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Learning legacyLessons learned from the London 2012 Games construction project

The procurement and use of sustainable concrete on the Olympic Park

AuthorKirsten Henson MEng MPhilMaterials Manager, CLM, KLH Sustainability

A concrete batching plant within the Park provided concrete to all projects on the Park

AbstractInitial estimates of concrete use on the Olympic Park indicated a requirement for 500,000m3 of ready-mixed concrete and an equivalent aggregate requirement of 1 million tonnes. Concrete has a high environmental impact and therefore improving the sustainability of the concrete was a key focus for the Olympic Delivery Authority (ODA).

The combination of a tight construction programme, the importance of a reliable supply and the opportunity to maximise the sustainability credentials, led to the procurement of a single ready-mix concrete batching plant to provide concrete to all projects on the Park.

The ODA worked with the concrete supplier and engaged with the supply chain to develop sustainable concrete mixes. This resulted in the use of

approximately 170,000 tonnes (almost 22 per cent) of recycled and secondary aggregate, a saving of approximately 30,000 tonnes (24 per cent) of embodied carbon and elimination of over 70,000 road vehicle movements.

Rationalisation and efficiency of design reduced concrete demand by 65,000 cubic metres, saving a further 120,000 tonnes of aggregate and 20,000 tonnes of embodied carbon.

Centralised procurement, early supply chain integration and extensive trials and testing of various sustainable concrete mixes, were key to reducing the overall environmental impact of concrete on the Park.

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IntroductionThis case study provides an overview of sustainable concrete on the Park, including: – the process of procuring sustainable

concrete and engaging the supply chain to use it;

– the barriers faced when driving the delivery of sustainable concrete and the strategies to overcome them;

– key outcomes and achievements; and

– recommendations for future projects.

It covers both the use of ready-mix concrete and precast concrete.

Appendix 1 highlights the sustainability initiatives related to ready-mix concrete and the resultant outcomes.

Figure 1 and Table 1 outline the embodied carbon of all concrete use and detail the contribution each sustainability initiative has made to the reduction of the embodied carbon.

Figure 1: Site-wide summary of impact on embodied carbon of concrete for a variety of sustainability initiatives

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

Stage A baseline

Rebaseline accounting for designefficiencies

Selection of cement production facility with low carbon

footprint

Cement substitution

Super-plasticiser

Mineral replacement

Sustainable transport

Embodied energy reduction initiative

Tota

l em

bodi

ed c

arbo

n (to

nnes

CO

2e)

20

30

40

50

60

70

80

90

100

Per cent of baseline embodied carbon

Precast (off-site)Ready mix

Percentage reduction against ‘high’ Institution of Civil Engineers (ICE) rebaseline accounting for design efficiencies

Percentage of low ICE rebaseline accounting for design efficiencies

NotesThe specification profile on the Park is indicated left. These volumes were used to calculate the baseline embodied carbon from concrete constituent component data contained in the Bath Inventory of Carbon and Energy (ICE) database v1.6a including a UK average of 18 per cent GGBS cement substitution.

The high Bath ICE baseline is shown for information only. It uses concrete data from the Bath ICE database and interpolates between data sets to account for 18 per cent GGBS substitution. The reduction shown using this data set is more than 32 per cent.

If comparison is made using the ‘Concrete Industry Sustainability Performance Report 2010’ average embodied carbon (including transportation) of 93.45kgCO2/tonne, the embodied carbon savings of the Park’s concrete are negligible. However, this is not considered to be an appropriate baseline for the London 2012 Games Construction project due to the unique nature and location of the construction.

Specification % Total volume (m3)

Nominal 2.7 11,414

GEN 3.1 12,982

ST 6.2 26,129

C10 to 20 3.3 13,935

C25 to 30 2.7 11,231

C35 to 45 58.9 247,780

C50 to 60 15.3 64,525

Prescribed 2.9 12,351

Precast (off-site) 4.9 20,620

Total 100 420,967

Table 1: Total embodied carbon (tonnes CO2e)

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Industry contextConcrete is the most widely used construction material in the world and has a significant environmental impact. The production of cement, one of the constituent materials in concrete, is accountable for around two per cent of UK carbon dioxide emissions 1 and on average has an embodied carbon of 830 kilograms CO2e/tonne 2. The other key materials, coarse and fine aggregates, while only making a small contribution to the embodied carbon of concrete, create other social and environmental impacts associated with extraction and distribution.

Typically one cubic metre of concrete requires one tonne of coarse aggregate, 800kg of fines and 320kg of Portland cement, although exact quantities vary due to technical and architectural requirements. Transportation of these raw materials to batching plants around the country resulted in approximately four million truck movements in 2009, with less than six per cent being delivered to batching plants by rail 3.

Park contextThe ODA set challenging sustainability targets for all projects on the Park. These are outlined in the ODA’s ‘Sustainable Development Strategy’ 4. The four material targets, which focused on responsible sourcing, embodied impacts, healthy materials and recycled content, together with

a planning requirement to deliver a minimum of 50 per cent of materials to site by sustainable means, had a major bearing on the sourcing and supply of concrete.

Initial estimates made for the Park indicated that 500,000 cubic metres of ready-mix concrete would be required to build both the sporting venues and supporting infrastructure, with an equivalent aggregate requirement of approximately one million tonnes.

The potential environmental impacts were significant, but so was the opportunity to achieve substantial reductions in carbon emissions, promote the use of recycled and secondary materials, ensure extensive use of responsibly sourced products and increase the use of non-road-based transport for the delivery of materials. The actual volumes of concrete used on the site were in fact approximately 15 per cent less than the original estimates, nearer 400,000 cubic metres. Over three-quarters of this reduction can be accounted for through design initiatives, with the remaining volumes attributed to conservatism in initial estimates.

These design initiatives include: – relocation of fencing to an off-site

venue and reduction in the number of bridges and structures;

Aerial view of the Olympic Park

400,000Cubic metres of ready-mix concrete used on the Park.

50%Minimum planning requirement to deliver materials to site by sustainable means.

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– conversion of the Basketball Arena to a lightweight temporary venue;

– innovative, lightweight Velodrome design: the original design concept of a steel arch roof structure was replaced with an innovative cable net arrangement with loading resolved at multiple levels – within the roof compression truss, via the bowl structure and through the ground;

– lightweight Olympic Stadium design: the solid roof in concept design was replaced by a cable net roof that is structurally independent from the bowl; and

– reduction in the Aquatics Centre’s building footprint by 30 per cent: achieved by integration of the adjacent pedestrian bridge from Stratford into the building structure and other design changes.

