Specifying Sustainable Concrete
Understanding the role of constituent materials 2 SPECIFYING SUSTAINABLE CONCRETE
BREEAM ‘Outstanding’ and 2014 Stirling Prize short listed London School of Economics utilised 50% GGBS in the cement to contribute to its About this publication sustainability strategy. © VIEW.
Concrete’s flexibility offers many opportunities for designers to influence the environmental, economic and social credentials of their projects, including performance credentials such as fire, durability, acoustics and adaptability. This publication is intended to assist designers in optimising the sustainable credentials of concrete through specification.
This guide focuses on concrete, its constituent materials and how the variation of specification can influence the sustainability performance of concrete. Sustainable credentials with the greatest scope for influence through specification include: the performance of fresh and hardened concrete (e.g. strength
gain, durability); embodied CO2 (ECO2); CO2 associated with transportation; responsible sourcing and use of recycled or secondary materials.
Aspects of sustainability, outside the scope of this document, are addressed in other Concrete Centre guidance. Readers should refer to www.concretecentre.com/ publications for titles including: Concrete and BREEAM, Material Efficiency, Concrete and Fire Safety, and Thermal Mass Explained.
Contents
Introduction 3
Key guidance 3
Responsible sourcing of materials 4
Aggregates 6
Cements and combinations 10
Water 15
Admixtures 16 Cover image: Reinforcement 18 White Collar Factory, London is a BREEAM ‘Outstanding’ 17-storey office building that has Specification of Concrete to BS 8500 20 extensive exposed concrete throughout. This provides the thermal mass that’s intrinsic Specification examples 21 to its passive cooling strategy. For more on this project, see page 9. References 22 Image © Timothy Soar SPECIFYING SUSTAINABLE CONCRETE 3
Introduction Key guidance
Concrete’s role in delivering a sustainable built Guidance that balances the desire to specify concrete environment through the performance benefits with low environmental impact, whilst ensuring of durability, robustness, fire resistance, thermal other performance parameters are optimised, can mass, acoustic performance and flood resilience – be summarised as follows: together with a reduced need for applied finishes – is ¢¢ Specify the strength required from producers with Product Conformity increasingly recognised and utilised by design teams in Certification. the delivery of the most sustainable projects. ¢¢ Consider specifying strength at 56 days rather than the conventional 28 days, where appropriate. Concrete is a versatile and natural material and designers can use it efficiently to deliver structure and other functions of integrated designs. ¢¢ Specify the largest maximum aggregate size conducive to achieving In addition, concrete and its constituents have strong sustainability placing and full compaction. credentials; for example, they are local to the UK and many have been ¢¢ Permit the use of recycled or secondary aggregates but do not certificated to the highest, most demanding responsible sourcing over specify. standards. These factors are resulting in designers choosing concrete on ¢¢ Specify concrete with a wider range of cement types/combinations sustainability grounds alone. selected from Table 1 from BS 8500-2: 2015.
Sustainability is optimising economic, social and environmental issues. Many ¢¢ Embodied CO2 (ECO2) of concrete should not be considered or assessment tools and methodologies have been developed to provide specified in isolation of other factors such as strength gain. measures and comparison tools. The shortcoming of generalised tools is ¢¢ Permit the use of admixtures. Admixtures can be used to enhance that - by definition - they are general, and specific geographical or project sustainability credentials and reduce the ECO2 of concrete, as well as constraints may not be accounted for. modifying its physical properties. ¢¢ Specify BES 6001 responsibly-sourced concrete and reinforcement A challenge for all assessments is weighting the different factors which often to gain credits under BREEAM. have different units of measurement; for example, how does one compare ¢¢ The specification of recycled and secondary aggregates is often biodiversity, health and safety and transportation CO2 emissions? Therefore it is accepted good practice for designers to not simply follow a tick box mentality not the most sustainable option, although it may gain most points. in their use of assessment tools but to understand the factors and take a holistic BS 8500 allows producers to use up to 20% of recycled aggregates in and whole-life view of sustainability when considering their project. many concretes. ¢¢ The BRE Green Guide does not recognise the availability or otherwise The European standard for concrete BS EN 206 [1] and its UK of recycled product when incentivising the use of recycled content. complementary Standard, BS 8500 [2] do not contain any provisions for Recycled aggregates should only be specified when they are locally specifying sustainability. This document aims to provide guidance over available, otherwise transportation impacts exceed the intended and above concrete standards, to enable the project team to balance the benefits. Using this assessment methodology, this should be discussed desire to specify concrete with low environmental impacts whilst ensuring with the client or project code assessor to prevent unfair penalisation. that its other performance parameters are optimised. These performance ¢¢ Use of cementitious additions can reduce the embodied CO parameters can affect overall environmental impacts, as well as other 2 (ECO ) of concrete and influence its visual appearance. When sustainability issues. 2 aesthetics are critical, specify the cement/combination to achieve colour consistency. 4 SPECIFYING SUSTAINABLE CONCRETE
Responsible sourcing of materials
The concrete industry was the first industry to link its sustainable construction strategy [3] to independent- certification scheme, BES 6001, and set targets for concrete and concrete products to be certified. BES 6001 provides a common benchmark for all construction products to demonstrate their responsible sourcing credentials. The full listing of products certified to BES 6001 is available at www.greenbooklive.com.
