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INVESTIGATION OF EFFICIENT ANCHORAGE SYSTEMS FOR SHEAR AND TORSION

RETROFITTING OF BOX GIRDER BRIDGES

By R. Kalfat, R. Al-Mahaidi, G. Williams

Synopsis: Carbon Fibre reinforced Polymers (CFRP) have become an effective solution to upgrade and strengthen existing box girder bridges in flexure, shear and torsion. The introduction of CFRP strain limitations to prevent premature delamination together with the increasing strengthening demands and the necessity for use of fibres of increasing stiffness and thickness has resulted in a very poor CFRP material utilisation levels achieved in practice. An effective method to increase CFRP material utilisation is by appropriately anchoring the ends of the CFRP. In this paper, a study into CFRP end anchorage solutions is presented which formed the basis of the experimental program. Both uni-directional and bi-directional fabric was applied to the ends of CFRP laminates and tested under direct shear loading. Uni-directional fabric was oriented both horizontally across and parallel to the direction of the laminate. In all cases it was found that the anchorages solutions tested resulted in a distribution of fibre-to-adhesive bond stresses over a greater length, width of concrete and could potentially result full CFRP utilisation and laminate rupture. Key words: CFRP, Concrete, Strengthening, Bond strength, Uni-directional, Bi-directional, Fabric, Anchorage

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Robin Kalfat is a practicing Structural Engineer for Structural Systems Limited and postgraduate PhD student at the Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, in Melbourne, Australia. His interests include: design of CFRP strengthening systems, post-tensioning, CFRP Anchorage research, experimental investigations and numerical modelling using finite element method. Riadh Al-Mahaidi is a Professor of Structural Engineering at Swinburne University of Technology and Adjunct Professor at Monash University in Melbourne, Australia. He received his MSc and PhD degrees from Cornell University. His research interests include rehabilitation of concrete and steel structures using FRP composites, finite element modelling of concrete structures, and strength assessment and rehabilitation of bridges. He is a member of ACI-ASCE Committee 447 and ACI-440. Grahme Williams is a practicing bridge engineer working for Sinclair Knight Merz (SKM) in Melbourne, Australia. His particular area of interest is in the use of CFRP strengthening systems having been involved with a range of activities including laboratory investigations, field monitoring of repaired structures, design and optimisation of systems and application to in-service bridges.

INTRODUCTION Many concrete bridges currently in use have exceeded their original design life. Meanwhile, allowable traffic loads have also increased over the last few decades. The increased demand placed on existing bridges and infrastructure has lead to the necessity of widening of existing bridges to accommodate additional lanes and a higher traffic loading. In addition, revisions to bridge deign codes and evaluation specifications have taken effect over the years, along with a better understanding of member behaviour. This has lead to more stringent design and assessment requirements for concrete girder bridges, particularly concerning shear capacity (AASHTO 1994; ACI318-95 1995; CAN/CSA-S6 1988; Drimoussis 1994). These combined factors have lead to an increasing number of bridge upgrade and restoration projects with a focus on shear and torsion. A good example of this is the Westgate Bridge in Melbourne, Australia, which is one of the largest CFRP strengthening projects in the world. The increasing challenges faced to strengthen existing concrete bridges and infrastructure has resulted in intensive research and the introduction of carbon fibre reinforced polymers (CFRP). These materials are excellent for external strengthening because of their high tensile strength, light weight, resistance to corrosion, superior durability, and cost-effective installation process (Khalifa 2000). Although the cost of FRP products remains high, the reduced labour costs and minimal traffic disturbances of a CFRP rehabilitation solution make this repair technique competitive compared to the traditional rehabilitation methods. Fibres are typically applied to the concrete after adequate surface preparation; which usually involves sandblasting, water jetting and application of a suitable primer. CFRP preformed laminates or fabrics are bonded to the concrete surface using high strength epoxy resin.

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Previous studies show that stresses in CFRP laminates are transferred to concrete beams via the adhesive interfaces and the effectiveness of the strengthening technique are largely determined by the bond behaviour between CFRP material and the concrete. It has been demonstrated that failure of concrete structures retrofitted with CFRP usually occurs by de-lamination of the CFRP from the concrete substrate. Extensive research has been undertaken to understand the mechanisms of CFRP application and failure which has resulted in several design guidelines introduced in recent years. (ACI 440.2R-08 2008; Concrete Society Technical Report No. 55 2004) are examples of such guidelines. The introduction of CFRP strain limitations to prevent premature delamination together with the increasing strengthening demands and the necessity for application of high modulus fibres has resulted in very low CFRP material utilisation levels achieved in practice; These can be as low as 10-25% of the ultimate CFRP UTS (Kalfat & Al-Mahaidi 2010). An effective method to increase CFRP material utilisation is by appropriately anchoring the ends of the CFRP. Design guidelines such as (ACI 440.2R-08 2008) recognise the benefits of anchorage systems and permit designers to utilise higher CFRP strain provided that the anchorage device is backed up by sufficient experimental testing (Refer section 11.4.1.2). Research into CFRP anchorage has largely been confined to flexural strengthening and has taken the form of U-straps, mechanical fastening and clamping devices (Al-Amery & Al-Mahaidi 2006; Duthinh 2001; Orton 2008; Pham & Al-Mahaidi 2006; Pornpongsaroj 2003; Sagawa 2001; Sawada 2003; Teng, JG, Smith, S.T, 2001). Limited research has been conducted for CFRP anchorages used in pure shear and torsion which are often a critical requirement for concrete T-beams and box girder bridges. Investigation in the use of anchorages at the web, flange and the ends of CFRP laminates has been conducted by: (Aridome 1998; Lee 2008; Melo 2003; Ortega Carlos. A 2009; Orton 2008; Sato 1997; Tanarslan 2008), which have taken the form of steel anchorages, flange embedment and spike anchorages. However, there remains a lack of sufficient experimental data to substantiate the widespread use of such anchors. Further more, very little testing has been conducted into the use of uni-directional and bi-directional fabric as a form of plate end anchorage. In this paper, an experimental study into CFRP end anchorage solutions, utilising uni-directional and bi-directional fabric applied to the ends of CFRP laminates, tested under direct shear loading will be presented. The following testing was conducted to investigate potential anchorage solutions for the Westgate Bridge strengthening project in Melbourne Australia.

