FIBRE COMPOSITES PILE REHABILITATION AND CONCRETE FORMWORK JACKET CONCEPT DEVELOPMENT AND FINITE ELEMENT ANALYSIS

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1 Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) December 2013, Melbourne, Australia 2013 International Institute for FRP in Construction FIBRE COMPOSITES PILE REHABILITATION AND CONCRETE FORMWORK JACKET CONCEPT DEVELOPMENT AND FINITE ELEMENT ANALYSIS A.C. Manalo, W. Karunasena, C. Sirimanna, and G. Maranan Centre of Excellence in Engineered Fibre Composites, Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, Queensland 4350, Australia ABSTRACT Fibre reinforced polymer (FRP) composites offer an extremely versatile option to strengthen or rehabilitate existing structures in harsh marine environment. However, most of the current composite repair systems are limited to applications in easy-to-assess sections or portions of the structure only at a certain depth underwater and requires longer time for people to install and consequently, work underwater. This paper presents the concept development of a new type of FRP jacket and the preliminary evaluation of the potential of this composite repair system for pile rehabilitation and concrete formwork. The novelty of this fibre composite strengthening system is that it is quick and safe to install due to the easy-fit and self-locking mechanical joint technology developed and patented by Joinlox. This paper focuses on the analysis and evaluation of the capacity and behaviour of an innovative FRP jacket system subjected to internal pressure using Strand7 finite element (FE) software to verify the practicability of the concept. Comparison with full-scale testing showed that the FE simulations can reasonably predict the expansion behaviour and failure of the FRP jacket. KEYWORDS Pile rehabilitation; jacket; joint; locking key; microfibre resins. INTRODUCTION In the last 20 years, fibre reinforced polymer (FRP) composite materials have become an extremely versatile option to strengthen or rehabilitate existing structures, especially in harsh marine environment. This material has been a preferred option because of their superior properties such as high strength, corrosion resistance, lightweight, high fatigue resistance, nonmagnetic, high impact resistance, and durability. In fact, reinforcement and strengthening of civil infrastructures using fibre composites has been a topic of research and development for years. Practical applications and implementations on using FRP composites in strengthening of existing bridges are now counted in hundreds (Monti et al. 2001). Among the possible upgrading strategies for deteriorated reinforced concrete (RC) piles in marine environment, the use of FRP wraps or jackets to prevent the lateral (radial) expansion of concrete, is gaining widespread acceptance. Considerable research has been reported in the last decade on the mechanical behaviour and failure of FRP wrapped RC columns. These are studies on FRP confinement of column structures not in marine environment which include the repair and retrofit of damaged columns using FRP system and strengthening of columns structures using FRP composites. In RC pile rehabilitation using FRP jacket, confinement is the key aspect that strengthens the columns or piles. This confinement effect of FRP wraps has been observed in numerous investigations to date, e.g. Sen and Mullins (2007). Columns wrapped with FRP composites exhibited an enhanced ductility, load carrying capacity, and lateral deflection capability. Also, hoop strength, longitudinal strength, and shear resistance of the wrapped piles are increased. Aside from meeting the strength requirements, the light weight characteristic of FRP makes it easy to install. Moreover, it serves as a shield that prevents deterioration due to environmental attacks. Pre-fabricated system is most of the time the preferred option in repairing structures using fibre composites located underwater. In the pre-fabricated system, the FRP composite jackets are produced in the manufacturing plants to achieve a high quality and uniform product. Available pre-fabricated FRP composite jackets require divers to do an underwater assembly using these repair system. Also, they lack the ability to be used as formwork in the construction of column structures. Further, the system requires a minimum of two FRP composite shells to achieve the needs for structural restoration and held together with circumferential metal

2 straps or temporary bands which are usually damaged and severed which lead to the opening of the composite shells. Thus, the ability of the pre-fabricated composite repair system to provide structural continuity and confinement in the circumferential direction is always a concern. There is a need therefore to develop an innovative and effective jointing system for an FRP jacket which can provide continuity and confinement. This paper presents the development of a new type of FRP jacket with an easy-fit and self-locking mechanical jointing system and the preliminary evaluation to demonstrate the application of this pre-fabricated composite repair system for pile rehabilitation and concrete formwork. This paper focuses on the investigation of the interlocking teeth and joint key of the FRP jacket including the characterisation of the mechanical properties of the composite resins, which are the main materials used in the joint. An assessment of the capacity and behaviour of the FRP jacket subjected to internal pressure using Strand7 finite element software