G. M. Chen

Finite-Element Modeling of Intermediate Crack Debonding in FRP-Plated RC Beams

Intermediate crack-induced debonding (IC debonding) is a common failure mode of RC beams strengthened with externally bonded fiber-reinforced polymer (FRP) reinforcement. Although extensive research has been carried out on IC debonding, much work is still needed to develop a better understanding of the failure mode and a more reliable strength model. This paper presents an advanced finite-element (FE) model on the basis of the smeared-crack approach for predicting IC debonding failure. Existing FE models of the same type are generally deficient in capturing localized cracks (both their pattern and widths). This deficiency is overcome in the proposed FE model through the accurate modeling of interfaces between the concrete and both the internal steel and the external FRP reinforcements. The capability and accuracy of the proposed model are demonstrated through comparisons of its predictions with selected test results. The importance of accurate modeling of localized cracking is also explained using numeric...

Interaction between Steel Stirrups and Shear-Strengthening FRP Strips in RC Beams

RC beams shear strengthened with either fiber-reinforced polymer (FRP) U-jackets/U-strips or side strips commonly fail due to debonding of the bonded FRP shear reinforcement. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding. Consequently, the yield strength of internal steel stirrups in such a strengthened RC beam cannot be fully used. In this paper, a computational model for shear interaction between FRP strips and steel stirrups is first presented, in which a general parabolic crack shape function is employed to represent the widening process of a single major shear crack in an RC beam. In addition, appropriate bond-slip relationships are adopted to accurately depict the bond behavior of FRP strips and steel stirrups. Numerical results obtained using this computational model show that a substantial adverse effect of shear interaction generally exists between steel stirrups and FRP strips for RC beams shear strengthened with FRP side strips. For RC beams shear strengthened with FRP U-strips, shear interaction can still have a significant adverse effect when FRP strips with a high axial stiffness are used. Therefore, for accurate evaluation of the shear resistance of RC beams shear strengthened with FRP strips, this adverse effect of shear interaction should be properly considered in design.

Shear Strength Model for FRP-Strengthened RC Beams with Adverse FRP-Steel Interaction

AbstractRC beams shear strengthened with externally bonded fiber-reinforced polymer (FRP) U strips or side strips usually fail owing to debonding of the bonded FRP shear reinforcement. Because such debonding usually occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups intersected by the critical shear crack may not have reached yielding at beam shear failure. Consequently, the yield stress of internal steel stirrups in such a strengthened RC beam cannot be fully utilized. This adverse shear interaction between the internal steel shear reinforcement and the external FRP shear reinforcement may significantly reduce the benefit of the shear-strengthening FRP but has not been considered explicitly by any of the shear strength models in the existing design guidelines. This paper presents a new shear strength model considering this adverse shear interaction through the introduction of a shear interaction factor. A comprehensive evaluation of the proposed model, ...

Behavior of RC Beams Shear Strengthened with Bonded or Unbonded FRP Wraps

Reinforced concrete (RC) beams shear-strengthened with fiber-reinforced polymer (FRP) fully wrapped around the member usually fail due to rupture of FRP, commonly preceded by gradual debonding of the FRP from the beam sides. To gain a better understanding of the shear resistance mechanism of such beams, particularly the interaction between the FRP, concrete, and internal steel stirrups, nine beams were tested in the present study: three as control specimens, three with bonded FRP full wraps, and three with FRP full wraps left unbonded to the beam sides. The use of unbonded wraps was aimed at a reliable estimation of the FRP contribution to shear resistance of the beam and how bonding affects this contribution. The test results show that the unbonded FRP wraps have a slightly higher shear strength contribution than the bonded FRP wraps, and that for both types of FRP wraps, the strain distributions along the critical shear crack are close to parabolic at the ultimate state. FRP rupture of the strengthened ...

