a short guide to - institution of structural engineers · 1 aim and scope 1.1 aim this short guide...

A short guide toembodied carbon inbuilding structures

AcknowledgementsFigures 2 and 4 courtesy of RambollFigure 5 courtesy of Arup and The Concrete CentrePhoto of velodrome courtesy of London 2012Photo of Bryghusprojektet copyright OMA

Published by The Institution of Structural EngineersInternational HQ, 47–58 Bastwick Street, London EC1V 3PST: þ44 (0)20 7235 4535E: [email protected]: www.istructe.orgFirst published 2011ISBN 978-1-906335-19-9

# 2011 The Institution of Structural Engineers

The Institution of Structural Engineers and those individuals who contributed to this Guide haveendeavored to ensure the accuracy of its contents. However, the guidance and recommendationsgiven in the Guide should always be reviewed by those using it in the light of the facts of theirparticular case and specialist advice obtained as necessary. No liability for negligence or otherwise inrelation to this Guide and its contents is accepted by the Institution, its servants or agents. Anyperson using this Guide should pay particular attention to the provisions of this Condition.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in anyform or by any means without prior permission of the Institution of Structural Engineers, who maybe contacted at International HQ, 47–58 Bastwick Street, London EC1V 3PS.

Contents

Foreword iv

1 Aim and scope 11.1 Aim 11.2 Scope 1

2 Introduction 2

3 What is embodied carbon and embodied energy? 3

4 Data for embodied carbon and energy – life cycleinventories (LCls) 4

5 Quantifying embodied carbon 5

6 Sources of data 6

7 Critical assessment of data 87.1 Transparency 87.2 Differentiation 87.3 Currency 87.4 Variability 87.5 Energy sources 97.6 Transport and site work 97.7 Allowance for recycling 97.8 Allowance for use of waste materials and by-

products 97.9 Allowance for materials wasted on site 107.10 Treatment of sequestered carbon 107.11 Carbonation 10

8 Some practical uses 118.1 The relative importance of embodied carbon 118.2 Baselines 118.3 Waste 128.4 Experience 128.5 Scope – what to include 12

9 Conclusions 13

References 14

Bibliography 16

Appendix A – Worked examples 17

The Institution of Structural Engineers A short guide to embodied carbon in building structures iii

Foreword

The first decade of the 21st Century was marked bythe rapidly growing acceptance that the world’sclimate was changing, and the results of thesechanges would have a profoundly damaging impacton the environment and large numbers of people’slives. Generally agreement developed recognising theseverity of the changes brought about by climatechange would be relative to the ability of the world’spopulations to mitigate the causes of these changesand in adapting to a changing environment. Alandmark agreement setting legally binding targets forcarbon emissions by industrialised countries wassigned by 174 countries in Kyoto in 1997, known asthe Kyoto Protocol, these agreements came intoforce in 2005. The UK government published theStern Report in 2006 which considered the economicimpact of climate change on the UK. The reportfound that the costs of action far outweighed thecosts of inaction. In the UK, November 2008 saw theClimate Change Bill enacted, with commitments toreduce by 80% of 1990 levels, carbon dioxide greenhouse gas emissions within a period of 42 years. Theworld forum – COP15 held in Copenhagen in 2009reinforced the depth of concern felt globally at thepotential of climate change to impact on the planetand its peoples.

The Council of the Institution of Structural Engineersopenly debated in October 2009, the contributionwhich structural engineering and specifically theInstitution could make to the challenge of globalclimate change and the UK Government’scommitment to reduce carbon dioxide emissions.The outcome from that debate was a request to theTrustees of the Institution to consider actions whichcould be taken which would empower structuralengineers to make an effective contribution toreducing the impact of climate change.

This short guide is a direct result of the Trusteesconsideration of the Councils request. The publicationwas commissioned directly by the Trustees of theInstitution of Structural Engineers from ProfessorRoger Plank, former Head of School of Architectureat Sheffield University and the then Senior Vice-President of the Institution.

In producing this guide the Trustees were mindful thatthe sustainability aspects of the built environment,best practice and legislative frameworks, differ acrossthe international spectrum and that all were evolvingvery rapidly. The guidance contained in thispublication should be considered as interim in thisrapidly evolving field of knowledge and the Trusteescontribution to furthering the necessary evolutions toour industry.

iv The Institution of Structural Engineers A short guide to embodied carbon in building structures

1 Aim and scope

1.1 Aim

This short guide is intended to provide structuralengineers with practical advice on how they cancontribute directly to reducing the embodied carbon/energy footprint of the projects they design.

Many structural engineers have ‘signed up’ to theprinciples of sustainable construction, but are notsure what they can do practically, and may befrustrated because the structural engineer is not thenatural leader for so many of the key issues – forexample, the services engineer will normally lead onthe design and specification of building environmentsystems (heating, cooling, ventilation) which areenergy intensive, whilst the architect will generally beresponsible for form, orientation, glazing, shading etc.Whilst the structural engineer should be aware of allof the relevant issues, most would be uncomfortabletrying to take responsibility for these. The intention ofthis guide is to concentrate on just that part of thedesign which the structural engineer can directlyinfluence, namely the embodied energy and carbonassociated directly with the structure.

Of course, embodied energy and carbon should notbe considered in isolation, and this guide will includereference to other relevant issues – notablyoperational energy – with some general commentabout their relative importance and potential trade-offs. However, in many cases it is possible to makeclear reductions in the embodied carbon withoutdetriment to other measures of sustainability; thisshort guide will provide some guidance on this.

1.2 Scope

The guide is not intended to provide any of thefollowing:– A direct comparison of different materials andsystems

– A comprehensive treatment of sustainableconstruction

– New data for the embodied carbon and energy ofdifferent materials and products

– A detailed commentary of life cycle assessment

The guide is short in length, therefore there is clearlyno room to consider the broader issues ofsustainability in any detail nor is it intended to providenew energy or carbon data for materials. The focus ison describing what information and tools are availableand how these can be applied to estimate, andsubsequently act as a vehicle to identify ways toreduce embodied carbon.

The Institution of Structural Engineers A short guide to embodied carbon in building structures 1

2 Introduction

There are a number of indicators and measureswhich need to be considered in a comprehensivetreatment of sustainable development. Many of theseare outside the direct influence of the structuralengineer, but recently attention has become centredon global warming and climate change, and what iswidely accepted as being one of the majorcontributors to this, namely the emission of carbondioxide – often abbreviated simply to carbon – intothe atmosphere. It should be noted that other gasessuch as methane, nitrous oxide and HFCs(hydrofluorocarbons) also contribute to globalwarming. In some circumstances, emissions of theseother greenhouse gases can be significant, forexample agricultural processes can often causemethane emissions (from animals or from soil), nitrousoxide is associated with the production of nylon, andHFCs may be used in the manufacture of someinsulants. The global warming potential (GWP) ofthese gases is much more severe than carbondioxide, but fortunately the corresponding quantitiesare very much lower. Where appropriate their effecton global warming is generally included in figures forembodied carbon, represented as a carbon dioxideequivalent.

