1. INTRODUCTION. (a) Sand/ Fabric-coated (b) Sand-coated deformed. (c) Helical wrapped/ribbed Fig.1 FRP anchors with different outer surfaces

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1 S2B3 Interface Bond Strength of Helical Wrapped GFRP Ground Anchors Weichen Xue Prof., Department of Building Engineering, Tongji University, Shanghai, China Yuan Tan PhD candidate, Department of Building Engineering, Tongji University, Shanghai, China ABSTRACT 63 pull-out specimens were tested to investigate the interface bond strength between helical wrapped GFRP ground anchors and different bonding agents. Test variables included types of bonding agents (normal concrete (NC) 2, 3 and 5, high performance concrete (HPC) 5, grout and epoxy resins) and types of anchors (helical wrapped GFRP anchor, deformed steel anchor, round GFRP anchor and steel anchor). Experimental results showed interface bond strength of helical wrapped GFRP anchors was 61% to 94% of that of steel anchors in concrete, which mainly depended on bond behavior between the inner cores and spirally wound ribs of GFRP anchors. With concrete strength increased, the interface bond strength of helical wrapped GFRP anchors increased but the trend of increasing slowed down. Compared to helical wrapped GFRP anchors, the interface bond strength of round GFRP anchors was very low which indicated that round GFRP anchors was not suitable for ground anchors in geotechnical engineering. The higher interface bond strength of helical wrapped GFRP anchors in grout and epoxy resin could provide references for the applications of GFRP anchors in post-tensioned bonded prestressed concrete structures and the development of helical wrapped GFRP anchor system. KEYWORD ground anchor; helical wrapped GFRP anchor; bonding agents; bond strength; pull-out test

2 1. INTRODUCTION A ground anchor is, in general term, a bar, which is inserted in a hole drilled in a rock/soil and then grouted [1]. Compared with steel anchors, FRP anchors have some different properties, such as excellent corrosion resistance, high tensile strength, lightweight (about 2% that of steel), magnetic transparency, ease of handling at job sites and cutting, thermal expansion coefficient similar to concrete and low shear strength which indicate that specific anchorages are need for FRP anchors [2,3]. Outer surfaces of FRP anchors could be treated in many ways, mainly including 1 sand/fabric-coated (Figure 1(a)), which was used to improve the bond behaviour between GFRP anchors and concrete; 2 sand-coated deformed (Figure 1(b)), which was used to further improve the bond behaviour of GFRP anchors; 3helical wrapped/ ribbed (Figure 1(c)), which exhibit many advantages, such as the relatively higher bond strength, the designable characteristic of rib heights, rib widths and rib pitches. Due to different outer surfaces treatments and substantial material differences in both the longitudinal and transverse directions, bond of FRP anchors performs differently from that of conventional steel reinforcements. The available studies mainly focus on sand/fabric-coated GFRP anchors and sand-coated deformed GFRP anchors, and little attention has been paid to the helical wrapped GFRP anchors. (a) Sand/ Fabric-coated (b) Sand-coated deformed (c) Helical wrapped/ribbed Fig.1 FRP anchors with different outer surfaces In 21, most of the findings of FRP ground anchors were reviewed and discussed by Zhang B., Benmokrane B., and Chennouf A [4,5]. Test results indicated that the pull-out behaviour, pull-out capacity and maximum bond stress of cement grouted FRP (AFRP and CFRP) anchors were influenced by the surface geometry of FRP anchors, the properties of the filling grout and the stiffness of the host medium (anchorage tube). The working loads of AFRP anchors and CFRP anchors were recommended as 4 and 5 percent of the strength of anchor tendons, respectively. Based on the descriptions of the mechanical coupling at the interface between the anchors and bonding agents, Li C., Stillborg B. [6] developed analytical models for ground anchors. Kilic A., Yasar E., Celik A. [7] carried out approximately 8 laboratory ground anchor pull-out tests in basalt blocks to explain and develop the relations between the bonding