ARTICLE pubs.acs.org/est
Material Nature versus Structural Nurture: The Embodied Carbon of Fundamental Structural Elements P. Purnell* Institute for Resilient Infrastructure, School of Civil Engineering, University of Leeds, Leeds LS2 9TJ, U.K.
bS Supporting Information ABSTRACT: The construction industry is under considerable legislative pressure to reduce its CO2 emissions. The current focus is on operational CO2 emissions, but as these are compulsorily reduced, the embodied CO2 of structural components, overwhelmingly attributable to the material from which they are manufactured, will become of greater interest. Choice of structural materials for minimal embodied CO2 is currently based either on subjective narrative arguments, or values of embodied CO2 per unit volume or mass. Here we show that such arguments are invalid. We found that structural design parameters (dimensions, section choice, and load capacity) for fundamental structural components (simple beams and columns) are at least as important as material choice with regard to their effect on embodied CO2 per unit load capacity per unit dimension, which can vary over several decades within and between material choices. This result demonstrates that relying on apparently objective analyses based on embodied CO2 per unit volume or mass will not lead to minimum carbon solutions; a formal definition of the correct functional unit for embodied CO2 must be used. In short, there is no such thing as a green structural material.
1. INTRODUCTION The construction of buildings and infrastructure is mankind’s most ubiquitous activity, the foundation for our modern urban way of life. It accounts for ∼10% of our economic output (£300 billion in the U.K., $7 trillion globally), provides employment for hundreds of millions of people, and consumes more than half of all the resources we extract from the earth annually.1,2 The impact of construction on the environment is considerable; for example, construction “potentially influences” half of all U.K. CO2 emissions.2 Pressure is mounting on the construction industry to adopt low-carbon or zero-carbon modes of operation. Direct pressure comes from legislative and regulatory processes, e.g., refs 3 and 4. Indirect pressure comes from shareholders, clients, and users demanding greener products. Achievement of true zero-carbon construction might be unachievable in the medium-term but is a worthwhile aspirational goal, forcing us to analyze the construction process and devise ways in which its impact with respect to CO2 emissions, or carbon footprint, can be minimized. There are two aspects to the carbon footprint of a built environment component, be it a brick, a bungalow, or a bridge. First, there is the CO2 produced as a consequence of its ongoing operation, associated with the energy used by heating, lighting, air conditioning, maintenance, and its eventual disposal. This operational CO2 emission (OC) increases with the lifetime of the component, analogous to a recurring running cost. OC is the current exclusive focus of U.K. zero-carbon construction legislation. r 2011 American Chemical Society
Second, there is the CO2 emitted as a consequence of the manufacture of the component, generated during mining, processing, and transport of raw materials, site operations, etc. This embodied CO2 emission (EC) is a set amount that does not vary over the lifetime of the component, analogous to a fixed capital cost. For most building and infrastructure projects, it is dominated by emissions associated with the extraction and processing of materials (steel, timber, concrete, aluminum, plastics, etc.), e.g., refs 5 and 6. The total CO2 emitted during the lifetime of the component [the lifetime CO2 emission (LC)] will be the sum of EC and OC. 1.1. EC, OC, and LC. Knowledge of the relative contributions of EC and OC to the LC of a component is crucial to making informed decisions regarding overall reduction of carbon emissions. For example, consider a traditional lightweight, thin-walled industrial building housing a workforce and equipment requiring heating, cooling, and lighting. We might reasonably assume that, for such a building, OC will overtake EC within a very short space of time and the OC component will dominate LC. We should then focus a greater proportion of the resource we have available for reducing carbon emissions on optimizing operational processes. We may even encourage increases in EC (e.g., the addition of heat insulation) if they can be shown to cut overall Received: June 27, 2011 Accepted: November 15, 2011 Revised: November 11, 2011 Published: November 15, 2011 454
