Wade Lucas

FRP-Reinforced Concrete Beams: Unified Approach Based on IC Theory

In general, steel-reinforced concrete involves a ductile steel material and a very strong and ductile bond between the steel reinforcement and concrete, so that debonding rarely governs the design. In contrast, fiber-reinforced polymer (FRP) reinforcement is a brittle material with a weak and brittle bond, making debonding a major issue. Consequently, there has been an extensive amount of research on FRP debonding and in particular intermediate crack (IC) debonding. This paper shows that the very good research by the FRP research community on the mechanics of IC debonding can be applied to a wide range of apparently disparate reinforced concrete behaviors to produce a unified approach. Hence, a single mechanism, or unified approach, based on IC debonding is proposed in this paper for dealing with moment rotation, tension stiffening and deflections, member ductility and moment redistribution, shear capacity, confinement, and fiber concrete for FRP RC beams.

Formulation of a Shear Resistance Mechanism for Inclined Cracks in RC Beams

The shear capacity of reinforced concrete (RC) members is often associated with sliding across inclined planes often referred to as critical diagonal cracks. However, quantifying the shear capacity of the RC member in terms of the sliding resistance of an inclined plane as a result of shear-friction has been found to be a very complex problem. This is because these sliding planes transcend both initially cracked and uncracked planes, their capacity is also a function of the separation between these sliding planes, and invariably the shear-friction sliding capacity overestimates the shear capacity of the member. In this paper, a structural mechanics model that incorporates shear-friction is developed for quantifying the various components of the shear resistance across a critical diagonal crack because of both longitudinal reinforcement and stirrups. It is shown that the shear resistance is less than would be anticipated from the direct application of shear-friction theory because the compressive force in the uncracked region of concrete is less than can be anticipated and because the shear resistance must provide shear forces to maintain equilibrium prior to resisting the direct shear force.

Our obsession with curvature in RC beam modelling

Much of the early research in reinforced concrete dealt with steel reinforcement that was both ductile and had a very strong bond with the concrete. Hence partial-interaction, that is slip between the reinforcement and concrete and subsequently debonding, has not been a major issue. This has allowed researchers to develop the two-dimensional full-interaction moment-curvature approach to model the three-dimensional behaviour of reinforced concrete. It is shown in this paper that this two-dimensional full-interaction moment-curvature approach relies on a large amount of empirical calibration to ensure a safe design. Furthermore, it is shown that a three-dimensional partial-interaction moment-rotation approach can lead to more advanced structural mechanics models of reinforced concrete behaviours and subsequently better accuracy and more versatile models.

Flexural Strength and Ductility of FRP-Plated RC Beams: Fundamental Mechanics Incorporating Local and Global IC Debonding

AbstractReinforced concrete (RC) beams and slabs are frequently strengthened or stiffened in flexure by adhesively bonding fiber-reinforced polymer (FRP) plates to their surfaces using a strain-based moment-curvature design technique. This design technique is generally based on the intermediate crack (IC) debonding strain of the FRP reinforcement, that is, on the start of IC debonding; from this analysis it is often deduced that FRP plating is ineffective at the ultimate limit state because FRP debonding occurs before yield of the steel reinforcement. In this paper, it is shown that the strain-based approach is generally a lower bound at the ultimate limit state. Instead, a displacement-based approach is described that shows that FRP plated beams can be designed to achieve a higher strength than that of the RC beam by itself no matter when IC debonding first occurs. The mechanics of the analysis approach developed here treat the FRP debonded plate as a FRP prestressing tendon with a force equal to the IC ...

The FRP reinforced shear-friction mechanism

The ability of reinforced concrete to resist shear forces across possible sliding planes is a well established area of research and is also recognised as an important aspect of the ability of reinforced concrete members to both resist loads and deformation. This characteristic of reinforced concrete is often referred to as the shear-friction or aggregate interlock mechanism and much of the previous research in this area has dealt with ductile steel reinforcement, which is assumed to yield prior to the shear-friction capacity being attained. In this paper, it is shown that, as FRP is an elastic material, the shear friction behaviour of FRP reinforced concrete is different to that with ductile steel reinforcement. However, and perhaps surprisingly, the shear-friction capacity of FRP reinforced concrete can be just as ductile and strong as steel reinforced concrete.

Fundamental Mechanics Governing FRP-Retrofitted RC Beams with Anchored and Prestressed FRP Plates

AbstractIt is often found that a major limitation to retrofitting RC beams by adhesive bonding fiber-reinforced polymer (FRP) plates is premature debonding, which restricts the FRP strains to well below that at fracture. Tests have shown that the FRP peak strains can be increased by anchoring the plate ends of adhesively bonded plates and also by prestressing the anchored plates before adhesive bonding, both of which make the system much more efficient. In this paper, a mechanics-based numerical approach is developed that quantifies the stiffness, strength, and ductility of FRP retrofitted RC beams, in which the FRP plates are anchored, prestressed, and adhesively bonded. The approach can cope with any degree of anchorage and prestress and whether adhesively bonded or not. The approach is generic as it can be applied to any type of RC beam, anchor, and FRP plate and any distribution of load. Furthermore, it allows for discrete flexural cracks and for the formation of softening hinges should these occur. W...

Laboratory testing of strengthened cavity unreinforced masonry walls

AbstractAn experimental campaign consisting of nine pressure-controlled quasi-static airbag tests on unreinforced masonry (URM) walls and accompanying material testing was completed to investigate ...

FRP Design using structural mechanics models

The application and expansion of FRP reinforced concrete has been hindered and obstructed through the misconception and misunderstanding that empirically derived rules developed for steel reinforced concrete in cracked regions can be used either directly or as a guidance for FRP reinforced concrete. This assumption is incorrect because the empirical rules developed for steel reinforced concrete in cracked regions, as with all empirical rules, should only be used within the bounds of the testing regimes from which they were developed, which for steel reinforced concrete is normal strength concrete with high ductile steel that has very good bond. As these bounds do not apply to FRP reinforced concrete, the steel RC empirical rules for cracked concrete are of little or no help for FRP RC. In fact, they are often misleading and as such prevent the widespread use of FRP reinforcement. It will be shown and illustrated in this presentation that generic mechanics based rules can be developed at all load conditions for RC beams that applies to both steel and FRP reinforcement. And, furthermore, that these generic mechanics based design rules allay many of the misconceptions inferred by the empirically based RC design rules such as that moment redistribution cannot occur with brittle FRP reinforced concrete which is simply not the case.