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Bonding performance of Portuguese Maritime pine glued laminated timber
Construction and Building Materials 223 (2019) 520–529
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Bonding performance of Portuguese Maritime pine glued laminated timber Carlos Martins a,⇑, Alfredo M.P.G. Dias a, Helena Cruz b a b
Institute for Sustainability and Innovation in Structural Engineering, Department of Civil Engineering, University of Coimbra, Coimbra, Portugal Structures Department, LNEC – National Laboratory for Civil Engineering, Lisbon, Portugal
h i g h l i g h t s Evaluation of the bonding performance of Portuguese Maritime pine. Delamination and shear strength tests. Performance of different adhesives for structural applications. Influence of the amount of adhesive and clamping pressure on bonding performance.
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 5 April 2019 Accepted 18 June 2019
Keywords: Glued laminated timber Maritime pine Bonding performance Delamination tests Shear strength tests Adhesives
a b s t r a c t The use of timber for construction purposes in Portugal has decreased during the last century. However, recent market requests aiming at sustainable constructions led to a rediscovery of timber structures. Glued laminated timber is one of the most used timber construction products and its production using Portuguese Maritime pine (Pinus pinaster Ait.) has been studied and validated concerning its economic and mechanical performance. This paper aims to present a state of the art of the research conducted so far on the use of Maritime Pine for glulam as well as new research conveying bonding with more recent adhesives. The ongoing study comprised an extended experimental campaign on gluing Maritime pine timber grown in Portugal, for glulam production. Eight different adhesives were considered, namely based on: i) Phenol-Resorcinol-Formaldehyde (PRF), ii) Melamine-Urea-Formaldehyde (MUF), iii) EmulsionPolymer-Isocyanate (EPI) and iv) Single Component Polyurethane (1C-PUR). The amount of adhesive used and the clamping pressure were considered as main variables. Delamination specimens were tested according to Method A of EN 14080 [1] and shear strength tests followed Annex D of the same standard. In terms of delamination it was observed an excellent performance of all PRF adhesives. MUF, EPI and 1C-PUR adhesives presented good bonding performance, however, it is important to emphasize the need to use primer (PR) in the case of 1C-PUR adhesive. The shear strength was not influenced significantly by the adhesive type but some differences were found on Wood Failure Percentage for EPI and 1C-PUR adhesives. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Maritime pine is the dominant softwood produced in Portugal, corresponding to 23% of 3.1 million ha of forests. Despite its large availability, the main actual uses of Maritime pine timber are still related with non-structural purposes like pallets, pellets, panels and flooring. During the last decades, several studies have shown
⇑ Corresponding author at: ISISE, Departamento de Engenharia Civil, Rua Luís Reis Santos – Pólo II da Universidade de Coimbra, 3030-788 Coimbra, Portugal. E-mail addresses: [email protected], [email protected] (C. Martins). https://doi.org/10.1016/j.conbuildmat.2019.06.143 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
its high mechanical properties allowing it to be used for structural purposes [2–4]. Moreover, its application as small diameter round wood was evaluated for timber-concrete floors, in terms of mechanical behavior [5], acoustics [6] and dynamics [7] and [8]. Glued laminated timber (glulam) is the most common engineered timber based product and it has been increasingly used by the Portuguese construction market mainly for roofs, floors and footbridges. However, the majority of the glulam used in this country is imported as final product or produced with imported raw-material, mostly Spruce (Picea abies) or Scots pine (Pinus sylvestris).
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Portuguese Maritime pine has also been proved to be a good option for glulam production for economic reasons [9] and mechanic behavior [10–11] and [9], used as untreated timber. Maritime pine is also recognised by EN 14080 [1] as a species suitable for glulam production. Gaspar et al. [12] pursued studies on gluing Maritime pine with PRF adhesives, assessing the influence of clamping pressure and cure temperature, both for untreated and preservative treated timber (two levels of retention). These authors observed that higher cure temperatures (45 °C) lead to less delamination, especially in the case of treated timber, as well as lower clamping pressures (0.6 MPa). Since ensuring 45 °C of curing temperature implies higher costs and is not ergonomic for labours, Lampreia [13] studied the influence of using a novolak based primer prior to adhesive application (PRF) for both untreated and treated timber. Despite a slight decrease of delamination when using the primer, the results obtained with the new adhesive formulation without primer also met the standard requirements. During the last two decades several new and improved adhesives came into the market. PRF and RF adhesives are still available and used for glulam, but nowadays their market share is lower than in the past, mainly due to their brown colour which is often seen as a disadvantage in aesthetical terms [14]. MUF adhesives have also been available for many years. It is known the use of a MUF adhesive by a Portuguese glulam company and some studies have been made regarding the mechanical behaviour of their beams using Maritime pine [15]. According to Lehringer and Gabriel [16] another adhesive with a significant share of engineered wood products is one-component polyurethane adhesive (1C-PUR), that was approved by German building authorities in 1994. The most recent adhesive system used for glulam is EPI. Grostad and Bredesen [17] refer a specific formulation that has been approved for gluing load bearing elements of Maritime pine (in France). The main advantage of EPI and 1C-PUR adhesives is the absence of formaldehyde release. Several studies were published regarding Maritime pine gluability with PRF and RF adhesives. However, there is still limited knowledge about how more recent adhesives behave and the best production parameters to achieve a reliable performance concerning delamination and shear strength. Thus, the present paper has the main goal to study the bonding performance of several commercially available adhesives used on untreated Maritime pine species. For that purpose, eight adhesives of four different types were considered: i) three PRF adhesives, ii) two MUF adhesives, iii) two EPI adhesives and iv) one 1C-PUR adhesive. Their Technical Data Sheets (TDS) were used as guide, together with some advices from the adhesive’s suppliers. Production parameters like the amount of adhesive and the clamping pressure were considered as variables. The influence of moisture content (MC) and Closed Assembly Time (CAT) were also observed through the results obtained. Delamination and shear strength tests were carried out to assess the bonding performance following the procedures of EN 14080 [1].
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board. MC was also measured with an electrical moisture meter according to Annex G of EN 14080 [1]. After planning each board, small clear specimens were collected to determine both the density and MC, by the oven dry method according to EN 13183-1 [18], to check specifications of EN 14080 [1]. The mean value and standard deviation were 13.9 ± 0.6% for MC and 631 ± 58 kg/m3 for density at 12% MC. These confirmed MC readings done with an electrical moisture meter (the mean value and standard deviation of MC were 13.3 ± 1.3%). 2.2. Bonding parameters and glulam beams preparation The bond quality depends on the procedure followed by the manufacturers during the gluing process. It is well documented in the literature that for the same specific adhesive, different wood species may present different performance [14]. The present study assessed the performance of four types of adhesives in a total of eight different adhesives: PRF-1; PRF-2; PRF-3; MUF-1; MUF-2; EPI-1; EPI-2, 1C-PUR and 1C-PUR + PR. In the case of MUF-1, two different ratios resin/hardener were considered (100/20 and 100/35) and 1C-PUR was considered without and with a primer (1C-PUR + PR) since previous work on Beech (Fagus sylvatica L.) [19] and Blue Gum (Eucalyptus globulus Labill.) [20] showed the benefits of using primer to obtain significant reduction of delamination. The primer used for bonding Maritime pine in the present study was developed specifically for bonding Blue gum. Each adhesive was evaluated in four different scenarios, by combining two fabrication variables: i) amount of adhesive (g/ m2) and ii) clamping pressure applied (MPa). Table 1 provides information relative to the adhesives specifications as well as the gluing parameters adopted. The study comprised a total of 50 glulam beams. Each glulam beam consists on 4 lamellas with 30 mm thickness each and 1 m long. The lamellas were sorted in order to have the top layer with the lowest density (usually below 600 kg/m3) and the bottom layer with the highest density (usually around 700 kg/m3). Intermediate density lamellas formed the inner layers (between 600 and 700 kg/ m3). The boards were selected in a way that the mean density of each glulam beam was close to the mean density of the sample. The lamellas were displayed in order to have the pith facing outwards, as recommended for Service Class 3 [1]. Prior to adhesive application (<6 h) the lamellas were planned to the target thickness (30 mm). The entire gluing and curing process occurred in a room with 20 ± 2 °C temperature and 65 ± 5% relative humidity. A manual spreader was used to uniformly distribute the adhesive and the amount applied was controlled by weighing. The primer – when considered – was sprayed uniformly over each mating surface. The amount applied (20 g/m2) was controlled by weighing. As recommended by each specific TDS the Open Assembly Time (OAT) was as shorter as possible and the CAT was followed. The pressing process (two glulam beams at the same time) was ensured by a series of hydraulic jacks spaced 500 mm and with maximum load capacity of 50 kN each (Fig. 1). After the pressing time, glulam beams were keep in the same controlled conditions during at least 7 days, prior to testing.
