GFRP-GLULAM-CONCRETE COMPOSITE BEAMS: AN EXPERIMENTAL EVALUATION Antonio Alves Dias 1, José Luiz Miotto 2 ABSTRACT: This paper focuses the contribution obtained by the association between reinforced concrete slab and glue-laminated-timber (glulam) beams reinforced with GRFP. In order to do this, experimental investigations were realized with four beams with structural dimensions, of which two of them were composite beams. The beams were tested using ASTM D 198 procedures, in four load point static bending test. The composite beams presented a 57% reduction on the vertical displacements average value and a 28% increase in the modulus of rupture (MOR) average value. The reduction on the stress level of the lumber tension, obtained by the addition of glass fibre reinforcement, was an important contribution of this combination of materials. KEYWORDS: Glulam-concrete composite structures, glulam, GFRP, experimental evaluation 1 INTRODUCTION 123 Over the last few decades, the structural products derived from wood such as glue-laminated-timber (glulam) have provided new application fields for wood, once they present compatible patterns with the modern demands of buildings. Although it is one of the oldest products from wood, glulam is still not an absolutely justifiable material to be used on Brazilian constructions, because of the lack of tradition on its use, high adhesives cost and reduced number of companies involved on its production. On the other hand, glulam advantages in comparison to sawn wood are relevant, especially concerning the possibility of producing devices with practically no dimensional boundaries, with higher strength and stiffness. In order to obtain a higher bending strength, desired in situations with large spans or with high loads, one of the solutions presupposes the adoption of fibre reinforcement on the tension side of glulam beams. Nonetheless, the relatively low longitudinal modulus of elasticity of the wood makes the deformations a limiting factor on the beams design. Furthermore, the use of fibres as reinforcements do not fully solve the glulam beams deformability problems, once the increases provided to their stiffness are slight. 1 Antonio Alves Dias, Department of Structural Engineering, São Carlos School of Engineering, University of São Paulo, Av. Trabalhador são-carlense, 400, 13.566-590 São Carlos São Paulo, Brazil. Email: dias@sc.usp.br 2 José Luiz Miotto, Department of Civil Engineering, State University of Maringá, Av. Colombo, 5.790, 87.020-900 Maringá Paraná, Brazil. Email: jlmiotto@uem.br On this way, in order to ensure a better bending performance, it is suggested on this research the association between reinforced concrete and glulam beams reinforced with synthetic fibres on the form of composites known as composite structures. This study focuses, particularly, the composite beams with a T- shaped cross-section, whose web is made of fibreglassreinforced (GFRP) glulam while the flange is made of reinforced concrete. Steel connectors were used between the materials. 2 LITERATURE REVIEW 2.1 GLULAM REINFORCED WITH SINTHETIC FIBRES Combining two materials with compatible and complementary physical and mechanical properties may revolution building techniques [1]. Wood consumption reduction is an advantage of the application of fibres reinforcements on timber beams. Researches show a 30% to 40% decrease on wood volume when fibres-reinforced glulam is used. [2] Glulam beams may have their mechanical properties improved with the gluing of reinforcements on the tension side, as shown on Figure 1. This association assembles the advantages of wood high performance with relatively low costs and an excellent relation between strength and density with the advantages of synthetic fibres, such as high resistance and stiffness, as well as versatility [1]. The impact of the strength decrease, due to the imperfections on the tension side of the glulam beams, declines with the adequate addition of the fibrereinforced polymers, usually designed by the acronym FRP, or GRFP in the case of glass fibres.
