Mechanical Properties of Ply-Lam Cross-Laminated Timbers Fabricated with Lumber and Plywood

doi:10.21203/rs.3.rs-2881708/v1

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Research Article

Mechanical Properties of Ply-Lam Cross-Laminated Timbers Fabricated with Lumber and Plywood

https://doi.org/10.21203/rs.3.rs-2881708/v1

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published 13 Nov, 2023

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The mechanical properties of four ply-lam cross-laminated timbers (CLTs) containing a plywood layer were compared with those of glued laminated timber (GLT) and CLT. The bending, out-of-plane shear, compression strengths were highest in the GLT, which was followed by the ply-lam CLTs and CLT. The modulus of elasticity values for three studied ply-lam CLT samples were 1–2.5 GPa higher than GLT; however, the bending strength of all ply-lam CLTs was 7.3–18.8 MPa lower than GLT. The length of the plywood product is 2,440 mm, and longitudinal bonding is required to manufacture ply-lam CLTs of length > 3m. The prediction of bending capabilities by shear analogy was compared with the bending properties when joints were included. The performances of all the pilot-scale ply-lam CLT samples exceeded the predicted bending performance standards for MOE (10 GPa) and modulus of rupture (30 MPa) All samples exceeded 10 GPa and 30 MPa, based on projected and experimental data.

bending performance

compression strength

cross-laminated timber

plywood

shear strength

Cross-laminated timber (CLT) is an engineered wood product that is made by gluing and nailing three or more layers of lumber or wood-based materials together at angles of 0° and 90° in accordance with ISO-16696-1 (2019). It is one of the most popular engineered wood panels of the 21st century (Li et al. 2020). CLT was first developed in Europe and has been used in various countries including Australia, China, Japan, the Republic of Korea, and the United States (Davids et al. 2017; Navaratnam et al. 2020; Komatsu et al. 2021; Pang et al. 2021; Choi et al. 2021; Li et al. 2021; Sikora et al. 2016). Various studies on CLT have been conducted using local wood species, coniferous trees, broadleaf trees, and a mixture of wood species (Navaratnam et al. 2020; Li et al. 2021; Sikora et al. 2016; Li et al. 2020; Sharifnia, and Hindman 2017; Liao et al. 2017; Mohd Yusof et al. 2019; Aicher et al. 2016; Lu et al. 2019; Sciomenta et al. 2020). Various types of CLTs have been developed using local woods to reduce the cost of materials. In terms of general local lumber usage, the value of local lumber resources has been increased by CLT development (Li et al. 2021a). Additionally, suitable CLTs for various regions have been developed to reduce carbon emissions and costs associated with long-distance transport of raw materials (Li et al. 2019).

Various types of CLTs have been developed to improve planar or rolling shear failure (RF), bending moments under out-of-plane load, and local lumber utilization (Wang et al. 2015). Among these types, more studies have been conducted on hybrid CLTs that utilize wood-based materials as the layer material (Li et al. 2020; Choi et al. 2021; Wang et al. 2015; Davids et al. 2017; Gong et al. 2019; Wang et al. 2017; Song et al. 2022; Choi et al. 2018; Fujimoto et al. 2021; Pang et al. 2019). Laminated strand lumber (LSL) (Wang et al. 2015; Davids et al. 2017; Gong et al. 2019), laminated veneer lumber (LVL) (Wang et al. 2017), and plywood (Sikora et al. 2016; Choi et al. 2018; Fujimoto et al. 2021; Pang et al. 2019) have been used as wood-based materials. Additionally, hybrid CLTs with LSL have been developed by researchers at University of New Brunswick’s Wood Science and Technology Center in Canada (Wang et al. 2015). Meanwhile, Wang et al. (Wang et al. 2015) have studied the mechanical properties of CLT with LSL as middle and outer layers. They showed that a CLT with LSL showed modulus of elasticity (MOE) and modulus of rupture (MOR) values that were 1.2–1.8 GPa and 8.5–12.8 MPa higher than those of conventional CLTs, respectively (Wang et al. 2015). To improve the bending properties, Wang et al. (Wang et al. 2017) studied the mechanical properties of a CLT obtained with LVL as middle and outer layers. The MOE and MOR values increased by 1.3 GPa and 1.5 MPa respectively, when the outer layer was LVL. Fujimoto et al. (Fujimoto et al. 2021) investigated the bending and shear characteristics of a hybrid CLT, which was made with Korean larch plywood and Japanese cypress lumber (Fujimoto et al. 2021). When plywood was used in place of wood, the MOE, MOR, and rolling shear strength values of the hybrid CLT were 49.0 MPa, 12.3 GPa, and 3.87 MPa, respectively. Song and Hong (Song, and Hong 2018) studied the bending properties of a general CLT with larch lumber. In the studies of Fujimoto et al. (Fujimoto et al. 2021) and Song and Hong (Song, and Hong 2018), plywood was found to increase the MOE and MOR values of a CLT by 0.8–1.7 GPa and 1.9–5.1 MPa, respectively. Choi et al. (Choi et al. 2020) created ply-lam CLT with plywood layer materials and demonstrated that the CLT layer improved significantly the bending and shear properties of the studied hybrid CLT (Li et al. 2020).

