experimental study of frp confined concrete cylinders

Engineering Structures 28 (2006) 1001–1008www.elsevier.com/locate/engstruct

Experimental study of FRP confined concrete cylinders

Guoqiang Li∗

Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, United StatesDepartment of Mechanical Engineering, Southern University, Baton Rouge, LA 70813, United States

Received 3 June 2005; received in revised form 17 October 2005; accepted 1 November 2005Available online 4 January 2006

Abstract

In this study, two types of fiber reinforced polymer (FRP) confined concrete cylinders were prepared. One was FRP jacketed concrete cylinders;the other was FRP tube encased concrete cylinders. A total of 24 jacketed cylinders and 15 encased cylinders were prepared. For the jacketedcylinders, six fiber orientations and two FRP wall thicknesses were used; for the encased cylinders, four batches of concrete with normal tohigh strength were used and both bonded and unbonded interfacial conditions were considered. It is found that insufficiently confined cylindersbehave similar to unconfined cylinders. FRP cannot confine the concrete core until the concrete is damaged (cracked or crushed) due to the largertransverse Poisson’s ratio and lower axial stiffness of FRP. The rate of increase in confinement effectiveness decreases nonlinearly as confinementratio increases. A considerable deviation is found between the prediction by existing design-oriented confinement models and test results.c© 2005 Elsevier Ltd. All rights reserved.

Keywords: FRP; Concrete; Cylinders; Compression; Confinement model; Poisson’s effect; Stress–strain curve

1. Introduction

For the past decade, there has been a continuouslyincreasing interest in the use of fiber reinforced polymer (FRP)in the repairing, retrofitting, strengthening, rebuilding, andnew construction of columns in engineering structures [1].Currently, FRPs are primarily used for two types ofapplications. One is a thin layer of FRP jacket for the repairing,retrofitting, and strengthening of damaged concrete columns;the other is a tubular FRP case for rebuilding and newconstruction. The difference between repairing and encasingis obvious. In repairing, the FRP is wrapped onto the surfaceof an existing and sometimes damaged concrete column; onlya small axial load is transferred to the FRP jacket throughinterfacial bonding or interfacial frictional force. Because ofthis, the fibers are aligned primarily along the hoop direction.In encasing, the axial load is applied directly to the tube wallin addition to the concrete core, thus the FRP tube is actuallysubjected to an axial compression and a hoop tension. This

∗ Corresponding address: Department of Mechanical Engineering, LouisianaState University, Baton Rouge, LA 70803, United States. Tel.: +1 225 578 5302;fax: +1 225 578 5924.

E-mail address: [email protected].

0141-0296/$ - see front matter c© 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2005.11.006

represents a distinct departure from the repairing case andfibers are generally aligned at a certain angle from the hoopdirection [2]. It is noted that most real-world columns aresubjected to an eccentric axial load. The small eccentricity willintroduce a bending moment to the columns. Therefore, evenfor FRP repaired concrete columns, there may be a need touse some fibers in a direction other than the hoop directionto provide a certain bending resistance. Indeed, some studieshave been conducted to use fibers aligned along a certain anglefrom the hoop direction [3]. From the mechanics of compositematerials [4], the fiber orientation influences the confinementefficiency of FRP primarily in four ways: (1) hoop tensilestrength; (2) hoop tensile stiffness; (3) transverse Poisson’sratio; and (4) axial compressive stiffness. While the effect ofhoop tensile strength and stiffness on the confinement efficiencyhas been well established in various confinement models, ourunderstanding of the effect of the transverse Poisson’s ratioand axial compressive stiffness is still limited. More testing andanalysis are desired.

The current dominant repair approach using FRP is handlay-up technology. The FRP laminate is fabricated in situ.While construction guidance or specifications have been inplace to better control the quality of the product, it cannotbe avoided that some marginal laminates are fabricated in

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1002 G. Li / Engineering Structures 28 (2006) 1001–1008

practice, including (1) incomplete wet-out of the fabric anddifficult release of trapped air bubbles, (2) non-uniformity ofresin distribution, (3) low compaction and poor fiber alignment(wrinkling), and (4) unreliable control of resin cure. As aresult of the marginal laminate, the sufficient confinementused in design may not be realized in construction. Theconfinement may become insufficient. Therefore, there is aneed to understand the structural behavior of insufficientlyconfined concrete cylinders.

