Green house gas emissions due to concrete manufacture

Abstract

Background, Aim and Scope

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.

Materials and Methods

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.

Results

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.

Discussion

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.

Conclusions

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.

Recommendations and Perspectives

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.