Buildings have an extremely important role to play inthe drive towards a low carbon economy. As anexample, in 2009 it is estimated that the domesticsector accounted for 28% of UK energy demand,consuming 43.6 million tonnes of oil equivalent andcausing 16% of UK CO2 emissions; this correspondsto 75.3 million tonnes CO2 equivalent

1. Estimatesvary, but it is generally agreed that between one thirdand a half of all carbon dioxide emissions areassociated with building construction and operationin service as illustrated in Figure 12.

A building’s carbon footprint describes its overallimpact in terms of carbon dioxide emissions.Estimating the carbon footprint of a building needsconsideration over its life cycle, including both what istermed the embodied carbon, which is associatedwith the construction of the building itself whichwould include the processing of materials,manufacture of products and components, andassembly on site, and the operational carbon, whichis associated with the energy used to service thebuilding, principally for heating, ventilation, coolingand lighting. Embodied carbon and how to quantify it,is the subject of this short guide.

If the structural design is to be informed by the needto reduce embodied carbon, it is important that someconsideration is given very early in the design process

to exploring potential reductions. During the initialdesign phases there is therefore a need to establish aquick estimate based on general information with littledetail. In the context of structural design, theengineer’s experience enables structural concepts tobe developed to define form, layout, and the principalmaterials which will be used. Without the sameexperience of doing this for carbon emissions,structural engineers need quick and easy tools toperform relatively crude comparisons of ideas, andthis guide provides some guidance on how to dothis. Over time benchmark data will help establishrules of thumb for concept design.

As the design develops, more detailed carboncalculations are also required. At the later stages, thecalculation of embodied carbon can require a largeinput of time and research into quantities and valuesfor materials used. Whilst this may be of less value inrefining the design concept, detailed issues such asmaterial specification, particularly for concrete, canhave a significant influence on the embodied carbon.A more detailed analysis of embodied carbon is alsolikely to become necessary as a means of certifyingthe carbon footprint of the building. Increasedexperience will, in time, help define how much detailwill be required, and identify those aspects which areof greatest importance, and others which may have anegligible influence.

2 The Institution of Structural Engineers A short guide to embodied carbon in building structures

Buildings 46%Agriculture 1%Industrial processes 23%Transport 30%

Figure 1 Sources of carbon dioxide emissions by sector

Planning

Concept

Potential for reduction

Repr

esen

tativ

enes

s of

EC

Resu

lts

Detailed Design

Tender

Construction

Use

Figure 2 In order to achieve the best results it is importantthat embodied carbon estimates are undertaken early in thedesign process, although a significant contribution can be madeat every stage

3 What is embodied carbon and embodied energy?

In the context of construction, embodied carbon(often abbreviated to EC or ECO2) is a measure of thecarbon dioxide emissions associated with creatingthe building fabric. It should therefore include thecarbon emitted when sourcing and processing rawmaterials, that emitted to manufacture them intoconstruction products, deliver them to site, andassemble them to form the building. Ideally it shouldalso allow for any replacement or refurbishmentduring the building’s life, and dismantling ordemolition at the end of life and recycling. Of coursethis may be difficult to predict precisely so is oftenaccounted for simply by including a percentageincrease in the initial calculation. How an appropriateallowance should be made for such replacement andrefurbishment is a central part of the work of theEuropean CEN committee TC 350 which isdeveloping a European standard for full building lifecycle analysis. This will hopefully lead to greater clarityand consistency of treatment.

Embodied energy (EE) is the corresponding measureof energy used in these processes. The shift fromenergy to carbon as a unit to measure the embodiedimpact of a product or material is due to the growingimportance attached to global warming and climatechange. In many instances there is a close correlationbetween carbon and energy since a high proportionof man-made carbon emissions results from theburning of fossil fuels. This is particularly the casewhen considering the operational impact of abuilding. However, in some cases significant carbonemissions occur as a result of the chemistry ofproduction for a particular process. The most obviousexample for the construction sector is the productionof cement which involves the conversion of calciumcarbonate to calcium oxide, with carbon dioxide as aby-product. In such cases the total embodied carbonis the sum of that associated with the energy used inmanufacture, and the carbon dioxide emitted as aresult of the chemical reactions. This is illustrated inTable 1 which compares embodied energy andembodied carbon figures for the most commonstructural materials.

In addition, different energy sources, for examplenuclear and fossil fuels, can have quite differentcarbon footprints, since carbon emissions are notnecessarily directly proportional to energyconsumption, and this is illustrated in Figure 3. Theexact relationship depends on the energy source andDEFRA in the UK publishes relevant factors4.

Table 1 Typical values for embodied energy andembodied carbon for common structural materials.(Source: University of Bath ICE v2.03)

Constructionmaterial

Embodied energy(MJ/kg)

Embodied carbon(kg CO2/kg)

Cement 4.5 0.73

Steel 20.1 1.37

Brick 3.0 0.23

Timber 10.0 0.3


1000900800700600500400300200100

0

Coal Oil

Natural

gas

Geother

mal

Nuclear

Hydroel

ectric

Photovo

ltaic

Wind

Figure 3 Indicative levels of carbon dioxide emissions per kWh associated with differentenergy sources

4 Data for embodied carbon and energy – life cycleinventories (LCIs)

The embodied carbon and energy for a basicmaterial such as steel or cement can be determinedby identifying all the relevant production processesincluding the extraction of raw materials,transportation and processing. For each singleprocess step, the raw materials used, energyconsumed etc. are quantified, and thecorresponding impacts are determined. These aretypically represented as MJ/kg and kg CO2/kg forembodied energy and carbon respectively. Clearlythere can be significant variations depending on theprocesses used by individual manufacturer/suppliers, and the particular material specification –for example stainless steel or carbon steel, andglulaminated or sawn timber. In the case ofconcrete, individual data for the constituentaggregates, cement and water is combined. Forbuilding components such as a steel beam orreinforcing bar, additional processing needs to beincluded, and for components such as precastconcrete units, data for different materials needs tobe combined. In this way embodied carbon andenergy data can be developed for the basicelements commonly used in construction. This dataforms what is known as a Life Cycle Inventory (LCI)– a database for a range of materials and basiccomponents, containing information such as theemissions of a variety of pollutants associated withthe product – and this is an essential part ofcalculating embodied carbon.

When considering alternative structural systems,direct comparison of these figures is of coursemeaningless, because of the variation in load carryingcapabilities of different materials – a steel beam forexample will generally be much lighter than theequivalent reinforced concrete beam to perform thesame function. It is therefore important to considerthe total embodied energy/embodied carbon for theequivalent functional units rather than simplycomparing these for the same quantity of differentmaterials.

Data for an LCI requires detailed consideration of allrelevant processes throughout every stage of the lifeof a product. This requires a quantitativeenvironmental analysis of the product to measure arange of impacts, often including global warmingpotential, ozone layer depletion, eutrophication, anddepletion of minerals and fossil fuels. It includes allthe production processes and services associatedwith the product through its life cycle.