In FRP repaired or encased concrete column design, thekey is to develop a confinement model to predict the ultimatestrength and strain of the confined columns. Currently, theconfinement models can be divided into two categories: design-oriented or analysis-oriented [5]. Design-oriented models aredirectly based on interpretation or curve-fitting of test results;while analysis-oriented models are based on incrementalnumerical procedures by accounting explicitly for equilibriumand radial displacement compatibility. The design-orientedmodels can be expressed generally as [5–17]:

f ′cc

f ′co

=[

1 + k

(frf ′co

)m](1)

where f ′cc is the compressive strength of the confined concrete,

f ′co is the compressive strength of the unconfined concrete, fr

is the lateral confining pressure, ( f ′cc/ f ′

co) is the confinementeffectiveness, ( fr/ f ′

co) is the confinement ratio, k is theeffectiveness coefficient, and m is the power coefficient ofthe confinement ratio. fr can be found using the followingequation:

fr = 2 fFRPt

D(2)

in which fFRP, t , and D are the hoop tensile strength and wallthickness of the FRP shell and diameter of the concrete core,respectively.

It is noted that almost all the design-orientated confinementmodels are based on normal strength concrete with sufficientconfinement. Because high strength concrete is frequently usednowadays, there is a need to understand the structural behaviorof FRP tube confined high strength concrete cylinders. Also,there is a need to evaluate the performance of the currentconfinement models on high strength concrete cylinders andinsufficiently confined concrete cylinders.

While a number of studies have concluded that interfacialbonding strength between the FRP jacket and the concrete corehas little effect on FRP repaired concrete cylinders [14], thereis currently a lack of understanding of the interfacial bondingeffect on FRP tube encased concrete cylinders. This maybecome an issue, because the FRP tube is subjected to a biaxialstress condition—axial compression and hoop tension. Coupledwith the Poisson’s effect, the effect may be considerable.

In summary, the purpose of this paper is to understand(1) Poisson’s ratio and axial stiffness effect; (2) structuralperformance of insufficiently confined concrete cylinders; (3)the effect of concrete strength on confinement efficiency; (4) theperformance of current design-oriented confinement models,

Table 1Physical/mechanical properties of the raw materials

Materials Viscosity at25 ◦C (cps)

Tensile strength(MPa)

Modulus ofelasticity (GPa)

UV curing vinyl ester 450 85 3.2E-glass 7715 fabric – 3000 70.0

and (5) the interfacial bonding effect on FRP tube encasedconcrete cylinders.

2. Experiments

2.1. Raw materials

Type I Portland cement, gravel, natural sand, water, andDARAVAIR 1000 (an air-entraining agent) were used toprepare the concrete. The maximum coarse aggregate diameterwas 25.4 mm. Five batches of concrete were prepared. Themix design followed ACI Standard 211.1 (“Standard” 1991).The concrete was cast, compacted, finished, demolded, andcured for 28 days in an ASTM standard curing room with100% relative humidity. The 28-day compressive strengthof the unconfined concrete was determined using standard150.4 mm by 300.8 mm cylinders per ASTM C39. The 28-daycompressive strength for each batch was found to be 31.1 MPa,35.2 MPa, 46.1 MPa, 49.5 MPa, and 82.0 MPa, respectively.

A unidirectional E-glass 7715 fabric, which had an arealweight of 200 g/m2 and a thickness of 0.2 mm/ply, and anultraviolet (UV) curing vinyl ester were used to prepare the FRPjacket. The physical/mechanical properties of the E-glass fiberand UV curing vinyl ester resin provided by the manufacturersare given in Table 1.