Despite the decreased volumes from initial estimates, in terms of mass, concrete was still the second most widely used material on the Park after engineered fills.

Introducing sustainability requirements for concrete to 17 Tier One contractors and a large number of architects, engineers, project managers and subcontractors was extremely challenging. Particularly as cost, programme and design quality were key drivers. There were approximately 30 main types of concrete used on the Park. However, subsequent contractor perception and knowledge of recycled materials, programme constraints and technical requirements have resulted in hundreds of different mix designs used on the Park, further adding to the complexity of the challenge.

Security of concrete supply was identified early on as a major risk to the London 2012 construction programme. Security of supply could be significantly impacted by local traffic congestion, batching plant breakdown and availability of raw materials. To mitigate this risk and maximise the opportunities for achieving sustainability credentials,

the ODA procured a single concrete supplier to provide concrete to all projects on the Park through the installation of a new concrete batching plant within the Park boundary, adjacent to the railhead.

The concrete supply contract was managed by the ODA’s Delivery Partner’s (DP’s) (CLM a) Logistics team. A dedicated Materials Manager within the DP’s sustainability team worked with the concrete supplier b and project teams to deliver ODA’s sustainability requirements and to push best practice in this area. This level of ownership and technical support was critical to maintaining the profile and raising awareness of the contribution concrete could make to the project’s sustainability targets.

Procurement process and contract awardReady-mix concreteThe ODA managed the procurement process for the license to supply the Park with concrete 5. The pre-qualification questionnaire and subsequent Invitation to Tender (ITT) used a balanced score-card approach to evaluate tenders. Sustainability requirements made up 20 per cent of the weighting of the technical assessment in the tender evaluation. This is much higher than standard contracts, where sustainability typically accounts for five per cent of the weighting, if it is included at all.

The tender questions were worded to encourage the supply chain to identify opportunities within the market-place and to deliver innovative solutions to meet the Park’s sustainability requirements.

The concrete supplier was awarded the framework supplier agreement for the site concrete in December 2007, in part because it was commercially competitive and in part because it stood out in terms of its understanding and commitment to sustainability 6.

a CLM is a consortium of CH2MHill, Laing O’Rourke and Mace.b London Concrete is a subsidiary business of Aggregate Industries.

To mitigate risk to security of concrete supply and influence sustainability outcomes, the ODA provided an on-site concrete batching plant.

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The concrete supplier proposed a number of mix designs that met the ODA sustainability requirements and tested the proposed mixes in accordance with the regulations of the Quality Scheme for Ready Mix Concrete 7 (QSRMC). Appendix 1 details the sustainability elements of the concrete supplier tender and what was subsequently achieved on the Park. This includes a discussion on the decision to promote pulverised fuel ash (PFA) c as the preferred cement substitute over ground granulated blast-furnace slag (GGBS) d, due to the broader environmental and social benefits.

Supply contracts were set up directly between the concrete supplier and the Tier One contractor, who were responsible for specifying the concrete to be used via their architects and engineers.

Precast concreteA framework supply agreement for precast concrete was not pursued as contractors were encouraged to maximise the use of ready-mix

concrete due to the outstanding sustainability credentials of the on-site batching plant. This resulted in only 25,000 cubic metres of precast concrete being used on site, or about five per cent of the total concrete used. Of this quantity, 5,000 cubic metres was precast on site using the concrete available from the concrete supplier.

On most construction sites precast is promoted due to faster construction programmes, reduced waste generated on site and quality consistency. However the London 2012 Games have demonstrated that all these benefits can be achieved, and more, through a well-managed ready-mix concrete supply chain.

Contractors undertook procurement on an individual basis, often using their standard procurement procedures. For the majority of contracts, this initially resulted in precast concrete being specified with virtually no recycled aggregate and very low, or non-existent, cement substitutions.

A single concrete supplier provided concrete to all projects on the Park which increased security of supply and sustainability credentials

c PFA is a waste by-product of coal-fired power generation, with an average embodied energy of 30kgCO2e/tonne.

d GGBS is a waste by-product from the steel industry with an average embodied energy of 89kgCO2e/tonne.

25,000Cubic metres of precast concrete used on site, about five per cent of the total concrete used.

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The DP encouraged contractors to challenge their supply chains to secure a cost-effective, local source of recycled aggregate, utilise cement substitutions without significantly increasing total cementitious content, and explore options for rail delivery to site. Where the use of recycled aggregate was successful, the source of the aggregate was always recycled concrete aggregate produced from old precast units within the supplier’s yard.

Team Stadium tendered their precast on a requirement for 25 per cent recycled aggregate and, through a period of testing and trials, successfully installed the first precast units on site with coarse aggregate substitution. The Infrastructure teams swiftly followed with recycled concrete aggregate replacing 33 per cent of coarse aggregates in the temporary precast bridge decks.

Supply chain engagementHaving procured a ready-mix concrete supplier who was fully committed to sustainability, there was a very significant task to encourage design teams and contractors to specify the use of sustainable concrete.

All contractors on the Park were contractually required to use the site-wide supplier which greatly assisted with this process. The majority of contractors initially raised issue over having to use a specified supplier, often on cost grounds.

The majority of the cost premium was associated with the imposed handling charge of raw material import at the Park’s railhead. These fees were an order of magnitude higher than those generally experienced by the concrete supplier for rail handling. A direct agreement was reached between the ODA and the railhead operator to reduce the handling fees passed on through the supply chain. From this

point forward, contractors were comfortable using the required concrete supplier.

The DP Sustainability team led numerous workshops at different project stages. An initial meeting was held with project teams and the concrete supplier technical team prior to the specification of ready-mix concrete and contract award.

The aim of the workshop was to introduce project teams to the potential contribution sustainable concrete mix designs could make to project sustainability targets such as recycled aggregate content, total recycled content, Building Research Establishment Environmental Assessment Method e (BREEAM) and sustainable transport.

The workshop also aimed to encourage the designers to specify clear sustainability requirements within concrete specifications. Design teams were urged to enter into early discussions around programme constraints, cost concerns and design requirements, and to raise any technical concerns about the proposed coarse aggregate substitutions or likely impacts on resulting finish quality from increased cement substitutions.

