Materials and Structures/Mat4riaux et Constructions, Vol. 36, June 2003, pp 282-290

Strengthening of RC wall-like columns with FRP under sustained loading

S. Tanwongsval, M. Maalej and P. Paramasivam Department of Civil Engineering, National University of Singapore, Singapore 117576

A B S T R A C T R I~ S U M I~

In recent years, increasing research efforts have been geared towards studying the repair and strengthening of concrete structures using fibre reinforced polymer (FRP) composites. Numerous studies have shown that structural members such as circular columns can experience a signif- icant increase in strength and ductility due to passive con- finement provided by the composite wraps. In rectangu- lar columns, however, limited strength increases have been achieved due to the nonuniform distribution of con- fining stress around the column. Despite the large num- ber of studies on FRP-confined concrete columns, studies on the strengthening of RC wall-like (i.e. high aspect ratio) columns have been limited. Furthermore, most studies did not consider the effect of sustained loading on the column during repair/strengthening. This paper reports the results of an experimental investigation where various schemes have been used to strengthen wall-like RC columns with and without sustained loading. The effects of sustained loading on the strengthening effi- ciency as well as the lateral strain distribution in the com- posite wrap have also been investigated.

De nombreux travaux ont ~td ddveloppds ces derni&es anne'es dam le but de renforcer et r@arer les structures en bdton armd utilisant des feuilles de polym&e renforcdes &fibres de carbone ou de verre (FRP). Plusieurs dtudes ont montrd que Ies colonnes circulaires ont subi une augmentation apprdciable de Ia capacitd ultime et la ductilitd en raison du confinement passif engendrd par les feuiUes composites. Cependant, dam les colonnes rectangulaires, l'augmentation de la capacitd ultime des membres a ~td limitde h cause de la distribution non uni-

forme de la contrainte du confinement autour de la colonne. Malgrd les nombreuses dtudes sur les colonnes en bdton armd utilisant des FRP, les essais sur les colonnes rectangulaires sont limitds. En outre, la plupart des dtudes faites n'ont pas pris en consid&ation l' effet de la charge soutenue par la colonne pendant la rdparation ou le renforcement. Cet article rapporte tes rdsuttats d'une ~tude exp&imentale oh plusieurs mdthodes ont dtd utilis&s pour renforcer des colonnes en bdton armd 2~ section rectangulaire avec et sans chargement soutenu. Les effets de la charge soutenue sur l'e~cacitd de renforcement ainsi que les distributions des tensions fat&ales dam les feuilles com- posites ont aussi dtd dtudids.

1. INTRODUCTION

Many concrete structures are in need of strengthening due to more stringent code design requirements, structu- ral deficiencies due to errors in calculation or plan execu- tion, adaptation of a structure for a different function, and/or poor construction practices. Traditionally, repai- ring or strengthening of reinforced concrete structures such as columns involved a time-consuming and disrup- tive process of removing and replacing the damaged concrete and/or steel reinforcement with new and stron- ger materials. However, with the introduction of exter- nally-bonded fibre-reinforced polymer (EB-FtLP) com-

posites, reinforced concrete columns can now be streng- thened effectively with minimum intervention by wrap- ping layers of FRP sheets around the columns. Strength and ductility were found to improve significantly due to the passive confinement provided by the F1KP [1-4]. However, one would not expect equal increases in strength and ductility for all types of columns. Miriman et al. [5] and Rochette et al. [6] studied the effectiveness of FRP-wrapped columns and concluded that columns with square and rectangular sections experienced less increase in strength and ductility than columns with circular sec- tions. This is because in circular-section columns, the dis- tribution of lateral confining pressure is uniform, while in

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Tanwongsval, Maalej, Paramasivam

square-section columns the confining pressure varies from a maximum at the corners to a minimum between the edges. Both authors also indicated that the corner radius of the square and rectangular columns affected the level of confining pressure on the concrete core. These results highlight the importance of rounding-off the corners and assert the fact that low confining pressure is to be expec- ted if the aspect ratio of the column's cross-section is high.

