Glulam-concrete composite structures: experimental investigation into the connection system José Luiz Miotto Doctorate student at São Carlos School of Engineering University of São Paulo, São Carlos, Brazil Antonio Alves Dias Professor at São Carlos School of Engineering Summary Timber-concrete composites work appropriately when a suitable connection system is included, because the degree of interaction between the materials stiffs the structural system. Thereby it is extremely important to improve the knowledge about their connection system. The purpose of this article is to show the results obtained by experimental investigation into interlayer shear tests of glulam-concrete specimens. The specimens were designed to simulate the behaviour of composite T beams. The connection system was constituted by steel pins getting by the division of steel bars used for reinforcement concrete and by perforated steel plates, both glued with epoxy adhesive. They were tested under shear forces with constant loading rate. Six specimens of each group were made, considering two different diameters of the reinforcement bar (8mm and 10mm). The stiffness reached by the steel pins confirms their suitability for use in composite timber-concrete systems. 1. Introduction The timber shortage contributes to the development of alternatives that consider the rationalization of its use and the application of reforestation wood, especially the pinus and eucalyptus. In this context, timber-concrete composite structures have been used with success in bridge structures and in floors of residential, industrial and sporting buildings, finding spaces for application in structural repairs of historical buildings as in new constructions. To reduce the dimensional limitations of timber, among other characteristics, have been studied structural solutions that presuppose the use of the glulam. It is indispensable the adoption of a connection system for that the timber-concrete composite system works appropriately. The connectors assure the transfer of shear forces and also avoid the vertical separation between the parts. Steel connectors, in different shapes, are usually used as connection elements. This work focuses on the flexible connection systems, particularly those that are constituted by pins obtained by the division of steel bars used in reinforced concrete (rebar), besides perforated flat steel plates. The results obtained from the push-out shear tests are presented and the tests were made in specimens specially designed for this purpose. 2. Brief Literature Review 2.1 Timber-concrete composite structures Timber-concrete composites systems are becoming popular more and more because it is taken advantage of the best qualities of these materials. In fact, the timber is positioned in the tension region of the composite section while the concrete is used practically in the compression ones, being obtained the best performance of these materials in strength and stiffness. Thereby it is possible to obtain a cross section structurally efficient, being rigid and light at same time. The structural performance of timber-concrete composite systems is a great motivation. CECCOTTI [1] affirms that the load carrying capacity of a traditional timber floor can be doubled and its out-ofplane rigidity improved three or four times. DAVIDS [2] points out that, when compared with noncomposite timber-concrete girders, bending strength increases of at least 40% and service-load stiffness increases of 200% or more are commonly reported for timber-concrete beams with partially composite concrete decks.
2.1.1 Connection system Timber-concrete composites work appropriately when an appropriate system of connection is included between the two materials, because their interaction stiffs the structural system. There is a great variety of elements that can be used for this purpose, such as those showed by CECCOTTI [1]. The strength and ductility of the connectors are experimentally obtained. For so much it has been frequent the use of push-out shear test specimens, as shown in Fig. 1. Because there is not normalization for the test, it is noticed that the researchers accomplish adaptations in the dimensions and configurations of the model, sometimes placing the concrete in the inside part of the specimen and, other times, inverting that arrangement. Fig. 1 View of specimen used in push-out shear tests Starting from experimental results, OEHLERS and BRADFORD [3] comment that the geometry and layout of the specimen can have significant influence in the resistance of the connectors, highlighting the parameters: the height (h) of the connectors above the base, the number of levels of connectors, the number of connectors for level and the link form to the base. It can be made the connection system, in an easy and economical way, through pieces of steel bars glued in holes usually using epoxy adhesive, which can be fastened perpendicular to the wood fibers or with some inclination. When the connectors are fastened with some inclination and alternately, the system is called connection in X, that, according to PIGOZZO [4], it presents stiffness two or three times larger when compared to the perpendicular connectors, depending on the considered diameter. Fig. 2 Connector type perfobond. Source: VALENTE e CRUZ [5] With the intention of accomplishing the connection between steel beams and concrete slabs, in the end of the eighties a connection element was developed. This connector was constituted by a perforated steel plate, also known by perfobond (Fig. 2). The "concrete pins" that are formed through the holes, associated with steel bars arranged in the transverse direction, supply shear strength and maintain joined the materials. The shear strength of connectors formed by perforated steel plates is usually high, independent of the opening shape. Thereby, this connector can become an efficient alternative for timber-concrete composite structures. 2.2 Slip modulus In the evaluation of the behaviour of connectors, in composite systems, is used the slip modulus, K, that is defined as the angular coefficient of the load-slip curve and obtained by means of specimens that represent the connection, with real dimensions. This coefficient considers all the elastic and mechanical parameters of the materials involved in the connection, such as: dimensions and stiffness of the connector, stiffness and embedding strength of the timber used, concrete embedding and cracking, besides all the imperfections of the specimen.
