BEHAVIOR OF CONCRETE CYLINDERS CONFINED

BY CARBON-COMPOSITE TAPES AND PRESTRESSED YARNS

1. EXPERIMENTAL DATA

V. Tamuzs,* R. Tepfers,**

Chi-Sang You,*** T. Rousakis,****

I. Repelis,* V. Skruls,* and U. Vilks*

Keywords: concrete, cylindrical specimens, composite confinement, compressive loading, plasticity, damage,

strength

The results of a comprehensive test program for the mechanical behavior of round concrete specimens con-

fined by carbon-fiber epoxy tapes and prestressed carbon yarns are reported. Five different concrete batches

of compressive strength from 20 to 100 MPa and the confinement of various thickness and pretension level

were investigated. The specimens, which were subjected to monotonic or repeated compressive loadings,

showed a nonlinear stress–strain behavior with a significant ductility and increased ultimate strength, in con-

trary to the brittle behavior of plain concrete. The limit of linearity on the stress–strain diagrams of the con-

fined concrete roughly coincided with the ultimate strength of plain concrete. Above this limit, the damage

accumulation and plastic deformations proceeded in the confined specimens. This fact was evidenced by the

increasing slope of deformation diagrams in unloadings and repeated loadings and by the pronounced resid-

ual strains. The limit of linearity could be raised significantly by pretension of the carbon yarns during their

winding.

Introduction

During the last decades, the field application projects have been concentrated on the repair and reinforcement of con-

crete columns by using composite materials. The repair and strengthening of reinforced concretes are some of the applications

where the FRP (fiber-reinforced plastic) reinforcement serves better than the conventional steel one. The use of composite ma-

terials can upgrade civil engineering structures and can be the most effective step to introduce composites into the common

civil engineering practice. Many industrial companies and university research groups have been active in research and demon-

stration works to assess the performance, durability, and cost of the composites for the repair and strengthening of reinforced

concrete structures [1-5].

130191-5665/06/4201-0013 © 2006 Springer Science+Business Media, Inc.

*Institute of Polymer Mechanics, University of Latvia, Riga, Latvia. **Chalmers University of Technology, Göteborg,

Sweden. ***Pohang University of Korea, visiting the Institute of Polymer Mechanics. ****Dept. of Civil Eng., Demokritus Uni-

versity of Thrace, Xanthi, Greece. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1,

pp. 21-44, January-February, 2006. Original article submitted November 21, 2005.

Mechanics of Composite Materials, Vol. 42, No. 1, 2006

FRP is being utilized for the flexural and shear strengthening of reinforced concrete, masonry, steel, and wood struc-

tures, but one of their most attractive applications is the use in confining concrete columns. The FRP reinforcement is linearly

elastic up to the final failure and exerts a growing confining pressure on the concrete core. This behavior results in a consider-

ably increased strength and ductility of concrete when loaded axially.

As a confinement of concrete, FRP can be used in the form of yarns, wraps, or tubes. In yarn winding, resin-impreg-

nated yarns are wound around columns by using mechanical devices. In the manual strengthening of columns, one or more lay-

ers of FRP tapes are glued to the resin-impregnated column surface. In our paper, experimental results obtained in testing round

concrete specimens of height 0.3 m and diameter 0.15 m are presented. The program comprised tests of five concrete batches

with different compressive strengths, confined by a unidirectional carbon-fiber composite of three different thicknesses. Tests

on the plain concrete were also performed. The influence of yarn pretension was investigated for all the five concretes, which

were confined by a carbon yarn at three different pretension levels. Altogether, 60 concrete specimens were tested.

Characteristic of Concrete and the CFRP used as Confinement

Composite properties. The concretes were confined by wrapping with a CFRP tape and by winding with carbon

yarns. As the carbon tape, the unidirectional Grafil Inc. 340-700 carbon reinforcement by was used, whose mechanical proper-

ties, as indicated by the supplier, are: strength 4500 MPa, elastic modulus 234 GPa, density 1.8 g/cm3, elongation 1.9%, yarn

diameter 7 �m, and thickness 0.17 mm.

