An innovative prefabricated timber-concrete composite system

Transcription

1 An innovative prefabricated timber-concrete composite system Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo 1 Abstract A novel type of timber-concrete composite floor, consisting of longitudinal glulam beams with a fibre reinforced concrete (FRC) slab on the top is proposed. In order to check some relevant mechanical properties of such a floor, full-scale laboratory tests along with numerical analyses were carried out. The shear connector system used in the investigation consisted of self-tapping screws driven at an angle of 45 to the grain direction of the glulam beams. The manufacture of the structure occurred according to the following steps: (a) the screws were inserted on the top of the glulam beams; (b) the beams were rotated 180 about the longitudinal axis and placed in a concrete formwork; (c) the FRC was cast into the formwork; (d) after curing of the FRC, the composite floor was again rotated 180 about the longitudinal axis into its right position, i.e. with the FRC slab on the top side. Long term tests and quasi-static bending tests were performed. It was found that the proposed connection system showed a very high degree of composite action both during the long-term testing and at load levels close to the failure load. Furthermore, the assembly of the prefabricated timber-concrete composite system revealed to be very fast and easy. Roberto Crocetti Department of Structural Engineering, Lund University, Sweden, roberto.crocetti@kstr.lth.se Tiziano Sartori Department of civil, environmental and mechanical engineering, University of Trento,, Italy tiziano.sartori@unitn.it Roberto Tomasi Department of civil, environmental and mechanical engineering, University of Trento,, Italy roberto.tomasi@unitn.it Jose L. F. Cabo ETS of Architecture, Polytechnic University of Madrid (UPM), Spain jose.fcabo@upm.es 1

2 2 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo 2 Introduction Timber-concrete composite structure consists of timber beams effectively interconnected to a concrete slab cast on top of the timber members. Most of the studies performed to date have focused on composite systems where wet ordinary concrete was cast on top of timber beams with mounted shear connectors. Even though such systems have proven to perform very well from the point of view of statics and dynamics, in-situ concrete casting has some clear disadvantages, for example, waste of time due to concrete curing, low stiffness and high creep, concrete shrinkage effects on the composite beam, high cost of cast-in-situ concrete slabs, etc. Recently, composite systems where the concrete slab is prefabricated off-site with shear connectors already embedded and then connected to the timber beams on site have been investigated ([4] and [7]). The research presented herein focus on the use of composite structure with very high prefabrication level, good performance and short construction time. Such composite structures are floor modules consisting of two glulam beams with a a concrete slab on the top. 3 Materials and methods In order to investigate the behaviour of the proposed composite system, three fullscale floors were built at the laboratory of Structural Engineering, Lund University. The main dimensions of the floor system are reported in Table 1. Table 1 Geometry of the A, B and C composite system (dimensions in [mm]). See also Fig. 1. Span (l) Slab width Slab thickness(h 1 ) Beam width (b 2 ) Beam depth (h 2 ) Beam spacing (i) Fig. 1 Geometry of the floor system The geometry of the three tested floors was nominally identical. The timber used for the manufacture of the floors was glulam GL30c. The moisture content of the

3 An innovative prefabricated timber-concrete composite system 3 beams was approximately 12%. For the production of 1 m3 of fiber reinforced concrete, 45 kg of steel fibres and 480 kg of cement were used, which gave a mean value of compression strength fc = 51 MPa for the concrete. In the following text, the three tested specimens will be referred to: A, B and C. Specimens A and B were tested on short term bending, whilst specimen C was tested on long term bending. During the short-term bending tests, the load was applied by an actuator in a displacement controlled manner. The load was distributed on four lines perpendicular to the longitudinal direction of the floor in order to induce stresses and deformations in the floor similar to those induced by a uniformly distributed load q, see Fig. 2. The total load applied to the specimen, the mid-span deflection, and the relative slip between slab and beam at the supports were continuously measured during testkn ing. For the long-term bending test, a uniformly distributed load of 1 m 2 was applied on the slab by means of sacks of cement. The mid-span deflection was measured over time in order to investigate the creep effects of both timber and concrete. 3.1 Shear connectors and manufacture of the floor systems In order to achieve composite action between the timber beams and the concrete slab, self-tapping screws with dimensions d = 11 mm and l = 250 mm were driven into the timber beams before the concrete was cast. The screws were driven at an angle of 45 to the longitudinal directions with a spacing of 200 mm close to the Fig. 2 Test setup

