Fiber Reinforced Polymer Hoop Research Articles

Seismic Retrofit Case Study of Shear-Critical RC Moment Frame T-Beams Strengthened with Full-Wrap FRP Anchored Strips in a High-Rise Building in Los Angeles

This paper discusses the iteration of a seismic retrofit solution for shear-deficient end regions of 19 reinforced concrete (RC) moment-resisting frame (MRF) T-beams located in a 12-story RC MRF building in downtown Los Angeles, California. Local strengthening with externally bonded (EB) fiber-reinforced polymer (FRP) fabric was chosen as the preferred retrofit strategy due to its cost-effectiveness and proven performance. The FRP-shear-strengthening scheme for the deficient end-hinging regions of the MRF beams was designed and evaluated through large-scale cyclic testing of three replica specimens. The specimens were constructed at 4/5 scale and cantilever T-beam configurations with lengths of 3.40 m or 3.17 m. The cross-sectional geometry was 0.98 × 0.61 m with a top slab of 1.59 m in width and 0.12 m in thickness. Applied to these specimens were three different retrofit configurations, tested sequentially, namely: (a) unanchored continuous U-wrap; (b) anchored continuous U-wrap with conventional FRP-embedded anchors at the ends; and (c) fully closed external FRP hoops made of discrete FRP U-wrap strips and FRP through-anchors that penetrate the top slab and connect both ends of the FRP strips, combined with intermediate crack-control joints. The strengthening concept with FRP hoops precluded the premature debonding and anchor pullout issues of the two more conventional retrofit solutions and, despite a more challenging and labor-intensive installation, was selected for the in-situ implementation. The proposed hooplike EB-FRP shear-strengthening scheme enabled the deficient MRF beams to overcome a 30% shear overstress at the end-yielding region and to develop high-end rotations (e.g., 0.034 rad [3.4% drift] at peak and 0.038 rad [3.8% drift]) at strength loss for a beam that, otherwise, would have prematurely failed in shear. These values are about 30% larger than the ASCE 41 prescriptive value for the Life Safety (LS) performance objective. Energy dissipation achieved with the fully closed scheme was 108% higher than that of the unanchored FRP U-wrap and 45% higher than that of the FRP U-wrap with traditional embedded anchors. The intermediate saw-cut grooves successfully attracted crack formation between the strips and away from the FRP reinforcement, which contributed to not having any discernable debonding of the strips up to 3% drift. This paper presents the experimental evaluation of the three large-scale laboratory specimens that were used as the design basis for the final retrofit solution.

Shear behavior of circular concrete short columns reinforced with GFRP longitudinal bars and CFRP grid stirrups

Although fiber-reinforced polymer (FRP) reinforcements have become popular, the post-pultrusion bending leads to significant deficiency in bent portions of FRP hoops and spirals. This study adopts flexible carbon FRP (CFRP) grids as stirrups, and investigates the shear behavior of circular short columns reinforced with GFRP bars and CFRP grid stirrups. Six specimens with different shear reinforcement ratios, axial load ratios and types of stirrups (CFRP grids and steel hoops) were tested to shear failures. The failure modes, crack and strain developments, and shear load-horizontal displacement responses were presented for discussion. All specimens experienced shear-compression failures, and the critical diagonal crack widths increased approximately linearly with shear loads. The typical shear load-horizontal displacement curve consisted of three branches: a linear elastic branch, a cracking branch with progressive slope deteriorations, and an almost flat failure branch. Increasing CFRP grid stirrup ratio could not only enhance strength and ductility performance, but also alleviate the slope degradations, while increasing axial load ratios could enhance the initial cracking loads, restrain crack propagations, and reduce the lateral displacements. In terms of strength performance, steel hoops could be replaced by CFRP grid stirrups even with smaller stirrup ratio. Additionally, analytical investigations suggested that the effective stresses and strains of CFRP grid stirrups at failure may be lower than code limits.

