Fiber Reinforced Polymer Shell Research Articles

Thermal-vibration ageing characteristics of three thin-walled cylindrical shells covered with a functionally graded protective coating: Modeling, analysis and test

In this study, the thermal-vibration ageing characteristics of three thin-walled cylindrical shells (TCSs) covered with a functionally graded protective coating (FGPC) are investigated for the first time, including fiber reinforced polymer (FRP), metal and fiber metal hybrid (FMH) shell structures with the FGPC. Initially, a dynamic model of an FGPC-TCS structure in a thermal-vibration ageing environment is developed by employing the first-order shear deformation principle enriched by the improved exponential function approach, the complex modulus method, the Rayleigh-Ritz method, etc. In this model, the ageing-related material parameters of the FGPC and the TCS structure are assumed to be affected by both environmental temperature and aging time. After the solution procedures of the vibration parameters of the above three coated shells are clarified, the determining method of fitting coefficients of the coating and different materials in the shells studied is provided. Also, the detailed experimental validation is undertaken on the uncoated and coated FRP, metal, and FMH shell specimens, which indicates that the present model is reliable for predicting the key dynamic ageing parameters when the complex thermal-vibration ageing effect is considered. From the measured and calculated data, it is proved that the suppression contribution of the FGPC is obvious no matter whether these shell structures are at room temperature or thermal degradation environment. Furthermore, some important application suggestions of the FGPC are summarized to improve the degradation resistance of such coated shell structures.

Performance of damaged concrete stub columns strengthened with post-tensioned CFRP under freezing-thawing environment

Reinforced concrete structure with directly bonded fiber reinforced polymer (FRP) on the surface showed obvious stress hysteresis under load, which caused that the strengthening materials cannot be fully utilized. The strengthening system formed by carbon fiber reinforced polymer (CFRP) and shrinkage compensating self-consolidating concrete (ESCC) applied posttensioning stress to the external CFRP shell as a stay-in-place strengthening method, which eliminated stress hysteresis and exerted the restraint effect immediately after being loaded. This paper explored whether this method can be applied to concrete compression members damaged by freeze–thaw environment and its effect. Experimental variables include the number of freezing-thawing cycles, FRP shell plies and types of strengthening concrete. The experimental results showed that the damage of concrete under freezing-thawing environment was an incremental process: the dynamic elastic modulus loss rate and the mass loss rate of specimens damaged by 50 freeze–thaw cycles were 5.9% and 0.4%, respectively. When the freezing-thawing cycles reached 125, the dynamic elastic modulus loss of concrete compression members reached 24.8% and the mass loss rate was 5.3%. Compared with concrete compression specimens after 125 freeze–thaw cycles, the load capacity of specimens strengthened with single-ply and double-ply CFRP were increased by 63.1% and 114.1% respectively, which fully compensated the damage caused by freeze–thaw cycles. It was indicated that when single-ply CFRP was used for strengthening, ordinary self-consolidating concrete specimens exhibited higher hoop strain and less loss of ultimate load capacity than ESCC. When two-ply CFRP was used for strengthening, concrete compression members strengthened with ESCC showed higher strength and less loss of ultimate load-bearing capacity than specimens strengthened with ordinary self-consolidating concrete (SCC). With the increase of freezing-thawing cycles, the normalized inflection stress, intercept stress as well as ductility coefficient of strengthened specimens showed decreasing trend. The theoretical value of normalized peak stress and corrected strain were in good agreement with experimental value and it shows that the maximum error were 10.74% and 24.88%, respectively

Experimental study on hybrid concrete-filled fiber reinforced polymer tube (HCFFTs) columns under simulated seismic loading

