Carbon Fiber Reinforced Laminated Veneer Lumber (LVL)

Wood is a traditional construction material widely used in the construction and furniture industries due to its natural appearance, aesthetic structure, and sustainability. However, its limited resistance to environmental factors, risk of biological degradation, and mechanical weaknesses restrict its use, particularly in load-bearing systems. To overcome these limitations, thermal treatment and composite reinforcement techniques have been developed to enhance the physical and mechanical performance of wood materials. In this context, carbon fiber-reinforced lamination stands out as a significant innovation in the production of high-performance wood composites.

^{Example of Laminated Coating (Generated by Artificial Intelligence)}

Thermal treatment is a physical process that modifies wood materials at temperatures between approximately 150–220 °C without the use of water or chemical additives. As a result of this process:

• Dimensional stability increases,
• Equilibrium moisture content decreases,
• Biological resistance (to decay and fungi) improves.

Conversely, mechanical properties such as modulus of rupture (MOR), modulus of elasticity (MOE), and compressive strength parallel to grain (CS//) decrease due to high temperatures. This reduction can limit the direct use of wood in load-bearing systems.

Lamination is a physical modification method that involves stacking wood veneers with adhesives and pressing them together. Through this method:

• Material defects are minimized,
• Homogeneous and strong structures are achieved depending on the number of layers, their orientation, and the type of adhesive,
• Structural composite materials such as LVL (Laminated Veneer Lumber) are produced.

In LVL production, wood species such as oriental beech (Fagus orientalis Lipsky) or black pine (Pinus nigra) are commonly used, with phenol-formaldehyde (PF), polyurethane (PU), or polyvinyl acetate (PVAc) selected as adhesives.

Carbon fiber is used as a reinforcing element in wood lamination systems due to its high tensile strength and low density. The reinforcement process is carried out as follows:

1. Thermal treatment: Applied for 2–3 hours at temperatures of 150, 175, and 200 °C.
2. Layer placement: Carbon fiber fabric (200 g/m², 95% pure carbon, 3800 MPa tensile strength) is inserted between wood veneers.
3. Adhesion and pressing: Layers are bonded with specified adhesive quantities and pressed at 10 kg/cm² pressure for approximately 180 minutes.
4. Curing: The composite structure is left to rest for 7 days to allow complete adhesive curing.

Significant increases in modulus of rupture (MOR) and modulus of elasticity (MOE) were observed in carbon fiber-reinforced LVL samples:

• In untreated samples, MOR was 103.45 N/mm², increasing to 121.16 N/mm² with carbon fiber reinforcement.
• MOE increased from 11665 N/mm² to 13299 N/mm² with carbon fiber reinforcement.

This improvement reaches its maximum at 150 °C thermal treatment temperature, with partial reductions observed at higher temperatures. Compressive strength varies depending on the type of fiber and the thermal treatment temperature.

For example:

• At 150 °C, CS// values for carbon fiber-reinforced samples reached 64.72 N/mm²,
• At 200 °C, this value decreased to 61.95 N/mm².

This indicates that although thermal treatment weakens the fiber structure, carbon fiber partially preserves the structural integrity. In all test groups, the bond strength of carbon fiber-reinforced samples was lower than that of unreinforced groups. This is due to the carbon fiber acting as a barrier that limits load transfer between layers.

Carbon fiber lamination is effectively used in the following areas:

• Structural engineering: Reinforcement of beams, columns, and floor systems,
• Historic building restoration: Enhancement of performance of load-bearing wooden elements,
• Furniture and outdoor applications: Garden furniture, exterior claddings, sauna and kitchen applications,
• Seismic reinforcement and repair: Increasing the load-bearing capacity of structural elements with reduced strength.

Carbon fiber-reinforced laminated veneer lumber (LVL) is a high-performance composite material that compensates for the mechanical weaknesses of thermally treated natural wood and imparts new structural properties. Thanks to the increases it provides in both flexural strength and modulus of elasticity, it offers a sustainable and reliable alternative in modern structural systems. Its growing potential for application plays a significant role in the future of wood construction technologies.