Axial compressive behavior and load carrying capacity prediction of prefabricated UHPC tube filled with seawater sea-sand coral aggregate concrete

To reduce construction costs, seawater, sea sand, and dead coral are often used in concrete buildings on distant islands. Such seawater-sea sand coral aggregate concrete (SSCAC) typically has insufficient mechanical properties and high chloride ion content. In this paper, a novel composite column, named SSCAC-filled ultra-high performance concrete (UHPC) tube (SFCT), is proposed and studied. The SFCT column combines a prefabricated fiber reinforced polymer (FRP) hoop reinforced thin-walled UHPC tube with the infill SSCAC. A total of 36 relative large-scale columns were conducted for axial compression tests. The SFCT columns showed a multi-crack failure mode on the surface of the tube without obvious peeling-off of the UHPC cover in the whole loading process. The test results reveal that the UHPC tube in the composite system effectively takes on the additional load released by the inner SSCAC during compacting due to the porous nature and low strength of coral aggregates. Therefore, the compressive behavior of the pre-peak stage of SFCT column is effectively improved compared with existing SSCAC confined by FRP tube or hoops, especially important is increased the axial stiffness. The proposed composite system effectively combines the ultra-high compressive performance of UHPC and the lateral confining effect from FRP hoops. A model was also proposed for predicting the axial load carrying capacity of SFCT columns. The analysis results indicate that selecting relatively thinner UHPC tube in SFCT composite system and using relatively flexible and low-cost basalt FRP (BFRP) and glass FRP (GFRP) hoops instead of carbon FRP (CFRP) hoops as the lateral confining component are economical and could be reasonable measures for manufacturing the composite columns.

FRP spiral strip-confined concrete columns: Stress-strain behavior and size effect

It is well-known that the hoop rupture strain of fiber-reinforced polymer (FRP) jacket in an FRP-confined concrete column is commonly much lower than that obtained from coupon tests. To improve the strain efficiency of the FRP jacket, FRP spiral strip wrapping scheme has been proposed. In the present study, totally 24 circular column specimens [including 16 FRP spiral strip-confined concrete (FSSCC) column specimens] of four different sizes, with a diameter ranging from 100 mm to 400 mm, were tested under uniaxial compression. The particle image velocimetry (PIV) technique was adopted to obtain the hoop and axial strains as well as the strain distributions of the test specimens. The test results showed that the FRP hoop rupture strains measured by the PIV technique in the FSSCC columns are close to that from the coupon tests. The size effect in terms of compressive strength and stress-strain behavior is obvious in the test FSSCC columns. An existing stress-strain model for FRP-confined concrete was then modified to predict the test results. Finally, a field application of the FRP spiral strip wrapping technique completed recently was also briefly introduced.

Effect of compression casting method on the axial compressive behavior of FRP-confined seawater sea-sand concrete columns

The application of the novel compression casting method is beneficial to improve the compressive strength and durability of concrete. However, the compressed concrete fails rapidly after the peak load and exhibits stronger brittleness than the normal concrete, which could impair the seismic performance of the columns. As one of the passive confining measures, wrapping fiber-reinforced polymer (FRP) could be a practical technique to improve the ductility of such brittle columns. In this regard, this study experimentally investigated the axial compression behavior of both the partially and fully FRP-confined compression-cast seawater sea-sand concrete (SSC) columns. The test results indicated that the compression casting method enhanced the compressive strength but weakened the axial deformation capacity of the unconfined and FRP-confined SSC specimens. The compressive strength of unconfined concrete was improved by 38.5% due to compression casting. The measured average FRP rupture strain of the FRP-confined compression-cast SSC specimens was 6.3% smaller than that of FRP-confined normal SSC specimens due to the sudden appearance of the main oblique cracks, indicating poorer FRP utilization efficiency. Meanwhile, the average ultimate axial strain enhancement ratio of FRP-confined SSC specimens decreased from 12.04 to 7.83 after compression casting. The FRP hoop strain of the fully FRP-wrapped columns distributed more uniformly compared to the partially FRP-wrapped columns as the mean value of FRP efficiency factor increased from 0.807 to 0.928. With increasing FRP strip spacing, the post-peak segment of the stress–strain curve transferred from strain-hardening to strain-softening regardless of the confining stiffness. Finally, several existing stress–strain models were employed to examine their accuracy in anticipating the ultimate conditions of the FRP-confined compression-cast SSC columns.