The application of composite sheets for columns is, however, not limited only to providing passive confine- ment. Neale et al. [7] and Chiew et al. [8] conducted studies on the strengthening of wall-like columns where glass and carbon composite wraps as well as conventional steel plates were used as compressive members. Various strengthening schemes were proposed and tested. After trial and error, both authors agreed that the use of glass fibre-reinforced polymer (GFRP) is preferable to that of carbon fibre-reinforced polymer (CFRP), as the former has a thicker section, which offers higher stability against buckling. Finally, both adopted a similar scheme where thick FRP C-channels were fabricated and attached to the ends of the columns with two horizontal FRP layers wrapped around the columns to prevent buckling of the channels. Test results showed an increase in axial capa- city by as much as 33 percent when eight vertical GFRP layers were used.

Although satisfactory increase in axial capacity was achieved in the above studies, the test programs did not simulate actual loading conditions of typical columns. In practice, columns would be subjected to service loads during and after strengthening. Kaluarachchi [9] repor- ted that when a RC column is repaired under sustained service loading, the repair material carries a smaller por- tion of the applied load in comparison to RC columns repaired without sustained loading. Hence, in FRP- strengthened columns, it is likely that the increase in axial load-carrying capacity would be less if the columns were strengthened under sustained loading.

While strengthening of circular- or square-section concrete columns by wrapping FRP sheets has been extensively studied in research, studies on rectangular- section columns, particularly those with high aspect ratios (wall-like), are limited. In the present study, two schemes for strengthening RC wall-like columns are investigated taking into consideration the effects of loads sustained during column strengthening. Results from this study should provide an insight as to how RC wall- like columns could be effectively strengthened using EB-FRP and how sustained loading affects the streng- thening ratio.

2. EXPERIMENTAL PROGRAM

The experimental program consisted of five half-scale models of typical wall-like RC columns found in building estates in Singapore. The dimensions of the columns are approximately 115 mm in thickness, 420 mm in width, and 1500 mm in height. The steel reinforcement consisted of eight 13 ram-diameter and four 10 ram-diameter axial

A

A

375

7'50

I

I l U

Section A-A

Elevation View

Fig. 1 - R e i n f o r c e m e n t details o f wall- l ike co lumn .

Table 1 - Mechanica l properties of materials

Material Properties

Concrete f'c = 32.4 MPa

Longitudinal reinforcement (T13) fy = 461 MPa, E s = 171 GPa

Longitudinal reinforcement (TIO) fy = 541 MPa, E s = 186 GPa

Transverse reinforcement (R6) fy = 365 MPa, E s = 211 GPa

FRP cured laminate tensile strength fau = 600 MPa, E a = 26.1 GPa, Eau = 2.24%, t a = 1.08 mm

Prestressingtendons fpu = 1860 MPa, Ep = 195 GPa

High strength repair mortar f'em = 59.5 MPa

reinforcing bars, with twenty 6 ram-diameter stirrups spa- ced at 100 mm in the central portion of each column as shown in Fig. 1. The spacing was reduced at the top and bottom to 60 rnm to ensure that premature failure does not occur at the column ends during the test. Only one type of FRP (GFRP tow sheets) was used for vertical and/or horizontal wraps in the two strengthening schemes as will be later discussed. It was imperative for this study that the materials used were of identical properties for all columns. Consequently, all specimens were cast simultaneously with ready-mixed concrete (f'c = 32.4 MPa) with steel reinfor- cement obtained from the same batch. The FRP manu- facturer's guidelines in the primer application and resin impregnation were strictly followed. Table 1 lists the mechanical properties of the materials used.

2.1 Strengthening schemes

The first strengthening scheme (Scheme 1) consisted of two vertical layers (90 ~ and two horizontal layers (0 ~ of FRP wraps that were externally bonded to the

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Materia/s and Structures/Mat~riaux et Constructions, Vol. 36, June 2003

Fig. 2 - F R P semi-cylindrical a t t a c h m e n t .

a Corner radius, b Al l dimensions are in ram.