The number of studies comparing bending, shear, and in-plane shear properties of engineered wood products (e.g., glued laminated timber (GLT) and CLT) has increased. Li et al. (Li et al. 2021b) investigated the bending characteristics of a CLT combined with a three-ply GLT, CLT, and bamboo-wood composite (B), which was fabricated from hem-fir lumber. The MOR values were 58.8, 47.3, 45.0, and 31.3 MPa for GLT, CLT, LBL, and BLB, respectively. The out-of-plane bending properties, shear characteristics, and joint performance of CLTs have also gained significant attention from researchers (Sikora et al. 2016; Shahnewaz et al. 2017; Wei et al. 2019). Here, we used plywood as a layer in CLT and evaluated the mechanical properties of ply-lam CLTs on the basis of the composition of lumber and plywood. The effect of plywood configuration on the mechanical properties of the ply-lam CLTs was determined, and the mechanical properties was compared with those of GLT and CLT, which are common structural materials in wooden edifices. Additionally, we used beam theory to predict the bending characteristics of ply-lam CLTs, and the predicted characteristics were compared with experimental data.

2.1 Materials

Larch lumbers (Japanese larch, Larix kaempferi (Lamb.) Carriere) of E9 and E10 grades and larch plywood made in the Republic of Korea were used as layers. Except for lumber with knots, larch lumbers of E9 and E10 mechanical grades were used to prevent failure from stress concentration at rotten and missing knots. The height (h), width (w), and length (l) dimensions of the lumber were 27, 140, and 2,700 mm, respectively, and the moisture content of the layer material was 10.8 (± 1.4)%. Two types of layer materials were processed into specimens with h, w, and l dimensions of 18, 120, and 2,400 mm, respectively. After surface grinding and cutting, the h, w, and l dimensions were 22.5, 120, and 3,000 mm, respectively. The larch plywood satisfying the structural first grade of the KS F 3113 standard was used KS F 3113 (2013). The h, w, and l dimensions of the larch plywood were 18.3, 1,220, and 2,440 mm, respectively. The moisture content and density values of the larch plywood were 8% and 0.57 g/cm³, respectively. A phenol resorcinol formaldehyde resin (PRF) (D-40) from Oshika Corporation (Tokyo, Japan) was used as adhesive.

2.2 Fabrication of GLT, CLT, and ply-lam CLTs

The basic properties and layer compositions in GLT, CLT, and ply-lam CLTs are summarized in Table 1. Representative photos of the samples are shown in Fig. 1. To simplify the material yield and manufacturing process, the GLT was manufactured in four layers. The larch lumber was prepared as a glued laminated board using the PRF adhesive. Considering a single-sided application, PRF at 150 g/cm² was used to prepare the glued laminated board and plywood. After applying the PRF, the material was cold pressed with pressure at 10 kgf/cm² that was maintained for 24 h. The prepared samples were then subjected to decompression curing for 7 days to obtain the GLT, CLT, and ply-lam CLTs. Twenty specimens of the four-ply GLT with h, w, and l dimensions of 90, 70, and 2,400 mm, respectively, were prepared. CLT and ply-lam CLT specimens with h, w, and l dimensions of 90, 1,200, 2,400 mm, respectively, were prepared with two panels per condition. The oven-dry density and moisture content values of the prepared specimens were measured as outlined in ASTM D2395 (2017). The measured values are summarized in Table 1. The specimens were cut as shown in Table 2, and their mechanical properties were evaluated.

Table 1

Basic properties and layer compositions of the GLT, CLT, and ply-lam CLTs.
	GLT	CLT	Ply-lam CLTs
	GLT	CLT	P-A	P-B	P-C	P-D
One-layer	L // (E10)	L // (E10)	L // (E10)	Plywood //	L // (E10)	Plywood //
Two-layer	L // (E9)	L ⊥ (E9)	Plywood //	L // (E9)	Plywood //	L // (E9)
Three-layer	L // (E9)	L // (E9)	L // (E9)	Plywood //	Plywood //	L // (E9)
Four-layer	L // (E10)	L ⊥ (E9)	Plywood //	L // (E9)	Plywood //	L // (E9)
Five-layer		L // (E10)	L // (E10)	Plywood //	L // (E10)	Plywood //
Thickness (mm)	89.9 (± 0.14)	88.9 (± 0.18)	90.3 (± 0.2)	91.3 (± 0.09)	90.2 (± 0.11)	90.3 (± 0.08)
Density (kg/cm³)	527.7 (± 14.3)	519.9 (± 15.9)	559.6 (± 2.86)	562.3 (± 17.0)	522.7 (± 9.4)	542.1 (± 15.7)
Moisture content (%)	13.7 (± 0.2)	11.3 (± 0.1)	9.9 (± 0.1)	9.6 (± 0.1)	9.8 (± 0.1)	10.9 (± 0.1)
* L, larch lumber; //, parallel grain direction; ⊥, perpendicular grain direction

Table 2

Measurements and number of specimens used in the experiments.
Test		Specimens
Test		Dimensions (mm) (height × width × length)	Number	Span-to-thickness ratio
Bending	Out-of-plane	90 × 70 × 2,100	15	21
Bending	In-plane	70 × 90 × 1,670	6	21
Shear	Out-of-plane	90 × 70 × 700	15	5
Shear	In-plane	70 × 90 × 450	10	5
Compression strength		90 × 70 × 1,200	12	-

