EXPERIMENTAL PROGRAM Test specimens and experimental parameters

The experimental program aimed to study the flex- ural strengthening of old-type nonseismically detailed RC columns with externally bonded FRP sheets, which are anchored at the columns’ end sections with fiber anchors in the form of spikes, and compare the effectiveness of different anchor schemes. A total of four large-scale RC column

Ioannis Vrettos received his Diploma in civil engineering and his MSc in seismic design of structures from the University of Patras, Patras, Greece, in 2007 and 2009, respectively. His research interests include advanced materials and seismic retrofitting of reinforced concrete structures.

Efstathia Kefala received her Diploma in civil engineering and her MSc in seismic design of structures from the University of Patras in 2007 and 2009, respectively. Her research interests include advanced materials and seismic retrofitting of reinforced concrete structures.

ACI member Thanasis C. Triantafillou is a Professor of civil engineering and Director of the Structural Materials Laboratory at the University of Patras. He received his Diploma in civil engineering from the University of Patras in 1985 and his MSc and PhD from the Massachusetts Institute of Technology, Cambridge, MA, in 1987 and 1989, respectively. He is a member of ACI Committee 440, Fiber-Reinforced Polymer Reinforcement. His research interests include the application of advanced polymer- or cement-based composites in combination with concrete, masonry, and timber with an emphasis on strengthening and seismic retrofitting.

specimens with the same geometry were constructed and tested under cyclic uniaxial flexure with constant axial load (Fig. 1(a)). The specimens were flexure-dominated cantile- vers (that is, slender and designed to fail by yielding of the longitudinal reinforcing bars) with a height to the point of application of the load (shear span) of 1.6 m (63 in.) (half a typical story height) and a cross section of 250 x 250 mm (9.84 x 9.84 in.). To represent old-type columns, speci- mens were reinforced longitudinally with four deformed bars 14 mm (0.55 in.) in diameter and 8 mm (0.32 in.) diam- eter deformed stirrups, closed with 90-degree hooks at both ends, at a spacing of 200 mm (7.87 in.). The geometry of a typical cross section is shown in Fig. 1(b).

The specimens were designed such that the effect of two basic parameters on the effectiveness of anchors—the number of anchors and the amount of fibers in each anchor— could be investigated. The specimens are described in the following, supported by Fig. 2.

• One specimen was tested without flexural strengthening as the control specimen. As in all strengthened speci- mens, however, longitudinal fiber sheets were confined at the base of the column with an FRP jacket so buckling of those fibers could be prevented; the same confining jacket was also used in the control specimen (Fig. 2(a)).

As a result, the only difference between the control specimen and any other specimen was due to the imple- mentation of flexural strengthening through the use of longitudinal sheets in combination with anchors. The jacket was made of a CFRP sheet that extended from the column base to a height of 600 mm (23.62 in.). • Specimen 2_1.5 was strengthened with a 200 mm

(7.87 in.) wide epoxy-impregnated carbon-fiber sheet on each of the two opposite sides of the column (those with the highest tension/compression). The CFRP sheet extended from the column base to a height of 1.4 m (55.12 in.) and was anchored at the base block with two carbon-fiber spike anchors on each side (Fig. 2(b)). The cross-sectional area of the fibers in each anchor was equal to 0.75 times the cross-sectional area of the fibers in the CFRP sheet; hence, the total cross-sectional area of the fibers in the two anchors was equal to 1.5 times that of the CFRP sheet. Finally, the column was confined with a jacket identical to that used in the control specimen.

• Specimen 3_1.5 was strengthened the same as 2_1.5 but with three instead of two anchors per side (Fig. 2(c)). Those anchors were 33% lighter than those in 2_1.5: each one had a cross-sectional area equal to 0.50 times the cross-sectional area of the fibers in the CFRP sheet; hence, the total cross-sectional area of the fibers in the three anchors was again equal to 1.5 times that of the CFRP sheet.

• Specimen 2_1.0 (Fig. 2(d)) was strengthened the same as 2_1.5 but with the light anchors used in Specimen 3_1.5. In summary, except for the control specimen, the speci- mens’ notation is as follows: the first number denotes the number of anchors on each side at the base of the column and the second number denotes the ratio of the fiber cross section in the anchors to that in the CFRP sheet.

Strengthening procedure

One unidirectional carbon-fiber sheet 1.4 m (55.12 in.) long and 200 mm (7.87 in.) wide was bonded on a properly prepared concrete surface on each of the two opposite sides

Fig. 1—(a) Schematic of test setup; and (b) cross section of columns. (Note: Dimensions in mm [in.].)

of the strengthened columns. The sheet was placed with fibers in a vertical configuration and was terminated at the column base.

Fiber anchor spikes were applied on top of the CFRP sheet at a spacing of 100 mm (3.94 in.) or 67 mm (2.64 in.) for columns with two or three anchors per side, respectively (Fig. 2). Spikes were formed from dry carbon fibers (half dry and half coated with epoxy). Holes were drilled into the

base of the column with a depth of 250 mm (9.84 in.) and a diameter of 14 or 16 mm (0.55 or 0.63 in.) for Specimens 3_1.5 and 2_1.0 or 2_1.5, respectively. The holes were filled with epoxy (Fig. 3(a)) to half of their depths. Each anchor spike was inserted into the holes after applying the CFRP sheets on the two opposite sides of the columns (Fig. 3(b)) and the protruding dry fibers were fanned out over the CFRP sheet (Fig. 3(c)). This method of anchoring was selected on

Fig. 2—Four columns tested: (a) control; (b) Specimen 2_1.5; (c) Specimen 3_1.5; and (d) Specimen 2_1.0. (Note: Dimensions in mm [in.].)