shown on each product BES 6001 certificate available at BES 6001– Framework Standard www.greenbooklive.com and is often higher than the generic score for the Responsible Sourcing of allocated to BES 6001 in BREEAM. The majority of certified concrete production attracts a RSCS score of 7 – currently the highest value for any Construction Products scheme.
The BRE responsible sourcing standard, BES 6001[4] first launched in The 2020 target for BES 6001 certification is 95% and the aspiration of the October 2008, provides a benchmark to compare responsible sourcing concrete industry is to achieve 100%. performance for all construction products on an equal basis.
The aim of BES 6001 was to integrate all of the activities associated Figure 1: The activities of the supply chain covered by the responsible with responsible sourcing, from the point at which a material is mined sourcing standard BES 6001 or harvested in its raw state through to manufacture and processing; together with a delivery mechanism using certified management systems. Supply chain The responsible sourcing standard encompasses social, economic and environmental dimensions and addresses aspects such as stakeholder engagement, labour practices and the management of supply chains TransportDelivery upstream of the manufacturer. Materials sourcing Product manufacture Energy management In 2014, BES 6001 was revised to significantly increase the minimum level of requirements to demonstrate compliance and to further encourage Life cycle thinking the adoption of best practice in the management of sustainable Ecotoxicity development. Figure 1 shows activities in the supply chain which are addressed by this standard. Traceability
Accreditation to BES 6001 enables products to gain credits under Legal compliance assessment schemes, including BREEAM and CEEQUAL (the assessment and awards scheme for improving sustainability in civil engineering and public Health and safety management realm projects). Environmental management
Management, measurement Production certified to BES 6001 and reporting of: • Greenhouse gas emissions responsible sourcing standard • Water usage, waste management • Employee training and skills Providing evidence of the responsible sourcing of building products and • Local community engagement • Transport and delivery impacts materials continues to be crucial with the need to demonstrate compliance with a recognised responsible sourcing scheme, certified by a third Resource use management party. Concrete industry indicators report the proportion of concrete and constituent materials production that are currently certified to BES 6001. Quality management
During 2017, certification of concrete products to BES 6001 reached 92% Ethical trading and employee rights of production tonnage [3]. Over 90% of this certified tonnage achieved a performance rating of ‘Very Good’ or ‘Excellent’. Of key interest to construction being rated to BREEAM assessment schemes, is a rating of responsible sourcing used in calculating credits. This is now demonstrated THE CONCRETE INDUSTRY HAS COMMITTED TO by the Responsible Sourcing Certification Scheme (RSCS) score which is LEADERSHIP IN THE RESPONSIBLE SOURCING OF MATERIALS. 92% OF CONCRETE IN THE UK WAS RESPONSIBLY SOURCED TO BES 6001, BASED ON 2017 PRODUCTION. SPECIFYING SUSTAINABLE CONCRETE 5
Specification of responsibly sourced concrete Quick Facts
The concrete industry welcomed a standard by which users could judge its Responsible sourcing level of responsible sourcing. Viewing BES 6001 as the most comprehensive responsible sourcing standard available, measuring the whole infrastructure of the supply chain, the industry quickly adopted independent certification to BES 6001 as a measure within its Sustainable Construction Strategy, launched in 2008.