Table 3

Density and moisture content values of the studied GLT, CLT, and ply-lam CLTs.
	GLT	CLT	P-A	P-B	P-C	P-D
Density (kg/cm³)	527.7 (± 14.3)	519.9 (± 15.9)	559.6 (± 2.86)	562.3 (± 17.0)	522.7 (± 9.4)	542.1 (± 15.7)
Moisture content (%)	13.7 (± 0.2)	11.3 (± 0.1)	9.9 (± 0.1)	9.6 (± 0.1)	9.8 (± 0.1)	10.9 (± 0.1)

2.3 Static tests

2.3.1 Four-point bending test

A four-point load bending test was performed on a test specimen in accordance with ASTM D 198 (Fig. 2) (2014). The lengths of the out-of- and in-plane bending specimens were 1,890 and 1,470 mm (span-to-depth ratio = 21), respectively. A displacement gage was installed between the load spans. Bending tests were performed using a 10-ton universal testing machine (KDPI-130-1 model) from company (Seoul, Republic of Korea). The loading rate was set such that failure occurred within 5 min. The MOE (GPa) and MOR (MPa) values were calculated using equations (1) and (2), respectively.

$$\text{M}\text{O}\text{E}= \frac{23P{l}^{3}}{108b{h}^{3}\varDelta }$$

$$\text{M}\text{O}\text{R}= \frac{{P}_{max}l}{b{h}^{2}}$$

The l, $\varDelta$, b, h, and P_max are the test span (mm), linear slope of the load–displacement response at 10–40% of the maximum load, specimen width (mm), specimen height (mm), and maximum load (kN), respectively.

2.3.2 Shear test

To compare the shear strength of the prepared GLT, CLT, and ply-lam CLTs, we conducted out-of- and in-plane shear tests with center point loading according to the ANSI/APA PRG 320 standard (2019). The lengths of the out-of- and in-plane shear specimens were 450 and 350 mm, respectively, with a span-to-depth ratio of 5. The spans of the specimens were separated by a displacement gage, and the 10-ton universal testing machine was used for shear tests. The loading rate was chosen so that failure would occur within 5 min, and the shear strength of a specimen (f_s) (MPa) was calculated using Eq. (3).

$${f}_{s}= \frac{3{P}_{max}}{4bh}$$

The P_max, b, and h are the maximum load (N), specimen width (mm), and specimen height (mm), respectively.

2.3.3 Compressive strength test

The compressive strength of a specimen was tested according to ASTM D 198 (2014) using a 100-ton hydraulic compression strength tester from Dong ah test machine (Seoul, Republic of Korea) (Shin et al. 2011). The h, w, and l dimensions of a specimen were 90, 70, and 1,200 mm, respectively, and the slenderness ratio (l/r; l-length of the column, r-the radius of gyration of the cross section) was ≥ 17. The loading rate was set to induce a failure within 5 min. The compression strength (f_c) was calculated using Eq. (4), which takes into account the cross-sectional area (A) (mm²) and maximum load (P_max) (N) used for a test specimen.

$${f}_{c}= \frac{{P}_{max}}{A}$$

2.3.4 Statistical analysis

The significance of the mechanical properties of the GLT, CLT, and ply-lam CLTs was verified using a SPSS 26 (V26.0.0) statistical software suite from IBM (NY, USA). The significance of a one-way analysis of variance (ANOVA) was determined using Duncan’s multiple test. Statistic significance was set at p < 0.05 (*), p < 0.01 (**), and p < 0.001 (***). According to the ANOVA results, the mean values of the groups denoted by the same superscript letter (i.e., a, b, or c) did not differ significantly at the 0.05 significance level (Srivaro et al. 2022).

2.4 Theoretical bending properties of ply-lam CLTs

The effective bending stiffness (EI_eff,s) (N/mm²) values of a CLT were calculated using equations (5), (6), and (8), which are based on the shear analogy method in the Cross-Laminated Timber Handbook [39], composite theory method (K-method) used in the plywood industry (Fellmoser, and Blaβ 2004), and gamma method in accordance with Eurocode 5 (2008), respectively. The mechanical properties of the lumber and plywood samples, which were used to predict the bending properties of the CLT, are summarized in Table 3.

Table 3

Properties of larch lumber and plywood used for design strength.
Materials	Mechanical grade/thickness (mm)	MOE (N/mm²)	MOR (N/mm²)	Shear modulus (N/mm²)
Larch lumber	E9	9,000^a	13.8^b	563^c
	E10	10,000^a	14.7^b	625^c
	E11	11,000^a	16.2^b	687^c
	E12	12,000^a	17.1^b	750^c
	E14	14,000^a	19^b	875^c
Structural plywood (first grade)	18 mm	5,000^d	9.5^e	100^f
Structural plywood (first grade)	24 mm	5,000^d	10.5^e	110^f
^a In KS F 3021 standard (2021), ^b bending strength/2.1 in standard (2021), ^c MOE/16 (2021), ^d In KS F 3113 standard (2013), ^e bending strength/2.1 in standard (Yoshihara 2012), and ^f MOE of plywood/50 (Xu et al. 1998; Yoshihara 2012)

$${EI}_{eff,s}= {B}_{A}+ {B}_{B}= \sum _{i=1}^{n}({E}_{i}{b}_{i}\frac{{{h}_{i}}^{3}}{12})+ \sum _{i=1}^{n}\left({E}_{i}{b}_{i}{h}_{i}{{z}_{i}}^{2}\right)$$