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a promising alternative to conventional reinforced concrete (RC) for the construction of bridge columns. While unreinforced CFFT columns are attractive for their ease of construction, their limited energy dissipation capabilities have restricted their application to seismic regions. The hybrid CFFT (HCFFT) system was introduced to address this shortcoming by incorporating steel reinforcement into the FRP shell in the form of 30-μm fibers. This system combines the ease of construction of conventional unreinforced CFFTs with the energy dissipation characteristics of lightly reinforced CFFT columns. This study reports the first experimental investigation on the seismic performance of HCFFT columns through large-scale testing of two columns under combined axial compression and cyclic lateral loading. Two shear loading conditions were studied. The flexural capacity, mode of failure, residual deformation, and energy dissipation capacity were evaluated. The low-shear specimen reached a drift ratio of 10.5% and the high-shear specimen reached a drift ratio of 6.5%. The specimens both reached displacement ductility demands above 5, demonstrating the system is appropriate for use as a bridge column in seismic regions. The curvature profiles for both specimens showed the hybrid shell prevented localized yielding of the steel fibers, which allowed for an efficient distribution of plasticity along the column height and enhanced displacement ductility. The results indicate that HCFFT columns can achieve high inelastic deformations under simulated seismic loading without substantial degradation of stiffness. It is anticipated that this large-scale experimental data will inform future research on the design and manufacturing process of HCFFT elements and encourage further experimental and analytical studies.

Experimental investigation of a simple shear connection to concrete-filled FRP tube (CFFT) columns

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a robust alternative to conventional reinforced concrete (RC) for column construction. Despite the superior structural performance of this column system in terms of durability, strength and ductility, its application in building construction is limited due to a lack of viable designs for beams-to-column connections. To address this gap, this study proposes a simple connection for a circular CFFT column and a steel I-shape beam. The connection consists of a curved end plate with a shear tab that is attached to the finished CFFT using commercially available concrete anchors. In this study, three connection assemblies with adhesive and expansion anchors were tested under half-cyclic loading. The connection assemblies varied based on the type of the anchors and installation torque. The performance was evaluated in terms of ease of construction, shear capacity, moment-rotation relationship, and observable damage. The FRP shell eliminated concrete breakout failure and the anchors exceeded the theoretical pullout capacity noted by the manufacturer. A methodology for estimating the proof load of the proposed connection detail is presented. The proposed connection provides a viable option for simple connection of steel beams to circular CFFT columns. The findings may inform future research on beam-column connections for CFFTs and lead to an increased use of CFFT systems in building construction.

Static and sustained loading behavior of a basalt FRP shell–concrete composite bridge deck: An experimental and numerical study

The fiber reinforced polymer (FRP)-concrete composite bridge deck is expected to be a competitive alternative to the reinforced concrete bridge deck owing to its great durability, low weight, and high loading efficacy. Based on previous studies, an optimized basalt FRP shell (BFRP)–concrete composite bridge deck was investigated experimentally and numerically in this study. Through the construction simulation test, the optimized BFRP shell was proved to be a qualified formwork with a 30.9% lower midspan deflection than the BFRP shell of previous studies and was lower than 1/800 of the net span. In the static test, the load capacities of deck specimens with depths of 200 mm and 180 mm reached 470 kN and 340 kN, respectively, with residual deflections lower than 7 mm. Meanwhile, the deck showed decent collaborative behavior, without visible interfacial debonding or slippage during the static test. Compared with the previously studied bridge deck, the bridge deck of the current study can achieve a higher short-term cost performance. After 175 days of three-point flexural sustained loading, the additional midspan deflections were 0.3 mm, 0.37 mm, and 0.46 mm, for load levels of 0.2, 0.25, and 0.3, respectively, which were less than 7% of their instantaneous elastic deflections. The finite element (FE) model was established using the Bailey-Norton model for the creep of the FRP and Bazant’s B-3 model for the creep of the concrete. The FE results were in good agreement with the experimental results, both in short- and long-term scenarios. Using numerical simulation with a load duration of 50 years, the allowable load level of this bridge deck reached 0.4, which meets the criteria for creep deflection in the ACI standard.

Flexural behavior of hybrid concrete-filled fiber reinforced polymer tube columns

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a durable alternative design for bridge columns. Research has shown that utilizing a small number of longitudinal steel bars in the concrete core provides the energy dissipation characteristics required for resisting extreme loads, such as seismic and blast. However, the construction complexity associated with the addition of rebar discourages the implementation of this system. In a novel approach, this study proposes and investigates the structural performance of CFFTs with a hybrid glass/steel fiber reinforced tube. This hybrid CFFT (HCFFT) system integrates the required longitudinal steel reinforcement into the FRP shell in the form of 30-µm fibers. This study details the experimental results from two CFFT and four HCFFT circular specimens tested under half-cyclic four-point bending. The effect of concrete slippage inside the tube was investigated. The flexural capacity, residual deformation, damage, and energy absorption capacity were experimentally evaluated. The HCFFT specimens exhibited higher energy absorption and lower degradations in stiffness compared to the CFFTs at comparable flexural deformations. Moment-curvature analysis was used to quantify the contribution of the concrete core when it is allowed to slide inside the tube.

