Locally Densified Timber Strengthens CLT With Wood
A research project from the University of Stuttgart uses controlled changes in wood density to reinforce cross-laminated timber (CLT). Locally Densified Timber applies heat, moisture and pressure to selected areas of a panel, while pressed and glued hardwood plugs become part of its cross-section. The result is a gradient-density reinforcement that strengthens critical zones without increasing the original panel height.
The method offers a wood-based alternative to conventional reinforcement systems that rely on stiff, carbon-intensive metal components. It could support thinner, more resource-efficient CLT floors while retaining the renewable material character of timber construction.
Designing Density As A Structural Property
Timber densification improves the material’s mechanical properties by compressing its cellular structure. Instead of densifying an entire element, the researchers apply this process only where higher strength and stiffness are needed. The wood fibres follow a smooth, continuous shape, allowing the surrounding material to retain its original structural capacity.
To compress and shape the timber, heat and moisture loosen the cellulose and hemicellulose within the lignin matrix. The team developed a direct steam-injection system with two layers of channels and matching perforations in the wood. The apparatus remains inside the press during lamination. It first supplies steam and then hot air to dry the timber. This sequence requires only a relatively simple addition to the existing CLT production process.
Reinforcing Column-To-Slab Connections
The main application is the column-to-slab joint in point-supported CLT floors. These joints experience concentrated forces and can determine the required thickness of an entire slab. Conventional reinforcement may puncture the timber with holes or transfer loads through rigid metal components.
The proposed system reinforces the timber around the most structurally critical area instead. This allows the existing wood to work in three directions and preserves the panel’s structural continuity. Where joint performance rather than self-weight determines slab thickness, the method could enable particularly thin, high-performance CLT floor systems.
Testing At Industrial Scale
Physical tests helped the researchers refine the pressing parameters and hardwood plug geometry. Small-scale bending and compression tests showed clear changes in strength, stiffness and failure behaviour. The observed failure mode shifted from punching shear towards the interface between the laminated layers. A two-dimensional finite element model using gradient anisotropic material properties complemented the experiments.
The team subsequently tested the fabrication process with an industrial-scale CLT press at the Tallwood Design Institute in Oregon. They adapted the system to the dimensions and production constraints of a five-layer, point-supported CLT slab. A full-scale demonstrator confirmed the feasibility of integrating local densification into established manufacturing processes.
Developed in 2024 by Chris Donghwi Kang and Otto Lindstam, the project received an Editor’s Choice distinction from the Green Concept Award 2026.
Source & photos: NOWRK / Green Concept Award