Very large self-supporting wooden roof. Built for Expo 2000, Hanover, Germany
Very large self-supporting wooden roof. Built for Expo 2000, Hanover, Germany
75-unit apartment building, made largely of wood, in Mission, British Columbia
75-unit apartment building, made largely of wood, in Mission, British Columbia

Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of wood, together with adhesives, or other methods of fixation[1] to form composite material. The panels vary in size but can range upwards of 64 by 8 feet (19.5 by 2.4 m) and in the case of cross-laminated timber (CLT) can be of any thickness from a few inches to 16 inches (410 mm) or more.[2] These products are engineered to precise design specifications, which are tested to meet national or international standards and provide uniformity and predictability in their structural performance. Engineered wood products are used in a variety of applications, from home construction to commercial buildings to industrial products.[3] The products can be used for joists and beams that replace steel in many building projects.[4] The term mass timber describes a group of building materials that can replace concrete assemblies.[5] Broad-base adoption of mass timber and their substitution for steel and concrete in new mid-rise construction projects over the next couple decades could help mitigate climate change.

Typically, engineered wood products are made from the same hardwoods and softwoods used to manufacture lumber. Sawmill scraps and other wood waste can be used for engineered wood composed of wood particles or fibers, but whole logs are usually used for veneers, such as plywood, medium-density fibreboard (MDF), or particle board. Some engineered wood products, like oriented strand board (OSB), can use trees from the poplar family, a common but non-structural species.

Wood-plastic composite, one kind of engineered wood
Wood-plastic composite, one kind of engineered wood

Alternatively, it is also possible to manufacture similar engineered bamboo from bamboo; and similar engineered cellulosic products from other lignin-containing materials such as rye straw, wheat straw, rice straw, hemp stalks, kenaf stalks, or sugar cane residue, in which case they contain no actual wood but rather vegetable fibers.

Flat-pack furniture is typically made out of man-made wood due to its low manufacturing costs and its low weight.

Types of products

Engineered wood products in a Home Depot store
Engineered wood products in a Home Depot store

Plywood

Plywood, a wood structural panel, is sometimes called the original engineered wood product.[6] Plywood is manufactured from sheets of cross-laminated veneer and bonded under heat and pressure with durable, moisture-resistant adhesives. By alternating the grain direction of the veneers from layer to layer, or “cross-orienting”, panel strength and stiffness in both directions are maximized. Other structural wood panels include oriented strand boards and structural composite panels.[7]

Densified wood

Densified wood is made by using a mechanical hot press to compress wood fibers and increase the density by a factor of three.[8] This increase in density is expected to enhance the strength and stiffness of the wood by a proportional amount.[9] Early studies confirmed this ends with a reported increase in mechanical strength by a factor of three.

Chemically densified wood

More recent studies[10] have combined chemical process with traditional mechanical hot press methods to increase density and thus mechanical properties of the wood. In these methods, chemical processes break down lignin and hemicellulose that are found naturally in the wood. Following dissolution, the cellulose strands that remain are mechanically hot compressed. Compared to the three-fold increase in strength observed from hot pressing alone, chemically processed wood has been shown to yield an 11-fold improvement. This extra strength comes from hydrogen bonds formed between the aligned cellulose nanofibers.

The densified wood possessed mechanical strength properties on par with steel used in building construction, opening the door for applications of densified wood in situations where regular strength wood would fail. Environmentally, wood requires significantly less carbon dioxide to produce than steel.[11]

Fibreboard

Medium-density fibreboard and high-density fibreboard (hardboard) are made by breaking down hardwood or softwood residuals into wood fibers, combining them with wax and a resin binder, and forming panels by applying high temperature and pressure.[12]

Particle board

Particle board is manufactured from wood chips, sawmill shavings, or even sawdust, and a synthetic resin or another suitable binder, which is pressed and extruded. Oriented strand board, also known as flakeboard, wafer board, or chipboard, is similar but uses machined wood flakes offering more strength. Particleboard is cheaper, denser, and more uniform than conventional wood and plywood and is substituted for them when the cost is more important than strength and appearance. A major disadvantage of particleboard is that it is very prone to expansion and discoloration due to moisture, particularly when it is not covered with paint or another sealer.