Experimental research on compressive behavior of seawater and sea sand concrete-filled RPC tubes

To address the dilemma of structures constructed with seawater and sea sand concrete (SWSSC), including insufficient durability and high cost, an innovative composite column, named SWSSC-filled reactive powder concrete (RPC) tube (SFRPC tube), is proposed and studied. This hybrid system is composed of a prefabricated RPC thin-wall hollow tube with fiber reinforced polymer (FRP) hoops and infilled SWSSC. In order to understand the basic compressive performance of the proposed composite structure, a total of 24 short columns, with 300 mm outer diameter and 600 mm height, were tested in axial compression. SFRPC tube columns exhibited a multi-crack failure mode on the surface of tube without obvious peeling-off of the RPC cover. The RPC tube can serve as the permanent formwork, and maintain its own integrity in the whole loading process, carrying considerable axial load acting on the composite column, due to its super-high compressive strength. The results reveal that the SFRPC tube system effectively utilizes the ultra-high mechanical properties of RPC and FRP confinement effect. Compressive performance of SFRPC tube columns can be further improved as the volumetric FRP hoop ratio in RPC tube increases. It is found that overlapping joint methods for single FRP hoop in RPC tube and the thickness of RPC tube have no significant effect on the mechanical performance of SFRPC tube columns under the current testing conditions. A special confinement model was also proposed for assessing the axial load carrying capacity of SFRPC tube columns.

PET FRP-concrete-high strength steel hybrid solid columns with strain-hardening and ductile performance: Cyclic axial compressive behavior

This paper presents the results of an experimental program on the behavior of fiber-reinforced polymer (FRP)-concrete-high strength steel solid columns (FCSSCs), with an outer polyethylene terephthalate (PET) FRP tube and an inner circular high-strength steel (HSS) tube, under cyclic axial compression. A PET FRP tube has a much larger rupture strain and a larger FRP hoop strain efficiency, leading to an excellent ductility of PET FRP-confined concrete. The HSS tube, which has a good deformation compatibility with the PET FRP-confined concrete in FCSSCs, provides a much larger longitudinal load carrying capacity and a larger confinement to the core concrete compared with a normal strength steel tube. The experimental results demonstrated that the axial load carrying capacity of an FCSSC is much larger than the summation of the axial load resistance of the hollow steel tube and that of the concrete-filled FRP tube; the buckling of the HSS tube is also prevented so that its post-yield material strength is effectively utilized. It is found that cyclic load-strain envelope curves lie closely to the corresponding monotonic load-strain curves, and repeated loading cycles increase the plastic strain while decrease the reloading new stress. The existing stress-strain model fails to provide an accurate prediction on the cyclic axial behavior of concrete under combined PET FRP-steel confinement.

