SEISMIC STRENGTHENING OF REINFORCED CONCRETE

COLUMNS WITH STRAIGHT CARBON FIBRE REINFORCED

POLYMER (CFRP) ANCHORS

Enrique del Rey Castillo1, Rhys Rogers2, Michael Griffith3 and Jason Ingham4…

ABSTRACT: The use of Externally Bonded Fibre Reinforced Polymer (EBR-FRP) systems is an established technique

for the structural improvement of existing buildings but the technique features disadvantages. Premature FRP-to-

concrete debonding has been commonly highlighted as one of the main problems, together with the difficulty of fully

wrapping the structural element when the structure presents complex geometries. FRP straight anchors are used to

transfer the forces from the FRP sheet into the structural element, ameliorating these 2 problems, but a comprehensive

design method for FRP anchors has not yet been established despite the increased use and research attention given to

FRP anchors.

A research project was undertaken with the ultimate aim of developing design equations suitable for wide

implementation of FRP anchors with EBR-FRP systems. The first stage of the project involved monotonically testing

single-anchors in tension to investigate the behaviour and capacity of isolated FRP anchors. However, a number of

factors that may have a significant influence on the capacity of the anchors could not be investigated with direct tensile single-anchor testing. Many of these factors are related to seismic response, such as the behaviour of the anchors when

subjected to tension-compression cycles and the effect of dynamic loads, which are highly relevant for the New Zealand

context.

To address some of the aspects not covered within the single-anchor tests, 6 full-scale reinforced concrete columns

replicating the ground-level columns of an existing understrength building in Wellington were tested. In addition to the

control column, 2 confinement schemes and 2 anchorage configurations were tested, all using pseudo-static loading,

with the aim being to verify that the peak moment developed during testing was forecasted accurately. Column test 6

was a proof-of-concept test, with the objective of the test being to verify whether the experimentally established column

moment and displacement ductility capacities could be predicted using a standardised design procedure. For column

test 6 the FRP sheet was installed without bonding the bottom part of the sheet to the column, and instead anchors were

used to hold the sheet in place, which allowed an increase in tensile strain of the FRP sheets above the value that would

cause debonding. This elevated strain enabled a larger column drift to occur when compared to the previous column tests, indicating that the displacement ductility capacity of the column could be designed by changing the height of the

debonded area.

1 Enrique del Rey Castillo, Department of Civil and Environmental Engineering, The University of Auckland, New Zealand.

Email: edel146@aucklanduni.ac.nz

2 Rhys Rogers, Structural Engineer, R&D and design at BBR Contech Ltd. Email: rrogers@contech.co.nz

3 Michael Griffith, School of Civil, Environmental and Mining Engineering, University of Adelaide, Australia.

Email: michael.griffith@adelaide.edu.au

4 Jason Ingham, Department of Civil and Environmental Engineering, The University of Auckland, New Zealand.

Email: j.ingham@auckland.ac.nz

1 Introduction

The use of Fibre-Reinforced Polymer (FRP) materials as Externally Bonded Reinforcement (EBR) is documented to be

more effective for strengthening and/or repairing Reinforced Concrete (RC) structures when compared to conventional

methods such as the use of metallic reinforcement or section enlargement (Bank 2006; Hollaway 2010). The main

advantages of the EBR-FRP system are low mass and high tensile capacity, plus greater durability when properly

installed. FRP sheets are the core material in the EBR-FRP system, which are saturated with epoxy resins to form a composite matrix that is adhered to the external surface of the structural member, which is typically composed of either

RC or masonry.

The primary deficiency of the EBR-FRP system is premature debonding of the FRP sheets from the concrete substrate

prior to development of the full tensile strength of the FRP fibres. While multiple methods to solve this problem have

been investigated, FRP anchors have been highlighted in the past as the most adequate technique to improve the FRP-

to-concrete bond strength and ensure the continuity of the load path from sheet to substrate (Kalfat et al. 2013). FRP

anchors consist of a bundle of fibres, or a fibre sheet, with one end of the bundle splayed out in a circular or fan shape

and bonded to an FRP sheet and the other end embedded into a pre-drilled hole using epoxy resin or passed through a

hole in the structural member and bonded to the other side of the RC member using a second splay. FRP anchors consist

of 3 components, as shown in Figure 1. The dowel is inserted into the structural member (either straight or bent at

insertion angle β), the fan is the portion where all the fibres from the FRP bundle are epoxy set to the FRP sheets, and

the key portion is the section where the fan transitions into the dowel. FRP anchors are typically divided into 2 types depending on the insertion angle β, as shown in Figure 1: straight anchors (a) and bent anchors (b and c).