static tests. The authors of the experiment observed that the association of the concrete slab provided a 26% increase on the loading capacity of the system. The researches regarding the behaviour of the GFRP glulam beams associated with a concrete slab, whose cross-section is being represented on Figure 3, are demonstrated on [6]. Figure 1: Cross section of glulam beam reinforced with GFRP 2.2 TIMBER-CONCRETE COMPOSITE STRUCTURES The direct and continuous exposition of timber structures to various weather conditions is a cause of constant worry, once it promotes wood decay and decreases the safety of these buildings and, consequently, of their users. One of the solutions consists in associating a concrete slab, as illustrated on Figure 2, generating the timber-concrete composite structures. These structures, besides guaranteeing an increase to the timber structures life cycles, are also capable of improving their mechanical behaviour. Figure 3: Cross-section of the composite beam. Source: Brody et al. (2000) On [7], the experimental performance of two GFRP glulam beams is demonstrated, considering the partial contribution of the reinforced concrete slab. The discontinuities on the timber-concrete interface, as shown on Figure 4, indicate the partial contribution of the reinforced concrete slab to the composite system stiffness. Figure 2: View of deck soffit made of timber-concrete composite structure In order for the composite system to work properly, it is indispensable the presence of a connector element between the materials, which ensures the horizontal shear transfer and also avoids the vertical detachment between the parts. Generally, steel connectors such as nails, screws, gang-nails, rings, pins and hooks are used for this purpose. Experimental investigations realized with hook-shaped steel connectors fixed with different kinds of epoxy resins, on a way that 0º, 45º and 90º were formed with the wood grain, demonstrating the efficiency of this type of connector [4]. Two composite T-beams, with a glulam web and reinforced concrete flange, using gang-nail connectors were submitted to experimental evaluations [5]. Initially, the glulam beams elasticity limits were obtained through Figure 4: Strain for a beam on the loading until failure. Source: Weaver (2002) 3 MATERIALS AND METHODS In order to evaluate the glulam-concrete composite beams, an experimental program was developed. This program consists on the following stages: (a) characterization of the materials; (b) determination of the slip modulus and ultimate load of connection system; (c) tests in structural dimensions of a T-beam with reinforced concrete slab and GRFP glulam beams. 3.1 MATERIALS CHARACTERIZATION The glulam beams were made using a Eucalyptus grandis/urophylla hybrid extracted from reforestation areas. On Table 1 some of its properties are shown.
These properties were obtained from tests using specimens and NBR 7190 [8] procedures. Table 1: Properties of wood Material properties Average values Moisture content 9.1 % Apparent density moisture content of 12% Compressive strength parallel to the grain, f c,0 Modulus of elasticity parallel to the grain, E c,0 Modulus of elasticity in bending, E M 0.79 g/cm³ 69.4 MPa 27,541 MPa 18,004 MPa Once the finger joints were done, the lumber was classified visually and mechanically. From the concrete, six cylindrical specimens with a 15 cm diameter and a 30 cm height were extracted. The following average values were found for the concrete: compression strength at 28 days f c,28 = 45.2 MPa and tangent modulus of elasticity E ci = 36436 MPa. The steel hooks used in the connection system, with a diameter of 8 mm, were obtained from rebars (f yk = 500 MPa) and fixed on the glulam beams, with the use of epoxy adhesive, making a 45º angle in relation to the wood grain. The fibreglass reinforcement was made from the impregnation of a fibreglass unidirectional textile, with a thickness of 0.50 mm, with the use of epoxy resin (AH- 30 and AR-300, both produced at Barracuda Advanced Composites). In order to determine the mechanical properties of the fibreglass, specimens were tested according to dimensions and experiment procedures established by the standard ASTM D 3039/D 3039M [9]. The average value found for tension strength was 956 MPa and for the longitudinal modulus of elasticity, 59463 MPa. 3.2 CONNECTION SYSTEM On [10], the performance of the connection system used between the