RESEARCH SIGNIFICANCE

In practical applications, the use of advanced composite materials is limited due to the ineffective bond between the composite and the concrete. The research presented therein investigates the use of CFRP fabric anchorage solutions to increase the efficiency and material utilisations of CFRP laminates when applied to concrete structures. The higher CFRP strain levels achieved warrants the adoption of less stringent strain limitations in design, resulting in both material and cost savings.

Investigation of Efficient Anchorage Systems for Shear and Torsion Retrofitting of Box Girder Bridges 42-3

EXPERIMENTAL PROGRAM Specimen Design The following experimental program presents anchorage types 2 – 6 of an investigation into CFRP anchorage systems. The reader is referred to (Kalfat & Al-Mahaidi 2010) for a description of type 1, where mechanical substrate strengthening as a form of plate end anchorage is presented. Prior to the construction of the anchorage specimens, an unanchored control specimen (type 0) was tested to form the benchmark for later studies. A full scale test set-up was designed using material properties prevalent on site for a comparative study. The factors which were considered in the current test program included: the depth of the concrete free edge dc, CFRP bond length Lf and the width ratio between the FRP strip and the concrete prism bf/bc. (Kalfat & Al-Mahaidi 2010) provides a detailed description of these parameters and how they affected the resulting specimen geometry.

Figure 1: Anchorage types 2 -5 applied to a box girder bridge.

Anchorage type 2 was developed for application at the web-flange interfaces (refer figure 1) of concrete box sections. The anchorage comprised of using 2 plies of 250mm (9.84 inches) wide uni-directional CFRP fabric applied horizontally across the laminate strip (refer figure 3). The direction of fabric fibres was orientated 90° to the direction of laminate. The first sheet overlayed the second, sandwiching the laminate strip in between. Anchorage type 3 is intended for use where combined shear and torsional strengthening is a requirement and utilised a unidirectional fibre orientation applied parallel to the direction of the laminate. In order to develop the tensile stresses, full wrapping of the concrete section is usually required for torsional strengthening. This can be achieved by using complete CFRP wrapping applied to all faces of the concrete section. However, due to the practical difficulties of achieving the complete wrapping of box girder bridges, CFRP U-wrapping with appropriate anchorage into the flanges is usually adopted in practice. This is often achieved through a reliance on the higher in plane flange capacity.

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Where the use of CFRP laminates in place of fabric is preferred due to strength, economy or practical requirements, a suitable detail to transfer the tensile forces around the section corners is required. The anchorage type 3 solution utilises L-shaped lengths of CFRP unidirectional fabric applied to the corners of a box section. These are appropriately lapped with a CFRP laminate which is applied to the main faces of the concrete prism (refer figure 3). In order to achieve a more efficient distribution of fibre - adhesive stresses over a greater area of concrete, two layers of a bi-directional fabric was implemented in anchorage types 4 and 5 to anchor the CFRP laminate. Anchorage type 6 was later developed to improve the performance of the type 3 anchor by adding a single layer of bi-directional fabric to the uni-directional fabric which continued around the corners of the concrete prism (refer figure 3).

Experimental Setup Many alternative experimental set-ups have been used by researchers for determining the FRP-to-concrete bond strength. The experimental design used in this study was based on the near end supported (NES) single pull test configuration. For a detailed description of the experimental setup, design and procedure refer to the first stage of works (Kalfat & Al-Mahaidi 2010). Figure 2 shows the specimen testing rig details used throughout the experimental program.

(a) (b)

Figure 2: Specimen testing rig details (a) configuration of test rig (front view); (b) configuration of test rig (side view)