In Eq. (5), E_i, b_i, h_i, and z_i are the MOE parallel to the i^th layer major or minor strength direction (MPa), width (mm), height (mm), and distance from the centroid of the i^th layer to the centroid of the cross section (mm), respectively.

$${EI}_{eff,k}= {E}_{0} {k}_{1}\frac{b{h}^{3}}{12}$$

k₁ is the value of the composition factor, which is calculated using Eq. (7).

$${k}_{1}=1-\left[\left(1-\frac{{E}_{90}}{{E}_{0}}\right)\left(\frac{{a}_{m-2}^{3} -{a}_{m-4}^{3}+ \cdots \pm {a}_{1}^{3}}{{a}_{m}^{3}}\right)\right]$$

In Eq. (7), b, E₀, h, and E₉₀ are the width (mm), MOE parallel to the grain in the longitudinal layer (MPa)$,$ height (mm)$,$and MOE perpendicular to the grain in the cross layer (${E}_{90}= {E}_{0}/30)$ (MPa). ${a}_{m}, {a}_{m-2}, {a}_{m-4}$ is the layer thickness (m = total number of layers) (mm) (Fig. 3).

$${EI}_{eff,G}= \sum _{i=1}^{n}({E}_{i}{b}_{i}\frac{{{h}_{i}}^{3}}{12})+ \sum _{i=1}^{n}\left({r}_{i}{E}_{i}{b}_{i}{h}_{i}{{z}_{i}}^{2}\right)$$

$${r}_{i}={\left(1+\frac{{\pi }^{2}{E}_{i}{b}_{i}{h}_{i}}{{L}_{eff}^{2}\frac{\left({G}_{90,j}{b}_{j}\right)}{{h}_{j}}}\right)}^{-1}$$

${E}_{i}$ , ${b}_{i}$, ${h}_{i}$, ${z}_{i}$, and ${r}_{i}$ are the MOE of the i^th layer that is parallel to grain ${E}_{0}$ for the longitudinal layers and equal to unity for transverse layers (MPa), width (mm), height (mm), distance from the centroid of the i^th layer to the centroid of the cross section (mm), and connection efficiency factor that is nonzero only for the longitudinal layer and equal to unity for the middle layer, respectively. ${L}_{eff}$, ${h}_{j}$, and ${G}_{80}$ are the effective length of the beam (mm), transverse layer connection to the central layer (mm), and shear modulus in the plane perpendicular to the grain and rolling shear modulus (MPa) (= 0.4 MPa (Kim et al. 2022)), respectively.

The bending stiffness values based on the three theories were calculated in terms of MOE and MOR using equations (10) and (11), respectively.

$$\text{M}\text{O}\text{E}= \frac{12{EI}_{eff}}{b{h}^{3}}$$

$$\text{M}\text{O}\text{R}=2.1 \left(\frac{6{\left({F}_{b}\frac{2{EI}_{eff}}{{E}_{1}h}\right)}_{eff}}{b{h}^{2}}\right)$$

Additionally, we intend to choose one of the three theories that can predict bending properties while taking into account the lumber and plywood composition of a given sample.

2.5 Pilot-scale ply-lam CLT

To compare the predicted and experimental bending properties of ply-lam CLTs that measured ≥ 3 m, including longitudinal plywood joints, pilot-scale fabrication of ply-lam CLT was conducted. The bending properties of the prepared ply-lam CLT were set at 10 and 30 MPa based on the bending properties of GLT, which is the most commonly used engineered wood in the Republic of Korea (Li et al. 2022). On the basis of the predicted bending properties, mechanical-grade E12 lumber and structural first-grade plywood were the selected constituent materials. The h × w × l dimensions of the lumber and structural plywood samples were 30 × 140 × 3,400 mm and 24 × 1,220 × 2,440 mm, respectively. The lumber was cut into specimens with h, w, and l dimensions of 24, 140, and 3,198 mm, respectively, by surface grinding and length cutting. The lumber and plywood for ply-lam CLTs were prepared in the same way as that for the P-C. To connect plywood longitudinally, we selected butt joint over half lap and tongue and groove joints. The butt joint simplifies the fabrication process while retaining the bonding method-dependent bending properties of larch ply-lam CLT (P-A type) (Liao et al. 2017). Niederwestberg et al. (Niederwestberg et al. 2018) evaluated the bending properties based on the longitudinal LSL position of a joint; the moment the resistance and bending stiffness of a joint are reduced by 15–23 kNm/m and 68–192 × 10⁹ Nmm²/m, respectively, compared with joint-free specimens; they configured joint placements in plywood samples as shown in Fig. 4 (Niederwestberg et al. 2018). The h, w, and l dimensions of the P-C were 120, 1,220, and 3,198 mm, respectively (span-to-depth ratio = 16.7). The prepared specimen was subjected to a four-point load bending test as outlined in Section 2.3.1. The displacement gage (Fig. 2) was installed to measure the strain across the length of the specimen.