combined with experimental verification was conducted to simulate the expansion due to the confinement effect and determine the viability of this new rehabilitation technique. THE PILEJAX TM COMPOSITE REPAIR SYSTEM The PileJax TM is a new type of a pile repair system and concrete formwork jacket manufactured from fibre composite materials with an innovative mechanical joining system (see Figure 1). The novelty of this repair system is that it is quick and safe to install due to the easy-fit and self-locking mechanical jointing system developed and patented by Joinlox TM Pty Ltd (Pilejax TM 2012). This joint system involves two interlocked edges that quickly mesh together like the teeth of a zipper. A locking key is placed between the interlocking teeth and slid or levered only one pitch length into place, wedging the joint edges together with a uniformly loaded force along the entire length or circumference of the joint, which makes it more reliable than strapped/lap joints. Figure 1. The PileJax TM FRP jacket The system works by wrapping the prefabricated and flexible FRP jacket around the pile above or below the waterline and placing the joint key vertically along the seam to lock the jacket producing a cylindrical confinement. Adhesive is then applied between the teeth of the jacket and the joint key to hold the key in place thereby improving its locking mechanism. The finished assembly is then lowered in the water up to the depth where repair is required. The jacket is lightweight and safe to install which does not require any cranes or rigging. A standard hose is fitted at the bottom of the installed jacket to up-fill annulus with water displacing grout, resin or concrete as required. After grouting, the hose is removed and the installation is done. JOINT MATERIALS The key component of the PileJax TM technology is the innovative jointing system which locks the FRP jacket and provides the structural continuity of the repair system. This joint (locking key and interlocking teeth) is made up of high strength composite resin which is moulded to appropriate geometry. Experimental investigation was conducted to determine the mechanical properties of composite resins and glass fibre reinforced polymer (GFRP) rod in order to select the most appropriate material for the joint. These mechanical properties are utilised as input into the finite element simulations of the behaviour of the jacket. Microfibre reinforced resin Three types of thermoset composite resins are considered in this study. These are the microfibre infused resin, high performance epoxy vinyl ester resin and microfibre reinforced high performance vinyl ester resin, which were labelled as Types 1, 2 and 3, respectively. The average mechanical properties and standard deviations of Types 1 to 3 resins determined from different tests of coupon are summarized in Table 1.

3 Test Table 1. Mechanical properties of microfiber composite resins Property Type 1 Type 2 Type 3 Average Std Dev Average Std Dev Average Std Dev Flexure Modulus (GPa) Peak Stress (MPa) Strain at Peak (%) Tensile Modulus (GPa) Peak Stress (MPa) Strain at Peak (%) Compression Modulus (GPa) Peak Stress (MPa) Strain at Peak (%) Shear Modulus (GPa) Peak Stress (MPa) Strain at Peak (%) GFRP bar One type of jointing system considered for Joinlox TM is a locking key with an embedded FRP rod as it is anticipated that the bending strength and shear strength of the key will be a governing factor in designing the joint. Thus, mechanical characterisation of a 10 mm diameter GFRP rod was conducted and the results are summarized in Table 2. Evaluation on the effect of the embedment of GFRP rod in the joint key is simulated using FE analysis and discussed in the succeeding sections. Table 2. Mechanical properties of GFRP bar Properties Measured values Average Std Dev Flexure Modulus (GPa) Strength (MPa) Strain at peak (%) Compression Strength (MPa) Shear Strength (MPa) FE SIMULATION OF THE FRP JACKET BEHAVIOUR Finite element (FE) analysis was performed to investigate the overall behaviour and the failure mechanism of the PileJax TM FRP jacket when subjected to internal pressure. The application of internal pressure is done to simulate the expansion of the FRP jacket under the effect of confining pressure in a repaired column. The FE simulation was performed using a 2-pitch model as shown in Figure 2. The diameter of the jacket is 650 mm with a height of 400 mm. The jacket is made up of 3.0 mm thick triaxial E-glass fibre-polyester composites and has an effective Young s modulus of 29 GPa. Similarly, the 0.5 mm thick SG 230 HV Methacrylate epoxy adhesive (E = 0.55 GPa) between the FRP jacket and joint is included in the development of the FE model. Four FE models with different joint and joint key materials were analysed. Models A, B and C represent the FRP jacket with Types 1, 2 and 3 composite resins for the joint, respectively, while Model D corresponds to the jacket with Type 3 resin and 10 mm diameter GFRP rod inserted in the locking key. In all FE models, the joint and joint key, GFRP rod and epoxy adhesive are modelled using Tetra4 brick elements while the FRP jacket is modelled using plate elements. Similarly, an internal pressure of 1 MPa is applied on the interior surface of the jacket elements. The restraint conditions for 2-pitch FE model are also shown in Figure 2. The top and bottom of the model are restrained to move up and down and to rotate about the radial and tangential directions. The jacket is allowed to move in the radial direction only.

