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 some 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.
Wood is a versatile and naturally occurring material. It has a relatively high strength-to-weight ratio and also offers high adaptability and workability. Therefore, it is not surprising that wood is one of the earliest and longest-used building materials.
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 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, wood also exhibits some disadvantageous properties that limit its use as a building material in load-bearing structures. 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 over-dimensioned.
To expand the range of applications for wood constructions, two innovative wood hybrid systems are being investigated, which can compensate for the unfavorable properties of wood. By strategically combining with other materials, the mechanical properties of the overall structure are significantly enhanced. 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
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, timber 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-fiber reinforced polymer (FRP) systems harness the strengths of synthetic fibers (such as glass or carbon) and natural fibers (like flax or basalt) in areas subjected to tensile stress. Depending on the application and load requirements, multiple layers of adhesive and fiber fabric are applied to the tensile side of wood structures. Various methods are used for applying FRP, such as vacuum infusion or the hand lay-up technique, which offers advantages for flexibility or in-situ reinforcements. In contrast, vacuum infusion provides high quality and reproducibility. By reinforcing the wooden beam with fiber-reinforced polymer, the tensile strength and stiffness of the component can be significantly increased, effectively managing the inherent variability of wood.
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. Due to its flexible processing, fiber-reinforced composite can even be used in existing timber structures to reinforce the load-bearing construction.
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. The team of researchers from Fraunhofer WKI and the Institute of Building Materials, Concrete Construction and Fire Safety (iBMB) at the Technical University of Braunschweig is examining the long-term behavior of the materials, including material degradation, under various climatic and mechanical load conditions.
The investigations are conducted on micro, meso, and macro levels and focus on the following two areas:
- Microstructure and bonding mechanisms within the two hybrid systems
- Long-term behavior and durability of the two hybrid systems under different climatic and mechanical stress conditions
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: Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI
Funding Provider: Bundesministerium für Ernährung und Landwirtschaft (BMEL)
Project Executor: Fachagentur Nachwachsende Rohstoffe e.V. (FNR)
Duration: December 1, 2018, to November 30, 2021