External Fiber-reinforced Polymer 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.

Flexural Performance and Stress Calculation of External Prestressed Fiber-Reinforced Polymer-Bar-Strengthened One-Way Concrete Slabs.

External prestressing is widely employed in structural strengthening engineering due to its numerous advantages. However, external prestressed steel bars are prone to corrosion when exposed to the service environment. This paper is dedicated to examining the use of fiber-reinforced polymer (FRP) bars as external prestressing materials to strengthen one-way concrete slabs. Five one-way concrete slabs were strengthened with externally prestressed FRP bars with different prestress levels and different amounts of FRP bars, while one non-strengthened slab was used for comparison. The effects of strengthening on the flexural behavior, specifically the cracking load, ultimate load, stiffness and failure mode, were analyzed systematically. Moreover, the ductility and cost-benefit optimizing properties of the reinforcing design were discussed. The results show that external prestressed FRP bars significantly improve the cracking load, ultimate load and stiffness of one-way concrete slabs. The absence of a bond between the concrete and FRP bars overcomes the brittleness of the FRP bars, while the strengthened slabs exhibit satisfactory ductility and a higher post-yield stiffness and bearing capacity. Additionally, the cost/benefit ratio is optimized by increasing the prestress level, while a higher number of prestressed FRP bars is beneficial to ductility. Finally, a method for calculating the stress in prestressed FRP bars at ultimate loads was proposed. Irrespective of the prestressing material, this method is applicable to both strengthened beams and one-way slabs.

Docking unmanned multi-rotor platform for the diagnostic of building structures strengthened with fiber-reinforced polymer overlays

External fiber-reinforced polymer overlays have become the most popular method of building structures' strengthening on a global scale. Since no widely used and reliable methods for diagnosing the connection of overlays with the strengthened surface have been developed so far, developing such a method with the entire system, enabling easy and reliable use, has become a vital need of the construction sector. The paper presents an original infrared-based diagnostic module placed on a drone, enabling docking under the structure and conducting precise, uninterrupted diagnostics. The article describes the solutions and configuration of the automatic drone control system and the diagnostic module in all phases of its operation. The embedded measurement and control system has a multi-level structure distributed between the onboard automation system and the ground control station. Laboratory and field tests have shown that the road viaduct solution accurately reproduces joint damage, enabling the assessment of the structure's safety.

Open issues on the use of carbon composite tendons as prestressed reinforcement of concrete members

In the last decades, fiber-reinforced polymer (FRP) composites gained popularity in the field of structural engineering thanks to their high mechanical properties and limited corrosion in harsh environments. These composites are used as reinforcement of new concrete members as well as for strengthening of existing structures. For existing reinforced concrete (RC) structures, different techniques are available to increase the strength and stiffness of the existing elements. Among them, a promising technology consists in inducing to the element a bending moment opposite to that induced by the applied load by using external prestressed FRP tendons. This technique increases the load carrying capacity of the beams and induces concrete crack closure, thus reducing the exposure of steel bars to the environment.In this paper, a literature review is carried out to investigate the open issues that hinder the diffusion of prestressing made with composite carbon fiber tendons. According to the literature, the main issue is represented by the anchor system that should guarantee high stress level into the composite material avoiding local ruptures. Another open issue is the definition of the maximum stress applied to the composite with respect to its tensile strength. In fact, high levels of stress may induce premature composite failure. Finally, the exposure of CFRP bars to some harsh environmental conditions and high temperatures may negatively affect the reinforcement effectiveness.

Behavior of hybrid FRP-concrete-steel double-skin tubular beams with ultra-high strength concrete and PBL shear connectors under bending