For the pretensioned confinement, carbon filament yarns were used. The yarn was PANEX 33 continuous carbon fi-

bers 7.2 �m in diameter produced by Zoltek. The mechanical properties of the yarn are: strength 3800 MPa, elastic modulus

228 GPa, density 1.8 g/cm3, average cross-sectional area of tow 1.86 mm2, fiber diameter 7.2 �m, and yield 278 m/kg.

The carbon reinforcements were impregnated with an epoxy resin consisting of two components — the base resin NM

BPE Composite System 417 and the hardener NM Härdare 417B, which were mixed in the ratio 100/44 by weight.

14

TABLE 1. Results of CFRP-Ring Tensile Tests

SpecimenStrength Reduction

factorUltimatestrain

Reductionfactor

Elastic modulus ReductionfactorkN/cm MPa kN/cm GPa

1-layer (à) 3.1 1827 0.41 0.0094 0.50 359 196 0.84

1-layer (b) 3.4 1985 0.44 0.0095 0.50 330 205 0.88

2-layer (à) 8.5 2506 0.56 0.0101 0.53 834 254 1.09

2-layer (b) 7.7 2272 0.50 0.0109 0.57 728 207 0.88

3-layer (à) 13.9 2730 0.61 0.0108 0.57 1277 252 1.08

3-layer (b) 13.9 2592 0.58 0.0121 0.64 1144 220 0.94

Yarn winding 16.8 1807 0.48 0.0085 2050 218 0.96

TABLE 2. Mechanical Properties of Plain Concrete Specimens

BatchDensity,kg/m3

Cubicstrength, ÌPà

Compressivestrength fc,

ÌPà

Ratio of the round tocubic strength

Ultimateaxial strain,

�cz

Ultimatelateral strain,

�c1

Elasticmodulus E,

GPaPoisson ratio �

20 2282 34.2 20.45 0.60 0.0026 –0.0003 24.40 0.14

40 2361 60.5 40.04 0.66 0.0017 –0.0007 34.03 0.17

60 2393 76.2 44.34 0.58 0.0017 –0.0006 37.83 0.19

80 2400 81.4 49.22 0.60 0.0017 –0.0007 36.65 0.18

100 2407 104.1 61.56 0.59 0.0018 –0.0004 39.09 0.19

It is well known that the strength of unidirectional composites depends not only on the strength of fibers, but also on

their volume fraction and adhesion [6]. Therefore, the true strength of the confinement was determined in a split-disc test ac-

cording to the ASTM standard D 2290.

The test specimens were fabricated in the shape of rings with a diameter of 150 mm, equal to that of the round concrete

specimens, and a width of 20 mm. For the wrapped confinements, three kinds of specimens (one, two, and three layers of the

CFRP tape) were prepared with a 150-mm overlap length. For the wound confinement, only one specimen was prepared by

hand-winding with a carbon yarn (5 cycles/cm) impregnated with the base resin. The results of the CFRP split-disc tests are

shown in Table 1.

The reduction factor is the ratio of the experimental tensile property of CFRP to the tensile property of CFRP in uniax-

ial tension following from the technical data given by the manufacturer. The strength given in the first column of Table 1 is the

direct test result. The second column was obtained assuming the cross section of carbon fibers per one cm according to the data

of the manufacturer. The intensity of winding was five yarns on one cm — the same as for the actual confined specimens. The

results obtained indicate that the strength of carbon fibers realized in the confinement was only about half of the fiber strength

indicated by the manufacturer, whereas the elastic moduli roughly coincided.

15

� �0 001 0 0.001 0.002 0.003

70

60

50

40

30

20

10

0

�33, MPa

A

BC

D

E

A

BC

D

E

�33� ��� ��

Fig. 1. The compressive behavior of cylindrical specimens of initial concretes of batches 100

(A), 80 (B), 60 (C), 40 (D), and 20 (E).

