Moisture conditions in coated glulam beams and columns during weathering

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1 Moisture conditions in coated glulam beams and columns during weathering Karin Sandberg 1 Anna Pousette 2 Lars-Olof Nilsson 3 T22 ABSTRACT Service-life assesment and life-cycle cost are becoming more and more important in investment and planning of construction work. Woodbuild is the name of a Swedish research programme with the goal of developing engineering tools based on models that can be used in the planning and design of timber structures with regard to improved service life.the purpose of the full-scale test is to verify models for prediction of wetting periods on wood surfaces and of moisture conditions dependent on climate loads, finishes, wood processing and design. The objective of the present work was to show results on moisture conditions in glued laminated timber (glulam) beams of Scots pine (Pinus Sylvestris) and Norway spruce (Picea abies) and glulam columns of spruce to demonstrate differences that can be ascribed to different coating types and colours and the influence of the varying positional orientation (north/south), up or down at columns and measurement depth. The glulam beams were 315 mm high, 140 mm, 215 mm or 90 mm wide and 2 m long. The columns had 100 x 100-mm cross-sections. Moisture content, relative humidity and temperature were measured using wireless sensors, measurement depth 5 mm and 15 mm. MC measurements of the beams undersides show small fluctuations over the time irrespective of temperature and measurement depth. MC measurements on the south and north sides of the beams fluctuated more compared to underside. The beam with oil treatment, follow climate changes more quickly compared to the painted ones. MC show small difference depending on the height of the columns. KEYWORDS Wireless sensors, Temperature, Moisture content, Relative humidity, outdoor exposure 1 SP Technical Research Institute of Sweden (SP Trätek), Skellefteå, SWEDEN, karin.sandberg@sp.se 2 SP Technical Research Institute of Sweden (SP Trätek), Skellefteå, SWEDEN, anna.pousette@sp.se 3 Div. of Building Materials, Lund Institute of Technology, Lund, SWEDEN, Lars-Olof.Nilsson@byggtek.lth.se

2 Sandberg, Pousette, Nilsson 1 INTRODUCTION Durability is essential to minimize life-cycle costs for outdoor structures. Degradation of wood is a complex process involving interaction between wood, microorganisms, climate and moisture content, exposure time, surrounding environment, irradiation, temperature, surface treatment, etc. Especially the presence of water in wood causes many problems, since it affects dimensional change through shrinking and swelling, which in turn influence crack formation, decay and strength reduction. To prevent water uptake, and thus reduce dimensional change, wood can be painted. A number of investigations show the importance of protection of endgrain with sealers and paint to reduce moisture uptake [Sell 1982; Öqvist 1988; Boxhall et al. 1992; Elowson et al. 2003]. However, after some year s exposure, the initial effect of protective coatings declined on sapwood. Generally, paint cannot make sapwood as durable as heartwood [Boxhall et al. 1992; Rydell et al. 2005]. Grüll [2010] found that degradation of paint led to decreasing moisture protection from less durable coating systems over time exposure. This means that paint not only protects wood, it might also cause problems if the moisture balance in wood is interfered with or if the painted surface cracks and water gets in, but not out. Outdoor constructions of wood, such as posts, beams and columns, often crack or delaminate, which can result in damage to the coating film and disturbances in the moisture balance of the wood, which in turn can lead to damaged wood and reduced service life. High moisture content can occur locally at and around cracks (Ekstedt 2002). Decay reduces strength and is thus a factor to consider, but other factors such as cracks also influence service life. Large cracks can hold water and dirt that are favorable for the growth of microorganisms. A field test with beams and columns is examining the effect of cracks on the prolongation of the life of structures [Pousette & Sandberg 2010], and this paper reports on a part of that study. Woodbuild is the name of a research program in Sweden with the goal of developing engineering tools that can be used in the planning and design of timber structures with regard to service life. These tools will be based on computational models. Climatic data and exposure conditions will be used as input in the models to calculate moisture content and risk of decay. The purpose of the in situ measurements being done over time is to verify such models for many different situations based on the duration of surface wetting, on moisture conditions dependent on climatic loads, coatings, wood processing and design and on the position of the object inside walls or floors. To verify models or provide a basis for assumptions, continuous measurements are performed on real test items. Moisture content (MC), relativ humidity (RH) and temperature (T) are monitored continuously using a wireless in situ measuring system. In order to measure a large amount of data, a wireless monitoring system was chosen with a gateway connected to a GSM modem that sends the information to an Internet-connected computer database from whence it is transferred to the users [Sandberg et al. 2011]. The objective of this paper is to show results on moisture conditions in glued laminated timber (glulam) beams and columns with different materials and paint coatings, and the influence of the different directional orientations (north/south, up/down) and measurement depth. 2 MATERIAL AND METHODS Measurements were performed on glulam beams and columns under natural weathering conditions in the northeast of Sweden. Moisture content (MC, %), relative humidity (RH, %) and temperature (T, C) were registered every hour with wireless electric resistance sensors and loggers [Sandberg et al. 2011] along with climate data. Associated with these data were measurement depth and orientation (north-/south-/underside). 2.1 Testing ground XII DBMC, Porto, PORTUGAL, 2011