In order to quantify the different impacts a verydetailed knowledge of the processes involved at thevarious stages of production, the energy (type andquantity) used and any chemical changes which maygive rise to carbon dioxide is clearly required.Considerable reliance has therefore been placed onmanufacturers to undertake this analysis or to engagelife cycle assessment experts to do so, on theirbehalf, and provide the necessary data. This hasbeen supplemented by independent studies, both togenerate new data and check the validity of industryfigures.

Precisely which stages of a product’s life are includedin LCI data sets can vary. This is what is referred toas the ‘system boundaries’. Most data is quoted as‘cradle-to-gate’ in which case it will include thecarbon emissions associated with all stages fromextraction through processing, but exclude anytransportation to site and subsequent assembly. Itwill, however, generally allow for transport associatedwith the production process itself, so for example, inthe case of concrete, the ‘gate’ is normally that of theready-mix plant. Many carbon calculators combinefigures for cradle-to-gate; this means that inevitablevariations in the impacts associated with transport tosite and disposal at the end of life are avoided; theseshould be considered separately, taking into accountrelevant information such as the specific projectlocation.

Alternative boundaries are sometimes used including:‘cradle-to-grave’ which includes all stages fromextraction (‘cradle’), manufacture, installation, and usethrough to disposal phase (‘grave’); and ‘cradle-to-cradle’, which extends cradle-to-grave assessment toallow for recycling (see Figure 4). These boundariesare more often applied to products which do nothave the complexity of a building or structure, andwhere the subsequent impacts are clearer (forexample consumer goods or products such as,fridge/freezers or computers). What is important isthat the system boundaries for the basic data areknown, and that all other stages through the life cycleare considered, ensuring that there is no duplicationor omission; although in some cases it may bereasonable to neglect a particular stage, or to use asimplified allowance, for example simply adding apercentage increase to account for transportation orsite construction.


EXTRACTION

MANUFACTURE

TRANSPORTATION

CONSTRUCTION

DISPOSAL

CRAD

LE T

O GR

AVE

CRAD

LE T

O SI

TE

CRAD

LE TOGA

TE

Figure 4 Schematic representation of different systemboundaries

5 Quantifying embodied carbon

At its simplest level, the essential componentsrequired to quantify embodied carbon of a buildingare a database of individual material emissions (kgs.of carbon per kg of material) – as detailed in a LifeCycle Inventory (LCI) – and quantities of materials.The total embodied carbon is then the sum of allbuilding quantities broken down by material, includingan appropriate allowance for site waste, multiplied bytheir unit impact.

In principle the process is therefore very simple, andcan be applied, for example, by entering totalquantities into a spreadsheet containing embodiedcarbon data for all materials or products – thedifficulty is assembling the embodied carbon data foreach material. A number of organisations havedeveloped LCIs based on the principles outlinedabove, and have made these available for moregeneral use. This data may be presented for thebasic materials and products of construction, butsome sets of data are presented for components andsystems. These might include, for example, aparticular precast floor system accounting forconcrete, steel and including the casting process, ora half brick masonry wall, accounting for both bricksand mortar. In such cases the data may beexpressed in a way which is consistent with standardmethods of measurement used for normal estimating,and using appropriate units such as kgs. of carbondioxide per sq. m.


6 Sources of data

There are only a few such inventories providingsources of this material data and some of these arenot readily available because they form part ofcomprehensive life cycle assessment software, or arefor internal use within a particular organisation.

One impartial, open and freely available inventory isthe Inventory of Carbon and Energy (ICE) developedby the University of Bath3, 5. This contains cradle togate data for both embodied energy and carbon andhas been largely assembled from publishedinformation and life cycle assessments provided by avariety of sources. The data presented is a synthesisof a much broader range of data collected by theauthors; with individual entries based on a number ofdifferent studies for each material. Inevitably data forthe same material from different sources showedsome variation – partly as a result of variations in LCAmethodologies, but more particularly because of evenlarger differences in production methods. The authorshave therefore had to use their expert judgement topresent explicit values rather than a data range, butthe results are consistent with other databases. Theauthors also acknowledge that the embodied carbondata has often had to be calculated based onpublished information on energy and this maytherefore compromise accuracy a little. Despite thesereservations the ICE database is generally recognisedas providing good quality data for the UK, and it isfreely available in a suitable format. It is thereforewidely used, and a number of organisations haveused it as the basis for developing their own tools forestimating embodied carbon, but as with anydatabase, its limitations should be understood. Theinventory appears to be actively updated withrevisions appearing roughly every year, so usersshould ensure they have the latest version. A briefextract of the ICE database is shown in Table 1.However, it should be noted that the databaseprovides a considerable degree of refinement, forexample quoting different data for particular concretestrengths and specifications.

Some companies have published their owninventories, sometimes based on the ICE, but refiningthe data to suit their own applications.

As an example, Mott MacDonald has used the ICE,supplementing it with independent data which theyhave sourced themselves and information frommanufacturers, to create a database of carbondioxide emissions for all activities of constructionwork. This is included alongside unit prices in thelatest edition of the annual price book which theyprepare6.The information is related to standardmethods of measurement for estimating materialquantities. Individual material impacts have thereforehad to be processed and combined as necessary toprovide data for standard elements of construction.All base data is cradle-to-gate, so transportation tosite and other downstream impacts are ignored, butappropriate plant on site is included. As with theUniversity of Bath database, where the authors had arange of data for a particular product, they usedsome judgement and presented a single figure. Alldata can be modified by the user if particular

circumstances suggest a different value should beused, and new information can be added, giving bothflexibility and transparency. A similar combination ofeconomic and carbon data for every material and unitof work is included in The Institution of CivilEngineer’s latest edition of its Civil EngineeringStandard Method of Measurement – CESMM3Carbon & Price Book7.

Arup has adopted a similar approach, and has alsoincluded figures relevant to product suppliersoutside the UK. Although this data is not publishedin full as a separate inventory, it is integrated intothe commercially available Oasys structuralsoftware8. The data includes more precise figuresfor different concrete specifications which Arup hasused in whole building studies in collaboration withthe Concrete Centre8–10. These studies showedjust how much variation there can be incalculations for embodied carbon as a result ofvariations in both material specification andmethodology as illustrated in Figure 5. This lednaturally to the conclusion that it is important forengineers to consider the detailed specification ofconcrete rather than using generic figures for thematerial, with reductions as high as 200kg CO2/m

2

achieved in extreme cases.

BRE has derived independent embodied carbonfigures for structural and architectural components,rather than basic materials and these are published inthe Green Guide to Specification11. The information iscategorised by building element or system, and usesa letter grading to represent the combination ofthirteen different environmental impacts, includingembodied carbon. The latest edition (4th, pub. 2009)of the guide does include absolute values ofembodied carbon per functional unit (typically onesquare metre of floor or wall) of different systems, but


Stru

ctur

al e

CO2

(kg/

m2 )

Range of dataassociated withmaterial specification

Range of dataassociated withmethodology

450

400

350

300

250

200

150

100

50

0

Figure 5 Variations in the embodied carbon of the structure ofa whole reinforced concrete framed building, principally as aresult of differences in concrete specification (Courtesy of Arup).(The methodology for accounting for recyclability of steel andthe variations in manufacturing methods used is discussed inSection 7)

as yet does not explicitly include similar data for basicstructural materials such as steel and concrete.

