J.G. Teng

Behavior and Modeling of Confined High-Strength Concrete

This paper presents a study on the behavior and modeling of the stress-strain behavior of confined high-strength concrete (HSC) without silica fume. The behavior of actively confined HSC is first examined, and a unified active-confinement model applicable to both HSC and normal-strength concrete (NSC) is then proposed based on a large test database assembled from the existing literature. An experimental study on fiber-reinforced polymer (FRP)-confined HSC is next presented and interpreted to examine its behavior, forming the basis for the subsequent modeling work. It is eventually shown that a recent analysis-oriented model developed by the writers’ group for NSC also provides close predictions for FRP-confined HSC. While the work is primarily concerned with HSC without silica fume, the effect of incorporating silica fume into HSC on the behavior of confined HSC is also given appropriate attention. The presence of silica fume in HSC is shown to reduce the effectiveness of confinement in term of strain cap...

Experimental Study on Intermediate Crack Debonding in FRP-Strengthened RC Flexural Members

A commonly observed failure mode of reinforced concrete (RC) flexural members strengthened with a fibre reinforced polymer (FRP) plate bonded to their tension face involves debonding that initiates at a major flexural or flexural shear crack and propagates towards a plate end, which has often been referred to as intermediate crack (IC) debonding. This paper first presents the results of IC debonding tests conducted on eighteen cantilever slabs in which IC debonding initiated at or near the fixed end, and four simply-supported slabs in three-point bending where IC debonding initiated at the load position. A number of factors are covered by these tests, including the type of FRP material, the width of FRP strip, the length of FRP strip, the slab width, the concrete strength, the longitudinal steel ratio, the concrete cover, and pre-cracking of concrete. The test results are next used to assess the accuracy of four available strength models that can be used to design against IC debonding. Of these four models, Teng et als (2003) model is shown to provide safe predictions of the test results in most cases but can lead to overly conservative results; the scatter of the predictions is also large. The other three models from recent design guidelines (JSCE, 2001, fib 2001, ACI 440 2002) are not sufficiently safe for use in design. This assessment clearly illustrates the need for much further research to develop a more accurate IC debonding strength model.

Stress-Strain Behavior of Concrete in Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns

Hybrid fiber-reinforced polymer (FRP) concrete-steel double-skin tubular columns are a new form of hybrid columns recently proposed by the second writer. The new column consists of an inner steel tube and an outer FRP tube, with the space between them filled with concrete. The new column possesses several advantages over existing columns including excellent ductility and corrosion resistance. This paper first examines the behavior of the confined concrete between the two tubes using a finite-element model to understand the effects of the key parameters including the stiffness of the FRP tube, the stiffness of the steel tube, and the size of the inner void. Based on the available experimental observations and the results from the finite-element model, a simple stress-strain model for the confined concrete is proposed for use in practical design. Comparisons with test results show that the proposed model provides reasonably accurate and conservative predictions.

Behaviour of GFRP-strengthened RC cantilever slabs

Abstract This paper presents an experimental study on reinforced concrete (RC) cantilever slabs bonded with glass fibre-reinforced polymer (GFRP) strips. Ten tests in three series were conducted on such slabs with different amounts of internal steel reinforcement and external FRP reinforcement. All FRP-strengthened test slabs were found to experience debonding of FRP strips from the slab, which started near the fixed end and propagated towards the free end, with the final failure mode being either complete debonding or FRP tensile rupture. The severity or likelihood of debonding is shown to depend on the thickness and width of the bonded FRP strips. It is further shown that debonding in these slabs does not have a serious detrimental effect on the ultimate strength, particularly when the FRP reinforcement ratio is low. Instead, it leads to a more ductile behaviour.

Advances in Adhesive Joining of Carbon Fibre/Polymer Composites to Steel Members for Repair and Rehabilitation of Bridge Structures:

Generally, it is relatively easy to obtain a high joint strength with most modern bonding systems if these are cured under ideal factory conditions. However, civil engineering construction joints are cured on site and are required to last the lifetime of the structural member in harsh environmental conditions. This implies that site joints might not be constructed and cured as well as those fabricated in the factory. The paper compares two possible methods for bonding an FRP composite patch/plate to a steel adherend suitable for construction sites. The first method utilises the accepted technique of bonding two dissimilar materials using a two-part cold cure adhesive. The second method employs a pre-impregnated FRP composite plate in conjunction with a compatible film adhesive; these comparisons are undertaken by examining the results of double-strap butt joint tests. A possible site technique using the pre-impregnated FRP composite to upgrade a steel beam whilst it is under a low frequency vibration load is investigated; this represents the repair of a steel bridge constantly under traversing traffic. The butt joint test results show that on average, the pre-impregnated composite in conjunction with the film adhesive leads to a failure load which is 15% higher than that of the cold setting adhesive technique. The results of the rehabilitated beam tests show that the bonded joint between the pre-impregnated CFRP composite and the steel adherend did not suffer any significant damage from the low frequency vibrations imposed upon the steel beam during the cure period.

