CONCRETE-FILLED SQUARE FRP TUBES UNDER AXIAL COMPRESSION

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1 Asia-Pacific Conference on FRP in Structures (APFIS 27) S.T. Smith (ed) 27 International Institute for FRP in Construction CONCRETE-FILLED SQUARE FRP TUBES UNDER AXIAL COMPRESSION T. Ozbakkaloglu *, J.C. Lim and D.J. Oehlers School of Civil and Environmental Engineering, The University of Adelaide, Australia. ABSTRACT Existing studies have proven the effectiveness of using fiber reinforced polymer (FRP) composites for retrofitting concrete columns. FRP composites also offer potential for use in new column construction in the form of concrete-filled FRP tubes, where the tubes fulfill multiple functions of: i) stay-in-place formwork, ii) confinement reinforcement, iii) protective shell against weathering and chemical attacks. This paper presents results of an experimental study on the behavior of concrete-filled square FRP tubes under concentric compression. The FRP tubes were designed as column confinement reinforcement and were manufactured using unidirectional carbon fiber sheets with fibers oriented in the hoop direction. The effects of the thickness and the corner radius of tubes on the axial behavior of concrete-filled FRP tubes were investigated experimentally. The test results indicate that FRP confinement may substantially improve the ductility of square columns. The confinement may also improve the axial load-carrying capacity of columns if the confinement effectiveness is sufficiently high. The results also indicate that the confinement effectiveness increases with the thickness and the corner radius of tubes. KEYWORDS Column; concrete; fiber reinforced polymer; confinement; compressive strength; stress-strain relations. INTRODUCTION It is well-established that lateral confinement can significantly enhance the both the strength and deformability of concrete columns (Richart et al. 1928; Ahmad and Shah 1982; Mander et al. 1988). Existing studies on the seismic behavior of FRP-confined concrete columns have shown that FRP confinement can substantially improve the inelastic deformability of columns (Seible et al. 1996; Zhou et al. 21; Yamakawa et al. 23; Shao and Mirmiran 25; Ozbakkaloglu and Saatcioglu 26, 27). In recent years, the compressive behavior of FRP-confined concrete has been studied extensively. However, the majority of these studies have been concerned with FRP-wrapped concrete specimens with circular cross-sections, and only a few tests have been reported on concrete-filled square or rectangular FRP tubes (Mirmiran et al. 1998; Hong and Kim 24; Fam et al. 25; Ozbakkaloglu and Oehlers 27a, b). In an FRP tube confined circular concrete column, the concrete is uniformly confined by the FRP tube, when the column is subjected to concentric compression. Unlike in circular columns, concrete in FRP-confined square columns is not subjected to uniform confining pressure, as the pressure provided by the FRP varies over the cross-section. Square FRP tubes develop high confining pressures near the corners where high lateral restraint is provided by transverse FRP, which diminish quickly and become very low between the edges where the restraint is limited to the flexural rigidity of the FRP tube. The confinement effectiveness of FRP tubes improves with the uniformity of confining pressure, and hence square tubes have lower effectiveness than circular tubes. However, it is now well understood that the effectiveness of square FRP tubes can be enhanced by rounding their corners, which makes the corner radius an important confinement parameter for square tubes (Rochette and Labossiere 2; Mirmiran et al. 21; Pessiki et al. 21; Lam and Teng 23; Ozbakkaloglu and Saatcioglu 27). 119

EXPERIMENTAL PROGRAM Test Specimens Extensive experimental and analytical research is underway at the Structures Laboratory of the University of Adelaide on the behavior concrete-filled FRP tubes.

The columns were prepared from the same batch of concrete and each of them had a 2 x 2 mm cross-section and a 6 mm height.