Other databases provide similar information for somelocations outside the UK. These include ELCD, theEuropean Reference Life Cycle Database, from theEuropean Commission12, and US LCI database13.Both are freely available, and are likely to grow intovaluable resources in the future.

Commercial Life Cycle Assessment packages suchas Athena (North America), GaBi (Germany),SimaPro, and RMIT (Australia)14–17 also includecomprehensive LCI data. However the use of suchpackages is a rather specialist area and they areunlikely to be of practical use for design engineers ona routine basis.

A number of engineering organisations such as Arup,Atkins, Buro Happold, Expedition, Jacobs andRamboll have used the ICE, as a basis for developingspecific tools for calculating embodied carbon andenergy. They generally adopt a simple spreadsheetapproach, applying the database to a set ofquantities, enabling a quick and easy estimate ofembodied carbon to be determined. In many casesthese tools are for internal use within theorganisation, but some are more widely available, forexample Arup’s Oasys structural software8. Jacobshas developed the Environment Agency CarbonCalculator18 for calculating embodied carbon (alongwith transport and site carbon) for the EnvironmentAgency’s flood defence schemes, and MottMacDonald markets a calculation tool19 using itsBlackbook data. A further tool is the ConstructionCarbon Calculator20. This is freely available on theinternet and provides a very simple calculator givingan approximate estimate of embodied carbon. Itappears to be based on data for North America,although the values used for individual items are notvisible and cannot be changed. There is currentlyinsufficient detail to enable a full design evaluation.

A more extensive list of tools is included on the Wikiavailable on the University of Bath ICE website21.


Sources of data

7 Critical assessment of data

There are a number of aspects which can affect thequality and value of the data presented in a life cycleinventory (LCI). These include transparency andvariability. The data should also be in meaningfulunits rather than based on a grading system. Thereare some issues such as how to allow for recycledmaterials, carbon sequestration, and carbonation ofconcrete, which are the subject of ongoing debate,and for which at present there is no clearconsensus. In such cases it is very important toknow the basis on which the LCI data has beendetermined, for example whether adjustments havebeen made to allow for recycling (this is particularlyimportant in the case of metals), or what materialspecification has been assumed (for example in thecase of concrete).

7.1 Transparency

It is important that the LCI data is visible to the user,and ideally can be modified to accommodate anyadditional information which may be available, ormore specific data which may be applicable in aparticular instance. The system boundaries should bespecified. It is normal for figures to be given on thebasis of a cradle-to-gate analysis, but often withsome adjustment to allow for recycling at the end oflife for highly recycled materials such as metals; thisallows additional impacts such as transportation andsite assembly to be included as appropriate for theparticular project. Without this transparency it is tooeasy for inappropriate data to be used, or worse, forparties with a vested interest to incorporate valueswhich are favourable to themselves.

7.2 Differentiation

Data needs to be sufficiently detailed to enabledifferentiation between similar products. For examplea generic figure for timber is inadequate because itdoes not show the differences between for example,softwood, plywood and glulaminated timberproducts. As the Arup study has shown9 and isevident from the ICE database3, the figures forconcrete need to reflect cement content andstrength, and both post tensioned and precastconcrete should be treated separately from normalreinforced concrete elements. Furthermore the use ofcement replacements such as pulverised fuel ash(PFA) and ground granulated blast furnace slag(GGBS) can have a very significant effect in reducingthe embodied carbon for concrete.

7.3 Currency

Data can quickly become out of date because ofcontinual improvements in processes. Its validity

should therefore always be questioned and clearly itis important to have current data. Generally in theUK, trade organisations and bodies representingdifferent material producers recognise theimportance of this and are increasingly providinggood quality information into the public domain. Theintention of the European committee CEN/TC 350 isto provide a formal mechanism for doing this in aregulated way.

7.4 Variability

A major issue with LCI data is the inevitablevariability, for example in the mix of constituentmaterials forming generic products. This can, inpart, be addressed by a more detailed database,such as the inclusion of data for concrete related tospecification.

Different suppliers may have quite differentprocesses, and these can result in very significantvariations. For example different processes for steelproduction can use quite varying amounts of energyand a greater amount of energy is needed for wet-kilnthan dry-kiln cement production. This is generallyimplicitly allowed for because most data reported inLCIs for a particular country reflects the proportion ofthe different processes used in that region. Of coursethis proportion can clearly change significantly withtime, reinforcing the need to maintain data to ensurecurrency.

A further source of variability is the different lifecycle assessment methodologies which may beused to obtain embodied impacts, and differentdatabases can therefore contain quite differentvalues for what is nominally the same product.Whilst such assessment methods are covered byvarious standards such as PAS 205022 and ISO1404023, some of these allow a wide range ofinterpretation. A series of European standards iscurrently being developed by CEN/TC 350 for thesustainability assessment of ‘construction works’,including environmental performance. This is likelyto have a major influence in construction,superseding current standards and providing aclear definition of how environmental profilesshould be determined for building materials andproducts.

There are potentially significant variations in the dataapplicable to different countries, and in some cases,notably China, India and the Middle East, there is littleor no information available. LCI data for Europe isreasonably accessible, although it may be embeddedwithin commercial LCA software.

Recognising these variations, it is clear that this is notan exact science and the base data is changing overtime. It may therefore be appropriate for all tools andreports to include an indication of the confidencelevels in the accuracy of the results, perhaps using‘error’ or variability bars.


7.5 Energy sources

LCI data should be based on primary energy; that is,the energy contained naturally in raw fuels withoutany conversion or transformation, rather thandelivered energy, most commonly electricity. In thisway the inefficiency and losses in transmission, whichcan be significant, are automatically included. Thesource of primary energy used in a particular processcan itself have a significant effect on the amount ofresulting carbon emissions. For example energy fromfossil fuels generates significant carbon dioxideemissions, but wind and solar energy produce verylittle.

7.6 Transport and site work

As discussed, LCI data is generally presented onthe basis of a ‘cradle-to-gate’ systems boundary.Clearly there are additional impacts (i.e. increasedenergy use and carbon emissions) associated withtransportation to site and site installation. The UKGovernment has published data for transport24, 25

enabling these impacts to be estimated based ontypical travel distances and the mode of transport,and the Arup/Concrete Centre9 study includesfurther details illustrating how this might beadapted and applied in practice. Of coursedetailed information may not be known until aspecific supplier has been engaged, making itdifficult to make a suitable allowance during initialdesign. However, studies by a number oforganisations have shown that for most materialsthe additional embodied carbon associated withtransport to site is relatively small (<10%). Theexception is aggregates for general use. In thiscase the embodied carbon data for extraction andprocessing (cradle-to-gate) is so small that theeffects of transport to site are much moreimportant than for other materials. It may also bean important consideration for precast concrete,but for other materials it can normally beaccounted for approximately by a generalallowance of 5–10%.

