Research on mechanical properties and fire resistance of flame- retardant laminated veneer lumber fabricated with fast-growing Poplar

Research on mechanical properties and fire resistance of flame- retardant laminated veneer lumber fabricated with fast-growing Poplar | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Research on mechanical properties and fire resistance of flame- retardant laminated veneer lumber fabricated with fast-growing Poplar Zhekui Cui, Qing Chun, Boxu Lin, Jian Sun, Zheng Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4603151/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Dec, 2024 Read the published version in European Journal of Wood and Wood Products → Version 1 posted 9 You are reading this latest preprint version Abstract To fully utilize Chinese fast-growing timber resources, fast-growing poplar was selected for manufacturing flame-retardant laminated veneer lumber (FRLVL). Firstly, orthogonal experiments were conducted to assess the impact of four factors (hot-pressing time, hot-pressing temperature, retardant concentration, and retardant types) on the mechanical properties and burning behavior of FRLVL. Subsequently, optimal manufacturing parameters were chosen based on statistical analysis. Finally, the fire performance of LVL manufactured with the optimal parameters was evaluated to investigate changes in physical-mechanical properties under high-temperature conditions. Results indicated that the addition of retardants led to a decrease in mechanical properties. In comparison to the control group, LVL composites impregnated with two retardants exhibited a higher limited oxygen index and longer fireproof time, with the effects of ammonium polyphosphate (APP) surpassing those of borax (BX). The optimal manufacturing parameters were a hot-pressing temperature of 140°C, a hot-pressing time of 1.3 min/mm, and concentrations of 15% for both retardant types. As the temperature increased, the mechanical properties of LVL manufactured with the optimal parameters decreased noticeably. However, under the conditions of a temperature of 200°C and a treatment time of 90 min, the mechanical properties of LVL composites still met the LVL-32P grade proposed in LVL handbook. Fast-growing Poplar Fire performance Laminated veneer lumber Orthogonal experiments Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Wood, one of the oldest building materials, is utilized in construction globally. Traditional timber buildings often employ logs as structural members to bear loads. However, this approach overlooks the impact of factors such as moisture content, knots, grain direction, and decay on the strength of wood (Lu et al. 2024 ; Pang et al. 2021 ; Zhang et al. 2022a ; Zhang et al. 2021 ). These defects often lead to deformations and cracks in wood members (Wang et al. 2023 ). In the present era where sustainability is a global development trend, many industries are embracing green transformation, and the construction industry is no exception. In this context, Engineered Wood Products (EWPs) have become a research hotspot due to their high strength and carbon sequestration capabilities (Li et al. 2023a ; Shi et al. 2023 ). Laminated Veneer Lumber (LVL) is a type of EWP. Thanks to its laminated structure, LVL effectively mitigates the influence of natural defects on mechanical properties and offers flexibility in size. Through techniques like finger jointing and layer addition, various sizes of LVL columns, beams, and panels can be manufactured as structural members in timber structures (Awaludin et al. 2021 ; Tapia and Aicher, 2023 ; Zhang et al. 2022b ), particularly in North America and Japan (Blanchet et al. 2023 ; Wang and Dai, 2013 ). The quality of veneers has a significant impact on the performance of LVL. The higher the quality of veneers, the better the performance of LVL. LVL is typically manufactured using softwoods such as Spruce, Pine, Hemlock-fir, Douglas-fir, and Larch(Colak et al. 2007 ; Duriot et al. 2021 ; Li et al. 2023b ; Wang and Dai, 2013 ). Consequently, some countries initially imported raw veneers. However, due to the high transportation costs, scholars are increasingly focusing on using local wood species to produce LVL (Awaludin et al. 2021 ; Duriot et al. 2021 ; Slabohm and Militz, 2023 ). China possesses abundant timber resources, especially fast-growing Poplar. Poplar is generally utilized in furniture and construction as nonstructural members due to its weak fire resistance and poor mechanical properties (Yue et al. 2023 ). Producing LVL from fast-growing Poplar proves to be an effective way to address these weaknesses. Although LVL possesses good mechanical properties, it remains combustible. Wood, being a natural polymer material, forms a layer of char on its surface when exposed to fire (Liu and Fischer, 2023 ). The charring layer has a much lower thermal conductivity than wood and isolates oxygen (Xu et al. 2015 ), thus preserving the thickness and strength of the remaining parts. Recognizing this unique characteristic, numerous studies and simulations on charring rate, fire resistance, and the limit-bearing capacity of timber members have been conducted (Xing et al. 2022 ; Zhang et al. 2012a ; Zhang et al. 2012b ). Considering the different charring performances of various materials, Xu et al. (Xu et al. 2015 ) conducted cone calorimeter tests to compare the combustion and charring properties of three softwood species and two hardwood species. Cui et al. (Cui et al. 2023 ) examined the combustion properties of four common Engineered Wood Products (EWPs) and assessed the AHP-based multi-level fire hazard, Qin et al. (Qin et al. 2021 ) investigated the structural performance and charring properties of three loaded wood: Douglas-fir, Radiata pine and Red cedar. The studies mentioned above focus on the characteristics of wood during combustion. In fact, before the formation of the charring layer, some organic substances inside wood have already started to pyrolyze, resulting in a permanent decrease in strength (Pan et al. 2024 ). Eurocode 5 (EN1995-1-2) defines 300°C as the starting point of charring. Investigating the physical-mechanical properties of wood before this point can provide a reference for calculating the residual capacity of timber members that are distant from the center of the fire and supporting structural safety evaluation after fire. Flame-retardant treatment is a typical method for EWPs to achieve flame retardancy, with two common approaches. The first method involves applying flame-retardant coatings to already prepared EWPs (Li et al. 2021 ; Song et al. 2022 ). In this case, retardants only exist on the surface of the members and have minimal influence on their strength. However, once the coating loses its function, the untreated interior parts become exposed to fire, and their flame retardancy reverts to the original level immediately. In comparison to coatings, impregnated modification is more commonly chosen (Tian et al. 2023 ; Wang et al. 2022 ). This method involves treating the raw materials directly, ensuring that retardants are present in every section of the prepared EWPs, thus guaranteeing overall flame retardancy. Although flame retardancy is improved, the addition of retardants still reduces the mechanical properties of veneers (Yang and Zhang, 2022 ). Moreover, the strength of LVL is significantly influenced by the adhesive as well (Sulaiman et al. 2009 ), which is closely related to manufacturing parameters. During the hot-pressing process, excessively high temperatures cause the outer adhesive to cure prematurely, hindering heat transfer to the core. Conversely, incomplete curing occurs at low temperatures, leading to delamination and a negative impact on bearing capacity. Therefore, finding a balance between flame retardancy and mechanical properties is necessary. In summary, existing research has shown the following problems. (1) research on utilizing low-quality fast-growing veneer to manufacture high-performance flame-retardant LVL composites is very lacking. (2) despite that impregnated treatment induces the mechanical properties of veneers, it is difficult to scientifically determine the balance points between mechanical properties and flame retardancy. (3) many scholars have evaluated the charring performance of wood, but research on changes of physical-mechanical properties before charring is less. Based on the aforementioned research background, this paper aims to improve the mechanical properties and flame retardancy of LVL composites by optimizing the manufacturing parameters and produce flame-retardant LVL composites with fast-growing poplar for structural use. The structure of this paper is as follows: In Section 2 , eight types of FRLVL composites were manufactured according to the orthogonal experiments. Then the mechanical properties and fire resistance tests of different FRLVL composites were carried out. In Section 3 , variance analysis and range analysis were employed to assess the significance and order of the four factors. A series of optimal manufacturing parameters was determined based on the results of the statistical analysis. In Section 4 , the fire performance of the optimal FRLVL composites was tested to examine the changes in physical-mechanical properties and identify typical failure modes in the condition of elevated temperature and increased time. 2. Materials and methods 2.1 Materials Fast-growing Poplar (Populus euramericana cv. I-214) veneers with dimensions of 1270 mm × 840 mm × 2 mm and a moisture content of 10 ± 2%, classified into Grade I and III according to GB/T 13010 − 2020, were provided by Guannan Yindelong Wood Industry Co., Ltd.Phenol-formaldehyde (PF) adhesive, with a solid content and viscosity of 47 ± 1% and 365 mPa.s, was purchased from Dare Wood-Based Panel Group (Jiangsu, China). Ammonium polyphosphate (APP, (NH 4 PO 3 ) n , n<30) and Borax (BX, (Na 2 B 4 O 7 ·10H 2 O)) were obtained from Zhengzhou Jiajie Chemical Products Co., Ltd., and Liaoning Shougang BoronIron Co., Ltd., respectively. 