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Environmentally friendly high performance timber–concrete panel
Construction and Building Materials 102 (2016) 1060–1069
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Environmentally friendly high performance timber–concrete panel Carlos Martins, Alfredo M.P.G. Dias ⇑, R. Costa, Pedro Santos Departamento de Engenharia Civil, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Rua Luís Reis Santos – Pólo II da Universidade de Coimbra, 3030-788 Coimbra, Portugal
h i g h l i g h t s Development of timber–concrete composite panels with natural resources. Lightweight concrete incorporating cork granules. Mechanical characterization of timber–concrete connections. Evaluation of acoustic and dynamic behaviour of full-scale timber concrete panels. Transversal load distribution and mechanical behaviour of a full-scale TCC panels.
a r t i c l e
i n f o
Article history: Received 14 November 2014 Received in revised form 27 July 2015 Accepted 31 July 2015 Available online 13 August 2015 Keywords: Timber–concrete composite panels Lightweight cork concrete Maritime pine logs Connections Transversal load distribution Acoustic and dynamic behaviour
a b s t r a c t This paper reports the development and the analysis of a new high performance timber–concrete composite (TCC) panel suitable for structural applications in floors. The new TCC panel is environmentally friendly, and combines low value natural resources with very low level of processing (small diameter Maritime pine logs) with industry sub products (cork granules) leading to a relatively low construction cost. The research program involved three stages: (i) development and characterization of the raw materials (cork lightweight concrete and small diameter Maritime pine logs), (ii) development and characterization of the structural connections and (iii) characterization and validation of the structural panel itself. The research program included an extensive experimental campaign and the results showed that (i) the production of high performance TCC panels is feasible and (ii) the developed TCC panel meets the structural and non-structural requirements for floor applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the last decade several timber–concrete composite (TCC) structural elements suitable for beams and floors for buildings and for footbridges have been developed [1–4]. The TCC panels usually have three main components: timber elements, concrete layer and connections. However, different materials can be used such as for example: solid wood, glued laminated wood or LVL, normal concrete or lightweight concrete [5–11]. Also different techniques for mounting can be considered, e.g. in situ concrete cast or the use of precast concrete members [12]. It has been shown [13–23] that TCC elements shown good performance for short and long-term behaviour, namely in terms of load carrying capacity and stiffness. These elements have also been evaluated in terms of their ability to meet other types of ⇑ Corresponding author. E-mail addresses: [email protected] (C. Martins), [email protected] (A.M.P.G. Dias), [email protected] (R. Costa), [email protected] (P. Santos). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.194 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
requirement such as vibration, acoustic, fire resistance or thermal performance [24–30] showing good results. Furthermore the transversal load distribution on TCC floors/bridges under concentrated loads has also been addressed [31]. Accordingly, it is clear that TCC developed so far are able to fulfil the most common mechanical requirements. However, the available solutions either are usually complex, costly or present some drawbacks such as being environmentally unfriendly [32]. Nowadays these are critical issues that in many situations govern the decision on the final solutions. The main objective of the research program herein described is to develop a new high performance, simple and environmentally friendly TCC structural panel suitable for floors. One of the key features of the developed TCC panel is the incorporation of Portuguese natural resources, namely the cork granules and small diameter round wood Maritime pine logs. The latter are simply unprocessed Maritime pine small diameter round wood members obtained from forest thinning. Maritime pine is the Portuguese main softwood specie and small diameter logs are abundant from forest
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management operations, necessary to improve the forest health and mitigate the risk of fire – these round wood members have already been tried for other applications such as for example, truss members or overhead line utility poles [33–35]. The cork is obtained from the external layer of the bark of oak trees (Quercus Suber L.). The harvesting of this layer of the bark does not introduce damage to the tree and allows a new layer to grow, being a renewable resource. The cork granules are a sub product from the national cork industry that discards up to 30% of raw cork collected. The use of cork granules in concrete leads to produce lightweight concrete. Lightweight concretes with cork aggregates in the composition have already been tested for other applications as non-structural material and revealed good acoustic [36] and thermal properties, which may help to reduce of energy consumption in a building [37]. In this work the concrete composition was optimized for the new TCC panels in order to address specific issues arising from the intended application. Later a deep mechanical characterization of the Maritime pine small diameter round wood logs was undertaken in order to assess their mechanical properties. After that, the connection between the concrete layer and the logs was evaluated and finally the behaviour of nine TCC isolated beams and a full scale TCC panel was analyzed.
2. Development and characterization of the materials Portugal is the world’s largest cork producer and the cork industry usually discards 20–30% of received cork, mainly in the form of small size particles, e.g. cork granules. Concrete with cork granules (CC) has several advantages, namely it, (i) is lighter than a normal concrete, (ii) is environmentally friendly because the cork granules are a sub product resulting from the cork industry without commercial value, and with no processing needed for incorporation in concrete. However, like the other types of lightweight concrete, CC has some disadvantages: (i) lower compressive strength and (ii) lower modulus of elasticity than normal concrete. However, according to Branco et al. [38], CC usually performs better than other types of lightweight concrete (aerated, no-fines, cellular and foamed) and exhibits better acoustic insulation, thermal isolation and shrinkage properties than other lightweight concretes produced from organic materials. The CC composition tested included sand, coarse aggregates, Portland cement 42.5 type I, water, and two types of expanded cork granules: (i) cork 0/3 (dimensions varying from 0 up to 3 mm), and (ii) cork 3/10 (dimensions varying from 3 up to 10 mm) with a density of 136 kg/m3 and 159 kg/m3, respectively. A super plasticizer ViscoCreteÒ 20 HE [39] was employed. In terms of mechanical performance the target for the developed CC was a density about 1800 kg/m3 with a minimum compressive strength about 20 MPa. The development and optimization of the CC composition was achieved by trial and error, adjusting the quantities of cork granulate, sand and coarse aggregates – the cork granules were introduced in the concrete composition simply by replacing part of the coarse and sand aggregates. During this stage, seven compositions were tested: (i) BR, the reference composition, with no cork granulates, (ii) BA3, a composition with 30% of sand replaced by cork 0/3, (iii) BB3, a composition with 30% of coarse replaced by cork 3/10, (iv) BA5, a composition with 50% of sand replaced by cork 0/3, (v) BA3B2, a composition with 30% of sand replaced by cork 0/3 and 20% of coarse replaced by cork 3/10, (vi) BA4B1, a composition with 40% of sand replaced by cork 0/3 and 10% of coarse replaced by cork 3/10 and (vii) BA4B2 a composition with 40% of sand replaced by cork 0/3 and 20% of coarse replaced by cork 3/10.
Nine concrete cubes with 150 mm side length and three quadrangular prisms with 600 mm length and cross sections with 150 mm side length were cast for each composition. Later these elements were tested to determine the compressive strength (cubes) and modulus of elasticity (prisms) of concrete. The compressive strength was experimentally assessed at the age of 28 days according to EN 12390-3 [40]. The modulus of elasticity was tested at the age of 28 days according to E397 [41]. In Table 1 are shown the average values of density, compressive strength and modulus of elasticity obtained for each composition. Table 1 shows that the composition BA4B2 is the one that best fulfils the intended mechanical properties. The composition for this CC is shown in Table 2. Fig. 1 shows a failure plan of a specimen of CC tested in compression. To date, there are no available technical documents to rule the use of small diameter Maritime pine logs for structural applications. To fulfill this gap, various studies have been carried out to characterize this wood based product [33,35]. These studies did not propose grading rules or visual grades but supplied the necessary information to allow that. The outcome of these research campaigns showed a very high potential of small diameter Maritime pine logs for structural application. For these reasons the use of Maritime pine round wood members for structural applications, namely beam members in composite systems, was not a main issue. A comprehensive characterization of the specimens used in the experimental work was, nevertheless, essential as an input for the analysis of the final results due the large variability that was observed in the mechanical properties of this wood product. The experimental campaign herein reported comprised the characterization of 16 small diameter Maritime pine logs from the coastal central area of Portugal divided in two samples: sample 1 included 9 logs and sample 2 included 7 logs. To begin with the logs were measured (length and cross sectional dimensions) and weighed. The moisture content was measured at three points (near both ends and at the middle cross section) using an electronic moisture meter. Later the logs were tested to determine their local modulus of elasticity according to EN 14251 [42] and their global modulus of elasticity according to EN 408 [43] – some adjustments were required for round wood elements. Table 3 shows a summary of the results of the experimental campaign where L is the length of the log, Dmean is the mean diameter of log, Dnom is the nominal diameter of log, W is the moisture content, q is the density of timber, Em.g is the global modulus of elasticity parallel to grain and Em.0 is the local modulus of elasticity parallel to grain.