3.1 Bending test results

3.1.1 Bending properties of GLT, CLT, and ply-lam CLTs by static test

The MOE and MOR values obtained for the GLT, CLT, and ply-lam CLTs are summarized in Table 4. The changes in the out-of- and in-plane MOE values with various loading directions were dependent on the type of material. The CLT and ply-lam CLTs exhibited significant out-of-plane MOE values, whereas the GLT showed a high in-plane MOE value, which is similar to the value reported by Li et al. (Li et al. 2022). The plywood used in CLTs and ply-lam CLTs showed a low in-plane MOE value because of the load along the cross section of the lumber. Compared with the value obtained for the GLT, the out-of-plane MOE values (F*** = 9.019) for three ply-lam CLTs (i.e., P-A, P-B, and P-C) were improved by 11.5–29.5%. However, the MOE value of the P-D ply-lam CLT was 8.2% lower than that of the GLT. Additionally, the MOE values obtained for the three ply-lam CLTs were 11.6–23.4% higher than those for CLT. When larch lumber was used as the outer layer of ply-lam CLTs, higher MOE values were obtained, particularly for P-A and P-C.

Wang et al. (Wang et al. 2015) studied the bending properties of CLT using a LSL and pine lumber. When the second layer was changed to LSL, MOE and MOR increased by 1,248 MPa and 8.58 MPa, respectively. Comparing P-A and CLT, the MOE and MOR increased by 2,257 MPa and 9.63 MPa, respectively. When LSL and plywood were applied as component materials of CLT, the increase in MOE and MOR in the fiber direction was consistent.

The in-plane MOE value obtained (F*** = 7.803) of GLT was the highest, followed by that of ply-lam CLTs and CLT. The average MOE values obtained for the GLT, ply-lam CLTs, and CLT were 13.5, 10.9–12.1, and 8.9 GPa, respectively. There were no significant differences in the MOE values found among the ply-lam CLTs in the significance test at 5% level (F = 1.566, p = 0.229). Thus, we concluded that the location of plywood unaffected the measured in-plane MOE values.

Table 4

Bending properties of the GLT, CLT, and ply-lam CLTs.
Panel type		Out-of-plane
Panel type		MOE (MPa) (F^*** = 9.019, p = 0.000)	MOR (MPa) (F^*** = 12.848, p = 0.000)	MOE (MPa) (F^*** = 7.803, p = 0.000)	MOR (MPa) (F^*** = 10.025, p = 0.000)
GLT		12,157^ab (12.4%)	56.56^d (21.7%)	13,502^c (16.3%)	53.98^c (9.3%)
CLT		11,839^ab (13.3%)	39.66^a (17.6%)	8,860^a (3.5%)	34.13^a (17.0%)
Ply-lam CLTs	P-A	14,096^cd (9.7%)	49.29^c (14.7%)	11,307^b (5.0%)	42.47^b (7.3%)
	P-B	13,210^bc (11.7%)	40.91^ab (10.0%)	12,142^bc (8.2%)	42.79^b (7.1%)
	P-C	14,612^d (13.6%)	46.37^bc (14.5%)	12,037^bc (9.3%)	42.9^b (11.2%)
	P-D	11,212^a (12.6%)	37.81^a (11.2%)	10,888^b (13.4%)	43.51^b (11.6%)
Wang et al. (Wang et al. 2015) (laminated strand lumber (T) and pine lumber (L))	LLL (∥-⊥-∥)	9,727 (7.37%)	35.37 (9.85%)
	LTL (∥-∥-∥)	10,975 (6.19%)	43.95 (9.94%)

The GLT and CLT showed higher out-of-plane MOR values than the ply-lam CLTs. The MOR values among the ply-lam CLTs were different, and the out-of-plane MOR value was high when lumber was used as the outer layer. Meanwhile, the in-plane MOR value was high when plywood was used as the outer layer. The out-of-plane MOR value (F^*** = 11.957, p = 0.000) of GLT was the highest, followed by those of ply-lam CLTs and CLT. The trend for the out-of-plane MOR at 5% lower limit was GLT (34.4 MPa) > P-B (32.4 MPa) > P-A (32.2 MPa) > P-C (31.6 MPa) > P-D (29.2 MPa) > CLT (26.1 MPa), which was similar to the average of MOR values.

The in-plane MOR value (F^*** = 9.99, p = 0.000) of GLT was the highest, followed by those of ply-lam CLTs and CLT. The significance test across the studied ply-lam CLTs exhibited no differences in their in-plane MOR values at the same level (F = 0.056, p = 0.982). The in-plane MOR values at 5% lower limit for P-A, P-B, P-D, P-C, GLT, and CLT were 34.9, 33.9, 32.4, 31.8, 43.2, and 21.7 MPa, respectively. The difference in the in-plane MOR values for the studied ply-lam CLTs was 3.1 MPa. When plywood was utilized as a layer, the homogeneity of the material was guaranteed because the coefficients of variation for the ply-lam CLTs were lower than those for GLT and CLT.