4 4 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo supports and 300 mm in the middle part of the floor respectively (Fig. 3). The main function of the inclined screws is to transfer the shear force from the concrete slab to the timber beam both by shear in the direction parallel to the slip interface and mainly by tension in the direction of the screw axis. The ultimate tensile strength of the screws was approximately f u = 1250MPa. Fig. 3 Screw positions The screws were inserted on the top of the glulam beams, then the beams were rotated 180 about the longitudinal axis and placed in a concrete formwork. The FRC was cast into the formwork, see Fig. 4. After curing of the FRC, the composite floor was again rotated 180 about the longitudinal axis into its right position, i.e. with the FRC slab on the top side. 4 Test 4.1 Preliminary tests Preliminary tests were performed in order to obtain modulus of elasticity of timber beam, strength of concrete, withdrawal resistance of screw to concrete connection and strength of steel screws. Standard compression tests were carried out on three concrete cubes. The geometry and the compression strength of the tested specimens are resumed in Table 2. Screws with different penetration length were inserted in the concrete cubes. The value of withdrawal tests on these specimens are reported in Table 3. Non destructive bending tests were performed on two timber beams in order to estimate the modulus of elasticity. The results are reported in Table 4.

5 An innovative prefabricated timber-concrete composite system 5 (a) (b) Fig. 4 (a) Timber beam with inserted screws- (b) upside down floor system (including the formwork) directly after concrete casting Table 2 Geometry and compression strength of the tested concrete cubes ID Side Side Depth Compression strength a b h σ m # [mm] [mm] [mm] [N/mm 2 ] A , 11 B , 89 C , 11 Table 3 Ultimate withdrawal capacity of the screws inserted into the concrete with different penetration lengths. ID specimen penetration lengths [mm] Ultimate tensile capacity [kn] A 50 27, 49 B 50 19, 75 C 50 23, 90 Mean value 23,71 A 75 40, 03 B 75 40, 04 C 75 38, 32 Mean value 39,46 A , 68 B , 92 C , 58 Mean value 41,39

6 6 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo Table 4 Modulus of elasticity and density of timber beams ID Lenght [mm] Depth [mm] Width [mm] ρ m [kg/m 3 ] E m [MPa] A B Short-term bending tests Load-deflection curves and load-slip curves are shown in Fig. 5 and Fig. 6 respectively. Equivalent uniformly distributed load q ( kn ) m Floor A Floor B EI max EI min Deflection at mid-span f (mm) Fig. 5 Equivalent uniformly distributed load vs mid span deflection The curves in Fig. 5 show the relationship between the equivalent uniformly distributed load q (i.e. the total load applied divided by the slab area) and the deflection f at mid-span. As it ca be observed, the behaviour is linear up to a load level of approximately 80 kn m 2, which is well above the design load used in design of common floor structures. The stiffness of both composite floors shows, after a slightly nonlinear initial part, a constant trend up to the failure of one of the two timber beams. The curve of Fig. 6 shows the slip at the support related to the equivalent distributed applied load. Failure of floor A occurred at a load q 84 kn m 2, with the propagation in one of the two beams of two large cracks in the direction parallel to the grain. The failure of the floor type B, on the other hand can be attributed to local failure of a finger joint of the lowest lamination located close to mid span of one of the two beams. The collapse of floor A occurred at q=80 kn m 2 firstly due to bending failure at a finger joint in one beam and secondly due to a shear failure located along a line running through the tips of the screws used as shear connectors.

7 An innovative prefabricated timber-concrete composite system Floor A Floor B Slip at the support (mm) Equivalent uniformly distributed load q ( kn m 2 ) Fig. 6 Load slip deflection vs equivalent uniformly distributed load 4.3 Long term bending test For the long-term bending test, a uniformly distributed load of 1 kn was applied m on the slab by means of sacks of cement(see Fig. 7). The purpose of 2 the long-term test was to investigate the time-dependent behaviour of the prefabricated timberconcrete composite system. The long-term test results for the specimen B is presented in Fig. 8 in terms of time vs mid-span deflection. The variables monitored during the entire test were the mid-span deflection through 2 inductive transducers, positioned at the mid-span of each glulam beam. Fig. 7 Long term test setup

8 8 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo Fig. 8 Increase in mid-span deflection of the floor C with time Also the temperature and the humidity in the laboratory was continuously monitored. The value of these parameters are presented in Fig. 9. Fig. 9 Relative humidity and temperature observed in the laboratory during the period /