Hybrid double-skin tubular beams (DSTBs) with the external fiber-reinforced polymer (FRP) tube-annular concrete-inner steel tube configuration have excellent mechanical performance and corrosion resistance, but are prone to slip problems. In this study, perfobond shear connectors (PBLs) are proposed to reduce the slip between infilled concrete and inner steel tube. Moreover, using ultra-high strength concrete (UHSC) as the annular material could further improve the structural performance and magnify the lightweight merit of hybrid DSTBs, while hybrid DSTBs with PBLs and UHSC have not been investigated yet. To this end, three-point bending experiments were conducted to hybrid DSTBs with PBLs and UHSC to study the flexural behavior. The mid-span load–deflection curves revealed a three-branched behavior: (i) a first linear ascending branch up to the first yielding at the extreme tension fiber of inner steel tube; (ii) a transition branch to the peak load with a significant slope deterioration when infilled concrete in the compression side reached the unconfined failure strain; (iii) a post-peak residual branch characterized by progressive load reductions over a number of load plateaus. The adoption of UHSC could significantly delay the plastic hinge formation in inner steel tube, and meanwhile improve the bond with PBLs. Thus, the slips between infilled concrete and inner steel tube were minimal (less than 0.2 mm) for most of the specimens, except for those with foamy sheet insertions between PBLs (at around 1 mm). In the end, the design loads of the hybrid DSTBs were predicted by a sectional analysis with high level of accuracy (average error at 6%).

Experimental and Analytical Study on the Axial Behavior of Circular High-Strength Concrete Columns with Hybrid Carbon Fibers and Steel Confinement System

This paper describes a work that examines a new solution to the problem that arises from the relatively high amount of transverse reinforcement required in HSC columns. It presents an alternative to common transverse steel reinforcement, a dual system comprising steel ties and a carbon-fiber mesh (CFM) applied internally together with steel ties. The behavior of the proposed system was examined in this work in a series of twelve laboratory tests of circular stub column specimens. The experiments performed in this work focused on the columns’ load and displacement capacities. The tests were planned with the aid of an analytical model that was originally developed for a hybrid system of external fiber-reinforced polymer (FRP) sheets and internal steel, and was adapted for the current system. An analysis of the results shows that for a given amount of conventional transverse steel, the application of the carbon-fiber meshes adds efficiency to the rebar confinement system, in terms of both the load bearing capacity and the ductility, and for specimens with the hybrid confinement system, the higher the carbon fiber amount the larger the ductility improvement. Furthermore, fair to good agreement was observed between the model and the measured stress–strain curves, especially those of the peak stresses. Based on the above findings and the added benefit of fire resistance, the hybrid method appears to be promising for confining HSC columns.

Damage and Defects in Fiber-Reinforced Polymer Reinforced and Strengthened Concrete Elements

Fiber-reinforced polymer (FRP) composites are becoming increasingly popular as an alternative to conventional civil engineering materials in existing structures as externally applied systems for strengthening purposes, and in new structures as internal reinforcements in the form of bars, meshes, and strands. The next phase of development in the application of FRPs in concrete structures could culminate in elements possessing both internal and external FRP systems simultaneously. With these advancements in the application of FRP in civil engineering, it is conceivable that traditional steel reinforcements could become obsolete in certain aggressive and hostile environments, such as coastal areas. Although higher durability and performance are associated with FRP materials in some respects when compared to steel, concerns remain regarding the damage and defects in this material, many of them related to their unique features. Regardless, similarly to other structural materials, it is necessary to understand the damage and defects associated with the use of FRP composites, as well as to identify their sources. This knowledge will support the positive prospects for FRP technology, promoting its application in civil infrastructure. Accordingly, this study investigated the types, characteristics, and identification of the observed or expected damage and defects associated with using FRP in reinforced and strengthened concrete elements. The damage was classified according to location and time of initiation. In addition, the sources of these defects and damage were determined, and a damage etiology was established for preventing the occurrence of such damage in future use. The results of this study are intended to provide the background for the development of a guide and training material for the inspection of structures that use FRP materials. The ability to inspect and properly maintain structures containing FRP will result in the enhanced durability and service life of concrete structures reinforced or strengthened with FRP materials.

Compressive behavior of FRP-confined cruciform steel-reinforced normal- and high-strength concrete columns

Fiber-reinforced polymer (FRP)-confined steel-reinforced concrete column (FCSRC), which consists of an external FRP tube and a steel-reinforced concrete (SRC) core, have been proposed to avoid the brittleness of SRCs. Capitalizing on the benefits of FRP-confine concrete, it is expected that full strength capacities of high-strength materials (i.e., high-strength concrete and high-strength steel) in FCSRCs can be exploited. Axial compression tests on FCSRCs with different strengths of concrete and steel were conducted in this study. A cruciform steel was adopted in FCSRCs. The parameters investigated in this study included the concrete strength, the steel strength and the FRP tube thickness. The test results demonstrated that the excellent performance of FCSRCs. Compared with normal-strength cross-shaped steel, the high-strength cruciform steel delays the development of hoop strains in the FRP tube for FCSRCs with normal-strength concrete. Additionally, the ultimate axial load of FCSRCs is generally larger than the direct summation of those of the steel section and FRP-confined concrete section (i.e., CFFT), demonstrating that the interaction between the three components (concrete, steel profile and FRP tube) in FCSRCs are in a benificial manner.

