Should FRP be Bonded to Concrete?

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Title: Should FRP be Bonded to Concrete?

Author(s): C. J. Burgoyne

Publication: Special Publication

Volume: 138

Issue:

Appears on pages(s): 367-380

Keywords: bonding; cracking (fracturing); ductility; prestressed concrete; fiber reinforced plastics; prestressing steels; reinforced concrete; General

Date: 9/1/1993

Abstract:
The question of whether it is right to bond tendons made of glass, aramid, or carbon fibers to concrete has not yet been addressed directly. Paper discusses the various issues involved and concludes that, in many cases, these tendons should remain unbonded. All the new materials that have a stiffness high enough and creep low enough show linear elastic response right up to failure. This contrasts with steel, even very high-tensile steel, which shows a considerable reduction in stiffness at high loads. In bonded beams, when cracks form on the tension face of the concrete, very high strains are generated across the crack. With a steel tendon, local yield must occur, with a consequent reduction in cross-sectional area, which leads to debonding of the bar on either side of the crack. This allows the strain at the crack to reduce below its theoretical maximum value. In calculation, average steel strains are used, which ignore any local increase at the crack positions, but there are some controversial code rules that limit the (average) steel strain to less than the material can actually sustain. When new materials are used, the local yielding mechanism is no longer available, and the concept of using average strains is no longer justified. In concrete reinforced with FRP, the entire strain capacity of the fibers is available, and it is unlikely that fiber failure will occur before the concrete strains become unacceptable. But in prestressed concrete, much of the fiber strain capacity is absorbed in the prestress, leaving a tendon very sensitive to high strains in the vicinity of cracks. There is a move to increase the ductility of beams reinforced or prestressed with FRP by using FRP cages in the compression zone. This will increase the chances of a bonded tendon snapping before concrete crushing occurs. These mechanisms are not present in unbonded tendons, where high local strains do not occur, and indeed the change in stress in the tendon is small. It has been argued that, for steel tendons, this is an economic disadvantage; however, for FRP tendons, it is shown here to be beneficial.