Clearly for materials sourced from outside the UK, orwhere very long travel distances by road are involved,impacts due to transport may also be higher andneed to be considered.

The embodied carbon associated with site works isalso generally small compared with the cradle-to-gatefigures. A small percentage may be added forconstruction, but again this is often ignored except inspecial cases such as the construction of deepbasements which may involve significant site activityand transportation, but use small quantities ofmaterials.

It should be recognised that some components arelikely to need replacing during the life of the building.Whilst this may not normally be relevant for thestructure itself, some components such as claddingmay be the responsibility of the structural engineer,and an allowance should be made for the additionalimpacts associated with replacement orrefurbishment.

7.7 Allowance for recycling

How recycling is accounted for is a complex andcontentious issue. The system boundaries for mostLCI databases are cradle-to-gate. Ideally a thoroughtreatment of embodied energy/embodied carbonshould consider the full lifecycle of the building orstructure, including end of life. As discussed above,the impacts associated with delivery and constructioncan be included. Impacts can also be introduced fordemolition/dismantling and disposal, which, liketransportation and site work, are generally small.However, for some materials and products there maybe possible benefits as a result of recycling or reuse,and for certain materials, notably metals, these canbe significant.

A number of studies9, 26 have been conducted in anattempt to provide a rigorous and logical method ofallowing for recycling, however although alternativeapproaches have been developed, none has beenformally accepted. Most commonly, recycled contentis reflected in the embodied figures throughreductions in virgin material and processing energyfor creating the final product – this is the recycledcontent approach. The recycled content approachreflects well the impact to produce a material. Forspecific products, some inventories cite impactsbased on what recycled content can be achieved(often called ‘recyclability’) – this is the substitutionmethod (closed loop system expansion). Thesubstitution method suggests how much benefitthere is from the end of life recyclability. These twomethods cannot be used together because it woulddouble count the benefit of recycling. However,neither approach deals adequately with all aspects ofthe problem, and the so-called 50:50 method hasbeen developed to share the benefits of recyclingbetween the initial and subsequent uses. Eachapproach has its advantages and disadvantages.These three approaches are summarised by theauthors of the ICE, and they state that regardless ofthe method chosen it should be consistent with thegoal and scope of study and that the method andresults should be transparently displayed.

7.8 Allowance for use of waste materialsand by-products

A number of construction products make use ofwaste materials or by-products. For example blastfurnace slag, a by-product of steel manufacturing,can be used as a cement replacement in theproduction of concrete. The question therefore arisesas to how the embodied energy/embodied carbonassociated with the waste product should beallocated between two products (in this example,steel and blast furnace slag). An approach sometimesadopted in a life cycle analysis is to do this on thebasis of economic value, although boundarystandards also generally allow other methods ofallocation based on mass or volume. In the case ofblast furnace slag, using allocation by value, thecarbon emissions associated with a quantity of steel,and coincidentally an associated quantity of blastfurnace slag, are divided between the steel and slagin relation to their monetary value.


Critical assessment of data 7.5

In practice the effects of waste products on theoverall embodied energy values for steel andconcrete are likely to be relatively small, but thereplacement of cement, with its high embodiedcarbon, by blast furnace slag (GBFS) or pulverisedfuel ash (PFA) – a waste product of coal fired powerstations, can have a significant effect in reducing theembodied carbon of concrete.

7.9 Allowance for materials wasted onsite

Standard data is available from the Waste andResources Action Programme (WRAP)27 for typicalwastage rates categorised by material and expressedas a percentage. Waste should generally beaccounted for by increasing the basic materialquantities by the corresponding waste percentage.Thus:

EC total¼ECunit�W(1þWR/100)

where:ECunit is the unit value of embodied carbon for the

material from the LCI (kg CO2/kg)W is the weight of material from Bill of Quantities

(kg)WR is the % allowance for waste for that material

7.10 Treatment of sequestered carbon

Some materials, notably timber, absorb carbondioxide, during growth. This is locked into the timberwhen it is harvested, and it is only released at the endof life when the timber is burned or as methane whenit is composted. The process whereby the timberremoves carbon dioxide from the atmosphere isknown as carbon sequestration. Some have arguedthat LCI data should therefore make an allowance forthis sequestered carbon, in which case the embodiedenergy for timber would often be negative. The ICEdatabase consciously excludes sequestration on thebasis of it being a cradle to gate study, but PAS 2050includes a simplified approach. Until this issue hasbeen resolved, some engineers quote values ofembodied carbon for timber based on both theinclusion and exclusion of carbon sequestration.

7.11 Carbonation

Part of the carbon dioxide emitted during cementproduction will be absorbed back into the material viaa process known as carbonation. In theory this canbe up to 90% for pure lime products, althoughexcessive carbonation in reinforced concrete can leadto problems of durability. For typical concreteconstruction the figure is nearer to 10–15% and theprocess is very slow. In concrete structures it isrestricted to a thin surface layer, but if the concrete iscrushed at the end of life, carbonation rates canincrease significantly, thereby offsetting some of thecarbon dioxide emissions during manufacture.However, if exposure is restricted, for example byusing the crushed concrete as fill, the process can be

severely affected, and as a consequence manypractitioners do not make any allowance forcarbonation.


7.9 Critical assessment of data

8 Some practical issues

The principles of carbon accounting can be appliedsimply to determine the approximate amounts ofembodied energy/embodied carbon of a buildingstructure. How the results might be used in practiceto influence design decisions is less straightforward,as other factors need to be considered, even in thecontext of sustainable construction, the mostsignificant of these is the efficiency of the buildingwith regard to operational carbon and energy.

It is also important to develop some perspective ofwhat might be considered reasonable benchmarks toavoid unrealistic targets. This section thereforediscusses some practical issues surrounding how theresults of embodied carbon calculations might beapplied in real projects.

8.1 The relative importance of embodiedcarbon

The carbon footprint of a building should include allcarbon emissions, embodied and incurred by itsoperation. Therefore to produce a reasonableassessment of this carbon footprint consideration willneed to be given to the full life cycle of the building.The relative importance of embodied carbon to theemissions associated with the operation depends onmany factors, notably, the design life of the building,its energy efficiency, the use of renewable energy, andthe intensity of the servicing.

For typical commercial buildings the proportion(embodied carbon:operational carbon) has beenestimated at between 1:5 and 1:10, depending ondesign life and specification28, 29. As buildingsbecome more energy efficient and use morerenewable energy particularly site generated, forexample by photovoltaic cells, this ratio is likely toreduce, increasing the relative importance ofembodied carbon. This conclusion is supported by arecent report – ‘Redefining Zero’30, which suggeststhat the embodied carbon of some types of buildingmay account for more than 50% of their total whole-life emissions. When embodied carbon becomes asignificant proportion of the carbon footprint of abuilding, the supporting structure being a ‘high mass’component, will be a significant portion of the totalembodied energy/embodied carbon of the wholebuilding fabric (which will include contributions fromelements such as the envelope and internal finishes),so the importance of the structural engineer’s designand specification increases further.

