This paper presents an experimental investigation to identify suitable indices to assess durability of glulam when subjected to freeze-thaw cycles in an exposed enviroenment. In this study, two types of glulam specimens were tested for their performance when subjected to different levels of aging due to freezing and thawing. Effect of aging treatment on various parameters including thickness swelling rate, static bending strength, elastic modulus, shear strength, and peeling rate of adhesive layer were studied. Obtained results showed that freeze-thaw aging treatment did not affect the water-resistance of the specimens as measured by thickness swelling rate and had little effect on the dimensional stability of the material. However, the applied aging treatment weakened the bending resistance of the glulam specimens with more pronounced effects on on low-density wood. On the other hand, bond strength of high-density wood was relatively more affected due to the appliedfreeze-thaw cycles. For high-density wood, it is suggested that the shear strength of the adhesive layer be taken as an important index to determine the durability of freeze-thaw cycles aging. For low-density wood, on the other hand, the static bending strength can be used as an index to determine the durability of glulam under freeze-thaw cycles aging.
Timber construction refers to engineering structures that use timber as the major structural system [
In order to ensure the safety of timber structures, the performance of wood components in different outdoor exposures has been widely investigated. Dhima et al. [
Glulam structures are widely used in many parts of the world with varying climatic conditions [
According to ANSI A190.1-2012 “Standard for wood products-Structural glued laminated timber” [
Wood species | Air-dried density |
Moisture content |
Specimen size |
Number of specimens |
---|---|---|---|---|
0.66 | 10.84 | 20 × 20 × 20 | 30 | |
0.40 | 11.09 | 20 × 20 × 20 | 30 |
Water-proof, weather-resistant phenol-resorcinol-formaldehyde resin adhesive was used in the current study;
Composition | Model | Appearance | Viscosity |
Solid content |
pH | Water solubility |
---|---|---|---|---|---|---|
Main agent | PR-1HSE | Reddish-brown |
15 | 65 | 7.5 | >5 |
Curing agent | PRH-10A | White powder | – | – | – | – |
Thickness swelling ratio due to freeze-thaw aging process and subsequent immersion in water, static bending strength and elastic modulus, shear strength, and peeling rate of the adhesive layer were selected as key indices to assess the durability of the specimens and the techniques used to determine those key indices are as follows:
(1) Thickness swelling rate
The thickness swelling ratio may be used to quantify the effect of moisture on the dimensional stability of glulam members [
(2) Static bending strength and elastic modulus
The static bending strength and elastic modulus were determined following GB/T 26899-2011 [
Within the elastic range, the deflection difference between the initial load and the final load was measured, and the elastic modulus (
where Δ
(3) Shear strength of the adhesive layer
The shear strength test of the adhesive layer was conducted according to ASTM D905-08 (2013) “Standard test methods of static tests of lumber in structural size” [
where
(4) Peeling rate of the adhesive layer
The peeling test was carried out according to CB/T 26899-2011 [
where
In the boiling peel test, the specimens were immersed in boiling water for 4 h, then kept at water at 10°C–15°C for 1 h. The specimens were taken out from the water and put into a constant temperature dryer with
The equipment used in freeze-thaw cycles aging test are as follows: Mechanical universal test analyzer (Model: AG-IC 100 kN; UTM4304); electric constant temperature water tank (Model: DK-600B); electro-thermostatic blast oven (Model: OH6-914385-II; DGG-9070B); freezer (Model: Midea BCD-135); temperature and humidity regulating box; electronic balance (accurate to 0.001g); electronic digital caliper (accurate to 0.01 mm); pH test pen (accurate to 0.01); and temperature and humidity meter.
