A timber-concrete composite (TCC) slab composed of nail-laminated timber (NLT) and topping concrete (TC) was developed for flooring applications. The NLT was laminated alternately with lumber and plywood. To investigate the nonstructural dimensional behavior of the TCC slab, the temperature, relative humidity (RH), and dimensional changes of the slab exposed to outdoor air were monitored for 205 days. Temperature change was transmitted directly to both components, and RH change was transmitted gradually to the NLT. Concrete pouring caused a sharp increase in NLT width, which was the laminating direction of the nails. This resulted from swelling of the wood owing to the moisture in the concrete mixture and loosening of the nail lamination. The member composition for the nail-laminating system, fastener type, and concrete volume help to secure the dimensional stability of the NLT. Cracks in the TC caused width deformation, which was recovered by drying shrinkage of the TC. Correlation analysis among temperature, RH, and strain indicated that dimensional changes in NLT correlated strongly with RH, while those in TC correlated strongly with temperature. The correlation between longitudinal strain in the TC and strain in the three directions of the NLT was attributed to the notches designed for mechanical connection.
Dimensional Behavior of Nail-Laminated Timber-Concrete Composite Caused by Changes in Ambient Air, and Correlation among Temperature, Relative Humidity, and Strain
Sung-Wook Hwang,a Hyunwoo Chung,b Taekyeong Lee,a Kyung-Sun Ahn,c Sung-Jun Pang,a Ji Yong Kim,c Junsik Bang,c Minjung Jung,c Jung-Kwon Oh,a,b,c Hyo Won Kwak,a,b,c and Hwanmyeong Yeo a,b,c,*
A timber-concrete composite (TCC) slab composed of nail-laminated timber (NLT) and topping concrete (TC) was developed for flooring applications. The NLT was laminated alternately with lumber and plywood. To investigate the dimensional behavior of the TCC slab, the temperature, relative humidity (RH), and dimensional changes of the slab exposed to outdoor air were monitored for 205 days. Temperature change was directly transmitted to both components, and RH change was gradually transmitted to the NLT. Concrete pouring caused a sharp increase in NLT width, which was the laminating direction of the nails. This resulted from swelling of the wood because of the moisture in the concrete mixture and loosening of the nail lamination. The member composition for the nail-laminating system, fastener type, and concrete volume help to secure the dimensional stability of the NLT. Cracks in the TC caused width deformation, which was recovered by drying shrinkage of the TC. Correlation analysis among temperature, RH, and strain indicated that dimensional changes in NLT correlated strongly with RH, while those in TC correlated strongly with temperature. The correlation between longitudinal strain in the TC and strain in the three directions of the NLT was attributed to the notches designed for mechanical connection.
Keywords: Correlation; Dimensional change; Moisture content; Monitoring; Nail-laminated timber; Slab
Contact information: a: Research Institute of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea; b: Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea; c: Department of Agriculture, Forestry and Bioresources, College of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea;
* Corresponding author: firstname.lastname@example.org
Mass timber, the fruit of the advancement of engineered wood, is a large-scale structural material formed by laminating dimension lumber with adhesives, nails, and dowels. Cross-laminated timber (CLT), dowel-laminated timber (DLT), glued-laminated timber (GLT), and nail-laminated timber (NLT) are representative mass timber products. Mass timber construction has turned wood, a material for small buildings, into a structural material suitable for large buildings, and a competitor for steel and concrete (Abed et al. 2022). Replacing steel and concrete with mass timber products reduces the potential for global warming by 18.0 to 26.5%, even without considering the carbon fixed in the wood itself (Pierobon et al. 2019; Liang et al. 2020). Hence, the use of mass timber products as sustainable materials can be a practical solution for achieving low carbon emission and energy efficiency.
The manufacture of NLT requires only the most basic building materials and tools, such as wood and nails. The ease of fabrication of NLT panels has made them useful as walls, elevator and stair shafts, shear walls, and roofs for more than a century (Binational Softwood Lumber Council 2017; Hong 2017). The emergence of environmental issues caused by global warming is now creating new opportunities for mass timber, such as NLT. A nail-laminated timber-concrete composite (TCC) is a structural system that integrates wood and concrete, maximizing the inherent strengths of concrete and wood, and complementing their shortcomings (Yeoh 2010; Hong 2017). Numerous studies on the structural performance and behavior of NLT and TCC have been reported (Lukaszewska et al. 2008; Yeoh et al. 2011; Fragiacomo et al. 2018; Derikvand et al. 2019; Pang et al. 2022). However, the behavior due to moisture changes was not the primary concern of these studies.