A subsequent workshop was then held with the principal contractor and, where the appointment had already been made, their concrete subcontractor. This early engagement with the supply chain allowed questions of quality, strike time and opportunity to be discussed prior to any concrete being poured on the project site. This workshop utilised samples of aggregate substitutions to reassure contractors of the consistent quality of the recycled materials proposed and often culminated in a DP sustainability-led site concrete visit.

e The bespoke BREEAM assessments included requirements for minimum cement substitutions. Forty per cent PFA or 70 per cent GGBS in substructure, 30 per cent PFA or 55 per cent GGBS in superstructure and 15 per cent PFA or 30 per cent GGBS in precast.

All contractors were contractually required to use the on-site concrete supplier which increased the use of sustainable concrete.

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The visit’s purpose was to provide assurance of achievable finish quality and to encourage the sharing of knowledge between contractors around the opportunities and challenges associated with working with sustainable concrete mixes. The site visits encouraged friendly competition between contractors to push the boundaries of sustainable concrete even further, while still delivering against quality, programme and cost expectations. The willingness of contractors to engage with their competitors to share best practice was paramount to the ongoing improvement in the sustainability credentials of the Park concrete.

Barriers to implementation and strategies to overcome themProject team lack of knowledge of stentA major concern for contractors was the proposed use of stent, a by-product of the china clay industry, as a substitution for coarse aggregate in the ready-mix concrete. Stent is widely used in Cornwall but there is little experience of it further afield.

Contractors initially had reservations about the material, voicing concerns around strike times and finish quality, despite there being no evidence to indicate that stent had any adverse effects on the concrete properties. This concern did not, however, have any impact on contractor use of stent within substructure concrete.

The majority of projects poured piling and foundations using concrete with at least 50 per cent coarse aggregate substitution – in the case of the Aquatics Centre this was 76 per cent, and at Eton Manor it was 100 per cent. On the other hand, it did become an issue when superstructure requirements were discussed. The conservatism of the UK construction industry and the high-profile nature of the project led early visible superstructure pours to be undertaken without stent substitution.

For example, Team Stadium were reluctant to use coarse aggregate substitute in the concrete columns and podium slab. Due to programme constraints they were encouraged to utilise an off-site quality benchmark determined by their subcontractor, Emirates Stadium. Therefore, mix designs were identical to those used in the Emirates Stadium construction and no site trials were undertaken to determine if the quality benchmark could be achieved with coarse aggregate substitution.

On the Aquatics Centre, the Tier One contractor, Balfour Beatty in particular refuted doubts about the use of stent in high-profile visible concrete, successfully pouring the internal walls of the Aquatics Centre with 76 per cent stent to an exceptional high quality finish. The Aquatics Centre became a showcase for other contractors concerned over the use of stent in superstructure concrete.

Volker Fitzpatrick, the Tier One contractor on the Greenway team, went on to successfully deliver a brushed concrete pathway with 100 per cent stent substitution and Carillion on the Media Press Centre (MPC) poured ground slab and structural cores with 100 per cent stent substitution.

Team Stadium, initially reluctant to use stent in the visible superstructure, went on to pour podium topping with 100 per cent coarse aggregate substitution. These increased coarse aggregate substitutes were not generally driven by a desire to showcase sustainable concrete, but were a result of the contractual obligation to achieve 25 per cent aggregate substitution in their project works (including other high value aggregate uses).

50%Coarse aggregate substitution used in concrete piles and foundations by the majority of projects at the Park.

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Aquatics Centre – pushing best practiceFollowing the design team workshop held with the Aquatics Centre team, it became apparent that due to the water retaining requirements of the pools (which constituted approximately one-third of the concrete requirements in the venue), a limestone aggregate would need to be stocked alongside the standard offering of granite aggregate and stent at the concrete batching plant. Limestone has a lower coefficient of thermal expansion, which reduces the chance of cracks forming during the exothermic hydration process of the concrete.

As only primary limestone would be supplied, the design team appreciated the need to raise the proportion of secondary aggregate in the remaining sub- and superstructures to ensure that the requirement of achieving 25 per cent recycled/secondary aggregate use in the project could be met. As a result, concrete mixes with 76 per cent coarse aggregate substitution were specified in the remaining concrete. This was the first team on the Park to push beyond the standard concrete supplier offering of 50 per cent coarse aggregate substitution.

Due to the complex nature of the Aquatics Centre’s superstructure, concrete and fair-faced finish requirements, this set an excellent precedent for follow-on projects. Concerns were raised by project teams about the impact of coarse aggregate substitution on finish quality – the Aquatics Centre’s finish clearly demonstrated beyond doubt that coarse aggregate substitution does not impact the achievable concrete finish.

The Aquatics Centre team also undertook numerous trials to establish the maximum ground granulated blastfurnace slag (GGBS) cement substitution that could be achieved while still maintaining a world-class concrete finish. Following trials at 70, 55, 40 and 30 per cent substitutions, the team finally settled on a 40 per cent cement substitution for the high-specification visible concrete.

76%Coarse aggregate substitution achieved in concrete mixes on the Aquatics Centre with high quality finish.

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High cement substitution – effect on strike times and finish qualityThere were widespread concerns among contractors about the impact of using cement substitutions above 30 per cent PFA and 40 per cent GGBS on strike times and finish quality, particularly in the super-structure. By contrast, all contractors were comfortable using between 30 and 40 per cent PFA substitution in piling.

High cement substitutions in below-ground concrete actually benefit the durability of concrete in contaminated ground conditions. The PFA or GGBS reduces the potential ingress of water and any aggressive chemical presence, thus protecting the structural rebar. These beneficial properties are a significant driving factor in the extensive use of cement substitutes in below-ground concrete.

The concerns within the superstructure are justified to some extent. The vast majority of superstructure concrete on the Park was specified as visible concrete with architects requiring GGBS cement substitute rather than PFA to achieve lighter finishes. Concrete with high cement substitutes tends to be left in the formwork for longer periods of time due to the slower strength gain.

This can cause two issues with GGBS substitution: blue-coloured blooms on the concrete surface and bleeding to a degree greater than for other cement types/blend levels. This is a feature phenomenon whereby water migrates to the top of the structure carrying finer particles and bits of sand with it. The result is an area of laitance with the further potential for sand runs and plastic surface cracking at the top of the pour.