Test Preparation and Material properties The experimental program utilised two differing concrete prism dimensions suitable for each anchorage type. Types 0, 2 and 4 utilised a total of 4 (type A) reinforced concrete blocks of dimension 250mm x 300mm x 600mm (9.84 inches x 11.81 inches x 23.62 inches) (Figure 3a, 3b, 3d). Types 3, 5 and 6 utilised 2 (type B) reinforced concrete blocks of dimension 200mm x 400mm x 600mm (7.87 inches x 15.75 inches x 23.62 inches) with a curved end recessed from the base of the prism (Figure 3c, 3e, 3f). Blocks were reinforced nominally with 4 x16mm (0.63 inches) diameter bars at 200mm (7.87 inches) centres each face to replicate the existing reinforcement present in the box girder webs. The reinforcement cover used was 30mm (1.18 inches). All specimens consisted of a single 120mmx2mmx1000mm (4.72 inches x 0.0787 inches x 39.37 inches) laminate strip bonded to the surface of the concrete block with a bond length of 500mm (19.68 inches) for concrete block type A and 425mm (16.73 inches) for block type B. The slight difference in bond length between each type is deemed negligible due to the concept of effective bond length (Chajes 1996; Chen 2001; Maeda 1997) which is defined as the length over which the majority of the bond stress is maintained (ACI 440.2R-08 2008). This assumption has been validated though our experimental results by the relatively low strain levels observed in figure 5 at distances of 400mm (15.75 inches) away from the loaded face. Application of both uni-directional and bi-directional fabric utilised 2 layers applied to the concrete prisms with the laminate placed in between each fabric layer for types 2-3 and 5 (1 plie used in type 4). Type 6 consisted of 2 layers of uni-directional fabric (laminate between each layer) together with a single layer of bi-directional fabric (refer figure 3). The first layer of fabric and its component saturant were applied (wet-lay-up) and allowed sufficient time to reach a tacky state. This was identified as the level of curing where by the saturant had set enough to hold the fabric in place and also not be contaminated with laminate adhesive when applied (approximately 30‐45 minutes curing). CFRP fabric was wrapped around the curved edges of the block and bonded a length of 150mm (5.9 inches) down the sides of the concrete block for type 2 (WG3 and WG4). The term WG is indicative of the individual specimen designation. The direction of fibres was 90° to the direction of laminate. Application was carried out similarly for type 3, but with the direction of the fibres being parallel to the direction of testing. Three specimens were constructed for testing of type 3 (WG5, WG6, WG7) with a “dry” method of application used for the last specimen (WG7). The alternative application method ensured that the interface between each layer of CFRP material had hardened sufficiently to ensure a “dry” join had occurred. This “cold” formed method was used to replicate possible work conditions/sequences on site. Types 5 and 6 were constructed using the bi-directional fabric with no bonding to the sides of the concrete block. Application was followed by curing which occurred in a temperature controlled chamber of 50°C for a period of 48 hrs, then further curing at room temperature (22°C) for a further 72 hours prior to testing. The accelerated curing was utilised to reduce the adhesive curing time from 7 days as specified by the manufacturer to 5 days to meet program time constraints. This form of curing was not a requirement for the performance

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of the system in field or laboratory conditions. Adhesion tests were carried out on additional adhesion samples to verify laminate adhesion and curing prior to testing.

(a) (b) (c)

(d) (e) (f)

Figure 3: Anchorage types 0 and 2-6 specimen geometry and material properties (a) type 0; (b) type 2 (WG35, WG4); (c) type 3 (WG5, WG6, WG7); (d) type 4 (WG12); (e) type 5 (WG10, WG11); (f) type 6 (WG8)

Tables 1 and 2 summarise the material properties used in the specimen construction as per manufacturers specifications.

Properties Laminate Adhesive Saturant Primer Units

Resin Type Epoxy Epoxy Epoxy -

Specific Gravity 1.8 1.12 1.08 -

Glass Transition Temperature >65 - - °C

Modulus of Elasticity 10 (1450) >3.0 (435) 0.7 (101) GPa (psi)

Lap Shear Strength to CFRP >17 (2466) - - MPa (psi)

Bond (to Concrete) >3.5 (508) >3.5 (508) >3.5 (508) MPa (psi)

Tensile Strength 32 (4641) >50 (7252) >12 (1740) MPa (psi)

Compressive Strength >60 (8702) >80 (11603) - (psi)

Flexural Strength >35 (5076) >120 (17404) >24 (3481) MPa (psi)

Full cure at:

25°C 7 7 0.208 Days

40°C 3 - 0.125 Days Table 1 – Adhesives and Saturant Properties data


Properties Bidirectional CFRP (±45°)

CFRP Laminate

Unidirectional CFRP

Bidirectional GFRP (±90°) Units

Tensile Strength 3.79 (5.49x105) 3.3 (4.79x105) 3.8 (5.51x105) 3.4 (4.93x105) GPa (psi)

Tensile Modulus 230 (3.33x107) 210 (3.05x107) 240 (3.48x107) 73 (1.06x107) GPa (psi)

Ult. Elongation 2.1 1.4 1.55 4.5 %

Density 1.8 (0.065) 1.56 (0.056) 1.7 (0.061) 2.6 (0.094) g/cm³

Thickness 0.55 (0.022) 2 (0.0788) 0.235 (0.0093) 0.067(0.0026) mm(inches)

Width 120 (4.73) 300 (11.82) mm(inches) Table 2 – CFRP Properties data

Instrumentation and loading procedure The specimens were loaded under displacement control at a load rate of 1mm/minute (0.0394inches/min). Strain and load results were obtained from surface mounted strain gauges and a 3D non-contact measuring technique based on image correlation photogrammetry (GOM mbH 2005). A series of strain gauges were placed to the CFRP laminate in the locations shown in figure 3. Gauges G1 and G2 were installed at the front and back of the laminate to monitor any bending in the CFRP plate during testing indicating the presence of tilting. G1 was placed at the back of the laminate and G2 at the front at the same location. The 3D photogrammetry measurements were taken using a pair of high resolution, digital CCD (charged couple device) cameras. A measuring step of 10 seconds was used between recording intervals. Experimental Results Quality control tests -- A total of 6 concrete cylinders were tested to assess the concrete compressive strength. After 53 days curing at room temperature, the average compressive strength of the concrete was 62MPa (8992 psi). Pull off tests conducted prior to testing of the specimens indicated that laminate bond failure also occurred within the concrete at a bond pressure of 3.6 MPa (522 psi). This suggested full bonding between laminate strip and concrete surface. Failure modes -- Most experimental data has shown that failure of unanchored CFRP bonded to concrete typically occurs a few millimetres beneath the concrete/adhesive interface (Maeda 1997; Van Gemert 1980). This was the critical mode of failure observed for the control specimen which failed by separation of the laminate from the concrete block with a layer of concrete still bonded to the plate. Two stages of delamination prior to ultimate failure were observed for type 2 specimens. Initially, cover separation of the laminate occurred in the unanchored zone. The failure occurred over a concrete zone which was greater than the width of the laminate. This can be seen by the exposed concrete aggregate in figure 4 and is attributed to the 50mm (1.97 inches) tapper of laminate adhesive which was applied to the edges of the laminate for the full bonded length. The data suggests that the use of adhesive tappers can distribute stresses from the CFRP laminate to a greater width of concrete. The second stage of

