The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015) Joint Conference, 14-16 December 2015, Nanjing,

DESIGN AND CONSTRUCTION OF LARGE-SCALE CFRP-BASED GROUND ANCHORS IN AIZHAI BRIDGE

Kuangyi Zhang1, Zhi Fang2, Antonio Nanni3

1 College of Civil Engineering, University , Hunan, 410082, China Email: [email protected]

2 College of Civil Engineering, Hunan University, Changsha, Hunan, 410082, China; Key Laboratory for Wind and Bridge Engineering of Hunan Province, Hunan University, Changsha, Hunan, 410082, China Email: [email protected]

3 Dept. of Civil, Architectural and Environmental Engineering, University of Miami Coral Gables, FL, 33146, United States E-mail: [email protected]

Keywords: Anchoring system, CFRP (Carbon Fiber-Reinforced Polymer), Ground anchor, Grout, RPC (Reactive Powder Concrete), Tendon

ABSTRACT A large-scale CFRP-based ground anchor system was developed and applied beneath the side hangers in the Aizhai Bridge. This is a suspension bridge with main span of 1,176 m located in Hunan, China. In this paper, design and construction of the ground anchor system is introduced. The system overall lock-off load as a superposition of seven ground anchors is up to 7,200 kN. Each ground anchor uses Carbon Fiber-Reinforced Polymer (CFRP) as the tendon, and Reactive Powder Concrete (RPC) grout as the bond medium in both above-ground anchor head and below-ground anchor part. The system is currently fulfilling its intended purpose as a component of the bridge structure.

1. PROJECT DESCRIPTION The Aizhai Bridge, a long-span suspension bridge with a main span of 1,176 m, located in west Hunan, China, was a critical part of the highway connecting Hunan and . The towers of bridge stood on top of the mountains, while its steel truss girders were connected to tunnels drilled through the mountains. In order to reduce the stress in the main cables and to increase the structural stiffness, several pairs of special side-hangers were used, as shown in Fig.1. Unlike the ordinary hangers that support stiffened truss girders in a suspension bridge, these special side-hangers were anchored into the ground. A corresponding large-scale ground anchor system was needed to handle the high tensile capacity under the special side-hangers. The location of the ground anchor beneath the side hangers was on top of a steep hillside. The rock at the spot mainly consisted of lightly disintegrated limestone with developed fractures and water- eroded grooves, as shown in Fig.2. Therefore, the ground anchors beneath the special hangers should have outstanding durability and efficiency. Frequently-used ground anchors consisting of ordinary steel tendon and cement grout cannot fulfill these purposes. On the one hand, the conventional materials face a serious problem of poor durability. They are vulnerable to corrosion attack in aggressive environments [1], especially in the water-rich mountain area where the bridge is constructed. This problem makes the conventional ground anchor incompatible with the overall bridge for long-term use. On the other hand, due to the poor performance of conventional materials, the below-ground length of ground anchor has to be long enough for anchoring tendons. Since the local below-ground geological condition is poor, the longer depth demands much more complex works to improve the condition before installation, and it also increases the costs of material and construction. Kuangyi Zhang, Zhi Fang and Antonio Nanni

Fig.1 Side hangers and ground anchor system in the Aizhai Bridge, China

(a) in southern side (b) in northern side Fig.2 Water-eroded cave/groove at the spot (before construction)

In order to overcome the flaws of traditional system, so as to form a durable and efficient ground anchor beneath the special hangers of the Aizhai Bridge, Carbon Fiber Reinforced Polymer (CFRP) rod was used as the tendon in the new system. Besides, ultra-high performance Reactive Powder Concrete (RPC) was selected as the grouting medium and an innovative bond-type anchoring system was developed. CFRP rods have advantages such as high uniaxial strength-to-weight ratio, corrosion resistance, good fatigue resistance and modularization, so they become ideal replacements for steel bars in concrete structures, cables or suspenders in bridges, and bolts in ground anchor systems [2]. RPC has higher strength, limited shrinkage and creep as well as remarkable durability, compared to epoxy resin or conventional cement mortar [3]. The new ground anchor was established based on these two new materials, and the behavior of the bond-type anchor as well as the ground anchor were systematically investigated by model tests in previous studies [4, 5]. On this basis, the ground anchors beneath the hangers of the Aizhai Bridge were determined. This paper introduced the design and construction of the ground anchors applied in the project.