Axial\u2013Flexural Interaction in FRP-Wrapped RC Columns

The study reported herein aims at investigating the behavior of medium-scale circular reinforced concrete columns wrapped with fiber reinforced polymer (FRP) sheets under concentric and eccentric axial loads. The experimental program was devised to assess the effects of loading conditions, absence/presence of an FRP jacket as well as the FRP wrapping system. To achieve the study objectives, four column groups were tested under axial compression at 0, 25, 50 and 65 mm loading eccentricities corresponding to eccentricity-to-diameter ratios of 0, 0.13, 0.26 and 0.34, respectively. Specimens in a fifth group were tested in pure bending simulating axial compression at infinite loading eccentricities. Three column subcategories were tested under each of the 5 loading eccentricities: unwrapped; wrapped with one ply of hoop FRP sheets; and wrapped with two FRP plies with fibers oriented at 0 and 90° to the longitudinal column axis thereby providing externally-bonded longitudinal reinforcement and hoop confinement, respectively. Tests confirmed that FRP confinement enhances the axial–flexural column resistance even at large eccentricities that exceed the balanced state of unconfined columns. Although axial column resistance decreased with increasing bending moments, relative enhancements (25–35%) in axial resistance provided by FRP confinement were found to be more significant under eccentric loading than in pure compression. Compared to hoop FRP-confined columns, using additional longitudinal sheets resulted in minor (7–9%) but stable enhancements in axial resistance that were unaffected by the increase in loading eccentricity. The FRP hoop wraps had a minor effect on the flexural resistance of specimens tested in pure bending but managed to double their resistance when combined with the externally-bonded longitudinal FRP sheets. Finally, three stress–strain models of FRP-confined concrete were used in conventional section analysis to assess the axial–flexural interaction in the FRP-jacketed columns. Strength predictions made using the stress–strain model proposed in ACI 440.2R-17 design guidelines did not agree with the test results of the eccentrically-loaded columns and underestimated the moment resistance at a given axial force even when considering higher confinement ratios than those permitted by the guidelines. Strength predictions made using eccentricity-dependent stress–strain models showed better results especially when accounting for the increase in ultimate axial strains under eccentric loading.

Confinement effects of unidirectional CFRP sheets on axial and bending capacities of square RC columns

The axial-flexural interaction of reinforced concrete columns wrapped with fiber reinforced polymer (FRP) sheets was investigated using 23 one-third scale specimens. The square column specimens, 1200 mm in height with a side dimension of 200 mm and a corner radius of 12.5 mm, were tested under eccentric axial compression with load eccentricities of 35, 50 and 65 mm and in pure bending. Under each of the four axial-flexural loading combinations three categories of columns were tested to examine the effects of different FRP wrapping systems: unwrapped; wrapped with one hoop carbon FRP ply; and wrapped with two plies of carbon FRP sheets with the main fibers oriented parallel to axial and hoop directions in the first and second plies, respectively. Although the use of the single ply of FRP confining wraps provided a relatively low confinement ratio, test results demonstrated the viability of using the FRP hoop confining system in enhancing axial resistance of the eccentrically-loaded columns. Despite the observed reductions in axial resistance of the control and FRP-confined columns with increasing load eccentricities, FRP confinement provided a stable increase in resistance of about 12% over that of the control unwrapped columns regardless of the level of applied bending moments. FRP confinement managed to provide considerable improvements in the ductility and toughness of the columns especially under largely eccentric loading. Combining the axially-oriented FRP sheets with the hoop wraps brought about further improvements (about 5%) in axial resistance. However, the increase in flexural stiffness of the FRP-jacketed columns provided by this arrangement of axial and hoop oriented fibers tended to counteract the enhancement in ductility and toughness of columns tested under large eccentricities bringing them to levels lower than those of the control specimens in the case of pure bending. Conventional section analysis and stress-strain models of FRP-confined concrete adopted by the 2017 ACI 440.2R design guidelines were used to generate theoretical P-M interaction diagrams. Compared with the experimental results, the prediction of axial-flexural interaction of the FRP-jacketed columns was found to be slightly conservative.