3. Development and characterization of the connection components In timber–concrete composite elements the connection between the concrete and the timber is a key component. The connection is required to be stiff in order to minimize the shear deformation between the two materials and lead to an efficient composite behaviour with adequate load carrying capacity [1,7].
Table 1 Compositions tested in the laboratory at 28 days (average values). Composition
Density (kg/m3)
Compression strength (N/mm2)
Modulus of elasticity (kN/mm2)
BR BA3 BB3 BA5 BA3B2 BA4B1 BA4B2
2338.1 2203.1 1974.7 2115.6 1974.0 2031.3 1964.1
46.1 30.0 19.8 27.1 22.8 23.6 20.7
32.7 23.6 18.6 22.7 18.6 21.2 18.1
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C. Martins et al. / Construction and Building Materials 102 (2016) 1060–1069 Table 2 Composition of CC BA4B2. Material
Quantity (kg/m3)
Sand Coarse aggregate Cement Water Cork 0/3 Cork 3/10 Super plasticizer
465.6 831.2 300.0 192.2 16.2 12.4 2.4
Gutkowski et al. [4] refers the notched connection as the most effective way of connecting the timber and concrete in a study with utility poles. Three connection systems were tested in the experimental campaign: (i) dowels, (ii) inclined cross screws and (iii) glued-in bars. These three systems are quite common [6,16,44–46] and, from past experience, different levels of difficulty to build, different costs and different mechanical performance are expected. The simplest and more economical are the dowel connections for which weaker mechanical performance (load carrying capacity and stiffness) is expected, on the other side the glued-in bars, more complex and expensive but for which high mechanical performance is expected. The inclined cross screws are somehow between the other two connection types either in terms of cost or in terms of mechanical performance. The connection system used in the tests comprised two lateral timber members connected to a central concrete member, Fig. 2. This test configuration was chosen mainly for two reasons: reliability of the results and simplicity of the test setup. Previous studies showed that there is an influence of the test setup in the test results, however, these differences are insignificant when compared with the variability of the mechanical properties of the connection [7] and, accordingly, are meaningless for engineering purposes. The dowel connection specimens were 250 mm long and the remaining ones (inclined cross screws and glued-in bars) were 300 mm long. The connection system consists of one connector on each shear plane. A scheme of the layout of the test setup for each type of connection is shown in Figs. 3–5. For each type of connection, 20 tests were performed. The preparation of the connection specimens was initiated with the sawing and drilling of the timber elements and the preparation of the steel fasteners. The timber elements were conditioned in a climatic room with 20 ± 2 °C temperature and 65 ± 5% relative humidity. The density (q) and the average diameter (Dmean) of the timber elements are shown in Table 4. The diameter varies from 119 mm up to 174 mm with a mean value of 137 mm.
Fig. 1. Failure plan of a cube of the CC BA4B2 after test.
Fig. 2. Test configuration for push out tests: (a) plan view, (b) side view.
Table 3 Physical properties and results from modulus of elasticity tests. Log 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
L (mm)
Dmean (mm)
Dnom (mm)
W (%)
q (kg/m3)
Em.g (MPa)
Em,0 (MPa)
3062 2970 2910 2930 2898 2972 2930 2940 2962 3523 3553 3536 3485 3554 3589 3530
152.5 131.0 147.8 135.0 153.8 148.0 147.5 138.0 147.0 118.3 133.0 128.5 128.0 115.3 123.0 130.8
156.7 132.0 148.3 125.0 152.0 142.6 131.0 150.6 148.0 118.0 139.0 130.0 132.0 113.0 122.0 137.8
15.0 15.0 15.8 15.4 15.8 16.1 16.9 15.1 16.4 20.6 33.5 19.3 50.5 20.0 27.4 22.3
529.6 551.4 550.2 586.2 584.5 555.8 554.0 567.8 654.0 673.0 622.9 663.5 615.2 545.9 507.3 614.6
8789.1 11724.1 14155.9 14827.3 12582.1 14728.3 17185.3 7993.9 11494.1 16752.1 9888.7 15784.9 10954.3 13753.6 11164.0 13985.2
9828.0 12174.7 15560.0 15143.1 12780.8 14207.2 17549.7 8488.0 10257.5 14719.8 10128.3 14083.8 9480.0 15928.3 11473.8 12520.5
The fasteners for the dowel connections were cut from 8 mm nominal diameter S500 grade hot rolled ribbed deformed reinforcement bars. To ensure a tight fitting of the fastener, the timber logs were predrilled with a 7 mm diameter drill and the fasteners were inserted in the timber members to a depth of 10 times their nominal diameter using a light hammer (Fig. 3). The inclined cross screws were produced from mild steel, 100 mm length with a head diameter of 12 mm. The threaded part
C. Martins et al. / Construction and Building Materials 102 (2016) 1060–1069
Fig. 3. Dowel connection.
of the screw had an external diameter of 6 mm and a length of 61 mm. The shank part of the screws had a diameter of 4 mm and a length of 33 mm. The screws were placed 20 mm between centres and applied at a 45° angle to the timber logs axis direction (Fig. 4). A pre drilling of 2 mm diameter was applied, to ensure an easier insertion of the screws with the correct angle. The fasteners used in glued-in bar connections were obtained from the same bars as the dowel connection. The steel bar was cut to the final length and then bent to the desired configuration (Fig. 5). In this case a pre drilling of 80 mm length and 10 mm diameter with an angle of 45° to the log axis was used. After that the fasteners were glued in the timber holes by an epoxide resin SikaÒ Icosit K-101TW [47]. The tests of the connection specimens were carried out 28 days after casting of the concrete. The load history was defined according to EN 26891 [48] for timber-to-timber connections. The load was applied vertically in the central concrete element making use of an hydraulic actuator with electronic control. The applied load was confirmed trough a load cell placed between the steel plate above the concrete and the hydraulic actuator, Fig. 2. The vertical relative movement between the concrete and the timber elements (the slip) was measured with four displacement transducers, two per shear plane, in the front and in the back of each connection specimen. All measurements were recorded by the acquisition data device once a second. The configuration was established in order to account for any rotation of the specimen during the test or a differential behaviour of connection in the two shear planes. Table 4 shows the most relevant mechanical properties of each connection system where: Dmean is the mean diameter of the logs, q is the density of the logs, Fmax is the load applied, Ks is the slip
Fig. 4. Inclined cross screw connection.
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modulus (stiffness), and dult is the ultimate slip measured immediately before the collapse, or if it did not occur up the 15 mm defined in the standard EN 26891 [48], 15 mm. The values shown in Table 4 refer to the entire specimen, i.e. two shear planes. Fig. 6 shows load-slip curves illustrative of the typical behaviour observed on the three connection systems tested and Fig. 7 shows scattering and illustrates the correlation of results for Fmax and Ks for all specimens. The results shown in Table 4 and Fig. 6 are in line with what was expected: (i) the glued-in bar connection is the stiffest connection while the dowel connection showed the lowest stiffness, (ii) the inclined cross screws showed the lowest load carrying capacity and the glued-in bar had the highest one and (iii) the dowel connection presented the highest ductility whereas the inclined cross screws had the lowest ductility. The relatively low values of load carrying capacity, stiffness and ductility of inclined cross screws could be justified by the small diameter used and was noted that the failure occurred mostly with the tension failure of one or more screws. Larger diameter screws would lead to significantly higher values as reported by others [15]. Fig. 7 shows a clear relation between stiffness and maximum load carrying capacity – the correlation coefficient (obtained by simple linear regression analysis) achieved between both properties for the three types of connections was 0.82. 4. Development and validation of the TCC panel The analysis included two types of tests, on full scale composite elements: composite beams and panel. The main purpose of the tests of beams was to obtain a base mechanical performance for the composite structure including, deformations and load carrying capacity for specimen configurations close to the final ones. On the other hand, the actual performance of the panel, such as for example transversal load distribution, acoustic and dynamic performance could only be assessed in bi-dimensional test specimens. For these reasons, one test specimen with plane dimensions of 3.48 3.34 m was prepared and tested in order to complement the tests of composite beams. The size of the panels were limited by the size of the acoustic chamber available. Additionally, these dimensions are compatible with the available material from small diameter Maritime pine logs available from forest thinning. On the other hand, in Portugal there is a significant number of ancient residential buildings with spans smaller than 3.5 m. Even for this range of spans, the use of a timber–concrete composite floor is better because is stiffer and shows much higher acoustic insulation properties.
Fig. 5. Glued-in-bar connection.
