Jämförelse av trä och betong i ett flerbostadshus

Sammanfattning: An ongoing discussion today is the environmental impact of concrete and whether Cementa should get to continue to mine limestone on Gotland despite its impact on the local ground water. Previous research has shown that half of the apartment buildings could be built with wood, which is considered to be a more environmentally friendly material than concrete. Environmental reports mostly focus of the time on the carbon dioxide emissions during the construction time, which gives a misleading result when concrete structures tend to have a longer lifetime than timber structures. The purpose of this report is to investigate which building material that is the most suitable for apartment buildings with focus on the environmental emissions and the properties of the materials. A drawback with wood is that it is a biological material which makes it sensitive to moisture and rot unlike the concrete. In an event of a fire the wooden house would take major damage compared to the one in concrete, since concrete counts as a non-combustible material. The concrete has a higher compressive- strength than wood, which results in a slimmer structure. In this report, two structures with the same floor plan have been modelled and compared: one building in concrete, and one in wood. The analysed structures have comparable U-values, the same building total height and the same building area. The concrete structure is built in a VST-system which means that factory-made and remaining form element are cast in place. The wooden house will be built in a column-beam system in combination with CLT, inspired from Trä-8. In apartment buildings built with wood a challenge is to get enough stability for the construction. To get enough stability crossbars have been placed on parts of the short sides of the building. The concrete house has a total living space of 814 m2, while the wooden house who has a bigger frame result in a living space of 790 m2. This is a reduction of 2.9% living space. The ceiling height is reduced on every level from 2,51 m to 2,26 m in the wooden house if the total building height is to stay the same. The environmental impact has been calculated with the help of lifecycle analyses where the houses GWP-impact has been approximated. The calculation was performed in two different ways per house: One product specific calculation with the help of EPD’s and one general calculation made with Byggsektorns miljöberäkningsberktyg. In the product specific calculation for the wooden house the tree’s absorption of carbon dioxide during its lifetime has been included and it gave a total negative impact of -296 000 kg CO2 eqv during the material production phase. This can give a somewhat misleading picture if there is no awareness of all the parameters included. It is the trees in the forest that absorbs carbon- dioxide, an infinite number of wooden houses would result in a negative environmental- impact. The absorption of carbon dioxide has not been included in the general calculation of the wooden house, and it resulted in a total carbon dioxide emissions of 83 000 kg CO2 eqv during the material production. The result from the concrete’s product specific and general calculation had a big variation. The product specific calculation gave a result of 432 200 kg CO2 eqv and the general calculation gave a result of 721 000 kg CO2 eqv during the material production. A source of error could be that in the product specific calculation a green concrete has been used, whilst in the general an average on normal concrete has been used. If the general calculations are compared for the different houses, the transportation of materials is more than four times bigger for the concrete house. These numbers do however not take the living space or the expected lifetime into consideration. The results indicate that the emissions from the material production would be reduced by 64.5% if the building was built of wood instead of concrete, considering its expected length and living area.