Shear resistance of RC circular members with FRP discrete hoops versus spirals

Nowadays, design guidelines and codes contain valuable shear provisions for the design of concrete bridge members reinforced with fiber-reinforced-polymer (FRP) bars. Limited researches seem to have assessed the shear strength of circular concrete members reinforced with FRP reinforcement. Therefore, these standards do not provide specific formulae for circular RC members designed with FRP bars, hoops and spirals under shear loads. This paper reports experimental data about the shear strength of circular concrete specimens reinforced with FRP bars, discrete hoops and continuous spirals. Full-scale circular concrete specimens with a total length of 3,000 mm and 508 mm in diameter were constructed and tested up to failure. The test parameters included the type of reinforcement (glass FRP and carbon FRP versus steel) and configuration of the shear reinforcement (discrete hoops versus continuous spirals). The investigation revealed that the specimen reinforced with FRP hoops exhibited high load-carrying capacity comparable to the counterpart reinforced with FRP spirals. The experimental shear strengths of the FRP-reinforced concrete specimens were compared to theoretical predictions provided by current codes, design guidelines. The results of this study can be used as a fundamental step toward code provisions for using GFRP or CFRP spirals and hoops as internal shear reinforcement.

Performance Evaluation of Concrete Columns Reinforced Longitudinally with FRP Bars and Confined with FRP Hoops and Spirals under Axial Load

AbstractNowadays, AASHTO LRFD Bridge Design Specifications and the Canadian Highway Bridge Design Code contain flexural and shear provisions for the design of concrete bridge members reinforced with fiber-reinforced polymer (FRP) bars. Because of a lack of research, these standards do not recommend using FRP bars to resist compressive stresses in compression members. This paper reports on 14 full-scale circular RC columns tested under concentric axial load. The columns were reinforced with longitudinal FRP bars and confined with circular FRP spirals or hoops. Sand-coated glass-FRP (GFRP) and carbon-FRP (CFRP) reinforcement was used. The test parameters included configuration of the confinement reinforcement (spirals versus hoops), hoop lap length, volumetric ratio, and FRP reinforcement type (glass versus carbon). The test results indicate that the GFRP and CFRP RC columns behaved similarly to columns reinforced with steel. Using GFRP and CFRP spirals or hoops according to the provisions of the Canadian S...

Variation of Hoop Strains in Concrete-Filled FRP Tubes with Concrete Strength, Amount of Confinement and Specimen Slenderness

This paper presents an experimental investigation on the variation of FRP hoop strains around specimen perimeter and along specimen height. A total of 24 concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) with circular cross-sections were tested under monotonic axial compression. The CFFT specimens were instrumented with numerous lateral strain gauges attached to the FRP tubes to examine the development of hoop strains along the specimen height and around specimen perimeter. Specimens were manufactured with height-to-diameter ratios (H/D) of 2 or 5, with all specimens maintaining a nominal diameter of 150 mm. Additional test parameters selected for this study included amount of confinement and concrete compressive strength. This paper focuses on the experimentally recorded hoop strains and their variation around specimen perimeter and along specimen height. The results indicate that hoop rupture strains along the height of CFFTs become more uniform for specimens with higher amounts of confinement. On the other hand, the variation of hoop strains around the perimeter of CFFTs was not observed to be significantly influenced by height-to-diameter ratio (H/D), concrete strength or amount of confinement.

STRAIN EFFICIENCY OF FRP JACKETS IN FRP-CONFINED CONCRETE-FILLED CIRCULAR STEEL TUBES

The confinement of concrete columns using fiber-reinforced polymer (FRP) jackets or wraps is a popular structural retrofitting technique. More recently, the benefits of FRP confinement of concrete-filled steel tubes have also been explored by researchers. Failure of such FRP-confined concrete-filled steel tubes is usually governed by the rupture of the FRP jacket in the hoop direction. However, the observed FRP hoop strain at failure (i.e. the hoop rupture strain) is typically lower than the ultimate tensile strain from a flat coupon test. Many factors may contribute to this phenomenon, one of which is the geometrical discontinuities at both the starting and finishing ends of the wrapping process commonly used to form an FRP jacket. This paper examines the effect of these geometrical discontinuities on the hoop rupture strain of FRP jackets in FRP-confined concrete-filled circular steel tubes. Detailed finite element (FE) analyses conducted using both linear elastic and elastic-perfectly plastic adhesive constitutive models are presented. Comparison between the FE predictions and available test results shows that the hoop rupture strains of FRP jackets predicted by FE analysis using an elastic-perfectly plastic adhesive model are in reasonable agreement with the test results. The influence of parameters such as the FRP thickness, FRP orthotropy, FRP elastic modulus, adhesive yield strength, adhesive thickness, and column size are examined.