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C. Martins et al. / Construction and Building Materials 102 (2016) 1060–1069 Table 4 Timber log properties and connection specimen test results. Connection specimen
Type of value
Dmean (mm)
q (kg/m3)
Fmax (kN)
Dowel
Mean Minimum Maximum CoV (%)
142.9 127.8 174.0 6.8
575.2 498.7 699.0 9.8
24.0 18.5 30.3 11.5
14.7 5.9 37.9 62.4
14.9 14.0 15.0 1.5
Inclined cross screws
Mean Minimum Maximum CoV (%)
129.7 119.5 136.0 3.6
516.2 471.7 566.9 5.3
21.6 14.3 30.0 19.7
22.9 5.2 88.0 92.7
8.9 5.8 15.0 29.4
Glued-in-bar
Mean Minimum Maximum CoV (%)
136.3 119.0 149.0 5.7
519.5 409.5 599.2 9.4
46.8 30.8 65.8 21.5
75.1 34.3 178.6 50.2
12.9 4.7 15.0 29.3
60 F (kN)
glued-in bar 50 40
tension failure of the first screw
inclined cross screws
30
dowel
20 10
δ (mm) 0 0
2
4
6
8
10
12
14
Fig. 6. Typical load-slip behaviour.
70.0 Fmax (kN) 60.0 50.0 40.0 Dowel
30.0
Inclined cross screws 20.0
Glued-in-bar
10.0 Ks (kN/mm) 0.0 0.0
50.0
100.0
150.0
200.0
Fig. 7. Scattering and correlation of the load carrying capacity and stiffness of the connections.
The preliminary design of the composite beams was made through the c-method available in Annex B of Eurocode 5 [49] assuming 500 mm distance apart between the logs centre lines and fulfilling the design requirements for residential buildings from Eurocode 1 [50]: 2.0 KN/m2 for live loads, 0.5 KN/m2 for permanent loads (self-weight of logs and concrete and finishing elements) and L/500 for deflection limits for short-term behaviour. The mechanical properties of the material and connection used in the analysis are the ones shown in Sections 2 and 3. Nine composite beams, three for each connection type, were cast and tested. A schematic configuration of the composite beams is shown in Fig. 8. The design of the beams with the three configurations was done to meet the same design requirements, i.e. optimizing the space between the connectors for the same level of stress.
Ks (kN/mm)
dult (mm)
Accordingly, the spacing between connectors was different, being equal to 100, 200, and 300 mm for dowel, inclined cross screws and glued-in bar, respectively. Fig. 9 represents two TCC beams ready to be cast. The free spaces between logs and formwork were filled with mastic sealant and the formwork was made in chipboards with a L shape in case of isolated beams and with an overturned U shape in case of full scale TCC panel. The logs were covered with a plastic membrane all around them in order to prevent the transfer of water from concrete to timber. The membrane was fixed to the logs with staples on their bottom side. The load procedure for the composite beam tests was defined according EN 26891 [48] and the test setup follows the indications presented in EN 408 [51]. Fig. 10 presents an illustration of the test setup and its instrumentation where 1 is a load cell; 2, 4, 5 and 7 are displacement gauges used to measure the slip between timber log and CC slab; 3 and 6 are displacement gauges used to measure the uplift between timber log and CC slab, and 8 is a displacement gauge used to measure the vertical displacement at mid span of the beams. To ensure the safety of the measurement equipment, the gauges were removed before the failure of the test specimen. Fig. 11 shows typical load vs. mid-span deflection curves for each of the three connection systems. Table 5 shows the results of the experimental tests of all of TCC beams where (EI)exp is the bending effective stiffness, Fmax is the maximum load, d0.4Fmax is the slip between the timber log and cork concrete slab at 40% of the maximum load; fcm is the compression strength and q is the density of cork concrete, DMV is the deflection measured at mid-span for loads equivalent to characteristic load combination and EFF is the efficiency of the connection system determined for 30% of Fmax. Fig. 11 shows that all three types of TCC beam tested exhibited significant nonlinear deformations. In all TCC beams the failure occurred in tension at the bottom part of the log. Accordingly, the load carrying capacity of the TCC beam was significantly influenced by the strength of the logs. Table 5 shows that, even though the TCC beams had different spacing between the connectors, the experimental effective stiffness was similar in TCC beams with dowel and inclined cross screws, and slightly higher in TCC beams with glued-in bars connections. This was stated through the calculations of the efficiencies of each connection system as proposed by Gutkowski et al. [4]. TCC beams with glued-in bar connections had higher average values of efficiency than the other two connection systems. Nevertheless, for the intended design conditions, any of the three connections can fulfil the requirements of load, being the maximum design load 8.1 KN for ULS (Ed = 1.35G + 1.5Q) lower than the load obtained by the experimental tests on TCC beams with inclined cross screws, 29.1 KN. This value was obtained through the quotient between minimum average value from TCC beams
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Fig. 8. Schematic representation of the composite beams with the three connection types (dimensions in mm).
Fig. 9. Composite beams before cast and formwork: glued-in bar connection (left) and inclined cross screws connection (right).
Fig. 10. Test arrangement and instrumentation of a TCC panel.
maximum loads and cM (1.3), multiplied by Kmod (0.8). Regarding the deflection limits for the load obtained through the characteristic combination (Ed = G + Q), the minimum value is 4.5 mm (L/500) higher than the experimental deflection obtained at that TCC beam for an equivalent load, 1.9 mm (Table 5). The maximum experimental deflection measured was 5.0 mm (TCC beam 3), nevertheless the maximum deflection allowed for this specific TCC beam is 5.4 mm.
As expected, Table 5 also shows that there is not a relation between the load carrying capacity and the effective stiffness of the TCC beams – as stated before, the load carrying capacity of the beams was not directly influenced by the connection but the stiffness was. From the previous results it was concluded that the dowel connection is the type of connection that offers the better compromise between mechanical performance, cost and mounting complexity,
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80 F (kN)
70
Fully composite
dowel
60 50
glued-inbar inclinedscrews
40 30
Non-composite
20 10
d (mm)
0 0
20
40
60
80
100
Fig. 11. Typical load vs. mid-span deflection curves for TCC beams.
i.e. the dowel connection type is cheaper, simpler to mount and leads to TCC elements with similar mechanical performance either in terms of load carrying capacity or stiffness. Accordingly, the dowel connections system was applied in the full scale TCC panel. Nevertheless is important to note that although for current practical conditions the dowel connection system seems the best choice, the glued-in bar connection stiffness produces a significant increase on the effective bending stiffness, being a valuable solution where larger spans are required. The design of the TCC panel was made using the same design conditions used for the single composite beams. Also the production of the test specimen was similar to the one used for the TCC beams: (i) 50 mm thickness of CC slab, (ii) 500 mm spacing between logs axis and (iii) 100 mm spacing between dowel connectors. The geometric configuration of the test specimen is shown in Fig. 12. The full scale panel construction was ensured through two auxiliary beams prepared with notches to receive the main beams. The formwork was done with chipboards prepared as a box in the middle of each two consecutive logs (Fig. 13). None additional devices were needed to support the beams or the formwork. The main purpose of the TCC panel test was to assess the bending stiffness and load carrying capacity of the panel as well as other phenomena that cannot be assessed in isolated composite beams. In terms of stiffness and load carrying capacity the transversal distribution of loads when various different locations of the point load are considered (B1 to B7), is essential to understand the mechanical behaviour of the TCC panel when the transverse direction is taken into account. In Fig. 12 are shown the locations where the loads were applied. The TCC panel was loaded with a free span of 3200 mm and one load cell was placed at the top of each timber log between it and the supporting steel frame. Also the deflections were measured with LVDTs placed under the timber beams at mid-span. The load was applied separately at each loading point
Fig. 12. Schematic representation of the TCC panel (dimensions in mm).
