Life cycle assessment of wood-concrete composite floor

Abstract

Initial Position

The studied project is the ETH House of Natural Resources. The building has two lower stories built in reinforced concrete and two upper stories built with an innovative timber structure. It was designed at Institute for Structural Engineering, ETH Zurich. The timber structure consists of a post-tensioned timber frame and a composite beech LVL concrete floor. The building prototype is erected on the ETH Hönggerberg Campus and serves as an office building for the Laboratory of Hydraulics, Hydrology and Glaciology. Further research on the object is done in collaboration between different groups at the ETH Zurich.

Aim of the Study

To examine the sustainability of the building a life cycle assessment (LCA) is applied to the timber structure. The results give information about the present ecological performance. Furthermore the ecological impact of the timber structure is compared with the impact of a similar concrete structure for making a relative statement. The study also identifies the most relevant building components, construction materials and processes. Finally strengths, weaknesses and potentials are illustrated.

Approach

The system boundary of the LCA study is cradle-to-construction site gate, considering the structural work including columns, beams, slab and if needed also the core. The study is carried out with regard to the global warming potential. As indicator for the green house gas emission the CO2-equivalent of the IPCC 2007 method is used. The timeframe is set to 100 years and the functional unit is square metre. The analysis is done on Aveny LCA tool (v1.3) with the background database from ecoinvent (v2.2). First the initial timber structure is compared with a similar concrete structure and with a similar composite structure. Based on the first comparison a variation study is made. For that purpose changing the initial inventory develops several scenarios. The variation study also includes the comparison between the wood-concrete composite slab of the House of Natural Resources and other slab types (reinforced concrete, composite slab, wooden slab). Finally a prospective scenario is created considering the findings of all scenarios before.

Conclusion

The overall green house gas emission of the timber structure is about 10% lower compared to a similar structure out of reinforced concrete. Whereas the wood-concrete composite slab accounts for the biggest impact of the building components. This fact also implies that from the construction materials the concrete has the largest fraction of the total emission. Although the concrete usage is cut by half and carbon reduced cement is used the relative impact is only reduced by 45%. This rebound effect results from the high cement ratio applied to the concrete in the composite slab. Either a reduction of the clinker ratio in the cement or the usage of concrete with a lower strength (i.e. lower cement ratio in concrete) would improve the ecological performance of the timber structure.