Constructional- and material technical optimization of concrete - possible environmental benefits

Detta är en Master-uppsats från Lunds universitet/Avdelningen för Konstruktionsteknik; Lunds universitet/Institutionen för byggvetenskaper

Sammanfattning: The greenhouse effect and the emission of greenhouse gases, namely carbon dioxide, is a global issue which today permeates alot of the societal consciousness and therefore also alot of the conducted research. The construction industry is not different in this regard. The construction sector in Sweden, for example, generates more than 12 million metric tonnes of carbon dioxide equivalents every year, this is around one fifth of all emissions in Sweden. Concrete is the most common construction material in the world. Reasons for why concrete has taken this position in construction projects stem from, for example, its low cost, the global availability of raw materials and its durability. Despite all its positive characterstics concrete is, and especially the manufacturing of cement, one of the largest producers of greenhouse gases originating from the construction sector. Currently it's reported that cement production amounts to between 3 and 8 \% of the global emissions of greenhouse gases. As urbanisation increases in third world countries, and with it the demand for cheap construction materials, studies have argued that the production of cement's share of global emissions can amount to as much as 30 \% by 2050. Something will have to be done to stop this development. Many research projects are currently operating with the intent of analyzing what possibilities there are to mitigate emissions from cement production. This master's thesis focuses on two possibilities: material technical, namely the substitution of portland cement with additives, and structural technical where, during design, larger importance is given to the reduction of concrete use. Another way of investigating CO$_{2}$-emission, that the industry has shown intrest in, is Carbon Capture Storage (CCS). This means that the carbon dioxide which is released during the firing of limestone amid cement production is collected and stored (for example in pockets in the bedrock under the sea). The material technical part is studied through casting and strength testing of concrete composed of different constituents. The types of concretes that were chosen were one reference recipe only containing portland cement as binder, one which represents todays additives; concrete where 20 \% of the cement has been substituted by blast furnace slag. In addition to this, two innovative types of concrete were chosen, which see limited use today. These were a concrete containing Calcium Sulfoaluminate (C\={S}A) cement and one containing cement composed of Limestone and Calcined Clay Cement (LCCC). The results showed that the properties of the two latter concretes, though different from one another, to a large degree displayed attributes that closely resembled that of the ordinary PC concrete and therefore also good opportunities for use. During discussion with currently practicing consultants in design engineering it was brought up that concrete constructions are often oversized. This with regard to size and quality of concrete as a way to pass acoustics requirements as well as to reduce drying time of the concrete. The structural technical part therefore studies the floor part of a reference project which, though it will not be named due to confidentiality, is taken from the real world. In this project the floors were constructed as partially prefabricated concrete slabs with casting. Different design alternatives were suggested and their respective reduction of cement was analyzed. These suggestions consist of a partially prefabricated concrete slab with less in-situ casting, T-beams and, to a lesser extent, pre-tensioned hollowcore slab. The results of the analyses showed that large improvements can be made to minimize the environmental impact of this part of a residencial building project. These solutions are currently associated with increased cost of the projects, for example as a result of longer construction times. It is concluded that there exist many possibilities regarding reduction of emission of greenhouse gases from the construction sector, but that it might be associated with increased cost. There are today very limited incentives to reduce the amount of cement used during construction. It might be advantageous for legislative bodies to examine the effect of, for example, excise duty on cement.

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