2. Materials and methods
2.3. Delamination tests
2.1. Wood
From each glulam beam several specimens of 75 110 120 mm3 (length width height) were cut to evaluate the resistance to delamination according to Method A from Annex C of EN 14080 [1]. Delamination test imposes internal stresses perpendicular to the glue lines between laminations through the introduction of a gradient of moisture into the specimens. The test procedure started by soaking the specimens with water in an autoclave (Fig. 2), and applying a vacuum period of 5 min
Maritime pine boards were collected from a region near Leiria in Portugal. A total of 109 boards of untreated Maritime pine were prepared. All boards were stored at 20 ± 2 °C temperature and 65 ± 5% relative humidity. Visual grading was performed, together with measuring width, thickness, length and weigh to determine the density of each
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Table 1 Summary of variables considered for gluing process. Adhesive system
PRF-1
Viscosity of resin (mPas) Viscosity of hardener (mPas) Assembly time (min) Amount of adhesive (g/m2) Resin/Hardener Clamping pressure (MPa) Pressing time (h)
5000–10000 5000–8000 701 501 350/450 350/450 100/20 100/20 0.6/0.8 0.6/0.8 4 3
Note: 1 Assembly 2 Assembly 3 Assembly 4 Assembly 5 Assembly 6 Assembly
time time time time time time
with with with with with with
PRF-2
PRF-3
MUF-1
MUF-2
EPI-1
EPI-2
1C-PUR
1C-PUR + PR
400–1500 4500–6000 451/ 602 350/450 100/20 0.6/0.8 4
3000–3500 3000 1503 400/500 100/20–100/35 0.6/0.8 9/4.5
3000–10000 3000–4000 1204 350/450 100/20 0.6/0.8 8
5000–10000 20–100 155 250/350 100/15 0.6/0.8 0.33
6000–10000 250–400
24 000 – 706 160/180 – 0.6/0.8 3
160/180 – 0.6/0.8 6
300/350 100/15 0.6/0.8 2
350 g/m2 at 20 °C/65% RH. 450 g/m2 at 20 °C/65% RH. 400–500 g/m2 at 20 °C/65% RH. 400 g/m2 at 20 °C/65% RH. 250 g/m2 at 20 °C/65% RH. 160–180 g/m2 at 20 °C/65% RH.
results of delamination are summarized and discussed further in detail.
2.4. Shear tests
Fig. 1. Glulam beams of Maritime pine under pressure (PRF adhesive).
(15–30 kPa absolute pressure) followed by a pressure period of 60 min (600–700 kPa absolute pressure). One complete cycle consists of two vacuum-pressure period, each one followed by a drying period of 21–22 h at a drying duct (Fig. 2) where the air circulates at 2 m/s–3 m/s at a temperature between 60 and 70 °C and a relative humidity below 15%. Method A establishes that the delamination limit after the 2nd cycle is 5% and an additional cycle (3rd) can be performed if the delamination measured is higher, the limit after the 3rd cycle being equal to 10%. In the present study three cycles of delamination were performed for all specimens. The
Shear strength test was performed according to Annex D of EN 14080 [1] and consists in the application of a shear load parallel to the glue line until failure, in a time not less than 20 s. The specimens were approximately 50 50 120 mm3 (width thickness length) and a displacement control with a rate of 0.06 mm/s was imposed. A hydraulic actuator of 50 kN maximum capacity was used to apply the loads under the displacement control. A device (Fig. 3) with a self-aligning part was used so that the test piece is uniformly loaded at the end grain, as specified in EN 14080 [1]. The glue line shear strength was measured by applying shear at a maximum distance of 1 mm from the glued joint plane; timber shear strength was measured approximately at half of thickness of each lamella. After testing, the wood failure percentage (WFP) of each glue-line was also measured using a transparent square mesh in which each square mesh corresponds to 1%. For white colour adhesives a mixture of Phloroglucinol (2%) with Chloridric acid solution (10%) was used to distinguish the adhesive failure from the wood failure (Fig. 4). Both faces were analysed and wood failure was considered when both zones presented wood fibres. Individual and mean values were assessed. According to
Fig. 2. Delamination test devices: left) autoclave, right) specimens displayed at drying duct.
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Fig. 3. Device used for shear strength tests.
the standard, an individual value of 4 MPa is acceptable if the corresponding WFP is 100%, whereas the minimum acceptable mean value is 6 MPa combined with 90% of WFP. 3. Results and discussion 3.1. Delamination test results Results from delamination tests of all adhesives are summarized in Table 2 as well as detailed information about the two variables considered in the present study (amount of adhesive and clamping pressure). Also the closed assembly time (CAT), the mean value of density and the respective number of specimens tested are presented. 3.1.1. PRF adhesives Results (Table 2) show that all PRF adhesives have a similar performance both concerning the mean and maximum delamination values after the 2nd and 3rd test cycles. It is also observed that the total delamination (mean and individual) values are lower than the specified limits for Service Class 3 (Method A). The best performance was achieved with PRF-3 – presenting mean values of 0.5% after the 2nd cycle and 0.6% after the 3rd cycle – for 0.8 MPa combined with 450 g/m2. In the present study it was observed that different adhesives of the same type (PRF) showed different behaviour for the same spe-
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cies. PRF-1 and PRF-2 (same resin but different hardeners) had similar behaviour, e.g., an increment of delamination for both mixtures was observed with an increase of the amount of adhesive (doubled the values in case of PRF-2) with both levels of clamping pressure. On the other hand, keeping the same amount of adhesive and increasing the clamping pressure produced different results being observed an increase of delamination for PRF-1 and a decrease of delamination for PRF-2. The most significant advantage of using PRF-2 is that it requires less 1 h of pressing time compared to PRF-1. Delamination observed on specimens glued with PRF-3 evidenced a different trend compared to the other two PRF adhesives. The increased amount of adhesive lead to a significant decrease of delamination (in both levels of clamping pressure). Increasing the clamping pressure did not produce significant changes of delamination. The main advantage of PRF-3 in relation to the other PRF adhesives is its longer storage life, 18 months for the resin and 36 months for hardener if stored at 20 °C. Slightly lower values of delamination were found by Lampreia [13] when gluing Maritime pine with the same PRF-1 in the same amount and using the same clamping pressure, namely 0.8% and 1.3% after the 2nd and 3rd cycles, respectively. Nevertheless these results were quite similar to the best results achieved in the present study with PRF-1, that is 0.9% and 1.0% after the 2nd and 3rd test cycles, respectively. 3.1.2. MUF adhesives A comparison between both MUF adhesives revealed some differences in terms of delamination. In the case of MUF-1 and for the higher clamping pressure (0.8 MPa) the mean delamination hardly varied with the amount of adhesive. For the lower level of clamping pressure, a significant increase of delamination was observed by increasing the amount of adhesive (three times more). Some specimens glued with MUF-1 adhesive showed excessive delamination after the 2nd cycle but none of them had delamination above 10% after the 3rd test cycle. The common scenarios considered a ratio between resin and hardener of 100/20 which required a total of 9 h pressing time. The TDS of the MUF-1 adhesive presents also different ratios that require less pressing time. This could be important for saving energy and increasing the production of a company providing a good bonding performance is guaranteed. Therefore, two glulam beams were produced (14 delamination specimens) and tested to assess the bonding behaviour for a ratio of 100/35, thus reducing
Fig. 4. Interface between lamellas after glue line shear strength test for WFP analysis.
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Table 2 Summary of delamination test results (mean and maximum values of total delamination).
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Table 2 (continued) Note: CAT is the closed assembly time registered. Bold values mean that more than one specimen had delamination in excess of the respective acceptable limit. Underlined values mean that one specimen had delamination higher than the respective limit. Highlighted values means that the values of delamination did not accomplish the limits. * means that the adhesive mixture had a ratio of 100/35. ** means that the adhesive mixture had a ratio of 100/15.
the pressing time to 4.5 h. The mean value of delamination was significantly higher (approximately 3 times) when compared to delamination measured on specimens glued with 100/20 ratio (equal amount of adhesive and the same clamping pressure). MUF-2 results show mean values of delamination (for all scenarios) lower than 1.4% and 1.9% after the 2nd and 3rd test cycles, respectively. Its behaviour was different from the previously discussed adhesives. The increase of adhesive amount produced an increase of delamination for 0.6 MPa, however for 0.8 MPa the delamination decreased. For the amount of adhesive equal to 350 g/m2 a higher clamping pressure lead to higher delamination whereas for 450 g/m2 a decrease of delamination was registered. In conclusion, a good bonding performance was obtained with both MUF adhesives when using an intermediate amount of adhesive and lower clamping pressure level with 100/20 formulation ratio. Comparing MUF with PRF adhesives a similar behaviour is observed, especially with MUF-2 adhesive. A qualitative analysis shows the MUF adhesive to be more interesting due to the white colour of glue lines. On the other hand, its extended pressing time could be a disadvantage related with energy consumption and production yields.