2. DESIGN OF THE CFRP-BASED GROUND ANCHOR 2.1 Description of the new ground anchor Figure 3 shows the lay-out of the new ground anchor. From top to bottom, there are an above- ground anchor head, a below-ground free length, a below-ground fixed length and a terminal unit. In the above-ground part, the anchor head for anchoring CFRP tendons consists of a steel tube and The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015) Joint Conference, 14-16 December 2015, Nanjing, China fasteners and uses RPC as the bond medium. In the below-ground part, the configuration is similar to that of traditional tension-type rock bolts, and RPC grout is also used as the bond medium to anchor CFRP tendons within the fixed length. Spacers installed at regular intervals along the below-ground tendons help keeping the minimum space among multiple tendons. In addition, an innovative terminal unit called “guide-anchorage combination capsule” is designed at the bottom of the bolt. During the insertion of the rock bolt, the unit works as a guiding cap with a conical head to prevent damage to the tendons or the borehole wall. Meanwhile, the configuration of the unit is similar to the above-ground anchorage with RPC grouted inside, and it can provide CFRP tendons with an efficient anchoring force. If debonding were to happen within the below-ground fixed length, the tension would transfer to the embedded terminal unit, and the rock bolt would turn into a pressure-type bolt. This design provides safety margin for the below-ground anchoring system.

Fig.3 General configuration of the CFRP-based ground anchor

2.2 CFRP tendon and RPC grout A kind of CFRP rod named CFCC (Carbon Fiber Composite Cable) was used as the tendon of the ground anchor. It had feature and configuration similar to the traditional steel strand, with seven twisted CFRP wires, a nominal diameter of 12.5 mm and a cross-sectional area of 76 mm2, as shown in Fig.4. According to tensile tests of mono-rod specimens, the breaking load, the tensile strength and the tensile modulus of elasticity of the 1×7∅12.5mm CFCC rod were measured 194 kN, 2558 MPa, 157 GPa, respectively. The CFCC rod did not suffer any degradation in strength or visible defects under highly corrosive environments [6].

Fig.4 1×7∅12.5mm CFCC rod

The constituent proportion of RPC was determined based on the minimum porosity of an optimized granular mixture [3]. In the above-ground and terminal anchors, the composition of RPC included cement, silica powder, quartz sand/powder and superplasticizer. To achieve early high strength and minimal shrinkage and creep, hot curing (24 hours at room temperature followed by 48 hours in hot water at 80±2 °C) was adopted for the RPC grout in the anchors [7]. Within the below-ground fixed length, nanometer silica and the expansion additive were added to improve the mechanical property and workability of the RPC grout. In the new ground anchor, the 3-day strength of the RPC under hot curing should be no less than 110 MPa, while the 14-day strength of the RPC grout within below-ground fixed length should be no less than 90 MPa. Because the improved mechanical properties of grout helped increasing the bonding Kuangyi Zhang, Zhi Fang and Antonio Nanni

