Resource conservation and energy efficiency will determine the building of the future. Wood is an environmentally friendly and versatile building material. In addition to the good ecological balance, timber constructions also offer various technical advantages. Innovative wood-hybrid systems have even better mechanical properties, greater durability and enable slender component structures. As a result, they are not only more resource-efficient than conventional construction methods, but also expand the architectural scope. In this project, we are investigating the long-term behaviour of such hybrid systems, optimizing them and thus creating the basis for their use in the construction industry. Our aim is to significantly increase the proportion of wood in future buildings.
Wood is a versatile and naturally occurring material. It has a relatively high strength in relation to its weight and also offers a high degree of adaptability and workability. It is therefore not surprising that wood is one of the earliest and longest used building materials. In addition, timber constructions are often aesthetically pleasing, which further favors their use.
Nowadays, however, masonry, steel and concrete dominate the market. Reinforced concrete in particular has been specially tailored to the high load conditions in multi-storey or long-span building construction and civil engineering. The combination of concrete (high compressive strength) and steel (high tensile strength) ensures high overall stability. In addition, steel and concrete are homogeneous. The mechanical properties of steel and concrete can be precisely predicted and specifically adjusted to the intended load. When constructed correctly, reinforced concrete is also very durable, even in changeable weather conditions.
However, the production, processing and recycling of reinforced concrete is very energy-intensive. The high energy input and chemical processes involved in cement production release large quantities of CO2. The long transportation routes of the raw materials also have a negative impact on the CO2 balance. Wood has a significantly lower energy requirement, is more climate-friendly as a rapidly renewable raw material and is also available locally. In view of the shortage of raw materials and rising energy prices, wood as a building material is once again becoming the focus of the construction industry, also from an economic perspective.
However, in addition to various advantages, wood also has some disadvantageous properties that have limited its use as a building material in load-bearing structures to date. Wood has a comparatively low tensile and compressive strength perpendicular to the grain direction and, depending on the type of wood, a relatively low dimensional stability and durability with fluctuating temperature and humidity. In addition, the mechanical properties of timber constructions are always subject to certain fluctuations due to the naturally grown wood structure. In order to ensure reliability despite the variability, the worst-case scenario is assumed. Therefore, timber constructions tend to be oversized.
In order to extend the range of applications for timber structures, two innovative timber hybrid systems are being investigated to compensate for the disadvantageous properties of timber. The targeted combination with other materials significantly improves the mechanical properties of the overall construction. The hybrid systems are particularly advantageous in areas subject to high loads, for example in the tensile stress area of a beam, in component connections or as sheathing for pillars. The variability of the mechanical properties of the overall structure is also reduced, making the behavior more predictable. The hybrid systems therefore enable slimmer structures, expand the scope for design and save costs.
Timber-concrete composite system
Compared to conventional reinforced concrete, timber-concrete composite systems (TCC) use wood instead of steel to absorb the tensile forces that occur in the composite. This hybrid system promotes the use of wood as a sustainable material in the construction industry. Furthermore, this system can offer advantages for use under bending loads, in which high tensile stresses occur on the underside of the composite system, such as in beams or ceiling slabs. In the latter case, a wooden beam construction with a top layer of wood-based panels is installed first. The top layer is an integral part of the construction and serves as both support and formwork. It is coated with an adhesive and then filled with fresh concrete. The concrete layer ensures high strength in the compression zone, while the timber absorbs tensile forces. This results in high flexural strength in the composite. Compared to reinforced concrete ceilings, large amounts of tensile reinforcement and concrete are saved. In addition, HBV systems facilitate processing on the construction site because, in contrast to conventional construction methods, the formwork is not removed once the concrete has hardened.
Combination of wood with fiber composite plastic
Wood-fiber-reinforced plastic systems utilize the strength of synthetic (e.g. glass or carbon) and natural (flax or basalt) fibers in the area subject to tensile stress. Depending on the application and stress, several layers of adhesive and fiber fabric are applied to the tensile side of wooden structures. There are various methods for the application of FRP, such as vacuum infusion or the so-called hand lay-up process, which offers advantages for high demands on flexibility or in-situ reinforcements. In return, vacuum infusion offers high quality and reproducibility. By reinforcing the wooden beam with fiber composite plastic, the tensile strength and rigidity of the component can be significantly increased and the high, natural variability of the wood can be better controlled. And to such an extent that the use of wood species and grades that have been little used to date is also conceivable. This could increase the scope for climate and environmentally friendly forestry. Due to its flexible processing, fiber composite plastic can even be used to reinforce the load-bearing structure in existing timber buildings.
So far, there is little knowledge about the long-term behavior of the two wood hybrid systems under different environmental conditions. Current studies are limited to the short-term behavior. A junior research group led by the Fraunhofer WKI is now investigating the long-term behavior of these hybrid timber construction systems for the first time. The team of scientists from the Fraunhofer WKI and the Institute for Building Materials, Solid Construction and Fire Protection (iBMB) at the Technical University of Braunschweig is looking at the long-term behavior of the materials, including material degradation under various climatic and mechanical load environments. The investigations are carried out at micro, meso and macro level and focus on the following two topics:
- Microstructure and the mechanisms of bonding within the two hybrid systems
- Long-term behavior and durability of the two hybrid systems under different climatic and mechanical loading conditions
The investigations will 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 safe construction. In this way, we are paving the way for their use in future buildings.
Project partner: Fraunhofer-Institut für Holzforschung, Wilhelm-Klauditz-Institut, WKI
Funding body: Bundesministerium für Ernährung und Landwirtschaft (BMEL)
Project sponsor: Fachagentur Nachwachsende Rohstoffe e.V. (FNR)