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debonding occurred in the anchorage zone where the horizontal fibres of the CFRP fabric wrap were observed to incline in angle toward the direction of loading. Failure in the anchored portion of the laminate occurred within the concrete adhesive interface, with the failure plane a few millimetres beneath the concrete surface. Debonding was first observed in type 3 specimens between the concrete and adhesive. This was followed by debonding of the CFRP laminate from between both layers of CFRP fabric. Failure within the concrete was localised to the width of the CFRP laminate with vertical splitting of the fabric occurring at the laminate edges.

(a) (b) (c) (d) (e) (f) Figure 4: Specimen failure summary; (a) Type 0-Control Specimen, WG9; (b) Type 2-Anchor, WG4; (c) Type 3-Anchor, WG5, (d) Type 4-Anchor, WG12; (e) Type 5-Anchor, WG10; (f) Type 6-Anchor, WG08

Type 4 specimens were the first of a series that were anchored with the bi-directional fabric and exhibited multiple stages of delamination prior to ultimate failure. Initially, debonding of the laminate/sheet to concrete interface occurred at the loading edge and was followed by a combination of laminate debonding, laminate rupture (along the direction of the fibres) and ±45° bidirectional fabric sheet rupture (along the direction of the fibres). A Multi-phase failure of both type 5 specimens (WG10 and WG11) was observed. The first stage of concrete-adhesive interfacial debonding of the laminate occurred in the vicinity of the loaded edge for both specimens (WG10 and WG11). Specimen WG10 went on to show progressive debonding of the sandwiched laminate structure from the concrete surface, which resulted in complete debonding of the laminate and bidirectional fabric structure from the concrete block. It is believed this mode of failure was induced by an inadequate surface roughness, caused by the recycling of the Type B concrete block (used in type 3 tests) and the need for secondary sand blasting to remove existing bonded fabric. The remaining stages of delamination for specimen WG11 were a combination of laminate debonding, laminate rupture (along the direction of the fibres) and ±45° bidirectional fabric sheet rupture (along the direction of the fibres). Failure of the type 6 specimen (WG8) was a result of laminate rupture. Tilt -- Tilting of the specimen during loading can occur as a result of miss-alignment of the CFRP to the position of load application. This can lead to a small unintended offset δ


in the position of the load, resulting in bending of the CFRP laminate. Another possible cause of tilting is rotation of the specimen due to inadequate clamping and can result in premature CFRP delamination. The potential for tilting was reduced by a vigorous alignment procedure and by using key clamping devices in the test setup (refer figure 2). Each specimen was tensioned to 25kN (5620 Ibf) to verify accuracy of specimen mounting. Gauges G1 and G2 were installed at the front and back of the laminate to monitor the presence of tilting. The readings from these two gauges were compared to ensure that the values were within an acceptable level of tolerance (±10% of each other). If readings were not within tolerance, the specimen was unloaded and re-aligned (Al-Mahaidi, R, Sentry, M & Williams, G 2009). CFRP strain distributions along length of laminate -- Table 3 summarises the failure loads and maximum CFRP elongations reached in types 0, 2-6 of the experimental program. In tables and figures which follow reference is made to AR (Photogrammetry) and SG (strain gauge). These refer to the two data acquisition techniques used in the experimental programme.

Type Ref Pmax

Max Laminate strain (µε)

Incre-se in Load

Max strain in CFRP ±45° Fabric (SG)

Max strain in CFRP ±45° Fabric (AR) Failure

Mode GA/AR kN (lbf)

GA (µε)

AR (µε) %

LS (µε)

RS (µε)

LS (µε)

RS (µε)

0 WG9 99.6

(22390) 2535 2706 - - - - - CSF

2

WG3 138.2

(31067) 3242 3212 27.9 - - - - CSF

WG4 142

(31921) 3142 3235 23.9 - - - - CSF/ ASF

3

WG5 156.5

(35181) 3470 3607 36.9 - - - - CSF/ ASF

WG6 146

(32820) 3239 3488 27.8 - - - - CSF/ ASF

WG7 145.3

(32663) 3245 3204 28 - - - - CSF/ ASF

4 WG12 218.3

(49074) 5800 4867 128.8 12896 13632 13136 - CSF / PLR/

5

WG10 213

(47882) 4900 5261 93.3 5228 5225 3982 - CSF

WG11 236.9

(53255) 5300 - 109.1 7433 12834 - - CSF

6 WG8 261.4

(58763) 7500 7589 195.9 4177 4372 4054 - PASF/LR Note: CFS (Cover separation failure); ASF (Adhesive separation failure); PASF (Partial Adhesive Separation Failure); LR (Laminate Rupture); FR (Fabric Rupture); LS (fabric right ride of laminate); RS (fabric right side of laminate)

