Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Long-term behavior of adhesively-bonded wood-hybrid systems for sustainable buildings

Resource conservation and energy efficiency determine the future of construction. Wood is an environmentally friendly and versatile building material. In addition to its ecological assessment, it also offers divers technical advantages. Innovative timber-hybrid systems have even better mechanical properties, higher durability and allow for slender structures. Therefore, they are not only more resource efficient but also expand the architectural scope. In this project, we investigate and optimize the long-term behavior of wood hybrid systems, thereby laying the foundation for their use in the construction industry. Our main goal is to significantly increase the use of wood in building construction.

© Fraunhofer WKI | Libo Yan
(a) Wood-concrete interface and (b) FRP-timber interface.
© Fraunhofer WKI | Christoph Pöhler
An example of timber-concrete composite (TCC) for slab application.
© Fraunhofer WKI | Christoph Pöhler
An example of FRP-timber composite (wood-FRP) for beam application.

Timber-concrete composite (TCC)

We are investigating timber-concrete composite systems (TCC systems) as an alternative to reinforced concrete. They are particularly suitable for use under bending loads, in which high tensile stresses occur on the underside of the composite system, for example in beams or floor slabs. Instead of steel, wood is used to absorb the tensile forces occurring in the composite.

For example, we develop ceiling slabs in which a beam structure is first installed with a top layer of wood-based panels. The top layer is an integral part of the structure and also serves as formwork and possible support for the ceiling. It is coated with an adhesive and then filled with fresh concrete. The concrete layer provides high strength in the compression zone, while the wood absorbs tensile forces. This results in a high bending strength within the compound. Compared with reinforced concrete floors, large amounts of tensile reinforcement and concrete are saved. In addition, TCC systems facilitate processing on the construction site as, in contrast to conventional construction methods, the formwork is not removed after the concrete has hardened.


Fiber-reinforced polymer-timber composite (Wood-FRP) 

Wood has a relatively high strength-to-weight ratio and offers high adaptability and workability. However, the tensile and compressive strength of wood is comparatively low, which means that its use in load-bearing structures has been limited until now. This disadvantage can be compensated for by combining it with fiber-reinforced polymer. We are developing fiber-reinforced polymer and manufacturing processes for fiber-reinforced polymer-timber composite (Wood-FRP).

One research approach is to incorporate several layers of polymer matrix and reinforcing fabric as a tension component in a wooden structure. We are testing various manufacturing methods for this purpose. High quality and reproducibility can be achieved by vacuum infusion. The hand lay-up process enables in-situ applications. Thus, fiber-reinforced polymer can even be used to reinforce existing wooden constructions, provided that the wooden components are exposed or can be exposed. 


Social relevance

Today, the market is dominated by masonry, steel and concrete. In particular steel-reinforced concrete is specially tailored to the high load conditions in multi-story buildings or wide-span building construction and civil engineering. The combination of concrete (high compressive strength) and steel (high tensile strength) ensures the overall stability of the structure. In addition, the mechanical properties of steel and concrete can be precisely predicted and specifically adjusted to the intended stress. When correctly executed, reinforced concrete is very durable, even under harsh weather conditions.

The production, processing and recycling of reinforced concrete is, however, highly energy intensive. Due to the high energy input and chemical processes during the cement production, large amounts of CO2 are released. Also, long transport distances for the raw materials have a negative impact on the CO2 balance. Wood, on the other hand, has a significantly lower energy requirement and, as a rapidly renewable raw material, is climate-friendly and also locally available. In the view of the scarcity of raw materials and rising energy prices, wood as a building material has regained the interest of the construction industry.

Until now, little knowledge has been gained concerning the long-term behavior of the two hybrid wood systems under different environmental conditions. The current studies are limited to the short-term behavior. A junior research group lead by the Fraunhofer WKI is now investigating for the first time the long-term behavior and durability of these hybrid timber construction systems, including material degradation under various climates and mechanical loads. The investigations help us to understand and assess the long-term behavior of adhesive-bonded wood-hybrid systems. Based on these findings, we will optimize the systems and develop guidelines for a safe construction design. We are thereby providing the basis for the use of wood-hybrid systems in future building construction.


Economic advantages

For real estate companies and the construction industry, hybrid systems offer a way to meet the increasing requirements for the eco-balance of buildings and at the same time build cost-efficiently.

Wood has comparatively low tensile and compressive strengths perpendicular to the grain and, depending on the species, relatively low dimensional stability and durability under fluctuating moisture and temperature conditions. Moreover, the mechanical properties of timber constructions are always subject to certain inconsistencies as a result of the naturally grown wood. In order to ensure the safety of a wooden structure despite the variability, the worst-case scenario is assumed. Timber constructions therefore tend to be overdimensioned.

Hybrid material systems compensate the above-mentioned disadvantages of wood and, through the targeted combination with other materials, provide the overall construction with considerably higher mechanical properties. This goes so far as to also enable and promote the deployment of less-used wood species and grading classes with lower mechanical properties. This could expand the scope for climate- and environmentally compatible forestry management.

Hybrid material systems are particularly advantageous in highly stressed areas, for example in beams with concentrated tensile and compression stresses, in component connections or in column encasements. The use of the hybrid system also reduces the natural variability of the structure and makes the performance more precisely predictable. Concludingly, the hybrid system allows for a more slender construction, expands the scope for design and increases resource efficiency.

Project partner

Institute of Building Materials, Concrete Construction and Fire Safety (iBMB) of the Technische Universität Braunschweig 


Funding body: Federal Ministry of Food and Agriculture

Project management: Fachagentur Nachwachsende Rohstoffe e. V. (German Agency for Renewable Resources)

FNR funding code: 22011617

Duration: 1.12.2018 to 30.11.2021

Funded by the Federal Ministry of Food and Agriculture by decision of the German Parliament
Fachagentur Nachwachsende Rohstoffe e. V. (Agency for Renewable Resources)