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Concrete In Australia : March 2013
46 Concrete in Australia Vol 39 No 1 FEATURE: ANCHORING & PRECAST associated with the production of different precast concrete products. e factors contributing to greenhouse gas emissions throughout the service life of a precast concrete product are generally similar and the information available in the public domain should be enough to properly understand the contribution of each of these factors to the overall "footprint" of precast concrete. However, looking at the range of literature from the last two decades, it is evident that the difference between footprint values is considerably wide. Embodied carbon values ranging from 75 kg CO2/t (MPA, 2009a), to 235 kg CO2/t (UKWIR, 2008) are reported in different literature. ere are a number of reasons why such a very wide range (or deviation) exists. Some of these factors are associated with the calculation methodology. Others are associated with precast products specifications and mix designs. However, one of the main under-researched areas might be factors associated with appropriate means of secondary data sourcing and the time and geographic relevance of the data used. 2.1 Precast product specifications and the carbon footprint e specifications of a precast concrete product will affect its mix design, level of reinforcement and method of manufacturing. All these elements should have a significant impact on that product s carbon footprint. 2.1.1 Precast concrete product mix Most emissions associated with a concrete product carbon footprint are associated with products and processes at the upstream of the supply chain for a precast concrete manufacturer: • Cement: e production of cement is an energy-intensive process. As noted by a number of studies and reports throughout the last two decades emissions associated with the production of cement contribute the most to the overall cradle-to-gate carbon footprint of a typical precast concrete product (Vares & Hakkinen, 1998; Börjesson & Gostavsson, 2000; Bijen, 2002; MPA, 2009b; Hammond & Jones, 2011). A factsheet by MPA (2009b) notes that the amount of carbon dioxide emissions associated with weighted average cementious material is around 720 kg CO2/t. is high impact is mainly associated with Portland cement and can be offset by the use of cement replacements such as fly ash and Ground Granulated Blastfurnace Slag which will reduce the overall footprint of concrete. • Steel Reinforcement: Steel has a high carbon footprint ranging usually between 500 to 3000 kg CO2/t. Despite the fact that the content of steel may not exceed 3 to 4% it can still contribute over 10% of a concrete product s cradle-to- gate carbon footprint. Other impacts from the upstream of the supply chain are usually very low: e carbon footprint of aggregates is believed to be no more than 8 kg CO2e/t (MPA, 2009b), transport of raw materials to a precast factory may not constitute more than 3% to 4% of a precast product cradle-to-gate footprint, water has a carbon footprint of no more than 0.6 per CO2e/t (0 CO2e/t if ground water is extracted), and the impact of admixtures may not reach 1% to 1.5%. 2.1.2 Manufacturing issues Methods of manufacturing at a precast factory can have a significant effect on the overall footprint of concrete. Highly standardised products manufactured at large-scale factories tend to have lower energy consumption levels and therefore lower carbon dioxide emissions. However, the most significant contributor to carbon dioxide emissions at a precast concrete factory is the method used to accelerate the heating of the product. Figure 1 is extracted from a PhD thesis (Elhag, 2006). It shows the considerable proportion of energy used in accelerated curing compared to other activities at three precast flooring factories in the UK. 2.2 Calculation methodology and precast products carbon footprint As noted above, there is no unified methodology to calculate carbon footprints and this has caused a significant amount of confusion within the sector on how different precast concrete products footprints (compared to other alternatives) should be interpreted. Despite consensus on the United Nation s International Panel for Climate Change (IPCC) Global Warming Potential (GWP) methodology, and despite the fact that most researchers in this discipline have a proper Figure 1. Energy breakdowns from PFF1 and PFF2 (MJ/t). Mixing, extrusio n, attached offices, sawing, etc. Internal Transport (including cranes) Accelerated curing PFF1 PFF2 Beams PFF2 hollowcore