Moisture conditions in coated glulam beams The glulam beams and columns were placed on the testing ground in Bygdsiljum (Lat. 64 20 57 N, Long.

For continuous measurements of MC, RH and temperature, glulam beams were chosen from the field and the measurement started in January 2009.

3 Moisture conditions in coated glulam beams The glulam beams and columns were placed on the testing ground in Bygdsiljum (Lat N, Long E) in the county of Skellefteå in northern Sweden in August September 2007 and are part of a larger test described in Pousette & Sandberg [2007] and Pousette & Sandberg [2010]. For continuous measurements of MC, RH and temperature, glulam beams were chosen from the field and the measurement started in January Climate, temperature and humidity at the test site were measured with a weather station at the test field. The beams were placed on supporting frames at various heights, with the lowest beam approximately 1 m above the ground (see Fig. 1). Five beams were placed on each frame. Beams were placed with one side facing south to receive the warmth of the sun and the other to the shaded and cooler north. The top surface was covered with a sheet of metal that was mounted in such a way as to prevent water from penetrating into the beam from above. Beam faces were exposed to rainfall, sunlight and the surrounding environment. The frames were placed so that they did not shade each other. The columns were fastened to a concrete foundation (see Fig. 1). a) b) Figure 1. a) Measurements at the middle of the glulam beams two meters long, one meter from the ground in the lowest row. Meaurments on the south side, north side and underneath. b) Measurment on the top and bottom of columns two meters heigh. Top surfaces are covered with sheet metal. 2.1 Sample preparation Glulam Beams Glulam beams of pine (Pinus Sylvestris L.) and spruce (Picea abies (L). Karst.) were studied. MC, RH and T were measured. The sizes of the beams were chosen in order to facilitate computer tomograpy (CT) scanning. They can easily be lifted and transported from the test site to the Luleå University of Technology, Campus Skellefteå. Beam width may be relevant to cracking and moisture content, and therefore widths of 90 mm and 215 mm were included as well. Surface treatments were chosen similar to what is used today for many timber bridges in Sweden. The painting was done at the glulam manufacturer. The amount of paint should correspond to a dry-film thickness of 60 µm according to the Swedish standards for construction of wooden bridges, TK Bro [Anon. 2009]. The surface treatments were - oil (raw linseed oil with aliphatic naphta as thinner). - Paint system 1: priming oil + two layers of water-based, alkyd oil-acrylic latex hybrid opaque glazing paint. All surfaces were treated with a water based priming oil, biocide Class 3, before painting. - Paint system 2: priming oil + primer + two layers of topcoat. All surfaces were treated with a solvent-based based priming oil, biocide Class 3, before painting. The primer was solvent-based, semigloss modified alkyd oil. The orginal trial consisted of 35 beams, but for this paper, seven beams were chosen for measurement of MC, RH and temperature. Table 1 shows the characteristics of the specimens. Table 1. Characteristics of glulam beams 2 meters long placed one meter above ground. XII DBMC, Porto, PORTUGAL,