Table 3 – Maximum FRP elongations and corresponding effective FRP strains and utilisation percentiles (types 0, 2-6)

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(a) (b)

(b) (d)

(e) (f) Figure 5: Strain vs. distance along Laminate; (a) Type 0 (Control) ; (b) Anchorage Type 2 (WG4); (c) Anchorage Type 3 (WG6); (d) Anchorage Type 4 (WG12); (e) Anchorage Type 5 (WG10); (f) Anchorage Type 6 (WG8);

CFRP elongation along the length of the laminate are reported in figures 5(a) to (f) for anchorage types 0 and 2-6. An examination of the experimental data shows that anchor type 2 was effective in increasing the ultimate failure load by 39-43% and resulted in an

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80kN (17984Ibf) (SG)80kN (17984Ibf) (AR)96.6kN (21715.68Ibf) (SG)96.6kN (21715.68Ibf) (AR)40kN (8992Ibf) (SG)40kN (8992Ibf) (AR)

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40kN (8992Ibf) (SG)40kN(AR)103.4kN (23244.32Ibf) (SG)100kN(AR)137kN (30797.6Ibf) (SG)137kN (30797.6Ibf) (AR)

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125kN (28100 Ibf) (SG)125kN (28100 Ibf) (AR)175kN (39340 Ibf) (SG)175kN (39340 Ibf) (AR)217.2kN (48827 Ibf) (SG)

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125kN (28100 Ibf) (AR)125kN (28100 Ibf) (AR)175kN (39340 Ibf) (AR)175kN (39340 Ibf) (AR)211.95kN (47646 Ibf) (AR)211.95kN (47646 Ibf) (AR)

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125.9kN (28302 Ibf) (SG)125.9kN (28302 Ibf) (AR)175kN (39340 Ibf) (SG)175kN (39340 Ibf) (AR)260.8kN (58628 Ibf) (SG)260.8kN (58628 Ibf) (AR)

(inches) 0 3.94 7.87 11.8 15.7

(inches)0 3.94 7.87 11.8 15.7

(inches) 0 3.94 7.87 11.8 15.7

(inches)0 7.87 15.7 23.6

(inches)0 3.94 7.87 11.8 15.7

(inches) 0 3.94 7.87 11.8 15.7


increase in the maximum laminate strain of 19-28% prior to failure. No increase in bond stress compared to the control specimen was recorded due to the anchor (refer figure 6b). The higher load carrying capacity of the anchorage was mainly attributed to the 50mm (1.97 inches) adhesive tapers distributing the laminate-adhesive stresses to a greater width and area of concrete; and the addition of the unidirectional fabric contributing to resist load through a strut-tie action resulting from the fabric fibres inclining towards the direction of loading prior to failure. Close correlations are observed between the photogrammetry and strain gauge measurements. The deviations in strain seen in figure 6(b) at location G7 prior to failure are due to the strain gauge G7 slipping. Photogrammetry data showed a continuous strain profile along the length of the laminate at each load increment. Examining the photogrammetry data, a slight dip in strain level is observed at a location of 50mm (1.97 inches); refer figure 5 (a), (b), (d), and (f). The location of strain depression corresponds to the edge of the concrete block (refer figure 3). The utilisation of uni-directional fabric applied parallel to the direction of the laminate (anchorage type 3) was effective in increasing the ultimate failure load by 46-57% compared to the unanchored control specimen. An increase in maximum laminate elongation of 18-37% was attributed to this form of anchorage. The increase in anchorage strength observed in table 3 was due to the transfer of bond stress to a greater distance away from the loaded edge, resulting in an increased effective anchorage length (Al-Mahaidi & Kalfat 2010). This is clearly observed in figure 5(c) by the higher level of strain recorded at a distance of 300mm (11.81 inches) away from the loaded face prior to failure. The presence of micro-cracking at the concrete-to-adhesive interface and consequent slip of the laminate during loading gradually forces a re-distribution of bond-stress further away from the loaded face. It is believed that this redistribution was greatly facilitated by the anchoring effect of the uni-directional fabric curved and anchored around the end of the concrete block. A comparison of the bond-slip curves yields maximum bond stresses of 5-5.5 MPa (725-798 psi) for both the control and type 3 specimens (refer figures 6a and 6c), the anchorage was therefore un-successful in increasing the strength of the CFRP-concrete contact bond strength. The use of bi-directional CFRP fabric to anchor the CFRP laminate was adopted in anchorage types 4 and 5. It is noted that an increase in failure load of 128% was observed for the type 4 anchor, due to the application of one ply of bi-directional fabric anchored 50mm (1.97 inches) down the sides of the concrete block (refer figure 2). A 93-109% increase in failure load was reached in type 5 which utilised two plies with no fabric anchorage. The maximum ±45° fabric elongations measured suggest that the fabric strain utilisations were 2-3 times greater when using a single fabric ply with anchorage compared with where no anchorage was provided. The 50mm (1.97 inches) anchorage was omitted in type 5 where the 2 plies of ±45° fabric were bonded across the full width of the type B concrete block. Anchorage type 6 showed the greatest increase in ultimate failure load (195%) and failed by rupture of the CFRP laminate at a load of 261.4 KN (58763 Ibf). The maximum ±45° fabric strain reached was 3762 - 4054 με with a corresponding laminate strain of approximately 7500 με. Examining the strain distribution prior to failure in figure 5f