(a) Straight anchor – front view (b) Bent anchor – front view (c) Bent anchor – side view

Figure 1 Attributes of FRP anchors

The main impediment to widespread implementation of FRP anchors in EBR-FRP systems is the lack of design

guidelines (Kalfat et al. 2013). To help in the development of these guidelines several failure modes were identified, and different models were proposed to calculate the capacity of the anchors depending on the failure mode. Kim (2010)

investigated concrete-related failure modes (concrete cone and dowel pull-out), Kanitkar (2016) investigated the fan-to-

sheet debonding failure mode, and the fibre rupture failure mode was investigated in an earlier stage of the current

research programme (del Rey Castillo, Griffith, et al. 2017c; del Rey Castillo, Griffith, et al. 2017a; del Rey Castillo,

Griffith, et al. 2017b). The last stage of the research programme was to verify that the design equations previously

developed for fibre rupture failure mode can be reliably implement in EBR-FRP strengthening schemes for existing

buildings, especially in the New Zealand context. Additionally, the effect of tension-compression cycles on the anchors

and the influence of confinement were investigated.

Different EBR-FRP intervention schemes have been investigated in the past, such as shear strengthening and/or repair

of beams (Kim et al. 2014), strengthening of slabs (Smith et al. 2013) and shear walls (Qazi et al. 2010), and seismic

strengthening of columns (Kim et al. 2011; Vrettos et al. 2013) for either deficient shear strength or deficient flexure

strength, see Figure 2, further details of each scheme can be found in (del Rey Castillo, Dizhur, et al. 2017). The seismic strengthening of columns was chosen as the most critical and representative example of the use of EBR-FRP systems to

be investigated, due to the seismicity of New Zealand. More information can be found in the literature regarding the

shear scheme represented in Error! Reference source not found.a than can be found regarding the flexural strengthening

scheme detailed in Error! Reference source not found.b, which has received less research attention. Additionally, after

consulting with local engineers it was found that engineers are more confident with the design of the EBR-FRP shear

scheme than with the design of the EBR-FRP flexural scheme. The flexural strengthening of columns was therefore

selected as the intervention of more urgent need of research.

(a) Shear strengthening (b) Flexural strengthening of column-foundation joint

Figure 2 Schemes for EBR-FRP strengthening of an RC column

2 Experimental programme

2.1 Material properties

Two concrete mixes were used, with one mix having a target compressive strength of 50 MPa for the foundation slab

and the second mix having a target compressive strength of 40 MPa for the column, delivered with a 15 day interval,

during which time the transverse reinforcement and the formwork for the column were installed. The average

compressive and tensile strength of each concrete mix reported in Table 1 was determined using NZS 3112-2(1986).

Table 1 Concrete mechanical properties

Column age (days) Foundation age (days)

7 23 28 333 7 28 43 353

Compressive strength (MPa) 34.3 36.6 38.9 44.3 43.4 49.9 52.5 56.2

Tensile strength (MPa) 2.4 3.3 3.5 4.0 4.5 5.2 5.4 5.8

One type of CFRP fabric and one type of CFRP rope (also known as CFRP bundles) were used to strengthen the

columns, with the net-fibre material properties for the FRP products (SIKA 2013b; SIKA 2014a; SIKA 2014b) and for

the epoxy resin (SIKA 2013a) used in this research project being reported in Table 2 and Table 3 respectively.

Table 2 FRP material net-fibre properties

Net fibre

thickness (mm)

Tensile Modulus E (GPa) Tensile strength σm (MPa) Ultimate strain εm (%)

x̅ Design x̅ Design x̅ Design1

CFRP fabric 0.331 75.7 68.1 968 833 1.3 1.1

CFRP Anchor† 28 mm2 - 230 - 2100 - 1.6

CFRP Anchor‡ 28 mm2 - 253 - 2479 - 1.0

† Manufacturer-specified properties

‡ Experimentally-obtained properties

Table 3 Manufacturer-specified resin material properties

Tensile Modulus (GPa) Tensile strength (MPa) Ultimate strain (%)

7 days at 23°C 3.5 45.0 1.5

72 hours at 60°C 3.2 72.5 4.8

2.2 Test columns

The six columns were designed to generally replicate the bottom half of a ground floor column of a RC structure in