and the percentage of load received by each beam are shown in Table 6. The loads applied were supposed to not surpass the elastic limit. However, during the tests a non-linear behaviour in the end of the test was observed; see Fig. 14 for the load point B1 at beam L11. Nevertheless no signals of failure were observed on the timber logs nor at the concrete slab. After the tests, the TCC panel was dismantled originating seven TCC beams. Each TCC beam was later tested until failure in a four-point bending test configuration scheme with a free span of 3200 mm. Table 7 shows a summary of these tests. Through the failure load of the TCC beams, gathered from the four point test configuration, the resisting bending moment of the TCC beams was computed. With that bending moment, an equivalent concentrated load applied at mid-span that causes that moment was computed. Dividing the equivalent concentrated load by the transversal distribution of loads determined from the TCC panel test, see Table 6, it could be obtained an approximation of the concentrated failure load of the TCC panel, assuming a linear and elastic behaviour up to failure. The relation between the maximum concentrated load necessary to cause the TCC panel failure
Table 5 TCC beams test results and CC properties. TCC beam
Log number
Fastener type
fcm (MPa)
q (kg/m3)
(EI)exp (kN.m2)
Fmax (kN)
d0.4Fmax (mm)
Dmid-span (mm)
EFF (%)
1 2 3 Average 4 5 6 Average 7 8 9 Average
5 7 8
Dowels
3 6 9
Inclined cross screws
1 2 4
Glued-in bars
11.8 13.8 13.9 13.2 11.5 12.7 12.1 12.1 12.3 12.2 11.7 12.1
1835.3 1835.9 1836.4 1835.9 1827.5 1819.5 1812.7 1819.9 1800.6 1795.7 1793.2 1796.5
1210.0 1251.9 953.7 1138.5 1459.1 1093.5 853.9 1135.5 1822.8 1143.0 1038.0 1334.6
65.5 64.6 50.9 60.3 47.8 51.0 43.1 47.3 58.7 49.6 54.4 54.2
1.0 1.7 1.3 1.5 0.8 1.4 1.6 1.3 0.4 0.3 0.3 0.3
3.6 2.4 5.0 3.7 2.5 3.1 4.7 3.4 2.7 2.0 1.9 2.2
33.3 –* 62.8 48.1 57.7 25.4 14.0 32.4 83.6 70.6 45.5 66.6
* The efficiency of TCC beam 2 is not presented because the mid-span deflection data was not measured owing to the fact that the devices were removed before that load level.
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Fig. 13. Schematic representation of the formwork application.
Table 6 Applied loads and transversal load distribution for the TCC panel. Loaded point
Applied loads (kN)
B1 B2 B3 B4 B5 B6 B7
Displacement at mid span (mm)
22.9 27.8 31.8 37.2 38.4 29.3 24.3
24.0 16.1 17.6 18.1 21.6 19.2 19.7
20
Transversal load distribution (%) L11
L14
L16
L10
L12
78 21 3 4 3 2 1
24 54 31 2 4 2 1
4 23 48 30 9 1 1
2
3 1 4 29 48 26 5
L13
0
1 2 4 4 3 20 80
1 0 2 28 52 19
Table 8 Results for the dynamic performance and acoustic insulation of the TCC panel.
F (kN) 15
Natural frequency (Hz) Airborne sound insulation Rw (dB)* Impact sound insulation Lnw (dB)**
10
5
δ (mm) 0 0
5 24 46 25 5 1
L15
5
10
15
20
25
Maximum
Minimum
17 63 67
40 46
* Minimum values obtained for the TCC panel without any other noise reducer, maximum values obtained for the TCC panel complemented with a suspended ceiling. ** Maximum values obtained for the TCC panel without any other noise reducer, minimum values obtained for the TCC panel complemented with a suspended ceiling.
Fig. 14. Load received at beam L11 vs. displacement at point B1.
Table 7 Results from failure test of TCC beams and predicted failure loads of TCC panel. TCC beam
Four-point bending test Failure load (kN)
Displacement at mid-span at failure (mm)
Load necessary for TCC panel failure (kN)
Concentrated loads – TCC panel TCC panel/ TCC beam (%)
L11 L14 L16 L10 L12 L15 L13
31.5 24.2 39.2 37.5 41.5 27.3 37.1
51.7 48.1 71.4 82.4 86.9 69.8 74.6
26.9 29.9 54.9 58.0 58.3 34.7 30.8
128 185 210 232 210 191 124
and the concentrated load predicted to cause the isolated composite beam failure is indicated in Table 7. From the results of failure tests in isolated composite beams and in concentrated load tests performed on the full scale panel it is also clear that the load carrying capacity of a full scale panel is significantly higher than the one for an isolated composite beam. The mean values of gain in load range between 26% for the limit beams L11 and L13, 88% for the beams L14 and L15, 110% for the beams L12 and L16 and 132% for the inner beam (L10).
The results shown in Table 7 clearly demonstrate that the actual performance of TCC panels is significantly better than the one anticipated from the analysis of TCC beams. Additionally to the mechanical tests the acoustic and dynamic performance of the TCC panel was assessed. In Table 8 a resume of the results obtained for acoustic and dynamic tests is shown. The acoustic tests were performed in a set of reverberant chambers designed to evaluate the sound insulation of slabs in what concerns airborne and impact sound. The improvement of acoustic behaviour using a suspended ceiling was assessed by acoustic tests performed in a similar timber–concrete composite panel [25]. Considering the use of a suspended ceiling with the same characteristics and behaviour we found that both the insulation to the airborne and impact sound insulation fulfils the requirements for normal multi-storey building constructions in Portugal, which are 60 dB and 50 dB respectively [52]. The dynamic tests were performed to assess the fundamental frequency of the TCC panel. The full panel was placed with both ends of each beam over the load cells used in the static tests. The free vibration response was produced through a hammer impact at the centre of the panel and was measured with an accelerometer in various positions, over the beams alignment and at mid span. Through a Fast Fourier Transform of the raw data, the power spectrum was obtained and the fundamental frequency was identified as 17.1 Hz, which is significantly higher than the limit (8 Hz)
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defined in Eurocode 5 [49], below which, specific analysis must be undertaken. The results obtained in the testing and analysis of the full scale panel demonstrate that the proposed solution is capable of fulfilling all the existing requirements, for building applications both in terms of safety and serviceability.
Science and Innovation Operational Program co-financed by the European Union Fund FEDER, for the support provided through the Research Project No. PTDC/ECM/099833/2008. A special acknowledgment to Cimpor – Cimentos de Portugal, SGPS, SA and Sika Portugal for the support provided. References
5. Conclusions This paper reports the results of the development of a new structural timber–concrete composite panel. It was shown that the developed TCC panel is inexpensive and environmentally friendly because it combines the advantages of using local small diameter Maritime pine round wood and lightweight concrete with cork granules (materials with low commercial value). The former are abundant in Portugal as result of forest thinning and latter are considered a sub product with no use by industry. Several compositions of lightweight cork concrete were developed in the laboratory. It has been shown that it is possible to produce a lightweight cork concrete suitable for structural applications with compressive strength around 21 MPa, modulus of elasticity around 18 GPa and density around 1964 kg/m3. Three connection systems, with quite different mechanical properties, were considered to connect the concrete layer and the timber logs: (i) dowel connection, (ii) inclined cross screws connection, and (iii) glued-in bar connection. As expected, the glued-in bar connection was the system showing the highest shear stiffness (75.1 kN/mm) and highest shear load (46.8 kN). The lowest stiffness was obtained with the dowel connection (14.7 kN/mm) and the lowest shear load was attained with the inclined cross screws (21.6 kN). However if the screws had the same diameter of the other connectors probably their strength would have been significantly higher. Nine TCC beams, three from each type of fastener, were produced and tested 28 days after cast in a four-point bending test layout. Different spacing between the connectors was considered in order to achieve a similar stiffness in all the TCC beams, using such arrangement it was possible to obtain similar mechanical performances for the three configurations. The failure of TCC beams was always governed by the bending strength of the logs. A full scale TCC panel was also included in the research program. The results of the tests obtained clearly demonstrated the capability of this structural element to fulfil the mechanical, acoustic and dynamic requirements for slabs in common multi-storey building constructions: the load carrying capacity requirements, the deformability requirements, the dynamic behaviour requirements and the sound transmission requirements were all fulfilled by a large margin. It is also clear from the tests that many critical phenomena in this type of structure cannot be assessed through tests on isolated beams, only on full scale panels, e.g. acoustic insulation and transversal load distribution and dynamic behaviour. This means that the TCC panel developed has the potential to be used in more demanding conditions such as for larger spans and higher loads. For those situations could be requested small diameter logs with longer lengths or could be used utility poles (usually with higher nominal diameters). It is also clear that the use of natural and local products coming from national forests could be used for structural purposes in residential buildings enhancing the sustainability of a timber–concrete solution. Acknowledgements The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) and the
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[39] Sika, Sika ViscoCrete 20 HE, 2011. [40] CEN, EN 12390-3 – Testing hardened concrete – Part 3: Compressive strength of test specimens, Brussels, 2001. [41] LNEC, E 397 – Hardened Concrete – Determination of modulus of elasticity of concrete in compression (in Portuguese) Lisbon, 1993. [42] CEN, EN 14251 – Structural round timber – Test methods, CEN, Brussels, 2003. [43] CEN, EN 408 – Timber structures – Structural timber and glued laminated timber – Determination of some physical and mechanical properties, CEN, Brussels, 2010. [44] A. Kenel, U. Meierhofer, Holz/Beton-Verbund unter langfristiger Beanspruchung (Long term performance of timber–concrete composite structural elements), EMPA Abteilung Holz, Dübendorf, Switzerland, 1998. [45] G. Turrini, M. Piazza, Static behavior of timber–concrete composite structures, Recuperare 2 (6) (1983) 214–225. [46] P. Gelfi, E. Giuriani, A. Marini, Stud shear connection design for composite concrete slab and wood beams, J. Struct. Eng. ASCE 128 (12) (2002) 1544– 1550. [47] Sika, Sika Icosit K 101 TW, 2011. [48] CEN, EN 26891 – Timber structures – Joints made with mechanical fasteners – General principles for the determination of strength and deformation characteristics, Brussels, 1991. [49] CEN, Eurocode 5 – Design of timber structures – Part 1-1: General – Common rules and rules for buildings, CEN, Brussels, 2004. [50] IPQ, NP EN 1991-1-1: Acções em estruturas: Part 1-1: Acções gerais – Pesos volúmicos, pesos próprios, sobrecargas em edifícios, Lisbon, 2009. [51] A. Ceccotti, M. Fragiacomo, S. Giordano, Behaviour of a timber–concrete composite beam with glued connection at strength limit state, in: 9th World Conference on Timber Engineering (WCTE), Portland, USA, 2006, p. 8. [52] República Dd. Decreto Lei n 96/2008 de 9 de Junho, in: Ministério do Ambiente dOdTedDR, editor, Lisbon, 2008.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Environmentally friendly high performance timber–concrete panel Carlos Martins, Alfredo M.P.G. Dias ⇑, R. Costa, Pedro Santos Departamento de Engenharia Civil, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Rua Luís Reis Santos – Pólo II da Universidade de Coimbra, 3030-788 Coimbra, Portugal
h i g h l i g h t s Development of timber–concrete composite panels with natural resources. Lightweight concrete incorporating cork granules. Mechanical characterization of timber–concrete connections. Evaluation of acoustic and dynamic behaviour of full-scale timber concrete panels. Transversal load distribution and mechanical behaviour of a full-scale TCC panels.