The Aquatics Centre team experienced these issues when using concrete with GGBS cement substitutions between 60 and 70 per cent. As the concrete was specified as fair-faced finish, the impact on the appearance of the concrete was not acceptable.

A number of remedial measures were introduced by the Aquatics Centre team to reduce this problem. These included the revibration of concrete after approximately one hour, the lowering of the target slump to 150 millimetres and the scooping out and replacement of the top section of the concrete pour. The blue blooms tended to fade over time without any remedial action. However, GGBS suppliers will not guarantee that the colour will fade to achieve a consistent colour finish.

As this level of reworking and waste is not consistent with a sustainable construction approach, extensive testing of subsequent fair-faced finish pours resulted in the GGBS cement substitution being reduced to 40 per cent.

Despite the Aquatics Centre project having some of the most challenging concrete requirements with exceptionally high finish quality requirements and complex curved forms, more than one-third of the superstructure was poured with GGBS substitutes between 55 and 70 per cent. It was therefore disappointing that other projects with less demanding concrete forms and finish quality requirements were not able to address the challenge of delivering concrete with more than 40 per cent GGBS substitution.

High cement substitutions in below-ground concrete actually benefit the durability of concrete in contaminated ground conditions.

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In a similar vein of eliminating additional finishes, a number of projects specified power-floated floor slabs. However, the slower setting characteristics impacted on the contractors’ ability to power float concrete on the same day as the pour. Consequently, the Olympic Stadium contractor had to work extended hours into the early morning to achieve the desired finish on a concrete slab poured with 36 per cent GGBS. On the back of this experience a number of other contractors did not utilise any cement substitution in power-floated floor slabs.

Even where non-visible concrete was poured, for example, on the MPC cores, projects defaulted to lower cement substitutes, in this case 30 per cent PFA. This is likely a factor of design teams requiring a given strength at 28 days, regardless of when an element is to become structurally active.

The strength gain of concrete with high cement substitutions is slower than conventional mixes, with strengths at 28 days likely to be five to six Newton per square millimetre less than Portland cement mixes and for which the revised compliance age of 56 days provides parity. To achieve higher cement substitutes, design teams need to understand how a building is to be constructed and when elements will be loaded so that due consideration can be given to the acceptability of specifying a 56-day characteristic strength.

This reduced level of substitution may also be driven by standard construction timeframes, which generally allow for construction of formwork in the morning, concrete pours scheduled for the afternoon and striking of concrete undertaken the following morning.

If the schedule could be rearranged so that concrete was poured in the early morning and struck the following morning, allowing an additional eight hours in the moulds, it is likely that higher cement substitutes could be accommodated without significantly impacting on the construction programme. This initiative would potentially negatively impact on the efficiency of formwork, which is often reused multiple times in non-visible concrete pours. Despite the potentially positive overall sustainability impact, this may not be a strategy that contractors are willing to accommodate.

Where exceptional finish quality is not a priority, a balance needs to be found between cement substitutions, efficiency of formwork and the construction programme.

Contractors and design teams have a responsibility to challenge convention in order to deliver more sustainable concrete mixes.

Contractors and design teams have a responsibility to challenge convention in order to deliver more sustainable concrete mixes.

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Use of post-tensioned slabs on the MPCThe team was the only team on the Park to use Post-Tensioned (PT) slabs in construction. PT slabs at One Brighton were used as a benchmark to engage with the supply chain. These contained 100 per cent stent and 50 per cent GGBS substitution.

The original MPC specification called for 30 per cent GGBS substitution. The team was also keen to utilise coarse aggregate substitution on the back of successful use of 50 per cent coarse aggregate substitution in other ready-mix concrete. However, concerns were raised by the subcontractor on achievement of the required strength for initial stressing. Hence the cement substitution was subsequently reduced to 15 per cent. Moreover, the post-tensioning subcontractor could not get their insurers to undersign the use of stent in the PT slabs.

While One Brighton provided a useful benchmark, the 4m greater span of the MPC presented a sticking point for the subcontractor and their insurers. Despite no evidence to indicate that the increased 13m span would be an issue for cement or aggregate replacements, the outcome clearly demonstrates how the overtly conservative nature of the construction industry can impact on the sustainability of the structures built.

The resulting total cementitious content of the PT concrete is 380kg per cubic metre. It is not clear how this compared to One Brighton and if high cement substitutions were achieved there due to an overall significant increase in cementitious component.

As the PT slabs were a significant use of concrete in the MPC, the lack of coarse aggregate substitution in the slabs had the potential to cause the team to miss their 25 per cent recycled aggregate target. Fortunately, careful monitoring and forecasting identified this issue early and the MPC was able to compensate by constructing a number of elements with 100 per cent stent, including the lift cores and areas of ground slab.

Use of cement or aggregate replacement in post-tensioned slabs was a sticking point for the subcontractor and their insurers.

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Specifications and standardsThe ODA required that all highways should be designed to adoptable standards. Series 1700 of the ‘Specification for Highways Works’ 8 states that recycled concrete aggregate and recycled aggregate shall not be used f. Fortunately stent is equivalent to primary granite and can therefore be used. However, the requirements of Series 1700 significantly impact the inclusion of recycled concrete aggregate into highways structures. While it can be understood that the integrity of highways structures needs to be controlled, having such rigid standards prevents innovation even where rigorous quality control procedures are in place.

The ODA was prepared to accept a deviation from the Highways Agency’s standards on temporary bridges. As a result, a precast temporary bridge deck supplier identified an opportunity to replace 25 per cent of the coarse aggregate fraction in the concrete with a locally-sourced recycled concrete aggregate.

Depending on how long the bridges remain in position, there is an opportunity to test whether the use of recycled aggregate has compromised the performance of these precast decks, and potentially to use the bridges as an example to encourage a relaxation in the Highways Agency’s specification.

There is an increasing interest in using recycled construction and demolition waste in concrete, although there are significant concerns around quality control and consistency of materials. While primary aggregates are still being used in sub-bases for road construction, engineered fill for embankments and structures, construction platforms and piling mats, it seems there is little need to focus on reprocessing construction and

demolition waste to a quality suited for inclusion in structural concrete. However, as infrastructure construction slows and demand for engineered fills reduces, it may be sensible to review the balance and initiate research and subsequent standards for the production and use of recycled construction and demolition materials in concrete.