2.2 Experiment design In this study, the hot-pressing pressure of each group was set to the same level in order to obtain the same compression ratio, keeping the thickness of each group being 20 mm. Besides, hot-pressing time (Parameter a), hot-pressing temperature (Parameter b), concentration (Parameter c), and types of retardants (Parameter d) were chosen as factors, with each factor having two levels. Table 1 illustrates the orthogonal experimental sets, where A represents the retardant APP, B represents the retardant BX, and C represents the control group. Table 1 Orthogonal experimental programs Group serial number Hot-pressing time Hot-pressing temperature Hot-pressing pressure Concentration retardant (min/mm) (°C) (MPa) (%) A 1 1.3 120 1.5 10 APP 2 1.3 140 1.5 15 3 1.5 140 1.5 10 4 1.5 120 1.5 15 B 1 1.3 120 1.5 10 Borax 2 1.3 140 1.5 15 3 1.5 140 1.5 10 4 1.5 120 1.5 15 C 1 1.3 120 1.5 / / 2 1.3 140 1.5 / 3 1.5 140 1.5 / 4 1.5 120 1.5 / 2.3 Flame-retardant LVL preparation The FRLVL designed in this paper, which consists of 12 layers, is categorized into three layers: the top surface, core, and bottom surface layers. In order to strength bending performance, the top and bottom surface layers comprise 2 Grade I veneers without impregnation, while the core layer consists of 8 Grade III veneers impregnated with retardants. Veneers were immersed in the APP or BX solution for 2 hours and then dried in the oven for 30 minutes. Following impregnation, roller coating was applied with a spread amount of 190 g/m 2 . As depicted in Fig. 1 , adhesive was coated onto the surfaces with lathe checks to achieve higher bonding performance (Li et al. 2020a ; Li et al. 2020b ). Subsequently, veneers laid up parallel to the grain were pre-pressed. The pre-pressing parameters included a pressure of 1.0 MPa, a time of 40 minutes, and a temperature of 20°C. Then the specimens were hot-pressed according to the orthogonal experimental programs. Finally, all specimens were conditioned for 12 hours to eliminate thermal stress and reach equilibrium moisture content. The specific manufacturing process is shown in Fig. 1 . 2.4 Mechanical properties and Fire resistance tests The Modulus of Elasticity (MOE), Modulus of Rupture (MOR), Compressive Strength (CS), and Horizontal Shear Strength (HSS) of FRLVL were tested according to the information provided in Table 2 . MOE, MOR, and HSS were tested in the flatwise direction, while CS was tested parallel to grain. MOE and MOR tests used the same specimens. All the mechanical properties tests were conducted using a universal testing machine. The information on fire resistance is also provided in Table 2 . Limited Oxygen Index was tested using a JF-3 oxygen index meter, with specimens recycled from the MOR test. During the flammability test, the nozzle was kept 100 mm away from the specimens, and the angle between the nozzle and the specimens was 45°. The flame was applied towards the center of the specimens with a tip temperature of 1300 ± 100°C. The reverse side temperature of specimens was measured every 30 seconds until a black spot emerged. The test process is illustrated in Fig. 2. Table 2 Information on test specimens Properties Dimensions of specimens Quantity Standards MOE 300mm×20mm×20mm 6×12 GB/T 1936.1–2009 [34] MOR GB/T 1936.2–2009 [35] HSS 120mm×40mm×20mm 6×12 GB/T 20241 − 2021 [36] CS 30mm×20mm×20mm 6×12 GB/T 1935–2009 [37] LOI 100mm×10mm×3mm 9×12 GB/T 2406 − 1993 [38] FT 250mm×90mm×20mm 3×12 GB/T 8626 − 2007 [39] 2.5 Statistical analysis IBM SPSS Statistics 26 was employed for statistical analysis. Variance analysis was used to estimate the significance (p-value < 0.05 and p-value < 0.01) of the four factors on the mechanical properties and fire resistance of FRLVL. Then, range analysis was conducted to determine the influence order of different levels. Taking into account the results of variance analysis and range analysis, the optimal manufacturing parameters of FRLVL were ultimately obtained based on the statistical analysis. 3. Results and disscussion 3.1 Mechanical properties As depicted in Fig. 3a, the average MOE for groups A and B was 12.27 GPa and 12.09 GPa, respectively, representing decreases of 4.7% and 6.1% compared to the control group. In Fig. 3c, the CS of each group is displayed, with the CS of A3 measuring 56.64 MPa, even higher than that of the control group. Conversely, P2 exhibits the lowest CS at 48.62 MPa, representing a decrease of 7.1% compared to the control group. These findings suggest that manufacturing parameters contribute to slight changes in both MOE and CS. Figure 3b and Fig. 3d present the results of MOR and HSS tests. In contrast to the MOE and CS test results, the changes in MOR and HSS are slightly larger. All the MOR and HSS specimens are tested flatwise, and the strength of the adhesive layer significantly impacts MOR and HSS. For instance, due to the lower hot-pressing temperature and incomplete cure of the adhesive, the MOR of A1 and A4 is noticeably lower than that of A2 and A3, while the MOR of the A2 group is lower than A3. This is because the hot-pressing time is too long, causing not only a decrease in adhesive layer performance but also the pyrolysis of cellulose, lignin, and other components in wood, leading to a decrease in material performance. Furthermore, empirical investigations demonstrated that impregnated veneers, when contrasted with standard veneers, exhibit increased brittleness. This aspect is considered one of the potential reasons for the divergence in the trends of MOR and HSS between Group P and Group A. Additionally, it should be noted that the phenomenon may be influenced by constraints in experimental conditions, including the impact of manual gluing. 3.2 Fire resistance Figure 4 illustrates the Limited Oxygen Index results for each specimen. The findings indicate that, in comparison to the control group, Group A exhibits the most substantial enhancement in the oxygen index, with increases of 25.7%, 34.6%, 23.9%, and 30.0%, respectively. Meanwhile, Group B shows an average oxygen index increase of 18.7%, 31.3%, 16.6%, and 22.1%. The overall trend among the groups is 2 > 4 > 1 > 3, indicating the effective functioning of both APP and BX. The LOI of Group A was all above 32%, which belonged to B1 level (nonflammable material) proposed in GB 8624 − 2012. In Fig. 5a and Fig. 5b, temperature-time curves for each group are presented. The maximum and minimum fireproof times in Group A were 1229s and 982s, while in Group B, they were 1081s and 840s. The average fireproof times for Group A and Group B were 1074.25s and 960.25s, respectively, representing increases of 74.3% and 55.8% compared to the control group. From the curves, it can be observed that due to the absence of retardants on the outer layer, the gradient of the FRLVL and the control group was initially similar. However, over time, the temperature rise rate of the FRLVL decreased significantly. Particularly in Group A, a noticeable pause in the curve occurred around 80°C, creating a "platform" shape. This phenomenon is attributed to the retardants in the core layers absorbing heat (Jiang et al. 2023 ; Oezcifci and Okcu, 2008 ), while the charring layer formed by the surface veneer also hinders heat transfer. In the final stage of the experiment, when the specimen thickness was nearly zero, the temperature rose sharply in a short time until the specimens burned through. As shown in Fig. 5, the specimens from the experiments of Group A (Fig. 5c), Group B (Fig. 5e), and Group C (Fig. 5d) were compared to illustrate the charring range. FRLVL exhibited a narrower charring range, whereas the control group had a much larger range, reflecting the impact of flame retardants. 3.3 Statistical analysis Table S1 presents the results of the range analysis, encompassing Ki, ki, and R. Here, Ki(i = 1,2) and ki represent the sum and average values of the experimental results containing a factor at the i level, respectively. R is the absolute value of the difference in ki, employed to reflect the varying influence of different levels. The influence trends of various parameters on the mechanical properties and fire resistance of FRLVL are illustrated in Fig. 6. For MOE, MOR, HSS, and CS, the order of influence of manufacturing parameters was b > c > d > a, b > c > a > d, c > b > a > d, and d > c > a > b, respectively. The optimal manufacturing parameters obtained based on the range analysis are presented in the "Optimal level" row of Table 2 , and it is noteworthy that the optimal level varied for the four mechanical properties. Concerning fire resistance, although the optimal level for LOI and fireproof time was both a2b1c2d, LOI was significantly impacted by concentration, while fireproof time was pronounced influenced by the type of retardant. Additionally, hot-pressing time and temperature exhibited minimal impact on fire resistance. The results of the variance analysis are presented in Table S2. According to the table, the hot-pressing temperature (P-value = 0.014) was the only factor that significantly influenced MOE. Compared with other factors, the hot-pressing temperature had a certain impact on MOR (P-value = 0.073), but it did not attain statistical significance. HSS was found to be significantly affected by the concentration of flame retardants (P-value = 0.031) exclusively. Moreover, the concentration of flame retardants had a significant impact on CS (P-value = 0.020), while the type of retardant exerted the most substantial influence on CS (P-value = 0.008). Regarding LOI, both the concentration and type of retardant demonstrated P-values less than 0.01, signifying a significant effect on LOI. It is founded that all four factors influenced fireproof time significantly, with P-values < 0.05. Remarkably, the P-values of hot-pressing temperature, concentration, and type of retardant were all less than 0.01. Through the analysis above, it can be concluded that the optimal manufacturing parameters vary according to different mechanical properties and fire resistance. Considering LVL members are easily damaged when exposed to fire, fire resistance is the most crucial property that should receive more attention. Therefore, based on the combination of range and variance analysis results, the final optimal manufacturing parameters were determined as a2b1c2d1, which means a hot-pressing temperature of 140°C, hot-pressing time of 1.3 min/mm, retardant concentration of 15%, and type of retardant as APP. 4. Fire Performance of LVL with optimal manufacturing parameters 4.1 Test Methods for fire performance To investigate the changes in the physical-mechanical properties of FRLVL after exposure to high temperatures, nine sets of tests were conducted, incorporating three temperatures (100°C, 150°C, 200°C) and three durations (30 minutes, 60 minutes, 90 minutes). These tests were performed on FRLVL manufactured using the optimal manufacturing parameters. Before the initiation of each test, the muffle furnace was preheated to the designated temperature, and then the specimens were placed into the furnace. To ensure the accuracy of the test data, only one group of specimens was treated at a time, with each group of specimens placed in the same position of the furnace. Following thermal treatment, the specimens were placed in a conditioning room for 24 hours before being tested for various properties. The MOE, MOR, and CS of the specimens were tested following the method outlined in section 2.4 , and the mass loss rate of the specimens was calculated by the following formula: $${m}_{L}=\frac{{m}_{0}-{m}_{T}}{{m}_{0}}$$ Where m L is the mass loss rate, m 0 is original mass, and m t is the mass after treatment. 