column's surface. The second strengthening scheme (Scheme 2) involved an application of a new strengthe- ning methodology. Hollow semi-cylindrical attach- ments (Fig. 2), which were prefabricated by epoxy impregnation of one vertical layer of GFRP tow sheet, were attached at the ends of the columns. The FRP semi-cylindrical attachments served as a permanent formwork for the extra semi-cylindrical sections as shown in Table 2. Two horizontal FRP layers were later wrapped around the enlarged column section to provide passive confinement to the cylindrical section as well as to prevent premature buckling of the external sections. Finally, the hollow semi-cylindrical attachments were filled with pourable, high strength, and non-shrinking repair mortar. The geometry of the attachment was expected to be conducive to the development of passive confining pressure in the semi-cylindrical sections pro- vided by the horizontal FRP wraps. This scheme was designed to offer a higher strengthening ratio compared to Scheme 1. Columns strengthened by two horizontal and two vertical FRP layers (Scheme 1) with and without sustained loading were designated as 2H2V-L and 2H2V-NL, respectively, whereas columns streng- thened by adding external semi-cylindrical sections (Scheme 2) with and without sustained loading were designated as 2H-VA-L and 2H-VA-NL, respectively. The testing program is summarized in Table 2.

2.2 Specimen preparation and test setup

The surfaces of the RC columns must be prepared prior to the application of FRP wraps. A hand-held grin- der was used to roughen the columns' surfaces and to round-off the columns' corners. Grinding was considered adequate once the coarse aggregates were exposed. The corners were ground to obtain a radius of approximately 30 mm. Roughening the surfaces and rounding-off the

corners is important in the application of FRP wrappings. Improper surface preparation leads to debonding at the interface while sharp corners introduce stress concentra- tion in the FRP wraps. Furthermore, the radius of the rounded corners affects the level of confinement provided by the horizontal FRP wraps [5-6].

Impregnated GFRP sheets were wrapped in discrete layers as the size and weight of the impregnated sheets made it difficult to wrap the columns in a continuous manner. An overlap length of 150 mm was recommen- ded by the manufacturer and was applied to all layers to prevent slippage and to allow full development of the confining pressure. After wrapping all the required layers, two additional 400 mm-wide GFRP strips (one in 0 ~ and one in 90 ~ direction) were wrapped at the top and bottom of the columns. These extra layers provided additional confinement and stiffness to ensure that the failure was contained within the test region.

To determine the effects of sustained loading on the strengthening efficiency, an external prestressing tech- nique was adopted to simulate service load on the column. The setup, shown in Fig. 3, comprised of two stiff built-up steel beams placed on the top and bottom of the column. Four holes were drilled at both ends of the beams to accommodate four 12.9 mm-diameter prestressing tendons. The tendons were jacked horizon- tally to generate on the column a total compressive force of 800 kN, which corresponded to approximately 40% of the unstrengthened column's load-carrying capacity. Upon attaining the required force, the column was pla- ced in an up-right position and subsequently strengthe- ned according to the two schemes. This setup proved to be more economical than utilizing the actuator to simu- late service load.

Strain gauges were installed on the horizontal FRP wraps along the strengthened column's perimeter to monitor the distribution of lateral FRP strains. The results enabled the strain distribution around the column to be plotted. The strain distribution plots are essential to a better understanding of the contribution of the FRP wraps to the strengthening of wall-like RC columns as well as their modes of failure. To optimize the number

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Tanwongsval, Maalej, Paramasivam

Fig. 3 - Experimental setup for specimen tested under sustained loading.

of strain gauges, the positions were selected at locations where stress concentration was likely to occur. The ove- rall deformation of the column in the test region was measured using two linear variable displacement trans- ducers (LVDTs), which were fixed on an aluminum frame with a gauge length of 700 mm.

3. RESULTS AND DISCUSSION

3.1 Failure of specimens

The failure of concentrically-loaded reinforced concrete columns can be caused by crushing of the confined concrete, rupture of the transverse reinforce- ment or buckling of the longitudinal reinforcement [10]. In the present series of tests, buckling of the longitudinal reinforcement appeared to be the primary cause of fai- lure. The phenomenon was visually observed in the control column and was believed to have controlled the failure of the strengthened columns (judging from the observed bulging of these specimens at failure as shown in Fig. 4). The buckling or instability of longitudinal reinforcement is related to the bar aspect ratio, which is the ratio of unsupported bar length between two ties to its diameter, and the lateral support provided by the concrete cover [11-12]. If the bar aspect ratio is high, the bar can lose its stability due to buckling prior to developing full strain hardening. Likewise, if the concrete cover spalls, the longitudinal reinforcement loses the lateral support and may be prone to buckling. Localized buckling of the longitudinal reinforcement - between two consecutive ties or the region where the concrete cover has spalled - would eventually trigger a global buckling failure of the column.

