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

Project start / December 01, 2021

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.

Last modified: December 01, 2021

Two microscope images each show two areas with different color and structure. — © Fraunhofer WKI
(a) Wood-concrete interface and (b) FRP-timber interface.

The computer graphic shows a wooden beam. On top of the beam is a slab composed of three layers (from bottom to top): wooden formwork, adhesive layer, concrete. — © Fraunhofer WKI | Christoph Pöhler
An example of timber-concrete composite (TCC) for slab application.

The computer graphic shows a wooden beam. On the underside of the beam there is a thin black layer. — © Fraunhofer WKI | Christoph Pöhler
An example of FRP-timber composite (wood-FRP) for beam application.

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 has a relatively high strength-to-weight ratio and additionally offers high adaptability and workability. However, the tensile and compressive strengths of wood vary considerably, 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 plastic. We develop suitable fiber-reinforced plastics and manufacturing processes for fiber-reinforced polymer-timber composite systems (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 accessible.

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.

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 over-dimensioned.

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.

Interim results (end of phase 1)

The photo shows a bending-test machine in which an elongated building component is clamped. The building component consists of three layers (from bottom to top): cross-laminated timber beam (85 mm), thin adhesive layer, concrete (50 mm). — © Fraunhofer WKI
Epoxy-bonded wood-concrete composite building component (floor slab) in bending test.

The close-up shows a building component consisting of three layers (from bottom to top): cross-laminated timber (80 mm), thin adhesive layer, concrete (50 mm). The cross-laminated timber has a crack that extends from the bottom of the beam to the top. — © Fraunhofer WKI
Various mechanical tests under consideration of environmental conditions provide information on the load limits and long-term behavior of adhesive-bonded wood hybrid systems.

The results from the first project phase are very promising. We were able to successfully accomplish the following development tasks:

Analysis and identification of the microstructure, the chemicals, the bonding behavior and the mechanical properties of the different phases within the two hybrid systems
Modification of the fabrics (in fiber-composite plastic) in order to improve the composite performance and long-term durability of the hybrid system
Evaluation of the long-term performance of the two hybrid systems by means of test specimens (small-scale components) under ambient conditions (including time-temperature, time-humidity, etc.) and development of a predictive model

In the first phase of this project, six master theses, six bachelor theses, and eight student theses were supervised and completed. Two other master theses as well as six doctoral theses were started. The project results were also used in the courses of the Institute of Building Materials, Concrete Construction and Fire Safety (iBMB) of the Technische Universität Braunschweig for bachelor and master students. In addition, a total of ten journal articles, five book chapters and three conference papers have been prepared and published based on the results from the first phase of the project.

As a result of the positive achievements, we were able to secure the extension of the project. In the second project phase, we are continuing the research as follows:

Evaluation of the long-term performance of the two hybrid systems by means of test specimens (small-scale components) under mechanical conditions (creep and fatigue) and development of a predictive model
Evaluation of the long-term performance by means of large-scale components under coupled mechanical and environmental conditions and determination of simplified analysis approaches and procedural instructions for the development of the two hybrid systems
Implementation in practice on an industrial scale

Project partner

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

Funding

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

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

FNR funding code: 22011617

Duration:

1.12.2018 to 30.11.2021
1.12.2021 to 30.11.2023

(German Agency for Renewable raw materials)

Research project

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

Timber-concrete composite

Fiber-reinforced polymer-timber composite

Economic advantages

Interim results (end of phase 1)

Project partner

Funding

Junior research group

Contact Press / Media

Prof. Dr. Libo Yan

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

Timber-concrete composite

Fiber-reinforced polymer-timber composite

Social relevance

Economic advantages

Interim results (end of phase 1)

Project partner

Funding