The FRP tubes were obtained by cutting FIBERBOND R©20 HV composite pipes. The pipes were produced by windingvinyl ester wetted E-glass fibers onto a rotating mandrel, withfibers orientated ±54◦ from the axial direction. The pipeshad two wall thicknesses: 8.10 mm and 6.35 mm. The innerdiameter of the pipe was 101.6 mm. Each tube of 304.8 mmlong was cut from the composite pipe using a diamond saw.

2.2. FRP confined cylinder preparation

Concrete with a compressive strength of 46.1 MPa was usedto prepare FRP jacketed cylinders. The wet lay-up techniquewas used to wrap the FRP layers. This technique started withapplying a layer of resin (about 200 g/m2) to the surface ofthe concrete column. Next, the unidirectional E-glass 7715fabric of 304.8 mm long (axial) by 504.6 mm wide (hoop) waswrapped to fully cover the resin. A 25.4 mm extra length wasused in the hoop direction to provide an overlap. It is noted thatno roller was used. The viscosity of the resin was appropriatefor wetting through the fabric without excessive running. Onlya brush was used to help wrap the fabric. On the top of thefabric, another layer of resin was applied. This completed oneFRP repair layer. The procedure was repeated for applyingsubsequent layers. Because unidirectional E-glass fabric wasused in this study, it was easy to form various winding angles.

G. Li / Engineering Structures 28 (2006) 1001–1008 1003

Table 2Fiber orientation and FRP thickness for each group of jacketed cylinders

Group ID JG1 JG2 JG3 JG4 JG5 JG6 JG7 JG8

Fiber orientation 90◦/90◦ 60◦/30◦ 45◦/45◦ −45◦/45◦ 90◦/0◦ 0◦/0◦ 90◦/90◦/90◦/90◦ 0◦/0◦/0◦/0◦Cured FRP thickness (mm) 0.738 0.738 0.738 0.738 0.738 0.738 1.476 1.476

Note: 0◦ stands for axial and 90◦ for hoop.

Table 3Details of each group of FRP tube encased concrete cylinders

Group ID EG1 EG2 EG3 EG4 EG5

Unconfined concretestrength (MPa)

31.1 35.2 49.5 82.0 31.1

FRP tube wall thickness(mm)

8.10 6.35 6.35 6.35 8.10

Interfacial condition Bonded Bonded Bonded Bonded Unbonded

A total of 24 cylinders were prepared with six fiber orientations(0◦ stands for the axial direction and 90◦ for the hoop direction)and two thicknesses, as summarized in Table 2. In Table 2,each group contains three identical cylinders. The 24 jacketedspecimens were cured using UV-A fluorescent bulbs. Twentyminutes was used to cure two layers of FRP and forty minuteswas used to cure four layers of FRP. Similar curing time andcuring set-up have been used previously [18].

The FRP tube-encased concrete cylinders were preparedusing four batches of concrete, with compressive strength of31.1 MPa, 35.2 MPa, 49.5 MPa, and 82.0 MPa, respectively.The encased cylinders were fabricated by directly pouringconcrete into the FRP tubes. In order to consider the effect ofthe interfacial bonding strength on the structural behavior, threetubes with 8.10 mm wall thickness were modified using a thinfilm of polyethylene liner, separating the concrete core fromthe FRP tube, producing an “abnormal” unbonded interfacialcondition. A total of 15 cylinders were prepared. They wereequally divided into five groups. Details for each group aregiven in Table 3.

2.3. FRP coupon tests

In order to determine the mechanical properties of the curedFRP jacket, flat FRP coupons using the same raw materialsand the same curing procedure were prepared. All the couponswere 25.4 mm wide by 254.0 mm long beam laminates. Fourplys were used to fabricate each coupon. The cured thicknessof the coupons was 1.476 mm. Each end of the coupons wastabbed using 25.4 mm wide by 50.8 mm long aluminum tabs.Therefore, the gage length of each coupon was 152.4 mm.Three fiber orientations were used in preparing the coupons:0◦, 45◦ and 90◦ from the axial direction. Five coupons wereprepared for each fiber orientation. The cured coupons weretested using an Instron QUEST 150 machine to determine theirtensile strength, modulus of elasticity, and ultimate strain perASTM D 3039. The test results are given in Table 4.

