Technology for Wood and Natural Fiber-Based Materials

Research project

HoTeSa – Development of sustainable sandwich elements from wood foam and textile-reinforced concrete

The exterior walls of energy-efficient buildings consist of complex components with several layers and therefore have high wall cross-sections. In order to reduce the wall cross-sections, RWTH Aachen University has designed slim sandwich elements. The outer layer consists of a thin layer of textile-reinforced concrete whilst the core is of PUR foam. In cooperation with the Institute for Joining and Welding Techniques (ifs) of the Technische Universität Braunschweig, we at the Fraunhofer WKI are developing a more ecological variant: a sandwich element with a wood foam core. 

The wood foam-textile-reinforced concrete sandwich element unites three significant advantages:

  • The foam core consists of a renewable raw material without the addition of a binder from fossil raw resources and is therefore a purely natural product.
  • The foam core can be made from the wood remnants of locally available wood species. This reduces transport distances and additionally preserves resources through the holistic utilization of the trees.
  • The foam can be processed considerably better than other insulation materials made from renewable raw resources, as the generation of dust is not high.
© Fraunhofer WKI | Manuela Lingnau
Sandwich element made from wood foam with a top layer made from textile-reinforced concrete
© Fraunhofer WKI | Manuela Lingnau
Principal composition of the textile-reinforced concrete structure

The outer layer of concrete can, as with the initial model with PUR foam, be kept very thin through the reinforcement with textile fibers. The result is a lightweight element with very good insulation properties and load bearing capacities. The lightweight construction saves material and transport energy. The sandwich element is recyclable and should be particularly flame-resistant, due to the materials applied

At the Fraunhofer WKI, we had already developed a wood foam in an earlier project, which can be utilized in the same ways as classic plastic foams. The application for which the new sandwich element can be used in construction depends on the adjustable properties of the wood foam. Particularly as regards compressive strength and thermal conductivity, the wood foams must exhibit certain properties in order to ensure fulfilment of the requirements for the load-bearing capacity and insulation value with the smallest possible component thicknesses. Through previous investigations at the Fraunhofer WKI, it is known that the species of wood from which the wood foam is produced and the bulk density of the foam heavily influence the thermal conductivity. Broadly speaking, the lower the bulk density, the better the thermal conductivity. Simultaneously, the compressive strength of the foam decreases with the bulk density. An optimal compromise needs to be found here in order to achieve the widest possible range of application for the sandwich element. For this reason, we initially produce wood foams using differing wood species and bulk densities and characterize the foam properties such as strength, thermal conductivity and moisture behavior.

We subsequently develop a manufacturing process for the sandwich element. The wood foam core and the textile concrete top layer must form a solid bond in order for the positive mechanical properties of these two materials to be utilized. We hereby follow two different approaches:

  1. The individual constituents (wood foam and textile concrete) are produced individually and then brought together. The bonding of the two materials following preceding surface treatment has the advantage that by means of selected adhesives, which are to be identified during the project, loads in the component can be specifically deduced.

  2. The concrete top layer is poured around the wood foam core following preceding surface treatment. It is hereby very important to examine the resistance of the wood foam to alkaline fluids.

The production and design of the textile concrete is obtained from the projects carried out at the Institute for Joining and Welding Techniques (ifs). We verify the strength of the composite by means of tensile and shear tests. Once a sufficient composite has been produced, we characterize the material properties of the sandwich element, such as thermal insulation, moisture protection, sound insulation and fire protection. From all the results obtained, we then identify possible applications for the sandwich element.

Project partners

  • Technische Universität Braunschweig, Institut für Füge- und Schweißtechnik (ifs)


We have developed sustainable sandwich elements that result in components with sufficient load-bearing capacity and thermal and acoustic effectiveness. The sandwich elements can be utilized in lightweight curtain-wall elements as well as for interior fittings.


Funding body:
German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety


Project management:
German Federal Institute for Research on Building, Urban Affairs and Spatial Development


1.10.2016 to 1.10.2018