Posted by Raphael Breiner - 07 August, 2018
The ceilings in a new residential complex in the Swiss Canton of Aargau were to be made from a composite of wood and concrete. But how do you effectively create a reliable, durable bond between these extremely different materials? It’s quite simple: with a good idea and the right concrete admixtures from Master Builders Solutions.
In the Rohr district of Aarau, a housing estate called “Quellengarten” (Spring Garden) built in the 1950s was to be replaced by a number of buildings with several flats. The “ugly blocks “, as they were called by the local newspaper, Aarauer Zeitung, were demolished and replaced by a high-grade urban planning solution with a total of 44 affordable flats with a character that is distinctive at the same time as being open to the surrounding district.
70 proposed designs and one sustainable winner
70 firms of architects entered a competition for the new development. Finally, “Tisch, Trompete, Trottinette“ (table, trumpet, scooter), a design by Zürich architect Jürgensen Klement, was selected. The plans included five three-storey residential blocks installed in a slightly offset configuration on an area of 5,900 square meters connected via their roofs around a courtyard. The objective was to obtain Minergie-A, Eco, and Minergie-P certification. This meant that the buildings were to be autonomous in terms of energy supply and constructed in an environmentally compatible way with maximum energy efficiency combined with the highest levels of comfort.
Combined construction: wooden houses with concrete components
The architects opted for a combined construction approach: the supporting structure and interior walls were constructed solely from timber while reinforced concrete was used for the basement, the underground car park and the staircases. Finally, it was planned to use timber concrete composite elements for the ceilings. These elements were to be prefabricated by a specialized timber construction contractor to avoid any need for curing on site. The idea was to ensure faster assembly and more rapid construction progress. This approach was also to prevent swelling of the wooden supporting structure and avoid the need for complex formwork installation on site.
Composite elements with easily compacted, shrinkage reduced concrete
For the production of the composite elements, the concrete supplier recommended easily compacted concrete combined with reinforcement mats. However, the concrete disbonded from the wooden boards on the first ceiling elements and it was necessary to reconsider the design. Firstly, it was decided that the timber should no longer be watered but should be sealed. Secondly, the concrete formulation, which had previously included the superplasticizer MasterGlenium SKY 587 and the viscosity-modifying admixture MasterMatrix SDC 100, was optimized using the shrinkage-reducing admixture MasterLife SRA 848 from Master Builders Solutions: MasterLife SRA 848 especially reduces the shrinkage of concrete during drying from a dosage of only one percent with reference to the cement content.
A model for the future projects
Both the concrete formulation and the procedure adopted proved their effectiveness in the production of the ceiling elements for the new building project. Disbonding and warping caused by shrinkage were significantly reduced and 328 timber composite elements meeting the highest architectural demands were finally delivered to Aarau. The result is a sustainable, durable construction procedure that is ideal for transfer to other situations and is to be used as a system solution on future projects of the timber construction company.