4.2 Physical properties Wood is a polymer material composed of cellulose, hemicellulose, lignin, and some extractives. With an increase in temperature, cellulose, hemicellulose, and lignin undergo a series of oxidation reactions, resulting in a gradual darkening of the color (Tomak et al. 2018 ). Color changes in specimens under different treatment conditions are shown in Fig. 7. It can be observed that the color changes in the core layer were less pronounced than in the top and bottom layers due to the addition of flame retardants. At 100°C, only the evaporation of water and the volatilization of some extractives occurred inside the wood, having a minimal effect on the color. At 150°C, attributed to the lack of flame retardant, the color of the outer layer became darker with increasing treatment time, while the core layer remained almost unchanged. Hemicellulose pyrolyzed when the temperature exceeded 200°C, causing a rapid color change. The average mass loss rate is depicted in Fig. 8, revealing an increase in the mass loss rate of FRLVL with temperature and time. The curve exhibited almost linearity at 100°C, during which the primary loss was moisture, absorbed through conditioning. In the 150°C stage, mass loss continued to increase, but the trend slowed when the treatment time exceeded 60 minutes. Upon reaching 200°C, due to the pyrolysis of the adhesive and organic matter inside wood, the mass loss rate increased rapidly, eventually surpassing the equilibrium moisture content (8%±0.2). 4.2.2 Mechanical properties From the results in Fig. 9a, it can be concluded that the MOE of the specimens changed little at a temperature of 100°C and even increased when treated for 30 minutes at 150°C. This could be explained by the further curing of the adhesive caused by the evaporation of water (Li et al. 2022 ). However, with the addition of treatment temperature and time, this brief strengthening couldn’t offset the overall strength decrease. Thus, when the specimens were treated for more than 30 minutes at a temperature above 200°C, the MOE decreased significantly, with the maximum reduction rate being 18.2%. Compared to MOE, the MOR of FRLVL experienced a more substantial change. The MOR of each specimen was all larger than 90 MPa at 100°C. However, when the temperature reached 200°C, the MOR of the specimens decreased to 60.31 MPa. The MOR of the specimens at 100°C and a treatment time of 30 minutes was 100.96 MPa, while the MOR of the specimen at 150°C and a treatment time of 90 minutes was 72.94 MPa. After the temperature exceeded 200°C, prolonged exposure to high temperatures led to the decomposition of hemicellulose and the further softening of wood fibers. Consequently, the mechanical properties of specimens decreased significantly. The MOR of specimens was only 45.14 MPa after 90 minutes of heating, representing a decrease of more than 55%. The CS of the specimens is depicted in Fig. 9(c). The CS of the specimens changed very little at 100°C and 150°C, which may be attributed to the evaporation of water having a certain effect on the mechanical properties parallel to grain. The CS of specimens declined significantly, falling below 45 MPa at 200°C. Although the MOR and CS of the specimens decreased substantially at high temperatures, the MOE of specimens treated for 90 minutes at 150°C was still sufficient to reach the LVL-35P level specified in the LVL handbook (LVL Handbook Europe. Finland), and the MOR and CS belonged to the LVL-50P level. The MOE of specimens treated for 90 minutes at 200°C could fulfill the standard of LVL-32P level, and the MOR and CS were able to reach the LVL-35P level. In general, FRLVL manufactured by the method in this research still retained excellent mechanical properties after exposure to high temperatures. 4.2.3 Typical failure mode Figure 10 revealed four typical failure modes in bending specimens. Types I, II, and III failures mainly occurred in specimens treated at temperatures of 100°C and 150°C. For Type I failure, when the specimens were bent, the veneer at the bottom of the LVL received tensile force, resulting in damage to the wood fibers. Type II and Type III failures mainly occurred in the compression and tension areas of the specimens. In these areas, there often existed tiny displacements between veneers, which applied shear force to the glueline and eventually led to the delamination of the adhesive layer. As the temperature increased, the flame retardants and adhesive decomposed, and wood fibers further softened. Thus, the brittleness of specimens kept increasing, and the failure mode of the FRLVL gradually changed to the brittle failure of Type IV, which was also accompanied by the shear failure of the adhesive layer. As depicted in Fig. 11, no significant relationships were identified between different typical failure modes and treatment temperature. Four distinct failure modes were consistently observed across all temperatures: diagonal cracks in the veneer surface (I), adhesive layer delamination (II), outer fiber bulging out (III), and middle fiber bulging out (IV). According to force analysis, the maximum shear stress occurred at a 45° angle when the specimens were subjected to pressure, causing diagonal cracks in the veneer surfaces due to an out-of-plane directional slip between fibers. Type II failure resulted from the opposing movement of adjacent veneers, creating a shear force perpendicular to the grain in the adhesive layer and leading to delamination. Type III and IV failures were attributed to the extrusion of wood fibers, causing them to bulge out in the in-plane direction. Certainly, it should be recognized that while the repetition number for each group was determined according to the Chinese national standard, the absence of a specific repetition number poses a challenge affecting the precision of the test results. Furthermore, in this paper, only nine groups of treatment time and temperature were designed, which is fewer compared to other research studies. Consequently, for the acquisition of more accurate and detailed test data, future work should include expanded sample size and the incorporation of additional treatment time and temperature groups. 5. Conclusion This paper utilized fast-growing Poplar to manufacture flame-retardant LVL composites. Orthogonal experiments were carried out to estimate the effects of four manufacturing parameters (hot-pressing time, hot-pressing temperature, retardant concentration, and retardant types) on the mechanical and flame-retardant properties of LVL composites. Then the optimal manufacturing parameters of FRLVL composites were obtained according to statistical analysis. Finally, the fire performance of FRLVL composites made by optimal manufacturing parameters was tested. The main conclusions can be drawn as follows: (1) The fire resistance of FRLVL composites was significantly influenced by the addition of two flame retardants compared to its mechanical properties. Only a few mechanical properties were found to be moderately susceptible to flame retardants. Both APP and BX demonstrated significant flame-retardant effects. But compared with BX, APP notably improved the flame retardancy of LVL composites. (2) The optimal manufacturing parameters of FRLVL composites under the conditions of this study were a2b1c2d1, which included a hot-pressing temperature of 140°C, hot-pressing time of 1.3 min/mm, retardant concentration of 15%, and retardant of APP. The MOE, Oxygen Index, and Fireproof time achieved the best levels in this condition. Although the CS and HSS were not at the optimal level, the gap between them and other groups was very small. Considering the MOR declined significantly at this condition, the FRLVL composites manufactured in this study might be recommended as columns instead of beams in timber buildings. (3) The temperature and exposed time significantly affected the mechanical properties of LVL composites. With the increase of time and temperature, wood experienced irreversible pyrolysis, leading to the massive loss of strength when the temperature reached 200°C and exposed time reached 90 min. However, although the mechanical properties dropped significantly, the LVL composites manufactured in this study still show high-strength. (4) When the temperature was below 150°C, the typical failure modes of bending specimens mainly contained delamination and fiber fracture. With the increase in temperature, wood fiber kept softening, and the failure mode of the specimens gradually switched to brittle failure. Compressive failure modes of specimens did not have a strong relation between temperature and exposed time. Four typical failure modes were observed under all conditions. Declarations Competing interests The authors declare no competing interests Author Contribution Z.C.: investigation, validation, formal analysis, software, writing-original draft; Q.C.: funding acquisition, supervision; B.L.: methodology, formal analysis; J.S.: investigation; Z.W.: project administration, conceptualization,. All authors reviewed the manuscript. Acknowledgements The authors acknowledge sponsorship by National Key R&D Program of China [2023YFF0906100] References Awaludin A, Irawati IS, Shulhan MA (2021) Two-dimensional finite element analysis of the flexural resistance of LVL Sengon non-prismatic beams (vol 10, e00225, 2019). Case Studies in Construction Materials 14 Blanchet P, Perez C, Cabral MR (2023) Wood Building Construction: Trends and Opportunities in Structural and Envelope Systems. Current Forestry Reports Colak S, Colakoglu G, Aydin I (2007) Effects of logs steaming, veneer drying and aging on the mechanical properties of laminated veneer lumber (LVL). 