At present, there is no specific standard for the structural glulam subjected to freeze-thaw cycles. Considering the winter average temperature in Northeast China and the actual test conditions, the freeze-thaw cycles aging treatment method, the following steps were applied in the current investigation:
(1) The specimens were immersed in water with
(2) After immersion, the specimens were taken out from the water, and the surface moisture was dried using a cotton cloth, and then immediately put into the freezer with
(3) After the specimen was taken out of the freezer, it was immediately immersed in water with
(4) Steps 2–3 were rpeated for the next freeze-thaw cycles. The total time for each freeze-thaw cycle was (16 ± 1) h;
(5) Every aging period consisted 7 freeze-thaw cycles, and the water in the test chamber was replaced after each period;
(6) The test was carried out for three aging periods. At the end of each period, the specimens were placed in the environment of
In this study, two types of glulam specimens were treated by freeze-thaw cycles aging. Experiemntal results were carefully analyzed to study the changes in thickness swelling rate, static bending strength, elastic modulus, shear strength, and peeling rate of the adhesive layer due to the aging treatment. This investigation suggests some indices for glulam produced from high and low density wood that could be used to assess durability against freeze-thaw cycles. A total of 320 specimens were tested in 2 groups as part of the current study. After the completion of each aging period, 10 specimens from each wood species were selected to be tested to evaluated key indices (Note: Due to the limitation of test conditions, the specimens for testing the peeling rate of the adhesive layer were also used to test the thickness swelling rate). To reduce the error, the average values were used in the study.
The specimens under freeze-thaw cycles were carefully inspected to understand the effects of simulated extreme weathering conditions. However, it was found that the freeze-thaw cycles had little effect on the shape and the appearance of the specimens. At the end of aging, small cracks occurred in the direction of annual ringsas shown in
Thickness swelling was measured at two stages to investigate the effect of freeze-thaw cycles on glulam’s durability. At the first stage, thickness swelling was measured after three periods of freeze-thaw cycles aging treatment. Once the this measurements were done, all specimens were immersed in water for 24 h and the second thickness swelling was measured to see the effect of freeze-thaw aging on water absorption.
Wood species | Thickness before aging, h1 (mm) | Thickness after aging, h2 (mm) | Swelling rate, (h2–h1)/h1 (%) | 24 h water absorption thickness, h3 (mm) | Swelling rate, (h3–h2)/h2 (%) |
---|---|---|---|---|---|
39.66 |
40.94 |
3.23 |
41.39 |
1.10 |
|
39.60 |
40.40 |
2.02 |
40.71 |
0.77 |
Note: The data in the table are the average values of test data after three freeze-thaw cycles aging, and the value within brackets are the corresponding coefficient of variation.
The thickness swelling rate due to freeze-thaw cycle aging and that after 24 h water immersion for
For
Note: Aging period 0 denotes the specimen without deterioration.
According to the requirements of GB/T 26899-2011 [
Observed changes in shear strength of the adhesive layer of
Note: Aging period 0 denotes the specimen without deterioration.
According to the requirements of GB/T 26899-2011 [
Wood species/peeling mode | Aging period | Single peeling | Two-cycle peeling | |||
---|---|---|---|---|---|---|
Immersion |
0 | 0 | 0 | 0.64 | 2.81 | |
1 | 10.76 | 22.37 | 15.58 | 41.10 | ||
2 | 13.71 | 100 | 18.87 | 100 | ||
3 | 19.43 | 100 | 26.13 | 100 | ||
Boiling |
0 | 0 | 0 | 2.73 | 8.40 | |
1 | 3.16 | 10.52 | 5.30 | 21.73 | ||
2 | 9.74 | 22.14 | 14.67 | 47.89 | ||
3 | 18.69 | 34.62 | 25.17 | 51.34 | ||
Immersion |
0 | 0 | 0 | 0 | 0 | |
1 | 0 | 0 | 3.79 | 45.50 | ||
2 | 2.68 | 19.13 | 5.21 | 62.57 | ||
3 | 4.42 | 24.51 | 8.64 | 51.32 | ||
Boiling |
0 | 0 | 0 | 0 | 0 | |
1 | 0 | 0 | 0 | 0 | ||
2 | 0 | 0 | 3.17 | 18.54 | ||
3 | 2.83 | 18.00 | 4.00 | 24.20 |
Note: Aging period 0 denotes the specimen without deterioration;
The above test results show that the changes in performance of different wood species are different after aging treatment. To assess the observed changes, the performance of the untreated specimen was considered as 100%, and the retention rate of various parameters at different stages of aging were used for relevant comparions in the current study. As shown in Section 3.2, thickness swelling indices after aging treatment as well as after 24 h water immersion were less than 3.5%. This clearly shows that the thickness swelling index can not be used to appropriately evaluate the durability of glulam against freeze-thaw aging process.