Managed by the Sustainable Concrete Forum (SCF), the strategy sets the vision that the UK concrete industry will be recognised as a leader in sustainable construction; taking a dynamic role in delivering a sustainable built environment in a manner that is profitable, socially responsible and functions within environmental limits. As a key part of the strategy, the concrete industry publishes an annual sustainability report [3]. This annual report is a vehicle for demonstrating active sustainable management across concrete manufacture and its supply chain, and the latest report can be ¢¢ Responsible sourcing is a holistic approach to the sustainable downloaded from www.sustainableconcrete.org.uk. assessment of materials. ¢¢ Responsible sourcing of materials (RSM) is demonstrated The SCF has published a guidance document, approved by BRE, which through an ethos of supply chain management and product provides interpretations of the requirements in BES 6001 specific to the stewardship and encompasses social, economic and concrete industry. It also gives details of suitable metrics, benchmarks environmental dimensions and is broader than the scope of and improvement targets which the industry has established and typical many stewardship schemes. examples of policies and management approaches. This document can ¢¢ The latest listing of responsibly sourced materials to BES 6001 be used by concrete producers, as an aid to assessment organisations can be found at www.greenbooklive.com. and is particularly valued by SMEs. The latest version, updated to provide guidance to Issue 3.1 of BES 6001 can be viewed at ¢¢ Eco-reinforcement is the certification scheme for responsibly www.sustainableconcrete.org.uk. sourced reinforcement steel to the standard BES 6001. www.eco-reinforcement.org. An alternative is the UKCARES Concrete is able to demonstrate the highest level of responsible sourcing, sustainable reinforcement scheme www.ukcares.com. based on the local availability of materials, short supply chains and regulated management systems. The industry’s high standards, achieved in To gain accreditation to BES 6001 the organisation must have areas such as employment rights, waste and environmental management as a minimum: and health and safety are also reflected. ¢¢ A responsible sourcing policy and comply with all relevant legislation. As a result of these high standards, the improvement strategy and full ¢¢ A quality management system that must follow the principles of commitment from the concrete producers, by 2017 the industry had ISO 9001. achieved 92% independent certification of concrete products to BES 6001 with over 90% of this being to performance level Very Good or higher (see ¢¢ Have a greenhouse gas reduction policy and measures using page 19). the principles of ISO 16064-1. ¢¢ Have policies that cover the efficient use of resources, water, Reinforcing steel for concrete can also be certified as responsibly sourced waste management, life cycle thinking, transport, training and either directly to BES 6001, through the ECO Reinforcement scheme based development and the local community. on BES 6001 [5] but with additional requirements or the CARES Sustainable ¢¢ Demonstrate that at least 60% of its constituent raw materials Reinforcing Steel Scheme based on BS 8902 [6]. are fully traceable (with best practice at greater than 90%). The specification of accredited concrete products enables designers ¢¢ Demonstrate that the supply chain has document to easily source certified materials and help gain maximum credits in environmental management systems that comply with sustainability assessment tools such as HQM and BREEAM [7]. ISO 14001. ¢¢ Demonstrate that the supply chain has documented Health and Safety system that are compliant with local legislation and Guidance for specification record incidents.
Responsible sourcing For more information download the Concrete Industry Guidance to Support BES 6001 [8] from www.sustainableconcrete.org.uk. Recommendation: Specify BES 6001 responsibly sourced concrete and reinforcement. 6 SPECIFYING SUSTAINABLE CONCRETE
Aggregates
Aggregates are the major component of concrete by volume and are inherently a low carbon product. Most are naturally occurring materials requiring little processing and are usually locally sourced, with the associated benefit of low transport CO2 emissions.
The standard BS EN 12620:2002 – Aggregates for concrete [9] does not Table 1: Designated concrete - allowable percentage of coarse CCA discriminate between different sources of material and permits aggregates from natural, recycled and manufactured sources. The focus is on fitness for Designated concrete Allowable CCA as a purpose, rather than origin of the resource. percentage of coarse aggregate
In addition to natural aggregates, suitable materials for use in concrete GEN 0 to GEN 3 100% include air cooled blastfurnace slag, crushed concrete aggregate (CCA), RC20/25 to RC40/50 20%* manufactured and lightweight aggregates, as well as some by-products from the china clay industry, sometimes referred to as stent. RC40/50XF 0%
The UK leads Europe in recycling rates for hard demolition waste, and PAV1 & PAV2 0% sources of secondary aggregates are utilised by the industry. Primary FND2 to FND4 0% aggregates are needed and as a resource are abundant. Their extraction is tightly regulated and sites of mineral extraction are restored, often to an * Except where the specification allows higher proportions to be used. enhanced state, delivering significant biodiversity benefits.