Reliable machine learning for the shear strength of beams strengthened using externally bonded FRP jackets

All over the world, shear strengthening of reinforced concrete elements using external fiber-reinforced polymer jackets could be used to improve building sustainability. However, reports issued by the American Concrete Institute called for heavy scrutiny before actual field implementation. The very limited number of proposed shear equations lacks reliability and accuracy. Thus, further investigation in this area is needed. In addition, machine-learning techniques are being implemented successfully to develop strength models for complex problems including shear, flexure, and torsion. This study aims to provide a reliable machine-learning model for reinforced concrete beams strengthened in shear using externally reinforced fiber polymer sheets. The proposed model was developed and validated against the experimental database and the very limited models in existing literature. The model showed better agreement with the experimentally measured strength compared to the previous models, which accounted for the effect of various parameters including but not limited to: the element geometry, strengthening details, and configurations. The model could guide the further developments of design codes and mechanical models.

Experimental and Numerical Study on Seismic Performance of PEN FRP-Jacketed Circular RC Columns

This paper presents an experimental study on the seismic behavior of polyethylene naphthalate (PEN) fiber–reinforced polymer (FRP)-jacketed circular reinforced-concrete (RC) columns. A total of seven specimens were tested under constant axial load and cyclic lateral load. The key parameters studied were the thickness (one, two, and three layers) and type (PEN FRP and CFRP) of FRP jacket. Test results indicated that the control column failed by buckling of the longitudinal reinforcement and shortage of ductility, while concrete spalling and bar buckling were effectively inhibited by the application of external FRP jacket. It is also observed that PEN FRP-jacketed specimens had more additional strain capacity compared with CFRP-jacketed specimens at the final condition. The additional strain capacity can serve as a safety reserve for structures and make PEN FRP more suitable for strengthening large or noncircular columns. Furthermore, it is found that 1-ply PEN FRP and 1-ply CFRP had a similar strengthening effect and thus the design of PEN FRP-strengthened column can simply refer to the design method of CFRP-strengthened columns in current codes. Different FRP thickness and types had marginal impact on the hysteretic curves of the specimens, which was attributed to the low axial load ratio (0.15) and high slenderness ratio (5.25) adopted in this paper. Finally, based on a cyclic stress–strain model for longitudinal reinforcement, including buckling effect, developed by the authors in a previous study, the test columns were simulated by the Open System for Earthquake Engineering Simulation (OpenSee)s to achieve an in-depth understanding of the experimental findings and associated strengthening mechanisms. Further parametric analyses showed that considering bar buckling played a significant role in accurately predicting the hysteretic response of FRP-jacketed columns.

Implementation of Modified Compression Field Theory to Simulate the Behavior of Fiber-Reinforced Polymer Shear-Strengthened Reinforced Concrete Beams under Monotonic Loading

The numerical modeling of structures is a widely preferable approach to investigate the structural behavior of RC beams since it delivers inexpensive predictions for confirming the required goals concurrently with reducing casting, testing time, and effort. Shear-strengthening of reinforced concrete (RC) beams using externally bonded (EB) fiber-reinforced polymers (FRPs) has attracted much attention due to the fact that the FRP strengthening technique has the ability to alter the distribution of stresses between the structural elements and increase the load-carrying capacity. A significant number of experimental studies have been carried out to test the monotonic behavior of FRP shear-strengthened RC beams. Conversely, limited numerical research has been performed to investigate such performance. The VecTor2 software is developed based on the modified compression field theory (MCFT) and is directed to examine the monotonic behavior of retrofitted specimens using fiber-reinforced polymer (FRP) composites. To the authors’ knowledge, the behavior of FRP shear-strengthened beams has not been explored in the literature using the MCFT modeling approach. The main objective of this study is to detect the software’s capability of predicting the experimental outcomes of FRP shear-strengthened RC beams. This research study is carried out in two stages. Initially, the numerical study involves the development of an accurate finite element model to simulate the control specimens. The quality of this model is assessed by comparing the numerical results with the experimental outcomes. In the second phase of the numerical study, the control beam model is modified to accommodate the presence of external FRP composites. The accuracy of this model is again measured by comparing its predictions with the experimental measurements. The goal of these phases is to ensure that the numerical model captures the actual behavior of the tested beams. Additionally, two distinctive modeling approaches are investigated to represent the behavior of FRP composites. The accuracy of the numerical models is verified through comparisons of numerical predictions to experimental results in terms of ultimate loading capacity, load–deflection relationships, and failure modes. It can be stated that the validated numerical model provides alternate means for evaluating the monotonic behavior of both strengthened and non-strengthened RC beams. The predicted results compare very well with the test results of the control specimens when discrete truss elements are employed for the FRP composites. Furthermore, the numerical model provides useful information on the crack patterns and failure modes.