Winding

Tensioning

Feeding

1

2

34

P

Fig. 2. Winding scheme: bobbin (1); application of resin (2); pretension weight (3); winding on

concrete cylinders (4).

Concrete Mixtures and Specimens

Five different concrete mixtures were designed to obtain strengths ranging from 20 to 100 MPa, so that five batches

could be distinguished by the concrete strength. For the mix, Portland cement and a granite aggregate with the maximum gravel

size of 16 mm were used. For fabricating the 80 and 100 MPa concretes, silica fume and some additives were used. The addi-

tives were the Sikament-56 superplasticizer and the Sika retarder, and both were added as a percent of the binder (cement and

silica fume). All the mixtures were prepared within a period of three weeks. After casting, every batch was placed into water for

one week and then cured under laboratory conditions (20C and 50% relative humidity) until testing, which was run after a

lapse of at least one month.1

Test Results for Plain Concrete Specimens

A total of 15 cylindrical specimens of plain concrete were tested in a monotonic compressive loading. The specimens

were divided into five batches of different strength, with three identical specimens in each.

All the cylindrical specimens were of diameter 150 mm and height 300 mm, but the cubic specimens had the size 150 �

150 � 150 mm.

In Table 2, a comparison between the compressive strength of concrete cylinders with teflon interlays at their ends

(four teflon sheets) and that of standard concrete cubes with a side length of 150 mm without an interlay is given. It is seen that

the strength of the round specimens with the interlays was about ~60% of the cubic strength. Typical stress–strain curves for the

round cylinders of all the plain concretes considered are shown in Fig. 1, where �33 is the axial stress, �33 is the axial strain,

and � �11 22 is the lateral strain.

16

Fig. 3. Experimental setup of a 5000-kN press and the data acquisition system.

TABLE 3. Values of the Parameter k for all the Tape-Confined Specimens Tested

ConfinementConcrete batches

20 40 60 80 100

1-layer 0.0213 0.0153 0.0138 0.0141 0.0133

2-layer 0.0426 0.0306 0.0276 0.0282 0.0266

3-layer 0.0639 0.0459 0.0414 0.0423 0.0399

1 All the cylindrical and cubic concrete specimens were produced at the Chalmers University of Technology by T. Rousakiswithin the frame of EC Network ConFibreCrete and then transported to the Institute of Polymer Mechanics for confining andtesting.

Most of the plain concretes showed a brittle behavior, except the specimens of batch 20, and failed with splitting in the

vertical direction, which may be explained by abrupt propagation of an internal crack into the axial direction with lateral dila-

tion. At the beginning of loading, there was no sign of internal damage. As the load grew, a sound caused by generation of an in-

ternal crack was heard, and the secant modulus decreased slightly. In the behavior of most plain concretes, the lateral strain in-

creased just before the final failure, which was caused by the growth of internal cracks. Then the full failure occurred with

vertical cracking.

Preparation of Confined Concrete Specimens

Before wrapping with a carbon-fiber tape, the surface of concrete specimens was treated — a primer resin was applied

to the cleaned concrete surface, so that a high performance of the CFRP confinement be achieved. A water-based NM

17

D

� � �0 010 0.005 0 0.005 0.010 0.015

A

B

C

A

B

C

70

60

50

40

30

20

10

0

àà�33, MPa

�33� ��� ��

100

80

60

40

20

0� � �0 010 0.005 0 0.005 0.010 0.015

A

B

C C

B

Abb

D

�33, MPa

�33� ��� ��

� � �0 001 0.005 0 0.005 0.010

120

100

80

60

40

20

0

A

BC

A

BC

cc

D

�33, MPa

�33� ��� ��

A

BC

A

BBC

� �0.0050 0.0025 0 0.0025 0.0050 0.0075

100

80

60

40

20

0

dd

D

�33, MPa

�33� ��� ��

� �0 004 0 002 0 0 002 0 004. . . .