performance at the rod-grout interface [8], the much higher strength of RPC brought improved anchoring capacities for CFRP tendons. 2.3 Ground anchor system beneath the hanger According to the design of Aizhai Bridge, the overall lock-off load for ground anchors beneath either side of the hanger was up to 7,200 kN. Therefore, a group of seven ground anchors including one N1-type anchor, and six N2-type anchors, were selected to form the ground anchor system. Figure 5 shows the lay-out of the ground anchor system on the anchor base plate. The design lock-off load Po of N1 and N2 ground anchors was 2100 kN and 850 kN, respectively, so 24 and 9 CFCC rods were selected as the tendon. The tendon overall capacity Pr as a summation of all 24 and 9 rods was equal to 4,656 kN and 1,746 kN, respectively, and the lock-off load Po was less than 0.50 times the tendon capacity Pr. For a bond-type anchoring system, the key parameter of bond length is determined by the bond strength at the interfaces. In the new ground anchor, the bond length within the anchorage is determined by the bond strength at the CFCC-RPC interface, while the below-ground fixed length is determined by the bond strength at both the CFCC–RPC interface and the RPC–rock interface. Considering the provisions of ground anchor standard [9], in practice, the above-ground anchor head of N1 and N2 was designed with a bond length of 0.5 m and 0.4 m and a much longer below-ground fixed length of 8 m, 6 m was selected to provide the large-tonnage anchor system with an additional safety margin. The below-ground free length was selected to avoid the wedge-shaped failure of the rock mass under tension. The free length of N1 was 12 m, while two types of N2 (i.e., N2-1 and N2-2) were designed with free lengths of 11 m and 10 m, in order to transfer the tension to the rock at different depth. The diameters of the holes were determined to provide enough grout cover to the below-ground CFRP tendons. The typical dimensions of ground anchors are summarized in Table 1.

Fig.5 Lay-out of ground anchors at one side

Table 1. Design dimensions of ground anchors (m) Anchorage Above-ground Below-ground Below-ground Depth of Borehole Type bond anchorage length free length fixed length borehole diameter length N1 0.74 0.5 12 8 21 0.19 N2-1 0.56 0.4 11 6 18 0.15 N2-2 0.56 0.4 10 6 17 0.15

3. FABRICATION AND CONSTRUCTION OF THE SYSTEM 3.1 Fabrication of rock bolts All rock bolts were fabricated in a factory. The system was designed according to the material properties of CFCC rods and RPC grout, with anchorage and accessories manufactured first. Then, the CFCC rods were cut into certain lengths and assembled with spacers in free length and anchorages at both ends (i.e., the above-ground anchor head and the terminal unit). RPC grout was injected into the The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015) Joint Conference, 14-16 December 2015, Nanjing, China bond-type anchorage tube and put into hot curing. All fourteen rock bolts including two N1, eight N2- 1 and four N2-2 bolts were prepared as the assemblies with bond-type anchorages at both ends of CFRP tendons, as shown in Fig.6. The properties of RPC grout in anchorages were tested using cubes 70.7-mm in side and the 3-day strength of RPC under hot curing reached 120.3 MPa. Before being transported to the project site, all rock bolts were pre-stressed under 1.2 times the service lock-off loads to verify the reliability of the prefabricated anchorages as a method of product inspection. The tendon-anchorage assemblies were put into a long trench for tensile tests as shown in Fig.7. During the tests, the load was monitored by a large-tonnage load cell while the anchor displacement was measured by linear variable displacement transducers (LVDTs). All fourteen rock bolts showed capacities more than 1.2 times the lock-off loads, and the measured residual anchor displacements after unloading were less than 0.1 mm. The prefabricated bond-type anchorages were proven reliable.

Fig.6 Tendon-anchorage assemblies Fig.7 Tensile test of tendon-anchorage assemblies

