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.