120

100

80

60

40

20

0

AB

C C

BA

ee�33, MPa

�33� ��� ��

D

Fig. 4. Compressive behavior of the concretes of batches 20 (à), 40 (b), 60 (c), 80 (d), and 100

(e) confined by three (A), two (B), and one (C) layers of CFRP under monotonic loading. D —

the initial concrete.

Grundering BP50 SUPER resin type, wetted with a NM Härdare 50 hardener, was used. After the treatment of concrete sur-

faces, a layer of the main NM BPE Composite System 417 resin was coated on them. Then, a carbon tape was attached to the

concrete surface. During the application, the tape was pressed and slightly stretched. In such a way, full impregnation and the

elimination of waving and air voids occurring underneath the tape were ensured. Another layer of the impregnation resin was

applied to the outer surface of the carbon tape and served as an underlay for the next tape or as a protective exterior resin lay.

Each lay was applied to the concrete in a continuous way. The overlap area of the external layer was impregnated with a special

care. The specimens after wrapping were left to stand under laboratory conditions (20C and 50% relative humidity) at least for

a week, so that the resin-carbon tape system be hardened completely. Specimens with one, two, and three layers of the carbon

tape were prepared.

In the winding process, the main epoxy resin was the same as that used in wrapping, namely NM BPE Composite Sys-

tem 417. The continuous reinforcement in the form of yarns was fed from a creel. The yarn from the creel went to a forcing de-

18

20 40 60 80 100

120

100

80

60

40

20

0

fcc, MPa

batch

àà

bb

20 40 60 80 100

3.5

3.0

2.5

2.0

1.5

1.0

f fcc c/

batch

cc

20 40 60 80 100

0.010

0.008

0.006

0.004

0.002

0

� lcc

batch

Fig. 5. Comparison of compressive strengths (a), normalized compressive strengths (b), and

ultimate lateral strains (c) of concretes confined by one (o), two ( ), and three ( ) layers of

CFRP in compression loading. (n) — initial concrete.

vice, where a weight creating the designed pretension level was suspended on the yarn, and then the winding process was con-

ducted over the rotating concrete specimen (Fig. 2).

The resin was applied to the concretes surface and yarns before winding. The placement of the yarns was controlled by

means of specially designed machines traversing at a step up to 4 mm per rotation. At the beginning of winding, the yarns were

fastened to the end of rotating disk. After the entire concrete surface had been covered two times, including the return path

19

� � �0 010 0.005 0 0.005 0.010 0.015 0.020

50

40

30

20

10

0

àà�33, MPa

� ��� �� ���

� � �0 010 0.005 0 0.005 0.010 0.015 0.020

70

60

50

40

30

20

10

0

bb�33, MPa

� ��� �� ���

� � �0 010 0.005 0 0.005 0.010 0.015 0.020

80

70

60

50

40

30

20

10

0

cc�33, MPa

� ��� �� ���

Fig. 6. Compressive behavior of the concrete of batch 20 confined by one (a), two (b), and three

(c) layers of CFRP under repeated loadings. (m) — stress–strain curves in monotonic loading.

(The compressive strain and stress are shown as positive quantities and tensile as negative.)

(which resulted in a final winding step equal to 2 mm), the end of the yarn was tied to its other end without losing the pretension

force. This process was conducted very carefully lest the yarns be damaged in the course of winding.

The regular tensile force applied to the yarn during winding was about 70 N, which corresponded to a weight P equal

to 14 kg. The maximum pretension level of the yarn was chosen equal to 400 N, which corresponded to the weight P = 80 kg. In

such a way, three winding confinements with a yarn step equal to 2 mm on the cylindrical concrete surface and yarn pretension

forces of about 150, 300, and 400 N were produced. Such pretension levels corresponded to pretension of the confinement

equal to 0.75, 1.5, and 2 kN/cm.

20

� � �0 010 0.005 0 0.005 0.010

90

80

70

60

50

40

30

20

10

0

bb�33, MPa

� ��� �� ���

� � �0 010 0.005 0 0.005 0.010

70

60

50

40

30

20

10

0

àà�33, MPa

� ��� �� ���

� � �0 010 0.005 0 0.005 0.010 0.015

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

Fig. 7. The same for the concrete of batch 40.