For the FRP tubes, an internal pressure rating test per ASTMD 2992 was conducted using an Instron 3200 series internalpressure tester. In this test, the internal pressure was increasedcontinuously until the tube burst. The loading rate was

0.14 MPa/s. This type of test has been used previously [19].Three identical specimens were tested for each type of tube(wall thickness 8.10 mm and 6.35 mm). The average burstingstrength for the 8.10 mm thick tube is 186.0 MPa; it is 182.5MPa for the 6.35 mm thick tube.

2.4. Testing

Uniaxial compression tests were conducted on the confinedcylinders and their corresponding control cylinders, i.e.,cylinders without FRP confinement. During the compressiontest, each specimen was uniaxially compressed to about 40%of the ultimate load of the unconfined concrete and unloaded toguarantee close contact between each component and to reduceerrors in displacement measurement. Then, the specimen wasreloaded until failure. The compression tests were conductedusing a FORNEY machine. This machine has a capacity of2688 kN. The assembled computer data acquisition systemcan directly record the load–displacement curves. The test wasconducted according to ASTM C 39. The loading rate was0.23 MPa/s.

3. Results and discussion

3.1. Behavior of insufficiently confined cylinders

The sufficient or insufficient confinement can be quantifiedby Spoelstra and Monti’s confinement ratio criterion [20].The confinement ratio is defined as fr/ f ′

co, where f ′co is the

unconfined concrete strength and fr is the confinement pressureby the FRP, as shown in Eq. (2). According to Spoelstraand Monti, if the confinement ratio is smaller than 0.07, theconfinement is insufficient.

Using the geometrical parameters and coupon test results,the confinement ratios for representative cylinders are given inTable 5. From Table 5, the encased cylinders (EG1, EG2, EG3,and EG4) are sufficiently confined. For the jacketed cylinders,JG7 (with four-layer hoop FRP wraps) is sufficiently confined.JG8 (with four-layer axial FRP wraps) is insufficientlyconfined. For the cylinders with two-layer FRP wraps (JG1,JG3, JG5, and JG6), they are insufficiently confined. Althoughcoupon testing was not conducted for fibers aligned along30◦, −45◦, and 60◦ from the axial direction, and thus theconfinement ratios for JG2 and JG4 cannot be estimated, it issafe to predict that they are insufficiently confined because JG1(also with two-layer wraps but having the highest hoop tensilestrength) is insufficiently confined. Therefore, all the two-layerFRP jacketed cylinders are insufficiently confined.

The insufficiently and sufficiently confined concretecylinders behave differently. Fig. 1 shows the axial stress–axial


Table 4FRP flat coupon test results

Fiber orientation Ultimate strain (%) Tensile strength (MPa) Modulus of elasticity (GPa)Average Standard deviation Average Standard deviation Average Standard deviation

0◦ 2.32 0.20 320.2 7.6 15.1 0.845◦ 1.40 0.16 64.8 2.8 5.6 0.590◦ 0.85 0.12 39.3 1.1 3.9 0.4

Note: 0◦ stands for longitudinal and 90◦ for transverse.

Table 5Confinement ratios

Group ID JG1 JG3 JG5 JG6 JG7 JG8 EG1 EG2 EG3 EG4

Confinement ratio 0.067 0.014 0.038 0.008 0.135 0.017 1.97 1.40 0.99 0.60

Fig. 1. Typical axial stress–axial strain curves of various cylinders.

strain behavior of several representative groups of confinedcylinders and a group of unconfined cylinder. For sufficientlyconfined cylinders (JG7, EG2, EG3, and EG4), the axialstress–axial strain behavior can be represented by a bi-linear curve with a transition zone. A significant increasein compressive strength and ultimate compressive strain isachieved, as given in Table 6. For insufficiently confinedcylinders (JG3 and JG8), the structural behavior is similar tothe unconfined cylinder (with unconfined concrete strength 46.1MPa), with only a slight or almost no increase in the peakcompressive stress, as given in Table 6. Actually, the failuremode can further validate this observation. From Fig. 2, thecore of the JG4 (insufficiently confined) failed in a cone shape,similar to unconfined concrete cylinders; the core of the EG1,on the other hand, showed concrete crushing at cylinder failure,which is an indication of sufficient confinement.

