INTERFACIAL STRESS DISTRIBUTION OF FRP-TO-CONCRETE JOINTS USING ADVANCED COMPOSITE PROCESSING TECHNIQUES

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1 INTERFACIAL STRESS DISTRIBUTION OF FRP-TO-CONCRETE JOINTS USING ADVANCED COMPOSITE PROCESSING TECHNIQUES Seyed Ali HADIGHEH Mr. RMIT University School of Civil, Environmental and Chemical Eng., RMIT University, Melbourne, VIC 3000, Australia Rebecca GRAVINA Senior Lecturer RMIT University School of Civil, Environmental and Chemical Eng., RMIT University, Melbourne, VIC 3000, Australia Sujeeva SETUNGE Associate Professor RMIT University School of Civil, Environmental and Chemical Eng., RMIT University, Melbourne, VIC 3000, Australia Abstract Fibre reinforced polymer (FRP) materials have been used to enhance the structural capability of deteriorated structures based on the high performance of composites. A sufficient strengthening methodology can be achieved by considering accurate bond behaviour between FRP and concrete substrate in RC structures externally bonded with FRP materials. Since, debonding of FRP materials from the substrate is a brittle failure, it is essential to study this phenomenon and propose significant ways to improve the behaviour of bond line. In this research, the fundamental characteristics of bond line in retrofitted specimens will be examined using advanced composite processing technique. An innovative processing method, called Sitecure, has been applied to joint fibre reinforced polymer strips to the concrete as substrate in the typical single shear push-pull test set-up. Results show that the stress in bond line for Sitecured specimens can be well distributed which leads to more appropriate bond characteristics of FRP jointed concrete. Keywords: Bondline, Concrete Prism, FRP, Single lap shear test, Sitecure. 1. Introduction The major concern in strengthening of structures with composite materials is utilizing their ultimate tensile strength during loading regime. It has been observed during the previous experiments that composite plates/sheets have been detached from the substrate before reaching the expected designed ultimate strength. This can be caused by improper surface preparation, insufficient curing time for the adhesive used between laminates and substrate, low tensile strength of epoxy, or concentrated stresses near the cracks. Based on past experiments, the main reason for debonding of FRP materials from the substrate is the low surface tensile strength of concrete, which can be proved by observing a thin layer of concrete (about 1 to 3 mm) beneath the composite materials is detached from the surface of concrete samples during loading. Hence, the behaviour of the bond line between FRP and substrate has Page 1 of 8

2 been investigated by different researchers during the past decade. Taljsten [1] performed simple lap shear tests on the concrete prisms on which steel and fibre reinforced polymer materials were attached. He considered different lengths for bonded area and studied the strain regime in the bond line (between concrete and FRP). Then, using the elastic theory of Volkersen [2], he compared the variation of the shear force per unit width in the bond line in pure shear with the experimental results. He suggested that there is a length beyond which no increase in the maximum shear load happens and based on energy theory he suggested a relation for calculating this effective bond length. Brosens and Van Gemert [3] suggested some relations for calculating the maximum transmissible shear force and the anchorage length of FRP in both the serviceability and the ultimate limit state based on the classical differential equation of Volkersen. Other researchers; such as, Chajes et al. [4], Nakaba et al. [5], De Lorenzis [6], Mazzotti et al. [7], Bizindavyi and Neale [8] and Pan and Leung [9] have studied the behaviour of the bond line between FRP materials and concrete surface. They have proposed that this interfacial behaviour depends on different factors; e.g. surface preparation, FRP bond length, width ratio between FRP to concrete, aggregate content and concrete strength. Nakaba et al. [5] showed that as the stiffness of FRP increases, the maximum load increases. They also concluded that the maximum local bond stress increases when the compressive strength of concrete increases but is independent from the type of FRP. Chen and Teng [10], assessed the empirical, fracture mechanics and simple design models based on a massive experimental results and proposed new simple design models using fracture mechanics model to predict the anchorage strength and the effective bond length. Recently, a new method, Sitecure, has been developed to achieve good bond using vacuum consolidation in comparison to standard resin application methods. Sitecure is an innovative evolved processing technique in which the air is removed from the bond line with vacuum consolidation leads to a good bonding. Through application of heat, a repeatable, reliable and fast cure can be achieved and FRP can be more resistant to long term thermal degradation. From technical observations, the initial quality of materials and consistency are high as well as moisture and chemical resistance of cured composites and overall quality assurance would be superior. Environmentally, there is no demolition or removal of defected material or construction of new concrete members. In addition, since neither heavy lifting equipment is required nor contractors are exposed to the chemical resins, the improved OHS & E is taken into account. Finally, considering the cost of repair, the labour force and job time are reduced, the cure time and process are fast and independent from weather or temperature condition and also there is no need to shutdown the structure during strengthening process takes place. In this article, the effects of bondline thickness on the interfacial performance are studied. For this reason, three different bond line thicknesses have been examined using Sitecure technique for applying FRP plates on the surface of concrete and the bond-slip behaviour was monitored during the experiment. The interfacial thickness was change by application of resin films between FRP plates and concrete surface. 2. Geometry of specimens and test setup 2.1 Specimens To study the intermediate flexural crack, 9 concrete prisms were tested in a direct pull out test. The dimensions of the concrete block specimens were 150mm in height, 150mm in width and 300mm in length, with a target concrete characteristic strength of 25 MPa (Figure1). Concrete was poured in the wooden forms under laboratory conditions and cured for 28 days covered by a plastic sheet. Prisms were vibrated using the shaking table and the top surface were Page 2 of 8