a r t i c l e
i n f o
Article history: Received 14 November 2014 Received in revised form 27 July 2015 Accepted 31 July 2015 Available online 13 August 2015 Keywords: Timber–concrete composite panels Lightweight cork concrete Maritime pine logs Connections Transversal load distribution Acoustic and dynamic behaviour
a b s t r a c t This paper reports the development and the analysis of a new high performance timber–concrete composite (TCC) panel suitable for structural applications in floors. The new TCC panel is environmentally friendly, and combines low value natural resources with very low level of processing (small diameter Maritime pine logs) with industry sub products (cork granules) leading to a relatively low construction cost. The research program involved three stages: (i) development and characterization of the raw materials (cork lightweight concrete and small diameter Maritime pine logs), (ii) development and characterization of the structural connections and (iii) characterization and validation of the structural panel itself. The research program included an extensive experimental campaign and the results showed that (i) the production of high performance TCC panels is feasible and (ii) the developed TCC panel meets the structural and non-structural requirements for floor applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the last decade several timber–concrete composite (TCC) structural elements suitable for beams and floors for buildings and for footbridges have been developed [1–4]. The TCC panels usually have three main components: timber elements, concrete layer and connections. However, different materials can be used such as for example: solid wood, glued laminated wood or LVL, normal concrete or lightweight concrete [5–11]. Also different techniques for mounting can be considered, e.g. in situ concrete cast or the use of precast concrete members [12]. It has been shown [13–23] that TCC elements shown good performance for short and long-term behaviour, namely in terms of load carrying capacity and stiffness. These elements have also been evaluated in terms of their ability to meet other types of ⇑ Corresponding author. E-mail addresses: [email protected] (C. Martins), [email protected] (A.M.P.G. Dias), [email protected] (R. Costa), [email protected] (P. Santos). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.194 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
requirement such as vibration, acoustic, fire resistance or thermal performance [24–30] showing good results. Furthermore the transversal load distribution on TCC floors/bridges under concentrated loads has also been addressed [31]. Accordingly, it is clear that TCC developed so far are able to fulfil the most common mechanical requirements. However, the available solutions either are usually complex, costly or present some drawbacks such as being environmentally unfriendly [32]. Nowadays these are critical issues that in many situations govern the decision on the final solutions. The main objective of the research program herein described is to develop a new high performance, simple and environmentally friendly TCC structural panel suitable for floors. One of the key features of the developed TCC panel is the incorporation of Portuguese natural resources, namely the cork granules and small diameter round wood Maritime pine logs. The latter are simply unprocessed Maritime pine small diameter round wood members obtained from forest thinning. Maritime pine is the Portuguese main softwood specie and small diameter logs are abundant from forest
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management operations, necessary to improve the forest health and mitigate the risk of fire – these round wood members have already been tried for other applications such as for example, truss members or overhead line utility poles [33–35]. The cork is obtained from the external layer of the bark of oak trees (Quercus Suber L.). The harvesting of this layer of the bark does not introduce damage to the tree and allows a new layer to grow, being a renewable resource. The cork granules are a sub product from the national cork industry that discards up to 30% of raw cork collected. The use of cork granules in concrete leads to produce lightweight concrete. Lightweight concretes with cork aggregates in the composition have already been tested for other applications as non-structural material and revealed good acoustic [36] and thermal properties, which may help to reduce of energy consumption in a building [37]. In this work the concrete composition was optimized for the new TCC panels in order to address specific issues arising from the intended application. Later a deep mechanical characterization of the Maritime pine small diameter round wood logs was undertaken in order to assess their mechanical properties. After that, the connection between the concrete layer and the logs was evaluated and finally the behaviour of nine TCC isolated beams and a full scale TCC panel was analyzed.
2. Development and characterization of the materials Portugal is the world’s largest cork producer and the cork industry usually discards 20–30% of received cork, mainly in the form of small size particles, e.g. cork granules. Concrete with cork granules (CC) has several advantages, namely it, (i) is lighter than a normal concrete, (ii) is environmentally friendly because the cork granules are a sub product resulting from the cork industry without commercial value, and with no processing needed for incorporation in concrete. However, like the other types of lightweight concrete, CC has some disadvantages: (i) lower compressive strength and (ii) lower modulus of elasticity than normal concrete. However, according to Branco et al. [38], CC usually performs better than other types of lightweight concrete (aerated, no-fines, cellular and foamed) and exhibits better acoustic insulation, thermal isolation and shrinkage properties than other lightweight concretes produced from organic materials. The CC composition tested included sand, coarse aggregates, Portland cement 42.5 type I, water, and two types of expanded cork granules: (i) cork 0/3 (dimensions varying from 0 up to 3 mm), and (ii) cork 3/10 (dimensions varying from 3 up to 10 mm) with a density of 136 kg/m3 and 159 kg/m3, respectively. A super plasticizer ViscoCreteÒ 20 HE [39] was employed. In terms of mechanical performance the target for the developed CC was a density about 1800 kg/m3 with a minimum compressive strength about 20 MPa. The development and optimization of the CC composition was achieved by trial and error, adjusting the quantities of cork granulate, sand and coarse aggregates – the cork granules were introduced in the concrete composition simply by replacing part of the coarse and sand aggregates. During this stage, seven compositions were tested: (i) BR, the reference composition, with no cork granulates, (ii) BA3, a composition with 30% of sand replaced by cork 0/3, (iii) BB3, a composition with 30% of coarse replaced by cork 3/10, (iv) BA5, a composition with 50% of sand replaced by cork 0/3, (v) BA3B2, a composition with 30% of sand replaced by cork 0/3 and 20% of coarse replaced by cork 3/10, (vi) BA4B1, a composition with 40% of sand replaced by cork 0/3 and 10% of coarse replaced by cork 3/10 and (vii) BA4B2 a composition with 40% of sand replaced by cork 0/3 and 20% of coarse replaced by cork 3/10.