agents and untensioned, fully ground anchors. Gao D [8]suggested the design method of GFRP ground anchors, but didn t conduct laboratory tests. The load carrying capacity and bond strength of cement grouted GFRP anchors were discussed by Benmokrane B., Xu H, Bellavance E [9]. The pull-out specimens were tested on four types of GFRP anchors and two types of steel anchors installed in concrete blocks and rock masses. Weichen Xue et al [1] carried out 48 pull-out tests to examine the bond properties of high strength carbon fiber reinforced polymer (CFRP) strands in different bonding agents, including normal concrete, high performance concrete, epoxy resin and grout, and proposed the bond stress versus slip models for high strength CFRP stands and steel strands. W.C.Tang, T.Y.Lo and R.V.Balendran [11] carried out pull-out tests to study the bond performance of GFRP anchors in polystyrene aggregate concrete(pac). The test results showed that the bond strength increased with the compressive strength and concrete density of PAC. Marta Baena, Lluis Torres, Albert Turon, Cristina Barris [12] carried out 88 concrete pull-out to analyze the influence of the rebar surface, rebar diameter and concrete strength on the bond-slip curves, new equations that accounted for the dependence on rebar diameter were presented to calibrate the analytical models. However, so far little attention has been paid to the interface bond behaviour between helical wrapped GFRP anchors and different bonding agents, including different types of concrete, grout and epoxy resin, etc. Considering types of bonding agents, embedment lengths and types of anchors as the main experimental parameters, this paper aims to investigate interface bond behavior between helical wrapped GFRP anchors and different bonding agents. The better understanding of bond behaviour will provide engineers references for

3 the application of helical wrapped FRP anchors in geotechnical engineering. 2. TEST SETUP Tests were conducted on 63 pull-out specimens and details of the specimens are shown in Table 1. Test variables included different types of bonding agents and types of anchors (helical wrapped GFRP anchor, deformed steel anchor, round GFRP anchor and steel anchor). The bonding agents composed of normal concrete 2, 3, 5 (short for NC 2, NC 3, NC 5), high performance concrete 5 (short for HPC 5), grout and epoxy resin. The round GFRP anchors and round steel anchors were used for comparison. The anchors hereinafter were helical wrapped GFRP anchors except special illuminations. The tested GFRP anchors included round and helical wrapped GFRP anchors, as shown in Figure 2. The tensile strength and elastic modulus of tested GFRP anchors were 54.2MPa and 41GPa, respectively. The tensile strength and elastic modulus of steel anchors were 55.3MPa and 167GPa, respectively. In HPC, blast furnace slag with fineness of cm 2 /g was substituted for part of cement, polypropylene fibers were added to improve the activity of mixture. The volume ratio of polypropylene fibers in HPC was.2%. The polypropylene fibers had a length of 19mm, diameter of 1μm, ultimate strength of 4MPa, elastic modulus of 6GPa, prolongation ratio of 8% and melting point of 16. The mix properties of normal concrete and high performance concrete as well as grout are shown in Table 2 and the corresponding mechanical properties of bonding agents are listed in Table 3. The Loading setup of pull-out tests is shown in Figure 3. The applied pullout force P and the corresponding slippages of anchors S, were measured at the free end of pull-out specimens. Fig.2 Photo of tested GFRP anchors Fig.3 Loading setup of pull-out tests Type of bar Diameter of bar (mm) Table 1 List of pull-out specimens Bonding agent Embedment length Quantity of specimens Average Bond strength (MPa) GFRP 9.5 NC 2 5d Steel 1 NC 2 5d GFRP (round) 9.5 NC 2 5d 6.9 Steel(round) 1 NC 2 5d GFRP 9.5 NC 3 5d Steel 1 NC 3 5d GFRP 9.5 NC 5 5d Steel 1 NC 5 5d GFRP 9.5 HPC 5 5d Steel 1 HPC 5 5d GFRP 9.5 Grout 5d GFRP (round) 9.5 Grout 5d 3.67 GFRP 9.5 Epoxy Resin 5d GFRP (round) 9.5 Epoxy Resin 5d Note: d= diameter of GFRP/steel anchors. All anchors in the table are helical wrapped GFRP anchors except these with description in the brackets.