reinforced concrete slab and the glulam beam made of hooks and perforated steel plates was evaluated through push-out tests. Even though the perforated plates present high initial slip modulus (K ser ), they also presented a fragile failure. Then, steel hooks with a diameter of 8 mm were used. They presented an average ultimate load of 131 kn and a slip modulus K ser = 142,936 N/mm. 3.3 BEAMS PRODUCTIONS AND TESTING On the production of the glulam beams, an isocyanate type adhesive was used Wonderbond EPI EL 70 with the catalyser EPI WS 742 both produced by Hexion Química do Brasil. The average dimensions of the four glulam beams produced were: 80 mm width, 312 mm height and 5400 mm length. Two of these beams, named V1 and V2, have only received the fibreglass reinforcement, while the other two beams, named V3 and V4, have received the reinforced concrete table as well as the fibreglass reinforcement. Twenty layers of fibreglass textile were applied on the glulam beams, resulting in a thickness of 10 mm, representing 3.1% of the total cross-section area. It s important to note that the total thickness of reinforcements layers were 16 mm, considering the space taken by the epoxy resin. In order to establish the cross-section dimensions of the composite beams, initially we sought analogies on [6], in which the concrete slab should have a width b c = 420 mm. However, the use of flange with exaggerate widths were avoided to decrease the shear lag effect. On this way, the chosen dimensions are the ones indicated on Figure 5, while the steel bars of the concrete slab are represented on Figure 6. Figure 5: Glulam-concrete beams cross-section Figure 6: Steel bars of concrete slab To evaluate the strains and stress distribution in the centre span of beams V1 and V2, strain gages KFG-10-120-C1-11, with a length of 10 mm, were installed on the positions shown on Figure 7, and were connected to a data acquisition system named System 5000, from Vishy Measurements Groups. Figure 7: Strain gages on beams V1 and V2 Regarding beams V3 and V4, strain gauges KFG-10-120- C1-11, with a length of 10 mm on the glulam parts and 20 mm on the reinforced concrete slabs, were installed on the positions shown on Figure 8 at the centre span.
Figure 8: Strain gages on beams V3 and V4 Modulus of elasticity (MOE) of the beams were obtained through static bending tests (according to ASTM D 198 [11] procedures), as shown in Figure 9. The loading was applied until it reached approximately 40% of the expected ultimate load, on two load cycles. After that, the beams were loaded until failure to obtain their respective modulus of rupture (MOR). Figure 11: Stress distribution on the central cross-section of the beam V2 The various glulam layers and the reinforcement layer are being indicated on these figures. Note that, in order to simplify the representation, the stress values were considered constant throughout each layer s height. Likewise, Figures 12 and 13 show stress distribution of the beams V3 and V4. Figure 12: Stress distribution on the central cross-section of the beam V3 Figure 9: Test setup for glulam-concrete beam Vertical displacements were measured at the centre of span, load points (1/3 of span) and supports. The slipping on the interface of the flange and web was also measured. 4 RESULTS AND DISCUSSIONS Based on the experimental data, it was possible to obtain the strain distribution on the central cross-section of the beams V1 and V2. Figures 10 and 11 show diagrams containing the stress distribution of these beams, based on the last loading level. Figure 10: Stress distribution on the central cross-section of the beam V1 Figure 13: Stress distribution on the central cross-section of the beam V4 One of the advantages on the addition o fibres reinforcements, emphasized on [12], is the reduction on the consumption of lumber of exceptional quality, which should be placed specially on the tension side of glulam beams. It s possible to verify this affirmation by analysing the stress-distribution graphics (Figures 10 to 13). Using the Transformed-Section Method, it was possible to obtain the theoretical bending stiffness of composite beams V3 and V4, named (EI) t on Table 2, without considering the slipping in the interface web flange. Later on, these values were compared to the experimental stiffness values observed on the loading level of 40.6 kn. The decrease on the experimental stiffness value was approximately 40%.