Oriented strand board

Oriented strand board (OSB) is a wood structural panel manufactured from rectangular-shaped strands of wood that are oriented lengthwise and then arranged in layers, laid up into mats, and bonded together with moisture-resistant, heat-cured adhesives. The individual layers can be cross-oriented to provide strength and stiffness to the panel. However, most OSB panels are delivered with more strength in one direction. The wood strands in the outmost layer on each side of the board are normally aligned into the strongest direction of the board. Arrows on the product will often identify the strongest direction of the board (the height, or longest dimension, in most cases). Produced in huge, continuous mats, OSB is a solid panel product of consistent quality with no laps, gaps, or voids.[13]

OSB is delivered in various dimensions, strengths, and levels of water resistance.

Laminated timber

Glued laminated timber (glulam) is composed of several layers of dimensional timber glued together with moisture-resistant adhesives, creating a large, strong, structural member that can be used as vertical columns or horizontal beams. Glulam can also be produced in curved shapes, offering extensive design flexibility.

Laminated veneer

Laminated veneer lumber (LVL) is produced by bonding thin wood veneers together in a large billet. The grain of all veneers in the LVL billet is parallel to the long direction. The resulting product features enhanced mechanical properties and dimensional stability that offer a broader range in product width, depth, and length than conventional lumber. LVL is a member of the structural composite lumber (SCL) family of engineered wood products that are commonly used in the same structural applications as conventional sawn lumber and timber, including rafters, headers, beams, joists, rim boards, studs, and columns.[14]

"Pakka wood", a common kitchen knife handle material, is a material with a cheap wood core and a laminated veneer surface made from more expensive wood. It is inexpensive, waterproof, strong, and durable.[15]

Cross laminated

Cross-laminated timber (CLT) is a versatile multi-layered panel made of lumber. Each layer of boards is placed cross-wise to adjacent layers for increased rigidity and strength. CLT can be used for long spans and all assemblies, e.g. floors, walls, or roofs.[16] CLT has the advantage of faster construction times as the panels are manufactured and finished off-site and supplied ready to fit and screw together as a flat pack assembly project.[citation needed]

Parallel strand

Parallel strand lumber (PSL) consists of long veneer strands laid in parallel formation and bonded together with an adhesive to form the finished structural section. A strong, consistent material, it has a high load-carrying ability and is resistant to seasoning stresses so it is well suited for use as beams and columns for post and beam construction, and for beams, headers, and lintels for light framing construction.[7] PSL is a member of the structural composite lumber (SCL) family of engineered wood products.[17]

Laminated strand

Laminated strand lumber (LSL) and oriented strand lumber (OSL) are manufactured from flaked wood strands that have a high length-to-thickness ratio. Combined with an adhesive, the strands are oriented and formed into a large mat or billet and pressed. LSL and OSL offer good fastener-holding strength and mechanical connector performance and are commonly used in a variety of applications, such as beams, headers, studs, rim boards, and millwork components. These products are members of the structural composite lumber (SCL) family of engineered wood products.[14] LSL is manufactured from relatively short strands—typically about 1 foot long—compared to the 2 foot to 8 foot long strands used in PSL.[18]

Finger joint

The finger joint is made up of short pieces of wood combined to form longer lengths and is used in doorjambs, moldings, and studs. It is also produced in long lengths and wide dimensions for floors.

Beams

I-joists and wood I-beams are "I"-shaped structural members designed for use in floor and roof construction. An I-joist consists of top and bottom flanges of various widths united with webs of various depths. The flanges resist common bending stresses, and the web provides shear performance.[19] I-joists are designed to carry heavy loads over long distances while using less lumber than a dimensional solid wood joist of a size necessary to do the same task. As of 2005, approximately half of all wood light framed floors were framed using I-joists.[citation needed]

Trusses

Roof trusses and floor trusses are structural frames relying on a triangular arrangement of webs and chords to transfer loads to reaction points. For a given load, long wood trusses built from smaller pieces of lumber require less raw material and make it easier for AC contractors, plumbers, and electricians to do their work, compared to the long 2x10s and 2x12s traditionally used as rafters and floor joists.[18]

Transparent wood composites

Transparent wood composites are new materials, currently only made at the laboratory scale, that combines transparency and stiffness via a chemical process that replaces light-absorbing compounds, such as lignin, with a transparent polymer.