On the basis of the MOE and MOR values, we believe that the MOE and MOR values obtained for P-A and P-C can be improved. Additionally, if plywood is used as the outer layer (e.g., P-B and P-D), we predict uniform bending characteristics. Specifically, the MOE and MOR values of P-D were similar or greater than those of regular CLT. Jang and Lee (Jang, and Lee 2019) maximized the use of small larch logs by designing a CLT panel that consisted of bonded plywood and glued laminated board made from small-diameter lumber as the outer and inner layers, respectively. They concluded that the P-D method is acceptable for the fabrication of structural panels from low-mechanical-grade lumber and small-diameter logs (Jang, and Lee 2019).

Niederwestberg et al. (Niederwestberg et al. 2018) compared the bending properties of LSL and lumber (L) as CLT (5-C1), 5-B2 (LSL∥-L∥-LSL∥-L∥-LSL∥), and 5-A1 (L∥-LSL∥-L∥-LSL∥-L∥). The bending moment and stiffness of 5-B2 were the highest, followed by those of 5-A1 and 5-C1. In this study, the bending properties of P-C were the highest, followed by those of P-B and CLT. This is judged to be the difference between plywood and LSL applied as a layer. LSL is a composite structural lumber consisting of oriented wood stands (Wang et al. 2015), and plywood is glued together with adjacent layer having their wood grain rotated up to 90º to one another (Shreeranga et al. 2017); thus, plywood has lower bending properties than LSL. Accordingly, it is judged that when plywood is used as the outer layer, it acts as a factor impairing the bending properties.

The failure modes were analyzed after the out-of- and in-plane bending tests, and the results are shown in Fig. 5. The out-of-plane failure modes were classified into tensile, tensile + shear, and shear to highlight the differences in the modes depending on the compositions of the specimens. All the GLT specimens failed during the bending tests, and the CLT, P-A, and P-C specimens exhibited tensile + shear failure. The P-B specimen exhibited tensile + shear and shear failure values of 13 ea and 2 ea, respectively. When plywood was used as the outer layer, the RF occurred between the veneers, which are arranged at the intersection that inhibited strength. The P-D specimen showed tensile, tensile + shear, and shear failure values of 9 ea, 4 ea, and 2 ea, respectively. When lumber was used continuously across two to four layers (P-D type), the tensile failure mode of the P-D specimen was identical to that of the GLT specimen.

The changes observed in a failure mode was closely related to the MOE and MOR values. We found that the ratio of shear failures existing in the material had an influence on the degradation rate of the bending qualities. Therefore, we concluded that the use of a layered material as the outer layer can ensure that the specimen would have better bending properties than plywood. When plywood was used as the outer layer to enable the use of low-mechanical-grade larch lumber, the specimen showed uniform bending properties. However, to avoid failure under shear of the outermost layer, it is necessary to consider both the thickness and shear properties of the layer.

3.1.2 Comparison of bending properties

The calculated MOE and MOR values obtained from the three beam theories were compared with MOE values from the out-of-plane bending test and MOR values at 5% lower limit, respectively. These are presented in Figs. 6 and 7. The average MOE values were higher than the corresponding values predicted using the theories. Among the calculated MOE values, the values for MOE_G were 15–30% lower than those for MOE_C and MOE_S. The MOE values obtained for the P-C and P-D specimens and the calculated MOE_C and MOE_S differed by 0.03 GPa. In all specimens, the third layer material was switched back and forth between plywood and larch lumber. The MOE values by the k-method were identical, whereas the values by the shear analogy method were different.

The MOR and MOE values exhibited similar trends. On the basis of beam theory, the MOE and MOR values were in the range of 1.3–2.4 GPa and 0.6–3.2 MPa, respectively, when the third layer in all specimens were changed. The type of layer showed a negligible impact on the bending ability of the specimens. Additionally, the calculated MOE and MOR values were lower than the 5% lower limit; thus, the three beam theories may be applied from a conservative view. The shear analogy method was most suitable for the ply-lam CLTs because it was able to verify any changes in the predicted value, which corresponded to changes in the composition of the inner layer.

3.2 Shear test results

The shear strength of the GLT, CLT, and ply-lam CLT specimens was compared, and the results are shown in Fig. 8. Both the out-of- and in-plane shear strength was GLT > ply-lam CLTs > CLT. The average values of out-of- and in-plane shear strength for GLT were comparable. However, the in-plane shear strength for the ply-lam CLTs was ~ 1.3 times higher than the out-of-plane shear strength. The out-of-plane shear strength (F^*** = 31.234, p = 0.000) of GLT was the highest, which was followed by that of ply-lam CLTs and CLT. The order of strength was P-D > P-A = P-B = P-C. The result of the significance test for P-A, P-B, and P-C, which was performed at an equivalent level (F = 2.603, p = 0.086), indicated no significant differences at the 5% significance level. We confirmed the improved shear strength of the P-D sample with a continuous arrangement of lumbers.

