Today, most markets are driven by innovation, and the construction market is no exception. New technologies allow building more flexible and efficient designs while also cutting the costs and construction time. Besides, one of the key concerns for many contemporary construction companies is sustainability. Developing technologies and materials that help to reduce environmental damage and are more sustainable is thus an important aspect of innovation in this area, requiring the attention of researchers and manufacturers alike.
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Cross-laminated timber is a relatively new material that was developed in Europe in the 1990s. Today, CLT is gaining popularity all over the globe, including the United States. CLT offers numerous benefits to construction companies, such as sustainability, cost reduction, and flexibility of design, which is why the CLT market is expected to grow further in the next decade. The present literature review will seek to introduce CLT by explaining its history, development, and the manufacturing process, while also providing information about its key benefits.
Definition of Cross-Laminated Timber (CLT)
Cross-laminated timber is a construction material consisting of layers of lumber stacked cross-wise (Crespell & Gagnon, 2010). The layers are typically placed at 90 degrees cross-wise, which increases the stability of CLT panels. In most cases, dimensional lumber is used to make CLT; as noted by Crespell and Gagnon (2010), low-grade timber can be used for interior layers, whereas higher-grade timber is required for external layers. In order to attach the layers to one another, the manufacturers use one of several options available. Gluing is the most popular method, which involves the use of glue to attack lumber board layers to one another (Karacabeyli & Douglas, 2013). Other techniques involve nailing or using wooden dowels to attach the layers. Recently, the development of interlocking cross-laminated timber has begun. It is a promising technology that avoids using adhesives or fasteners (Karacabeyli & Douglas, 2013). Every CLT panel consists of at least three layers of lumber boards of variable thickness. According to Karacabeyli and Douglas (2013), “thickness of individual lumber pieces may vary from 5/8 inch to 2.0 inches (16 mm to 51 mm), and the width may vary from about 2.4 inches to 9.5 inches (60 mm to 240 mm)” (p. 3). The length of pieces is determined by the panel size requirements and can be anywhere between 2 to 60 feet.
Brief History of CLT
The history of CLT begins in Europe in the 1990s. Initially, the material was developed in Switzerland; however, in 1996, Austrian researchers undertook a major joined effort with the construction industry to finalize the idea of Swiss scientists and produce what is now called cross-laminated timber (Crespell & Gagnon, 2010). Although the new material was promising and could be used for building a wide variety of structures, the popularization of CLT took several years. In the early 2000s, construction companies in some European countries, such as Sweden and the Netherlands began using CLT (Crespell & Gagnon, 2010).
One of the main motivations behind popularizing CLT were environmental concerns. As part of the green efforts, construction and manufacturing companies attempted to find new, sustainable alternatives that would also be cost-efficient. CLT proved to be beneficial in construction and became popular in Europe in the early 2000s. It was widely perceived to be a heavy construction system, similar to concrete, and was thus used to produce mid-rise and high-rise buildings, such multi-family homes, university buildings, and more (Karacabeyli & Douglas, 2013). The introduction of CLT to North America occurred a little later, and the material is still gaining trust and popularity among Canadian and American construction companies.
Development of CLT in North America
The successful experience of European construction companies prompted for the development of CLT in North America. As explained by Mallo and Espinoza (2015), the introduction of CLT to new markets follows the same steps of product adoption as any other innovation, including awareness, interest, evaluation, trial, and adoption. In Canada and the United States, the awareness about CLT was largely tied to European research and the publication of CLT handbooks in 2011 and 2013, respectively. At the moment, the development of CLT in North America remains in the early stages of product adoption.
Nevertheless, there is growing interest and initiative with regards to CLT use in North America, primarily in the United States and in Canada. Espinoza, Buehlmann, and Mallo (2015) note the activity of American researchers in studying and evaluating CLT. Besides, the U.S. government has issued grants to investigate the CLT and its potential effect on construction and the industry, and some demonstration projects were also produced in the country (Mallo & Espinoza, 2015). Given the current status of the forest industry in the United States and Canada, it is likely that the development of CLT in North America will continue at a steady pace. Jones, Stegemann, Sykes, and Winslow (2016) predict a similar trend, stating that CLT development in new markets represents an opportunity for innovative designers and companies to occupy the developing niche, thus improving motivation for adoption. With the support of the government and a solid body of research, more and more companies will be driven to adopt the technology in the next few years.
The manufacturing of CLT is a rather complex process involving 8 or 9 steps, depending on the facility. The first step is lumber drying. To ensure the stability of panels, it is imperative to use lumber boards dried to 10-14% of moisture content (Crespell & Gagnon, 2010). Next, the boards are trimmed to the desired length, whereas finger jointing is used to check the quality of boards (Karacabeyli & Douglas, 2013). After the trimming, the panels are assembled using one of the assembly options: gluing, nailing, or attaching wooden dowels. Next, the panels undergo vertical and horizontal pressing using either a hydraulic press or a compressed air press (Crespell & Gagnon, 2010).
The fifth step in the manufacturing process is designed to ensure that the panels are smooth. This is achieved by using planers or sanders on the surface of the CLT panel. Depending on the target structure, the panels are then trimmed and cut to make openings for doors, windows, and other features. This is done using CNC routers, which allow for high precision (Crespell & Gagnon, 2010). At this stage, the panels get their final look. The final stages include quality control, packaging, and shipping to the destination. At some factories, the panels also go through a carpentry room, where the insulation is installed (Crespell & Gagnon, 2010). A multi-step manufacturing process allows to enhance operations and improve the quality of the product. In addition, quality control assists in ensuring the quality of goods and prevents incidents such as cracking of the panels.