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shows the specimen to have the highest effective anchorage length where laminate strains of 3458 με were recorded at a distance of 400mm (15.75 inches) away from the loaded face. By introducing the ±45° bidirectional fabric sheet in addition to the unidirectional fibres installed parallel to the direction of the laminate. The anchorage has combined the benefits of anchor types 3 and 5, which has resulted in a distribution of fibre-to-adhesive bond stresses over a greater length and width of concrete. The strains in the ±45° bidirectional fabric reached similar levels to those recorded in type 5, 3762-4054 με which indicated a comparable distribution of stress across the width of the concrete block between the two samples. Load – Displacement curves -- The stages of delamination can be observed from the load displacement curves presented in figure 6. The onset of de-bonding of the CFRP-to-concrete interface occurs with an increase in transverse micro cracking and a local reduction of bond stiffness which results in a flattening of the load-displacement curve and a redistribution of strain and consequent increase in the strain levels in areas further away from the loaded edge. This can be clearly observed in all specimens of figure 6.

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Micro strain (µε)

TYPE_2.AR/SG.4A


0

5000

10000

15000

20000

25000

30000

35000

0

20

40

60

80

100

120

140

0 1000 2000 3000

Load

(kN

)

Micro strain (µε)

TYPE_3A.AR/SG.WG6

G4 (SG)G3 (AR)G3 (SG)G4 (AR)G5 (SG)G6 (SG)G5 (AR)G6 (AR)G7 (SG)G7 (AR)

0

10000

20000

30000

40000

50000

020406080100120140160180200220240

0 1500 3000 4500 6000

Load

(kN

)

Micro strain (µε)

TYPE_4.AR/SG.WG12

G3 (SG)G4 (SG)G3 (AR)G4 (AR)G5 (SG)G6 (SG)G5 (AR)G6 (AR)G7 (SG)G7 (AR)

(Ibf)

(Ibf)

(Ibf)

(Ibf)


(e) (f) Figure 6: Load vs. strain distribution; (a) Type 0 (Control) ; (b) Anchorage Type 2 (WG4); (c) Anchorage Type

3 (WG6); (d) Anchorage Type 4 (WG12); (e) Anchorage Type 5 (WG10); (f) Anchorage Type 6 (WG8);

Experimental bond slip curves -- An understanding of the local bond–slip behaviour of the CFRP-concrete interface is of fundamental importance to the accurate modelling of debonding failures in FRP-strengthened RC structures. The bond slip data can be computed from the axial strains of the CFRP plate measured at discrete locations. The strain measurements obtained at gauge locations and from photogrammetry measurements can be used to obtain bond-slip information. A comparison of the results obtained using both data acquisition techniques is presented in figures 7 and 8. The shear stress of a particular location along the length of the laminate can be found using a difference formula, while the corresponding slip can be found by a numerical integration of the measured axial strains of the plate (Lu et al. 2005). This is expressed in equations 1 and 2:

τE ε , ε ,

∆L (1) s

ε , ε , ∆L s (2)

Where Ef and tf are CFRP elastic modulus and thickness; εf,i+1 and εf,i are CFRP strains; and ΔL is the distance between strain gauges. In general, the bond–slip curves have a non-linear ascending and descending trends. It was found that these trends can be approximately described using Popovics (Popovics 1973) equation given by:

⁄ (3)

Where : τ and s1 are the maximum bond stress and corresponding slip. The value (a) controls the slope of the ascending and descending branches of the bond slip curve. A value of a = 3 was established by (Nakaba 2001) .

0

10000

20000

30000

40000

50000

60000

020406080100120140160180200220240

0 1000 2000 3000 4000 5000

Load

(kN

)

Micro strain (µε)

TYPE_2A.AR/SG.WG10

G3 (SG)G4 (SG)G3 (AR)G4 (AR)G5 (SG)G6 (SG)G5 (AR)G6 (AR)G7 (SG) 0

10000

20000

30000

40000

50000

60000

70000

020406080

100120140160180200220240260280

0 2000 4000 6000

Load

(kN

)

Micro strain (µε)

TYPE_3B.AR/SG.WG8


(Ibf)

(Ibf)

42-14 Kalfat et al.

Type

Ref

Distance along laminate from Strain Gauge G1

100-125mm (3.94-4.92 inches) 175mm (6.98 inches)

τ MPa (psi) so

τ MPa (psi) so

0 WG9

SG 4.9 (711) 0.036 5.02 (728) 0.06

AR 3.25 (471) 0.06 5.83 (846) 0.11

2 WG3

SG 2.63 (381) 0.05 9.57 (1388) 0 .16

AR 3.82 (554) 0.071 8.73 (1266) 0.15

WG4

SG 3.94 (571) 0.087 4.35 (631) 0.06

AR 4.05 (587) 0.076 4.11 (596) 0.053

3

WG5

SG 3.26 (473) 0.07 4.59 (666) 0.083

AR 4.46 (647) 0.06 5.14 (745) 0.08

WG6

SG 7.94 (1152) 0.36 5.34 (775) 0.14

AR 5.93 (860) 0.25 5.22 (757) 0.15

WG7

SG 6.16 (893) 0.30 5.37 (779) 0.10

AR 7.71 (1118) 0.22 5.34 (775) 0.18

4

WG12 SG 6.99 (1014) 0.19 7.73 (1121) 0.15

AR 6.47 (938) 0.097 7.24 (1050) 0.26

5

WG10 SG 14.64 (2123) 0.87 7.18 (1041) 0.30

AR 12.95 (1878) 0.99 5.9 (856) 0.24

WG11 SG 15.94 (2312) 0.43 7.32 (1062) 0.17

AR - - - - 6

WG8 SG 15.46 (2242) 1.92 7.54 (1094) 0.11

AR 14.97 (2171) 1.28 4.01 (582) 0.12 Table 4 – Bond stress and corresponding slip results summary (type 0, 2-6)