3.1.3. EPI adhesives Regarding EPI adhesives, it was clearly observed that EPI-1 is not adequate for bonding Maritime pine, as the mean values of delamination were higher than 10% (established limit for the 3rd test cycle) already after the 2nd cycle. Despite that there was a decrease of delamination with the increase of the amount of adhesive. Also a significant decrease was observed with the increase of clamping pressure for the lower amount of adhesive tested. Regarding the EPI-2 adhesive mixture a different performance was observed. For the lower clamping pressure (0.6 MPa) the use of this adhesive did not meet the established limits after the 2nd test cycle of delamination tests and after the 3rd test cycle the mean values were slightly higher or close to 10%. On the other hand, promising results were found for 0.8 MPa with a significant increase on its delamination performance, bringing the mean values of delamination below the maximum allowed for both cycles.
This tendency of decreasing the delamination with the increase of pressure was already observed with EPI-1 adhesive. Nevertheless, for 350 g/m2 combined with 0.8 MPa one of the specimens presented excessive delamination after both cycles being evident the presence of tangential growth rings in the adjacent lamellas (both faces) (Fig. 5). The influence of the tangential growth rings was observed also on the remaining specimens from all elements glued with EPI adhesives. The short pressing time required for EPI adhesives is an advantage comparing with the other adhesives considered within the present study.
3.1.4. 1C-PUR adhesives Also the use of 1C-PUR adhesive was considered in the present study. The analysis of the results show that none of the considered scenarios of 1C-PUR (without primer) resulted in a reliable performance of bonded untreated Maritime pine. However, a trend was found showing that the increase of clamping pressure produced a significant decrease of delamination (approximately 10%). As this adhesive has a significant share of the European market both for glulam and CLT (Cross Laminated Timber), it was decided to use a primer specifically developed for Blue gum timber by the same producer. It was found that the combination 1C-PUR + PR results in a significant improvement concerning delamination that was satisfactory. The benefits of using primer have been demonstrated by Lopez-Suevos and Richter [20] for Blue gum glulam. All four scenarios presented mean delamination values below 2.4% and 3.0% after the 2nd and 3rd test cycles, respectively. The trend identified before with 1C-PUR, regarding the increase of amount of adhesive, was also evident for 0.6 MPa when the primer was used. The increase of clamping pressure also conducted to a reduction of delamination in both amount of adhesive values. From a total of 56 specimens collected from the elements glued with 1CPUR + PR only one specimen had excessive delamination after both delamination cycles. A detailed analysis of that specimen (located at one end of the glulam beam) showed that the lack of primer at the lower glue line combined with the presence of tangential growth rings adjacent to the glue line was probably the reason for the excessive delamination registered. Fig. 6 illustrates both
Fig. 5. Delamination observed coincident with tangential growth rings located in the adjacent lamellas (EPI-2 adhesive: 350 g/m2 and 0.8 MPa).
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Fig. 6. Delamination observed at the specimen D7 with excessive delamination caused by lack of primer combined with tangential growth rings adjacent to glue line.
Fig. 7. Delamination observed at the specimen D6 (1C-PUR + PR adhesive) with presence of tangential growth rings adjacent to glue line.
Table 3 Summary of shear strength test results on glue lines and timber. Adhesive
PRF-1
Amount of adhesive (g/m2)
Clamping pressure (MPa)
Density (kg/m3)
Glue line N° of specimens
Shear strength (std. dev.) (MPa)
WFP (%)
N° of specimens
Shear strength (std. dev.) (MPa)
350
0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8
647 642 653 631 647 653 650 646 649 655 659 644 654 656 652 649 661 636 647 656 640 649 645 653 651 644 635 644 634 666 641 659 634 655 643 645 643
22 84 24 26 60 28 28 29 27 22 27 60 84 27 29 29 57 26 23 52 27 28 27 25 24 29 22 26 22 29 44 25 25 52 51 55 50
13.8 13.5 14.7 13.7 14.6 14.0 14.1 14.5 13.4 13.9 13.6 14.4 13.9 14.2 13.0 14.8 13.2 13.3 12.3 13.1 11.7 12.5 12.2 11.7 13.0 12.5 12.8 13.1 12.9 11.6 13.5 11.4 13.8 14.1 13.7 13.7 13.5
99 95 98 97 99 99 99 99 99 99 99 100 96 98 98 99 95 94 93 93 91 83 91 74 79 86 96 89 94 71 87 61 80 89 93 93 93
15 44 16 14 32 15 16 12 16 14 15 30 47 16 15 15 30 13 16 28 15 14 12 16 14 14 16 14 14 15 29 15 16 31 23 30 30
14.9 14.2 15.1 14.1 14.6 14.0 15.1 14.5 14.2 15.5 13.5 15.1 13.3 15.1 15.0 14.3 13.8 14.3 12.8 13.7 12.4 15.7 15.6 15.1 14.5 14.7 14.6 14.2 13.6 14.3 14.4 14.0 15.0 15.0 14.3 15.5 14.4
450 PRF-2
350 450
PRF-3
350
MUF-1
400
450
500
MUF-2
400* 350 450
EPI-1
250** 350**
EPI-2
300**
1C-PUR
160
350**
180 1C-PUR + PR
160 180
Note: * means that the adhesive mixture had a ratio of 100/35. ** means that the adhesive mixture had a ratio of 100/15.
Timber
(1.5) (1.3) (2.0) (1.1) (1.1) (2.0) (1.4) (2.0) (1.8) (1.7) (1.3) (1.4) (1.2) (1.5) (1.1) (1.2) (1.6) (1.5) (1.4) (1.6) (1.0) (1.4) (1.9) (2.7) (1.9) (1.3) (1.2) (1.0) (0.9) (1.9) (1.4) (1.5) (1.2) (1.6) (1.0) (1.4) (1.1)
(0.9) (1.2) (1.3) (0.8) (1.5) (1.3) (1.2) (1.7) (1.9) (1.5) (1.8) (1.7) (1.2) (2.0) (1.6) (1.2) (0.9) (2.7) (1.3) (1.7) (1.5) (2.4) (1.9) (1.2) (1.8) (1.8) (2.1) (1.6) (1.8) (1.8) (1.1) (2.4) (1.5) (1.4) (1.6) (1.8) (1.9)
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faces of the specimen after the 3rd delamination test cycle whereas in Fig. 7 is illustrated an inner element collected from the same element and significantly lower delamination. The lowest delamination between all adhesives was obtained for 1C-PUR + PR (180 g/m2 and 0.8 MPa) with mean values of 0.1% and 0.4% after the 2nd and 3rd test cycles, respectively. The use of primer has the disadvantage of doubling the pressing time compared with 1C-PUR adhesive only. 3.2. Shear strength test results Shear strength results are summarized in Table 3 with detailed information on the production variables considered within the present study. Mean and standard deviation values are listed both for glue lines and timber shear strength as well as WFP values measured on glue lines. Fig. 8 presents the individual values of shear strength and WFP of glue line tests from specimens glued with PRF and MUF adhesives whereas Fig. 9 presents the individual test results of specimens glued with 1C-PUR and EPI adhesives. As Maritime pine is denser than the most common softwoods used for glulam production (Spruce and Scots pine) higher values of shear strength were expected as referred by Vick [21]. Slightly higher values of shear strength were obtained on timber as compared to glue line values. In the previous study done with PRF adhesives by Lampreia [13], similar values of shear strength on glue lines were found (13.4 MPa). Generally, the shear strength values were quite similar for all adhesives with the exception of both EPI and 1C-PUR adhesives that have shown slightly lower values but still around 12 MPa. Regarding the WFP, it was clear that PRF and MUF adhesives provided excellent results. EPI-1 glue lines had lower strength which
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is in line with the previously mentioned inadequate performance observed through delamination tests. As the delamination performance improved using EPI-2 adhesive, also the shear strength (mean values) and WFP have increased. The same trend was obvious for 1C-PUR + PR adhesive. The mean WFP obtained in the case of 1C-PUR + PR was around 90% having some individual values being the lower WFP clearly related with some absence of primer on the surface during its application. From the individual shear strength results it was clear that only EPI-1 and 1C-PUR had some glue lines (maximum 2 per specimen) that did not meet the minimum required values of shear strength combined with WFP. The use of primer in 1C-PUR + PR significantly improved the glue lines performance, as compared to 1C-PUR, especially regarding the WFP thus meeting the acceptable limits. PRF and MUF adhesives had 100% of glue lines above the limits in all scenarios. By using load and displacement data from the hydraulic actuator, typical load vs displacement curves obtained in shear tests were drawn. Figs. 10 and 11 present typical curves (corresponding to specimens with shear strength closer to the average value) of each adhesive type. These curves show an initial linear elastic behaviour followed by a non-linear behaviour – more evident in the case of MUF-2 adhesive, what is probably related to failure conditioned by the wood fibres as seen by the high WFP values measured in this adhesive. In general, load-displacement curves have similar shapes, both within each adhesive and between different adhesives. 3.3. Moisture content It was observed that some boards (7 in 109) had moisture content above the defined limits for untreated timber (15%). Despite
Fig. 8. Shear strength and Wood Failure Percentage results for PRF and MUF adhesives (individual values).
