J G Teng

Intermediate crack-induced debonding in RC beams and slabs

Abstract A variety of debonding failure modes have been observed in tests on reinforced concrete (RC) beams strengthened by bonding fibre reinforced polymer (FRP) plates to their tension face. These can be broadly classified into two types: (a) those associated with high interfacial stresses near the ends of the bonded plate; and (b) those induced by a flexural or flexural-shear crack away from the plate ends. The first type of failures has been extensively studied, leading to many strength models. By contrast, the second type of failures has received much less attention and no strength model appears to have been developed for it. This paper is, therefore, concerned with the second type of failures induced by a flexural or flexural-shear crack, which is referred to as intermediate crack-induced debonding. The mechanism of this type of debonding failures is first examined briefly, highlighting the similarity between such debonding failures and those in simple shear tests of FRP-to-concrete bonded joints used for determining bond strengths. Based on this similarity, the recent Chen and Teng bond strength model is combined with a simple section analysis for predicting the strength of beams and slabs, which fail by intermediate crack-induced debonding. It is shown that through a simple modification of the Chen and Teng bond strength model, the proposed debonding strength model provides a good first approximation to the strengths of such beams. A design procedure for the flexural strengthening of beams incorporating the proposed debonding strength model is finally proposed.

Refinement of a Design-Oriented Stress–Strain Model for FRP-Confined Concrete

This paper presents the results of a recent study conducted to refine the design-oriented stress–strain model originally proposed by Lam and Teng for fiber-reinforced polymer (FRP)-confined concrete under axial compression. More accurate expressions for the ultimate axial strain and the compressive strength are proposed for use in this model. These new expressions are based on results from recent tests conducted by the writers’ group under well-defined conditions and on results from a parametric study using an accurate analysis-oriented stress–strain model for FRP-confined concrete. They allow the effects of confinement stiffness and the jacket strain capacity to be separately reflected and accounts for the effect of confinement stiffness explicitly instead of having it reflected only through the confinement ratio. The new expressions can be easily incorporated into Lam and Teng’s model for more accurate predictions. Based on these new expressions, two modified versions of Lam and Teng’s model are present...

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...

Bond-Slip Model for FRP Laminates Externally Bonded to Concrete at Elevated Temperature

AbstractThis paper presents a nonlinear local bond-slip model for fiber reinforced polymer (FRP) laminates externally bonded to concrete at elevated temperature for future use in the theoretical modeling of fire resistance of FRP-strengthened concrete structures. The model is an extension of an existing two-parameter bond-slip model for FRP-to-concrete interfaces at ambient temperature. The two key parameters employed in the proposed bond-slip model, the interfacial fracture energy, Gf, and the interfacial brittleness index, B, were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperature. In the interpretation of test data, the influences of temperature-induced thermal stress and temperature-induced bond degradation are properly accounted for. As may be expected, the interfacial fracture energy, Gf, is found to be almost constant initially and then starts to decrease when the temperature approaches the glass transition temperature of the bonding adhesive; the int...

Effect of Temperature Variation on the Full-Range Behavior of FRP-to-Concrete Bonded Joints

AbstractService temperature variations (thermal loadings) may significantly affect the behavior of the bond between externally bonded fiber reinforced polymer (FRP) and concrete. This paper presents an analytical solution for the full-range deformation process of FRP-to-concrete bonded joints under combined thermal and mechanical loadings. The solution is based on a bilinear bond-slip model and leads to closed-form expressions. The validity of the solution is demonstrated through comparisons with both experimental results and finite-element predictions. Numerical results from the solution are presented to illustrate the effect of thermal loading on the interfacial shear stress and slip distributions in addition to the global load-displacement response. Provided the material properties are not affected by temperature variations, a temperature rise is shown to increase the ultimate load, whereas a temperature reduction decreases the ultimate load; the latter can have serious implications for the safety of t...

Preparation and characterization of steel surfaces for adhesive bonding

AbstractIn fiber-reinforced polymer (FRP) strengthened steel structures, debonding of the bonded FRP reinforcement from the steel substrate may result from adhesion failure at the steel/adhesive interface or the FRP/adhesive interface, cohesion failure in the adhesive, or a combination of these two modes. Of these failure modes, cohesion failure in the adhesive is the preferred mode of failure as it facilitates the development of a design theory based on the adhesive properties; the other two failure modes should be avoided if at all possible. This paper presents a systematic experimental study to identify a surface-adhesive combination that will avoid adhesion failure at the steel/adhesive interface. Different steel surface preparation methods, including solvent cleaning, hand grinding, and grit blasting, and different commonly used adhesives were examined in the study. Surface characterization using three key parameters (namely surface energy, surface chemical composition, and surface roughness and topo...

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 CFRP Laminates Bonded to a Steel Substrate Using a Ductile Adhesive

Strengthening of steel structures with adhesively bonded carbon fiber–reinforced polymer (CFRP) laminates (or plates) has received extensive research attention over the past few years. Existing studies have revealed that debonding of CFRP plates from a steel substrate is one of the main failure modes in such CFRP-strengthened steel structures. To better understand and model debonding failures, the behavior of CFRP-to-steel bonded joints needs to be well understood. Recent tests conducted by the authors’ have demonstrated that the bond strength of such bonded joints depends significantly on the properties of the adhesive used, and more specifically on the interfacial fracture energy rather than the tensile strength of the adhesive. The study has also showed that the bond-slip curves for nonlinear ductile adhesives have an approximately trapezoidal shape. This paper resents an analytical solution for the full-range behavior of CFRP-to-steel bonded joints with a ductile nonlinear adhesive. Predictions from the analytical solution are presented to explain the different stages of debonding failure and are compared with test data to demonstrate the validity of the analytical solution.

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 ...

Axisymmetric shells and plates on tensionless elastic foundations

This paper first describes a finite element method for the large deflection analysis of axisymmetric shells and plates on a nonlinear tensionless elastic foundation. Through the use of discrete data points, any form of nonlinear elastic foundation behaviour can be easily modelled. The analysis is then validated by comparison with existing results for circular plates and beams as the only existing results for shells on tensionless foundations are found to be in error. Following this verification, the analysis is applied to investigate the behaviour of shallow spherical shells subject to a central concentrated load on tensionless linear elastic foundations. A number of insightful conclusions regarding the behaviour of such structure-foundation systems are drawn. The numerical results for shells are believed to be the first correct results, which may be useful in benchmarking results from other sources in the future.