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Table 1. Embodied CO2 of Selected Structural Materialsa
e.g., refs 712. Sustainability can be defined in terms of many impacts (environmental toxicity, resource depletion, loss of biodiversity, etc.), but the political focus on reduction of CO2 emissions3 makes carbon footprint the dominant metric. These arguments are frequently confused, distorted by vested interests or deficient in transparent quantitative analysis. Some among the timber lobby suggest that wood has a lower carbon footprint because it is a natural, renewable material that “locksup” carbon, e.g., refs 9 and 11, yet deforestation (both direct, and indirect, in which land conversion for, e.g., agricultural use is funded by timber sales from the forest thus cleared) remains a high-profile global environmental concern, contributing ∼17% of global CO2 emissions.13 The energy requirements and greenhouse gas emissions of forestry operations, intercontinental timber transport, kiln drying, glulam production, and waste wood disposal are rarely mentioned. Many parties with interests in steel maintain that since it is (in theory) 100% recyclable, it is inherently low-carbon.7,10 This takes no account of the energy requirement and primary CO2 emissions associated with smelting and refining primary steel; recycling involves significant energy use to remelt the steel (∼1020 MJ/kg).14 Those advancing the low-carbon case for concrete tend to focus on either reducing air conditioning and/or heating requirements by exploiting its thermal inertia, or the use of waste materials as supplementary binders, e.g., refs 14 and 12, yet we are reminded that cement production accounts for ∼5 10% of global CO2 emissions.15 So, which of these materials is “greenest”? In fact, none of these arguments actually have much practical relevance; they are rather elements of propaganda used by lobbyists to their own advantage. Fortunately, those without axes (or welders or trowels) to grind have been quantifying the EC of structural materials for some years; their numbers can lead us toward clarity. The main sources of data on EC of materials are the proprietary databases within commercial LCA software and not generally in the public domain. Recognizing the need for a more open source, Hammond and Jones14 compiled an “Inventory of Carbon and Energy” (ICE) from public domain data on the embodied energy and carbon in a wide range of materials used in construction, taking due account of the wide range of variation and uncertainty in the figures. The ICE has limitations, openly discussed by its authors and not repeated here; nonetheless, it is the most authoritative single freely available source and has become a de facto standard for many studies. Typical data for a range of structural materials (Table 1) appears to suggest that we should use medium-strength low-CO2 concrete for all structural work; its EC is an order or magnitude lower than that of steel or timber, and 2 orders below that of aluminum and the composite material. Yet these figures are expressed per kilogram; 1 kg of concrete cannot do the same job as 1 kg of steel, or 1 kg of timber, and we are not comparing like with like. To compensate, we might adopt the approach popularised by Ashby16 and normalize with respect to a mechanical property of interest. In considering a column, we would normalize with respect to compressive strength σc to give “EC per unit compressive strength” (Table 1). On this basis, the steel compares more favorably to the concrete. Yet this takes no account of geometry, which we know to be crucial with regard to the behavior of columns. To resist a given compressive load, a different cross-sectional area of each material would be required,
EC per unit of
material
EE,
EC,
compressive strength,
MJ/kg
kg-CO2/kg
(kg-CO2/kg)/MPa
Steel virgin (v)
35
2.8
recycled (r)
9.5
0.43
0.0079 0.0012
typical (60% r, 40% v)*
20
1.4
0.0039
7.4 12
0.45 0.7
∼0.02 0.026
Timber sawn softwood glulam structural composite*
Concrete low strength (C12)
0.35
0.05
0.0042
medium strength (C50)
0.87
0.15
0.0030
“low-CO2” (C50, 40% PFA)*
0.56
0.09
0.0018
high strength (C90)*
1.8
0.32
0.0036
Aluminum virgin
218
11.5
∼0.1
recycled
29
1.7
∼0.01
Injection-Molded PVC
95
2.2
∼0.03
polypropylene
115
3.9
∼0.1
glass-fiber reinforced polymer
100
8.1
∼0.1
glass
15
0.58
Other ∼0.10.01
a
Data taken or derived from ref 14. * Denotes values corresponding to the analysis in section 2 of this paper below.