Methodology of Experiments

In our test, a vertical hydraulic column-testing machine with a capacity of 5000 kN (Fig. 3) was used, which had been

checked and certified by the Latvian Certification Agency before the test program was carried out.

The specimens were centered on the plate of the loading machine to avoid any loading eccentricity. Four teflon sheets

of dimensions 200 � 200 mm were used to reduce the friction between specimen end faces and the loading plates. Three linear

variable displacement transducers were employed to determine variations in the measuring length, which was equal to 200 mm.

For measuring the axial and lateral strains, four strain gages of type PL-60-11 (by Tokyo Sokki Kenkyujo Co., Ltd.)

with a resistance of 120 were glued with a P-2 adhesive to each specimen after a proper treatment of the rough surface of the

21

� � �0 004 0.002 0 0.002 0.004 0.006

90

80

70

60

50

40

30

20

10

0

bb�33, MPa

� ��� �� ���

������ ������ � �0.004 0.002 0 0.002 0.004 0.006

80

70

60

50

40

30

20

10

0

àà�33, MPa

� ��� �� ���

� � �0 008 0.004 0 0.004 0.008 0.012

120

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

Fig. 8. The same for the concrete of batch 60.

confinement. Two of the strain gages were located perpendicularly to each other in order to measure the axial and lateral strains

at the specimen midheight, and the other two were arranged on the opposite side. The strain gages were placed away from the

CFRP sheet overlap layer to acquire reliable measurement data.

All signals from the strain gages were amplified by a Spider 8 amplifier system, and then the data were collected by a

Cat software.

The loading rate was 10 MPa/min according to the ASTM C 39/C39M-99 standards. The cyclic response of the

CFRP-sheet-confined concrete was investigated by applying load-unload cycles related to the compressive strengths of plain

concrete, fc , and confined concrete, fcc . The load in each cycle was increased successively to 0.5 fc , 0.8 fc , fc , fc + 0.33( fcc –

fc), fc + 0.66( fcc + fc), and finally up to the full failure. For the case of concrete wound with pretensioned yarns, the load lev-

22

� � � �0 003 0.002 0.001 0 0.001 0.002 0.003 0.004

100

80

60

40

20

0

bb�33, MPa

� ��� �� ���

������ ������ � �0.004 0.002 0 0.002 0.004 0.006

90

80

70

60

50

40

30

20

10

0

àà�33, MPa

� ��� �� ���

� � � �0 006 0.004 0.002 0 0.002 0.004 0.006 0.008

120

100

80

60

40

20

0

�33, MPa

� ��� �� ���

cc

Fig. 9. The same for the concrete of batch 80.

els were assigned with only fc , followed by 0.5 fc , 0.8 fc , fc , 2 fc , 3 fc ,… until the total failure. The loading rate in the repeated

and monotonic loadings was the same.

Experimental Results

a) Concrete confined by CFRP tapes. Monotonic loading. The experimental investigations were performed on

concrete specimens confined by CFRP tapes. In all, 30 specimens (150 mm in diameter and 300 mm in height) with different

concrete strengths and volumetric ratios of carbon layers ranging from 0.45% (one layer) to 1.35% (three layers) were tested.

The efficiency of the confinement can be characterized by the parameter

23

� � � �0 006 0.004 0.002 0 0.002 0.004 0.006

120

100

80

60

40

20

0

�33, MPa

� ��� �� ���

cc

� �0.002 0.001 0 0.001 0.002 0.003

100

80

60

40

20

0

bb�33, MPa

� ��� �� ���

� �0.002 0.001 0 0.001 0.002 0.003

90

80

70

60

50

40

30

20

10

0

àà�33, MPa

� ��� �� ���

Fig. 10. The same for the concrete of batch 100.

kE

E

h

R

c

b

c ,

where hc and Ec are the thickness and the elastic modulus of the composite confinement, Eb is the elastic modulus of concrete,

and R is the radius of specimens; k � � if the confinement is absolutely stiff, and k = 0 in the absence of confinement. Using

data of the carbon sheet and the elastic modulus of different concrete batches, the values of k were obtained, which are shown in

Table 3.

