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Vella, R., Heitzmann, M. T., Redman, A., and Bailleres, H. (2019). "Comparison of test methods for the determination of delamination in glued laminated timber," BioRes. 14(4), 7920-7934.

Abstract

Engineered wood products are becoming more prevalent in the residential and commercial construction industries and need to be regulated by standards that ensure their classification and performance requirements. The applicable standard in Australia for the qualification of glued laminated timber is AS/NZS 1328.1:1998 (2011), last revised in 2011. Confusion exists with the specific interpretation of the testing procedures and subsequent methods used for the delamination process. Two methods currently used for product qualification were compared: one using a drying chamber and the other using a standard laboratory dehydrating oven. The trials conducted in accordance with the standard showed a significant difference in glue line delamination between the two testing methods. This was attributed to the lack of humidity and airflow control within the laboratory dehydrating oven and an incomplete drying of the test samples, resulting in different stresses on the glue line. It is evident that the current standard procedure requires clarification to ensure consistency in the test methods and comparable test results.


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Comparison of Test Methods for the Determination of Delamination in Glued Laminated Timber

Rodney Vella,* Michael T. Heitzmann, Adam Redman, and Henri Bailleres

Engineered wood products are becoming more prevalent in the residential and commercial construction industries and need to be regulated by standards that ensure their classification and performance requirements. The applicable standard in Australia for the qualification of glued laminated timber is AS/NZS 1328.1:1998 (2011), last revised in 2011. Confusion exists with the specific interpretation of the testing procedures and subsequent methods used for the delamination process. Two methods currently used for product qualification were compared: one using a drying chamber and the other using a standard laboratory dehydrating oven. The trials conducted in accordance with the standard showed a significant difference in glue line delamination between the two testing methods. This was attributed to the lack of humidity and airflow control within the laboratory dehydrating oven and an incomplete drying of the test samples, resulting in different stresses on the glue line. It is evident that the current standard procedure requires clarification to ensure consistency in the test methods and comparable test results.

Keywords: Engineered wood products; Glued laminated timber; Glue line delamination; Australian standard; Drying oven; Dehydrating oven

Contact information: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, 50 Evans Road, Salisbury, Queensland 4107 Australia;

* Corresponding author: Rodney.vella@daf.qld.gov.au

INTRODUCTION

Engineered wood products (EWPs) are rapidly finding favour in the construction and furniture-manufacturing sectors on a global level, with the main driving factor behind this transition being an increase in timber demand compared to supply capacity. In Queensland, Australia, demand is projected to increase by 33% by 2020, with production capacity predicted to supply only 65% of the domestic market requirement (Timber Queensland 2011). Products such as plywood, glued laminated timber (glulam), and cross-laminated timber (CLT) use previously underutilized material for purpose-made products, which are dimensionally accurate and stable with superior mechanical properties in comparison to raw timber equivalents. Structurally rated formaldehyde-based adhesives such as resorcinol formaldehyde are used in the manufacture of EWPs and remain an industry standard. Isocyanate-based adhesives (IBAs), including polyurethanes (PUR), are a relatively recent adhesive technology and are also finding use in EWP production. These rapid-cure adhesives have gained approval for load-bearing applications in external conditions in both Europe and the United States (Aicher et al. 2015).

For EWPs such as glulam and CLT to replace solid timber as structural elements, they need to conform to the minimum mechanical property requirements of their intended applications. These are dictated by a series of manufacturing and performance standards, either global or country specific, that establish specifications and procedures to ensure that products, services, and systems are safe, reliable, and consistently performing as intended. They establish the minimum requirements defining the quality and safety criteria, and they are usually adopted into legislation. Some standards that regulate the production of glued laminated timber include the international standard ISO 12580 (2007), the American standard ASTM D2559-12a (2012), the European standard BS EN 14080 (2014), and the Australian / New Zealand standard AS/NZS 1328.1:1998 (2011).

All of these standards outline the timber and adhesive requirements for glulam element production, product verification processes, qualification, and routine test procedures. Whenever a new product, a new process, or a process change, such as a new adhesive or species, is to be introduced into the market, preliminary qualification tests have to be conducted to ensure compliance. Routine tests are a means of monitoring production and are conducted as a means of quality assurance.