Fig. 3—(a) Filling of holes in anchorage region with epoxy resin; (b) place- ment of carbon-fiber anchor; (c) fanning out of fiber anchors over CFRP sheet; (d) local jacketing with CFRP; and (e) position of displacement transducers. (Note: Dimensions in mm [in.].)

the basis of transferring the tension forces from the CFRP sheet terminating at the bottom of each column into the concrete base. Finally, all columns received jacketing by wrapping a single layer of a 600 mm (23.62 in.) wide carbon sheet, identical to that used in the columns’ longitudinal direction (Fig. 3(d)). The effectiveness of confinement was improved by rounding the four corners near the base of each column to a radius equal to 25 mm (0.98 in.).

Test setup and materials

The columns were fixed into a heavily reinforced 0.5 m (19.68 in.) deep base block 1.2 x 0.5 m (47 x 19.7 in.) in plan, within which the longitudinal bars were anchored with 50 mm (1.97 in.) radius hooks at the bottom. The longitudinal bars 14 mm (0.55 in.) in diameter had a yield stress of 545 MPa (79.0 ksi), a tensile strength of 652 MPa (94.5 ksi), and an ultimate strain equal to 13.7% (average values from six specimens). The corresponding values for the steel used for the stirrups were 351 MPa (50.9 ksi), 444 MPa (64.4 ksi), and 19.5%. To simulate field conditions, the base blocks and the columns were cast with separate batches of ready mixed concrete (on 2 consecutive days). Casting of the columns was also made with separate batches due to the unavailability of a large number of molds. The average compressive strength and standard deviation on the day of testing the columns—measured on 150 x 150 mm (5.9 x 5.9 in.) cubes (average values from three specimens)—were equal to 17.1 and 0.95 MPa (2478 and 138 psi), respectively, suggesting that the variability in concrete strength would not affect the column test results. Cylinders with a diam- eter of 150 mm (5.9 in.) and a height of 300 mm (11.81 in.) were also used to obtain the splitting tensile strength of the concrete; the average tensile strength that was obtained from six specimens on the day of testing was equal to 2.2 MPa (319 psi).

The carbon-fiber sheet used as both longitudinal reinforce- ment (vertical fibers) and confinement (horizontal fibers) was a commercial unidirectional fiber product with a weight of 644 g/m2 _{(2.62 × 10}–6 _lb/in.2_{) and a nominal thickness}

(based on the equivalent smeared distribution of fibers) of 0.37 mm (0.0146 in.). The mean tensile strength and elastic modulus of the fibers (as well as of the sheet when the nominal thickness is used) was taken from data sheets equal to 3790 MPa (549.27 ksi) and 230 GPa (33,333 ksi), respectively. The carbon-fiber sheet was impregnated with a commercial low-viscosity structural adhesive (two-part epoxy resin with a mixing ratio of 3:1 by weight) with a tensile strength of 70 MPa (10.15 ksi) and an elastic modulus of 3.2 GPa (464 ksi) (cured for 7 days at 23°C [73°F]). The values of the tensile strength and elastic modulus for the epoxy-impregnated sheet were taken from data sheets equal to 986 MPa (142.9 ksi) and 95.8 GPa (13,884 ksi), respectively, corresponding to a thickness equal to 1 mm (0.039 in.).

Each anchor comprised a tow of carbon fibers of the same type used in the unidirectional sheets. The weight of the fibers for the anchors used in Specimens 3_1.5 and 2_1.0 was 63 g/m (0.0035 lb/in.); the anchors used in Specimen 2_1.5 were 50% heavier and the respective weight of the fibers was 94.5 g/m (0.0053 lb/in.). Impregnation and bonding of the fiber anchors was done using the same epoxy adhesive used for the impregnation of the carbon sheets.

The columns were subjected to lateral cyclic loading, which consisted of successive cycles progressively increasing

by 5 mm (0.20 in.) of displacement amplitudes in each direc- tion. The loading rate was in the range of 0.2 to 1.1 mm/s (0.008 to 0.043 in./s)—the higher rate corresponding to a higher displacement amplitude—all in displacement- control mode. At the same time, a constant axial load was applied to the columns, corresponding to 25.4% of the members’ compressive strength, which was calculated by multiplying the gross section area by the strength of the concrete. The lateral load was applied using a horizontally positioned 250 kN (56.2 kip) MTS actuator. The axial load was exerted by a set of four hydraulic cylinders with auto- mated pressure self-adjustment acting against two vertical rods connected to the strong floor of the testing frame through a hinge (Fig. 1(a)). As a result of this loading scheme, the variation of axial load during each test was negligible. With this setup, the P-D moment at the base section of the column is equal to the axial load times the tip displacement (that is, at the piston fixing position) of the column times the ratio of the hinge distance from the base (0.25 m [9.84 in.]) and the top (0.25 + 1.60 = 1.85 m [72.83 in.]) of the column (that is, times 0.25/1.85 = 0.135).

The displacements and axial strains at the plastic hinge region were monitored using six displacement transducers (three on each side, perpendicular to the loading direction) fixed at the cross sections 130, 260, and 450 mm (5.12, 10.24, and 17.72 in.) from the column base, as shown in Fig. 1(a) and 3(e).

EXPERIMENTAL RESULTS