EXPERIMENTAL INVESTIGATION ON EPOXY BONDED SHEAR CONNECTION FOR TIMBER-CONCRETE COMPOSITES

Transcription

1 EXPERIMENTAL INVESTIGATION ON EPOXY BONDED SHEAR CONNECTION FOR TIMBER-CONCRETE COMPOSITES Rijun Shrestha, James Mak, Keith Crews ABSTRACT: This paper presents the results of an experimental investigation on epoxy bonded shear connections for timber-concrete composites, with and without mechanical fasteners. The results of the tests showed that connections with high stiffness and strength, comparable to that of notch type connections, could be achieved by using epoxy for bonding timber and concrete. Addition of a mechanical fastener to the epoxy bonded connection had the advantage of delaying the failure of the connection and thereby avoiding brittle failure mode associated with epoxy bonded connections. The coach screw also facilitated in maintaining good contact between concrete and LVL. Observation of the failed specimens showed that failure in all connections was concentrated either in the LVL or concrete close to the interface and no interface failure was observed. KEYWORDS: epoxy bonded, shear connection, timber-concrete composite, push-out 1 INTRODUCTION 12 Timber-concrete composite (TCC) floor combines the with underlying timber joist to effectively utilise the favourable properties of both materials (Figure 1). The timber joist can be either sawn timber or engineered wood products such as glue laminated timer (glulam) or laminated veneer lumber (LVL). Shortage of steel between the first and second world wars led to the development of TCC structures in Europe during 190s which were essentially limited to upgrade of old timber flooring [1, 2]. Such construction resulted in improved structural and acoustic performance; however, the composite behaviour between concrete and timber was not fully understood. Newmark, Siess and Viest (1952) were amongst the first researchers to undertake detailed investigations to study and quantify the composite interaction of steel beam acting compositely with a concrete deck. A key finding of their research was recognition of the fact that the performance 1 Rijun Shrestha, School of Civil and Environmental Engineering, University of Tecnology Sydeny (UTS), Sydney, Australia. rijun.shrestha-1@uts.edu.au 1 James Mak, School of Civil and Environmental Engineering, University of Tecnology Sydeny (UTS), Sydney, Australia. james.mak@student.uts.edu.au 1 Keith Crews, School of Civil and Environmental Engineering, University of Tecnology Sydeny (UTS), Sydney, Australia. keith.crews@uts.edu.au of the shear connectors was critical to achieve predictable composite behaviour between dissimilar structural materials [2]. It was not until the 1980 s that comprehensive research investigations into the performance of timber concrete composite floor systems commenced with a view to applying the technology to new structures. Subsequent research efforts led to new types of connectors being developed that made it possible during the 1990 s to apply TCC systems for both floors in new buildings (especially in Europe) and road decks on short span bridges (mainly in North America and New Zealand) []. Concrete slab Timber joist Figure 1 Typical TCC cross-section In recent years there has been an increasing trend for the use of TCC systems in new buildings and construction. Two important benefits of using TCC system are fire resistance and low noise transfer (particularly impact noises) which have enabled timber concrete composite floor systems to meet the rigorous requirements of modern construction for multi- storey buildings and therefore have led to increased use of TCC systems in

2 new buildings. Frangi and Fontana [4] note that combining timber and concrete improves the fire performance of the floors as burning wood protects the concrete and connections from high temperatures while the concrete provides protection against the spreading of the fire. Other benefits of TCC system include lower self-weight compared to equivalent structural performance of traditional reinforced concrete floor system, efficient use of the structural elements in the composite floor with timber elements under tension and concrete members under compression, spalling deteriorations observed in steel-reinforced concrete being less of an issue and favourable range of vibration responses for human comfort [5]. Use of timber for building is attractive from an environmental point of view as timber stores carbon absorbed from the atmosphere during photosynthesis. Moreover, energy required for timber production is significantly lower in comparison to concrete and steel and thus making timber a sustainable construction material. As have been highlighted by previous studies, the shear connection between timber and concrete is a key aspect of TCC systems. Lack of good bond between timber and concrete means little or no composite action between the two materials can be achieved, resulting in larger deflections, which in turn limit the application of such floor systems to short spans with smaller joist spacing. Providing a suitable shear connection between timber and concrete, on the other hand, makes composite action between the two materials possible and therefore has the advantage of utilising the strengths of the two materials in an effective way such that the and timber joist are under compressive and tensile stresses, respectively, under normal loading conditions. Given their significance in achieving composite action between concrete and timber in TCC floors, shear connectors have been the subject of research for a considerable time and several alternatives have been studied such as screws, nails, studs, concrete notches and glued connection [5][6]. Early connection types for TCC were adapted from the already developed timber to timber connections e.g. nails. Ahmadi and Saka [5] studied further into this by varying the mechanical connectiors from nails to screws and bolts and subsequently performed bending tests. Similarly, Gutkowski and Tser-Ming [7] conducted bending and shear testing on nail connected TCC members and focused on different nail sizes and penetration depth. Mechanical shear connectors only, however, provided low slip modulus or stiffness which resulted in partially composite action [8]. connections with combination of mechanical fastener with epoxy resin were also studied. [10]. This combination method utilized dowels that were inserted into pre-drilled holes that were partially filled with epoxy resin, where pre-drilled holes were slightly larger in diameter to ensure dowel insertion would not be hindered. By combining epoxy with a mechanical anchor results showed increased strength and stiffness around the dowel connection, but the only drawback was that the holes had to be pre-drilled. In order to achieve a low to non-slip modulus from a connection, it is necessary to develop a connection where full composite action could be achieved between the timber and concrete members. Van der Linden [2] mentions through his literature that previous work was studied in bonding concrete directly to timber by an adhesive glue - epoxy resin, and described the end results as close to composite action achieved and that failure only occurred from the concrete in shear or crushing and or the timber failing in tension and that the bond was intact. Typical load-slip behaviour of different types of shear connectors (Figure 2) also shows that close to composite action can be achieved by using adhesive bonded shear connector. Figure 2 Typical load-slip behaviour for different types of joints [6] This paper presents the results of an experimental investigation on epoxy bonded shear connection for TCC, with and without mechanical fasteners. The aim of the experimental study was to investigate the behaviour of epoxy bonded shear connections in term of strength, stiffness and failure mode. Other variations were tested with horizontal mechanical shear joints [9], where steel rods were passed through the timber members and a part of the timber member is slotted into the concrete. These connections had almost no slip for loads up to half of the maximum load. Shear