Table 2: Bending stiffness beams V3 and V4 Beam Bending stiffness (EI) t (N.mm 2 ) Experimental stiffness (EI) exp (N.mm 2 ) (EI) (EI) V3 2.1118E+13 1.1796E+13 0.56 V4 2.1338E+13 1.3330E+13 0.62 Table 3 shows a comparative between the average deflection of beams V1 and V2 with the composite beams V3 and V4, at the same loading level. Table 3: Comparative between the average deflection of beams (Load = 40.6 kn) Beams Deflection average values (mm) exp Relation to V3 and V4 (%) V3 and V4 7.2 --- V1 and V2 16.8 133.3 Table 4 shows beam's modulus of rupture (MOR). A smaller dispersion on the results was observed when compared to similar beams which did not receive a fibreglass reinforcement, as presented in [10]. On this way, one of the greatest advantages on the application of fibreglass reinforcements consists on the decrease on the strength variability. Table 4: Performance of the beams tested until failure Type of beam Designation 5 CONCLUSIONS Average MOR (kn) Relation to V1 and V2 (%) V1 and V2 66.6 --- V3 and V4 85.3 28.0 The addition of a reinforced concrete slab provided an average increase of 28% on the modulus of rupture (MOR), and an average decrease of 57% on the deflections. Besides the strength increase, the addition of fibreglass reinforcements provide other advantages, such as a reduction on the stress level of the glulam s bottom lumber which means wood saving and a decrease on the results dispersion, providing more reliability to the system. On the other hand, it was also observed that the experimental bending stiffness of the composite glulamconcrete beams was equivalent to 60% of the theoretical t values considering total composition, due to the slip between the slab and web, allowed by the connection flexibility. ACKNOWLEDGEMENTS The authors thank to Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq by the financial support. REFERENCES [1] Dagher H. J.: High performance wood composites for construction. In: Annals of VII Encontro Brasileiro em Madeiras e em Estruturas de Madeira, São Carlos, Brazil, 2000. [2] Dagher H. J.: FRP reinforced wood in bridge applications. Proceedings of 1st RILEM Symposium Timber Engineering, 1999. Stockholm, Sweden, p.591-598. [3] Lindyberg R. F.: The volume effect in FRP-glulams. Proceedings of 2nd International Conference on Advanced Engineered Wood Composites, 2001. Bethel, USA. [4] Pigozzo J. C.: Estudos e aplicações de barras de aço coladas como conectores em lajes mistas de madeira e concreto para tabuleiros de pontes. Thesis of doctorate, São Carlos School of Engineering, University of São Paulo, Brazil, 358p, 2004. [5] Mantilla Carrasco E. V. et al.: Viga mista de madeira laminada colada de Eucalyptus Grandis e concreto armado uma avaliação experimental e numérica. In: Annals of IX Encontro Brasileiro em Madeiras e em Estruturas de Madeira, Cuiabá, Brazil, 2004. [6] Brody J. et al.: FRP-wood-concrete composite bridge girders. Proceedings of Structures Congress 2000 Advanced Technology in Structural Engineering, 2000. Philadelphia, USA, Section 53, chapter 1. [7] Weaver C. A.: Behavior of FRP-reinforced glulamconcrete composite bridge girders. Dissertation of Master of Science in Civil Engineering, The University of Maine, 236p., 2002. [8] Associação Brasileira de Normas Técnicas: NBR 7190 Projeto de Estruturas de Madeira. Rio de Janeiro, Brazil, 1997. [9] American Society for Testing and Materials: ASTM D 3039/D 3039M Standard test method for tensile properties of polymer matrix composite materials. Philadelphia, USA, 2006. [10] Miotto J. L.: Estruturas mistas de madeira-concreto: avaliação das vigas de madeira laminada colada reforçadas com fibras de vidro. Thesis of doctorate, São Carlos School of Engineering, University of São Paulo, Brazil, 325p, 2009. [11] American Society for Testing and Materials: ASTM D198-05a Standard tests methods of static tests of lumber in structural sizes. Philadelphia, USA, 2005. [12] Davids W. G.: Nonlinear analysis of FRP-glulamconcrete beams with partial composite action. Journal of Structural Engineering, ASCE, 127(8):967-971, 2001.