Uplift Behaviour of External Fibre-Reinforced Polymer Wrapping on RC Piles in Dry and Submerged Sandy Soil

The sudden occurrence of an earthquake induces a liquefaction effect on foundation soil, which causes a substantial increase in the uplift pressure acting on piles and causes structural damage to superstructures. This forms the basis of the necessity of experimenting with the behaviour of piles subjected to uplift loads and predicting their load-carrying capacity or resistance. Fibre-reinforced polymer (FRP) wraps are widely used for strengthening and retrofitting piles subjected to damage. The current study is aimed at determining the uplift load-carrying capacity or resistance of piles wrapped with basalt fibre-reinforced polymer (BFRP) and glass fibre-reinforced polymer (GFRP) sheets by experiment. Preliminary tests were conducted to identify the influence of BFRP and GFRP wraps on the mechanical strength properties of concrete. The mechanical strength of the specimen with the double wrapping of basalt and glass fibres in the perpendicular direction outperformed all other specimens. Moreover, the piles were wrapped with laminates and experimented on for their uplift capacity in dry and submerged conditions. The results indicate a considerable improvement in the uplift resistance of the piles compared with the unconfined piles. The BFRP and GFRP wraps improved the uplift resistance of the piles by 35.56% and 15.56%, respectively, higher than the unconfined pile for dry conditions. The angle of the interfacial friction in dry and submerged states was observed to be the maximum for the perpendicular direction for both of the FRP wraps, and the failure modes were compared. The simulated model showed a significant correctness for determining the uplift resistance of FRP-wrapped piles in dry and submerged states. The degree of agreement in the dry condition for the experimental results and finite element method was more than 94% for all fibre wraps.

Seismic Performance of Recycled Aggregate Geopolymer Concrete-Filled Double Skin Tubular Columns with Internal Steel and External FRP Tube

The large production of cement is resulting in a high-carbon footprint, which is essential to minimize for sustainable concrete construction. Moreover, the large quantity of recycled coarse aggregate (RCA) from the demolition of old concrete structures is creating problems for landfill and disposal. The primary goal of this study is to investigate the seismic efficiency of innovative fiber-reinforced polymer (FRP)-recycled aggregate geopolymer concrete (RAGC) steel-tubed columns (FGSTCs) with an internal steel tube (STT), an external glass-FRP tube (GLT), and RAGC located between the two-tubed components to develop a serviceable structural element. To study their seismic functioning under axial load and lateral repeated load, five FGSTC specimens were manufactured and analyzed under quasi-static loads. The influence of three variables on the performance of FGSTC specimens, consisting of STT reinforcing ratio, compression ratio, and recycled coarse aggregate (RCA) replacement ratio, was investigated in this investigation. The produced specimens' ductility, hysteretic loops, strain distribution, skeleton curves, stiffness functioning, energy capacity dissipation, damaging functioning, and strength loss were all assessed and discussed. The results of this investigation revealed that percentage substitution of RCA had a minor impact on the seismic functioning of FGSTCs; however, the compression-load ratio depicted a substantial impact. The energy loss of the FGSTCs was 24.5% higher than that of their natural aggregate equivalents. FGSTCs may have a 16.9% lower cumulative failure rate than their natural aggregate counterparts.