9 An innovative prefabricated timber-concrete composite system 9 As it can be seen in Fig. 8 the mid-span deflection increase to about three times the instantaneous deflection after three months. In March the floor has been downloaded for two days. When the floor was reloaded it achieved again the same level of deflection as before unloading. This operation was performed again in the first days of May and similar results were obtained. 5 Efficiency of the composite beams Deformations at the shear connectors generate horizontal movement, i.e. slip at the interface between concrete and timber. Such a behaviour is due to as partial composite action and, as the slip increases it reduces the efficiency of the cross section. The efficiency of a shear connection for a composite beam can be estimated using the following equation, see [9] and [10]. η = EI real EI min EI max EI min (1) where η is the efficiency, EI max is the bending stiffness of the floor with full composite action, EI min is the bending stiffness of the floor with no composite action and EI real is the actual bending stiffness of the floor. At load levels comparable to those at the serviceability limit state (i.e. 1 kn m 2 ) the efficiency η is approximately 1.0. The efficiency at a load of 20 kn m 2 or more remains constant,i.e. η Conclusions This paper presents the main results of a research project conducted on a novel prefabricated timber-concrete composite system. The system provides several advantages compared to cast in-situ concrete slabs, e.g. reduced time of construction and considerable reduction of the effects of concrete shrinkage. Experimental tests were carried out on three 7.2 m long strip floor specimens to investigate on stiffness and the strength and stiffness of the prefabricated systems, of which two tests to failure and one long-time test. The principal observations from the experimental investigations are: The tested system showed considerably higher stiffness and strength properties than a similar system with concrete deck not able to transfer shear stress The load carrying capacity was very high. The equivalent uniformly distribute load at failure was approximately 80 kn/m 2, which is considerably larger than common designs load for floor structures. The stiffness of the system was also very high. This depends primarily on the ability of the shear connectors to transmit shear without (or with minor) slipping. In the tested specimens the efficiency of the system was approximately 1 for

10 10 Roberto Crocetti, Tiziano Sartori, Roberto Tomasi, Jose L. F. Cabo loads well above common design loads. For extremely high loads (q 20kN/m 2 ) the efficiency of the system was approximately 0.85, which is also very high compared to similar composite system with more traditional shear connectors, i.e. screws or bars inserted perpendicularly to the plane of the slab. The instantaneous deflection of the floor increased roughly by a factor 3 after a period of approximately 7-8 months. This relatively large increase in deflection is believed to be due mainly to the creep of the concrete slab and of the timber beam and - in some minor extent - to the long-term deformation of the shear connectors. Last but not least, the easiness of manufacture of the proposed system should not be underestimated, since it allows for a quick construction whit a reduced possibility of human errors. 7 Acknowledgements The authors wish to gratefully acknowledge the Mr. Franco Moar who has performed his Master s thesis on this topic. The timber material was supplied by the glulam mill Moelven Töreboda AB, Treboda, Sweden. The screws were supplied by Rotho Blaas srl, Cortaccia, Italy. The fibers for the FR concrete were supplied by Bekaert Svenska A.B. All the suppliers are kindly acknowledged. References 1. Bathon, L. A., Bletz, O. and Bahmer R. Concrete bearings a new design approach in woodconcrete-composite applications. Proceedings of World conference timber engineering, Portland - Oregon-USA (2006). 2. Blass, H. J., Bejtka I. Screws with continuous threads in timber connections. International RILEM Symposium on Joints in Timber Structures: Moar, F. Prefabricated timber-concrete composite system Master thesis, Lunds Tekniska Hgskola, Lunds Universitet (2012). 4. Crocetti R., Sartori T., Flansbjer M.. Timber-Concrete Composite Structures with Prefabricated FRC Slab, Proceedings of World conference timber engineering, Riva del Garda, Italy (2010). 5. Gutkowski, R., Brown, K., Shigidi, A., Natterer, J. Laboratory tests of composite wood concrete beams, Construction and Building Materials, 22 (6), (2008). 6. Lukaszewska E. Development of Prefabricated Timber-Concrete Composite Floors. PhD thesis, Department of Civil, Mining and Environmental Engineering Division of Structural Engineering, Lule (2009). 7. Lukaszewska, E., Johnsson, H. and Sthen L. Connections for Prefabricated Timber Concrete Composite Systems. Proceedings of World conference timber engineering, Portland - Oregon- USA. (2006). 8. A. Sjöström, J. N.-Montero, D. Bard, R. Crocetti Vibratory investigation of a fiber reinforced concrete floor supported by wooden beams: part I. Proceedings of Joint Baltic-Nordic Acoustics Meeting, Odense - Denmark. (2012).

11 An innovative prefabricated timber-concrete composite system M. Piazza, G. Turrini Sulle strutture composte legno-legno. Proceedings Italian Workshop on Composite Structures, Department of Mechanical and Structural Engineering, University of Trento, Villa Madruzzo, Trento, Italy, 1993: pp M. Piazza Restoration of timber floors via a composite timber-timber solution. RILEM Workshop Timber: a Structural Material from the Past to the Future, Trento, 1994: pp