24

�0 01 0 0 01 0 02 0 03. . . .

120

100

80

60

40

20

0

bb�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015 0 020 0 025 0 030. . . . . . . .

120

100

80

60

40

20

0

àà�33, MPa

�� ������ ���� ���

� �0 01 0 0.01 0.02 0.03

120

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

Fig. 11. Compressive behavior of the concrete of batch 20 wound with carbon yarns under

repeated loadings. Tensioning levels are 80 (a), 160 (b), and 210 MPa (c). (m) — stress–strain

curves of the initial concrete. (The compressive strain and stress are shown as positive

quantities and the tensile ones as negative.)

From the test results it follows that the strength increased 1.3-3.1 times, the axial strain 1.5-7.8 times, and the lateral

strain 5-26 times in comparison with the corresponding values for the plain concrete. Figure 4 shows the compressive behavior

of the tape-confined concrete samples.

In the early loading stage, up to the elastic limit, the behavior of the confined concretes was almost the same as that of

the plain concretes. The difference in the elastic modulus and the Poisson ratio was negligible.

When the load exceeded the elastic limit, a sound caused by the initiation of internal damage of concrete was heard,

the slope of the deformation diagram increased, and the stress–strain relationship in loading became nonlinear. Thus, the con-

fined concretes displayed a distinct nonlinear response, with a transition zone at the level of ultimate strength of the unconfined

concrete.

25

�0 01 0 0 01 0 02 0 03. . . .

160

140

120

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

�0 01 0 0 01 0 02 0 03. . . .

160

140

120

100

80

60

40

20

0

bb�33, MPa

�� ������ ���� ���

�0 01 0 0 01 0 02 0 03. . . .

160

140

120

100

80

60

40

20

0

àà�33, MPa

� ��� �� ���

Fig. 12. The same for the concrete of batch 40.

The same CFRP-sheet confinement seems to give about the same strength rise for all the concretes considered, as seen

in Fig. 5a, which also means that it is less effective for stronger concretes, Fig. 5b.

It could be expected that the specimens would fail at the same lateral strain of the CFRP sheet as that measured in the

ring tests (Table 1). However, the measured lateral strains at the failure of cylinders were less than those in the ring tests and

showed a wide scatter, which can be explained by the improper failure modes occurring in some cases (the failure began close

to specimen ends, away from the gage of lateral strains). The nonuniform pressure from the fragmented concrete within the

CFRP confinement, leading to stress concentrations, might also cause the increased scatter and the early failure.

b) Repeated loading. Two specimens of the same type in each batch were prepared for testing in monotonic and re-

peated compressive loadings, respectively. The load levels were assigned based on the results of previous tests, namely the

26

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

160

140

120

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

160

140

120

100

80

60

40

20

0

bb�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

160

140

120

100

80

60

40

20

0

àà�33, MPa

� ��� �� ���

Fig. 13. The same for the concrete of batch 60.

compressive strengths of plain concrete fc and the confined concrete of the same type fcc , followed by 0.5 fc , 0.8 fc , fc ,

f f fc cc c� �033. ( ), f f fc cc c� �066. ( ) and up to the final failure. Figures 6-10 show the compressive behavior of concrete

specimen confined by CFRP tapes under repeated load-unload cycles for all the concretes considered. For a better comparison,

the stress–strain relationships for both the loading modes (monotonic and repeated) are presented in the same figure.

Under repeated loading, the envelope stress–strain curves usually followed the curves of monotonic loading (Figs.

6-10). Only some specimens showed a small degradation in the strength and ductility as a result of the improper fracture of

specimens. The unloading below the yield point revealed no changes in the elastic modulus, but some decrease in it was ob-

served above the elastic limit during unloading.

27

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

180

160

140

120

100

80

60

40

20

0

cc�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

180

160

140

120

100

80

60

40

20

0

bb�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015 0 020. . . . . .