4 Table 2 Mix properties of bonding agents Bonding agent NC 2 NC 3 NC 5 HPC 5 Grout Coarse aggregate (kg/m 3 ) Fine aggregate (kg/m 3 ) Cement (kg/m 3 ) Water (kg/m 3 ) Blast furnace slag (kg/m 3 ) 26 Water reducing agent (kg/m 3 ) Polypropylene fibre (kg/m 3 ) 1.8 Note: = This kind of materials not included in corresponding bonding agents. Watercement ratios (w/c)=.4 Table 3 Mechanical properties of bonding agents Bonding agents Elastic modulus E c Cubic compressive strength Tensile strength (MPa) f cu (MPa) f t (MPa) NC NC NC HPC Epoxy Resin Grout Note: = This mechanical property not measured in this paper. 3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1 Failure mode The failure mode showed that concrete in the anchoring zone of helical wrapped GFRP anchors was not crushed during the whole loading process, nearly all specimens failed due to detaching of ribs from the inner cores of GFRP anchors or damage of the ribs, as shown in Figure 4. Accordingly, bond strength of GFRP anchors in different bonding agents mainly depended on bond behaviors of the bar itself, meaning bond strength between the inner cores and spirally wound ribs. plotted through Figure 5 to Figure 8. The slip in the average bond stress-slip curves means the slip at the free end. As shown in Figure 5 to Figure 8, when GFRP anchors started to slip, the average bond stress of GFRP anchors was a little higher than that of the steel anchors. In comparison with steel anchors, the free end slippage of GFRP anchors at the ultimate load point was lower. The descending branch after the peak point in τ-s curves of GFRP anchors was smoother than that of steel anchors. Compared with steel anchors, the free end slippage of GFRP anchors at ultimate state was much lower. Before the bond between concrete and the embedded anchors was damaged, ribs were quickly detached from the inner cores of GFRP anchors without obvious indications. By contrast, bond failure of steels was characterized by crushing of concrete between ribs with large slips. Fig.4 Failure patterns of pull-out specimens 15 1 τ/mpa Average bond stress-slip relationships Typical average bond stress-slip (τ-s) relationships of the helical wrapped GFRP/steel specimens are Fig.5τ-S curves in NC2

5 τ/mpa τ/mpa Fig.6 τ-s curves in NC τ/mpa Fig. 7τ-S curves in NC Fig.8τ-S curves in HPC5 3.3 Bond strength Interface bond strengths between different bonding agents and helical wrapped GFRP/steel anchors are reported in Table 1. The conclusions could be got as follows: (1) The interface bond strength of helical wrapped GFRP anchors was 61%, 77%, 83% and 94% of that of steel anchors in NC 2, NC 3, NC 5 and HPC 5, respectively, which could give better understandings to the application of GFRP ground anchor. With the concrete strength increased, the interface bond strength ratios between the helical wrapped GFRP anchors and steel anchors were increased. (2) The interface bond strength between the helical wrapped GFRP anchors in different concrete increased with the concrete strength increased but the trend of increasing slowed down. Bond strength of GFRP anchors embedded in HPC 5 was 84.9%, 14.4%, 3.8% higher than that in NC 2, NC 3, NC 5, respectively. (3) Interface bond strength between round GFRP anchors and concrete, grout as well as epoxy resin was about 7.% to 36.6% of that of helical wrapped GFRP anchors, which indicated that round GFRP anchors were not suitable for ground anchors in geotechnical engineering. (4) Bond strength between helical wrapped GFRP anchors and grout was 9.3MPa, which made and cement grout work together and provided references for applications of GFRP in post-tensioning bonded prestressed concrete structures. (5) Bond strength between helical wrapped GFRP anchors and epoxy resin was as high as 15.1MPa, which provided references for the development of helical wrapped GFRP anchor system. 