Increase of cementitious component to accommodate cement substitutions or recycled aggregatesIncreasing the total cementitious content of concrete to accommodate cement substitutions is largely unavoidable to some extent. PFA and GGBS are effectively inert materials which are ‘activated’ by Portland cement. The different hydration processes for each combination slows the strength gain. As previously discussed, strength compliance is generally required at 28 days and, therefore, the increased cementitious content is necessary to maintain equivalence with Portland cement for attainment of strength at 28 days. Water reducing admixtures and superplasticisers can help to minimise the increase.

Precast concrete tends to be more sensitive to cement substitution. Precast yards require early strength gain to maintain their 18-hour mould turn-out time as their profitability is heavily dependent on mould productivity.

On occasion, precast yards will also increase the total cementitious component to mitigate against the perceived risk of including recycled aggregates in the concrete mixes. A more carbon efficient mitigation strategy is to impose source control to ensure a high-quality recycled aggregate. Contractors were sufficiently aware to ensure that the overall carbon footprint of the concrete did not undermine the benefits.

f There is a section within the ‘Design Manual for Roads and Bridges’ which provides guidance for use of recycled aggregates in concrete at a maximum of 20 per cent substitution for certain low-level applications. Use in higher grade applications requires extensive testing in extreme conditions which, while impractical for many recycled aggregates, stent has largely undergone.

There is an increasing interest in using recycled construction and demolition waste in concrete.

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Concrete grade C40 C40 C60 C60 C50 C50

Aquatics rakers

Aquatics rakers

Stadium rakers

Stadium rakers

Velo rakers SBH bridge decks

Cement 380 280 315 309 325 289

GGBS 95 120 190 132 185

PFA 163

Limestone Filler 435 100

Granite 750 1,005 680 1,545 742 733

Recycled 255 170 205 244

Sand 698 698 435 903 800

Total Cement Content kg/m3

475 400 505 441 510 452

Cement Replacement

20% 30% 38% 30% 36% 36%

Coarse Aggregate Replacement

34% 0% 25% 13% 0% 33%

Embodied Energy Tonnes CO2/m3

0.335 0.254 0.295 0.274 0.300 0.258

Table 2: Example precast mix designs used on the Park

Comparison between the design mix on two different rakers on the Aquatics Centre’s stands (Table 2) demonstrates how a supplier may increase the cementitious component if requested to include recycled aggregates in the mix design. The second mix design was selected for the seating rakers because the project team considered, on balance, that the latter represented a more sustainable mix design.

The two mix designs utilised for stadium rakers (two different precast yards were used to meet the demand) clearly indicates the careful balance that has to be sought when considering cement substitutions. Despite apparently superior sustainability credentials (higher percentage cement replacement and coarse aggregate replacement), the first stadium mix design has a significantly higher embodied carbon than the latter.

This is predominantly due to a significantly increased total cementitious component. However, Team Stadium was the first to engage their supply chain in the use of locally-sourced recycled concrete aggregate to produce high-quality precast concrete and set a good benchmark for others to follow.

The precast yard that produced the temporary bridge decks successfully optimised the balance between recycled content and embodied energy. At the request of the London 2012 contractor to secure a source of recycled aggregate, Evans Concrete invested in clearing and reprocessing the precast units dumped by the previous owner. The operation provided clean and cheap material and recovered land for redevelopment. This contractor has subsequently been offering ‘Olympic Park mix concrete’ to other clients. They were also the only precast supplier to utilise rail delivery to site, further reducing the embodied energy of the bridge decks.

It is also interesting to note that the trend to push precast over ready-mix concrete to achieve sustainable construction is not necessarily appropriate. The cementitious contents in the precast are significantly higher than the equivalent ready-mix specification with a resultant increase in embodied energy.

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The impact of material substitution on costThe cost of GGBS is slightly lower than Portland cement and PFA is significantly lower in cost. Aggregates are, by comparison, relatively cheap and energy efficient to produce. Transportation distances are the driving factor of both cost and embodied energy of aggregates g.

The selected source of secondary aggregate for the Park’s batching plant was Cornwall, some 250 kilometres further away than the primary aggregate source. The secondary aggregate therefore carried a small cost premium of £2.70/tonne. Many organisations, including Waste and Resources Action Programme promote primary aggregate over recycled aggregate when the recycled aggregate source is more than 30km further away from the demand site than the primary aggregate source. This is a useful rule of thumb, particularly where there are other, closer demands for the identified recycled aggregate source.

However, Cornwall has a vast excess of stent with London and the south east being significant and suitable markets h. If this market is to be exploited, the cost of delivering the material to the London area needs to be comparable with primary aggregates from the Midlands and Somerset, London’s major source of virgin aggregates. The concrete supplier has invested in stent and is aware that to build the market in the South East and further afield, cost-competitive transport solutions need to be found.

The increased cost of stent became an issue where stent substitutes at percentages greater than the concrete supplier standard offering were not explicitly stated in the concrete tender specifications and which were subsequently required by contractors to meet recycled aggregate targets.

Despite the target forming part of the contract, contractors argued that design teams had the same targets and therefore should have ensured that specifications delivered these targets. Unfortunately, a number of design teams were blocked by project managers from specifying higher substitutes due to the tight budgets and perceived increased cost.

This situation is rather perverse. Where stent substitutions were clearly specified, the cost premiums associated with the aggregate substitution were minimal, as contractors competed to deliver the best tender price to secure the contract.

The price differential was contentious if coarse aggregate substitutions were increased after the award of the tender, as the contractor would now have to absorb the full increase in cost or alternatively raise a compensation event to the Project Management team. Better outcomes and a better price were generally achieved through upfront specification of sustainability requirements, enabling partnership and a supportive contractor/client relationship.

g As Figure 1 demonstrates, the increase in transportation distance for secondary aggregates is largely insignificant in concrete (provided rail delivery is used). The embodied energy of the cement in concrete dwarfs the contribution made by aggregates.

h It is important to note that the use of stent may not currently be a suitable strategy for projects beyond London and the south west – the supply chain must be invited to find cost-effective aggregate substitutions rather than design teams or clients dictating a source.

Primary aggregate is promoted over recycled aggregate when the recycled aggregate source is more than 30km further away than the primary source.

Cornwall has a vast excess of stent, however to build the market in the South East, cost-competitive transport solutions need to be found.

