Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

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

Timber structures with fiber-reinforced polymer (FRP) and timber-concrete composite (TCC) structures

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.

Wood is a versatile, sustainable and locally available material. It has a relatively high strength-to-weight ratio and offers high adaptability and workability. Therefore, it is not surprising that wood has been used as a building material since ancient times. Moreover, wooden structures are also often aesthetically pleasing, which further favors their use. Today, however, 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.

In addition to various advantages, however, wood also has some disadvantageous properties, which have until now limited its use as a load-bearing structural material. 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.

In order to widen the application range of wooden materials in construction, we are investigating two innovative wood-hybrid systems in this project. These 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.


Timber-concrete composite (TCC)

Compared to conventional reinforced concrete, timber-concrete composite systems (TCC) use wood instead of steel to absorb the tensile forces occurring in the composite. This hybrid system promotes the use of wood as a more sustainable construction material. Furthermore, it offers advantages for use under bending stress, in which the high tensile stresses occur on the underside of the composite system, such as in beams or ceiling slabs. For slab panels, wooden plates are initially installed on top of the wooden beams. The top layer is an integral part of the construction and, at the same time, serves as a formwork and possible support for the ceiling. It is coated with an adhesive and then covered with fresh concrete. The concrete layer ensures high strength in the compression zone, whilst the wood absorbs tensile forces. This results in a high bending strength within the compound. Compared to reinforced concrete slabs, 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)

Fiber-reinforced polymer-timber composite systems utilize the strengths of synthetic fibers, such as glass or carbon, and natural fibers, such as flax or basalt, in the areas with tensile stress. Depending on the application and stress level, several layers of fabric and matrix are hereby used and applied as a tension component in a wooden structure. Various methods, such as hand lay-up or vacuum infusion procedures, are suitable for the application of the fiber-reinforced polymer to the wood. The hand lay-up method is favorable in cases with high demands concerning the flexibility or in in-situ reinforcements. In contrast, vacuum infusion offers better quality and reproducibility. Due to its flexible application, fiber-reinforced polymer can even be used in existing timber structures to reinforce the load-bearing structure, provided that the components concerned are exposed or can be exposed.

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 team consists of scientists from the Fraunhofer WKI and the Institute of Building Materials, Concrete Construction and Fire Safety (iBMB) of the Technische Universität Braunschweig. Together, we investigate the structure and mechanisms at the micro, meso and macro levels.

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.

Project partner#

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


Funding body: German Federal Ministry of Food and Agriculture (BMEL)

Project management: Fachagentur Nachwachsende Rohstoffe e.V. (German government 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)