CECCOTTI [1] proposed a model based on the indications of EUROCODE 4 [6], in that is defined the slip modulus of service, K ser, corresponding to the initial levels of loading, and the ultimate slip modulus, K u. The K ser value is obtained by the inclination of the secant straight line that cross the beginning of the load-slip curve and the point corresponding to 40% of the rupture load, with its respective slipping. In agreement with EUROCODE 5 [7], the ultimate slip modulus is calculated according to Eq. (1): 2 K u = K ser (1) 3 PIGOZZO [4] emphasizes that, in the slip modulus determination, the limits that are used for the evaluation vary a lot among the authors and normative codes. Thereby, this can result in significant variations in the calculated value. For pins fastened perpendicular to the grain, the slip modulus of service is calculated by Eq. (2), based on the indications of EUROCODE 5 [7], that it takes into account the equivalent density of the materials, in kg/m³, according to Eq. (3), and the diameter of the pins, in mm. However, according to that normative code, for connections between timber and concrete, K ser must be multiplied by 2. K ser 1,5 ρk φ = with ρ k = ρk1 ρ (2) and (3) k2 23 3. Materials and Methods The experimental program was constituted by the stages: (a) characterization of the involved materials; (b) determination of the slip modulus of all connection systems proposed. All the tests were carried out at the laboratories of the Department of Structures, at São Carlos School of Engineering of the University of São Paulo (Brazil). 3.1 Push-out shear tests: description of the specimens The slip modulus determination and rupture load, corresponding to each connection system, was made by means of push-out shear tests. Specimens were designed with the expectation of reproducing the solicitations that will be experienced for the connectors in the applications of glulam-concrete composite beams. In specimens of type SP-I and SP-II, represented in Fig. 3, the connection system is composed by steel pins, which were obtained from the division of corrugated steel bars used for reinforced concrete (rebar). It was made six specimens for each group of connectors. The pins were fastened in glulam pieces by means of epoxy adhesive (Compound gel adhesive, manufactured by Otto Baumgart), in holes previously accomplished and whose diameters are related in Tab.1. The anchorage length of the pins in glulam equivalent to 11 d and that also are related in Tab. 1 are based on the results of AHMADI and SAKA [8]. Specimen Type Diameter of pin (mm) Tab. 1 Pins characteristics Diameter of hole (mm) Anchorage length in glulam (mm) SP-I 8 10.0 88 SP-II 10 12.5 110
Fig. 3 Specimens type SP-I and SP-II The pins were fastened with an inclination of 45º in relation to the grain, being submitted, in this case, mainly to tension forces. The results obtained by PIGOZZO [4] justify this type of positioning. The two lateral pieces of reinforced concrete, that compose the specimens, also have the function of providing stability to the specimen. To prevent the cracking of concrete were properly positioned a layer of reinforcing bars, which were obtained by cuttings of steel mesh reinforcement (10 x 10cm), with characteristic yield strength of 600 MPa. In Fig. 4 is possible to see details of the specimens type SP-III. They were made with perforated steel plates, serving as connection element between the concrete and glulam. It was also produced six specimens for this group. The flat steel plates have 4.75 mm thickness and were glued to glulam with the same epoxy adhesive used for gluing the pins, after be carried out openings in glulam with thickness of 6 mm. The lateral pieces of reinforced concrete also received a layer of mesh reinforcement, previously mentioned.