Whilst the structural engineer may have little directinfluence over the operational carbon and energy, theimportance of these impacts should not be ignored,and it is essential to maintain a holistic view of thedesign. It would be counterproductive to minimiseembodied carbon if this were offset by an equal orgreater increase in operational emissions. Ideally thetwo should be considered together, but thecalculations for operational carbon will require adifferent approach. At its simplest these might be, for

example, typical figures for construction that achievethe minimum performance specified in nationalregulations, using published energy benchmarks, orby using hand calculations. More precise figures willrequire some environmental modelling to predictenergy usage over the expected design life of thebuilding.

A number of software tools have been developedspecifically for the environmental impact assessmentof buildings – for example, BREEAM31 and LEED32.Both these tools include for elements of operationaland embodied carbon, along with a range of otherindicators, to give an overall rating for a buildingthroughout its design life, but do not provide specificdata for embodied carbon.

Commercial life cycle assessment software packagessuch as GaBi and SimaPro may be of interest to thespecialist, but are not intended for routine designwork and require expert use. They do howeverprovide a potentially rich source of data for basicmaterials and products which can then beincorporated into simpler design assessments.

8.2 Baselines

At present there is insufficient experience available toestablish good benchmarks on which to comparebuildings/structural techniques. Consequently theprocess of calculating embodied carbon can only beused to compare options. However as experiencegrows, and more data become available, it is likelythat it will be possible to identify typical figures forembodied carbon and establish baseline data. Somecompanies are working towards doing this by backcalculating the embodied carbon for a number of


Olympic Velodrome, London. Image courtesy of London 2012. During the design of thevelodrome, an investigation into options for the roof structure was undertaken by Expeditionin collaboration with the architects Hopkins. Four different options for the distinct doublecurved roof were evaluated on cost, carbon, risk and programme impact. The steel cablenet roof (Option C), was shown to have the lowest embodied carbon and also performedwell on cost and programme, and it was this solution that was taken forward toconstruction. The lightweight design of the Velodrome means that it has a very lowembodied carbon for the scale of structure and building typology.

completed buildings. This also helps reveal thosebuilding elements which contribute most to embodiedcarbon.

Ideally such a benchmarking exercise will beundertaken by a number of companies, using aconsistent methodology and reported in atransparent, accessible way, so that the outcomescan be shared with the profession. In the absence ofsuch benchmarks, consulting engineers Jacobs hasadopted an approach for its in-house assessmenttool which determines a baseline through theselection of ‘typical constructions’ for each of themajor building elements (foundations, external walls,roof, etc.) for a project. For the external envelope, the‘typical constructions’ available comply with thelimiting U-value standards specified in the BuildingRegulations Approved Document Part L.

8.3 Waste

The net embodied carbon of a building can beimproved by avoiding over-specification of materialsand reducing waste. This requires careful design andplanning, but is no more than the good practicewhich should be adopted on any project. Studiesindicate that most waste arisings result from thefitting out of a building rather than structure, and boththe BRE SMART Waste program33 and WRAP27

provide useful waste benchmarking data, forexample; typical quantities of each material (byweight) generated as waste for each unit area ofconstruction type.

Some notable projects such as Heathrow AirportTerminal 5, and the London 2012 Olympic Park MainStadium have demonstrated what can be achieved inreducing waste and re-using materials, therebycontributing effectively to a reduction in the carbonfootprint.

8.4 Experience

Experience of calculating embodied energy/carbonfor buildings is at present limited, although someindependent studies34 have been conducted,particularly comparing the performance of differentstructural systems. Preliminary indications are that ingeneral there is relatively little variation in embodiedenergy/embodied carbon for different forms ofstructure using the same basic structural grid,particularly when regard is taken of the uncertaintiesand variations in data for nominally the samematerials. However, varying the structural layout itselfcan have a significant beneficial effect, as can thedetailed design and specification, for example of theconcrete mix. Design forms which are relativelyinefficient, very material intensive and/or usecomponents with high embodied energy values willnaturally result in higher embodied carbon howeverthis has not been found to correlate with absoluteweight or cost when comparing between differentmaterial. Long span and some specific types of floorconstruction such as flat slab and slim deck usemore material than some other types of floorconstruction and hence will probably have a highercarbon footprint than other systems.

It is very important that embodied energy/embodiedcarbon is considered in the context of the overalldesign – as an example, longer spans have becomepopular because they enable more efficient use offloor space, and hence this could potentially increasethe design life of a building by improving its flexibility,and as a result the effective carbon footprint isreduced.

8.5 Scope – what to include

Ideally, an embodied energy /embodied carbonassessment should include all major elements of thebuilding. Considering just the structure – which mighttypically account for between 15% and 50% of thetotal embodied energy for the building35, can distortthe picture because of potential interactions andeffects on other elements of the building. Forexample, deeper floor construction will call forincreased cladding, fair faced concrete can eliminatethe need for finishes, and some design layouts maylend themselves to reductions in applied fireprotection to the structure. Ideally the process shouldtherefore involve the entire design team, although asthe structure is a key aspect, structural engineerscould play the lead role, applying the same principlesoutlined here to other elements of the building fabric.


8.3 Some practical issues

9 Conclusions

Calculating embodied carbon is in principle a simpleprocess, requiring a comprehensive set of data (theLCI) and a breakdown of quantities. LCI data isavailable, with new sources appearing and existingdata being refreshed. Whilst at present there can, insome cases, be considerable variation in the data fornominally the same basic material, this situation islikely to improve as more data becomes available,and consistent methodologies are agreed. So thereare no obstacles in principle to the structural engineerundertaking these calculations. At present there is norequirement to determine a building’s carbonfootprint, and little benchmark data to use as a basisfor comparison. However, with a UK Governmentcommitment to a reduction of 80% of carbon dioxideemissions from 1990 levels by 2050, this is likely tochange in the near future. This presents anopportunity for structural engineers to assumeresponsibility for a central role in achieving low carbonbuilding construction by minimising the associatedembodied carbon.

This may start by just calculating the embodiedcarbon of structures in the simplest way possible.With a little time, the experience gained might thenlead to an improved understanding of how initialdesign decisions can influence the embodied carbon,and the data generated can help to establishreasonable benchmarks, particularly if these areshared across the structural engineering community.

Embodied carbon is just one aspect of sustainableconstruction; it is that which the structural engineercan most directly influence, but it is important to takea holistic approach recognising that in some casesdesign decisions to minimise embodied carbon couldresult in increasing other impacts. However, there arelikely to be a number of aspects of the structurewhich can be controlled without any adverse effectson other measures of sustainability.


Bryghusprojektet, Copenhagen. Image courtesy of OMA. Arup carried out embodied carboncalculations as part of a broad response to the client’s sustainability objectives. The teamused those calculations to set a carbon target for the next stage of the design. Thisapproach has ensured that the significant elements for this particular project have beenidentified and can now be targeted.