Stress-strain behavior of concrete columns confined with hybrid composite materials

A new type of concrete columns was developed at the University of Alabama in Huntsville for new construction to achieve more durable and economical structures. The columns are made of concrete cores encased in a PVC tube reinforced with fiber reinforced polymer (FRP). The PVC tubes are externally reinforced with continuous impregnated fibers in the form of hoops at different spacings. The PVC acts as formwork and a protective jacket, while the FRP hoops provide confinement to the concrete so that the ultimate compressive strength and ductility of concrete columns can be significantly increased. The volume of fibers used in this hybrid column system is very modest compared to other existing confinement methods such as FRP tubes and FRP jackets. This paper discusses the stress-strain behavior of these new composite concrete cylinders under axial compression loading. Test variables include the type of fiber, volume of fiber, and the spacing between the FRP hoops. A theoretical analysis was performed to predict the ultimate strength, failure strain and the entire stress-strain curve of concrete confined with PVC-FRP tubes. Test results show that the external confinement of concrete columns by PVC-FRP tubes results in enhancing compressive strength, ductility and energy absorption capacity. A comparison between experimental and analytical results indicates that the models provide satisfactory predictions of ultimate compressive strength, failure strain and stress-strain response.

Durability studies on concrete columns encased in PVC–FRP composite tubes

A new hybrid concrete column was developed for new construction. The proposed hybrid column, cast in place, consists of an exterior PVC–fiber reinforced polymer (FRP) shell with a concrete core. The exterior shell is commercially available cylindrical PVC pipe externally reinforced with impregnated continuous fiber in the form of hoops at different spacing. The proposed system (PVC–FRP) uses less fibers than current FRP confining methods but has similar strength and toughness characteristics. This paper present the results of an experimental study on the performance of hybrid concrete columns subjected to different environmental conditions such as room temperature, freeze and thaw, wet and dry conditions. Test variables included the type of fiber (carbon, aramid and glass), the spacing between the FRP hoops and the environmental exposure conditions. The specimens were subjected to 200 and 400 freeze/thaw and wet/dry cycles. At the end of each exposure, the specimens were instrumented and tested under a uniaxial compressive test load. The stress–strain behavior was used to evaluate the effect of exposure conditions on the strength, stress–strain behavior and ductility of the confined specimens. Test results show that the external confinement of concrete columns by PVC–FRP tubes results in enhancing compressive strength, ductility and energy absorption capacity. Durability tests show that the PVC–CFRP confined specimens performed quite well under the severe exposures used in this study, whereas the PVC–GFRP and PVC–AFRP confined specimens experienced reductions in both strength and ductility.

Design equations for concrete columns confined with hybrid composite materials

Retrofitting of existing concrete columns by wrapping them by fiber reinforced polymer (FRP) sheets, straps, or pultruded FRP shells has been gaining great interest in the research community and in industry. New hybrid concrete columns are developed in this study for new constructions. The basic structure of the proposed hybrid column consists of an exterior PVC-FRP shell with a concrete core. The exterior shell is commercially available cylindrical PVC pipe externally reinforced with impregnated continuous fiber in the form of hoops at different spacing. This paper presents the results of experimental and analytical studies of the performance of axially loaded concrete columns confined with PVC-FRP tubes. Test variables include type of confinement, volume of fiber and FRP hoop spacing. The results indicate that the new hybrid columns exhibited high strength and ductility. A new design procedure for concrete confined with PVC-FRP is proposed. Comparisons between experimental and theoretical results showed good agreement.