Wellington strengthened with EBR-FRP and FRP anchors, with the design parameters being consistent with similar

structures in New Zealand. The details of column size, reinforcement and FRP materials installed in each column can be

Straight

anchor

FRP

sheet RC slab

FRP sheet

Straight

anchor

Obstruction wall

seen in Figure 3. The transverse reinforcement was artificially increased to prevent undesired failure modes such as

shear failure or buckling of the longitudinal bars. The objective of column test 1 was to characterize the behaviour of

the as-built column to be able to later compare with the results from the strengthened columns. Column 2, featuring

only longitudinal FRP and no supplementary FRP confinement reinforcement, was the first attempt to design and

calculate the maximum moment of the FRP strengthened column with FRP anchors implemented into the design, in

addition to verifying the influence of compression-tension loading cycles on anchor behaviour. Column 3 featured the

same anchor configuration and strength as for column 2, but with FRP transverse confinement also provided to

investigate the influence of this confinement on the system response. Column 4 was a second attempt to design the

anchors and elevate the flexural strength of the column at higher values than for columns 2 and 3. Columns 5 and 6

were designed to achieve the same flexural strength as for column 4, but column 5 was designed to shift the failure

location from the anchors to the sheets and column 6 was designed to improve the ductility capacity of the column by installing a bond breaking layer at the bottom of the column.

Figure 3 Reinforcing and EBR-FRP details

2.3 Testing set-up and loading protocols

The column was secured to the strong floor using 8 post-tensioned rods that passed through the foundation slab, and 2

additional post-tensioned rods were used to apply horizontal prestressing to the foundation to further increase the

foundation strength and reduce the size of the foundation, with an external frame installed around the test for safety. An

axial load of 500±50 kN was applied to the top of the column using two hydraulic jacks that reacted against a double C-

channel beam and applied tension to threaded rods that passed through the strong floor. A load cell was placed between

each jack and the beam to measure the magnitude of the axial load, while the load cell from the horizontal actuator

measured the applied lateral load. Draw wires plus the Linear Variable Differential Transformer (LVDT) installed

inside the actuator measured the lateral displacement of the column, while additional LVDTs and draw wires were used

to monitor the displacement and/or uplift of the foundation. The strain in the longitudinal bars was calculated with

displacement gauges connected to the corner longitudinal bars while Digital Single-Lens Reflex (DSLR) cameras were used to capture the displacements and strains in the FRP material surface through the Digital Image Correlation (DIC)

technique. The testing set-up can be seen in Figure 4.

Figure 4 Testing set-up

The loading protocol used in columns 1 and 2 was based on the protocol described in ACI 374 (2013), which specifies a

minimum of 2 cycles for each drift increment, with 2 increments up to the yield point and increments equal to the yield

drift from that point, see Figure 5. The increment in the non-elastic range of column 2 was reduced to better capture the

failure of the anchors, resulting in a large number of loading cycles. A different loading protocol was used for the

remaining tests, based on previous studies (Bournas & Triantafillou 2009; Goksu et al. 2012) and also the behaviour

observed in column 2. The loading protocol for columns 4 to 6 consisted of a 0.125% incremental drift up to 1% drift,

followed by a 0.25% increment in drift from a drift level of 1% to a drift of 2.5% and a 0.5% increment in drift from

2.5% to 5% drift, see Figure 5. Three cyclic steps were applied for each drift increment, due to the significant damage observed in the second and third cycles when column 2 was tested, with the loading rate being always maintained below

1 mm/sec.

Actuator

Draw wires

Displacement gauges

Safety frame

Cameras

(a)

Loading protocol according to ACI 374 (2013) (b) Loading protocol for columns 3 to 6

Figure 5 Loading protocols used in the project

2.4 Strengthening process

The first step of the FRP installation process was to drill holes all the way through the foundation, to grind the column

surface to expose the aggregate and open the concrete pores to improve the FRP-to-substrate bond strength, and to

thoroughly remove any remaining dust particle from the concrete surface and aggregate pits. The FRP materials were

then installed starting with a layer of FRP sheet, followed by the FRP anchors and the second (and third when

necessary) layers of FRP sheets, and finishing with the confinement layer, see details in Figure 6. Column 6 featured a

plastic sheet to act as a bond breaking layer around the concrete over the bottom half metre of the column. More details

of the anchor installation process can be found in (del Rey Castillo, Griffith, et al. 2017b).