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Wood Mater Sci Eng 18:76–81 Song F, Liu T, Fan Q, Li D, Ou R, Liu Z, Wang Q (2022) Sustainable, high-performance, flame-retardant waterborne wood coatings via phytic acid based green curing agent for melamine-urea-formaldehyde resin. Prog Org Coat 162 Sulaiman O, Salim N, Hashim R, Yusof LHM, Razak W, Yunus NYM, Hashim WS, Azmy MH (2009) Evaluation on the suitability of some adhesives for laminated veneer lumber from oil palm trunks. Mater Design 30:3572–3580 Tapia C, Aicher S (2023) A new concept for column-to-column connections for multi-storey timber buildings-Numerical and experimental investigations. Eng Struct 295 Tian F, Mao W, Xu X (2023) Effect of a layered combination of APP and TBC on the mechanics and flame retardancy of poplar strandboards. Constr Build Mater 401 Wang BJ, Dai CP (2013) Development of structural laminated veneer lumber from stress graded CrossMuk short-rotation hem-fir veneer. Constr Build Mater 47:902–909 Wang H, Chun Q, Zhang C, Ma S, Cao G, Dong Q (2023) Experimental study on the compression behavior of long timber column strengthened with the novel hybrid fiber sheets. J Wood Sci 69 Wang K, Meng D, Wang S, Sun J, Li H, Gu X, Zhang S (2022) Impregnation of phytic acid into the delignified wood to realize excellent flame retardant. Ind Crops Prod 176 Xing ZY, Zhang J, Zheng CY (2022) Material model development and fire resistance research on CLT wall-floor angle bracket connections in OpenSees. Constr Build Mater 347 Xu Q, Chen L, Harries KA, Zhang F, Liu Q, Feng J (2015) Combustion and charring properties of five common constructional wood species from cone calorimeter tests. Constr Build Mater 96:416–427 Yang G, Zhang Q (2022) In Situ Polymerization and Flame Retardant Mechanism of Bio-Based Nitrogen and Phosphorus Macromolecular Flame Retardant in Plywood. Macromol Rapid Commun 43 Yue K, Qian J, Wu P, Jiao X, Lu D, Song X (2023) Experimental analysis of thermally-treated Chinese poplar wood with focus on structural application. Ind Crops Prod 197 Zhang C, Chun Q, Lin Y, Wang H, Li P (2022a) Experimental and nonlinear finite-element analysis study on lateral push resistance of San-dou components perpendicular and parallel to grain in traditional timber buildings. J Wood Sci 68 Zhang CW, Chun Q, Lin YJ, Han YD, Jia XH (2021) Quantitative assessment method of structural safety for complex timber structures with decay diseases. J Building Eng 44 Zhang J, Liu Z, Xu Y, Ma S, Xu Q (2012a) An experimental and numerical study on the charring rate of timber beams exposed to three-side fire. Sci China-Technological Sci 55:3434–3444 Zhang J, Xu Q-f, Xu Y-x, Wang B, Shang J-x (2012b) A numerical study on fire endurance of wood beams exposed to three-side fire. J Zhejiang University-Science A 13:491–505 Zhang RZ, He YH, Dauletbek A, Shen ZY, Zhou YH, Wang Z (2022b) Design and Manufacture of Laminated Veneer Lumber Packaging Boxes and Pallets and Evaluation of Their Mechanical Properties. BioResources 17:6910–6925 Chinese National Standard GBT (1936) 1-2009. Method of testing in bending strength of wood. Beijing Chinese National Standard GB/T 20241 – 2021. Laminated veneer lumber. Beijing Chinese National Standard GBT 1935–2009. Method of testing in compressive strength parallel to grain of wood. Beijing Chinese National Standard GB/T 2406 – 1993 Plastic-Determination of flammability of oxygen index. Beijing Chinese National Standard GB/T 8626 – 2007 Test method of flammability for building materials. Beijing Chinese National Standard GB 8624 – 2012. Classification for burning behavior of building materials and products. Beijing Jiang X-C, Li P, Liu Y, Yan Y-W, Zhu P (2023) Preparation and properties of APP flame-retardant ramie fabric reinforced epoxy resin composites. Ind Crops Prod 197 Oezcifci A, Okcu O (2008) Impacts of some chemicals on combustion properties of impregnated laminated veneer lumber (LVL). J Mater Process Technol 199:1–9 Tomak ED, Ustaomer D, Ermeydan MA, Yildiz S (2018) An investigation of surface properties of thermally modified wood during natural weathering for 48 months. Measurement 127:187–197 Li X, Mou Q, Ji S, Li X, Chen Z, Yuan G (2022) Effect of elevated temperature on physical and mechanical properties of engineered bamboo composites. Ind Crops Prod 189 Finnish Woodworking Industries LVL Handbook Europe. Finland: Federation of the Finnish Woodworking Industries Additional Declarations No competing interests reported. Supplementary Files SupplementaryMaterial.docx Cite Share Download PDF Status: Published Journal Publication published 26 Dec, 2024 Read the published version in European Journal of Wood and Wood Products → Version 1 posted Editorial decision: Revision requested 08 Nov, 2024 Reviewers agreed at journal 17 Aug, 2024 Reviewers agreed at journal 30 Jun, 2024 Reviews received at journal 27 Jun, 2024 Reviewers agreed at journal 27 Jun, 2024 Reviewers invited by journal 27 Jun, 2024 Editor assigned by journal 23 Jun, 2024 Submission checks completed at journal 19 Jun, 2024 First submitted to journal 19 Jun, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4603151","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":321610354,"identity":"84c14c97-5b4e-44d5-90f7-72af55d483bb","order_by":0,"name":"Zhekui Cui","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Zhekui","middleName":"","lastName":"Cui","suffix":""},{"id":321610357,"identity":"eb804021-04f4-4ee5-a2b3-77b231459d34","order_by":1,"name":"Qing Chun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwUlEQVRIiWNgGAWjYJACg4SK/3L8zMyHHxCv5cEZZmPJdrY0A6K1MD5sY07ccJ5HQYIo5fztuQcKEtvYEjcf5mEwYKixiSaoReLMuwSDhHM8xtsO8x54wHAsLbeBkBYDiRwDg4QyCdlth/kSDBgbDhOrhc2AcXMzj4EECVraEhQ3MBOrReLMG6CWMweMJQ4DAzmBGL/wt+eYGf6oOCDH33/48IMPNTaEtTAwAD2CYBNWDlbG/IA4haNgFIyCUTBiAQDo9T9X5GPnxAAAAABJRU5ErkJggg==","orcid":"","institution":"Southeast University","correspondingAuthor":true,"prefix":"","firstName":"Qing","middleName":"","lastName":"Chun","suffix":""},{"id":321610359,"identity":"67f13e60-5ff2-4595-b6d0-22133fe25d18","order_by":2,"name":"Boxu Lin","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Boxu","middleName":"","lastName":"Lin","suffix":""},{"id":321610363,"identity":"a194fcb2-4742-4924-bfb4-8e25c148ee3a","order_by":3,"name":"Jian Sun","email":"","orcid":"","institution":"Southeast University","correspondingAuthor":false,"prefix":"","firstName":"Jian","middleName":"","lastName":"Sun","suffix":""},{"id":321610369,"identity":"84f1343f-3416-4088-8fe6-e86cdd77ea6c","order_by":4,"name":"Zheng Wang","email":"","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":false,"prefix":"","firstName":"Zheng","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-06-19 04:38:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4603151/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4603151/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00107-024-02158-z","type":"published","date":"2024-12-26T15:57:51+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":59641557,"identity":"5a37ed7e-42d2-4a04-822e-956b0e86b026","added_by":"auto","created_at":"2024-07-04 07:59:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":393035,"visible":true,"origin":"","legend":"\u003cp\u003eProcess of manufacture of flame-retardant LVL\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/6b05f926b2a2005bf3e151bb.png"},{"id":59641575,"identity":"f5638418-c947-4d52-aa5f-3722667a8796","added_by":"auto","created_at":"2024-07-04 07:59:19","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":279900,"visible":true,"origin":"","legend":"\u003cp\u003eFlammability test:Setup diagram(a); Field test(b)\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/028e6d2ccd625ddca792d5f1.png"},{"id":59641577,"identity":"a188fcbc-a22d-4fcc-bd20-3cdf7165bc7a","added_by":"auto","created_at":"2024-07-04 07:59:19","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":543154,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical properties: MOE(a); MOR(b); CS(c); HSS(d)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/508484a9b43ae56a2b6bf088.png"},{"id":59641556,"identity":"a17a34c4-84d7-4d37-94cf-c6a935941004","added_by":"auto","created_at":"2024-07-04 07:59:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":188580,"visible":true,"origin":"","legend":"\u003cp\u003eResults of Limited Oxygen Index\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/c0343b5c0ee53eb423d183e5.png"},{"id":59641578,"identity":"bab01dea-b512-4be5-b98c-9c98c01a377d","added_by":"auto","created_at":"2024-07-04 07:59:19","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":341365,"visible":true,"origin":"","legend":"\u003cp\u003eResults of flammability test: Temperature-time curve of APP group (a); Temperature-time curve of Borax group (b); APP treated specimen after test (c); Control group specimen after test (d); Borax treated specimen after test (e)\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/51d33f673f94f6c5b4ae8541.png"},{"id":59641568,"identity":"0361288d-6e96-414c-b11e-fc3b67367ae6","added_by":"auto","created_at":"2024-07-04 07:59:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":73277,"visible":true,"origin":"","legend":"\u003cp\u003eResults of range analysis of different manufacturing parameters\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/d20787bfe8dc67afe328e80c.png"},{"id":59642409,"identity":"40c352e9-9428-459a-bf68-3e5c66237f29","added_by":"auto","created_at":"2024-07-04 08:07:17","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":121510,"visible":true,"origin":"","legend":"\u003cp\u003eColor changes of specimens\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/7188a5148e4db34ee4b7f618.png"},{"id":59641573,"identity":"33e27567-7ca5-4371-9b70-83f6596f162c","added_by":"auto","created_at":"2024-07-04 07:59:18","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":55167,"visible":true,"origin":"","legend":"\u003cp\u003eMass loss rate\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/44f5d9307e98399b45abad2a.png"},{"id":59641565,"identity":"5ee08c20-b015-45be-8f81-24862015b42a","added_by":"auto","created_at":"2024-07-04 07:59:17","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":127125,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical properties of specimens\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/c61eee32c458c061dc7f3b08.png"},{"id":59641580,"identity":"565db7ca-e56d-40af-8ffb-faa5e16191d6","added_by":"auto","created_at":"2024-07-04 07:59:19","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":305801,"visible":true,"origin":"","legend":"\u003cp\u003eTypical failure modes of bending specimens\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/efb3807001d1998440c49e21.png"},{"id":59641563,"identity":"e5bc255f-5fbb-4a57-a99e-7b9459680bc4","added_by":"auto","created_at":"2024-07-04 07:59:17","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":125768,"visible":true,"origin":"","legend":"\u003cp\u003eTypical failure modes of compressive specimens\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/83d9d12afa37abeb144817c6.png"},{"id":72640690,"identity":"1e3edaac-3c96-4516-ae19-27f94b4a689b","added_by":"auto","created_at":"2024-12-30 16:08:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3529933,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/4f3aeb24-3756-4c49-b8d4-658a6c009270.pdf"},{"id":59641551,"identity":"097b17a8-e892-4ef5-b579-42d3688f9215","added_by":"auto","created_at":"2024-07-04 07:59:15","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":17656,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-4603151/v1/8656a8962d2ea59e8128a095.