4 Sandberg, Pousette, Nilsson Name Material/dimensions (width x hight) Surface treatment/ Colour [mm] Thinner H59 Pine, pressure treated/140 x 315 Oil/solvent based - H63 Pine, pressure treated/140 x 315 Paint system 1/water based White H69 Pine, pressure treated/140 x 315 Paint system1/ water based Red H73 Spruce/140 x 315 Paint system 1/ water based Red H76 Spruce/140 x 315 Paint system 2/ solvent based Red H81 Spruce/90 x 315 Paint system 1/ water based Red H86 Spruce/215 x 315 Paint system 1/ water based Red Columns Five columns (H51, H52. H53 H54, H55) of solid spruce (Picea abies (L). Karst.) 100 mm x 100 mm, length two meters, were included in the test. The columns were documented with tomographs through the cross section and scanning of the side surfaces before they were placed on the test site. All columns were covered with sheet metal or wood on top. The columns were painted with the same system that was used for the beams, paint system 1 (opaque glazing), red colour. 2.2 Measurement system Documentation at the start included dimensions of beams and columns, visual inspection, measurement of moisture content, measurement of dry paint-film thickness, CT scannning (Siemens SOMATOM AR.T ) through the cross-section and scanning image of the surfaces. The specimens were scanned in an SP Trätek laboratory in Skellefteå with a camera equipped with a sensor line for the best colour reproduction. An LED (light emitted diod) ramp was used to minimize the need to calibrate for uneven illumination. White balance was calibrated against a gray card. Colour was measured with a Minolta Chroma Meter CR-310 to quantitatively describe colour by measuring the reflected colour of the surfaces. The Chroma Meter illuminates the surface and measures the colour of an area of diameter 50 mm. The chosen colour coordinates were expressed in the L * C * h colour space, where L is lightness from 0 (black) to 100 (white), C is the chroma from 0 (grey) to more saturated colour the higher value, and h is the hue angle where 0º is red, 90º is yellow, 180º is green and 270º is blue. The measurements were done on the north and south sides of the specimens. Dry paint-film thickness was measured using a paint thickness gauge, DeFelsko Model 200, that measures with high frequency ultrasound to reflect from surfaces with different densities, in this case the layer between paint and wood. The measurements were done on the norh and south sides of the specimens. Beams and columns were inspected every year during the summer using non-destructive methods. Visual inspections were done of cracks, biological attacks and other visible defects. Measurements of colour and paint thickness were made once a year in the field at the same spot (area). Width, length and depth of cracks were measured, but are not shown in this work. For continuous measurement of MC (%), RH (%) and T ( C), wireless sensors and the OmniSense Facility Monitoring System (FMS) [OmniSense 2010] were used. The sensors send measured values to a gateway connected to a broadband socket or GSM modem that sends the information on to an Internet-connected computer database. Through a website, data were transferred to an SP Trätek (SP Technical Research Institute of Sweden) database. Measured data were then compensated for wood species and temperature [Samuelsson 1990] and transferred to the researchers. The system is decsribed in [Sandberg et al. 2011]. MC, RH and T were measured in the middle of the glulam beams, on the north and south surfaces and on the underside. The measurements were made at the three depths on the surface and 5 mm and 15 mm below the surface at 49 measurement points. XII DBMC, Porto, PORTUGAL, 2011

Moisture conditions in coated glulam beams Continuous measurements of MC, RH and T were performed on the top and bottom on the south sides of the columns on the surface and 5 mm and 15 mm below the