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Limited space available for aggregate stockSpace constraints within the Park’s batching site restricted the number of coarse aggregates that could be stocked. Initially, single-sized 10 and 20mm stent and granite were intended to be stocked. However, the Aquatics Centre’s demand for limestone aggregate (see case study on page 8) resulted in graded four to 20mm stent and granite being stocked. Subsequent requests for a 10mm aggregate were made due to contractors struggling to obtain sufficient rebar coverage and good compaction on some bridge abutments using the 20mm stone. In addition, the Velodrome piling contractors were also experiencing issues with the blocking of pumps using the 20mm stone.

Unfortunately, there were insufficient bays to stock both a 10mm stent and a 10mm primary granite, and therefore only 10mm primary granite was stocked. This resulted in an additional quantity of concrete being poured without any aggregate substitution. This highlights the importance of design teams and contractors understanding the limitations of aggregates available at concrete batching plants and ensuring that the design and site plant can accommodate the available aggregates.

AchievementsThe results discussed in this section are represented graphically in Figure 1. – The first step towards sustainability

must always be resource efficiency and the ODA have succeeded on this front. Initial estimates of ready- mix concrete demands on the Park were in the order of 500,000 cubic metres. The actual volume poured will be nearer 400,000 cubic metres, with a further 20,000 cubic metres precast off-site. More than three-quarters of this difference can be accounted for through design initiatives and masterplan rationalisation. The remainder is attributed to initial over-estimation.

– Efficient design solutions have led to an 11.0 per cent decrease in concrete volume used. A further 2.3 per cent reduction in concrete usage has been achieved through rationalisation of buildings and infrastructure on the Park. The total saving delivered is more than 20,000 tonnes of CO2 and 120,000 tonnes of primary aggregate use i.

– The ready-mix concrete supplier has achieved a 2.2 per cent reduction in the carbon footprint of the concrete used against the UK average – a saving of 2,500 tonnes CO2 by selecting an energy-efficient cement supplier j.

i Reductions in concrete volumes used are determined from the relevant design reports. The embodied carbon savings are given against a baseline calculated utilising component data (aggregates, cement, etc) from the Bath Inventory of Carbon and Energy (ICE) database v1.6a, assuming an industry average of 18 per cent GGBS cement substitution in ready mix and 0 per cent in precast. Relevant quantities of components are determined from Olympic Park mix designs. The ‘Concrete Industry Sustainable Performance Report 2009’ indicates that the average cement substitution in the UK was in the region of 18 per cent and, given the observed preference of designers for using GGBS it was assumed that the majority of the 18 per cent substation would be GGBS. Note that the 18 per cent substitution does not compare well to the 32 to 36 per cent substitution subsequently reported in the ‘Concrete Industry Sustainable Performance Report 2010’. Carbon reductions against alternative baselines are briefly discussed in Figure 1.

j This figure is calculated using the Bath ICE v1.6a data for UK cement and GGBS production (with an additional allowance made for road transportation to the batching plant) and comparing it to the carbon footprint data for the concrete supply to site, assuming road delivery. The cement supplier was not selected for these credentials but for the ability to make deliveries by rail.

2,500Tonnes of CO2 saved by selecting an energy-efficient cement supplier.

11%Decrease in concrete volume used.

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– Cement substitution across the Park averaged 32 per cent. This figure would have been higher if GGBS had been promoted as the preferred cement substitute. However, for the reasons explained in Appendix 1, PFA was considered to be more sustainable. Cement substitution resulted in embodied carbon savings of approximately 14,200 tonnes – an 11.6 per cent reduction.

– The use of superplasticiser resulted in a reduction in the total cementitious content of the concrete across the Park with an equivalent carbon saving of 8,900 tonnes, a 7.3 per cent reduction. The average total cementitious content at 372kg per cubic metre (an average of 368kg per cubic metre for ready-mix concrete, 444kg per cubic metre for precast concrete) was still considerably higher than the commonly accepted UK average of 320kg per cubic metre. This was largely due to the high demand for below-ground concrete (approximately 65 per cent k) in contaminated ground l and the large percentage of high-strength concrete used in the Park m.

– Overall, as a result of the various constraints placed on the use of coarse aggregate substitution and despite the use of 60 to 100 per cent coarse aggregate substitutions in some situations, the total aggregate substitution achieved was 21.9 per cent, or 169,000 tonnes – just under the target of 25 per cent n. The majority of this was achieved through the use of stent,

although glass sands and recycled concrete aggregate in precast have made a small contribution. The use of stent contributes a slightly higher carbon footprint than the primary equivalent due to transportation distances. The increase is a marginal 2,800 tonnes of CO2, a 2.3 per cent increase.

– Carbon savings of 6,200 tonnes, a 5.1 per cent reduction, are allocated to use of sustainable transport. In addition, more than 70,000 heavy vehicle movements were removed from the motorways and local roads.

– Overall, the Park can claim to have reduced the embodied carbon associated with the Park concrete by approximately 24 per cent, equivalent to 29,000 tonnes, compared to an industry average concrete with 18 per cent GGBS cement substitution. An additional reduction in the carbon footprint may be attributed to the rationalisation of the masterplan and design efficiencies.

– More than 95 per cent of the complete concrete supply chain, including raw material suppliers, operate under an externally accredited responsible sourcing schemes.

The innovation of the Park planners, engineers, contractors and their supply chain has eliminated the quarrying of over 289,000 tonnes of primary material and saved more than 46,500 tonnes of carbon, equivalent to almost six years of the Park’s operation.

k Based on known volumes of below-ground and above-ground concrete for Stadium, MPC, Aquatics Centre and Velodrome.

l Due to contaminated ground below the human health zone (300–800mm below ground level across the Park), DC3 and DC4 mixes were used, rather than DC1 and DC2 specifications which are more normal in piling and below-ground structures. On average DC3 and DC4 mixes contain 50kg per cubic metre more cement than the equivalent DC2 mix.

m High strength concrete, C50 and above, has approximately 75kg per cubic metre more cement in than the C35 strength class associated with buildings.

n The ODA’s target for 25 per cent aggregate substitution is for all high-value aggregate uses including engineered fills and gabion stone, not just concrete. Due to the high use of site-won recycled materials for engineered fills, the Park-wide target of 25 per cent recycled aggregate use has been exceeded.

289,000Tonnes of primary material saved from quarrying.