References

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2 Harvey, L.D.D. et al. ‘Mitigating CO2 emissions fromenergy use in the world’s buildings’. Building Researchand Information, 35(4), 2007, pp379–398

3 Hammond, G. and Jones, C. Inventory of carbon & energy(ICE) v2.0. Available at: http://www.bath.ac.uk/mech-eng/sert/embodied [Accessed: 9 March 2011]

4 Department for Environment, Food and Rural Affairs.Greenhouse gas (GHG) conversion factors. Available at:http://www.defra.gov.uk/environment/business/reporting/conversion-factors.htm [Accessed: 9 March 2011]

5 Hammond, G. and Jones, C. Embodied carbon: theinventory of carbon and energy (ICE). BSRIA BG 10/2011.Bracknell: BSRIA, 2011

6 UK building costs blackbook: the capital cost andembodied CO2 Guide. Hutchins 2010. Volume 2: majorworks. London: Franklinþ Andrews, 2010

7 Franklinþ Andrews. ed. CESMM3 Carbon & Price Book2011. London: ICE Publishing, 2011

8 Arup. Oasys website. Available at: http://www.oasys-software.com [Accessed: 9 March 2011]

9 Kaethner, S.C. and Yang, F. ‘Environmental impacts ofstructural materials: finding a rational approach todefault values for software’. The Structural Engineer,89(13), 5 July 2011, pp24–30

10 Burridge, J. ‘Embodied CO2 in construction’. TheStructural Engineer, 88(18), 21 September 2010, pp10,12

11 BRE Global. Green book live website. Available at: http://www.greenbooklive.com [Accessed: 9 March 2011]

12 European Commission – Joint Research Centre. ELCDcore database version II. Available at: http://lca.jrc.ec.europa.eu/lcainfohub/datasetArea.vm [Accessed:9 March 2011

13 National Renewable Energy Laboratory. U.S. life cycleinventory database. Available at: http://www.nrel.gov/lci[Accessed: 9 March 2011]

14 The Athena Institute. ATHENA impact estimator forbuildings. Available at: http://www.athenasmi.org/tools/impactEstimator [Accessed: 9 March 2011]

15 PE International. GaBi software website. Available at:http://www.gabi-software.com [Accessed: 9 March 2011]

16 PRe Consultants. SimaPro website. Available at: http://www.pre.nl/simapro/default.htm [Accessed: 9March 2011]

17 RMIT University. Australian lca inventory data project.Available at: http://simapro.rmit.edu.au/LCA/datadownloads.html [Accessed: 9 March 2011]

18 Environment Agency. Carbon calculator for constructionactivities. Available at: http://www.environment-agency.gov.uk/business/sectors/37543.aspx [Accessed: 9March 2011]

19 Franklinþ Andrews. CapIT carbon and cost tool. Availableat: http://www.franklinandrews.com/publications/capittool[Accessed: 9 March 2011]

20 BuildCarbonNeutral. Construction carbon calculator.Available at: http://buildcarbonneutral.org [Accessed:9 March 2011]

21 University of Bath. Inventory of Carbon and Energy wiki.Available at: https://wiki.bath.ac.uk/display/ICE/Tools[Accessed: 9 March 2011]

22 PAS 2050: 2008: Specification for the assessment of thelife cycle greenhouse gas emissions of goods andservices. London: BSI, 2008 [due for revision in 2011;see also Guide to PAS 2050: how to assess the carbonfootprint of goods and services. London: BSI, 2008]

23 BS EN ISO 14040: 2006: Environmental management –Life cycle assessment – Principles and framework.London: BSI, 2006

24 Department for Transport. Road freight statistics 2008.Available at: http://www.dft.gov.uk/pgr/statistics/datatablespublications/freight/goodsbyroad/roadfreightstatistics2008 [Accessed: 9 March 2011]

25 Department for Environment, Food and Rural Affairs. 2008guidelines to Defra’s GHG conversion factors: methodologypaper for transport emission factors. Available at: http://www.defra.gov.uk/environment/business/reporting/pdf/passenger-transport.pdf [Accessed: 9March 2011]

26 Jones, C.I. ‘Embodied impact assessment: themethodological challenge of recycling at the end ofbuilding lifetime’. Construction Information Quarterly,11(3), 2009, pp140–146.

27 WRAP. The Net waste tool. Available at: http://www.wrap.org.uk/construction/tools_and_guidance/net_waste_tool [Accessed: 9 March 2011]

28 Amato, A. and Eaton, K.J. A Comparative life cycleassessment of modern office buildings. SCI Publication182. Ascot: SCI. 1998

29 Lane, T. ‘Our dark materials’. Building, 272(45),9 November 2007

30 Sturgis, S. and Roberts, G. Redefining zero: carbonprofiling as a solution to whole life carbon emissionmeasurement in buildings. RICS Research Report.London: RICS, 2010. Available at: http://www.rics.org/site/download_feed.aspx?fileID=6878&fileExtension=PDF[Accessed: 9 March 2011]

31 BRE Global. BREEAM website. Available at: http://www.breeam.org [Accessed: 9 March 2011]

32 U.S. Green Building Council. LEED website. Available at:http://www.usgbc.org/DisplayPage.aspx?CategoryID=19[Accessed: 9 March 2011]


33 BRE. SMARTWaste website. Available at: http://www.smartwaste.co.uk [Accessed: 9 March 2011]

34 Kelly, F. ‘Steel tops sustainability study’. New SteelConstruction, 15(4), April 2007, pp14–15

35 Symons, K. and Symons, D., ‘Embodied energy andcarbon – what structural engineers need to know’. TheStructural Engineer, 87(9), 5 May 2009, pp19–23


References

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Anderson, J.E. and Silman, R. ‘The Role of the structuralengineer in green building’. The Structural Engineer, 87(3),3 February 2009, pp28–31

Anderson, J. et al. The Green guide to specification: anenvironmental profiling system for building materials andcomponents. BR 501. 4th ed. Watford: IHS BRE Press, 2009

Atkins. Carbon critical design. Available at: http://www.atkinsglobal.com/corporate_responsibility/carbon_reduction/carbon_critical_design/index.aspx[Accessed: 9 March 2011]

Cobb, F. Structural engineer’s pocket book. 2nd ed. Oxford:Butterworth–Heinemann, 2009

Klettner, A. ‘London Olympics: cleaning up highly contaminatedland’. Contract Journal, 3 September 2009

Low Carbon Construction Innovation and Growth Team. FinalReport. Available at: http://www.bis.gov.uk/assets/biscore/business-sectors/docs/l/10-1266-low-carbon-construction-igt-final-report.pdf [Accessed: 9 March 2011]

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Rawlinson, S. and Weight, D. ‘Sustainability – embodiedcarbon’. Building, 272(41), 12 October 2007, pp88–91.Available at: http://www.tinyurl/yl9wuu5 [Accessed: 9 March2011]

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U.S. Environmental Protection Agency. LCA resources. Availableat: http://www.epa.gov/ORD/NRMRL/lcaccess/resources.html[Accessed: 9 March 2011]

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Wise, C. ‘What if everything we did was wrong?’. Building,275(22), 4 June 2010, pp24–25


Appendix A – Worked examples

This Appendix includes two simple examples toillustrate how the approach outlined in this shortguide may be used in practice.