Wood, a hygroscopic material, undergoes repeated moisture absorption and desorption depending on the surrounding climate, and this causes dimensional changes. A continuous high-humidity environment and the resulting high moisture content of wood can threaten structural stability by causing deterioration of wood strength, biological damage, and swelling (Franke et al. 2015; Dietsch and Winter 2018; Bobadilha et al. 2020; Sinha et al. 2020). Therefore, precise moisture determination is essential for wooden materials, and long-term observations of moisture and dimensional changes are required for structures using mass timber products.
Electrical resistance-based methods are standard approaches for moisture determination in wood used in large structures (Brischke et al. 2008; Dietsch et al. 2015; Björngrim et al. 2016; Hwang et al. 2021). However, electrical methods must control factors such as wood species, internal stress, preservatives, the position and distance of the inserted electrodes, and incomplete electrode contact (Hwang et al. 2021). High-frequency and near-infrared spectroscopy measurements, another approach for moisture determination, must consider the high cost and stability of long-term field observations. A hygrometric method that determines moisture content using wood properties in equilibrium with the surrounding climate can be a good alternative. The equilibrium moisture content (EMC) can be calculated from the temperature and relative humidity (RH) of the enclosed space inside wood using the sorption equation (Hailwood and Horrobin 1946). This method has fewer limitations in terms of internal moisture measurements and long-term observation.
Through long-term observation of a TCC slab developed for flooring applications, this study describes the moisture fluctuation and the resulting dimensional behavior of NLT and concrete, which are elements of the TCC slab. Long-term monitoring of the moisture and dimensional behavior of NLT panels has been reported (Hwang et al. 2022), but the composite behavior of NLT and concrete has yet to be investigated. To the best of our knowledge, this is the first report on the correlation between NLT and concrete for the dimensional behavior of TCC systems. The findings of this study can be used as primary data for TCC-based structure design and structural safety evaluation.
NLT-Concrete Composite Slab
A TCC slab for observing moisture and dimensional changes was designed with an underlying NLT panel connected to a concrete slab. As shown in Fig. 1, the NLT consists of 24 mm × 140 mm × 2000 mm (radial × tangential × longitudinal directions) lumber and 24 mm × 245 mm × 2000 mm (thickness × height × length) 9-ply plywood. All lumber was flat sawn wood. The wood species used for lumber and plywood was Japanese larch (Larix kaempferi). A total of 42 elements, 21 each of lumber and plywood, were used to fabricate the NLT. The lumber and plywood were alternately fastened with nails with head diameter, shank diameter, and length of 7 mm, 2.9 mm, and 75 mm, respectively. The end distance, edge distance, and spacing of nails for lamination nailing were 45, 25, and 120 mm, respectively. The end distance and edge distance were determined by EN 1995-1-1 (2004), and the spacing of the nails followed the recommendations of the Nail-laminated Timber U. S. Design & Construction Guide (Binational Softwood Lumber Council 2017). The air-dry densities of the lumber and plywood were 0.55 and 0.66 g/cm3, respectively. Plywood higher than the lumber formed bulkheads on the upper surface of the NLT. Additionally, the plywood had five notches with a height of 35 mm, length of 15 mm, and edge angle of 90° at the center, spaced 90 mm apart, to improve the slip modulus.
Fig. 1. Elements and lamination schemes of the nail-laminated timber panel used to fabricate the slab specimen
Topping concrete (TC) was formed by pouring ready-mixed concrete composed of Portland cement onto the NLT with a 1000 mm × 200 mm × 2000 mm (width × height × length) mold. The nominal strength, the nominal maximum size of the coarse aggregate, and the slump flow of the concrete mixture were 24 MPa, 25 mm, and 120 mm, respectively. The TC was 185-mm-thick based on the height of the underlying lumber and 80-mm-thick based on the height of the plywood, not including the notch height. The wet concrete was cured while exposed to ambient conditions. Finally, a TCC slab with dimensions of 1000 mm (width, X-direction) × 320 mm (height, Y-direction) × 2000 mm (length, Z-direction) was manufactured (Fig. 2a) and used to observe moisture and dimensional changes.
