structural composite lumber

Structural Composite Lumber (SCL)

Structural Composite Lumber (SCL) is a family of solid, highly predictable and uniform engineered wood products designed for structural use. SCL is created by layering dried and graded wood veneers, strands or flakes with exterior type adhesives into blocks of material known as billets. The billets are cured in a heated press and sawn to consistent sizes that are easily worked in the field using conventional construction tools.SCL products include-

1)laminated veneer lumber (LVL) 2) parallel strand lumber (PSL),

3)laminated strand lumber (LSL) 4) Oriented strand lumber (OSL).

These products are commonly used in the same strucPtural applications as conventional sawn lumber and timber, including rafters, headers, beams, joists, rim boards, studs and columns. SCL products are also used for wood I-joist flanges and truss chords.

Strong, Reliable and Consistent

Virtually free from warping, splitting and shrinking, SCL products out-perform conventional lumber when either face- or edge-loaded. SCL is generally manufactured with the grain of each layer of veneers, strands or flakes oriented parallel to the length of the billet. Because the grade and quality of each individual layer can be closely controlled, variations in product properties are lower than in conventional sawn lumber products. As a result, the properties and performance of SCL can be more confidently predicted.

Making the Best Use of Resources

The manufacture of SCL represents an efficient utilization of wood resources. Large SCL billets are produced from a variety of tree species, including species that are relatively small, underutilized, fast-growing, and sometimes considered low-grade.

Laminated Veneer Lumber:- LVL is the most widely used SCL product. Laminated veneer lumber (LVL) is an engineered wood product that uses multiple layers of thin wood assembled with adhesives. The grain of all veneers is parallel to the long direction. The LVL billet is then sawn to desired dimensions depending on the construction application. The resulting product features enhanced mechanical properties and dimensional stability that offer a broader range in product width, depth and length than conventional lumber. The many uses of LVL include headers and beams, hip and valley rafters, rim board, scaffold planking, studs, flange material for prefabricated wood I-joists, and truss chords, planks aircraft part. LVL offers several advantages over typical milled lumber: Made in a factory under controlled specifications, it is stronger, straighter, and more uniform. Due to its composite nature, it is much less likely than conventional lumber to warp, twist, bow, or shrink. LVL is similar in appearance to plywood without cross-bands, and is typically rated by the manufacturer for elastic modulus and allowable bending. The distinguishing difference between Plywood and LVL is in orientation of the plies. The grain of the each veneer assembled into LVL runs parallel with each adjacent ply. These results in a veneer based product that is strong when either edge loaded as a beam or face loaded as a plank.

Manufacturing Process of LVL:-

· Raw material: - Evaluating raw material is essential part of manufacturing process. Dry veneers suitable for L.V.L are visually as well as electronically graded. Ultrasonic grading is also practised in some industries. It is a non- destructive method. Selected veneers are then fed into the LVL assembly systems.

LVL assembly systems:

· Batch assembly

· Continuous assembly

· Prepared veneers are glues end to end forming thin ribbons four feet wide up to 60 feet long in batch processing. Glued veneers end joints may be scarfed or lapped. Complete ribbons pass through a curtain coater for adhesive application. Once the desired number of layers is assembled, the unit is transferred to prepress. After prepress, assembly is pressed. Press time varies from 12-14 minutes.

Laminated wood: -

Growth defects such as knots are localised and often form the weakest part of a wooden structure when solid wood is used. Such failures can be reduced by dividing the wood into thin lamellae which are again glued together in such a manner that localised defects are distributed. Through the presence of a knot, the cross section of a member is reduced by 60% while by cutting the piece into several lamellae and re-arranging them, the strength is reduced by only 5 % to 10%.

Laminated wood can be defined as built-up product made of wood layers called lamellae, all laid with their grain parallel and glued or otherwise fastened together. Laminated wood is useful for aircraft parts (spars, propellers etc.), textile mill auxiliaries, sports goods, gunstocks, furniture component, arches in buildings etc.

Suitability of Indian Timbers for laminated wood:

Systematic study to evaluate the strength properties of about 20 indigenous timbers has been carried out at FRI. Laminated wood has been studied from Bombax ceiba, Zanthoxylum rhetsa, Michelia champaca, Cedrela toona, Dalbergia latifolia, Dysoxylum malabaricum, Mangifera indica, etc

Comparison to solid wood:

Laminated wood products /engineered wood products are used in a variety of ways, often in applications similar to solid wood products. These products may be preferred over solid wood in some applications due to certain comparative advantages:

· Because engineered wood is man-made, it can be designed to meet application-specific performance requirements.

· Engineered wood products are versatile and available in a wide variety of thicknesses, sizes, grades, and exposure durability classifications, making the products ideal for use in unlimited construction, industrial and home project application.

· Engineered wood products are designed and manufactured to maximize the natural strength and stiffness characteristics of wood. The products are very stable and some offer greater structural strength than typical wood building materials.

· Glued laminated timber (glulam)/LVL has greater strength and stiffness than comparable dimensional lumber and, pound for pound, is stronger than steel. Glulam products are also a better environmental choice than steel because they have less embodied energy.

· Some engineered wood products offer more design options without sacrificing structural requirements.