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Research on mechanical properties and fire resistance of flame- retardant laminated veneer lumber fabricated with fast-growing Poplar","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWood, one of the oldest building materials, is utilized in construction globally. Traditional timber buildings often employ logs as structural members to bear loads. However, this approach overlooks the impact of factors such as moisture content, knots, grain direction, and decay on the strength of wood (Lu et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Pang et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These defects often lead to deformations and cracks in wood members (Wang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In the present era where sustainability is a global development trend, many industries are embracing green transformation, and the construction industry is no exception. In this context, Engineered Wood Products (EWPs) have become a research hotspot due to their high strength and carbon sequestration capabilities (Li et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e; Shi et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Laminated Veneer Lumber (LVL) is a type of EWP. Thanks to its laminated structure, LVL effectively mitigates the influence of natural defects on mechanical properties and offers flexibility in size. Through techniques like finger jointing and layer addition, various sizes of LVL columns, beams, and panels can be manufactured as structural members in timber structures (Awaludin et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tapia and Aicher, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022b\u003c/span\u003e), particularly in North America and Japan (Blanchet et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wang and Dai, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe quality of veneers has a significant impact on the performance of LVL. The higher the quality of veneers, the better the performance of LVL. LVL is typically manufactured using softwoods such as Spruce, Pine, Hemlock-fir, Douglas-fir, and Larch(Colak et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Duriot et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e; Wang and Dai, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Consequently, some countries initially imported raw veneers. However, due to the high transportation costs, scholars are increasingly focusing on using local wood species to produce LVL (Awaludin et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Duriot et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Slabohm and Militz, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). China possesses abundant timber resources, especially fast-growing Poplar. Poplar is generally utilized in furniture and construction as nonstructural members due to its weak fire resistance and poor mechanical properties (Yue et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Producing LVL from fast-growing Poplar proves to be an effective way to address these weaknesses.\u003c/p\u003e \u003cp\u003eAlthough LVL possesses good mechanical properties, it remains combustible. Wood, being a natural polymer material, forms a layer of char on its surface when exposed to fire (Liu and Fischer, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The charring layer has a much lower thermal conductivity than wood and isolates oxygen (Xu et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), thus preserving the thickness and strength of the remaining parts. Recognizing this unique characteristic, numerous studies and simulations on charring rate, fire resistance, and the limit-bearing capacity of timber members have been conducted (Xing et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e). Considering the different charring performances of various materials, Xu et al. (Xu et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) conducted cone calorimeter tests to compare the combustion and charring properties of three softwood species and two hardwood species. Cui et al. (Cui et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) examined the combustion properties of four common Engineered Wood Products (EWPs) and assessed the AHP-based multi-level fire hazard, Qin et al. (Qin et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) investigated the structural performance and charring properties of three loaded wood: Douglas-fir, Radiata pine and Red cedar. The studies mentioned above focus on the characteristics of wood during combustion. In fact, before the formation of the charring layer, some organic substances inside wood have already started to pyrolyze, resulting in a permanent decrease in strength (Pan et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Eurocode 5 (EN1995-1-2) defines 300\u0026deg;C as the starting point of charring. Investigating the physical-mechanical properties of wood before this point can provide a reference for calculating the residual capacity of timber members that are distant from the center of the fire and supporting structural safety evaluation after fire.\u003c/p\u003e \u003cp\u003eFlame-retardant treatment is a typical method for EWPs to achieve flame retardancy, with two common approaches. The first method involves applying flame-retardant coatings to already prepared EWPs (Li et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In this case, retardants only exist on the surface of the members and have minimal influence on their strength. However, once the coating loses its function, the untreated interior parts become exposed to fire, and their flame retardancy reverts to the original level immediately. In comparison to coatings, impregnated modification is more commonly chosen (Tian et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This method involves treating the raw materials directly, ensuring that retardants are present in every section of the prepared EWPs, thus guaranteeing overall flame retardancy. Although flame retardancy is improved, the addition of retardants still reduces the mechanical properties of veneers (Yang and Zhang, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Moreover, the strength of LVL is significantly influenced by the adhesive as well (Sulaiman et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), which is closely related to manufacturing parameters. During the hot-pressing process, excessively high temperatures cause the outer adhesive to cure prematurely, hindering heat transfer to the core. Conversely, incomplete curing occurs at low temperatures, leading to delamination and a negative impact on bearing capacity. Therefore, finding a balance between flame retardancy and mechanical properties is necessary.\u003c/p\u003e \u003cp\u003eIn summary, existing research has shown the following problems. (1) research on utilizing low-quality fast-growing veneer to manufacture high-performance flame-retardant LVL composites is very lacking. (2) despite that impregnated treatment induces the mechanical properties of veneers, it is difficult to scientifically determine the balance points between mechanical properties and flame retardancy. (3) many scholars have evaluated the charring performance of wood, but research on changes of physical-mechanical properties before charring is less. Based on the aforementioned research background, this paper aims to improve the mechanical properties and flame retardancy of LVL composites by optimizing the manufacturing parameters and produce flame-retardant LVL composites with fast-growing poplar for structural use.\u003c/p\u003e \u003cp\u003eThe structure of this paper is as follows: In Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, eight types of FRLVL composites were manufactured according to the orthogonal experiments. Then the mechanical properties and fire resistance tests of different FRLVL composites were carried out. In Section \u003cspan refid=\"Sec8\" class=\"InternalRef\"\u003e3\u003c/span\u003e, variance analysis and range analysis were employed to assess the significance and order of the four factors. A series of optimal manufacturing parameters was determined based on the results of the statistical analysis. In Section \u003cspan refid=\"Sec12\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the fire performance of the optimal FRLVL composites was tested to examine the changes in physical-mechanical properties and identify typical failure modes in the condition of elevated temperature and increased time.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Materials\u003c/h2\u003e\n \u003cp\u003eFast-growing Poplar (Populus euramericana cv. I-214) veneers with dimensions of 1270 mm \u0026times; 840 mm \u0026times; 2 mm and a moisture content of 10\u0026thinsp;\u0026plusmn;\u0026thinsp;2%, classified into Grade I and III according to GB/T 13010\u0026thinsp;\u0026minus;\u0026thinsp;2020, were provided by Guannan Yindelong Wood Industry Co., Ltd.Phenol-formaldehyde (PF) adhesive, with a solid content and viscosity of 47\u0026thinsp;\u0026plusmn;\u0026thinsp;1% and 365 mPa.s, was purchased from Dare Wood-Based Panel Group (Jiangsu, China).\u003c/p\u003e\n \u003cp\u003eAmmonium polyphosphate (APP, (NH\u003csub\u003e4\u003c/sub\u003ePO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003en\u003c/sub\u003e, n\u0026lt;30) and Borax (BX, (Na\u003csub\u003e2\u003c/sub\u003eB\u003csub\u003e4\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u0026middot;10H\u003csub\u003e2\u003c/sub\u003eO)) were obtained from Zhengzhou Jiajie Chemical Products Co., Ltd., and Liaoning Shougang BoronIron Co., Ltd., respectively.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Experiment design\u003c/h2\u003e\n \u003cp\u003eIn this study, the hot-pressing pressure of each group was set to the same level in order to obtain the same compression ratio, keeping the thickness of each group being 20 mm. Besides, hot-pressing time (Parameter a), hot-pressing temperature (Parameter b), concentration (Parameter c), and types of retardants (Parameter d) were chosen as factors, with each factor having two levels. Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the orthogonal experimental sets, where A represents the retardant APP, B represents the retardant BX, and C represents the control group.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eOrthogonal experimental programs\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"7\"\u003e\u003c/colgroup\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eGroup\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eserial number\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHot-pressing time\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHot-pressing temperature\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHot-pressing pressure\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConcentration\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eretardant\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(min/mm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(\u0026deg;C)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(%)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003eA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003eAPP\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003eB\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003eBorax\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"4\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e140\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e/\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 Flame-retardant LVL preparation\u003c/h2\u003e\n \u003cp\u003eThe FRLVL designed in this paper, which consists of 12 layers, is categorized into three layers: the top surface, core, and bottom surface layers. In order to strength bending performance, the top and bottom surface layers comprise 2 Grade I veneers without impregnation, while the core layer consists of 8 Grade III veneers impregnated with retardants. Veneers were immersed in the APP or BX solution for 2 hours and then dried in the oven for 30 minutes. Following impregnation, roller coating was applied with a spread amount of 190 g/m\u003csup\u003e2\u003c/sup\u003e. As depicted in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, adhesive was coated onto the surfaces with lathe checks to achieve higher bonding performance (Li et al. \u003cspan class=\"CitationRef\"\u003e2020a\u003c/span\u003e; Li et al. \u003cspan class=\"CitationRef\"\u003e2020b\u003c/span\u003e). Subsequently, veneers laid up parallel to the grain were pre-pressed. The pre-pressing parameters included a pressure of 1.0 MPa, a time of 40 minutes, and a temperature of 20\u0026deg;C. Then the specimens were hot-pressed according to the orthogonal experimental programs. Finally, all specimens were conditioned for 12 hours to eliminate thermal stress and reach equilibrium moisture content. The specific manufacturing process is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4 Mechanical properties and Fire resistance tests\u003c/h2\u003e\n \u003cp\u003eThe Modulus of Elasticity (MOE), Modulus of Rupture (MOR), Compressive Strength (CS), and Horizontal Shear Strength (HSS) of FRLVL were tested according to the information provided in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. MOE, MOR, and HSS were tested in the flatwise direction, while CS was tested parallel to grain. MOE and MOR tests used the same specimens. All the mechanical properties tests were conducted using a universal testing machine.\u003c/p\u003e\n \u003cp\u003eThe information on fire resistance is also provided in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Limited Oxygen Index was tested using a JF-3 oxygen index meter, with specimens recycled from the MOR test. During the flammability test, the nozzle was kept 100 mm away from the specimens, and the angle between the nozzle and the specimens was 45\u0026deg;. The flame was applied towards the center of the specimens with a tip temperature of 1300\u0026thinsp;\u0026plusmn;\u0026thinsp;100\u0026deg;C. The reverse side temperature of specimens was measured every 30 seconds until a black spot emerged. The test process is illustrated in Fig. 2.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eInformation on test specimens\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eProperties\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003eDimensions of specimens\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003eQuantity\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eStandards\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eMOE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"2\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003e300mm\u0026times;20mm\u0026times;20mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"2\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003e6\u0026times;12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 1936.1\u0026ndash;2009 [34]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eMOR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 1936.2\u0026ndash;2009 [35]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eHSS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003e120mm\u0026times;40mm\u0026times;20mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003e6\u0026times;12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 20241\u0026thinsp;\u0026minus;\u0026thinsp;2021 [36]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eCS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003e30mm\u0026times;20mm\u0026times;20mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003e6\u0026times;12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 1935\u0026ndash;2009 [37]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eLOI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003e100mm\u0026times;10mm\u0026times;3mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003e9\u0026times;12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 2406\u0026thinsp;\u0026minus;\u0026thinsp;1993 [38]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" style=\"width: 15.6436%;\"\u003e\n \u003cp\u003eFT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 36.6337%;\"\u003e\n \u003cp\u003e250mm\u0026times;90mm\u0026times;20mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 13.6634%;\"\u003e\n \u003cp\u003e3\u0026times;12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" style=\"width: 34.0594%;\"\u003e\n \u003cp\u003eGB/T 8626\u0026thinsp;\u0026minus;\u0026thinsp;2007 [39]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5 Statistical analysis\u003c/h2\u003e\n \u003cp\u003eIBM SPSS Statistics 26 was employed for statistical analysis. Variance analysis was used to estimate the significance (p-value\u0026thinsp;\u0026lt;\u0026thinsp;0.05 and p-value\u0026thinsp;\u0026lt;\u0026thinsp;0.01) of the four factors on the mechanical properties and fire resistance of FRLVL. Then, range analysis was conducted to determine the influence order of different levels. Taking into account the results of variance analysis and range analysis, the optimal manufacturing parameters of FRLVL were ultimately obtained based on the statistical analysis.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and disscussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Mechanical properties\u003c/h2\u003e\n \u003cp\u003eAs depicted in Fig.\u0026nbsp;3a, the average MOE for groups A and B was 12.27 GPa and 12.09 GPa, respectively, representing decreases of 4.7% and 6.1% compared to the control group. In Fig.\u0026nbsp;3c, the CS of each group is displayed, with the CS of A3 measuring 56.64 MPa, even higher than that of the control group. Conversely, P2 exhibits the lowest CS at 48.62 MPa, representing a decrease of 7.1% compared to the control group. These findings suggest that manufacturing parameters contribute to slight changes in both MOE and CS.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;3b and Fig.\u0026nbsp;3d present the results of MOR and HSS tests. In contrast to the MOE and CS test results, the changes in MOR and HSS are slightly larger. All the MOR and HSS specimens are tested flatwise, and the strength of the adhesive layer significantly impacts MOR and HSS. For instance, due to the lower hot-pressing temperature and incomplete cure of the adhesive, the MOR of A1 and A4 is noticeably lower than that of A2 and A3, while the MOR of the A2 group is lower than A3. This is because the hot-pressing time is too long, causing not only a decrease in adhesive layer performance but also the pyrolysis of cellulose, lignin, and other components in wood, leading to a decrease in material performance.\u003c/p\u003e\n \u003cp\u003eFurthermore, empirical investigations demonstrated that impregnated veneers, when contrasted with standard veneers, exhibit increased brittleness. This aspect is considered one of the potential reasons for the divergence in the trends of MOR and HSS between Group P and Group A. Additionally, it should be noted that the phenomenon may be influenced by constraints in experimental conditions, including the impact of manual gluing.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Fire resistance\u003c/h2\u003e\n \u003cp\u003eFigure\u0026nbsp;4 illustrates the Limited Oxygen Index results for each specimen. The findings indicate that, in comparison to the control group, Group A exhibits the most substantial enhancement in the oxygen index, with increases of 25.7%, 34.6%, 23.9%, and 30.0%, respectively. Meanwhile, Group B shows an average oxygen index increase of 18.7%, 31.3%, 16.6%, and 22.1%. The overall trend among the groups is 2\u0026thinsp;\u0026gt;\u0026thinsp;4\u0026thinsp;\u0026gt;\u0026thinsp;1\u0026thinsp;\u0026gt;\u0026thinsp;3, indicating the effective functioning of both APP and BX. The LOI of Group A was all above 32%, which belonged to B1 level (nonflammable material) proposed in GB 8624\u0026thinsp;\u0026minus;\u0026thinsp;2012.