EXPERIMENTAL EVALUATION OF FULL-SCALE FRP-CONFINED REINFORCED

CONCRETE COLUMNS

A. De Luca*,1, F. Matta1, A. Nanni,1,2 F. Nardone2, G. P. Lignola2, and A. Prota2 1Civil, Architectural and Environmental Engineering Department, University of Miami

2Department of Structural Engineering, University of Naples

*Civil, Architectural and Environmental Engineering Department, University of Miami EB-105 McArthur Engineering Building, 1251 Memorial Dr., Coral Gables, FL 33146-0630, US

E-mail: adeluca@miami.edu; Phone: 305-284-3391, Fax: 305-284-3492 ABSTRACT The external confinement of reinforced concrete (RC) columns by means of externally bonded fiber reinforced polymer (FRP) laminates is a well established technique for strengthening and retrofitting purposes. This paper presents a pilot research that includes laboratory testing of full-scale square and rectangular RC columns externally confined with glass and basalt-glass FRP laminates, and subjected to pure axial load. The objectives were: to understand the effectiveness of confinement as it relates to the cross-sectional geometry and size; to evaluate the efficiency of corner-bent FRP sheets; to analyze the contribution of internal longitudinal and transverse steel reinforcement; and, to provide much needed and currently unavailable experimental data. Specimens that are representative of full-scale building columns were designed according to dated codes (i.e., prior to 1970) for gravity loads only. In particular, the specimens were designed and constructed using the ACI 318-63 code-mandated minimum amount of longitudinal reinforcement, and minimum area of ties at maximum spacing. The length of the column specimens was 3.05 m. Three series of specimens (S1, S2 and S3) were tested. Series S1 corresponds to a side aspect ratio of 1, series S2 and S3 to a side aspect ratio of about 1.45. The area aspect ratio of series S2 and S3 evaluated with respect to series S1 is, respectively, 1 and 0.5. Each series included a confined specimen and an unconfined (control) counterpart. The discussion of the results includes a comparison with the values obtained using existing analytical models. KEYWORDS confinement, deformability, FRP laminates, full-scale RC columns, strengthening. INTRODUCTION Fiber reinforced polymer (FRP) composites are widely used for rehabilitation of existing reinforced concrete (RC) structures. In particular, confinement of RC columns is one of the most attractive applications of FRP laminates. Many studies have been carried out on FRP-confined RC prismatic columns in the past years, and several analytical models have been proposed (Mirmiran et al., 1998; Campione and Miraglia, 2003; Lam and Teng, 2003b; Kumutha et al., 2007; Wu and Wang, 2009). These models, however, do not converge to the same predicted values, and their validity for full-scale columns still has to be proven. It is herein defined as full-scale column an element with a minimum side larger than 0.3 m and a height versus minimum side greater than five. Also, predictive equations found in current design guidelines are based on models created for circular columns and modified by means of factors intended to account for the change in cross-sectional shape and its effect on the confining pressure. In circular concrete columns, the effectiveness of the FRP confinement is optimal since the geometric configuration allows the fibers to be effective around the entire cross section. Prismatic cross-sections behave differently, being well-recognized that the confining pressure is higher at the corners and lower along the flat sides, so that the cross-section is only partially confined. The research program presented in this paper aims at providing experimental evidence on the behavior of large-size FRP-strengthened RC columns subjected to pure compressive load. Nowadays, very limited data on full-scale column specimens is available due to high costs and lack of high-capacity testing equipments (Rocca et al., 2008). Experimental data on full-scale specimens is of critical importance to better address the main issues governing the confinement of concrete with composite materials and to validate new analytical and/or design models. The objectives of this study are to: study the behavior of full-scale RC columns; investigate the effectiveness of the FRP confinement in relation to the cross-sectional geometry and size; evaluate the efficiency

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of corner-bent FRP laminates; study deformability enhancement due to FRP confinement; and, compare the performance of basalt fibers to that of glass fibers. EXPERIMENTAL PROGRAM