Advantages

Engineered wood products are used in a variety of ways,[20] often in applications similar to solid wood products. Engineered wood products may be preferred over solid wood in some applications due to certain comparative advantages:

Disadvantages

Environmental benefits

Engineered wood has the potential to help reduce carbon emissions by replacing cement and steel as a primary material in the construction of buildings. Not only do buildings made from engineered wood act as a carbon sink, but they also produce less emissions in the manufacturing process than steel and cement, which both emit a lot of CO2 due to the chemical processes involved in their manufacturing. For example, in 2014, steel and cement production accounted for about 1320 Mt CO2e and 1740 Mt CO2e respectively, which made up about 9% of global CO2 emissions that year.[26] In a study that didn’t even take the carbon sequestration potential of engineered wood into account, it was found that, on average, around 50 Mt CO2e could be eliminated in a scenario in which the full uptake of the hybrid construction system, which involves utilizing both steel and timber, was accomplished by 2050.[27] When considering the added effects that carbon sequestration can have over the lifetime of the material, engineered wood is even more favorable, as laminated wood that is not incinerated at the end of its lifecycle absorbs around 582 kg of CO2/ m3., while reinforced concrete emits 458 kg CO2/m3 and steel 12.087 kg CO2/m3.[28]

Properties

Plywood and OSB typically have a density of 560–640 kg/m3 (35–40 lb/cu ft). For example, 9.5 mm (38 in) plywood sheathing or OSB sheathing typically has a surface density of 4.9–5.9 kg/m2 (1–1.2 lb/sq ft).[29] Many other engineered woods have densities much higher than OSB.

Engineered wood flooring manufacturing

Lamella

The lamella is the face layer of the wood that is visible when installed. Typically, it is a sawn piece of timber. The timber can be cut in three different styles: flat-sawn, quarter-sawn, and rift-sawn.

Types of core/substrate

  1. Wood ply construction ("sandwich core"): Uses multiple thin plies of wood adhered together. The wood grain of each ply runs perpendicular to the ply below it. Stability is attained from using thin layers of wood that have little to no reaction to climatic change. The wood is further stabilized due to equal pressure being exerted lengthwise and widthwise from the plies running perpendicular to each other.
  2. Finger core construction: Finger core engineered wood floors are made of small pieces of milled timber that run perpendicular to the top layer (lamella) of wood. They can be 2-ply or 3-ply, depending on their intended use. If it is three-ply, the third ply is often plywood that runs parallel to the lamella. Stability is gained through the grains running perpendicular to each other, and the expansion and contraction of wood are reduced and relegated to the middle ply, stopping the floor from gapping or cupping.
  3. Fibreboard: The core is made up of medium or high-density fibreboard. Floors with a fibreboard core are hygroscopic and must never be exposed to large amounts of water or very high humidity - the expansion caused by absorbing water combined with the density of the fibreboard, will cause it to lose its form. Fibreboard is less expensive than timber and can emit higher levels of harmful gases due to its relatively high adhesive content.
  4. An engineered flooring construction that is popular in parts of Europe is the hardwood lamella, softwood core laid perpendicular to the lamella, and a final backing layer of the same noble wood used for the lamella. Other noble hardwoods are sometimes used for the back layer but must be compatible. This is thought by many to be the most stable of engineered floors.

Adhesives

The types of adhesives used in engineered wood include:

A more inclusive term is structural composites. For example, fiber cement siding is made of cement and wood fiber, while cement board is a low-density cement panel, often with added resin, faced with fiberglass mesh.