3 2 EXPERIMENTAL INVESTIGATION 2.1 TEST SPECIMENS A set of ten connection specimens (Table 1) with LVL joist and were fabricated. Five of specimens (E series) had the joist and slab bonded together using epoxy only (Figure ) whilst for the remaining five specimens (EC series), a 12 mm diameter and 200 mm long coach screw was subsequently installed through a pre-drilled hole in the and into the joist following the application of the epoxy (Figure 4). Table 1 Summary of tested specimens cylinders as per Australian standard [11] was 6.6 MPa. The elastic modulus for concrete based on tests on three 150 mm diameter cylinders as per Australian standard [12] was 24.6 GPa. The density, tensile strength and elastic modulus for the LVL was 640 kg/m, 0 MPa and 1.2 GPa, respectively Series Specimen Connection type E E-1 Epoxy only E-2 E- E-4 E-5 EC EC-1 Epoxy and 12 mm EC-2 EC- EC-4 diameter coach screw EC-5 Sikadur-0, which is an epoxy based two part thixotropic resin, was used for bonding the with an. While the epoxy was expected to provide composite action between the and the, the purpose of the coach screw was to delay the failure in the connection once the bond between the two materials is broken. The coach screws were also found to be effective to maintain good contact between the concrete and LVL. The, which was pre-fabricated, was 00 mm wide and 450 long and had a depth of 75 mm while the s were 450 mm long, 200 mm deep and had a thickness of 48 mm. Prior to bonding the slab and joist, the weak cement lattice on the concrete surface to be bonded to the LVL was removed to expose the aggregates using a needle gun and all dust particles were removed using compressed air. The and were bonded over a length of 50 mm (Figures and 4). For connections with coach screw, the coach screw was installed at the centre of the bonded area between the and (Figure 4). Holes of 12. mm and 10 mm diameter were drilled into the and, respectively, prior to application of the epoxy. The coach screw was installed 0 to 60 minutes after the application of the epoxy. The coach screws were therefore effective in maintaining good contact between the and. Any void in the around the coach screw was filled with epoxy. 2.2 MATERIAL PROPERTIES The mean 28 day concrete cylinder compressive strength based on compressive tests on three mm diameter Figure Details of epoxy bonded TCC connection mm dia 200 mm long coach screw Figure 4 Details of epoxy bonded connection with coach screw 2. TEST PROCEDURE Tests on the connections were carried out using an asymmetrical push-out test (referred to as push-out test hereafter). Details of the test setup and the test rig are shown in Figure 6. The specimen was placed on the test rig and load was applied through the cross-head of the universal testing machine which had a maximum load capacity of 500 kn. Three LVDTs were attached to the test specimen to record relative displacements (or slips)