Influence of fiber orientation and shell thickness on the axial compressive behavior of concrete-filled fiber-reinforced polymer tubes

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) present a viable alternative column system to conventional reinforced concrete for new construction. A CFFT system consists of two parts: the prefabricated FRP tube and the concrete core. The exterior shell provides longitudinal and transverse reinforcement for the concrete core. Many design variables of the FRP shell, including its thickness and winding angle, can be altered to achieve desirable confinement, axial capacity, and ductility in CFFT columns. This research proposes a methodology to determine the minimum fiber angle for filament wound FRP tubes to provide confinement to the concrete core in CFFTs. The proposed approach was used to calculate the threshold winding angle for glass FRPs, which was found to be ±51.5°. To evaluate the proposed fiber angle threshold, 11 circular specimens with a concrete core diameter of 152 mm, a height of 597 mm, and glass fibers at angles of ±45°, ±55°, or ±65° were tested under cyclic axial compression. The effect of fiber angle and thickness of the shell were studied. The experimental results show that the level of confinement increases with winding angle and number of layers of fiber reinforcing, and the specimens with wind angles below the threshold, i.e. ±45°, behave similarly to unconfined concrete.

Development of Lateral Prestress in High-Strength Concrete-Filled FRP Tubes

This paper reports on an experimental investigation into the axial and lateral strain development of fiber reinforced polymer (FRP) confined high-strength concrete (HSC) with prestressed FRP shells. A total of 24 aramid FRP (AFRP)-confined concrete specimens were manufactured as concrete-filled FRP tubes (CFFTs) with instrumentation to measure the strain variations during application of prestress, removal of end constraints and progressive prestress losses. Prestressed CFFT specimens were prepared with three different dose rates of expansive mineral admixture to create a range of lateral prestress applied to AFRP tubes manufactured with sheet thicknesses of 0.2 or 0.3 mm/ply and referred to as lightly- or well-confined, respectively. In addition to these three levels of prestress, non-prestressed companion specimens were manufactured and tested to determine baseline performance. The experimental results from this study indicate that lateral prestressing of CFFTs manufactured with HSC can be achieved by varying the expansive mineral admixture dose rate with a lateral prestress of up to 7.3 MPa recorded in this study. Significant strain variations were measured during removal of the end constraints with up to 700 microstrain recorded in the axial direction. Finally, the measurement of prestress losses for the month following prestress application revealed minimal progressive losses, with only 250 and 100 με recorded for the axial and hoop strains, respectively.

Durability assessment of hybrid FRP composite shell and its application to prestressed concrete girders

This paper presents the results of an experimental-analytical research program aimed at assessing the durability of a hybrid fiber-reinforced polymer (FRP) composite shell, incorporated as a protective barrier during construction to impede the ingress of deleterious elements into the lower flange of a prestressed concrete girder. A series of tests were performed on the FRP composite shell and FRP-concrete specimens to examine the effectiveness of the FRP shell as a barrier against ingress of aggressive elements, such as moisture and chlorides in controlled environments. In addition, scanning electron microscopy (SEM) was used to determine the effects of aging, temperature variation and chloride diffusion in the FRP-concrete specimens. These experimental results were used to model the ingress of chlorides and moisture into the FRP-concrete system to predict the service life of prestressed concrete bridge girders reinforced with FRP composite shells. The results showed that the FRP composite shell can provide significant resistance against chloride ingress by impeding the rate of its ingress and the total amount of chloride ions, thus considerably help in retarding the resulting deterioration reactions and significantly enhancing their service life.