Health concerns

While formaldehyde is an essential ingredient of cellular metabolism in mammals, studies have linked prolonged inhalation of formaldehyde gases to cancer. Engineered wood composites have been found to emit potentially harmful amounts of formaldehyde gas in two ways: unreacted free formaldehyde and the chemical decomposition of resin adhesives. When exorbitant amounts of formaldehyde are added to a process, the excess will not have any additive to bond with and may seep from the wood product over time. Cheap urea-formaldehyde (UF) adhesives are largely responsible for degraded resin emissions. Moisture degrades the weak UF molecules, resulting in potentially harmful formaldehyde emissions. McLube offers release agents and platen sealers designed for those manufacturers who use reduced-formaldehyde UF and melamine-formaldehyde adhesives. Many oriented strand board (SB) and plywood manufacturers use phenol-formaldehyde (PF) because phenol is a much more effective additive. Phenol forms a water-resistant bond with formaldehyde that will not degrade in moist environments. PF resins have not been found to pose significant health risks due to formaldehyde emissions. While PF is an excellent adhesive, the engineered wood industry has started to shift toward polyurethane binders like pMDI to achieve even greater water resistance, strength, and process efficiency. pMDIs are also used extensively in the production of rigid polyurethane foams and insulators for refrigeration. pMDIs outperform other resin adhesives, but they are notoriously difficult to release and cause buildup on tooling surfaces.[30]

Other fixations

Some engineered products such as CLT Cross Laminated Timber can be assembled without the use of adhesives using mechanical fixing. These can range from profiled interlocking jointed boards,[31][32] proprietary metal fixings,[33] nails or timber dowels (Brettstapel - single layer or CLT[34]).

Standards

The following standards are related to engineered wood products:

Lightweight, strong, moldable wood via cell wall engineering

There are many ways to make wood fix the physical needs in industry, however, the methods are usually in a macro scale which does not change the micro-structure or material properties. Therefore, fails in simultaneously achieve high mechanical strength and good moldability. To make wood material which has high mechanical strength and good moldability, there are ways to achieve.

Process

The process[35] is achieved by partially delignify[36] and soften natural wood, then shrink its vessels and fibers by drying, followed by “shocking” the material in water to selectively open the vessels. The water shock process forms partially open vessels and wrinkled fiber cell wall. This microstructure makes the wood able to bend and mold. Observing by Scanning Electron Microscopy (SEM), the moldable wood has a structure that the fibers are close packed together and there are vessels that are partially opened because of the water shock process.

Mechanical properties

Mechaical properties include:[35]

Bending test

Compare moldable wood and Al-5052 by folding the two materials, moldable wood does not fracture after folding and unfolding process for 100 times, while Al-5052 breaks after 3 cycles. The dislocations in metals are able to slip and move due to the process of folding and unfolding, the dislocations move and aggregate at the center of bending then cause fracture. On the other hand, the moldable wood will not have the issue of dislocations aggregation, since it is a polymer. The partially opened, wrinkled cell wall structure enables the flexibility of the wood. Fiber-scale mechanics modeling shows that the strain level in all cell walls of the moldable wood is extremely low (with a maximum principal tensile strain of 0.23% and compressive strain of 0.31%) even when the moldable wood is subjected to a 60% nominal strain deformation (tensile or compressive).

Tensile strength

The low density of moldable wood (0.75 g/cm3) gives it a high specific tensile strength of 386.8 MPa /(g/cm3), which is about five times greater than that of Al-5052 (84.4 MPa / (g/cm3)).The low density, high mechanical strength,[37] and excellent formability of the 3D-molded wood offers broad versatility in designing and manufacturing large, lightweight, load-bearing designs.

Compression strength

Testing the specific compressive strength of moldable wood and Al-5052, both materials are made into a honeycomb shape and applied stress along the Z-direction. The moldable wood has a specific compression strength for about 55 MPa/(g/cm3), which is slightly higher then the aluminum honeycomb with compression strength about 50 MPa/(g/cm3). This shows that if only consider the material property of strength, moldable wood is able to replace Al.

Advantages

Some advantages are:[35]

See also

References

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