The load versus deformation curves of tested speci- mens is given in Fig. 5. It was noticed that as the streng- thened specimens approached their respective ultimate load, the FP, P wraps started to bulge in the test region. Bulging was believed to be triggered by &lamination of the concrete cover and subsequent buckling of the longi- tudinal reinforcement. Fig. 5 shows that all strengthened columns exhibited higher ductility compared to the control column. This is probably due to the FRP wraps, which provided additional stiffness to the unconfined concrete cover; thus, delaying it from spalling. Evidence

on the foregoing can be seen from the difference in average vertical strain in the longi- tudinal reinforce- ment at ult imate load (Table 3) bet- ween the FR.P- wrapped speci- mens and the control specimen (wi thout FRP wraps). Specimens with FP, P wraps ( 2 H 2 V - N L , 2H2V-L, 2H-VA- NL and 2H-VA-L) have higher ave- rage vertical strain in the longitudinal

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Materials and Structures/Mat~riaux et Constructions, Vol. 36, June 2003

3500

3 0 0 0 ~ N L

2500 �9 , ~ ~ V - N L

~z~ A2000 / ~ t r o ~

2 15oo

1000 - 700 mm

500 �9

0 ' ' ' ' I ' ' ~ ' I ' ' ' ' I ' ' ~ ' I ' ' - ' ' I ' ' ' '

0 0.1 02 0.3 0.4 0.5 0.6 Axial Strain (%)

Fig. 5 - Load-axial strain curves for test specimens.

Table 3 - Experimental results

3.3 Effects of sustained loading

As descr ibed earlier, the exper imen ta l setup employed to study the effects of sustained loading on the strengthening efficiency involved the utilization of external prestressing. In order to obtain the load-axial strain response of the unstrengthened column during prestressing, a series of strain gauges were installed on the tendons to measure the prestressing strain, while a displacement transducer was placed on the column to measure the deformation. After strengthening, the whole setup was placed in the testing frame in an up- right position. The top built-up steel beam served as a loading platen to transfer the load from the actuator to the strengthened column as well as to transfer the force from the prestressing tendons to the column. It is essen- tial to note that when the load was gradually applied to

the strengthened column, the prestressing tendons started to gradually lose their pres-

Specimen Ultimate load (kN)

Contml

2H2V-NL

2H2V-L

2H-VA-NL

2H-VA-L

Average axial strain at ultimate load (%)

Increase in ultimate load (%)

Increase in axial strain at ultimate load (%)

2067 0.28

2657 0.43 28.5 53.6

2504 0.35 21.1 25.0

3311 0.33 60.2 17.9

2849 0.30 37.8 7.1

reinforcement at ultimate load than the control specimen.

3.2 Strengthening efficiency

A summary on the increase in axial capacity of the s t reng thened specimens is given in Table 3. Strengthening Scheme 2, where external semi-cylindri- cal sections were cast at the ends of the column, proved to be extremely effective. The external sections cast from a 60 MPa high strength mortar were capable of increasing the axial capacity by 60%. This was twice the increase given by strengthening Scheme 1, where two horizontal and two vertical FRP layers were wrapped around the column. In addition, it was observed that columns strengthened by Scheme 1 showed a softer res- ponse but higher peak strain (defined as the strain cor- responding to the ultimate load). This was expected because the semi-cylindrical sections (Scheme 2) were m u c h stiffer than the two vertical FRP layers (Scheme 1). However, the two vertical and two hori- zontal layers used in Scheme 1 were more effective than the two horizontal layers alone (Scheme 2) in delaying the concrete cover from spalling, thus, resulting in a high peak strain (Scheme 1).

Average vertical strain at ultimate

load ( ~ ) FRP Steel

4254

3507 7241

2905 6316

5896

4289

tressing forces. Consequently, the prestres- sing strain in each tendon was monitored so that the actual load-axial strain response could be reconstructed.