Although all the FRP jackets with two-layer wraps areinsufficiently confined, their confinement ratios are different,depending on their fiber orientations. As fibers alignedmore and more toward the axial direction, the jackets havelower and lower hoop tensile strength, leading to lower andlower confinement ratio, as given in Table 5. Also, the fiberorientation influences the failure mode of the FRP jacket. Asshown in Fig. 3, the FRP jacket fails in the form of longitudinaltension, transverse tension or in-plane shear. From left to right,

(a) JG4

(b) EG1

Fig. 2. Failure modes of concrete core.

the specimens in Fig. 3 are JG2, JG3, JG4, JG1, JG5, and JG6.It is noted that the fiber angles marked in Fig. 3 have a 90◦difference from the fiber angles used in this study. For instance,the fiber angle 0◦/0◦ marked in Fig. 3 actually represents90◦/90◦ (JG1). Obviously, except for the hoop jackets, whichfail by longitudinal (hoop) tension, jackets with other fiberorientations fail by excessive transverse tensile stress or in-plane shear stress, as evidenced by the crack propagation path.

3.2. Poisson’s ratio and axial stiffness effect

Subjected to a uniaxial compressive load, both the core andthe jacket expand laterally due to the Poisson’s effect. Beforethe unconfined concrete strength is reached, Poisson’s ratio of


Table 6Summary of compressive strength and ultimate strain

Group ID JG1 JG2 JG3 JG4 JG5 JG6 JG7 JG8 EG1 EG2 EG3 EG4

Strength of confinedcylinders (MPa)

Average 49.4 46.1 46.8 46.7 48.7 47.6 54.6 48.9 141.2 152.1 175.5 180.2Standard deviation 1.2 0.9 0.9 1.1 1.3 1.0 0.6 1.1 4.2 4.3 4.6 3.8

Strain of confinedcylinders (%)


Strength of unconfinedcylinders (MPa)


Strain of unconfinedcylinders (%)


Fig. 3. Failure modes of FRP jackets.

concrete is about 0.18. Once the unconfined concrete strength isreached, extensive microcracks will be induced in the core andthe core will dilate laterally, with a significant increase in thecoefficient of lateral expansion [17]. Obviously, in order for thejacket to confine the core, the transverse (radial) Poisson’s ratioof the jacket should be smaller than that of the core. However,this is not the case before the unconfined compressive strengthof the core is reached. The transverse Poisson’s ratio of thejacket is larger than that of the core. This can be validated usingthe mechanics of composite materials. When it is subjected toan axial compressive stress σa , the transverse (out-of-plane)Poisson’s ratio of the FRP jacket νar (where subscript “a”denotes axial and “r” radial, as shown in Fig. 4) is [4]:

νar = Ea

(ν12

E1cos2 θ + ν23

E2sin2 θ

)(3)

where the modulus of elasticity in the axial direction, Ea , is:

1

Ea= 1

E1cos4 θ + 1

E2sin4 θ +

(1

G12− 2

ν12

E1

)sin2 θ cos2 θ

(4)

in which E1 is the longitudinal modulus of elasticity, E2 isthe transverse modulus of elasticity, G12 is the in-plane shearmodulus; ν12 is the in-plane major Poisson’s ratio; ν23 is theout-of-plane Poisson’s ratio; and θ is the fiber orientation withrespect to the axial direction.

Fig. 4. Coordinate system.

From the coupon test results given in Table 4, E1 =15.1 GPa and E2 = 3.9 GPa. During the coupon tests, thePoisson’s ratios ν12 and ν23 were not measured. Based onmany previous studies on E-glass fiber reinforced vinyl estercomposites, it is assumed that ν12 = 0.3 and ν23 = 0.35 inthis study. From Table 4, Ea = 5.6 GPa when θ = 45◦. UsingEq. (4), the shear modulus G12 is found to be 2.32 GPa. Basedon the above data preparation, the variation of the transversePoisson’s ratio νar and the ratio of the axial modulus (Ea)

to longitudinal modulus (E1) with the fiber orientation θ isshown in Fig. 5. The Poisson’s ratio of the concrete core is alsopresented for comparison.