Fig. 2. Configuration of the slab specimen and installed sensors for observation: (a) Nail-laminated timber-concrete composite, (b) nail-laminated timber, and (c) topping concrete
Observation site and period
The TCC slab was placed outdoors on the Seoul National University campus to investigate the dimensional behavior of the slab resulting from fluctuations in the outdoor air. A canopy was built over the slab to prevent direct contact with sunlight and rain. Because the observation site (37°45’79.03” N, 126°94’79.62” E) was located near forests and streams, the RH of the site was somewhat higher than that of the reference point in Seoul (Hwang et al. 2022). The climatic conditions of the observation site are harsher than those where TCC is typically used.
Table 1. Details for Observation of Moisture and Dimensional Behavior of the TCC Slab
This study dealt with temperature, RH, and dimensional data collected from March 7 to September 27, 2022 (205 days) for a TCC slab. Because the concrete was poured (May 30, 2022) and cured during the observation period, the observation period for each element of the TCC slab was different, as listed in Table 1.
The sensors used to observe the temperature, RH, and dimensional changes in the TCC slab are listed in Table 1. A probe-type thermo-hygrometer (HMP 60, Vaisala, Vantaa, Finland) was placed in an instrument shelter in the canopy to measure the temperature and RH of the air.
Nine K-type thermocouples were installed inside the lumber elements in the bottom quadrant to measure the temperature of the NLT, as shown in Fig. 2b. Meanwhile, nine coin-type thermo-hygrometers (DS 1923, Maxim Integrated, San Jose, CA, USA) for measuring temperature and RH were installed at points opposite to where the thermocouples were installed. All DS 1923 sensors used in this study were sealed with a waterproof/breathable fabric made of expanded polytetrafluoroethylene (ePTFE), known as Gore-Tex, to prevent moisture ingress into the sensor. In addition, a thermo-hygrometer (HMP60) was installed at the center of the NLT as a spare. All thermocouples and thermo-hygrometers were placed at a depth of 7 cm in the middle of the lumber height. The holes where the sensors were located were double-sealed with a polyvinyl acetate adhesive and silicone sealant to prevent the inflow of ambient air.
Three thermo-hygrometers (DS1923) were installed inside the concrete layer to measure the temperature and relative humidity (RH) of the TC (Fig. 2c). It is difficult to accurately position the sensors in the middle of the TC height during concrete pouring. Hence, concrete bars with dimensions of 24 mm × 180 mm × 24 mm (width × height × length) were prefabricated with an ePTFE-sealed sensor positioned at the center of the bar. Concrete bars were placed on the central lumber of the NLT, and concrete was poured over them.
Wire tension-type displacement transducers (DP-500G, Tokyo Measuring Instruments Laboratory, Tokyo, Japan) were used to measure dimensional changes in the TCC slab elements. Dimensional changes in the width (X), height (Y), and length (Z) of the elements were determined using six displacement transducers installed on the surfaces of the NLT and TC.
All sensors were monitored by a data logger (CR1000, Campbell Scientific, Logan, UT, USA) equipped with a relay multiplexer (AM16/32B, Campbell Scientific), except for the DS 1923 thermo-hygrometer with a built-in data logger. The DS 1923 sensors stored the temperature and RH at 1-h intervals. The other sensors recorded measurements at 30-s intervals, and the data logger stored 1-h and 24-h average measurements. The dedicated software of the data logger provided data management and real-time monitoring.
Evaluation of Changes in Moisture and Dimension
To evaluate the moisture in the NLT, the temperature and RH data acquired from the thermo-hygrometers were converted into EMC using Eq. 1 (Glass and Zelinka 2010) based on the Hailwood–Horrobin sorption model (Hailwood and Horrobin 1946),
where h is the RH (%); T is the temperature (°C); and W, k, k1, and k2 are variables related to temperature. Plywood is known to have a slightly lower EMC than solid wood (APA 2010), but the difference decreases as RH increases. Hence, this study assumed that the EMC of both components was similar.
The measurements in the three orthogonal directions obtained from the displacement transducers were converted to strain using Eq. 2 to evaluate dimensional changes in the TCC elements,
where is the linear strain in a single direction, and and are the initial and deformed lengths, respectively.
The Pearson correlation coefficient was used to quantify the relationship between the variables related to moisture and the dimensional behavior of TCC elements. The correlation coefficient indicating a linear relationship between the two variables was calculated using Eq. 3,
where r is the correlation coefficient, n is the sample size, and a and b are the observations of the two variables.