Depending on the type of recycled or secondary aggregates used, there CCA is also permitted in designed concrete, although no direct guidance is may be increased water demand and a need to increase the cement given on limiting proportions. BS 8500-2 does, however, provide guidance content of the concrete to achieve the specified characteristic strength, on limiting concrete strength and exposure classes for CCA use, as shown in Table 2. with a consequential increase in ECO2. When assessing the broader sustainability aspects it will, in many cases, prove to be better if recycled aggregates are used in other applications (in lieu of primary aggregate) in Table 2: Permitted use of CCA in designed concretes preference to their use in concrete.
If exposed aggregates are a requirement for a visual concrete finish, the Exposure Use of CCA permitted architect and concrete frame contractor should agree the specification; a XO Yes test panel of the required finish is recommended. XC1, XC2 & XC3/4 Yes
XD1, XD2 & XD3 Possibly**
XS1, XS2 & XS3 Possibly**
XF1 Yes
XF2, XF3 & XF4 Possibly**
DC1 Yes
DC-2, DC-3 & DC-4 Possibly**
**CCA may be used if it can be demonstrated that it is suitable for the exposure condition.
Note: The maximum strength class should be C40/50, unless the CCA comes from previously unused concrete of known composition, for example from a precast factory. Recycled aggregates (RA) Provisions for the use of fine CCA and fine RA are not given in BS 8500 but BS 8500 permits the use of coarse RA and CCA in concrete, providing this does not preclude their use when it is demonstrated that, due to the certain quality and performance criteria are met. RA is aggregate resulting source of material, significant quantities of deleterious materials are not from the reprocessing of inorganic material previously used in construction, present and their use has been agreed. while CCA principally comprises crushed concrete. Constraining factors for the use of CCA include consistency of supply and Clauses 4.3.3 and 4.3.6 of BS 8500-2:2015 [2] and clause A.7.10 of BS 8500-1: the original source. Due to their inherent variability, testing regimes for 2015 provide guidance on RA and CCA use in designated concrete, as quality control of the RA or CCA aggregates may need to be more rigorous shown in Table 1. than for natural/primary aggregates. SPECIFYING SUSTAINABLE CONCRETE 7
Secondary and manufactured aggregates Quick Facts
Secondary or manufactured aggregates may also be specified for use in The aggregates sector structural concrete. These materials are typically industrial by-products not previously used in construction. These aggregate types are derived from a very wide range of materials; many having a strong regional character. Examples include china clay waste in South West England and air cooled blastfurnace slag in South Wales, Yorkshire and Humberside.
Materials such as china clay sand and stent have similar properties to primary aggregates. As such they conform to BS EN 12620:2002 [9] and their use is well established for fine and coarse aggregate substitution in concrete. However, it is important to ensure that the aggregates conform with all requirements of the specification and an appropriate mix design is used, while an enhanced level of testing may be required.
¢¢ Primary aggregates are predominantly UK-sourced, their Guidance for specification extraction is tightly regulated and adverse environmental impacts - such as noise and dust - are minimised. Recycled and secondary aggregates ¢¢ Regulators such as the Environment Agency work closely with industry to ensure the life cycle of a quarry is Recommendation: Permit the use of recycled or secondary environmentally positive. aggregates but do not over-specify. ¢¢ Over 700 sites of special scientific interest are current When specifying recycled and secondary aggregates, the factors and former mineral extraction sites. The significant
to balance are resource depletion, transportation CO2 impacts and contribution to UK biodiversity from the minerals sector implications on concrete mix design. These are all impacted by is increasingly recognised. availability, and concrete producers are well placed to ensure the ¢¢ Approximately a third of all UK aggregates used in concrete and most sustainable aggregates for each project are used. other applications are either recycled or secondary aggregates. Urban regions provide the principal share of recycled materials and construction and demolition waste. Aggregate size For more information visit www.mineralproducts.org
Aggregate size can have a significant impact on the cement content of concrete; larger aggregate sizes generally requiring lower cement content for the same or similar strength class. Guidance for specification
As an example, the limiting mix design requirements for designated Specification of aggregates concretes are given in BS 8500-2: 2015 (Table 6). It should be noted that each designation class is assigned minimum cement contents Recommendation: Specification of natural aggregates for concrete, (kg/m3) for different maximum aggregate sizes. For an RC32/40 designation, which may contain up to 20% of recycled content, as permitted for example, the minimum cement content for concrete with maximum under BS 8500, is a practical alternative to overly prescriptive aggregate sizes of 10mm and 20mm is 340 and 300kg/m3 respectively. specifications.