\u003c/p\u003e\n \u003cp\u003eIn Fig. 5a and Fig. 5b, temperature-time curves for each group are presented. The maximum and minimum fireproof times in Group A were 1229s and 982s, while in Group B, they were 1081s and 840s. The average fireproof times for Group A and Group B were 1074.25s and 960.25s, respectively, representing increases of 74.3% and 55.8% compared to the control group. From the curves, it can be observed that due to the absence of retardants on the outer layer, the gradient of the FRLVL and the control group was initially similar. However, over time, the temperature rise rate of the FRLVL decreased significantly. Particularly in Group A, a noticeable pause in the curve occurred around 80\u0026deg;C, creating a \u0026quot;platform\u0026quot; shape. This phenomenon is attributed to the retardants in the core layers absorbing heat (Jiang et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e; Oezcifci and Okcu, \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e), while the charring layer formed by the surface veneer also hinders heat transfer. In the final stage of the experiment, when the specimen thickness was nearly zero, the temperature rose sharply in a short time until the specimens burned through.\u003c/p\u003e\n \u003cp\u003eAs shown in Fig. 5, the specimens from the experiments of Group A (Fig. 5c), Group B (Fig. 5e), and Group C (Fig. 5d) were compared to illustrate the charring range. FRLVL exhibited a narrower charring range, whereas the control group had a much larger range, reflecting the impact of flame retardants.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Statistical analysis\u003c/h2\u003e\n \u003cp\u003eTable \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e presents the results of the range analysis, encompassing Ki, ki, and R. Here, Ki(i\u0026thinsp;=\u0026thinsp;1,2) and ki represent the sum and average values of the experimental results containing a factor at the i level, respectively. R is the absolute value of the difference in ki, employed to reflect the varying influence of different levels. The influence trends of various parameters on the mechanical properties and fire resistance of FRLVL are illustrated in Fig.\u0026nbsp;6. For MOE, MOR, HSS, and CS, the order of influence of manufacturing parameters was b\u0026thinsp;\u0026gt;\u0026thinsp;c\u0026thinsp;\u0026gt;\u0026thinsp;d\u0026thinsp;\u0026gt;\u0026thinsp;a, b\u0026thinsp;\u0026gt;\u0026thinsp;c\u0026thinsp;\u0026gt;\u0026thinsp;a\u0026thinsp;\u0026gt;\u0026thinsp;d, c\u0026thinsp;\u0026gt;\u0026thinsp;b\u0026thinsp;\u0026gt;\u0026thinsp;a\u0026thinsp;\u0026gt;\u0026thinsp;d, and d\u0026thinsp;\u0026gt;\u0026thinsp;c\u0026thinsp;\u0026gt;\u0026thinsp;a\u0026thinsp;\u0026gt;\u0026thinsp;b, respectively. The optimal manufacturing parameters obtained based on the range analysis are presented in the \u0026quot;Optimal level\u0026quot; row of Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, and it is noteworthy that the optimal level varied for the four mechanical properties. Concerning fire resistance, although the optimal level for LOI and fireproof time was both a2b1c2d, LOI was significantly impacted by concentration, while fireproof time was pronounced influenced by the type of retardant. Additionally, hot-pressing time and temperature exhibited minimal impact on fire resistance.\u003c/p\u003e\n \u003cp\u003eThe results of the variance analysis are presented in Table S2. According to the table, the hot-pressing temperature (P-value\u0026thinsp;=\u0026thinsp;0.014) was the only factor that significantly influenced MOE. Compared with other factors, the hot-pressing temperature had a certain impact on MOR (P-value\u0026thinsp;=\u0026thinsp;0.073), but it did not attain statistical significance. HSS was found to be significantly affected by the concentration of flame retardants (P-value\u0026thinsp;=\u0026thinsp;0.031) exclusively. Moreover, the concentration of flame retardants had a significant impact on CS (P-value\u0026thinsp;=\u0026thinsp;0.020), while the type of retardant exerted the most substantial influence on CS (P-value\u0026thinsp;=\u0026thinsp;0.008). Regarding LOI, both the concentration and type of retardant demonstrated P-values less than 0.01, signifying a significant effect on LOI. It is founded that all four factors influenced fireproof time significantly, with P-values\u0026thinsp;\u0026lt;\u0026thinsp;0.05. Remarkably, the P-values of hot-pressing temperature, concentration, and type of retardant were all less than 0.01.\u003c/p\u003e\n \u003cp\u003eThrough the analysis above, it can be concluded that the optimal manufacturing parameters vary according to different mechanical properties and fire resistance. Considering LVL members are easily damaged when exposed to fire, fire resistance is the most crucial property that should receive more attention. Therefore, based on the combination of range and variance analysis results, the final optimal manufacturing parameters were determined as a2b1c2d1, which means a hot-pressing temperature of 140\u0026deg;C, hot-pressing time of 1.3 min/mm, retardant concentration of 15%, and type of retardant as APP.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Fire Performance of LVL with optimal manufacturing parameters","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 Test Methods for fire performance\u003c/h2\u003e\n \u003cp\u003eTo investigate the changes in the physical-mechanical properties of FRLVL after exposure to high temperatures, nine sets of tests were conducted, incorporating three temperatures (100\u0026deg;C, 150\u0026deg;C, 200\u0026deg;C) and three durations (30 minutes, 60 minutes, 90 minutes). These tests were performed on FRLVL manufactured using the optimal manufacturing parameters. Before the initiation of each test, the muffle furnace was preheated to the designated temperature, and then the specimens were placed into the furnace. To ensure the accuracy of the test data, only one group of specimens was treated at a time, with each group of specimens placed in the same position of the furnace. Following thermal treatment, the specimens were placed in a conditioning room for 24 hours before being tested for various properties. The MOE, MOR, and CS of the specimens were tested following the method outlined in section \u003cspan class=\"InternalRef\"\u003e2.4\u003c/span\u003e, and the mass loss rate of the specimens was calculated by the following formula:\u003c/p\u003e\n \u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$${m}_{L}=\\frac{{m}_{0}-{m}_{T}}{{m}_{0}}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere m\u003csub\u003eL\u003c/sub\u003e is the mass loss rate, m\u003csub\u003e0\u003c/sub\u003e is original mass, and m\u003csub\u003et\u003c/sub\u003e is the mass after treatment.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 Physical properties\u003c/h2\u003e\n \u003cp\u003eWood is a polymer material composed of cellulose, hemicellulose, lignin, and some extractives. With an increase in temperature, cellulose, hemicellulose, and lignin undergo a series of oxidation reactions, resulting in a gradual darkening of the color (Tomak et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). Color changes in specimens under different treatment conditions are shown in Fig. 7. It can be observed that the color changes in the core layer were less pronounced than in the top and bottom layers due to the addition of flame retardants. At 100\u0026deg;C, only the evaporation of water and the volatilization of some extractives occurred inside the wood, having a minimal effect on the color. At 150\u0026deg;C, attributed to the lack of flame retardant, the color of the outer layer became darker with increasing treatment time, while the core layer remained almost unchanged. Hemicellulose pyrolyzed when the temperature exceeded 200\u0026deg;C, causing a rapid color change.\u003c/p\u003e\n \u003cp\u003eThe average mass loss rate is depicted in Fig. 8, revealing an increase in the mass loss rate of FRLVL with temperature and time. The curve exhibited almost linearity at 100\u0026deg;C, during which the primary loss was moisture, absorbed through conditioning. In the 150\u0026deg;C stage, mass loss continued to increase, but the trend slowed when the treatment time exceeded 60 minutes. Upon reaching 200\u0026deg;C, due to the pyrolysis of the adhesive and organic matter inside wood, the mass loss rate increased rapidly, eventually surpassing the equilibrium moisture content (8%\u0026plusmn;0.2).\u003c/p\u003e\n \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\n \u003ch2\u003e4.2.2 Mechanical properties\u003c/h2\u003e\n \u003cp\u003eFrom the results in Fig. 9a, it can be concluded that the MOE of the specimens changed little at a temperature of 100\u0026deg;C and even increased when treated for 30 minutes at 150\u0026deg;C. This could be explained by the further curing of the adhesive caused by the evaporation of water (Li et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, with the addition of treatment temperature and time, this brief strengthening couldn\u0026rsquo;t offset the overall strength decrease. Thus, when the specimens were treated for more than 30 minutes at a temperature above 200\u0026deg;C, the MOE decreased significantly, with the maximum reduction rate being 18.2%.\u003c/p\u003e\n \u003cp\u003eCompared to MOE, the MOR of FRLVL experienced a more substantial change. The MOR of each specimen was all larger than 90 MPa at 100\u0026deg;C. However, when the temperature reached 200\u0026deg;C, the MOR of the specimens decreased to 60.31 MPa. The MOR of the specimens at 100\u0026deg;C and a treatment time of 30 minutes was 100.96 MPa, while the MOR of the specimen at 150\u0026deg;C and a treatment time of 90 minutes was 72.94 MPa. After the temperature exceeded 200\u0026deg;C, prolonged exposure to high temperatures led to the decomposition of hemicellulose and the further softening of wood fibers. Consequently, the mechanical properties of specimens decreased significantly. The MOR of specimens was only 45.14 MPa after 90 minutes of heating, representing a decrease of more than 55%.\u003c/p\u003e\n \u003cp\u003eThe CS of the specimens is depicted in Fig.\u0026nbsp;9(c). The CS of the specimens changed very little at 100\u0026deg;C and 150\u0026deg;C, which may be attributed to the evaporation of water having a certain effect on the mechanical properties parallel to grain. The CS of specimens declined significantly, falling below 45 MPa at 200\u0026deg;C.\u003c/p\u003e\n \u003cp\u003eAlthough the MOR and CS of the specimens decreased substantially at high temperatures, the MOE of specimens treated for 90 minutes at 150\u0026deg;C was still sufficient to reach the LVL-35P level specified in the LVL handbook (LVL Handbook Europe. Finland), and the MOR and CS belonged to the LVL-50P level. The MOE of specimens treated for 90 minutes at 200\u0026deg;C could fulfill the standard of LVL-32P level, and the MOR and CS were able to reach the LVL-35P level. In general, FRLVL manufactured by the method in this research still retained excellent mechanical properties after exposure to high temperatures.