(a) Anchors installed onto first layer

of longitudinal sheets

(b) Second and third layers of

longitudinal sheets installed on top

of the anchors

(c) Confinement sheets being

installed on top of the longitudinal

sheets

Figure 6 FRP materials installation details

3 Experimental Results

Despite the large amount of data collected from the 6 columns, only a summary of the results can be reported herein due

to space constrains. The reported data is limited to the applied lateral force, the displacement measured at the point

where the load was applied, the moments and drifts calculated from these loads and displacements, photographic

records of the observed failure, and a few examples of the strain fields obtained using DIC. No discussion regarding

rebar or FRP material strain, rotation or curvature is included herein.

In Figure 7 the force-displacement hysteresis curves are plotted in red for each column, together with the backbone

curves plotted as dashed black lines. During the testing of column 1, at the displacement where inelastic response was

expected to commence, an equipment failure triggered the actuator to rapidly push the column to the maximum stroke. Consequently only the first part of the test has been plotted in red. The backbone curve of column 1 was obtained from

moment-curvature analysis using Response 2000 (2000) and validated with the readings from column 2 after the FRP

anchors failed (represented in purple in Figure 7), which corresponds to the predicted as-built behaviour. The maximum

drift for column 2 was 3%, at which point the vertical post tensioning bars used to apply the column axial load were

exhibiting significant curvature and the test was stopped for safety reasons. This problem was resolved for the

remaining column tests by installing a rotating swivel that prevented curvature from developing in the post-tensioning

bars.

The design of the FRP strengthening scheme gave accurate results for peak moment, see Table 4, but further work is

necessary to accurately calculate the expected drift. The drift capacity has been typically defined as the drift at the point

when the peak load dropped 20% (represented as a black-dotted line in Figure 7), which is the point considered to lead

to either partial or total lateral instability and potential collapse failure. A 20% drop in moment was achieved after

either the FRP anchors or the FRP sheets started to fail, depending on the strengthening scheme deployed. A suggestion

is made that the current interpretation of failure for RC columns strengthened with FRP materials should be reviewed,

because of the high cost of repairing the column after FRP materials fail. The engineer might design a strengthening

scheme that is based upon the peak capacity of the column at the point when the FRP debonds, rather than when the

FRP fails, although this decision will result in a higher cost because more FRP material would be necessary. The behaviour of the columns was highly asymmetrical, especially after the peak load was reached and the fibres started to

fail, as can be seen in Table 4, Figure 7 and Figure 8.

Column 1

Column 2

Column 3

Column 4

Column 5 Column 6 Where:

Red continuous line represents the hysteresis loops,

Black dash line represents the backbone curve,

Black dotted line represents 80% of the peak load,

Purple line in column 1 plot represents the post FRP rupture behaviour of column 2, and

Cyan dash line represents the as-built behaviour in columns 2 to 6

Figure 7 Force-displacement hysteresis loops for each column

The peak moment was calculated using section analysis and the New Zealand Concrete Structures Standard (NZS 3101-

1 2006), two internationally recognized FRP design guidelines (ACI 440.2R 2008; CNR-DT 200 2013), and the design

equations developed by the authors for straight FRP anchors exhibiting fibre rupture (del Rey Castillo, Griffith, et al.

2017a). The calculated and measured peak moment in kN and the drift ratio at that point are reported in Table 4, together with the strengthening ratio and the ductility ratio. The strengthening ratio was defined as the ratio of the peak

moment of the strengthened column (Mn) over the peak moment of the as-built column (M1), while the ductility ratio

was defined as the ratio of the drift ratio at peak moment of the strengthened column (δn) over the drift ratio at peak

moment of the as-built column (δ1). The drift ratio at peak moment for column 1 was assumed to be the point where

inelastic behaviour commenced, see Figure 8.

Column 2 and column 3, which featured the same longitudinal FRP configuration, performed comparably in terms of

strength but the transverse confinement installed on column 3 stiffened the column significantly. The strength of

columns 4, 5 and 6 was nominally the same, but while column 4 was designed to exhibit FRP anchor rupture, column 5

and column 6 were designed to exhibit FRP sheet rupture. The strengthening ratio of the three columns was therefore

similar, and so was the ductility ratio of columns 4 and 5. However, the ductility of column 6 was increased using a

bond-breaking layer at the bottom section of the column, and by doing so the test results were more homogeneous and less asymmetrical between the push and pull loading directions. The objective was to provide engineers a design tool to

calculate and control not only the peak moment but also the drift corresponding to that moment. Note that the design

process is not included herein due to space constraints.