2 EXPERIMENTAL PROGRAM Test Specimens Extensive experimental and analytical research is underway at the Structures Laboratory of the University of Adelaide on the behavior concrete-filled FRP tubes. This paper presents the results of an experimental investigation on the behavior of axially loaded concrete-filled square FRP tubes, where a total of nine columns were tested. The columns were prepared from the same batch of concrete and each of them had a 2 x 2 mm cross-section and a 6 mm height. Three of the columns were tested without FRP confinement as control specimens. The remaining six columns were confined by carbon FRP tubes, with either 3 or 5 layers of FRP. In order to investigate the effect of corner radius on the confinement effectiveness of FRP tubes, the tubes were manufactured with three different corner radii, that is, 2, or 4 mm. The FRP tubes were manufactured by manually wrapping impregnated FRP sheets around high-density styrofoam templates in the hoop direction. Details on the manufacturing process of the tubes can be found elsewhere (Ozbakkaloglu and Oehlers 27b). Figure 1 shows the FRP tubes before concrete casting. Material Properties Figure 1. FRP tubes before casting The columns were cast from a same batch of ready-mix concrete with specified target strength of 25MPa. All specimens were cast vertically. The development of concrete compressive strength was monitored through testing of x 2 mm cylinders. The average cylinder compressive strength (f c) at the time of column testing was found to be 28 MPa. The properties of the unidirectional carbon fiber sheets used to manufacture the FRP tubes are shown in Table 1. The tensile properties of the FRP formed from these carbon fiber sheets and epoxy resin were determined from flat coupon tests in accordance with ASTM D 39 (21), and the results were found to be consistent with the fiber properties provided by the manufacturer. Instrumentation and Testing Table 1. Properties of carbon fibers reported by the manufacturer Property Typical value Nominal thickness.117 mm/ply Elastic modulus 24 GPa Tensile strength 38 MPa Ultimate tensile strain 1.55% Weight 2 g/m 2 The columns were instrumented with linear variable displacement transducers (LVDTs) and unidirectional strain gauges. A total of eight LVDTs were used to measure the axial deformations of a column. Four of these LVDTs were mounted one on each face of a column and covered a height of 2 mm at the mid-height region. Another four LVDTs were mounted at the corners between the loading and supporting steel platens of the test machine to measure average axial strains along the 6mm height of a column. In addition, four longitudinal strain gauges that were bonded at the mid-width of each face of a column to measure the axial strains at the mid-height. APFIS 27 12

Transverse strains were measured by unidirectional strain gauges that were installed at the mid-height of the columns.

Four of these strain gauges were installed at the mid-width of each face and the other two were placed at two opposite corners as shown in Fig.2.

In addition, a 2 mm thick steel plate, with slightly smaller dimensions than the cross-section of FRP tubes, was placed at each end of the column to ensure that the load was applied only to the

The test data were recorded using a data acquisition system. The test setup and specimen instrumentation are shown in Figure 2.

Test setup and specimen instrumentation Specimen designation The columns were labeled as follows: In FRP-confined columns, letter R (for radius) was followed by the tube corner radius and L (for

3 Transverse strains were measured by unidirectional strain gauges that were installed at the mid-height of the columns. A total of six strain gauges were installed on each of the FRP-confined columns to obtain the distribution of the transverse strains along the column perimeter. Four of these strain gauges were installed at the mid-width of each face and the other two were placed at two opposite corners as shown in Fig.2. Prior to testing, all columns were capped with dental stones at both bearing ends to ensure uniform distribution of the applied axial pressure. In addition, a 2 mm thick steel plate, with slightly smaller dimensions than the cross-section of FRP tubes, was placed at each end of the column to ensure that the load was applied only to the concrete core to fully utilize the tubes as confinement reinforcement. The columns were loaded under axial compression using a universal testing machine with 5 kn capacity. The test data were recorded using a data acquisition system. The test setup and specimen instrumentation are shown in Figure 2. H/2 LVDTs H Strain gauges mm Strain Gauge Steel platen 2 mm Strain Gauge Figure 2. Test setup and specimen instrumentation Specimen designation The columns were labeled as follows: In FRP-confined columns, letter R (for radius) was followed by the tube corner radius and L (for layer) was followed by the number of FRP layers. In unconfined columns, letter P (for plain) was followed by the test number. TEST RESULTS AND DISCUSSION Observed Behavior and Failure Mode All of the confined columns were failed by the rupture of the FRP tube at the tube corners as shown in Figure 3. Once the tube failed, the concrete core was no longer able to sustain the axial load. Although steel plates were used to ensure that the axial load was applied only to the concrete core, the recorded longitudinal strain data indicated that significant portion of the axial stresses were transferred from the concrete to the FRP tube. A similar phenomenon was previously reported by Zhu et al. (25), who tested concrete-filled circular tubes under concentric compression. (a) RL5 (b) R2L5 (c) R4L3 Figure 3. Failure modes of test specimens APFIS