LC by reducing OC. On the other hand, a massive infrastructure component such as a bridge or a dam will have comparatively small operational energy requirements but much larger material usage, and thus, we would expect the contribution of EC to LC to be much higher. For most real structures, it will be necessary to perform a life-cycle assessment (LCA, as defined by the ISO 14040 series of international standards) to determine the relative magnitudes of EC, OC, and LC and thus formulate detailed carbon reduction policy. Nonetheless, given the millions of tonnes of materials used in construction projects and the dominance of EC by emissions associated with materials, the contribution of EC to LC is always likely to be significant. The LCC-IGT report2 suggests that, for the built environment as a whole, EC is currently ∼1520% of LC. As we move toward the zerocarbon construction targets,3 which aim to reduce OC to zero by 2019, EC will increase toward 100% of LC. For physical infrastructure assets (bridges, large structures, roads) OC is normally attributable to the users (e.g., vehicle operators) and not associated with the system itself; thus, EC will already be of greater interest to infrastructure stakeholders. In this paper, we therefore concentrate on quantifying EC to prepare us for this second stage of carbon reduction. 1.2. EC of Materials. The debate over the “best” material to use for a given structural solution has been raging for many years. Claims advanced by proponents of a given material, previously based on cost, buildability, or durability, are now being augmented by simplistic arguments regarding its sustainability, 455
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leading to a different maximum height to prevent buckling failure; the ratio between the maximum heights LA, LB for two different materials A, B is a function of σc and Young’s moduli E, which (given the same cross-sectional shape) can be expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffi LA EA σ2B ð1Þ ¼ LB EB 3 σ 2A
structures. Other top-down studies almost exclusively deal with housing and thus pick an apartment building20 or house5 as the functional unit. These report that timber houses often, but not always, have lower EC than concrete. Some report that items common to both systems (e.g., carpets, air-conditioning systems, and roofing) tend to dominate the EC of the house,5 obscuring the contribution of structural materials. Such studies are limited to relatively small, light components 10% of global anthropogenic CO2 emissions; this work is a first step toward its coherent reduction.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed design procedures adopted for the RC columns and beams; the rationale for optimization and choice of concrete grades; and their EC vs strength curves. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +44 113 343 0370; e-mail:
[email protected].
’ ACKNOWLEDGMENT Prof. Nigel Wright; Drs. Julia Steinberger, Mark Tyrer, and Leon Black; and Ms. Katy Roelich are thanked for their comments on the manuscript. ’ REFERENCES (1) Krausmann, F.; Gingrich, S; Eisenmenger, N.; et al. Growth in global materials use, GDP and population during the 20th century. Ecological Economics 2009, 68 (10), 26962705. DOI:10.1016/j.ecolecon. 2009.05.007. (2) Low Carbon Construction—Innovation and Growth Team 2010, Final Report; HM Government Department for Business, Innovation and Skills, Crown Copyright BIS/11/10/NP, URN 10/1266, 2010; http://www.bis.gov.uk/assets/biscore/business-sectors/docs/l/10-1266low-carbon-construction-igt-final-report.pdf. (3) Zero Carbon for New Non-Domestic Buildings—Consultation on Policy Options; Department for Communities and Local Government. Communities and Local Government Publications, Crown Copyright 09BD06162, 2009; http://www.communities.gov.uk/documents/ planningandbuilding/pdf/1391110.pdf. (4) Climate Change Act 2008; HMSO: U.K., 2008; http://www. legislation.gov.uk/ukpga/2008/27/contents. (5) Hacker, J. N.; De Saulles, T. P.; Minson, A. J.; Holmes, M. J. Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change. Energy Build. 2008, 40 (3), 375384. DOI:10.1016/j.enbuild.2007.03.005. (6) Harrison, G. P.; Maclean, E. J.; Karamanlis, S.; Ochoa, L. F. Life cycle assessment of the transmission network in Great Britain. Energy Policy 2010, 38 (7), 36223631. DOI:10.1016/j.enpol.2010.02.039. (7) Sustainable Construction—The Bigger Picture; Corus/CMP Information, U.K., undated (retrieved March 2010); www.corusconstruction.com/en/reference/publications/sustainability_and_environment/. (8) Increased Temperature; The Concrete Centre, U.K., undated (retrieved September 2010); www.sustainableconcrete.org.uk/main. asp?page=68. 460
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