5 Moisture conditions in coated glulam beams Continuous measurements of MC, RH and T were performed on the top and bottom on the south sides of the columns on the surface and 5 mm and 15 mm below the surface at 22 measurement points. 3 RESULTS AND DISCUSSION 3.1 Materials The beams mean MC at the start in 2007 was 13.4 %±1.4. Mean dry paint-film thickness was measured at the start in 2007 and then once a year. The mean dry paint-film thickness was 77 ±7.7, 73 ±5.9 and 77 ±8.2 and 70 ±4.4 µm respectively for the years 2007 to The beam with alkyd paint had a somewhat grainy surface. The columns had a mean MC of 15.3% ±0.9 at the start in 2007 (mean value of four measured points on five columns). The mean dry paint-film thickness was 69 ±17.7, 59 ±7.7 and 60 ±6.9 µm respectively for 2008, 2009 and Table 2 shows the mean value of colour coordinates before and after three years of exposure. Table 2. Mean value of colour coordinates measured on south and north sides. L is lightness, C is chroma, h is the hue angle. No. of specimen Oiled (1) White (1) Red (5) L start C start H start [ ] ± ± ± ± ± ± ± ± ±0.73 L C H [ ] ± ± ± ± ± ± ± ± ±0.42 The results show a small average decrease in paint film thickness and change in colour coordinates during the three years. The method for measuring paint-film thickness can suffice as a method for comparison between the years. The increased value for film thickness on the beams in 2009 may be related to the fact that a different person did the measurements that year. 3.2 MC glulam beams during exposure Fig 2. shows moisture conditions in one glulam beam (H69) during outdoor exposure. Under the beam, the MC condition are rather even during the measured time. Figure 2. MC measured at 5 mm depth on the underside of beam H69. Values measured every hour. The gaps in the graph correspond to values missing as a result of malfuncion of the measuring system. XII DBMC, Porto, PORTUGAL,

6 Sandberg, Pousette, Nilsson Fig. 3 shows measurements on the south side of beam H59. There are larger variation over the year compared to Fig. 2. The beam is an impregnated beam with oil treatment, and it follows climate changes rather quickly. There is a crack about 10 mm below the measured point that may influence the measurements. Figure 3. MC variations over a year in a beam H59 measured once every hour. Gaps in the graph correspond to values missing as a result of malfuncion of the measuring system. Above fiber saturation point (FSP), there are values that showed some dissolution, but these have not been calibrated. Over time, the MC measurements show fluctuations that can be difficult to explain without a thorough investigation. For example, there were very high MC values in December in a winter climate. It was found that for a couple of days, the temperature was above freezing point, and probably the sun melted the snow or frost on the surface for a few hours during those days. After that, the weather turned cold again. Some sensors registered MC above 26% during a long period when temperature was around or below zero, which can be seen in Table 3, footnot 2. Unexpected high MC were found during this period. A possible reason might be measuring failure due to condensation or snow on the device. If there is a natural reason for high MC values during periods with low temperatures or measuring failure must be further investigated. Cracks and delamination occur in the beams and columns during the first year and then increase. There are differences between the beams. For beam H59, the sensor is placed approximately 10 mm under a delamination gap showing on both the north and south sides. On beam H69, the sensor is placed approximately 10 mm above a crack on south side. On beam H86, the sensor is placed approximately 10 mm above a crack on the south side. In this case the cracks break the paint coat and therfore the paint gives limited protection. 3.3 MC in different orientations and measurement depths As shown in Fig. 3, there are fluctuations over the year. The data was treated in the following way. MC below 11% and above 50% have been regarded as errors in measurements or as a result of the sensors not working. These values have been excluded from the statsitics. MC values within the range of 12% and 26% are used in mean-value calculation and standard deviation based on measurements made at night (from 01:00 to 03:00) to avoid influence from the sun. Values above FSP (>26% and <50%) are shown as percent of total measured values. Table 3 shows data from January 2009 until October 1, XII DBMC, Porto, PORTUGAL, 2011