The Student’s t-test was performed using Python 3.8 with the open-source library SciPy to determine whether the relationship between the variables was significant. The t-score was calculated from the correlation coefficient using Eq. 4, and the significance of the association was evaluated with significance levels of 0.05 and 0.01 for the two-tailed p-value.
RESULTS AND DISCUSSION
Temperature and RH Changes
Figure 3 shows the temperature changes at nine points inside the NLT and in the air. Initially, the NLT temperatures were lower than the air temperature. After the air temperature increased to approximately 15 °C, the NLT and air temperatures remained at a similar level. Fluctuations in the air temperature were directly transferred into the NLT. The inset in Fig. 3 shows that the temperatures of the NLT at nine points fluctuated by forming two clusters. The two clusters correspond to different sensor positions, that is, the edges and center of the NLT. This indicates that the temperature transfer rate differed for each part of the NLT. The temperature difference inside the NLT was monitored throughout the observation period, including when the concrete was poured. Descriptive statistics of the air and NLT temperature data are presented in Table 2.
Fig. 3. Changes in temperature of ambient air and inside nail-laminated timber
Table 2. Descriptive Statistics of Temperature, RH, and EMC Acquired from Elements of the TCC Slab and Ambient Air
In contrast to the temperature change, the noticeable fluctuation in the RH of the air was transmitted gently into the NLT because of the humidification effect of the wood (Fig. 4a). However, the variation pattern of RH could be divided into two clusters, similar to the behavior of the temperature. In the early stages of observation, the RH values of the sensors located at the edges were higher than those of the central sensors. However, this behavior was reversed when the concrete was poured. Within 24 h after concreting, the RH of sensor no. 9, closest to the centroid of the NLT, increased from 55.9% to 62.0%, and after a week, the value increased to 67.2%. This sharp increase was observed in the sensors at the center but not at the edge. The RH change at the edge seemed to be dominated by air RH rather than moisture introduced by the concrete.
Fig. 4. Changes in relative humidity (a) and equilibrium moisture content (b) at nine measuring points inside nail-laminated timber
The EMC of the NLT and air calculated from the temperature and RH are depicted in Fig. 4b. The EMC is the average of nine measurements. The humid climate in the summer increased the air EMC, which caused an increase in the EMC of the NLT. The EMC of the NLT started at a low level of 9.3% on the first day of observation and peaked at 16.3% on September 5 (Table 2). The minimum and maximum values at the center were 8.7% and 16.7%, respectively. The EMCs are within a safe range for strength degradation and biodegradation (Shupe et al. 2008; Kirker et al. 2016). Concrete pouring caused an increase in the EMC of the NLT. A week after concreting, the EMC had increased 1.5% from 10.3% to 11.8%. Since then, the EMC continued to grow, influenced by a large amount of water in the concrete and an increasing ambient RH. Although the increase in EMC by concreting did not threaten the safety of the NLT, it is worth considering the formation of a moisture barrier using waterproof paint to minimize the increase in internal moisture (Fragiacomo et al. 2018; Ou et al. 2020).
Figure 5 shows the temperature and RH trends in the TC. Similar to NLT, temperature fluctuations in the air were reflected inside the TC. Although not exceptionally high, a temperature slightly higher than that of air was seen in the TC at the beginning of curing, which was likely because of the heat of hydration of concrete (Kaleta-Jurowska and Jurowski 2020). The internal RH of the TC during curing was maintained at 100% and then decreased from the edge after 15 d, and from the center after a month. Temperature variation is a factor that induces the thermal expansion or contraction of concrete (Ng and Kwan 2016), and a decrease in RH during curing, i.e., evaporation of free water, can lead to drying shrinkage (Sakata 1983). Displacement transducers captured the deformation of the TC because of temperature and RH fluctuations.
Fig. 5. Temperature and relative humidity changes in topping concrete
The NLT strain in three directions versus time is plotted in Fig. 6a. The strain in the height (Y-direction) corresponding to the tangential direction of the wood was the highest, followed by that in the width (X-direction) corresponding to the radial direction. The maximum strain in each direction was 1.46% and 0.80%, respectively (Table 3).
Table 3. Descriptive Statistics for Strains of Elements of the TCC Slab
The longitudinal (Z-direction) strain was negligible with a maximum value of 0.02%. The normalized strain in the three directions shows a pattern resembling the internal RH fluctuations of the NLT (Fig. 6b), indicating that moisture fluctuations induce dimensional changes.