The asymmetric behaviour observed in the hysteresis loops reported in Figure 7 is partially quantified in Table 4, with

the largest moment asymmetry of 16% being observed in column 5. The maximum asymmetry in terms of drift was

observed in column 5 with a 65% difference between the push and pull loading difference, although such a large

difference was not observed in any other column and may be considered as an anomaly.

Table 4 Summary of results

Peak moment

(kNm)

Drift ratio at peak

(%)

Strengthening ratio

(Mn/M1)

Ductility ratio

(δn/δ1)

Column Calculated Push Pull Push Pull Push Pull Push Pull

1 367† 367† 3671 1.15‡ 1.15‡ 1 1 1 1

2 516 485 512 1.32 1.41 1.32 1.40 1.15 1.23

3 527 500 442 0.72 0.76 1.36 1.20 0.63 0.66

4 721 690 643 1.98 1.76 1.88 1.75 1.72 1.53

5 721 622 626 1.95 3.01 1.69 1.71 1.70 2.62 6 721 685 688 2.49 2.50 1.84 1.87 2.17 2.17

†Obtained with moment curvature analysis

‡ Point where the ductility of the curve change to ductile

4 Behaviour and failure mode

The moment-drift backbone and the idealised bilinear elastic-perfectly plastic curves are reported in Figure 8. Due to

the transverse steel of the columns being intentionally over-reinforced in order to prevent column shear failure, the first

column test did not experience the typical behaviour expected in RC columns where a post-peak strength decay is

expected when the cover concrete spalled, the longitudinal bars buckled and the stirrups failed. Instead, the strength

continued to increase as the longitudinal bars yielded progressively. In Figure 8 see the bilinear curve for column 1,

which was obtained by matching the area above and below the curve up to a 2.5% drift (maximum permissible value in (NZS 1170-5 2004)). The point identified as peak load was selected as the point where the column transitioned from

elastic to inelastic behaviour, and this point is marked by a red circle in Figure 8. The bilinear curves for columns 2 and

3 are coincident, as the FRP confinement did not significantly influence the capacity of the column. Similarly, the

backbone curves for columns 4, 5 and 6 are coincident, as the strengthening scheme for these three columns was

designed to achieve a similar peak strength. Loss of gravity load carrying capacity was not observed in any of the six

column tests.

Figure 8 Back bone and idealised bilinear elastic-perfectly plastic curves for each column

The idealised bilinear elastic-perfectly plastic behaviour typically used for RC design did not correctly represented the

behaviour of FRP strengthened RC columns, as the columns featured 3 different behaviour stages, being (1) an elastic

behaviour, (2) inelastic hardening, and (3) inelastic degradation.

Stage 1 - Elastic behaviour. The duration of the first stage of response was from the start of the test to when FRP-to-

concrete debonding commenced, at a point where the

moment was approximately 80% of the peak moment. The

hysteresis loops were mostly thin with no significant

degradation in second and third cycles and no residual

displacement that would imply damage in the column or the

FRP strengthening materials. This observation was

corroborated by visual inspection, with a single concrete

crack appearing at the column-base joint but no visible

damage being observed in the FRP, and no cracking noises

being heard. The longitudinal strain fields from -2% strain

(compression) to +2% strain (tension) obtained with DIC and reported in Figure 10 were smooth and below the ultimate

strain of the FRP, although high strains were starting to

concentrate at parts where damage eventually occurred.

Figure 9- Hysteresis loops in the first stage of testing

Point selected as

yield point

2.5% drift as per

NZS 1170.5

2.5% drift as per

NZS 1170.5

(a) Results on sheets – Column 5

cycle to ±1% drift

(b) Results on anchors – Column 2 cycle to ±1%drift

Figure 10-Longitudinal strain DIC results from the first stage of testing

Stage 2 - Inelastic hardening. The second stage of response ranged from approximately 80% of the peak moment, where FRP-to-concrete debonding commenced, until the peak moment was reached, where the first FRP rupture occurred after

the FRP completely debonded from the concrete substrate, see Figure 12, with the blue hysteresis loops representing the

behaviour of stage 1 and the black hysteresis loops representing the behaviour of stage 2.The damage in the FRP

material and the strain fields obtained with DIC can be observed in Figure 12 and Figure 13 respectively. The FRP

debonded from the concrete substrate in the cycles that resulted in the FRP material being subjected to compression

forces, rather than in those cycles when the FRP material was subjected to tension forces.

