Modified Compression-Field Theory. As result of the experiment and analysis, it was confirmed that (1) pure shear and pure tensile strengths of the strengthened panels are in proportion to the tensile strength of the fiber, (2) analysis based on the Modified Compression-Field Theory can express the experimental results excellently, and (3) in the case of fibers with small elastic modulus, there is a vast difference between local shear strains and average strains. Further analysis was conducted with the elastic modulus and weight per unit area set as the variable factors.