180

160

140

120

100

80

60

40

20

0

àà�33, MPa

� ��� �� ���

Fig. 14. The same for the concrete of batch 80.

The experimental graphs obtained give a basis for estimating the plastic deformation and the change in the characteris-

tics of concrete under the lateral compression caused by the confinement. The corresponding model of plasticity coupled with

damage will be the subject of next papers.

c) Repeated loading of concrete cylinders confined by a prestressed composite shell. The pretension of a yarn was

achieved by suspending a weight on it, as shown in Fig. 2. The maximum suspended weight applied for pretension level 3 was

80 kg, which gave the yarn a tensile stress of 210 MPa. The weights of 30 and 60 kg were used for the level 1 and 2 tensile

stresses 80 and 160 MPa, respectively. The volumetric ratio of carbon yarns in all the confined concretes was 2.48%, and they

were divided into five batches according to the concrete strength. Thus, the tests were performed altogether on 15 specimens of

5 concretes, with three different pretension levels in each batch. The specimens were compressed under a successively increas-

28

� �0 010 0 005 0 0 005 0 010 0 015. . . . .

180

160

140

120

100

80

60

40

20

0

c�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015. . . . .

180

160

140

120

100

80

60

40

20

0

b�33, MPa

� ��� �� ���

� �0 010 0 005 0 0 005 0 010 0 015. . . . .

180

160

140

120

100

80

60

40

20

0

à�33, MPa

� ��� �� ���

Fig. 15. The same for the concrete of batch 100.

ing repeated load, assigned based on the compressive strength of plain concretes, followed by 0.5 fc , 0.8 fc , fc , 2 fc , 3 fc , …. up

to the final failure. The results obtained are shown on Figs. 11-15 for all the concrete batches tested.

The behavior of plain concrete specimens on Figs. 8-12 is marked by open dots. It is remarkable that the limit of linear-

ity for the specimens significantly exceeds the strength of plain concrete, in contrary to that for the specimens confined by a car-

bon composite tape without pretensioning (see for comparison Fig. 4).

The pretensioning did not give a detectable growth in ultimate strength. The reason for this is that the ultimate strength

of confined concrete is determined by the strength of the composite shell; but this strength depends on the thickness of confine-

ment, which was the same for all the pretension levels used.

The measured ultimate lateral strain (Fig. 16) for the prestressed specimens occured in the range of that measured in

the ring tests, Table 1. The data scatter was less than that observed for the CFRP-tape-confined specimens (Fig. 5c). The pre-

29

20 40 60 80 100

200

180

160

140

120

100

80

0

fcc , MPa

batch

à

20 40 60 80 100

6

5

4

3

2

1

f fcc c/

b

batch

20 40 60 80 100

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0

�lcc c

batch

Fig. 16. Comparison of compressive strengths (a), normalized compressive strengths (b), and

ultimate lateral strains (c) of concretes wound with tensioned carbon yarns at various tensioning

levels: low ( o ), moderate ( ), and high ( ).

stress of the composite equalizes the stretch in fibers, whereas the manual wrapping with tapes most likely leads to

nonuniformly stressed fibers under a load.

Repeated Loading After Tearing off the Composite Shell

The acoustic sound arising in the confined specimens at a compression stress exceeding the elastic limit points to the

formation of damage in concrete. The damage level can be estimated by the decreased elastic modulus and the residual

strength. Special tests were carried out to register these changes. The reduction in the modulus of the confined specimens can

be obtained from loading-unloading curves (Figs. 5-15), but for determining the residual strength of concrete, it is necessary to

test the damaged concrete after tearing off the composite confinement. A specimen after the removal of the shell is shown in

Fig. 17. Two special tests were carried out on cylindrical concrete specimen with initial cubic strengths 50 (batch 50) and 25

MPa (batch 25).