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Recommendations for future projects1 Delivering sustainable concrete

requires a client with strong sustainability ambitions, an informed client representative, a sustainable concrete supplier and supply chain, and designers and contractors who are engaged with the sustainability agenda. The design team are often responsible for specifying the concrete strengths and finishes required. They, therefore, have a critical role in setting standards and need to be engaged in setting minimum substitution levels of aggregate and cement in specifications for tender to drive the market and to encourage contractors and suppliers to invest in the skill, know-how and materials that are essential to achieve high-quality sustainable concrete. A decision can then be made on a broad sustainability basis, including carbon footprinting, as to whether the offer from the supply chain represents a sustainable solution. Design teams need to explore options for specifying 56-day strength characteristics, based on a clear understanding of construction programming, to enable increased cement substitutions to be met without an excessive increase in total cementitious component.

2 Initially, as the industry tests new specifications, there will be a need to trial different mixes. This can be costly and take time, but as the market becomes more confident in its use of sustainable mixes, this requirement will reduce. It is also important to engage with the entire supply chain, including the design team, concrete contractor, formwork and form release agent supplier and concrete supplier. The dominating factor in achieving excellence in concrete is not in the levels of sustainable substitutes (particularly in the case of coarse aggregate substitution) but in discussing clearly and openly all aspects of a concrete pour with

the entire supply chain to ensure that all elements work in unison to deliver the required end product. In this respect, the London 2012 Games benefited from the willingness of the Tier One contractor in particular to trial different mixes on the Aquatics Centre and to then share the findings with other contractors. Team Stadium led the way on precast concrete with their supplier, Tarmac, and again demonstrated that it was possible to achieve excellent results with sustainable mixes. Across the industry, increased use of cement substitution and recycled aggregate can be achieved, but designers and contractors will need to be actively engaged and persuaded that this is possible.

3 While all of the opportunities detailed may not be available on smaller-scale projects, the range of initiatives demonstrated on the Park is such that one or more opportunities are likely to be scalable and transferable. The single most significant opportunity in delivering sustainable concrete to all projects of any size and location is that of reducing the quantity of concrete required in the construction in the first instant through intelligent and efficient design.

4 The benefits of forecasting material use within a construction project are considerable. Those projects that understood concrete and aggregate use at the early design stage were able to write specifications to support delivery of sustainability objectives and to engage with the concrete supplier, to ensure concrete requirements could be met. Early clarity on material demands and requirements allowed designers and contractors to take a proactive approach to delivering given sustainability targets, while ongoing update and review enabled contractors to employ remedial action as necessary.

The benefits of forecasting material use within a construction project are considerable.

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Appendix 1: The concrete supplier’s sustainability initiatives and resulting outcomes

The concrete supplier’s sustainability proposal

Available alternatives

Park outcome Park achievement

Five to 50 per cent substitution of the coarse aggregate fraction with stent, depending on strength designation of concrete. Stent, a weathered granite, is a waste by-product of the Cornish clay industry and is classified as a secondary aggregate which does not attract the Aggregate Levy.

On- and off-site recycled concrete aggregate, recycled track ballast, incinerator bottom ash, crushed rock fines, and stent.

Stent is an industrial by-product of consistent quality. The nature of the Cornish clay industry results in 100 tonnes of stent being produced for every tonne of china clay and therefore there is ample availability of the material. In addition, use of stent provides much needed financial revenue for a county that is identified as a European Union (EU) Objective 1 regiono. The concrete supplier was concerned about the variable quality and supply of the alternatives considered.

Substitutions varied from zero to 100 per cent stent resulting in an average coarse aggregate substitution of approximately 40 per cent Park-wide.

Total average aggregate substitution by mass, including fines of 22.5 per cent.

Zero to 15 per cent substitution of the fines fraction with glass sand, depending on the strength designation of concrete.

N/A Glass sand is a consistent, high quality, fine aggregate that is able to replace primary sands.

Substitution varied from zero to 15 per cent glass, resulting in an average fines substitution of approximately two per cent Park-wide.

Thirty per cent substitution of Portland cement with PFA.

GGBS. Cement substitutions of up to 70 per cent are possible with GGBS, enabling a lower carbon footprint than the PFA substitution equivalent.

At the time of tender it was estimated that the UK market imports approximately 50 per cent of GGBS to meet demands while PFA produced by the UK’s coal-fired power stations generally ends up in landfill. These broader environmental and social benefits associated with the use of PFA as a cement substitute were recognised by the ODA and the decision was taken to promote PFA as the cement substitute.

However, the concrete supplier also stocked GGBS at the Park’s batching plant as it was apparent that architects preferred the lighter concrete finish achieved with GGBS substitution for visible elements.

Between 30 and 70 per centp PFA substitution for piling and substructure concrete, non-visible superstructure concrete and all highways structures.

Between zero and 70 per cent GGBSq substitution for visible superstructure concrete.

Resulting average cement substitution of 32.1 per cent.

Total average recycled content of concrete, by materials value is 22.4 per cent.

o EU Objective 1 regions are defined as having a number of key indicators in the red including: low level of investment, higher than average unemployment rate, lack of services for businesses and individuals, and poor basic infrastructure.

p A PFA substitution of 70 per cent was used for the female piles in the handball secant piled wall where very low strength gain was desirable. All other concrete on the Park contained a maximum PFA substitution of 40 per cent.

q Cement substitution levels of 36 to 40 per cent GGBS were more common for walls and columns requiring and early strike time.

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The concrete supplier’s sustainability proposal

Available alternatives

Park outcome Park achievement

Deliver all bulk materials by rail.

Road delivery. The railhead at the Park did not have the faculty to handle powdered materials such as cement, GGBS and PFA. Consequently these materials were delivered to the site by road. Cement was transported from Ketton to King’s Cross by rail and undertook only the final leg of the journey by road.

94.6 per cent of materials, by mass delivered to site by rail (this figure includes cement).

Use of superplasticiser admixture polycarboxylate (PCE). PCE has numerous benefits over traditional superplasticisers and water reducing admixtures; a reduction of total cementitious content of approximately 10 to 15 per cent by massr with high cement substitutions (40 per cent PFA) can be achieved while maintaining consistency, workability and strength gain characteristics.

Various admixtures available.

A range of admixtures were utilised on the Park, depending on specific contract requirements. The use of superplasticiser enabled a reduction in the total cementitious content of the concrete across the Park; the average content being 368kg per cubic metre.