Test matrix The test matrix (Table 1) is designed considering different factors, namely: shape factor (side-aspect ratio), volume factor (volume-aspect ratio based on a benchmark volume of 0.61×0.61×3.05 m3), type and amount of FRP sheet plies. Specimens are intended to represent real size building columns designed according to dated codes (i.e., prior to 1970) for gravity loads only. In particular, column specimens are designed using the ACI 318-63 code-mandated minimum amount of longitudinal reinforcement and minimum tie area at maximum spacing. Three series of specimens are considered: series S1 corresponds to a shape factor of 1.0 and a volume factor of 1.0; series S2 to a shape factor of 1.45 and a volume factor of 1.0; series S3 to a shape factor of 1.43 and a volume factor of 0.5. For each series (S1, S2 and S3), one column is kept unstrengthened and used as benchmark, whereas the other two specimens are confined with a glass and a hybrid glass-basalt FRP system, respectively, whose salient properties are reported in Table 2.

Table 1 – Test matrix

Specimen code

Cross-section geometry

Internal steel Reinforcement

Shape factor

Volume factor

Type of fibers

Yield of plies

S1-control

corners chamfered with a radius of about 1 in

ACI 318-63 minimum amount of

longitudinal reinforcement (1% of the gross area) and minimum tie area at maximum

spacing: 8 #8 longitudinal

bars and #4 ties at a spacing of 16 in

1.0 1.0 as-built benchmark specimen

S1-G 1.0 1.0 GLASS 5

S1-H 1.0 1.0 HYBRID 8

S2-control

corners chamfered with a radius of about 1 in

1.45 1.0 as-built benchmark specimen

S2-G 1.45 1.0 GLASS 5

S2-H 1.45 1.0 HYBRID 8

S3-control

corners chamfered with a radius of about 1 in

4 #8 longitudinal bars and #4 ties at a

spacing of 14 in

1.43 0.5 as-built benchmark specimen

S3-G 1.43 0.5 GLASS 5

S3-H 1.43 0.5 HYBRID 8

Table 2 – FRP system properties

Filament yarn properties Glass fabric Hybrid fabric

Type of fibers E Glass – HP Glass Basalt – Glass

Tensile modulus (GPa) 77 – 77 89 – 89

Tensile strength (MPa) 3,400 – 3,400 4,840 – 3,400

Tensile strain (%) 4.7 – 4.7 3.15 – 4.7

Laminate properties Glass fabric Hybrid fabric Thickness (mm) 0.25 0.20

Weight (g/m2) 600 320 The number of glass FRP sheet plies (5), reported in the last column in Table 2, is chosen with the intent to reproduce a real field application: in a common field application the average number of plies ranges between 3 and 6 in case of 600 grams per square meter fiber laminates or between 2 and 3 in case of 900 grams per square meter fiber laminates. The amount of hybrid FRP sheet plies (8) is designed to have approximately the same confinement ratio (defined as the fiber reinforcement volume ratio multiplied by the fiber modulus of elasticity) provided by the glass FRP laminates.