3.2 Materials and their characteristics Fig. 4 Specimens type SP-III Timber used for making the glulam pieces that constitute the central part of the specimens is a hybrid of Eucalyptus grandis and Eucalyptus urophylla, which is sold in Brazil with the name of Lyptus. The tested lot showed the following properties: average moisture content of 9.1% and apparent density ρ ap, 12% = 0.79 g/cm³. From the glulam pieces were also extracted specimens for the verification of the material mechanical properties, being found the following average values corresponding to 12% of moisture content: compression strength parallel to grain f c,0 = 69.4 MPa; tension strength parallel to grain f t,0 = 82.9 MPa; shear strength in the glue interface f v,0 = 7.7 MPa; bending strength f M = 103.8 MPa; embedding strength f h,0 = 60.9 MPa and modulus of elasticity parallel to grain E c,0 = 27,541 MPa. All the tests were carried out in conformity with the recommendations of NBR 7190 [9] Annexe B. After the machining, the laminations resulted in approximately 30 mm thickness. It was necessary to make finger-joints into the laminations for the production of the glulam beams, which were cut, later, for production of the specimens. In production of the beams and finger-joints an aqueous emulsion polymer isocyanate adhesive was used Wonderbond EPI EL 70, made by Hexion Química in Brazil. From the prepared concrete for specimens production was extracted enough amount for moulding twelve cylindrical specimens (15 x 30 cm), from which were obtained the following average values: compression strength f c,28 = 33.9 MPa and modulus of elasticity (initial tangent) E= 31,358 MPa. The specimens were moulded according to NBR 5738 [10] and the tests were carried out in conformity with the codes NBR 5739 [11] and NBR 8522 [12]. The pins that were used as connection system were produced from steel bars whose characteristic yield strength, f yk, is 500 MPa with nominal diameters of 8 mm and 10 mm. The perforated steel
plates, also used as connectors, were cut out of flat steel plates with 4.75 mm thickness and the holes were made at Laboratory of Structures of São Carlos School of Engineering. It is pointed out that, in daily applications of the composite systems, these connectors must be galvanized. 4. Discussion and Results The specimens were submitted to constant rate of loading, in a single loading cycle, until they reach the collapse. Equipments were installed, as shown in Fig. 5(a), for the acquisition of the following data: (a) applied loads; (b) slip in the timber-concrete interfaces; and (c) rupture loads. (a) (b) (c) Fig. 5 (a) test arrangement and data acquisition; (b) SP-I after rupture; (c) detail from the concrete SP-I rupture lateral view The slips in the glulam-concrete interface were obtained through measurement instruments manufactured by Kyowa Electronic Instruments, which were installed in opposite faces of the specimens, as it is illustrated in Fig. 5(a). In the upper end of the specimens was installed a load cell with capacity for 250 kn. All the instruments were connected to a system for data acquisition System 5000 of Vishy Measurements Groups. During the tests, the specimens showed an excellent behaviour relatively to stability. It was perceptible the cracking of concrete particularly along the line that contains the connectors, as it is seen in Fig. 5(c) and in the proximities of the connectors. The cracking of concrete in the line that contains the connectors was the predominant way of rupture, accompanied by a little timber embedding in the contact area with the pins. The load-slip relationship of type SP-I specimens is illustrated in Fig. 6. In the fastening of connectors it was verified, in tests previously carried out, that the use of epoxy adhesive with high consistence caused considerable difficulty and, consequently, it was the reason for the detachment of one of the tested pins. On the other hand, the epoxy adhesive used in the production of these specimens, for having a flowing consistence, it reached fully its aim. It was not observed any detachment of the pins or perforated plates during the tests. The glulam also had an excellent performance during the tests and was not observed any delaminations or splitting in the glued surfaces.