Reject concrete volumes were extremely low, with less than 0.5 per cent (1,412 cubic metres) returned to the batching plant for recycling. Reject rates within London are approximately three per cent (according to the concrete supplier data). The lower than average reject rates on the Park may be contributed to the proximity of the batching plant to the end user and the use of superplasticisers.

An approximately 11.5 per cent reduction in total cementitious content achieved through the use of super-plasticisers.

Avoidance of 17,000 tonnes of concrete waste.

Installation of an EcoFrog to allow rejected and returned concrete to be washed, reclaiming the aggregate for reuse in new concrete.

Allow reject concrete to set in yard, then break out and recycle for use in engineered fills.

All of the rejected 1,412 cubic metres of concrete and an additional returned 143 cubic metres (often as part of partial loads) were reprocessed at the batching plant through the EcoFrog enabling the salvage and reuse of the aggregates in new concrete batches. The EcoFrog is not prevalent at batching plants due to the significant upfront capital cost and the attention to maintenance required. It does however contribute to the sustainability agenda by allowing the recycling, rather than down-cycling, of rejected and returned concrete.

The 143 cubic metres is not considered representative of total concrete wastage. The cost of returning concrete to the batching plant is significant. The majority of contractors poured excess loads within their site compounds and allowed it to set before breaking it out and sending it to the on-site reprocessing facility (soil hospital).

In-situ reclamation and reuse of approximately 3,000 tonnes of aggregates.

r A higher dosage of admixture can lead to more water reduction; however, excess retardation of the hydration mechanism and/or excessive entrainment of air may be encountered leading to quality issues. Water retaining admixtures are fairly commonplace on the market, but new generation superplasticisers are not so widely used as it is often cheaper to increase the cementitious component and use a lower cost plasticiser.

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The concrete supplier’s sustainability proposal

Available alternatives

Park outcome Park achievement

Harvesting and use of rainwater and concrete truck wash-out water to supplement potable water demands for concrete batching.

Potable water. The rainwater harvesting initiative at the site batching plant reduced potable water consumption by nine per cent, with more than 4,125 cubic metres of rainwater and recycled water used in concrete batching.

Nine per cent reduction in potable water consumption.

The concrete supplier offered a supply chain whereby all key suppliers were operating under an ISO 14001 accredited Environmental Management System.

N/A Part way through the programme, Aggregate Industries, the parent company of the concrete supplier, obtained the first awarded responsible sourcing certificate BES 6001, achieving a ‘good’ rating.

BREEAM and CEEQUAL credits achieved for responsible sourcing of concrete.

Use of the Aggregate Industries’ concrete tool to provide bespoke Green Guide ratings for any given building elements that are cognisant of actual concrete mix designs.

N/A Re-evaluation of a number of key building elements that utilised high levels of coarse aggregate and cement substitutes enabled venues to increase the Green Guide rating of their building elements by up to two gradess.

Increase in the number of key building elements achieving Green Guide A+ to C ratings.

s It is interesting to note that aggregate substitutions appear to have a more significant impact on the Green Guide rating than cement substitutes, while the design, construction and material supply industries tend to focus on cement as the component of concern in terms of environmental impact.

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References1 British Standards Institution. Concrete, aggregates and masonry.

Available from: shop.bsigroup.com/en/Browse-by-Sector/Building--Construction/Construction-Materials/Concrete-and-Aggregates/ (accessed 6 October 2011).

2 Mineral Products Association (MPA). A carbon reduction strategy. Available from: cement.mineralproducts.org/special_features/a_carbon_reduction_strategy.php (accessed 31 August 2011).

3 University of Bath, Sustainable Energy Research Team. ICE version 1.6a. Available from: www.bath.ac.uk/mech-eng/sert/embodied/ (accessed 31 August 2011).

4 MPA – The Concrete Centre. Concrete Industry Sustainability Performance Report, based on 2009 production. Surrey; 2010. Available from: 86.54.106.20/PDF/MB_3rd_Concrete_Industry_Sustainability_Performance_Report.pdf (accessed 31 August 2011).

5 ODA. Sustainable Development Strategy. London; 2007. Available from: www.london2012.com/documents/oda-publications/oda-sustainable-development-strategy-full-version.pdf (accessed 31 August 2011).

6 London 2012. Concreters bid to cement ODA deal. London; 2007. Available from: www.london2012.com/news/2007/05/concreters- bid-to-cement-oda-deal.php (accessed 31 August 2011).

7 London 2012. Concrete contract for Olympic Park cements sustainability. London; 2007. Available from: www.london2012.com/press/media-releases/2007/12/concrete-contract-for-olympic-park-cements-sustainability.php (accessed 31 August 2011).

8 QSRMC. The Quality Scheme for Ready Mixed Concrete. Available from: www.qsrmc.co.uk/ (accessed 31 August 2011).

9 Standards for Highways. Manual of Contract Documents for Highway Works; Volume 1 – Specification for Highway Works. England; 2006. Available from: www.standardsforhighways.co.uk/mchw/vol1/pdfs/series_1700.pdf (accessed 31 August 2011).

10 Waste and Resources Action Programme. Available from: www.wrap.org.uk (accessed 13 September 2011).

AcknowledgementsContributors – Kirsten Henson (KLH Sustainability) – David Barrett (London Concrete) – Shaun Roche (London Concrete) – Tim Beeson (Balfour Beatty) – Evelina Maier (Balfour Beatty) – Anna Baker (Team Stadium) – Michael Kerverne (Team Stadium) – Chloe Soque (Carillion) – Samantha Connolly (CLM)

Sustainability research coordinators – Dan Epstein (ODA) – Jo Carris (ODA) – Karen Elson (ODA)

Peer reviewers – Andrew Minson (Concrete Centre) – Peter Walker (University of Bath) – John Burland (Imperial College London) – Chris Clear (British Ready-mixed Concrete Association)

Principal participants – London Concrete – Concrete supplier

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© 2011 Olympic Delivery Authority. The official Emblems of the London 2012 Games are © London Organising Committee of the Olympic Games and Paralympic Games Limited (LOCOG) 2007. All rights reserved.

The construction of the venues and infrastructure of the London 2012 Games is funded by the National Lottery through the Olympic Lottery Distributor, the Department for Culture, Media and Sport, the Mayor of London and the London Development Agency.

For more information visit: london2012.com/learninglegacy Published October 2011 ODA 2011/031