International Concrete Abstracts Portal

The use of Fiber-reinforced polymer (FRP) composite materials in new construction and repair of concrete structures has been growing rapidly in recent years. FRP provides options and benefits not available using traditional materials. The promise of FRP materials lies in their high-strength, lightweight, noncorrosive, nonconducting, and nonmagnetic properties. ACI Committee 440 has published several guides providing recommendations for the use of FRP materials based on available test data, technical reports, and field applications. The aim of these document is to help practitioners implement FRP technology while providing testimony that design and construction with FRP materials systems is rapidly moving from emerging to mainstream technology.

Near surface mounted (NSM) is a promising strengthening technique provided that the full bond between the strengthening material and the existing structure develops. Wahab et al. (2011 and 2012) tested reinforced concrete beams to asses the bond strength between the prestressed NSM carbon fibre reinforced polymers (CFRP) rods and the concrete. The beams were tested in four points bending under different fatigue load levels. Failure was by slipping between the CFRP rod and the epoxy that started at the support and traveled to the loading point as the number of cycles was increased. Modeling of the failure of the beams is presented in this paper. When the CFRP rod is prestressed, the shear stress is largest at the free end and decreases along the transfer length. The prestressing shear stresses are taken equal to the cracking shear stress under monotonic load for non-prestressed beams. During loading, following flexural cracking, debonding occurs at the loading point and progresses towards the supports as the load is cycled. The debonding is modeled as a crack growing at the interface between the CFRP rod and the epoxy. A bond stress versus slip model is proposed. It consists of an ascending branch representing the strains and stresses ahead of the crack tip and a descending branch representing the region behind the crack tip. The driving force for the crack is the shear stresses at the interface between the CFRP rod and the epoxy. The overlapping of the two shear stress distributions increases the shear stress near the end of the beam, which becomes the critical shear stress that causes failure. The failure shear stress at the end is higher than the prestressing shear stress and is estimated to be 25.5 MPa (3.698 ksi) for the spirally wound rods and 30.4 MPa (4.409 ksi) for the sand coated rods. The model also predicts the number of cycles and the forces in the CFRP rod at all locations in the shear span at the onset of failure with reasonable accuracy.

This paper introduces a rectangular concrete-filled fiber reinforced polymer (FRP) tube (CFFT) hybrid beam with an inner voided tube. The beam contains an outer rectangular filament-wound glass fiber reinforced polymer (GFRP) tube, and an inner voided circular filament-wound GFRP tube shifted toward the tension zone. The space between the tubes is filled with concrete. Steel bars, at the tension side, were provided to enhance both stiffness and serviceability of the beam. The flexural behavior of this voided CFFT beam was compared with a fully CFFT beam and another conventional steel reinforced concrete (RC) beam having identical dimensions. The results showed that the new hybrid composite beam behaves positively in terms of strength, ductility, and failure propagation, in addition to its high durability. The results also showed that, while the weight of the voided CFFT beam is 30% lighter than the weight of the conventional RC beam or the fully CFFT beam, its flexural capacity is 141% and 6% higher than their flexural capacities, respectively.

Lap splices are an easy to implement low cost method of transferring force between reinforcing bars in concrete structures. However, the bond between lap spliced bars is usually the weakest region in a reinforced concrete structure. Fiber reinforced polymer materials (FRP) are widely used to strengthen and repair lap splices because of their high strength, durability and ease of handling. Researchers have found that increased concrete cover provides an increase in bond strength similar to that supplied by wrapping with FRP sheets. Currently the FRP industry produces a new generation of high stiffness FRP sheets that provide a high degree of confinement and large increases in bond strength to lap splices.

This paper compares the effectiveness of wrapping with very high stiffness carbon FRP sheets (CFRP 900), wrapping with low stiffness glass FRP sheets (GFRP 430) and no wrapping on the bond strength of lap splice connections for various concrete covers. The test variables were the amount of concrete cover and the wrapping condition. The results showed that the GFRP wrapped beams had an increased in bond strength of approximately 25% compared to the unwrapped beams for each of the concrete covers. However, the CFRP wrapped beams had a percentage increase in bond strength that decreased as the concrete cover increased. The CFRP wrapped beams had increases in bond strength of 71%, 60% and 44% compared to the unwrapped beams for concrete covers of 20mm, 30mm and 50 mm, respectively.

This study investigates the effect of aggressive regime of 300 freeze-thaw (FT) cycles, at a core temperature range of +5 oC (+41 oF) to -18 oC (-0.4 oF) on the structural behaviour and bond integrity of concrete beams cast onto glass fiber reinforced polymer (GFRP) stay-in-place (SIP) structural forms. The study aims at comparing two configurations of the SIP forms, namely a flat plate with T-shape ribs and a corrugated plate, under the potential ‘frostjacking’ effect arising from FT cycles. The study explored specimens with no surface treatment, wet adhesive bonding to freshly cast concrete, and bonded coarse aggregates to enhance roughness of SIP form. It was clearly shown that flat-ribbed form specimens are superior to the corrugated form ones, as no loss in strength occurred after the FT exposure, whereas the corrugated form specimens lost 18-21%. This is attributed to the anchorage advantage provided by the T-shape rib embedment into concrete. Specimens with untreated corrugated forms showed strengths that are only 21-26% of treated ones. For flat-ribbed form specimens, the one with untreated form had 44% the strength of that with bonded aggregates.