Debonding of the FRP when subjected to compression forces,

identified by cracking noises and coin-tapping, was related to

the stiffness of the material, which cannot accommodate the

deformed column curvature. This premature debonding of the

FRP materials when subjected to compression was not observed in column 6, where FRP debonding occurred when

the FRP material was subjected to tension forces and the FRP-

to-concrete debonding progressively developed in a more

uniform, symmetric and homogeneous manner up the height

of the column when compared to the FRP-to-concrete

debonding observed in the other columns. Premature

debonding of the FRP material when subjected to compression

forces could potentially be alleviated by not bonding the FRP

materials to the bottom part of the column. During the second

and third cycles of each drift increment the strength degraded

and the residual displacement increased when compared to the

response of the first behaviour stage and FRP-to-concrete debonding was observed to incrementally progress towards

the top of the column.

Figure 11- Hysteresis loops in the second stage of

testing – Column 5

(a) First longitudinal FRP sheet damage - Column 6 (b) Concrete crack at the bottom of the column and first FRP

crack in the anchor – Column 2

Figure 12-Damage observed in the FRP materials

(a) Results from sheets –

Column 5 cycle to ±2.5%

drift

(b) Results from anchors – Column 2 cycle to ±2.5% drift

Figure 13- Longitudinal strain DIC results from the second stage of testing

Stage 3 - Inelastic degradation. Progressive rupture of the

FRP materials occurred at this stage, with the duration of

this rupture varying highly from column to column, and even from the push to the pull directions within the same

column. The variation was more obvious for column 5,

which featured a short duration of stage 3 in the pull

direction but a long duration in the push direction. The

lateral capacity decreased significantly in this stage as the

displacement increased, with sudden drops in strength as

fibres ruptured, see Figure 14, with the hysteresis from the

first stage in blue, those from the second stage in green and

those from the third stage in black. Degradation in the

second and third cycles within the increment and FRP

damage was observed, see Figure 15 and Figure 16. Loud cracking noises were heard as the fibres ruptured,

sometimes after the loading stage was completed, with the

residual displacement increasing significantly after the first

fibre rupture. Once all the longitudinal fibres were broken

the column resumed the behaviour of a column without

longitudinal fibres.

Figure 14- Hysteresis loops in the third stage of

testing – Column 5

(a) Results from sheets –

Column 5 cycle to ±5% drift

(b) Results from anchors – Column 2 cycle to ±5% drift

Figure 15-DIC results from the third stage of testing

(a) General rupture of the longitudinal FRP sheets –

Column 5

(b) Column 2 with all the anchors broken

Figure 16-Damage in the third stage

5 Summary and future work

A total of 6 RC columns, one being an as-built and 5 being flexurally strengthened with FRP sheets and anchors, were

tested by applying pseudo-static cyclic loads. The peak moment was accurately calculated using the New Zealand

Concrete Structures standard (NZS 3101 2006), EBR-FRP strengthening design guidelines (ACI 440.2R 2008; CNR-

DT 200 2013) and the FRP anchor design equations previously developed by the same authors (del Rey Castillo,

Griffith, et al. 2017a). The influence of tension-compression cycles and fatigue degradation originating from the cyclic

loading on the anchor capacity was negligible, and the retrofitted columns did not experience a loss of gravity load

bearing capacity. The behaviour was succinctly described using the recorded lateral load and displacement at the point where the lateral load was applied, the moment and drift calculated from these loads and displacements, and a few DIC

results and photographs. Three different behaviour stages were identified that represent the column behaviour.

Further experimental and analytical research should be undertaken to fully understand the behaviour of the FRP

strengthened RC columns, especially regarding the behaviour of the FRP sheets and anchors, the curvature of the

column and how the curvature affects the ductility.

6 Acknowledgments

The authors acknowledge the support of the technical staff of the Department of Civil and Environmental Engineering

at the University of Auckland. The materials used in this project were supplied by Contech Limited and Sika (NZ)

Limited and their contribution to the success of this project is gratefully acknowledged. Funding support provided by

the New Zealand Earthquake Commission is highly appreciated. This project was (partially) supported by QuakeCoRE,

a New Zealand Tertiary Education Commission-funded Centre.

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