4 Analysis was conducted using the linear static solver in Strand7. The prediction of failure of the jacket was performed by automatically load stepping the applied internal pressure of 1 MPa. The level of stresses in each component of the jacket was then checked at each load step and compared with the strength properties of materials established from test of coupons. The jacket is considered to fail when the allowable strength of the joint materials, summarized in Tables 1 and 2, is/are exceeded. The possible failure mechanisms of the FRP jacket with and without FRP rod in the joint key were also determined. Similarly, the radial deformation of the jacket was determined. This is done to analyse the overall behaviour of the FRP jacket when subjected to internal pressure and determine the effect of the internal expansion of FRP jacket due to confining effect. Figure 2. FEM model of PileJax TM FRP jacket (left) and node restraints in the symmetric end (right) RESULTS OF FE SIMULATIONS AND EXPERIMENTAL EVALUATION Failure behaviour of FRP jacket at varying internal pressure The level of stresses in the interlocking teeth and key of the joint is analysed at each load step and compared with the strength properties of materials to determine the level of applied internal pressure where failure will occur in the FRP jacket. In the FE simulation, it was deliberately assume that no failure will occur in the FRP jacket and limit only on the joints to readily evaluate the suitability of the composite resins for the joint and compare the capacity of the joints for the different materials considered. The stress profile of the interlocking teeth and locking key and GFRP rod for Model D is shown in Figure 3. Table 3 summarizes the failure mechanism and the magnitude of internal pressure that induces this type of failure in FRP jacket. The results of the FE simulation indicated that high stress levels in the FRP jacket can be found in the interlocking teeth and locking key of the joint. The results also showed that the tensile stress in the interlocking teeth and the shear stress in the locking key will exceed the strength of the composite resin at an applied internal pressure of 14 MPa. On the other hand, the bending stress in the locking key will exceed the strength of the composite resin at an applied pressure of 20 MPa. Similarly, the shear stress in the GFRP rod will exceed 50 MPa when an internal pressure of 20 MPa is applied while it will fail in bending at a pressure of 42 MPa. In all FE models, debonding failure of the adhesive layer will occur at an applied internal pressure of 22 MPa. Based on this level of stresses, it anticipated that the failure of the PileJax TM FRP jacket will more likely occur due to the tensile failure of the interlocking teeth or shear failure of the locking key. The results of the FE simulation indicate that the FRP jacket without any GFRP rod inserted in the locking key will most likely fail due to shear failure. This type of failure will occur at an applied pressure of 4, 6 and 14 MPa for Models A, B and C, respectively. This finding is concurrent with the results of the test of coupons, wherein the three types of composite resin exhibited low shear strength. Interestingly, Model C or the FRP jacket with joint made from high performance epoxy vinyl ester resin provide the best structural continuity for the PileJax TM FRP jacket and a high confining pressure to the repaired column. The results also indicated that no edge debonding will occur on the epoxy adhesive when failure of the joint occurs indicating that a reliable structural bond between the joint and the jacket can be achieved using SG230 HV Methacrylate adhesive. The results of the FE simulations showed that the shear strength of the locking key is improved with the insertion of a GFRP rod. Based on the results, the key joint in Model D will fail in shear at an applied internal pressure of 20 MPa. However, the positive effect of inserting the GFRP rod did not translate to a better structural performance of the Joinlox TM FRP jacket as its overall behaviour was governed by the tensile strength of the interlocking teeth.