Nine concrete cubes with 150 mm side length and three quadrangular prisms with 600 mm length and cross sections with 150 mm side length were cast for each composition. Later these elements were tested to determine the compressive strength (cubes) and modulus of elasticity (prisms) of concrete. The compressive strength was experimentally assessed at the age of 28 days according to EN 12390-3 [40]. The modulus of elasticity was tested at the age of 28 days according to E397 [41]. In Table 1 are shown the average values of density, compressive strength and modulus of elasticity obtained for each composition. Table 1 shows that the composition BA4B2 is the one that best fulfils the intended mechanical properties. The composition for this CC is shown in Table 2. Fig. 1 shows a failure plan of a specimen of CC tested in compression. To date, there are no available technical documents to rule the use of small diameter Maritime pine logs for structural applications. To fulfill this gap, various studies have been carried out to characterize this wood based product [33,35]. These studies did not propose grading rules or visual grades but supplied the necessary information to allow that. The outcome of these research campaigns showed a very high potential of small diameter Maritime pine logs for structural application. For these reasons the use of Maritime pine round wood members for structural applications, namely beam members in composite systems, was not a main issue. A comprehensive characterization of the specimens used in the experimental work was, nevertheless, essential as an input for the analysis of the final results due the large variability that was observed in the mechanical properties of this wood product. The experimental campaign herein reported comprised the characterization of 16 small diameter Maritime pine logs from the coastal central area of Portugal divided in two samples: sample 1 included 9 logs and sample 2 included 7 logs. To begin with the logs were measured (length and cross sectional dimensions) and weighed. The moisture content was measured at three points (near both ends and at the middle cross section) using an electronic moisture meter. Later the logs were tested to determine their local modulus of elasticity according to EN 14251 [42] and their global modulus of elasticity according to EN 408 [43] – some adjustments were required for round wood elements. Table 3 shows a summary of the results of the experimental campaign where L is the length of the log, Dmean is the mean diameter of log, Dnom is the nominal diameter of log, W is the moisture content, q is the density of timber, Em.g is the global modulus of elasticity parallel to grain and Em.0 is the local modulus of elasticity parallel to grain.
3. Development and characterization of the connection components In timber–concrete composite elements the connection between the concrete and the timber is a key component. The connection is required to be stiff in order to minimize the shear deformation between the two materials and lead to an efficient composite behaviour with adequate load carrying capacity [1,7].
Table 1 Compositions tested in the laboratory at 28 days (average values). Composition
Density (kg/m3)
Compression strength (N/mm2)
Modulus of elasticity (kN/mm2)
BR BA3 BB3 BA5 BA3B2 BA4B1 BA4B2
2338.1 2203.1 1974.7 2115.6 1974.0 2031.3 1964.1
46.1 30.0 19.8 27.1 22.8 23.6 20.7
32.7 23.6 18.6 22.7 18.6 21.2 18.1
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Quantity (kg/m3)
Sand Coarse aggregate Cement Water Cork 0/3 Cork 3/10 Super plasticizer
465.6 831.2 300.0 192.2 16.2 12.4 2.4
Gutkowski et al. [4] refers the notched connection as the most effective way of connecting the timber and concrete in a study with utility poles. Three connection systems were tested in the experimental campaign: (i) dowels, (ii) inclined cross screws and (iii) glued-in bars. These three systems are quite common [6,16,44–46] and, from past experience, different levels of difficulty to build, different costs and different mechanical performance are expected. The simplest and more economical are the dowel connections for which weaker mechanical performance (load carrying capacity and stiffness) is expected, on the other side the glued-in bars, more complex and expensive but for which high mechanical performance is expected. The inclined cross screws are somehow between the other two connection types either in terms of cost or in terms of mechanical performance. The connection system used in the tests comprised two lateral timber members connected to a central concrete member, Fig. 2. This test configuration was chosen mainly for two reasons: reliability of the results and simplicity of the test setup. Previous studies showed that there is an influence of the test setup in the test results, however, these differences are insignificant when compared with the variability of the mechanical properties of the connection [7] and, accordingly, are meaningless for engineering purposes. The dowel connection specimens were 250 mm long and the remaining ones (inclined cross screws and glued-in bars) were 300 mm long. The connection system consists of one connector on each shear plane. A scheme of the layout of the test setup for each type of connection is shown in Figs. 3–5. For each type of connection, 20 tests were performed. The preparation of the connection specimens was initiated with the sawing and drilling of the timber elements and the preparation of the steel fasteners. The timber elements were conditioned in a climatic room with 20 ± 2 °C temperature and 65 ± 5% relative humidity. The density (q) and the average diameter (Dmean) of the timber elements are shown in Table 4. The diameter varies from 119 mm up to 174 mm with a mean value of 137 mm.
Fig. 1. Failure plan of a cube of the CC BA4B2 after test.
Fig. 2. Test configuration for push out tests: (a) plan view, (b) side view.
Table 3 Physical properties and results from modulus of elasticity tests. Log 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
L (mm)
Dmean (mm)
Dnom (mm)
W (%)
q (kg/m3)
Em.g (MPa)
Em,0 (MPa)
3062 2970 2910 2930 2898 2972 2930 2940 2962 3523 3553 3536 3485 3554 3589 3530
152.5 131.0 147.8 135.0 153.8 148.0 147.5 138.0 147.0 118.3 133.0 128.5 128.0 115.3 123.0 130.8
156.7 132.0 148.3 125.0 152.0 142.6 131.0 150.6 148.0 118.0 139.0 130.0 132.0 113.0 122.0 137.8
15.0 15.0 15.8 15.4 15.8 16.1 16.9 15.1 16.4 20.6 33.5 19.3 50.5 20.0 27.4 22.3
529.6 551.4 550.2 586.2 584.5 555.8 554.0 567.8 654.0 673.0 622.9 663.5 615.2 545.9 507.3 614.6
8789.1 11724.1 14155.9 14827.3 12582.1 14728.3 17185.3 7993.9 11494.1 16752.1 9888.7 15784.9 10954.3 13753.6 11164.0 13985.2
9828.0 12174.7 15560.0 15143.1 12780.8 14207.2 17549.7 8488.0 10257.5 14719.8 10128.3 14083.8 9480.0 15928.3 11473.8 12520.5
The fasteners for the dowel connections were cut from 8 mm nominal diameter S500 grade hot rolled ribbed deformed reinforcement bars. To ensure a tight fitting of the fastener, the timber logs were predrilled with a 7 mm diameter drill and the fasteners were inserted in the timber members to a depth of 10 times their nominal diameter using a light hammer (Fig. 3). The inclined cross screws were produced from mild steel, 100 mm length with a head diameter of 12 mm. The threaded part
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Fig. 3. Dowel connection.
of the screw had an external diameter of 6 mm and a length of 61 mm. The shank part of the screws had a diameter of 4 mm and a length of 33 mm. The screws were placed 20 mm between centres and applied at a 45° angle to the timber logs axis direction (Fig. 4). A pre drilling of 2 mm diameter was applied, to ensure an easier insertion of the screws with the correct angle. The fasteners used in glued-in bar connections were obtained from the same bars as the dowel connection. The steel bar was cut to the final length and then bent to the desired configuration (Fig. 5). In this case a pre drilling of 80 mm length and 10 mm diameter with an angle of 45° to the log axis was used. After that the fasteners were glued in the timber holes by an epoxide resin SikaÒ Icosit K-101TW [47]. The tests of the connection specimens were carried out 28 days after casting of the concrete. The load history was defined according to EN 26891 [48] for timber-to-timber connections. The load was applied vertically in the central concrete element making use of an hydraulic actuator with electronic control. The applied load was confirmed trough a load cell placed between the steel plate above the concrete and the hydraulic actuator, Fig. 2. The vertical relative movement between the concrete and the timber elements (the slip) was measured with four displacement transducers, two per shear plane, in the front and in the back of each connection specimen. All measurements were recorded by the acquisition data device once a second. The configuration was established in order to account for any rotation of the specimen during the test or a differential behaviour of connection in the two shear planes. Table 4 shows the most relevant mechanical properties of each connection system where: Dmean is the mean diameter of the logs, q is the density of the logs, Fmax is the load applied, Ks is the slip
Fig. 4. Inclined cross screw connection.
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modulus (stiffness), and dult is the ultimate slip measured immediately before the collapse, or if it did not occur up the 15 mm defined in the standard EN 26891 [48], 15 mm. The values shown in Table 4 refer to the entire specimen, i.e. two shear planes. Fig. 6 shows load-slip curves illustrative of the typical behaviour observed on the three connection systems tested and Fig. 7 shows scattering and illustrates the correlation of results for Fmax and Ks for all specimens. The results shown in Table 4 and Fig. 6 are in line with what was expected: (i) the glued-in bar connection is the stiffest connection while the dowel connection showed the lowest stiffness, (ii) the inclined cross screws showed the lowest load carrying capacity and the glued-in bar had the highest one and (iii) the dowel connection presented the highest ductility whereas the inclined cross screws had the lowest ductility. The relatively low values of load carrying capacity, stiffness and ductility of inclined cross screws could be justified by the small diameter used and was noted that the failure occurred mostly with the tension failure of one or more screws. Larger diameter screws would lead to significantly higher values as reported by others [15]. Fig. 7 shows a clear relation between stiffness and maximum load carrying capacity – the correlation coefficient (obtained by simple linear regression analysis) achieved between both properties for the three types of connections was 0.82. 4. Development and validation of the TCC panel The analysis included two types of tests, on full scale composite elements: composite beams and panel. The main purpose of the tests of beams was to obtain a base mechanical performance for the composite structure including, deformations and load carrying capacity for specimen configurations close to the final ones. On the other hand, the actual performance of the panel, such as for example transversal load distribution, acoustic and dynamic performance could only be assessed in bi-dimensional test specimens. For these reasons, one test specimen with plane dimensions of 3.48 3.34 m was prepared and tested in order to complement the tests of composite beams. The size of the panels were limited by the size of the acoustic chamber available. Additionally, these dimensions are compatible with the available material from small diameter Maritime pine logs available from forest thinning. On the other hand, in Portugal there is a significant number of ancient residential buildings with spans smaller than 3.5 m. Even for this range of spans, the use of a timber–concrete composite floor is better because is stiffer and shows much higher acoustic insulation properties.
