International Concrete Abstracts Portal

Upgrading often becomes a necessity due to changes in usage of buildings due to factors such as deterioration and aging, change in occupancy, or the need for installation of facilities such as air-conditioning, heating, escalators, elevators, additional skylights, or new façade structures. In a number of cases upgrading is related to changes which affect the load bearing components of the structure. Fiber reinforced polymer matrix composites provide an efficient means of both strengthening slabs for enhanced load carrying capacity and for strengthening slabs after installation of cut-outs. This paper reports on a series of tests conducted to assess the comparative efficiencies of a commercially available strip form and a fabric form of material vis-à-vis strengthening ability and ductility. It is shown that material tailoring can result in significant changes in efficiencies. The extension of this to the rehabilitation of cut-outs is also detailed and aspects of an on-going full-scale test program in that area are elucidated.

An experimental study was conducted to determine the transfer length, development length and flexural behavior of fiber-reinforced polymer (FRP) tendons in prestressed concrete beams. Three kinds of nominally 5/16 in (8 mm) diameter FRP tendons were included in the study: Carbon Leadline, Aramid Technora and Carbon Strawman. Thirty beams were pretensioned using a single FRP tendon. In addition, twelve control beams were pretensioned with a seven-wire steel strand (ST). Transfer length observations from this study were based on concrete strain measurements with a DEMEC gage system. Development length observations were based on three-point flexural tests. Four-point flexure tests were also performed on each material to gain additional understanding of the bond behavior between concrete and the PC reinforcing materials. The "95% average plateau strain" method of using concrete strain results was shown to be an effective way to determine transfer length. By using an appropriate flexural model and extrapolating results from over-reinforced tests to situations where the tendon would actually fail, it was possible to determine development length in this investigation. Despite differences in tendon material properties and prestressing forces, both the measured transfer lengths and the development lengths were almost identical for all tendon materials tested. The development length for FRP tendons was reasonably predicted by the ACI design equation, although transfer length appears to be underestimated.

The bond behavior of carbon FRP tendons for concrete is characterized with an interface model. In particular tendons with a surface structure that produce significant mechanical interlocking with the adjacent concrete are considered. This type of mechanical interaction can produce damage in the adjacent concrete and within the surface structure of the reinforcing element. The combination of these mechanisms is characterized with an elastoplasticity model that fully couples the longitudinal and radial response; the model calibration is based upon a series of bond tests under differing stress states. The model does not provide a detailed description of the underlying mechanics associated with the progressive bond failure, and it will generally require recalibration when applied to significantly different FRP bars or tendons. However, using a calibration for a GFRP bar, the model gives acceptable estimates of the bond strength for several tests of a particular CFRP tendon, even though the specimens have significantly different attributes. Additional validation tests (using data with measures of the experimental scatter) are needed to define the predictive limits of the model; nonetheless the transfer length problem further demonstrates the potential application of the model to help predict and understand the behavior of FRP-reinforced structural components.

The bond between an aramid fibre reinforced plastic (AFRP) tendon and concrete has a significant effect on the flexural behaviour of a concrete beam pre-tensioned with AFRP. In particular, the performance of beams with prestressed AFRP tendons can be enhanced by the use of partially-bonded tendons. Two types of partial bond are possible; intermittent bond, where sections of the tendon are alternately bonded and debonded from the concrete, and adhesive bond, where the tendon is coated with a resin of known, low shear strength. However, the choice between these methods, and the determination of the values of the various parameters required, are not trivial problems. It is found that a major obstacle in the development of a generalised design procedure for the partially-bonded beams is the uncertainty regarding the rotation at which the concrete will fail. Nevertheless, insight into design aspects of the intermittently-bonded and adhesively-bonded beams is gained and a design methodology is proposed.

The use of FRP for reinforcing is not as popular as its use for prestressing because the modulus of elasticity to strength ratio of most FRP bars is relatively small compared to steel, and the unit price is significantly higher than steel. Therefore, to control deflection and crack width under service conditions, FRP reinforced sections often need to be greatly over-reinforced, which increases the overall cost of the structure. This paper offers an innovative solution to the latter problem by suggesting the use of tension elements as reinforcement. The CFRP tension elements developed in the present investigation are concentrically pretensioned prisms of small cross-section. Such elements would reduce the need for high reinforcement ratio while simultaneously endowing the member with greater flexural rigidity. This paper will briefly explain the concept of FRP prestressed tension elements used for reinforcement of concrete beams, followed by the description of an experimental investigation related to the development of CFRP tension elements and their use as flexural reinforcement in concrete beams. The effectiveness of the tension elements in controlling crack width and deflections under service loads is demonstrated.