Parametric Study on Steel–Concrete Composite Beams Strengthened with Post-Tensioned CFRP Tendons

A sustainable environment can be achieved by strengthening the existing building to avoid new construction and by replacing the construction materials with long-lasting sustainable materials such as a fiber-reinforced polymer (FRP). Using post-tensioned (PT) FRP systems has proven to be an effective technique in strengthening the structure and decreasing cracks and deformability. In this study, a 3-D finite element model was built to investigate the flexural behavior of composite beams strengthened with external PT FRP tendons. Limited research studied the use of FRP tendons to enhance the structural behavior of composite beams. This paper represents a comprehensive study of the effect of several parameters that control the design of the FRP tendons. Parameters such as PT level, tendon material, tendon length, degree of shear connection (DOSC), and tendon profile shape were considered under loading. The 3-D model’s correctness is validated using published experimental data. It was observed that of all FRP materials, carbon FRP is the best type for upgrading the beam strength, and it was recommended to use a 30 to 40% PT level. In addition, applying external PT over the full length of the beam increases the ultimate load capacity significantly. However, due to the difficulty of construction, it was recommended to use 90% of the beam span length since the difference in beam capacity does not exceed 5%. Finally, adding PT tendons with a trapezoidal and parabola profile to composite beams significantly increases the yield load and the beam capacity.

Vehicular Impact Resistance of FRP-Strengthened RC Bridge Pier

As-built bridge piers are likely to be subjected to vehicular collisions, causing severe damage to the bridge substructure and even resulting in the collapse of the entire bridge. This study aimed to evaluate the effectiveness of fiber-reinforced polymer (FRP) strengthening in enhancing the vehicular impact resistance of as-built bridge piers. A typical local, refined finite-element (FE) model of a simply supported two-span, double-column, reinforced-concrete (RC) bridge was established. The adopted material constitutive models and the FE analytical approaches were then validated through reduced vehicle model lateral impact tests on bare and carbon fiber–reinforced polymer (CFRP)-strengthened RC columns. The effective schemes for FRP strengthening of bridge piers against vehicle impact are discussed from the aspects of fiber orientation, number of layers, and strengthened height as well as by FRP type. Based on the FRP strengthening scheme, 48 vehicle–bridge collision scenarios were designed to evaluate the collapse of bridges with/without FRP-strengthened piers. It was found that (1) external FRP wrapping can effectively enhance vehicular impact-resistance of as-built bridge piers (e.g., reducing the degree of damage and deformation of the impacted pier); (2) for typical bridge piers, the effective strengthening scheme comprised a fiber orientation of 0° in reference to the circumferential direction, a 3-m strengthened height, and four-layer CFRP wrapping; (3) compared with bare bridge piers, FRP-strengthened piers could improve the vehicular impact-resistant safety redundancy of as-built bridges and, to some extent, avoid bridge collapses under truck collisions at high speed. Furthermore, through introducing the dynamic increase factors of concrete strength, reinforcement rebars, and FRP, and considering the nonuniform distributions of the strain rate of FRP-strengthened bridge piers caused by vehicular collision, the dynamic shear capacity of strengthened bridge pier was formulated.

Optimization of anchorage and deviator for concrete beams prestressed with external fiber-reinforced polymer tendons

This paper focuses on the optimization of anchorage and deviator for concrete beams prestressed with external basalt fiber-reinforced polymer (BFRP) tendons. A composite-wedge (CW) anchorage for BFRP tendons was first developed. The dimensions of the wedge were optimized using finite element (FE) analysis. Additionally, a three-dimensional deviator-tendon FE model was established, and the ratio of the curvature radius of the deviator to the radius of the tendon (Rd/Rp) and the bending angle (θ) of the BFRP tendon were optimized. Furthermore, an experimental study on the flexural behaviors of a non-prestressed concrete beam and a beam prestressed with external BFRP tendons was conducted. The results show that the proposed CW anchorage achieved an anchor efficiency factor of 85%. The recommended lower limit value of Rd/Rp is 200, and the upper limit value of θ is 5°. The RC beam prestressed with external BFRP tendons exhibited enhanced flexural behaviors, compared with the non-prestressed RC beam. The BFRP tendons at the anchor zones and tendons at the deviators remained intact at the ultimate load of the beam. The results validate the effectiveness of the anchorage and deviator parameters.