The reconstruction of the load versus axial strain curve for specimen 2H2V-L is shown in Fig. 6. The actual load-axial strain curve (Curve A) was recons t ruc ted by superimposing the load-axial strain curve obtained during the test (Curve B) with the load-axial strain curve of the residual pres- tressing compressive force (Curve C). The

origin of Curve B was shifted to the axial strain that cor- responded to the magnitude of the sustained load. Thus, the initial portion of Curve A was identical to the ascen- ding portion of Curve C. The ascending portion of Curve C was the load-axial strain response of the uns- trengthened column obtained during prestressing with the peak load corresponding to the level of sustained loa- ding on the column prior to strengthening. Later, the total load on the column at other axial strains was deter- mined by combining the residual compressive force applied by the prestressing tendons and the applied load

3000

2500

20OO Z

1500 O

1000

500

Load-axial Superimposed load- strain curve /a/ xial strain curve

: during A ~ ~ r e s t r e s s i n g /

/ B J L~ strain curve of / / strengthened column

/ / Total residual compressiv, / / _ /force in the prestressing

i i p i

0 0.1 0.2 0.3 0.4 0.5 0.6

Axial Strain (%)

Fig. 6 - Reconstruction of load-axial strain curves for specimens strengthened under sustained loading.

286

Tanwongsva l , M a a l e j , P a r a m a s i v a m

4000 3000

70001 , 0.2Pu 6 0 0 0 i r " - - I - 0 . 4 P u

i i - - , -o .8~o 6ooo . . . . . 4 ................. i . . . . . . . . . . . . . . . . . . . . . . . o ,~o

.~ ! i ~o.gPo

40oo ...... i . . . . . i . . . . . . . -*-og~Pu i ! ---~opo

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i i i i

S t r a i n Gauge

: 57.5 , ~

II1)t: ,I 2000 1000 0

~ / 0 1000 2000 3000 4000

-210 -140 i

:

i :

: ;

70 140 210 0

blO00

2000

3000 ~ '

4000 i

5000

6000

Fig. 7 - Lateral FRP strain distribution of column 2H2V-NL.

measured by the testing frame's load cell. The experimental results showed that columns

strengthened under sustained loading had lower streng- thening ratios than columns strengthened without sus- tained loading. In the latter case, all materials were sub- jected to the same strain from the initial loading until failure, assuming that the retrofit material and the column behaved compositely. Columns strengthened under sustained loading, on the other hand, received smaller contributions from the retrofit material. The retrofit material, in this case, was engaged only from the level of the sustained loading until failure. Thus, the retrofit material was strained to a lesser extent than the retrofit material in columns strengthened without sustai- ned loading. This difference in mechanical response can be observed from the average vertical FP,.P strain rea- dings given in Table 3 for columns 2H2V-NL and 2H2V-L. As expected, the vertical FRP strain at ulti- mate load for column 2H2V-L is less than that for column 2H2V-NL. The overstraining of the original material, however, triggered failure in both columns.

Since both specimens, 2H-VA-L and 2H2V-L, were subjected to similar levels of sustained loading, the speci- men with stiffer and higher amount of retrofit material (2H-VA-L) was expected to experience a greater reduc- tion in the strengthening ratio. This was found to be the case as the measured reductions in strengthening ratios due to sustained loading were 7.4 and 22.4 percent for

2H2V-L and 2H-VA-L, respectively. This result indi- cates that the decrease in the strengthening ratio due to sustained loading must be considered in design when repairing or strengthening a P,.C column. Otherwise, the R.C column may not be able to withstand targeted maximum load after repair/strengthening.

3 . 4 L a t e r a l F R P s t r a i n d i s t r i b u t i o n

The objectives of monitoring the FRP strain in the horizontal layers were to obtain the lateral FR.P strain distribution around the column at various loading levels and to identify regions of high stress concentration. The locations and the measured FRP strain distributions for columns 2H2V-NL, 2H2V-L, 2H-VA-NL and 2H-VA- L were given in Figs. 7-10, respectively. The strain dis- tributions shown were selected at loading levels corres- ponding to 0.2P u, 0.4Pu, 0.6P u, 0.8Pu, 0.9P u, 0.95P u, Pu and 0.95Pu (post-peak), where Pu is the peak load. The strain profiles enabled a better understanding of the effectiveness of the Flq.P wraps in confining high aspect ratio lq.C columns.

