M.S. Mohamed Ali

The Tension Stiffening Mechanism in Reinforced Concrete Prisms

Tension stiffening is an important phenomenon in reinforced concrete because it controls not only deflections but also crack spacings, crack widths and the formation of multiple cracks. It is now common practice to study the effects of tension stiffening in concentrically loaded prisms, which is the subject of this paper, and use these behaviours as guidance for the effects of tension stiffening in reinforced concrete beams. As tension stiffening is a mechanism for stress transfer between the concrete and reinforcement, the interface bond stress-slip (τ–δ) properties are of utmost importance. In this paper, partial interaction theory is used to develop generic closed form solutions for crack spacings and widths, the load to cause primary, secondary cracks and subsequent cracks. Four different types of interface bond characteristics (τ–δ) are considered: a linear ascending bond slip which is useful at serviceability; a linear descending bond slip which is useful at the ultimate limit state; a nonlinear bond slip characteristic which closely resembles material bond slip behavior at all limits; and the CEB-FIP Model Code 90 (CEB 1992).

A structural engineering approach to adhesive bonding longitudinal plates to RC beams and slabs

Retrofitting by plating structures has been found to be very efficient. However, tests have shown that externally bonded longitudinal plates are prone to premature debonding. Furthermore, a comprehensive study of published research has also shown that there can be large discrepancies between debonding mathematical models and tests. To overcome this problem and to allow structural engineers to adhesively bond plates with safety and efficiently, a structural engineering approach is suggested whereby many of the debonding mechanisms can be prevented by judicious detailing; this approach can be applied to tension face plates, compression face plates, side plates, U-sectioned plates, and angle-sectioned plates. Four continuous reinforced concrete beams have been retrofitted with adhesively bonded longitudinal plates and tested in order to illustrate this design approach, to directly compare the performance of longitudinal side plates with longitudinal tension face plates, to compare FRP plating with steel plating, and in particular to illustrate the effect of debonding on the sectional ductility of longitudinally plated continuous RC beams.

The Residual Strength of Confined Concrete

Structural engineers have long recognised the importance of member ductility in the design of reinforced concrete members to overcome uncertainties in the design procedure as well as in absorbing energy due to dynamic loads such as earthquakes, impact and blast loads. This has lead to much research on quantifying the rotational capacities of reinforced concrete hinges where intrinsic to the analysis is the behaviour of the concrete compressive stress-strain softening branch, of which the residual strength and strain capacities are important parameters. Much of the softening branch research has been based on careful empirical analyses of confined concrete cylinders. This paper shows that shear-friction theory, which is an established area of research, can be used to quantify the residual strength of hydrostatically and spirally-reinforced confined concrete and provide a lower bound to the residual strength of FRP confined concrete. Hence shear-friction theory is shown to provide an additional structural mechanics tool in the analysis and application of confined concrete.

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.

Concrete Component of the Rotational Ductility of Reinforced Concrete Flexural Members

Reinforced concrete flexural members inherently rely on member ductility to ensure a safe design by allowing for: redistribution of applied stress resultants; quantification of drift for determining magnified moments; and for the absorption of seismic, blast and impact energy. Structural engineers have recognised that much of the member rotation is concentrated in a small region referred to as the plastic hinge and because of the complexity of the problem this has been quantified mainly through testing. In this paper, a new plastic hinge approach that is based on well established shear-friction theory is postulated. The generic behaviour of this novel shear-friction hinge is shown to agree with that exhibited in tests. Furthermore, the shear-friction hinge explains the mechanics of the benefits of confinement, such as that due to FRP encasement or steel stirrups, on the rotational capacity of RC members.

Comparison between FRP and steel plating of reinforced concrete beams

Very advanced design rules have already been developed for adhesive bonding steel plates to reinforced concrete beams in order to prevent premature debonding by either shear peeling or flexural peeling. The aim of this study is to determine experimentally whether these design rules that were developed for steel plated beams and slabs, can be applied to fiber reinforced plastic (FRP) plated beams. This paper compares, with the help of test results, the shear and flexural debonding mechanisms of steel and FRP plated 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.

Interaction Between Flexure and Shear on the Debonding of RC Beams Retrofitted with Compression Face Plates

Steel and FRP plating reinforced concrete structures is increasingly being used for retrofitting. Plates can be bonded to any surface of a beam or slab, although it is common practice to adhesively bond plates to the tension faces. The addition of these tension face plates reduces the sectional ductility of the beam. Furthermore, these tension face plates are prone to premature debonding because the stress concentrations induced by these plates overlap with those induced by the tension reinforcing bars adjacent to the plate. Solutions to these two problems, which are the subject of this paper, consist of: adhesively bonding plates to the compression faces to counterbalance the tension face plates and, hence, improve the beam sectional ductility; and to extend the tension face plates, in continuous beams, past the points of contraflexure so that they terminate in a compression face. In this paper, eleven new tests on 340 mm deep beams are presented that show that compression face plates are less prone to debonding than tension face plates.

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.

Design for Moment Redistribution in RC Beams Retrofitted with Steel Plates

It is now common practice to retrofit reinforced concrete members by adhesively bonding steel or fibre reinforced polymer plates to their surfaces. However, tests have shown that these plated RC structures tend to have less member ductility, or rotational capacity, than the unplated structure because of premature plate debonding. In this paper, structural mechanics approaches are described for both: quantifying the moment rotation capacity, or member ductility, of steel plated RC flexural members; and quantifying the moment redistribution capacity from the moment rotation capacity. It is shown how the moment redistribution structural mechanics model can be used to design for member ductility directly and, furthermore, it is applied to both externally bonded and near surface mounted steel plates. As would be expected, it is shown that steel plating produces more ductile members than fibre reinforced polymer plating.