A versatile continuous model for predicting various post-peak patterns of FRP-confined concrete

The utilization of fiber reinforced polymers (FRPs) is currently recognized as an effective solution against concrete deterioration. The external FRP not only provides strength and durability to the concrete structure but also propels the improvement of the internal concrete performance due to the confinement effect. To accurately predict the true response of the concrete core represented in various patterns caused by the confinement effect, a versatile continuous model is inevitable for analyzing the mechanical performance of confined concrete. To achieve this, in the present study, a comprehensive database has been established and reviewed. The confinement mechanisms and four post-peak patterns in terms of stress–strain relationship to the intensification of confinement effect are summarized. The ultimate state predictions and the mathematical expressions of stress–strain models are evaluated. Accordingly, the strain efficiency factors of different types of FRPs, and the methodology for four major coefficient parameters have been proposed to develop a versatile continuous model for FRP-confined concrete. Notably, the proposed model is a derivable, and integrable single-segment expression, which is capable of describing all the post-peak patterns associated with different types of FRPs.

A State-of-the-Art Review of FRP-Confined Steel-Reinforced Concrete (FCSRC) Structural Members.

Fiber-reinforced polymer (FRP) composites have been widely used for strengthening or constructing structures due to their excellent corrosion resistance and high tensile strength. An emerging hybrid structural member form with FRP composites—which consist of a steel section as internal reinforcement, an external FRP wrap/tube, and concrete filled between them (referred to as FRP-confined steel-reinforced concrete (FCSRC) systems)—has attracted increasing research interest. To date, the concept has been adopted to strengthen/repair steel structures or used as new hybrid structural members (e.g., hybrid columns or beams, including buckling restrained braces (BRBs)). The FRP confinement and composite action between the three components in FCSRCs result in the excellent performance of the hybrid member. This paper presents a state-of-the-art review of FCSRCs for structural applications. The gaps in knowledge and future research opportunities on FCSRC structural members are also identified.

Effect of Internal Stirrups on the Eccentric Compression Behavior of FRP-Confined RC Columns Based on Finite-Element Analysis

Fiber-reinforced polymers (FRPs) have been widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can provide substantial lateral confinement to concrete. Conventionally, the presence of internal stirrups is neglected in the retrofitting design due to a lack of understanding on the effect of internal hoops when both external FRP and internal steel confinement exist, particularly under eccentric loading. In this study, a finite-element (FE) method is developed to investigate the eccentric compression behavior of FRP-confined RC columns. A reliable concrete plastic-damage model that accurately considers the confining stiffness ratio between the FRP and steel is proposed for the first time, which facilitates the study of a difficult problem: the behavior of RC columns with the dual confinement system under eccentric loading. The accuracy of the model is verified by test results. An extensive parametric study of various affecting factors is carried out to investigate the confinement mechanisms of the concrete columns in terms of the axial stress and confining pressure distributions, lateral principal stress ratio, local stress–strain response, and interaction between the FRP/steel hoops and concrete. It is found that the nonuniform confining pressures under eccentric compression are significantly affected by the internal stirrups. With the existence of discrete transverse steel reinforcement, the axial resistance of the concrete core is increased, thereby improving the ultimate load capacity of the columns. An ultimate axial load model for RC columns that considered combined FRP–steel confinement is subsequently proposed. More accurate results are obtained using the proposed model compared with those calculated from existing design codes.

Novel FRP micro-bar reinforced UHPC permanent formwork for circular columns: Concept and compressive behavior

In this study, a novel form of tubular permanent formwork that is made of ultra-high-performance concrete (UHPC) internally reinforced with fiber-reinforced polymer (FRP) micro-bars (herein referred to as FRP-UHPC tubular permanent formwork or simply FRP-UHPC tubular column) is developed. The axial compression test results of FRP-UHPC tubular columns with and without in-filled concrete are presented and discussed. Effects of the FRP micro-bar spiral pitch, the steel fiber addition in the UHPC, the tube thickness and the presence of external FRP confinment are investigated. The test results confirmed that the FRP-UHPC tubular columns have an excellent compressive strength, and the strength and ductility of existing concrete columns jacketed with an FRP-UHPC composite tube are substantially enhanced due to the confinement and axial contribution of the FRP-UHPC tube. The proposed FRP micro-bar-reinforced UHPC composite tubes are attractive in structural applications as pipelines or permanent formworks for columns, as well as external jackets (can be prefabricated in the form of two halves of composites tubes) for strengthening deteriorated reinforced concrete columns.