Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Lightweight concrete with beech wood additives

Prefabricated concrete products exhibit a very high mass with simultaneous high thermal conductivity; optimization of both properties would therefore seem reasonable. In order to realize an improvement in these properties, this project aims to replace the mineral components with wood constituents.

© Fraunhofer WKI
Maximum compessive force of the lightweight concrete samples, plotted against the bulk density of the samples.
© Fraunhofer WKI
Maximum compressive force of the lightweight concrete samples after 24 hours, plotted against the content of CaCl² per liter of mixing water.

The aim of the project should be achieved with the help of systematic investigations into

  • the optimum particle shape and size,
  • the curing process regarding interaction with the wood components,
  • the practicability and manufacturing parameters within the production process and
  • the behavior under hygrothermal and mechanical load.

The investigations within the production process regarding the optimal shape and size of the wood particles show that shapes which are as globular or cubic as possible can be processed best. With strands or even Scrimber, very good strength properties - in particular flexural strengths - can be achieved in the laboratory using hand-made samples. However, the elongated shape prevents smooth processing and leads to problems in the production, both in the mixture and during transportation. Figure 1 shows the influence of the bulk density on the compressive strength. It can thereby be clearly seen that within a specific range, minor alterations to the bulk density have a pronounced effect on the compressive strength. Wood components such as sugar are known to have such an extreme influence on the curing process that measures must be taken to minimize or even reverse this influence.

© Fraunhofer WKI
Maximum compressive force of the lightweight concrete samples after 24 hours, plotted against the type of pre-treatment of the wood particles.
© Fraunhofer WKI
Weak point: Adhesive strength between cement (left) and corresponding wood (right); thr lower images show the wood fibers stained yellow.
© Fraunhofer WKI
Weak point: Adhesive strength on flat surfaces (oval marker); rough surface with faultless adhesion (arrow).

Tests were therefore carried out

  • using CaCl2 to accelerate the curing process,
  • using hydrophobic agents to isolate the wood components and
  • with pre-treatments to release the water-soluble wood components from the particles.

Whilst treatment with hydrophobic agents and dissolving through immersion in water brought little success, the addition of CaCl2 had a significant acceleration effect on the curing. Figure 2 shows the influence of CaCl2 on the early strength of the compressive strength, i.e. 24 hours after production of the samples. Dissolving away of the wood sugar through immersion in water proved to have little success in the tests performed here (Fig. 3). Figure 4 shows the problem of the adhesive strength of cement on the wood surface. On smooth surfaces, the cement dissolves without a significant fiber layer; with very rough surfaces, an interlocking of the cement and the wood apparently occurs, which results in a higher strength (Fig. 5).

The application of wood particles enables the production of lightweight concrete and individual characteristics, such as thermal conductivity, can be improved. The problems caused by the wood components and the related delay in curing can also be mastered through the addition of curing accelerators. The hygric properties and the durability following several hygrothermal load changes have not yet been investigated, nor have the possibilities for the utilization of reinforcements for increasing the tensile strength. Further research is needed here. However, the technical objectives of the project have been fully achieved and several demonstrators in the form of wall and ceiling elements have been manufactured which fully meet the application criteria.

 

Publication of the project results:

Entwicklung eines Leichtbetons mit Zuschlägen aus Buchenholz
Rüther, N.; Keilholz, M.; Wolf, M.; Yan, L.

Ibausil. 20. Internationale Baustofftagung, 12. bis 14. September 2018, Weimar, Deutschland.
Tagungsband 2, Seite 261-273

ISBN 978-3-00-059951-4

Project partners

Universalbeton Heringen GmbH & Co. KG

Funding

Funding body:
Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF) within the framework of the BioEconomy Cluster

 

Project coordinator:
Forschungszentrum Jülich

 

Project duration:
01.09.2014 to 31.08.2017