Influence of shrinkage on compressive behavior of concrete-filled FRP tubes: An experimental study on interface gap effect

Fiber reinforced polymer (FRP)-confined concrete has received significant research attention over the last two decades, with recent experimental studies identifying significant benefits of FRP-confined high-strength concrete (HSC). However, studies examining the influence of concrete shrinkage on the behavior of FRP-confined concrete remain limited, with no studies examining this effect for HSC specimens. Concrete shrinkage may pose a concern for concrete-filled FRP tubes (CFFTs), as in these members the curing of concrete takes place inside the FRP tube. This paper presents an experimental investigation on the influence of FRP-to-concrete interface gap, caused by concrete shrinkage, on axial compressive behavior of FRP-confined normal- and high-strength concrete (NSC and HSC). A total of 30 aramid FRP (AFRP)-confined concrete specimens with circular cross-sections were manufactured. Six of the specimens were instrumented to monitor long term shrinkage strain development of the FRP-confined NSC and HSC, with three specimens allocated to each mix. The remaining 24 specimens were tested under axial compression, where nine of these specimens were manufactured with NSC and the remaining 15 with HSC. The influence of concrete shrinkage was examined by applying a gap of up to 0.12mm thickness at the FRP-to-concrete interface, simulating 1600 microstrain of shrinkage in the radial direction. The results of this experimental study indicate that the influence of interface gap on axial strain enhancement is significant, with an increase observed as the gap increased. Conversely, the influence of interface gap on axial strength enhancement is found to be small with a slight reduction observed with increased gap. The results also indicate that an increase in interface gap causes an increase in strength loss during the transition region of the stress–strain curve as a result of the delayed activation of the FRP shell. Finally, it was observed that an increase in FRP-to-concrete interface gap leads to an increased difference between axial deformation of the concrete core and FRP shell, indicating the presence of interface slip.

A Simple Design-Oriented Model for FRP-Confined High-Strength Concrete

This paper presents a study on the axial compressive behavior of fiber reinforced polymer (FRP)-confined high-strength concrete (HSC). A large experimental test database assembled from the published literature was used to investigate and quantify factors influencing the compressive behavior of FRP-confined HSC. The database consisted of 976 test data having unconfined concrete strength ranging from 6.2 to 169.7MPa. Based on the analysis results of the database, it was found that the threshold confinement stiffness increases significantly with an increase in concrete strength, which in turn adversely affects the strength enhancement of confined concrete. It was also observed that the hoop rupture strain of FRP shell decreases with an increase in concrete strength. Existing confinement models that are applicable to FRP-confined HSC were assessed using the database. Finally, a new simple design-oriented model for FRP-confined HSC developed on the basis of the database is presented.

Confinement Model for FRP-Confined High-Strength Concrete

It is well understood that confining concrete with fiber-reinforced polymer (FRP) composites can significantly enhance its strength and deformability. However, the confinement demand of concrete increases proportionally with its strength, resulting in higher confinement requirements for high-strength concrete (HSC). This paper reports on a study on the axial compressive behavior of FRP-confined HSC. A large experimental test database, which consists of 237 axial compression tests results for FRP-confined HSC, was assembled from the published literature and presented in this paper. This database was augmented with another database of FRP-confined normal-strength concrete (NSC), which consists of 739 test results. The combined database of 1063 test results, which cover specimens with unconfined concrete strengths ranging from 6.2 to 169.7 MPa, was used to investigate and quantify the factors that influence the compressive behavior of FRP-confined HSC. Analysis of the test results reported in the database indicates that the confinement requirement increases significantly with an increase in concrete strength, which adversely affects the observed strength enhancement through confinement. In addition, it was also observed that the hoop rupture strain of the FRP shell decreases as the concrete strength increases. Many existing stress-strain models developed for FRP-confined concrete were assessed by using the HSC database. A close examination of the results of the model assessment led to many important conclusions regarding the strengths and weaknesses of existing stress-strain models. Finally, a novel design-oriented model for FRP-confined concrete is presented that was developed on the basis of the database summarized in the paper. It is shown that the proposed model performs significantly better than any of the existing stress-strain models of FRP-confined concrete in predicting the ultimate conditions of FRP-confined HSC.