4 between the and two on either side of the and one at the centre, as shown in Figure 6. An additional LVDT was used to measure the cross head movement of the testing machine. The loading procedure in European Standard [1] and shown in Figure 5 was closely adopted for all tests. The basic loading procedure is as follows. 1. Specimen is loaded to 0.4 Fest at a gradual rate within roughly a timeframe of 120seconds 2. After reaching the load of 0.4 Fest, the load was held for 0 seconds. After 0 seconds, the load was released to 0.1 Fest within a timeframe of roughly 90 seconds. 4. After reaching the load of 0.1 Fest, the load was again held at the same level for 0 seconds 5. The specimen was then loaded at a constant rate to failure. mode. The strength of the connection specimens is defined as the maximum load that can be applied in the push-out tests before failure. The connection stiffness or slip modulus, which represents the resistance to the relative displacement between the timber joist and the, is one of the key parameters that define the efficiency of a shear connection. Stiffness for the serviceability limit state (SLS) calculations and ultimate limit state (ULS) calculations are essential to characterise a shear connection. The stiffness for SLS (K s ) was calculated based on equation 1 [1]. The stiffness for ULS (K u ) is calculated from equation 2 [1]. 0.4Fest K s (1) Where F est is the estimated peak load and v 40 and v 10 are slip corresponding to 40% and 10% F est. 2 (2) K u K s Figure 5 Loading regime as per [1] EXPERIMENTAL RESULTS.1 FAILURE MODE Failure modes for all connections with epoxy only (E series) were similar, characterised by sudden and brittle failure. The failure in all connections was located either in concrete or in LVL and chunks of concrete attached to and LVL attached to could be seen (Figures 7 and 8). For connections E5, closer inspection of the failed specimen indicated that the LVL joist was not fully bonded to the (Figure 9) and it therefore failed at a much lower load Figure 7 Failed connection E-1 showing layer of concrete attached to (a) (b) Figure 6 Push-out test setup (1 test rig, 2 brackets for LVDT LVDT 4 cross-head 5 safety screen 6 7 steel packing 8 ) 2.4 TEST CRITERIA The behaviour and effectiveness of the tested shear connections were assessed based on their strength (failure load or maximum load), stiffness and failure

5 Figure 11 Split in away from the bond surface for connection EC-4 Figure 8 Failed connection E-2 showing layer of concrete attached to and layer of LVL attached to concrete. Figure 9 Failed connection E-5 showing inadequate bond. The failure mode for epoxy bonded connections with coach screw (EC series) was governed by the strength of the. The had failed in shear and splits in the could be seen in the failed connections (Figures 10 and 11). The coach screw prevented the slab from separating from the joist and the connections still had some load carrying capacity following the peak load. Similar to the E series connections, no failure was observed in the bond interface..2 LOAD-SLIP BEHAVIOUR Load-slip results for E and EC series connections are shown in Figures 12 and 1, respectively. The load-slip behaviour of EC series connections was more consistent when compared to the E series. Linear load-slip relationship for most part of the curves and immediate loss of strength following the peak load could be seen for E series connections. Unlike the E series connections, the load carrying capacity of the EC series connections were generally not completely lost following the peak load and the specimens could be further loaded. Connection EC- was an exception to this kind of behaviour. Load (F) kn E Series Slip, mm Figure 12 Load-slip behaviour for E series connections E-1 E-2 E- E-4 E EC Series 140 Load (F) kn EC-1 EC-2 EC- EC-4 EC Figure 10 Split in away from the bond surface for connection EC Slip, mm Figure 1 Load-slip behaviour for EC series connections

6 . STRENGTH AND STIFFNESS RESULTS Strength and stiffness results for all connections are summarised in Table 2. The mean strength for E and EC series were 79.7 kn and kn, respectively. The strength results for EC series were more consistent as indicated by a low coefficient of variation (CoV). This attributed to the effectiveness of the coach screws in maintaining good contact between concrete and LVL as the epoxy dried. The mean stiffness for serviceability and ultimate limit state calculations (K s and K u ) for the E series connections was 10.2 kn/mm and kn/mm, respectively, while the same for the EC series connections was 46. kn/mm and 20.9 kn/mm, respectively. Variation in stiffness results were again relatively low for the EC series compared to the E series connections. The stiffness of the E and EC series were not significantly different as the stiffness for serviceability calculation is mostly governed by the epoxy bond between the two materials. Table 2 Summary of strength and stiffness results for tested connections between concrete and LVL. Observation of the failed specimens revealed that the epoxy provided good bond between LVL and concrete as all failures were concentrated either in the LVL or concrete close to the interface and no interface failure was observed. The linear elastic behaviour of the connections was observed for significant portion of the load-slip curve. Epoxy bonded shear connections are therefore considered to be effective in achieving significant improvements in composite action between concrete and timber in TCC and in efficiently utilising the strengths of both materials. Such connections have potential to be both advantageous and effective, especially for pre-fabricated systems, where a pre-cast can be bonded to timber joist. Maintaining good contact between the two materials was found to be an issue when bonded with epoxy only but such issues were resolved by using a combination of coach screw and epoxy. Peak Load Stiffness P L Ks Ku kn kn/mm kn/mm E E E E E Mean Stdev Cov 1% 55% 47% EC EC EC EC EC Mean Stdev CoV 12% 21% 21% 4 CONCLUSIONS Epoxy bonded shear connections for timber-concrete composites were investigated in this study. The connections were studies in terms of strength, stiffness and failure behaviour. The strength and stiffness results show that connections with high strength and stiffness, comparable to that of notch type connections, could be achieved by using epoxy for bonding timber and concrete. The addition of a mechanical fastener in the form of coach screw to the epoxy bonded connections had the advantage of delaying the failure of the connection as well as avoiding brittle failure mode associated with epoxy bonded connections. The coach screw also facilitated in maintaining good contact

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