Fig. 10. Typical load vs displacement curves of glue line shear strength tests for PRF and MUF adhesives.
Fig. 9. Shear strength and Wood Failure Percentage results for 1C-PUR and EPI adhesives (individual values).
Fig. 11. Typical load vs displacement curves of glue line shear strength tests for EPI and 1C-PUR adhesives.
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-
-
Fig. 12. Mean values of delamination after the 2nd cycle as a function of CAT.
the maximum value recorded was 16.1% this higher moisture content could not be related with higher delamination. The mean values of delamination were always well below the standard limits even if one of the lamellas had moisture content above 15%.
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sive (1C-PUR) was not efficient. The high content of resin of Maritime pine boards can be pointed out as the reason for that. However, an inadequate application of the primer may also result in excessive delamination, as observed; EPI adhesive systems gave inconsistent results regarding delamination. EPI-1 did not prove suitable to glue Maritime pine. On the other hand, a good performance was achieved with EPI-2 in two of four scenarios; Lamellas with growth rings displayed tangential to glue lines are more susceptible to present delamination if located adjacent to each other; Regarding the glue line shear strength similar values were obtained for all adhesives. Generally, the minimum requirements were accomplished for all adhesives systems. However lower values of WFP were observed with 1C-PUR and EPI-1 adhesives. These two adhesives produced also some glue lines that did not meet the minimum requirements established in EN 14080 [1]; A clear relation between low values of WFP and inadequate performance according to delamination tests was found.
3.4. Closed Assembly time (CAT) The main production variables considered within the present study were the amount of adhesive and the clamping pressure. However, in some of the 50 glulam beams produced the CAT varied, for the same amount of adhesive and clamping pressure. For those beams it was checked if the CAT have influenced delamination performance (Fig. 12). The results point out a trend for a general decrease of delamination with the increase of CAT from approximately 5 min (minimum recommended value) to 10 min or 15 min. The only exception was PRF-2 adhesive where the mean values of delamination doubled with the increase of CAT from 10 min to 14 min. The decrease of delamination with the increase of CAT was also observed by Knorz et al. [22] who studied the effect of CAT (between 15 min and 120 min) on Ash (Fraxinus excelsior L.) with PRF and different MUF adhesives. However, different trend was observed by Bourreau et al. [23] which have obtained better results of delamination for 5 min of CAT as compared to the ones obtained for 15 min when gluing several tropical hardwoods with a PRF adhesive. Nevertheless, those authors also found that 20 min of CAT gave less delamination when compared with 10 min of CAT. Regarding shear strength values no influence of CAT was observed on the results. 4. Conclusions The bonding performance of Maritime pine grown in Portugal was the focus of the present study. Four adhesive systems were considered with a total of eight different adhesives. A total of 50 glulam beams of 1 m length and 4 lamellas each with 30 mm thickness were glued. The bonding performance was assessed in terms of delamination and shear strength. The main conclusions obtained from the present work are listed below. - An excellent bonding performance was observed with all adhesive systems at least for one of the studied scenarios (combination of the amount of adhesive and clamping pressure); - Generally, PRF adhesives gave consistent results, the best results being obtained for PRF-3 adhesive; - MUF adhesives are a good alternative to the well-established PRF adhesives. In comparison to PRF adhesives, the disadvantage of MUF (100/20 mix ratio) can be its longer pressing time; - 1C-PUR + PR adhesive (involving a primer developed for gluing Blue gum timber), presented the lowest delamination of all adhesives considered in the study. It was clear that gluing Maritime pine without the application of a primer with this adhe-
Declaration of Competing Interest None. Acknowledgements This work was partly financed by FEDER funds through the Competitiveness Factors Operational Programme – COMPETE and by national funds through FCT – Foundation for Science and Technology, within the scope of the Project POCI-01-0145-FEDER-007633 as well as, by the Operational Program Competitiveness and Internationalization R&D Projects Companies in Co-promotion, Portugal 2020, within the scope of the project OptimizedWood – POCU-010247-FEDER-017867. The authors wish to thank Foundation for Science and Technology for the PhD grant (PD/BD/52656/2014) given to Carlos Martins, to Dynea AS and Henkel for providing the adhesives and finally to Pedrosa Irmãos for the collaboration given concerning the timber. References [1] CEN (2013) EN 14080 – Timber structures – Glued laminated timber and glued solid timber – Requirements. Brussels. [2] C. Martins, A. Dias, Bending strength and stiffness of portuguese maritime pine utility poles forest, Prod. J. 62 (2012) 114–120. [3] T.F.M. Morgado, A.M.P.G. Dias, J.S. Machado, J.H. Negrão, A.F.S. Marques, Grading of portuguese maritime pine small-diameter, Roundwood J. Mater. Civ. Eng. 7 (2016), https://doi.org/10.1061/(ASCE)MT.1943-5533.0001721. [4] T.F.M. Morgado, A. Dias, J.S. Machado, J.H. Negrao, Structural connections for small-diameter poles, J. Struct. Eng. 139 (2013) 2003–2009, https://doi.org/ 10.1061/(asce)st.1943-541x.0000752. [5] C. Martins, A.M.P.G. Dias, R. Costa, P. Santos, Environmentally friendly high performance timber-concrete panel, Constr. Build. Mater. 102 (2016) 1060– 1069, https://doi.org/10.1016/j.conbuildmat.2015.07.194. [6] C. Martins, P. Santos, P. Almeida, L. Godinho, A. Dias, Acoustic performance of timber and timber-concrete floors, Constr. Build. Mater. 101 (2015) 684–691, https://doi.org/10.1016/j.conbuildmat.2015.10.142. [7] C. Martins, P. Santos, P. Almeida, L. Godinho, A.M.P.G. Dias, Modal frequencies of a reinforced timber-concrete composite floor: testing and modeling, J. Struct. Eng. 141 (2015), https://doi.org/10.1061/(ASCE)ST.1943541X.0001275. [8] J. Skinner, C Martins, J. Bregulla, R. Harris, K. Paine, P. Walker, A.M.P.G. Dias (2014) Concrete upgrade to improve the vibration response of timber floor Structures and Buildings 167. [9] P.M. Pontífice de Sousa, Structures of Glued Laminated Timber, LNEC, Feasibility of using Maritime pine, 1990 (in Portuguese). [10] J. Franco da Costa, Application of Maritime pine species in glued structures, Bonding Feasibility, LNEC, Lisboa, 1978 (in Portuguese). [11] H. Cruz, Application of Maritime pine species in glulam structures in Portuguese, Bonding Tests for External Environmental Conditions, LNEC, Lisboa), 1985.
C. Martins et al. / Construction and Building Materials 223 (2019) 520–529 [12] F. Gaspar, H. Cruz, A. Gomes, L. Nunes, Production of glued laminated timber with copper azole treated maritime pine, Eur. J. Wood Wood Prod. 68 (2009) 207–218, https://doi.org/10.1007/s00107-009-0373-6. [13] N. Lampreia, Use of adhesion primers for glued laminated timber production with Maritime pine (in Portuguese), Instituto Superior de Engenharia de Lisboa (2010). [14] Y. Jiang, J. Schaffrath, M. Knorz, S. Winter, in: Bonding of Various Wood Species – Studies about Their Applicability in Glued Laminated Timber, Rilem Bookseries, 2014, pp. 365–374. doi:Doi 10.1007/978-94-007-7811-5_33. [15] J.G. Ferreira, H. Cruz, R. Silva, Failure behaviour and repair of delaminated glulam beams, Constr Build Mater 154 (2017) 384–398, https://doi.org/ 10.1016/j.conbuildmat.2017.07,200. [16] C. Lehringer, J. Gabriel, Review of Recent Research Activities on OneComponent PUR-Adhesives for Engineered Wood Products, in: S. Aicher, H. W. Reinhardt, H. Garrecht (Eds.), Materials and Joints in Timber Structures: Recent Developments of Technology, RILEM Bookseries. Springer, Dordrecht, 2014, pp. 405–420. doi:10.1007/978-94-007-7811-5_37. [17] K. Grostad, R. Bredesen, EPI for Glued Laminated Timber, in: S. Aicher, H.W. Reinhardt, H. Garrecht (Eds.), Materials and Joints in Timber Structures: Recent Developments of Technology, RILEM Bookseries Springer, Dordrecht, 2014, pp. 355–364. doi:10.1007/978-94-007-7811-5_32.