Niederwestberg et al. (Niederwestberg et al. 2018) compared the out-of-plane shear strength of LSL and lumber as CLT (5-C1), 5-B2, and 5-A1. The shear strength results of this study is in good agreement with those reported by Niederwestberg et al. (Niederwestberg et al. 2018). The shear strength of 5-B2 (3.6 MPa) was the highest, followed by 5-A1 (2.9 MPa) and CLT (5-C1: 1.3 MPa). Continuously arranged 5-A1 specimen showed failure at the interfacial glue bond and 5-B2 showed tensile and interfacial glue bond failure. Ply-lam CLTs also showed that rolling shear failure was increased when the plywood was continuously arranged. Although there was no substantial difference in the shear strength results of P-A, P-B, and P-C, it is judged that further research on shear stiffness is needed considering the type of failure mode.

The in-plane shear strength (F^***= 7.314, p = 0.000) for all the specimens except CLT was at the same level. They followed the order GLT = ply-lam CLTs > CLT. Therefore, the significance of GLT and ply-lam CLTs was verified. Consequently, there was no significant difference at the 5% significance level (F = 0.580, p = 0.68). On the basis of the out-of- and in-plane shear strength results, the shear strength of the ply-lam CLTs followed the order P-D > P-B = P-C = P-A.

We concluded that the placement of plywood and lumber in the third layer had no significant effect on the shear strength of the ply-lam CLTs. Nonetheless, we found that continuous staking of lumber as an inner layer in the P-D was preferred for the intersectional or continuous arrangement of plywood as an inner layer that enhanced the out-of-plane shear strength.

The shear failure modes of the specimens were investigated, and the results are shown in Fig. 9. The planar shear failure modes were divided into two types including bending failure due to shear (BF) and RF. The bending failure of the GLT and P-D specimens was completely caused by the shear failure mode. CLT (BF = 3 ea, RF = 12 ea), P-A (BF = 8 ea, RF = 7 ea), P-B (BF = 13 ea, RF = 2 ea), and P-C (BF = 8 ea, RF = 7 ea) failed because of a combination of bending failure due to BF and RF. The ply-lam CLTs showed a weaker RF mode compared with the CLT. When plywood was arranged in the outermost layer (P-B and P-D), RF was not observed in the layers. Meanwhile, when plywood was arranged in 2, 3, and 4 layers (P-A, P-B, and P-D), RF was observed. According to the in-plane shear failure mechanism, all the specimens experienced bending failure due to shear.

3.3 Compression test results

The compression strength and failure modes of the GLT, CLT, and ply-lam CLTs are summarized in Table 5 and Fig. 10, respectively. The compression strength (F*** = 53.987, p = 0.000) of GLT was the highest, followed by those of ply-lam CLTs and CLT. The compression strength of ply-lam CLTs decreased as the plywood content in the layered composition was increased. The significance verification test on the compression strength of the ply-lam CLTs (F = 0.676, p = 0.516) indicated no difference at 5% significance level. Therefore, we concluded that it is possible to increase the compression strength by using plywood as the third layer.

The failure modes followed the order P-A > P-B > GLT > P-D > P-C > CLT due to the 5% lower limit (Table 5). The coefficients of variation were 5.6%, 3.1%, and 10% for P-A, P-B, and GLT, respectively. Therefore, the failure modes obtained for P-A and P-B with 5% lower compression strength were greater than those for GLT. The P-A and P-B specimens can be utilized as compression members. However, further research on their buckling and compression characteristics are required before they can be utilized as suitable building columns.

Table 5

Compression strength values for the general GLT, CLT, and ply-lam CLTs.
Panel type	GLT	CLT	Ply-lam CLTs
Panel type	GLT	CLT	P-A	P-B	P-C	P-D
Compression strength (MPa) (F^*** = 53.987, p = 0.000)	52.1^d (10.3%)	30.9^a (6.4%)	48.4^c (5.6%)	44.8^b (3.1%)	44.2^b (7.4%)	45.6^bc (7.9%)
5% percentile value (Weibull distribution parameter; η, β)	41.2 (η: 55.39, β: 10.03)	26.5 (η: 31.84, β: 16.04)	42.2 (η: 49.76, β: 17.91)	41.7 (η: 45.52, β: 33.57)	37.0 (η: 45.76, β: 13.97)	37.8 (η: 47.34, β: 13.25)

All the compression failure modes of ply-lam CLT were comparable. The layer material under compression load first failed because of load concentration at knots and defects, which was followed by a transfer to the plywood, where the failure occurred along the veneer placed at the intersection. In particular, we confirmed that when the plywood was arranged the outer layer, the delamination of the plywood caused by the compressive load was reduced.

After comparing the bending properties, shear strength, and compression strength, we were able to confirm the better performance of ply-lam CLTs compared with CLT, indicating their suitability as structural panels. Additionally, the MOE values of the ply-lam CLTs were superior to those of GLT. The MOR values at 5% lower limit for the ply-lam CLTs were 2–5.2 MPa lower than those for GLT. The compression strength values at 5% lower limit for the P-A and P-B were 0.5–1 MPa higher than those for GLT. Meanwhile, the P-C and P-D values were 3.6–4.2 MPa lower than those of GLT. P-C was chosen as the representative type of ply-lam CLT because it did not show yielding of the raw materials and shear failure in its outer layer.