Where possible, therefore, reduced ECO2 levels will be achievable by specifying increased maximum aggregate sizes. It should be noted that Any recycled content used in this approach will be at the discretion most plants and factories do not stock aggregate sizes greater than 20mm. of the concrete producer based on availability and cost (with aggregates levy and landfill tax in-built to any cost comparison).
Guidance for specification
Aggregate size
Recommendation: Do not specify aggregate sizes below 10mm unless necessary. 8 SPECIFYING SUSTAINABLE CONCRETE
Transportation impact from Life Cycle Analysis and BREEAM
the utilisation of recycled/ BREEAM New Construction 2014 further encourages the use of recycled and secondary aggregate with a credit awarded for certain percentage replacements secondary aggregates of aggregate, with an additional credit for exemplary performance. The quantities The UK construction industry is very efficient at recycling hard construction and criteria required to be met vary considerably between editions of BREEAM. and demolition waste in non-concrete applications, and there is very little The BRE’s Green Guide to Specification is still a commonly used source of evidence that any material is being land-filled as waste [10]. Approximately Life Cycle Analysis (LCA) ratings for generic forms of construction and is a a third of all UK aggregates used in concrete and other applications are component of many BREEAM assessments in its Materials sections. It rewards either recycled or secondary aggregates. Urban regions provide the the use of recycled and secondary aggregates in concrete, but does not principal share of recycled materials and construction and demolition acknowledge that aggregates are plentiful in the UK and available recycled waste. aggregate stocks are usefully employed in other non-concrete applications. Its Given that recycling is already efficiently undertaken, most available methodology is based on the principle that extraction of a certain tonnage of recyclable materials are already in the market and future growth is likely material has the same impact whether it is abundant or scarce [11]. This is in to be incremental and linked to the future amounts of construction and contrast to more recently established European standards for LCA (CEN/TC 350 demolition activity. As such, primary aggregate extraction is unlikely to series) which do take into account the rate of mineral depletion ( i.e. volumes be reduced by further encouragement of use of recycled aggregates on used in relation to the size of reserves, also known as abiotic depletion). particular concrete projects. Instead, overly prescriptive specifications Environmental Product Declarations can be used as an alternative to, or in would result in recycled aggregates being transported longer distances to association with Green Guide LCA ratings in the Materials section of BREEAM meet specific project requirements rather than being used more efficiently New Construction 2014, offering an opportunity for higher scores in in the locations where the materials are generated. Such a distortion of acknowledgement of the more recent and specific data and standards used. markets could be less environmentally-friendly than using locally available primary aggregates due to the related increase in the delivery distances. For those using BREEAM 2018, the BRE’s approach to Life Cycle Assessment There will be no net reduction in primary aggregate use; just increased in BREEAM is now designed to ensure that all life cycle greenhouse gas transportation of material. As a bulk material road transport is a significant emissions are considered. BRE EN EcoPoints have replaced BRE’s Green element of delivered aggregates carbon emissions relative to the typically Guide to Specification in the calculation method for life cycle assessment low per tonne emissions associated with aggregates, extraction and in BREEAM. LCA assessments submissions are either made via the BREEAM processing. Simplified Building LCA Tool or via IMPACT Compliant tools and other building LCA tools recognised by BREEAM. The impact of this is illustrated in Table 3, which provides indicative ECO2 for the extraction and production of virgin and recycled aggregates, as well For more information download Concrete and BREEAM [7] from as their delivery to site. Table 3 demonstrates that ECO2 values for recycled www.concretecentre.com/publications. aggregates may be higher than for virgin materials if delivery distances are longer than around 15km (10 miles). Furthermore, as recycling rates are so high, no tangible benefits in terms of resource depletion will have been Environmental Product Declarations achieved. An Environmental Product Declaration, EPD, is a document that reports environmental data of products based on life cycle assessment (LCA) Table 3: Indicative CO2 for virgin and recycled aggregates and other relevant information. Standards such as ISO 14025 and EN 15804 exist to ensure consistency of format and comparability of kg CO Cradle to gate Transport kg CO Total kg CO +/– % CO Material and data. EPDs can be verified by accredited third parties so that users have delivery distance assurance that the EPD complies with the requirements of the standard 2 /tonne