Fig. 6 Load-slip relationship for specimens type SP-I The behaviour of the connection systems proposed in this research is registered in Tab. 2, with their respective rupture loads and slip modulus (initial stiffness). Specimen Type Tab. 2 Behaviour of connection systems Connection Ultimate Loads medium values (kn) K ser (N/mm) K ser Eurocode 5 (N/mm) SP-I Pins ø 8mm 131.0 142,936.4 35,206.37 SP-II Pins ø 10mm 135.2 112,998.3 44,007.97 SP-III Perforated plates # 4.75mm 153.4 339,406.2 - - - - - - - It was noticed, on the other hand, that the specimens type SP-III presented small slips in the glulamconcrete interface, what resulted in a high slip modulus. Though these specimens had an abrupt collapse (fragile rupture) and this behavior, in general, is not very interesting for the structures. Being removed the concrete, after the tests, it was observed that there was a small embedding of the timber in contact with the pins, in specimens type SP-I and SP-II, besides a bending portion. 5. Conclusions and Acknowledgements Undoubtedly, timber-concrete composites have a vast field of applications in structural engineering. Their structural performance, associated with other advantages, allow their use in the refurbishment of historical building and in new constructions. To enlarge the domain of applications of timber-concrete composite systems, an important alternative consist in the substitution of logs or solid timber for glulam pieces, which offer the requested dimensional freedom and motivate the use of timber coming from planted forests. This system works appropriately when appropriated connectors are included between the materials, which impede the separation between the parts and also stiff the structural system. There is a great variety of elements that can be used for this purpose. However, the flexible connections occupy a prominence place due to their versatility and low cost. The performance of the connection systems is evaluated experimentally in models with real size and their respective slip modulus are calculated starting from the tests.
In this work were built eighteen specimens, divided in three groups, whose connectors were obtained starting from corrugated steel bars and perforated steel plates. The results obtained for the steel pins demonstrate the efficiency of this connection system. It is noticed that the diameter of pins has influence on their respective slip modulus. On the other hand, the perforated steel plates are more efficient in terms of initial stiffness, with high slip modulus when compared to the values obtained for the pins. In spite of the high slip modulus reached by the perforated plates, their way of rupture was fragile. This verification suggests the use of steel pins for the production of timber-concrete composite beams with cross section in T. The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq for the financial resources provided for this work. 6. References [1] CECCOTTI A., Timber-concrete composite structures. In: BLASS H. J. et al., Timber Engineering STEP 2 (Structural Timber Education Programme). The Netherlands: Centrum Hout, p.e13/1-e13/12. [2] DAVIDS W.G., Nonlinear analysis of FRP-glulam-concrete beams with partial composite action. Journal of Structural Engineering, ASCE, v.127, n.8, p.967-971, aug. [3] OEHLERS D.J. and BRADFORD M.A., Composite steel and concrete structural members: fundamental behaviour. 1st ed. Oxford: Elsevier Science Ltd. 549 p. [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 (Studies and applications of steel bars glued as connectors in timber-concrete composite slabs of bridges). Thesis (Doctorate in Structural Engineering) São Carlos School of Engineering, University of São Paulo, São Carlos, 2004. [5] VALENTE I. and CRUZ P.J.S., Experimental analysis of Perfobond shear connection between steel and lightweight concrete. Journal of Constructional Steel Research, v.60, 2004, p.465-479. [6] EUROPEAN COMMITTEE FOR STANDARDIZATION. EUROCODE 4 (DDENV 1994-1-1): Part 1: Design of composite steel and concrete structures. Draft for development. Brussels, 1994. [7]. EUROCODE 5 (pren 1995-1-1): Design of timber structures. Part 1-1: General rules and rules for buildings. Brussels, 2002. [8] AHMADI B.H. and SAKA M.P., Behaviour of composite timber-concrete floors. Journal of Structural Engineering, ASCE, v.119, n.10, 1993, p.3111-3130, nov. [9] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 7190: Projeto de Estruturas de Madeira (Design of Timber Structures). Rio de Janeiro, 1997. [10]. NBR 5738: Concreto Procedimento para moldagem e cura de corpos-de-prova (Concrete Proceedings for molding and curing of specimens). Rio de Janeiro, 2003. [11]. NBR 5739: Concreto Ensaio de compressão de corpos-de-prova cilíndricos (Concrete Compression tests in cylindrical specimens). Rio de Janeiro, 1994. [12]. NBR 8522: Concreto Determinação dos módulos estáticos de elasticidade e de deformação e da curva tensão-deformação (Concrete Determination of static and strain elasticity modulus and of stress-strain curve). Rio de Janeiro, 2003.