3.2 Construction of ground anchor system The process to build up the ground anchor system in-situ includes the following steps: excavating foundation pit, drilling holes, installing reinforcements and pre-embedded accessories, casting concrete podium, inserting rock bolts, grouting, installing steel anchor base plate, tensioning rock bolts, and sealing the anchors. The diameter of the borehole for N1 and N2 ground anchors was 190 mm and 150 mm, respectively. After drilling, borehole testing was performed to ensure the good condition of the hole so that the fixed length could be fully grouted. After being transported to the construction site, the rock bolt was inserted into the borehole in a controlled manner. Because the depth of the hole was longer than the below-ground CFRP tendons, the tendons remained suspended before grouting. Guided by the borehole testing, RPC grout was pumped into the below-ground fixed length starting from the lower end under a pressure not less than 1.0 MPa. In order to study the strength development of below- ground grout, batches of RPC cubes were cast along with the ground anchors and were put into a natural cave nearby the construction site simulating the environment in the below-ground borehole. The test results showed that the 14-day and 28-day strengths of the below-ground RPC grout reached 93.7 MPa and 109.6 MPa, respectively. After 14 days of curing, the ground anchor was tensioned to the design load and locked off. The tensioning and monitoring setup is shown in Fig.8. At the top, a large-tonnage hollow hydraulic jack was used to apply tension through a steel rod connecting to the anchor head, and the load was monitored by a pressure sensor. Around the anchor head, two LVDTs were used for monitoring the vertical displacement of the anchor. According to specification requirements [9], the lower limit of the anchor displacement is the tendon elastic elongation in 0.8 times below-ground free length, and the upper limit is the tendon elastic elongation in the length of below-ground free length plus 0.5 times below-ground fixed length. During the tensile process, every anchor showed displacements inside the acceptance limits. The locking off method is shown in Fig.9. Upon tensioned to about 1.1 times the lock-off load, the locking nut around the anchor head was tightly turned to hold the tensile force and Kuangyi Zhang, Zhi Fang and Antonio Nanni

allow the hydraulic jack to be released. In this way, the design lock-off load of each ground anchor was achieved.

Fig.8 Tensioning and monitoring setup

(a) before tensioning (b) locking off (c) after locking off Fig.9 Locking off method

4. CONCLUSIONS This paper introduced the design and construction of an innovative ground anchor system for the Aizhai suspension bridge. Each ground anchor used CFRP rods as the tendons and RPC grout as the bond medium in both above-ground and below-ground anchor parts. The system beneath either hanger consisted of one N1 and six N2 ground anchors, with overall lock-off load up to 7,200 kN. Fourteen rock bolts were designed and fabricated in the factory, and transported to project site to build the ground anchors. They displayed reliable behaviours both during the assembly tensile tests performed at the factory and during the tensioning process at the construction site. Now the system is fulfilling its intended purpose as a component of the bridge structure, and its long-term behavior is being monitored.

REFERENCES [1] M. Sentry, A. Bouazza, R. Al-Mahaidi, D. Loidl, C. Bluff, L. Carrigan, Use of Carbon Fibre Reinforced Polymer (CFRP) as an alternative material in permanent ground anchors, Australian Geomechanics Journal, 2009, 44(3):47-56. [2] A. Nanni, Flexural behavior and design of RC members using FRP reinforcement, Journal of structural engineering, 1993, 119(11): 3344-3359. [3] P. Richard, Composition of Reactive Powder Concrete, Cement and Concrete Research, 1995, 25(7):1501-1511. [4] Z. Fang, K. Zhang, B. Tu, Experimental investigation of a bond-type anchorage system for multiple FRP tendons, Engineering Structures, 2013, 57:364-373. The 12th International Symposium on Fiber Reinforced Polymers for Reinforced Concrete Structures (FRPRCS-12) & The 5th Asia-Pacific Conference on Fiber Reinforced Polymers in Structures (APFIS-2015) Joint Conference, 14-16 December 2015, Nanjing, China

[5] K. Zhang, Z. Fang, A. Nanni, J. Hu, G. Chen, Experimental study of a large-scale ground anchor system with FRP tendon and RPC grout medium, J Compos Constr, 2015, Accepted for publication. [6] T. Enomoto, T. Sugisaki, K. Suga, T. Sueyoshi, Ground anchor made of new materials applicable for highly corrosive environments, International Conference on Ground Anchorages and Anchored Structures in Service, 2007, 341-350. [7] A. Feylessoufi, F. Villieras, P. Richard, Water environment and nonstructural network in a reactive powder concrete, Cement and Concrete Composites, 1996, 18(6):203-209. [8] J. Martí-Vargas, E. García-Taengua, P. Serna, Influence of concrete composition on anchorage bond behavior of prestressing reinforcement, Construction and Building Materials, 2013, 48:1156-1164. [9] China Association for Engineering Construction Standardization (CECS), Technical Specification for Ground Anchors, CECS 22-2005, Beijing, China. 2005.