4. CONCLUSIONS Based on 63 pull-out tests, the interface bond strength between helical wrapped GFRP anchors and different bonding agents are systematically investigated, main conclusions are reported as: (1) The interface bond strength of helical wrapped GFRP ground anchors mainly depends on bond behavior between the inner cores and spirally wound ribs of GFRP anchors. (2) The average bond stresses of helical wrapped GFRP anchors are a little higher than those of deformed steel anchors. Free end slippage of GFRP anchors at the peak load point and ultimate state is lower than that of steel anchors. The descending branch after the peak load point in τ-s curves of GFRP anchors is smoother than that of steel anchors. (3) The interface bond strength of the helical wrapped GFRP was 61% to 94% of that of steel anchors. With concrete strength increased, the interface bond strength ratios between helical wrapped GFRP anchors and steel anchors were increased. (4) The interface bond strength between helical wrapped GFRP anchors in different concrete increased with concrete strength increased but the trend of increasing slowed down. (5) The interface bond strength between round GFRP anchors and normal concrete, grout and epoxy resin was about 7.2% to 36.6% of that of helical wrapped GFRP anchors which indicated that round GFRP anchors were not suitable for ground anchors in geotechnical engineering. (6) The interface bond strength of helical wrapped GFRP anchors in grout and epoxy resin was 9.31MPa and 15.1MPa, respectively, which provided references for the applications of GFRP anchors in post-tensioned bonded prestressed concrete structures and the

6 development of helical wrapped GFRP anchor system. ACKNOWLEDGEMENT The authors acknowledge the supports of Fund of Western Communications Construction Scientific and Technological Project by the Ministry of Communications of the P.R. China (No ) and the Fund of National Natural Science Foundation of China (No ). REFERENCES [1] E. Gamboa, A. Atrens: Environmental influence on the stress corrosion cracking of rock bolts, Engineering Failure Analysis, Vol.1, No.5, pp , Oct. 23 [2] W. Xue: Design of modern prestressed structure, Beijing: China Architectures and Building Press, 23 [in Chinese] [3] W. Xue: Study progress of FRP rebars in concrete structures, Science Foundation in China, Vol.13, No.1, pp.2-22, 25 [in Chinese] [4] B. Zhang, B. Benmokrane and A. Chennouf: Tensile behaviour of FRP t endons for prestressed ground anchors, Journal of Composites for Construction, Vol.5, No.2, pp.85-93, May. 21 [5] B. Benmokrane, B. Zhang and A. Chennouf: Tensile properties and pull-out behaviour of AFRP and CFRP rods for grouted anchor applications, Construction and Building Materials, Vol.14, No.3, pp , Apr. 2 [6] C. Li and B. Stillborg: Analytical models for rock bolts, International Journal of Rock Mechanics and Mining Sciences, Vol.36, No.8, pp , Dec.1999 [7] A. Kilic, E. Yasar and A. Celik: Effect of grout properties on the pull-out load capacity of fully grouted rock bolt, Tunneling and Underground Space Technology, Vol.17, No.4, pp Oct. 22 [8] D. Gao, Q. Zhang and J. Xie: Design method of bonded FRP anchor bolts, Journal of Hydraulic Engineering, pp.5-9, 22 [in Chinese] [9] B. Benmokrane, H. Xu and E. Bellavance: Bond strength of cement grouted glass fibre reinforced plastic (GFRP) anchor bolts, International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts Vol.33, No.6, pp , July [1] Weichen Xue, Xiaohui Wang, Shulu Zhang: Bond Properties of High-Strength Carbon Fiber-Reinforced Polymer Strands, ACI Materials Journal, Vol.15, No.1, pp.11-19, Feb. 28 [11] W. C. Tang, T. Y. Lo, R.V. Balendran: Bond performance of polystyrene aggregate concrete (PAC) reinforced with glass- fibre -reinforce polymer(gfrp) bars, Building and environment, Vol.43, No.1, pp Nov. 28 [12] Marta Baena, Lluis Torres, Albert Turon, Cristina Barris: Experimental study of bond behavior between concrete and FRP bars using a pull-out test, Composites Part B: Engineering, Vol.4, No.8, pp , July. 29