4 Axial Stress-strain Behavior The summary of the experimental results and calculated capacities of unconfined columns and FRP-confined columns are presented in Tables 2 and 3, respectively. As shown in Table 2, the average peak axial stress (f co ) of unconfined columns was 26.7 MPa, and the average axial strain corresponding to this stress (ε co ) was.22%. For FRP tube confined columns, two axial stresses were calculated, one corresponding to the maximum load recorded during the test (f cc ), and the other to the load at failure (f cu ). Table 3 shows that the ultimate compressive strength of concrete (f cu ) was significantly affected by the corner radius of the tubes. The axial strains recorded on the FRP-confined test columns at failure are listed in Table 3. The reported ultimate axial strains (ε cu ) are the average values along the height of the columns, which are obtained using the four displacement transducers with 6 mm gauge lengths. As evident from the ε cu / ε co ratios shown in Table 3, FRPconfinement resulted in a significant enhancement in the ultimate strain of concrete in all tested columns. Table 2. Test results of unconfined columns Column f' c f' co f' co Average ε co (%) ε co (%) Average f co / f c P P f co / f c Average.95 Column f' c f' co Table 3. Test results of confined columns f' cc f' cu ε cu (%) f cc / f co f cu / f co ε cu / ε co RL R2L R4L RL R2L R4L It has been well-established that a FRP-confined concrete exhibits a monotonically ascending curve, which consists of a parabolic first portion and a nearly straight-line second portion, if the amount of confinement exceeds a certain threshold. On the other hand, when the amount of confinement is below this threshold the first ascending branch may be followed by a descending second branch. In order to illustrate the distinction in between these two types of curves, the stress-strain curves of columns R4L5 and RL3 are shown in Figures 4 and 5, respectively. As can be seen in Figure 4, the peak axial strength of confined concrete (f cc ) is equal to the ultimate concrete strength (f cu ), when the stress-strain curve exhibits an ascending second branch. On the other hand, the ultimate concrete strength (f cu ) may be significantly lower than the peak axial strength (f cc ) if the stress-strain curve exhibits a descending second branch as shown in Figure 5. In such specimens, the peak axial strength of confined concrete (f cc ) is close to the unconfined member strength (f co ). Axial Stress f'cc = f'cu Axial Stress f'cc f'cu Figure 4. Axial stress-strain curves of Column R4L Figure 5. Axial stress-strain curves of Column RL3 APFIS

5 The axial stress-strain curves of confined test columns are shown in Figures 6 and 7. These figures illustrate the effect of corner radii and FRP tube thickness on the stress-strain behavior of FRP tube confined concrete, which are discussed in the following sections. Effect of Corner Radius Figure 6 illustrates the stress-strain curves of the columns confined by FRP tubes with, 2 or 4 mm corner radius and with 3 or 5 layers of FRP. The figure shows that the axial strengths of the columns increase significantly with the corner radius of the tubes. The second branch of the stress-strain curve changes from descending to ascending type as the corner radius is increased from to 4 mm. Axial Stress 4 2 RL3 R4L3 R2L3 Axial Stress R4L5 R2L5 RL Effect of Tube Thickness (a) 3 layers of FRP (b) 5 layers of FRP Figure 6. Axial stress-strain curves of specimens with different corner radii Figure 7 illustrates the effect of FRP tube thickness on the stress-strain behavior of FRP-confined concrete. Columns shown in each of the Figures 7(a) and 7(b) had the same corner radius (R) and were confined with either 3 or 5 layers of FRP. It is clear from these figures that the tube thickness has a significant influence on the ultimate strain of confined concrete, and the ultimate strain increases with the tube thickness. Furthermore, the tube thickness also has some influence on the trend of the second branch of the stress-strain curve, and increased tube thickness results in a significant increase in the axial strength if the confinement effectiveness of the tube is high, as in Fig. 7(b). 5 Axial Stress RL3 RL5 Axial Stress 4 2 R4L3 R4L CONCLUSIONS (a) R = mm (b) R = 4 mm Figure 7. Axial stress-strain curves of specimens with different FRP tube thickness An experimental study on the behavior of axially loaded concrete-filled square FRP tubes has been described. The following conclusions can be drawn from this study: Confinement of square columns with FRP tubes leads to substantial improvement in the ductility of the columns. Confinement provided by the FRP tube may also improve the axial load-carrying capacity of the square columns if the confinement effectiveness of the FRP tube is sufficiently high. The corner radius of the tube has a significant influence on the confinement effectiveness of the square FRP tubes, and the effectiveness of confinement increases with the corner radius. As a result, the corner APFIS