· Engineered wood panels are easy to work with using ordinary tools and basic skills. They can be cut, drilled, routed, jointed, glued, and fastened. Plywood can be bent to form curved surfaces without loss of strength. And large panel size speeds construction by reducing the number of pieces to be handled and installed.

· Engineered wood products provide the natural warmth and beauty of wood. Many products are available in a variety of surface textures and treatments for nearly every aesthetic taste, from rustic to elegant. The products can be easily and beautifully finished with paints, stains, and varnishes.

· Engineered wood products make more efficient use of wood. They can be made from small pieces of wood, wood that has defects or underutilized species.

· Wooden trusses are competitive in many roof and floor applications, and their high strength-to-weight ratios permit long spans offering flexibility in floor layouts.

Engineered wood products also have some disadvantages:

· Some products may burn more quickly than solid lumber.

· They require more primary energy for their manufacture than solid lumber. · The adhesives used in some products may be toxic. A concern with some resins is the release of formaldehyde in the finished product, often seen with urea-formaldehyde bonded products.

· Cutting and otherwise working with some products can expose workers to toxic compounds.

· Some engineered wood products, such as those specified for interior use, may be weaker and more prone to humidity-induced warping than equivalent solid woods. Most particle and fiber-based boards are not appropriate for outdoor use because they readily soak up water.

Bamboo laminates:-

One of the most exciting application areas for value added application of bamboo is the manufacture of bamboo laminates. Bamboo laminates can replace timber in many applications such as doors and windows frames, partitions, furniture, flooring and some structural applications. Bamboo needs to undergo certain processing to convert to value added applications by establishing the process technology parameters at different stages of conversion to laminates after determining the anatomical, physio-mechanical and chemical properties and anti-fungal treatment.

Species suitable for making Bamboo laminates:

Initially two species of bamboo, namely Dendrocalamus strictus and Bambusa bamboo which are available in plenty all over the country were tried for making laminates. As wall thickness and the outer diameter of D. Strictus were less, it was not suitable for making laminates. Bambusa tulda and B. hamiltonni were found suitable for making bamboo laminates.

Raw Material:- The quality of bamboo required for manufacturing of laminates should be selected with reference to hardness,girth,wall thickness,straightness,age,moisture content etc.

· Hardness: softer species can be cut better than the harder ones. High silica content accelerates blunting of the cutting knifes/ blades. It has been found that knot removal with width sizing is found to be difficult with hard bamboo due to frequent jamming of the machines and also fibers rising in higher quantity.

· Girth: one of the important factors controlling the yield and quantity of strips is girth. Higher the girth easier to make strips and higher is the yield. Bamboo having lower girth is difficult to process using the available machines and the yield is low.

· Wall thickness- it is one property of bamboo which affects both processing as well as yield of strips. Bamboo having wall thickness 10-15 mm have been found suitable for processing to obtain higher yield. Strips having wall thickness less than 6 mm is difficult to process because the cutter system after removing epidermal and endodermal layers of the strips leaves a very thin strip.

· Straightness: - this property affects yield and quality strips. During radial splitting of bamboo the splitting takes place along the grain direction of the bamboo. If bamboo is curved in nature, all strips will be curved. Such strips are difficult to process further. The same thing will also happen if splitting is done manually. In the case of parallel splitting, yield will come down as waste will be more during cutting of straight strips from a curved bamboo.

· Age of bamboo: Bamboo of age group 2-4 years has been found more suitable for making strips. As strength of bamboo at lower age is less. More than 4 yrs of bamboo starts drying and deteriorating and hence difficult to process and quality and yields of strips are also poor.

· Moisture content: - the ideal moisture content for trip making is 50-60 %. The processing of dry bamboo results in more fiber on the surface of the strips.

Machinery required for manufacture of bamboo laminates:-

Primary processing

· Bamboo storage tank

· Cross cutting machine

· Radial splitting machine, Horizontal splitting machine

· Knot removing machine

· Slab making machine

· Boiling vat

· Steam treatment chamber

· Four side planner

· Drying chamber

Secondary processing:-

· Resin kettle and chemical storage tank

· Glue mixer

· Glue spreader

· Steam boiler

· Hot press

· Trimming machine

· Four side planer

· Sanding machine

Manufacturing process of bamboo strips and yield:-

Bamboo culms are cross cut to pre-determined length in a cross cutting machine. Branches in nodal portion, if any, are removed by manual process and mounted on splitting machines. The split bamboo are then mounted on knot removing and width sizing machine so as to get rid of external bulging and to get bamboo strips having not more than two nodes in a given length to facilitate further processing. The bamboo strips are then immersed in boiling water containing preservatives so as to remove the starch so that bamboo become resistant to attack by insect. The strips are then dried to 8 to 10% moisture content in a hot air chamber. Strips are then passed through the four side planner to get rectangular bamboo strips of smooth and uniform thickness. The rough planed strips are boiled in boiling vats containing chemicals like sodium pentachlorophenate, boric acid-borax mixture. Hydrogen peroxide is added in water and boiled for 6-8 hours whereby the strips become lighter in color. After boiling strips are dried properly. The strips are then passed through four side planner