When the axial strain is the same, the Poisson’s ratio νar

directly reflects the radial strain. From Fig. 5, the νar of the FRPjackets is larger than that of the concrete core. The differencebecomes larger and larger as the fibers align more and moretoward the hoop direction. Therefore, the FRP jackets usedin this study cannot confine the core unless the unconfined


Fig. 5. Variation of axial modulus ratio and transverse Poisson’s ratio with fiberorientations.

compressive strength of the concrete is reached, i.e., unless thecore is cracked or crushed with considerable lateral expansion.

In addition to the Poisson’s ratio effect, the axial stiffnessalso influences the radial expansion. From Fig. 5, the axialstiffness decreases as the fibers align more and more towardthe hoop direction. Usually, the modulus of elasticity of theconcrete core is about 30 GPa, which is much larger than theaxial stiffness of the FRP (in this study between 3.9 and 15.1GPa). Subjected to the same axial stress, the FRP jacket haslarger axial strain and thus larger transverse (radial) expansion,leading to poor or no confinement at all.

From the above discussion, both the transverse Poisson’sratio and the axial stiffness of the jackets cause larger radialexpansion when fibers are aligned toward the hoop direction.It is thus inferred that fibers aligned along the axial directionshould have the smallest lateral expansion. This may explainwhy the jacket with fibers aligned along the axial direction(for instance JG6) does not result in the lowest compressivestrength, as given in Table 6, although the jacket has the lowestconfinement ratio, as given in Table 5.

For FRP tube encased concrete cylinders, a similarquantitative analysis cannot be conducted because the coupontest results are incomplete. However, it is safe to predictthat similar mechanisms take effect. The FRP tube cannotconfine the concrete core before the unconfined concretestrength is reached due to the higher transverse Poisson’s ratioand smaller axial stiffness of the FRP tube. The typical bi-linear stress–strain behavior, as shown in Fig. 1, supports thisinference.

The inability for the FRP to confine the concrete core beforethe unconfined concrete strength is reached should be treated asa limitation of FRP because, in order to display the confinementeffect, the concrete must be “damaged” first. Although the FRPconfined concrete has been cited as having higher compressivestrength and ductility, it cannot be achieved unless there is aconsiderable axial strain, in the range of 0.02 or higher, whichis much higher than the current design strain 0.003, as specifiedin ACI 318.

Fig. 6. Comparison of test results with confinement models for jacketedcylinders.

3.3. Comparison with existing design-oriented confinementmodels

There are various design-oriented confinement modelsavailable based on Eq. (1). Some representative models arelisted in Table 7. The confinement effectivenesses as a functionof the confinement ratio for the listed confinement models areshown in Fig. 6 for jacketed cylinders and in Fig. 7 for encasedcylinders.

From Figs. 6 and 7, it is clearly seen that the existing design-oriented confinement models vary significantly. This is becauseeach model is based on an interpretation or curve-fitting of aparticular set of data. It is thus understandable that they behavedifferently. Therefore, there is a need to continuously expandthe current database.

Compared with the test results in this study, it is seen thatmost models overestimate the confinement effectiveness. Thismay be due to the fact that the FRP jacketed specimens wereinsufficiently confined and the FRP tube encased specimensused higher strength concrete. Both insufficient confinementand higher strength concrete may lead to lower confinementeffectiveness. For the jacketed cylinders, the model by Lam andTeng [15] and Lin and Liao [16] (model No. 9) yields the closestprediction; for the encased cylinders, models No. 3, No. 4, No.8, No. 9, and No. 10 yield reasonable predictions. It is noted thatmodel No. 9 is among the best models in both cases. Therefore,at least for the data examined in this study, which covers bothFRP jacketed and encased cylinders, various fiber angles andFRP wall thicknesses, and different concrete strength, modelNo. 9 is an acceptable model because it is based on a largerdatabase.