3 flattened by steel-trowel. Three standard concrete cylinders, 100mm in diameter and 200mm in height, were made for each batch and cured in a water tank for 28 days. At the 28 th day, curing was stopped for both cylinders and concrete blocks and kept in the room ambient condition until the day of testing. Figure 1. Geometry of specimens in the single pull out test (dimensions are in mm). Pre-impregnated FRP laminates consist of 4 plies of unidirectional carbon fibres with the weight of 300 grams per square metre (gsm). The properties of FRP materials, which are mentioned here, are reported by manufacturer. The fibre content of FRP laminates is 69.17% with Young modulus of elasticity E = MPa and tensile strength of 1601 MPa. The number of resin film layers was varied in the experimental program and consisted of 2, 4 and 8 layers of resin film, with lap shear strength of 34 MPa between the outer surface of concrete and FRP laminates. 2.2 Sitecure Process and FRP position on the prisms CFRP plates are bonded on the centreline of the top surface of concrete prisms using the Sitecure technique (Figure 2). Application of vacuum on the samples while inducing steam leads to a higher quality of bondline for plates compared to traditional methods such as, wet lay-up or pultrusion systems, in which the labour is not able to control the condition of bonding at the time of applying or cure [11] Because of this superiority of Sitecure technique, the authors were able to change the thickness of bond line. FRP laminates were initiated 25 mm away from the loaded end to assure the prevention of concrete splitting in the front side of concrete blocks in which high tensile stresses occur (Figure 1). This initial gap is used successfully by other researchers; such as, [7 and 8]. Figure 2. Application of Sitecure system to attach the laminates on the concrete. To study the effects of thickness on the bond line behaviour, three different thicknesses for FRP plates were examined applying 2, 4 and 8 layers of resin film. The resin film was placed between the pre-impregnated laminates and concrete surface in order to increase the bond Page 3 of 8

4 thickness. For each feature, three repeated samples were tested to gain reliable results. Table 1 represents the geometry of FRP plates attached on the samples using different layers of resin film. The average bondline thickness consists of the pre-impregnated laminate thickness and the resin film layer. Table 1. Details of the bondline. Specimens ID S1-1 S1-2 S1-3 S2-1 S2-2 S2-3 S3-1 S3-2 S3-3 Initial Gap Bond Length Layers of Resin Film Bonded Width Ave. Laminate Thickness Ave. Resin Film Thickness Ave. Bondline Thickness Instrumentation and measurements The concrete blocks were placed tightly on a platform which was designed specifically for these experiments. The vertical and horizontal movement were prevented using reaction and supporting frames in front and the back of blocks, respectively. The height of reaction plate was chosen 100 mm based on the recommendations of [12]. According to [13], the test equipment have been aligned in the way that the maximum eccentricity was ±2 mm. A monotonic shear load was applied by means of an actuator. Since, the ultimate displacements are small; the speed for applying the load was 0.2 mm/min. The slip of FRP was monitored by three linear variable displacement transducers (LVDTs), two on the loaded end and far end and one at the back of concrete sample (Figure 3). Meanwhile, the data was gathered with an automatic data logger system. Figure 3. Test set-up for pull out tests. 3. Results of single shear pull out test To examine the effects of the FRP thickness on the bond behaviour between concrete and FRP laminates, 9 concrete samples with different bondline thickness were tested under tensile load up to failure. The failure mechanism and load-slip relationships were investigated. 3.1 Failure mechanism The mechanism of failure was similar for all of the samples in the way which was occurred with a high sound at the end of tests. A thin layer of concrete was detached from the samples in all of Page 4 of 8

5 the tests, about 1.1mm, 1.26mm and 0.93mm for S1, S2 and S3, respectively (Figure 4). It can be seen from this figure that the melted resin film was penetrated well into the voids on the surface of concrete and made a good bond between FRP and concrete. Figure 4. S1.3 sample after single shear pull out test. For the most of samples, the maximum displacement before debonding was less than 1mm, except sample S1.2 which was 1.4 mm, that indicates the behaviour of bondline is brittle (Table 2). Table 2. Test results for different bondline thickness. Specimen Max Dis., X max Max Load, F max (kn) Test Failure Mode S CD S CD, NEF S CD Mean S.D COV. (%) S CD, NEF S CD, NEF S CD Mean S.D COV. (%) S CD S CD, NEF S CD, NEF Mean S.D COV. (%) CD: Concrete debonding, NEF: near end failure 3.2 Maximum load The maximum load of samples is presented in the Table 2. According to this table, when the thickness of bondline increases, the load carrying capacity of the FRP bonded to the concrete substrate will improve. However, considering the mean value for S2 and S1 groups, it can be seen that the maximum load for S2 series, with 4 layers of resin film, are almost the same, as S3 series consists of 8 layers of resin film (Figure 5). In addition, if a comparison is made between the maximum displacement of these two series, the laminates with 4 layers of resin film shows higher displacement than the third series (Figure 6). Therefore, there should be an optimum thickness and/or effective bondline (resin) thickness, through which the bond characteristics are constant. This concept is similar to the effective bond length beyond which no increase can be Page 5 of 8