5 (a) Axial stress in the teeth (b) Shear stress in key Figure 3. Axial stress profile in the teeth and joint key at an applied internal pressure of 14 MPa Full-scale testing and verification Table 3. Level of internal pressure (in MPa) to initiate failure Model Possible failure mechanism A B C D Tensile failure of the teeth Shear stress in the key Bending stress in the key Shear stress in the adhesive Shear stress in the FRP rod Bending stress in the FRP rod Full-scale FRP jackets (models C and D) with an internal diameter of 650 mm and length of 1.0 m were fabricated and tested to demonstrate the capacity of the joints to resist an applied internal pressure. The test was conducted to determine if the PileJax TM FRP jackets will safely satisfy the industry design criteria for repair jackets of an internal pressure 2 MPa without failure. Hoop load testing was conducted to simulate internal expansion of FRP jacket. This was done by placing two steel half shells inside the FRP jacket. The shells are expanded outward using a hydraulic machine producing internal loads. The specimen is instrumented to obtain the radial deformations. The load is applied in increments of 0.5 MPa and hold for 30 sec to observe for any failure. The FRP jacket with interlocking joint made purely from composite resin (Model C) was tested up to failure while the jacket with inserted GFRP rod in the joint (Model D), the application of load pressure was applied in 0.5 MPa increments up to 6 MPa, then subsequently increased steadily to 50 MPa and stopped without any signs of failure. The actual test set-up is shown in Figure 4. Figure 4. Hoop load test set-up

6 Comparison between FE simulations and full-scale test Figure 5 shows the radial displacement of the FRP jacket under the applied internal pressure of 2 MPa or the industry design criteria for repair jackets. The radial displacement of the FRP jacket based on FE simulation is comparable with the results obtained from the actual experiment up to 1.5 MPa. This means that the FE modelling is validated by the test results. At 2.0 MPa, the experiment resulted to a lower radial deformation than FE simulation as the FRP jacket slide from the test set-up which resulted to lower measured deformation. Also, this can be due to the difference in the application of load in the experiment and FE simulation. Applied pressure (MPa) Expt FEM Radial displacement (mm) Figure 5. Comparison of the applied pressure and radial displacement of the FRP jacket The FEM analysis indicated that the shear failure at the locking key for Model C will occur at an applied pressure of around 14 MPa. Based on the full-scale testing, cracking was heard at an applied pressure of 14.5 MPa and final failure of the joint occurred at an applied pressure of 16.2 MPa. This level of applied pressure is at 8 times the design pressure of 2 MPa indicating that the efficiency of the PileJax TM FRP jacket for pile repair. There was no observed failure in the joint of Model D when the test was stopped at 50 MPa. In Figure 2, the tensile stress in the outer face of the interlocking teeth is higher than the tensile strength of the materials at 14 MPa with the stress at the back of the teeth lower than 60 MPa. Results of the FE simulation showed that this level of stress will be reached at an applied internal pressure of around 54 MPa. CONCLUSIONS The behaviour of a new type of FRP jacket with an innovative joining was evaluated using FE analysis and verified from full-scale pressure testing. The following are the major findings of the investigation: High stress levels were found in the locking key. Results of the FE simulations and full-scale testing showed that the joint will fail due to shear failure of the locking key. High performance vinyl ester resin was found to have the highest capacity among the considered microfiber resin. A higher joint capacity can be achieved by a locking key with an embedded GFRP rod. Comparisons between theoretical and full-scale experimental evaluations showed that the FE simulations can reasonably predict the expansion behaviour and failure of the FRP jacket. The performance of the FRP jacket exceeds considerably the industry standard indicating that this system is an effective strengthening method for pile and a permanent concrete formwork jacket. ACKNOWLEDGEMENT The authors gratefully acknowledge Joinlox TM Pty. Ltd., Australia for the financial, materials and technical support in conducting this study. REFERENCES Monti, G., Nistico, N., and Santini, S. (2001). Design of FRP jackets for upgrade of circular bridge piers, ASCE Journal of Composites for Construction, 5(2), Sen, R. and Mullins, G. (2007). Application of FRP composites for underwater piles repair, Composites, Part B, 38, PileJax TM - Pile Repair Jackets, website: viewed: 10 November 2012.