Analysis-oriented stress-strain model for concrete under combined FRP-steel confinement

AbstractExtensive research has been conducted on fiber-reinforced polymer (FRP)-confined plain and RC columns, leading to a large number of stress–strain models. Most of these models have been developed for FRP-confined plain concrete and are thus applicable only to concrete in FRP-confined RC columns with a negligible amount of transverse steel reinforcement. The few models that have been developed for concrete under the combined confinement of FRP and transverse steel reinforcement are either inaccurate or too complex for direct use in design. This paper presents an accurate design-oriented stress–strain model for concrete under combined FRP-steel confinement in FRP-confined circular RC columns. The proposed model is formulated on the basis of extensive numerical results generated using an analysis-oriented stress–strain model recently proposed by the authors and properly captures the key characteristics of FRP-steel-confined concrete as revealed by existing test results. The model strikes a good balanc...

Design of Concrete-Filled FRP Tubular Columns: Provisions in the Chinese Technical Code for Infrastructure Application of FRP Composites

In recent years, fiber-reinforced polymer (FRP) composites have found wide applications in new construction. A popular type of hybrid member incorporating FRP is the concrete-filled FRP tube (CFFT), consisting of an FRP tube filled with plain or steel-reinforced concrete. CFFTs have several advantages over traditional column forms, including their excellent corrosion resistance and ductility. Much research has been conducted on CFFTs over recent years, but no systematic procedure for designing such members has been developed, which has been one of the factors preventing CFFTs from becoming more widely accepted in practice. This paper presents a summary of the design provisions for CFFTs given in the Chinese Technical Code for Infrastructure Application of FRP Composites and explains the rationale behind these provisions. These provisions have been developed on the basis of available research and work undertaken at The Hong Kong Polytechnic University.

Behavior of hybrid FRP-concrete-steel double-skin tubular columns subjected to cyclic axial compression

Abstract Hybrid FRP-concrete-steel double-skin tubular columns (hybrid DSTCs) are a new form of hybrid columns which consists of a layer of concrete sandwiched between an inner steel tube and an outer FRP tube. While a large amount of research has been conducted on the monotonic behavior of this novel form of columns, only a limited amount of work has been conducted on their behavior under cyclic loading. This paper presents the first ever study on the behavior of circular hybrid DSTCs under cyclic axial compression. Results from a series of stub column tests, where the hybrid DSTCs were subjected to cyclic axial compression, are first presented and discussed. The test results show that hybrid DSTCs are very ductile under cyclic axial compression, with the envelope axial load-strain curve being almost the same as the axial load-strain curve of a corresponding DSTC under monotonic compression. It is also shown that repeated unloading/reloading cycles have a cumulative effect on the permanent strain and the stress deterioration of the confined concrete in hybrid DSTCs. The experimental stress–strain curves of the confined concrete in hybrid DSTCs are then compared with predictions from two existing models: (1) a monotonic stress–strain model for the confined concrete in hybrid DSTCs; and (2) a cyclic stress–strain model for the concrete in FRP-confined solid columns. The comparison suggests that the combined use of the two models can give reasonably accurate predictions of the test results.

Elastic buckling of cone-cylinder intersection under localized circumferential compression

Conical shells are often joined to cylindrical shells as end closures, reducers or roofs. Under a variety of loading conditions, the intersection between the large end of a cone and a cylinder is subject to a large circumferential compressive force which can lead to its failure by buckling. The problem may be idealized as a cone-cylinder intersection under a radial inward ring load. This paper first investigates the elastic buckling strength of thin cone-cylinder intersections under a radial inward ring load and develops simple and accurate equations for the prediction of buckling mode and strength. The ability of ring-loaded intersections to conservatively represent intersections under a variety of other loading conditions for their buckling behaviour is then explored. The ring load idealization is shown to be generally conservative, but may become rather conservative for some loading conditions such as uniform internal pressure. The strength of cone-cylinder intersections under uniform internal pressure is examined in detail in the final part of the paper and approximate strength equations are also developed, as this loading condition is important for pressure vessel and piping applications.

Stress-Strain Behavior of FRP-Confined Recycled Aggregate Concrete

AbstractA large amount of research has been conducted on recycled aggregate concrete (RAC) due to its social, environmental, and economic significance. However, the in situ application of RAC has so far been mainly limited to nonstructural purposes, as the performance of RAC, in both the short and long term, is inferior to its normal concrete counterpart. Existing research has shown that the performance of concrete in compression members can be significantly enhanced through external confinement using steel tubes and fiber-reinforced polymer (FRP) tubes/wraps. Some recent research has examined the behavior of steel tubes filled with RAC, but the research on the behavior of RAC confined with FRP has been rather limited. Research is therefore needed to better understand the stress-strain behavior of and develop a reliable stress-strain model for FRP-confined RAC to facilitate the design of members with FRP-confined RAC. This paper presents the results of the first systematic experimental study on the axial ...