Experiments on a Hybrid-Composite Beam for Bridge Applications

This paper details a study of the structural behavior of hybrid-composite beams (HCBs), which consist of a fiber-reinforced polymer (FRP) shell with a tied concrete arch. The HCB offers advantages in life-cycle costs through reduced transportation weight and increased corrosion resistance. Through a better understanding of system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces. A long-term outcome of this research will be a general, structural analytical framework, which can be used by transportation departments to design HCBs as rapidly constructible bridge components. This study focused on the identification of the load paths and load sharing between the arch and FRP shell in an HCB and on the test of an HCB with a composite bridge deck. Tests were performed through the application of point loads on simple span beams (before the bridge deck was placed) and with a three-beam, skewed composite bridge system, which resulted in strain data for the arch and FRP shell. The test results showed that strain behavior was linear elastic at service loads, and the FRP shell had a linear strain profile. Curvature from strain data was used to find internal bending forces, and the proportion of load within the arch was found. A stress integration method was used to confirm the internal force contributions. The arch carried about 80% of the total load for the noncomposite case without a bridge deck. When composite with a bridge deck, the arch made a minimal contribution to the HCB stiffness and strength, because most of the arch was below the neutral axis and cracked under the maximum live load expected for the bridge. For this composite case, the FRP shell and prestressing strands resisted about 80% of the applied load, while the bridge deck carried the remaining 20% to the end diaphragms and bearings.

콘크리트 충진 FRP Tube의 실험적 휨거동 분석

본 연구에서는 FRP 재료를 활용한 구조시스템인 콘크리트 충진 FRP 튜브 시스템을 대상으로 동특성 시험 및 정적 휨재하 시험을 실시하여 구조거동을 실험적으로 분석하였다. 실험체는 원통형 FRP 쉘에 철근을 삽입하고 콘크리트가 충진되어 합성된 하이브리드 구조체이다. 실험 프로그램은 2개의 단계로 구분하였다. 첫번째 단계는 콘크리트 충진 FRP 튜브 시스템을 대상으로 동특성 실험을 실시하여 고유진동수 및 모드를 분석하였으며, 두번째 단계는 하이브리드 형태의 대상 구조물의 구조거동 및 파괴형태를 파악하기 위해 4점 재하실험을 실시하고 파괴거동을 분석하였다. 그 결과, 콘크리트 충진 FRP 튜브시스템은 다양한 재료가 사용된 하이브리드 시스템이나 전형적인 보 구조물의 동특성과 파괴거동을 나타내고 있음을 확인하였다. 콘크리트 충진 FRP 튜브는 FRP 재료의 고강성, 고강도 등의 구조적 거동을 보일 뿐만 아니라 철근콘크리트의 파괴거동과 유사한 연성파괴 형태를 나타내었다. This study presents the experimental investigation of concrete filled FRP (Fiber Reinforced Polymer) tube system. A new structural system was developed that combines a FRP shell with a reinforced concrete core for use as flexural members in civil structural applications. The experimental program involved two phases. First, the dynamic characteristics were identified. The second phase involved the measurement of the nonlinear static behavior until failure. The failure mode of FRP only was known to be brittle. However, the results of the proposed system showed that the failure mode was ductile as the system was a hybrid system with reinforced concrete. The proposed system takes advantages of structural advantages of FRP structure as well as the failure mode of reinforced concrete material.

Development of damage-controlled latter cast FRP–RC hybrid girders

A type of fiber reinforced polymer (FRP)–reinforced concrete (RC) hybrid structure as one kind of novel construction with integrated structural performances is addressed. The FRP–RC hybrid structure designed under a damage-controlled methodology is composed of external hybrid FRP shells and a RC core. The bonding of interior walls of the hybrid FRP shell and cast RC core is realized through epoxy resins under wet condition. The external hybrid FRP shell consists of carbon FRP with high strength and high modulus as longitudinal reinforcements and glass FRP with high ductility as both longitudinal and shear reinforcements. In addition, the external hybrid FRP shell provides the concrete core and steel rebars with an effective protection against corrosion. A series of 4-point bending experiments are performed to study the structural performance of FRP–RC hybrid girders designed and fabricated under the concept of damage-controlled methodology. Based on the bending theories, the flexural properties of the developed structures are also evaluated. The investigation reveals that the damage of FRP–RC hybrid structure is controllable and that the wet bonding is effective.