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[18] CEN (2002) EN 13183-1 – Moisture content of a piece of sawn timber – Part 1: Determination by oven dry method. Brussels. [19] D. Ohnesorge, K. Richter, G. Becker, Influence of wood properties and bonding parameters on bond durability of European Beech (Fagus sylvatica L.) glulams, Ann. Forest Sci. 67 (2010), https://doi.org/10.1051/Forest/2010002. [20] F. Lopez-Suevos, K. Richter, Hydroxymethylated Resorcinol (HMR) and Novolak-Based HMR (n-HMR) primers to enhance bond durability of eucalyptus globulus Glulams, J. Adhes. Sci. Technol. 23 (2009) 1925–1937, https://doi.org/10.1163/016942409x12508517390914. [21] C.B. Vick, Adhesive Bonding of Wood Materials – Chapter 9, Wood Handbook – Wood as an engineering material, Forest Products Laboratory, Madison, 1999. [22] M. Knorz, M. Schmidt, S. Torno, J.W. van de Kuilen, Structural bonding of ash (Fraxinus excelsior L.): resistance to delamination and performance in shearing tests, Eur. J. Wood Wood Prod. 72 (2014) 297–309, https://doi.org/ 10.1007/s00107-014-0778-8. [23] D. Bourreau, Y. Aimene, J. Beauchene, B. Thibaut, Feasibility of glued laminated timber beams with tropical hardwoods, Eur. J. Wood Wood Prod. 71 (2013) 653–662, https://doi.org/10.1007/s00107-013-0721-4.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Bonding performance of Portuguese Maritime pine glued laminated timber Carlos Martins a,⇑, Alfredo M.P.G. Dias a, Helena Cruz b a b
Institute for Sustainability and Innovation in Structural Engineering, Department of Civil Engineering, University of Coimbra, Coimbra, Portugal Structures Department, LNEC – National Laboratory for Civil Engineering, Lisbon, Portugal
h i g h l i g h t s Evaluation of the bonding performance of Portuguese Maritime pine. Delamination and shear strength tests. Performance of different adhesives for structural applications. Influence of the amount of adhesive and clamping pressure on bonding performance.
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 5 April 2019 Accepted 18 June 2019
Keywords: Glued laminated timber Maritime pine Bonding performance Delamination tests Shear strength tests Adhesives
a b s t r a c t The use of timber for construction purposes in Portugal has decreased during the last century. However, recent market requests aiming at sustainable constructions led to a rediscovery of timber structures. Glued laminated timber is one of the most used timber construction products and its production using Portuguese Maritime pine (Pinus pinaster Ait.) has been studied and validated concerning its economic and mechanical performance. This paper aims to present a state of the art of the research conducted so far on the use of Maritime Pine for glulam as well as new research conveying bonding with more recent adhesives. The ongoing study comprised an extended experimental campaign on gluing Maritime pine timber grown in Portugal, for glulam production. Eight different adhesives were considered, namely based on: i) Phenol-Resorcinol-Formaldehyde (PRF), ii) Melamine-Urea-Formaldehyde (MUF), iii) EmulsionPolymer-Isocyanate (EPI) and iv) Single Component Polyurethane (1C-PUR). The amount of adhesive used and the clamping pressure were considered as main variables. Delamination specimens were tested according to Method A of EN 14080 [1] and shear strength tests followed Annex D of the same standard. In terms of delamination it was observed an excellent performance of all PRF adhesives. MUF, EPI and 1C-PUR adhesives presented good bonding performance, however, it is important to emphasize the need to use primer (PR) in the case of 1C-PUR adhesive. The shear strength was not influenced significantly by the adhesive type but some differences were found on Wood Failure Percentage for EPI and 1C-PUR adhesives. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Maritime pine is the dominant softwood produced in Portugal, corresponding to 23% of 3.1 million ha of forests. Despite its large availability, the main actual uses of Maritime pine timber are still related with non-structural purposes like pallets, pellets, panels and flooring. During the last decades, several studies have shown
⇑ Corresponding author at: ISISE, Departamento de Engenharia Civil, Rua Luís Reis Santos – Pólo II da Universidade de Coimbra, 3030-788 Coimbra, Portugal. E-mail addresses: [email protected], [email protected] (C. Martins). https://doi.org/10.1016/j.conbuildmat.2019.06.143 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
its high mechanical properties allowing it to be used for structural purposes [2–4]. Moreover, its application as small diameter round wood was evaluated for timber-concrete floors, in terms of mechanical behavior [5], acoustics [6] and dynamics [7] and [8]. Glued laminated timber (glulam) is the most common engineered timber based product and it has been increasingly used by the Portuguese construction market mainly for roofs, floors and footbridges. However, the majority of the glulam used in this country is imported as final product or produced with imported raw-material, mostly Spruce (Picea abies) or Scots pine (Pinus sylvestris).
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Portuguese Maritime pine has also been proved to be a good option for glulam production for economic reasons [9] and mechanic behavior [10–11] and [9], used as untreated timber. Maritime pine is also recognised by EN 14080 [1] as a species suitable for glulam production. Gaspar et al. [12] pursued studies on gluing Maritime pine with PRF adhesives, assessing the influence of clamping pressure and cure temperature, both for untreated and preservative treated timber (two levels of retention). These authors observed that higher cure temperatures (45 °C) lead to less delamination, especially in the case of treated timber, as well as lower clamping pressures (0.6 MPa). Since ensuring 45 °C of curing temperature implies higher costs and is not ergonomic for labours, Lampreia [13] studied the influence of using a novolak based primer prior to adhesive application (PRF) for both untreated and treated timber. Despite a slight decrease of delamination when using the primer, the results obtained with the new adhesive formulation without primer also met the standard requirements. During the last two decades several new and improved adhesives came into the market. PRF and RF adhesives are still available and used for glulam, but nowadays their market share is lower than in the past, mainly due to their brown colour which is often seen as a disadvantage in aesthetical terms [14]. MUF adhesives have also been available for many years. It is known the use of a MUF adhesive by a Portuguese glulam company and some studies have been made regarding the mechanical behaviour of their beams using Maritime pine [15]. According to Lehringer and Gabriel [16] another adhesive with a significant share of engineered wood products is one-component polyurethane adhesive (1C-PUR), that was approved by German building authorities in 1994. The most recent adhesive system used for glulam is EPI. Grostad and Bredesen [17] refer a specific formulation that has been approved for gluing load bearing elements of Maritime pine (in France). The main advantage of EPI and 1C-PUR adhesives is the absence of formaldehyde release. Several studies were published regarding Maritime pine gluability with PRF and RF adhesives. However, there is still limited knowledge about how more recent adhesives behave and the best production parameters to achieve a reliable performance concerning delamination and shear strength. Thus, the present paper has the main goal to study the bonding performance of several commercially available adhesives used on untreated Maritime pine species. For that purpose, eight adhesives of four different types were considered: i) three PRF adhesives, ii) two MUF adhesives, iii) two EPI adhesives and iv) one 1C-PUR adhesive. Their Technical Data Sheets (TDS) were used as guide, together with some advices from the adhesive’s suppliers. Production parameters like the amount of adhesive and the clamping pressure were considered as variables. The influence of moisture content (MC) and Closed Assembly Time (CAT) were also observed through the results obtained. Delamination and shear strength tests were carried out to assess the bonding performance following the procedures of EN 14080 [1].