Both qualification and routine procedures test the bond integrity of the glulam element by the introduction of a moisture gradient within the element. This induces an associated stress gradient with high tensile stresses perpendicular to the glue line, which will either result in the fracture of the timber lamella or a delamination of glue lines if the bond strength is inadequate. Delamination is assessed as part of the individual glue line as well as the total glue line of the element. All standards involve a procedure that fully impregnates test samples with water followed by a drying cycle. While the technique for the water impregnation cycle is shared among standards, there are differences in how the drying cycles are conducted. All have similar requirements for temperature and humidity within the drying chamber, but determination of the circulating airflow varies among the standards. AS/NZS 1328.1:1998 (2011) requires test specimens to be dried for a period of 21 h to 22 h at 60 °C to 70 °C with a relative humidity (RH) not greater than 15% and a circulating air velocity over the samples of 2 m/s to 3 m/s. ASTM D2559-12a (2012) also stipulates a drying temperature of 65.5 °C ± 2 °C for a period of 21 h to 22 h, but the air circulation is defined as that which is sufficient to reduce the sample weight to within 15% of the original test specimen mass. The European standard was updated in 2004 to reflect the drying cycle of the American standard, but prior to this it was similar to the Australian standard in that it specified a required air velocity based on a time.

While maintaining the correct temperature and humidity levels within the drying chamber can be digitally monitored and controlled, measuring the air velocity (specifically, how and where to measure the air velocity within the drying chamber) has proven to be a point of confusion when testing to the AS/NZS 1328.1:1998 (2011) standard. The ambiguity on this matter has resulted in several different procedures being used to measure and set airflow within the chamber. These include determination of air velocity in an empty chamber prior to loading the samples, velocity measured in front of the samples, velocity measured between the samples, and velocity measured behind the samples. One testing body uses a laboratory dehydrating oven and has suggested that the air velocity inside the chamber is not a critical factor in the drying process but is more a means of venting excess moisture from the chamber, with velocity measured to standard requirements at the exhaust vent.

While the dehydrating oven method and drying chamber method meet the requirements of the standard through temperature control, minimised humidity, and airflow control, they do so in different ways, depending on the interpretation of the standard. This study compared the method using a standard laboratory dehydrating oven with that of a fully controlled drying chamber to determine whether there are differences in the delamination outcomes as outlined in AS/NZS 1328.1:1998 (2011).

EXPERIMENTAL

Materials

Material supplied for the study was a randomized mix of two exotic pine plantation species, slash pine (Pinus elliottii) and Caribbean pine (Pinus caribaea). The test material comprised strength-graded Machine graded pine (MGP) 15 backsawn boards sized at 90 mm × 35 mm × 5400 mm with an average timber moisture content of 12%. Boards that presented a high frequency of high-density late wood on the face and had minimal defects were selected for the trial. These were reduced to 400-mm lengths and allowed to condition at 20 °C and 65% RH (12% equilibrium moisture content, EMC) in a constant environment chamber until constant mass was achieved.

Sample Manufacture

Eight glulam elements of six lamellae were manufactured for the study as follows. Forty-eight 400-mm-long boards were passed through a rotary planer fitted with newly sharpened blades to remove 2 mm from all contact surfaces in 0.5 mm increments. The boards were randomly presented for adhesion within 60 min of planing and adhered using a structural rated, single-component polyurethane adhesive (1C-PUR, Jowat Jowapur 686.70, Jowat Klebstoffe, Detmold, Germany). This was applied at a spread rate of 200 g/m2, applied to one contact surface in each glue line and pressed on a dual-axis gluing bench at 1.2 MPa, ensuring that the whole process did not exceed the specified open time of the adhesive. The total press time was 180 min at 20 °C and 50% ambient RH. The glulam elements had a final size of 400 mm x 190 mm x 90 mm. All samples were stored at ambient conditions for 7 d to ensure that the adhesive had achieved full cure.

The centrepoint of each glulam element was determined and marked. One side of each of the eight elements was labelled from 1 to 8, and the other side was labelled from 9 to 16. The elements were cut along the centreline to yield 16 separate test samples (Fig.3).

Testing Procedure

Each test sample was treated according to the requirements of AS/NZS 1328.1:1998 (2011), Appendix C, test cycle A, as follows. A 75 mm test sub-sample was obtained from the centre of each 200 mm test sample. Each glue line was labelled from 1 to 10, and the length of each individual glue line across the grain was determined using digital calipers under a magnifying lens and recorded. Water impregnation was performed in a treatment cylinder by total immersion of the test samples in water at 20 °C and application of a vacuum at -78 kPa held for 5 min, followed by a pressure cycle of 550 kPa for 1 h. While still immersed, the vacuum/pressure cycle was repeated, resulting in a two-cycle impregnation period.

Samples 1 to 8 were placed in a computer-controlled laboratory drying chamber, arranged approximately 60 mm apart, and oriented so that the end grain was perpendicular to the flow of air. The chamber was baffled to ensure that all airflow was directed over the samples. An initial airflow of 2.4 m/s was set in the centre of the sealed chamber and 50 mm in front of the samples using a vane anemometer calibrated by the National Association of Testing Authorities (NATA) (Fig. 1). Initial airflow measurements were recorded in front of each sample before starting the drying cycle. All samples were dried under constantly monitored and controlled conditions for 21 h at 65 °C and less than 15% RH, with an air velocity of 2.4 m/s. Both temperature and RH were monitored over the duration of the run with two HOBO MX1101 temperature/RH data loggers (Onset Computer Corporation, Bourne, MA, USA). These were placed in the centre of the chamber at two locations within the airflow, 100 mm in front of the samples, and 100 mm behind the samples, and set to log at 5-min intervals. The water impregnation and drying cycles were performed twice before the samples were inspected for delamination, as outlined in AS/NZS 1328.1:1998 (2011).

