Green house gas emissions due to concrete manufacture

Abstract

The issues of environmental impacts of concrete have become important since many major infrastructure owners are now requiring environmentally sustainable design (ESD). The carbon dioxide (CO2) emissions are often used as a rating tool to compare the environmental impact of different construction materials in ESD. Currently, the designers are forced to make estimates of CO2 emissions for concrete in ESD based on conjecture rather than data. The aim of this study was to provide hard data collected from a number of quarries and concrete manufacturing plants so that accurate estimates can be made for concretes in ESD. This paper presents the results of a research project aimed to quantify the CO2 emissions associated with the manufacture and placement of concrete. The life cycle inventory data was collected from two coarse aggregates quarries, one fine aggregates quarry, six concrete batching plants and several other sources. The results are presented in terms of equivalent CO2 emissions. The potential of fly ash and ground granulated blast furnace slag (GGBFS) to reduce the emissions due to concrete was investigated. A case study of a building is also presented. Portland cement was found to be the primary source of CO2 emissions generated by typical commercially produced concrete mixes, being responsible for 74% to 81% of total CO2 emissions. The next major source of CO2 emissions in concrete was found to be coarse aggregates, being responsible for 13% to 20% of total CO2 emissions. The majority contribution of CO2 emissions in coarse aggregates production was found to from electricity, typically about 80%. Blasting, excavation, hauling and transport comprise less than 25%. While the explosives had very high emission factors per unit mass, they contribute very small amounts (<0.25%) to coarse aggregate production, since only small quantities are used. Production of a tonne of fine aggregates was found to generate 30% to 40% of the emissions generated by the production of a tonne of coarse aggregates. Fine aggregates generate less equivalent CO2 since they are only graded, not crushed. Diesel and electricity were found to contribute almost equally to the CO2 emissions due to fine aggregates production. Emission contributions due to admixtures were found to be negligible. Concrete batching, transport and placement activities were all found to contribute very small amounts of CO2 to total concrete emissions. The CO2 emissions generated by typical normal strength concrete mixes using Portland cement as the only binder were found to range between 0.29 and 0.32 t CO2-e/m3. GGBFS was found to be capable of reducing concrete CO2 emissions by 22% in typical concrete mixes. Fly ash was found to be capable of reducing concrete CO2 emissions by 13% to 15% in typical concrete mixes. The results presented are based on typical concrete manufacturing and placement methods in Australia. The data presented in this paper can be utilized to compare green house gas emissions due to concrete with those associated with alternative construction materials. The various rating schemes used to compare alternative construction materials should use models such as the one presented in this paper, based on hard data so that reliable comparisons can be made. A case study is presented in this paper demonstrating how the results may be utilized.