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board. MC was also measured with an electrical moisture meter according to Annex G of EN 14080 [1]. After planning each board, small clear specimens were collected to determine both the density and MC, by the oven dry method according to EN 13183-1 [18], to check specifications of EN 14080 [1]. The mean value and standard deviation were 13.9 ± 0.6% for MC and 631 ± 58 kg/m3 for density at 12% MC. These confirmed MC readings done with an electrical moisture meter (the mean value and standard deviation of MC were 13.3 ± 1.3%). 2.2. Bonding parameters and glulam beams preparation The bond quality depends on the procedure followed by the manufacturers during the gluing process. It is well documented in the literature that for the same specific adhesive, different wood species may present different performance [14]. The present study assessed the performance of four types of adhesives in a total of eight different adhesives: PRF-1; PRF-2; PRF-3; MUF-1; MUF-2; EPI-1; EPI-2, 1C-PUR and 1C-PUR + PR. In the case of MUF-1, two different ratios resin/hardener were considered (100/20 and 100/35) and 1C-PUR was considered without and with a primer (1C-PUR + PR) since previous work on Beech (Fagus sylvatica L.) [19] and Blue Gum (Eucalyptus globulus Labill.) [20] showed the benefits of using primer to obtain significant reduction of delamination. The primer used for bonding Maritime pine in the present study was developed specifically for bonding Blue gum. Each adhesive was evaluated in four different scenarios, by combining two fabrication variables: i) amount of adhesive (g/ m2) and ii) clamping pressure applied (MPa). Table 1 provides information relative to the adhesives specifications as well as the gluing parameters adopted. The study comprised a total of 50 glulam beams. Each glulam beam consists on 4 lamellas with 30 mm thickness each and 1 m long. The lamellas were sorted in order to have the top layer with the lowest density (usually below 600 kg/m3) and the bottom layer with the highest density (usually around 700 kg/m3). Intermediate density lamellas formed the inner layers (between 600 and 700 kg/ m3). The boards were selected in a way that the mean density of each glulam beam was close to the mean density of the sample. The lamellas were displayed in order to have the pith facing outwards, as recommended for Service Class 3 [1]. Prior to adhesive application (<6 h) the lamellas were planned to the target thickness (30 mm). The entire gluing and curing process occurred in a room with 20 ± 2 °C temperature and 65 ± 5% relative humidity. A manual spreader was used to uniformly distribute the adhesive and the amount applied was controlled by weighing. The primer – when considered – was sprayed uniformly over each mating surface. The amount applied (20 g/m2) was controlled by weighing. As recommended by each specific TDS the Open Assembly Time (OAT) was as shorter as possible and the CAT was followed. The pressing process (two glulam beams at the same time) was ensured by a series of hydraulic jacks spaced 500 mm and with maximum load capacity of 50 kN each (Fig. 1). After the pressing time, glulam beams were keep in the same controlled conditions during at least 7 days, prior to testing.
2. Materials and methods
2.3. Delamination tests
2.1. Wood
From each glulam beam several specimens of 75 110 120 mm3 (length width height) were cut to evaluate the resistance to delamination according to Method A from Annex C of EN 14080 [1]. Delamination test imposes internal stresses perpendicular to the glue lines between laminations through the introduction of a gradient of moisture into the specimens. The test procedure started by soaking the specimens with water in an autoclave (Fig. 2), and applying a vacuum period of 5 min
Maritime pine boards were collected from a region near Leiria in Portugal. A total of 109 boards of untreated Maritime pine were prepared. All boards were stored at 20 ± 2 °C temperature and 65 ± 5% relative humidity. Visual grading was performed, together with measuring width, thickness, length and weigh to determine the density of each
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Table 1 Summary of variables considered for gluing process. Adhesive system
PRF-1
Viscosity of resin (mPas) Viscosity of hardener (mPas) Assembly time (min) Amount of adhesive (g/m2) Resin/Hardener Clamping pressure (MPa) Pressing time (h)
5000–10000 5000–8000 701 501 350/450 350/450 100/20 100/20 0.6/0.8 0.6/0.8 4 3
Note: 1 Assembly 2 Assembly 3 Assembly 4 Assembly 5 Assembly 6 Assembly
time time time time time time
with with with with with with
PRF-2
PRF-3
MUF-1
MUF-2
EPI-1
EPI-2
1C-PUR
1C-PUR + PR
400–1500 4500–6000 451/ 602 350/450 100/20 0.6/0.8 4
3000–3500 3000 1503 400/500 100/20–100/35 0.6/0.8 9/4.5
3000–10000 3000–4000 1204 350/450 100/20 0.6/0.8 8
5000–10000 20–100 155 250/350 100/15 0.6/0.8 0.33
6000–10000 250–400
24 000 – 706 160/180 – 0.6/0.8 3
160/180 – 0.6/0.8 6
300/350 100/15 0.6/0.8 2
350 g/m2 at 20 °C/65% RH. 450 g/m2 at 20 °C/65% RH. 400–500 g/m2 at 20 °C/65% RH. 400 g/m2 at 20 °C/65% RH. 250 g/m2 at 20 °C/65% RH. 160–180 g/m2 at 20 °C/65% RH.
results of delamination are summarized and discussed further in detail.
2.4. Shear tests
Fig. 1. Glulam beams of Maritime pine under pressure (PRF adhesive).
(15–30 kPa absolute pressure) followed by a pressure period of 60 min (600–700 kPa absolute pressure). One complete cycle consists of two vacuum-pressure period, each one followed by a drying period of 21–22 h at a drying duct (Fig. 2) where the air circulates at 2 m/s–3 m/s at a temperature between 60 and 70 °C and a relative humidity below 15%. Method A establishes that the delamination limit after the 2nd cycle is 5% and an additional cycle (3rd) can be performed if the delamination measured is higher, the limit after the 3rd cycle being equal to 10%. In the present study three cycles of delamination were performed for all specimens. The
Shear strength test was performed according to Annex D of EN 14080 [1] and consists in the application of a shear load parallel to the glue line until failure, in a time not less than 20 s. The specimens were approximately 50 50 120 mm3 (width thickness length) and a displacement control with a rate of 0.06 mm/s was imposed. A hydraulic actuator of 50 kN maximum capacity was used to apply the loads under the displacement control. A device (Fig. 3) with a self-aligning part was used so that the test piece is uniformly loaded at the end grain, as specified in EN 14080 [1]. The glue line shear strength was measured by applying shear at a maximum distance of 1 mm from the glued joint plane; timber shear strength was measured approximately at half of thickness of each lamella. After testing, the wood failure percentage (WFP) of each glue-line was also measured using a transparent square mesh in which each square mesh corresponds to 1%. For white colour adhesives a mixture of Phloroglucinol (2%) with Chloridric acid solution (10%) was used to distinguish the adhesive failure from the wood failure (Fig. 4). Both faces were analysed and wood failure was considered when both zones presented wood fibres. Individual and mean values were assessed. According to
Fig. 2. Delamination test devices: left) autoclave, right) specimens displayed at drying duct.
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Fig. 3. Device used for shear strength tests.
the standard, an individual value of 4 MPa is acceptable if the corresponding WFP is 100%, whereas the minimum acceptable mean value is 6 MPa combined with 90% of WFP. 3. Results and discussion 3.1. Delamination test results Results from delamination tests of all adhesives are summarized in Table 2 as well as detailed information about the two variables considered in the present study (amount of adhesive and clamping pressure). Also the closed assembly time (CAT), the mean value of density and the respective number of specimens tested are presented. 3.1.1. PRF adhesives Results (Table 2) show that all PRF adhesives have a similar performance both concerning the mean and maximum delamination values after the 2nd and 3rd test cycles. It is also observed that the total delamination (mean and individual) values are lower than the specified limits for Service Class 3 (Method A). The best performance was achieved with PRF-3 – presenting mean values of 0.5% after the 2nd cycle and 0.6% after the 3rd cycle – for 0.8 MPa combined with 450 g/m2. In the present study it was observed that different adhesives of the same type (PRF) showed different behaviour for the same spe-
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cies. PRF-1 and PRF-2 (same resin but different hardeners) had similar behaviour, e.g., an increment of delamination for both mixtures was observed with an increase of the amount of adhesive (doubled the values in case of PRF-2) with both levels of clamping pressure. On the other hand, keeping the same amount of adhesive and increasing the clamping pressure produced different results being observed an increase of delamination for PRF-1 and a decrease of delamination for PRF-2. The most significant advantage of using PRF-2 is that it requires less 1 h of pressing time compared to PRF-1. Delamination observed on specimens glued with PRF-3 evidenced a different trend compared to the other two PRF adhesives. The increased amount of adhesive lead to a significant decrease of delamination (in both levels of clamping pressure). Increasing the clamping pressure did not produce significant changes of delamination. The main advantage of PRF-3 in relation to the other PRF adhesives is its longer storage life, 18 months for the resin and 36 months for hardener if stored at 20 °C. Slightly lower values of delamination were found by Lampreia [13] when gluing Maritime pine with the same PRF-1 in the same amount and using the same clamping pressure, namely 0.8% and 1.3% after the 2nd and 3rd cycles, respectively. Nevertheless these results were quite similar to the best results achieved in the present study with PRF-1, that is 0.9% and 1.0% after the 2nd and 3rd test cycles, respectively. 3.1.2. MUF adhesives A comparison between both MUF adhesives revealed some differences in terms of delamination. In the case of MUF-1 and for the higher clamping pressure (0.8 MPa) the mean delamination hardly varied with the amount of adhesive. For the lower level of clamping pressure, a significant increase of delamination was observed by increasing the amount of adhesive (three times more). Some specimens glued with MUF-1 adhesive showed excessive delamination after the 2nd cycle but none of them had delamination above 10% after the 3rd test cycle. The common scenarios considered a ratio between resin and hardener of 100/20 which required a total of 9 h pressing time. The TDS of the MUF-1 adhesive presents also different ratios that require less pressing time. This could be important for saving energy and increasing the production of a company providing a good bonding performance is guaranteed. Therefore, two glulam beams were produced (14 delamination specimens) and tested to assess the bonding behaviour for a ratio of 100/35, thus reducing
Fig. 4. Interface between lamellas after glue line shear strength test for WFP analysis.