Fig. 1. Airflow measured in dehydrating oven with flow controlled through exhaust vent

Samples 9 to 16 were dried in a Thermoline Scientific TO-235F laboratory dehydrating oven (Thermoline Scientific, Wetherill Park, NSW, Australia) with the temperature control set at 65 °C. Four rack shelves were evenly placed in the oven chamber, and two samples were placed on each shelf approximately 300 mm apart. Air velocity was measured at the exhaust vent of the kiln until a final exhaust velocity of 2.64 m/s was achieved. Adjustment was via an adjustable flap on the air inlet fixture. Initial airflow measurements were recorded at three locations (left, centre, and right) on each shelf before starting the drying cycle (Fig. 2).

Fig. 2. Airflow measured in dehydrating oven with flow controlled through exhaust vent

All samples were dried for 21 h at 65 °C, with no control of RH. Both temperature and relative humidity were monitored with three HOBO MX1101 temperature/RH data loggers (Onset Computer Corporation, Bourne, MA, USA). These were placed in the centre of the top, middle, and lower shelves and set to log at 5-min intervals. The water impregnation and drying sequence was repeated twice before the samples were inspected for delamination as outlined in AS/NZS 1328.1:1998 (2011). A schematic representation of the sample manufacture and testing process is shown in Fig. 3.

Fig. 3. Sample manufacture and testing process

Delamination of gluelines was measured within 1 hour of completion of the drying cycle using digital calipers and an illuminated desk stand magnifier. Delamination was considered to be areas with an open glueline that was as a result of adhesive failure. Delamination on areas of defects, such as knots and knotty areas, were not measured and were excluded from the assessment. Delamination as a result of timber fiber failure, checking, or other causes was also not included in the measurements of calculations.

The acceptance criteria established in the standard for a Type 1 adhesive delamination test allow a maximum delamination of 40% for each individual glue line and no more than 5% total delamination of the sum of all glue lines. An extra test cycle should be performed if the total delamination is greater than 5% but less than 10%. The total delamination percentage should not exceed 10% after this final cycle for the sample to pass.

Moisture Content Determination

After delamination measurements were completed, moisture content determination samples were removed from two positions within each of the 16 glulam elements. A 15-mm cube was cut 15 mm from the bottom edge of the bottom lamella at a position 35 mm in towards the centre, along the grain. A second 15-mm cube was obtained from the centre of the glulam element, also 35 mm in, along the grain (Fig. 4). After initial weights for each block were recorded, all samples were placed in a dehydrating oven at 103 C overnight.

The samples were allowed to cool in a desiccator before obtaining an oven-dry mass. All samples were then returned to the dehydrating oven at 103 C and dried for a further 2 h. After cooling in a desiccator, the samples were weighed, ensuring constant mass and comparing to the previous oven-dry weight (in accordance with AS/NZS 1080.1:2012 (2012).

Fig. 4. Moisture content samples (A) from the middle outer edge and (B) from the centre of the glulam block

RESULTS AND DISCUSSION

Comparison of airflow measurements within the loaded chambers revealed a distinct difference in the measured air velocities of the two drying methods. The computer-controlled drying chamber delivered a consistent airflow across all samples, ranging from 2.15 m/s to 2.56 m/s, within the parameters set out by the standard. The dehydrating oven showed a much less consistent airflow throughout the chamber. Greater velocities were measured along the left side of the chamber where the air entered the oven, while lower velocities were recorded along the right side of the chamber. The lowest velocities were recorded in the centre of the chamber, with the bottom of the oven registering no airflow (Fig. 5). None of the velocities measured in the dehydrating oven met the requirements of the standard.

Fig. 5. Measured air velocities in (a) drying chamber and (b) dehydrating oven

Although the temperatures of the drying chamber and the dehydrating oven were well controlled within the parameters of the standard (Fig. 6), the RH recorded over the treatment period showed a marked difference in the regulation capacities of the two methods, as displayed in Fig. 7. The drying chamber accurately controlled the temperature and humidity within the standard’s specifications. As expected, the dehydrating oven accurately controlled the temperature but was outside of the limits specified in the standard for the RH, which should remain less than 15% during the treatment period. Instead, the RH was at 26% when the drying cycle began and did not decrease below 15%, as required by the standard, until 12 h into the drying cycle. This indicates that the humidity was not adequately evacuated, possibly due to the high water content and high evaporation rate of the specimens.