Fig. 5. Glued-in-bar connection.
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C. Martins et al. / Construction and Building Materials 102 (2016) 1060–1069 Table 4 Timber log properties and connection specimen test results. Connection specimen
Type of value
Dmean (mm)
q (kg/m3)
Fmax (kN)
Dowel
Mean Minimum Maximum CoV (%)
142.9 127.8 174.0 6.8
575.2 498.7 699.0 9.8
24.0 18.5 30.3 11.5
14.7 5.9 37.9 62.4
14.9 14.0 15.0 1.5
Inclined cross screws
Mean Minimum Maximum CoV (%)
129.7 119.5 136.0 3.6
516.2 471.7 566.9 5.3
21.6 14.3 30.0 19.7
22.9 5.2 88.0 92.7
8.9 5.8 15.0 29.4
Glued-in-bar
Mean Minimum Maximum CoV (%)
136.3 119.0 149.0 5.7
519.5 409.5 599.2 9.4
46.8 30.8 65.8 21.5
75.1 34.3 178.6 50.2
12.9 4.7 15.0 29.3
60 F (kN)
glued-in bar 50 40
tension failure of the first screw
inclined cross screws
30
dowel
20 10
δ (mm) 0 0
2
4
6
8
10
12
14
Fig. 6. Typical load-slip behaviour.
70.0 Fmax (kN) 60.0 50.0 40.0 Dowel
30.0
Inclined cross screws 20.0
Glued-in-bar
10.0 Ks (kN/mm) 0.0 0.0
50.0
100.0
150.0
200.0
Fig. 7. Scattering and correlation of the load carrying capacity and stiffness of the connections.
The preliminary design of the composite beams was made through the c-method available in Annex B of Eurocode 5 [49] assuming 500 mm distance apart between the logs centre lines and fulfilling the design requirements for residential buildings from Eurocode 1 [50]: 2.0 KN/m2 for live loads, 0.5 KN/m2 for permanent loads (self-weight of logs and concrete and finishing elements) and L/500 for deflection limits for short-term behaviour. The mechanical properties of the material and connection used in the analysis are the ones shown in Sections 2 and 3. Nine composite beams, three for each connection type, were cast and tested. A schematic configuration of the composite beams is shown in Fig. 8. The design of the beams with the three configurations was done to meet the same design requirements, i.e. optimizing the space between the connectors for the same level of stress.
Ks (kN/mm)
dult (mm)
Accordingly, the spacing between connectors was different, being equal to 100, 200, and 300 mm for dowel, inclined cross screws and glued-in bar, respectively. Fig. 9 represents two TCC beams ready to be cast. The free spaces between logs and formwork were filled with mastic sealant and the formwork was made in chipboards with a L shape in case of isolated beams and with an overturned U shape in case of full scale TCC panel. The logs were covered with a plastic membrane all around them in order to prevent the transfer of water from concrete to timber. The membrane was fixed to the logs with staples on their bottom side. The load procedure for the composite beam tests was defined according EN 26891 [48] and the test setup follows the indications presented in EN 408 [51]. Fig. 10 presents an illustration of the test setup and its instrumentation where 1 is a load cell; 2, 4, 5 and 7 are displacement gauges used to measure the slip between timber log and CC slab; 3 and 6 are displacement gauges used to measure the uplift between timber log and CC slab, and 8 is a displacement gauge used to measure the vertical displacement at mid span of the beams. To ensure the safety of the measurement equipment, the gauges were removed before the failure of the test specimen. Fig. 11 shows typical load vs. mid-span deflection curves for each of the three connection systems. Table 5 shows the results of the experimental tests of all of TCC beams where (EI)exp is the bending effective stiffness, Fmax is the maximum load, d0.4Fmax is the slip between the timber log and cork concrete slab at 40% of the maximum load; fcm is the compression strength and q is the density of cork concrete, DMV is the deflection measured at mid-span for loads equivalent to characteristic load combination and EFF is the efficiency of the connection system determined for 30% of Fmax. Fig. 11 shows that all three types of TCC beam tested exhibited significant nonlinear deformations. In all TCC beams the failure occurred in tension at the bottom part of the log. Accordingly, the load carrying capacity of the TCC beam was significantly influenced by the strength of the logs. Table 5 shows that, even though the TCC beams had different spacing between the connectors, the experimental effective stiffness was similar in TCC beams with dowel and inclined cross screws, and slightly higher in TCC beams with glued-in bars connections. This was stated through the calculations of the efficiencies of each connection system as proposed by Gutkowski et al. [4]. TCC beams with glued-in bar connections had higher average values of efficiency than the other two connection systems. Nevertheless, for the intended design conditions, any of the three connections can fulfil the requirements of load, being the maximum design load 8.1 KN for ULS (Ed = 1.35G + 1.5Q) lower than the load obtained by the experimental tests on TCC beams with inclined cross screws, 29.1 KN. This value was obtained through the quotient between minimum average value from TCC beams
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Fig. 8. Schematic representation of the composite beams with the three connection types (dimensions in mm).
Fig. 9. Composite beams before cast and formwork: glued-in bar connection (left) and inclined cross screws connection (right).
Fig. 10. Test arrangement and instrumentation of a TCC panel.
maximum loads and cM (1.3), multiplied by Kmod (0.8). Regarding the deflection limits for the load obtained through the characteristic combination (Ed = G + Q), the minimum value is 4.5 mm (L/500) higher than the experimental deflection obtained at that TCC beam for an equivalent load, 1.9 mm (Table 5). The maximum experimental deflection measured was 5.0 mm (TCC beam 3), nevertheless the maximum deflection allowed for this specific TCC beam is 5.4 mm.
As expected, Table 5 also shows that there is not a relation between the load carrying capacity and the effective stiffness of the TCC beams – as stated before, the load carrying capacity of the beams was not directly influenced by the connection but the stiffness was. From the previous results it was concluded that the dowel connection is the type of connection that offers the better compromise between mechanical performance, cost and mounting complexity,
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80 F (kN)
70
Fully composite
dowel
60 50
glued-inbar inclinedscrews
40 30
Non-composite
20 10
d (mm)
0 0
20
40
60
80
100
Fig. 11. Typical load vs. mid-span deflection curves for TCC beams.
i.e. the dowel connection type is cheaper, simpler to mount and leads to TCC elements with similar mechanical performance either in terms of load carrying capacity or stiffness. Accordingly, the dowel connections system was applied in the full scale TCC panel. Nevertheless is important to note that although for current practical conditions the dowel connection system seems the best choice, the glued-in bar connection stiffness produces a significant increase on the effective bending stiffness, being a valuable solution where larger spans are required. The design of the TCC panel was made using the same design conditions used for the single composite beams. Also the production of the test specimen was similar to the one used for the TCC beams: (i) 50 mm thickness of CC slab, (ii) 500 mm spacing between logs axis and (iii) 100 mm spacing between dowel connectors. The geometric configuration of the test specimen is shown in Fig. 12. The full scale panel construction was ensured through two auxiliary beams prepared with notches to receive the main beams. The formwork was done with chipboards prepared as a box in the middle of each two consecutive logs (Fig. 13). None additional devices were needed to support the beams or the formwork. The main purpose of the TCC panel test was to assess the bending stiffness and load carrying capacity of the panel as well as other phenomena that cannot be assessed in isolated composite beams. In terms of stiffness and load carrying capacity the transversal distribution of loads when various different locations of the point load are considered (B1 to B7), is essential to understand the mechanical behaviour of the TCC panel when the transverse direction is taken into account. In Fig. 12 are shown the locations where the loads were applied. The TCC panel was loaded with a free span of 3200 mm and one load cell was placed at the top of each timber log between it and the supporting steel frame. Also the deflections were measured with LVDTs placed under the timber beams at mid-span. The load was applied separately at each loading point
Fig. 12. Schematic representation of the TCC panel (dimensions in mm).