6 observed in the maximum load of bonded FRP. Figure 5. Maximum load vs bondline thickness for samples. Figure 6. Maximum dislacement vs bondline thickness for samples. 3.3 Load-slip relationship During the experiment, the applied load and slip of FRP relative to the concrete substrate was collected by means of a data logger. The load vs slip for S1.2, S2.1 and S3.1 samples are plotted in Figure 7 and 8. The maximum load for S2.1 and S3.1 with 4 and 8 resin films, respectively, is almost the same. In addition, the initial stiffness of the bond for these two samples is higher than the FRP attached on the concrete with 2 layers of resin film. As it can be seen from Figure 8 and Table 2, as long as the bondline thickness increases the maximum ultimate displacement for the FRP decreases. It shows that if the bond thickness increases, the bond behaviour between FRP and substrate tends to be brittle which can be a deficiency for the retrofitting of members with fibre reinforced polymers. Page 6 of 8

7 Figure 7. Load-slip relationship for samples with 2 layers of resin film. 4. Conclusion Figure 8. Load-slip relationship for FRP with, 2, 4 and 8 layers of resin film. In this experimental program, the pull out test was done on nine samples in which FRP materials were attached to the concrete cubes using resin films in order to study the effects of thickness on the behaviour of bondline. The results showed that the bondline thickness of FRP is one of the important factors which has to be considered in the shear stress between FRP and concrete surface. Investigating the maximum load-slip diagrams for samples indicates that like the effective bond length, there is an effective bond thickness beyond which the maximum load does not increase dramatically and the behaviour of the joint tends to be brittle. 5. Acknowledgment Authors would like to thank Industrial Composite Contractors Co. (ICC) for helping to provide material and technical supports. Also, authors are grateful for the financial support provided by RMIT University. 6. References [1] TALJSTEN, B., Defining Anchor Length of Steel and CFRP Plates Bonded to Concrete, Int. J. Adhesion and Adhesives, Vol. 17, 1997, pp Page 7 of 8

8 [2] VOLKERSEN, O., Die Nietkraftverteilung in Zugbean Spruchten Nietverbindungen Mit Konstanten Laschenquerscnitten, Luftfahrforschung, 1938, 15, pp [3] BROSENS, K., VAN GEMERT, D., Plate End Shear Design for External CFRP Laminates, Proceedings of Fracture Mechanics of Concrete Structures (FRAMCOS-3), Vol. 3, 1998, pp [4] CHAJES, M., FINCH, W.W., JANUSZKA, T.F., THOMSON, T.A., Bond and Force Transfer of Composite Material Plates Bonded to Concrete, ACI Structural Journal, Vol. 93, No. 2, 1996, pp [5] NAKABA, K., KANAKUBO, T., FURUTA, T., YOSHIZAWA, H., Bond Behavior between Fiber-reinforced Polymer Laminates and Concrete, ACI Structural Journal, Vol. 98, No. 3, 2001, pp [6] DE LORENZIS, L., MILLER, B., NANNI, A., Bond of FRP Laminates to Concrete, ACI Material Journal, Vol. 98, No. 3, May-June 2001, pp [7] MAZZOTTI, C., SAVOIA, M., FERRACUTI, B., An Experimental Study on Delamination of FRP Plates Bonded to Concrete, Construction and Building Materials, Vol. 22, 2008, pp [8] BIZINDAVYI, L., NEALE, K.W., Transfer Lengths and Bond Strengths for Composites Bonded to Concrete, Jounal of Composites for Construction, Vol. 3, No. 4, November 1999, pp [9] PAN, J., LEUNG, C.K.Y., Effects of Concrete Composition on FRP/Concrete Bond Capacity, Jounal of Composites for Construction, Vol. 11, No. 6, December 2007, pp [10] CHEN, J.F., TENG, J.G., Anchorage Strength Models for FRP and Steel Plates Bonded to Concrete, Jounal of Structural Engineering, Vol. 127, No. 7, July 2001, pp [11] GRAVINA, R., HADIGHEH, S.A., SETUNGE, SUJEEVA, Bond and Force Transfer of FRP Materials Bonded to Concrete Using Sitecure System, The Third Asia Pacific Conference on FRP in Structures (APFIS 2012), Japan, Feb 2012, Full Paper is Accepted. [12] YAO, J., TENG, J.G., CHEN, J.F, Experimental Study on FRP-to-concrete Bonded Joints, Journal of Composites: Part B, Vol. 36, 2005, pp [13] HB 305, Design Handbook for RC Structures Retrofitted with FRP and Metal Plates: Beams and Slabs, Standards Australia, 2008, p. 67. Page 8 of 8