HANDBOOK OF LOW CARBON CONCRETE

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Concrete is the most widely used construction material. Current average consumption of concrete is about 1 t/year per every living human being. Human beings do not consume any other material in such tremendous quantities except for water. Due to its large consumption, even small reductions of greenhouse gas emissions per ton of manufactured concrete can make a significant global impact. This chapter presents a systematic approach to estimate carbon dioxide (CO2) emissions due to the various components of concrete manufacture. Reliable estimates of greenhouse gas emission footprints of various construction materials are becoming important, because of the environmental awareness of the users of construction material. Life cycle assessment of competing construction materials (e.g., steel and concrete) [1] can be conducted before the type of material is chosen for a particular construction. This chapter provides greenhouse gas emissions data collected from typical concrete manufacturing plants for this purpose. The basic constituents of concrete are cement, water, coarse aggregates, and fine aggregates. Extraction of aggregates has considerable land use implications [2]. However, the major contributor of greenhouse emissions in the manufacture of concrete is Portland cement. It has been reported that the cement industry is responsible for 5% of global anthropogenic
CO2 emissions [3]. As a result, emissions due to Portland cement have often become the focus when assessing the greenhouse gas emissions of concrete. However, as demonstrated by the data presented in this chapter, there are also other components of concrete manufacture that are responsible for greenhouse gas emissions that need consideration. With users beginning to require detailed estimates of the environmental impacts of the materials in new construction projects, this study was intended to provide the basis for a rating tool for concrete, based on CO2 emissions. Other cementitious components considered include ground granulated blast furnace slag (GGBFS), a byproduct of the steel industry, and fly ash, a byproduct of burning coal. These two materials are generally used to replace a portion of the cement in a concrete mix. The use of water in concrete leads to minimal CO2 emissions, which leaves cement, coarse
and fine aggregates, GGBFS, and fly ash as the main material contributors to the environmental impacts of concrete. In addition to the production of materials, the processing components of concrete production and placement were considered. Transport, mixing, and in situ placement of concrete all require energy input leading to CO2 emissions. Fig. 1.1 shows the system that was considered for this research. The CO2 emissions from most of the activities involved in concrete production and placement result from the energy consumed to accomplish them. Hence, to find the CO2 emissions associated with an activity, the
energy consumption per unit of material produced had to be audited. The exception to this rule is cement, where approximately 50% of the emissions are process based, due to the decomposition of limestone in the kiln with the remainder associated with kiln fuels and electricity [3,4]. Previous research into the environmental impacts of cement production has alread yielded several estimates of the CO2 emissions per ton of cement produced. Similarly, fly ash and GGBFS have also both been investigated previously, and their emissions quantified. So the research that was conducted for this chapter covered the production of coarse and fine aggregates and admixtures, raw materials transport, concrete batching and transport, and onsite placement activities. The procedures used to calculate CO2 emissions due to various energy sources in this study were obtained from the Australian Greenhouse Office Factors and Methods Workbook [5]. Table 1.1 shows the emission factors that were sourced from this publication.

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