At low loading levels, 0.2P~ to 0.6P u, the strains developed in the wraps of columns 2H2V-NL and 2H2V-L were very much uniform. No region of high stress concentration was observed. At higher loading levels, however, stress concentrations were noticed at the

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Materials and Structures/Mat6riaux et Constructions, Vol. 3 6 , June 2 0 0 3

~2000

e .~ 15oo

' i Ioo0~

500

o! -210

" " ~ =-- 575 i i M i

i

o I

2000 1500 1000 800 0 575

-210

: -~- o.2Pu i i : ~ 0.4Pu

i ! --A- O,6Pu -~'~-0 8Pu --~-o 9pu

i -.-oo6Pu i i L - - - 1 oPu i i --095PU (post-peak)

-140 -70 70 140

-140 -70 0 70 140

Strain Gauge 215

~ 7 0 ~ 500 1~ 1500 2~

2100

1ooo !

15oo ~'

2000 ~

Fig. 8 - Lateral FRP strain distribution of column 2H2V-L.

Fig. 9 - Lateral F1LP strain distribution of column 2H-VA-NL.

2 8 8

Tanwongsval, Maalej, Paramasivam

Fig. 10 - Lateral FRP strain distribution of column 2H-VA-L.

middle of the columns' longer dimension. The stress concentrations were probably due to the &lamination of the concrete cover and subsequent buckling of the longi- tudinal reinforcement as described earlier. Also, it was interesting to note that the strains at the columns' cor- ners were not very high. The lower than expected strains at the corners were due to the failure mode of these columns. If the primary failure mode were due to the crushing of concrete core rather than the buckling of the longitudinal reinforcement, the FRP wraps would have been more fully utilized as they would have tried to prevent the concrete core from disintegrating. This would have resulted in an overall higher lateral strain and stress concentration at the corners. In fact, if the steel reinforcement was not present, as in the case of plain

rectangular concrete columns confined by FRP wraps, the FRP wraps would have ruptu- red at the corners due to stress concentration, as reported by Rochette et al. [6]. The effect of sustained loading on the strain distribution was gene- rally marginal except for the noticeable difference in magnitude on the side where the FRP wraps had bulged.

For columns strengthened by Scheme 2, 2H-VA-NL and 2H-VA-L, the overall beha- viour was similar to that of Scheme 1. At low loading levels, the strains in the hori- zontal FRP wraps were uni- form but as the load approached the peak load, stress concentra- tions were noticed at the middle of the longer dimension and at the interface between the verti- cal attachments and the column. The high strains at the interface indicated that the bond between the vertical attachments and the column was insufficient, despite the efforts in grinding the column's surface until the coarse aggre- gates were exposed. Thus, the column and the vertical attach- ments may not exhibit compo- site action at high load levels. The horizontal wraps, there- fore, provide not only passive confinement to the vertical attachments, but also lateral support to prevent the attach- ments from buckling. The strains around the circular sec- tion of the vertical attachment were very uniform except at

peak load where higher strains were observed on the side adjacent to where the FRP wraps had bulged. The effect of sustained loading on the strain distribution was noticeable. The strains of column 2H-VA-L were considerably lower than those of column 2H-VA-NL. This was because the former failed at a much lower load due to sustained loading; thus, the FRP wraps were less utilized.

4. PRACTICAL CONSIDERATIONS AND FURTHER RESEARCH

To evaluate the performance and the effectiveness of any strengthening scheme, it is very important that the strengthened member be subjected to conditions that are

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Materials and Structures/Mat4riaux et Constructions, Vol. 36, June 2003

more similar to what is observed in the field. This includes the effects of member boundary conditions, the effects of load- and environmentally-induced damage as well as the influence of loading sustained during streng- thening. For column members, the strengthening of the co lumn may in turn require s t rengthening of the beam/slab regions immediately below and above the column so that premature failure does not take place at these locations. Further, certain strengthening schemes may require additional work on immediate neighbou- ring members. For example, if strengthening Scheme 2 is to be used, the external semi-cylindrical attachments have to be integrated into the beams/slabs immediately below and above the column and this will unavoidably involve extra work. Finally, when strengthening buil- ding structural elements, it should be realized that FRP composites generally do not have sufficient fire resis- tance and, therefore, often need additional protection.

This paper reported on tests of RC wall-like columns strengthened with externally-bonded FRP taking into consideration the effects of sustained loading. The objective of these tests was to establish the structural per- formance of the strengthened columns in the short term as an indicator of the general effectiveness. The issues of how real conditions, such as those mentioned above and not considered in the present study, need to be addressed in future research. Further tests are planned at the National University of Singapore to address these issues.