(a) (b)

(b) Figure 7 – Bond-slip curves fitted with Popovics equation at bond critical regions- (a) Type 0 (Control) ; (b) Anchorage Type 2 (WG4)

0

1

2

3

4

5

6

0 0.1 0.2 0.3 0.4

Bond Stress (MPa)

Slip (mm)

0

1

2

3

4

5

0 0.1 0.2 0.3 0.4

Bond Stress (MPa)

Slip (mm)

175 mm (6.89inch) AR

Popovics

175 mm (6.89inch) AR

870

725

580

435

290

145

0

(psi)

725

580

435

290

145

0

(psi)

(inches) 0 0.00394 0.00787 0.011 0.0157

(inches)0 0.00394 0.00787 0.011 0.0157


. (a) (b)

(c) (d)

Figure 8 – Apparent Bond-slip curves fitted with Popovics equation at bond critical regions (measured 175mm (6.89 inches) away from strain gauge G1) - (a) Anchorage Type 3 (WG6); (b) Anchorage Type 4 (WG12); (c) Anchorage Type 5 (WG10); (d) Anchorage Type 6 (WG8); A review of the bond-slip curves shows a comparable relationship between the two data acquisition techniques. A distinction has been drawn between true and apparent bond stress which is presented in figures 7 and 8. True bond stress can be defined as the stress induced in the concrete as a result of a differential in strain measured across a finite length along the CFRP laminate. The true bond stress must be calculated from the laminate strain and relies on the assumption of perfect strain compatibility between the laminate, epoxy and concrete. Due to the presence on uni-directional and bi-directional fabric layers for anchorage types 3-6, laminate strain readings were taken from uppermost layer of CFRP fabric in the locations shown in figure 3. It is estimated that the strains measured from the uppermost fabric layer will be higher than the actual strain in the laminate. This is due to the effects of interface slip between the fabric, laminate and concrete layers during loading and laminate relaxation. In addition, shedding strains are induced in the ±45° bidirectional fabric from the shedding of laminate forces to a wider area concrete, which will not be felt by the CFRP laminate. The true bond stress in the concrete for anchorage types 3-6 is expected to be significantly lower. The bond stresses presented in figure 8 have been defined as apparent stresses as a result of the strains measured from the uppermost CFRP fabric layer not corresponding to the true strain in the laminate and concrete.

0

1

2

3

4

5

6

0 0.1 0.2 0.3 0.4

Bond Stress (MPa)

Slip (mm)

0

1

2

3

4

5

6

7

8

0 0.1 0.2 0.3 0.4

Bond Stress (MPa)

Slip (mm)

0

1

2

3

4

5

6

7

8

0 0.2 0.4 0.6 0.8

Bond Stress (MPa)

Slip (mm)

0

1

2

3

4

5

6

7

8

0 0.1 0.2 0.3 0.4

Bond Stress (MPa)

Slip (mm)

870

725

580

435

290

145

0

(psi)

1160

1015

870

725

580

435

290

145

0

1160

1015

870

725

580

435

290

145

0

1160

1015

870

725

580

435

290

145

0

(psi)

(psi)

(psi)

(inches) 0 0.00394 0.00787 0.011 0.0157

(inches) 0 0.00394 0.00787 0.011 0.0157

(inches)0 0.00394 0.00787 0.011 0.0157

(inches)0 0.00394 0.00787 0.011 0.0157

42-16 Kalfat et al.

The softening branches of the bond slip curves follow comparable descending gradients for anchorage specimens 2 and 5, with photogrammetry estimating a lower degree of softening and a higher fracture energy and slip for specimens 0, 3 and 4. The difficulty of obtaining accurate bond-slip curves is largely attributed to local variations in the strains along the length of the laminate. This is clearly observed in figure 5 and due to the discrete nature of the concrete cracks, the heterogeneity of concrete and the roughness of the underside of the debonded CFRP plate (Teng, JG et al. 2006). This variation in strain is more pronounced in the photogrammetry measurements which required a high degree of filtering to smooth out irregularity and noise in the raw data. It is noted that Table 5 presents peak apparent bond stresses of up to 12.95-15.94 MPa (1878-2312 psi) within the zone of 100-125mm (3.94-4.92 inches) from strain gauge G1 for anchorage types 5 and 6. This zone corresponds to 50-75mm (1.97-2.95 inches) from the face of the concrete block. The stress slip distribution within the zone demonstrated a linear trend with no indication of softening. It is believed that the high level of apparent bond stress and the lack of softening are due to the pronounced effects of interfacial slip between multiple fibre layers within this zone. The effects of interfacial slip become less apparent at a distance of 175mm (6.89 inches) away from strain gauge G1. The bond-slip curves within this zone indicate a softening tend comparable with current prediction models. Two or more peaks in the apparent bond stress distributions of figure 8 during loading possibly related to the presence of transverse concrete cracks which introduce local disturbances to the bond behaviour.