In order to analyze the influence of the aging period and the wood species under freeze-thaw cycles aging treatment on its bending strength (
Mechanicalproperties | Source | SS | df | MS | Significant | |
---|---|---|---|---|---|---|
Intercept | 39660.38 | 1 | 39660.38 | 13.08 | 0.172 | |
3031.26 | 1 | 3031.26 | ||||
Aging periods | 1578.47 | 3 | 526.16 | 9.64 | 0.047 | |
163.69 | 3 | 54.56 | ||||
Wood species | 3031.26 | 1 | 3031.26 | 55.56 | 0.005 | |
163.69 | 3 | 54.56 | ||||
Intercept | 7.76 × 108 | 1 | 7.76 × 108 | 10.71 | 0.189 | |
7.25 × 107 | 1 | 7.25 × 107 | ||||
Aging periods | 1.33 × 107 | 3 | 4.44 × 106 | 18.34 | 0.020 | |
7.26 × 105 | 3 | 2.42 × 105 | ||||
Wood species | 7.25 × 107 | 1 | 7.25 × 107 | 299.55 | 0.000 | |
7.26 × 105 | 3 | 2.42 × 105 | ||||
Intercept | 205.57 | 1 | 205.57 | 105.59 | 0.062 | |
1.95 | 1 | 1.95 | ||||
Aging periods | 5.50 | 3 | 1.83 | 6.76 | 0.075 | |
0.81 | 3 | 0.27 | ||||
Wood species | 1.95 | 1 | 1.95 | 7.18 | 0.075 | |
0.81 | 3 | 0.27 |
320 glulam specimens manufactured from 2 types of wood species, i.e., Continuous changes in temperature and humidity during the freeze-thaw cycles showed insigficant effect on the water absorption behavior of glulam; the thickness swelling rate was less than 3.5%, which seemed to indicate that glulam specimens met the requirements of relevant standards. It showed that the aging process due to freeze-thaw cycles had no significant effect on the dimensional stability as well as on the water-resistance ability of the glulam specimens. Hence, the thickness swelling rate should not be used a key indicator to assess the overall durability perfomrnace of glulam when subjected to freeze-thaw cycles. Test results showed that freeze-thaw cycles can considerably weaken the bending performance of glulam specimens, and the influence on low-density wood is more signficant when the same adhesive is used to manufacture. The shear strength of the adhesive layer for both types of specimens met relevant strength standards when subjected to freeze-thaw cycles. However, the bond strength of high-density wood was more affected than that of low-density wood. Changes in mechanical properties due to the applied aging treatment were critically analyzed to find indices that can be reliably used to assess the effect of freeze-thaw cycles on glulam. For high-density wood, it is suggested that the shear strength of the adhesive layer can be taken as an important index to determine the durability of wood during freeze-thaw cycles aging, whereas for low-density wood, the static bending strength can be used as a reliable index. However, it is worth noting that wider variations in wood specimen densities should be considered in future research to evaluate the effect of wood density on durability of glulam under freeze-thaw cycles. Based on obtained test results and subsequent analysis it is obvious that freeze-thaw cycles could significantly weaken the mechanical properties of glulam. Therefore, it is suggested that some thermal insulation materials should be added outside the glulam members, and use of glulam must be carefully and thoroughly assessed prior to using for construction in places with large temperature differences.