5. CONCLUSIONS

An experimental program was designed to study two strengthening schemes for wall-like (high aspect ratio) 1KC columns, taking into account the effects of sustained loa- ding. The experimental results showed that proposed Scheme 2, which utilized semi-cylindrical attachments, was very effective in increasing the axial capacity of wall- like columns. The increase was twice the increase obtai- ned from strengthening Scheme 1, where two horizontal and two vertical FRP wraps were used around the column.

Increasing the cross sectional area of the column may not be preferable due to space constraints. However, it would likely be an unavoidable solution if strength increases greater than 30% were sought. In this case, and with the use of higher strength retrofit material, the size of the semi-cylindrical attachments can be optimi- zed to meet targeted strengthening ratio for the wall-like 1LC columns. It was also found that columns strengthe- ned under sustained loading showed 7 to 22 percent less increase in axial load capacity compared to columns strengthened without sustained loading, indicating the need to consider the effect of sustained loading in design when repairing or strengthening a tLC column.

Singapore. Useful comments from anonymous revie- wers are gratefully acknowledged.

NOTATIONS

f'c - cylinder compressive strength of concrete f'cm-cylinder compressive strength of high strength

repair mortar = yield strength of reinforcing steel

s = elastic modulus of reinforcing steel fan = ultimate tensile strength of FRP cured laminate Eaa = elastic modulus of FtLP cured laminate 8au = ultimate tensile strain of FRP cured laminate t a = thickness of one layer of FRP cured laminate ~ = ultimate tensile strength of prestressing tendon

elastic modulus of prestressing tendon

REFERENCES

[1] Toutanji, H.A., 'Stress-strain characteristics of concrete columns externally confined with advanced fiber composites sheets', ACI Materials Journal 96 (3) (1999) 397-404.

[2] Spoelstra, M.R. and Monti, G., 'FRP-confined concrete model', ASCEJoumal of Composites for Construction 3 (3) (1999) 143-150.

[3] Saadatmanesh, H., Ehsani, M.R. and Li, M.W., 'Strength and ductility of concrete columns externally reinforced with fiber composite straps', ACI StructumUouma191 (4) (1994) 434-447.

[4] Saadatmanesh, H., Ehsani, M.R. and Jin, L., 'Repair of earth- quake-damaged RC columns with FRP wraps', ACI Structural

Journal 94 (2) (1997) 206-215. [5] Mirmiran, A., Shahawy, M., Samaan, M., Echary, H.E.,

Mastrapa, J.C. and Pico, O., 'Effect of column parameters on FRP-confined concrete', ASCE Journal of Composites for Construction 2 (4) (1998) 175-185.

[6] Rochette, P. and Labossiere, P., 'Axial testing of rectangular column models confined with composites', ASCEJournal of Composites for Construction 4 (3) (2000) 129-136.

[7] Neale, K.W., Demers, M., DeVino, B. and Ho, N.Y., 'Strengthening of wall-type reinforced concrete columns with fiber reinforced composite sheets', in 'Structural Failure Durability and Retrofitting', Proceedings of the Fifth International Conference, Singapore Nov. 1997 (Singapore Concrete Institute, Singapore,1997) 410-417.

[8] Chiew, S.P., Lau,J.M. and Ho, N.Y., 'Testing of wall-type rein- forced concrete column strengthening with advanced composite materials', Seminar on Strengthening and Upgrading Structures using Advanced Composite Materials, Singapore May 1999 (Singapore Concrete Institute, Singapore, 1999) 1-16.

[9] Kaluarachchi, D.M., 'Performance of repaired axially loaded members', M. Eng Thesis, Department of Civil Engineering, National University of Singapore, 1997.

[10] Papia, M., Russo, G. and Zingone, G., 'Instability of longitudi- nal bars in RC columns', ASCEJournal of Struaural Engineering 114 (2) (1988) 445-461.

[11] Yalcin, C. and Saatcioglu, M., 'Inelastic analysis of reinforced concrete columns', Computers and Structures 77 (2000) 539-555.

[12] Mau, S.T., 'Numerical simulation of the behavior of axially loa- ded reinforced concrete columns', Computers and Structures 35 (4) (1990) 361-368.

ACKNOWLEDGEMENT

Part of this study was supported by a research grant (R-264-000-105-112) from the National University of Paper received: Au2ust 20, 2001; Paper accepted: February 25, 2002

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