0.36 by 0.51 m2

0.51 by 0.74 m2

0.61 by 0.61 m2

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Instrumentation and test procedure The instrumentation in all the specimens consists of electrical strain gauges located on the longitudinal and transverse steel reinforcement, and on the FRP jacket at critical locations (corner areas and mid-section on each face of the prismatic specimens) along the perimeter of the cross-section, at the mid-height section of the strengthened specimens. Additionally, a total of twelve linear variable differential transformer (LVDT) sensors are used to measure the vertical displacement of the specimen, and to evaluate the horizontal (in-plane) dilation at the mid-height cross-section, along the two side and the two diagonal directions. The load is applied concentrically under a displacement control rate of 0.15 mm/min and the loading is conducted in five cycles (each one repeated) at increments of one fifth of the expected capacity of each specimen. RESULTS AND DISCUSSIONS The test results are summarized in Table 3. The following is reported: average concrete compressive strength, fc; maximum load applied to the column specimen, Fc,peak; average axial concrete stress (defined as the ratio between the maximum applied load and the gross cross sectional area), σc,peak; normalized axial stress (defined as the ratio between the average axial stress and the average concrete compressive strength), σc,peak/fc; ratio between the average axial confined concrete stress with respect to the control specimen for the reference series, σc,p /σc,pC; axial deformation at the peak load, ∆c,peak; post-peak axial deformation corresponding to a load equal to about 75% of the maximum load, ∆c,max; ratio between the post-peak axial deformation and the axial deformation at peak, ∆max/∆c,peak. Control specimen failure initiated with vertical cracks followed, first, by lateral deflection of the longitudinal bars contributing to the splitting of the concrete cover and, finally, by crushing of the concrete core and buckling of the longitudinal bars. All FRP-confined column specimens ultimately failed due to rupture of the FRP jacket. Cracking of the concrete core was heard during the post-peak phase and longitudinal bar buckling was visible after peeling off the ruptured FRP-jacket and removing the concrete cover. In Figure 1a, Figure 2a and Figure 3a, the normalized axial stress is plotted versus the axial deformation for series S1, series S2 and series S3 specimens, respectively. While testing specimen S1-G, a problem on the data acquisition system occurred and this caused the loss of the data of the post-peak behavior. Specimen S2-G experienced a premature failure localized on the top of the specimen due to stress concentration and has not been taken into account in this study. Photographs in Figure 1b-d, Figure 2b-c and Figure 3b-d document the failures of column specimens from series S1, series S2 and series S3, respectively. Splitting of the concrete cover and buckling of vertical steel bars accompanied the failure of all unconfined control specimens. Rupture of the FRP laminate characterized the failure of the confined specimens, as shown in Figure 1c-d, Figure 2c and Figure 3c-d. Fiber rupture always initiated in the proximity of a corner.

Table 3 – Test results

Spec. ID fc [MPa]

Fc,peak [kN]

σc,peak [MPa] σc,peak/fc σc,p /σc,pC ∆c,peak

[mm] ∆c,max [mm] ∆max/∆c,peak

S1-control 37.2 12,500 33.6 0.902 - 6.54 8.95 1.37 S1-G 48.6 17,800 47.9 0.986 1.09 7.01 - - S1-H 47.9 15,600 45.3 0.945 1.05 6.09 17.6 2.89

S2-control 46.9 16,400 44.1 0.926 - 7.09 7.73 1.09 S2-H 47.6 16,900 45.5 0.957 1.03 8.64 14.6 1.69

S3-control 34.5 5,520 30.6 0.886 - 8.00 8.46 1.06 S3-G 53.8 8,650 47.9 0.891 1.01 7.67 15.4 2.01 S3-H 46.8 7,970 44.1 0.943 1.06 8.00 27.7 3.46

No significant increment in concrete strength due to confinement has been observed. The normalized axial stress varies between 0.886 and 0.926 for the control specimens and between 0.891 and 0.986 for the confined ones. The increment in concrete strength due to confinement evaluated with the respect to the corresponding control specimen ranges between about 1.03 and 1.09 for all confined specimens, with the exception of specimen S3-G, which did not experience any gain in concrete strength. Factors contributing to the result for specimen S3-G may include those affected by preparation, setup and execution of the test itself. On the other hand, a noteworthy ductility enhancement has been noticed for all FRP-confined column specimens from series S-3 and an important improvement in deformability in the post-peak behavior has been experienced by the two specimens from series S-1 and S-2. The ratio between the post-peak axial deformation and the peak axial deformation ranges between 1.69 and 3.46 for the confined column specimens and between 1.06 and 1.37 for the control ones. Cross-sectional shape and size seem not to affect confinement effectiveness. Square and rectangular columns behaved similarly and no size effect has been observed comparing the results from series S2 and S3. Also, similar results are obtained in terms of ultimate capacity when different types of fabric are used.

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0

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axi

al s

tres

s S1-G

S1-HS1-control

0.51m

0.51m

a)

b)

c) d)

Figure 1 – Normalized axial stress- axial deformation response (a), photograph of failed specimens S1-control (b), S1-G (c) and S1-H (d).

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0.51m

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Figure 2 – Normalized axial stress – axial deformation response (a), photograph of failed specimens S2-control (b), S2-H (c).

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axi

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tres

sS3-G S3-H

S3-control

0.51m

0.36m

a)

b)

c) d)

Figure 3 – Normalized axial stress- axial deformation response (a), photograph of failed specimens S3-control (b), S3-G (c) and S3-H (d).