The first example is for a unit area of a typical cavitywall; the second is for a fictitious building includingelements of steel, concrete and timber construction,and using assumed quantities which in practicewould need to be estimated or taken directly from theBill of Quantities.

Example 1: Cavity wall construction

The quantities (kg) of materials required for theconstruction of 1m2 of a standard cavity wall areshown in Table 2, together with the unit values for theembodied carbon for each material taken from theICE database3. The contribution from eachcomponent is simply the product of the respectivevalues, and the total embodied carbon is the sum ofthese values as shown.

These quantities are based on specified requirementsand some allowance for waste on site shouldnormally be included. The WRAP Net Waste tool27

gives ‘typical’ and ‘good’ figures which can be usedto amplify the figures as appropriate. These areshown in the two right hand columns.

Clearly this unit value of embodied carbon per m2 ofwall can then be used as a convenient way ofestimating the total embodied carbon for a specifiedwall of given length and height, using the sameconstruction.

Example 2: A hybrid structure, with someareas framed in reinforced concrete, someusing steel framing with composite floors,and some use of structural timber

In the early stages of design, the quantities (tonnes)of materials can be estimated; towards the end of themore detailed design stage, more precise informationmay be available through the bill of quantities.Assumed quantities for concrete, structural steel,rebar, decking, and structural timber are shown inTable 3 together with the unit values for theembodied carbon for each material, taken from theICE database. As in the previous example, thecontribution from each material is simply the productof the respective values, and the total embodiedcarbon is the sum of these values as shown.

It is clear that there is a great deal of uncertaintysurrounding much of the data used in the calculationsfor embodied carbon, and it is important thatengineers recognise this. The results should be


Table 2 Example of calculating embodied carbon for a unit area of cavity wall

Material Quantity(kgs)

Unit embodied carbon(kg CO2/kg)

Embodied carbon(kg)

Typicalwastage27

Adjusted embodiedcarbon

Bricksa 120 0.23 27.6 20% 33.1

Blocksb 180 0.059 10.6 20% 12.7

Mortarc 30 0.163 4.9 5% 5.2

Insulationd 1.2 1.05 1.3 15% 1.5

Plastere 14 0.12 1.7 5% 1.8

Total 46.1 54.3

Notes:aThe manufacture of clay products, including bricks, release carbon dioxide emissions, depending on the type of product. The figureused here is for general clay bricks, for which there is a very large data range.bThe embodied carbon for concrete blocks is largely dependent on the specification of the concrete mix used. The figures quoted inthe ICE database are estimated from assumed concrete block mix proportions, plus an allowance for curing, plant operations andtransport of materials to factory gate. The value used in this case is for concrete blocks with a strength of 8N/mm2.cThe figure used is for a 1 :1 :6 (cement : lime : sand) mix, which assumes a typical cement. The data for mortar covers a wide rangeas it is highly dependent upon the clinker content of cement, manufacturing technology and the possible use of additions orreplacements, such as fly ash, slag . . . etc. Cement is an important building material and is important in the manufacture of concrete.dThe ICE reports that data for insulation generally is poor with a considerable variation (�40%). The figure used here is for rockwool,for which there is good data available.eThe ICE notes that there is little good quality data available for plaster, particularly for embodied carbon. The figure used here is forgeneral plaster

No allowance has been made for transportation, but this can be included by using published unit data4 with estimated total journeydistances and mode of transport.

interpreted accordingly, and although they do notprovide precise information, used sensibly they canhelp inform design decisions. In doing so engineersshould consider the ‘range of uncertainty’ which canto some extent be quantified by using upper andlower bound values for the basic data. The figures willalso help identify those elements which contributemost to the total embodied carbon, and those whichare relatively unimportant, allowing attention to befocused on making reductions where they will havemost effect.

Further examples

A number of examples and case studies have beenpublished, illustrating the process of calculatingembodied carbon, and how the results may be usedto inform the design5, 10, 30, 34, 35.

Table 3 Example of calculating embodied carbon for a whole structure

Material Quantity (t) Unit EC (kg CO2/t) EC (t)

Concretea 2737 153 418

Steelb 128 1420 182

Rebarc 163 1310 214

Deckingd 15 1450 22

Timbere 32 190 6

Total 842

Notes:aThe figure used is for concrete strength R32/40 using no cement replacements. The embodied carbon is directly related tostrength, with figures quoted in ICE ranging from 124kgs CO2/t for R20/25 (even lower for weaker mixes) to 176 for R40/50 (andmore for stronger mixes). The variation can be even bigger in practice because of differences in cement content for the same designstrength. The use of cement replacements has an even more dramatic effect. Using 30% GGBS in place of cement reduces the unitEC value from 153 to 125 and 50% replacement reduces this further to 94. This clearly demonstrates the importance of carefulspecification of the concrete in minimising EC, although it is important to ensure that the proportion of cement replacement issuitable for the particular application.bThe figure used is that quoted for Sections, using the data listed for ‘UK Typical – EU 59% recycled’. Other figures are included inthe ICE database are for the ‘Rest of the World typical – 35% recycled’, ‘World typical – 39% recycled’, and ‘Primary’ (0%recycled). The corresponding data for embodied carbon is listed as 2010, 1930, and 2880kg CO2/kg. The proportion of recycledfeedstock has a very significant influence on the unit EC. The ICE does not include a figure for 100% recycled content, but this istypically quoted as about 430kgs CO2/t.

Unlike concrete, it is acknowledged that specifying the precise constituent materials or production route for steel is not appropriate.This is partly because of the high recycling rates and fact that some production of steel from raw materials is necessary to satisfydemand as well as to provide a source of material for subsequent production through recycling. How this is allowed for is a complexissue, and is discussed in some detail by Hammond and Jones3 in Annex B to the ICE database.cThe figures used are for ‘UK typical’, and similar comments apply as for structural steel above. However it should be noted that UKmanufacture of rebar uses 100% scrap feedstock.dThe figures used are for galvanised coil (UK typical), and similar comments apply as for structural steel above. However, as can beseen from this example, the contribution of decking to the overall EC for the structure is relatively small, so the overall results are notsensitive to the data used.eThe figures are for sawn softwood, and exclude the element from biomass energy. The ICE does not include an allowance forsequestration. There is little good quality data for timber in the UK and EU, although there is better information for the USA. However,the data should be considered as region specific, so is not applicable elsewhere. There are often significant variations in the energyused in the manufacture of timber products. Moreover, the fuel mix may include timber off-cuts. This is allowed for in the ICE byseparating the contributions of this biomass fuel from that of fossil fuel. If the timber is from a sustainably managed source theembodied carbon from the biomass fuel may be ignored.


Appendix A – Worked examples

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