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Table 2 Summary of delamination test results (mean and maximum values of total delamination).
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Table 2 (continued) Note: CAT is the closed assembly time registered. Bold values mean that more than one specimen had delamination in excess of the respective acceptable limit. Underlined values mean that one specimen had delamination higher than the respective limit. Highlighted values means that the values of delamination did not accomplish the limits. * means that the adhesive mixture had a ratio of 100/35. ** means that the adhesive mixture had a ratio of 100/15.
the pressing time to 4.5 h. The mean value of delamination was significantly higher (approximately 3 times) when compared to delamination measured on specimens glued with 100/20 ratio (equal amount of adhesive and the same clamping pressure). MUF-2 results show mean values of delamination (for all scenarios) lower than 1.4% and 1.9% after the 2nd and 3rd test cycles, respectively. Its behaviour was different from the previously discussed adhesives. The increase of adhesive amount produced an increase of delamination for 0.6 MPa, however for 0.8 MPa the delamination decreased. For the amount of adhesive equal to 350 g/m2 a higher clamping pressure lead to higher delamination whereas for 450 g/m2 a decrease of delamination was registered. In conclusion, a good bonding performance was obtained with both MUF adhesives when using an intermediate amount of adhesive and lower clamping pressure level with 100/20 formulation ratio. Comparing MUF with PRF adhesives a similar behaviour is observed, especially with MUF-2 adhesive. A qualitative analysis shows the MUF adhesive to be more interesting due to the white colour of glue lines. On the other hand, its extended pressing time could be a disadvantage related with energy consumption and production yields.
3.1.3. EPI adhesives Regarding EPI adhesives, it was clearly observed that EPI-1 is not adequate for bonding Maritime pine, as the mean values of delamination were higher than 10% (established limit for the 3rd test cycle) already after the 2nd cycle. Despite that there was a decrease of delamination with the increase of the amount of adhesive. Also a significant decrease was observed with the increase of clamping pressure for the lower amount of adhesive tested. Regarding the EPI-2 adhesive mixture a different performance was observed. For the lower clamping pressure (0.6 MPa) the use of this adhesive did not meet the established limits after the 2nd test cycle of delamination tests and after the 3rd test cycle the mean values were slightly higher or close to 10%. On the other hand, promising results were found for 0.8 MPa with a significant increase on its delamination performance, bringing the mean values of delamination below the maximum allowed for both cycles.
This tendency of decreasing the delamination with the increase of pressure was already observed with EPI-1 adhesive. Nevertheless, for 350 g/m2 combined with 0.8 MPa one of the specimens presented excessive delamination after both cycles being evident the presence of tangential growth rings in the adjacent lamellas (both faces) (Fig. 5). The influence of the tangential growth rings was observed also on the remaining specimens from all elements glued with EPI adhesives. The short pressing time required for EPI adhesives is an advantage comparing with the other adhesives considered within the present study.
3.1.4. 1C-PUR adhesives Also the use of 1C-PUR adhesive was considered in the present study. The analysis of the results show that none of the considered scenarios of 1C-PUR (without primer) resulted in a reliable performance of bonded untreated Maritime pine. However, a trend was found showing that the increase of clamping pressure produced a significant decrease of delamination (approximately 10%). As this adhesive has a significant share of the European market both for glulam and CLT (Cross Laminated Timber), it was decided to use a primer specifically developed for Blue gum timber by the same producer. It was found that the combination 1C-PUR + PR results in a significant improvement concerning delamination that was satisfactory. The benefits of using primer have been demonstrated by Lopez-Suevos and Richter [20] for Blue gum glulam. All four scenarios presented mean delamination values below 2.4% and 3.0% after the 2nd and 3rd test cycles, respectively. The trend identified before with 1C-PUR, regarding the increase of amount of adhesive, was also evident for 0.6 MPa when the primer was used. The increase of clamping pressure also conducted to a reduction of delamination in both amount of adhesive values. From a total of 56 specimens collected from the elements glued with 1CPUR + PR only one specimen had excessive delamination after both delamination cycles. A detailed analysis of that specimen (located at one end of the glulam beam) showed that the lack of primer at the lower glue line combined with the presence of tangential growth rings adjacent to the glue line was probably the reason for the excessive delamination registered. Fig. 6 illustrates both
Fig. 5. Delamination observed coincident with tangential growth rings located in the adjacent lamellas (EPI-2 adhesive: 350 g/m2 and 0.8 MPa).
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Fig. 6. Delamination observed at the specimen D7 with excessive delamination caused by lack of primer combined with tangential growth rings adjacent to glue line.
Fig. 7. Delamination observed at the specimen D6 (1C-PUR + PR adhesive) with presence of tangential growth rings adjacent to glue line.
Table 3 Summary of shear strength test results on glue lines and timber. Adhesive
PRF-1
Amount of adhesive (g/m2)
Clamping pressure (MPa)
Density (kg/m3)
Glue line N° of specimens
Shear strength (std. dev.) (MPa)
WFP (%)
N° of specimens
Shear strength (std. dev.) (MPa)
350
0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8 0.6 0.8
647 642 653 631 647 653 650 646 649 655 659 644 654 656 652 649 661 636 647 656 640 649 645 653 651 644 635 644 634 666 641 659 634 655 643 645 643
22 84 24 26 60 28 28 29 27 22 27 60 84 27 29 29 57 26 23 52 27 28 27 25 24 29 22 26 22 29 44 25 25 52 51 55 50
13.8 13.5 14.7 13.7 14.6 14.0 14.1 14.5 13.4 13.9 13.6 14.4 13.9 14.2 13.0 14.8 13.2 13.3 12.3 13.1 11.7 12.5 12.2 11.7 13.0 12.5 12.8 13.1 12.9 11.6 13.5 11.4 13.8 14.1 13.7 13.7 13.5
99 95 98 97 99 99 99 99 99 99 99 100 96 98 98 99 95 94 93 93 91 83 91 74 79 86 96 89 94 71 87 61 80 89 93 93 93
15 44 16 14 32 15 16 12 16 14 15 30 47 16 15 15 30 13 16 28 15 14 12 16 14 14 16 14 14 15 29 15 16 31 23 30 30
14.9 14.2 15.1 14.1 14.6 14.0 15.1 14.5 14.2 15.5 13.5 15.1 13.3 15.1 15.0 14.3 13.8 14.3 12.8 13.7 12.4 15.7 15.6 15.1 14.5 14.7 14.6 14.2 13.6 14.3 14.4 14.0 15.0 15.0 14.3 15.5 14.4
450 PRF-2
350 450
PRF-3
350
MUF-1
400
450
500
MUF-2
400* 350 450
EPI-1
250** 350**
EPI-2
300**
1C-PUR
160
350**
180 1C-PUR + PR
160 180
Note: * means that the adhesive mixture had a ratio of 100/35. ** means that the adhesive mixture had a ratio of 100/15.