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\n \u003ch2\u003e4.2.3 Typical failure mode\u003c/h2\u003e\n \u003cp\u003eFigure\u0026nbsp;10 revealed four typical failure modes in bending specimens. Types I, II, and III failures mainly occurred in specimens treated at temperatures of 100\u0026deg;C and 150\u0026deg;C. For Type I failure, when the specimens were bent, the veneer at the bottom of the LVL received tensile force, resulting in damage to the wood fibers. Type II and Type III failures mainly occurred in the compression and tension areas of the specimens. In these areas, there often existed tiny displacements between veneers, which applied shear force to the glueline and eventually led to the delamination of the adhesive layer. As the temperature increased, the flame retardants and adhesive decomposed, and wood fibers further softened. Thus, the brittleness of specimens kept increasing, and the failure mode of the FRLVL gradually changed to the brittle failure of Type IV, which was also accompanied by the shear failure of the adhesive layer.\u003c/p\u003e\n \u003cp\u003eAs depicted in Fig.\u0026nbsp;11, no significant relationships were identified between different typical failure modes and treatment temperature. Four distinct failure modes were consistently observed across all temperatures: diagonal cracks in the veneer surface (I), adhesive layer delamination (II), outer fiber bulging out (III), and middle fiber bulging out (IV). According to force analysis, the maximum shear stress occurred at a 45\u0026deg; angle when the specimens were subjected to pressure, causing diagonal cracks in the veneer surfaces due to an out-of-plane directional slip between fibers. Type II failure resulted from the opposing movement of adjacent veneers, creating a shear force perpendicular to the grain in the adhesive layer and leading to delamination. Type III and IV failures were attributed to the extrusion of wood fibers, causing them to bulge out in the in-plane direction.\u003c/p\u003e\n \u003cp\u003eCertainly, it should be recognized that while the repetition number for each group was determined according to the Chinese national standard, the absence of a specific repetition number poses a challenge affecting the precision of the test results. Furthermore, in this paper, only nine groups of treatment time and temperature were designed, which is fewer compared to other research studies. Consequently, for the acquisition of more accurate and detailed test data, future work should include expanded sample size and the incorporation of additional treatment time and temperature groups.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThis paper utilized fast-growing Poplar to manufacture flame-retardant LVL composites. Orthogonal experiments were carried out to estimate the effects of four manufacturing parameters (hot-pressing time, hot-pressing temperature, retardant concentration, and retardant types) on the mechanical and flame-retardant properties of LVL composites. Then the optimal manufacturing parameters of FRLVL composites were obtained according to statistical analysis. Finally, the fire performance of FRLVL composites made by optimal manufacturing parameters was tested. The main conclusions can be drawn as follows:\u003c/p\u003e \u003cp\u003e(1) The fire resistance of FRLVL composites was significantly influenced by the addition of two flame retardants compared to its mechanical properties. Only a few mechanical properties were found to be moderately susceptible to flame retardants. Both APP and BX demonstrated significant flame-retardant effects. But compared with BX, APP notably improved the flame retardancy of LVL composites.\u003c/p\u003e \u003cp\u003e(2) The optimal manufacturing parameters of FRLVL composites under the conditions of this study were a2b1c2d1, which included a hot-pressing temperature of 140\u0026deg;C, hot-pressing time of 1.3 min/mm, retardant concentration of 15%, and retardant of APP. The MOE, Oxygen Index, and Fireproof time achieved the best levels in this condition. Although the CS and HSS were not at the optimal level, the gap between them and other groups was very small. Considering the MOR declined significantly at this condition, the FRLVL composites manufactured in this study might be recommended as columns instead of beams in timber buildings.\u003c/p\u003e \u003cp\u003e(3) The temperature and exposed time significantly affected the mechanical properties of LVL composites. With the increase of time and temperature, wood experienced irreversible pyrolysis, leading to the massive loss of strength when the temperature reached 200\u0026deg;C and exposed time reached 90 min. However, although the mechanical properties dropped significantly, the LVL composites manufactured in this study still show high-strength.\u003c/p\u003e \u003cp\u003e(4) When the temperature was below 150\u0026deg;C, the typical failure modes of bending specimens mainly contained delamination and fiber fracture. With the increase in temperature, wood fiber kept softening, and the failure mode of the specimens gradually switched to brittle failure. Compressive failure modes of specimens did not have a strong relation between temperature and exposed time. Four typical failure modes were observed under all conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003e Competing interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eZ.C.: investigation, validation, formal analysis, software, writing-original draft; Q.C.: funding acquisition, supervision; B.L.: methodology, formal analysis; J.S.: investigation; Z.W.: project administration, conceptualization,. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors acknowledge sponsorship by National Key R\u0026amp;D Program of China [2023YFF0906100]\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAwaludin A, Irawati IS, Shulhan MA (2021) Two-dimensional finite element analysis of the flexural resistance of LVL Sengon non-prismatic beams (vol 10, e00225, 2019). 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Finland: Federation of the Finnish Woodworking Industries\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"european-journal-of-wood-and-wood-products","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"harw","sideBox":"Learn more about [European Journal of Wood and Wood Products](http://link.springer.com/journal/107)","snPcode":"107","submissionUrl":"https://submission.nature.com/new-submission/107/3","title":"European Journal of Wood and Wood Products","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fast-growing Poplar, Fire performance, Laminated veneer lumber, Orthogonal experiments","lastPublishedDoi":"10.21203/rs.3.rs-4603151/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4603151/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo fully utilize Chinese fast-growing timber resources, fast-growing poplar was selected for manufacturing flame-retardant laminated veneer lumber (FRLVL). Firstly, orthogonal experiments were conducted to assess the impact of four factors (hot-pressing time, hot-pressing temperature, retardant concentration, and retardant types) on the mechanical properties and burning behavior of FRLVL. Subsequently, optimal manufacturing parameters were chosen based on statistical analysis. Finally, the fire performance of LVL manufactured with the optimal parameters was evaluated to investigate changes in physical-mechanical properties under high-temperature conditions. Results indicated that the addition of retardants led to a decrease in mechanical properties. In comparison to the control group, LVL composites impregnated with two retardants exhibited a higher limited oxygen index and longer fireproof time, with the effects of ammonium polyphosphate (APP) surpassing those of borax (BX). The optimal manufacturing parameters were a hot-pressing temperature of 140\u0026deg;C, a hot-pressing time of 1.3 min/mm, and concentrations of 15% for both retardant types. As the temperature increased, the mechanical properties of LVL manufactured with the optimal parameters decreased noticeably. However, under the conditions of a temperature of 200\u0026deg;C and a treatment time of 90 min, the mechanical properties of LVL composites still met the LVL-32P grade proposed in LVL handbook.\u003c/p\u003e","manuscriptTitle":"Research on mechanical properties and fire resistance of flame- retardant laminated veneer lumber fabricated with fast-growing Poplar","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-04 07:59:09","doi":"10.21203/rs.3.rs-4603151/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-08T10:38:07+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"116050117011130376772112913939063492920","date":"2024-08-18T02:40:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"335493381177243613783017189050132964587","date":"2024-06-30T18:33:16+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-28T03:22:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"189397015869725265629243922120335621866","date":"2024-06-27T09:12:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-27T08:58:31+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-23T13:32:30+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-06-19T05:25:13+00:00","index":"","fulltext":""},{"type":"submitted","content":"European Journal of Wood and Wood Products","date":"2024-06-19T04:37:25+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"european-journal-of-wood-and-wood-products","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"harw","sideBox":"Learn more about [European Journal of Wood and Wood Products](http://link.springer.com/journal/107)","snPcode":"107","submissionUrl":"https://submission.nature.com/new-submission/107/3","title":"European Journal of Wood and Wood Products","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"dce4796f-b5e0-4997-b454-e1329c13db9d","owner":[],"postedDate":"July 4th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-12-30T16:03:29+00:00","versionOfRecord":{"articleIdentity":"rs-4603151","link":"https://doi.org/10.1007/s00107-024-02158-z","journal":{"identity":"european-journal-of-wood-and-wood-products","isVorOnly":false,"title":"European Journal of Wood and Wood Products"},"publishedOn":"2024-12-26 15:57:51","publishedOnDateReadable":"December 26th, 2024"},"versionCreatedAt":"2024-07-04 07:59:09","video":"","vorDoi":"10.1007/s00107-024-02158-z","vorDoiUrl":"https://doi.org/10.1007/s00107-024-02158-z","workflowStages":[]},"version":"v1","identity":"rs-4603151","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4603151","identity":"rs-4603151","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}