3.4 Bending properties of the pilot-scale ply-lam CLT by static test and theoretical method

To fabricate panels with lengths ≥ 2,400 mm, the plywood was bonded longitudinally. The degradation effect caused by the longitudinal joint does not take into consideration the bending properties predicted using the shear analogy method. Therefore, we compared the characteristics of the longitudinal plywood joint-equipped pilot-scale beam. The predicted bending characteristics based on the mechanical grade of layered materials are summarized in Tables 6 and 7. The layered materials of E12 and E14 grades satisfied the established bending performance standards at 10 GPa and 30 MPa. Kim (Kim 2020) studied the adhesive strength and bending performance of CLT layers made of larch (Larix kaempferi) and pine (Pinus densiflora). A total of 650 larch lumbers were used, and the MOE values were measured and classified into mechanical grades. Among them, the E12 and E14 grades accounted for 20% and 10%, respectively; thus, we conclude that it is appropriate to use the E12 grade layer material with a high layer material composition rate.

Table 6

Prediction of the bending properties of P-C according to mechanical grades of lumber.
Layer composition		L-P-P-P-L
Lumber grade		E10	E11	E12	E14
First structural plywood		24 mm
Bending properties	MOE	9.03	9.81	10.6	12.2
Bending properties	MOR	27.9	30.35	31.71	34.67

The bending characteristics of the 3,198-mm pilot-scale P-C are presented in Table 7. The average MOE and MOR values were 11,045 and 41.89 MPa, respectively, which are comparable with the values predicted using the shear analogy method. Thus, the shear analogy method is useful in estimating the bending properties for the configuration illustrated in Fig. 4. Additionally, the pilot-scale P-C presented the same failure mode of tension + shear.

Table 7

Bending characteristics of the pilot-scale P-C.
Specimen	MOE (MPa)	MOR (MPa)	Failure mode
P-C-1	11,620	45.88	Tension + shear
P-C-2	10,364	59.32
P-C-3	10,993	32.44
P-C-4	10,974	43.44
P-C-5	11,116	32.96
P-C-6	11,205	37.29
Mean value	11,045 (3.4%)	41.89 (22.1%)

We compared the mechanical properties of ply-lam CLTs, which were composed of plywood as a layer material, with GLT and CLT. The predicted bending properties of ply-lam CLTs by beam theory were compared with the experimental values. The bending properties of the pilot-scale ply-lam CLT were also evaluated. The key results are summarized as follows.

On the basis of the out-of- and in-plane bending properties of GLT, CLT, and ply-lam CLTs, the MOE values of P-A, P-B, P-C ply-lam CLTs were the highest, followed by those of GLT and CLT. The MOR values of GLT were the highest, followed by those of ply-lam CLTs and CLT. The bending properties of the ply-lam CLTs with plywood as a layer were superior to those of CLT; thus, they can be used appropriately as panels. Furthermore, the P-D type is made up of a continuous stack of layered materials; hence, it can be used to fabricate small-diameter wood and low-grade materials.

The out-of- and in-plane shear strength of the GLT, CLT, and ply-lam CLTs was compared, and showed similar in-plane shear strength to GLT, whereas CLT and ply-lam CLTs showed higher in-plane shear strength than out-of-plane shear strength. The in-plane shear strength of the ply-lam CLTs was similar to that of GLT. Ply-lam CLTs in panel form can be applied as beam members that are used for GLT.

The compression strength at 5% lower limit was in the following order: P-A > P-B > GLT > CLT > P-D > P-C. Two types (P-A, P-B) of ply-lam CLTs were stronger than the GLT.

From a conservative point of view, the shear analogy method is most suitable among the various methods used for predicting the bending properties of ply-lam CLTs. The bending properties and the predicted bending performances (MOE = 10 GPa and MOR = 30 MPa) of the pilot-scale P-C specimen containing plywood joints were compared, and the bending properties obtained for all specimens passed the predicted bending performance standards.

The ply-lam CLTs showed higher strength than CLT; however, they showed poor strength than GLT. Their strength performances were better than those of CLT. Additional studies on joint and design concepts are needed to apply the ply-lam CLTs as structural materials in buildings.

Competing Interests: The authors declare that there are no financial or commercial conflicts of interest.

Funding

This study was conducted with support of R&D Program for Forest Science Technology (Project No. 2022432C10-2222-0101) provided by Korea Forest Service (Korea Forestry Promotion Institute).

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were preformed by Seungmin Yang, Hyunjae Lee, Gyuwoong Choi, and Seoggoo Kang. The first draft of the manuscript was written by Seungmin Yang and all authors commented on previous versions of the manuscript. All author authors read and approved the final manuscript.

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No competing interests reported.

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Mechanical Properties of Ply-Lam Cross-Laminated Timbers Fabricated with Lumber and Plywood

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Materials

2.2 Fabrication of GLT, CLT, and ply-lam CLTs

2.3 Static tests

2.3.1 Four-point bending test

2.3.2 Shear test

2.3.3 Compressive strength test

2.3.4 Statistical analysis

2.4 Theoretical bending properties of ply-lam CLTs

2.5 Pilot-scale ply-lam CLT

3. Results and discussion

3.1 Bending test results

3.1.1 Bending properties of GLT, CLT, and ply-lam CLTs by static test

3.1.2 Comparison of bending properties

3.2 Shear test results

3.3 Compression test results

3.4 Bending properties of the pilot-scale ply-lam CLT by static test and theoretical method

4. Conclusions

Declarations

References

Additional Declarations

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