Concrete Cover Separation in FRP-Plated RC Beams: Mitigation Using FRP U-Jackets

AbstractConcrete cover separation is a common failure mode of RC beams strengthened with a fiber-reinforced polymer (FRP) plate externally bonded to the tension face (FRP-plated RC beams). This failure mode initiates at the critical plate end and then propagates at the level of steel tension reinforcement in the direction of increasing moment. Plate-end anchorage by FRP U-jackets has been specified in some design guidelines as a mitigation measure to delay or suppress concrete cover separation, although its effectiveness is far from clear. This mitigation method is more attractive than other options because the same strengthening material is used and the installation procedure is simple. This paper presents the first systematic experimental study on the use of FRP U-jackets for mitigating plate-end concrete cover separation failure, with particular attention to the effect of inclination angle between the U-jacket and the beam axis. A total of 10 full-scale FRP-plated RC beams were tested. The test results...

Experimental Study on the Welding Residual Stresses of Integral Joint Using Full-Scale Joint Model of a Steel Truss Bridge:

In order to study the residual stress distribution of the welded integral joint of steel truss bridge, a full-scale model was fabricated with the same welding process, same material as the real bridge in the bridge workshop. The residual stress state and distributions of residual stresses in the full-scale model were measured using blind-hole strain-gage method. The tested results show that high residual stress is caused during the welding process and the maximum tensile residual stress may reach the yield strength of the steel. Longitudinal tensile residual stress appears in the regions near the welding bead and changes to compressive stress at a certain distance from the welding bead, with the maximum tensile residual stress being in the centre of the welding bead. In butt-welding between two plates with different thicknesses, the value of welding residual stress in the thinner plate is lower than that of the thicker one. The results presented in this paper would be useful as references for developing measures of reducing the residual stress of integral joint in the fabrication of a real bridge.

Role of interfacial damage modeling in finite element analysis of intermediate crack debonding failure of FRP-plated reinforced concrete beams: A numerical investigation

This study presents a numerical investigation into the effects of interfacial damage on the bond behaviors of the interfaces between two adjacent cracks in a fiber-reinforced polymer-plated reinforced concrete beam. The interfaces investigated herein refer to both fiber-reinforced polymer-to-concrete interface and steel-to-concrete bonded interface which were well represented in this study by a simplifying fiber-reinforced polymer-to-concrete bonded joint or steel bar-to-concrete bonded jointed joint loaded at their two ends, aiming to simulating the interaction between two adjacent flexural cracks in the fiber-reinforced polymer-plated reinforced concrete beams. Parametric studies were carried out using nonlinear finite element model built in ABAQUS to investigate the effects of a number of significant factors such as bond length, ratio of loads applied at the two ends (i.e. load ratio), and types of bond–slip models, with the main objective being to clarify the effect of interfacial damage (i.e. bondline damage) during slip reversals on the bond behaviors of the interfaces. The numerical results show that in finite element analysis of intermediate crack debonding of fiber-reinforced polymer-plated reinforced concrete beams where multiple cracks may exist, an appropriate consideration of the interfacial damage during slip reversals is necessary in order to achieve accurate predictions on the behavior of fiber-reinforced polymer-to-concrete bonded interfaces, while for the steel bar-to-concrete interface, the consideration of the interfacial damage has insignificant effect on the numerical results if the yield strength and bar diameter are in their practical range.

Effect of Load Distribution on IC Debonding in FRP-Plated RC Beams

Intermediate crack debonding (IC debonding) is a common failure mode for RC beams flexurally-strengthened with externally bonded FRP reinforcement. Although numerous studies have been carried out on IC debonding, the vast majority of them have been concerned with beams subjected to three- or four- point bending despite the fact that a uniformly distributed load (UDL) is a more common loading condition in practice. This paper presents the first ever finite element study into the effect of load distribution on IC debonding failure. A recently developed FE model was employed to simulate the IC debonding failure of three beams tested by other researchers under different load distributions and then to simulate the IC debonding failure of a beam under UDL. The numerical predictions are found to be in close agreement with the test re-sults and confirm the experimental observation that the IC debonding strain in the FRP plate (and hence the IC debonding moment of the strengthened section) increases as the load distribution becomes more uniform.