Timber
(1.5) (1.3) (2.0) (1.1) (1.1) (2.0) (1.4) (2.0) (1.8) (1.7) (1.3) (1.4) (1.2) (1.5) (1.1) (1.2) (1.6) (1.5) (1.4) (1.6) (1.0) (1.4) (1.9) (2.7) (1.9) (1.3) (1.2) (1.0) (0.9) (1.9) (1.4) (1.5) (1.2) (1.6) (1.0) (1.4) (1.1)
(0.9) (1.2) (1.3) (0.8) (1.5) (1.3) (1.2) (1.7) (1.9) (1.5) (1.8) (1.7) (1.2) (2.0) (1.6) (1.2) (0.9) (2.7) (1.3) (1.7) (1.5) (2.4) (1.9) (1.2) (1.8) (1.8) (2.1) (1.6) (1.8) (1.8) (1.1) (2.4) (1.5) (1.4) (1.6) (1.8) (1.9)
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faces of the specimen after the 3rd delamination test cycle whereas in Fig. 7 is illustrated an inner element collected from the same element and significantly lower delamination. The lowest delamination between all adhesives was obtained for 1C-PUR + PR (180 g/m2 and 0.8 MPa) with mean values of 0.1% and 0.4% after the 2nd and 3rd test cycles, respectively. The use of primer has the disadvantage of doubling the pressing time compared with 1C-PUR adhesive only. 3.2. Shear strength test results Shear strength results are summarized in Table 3 with detailed information on the production variables considered within the present study. Mean and standard deviation values are listed both for glue lines and timber shear strength as well as WFP values measured on glue lines. Fig. 8 presents the individual values of shear strength and WFP of glue line tests from specimens glued with PRF and MUF adhesives whereas Fig. 9 presents the individual test results of specimens glued with 1C-PUR and EPI adhesives. As Maritime pine is denser than the most common softwoods used for glulam production (Spruce and Scots pine) higher values of shear strength were expected as referred by Vick [21]. Slightly higher values of shear strength were obtained on timber as compared to glue line values. In the previous study done with PRF adhesives by Lampreia [13], similar values of shear strength on glue lines were found (13.4 MPa). Generally, the shear strength values were quite similar for all adhesives with the exception of both EPI and 1C-PUR adhesives that have shown slightly lower values but still around 12 MPa. Regarding the WFP, it was clear that PRF and MUF adhesives provided excellent results. EPI-1 glue lines had lower strength which
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is in line with the previously mentioned inadequate performance observed through delamination tests. As the delamination performance improved using EPI-2 adhesive, also the shear strength (mean values) and WFP have increased. The same trend was obvious for 1C-PUR + PR adhesive. The mean WFP obtained in the case of 1C-PUR + PR was around 90% having some individual values being the lower WFP clearly related with some absence of primer on the surface during its application. From the individual shear strength results it was clear that only EPI-1 and 1C-PUR had some glue lines (maximum 2 per specimen) that did not meet the minimum required values of shear strength combined with WFP. The use of primer in 1C-PUR + PR significantly improved the glue lines performance, as compared to 1C-PUR, especially regarding the WFP thus meeting the acceptable limits. PRF and MUF adhesives had 100% of glue lines above the limits in all scenarios. By using load and displacement data from the hydraulic actuator, typical load vs displacement curves obtained in shear tests were drawn. Figs. 10 and 11 present typical curves (corresponding to specimens with shear strength closer to the average value) of each adhesive type. These curves show an initial linear elastic behaviour followed by a non-linear behaviour – more evident in the case of MUF-2 adhesive, what is probably related to failure conditioned by the wood fibres as seen by the high WFP values measured in this adhesive. In general, load-displacement curves have similar shapes, both within each adhesive and between different adhesives. 3.3. Moisture content It was observed that some boards (7 in 109) had moisture content above the defined limits for untreated timber (15%). Despite
Fig. 8. Shear strength and Wood Failure Percentage results for PRF and MUF adhesives (individual values).
Fig. 10. Typical load vs displacement curves of glue line shear strength tests for PRF and MUF adhesives.
Fig. 9. Shear strength and Wood Failure Percentage results for 1C-PUR and EPI adhesives (individual values).
Fig. 11. Typical load vs displacement curves of glue line shear strength tests for EPI and 1C-PUR adhesives.
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-
-
Fig. 12. Mean values of delamination after the 2nd cycle as a function of CAT.
the maximum value recorded was 16.1% this higher moisture content could not be related with higher delamination. The mean values of delamination were always well below the standard limits even if one of the lamellas had moisture content above 15%.
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sive (1C-PUR) was not efficient. The high content of resin of Maritime pine boards can be pointed out as the reason for that. However, an inadequate application of the primer may also result in excessive delamination, as observed; EPI adhesive systems gave inconsistent results regarding delamination. EPI-1 did not prove suitable to glue Maritime pine. On the other hand, a good performance was achieved with EPI-2 in two of four scenarios; Lamellas with growth rings displayed tangential to glue lines are more susceptible to present delamination if located adjacent to each other; Regarding the glue line shear strength similar values were obtained for all adhesives. Generally, the minimum requirements were accomplished for all adhesives systems. However lower values of WFP were observed with 1C-PUR and EPI-1 adhesives. These two adhesives produced also some glue lines that did not meet the minimum requirements established in EN 14080 [1]; A clear relation between low values of WFP and inadequate performance according to delamination tests was found.
3.4. Closed Assembly time (CAT) The main production variables considered within the present study were the amount of adhesive and the clamping pressure. However, in some of the 50 glulam beams produced the CAT varied, for the same amount of adhesive and clamping pressure. For those beams it was checked if the CAT have influenced delamination performance (Fig. 12). The results point out a trend for a general decrease of delamination with the increase of CAT from approximately 5 min (minimum recommended value) to 10 min or 15 min. The only exception was PRF-2 adhesive where the mean values of delamination doubled with the increase of CAT from 10 min to 14 min. The decrease of delamination with the increase of CAT was also observed by Knorz et al. [22] who studied the effect of CAT (between 15 min and 120 min) on Ash (Fraxinus excelsior L.) with PRF and different MUF adhesives. However, different trend was observed by Bourreau et al. [23] which have obtained better results of delamination for 5 min of CAT as compared to the ones obtained for 15 min when gluing several tropical hardwoods with a PRF adhesive. Nevertheless, those authors also found that 20 min of CAT gave less delamination when compared with 10 min of CAT. Regarding shear strength values no influence of CAT was observed on the results. 4. Conclusions The bonding performance of Maritime pine grown in Portugal was the focus of the present study. Four adhesive systems were considered with a total of eight different adhesives. A total of 50 glulam beams of 1 m length and 4 lamellas each with 30 mm thickness were glued. The bonding performance was assessed in terms of delamination and shear strength. The main conclusions obtained from the present work are listed below. - An excellent bonding performance was observed with all adhesive systems at least for one of the studied scenarios (combination of the amount of adhesive and clamping pressure); - Generally, PRF adhesives gave consistent results, the best results being obtained for PRF-3 adhesive; - MUF adhesives are a good alternative to the well-established PRF adhesives. In comparison to PRF adhesives, the disadvantage of MUF (100/20 mix ratio) can be its longer pressing time; - 1C-PUR + PR adhesive (involving a primer developed for gluing Blue gum timber), presented the lowest delamination of all adhesives considered in the study. It was clear that gluing Maritime pine without the application of a primer with this adhe-
Declaration of Competing Interest None. Acknowledgements This work was partly financed by FEDER funds through the Competitiveness Factors Operational Programme – COMPETE and by national funds through FCT – Foundation for Science and Technology, within the scope of the Project POCI-01-0145-FEDER-007633 as well as, by the Operational Program Competitiveness and Internationalization R&D Projects Companies in Co-promotion, Portugal 2020, within the scope of the project OptimizedWood – POCU-010247-FEDER-017867. The authors wish to thank Foundation for Science and Technology for the PhD grant (PD/BD/52656/2014) given to Carlos Martins, to Dynea AS and Henkel for providing the adhesives and finally to Pedrosa Irmãos for the collaboration given concerning the timber. References [1] CEN (2013) EN 14080 – Timber structures – Glued laminated timber and glued solid timber – Requirements. Brussels. [2] C. Martins, A. Dias, Bending strength and stiffness of portuguese maritime pine utility poles forest, Prod. J. 62 (2012) 114–120. [3] T.F.M. Morgado, A.M.P.G. Dias, J.S. Machado, J.H. Negrão, A.F.S. Marques, Grading of portuguese maritime pine small-diameter, Roundwood J. Mater. Civ. Eng. 7 (2016), https://doi.org/10.1061/(ASCE)MT.1943-5533.0001721. [4] T.F.M. Morgado, A. Dias, J.S. Machado, J.H. Negrao, Structural connections for small-diameter poles, J. Struct. Eng. 139 (2013) 2003–2009, https://doi.org/ 10.1061/(asce)st.1943-541x.0000752. [5] C. Martins, A.M.P.G. Dias, R. Costa, P. Santos, Environmentally friendly high performance timber-concrete panel, Constr. Build. Mater. 102 (2016) 1060– 1069, https://doi.org/10.1016/j.conbuildmat.2015.07.194. [6] C. Martins, P. Santos, P. Almeida, L. Godinho, A. Dias, Acoustic performance of timber and timber-concrete floors, Constr. Build. Mater. 101 (2015) 684–691, https://doi.org/10.1016/j.conbuildmat.2015.10.142. [7] C. Martins, P. Santos, P. Almeida, L. Godinho, A.M.P.G. Dias, Modal frequencies of a reinforced timber-concrete composite floor: testing and modeling, J. Struct. Eng. 141 (2015), https://doi.org/10.1061/(ASCE)ST.1943541X.0001275. [8] J. Skinner, C Martins, J. Bregulla, R. Harris, K. Paine, P. Walker, A.M.P.G. Dias (2014) Concrete upgrade to improve the vibration response of timber floor Structures and Buildings 167. [9] P.M. Pontífice de Sousa, Structures of Glued Laminated Timber, LNEC, Feasibility of using Maritime pine, 1990 (in Portuguese). [10] J. Franco da Costa, Application of Maritime pine species in glued structures, Bonding Feasibility, LNEC, Lisboa, 1978 (in Portuguese). [11] H. Cruz, Application of Maritime pine species in glulam structures in Portuguese, Bonding Tests for External Environmental Conditions, LNEC, Lisboa), 1985.
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