Fig. 6. Original and normalized strain in three orthogonal directions of nail-laminated timber (a, b) and topping concrete (c, d)
Concrete pouring resulted in a sharp increase in width. The nature of the NLT system, in which nails mechanically fasten the elements in the X-direction, suggests relatively weak dimensional stability in this direction (Binational Softwood Lumber Council 2017). In other words, the sharp increase after concreting indicates that the nail-laminated structure loosened because of the load and moisture of the concrete (Fig. 7).
Fig. 7. The space created between the lamination of nail-laminated timber by concrete pouring
One week after concreting, the width strain increased from 0.1% to 0.43%, corresponding to a strain of 0.28% per 1% EMC. Because the reported shrinkages per 1% MC in the radial direction of Japanese larch lumber and plywood are approximately 0.142% (Kim and Lee 2002; Park et al. 2015) and 0.126% (Hwang et al. 2022), respectively, it is considered that the moisture and concrete load contributes similarly to the width swelling.
The RH continuously rose, starting in mid-June, and the strain in the X-direction and its fluctuation range were smaller than those in the Y-direction (Fig. 6a). This can be explained by the swelling anisotropy of wood. Another factor to be considered in interpreting the phenomenon is that the gap between the elements caused by the rapid increase in width after concreting offsets the actual swelling of the elements in the X-direction.
Hwang et al. (2022) reported that the width strain of an NLT with a flat surface increased 0.23% after concreting. In addition, they stated that the moisture inside the NLT was not significantly affected by concreting and that the load of the concrete dominated the rapid width increase. The width strain observed in this study was higher than the values reported by Hwang et al. The higher strain is a result of a combination of the following three factors:
(1) Higher concrete volume: The volume of concrete poured per square meter of NLT surface area was 0.122 m3, which is approximately 22% higher. The higher concrete load applied to the NLT results in further loosening of the nail-laminated structure.
(2) Bulkhead structure of NLT: Concrete pouring applies a flexural load to the NLT, resulting in width deformation. However, the bulkheads of the plywood elements accelerate this deformation because they create an additional moment.
(3) Fasteners: This study used nails to fasten the elements, whereas screws were used in the Hwang study. The withdrawal resistance force of the nails and screws used, calculated according to EN 1995-1-1 (2004) are approximately 1500 N and 16,000 N, respectively, indicating that the screws were more than ten times stronger. Fasteners are an essential factor in mechanical bonding between members. Thus, nailed lamination resulted in a more significant deformation than screwed lamination.
Therefore, the width strain of the NLT in this study was greater than the value reported by Hwang. Additionally, the wider gap between the elements created by width swelling may have contributed to the increase in internal moisture. The width swelling immediately after concreting is a permanent deformation that does not recover, even if the RH of the air is sufficiently low (Hwang et al. 2022).
Dimensional changes in the TC were recorded by installing displacement transducers two weeks after concreting. The deformation of the TC was based on its width (Fig. 6c). At an early stage, the strain in the X-direction increased rapidly to a maximum of 1.72%, and then it continuously decreased. Meanwhile, the maximum strain in the Y- and Z-directions was only 0.16% and 0.05%, respectively (Table 3).
Considering that concrete is an isotropic material, remarkable deformation in the X-direction is unusual. Several surface cracks that developed during curing were observed. Cracks can occur because of a thermal gap between the inside and outside of the structure and rapid water loss during curing (Holt and Leivo 2004; Ha et al. 2014). In this study, hairline cracks were observed on the surface of the TC. Additionally, the gap widened, while the internal RH of the TC was maintained at 100%, because of the difference in dryness between the surface and center. These factors likely affected the increase in width. The width recovery was attributed to drying shrinkage, as a decrease in the X-direction strain and a decrease in the internal RH to less than 100% coincided. The decrease in the strain of NLT in the three directions observed during the rising phase of RH at the end of the observation may suggest partial composite behavior of TCC due to the drying shrinkage of TC.
Although the strain in the Y-direction deviated from the pattern in some sections, the variation pattern of the normalized strain plots in the three directions resembles that of the TC temperature (Fig. 6d). These results indicate that the temperature is a factor in the dimensional change of concrete.
Correlation coefficients (r) were calculated to determine the relationships between TCC slab parameters (Fig. 8). The internal RH of the NLT showed the strongest correlation with the dimensional change in the NLT. The correlations between RH and strain of the NLT were very strong for the X-direction (r = 0.94), strong for the Y-direction (r = 0.74), and moderate for the Z-direction (r = 0.65). The correlations were considered statistically significant at p < 0.01.