6 radius significantly influences the trend of the second branch of the stress-strain curve and hence the ultimate strength of confined concrete. FRP tube thickness has a significant influence on the ultimate strain of confined concrete, and the ultimate strain increases with the tube thickness. The tube thickness also has some influence on the trend of the second branch of the stress-strain curve, and increased tube thickness results in an increase in the column load carrying capacity if the confinement effectiveness of the tube is high. REFERENCES Ahmad, S.H., and Shah, S.P. (1982). Complete triaxial stress-strain curves for concrete J. Struct. Div. ASCE, 8(4), ASTM. (21). Standard test method for tensile properties of polymer matrix composite materials, ASTM Standards, 15.3, ASTM D 39, West Conshohocken, Pa., Fam, A., Schnerch, D. and Rizkalla, S. (25). Rectangular filament-wound GFRP tubes filled with concrete under flexural and axial loading: experimental investigation, Journal of Composites for Construction, 9(1), Hong, W.K., and Kim, H.C. (24). Behavior of concrete columns confined by carbon composite tubes, Canadian Journal of Civil Engineering, 31(2), Lam, L., and Teng, J.G. (23). Design-oriented stress-strain model for FRP-confined concrete in rectangular columns, Journal of Reinforced Plastics and Composites, 22(13), 23, Mander, J.B., Priestley, M.J. N., and Park, R. (1988). Theoretical stress-strain model for confined concrete, Journal of Structural Engineering, ASCE, 114(8), Mirmiran, A., Shahawy, M., Samaan, M., El Echary, H., Mastrapa, J.C., and Pico, O. (1998). Effect of column parameters on FRP-confined concrete, Journal of Composites for Construction, ASCE, 2, 4, pp Mirmiran, A., Shahawy, M., and Beitleman, T. (21). Slenderness limit for hybrid FRP-concrete columns, Journal of Composites for Construction, ASCE, 5, 1, Ozbakkaloglu, T., and Oehlers, D.J. (27a). Concrete-filled square and rectangular FRP tubes under axial compression, Journal of Composites for Construction, ASCE, accepted. Ozbakkaloglu, T., and Oehlers, D.J. (27b). Behavior of axially loaded concrete-filled rectangular FRP tubes, Proceedings of the 8th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS-8), Patras, Greece. Ozbakkaloglu, T., and Saatcioglu, M. (26). Seismic behavior of high-strength concrete columns confined by fiber reinforced polymer tubes, Journal of Composites for Construction, ASCE, (6), Ozbakkaloglu, T., and Saatcioglu, M. (27). Seismic performance of square high-strength concrete columns in FRP stay-in-place formwork, Journal of Structural Engineering, ASCE, 133(1), Pessiki, S., Harries, K.A., Kestner, J.T., Sause R., and Ricles, J.M. (21). Axial behavior of reinforced concrete columns confined with FRP jackets, Journal of Composites for Construction, ASCE, 5, 4, Richart, F.E., Brandtzaeg, A., and Brown, R.L. (1928). A study of the failure of concrete under combined compressive stresses, Engineering Experiment Station, University of Illinois, Urbana, Illinois. Rochette, P., and Labossiere, P. (2). Axial testing of rectangular column models confined with composites, Journal of Composites for Construction, ASCE, 4, 3, Seible, F., Burgueño, R., Abdallah, M.G., and Nuismer, R. (1996). Development of advanced composite carbon shell systems for concrete columns in seismic zones. Proceedings of the 11th World Conference on Earthquake Engineering, Elsevier Science, Paper No Shao, Y., and Mirmiran, A. (25). Experimental investigation of cyclic behavior of concrete-filled fiber reinforced polymer tubes, Journal of Composites for Construction, ASCE, 9(3), Yamakawa, T., Zhong, P., and Ohama, A. (23). Seismic performance of aramid fiber square tubed concrete columns with metallic and/or non-metallic reinforcement, Journal of Reinforced Plastics and Composites, 22, 13, Zhou, W., Fan, L., and Xue, Y. (21). Shaking table testing of simply supported bridges with prefabricated GFRP tube jacketed RC columns, Proceedings of the International Conference on FRP Composites in Civil Engineering, Hong Kong, Vol. II, Zhu, Z., Ahmad, l., and Mirmiran, A. (25). Effect of column parameters on axial compression behavior of concrete-filled FRP tubes, Advances in Structural Engineering, 8, 4, APFIS

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