Benefits of CLT
CLT provides a number of benefits to construction companies. For instance, it can help to reduce costs and provides significant savings in construction time compared to poured concrete (Crespell & Gaston, 2011). Also, it enhances structural performance and livability of structures while also allowing for design flexibility, which is why it is an attractive opportunity for many companies (Finch, Wang, & Ricketts, 2013). However, besides these benefits, CLT also has environmental and seismic advantages, as well as good fire and acoustical performance. The following sections will review each of these advantages in detail.
Wooden materials receive a lot of attention in the construction industry due to their environmental benefits. This is also the case with CLT. According to Wilson (2006), using wood products in construction can help to reduce greenhouses gases. Thus, by using CLT in construction, companies can avoid contributing to global warming. In addition, Peñaloza, Erlandsson, and Falk (2016) study the effect of using bio-materials in construction on climate. The researchers compared a CLT building with a building without bio-materials and found that buildings made from biobased materials have a lower impact on climate throughout their life cycle, thus supporting the findings of the previous article. Karacabeyli and Douglas (2013) also consider environmental benefits of CLT, concluding that it reduces CO2 emissions and thus contributes to global environmental protection efforts.
Another important factor affecting the adoption of new building materials are their seismic characteristics. Overall, research points to the fact that, although CLT buildings can be damaged during earthquakes, there are a lot of opportunities for improving seismic characteristics of CLT buildings. For example, a seismic analysis of CLT buildings by Sustersic, Fragiacomo, and Dujic (2016) showed that the seismic resistance of CLT buildings could be enhanced by dividing monolithic walls into narrower parts and ensuring appropriate connections between adjacent panels. Similarly, Hashemi, Valabeigi, Masoudnia, Quenneville, and Zarnani (2016) studied seismic performance of large buildings. The researchers state that previous studies of CLT buildings showed high accelerations of up to 3.8 g at higher floors during seismic activity. Although such accelerations are still safe for humans, they can be uncomfortable; one opportunity to reduce accelerations is to use resilient slip friction (RSF) damping devices in construction. These devices allow improving seismic characteristics of CLT buildings, lowering the accelerations to 0.9-1.8 g (Hashemi et al., 2016). Another option for enhancing seismic characteristics of CLT is displacement-based design. Hummel (2017) studies the application of displacement-based seismic design in multi-story buildings made from CLT, concluding that such designs offer increased stability and seismic performance compared to traditional CLT designs.
Fire performance also poses significant concerns for construction companies seeking to adopt the use of wooden materials. Hence, researchers attempted to study fire performance of CLT compared to other materials. Suzuki, Mizukami, Naruse, and Araki (2016) performed standard fire tests on CLT panels and joints, finding that the material provided sufficient insulation and integrity over 90 minutes, although the buckling strength of CLT panels decreased initially during loaded fire tests. Delamination and char fall-off were also evident in some of the earlier fire tests; however, as noted by Barber and Gerard (2015), most of the tests studied unprotected CLT panels directly exposed to the fire, which is not the case in many structures. Karacabeyli and Douglas (2013) also point to the need for considering structural resistance and integrity of assembly when accounting for CLT fire performance, as these factors can help to avoid excessive damage to the panels and structures. Overall, although evidence points to the need for further fire testing to determine fire performance of CLT, current research shows that the material provides sufficient fire protection.
For multi-story homes, acoustic insulation is among the critical factors for choosing the right material. As shown by Karacabeyli and Douglas (2013), acoustic insulation in CLT structures depends on the panels’ thickness, the number of layers, and assembly type. However, CLT has been rated sound class B and A in Europe for good acoustic insulation. Crespell and Gagnon (2010) state that sound insulation of exterior CLT walls can be between 47 and 52 dB, whereas partition walls and ceilings provide acoustic insulation of 65-75 dB and 40 dB, respectively. Given that CLT is widely used for building multi-family homes, its acoustic characteristics are beneficial both for the construction companies and for future users.
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Building enclosure design affects the performance and durability of structures and includes controlling the flow of heat, air, and moisture, as well as prevention of water intrusion. Again, due to the fact that CLT is used in multi-family homes, these characteristics are vital to ensuring the comfort of residence as well as for improving the durability of non-residential structures. In general, CLT panels provide optimal conditions for heat and air circulation and can prevent water intrusion using unique protective technologies, such as wall water management systems, cladding systems, and water-resistive barriers (Karacabeyli & Douglas, 2013). Another critical aspect of building enclosure design is its energy efficiency. At some levels, it can be improved using a variety of measures to enhance heat and air circulation, such as insulation and ventilation (Finch et al., 2013). With CLT, the flexibility of materials allows for improving heat and air circulation in cost-efficient ways, thus improving the structure’s energy efficiency.
Cross-laminated timber is a promising new material that can earn popularity in all areas of the world. It was first developed in Europe just over 30 years ago but has already gained the attention of construction companies worldwide due to the extensive benefits it provides. One of the main factors that can affect the adoption of CLT in new markets is its cost-efficiency. The manufacturing costs of CLT are relatively low, which allows manufacturing companies to set competitive prices for CLT panels. In addition, the use of CLT panels offers several important advantages for construction companies, such as the reduced time required to build a structure and decreased labor requirements resulting from it. The material is also environmentally-friendly, which is why the research and use of CLT will likely be supported by governments. CLT is especially suitable for building large multi-family homes due to its acoustic performance and building enclosure characteristics. Future development of CLT technologies will improve the application of the material further by contributing to its fire protection and seismic resistance properties, making CLT structures safe, sustainable, and energy-efficient. Overall, CLT could potentially become one of the key construction materials used for building new structures, thus promoting a green approach to construction.
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