A confined specimen was loaded above the limit of linearity without reaching the total failure. Then the specimen was

unloaded, the composite shell was cut and torn off, and the specimen was prepared for a repeated compression test, which was

continued up to the complete failure. The results obtained are displayed in Fig. 18, where the registered stress–strain behavior

of the undamaged plain concrete from the same batch is also seen. Curves 1 and 2 show the behavior of two different confined

specimens, and curves 3 and 4 correspond to the unloading and repeat loading of the specimen after tearing off the confine-

ment. Curve 5 shows a typical stress–strain diagram in compression for the undamaged plain concrete.

It is remarkable that reduction in the elastic modulus for both the specimens is roughly the same, and the ultimate com-

pression strain after the damage is the same as for the undamaged material, or even slightly greater.

30

Fig. 17. A concrete specimen after the first loading above the limit of linearity and tearing off

the composite shell.

Summary and Conclusions

The objective of this study was to investigate the compressive behavior of concrete confined by CFRP. The investiga-

tions were carried out on concretes of five different strengths, and the varied parameters of the confinement were the number of

layers for wrapping with a carbon tape and the pretension level for winding with pretensioned carbon yarns.

From test results for the CFRP-sheet-confined concretes, an increase in the strength and ductility was seen for all the

confined concretes, which resulted in a higher load-carrying capacity. The confined concretes displayed a distinct nonlinear re-

sponse with a transition zone around the ultimate strength of the unconfined concrete.

The maximum increase in strength caused by the confinement was obtained for the concrete specimen of batch 20

wrapped with three layers of a tape with a normalized strength of 3.08; this specimen showed the maximum ratio of lateral

stress to the axial stress. The normalized effect of confinement, at the same number of CFRP tapes, decreased with increasing

concrete strength, which is natural, because the ultimate strength of a specimen is determined by the strength of the composite

confinement.

The confined concretes failed by fracture of the FRP confinement, which occurred at a lower lateral strain of concrete

specimens than the ultimate uniaxial tensile strain of carbon yarns indicated by the producer. The CFRP-ring test (ASTM stan-

dard D2290) revealed the actual ultimate tensile strain of the confinement, which also exceeded the ultimate lateral strain. The

lower values can be explained by the improper failure modes occurring in some cases.

The repeated loading revealed the plastic behavior of confined concrete. The stress–strain curve in the monotonic

loading formed the envelope of that in the repeated loading.

The confining action of pretensioned carbon yarns was much more effective than that of CFRP tapes, but this could

partially be caused by the larger carbon confinement ratio, 2.48%, while the three-layer tape confinement contained only

1.35% carbon.

A significantly higher elastic limit was observed for the specimens confined under pretension in comparison with that

for the plain concrete and the specimens confined by a carbon tape.

The repeated loading revealed the plastic behavior of confined concrete specimens, which was coupled with a reduc-

tion in the elastic modulus and with damage accumulation. The creation of an appropriate mechanical model describing these

effects is the subject of next papers.

The present paper comprises only concise results of the experimental study. The full text and the results in a digital

form are given in [7] and are also available in the internet http://www.pmi.lv/... .

31

0.1 0.2 0.3 0.4 0.5 0.6 0.70

80

70

60

50

40

30

20

10

�33, MPa

1

2

3

45

�33, %

Fig. 18. Stress–strain curves for the concrete of batch 50: 1 and 2 — compression of confined

concrete specimens above the limit of linearity; 3 and 4 — repeated compression of specimens

1 and 2 after unloading and tearing off the composite shell; 5 — compression of the initial

concrete up to the final failure.

Acknowledgment. This investigation is a joint project of the Division of Building Tehnology, Chalmers University of

Technology in Göteborg, Sweden and of the Institute of Polymer Mechanics, University of Latvia, Riga, Latvia. The project

was sponsored by Åke och Greta Lissheds Stiftelse, SEB Enskilda Banken SE 10640, Stockholm. The investigation was coor-

dinated within the frames of European Comission TMR Network ConFibreCrete and Marie Curie Network En-Core. The other

contributors are mentioned in [7]. We express most sincere thanks to all contributors who supported this research.

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