Concrete column shape modification with FRP shells and expansive cement concrete

Fiber Reinforced Polymer (FRP) composites are an effective material for strengthening circular concrete columns. Confinement of square and rectangular columns is not as effective as confinement of circular columns. The flat sides of square and rectangular columns remain largely unconfined and the FRP jacket is effective only along the two diagonals of the cross-section. Shape modification of square and rectangular columns to circular and elliptical columns is implemented. Two methods are used to achieve shape modification: (a) modifying the cross-section with non-shrink grout and applying an FRP jacket using wet layup, and (b) filling the annular gap between the original section and a prefabricated FRP shell with expansive cement concrete. Several mix designs of expansive cement and Portland cement were used for obtaining the desired level of expansion. An analytical model and a finite element model are used to predict the stress–strain behavior of the test specimens. Parametric studies regarding the optimal geometry of shape-modified cross-sections are presented utilizing the analytical model.

Improved shell finite element for piezothermoelastic analysis of smart fiber reinforced composite structures

Present article deals with the development of an improved eight noded layered shell finite element formulation for piezothermoelastic analysis of smart fiber reinforced polymer (FRP) composite shell structures with bonded piezoelectric sensors and actuators. Stress resultant-type Koiter's shell theory has been used and twist curvature component has been incorporated to keep the strain equations complete. The transverse shear effect has also been considered according to Mindlin's hypothesis. Pyroelectric effect has been considered in the formulation. The developed formulation has been observed to give accurate results for both deep and shallow shells and will be useful for analysis of smart FRP shell structures. It is observed that pyroelectric effect has a significant influence on the response of such shells under piezothermoelastic loading. Different types of smart shell panels viz. spherical, ellipsoidal, doubly curved and cylindrical have been analyzed and the coupled thermo-electro-mechanical responses have been presented.

Genetic algorithm based optimal control of smart composite shell structures undermechanical loading and thermal gradient

In the present paper an improved genetic algorithm (GA) based linear quadratic regulator(LQR) control scheme has been proposed for active vibration control of smart fiberreinforced polymer (FRP) composite shell structures under combined mechanicaland thermal loading. A layered shell finite element formulation has been done toobtain the electro-thermo-mechanical response of fiber reinforced polymer (FRP)composite shell structures bonded with piezoelectric patches. Based on the responsesobtained from finite element analysis, a real coded GA based improved LQRcontrol scheme has been incorporated, which maximizes the closed loop dampingwhile keeping the actuator voltages within limit. It has been observed that thedeveloped FE code can be used for determination of the accurate response ofsmart FRP shell structures for the simulation of active vibration control of suchstructures. The proposed GA based LQR control scheme could control both dynamicoscillation due to mechanical load as well as the static displacement due to athermal gradient, which was not possible with conventional LQR control scheme.

Confinement Model of Concrete with Externally Bonded FRP Jackets or Posttensioned FRP Shells

A design-oriented confinement model for square and rectangular columns confined with bonded fiber-reinforced polymer (FRP) jackets, and shape-modified square and rectangular sections confined with posttensioned FRP shells is developed. The proposed design model for FRP-confined concrete columns is based on the bilinear four-parameter formulation by Richard and Abbott. The axial compressive strength of FRP-confined concrete is obtained using concrete plasticity theory based on the five parameter Willam and Warnke model. The ultimate axial strain of FRP-confined concrete is obtained from a strain-based approach depending on either passive or active confinement; the ultimate axial strain is based on the concepts of secant concrete modulus and strain dependent stiffness established by Pantazopoulou and Mills. Comparisons of the proposed stress-strain model with uniaxial compression experiments for columns with bonded FRP jackets or posttensioned FRP shells, performed by the writers and other researchers, show satisfactory agreement for the entire stress-strain curve.

Posttensioned FRP Composite Shells for Concrete Confinement

Experiments have shown that externally bonded fiber-reinforced polymer (FRP) jackets for square and rectangular columns are not as effective as they are for circular columns. The results of experiments on shape-modified concrete columns using posttensioned FRP shells are presented. Posttensioning was achieved by radially straining the precured FRP shell outwards to a substantial strain level, using expansive cement concrete, over a period of 60days. The prefabricated FRP shell was also used as a stay-in-place formwork. The effectiveness of shape modification using posttensioned FRP shells is compared to FRP-confined original square and rectangular columns, as well as shape-modified columns with nonshrink grout and externally bonded FRP jackets. It is shown that shape modification with posttensioning of FRP shells, using expansive cement concrete, can change the confinement from passive to active and improve significantly the axial strength and ultimate compressive axial strain capacity of square and rectangular columns.