Figure 4 compares the experimental values of the normalized axial strength with those obtained using selected analytical models including those proposed by Mirmiran et al. (1998), Campione and Miraglia (2003), Lam and Teng (2003b), Kumutha et al. (2007), and Wu and Wang (2009).

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Figure 4 – Comparison of predicted normalized axial stress with experimental values

All models tend to overestimate the increment in concrete capacity in terms of normalized axial strength (first column in each group, Figure 4). However, the predictions from Mirmiran et al.’s, Campione and Miraglia’s and Wu and Wang’s models are quite accurate for specimens from series S1 and S2, whereas the discrepancy between experimental results and predicted values becomes higher for specimens from series S3. All models rely on the assumption that the concrete strength of an as-built unconfined column is equal to that of a control

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cylinder, f’c. This assumption was not verified in this study as f’c was found to be about 10% higher than the axial concrete strength of an as-built column. For this reason, in evaluating the increment in concrete strength due to confinement, instead of using the cylinders as a benchmark, after normalizing with respect to f’c, the strength of the control column specimen was used. This issue had never been addressed before because the models base their predictions on cylinders with height-to-diameter ratio equal to 2, while the work presented herein refers to elements with a height-side ratio equal or larger than 5 as typical of building columns. CONCLUSIONS Based upon the experimental evidence gained through the full-scale experiments presented in this paper, the following conclusions are drawn.

• Effective confinement of concrete columns with FRP composite laminates does not result in any significant enhancement of the compressive strength.

• FRP jackets provide excellent confinement, thereby enhancing deformability by offsetting buckling of the vertical reinforcing bars, and delaying unstable crack propagation in the concrete core.

• Existing semi-empirical models, which are typically developed and assessed on the basis of small-scale tests on confined cylinders, show their limitations in predicting the ultimate capacity of full-scale FRP-confined concrete columns representative of real-case scenarios.

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the NSF Industry/University Cooperative Research Center for “Repair of Buildings and Bridges with Composites” (RB2C) at the University of Miami, of the “REte dei Laboratori Universitari di Ingegneria Sismica” (RELUIS) at the University of Naples “Federico II”, and of Mapei S.p.A. Special thanks are extended to the Fritz Engineering Laboratory at Lehigh University, in particular to Mr. Frank Stokes and Mr. Gene Matlock, and to the Building and Fire Research Laboratory at the National Institute for Standards and Technology (NIST), in particular to Mr. Steven Cauffman and Mr. Frank Davis, for the assistance in planning and conducting the tests. REFERENCES ACI Committee 318, 1963, “Building Code Requirements for Structural Concrete” (ACI 318-63), American

Concrete Institute. ACI Committee 440, 2008, “Guide for the Design and Construction of Externally Bonded FRP Systems for

Strengthening Concrete Structures (ACI 440.2R-08 American Concrete Institute, Farmington Hills, MI, 76 pp.

Campione, G., and Miraglia, N., 2003, “Strength and Strain Capacities of Concrete Compression Members Reinforced with FRP,” Cement & Concrete Composites, 25(1), pp. 31–41.

Kumutha, R., Vaidyanathan, R., and Palanichamy, M. S., 2007, “Behaviour of Reinforced Concrete Rectangular Columns Strengthened Using GFRP,” Cement & Concrete Composites, 29(8), pp. 609–615

Lam, L., and Teng, J., 2003b, “Design-Oriented Stress- Strain Model for FRP-Confined Concrete in Rectangular Columns,” Journal of Reinforced Plastics and Composites, 22(13), pp. 1149–1186.

Mirmiran, A., Shahawy, M., Samaan, M., and El Echary, H., 1998, “Effect of Column Parameters on FRP-Confined Concrete,” Journal of Composites for Construction, 2(4), pp. 175–185.

Rocca, S.; Galati, N.; and Nanni, A., 2008, “Review of Design Guidelines for FRP Confinement of Reinforced Concrete Columns of Noncircular Cross Sections,” Journal of Composites for Construction, 12(1), pp. 80-92.

Wu, Y.-F., and Wang, L.-M., 2009, “Unified Strength Model for Square and Circular Concrete Columns Confined by External Jacket,” Journal of Structural Engineering, 135(3), pp. 253–261.

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