and the percentage of load received by each beam are shown in Table 6. The loads applied were supposed to not surpass the elastic limit. However, during the tests a non-linear behaviour in the end of the test was observed; see Fig. 14 for the load point B1 at beam L11. Nevertheless no signals of failure were observed on the timber logs nor at the concrete slab. After the tests, the TCC panel was dismantled originating seven TCC beams. Each TCC beam was later tested until failure in a four-point bending test configuration scheme with a free span of 3200 mm. Table 7 shows a summary of these tests. Through the failure load of the TCC beams, gathered from the four point test configuration, the resisting bending moment of the TCC beams was computed. With that bending moment, an equivalent concentrated load applied at mid-span that causes that moment was computed. Dividing the equivalent concentrated load by the transversal distribution of loads determined from the TCC panel test, see Table 6, it could be obtained an approximation of the concentrated failure load of the TCC panel, assuming a linear and elastic behaviour up to failure. The relation between the maximum concentrated load necessary to cause the TCC panel failure
Table 5 TCC beams test results and CC properties. TCC beam
Log number
Fastener type
fcm (MPa)
q (kg/m3)
(EI)exp (kN.m2)
Fmax (kN)
d0.4Fmax (mm)
Dmid-span (mm)
EFF (%)
1 2 3 Average 4 5 6 Average 7 8 9 Average
5 7 8
Dowels
3 6 9
Inclined cross screws
1 2 4
Glued-in bars
11.8 13.8 13.9 13.2 11.5 12.7 12.1 12.1 12.3 12.2 11.7 12.1
1835.3 1835.9 1836.4 1835.9 1827.5 1819.5 1812.7 1819.9 1800.6 1795.7 1793.2 1796.5
1210.0 1251.9 953.7 1138.5 1459.1 1093.5 853.9 1135.5 1822.8 1143.0 1038.0 1334.6
65.5 64.6 50.9 60.3 47.8 51.0 43.1 47.3 58.7 49.6 54.4 54.2
1.0 1.7 1.3 1.5 0.8 1.4 1.6 1.3 0.4 0.3 0.3 0.3
3.6 2.4 5.0 3.7 2.5 3.1 4.7 3.4 2.7 2.0 1.9 2.2
33.3 –* 62.8 48.1 57.7 25.4 14.0 32.4 83.6 70.6 45.5 66.6
* The efficiency of TCC beam 2 is not presented because the mid-span deflection data was not measured owing to the fact that the devices were removed before that load level.
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Fig. 13. Schematic representation of the formwork application.
Table 6 Applied loads and transversal load distribution for the TCC panel. Loaded point
Applied loads (kN)
B1 B2 B3 B4 B5 B6 B7
Displacement at mid span (mm)
22.9 27.8 31.8 37.2 38.4 29.3 24.3
24.0 16.1 17.6 18.1 21.6 19.2 19.7
20
Transversal load distribution (%) L11
L14
L16
L10
L12
78 21 3 4 3 2 1
24 54 31 2 4 2 1
4 23 48 30 9 1 1
2
3 1 4 29 48 26 5
L13
0
1 2 4 4 3 20 80
1 0 2 28 52 19
Table 8 Results for the dynamic performance and acoustic insulation of the TCC panel.
F (kN) 15
Natural frequency (Hz) Airborne sound insulation Rw (dB)* Impact sound insulation Lnw (dB)**
10
5
δ (mm) 0 0
5 24 46 25 5 1
L15
5
10
15
20
25
Maximum
Minimum
17 63 67
40 46
* Minimum values obtained for the TCC panel without any other noise reducer, maximum values obtained for the TCC panel complemented with a suspended ceiling. ** Maximum values obtained for the TCC panel without any other noise reducer, minimum values obtained for the TCC panel complemented with a suspended ceiling.
Fig. 14. Load received at beam L11 vs. displacement at point B1.
Table 7 Results from failure test of TCC beams and predicted failure loads of TCC panel. TCC beam
Four-point bending test Failure load (kN)
Displacement at mid-span at failure (mm)
Load necessary for TCC panel failure (kN)
Concentrated loads – TCC panel TCC panel/ TCC beam (%)
L11 L14 L16 L10 L12 L15 L13
31.5 24.2 39.2 37.5 41.5 27.3 37.1
51.7 48.1 71.4 82.4 86.9 69.8 74.6
26.9 29.9 54.9 58.0 58.3 34.7 30.8
128 185 210 232 210 191 124
and the concentrated load predicted to cause the isolated composite beam failure is indicated in Table 7. From the results of failure tests in isolated composite beams and in concentrated load tests performed on the full scale panel it is also clear that the load carrying capacity of a full scale panel is significantly higher than the one for an isolated composite beam. The mean values of gain in load range between 26% for the limit beams L11 and L13, 88% for the beams L14 and L15, 110% for the beams L12 and L16 and 132% for the inner beam (L10).
The results shown in Table 7 clearly demonstrate that the actual performance of TCC panels is significantly better than the one anticipated from the analysis of TCC beams. Additionally to the mechanical tests the acoustic and dynamic performance of the TCC panel was assessed. In Table 8 a resume of the results obtained for acoustic and dynamic tests is shown. The acoustic tests were performed in a set of reverberant chambers designed to evaluate the sound insulation of slabs in what concerns airborne and impact sound. The improvement of acoustic behaviour using a suspended ceiling was assessed by acoustic tests performed in a similar timber–concrete composite panel [25]. Considering the use of a suspended ceiling with the same characteristics and behaviour we found that both the insulation to the airborne and impact sound insulation fulfils the requirements for normal multi-storey building constructions in Portugal, which are 60 dB and 50 dB respectively [52]. The dynamic tests were performed to assess the fundamental frequency of the TCC panel. The full panel was placed with both ends of each beam over the load cells used in the static tests. The free vibration response was produced through a hammer impact at the centre of the panel and was measured with an accelerometer in various positions, over the beams alignment and at mid span. Through a Fast Fourier Transform of the raw data, the power spectrum was obtained and the fundamental frequency was identified as 17.1 Hz, which is significantly higher than the limit (8 Hz)
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defined in Eurocode 5 [49], below which, specific analysis must be undertaken. The results obtained in the testing and analysis of the full scale panel demonstrate that the proposed solution is capable of fulfilling all the existing requirements, for building applications both in terms of safety and serviceability.
Science and Innovation Operational Program co-financed by the European Union Fund FEDER, for the support provided through the Research Project No. PTDC/ECM/099833/2008. A special acknowledgment to Cimpor – Cimentos de Portugal, SGPS, SA and Sika Portugal for the support provided. References
5. Conclusions This paper reports the results of the development of a new structural timber–concrete composite panel. It was shown that the developed TCC panel is inexpensive and environmentally friendly because it combines the advantages of using local small diameter Maritime pine round wood and lightweight concrete with cork granules (materials with low commercial value). The former are abundant in Portugal as result of forest thinning and latter are considered a sub product with no use by industry. Several compositions of lightweight cork concrete were developed in the laboratory. It has been shown that it is possible to produce a lightweight cork concrete suitable for structural applications with compressive strength around 21 MPa, modulus of elasticity around 18 GPa and density around 1964 kg/m3. Three connection systems, with quite different mechanical properties, were considered to connect the concrete layer and the timber logs: (i) dowel connection, (ii) inclined cross screws connection, and (iii) glued-in bar connection. As expected, the glued-in bar connection was the system showing the highest shear stiffness (75.1 kN/mm) and highest shear load (46.8 kN). The lowest stiffness was obtained with the dowel connection (14.7 kN/mm) and the lowest shear load was attained with the inclined cross screws (21.6 kN). However if the screws had the same diameter of the other connectors probably their strength would have been significantly higher. Nine TCC beams, three from each type of fastener, were produced and tested 28 days after cast in a four-point bending test layout. Different spacing between the connectors was considered in order to achieve a similar stiffness in all the TCC beams, using such arrangement it was possible to obtain similar mechanical performances for the three configurations. The failure of TCC beams was always governed by the bending strength of the logs. A full scale TCC panel was also included in the research program. The results of the tests obtained clearly demonstrated the capability of this structural element to fulfil the mechanical, acoustic and dynamic requirements for slabs in common multi-storey building constructions: the load carrying capacity requirements, the deformability requirements, the dynamic behaviour requirements and the sound transmission requirements were all fulfilled by a large margin. It is also clear from the tests that many critical phenomena in this type of structure cannot be assessed through tests on isolated beams, only on full scale panels, e.g. acoustic insulation and transversal load distribution and dynamic behaviour. This means that the TCC panel developed has the potential to be used in more demanding conditions such as for larger spans and higher loads. For those situations could be requested small diameter logs with longer lengths or could be used utility poles (usually with higher nominal diameters). It is also clear that the use of natural and local products coming from national forests could be used for structural purposes in residential buildings enhancing the sustainability of a timber–concrete solution. Acknowledgements The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) and the
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