CONCLUSION The experimental study was conducted to improve the efficiency and strain utilisations of CFRP bonded to concrete using uni-directional and bi-directional fabric anchorage systems. The anchorages tested were successful in improving the degree of CFRP strain utilisation. The results and discussions presented allow the following conclusions to be made: Anchoring the ends of CFRP laminates using uni-directional CFRP fabric wrap

applied horizontally across the laminate strip (anchorage type 2) was effective in increasing the ultimate failure load by 39-43% and resulted in an increase in the maximum laminate strain of 19-28%.

The use of 50mm (1.97 inches) adhesive tappers increase along the length of the laminate was found to distribute the laminate-adhesive stresses to a greater width of concrete.

CFRP fabric applied horizontally across the laminate strip does not provide an effective level of confinement to uniformly increase the bond strength between the adhesive and concrete layer.

The application of unidirectional fibres with an orientation parallel to the direction of the laminate (anchorage type 3) was effective in increasing the ultimate failure load by 46-57%. The overall increase in strength of this anchorage system was attributed to the transfer of bond stress to a greater distance away from the loaded edge, which was facilitated by the anchoring effect of the uni-directional fabric curved and anchored around the end of the concrete block.


One ply of bi-directional fabric anchored 50mm (1.97 inches) down the sides of the concrete block used to anchor the laminate in type 4 of the program was effective in increasing the ultimate failure load by 128%.

The use of 2 plies of bidirectional fabric with no anchorage down the side of the concrete block was effective in providing a 93-109% increase in failure load.

Bi-directional fabric applied to the ends of CFRP laminates resulted in a more efficient distribution of CFRP-adhesive stresses over a greater width of concrete.

Utilising the properties of anchorage types 3 and 5 resulted in a distribution of fibre-to-adhesive bond stresses over a greater length and width of concrete achieving an increase in failure load of 195% and resulting in laminate rupture.

REFERENCES

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42-18 Kalfat et al.

Chen, JF, Teng, J.G, 2001, 'Anchorage Strength Models for FRP and Steel Plates Bonded to Concrete', Journal of Structural Engineering, vol. 127, no. 7, pp. 784-91. Concrete Society Technical Report No. 55 2004, 'Design guidance for strengthening concrete structures using fibre composite materials, Second Edition', ISBN 1904482 14 7. Drimoussis, E, and Cheng, J.J.R. 1994, 'Shear strengthening of concrete girders using CFRP sheets', Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alta. Structural Engineering Report No. 205, p. 177. Duthinh, D, Starnes, M, 2001, 'Strengthening of Reinforced Concrete Beams with Carbon FRP', Composites in Constructions, Figueiras et a1 (eds), vol. Swets & Zeitlinger, Lisse. GOM mbH 2005, 'ARAMIS v5.4.1, Gom Optical Measuring Techniques'. Kalfat, R & Al-Mahaidi, R 2010, 'Investigation into bond behaviour of a new CFRP anchorage system for concrete utilising a mechanically strengthened substrate. ', Compos Struct (2010), doi:10.1016/j.compstruct.2010.04.004. Khalifa, A, Belarbi, A, and Nanni, A, 2000, 'Shear Performance of RC Members Strengthened with Externally Bonded FRP Wraps', Proc., 12th World Conference on Earthquake Engineering, Auckland, New Zealand, pp. paper 305,10 pp. Lee, TK, Al-Mahaidi, R, 2008, 'An experimental investigation on shear behaviour of RC T-beams strengthened with CFRP using photogrammetry', Composite Structures, vol. 82, no. 2, pp. 185-93. Lu, XZ, Teng, JG, Ye, LP & Jiang, JJ 2005, 'Bond-slip models for FRP sheets/plates bonded to concrete', Engineering Structures, vol. 27, no. 6, pp. 920-37. Maeda, T, Asano, Y, Sato, Y, Ueda, T, and Kakuta, Y, 1997, 'A study on bond mechanism of carbon fiber sheet', Non-Metallic (FRP) Reinforcement for Concrete Struct., Proc., 3rd Int. Symp., Japan Concrete Institute, Sapporo, vol. 1, pp. 279-85. Melo, GS, Araujo, A.S, Nagato, Y, 2003, 'Strengthening of RC T-Beams in Shear with Carbon Fibre Sheet Laminates (CFRP)', FRPRCS-6, Fibre reinforced polymer Reinforcement for Concrete Streuctures, World Scientific Pubishing Company, pp.477-86. Nakaba, K, Kanakubo, T, Furuta, T, Yoshizawa, H, 2001, 'Bond behaviour between fibre-reinforced polymer laminates and concrete', ACI Structural Journal, vol. 98-S34, pp. 1-9. Ortega Carlos. A, BA, Bae Sang-Wook 2009, 'End Anchorage of Externally Bonded FRP Sheets for the case of Shear Strengthening of Concrete Girders', FRPRCS-9, Fiber Reinforced Polymer Reinforcement for Concrete Structures, Sydney, Australia, pp. 1-4.


Orton, SL, Jirsa, J.O, Bayrak, O, 2008, 'Design Considerations of Carbon Fiber Anchors', Journal of Composites for Construction, vol. 12, no. 6, pp. 608-16.

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ACKNOWLEDGEMENTS The authors acknowledge the financial support provided by the Westgate Bridge Strengthening Alliance and the services provided by of the Department of Civil Engineering at Monash University

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