7 Moisture conditions in coated glulam beams Table 3. Mean value of MC measurement (depth 5 mm and 15mm) on glulam beams and columns. Measured from January 2009 until October 1, Measured every night for three hours. H59 H63 H69 H73 H76 H81 H86 South Mean (%) mm SD >FSP (%) North Mean (%) mm SD >FSP (%) Under Mean (%) mm SD >FSP (%) South Mean (%) mm SD >FSP (%) North Mean (%) mm SD >FSP (%) Under Mean (%) mm SD >FSP (%) H51 H52 H53 H54 H55 Down Mean (%) mm SD >FSP (%) Upper Mean (%) mm SD >FSP (%) H69 appeared to have developed a resin pocket next to the sensor, which may have influence measurements made after the summer of MC >26% for a long time during winter when wood is frozen, maybe due to snow or condensation. 3 Many values are missing due to malfunction of the measuring sensor. It should be noted that several of the beams are impregnated and that measurements made using resistive moisture-quotient meters do not compensate for this fact. The only way to measure moisture quotient in impregnated timber is to use the dry-weight method, which is not possible in the present application. Therefore, the measured values can be seen as a comparison of moisture changes over time, rather than absolute values. 4. CONCLUSIONS Exposure of beams and columns outdoors show that MC measurements; - of the beams undersides had small fluctuations over the time irrespective of temperature and measurement depth (5 mm or 15 mm). - on the south and north sides of the beams have larger fluctuations than underside. - on the same beam can differ between the south and north side. -show small difference depending on the height of the columns. Some peaks or deviations were difficullt to explain and can be due to condensation, sensor malfunction, the influence of sunshine, cracks, resin, etc. Future work is to calibrate and verify moisture measurements above FSP. XII DBMC, Porto, PORTUGAL,

8 Sandberg, Pousette, Nilsson ACKNOWLEDGMENTS This project is part of WoodBuild, a research programme within the Sectoral R&D Programme for the Swedish forest-based industry. This Programme is jointly funded by government, industry and other stakeholders with interests related to Swedish forest-based industry. A thanks to TCN (Wood Centre North) for supporting the work. REFERENCES Anon TK Bro. Publikation Vägverket 2009:27, July ISSN (Technical Requirements for Bridges. In Swedish). Boxhall, J., Carey, J. K. and Miller, E. R The effectiveness of end-grain sealers in improving paint performance on softwood joinery, Part 3.: Influence of coating type and wood species on moisture content and fungal colonization. Holz Roh. Werkst. 50: Ekstedt, J Studies on the barrier properties of exterior wood coatings. Doctoral thesis, KTH-Royal Institute of Technology Department of Civil and Architectural Engineering Stockholm, Sweden, Thesis No. TRITA-BYMA 2002:5. Elowson, T., Bergström, M. and Hämäläinen, M Moisture dynamics in Norway spruce and Scots pine during nine years of outdoor exposure above ground in relation to different surface treatments and handling conditions. Holzforschung 57: Grüll, G. Truskaller, M. Podgorski, L. Bollmus, S. De Windt, I. and Suttie, E Moisture conditions in coated wood panels during 18 months natural weathering at five sites in Europe. Proceedings 7th Woodcoatings Congress 2010, PRA Hampton, Amsterdam OmniSense info@omnisense.com Pousette, A. and Sandberg, K Träbalkar och stolpar i utomhusförsök planering och utplacering. SP Rapport 2007:35, SP Technical Research Institut of Sweden (Beams and columns in outdoor tests, planning and placement. In Swedish). Pousette, A. and Sandberg, K Outdoor tests of timber beams and columns. Proc. International Conference on Timber Bridges, ICBT2010, Lillehammer, Norway, September, 2010, pp Rydell, Å. Bergström, M. and Elowsson, T Mass loss and moisture dynamics of Scots pine (Pinus sylvestris L.) exposed outdoors above ground in Sweden. Holzforschung 59: Samuelsson, A Resistanskurvor för elektriska fuktkvotsmätare. TräteknikCentrum, Rapport L , Stockholm. (Resistance curves of electrical moisture-quotient meters. In Swedish). Sandberg, K, Pousette, A. and Dahlquist, S Wireless in situ measurements of moisture content and temperature in timber constructions. XII DBMC, Porto, PORTUGAL, April, 2011 (not yet published). Sell, J Untersuchungen zur Optimerung des Oberflächeneschutzes von Holzbauteilen, Teil 1: Bewitterungsversuche mit Fensterrahmen-Abschnitten. Holz Roh. Werkst. 40: Öqvist, H Utomhusvirkets beständighet. Fältförsök: Ovanjordexponering av Träpaneler. Sveriges lantbruksuniversitet, Institutionen för virkeslära, Rapport nr 204. Uppsala: Sveriges lantbruksuniversitet. XII DBMC, Porto, PORTUGAL, 2011

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