It is worth mentioning that, although model No. 9 yieldsa better prediction, it has a fundamental difference from


Table 7A list of various representative design-oriented confinement models

Model Richart et al.[6] and Fardisand Khalili [7]

Newmanand New-man [8]

CussonandPaultre [9]

KarbhariandGao [10]

Samaanet al.[17]

Toutanji[11]

Miyauchiet al. [12]

Saafi[13]

Chenget al.[14]

LamandTeng [5]

Lam and Teng [15]and Lin andLiao [16]

m 1.0 0.86 0.7 0.87 0.7 0.85 1.0 0.84 1.0 1.0 1.0k 4.1 3.7 2.1 2.1 3.38 3.5 2.98 2.2 2.4 3.3 2.0

Fig. 7. Comparison of test results with confinement models for encasedcylinders.

the test results. Model No. 9 shows a linear dependenceof confinement effectiveness on confinement ratio, i.e., theconfinement effectiveness increases linearly as the confinementratio increases. The test results from this study show thatthis dependence is nonlinear; see the dotted line in Fig. 7. Asthe confinement ratio increases, the rate of increases in theconfinement effectiveness is reduced.

3.4. Interfacial bonding effect

Groups EG1 and EG5 are exactly the same, except thatthe EG1 is bonded and the EG5 is unbonded by insertinga plastic sheet in between the FRP tube and the concretecore. Fig. 8 shows a typical axial stress–strain behavior ofboth types of cylinders. From Fig. 8, the bonded cylinder hasa considerably larger compressive strength and strain thanthe unbonded counterpart. Therefore, increasing the interfacialbonding strength has a positive effect. The reason for this isthat the FRP tube is subjected to a biaxial stress condition— axial compression and hoop tension. Both the axial stressand the hoop stress are considerably large. This presents adistinct departure from FRP jacketed cylinders, where the axialstress applied to the jacket is small. With a sufficient interfacialbonding strength, the force applied to the FRP tube can betransferred effectively to the concrete core and vice versa. In

Fig. 8. Axial stress–strain curves of FRP tube encased concrete cylinders.

addition, the bonded FRP tube can more effectively blunt,contain, and arrest the microcrack propagation within the core.All these contribute to increasing the compressive strength andductility of the encased cylinders.

4. Conclusion

Based on the test results and analysis, the followingconclusions are obtained:

• A limitation with FRP confinement is that the FRP cannoteffectively confine the concrete core before the unconfinedconcrete strength is reached, due either to its highertransverse Poisson’s ratio and/or to its lower axial stiffness.

• In order to fully display the confinement efficiency, theconcrete must work in a “damaged” status or with aconsiderable axial strain.

• The FRP jacket with axial fibers shows a slightly higherconfinement effectiveness than other angle-ply jackets(fibers oriented 30◦, 45◦, and 60◦ with respect to the axialdirection), due to its smaller radial expansion.

• Insufficient confinement results in almost no increasein strength. Insufficient confinement must be avoided inpractice. Otherwise, it is a waste of materials and may leadto premature structural failure.

• Most existing design-oriented confinement models overesti-mate the confinement efficiency. The rate of increase in con-finement efficiency decreases nonlinearly as the confinementratio increases.

• For the encased cylinders, higher interfacial bondingstrength results in higher cylinder compressive strengthand ductility. This is different from the jacketed cylinders,on which the interfacial bonding strength has little effect,probably due to lower axial stress in the FRP jacket.


Acknowledgments

This study was partially sponsored by the LouisianaBoard of Regents and EDO Fiber Science under contractnumber LEQSF(2004-07)-RD-B-05. The concrete cylinderswere prepared and the tests were conducted in the ConcreteLaboratory at the Louisiana Transportation Research Center(LTRC). The support of Mr. Randy Young, Mr. John Eggers,and Mr. Sadi Torres from LTRC is greatly appreciated.

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experimental study of frp confined concrete cylinders

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