2 to which it is claimed to follow. The methodology for LCA (based 2
/tonne on the European and International Standards) varies from the BRE’s
2 Environmental Profiles Methodology upon which the Green Guide is /tonne based. The concrete industry has produced generic EPDs to EN 15804 for a range of commonly used precast concrete products and ready- mixed specifications and these can be used in BREEAM assessment Virgin aggregates and for inclusion with BIM software. Manufacturers are also producing product specific EPDs. +58.5km (delivery and return 6.6 2.7 9.3 – See www.concretecentre.com/EPD for more details. distance by road)
Recycled C&D aggregates compared to the use of virgin aggregates Used on-site, 0 km transport 7.9 0.0 7.9 –15% Guidance for specification +5km (delivery distance by road) 7.9 0.5 8.4 –10%
+10km (delivery distance by road) 7.9 0.9 8.8 –5% Use of recycled and secondary aggregate
+15km (delivery distance by road) 7.9 1.4 9.3 0% Recommendation: Recycled and secondary aggregates should only be specified when they are locally available, or delivered using +20km (delivery distance by road) 7.9 1.8 9.7 5% low-carbon transportation. Availability of resource and technical implications should be discussed with the client and contractor. +58.5km (delivery and return 7.9 2.7 10.6 14% distance by road) Within the current assessment method, discussion with the client or project code assessor is recommended to prevent unfair penalisation. *C&D - Construction and Demolition SPECIFYING SUSTAINABLE CONCRETE 9
Case study Case study
White Collar Factory, London Simon Sainsbury Centre, Cambridge
Image © Timothy Soar. Image © Hufton and Crow
This BREEAM Outstanding 17-storey office building has extensive Concrete is integral to the sustainability strategy of the Simon exposed concrete throughout, which provides the thermal Sainsbury Building, providing a robust structure with thermal mass that’s intrinsic to its passive cooling strategy. The concrete mass. An innovative natural ventilation system uses localised heat flat slabs that house the plastic water pipes benefit from an exchangers in window reveals in combination with the thermal advanced building management system (BMS) to optmise the flow mass and night time cooling to minimise energy consumption in temperature. the building system. Only the lecture theatre is mechanically cooled.
The concrete mix used for the project replaced 50% of the cement The project has extensive exposed concrete, which used 50% GGBS with ground granulated blast-furnace slag (GGBS) to reduce the in its mix. The building attained a BREEAM rating of Excellent. carbon content. The resulting concrete was light in colour, due to the GGBS, so 200kgm3 of fly ash was added to darken the mix for a Project team: warmer, more traditional concrete look. Client: University of Cambridge Architect: Stanton Williams Project team Structural engineer: AKT II Client: Derwent London M&E Engineer: Arup Architect: AHMM Structural engineer: AKT II M&E Engineer: Arup 10 SPECIFYING SUSTAINABLE CONCRETE
Cements and combinations
The cementitious component of concrete represents the majority of the embodied CO2 (ECO2) of concrete. ECO2 is the carbon emissions associated with the production and manufacture of a product (cradle to gate). The UK cement industry, through the implementation of a range of measures – including using waste-derived fuel and incorporating mineral additions - has made significant progress in reducing the ECO2 of cement.
The use of alternative fuels not only diverts waste from landfill and saves Table 4: Cement and combination types from BS 8500 [2] on the need for fossil fuels, but can reduce the need for raw materials; for example, the use of waste tyres provides a fuel and minimises the need to Broad Composition Cement/ add iron-oxide to cement due to the steel wire content of the tyres. designationa,b combination types (BS 8500) It is important to note that ECO of concrete should not be considered or 2 CEM I Portland cement CEM I specified in isolation of other sustainability factors such as strength gain. SRPC Sulfate-resisting Portland CEM I, SR0 or SR3 Cementitious additions cement IIA Portland cement with 6–20% CEM II/A-L, A number of by-products from other industries can be blended with fly ash, ground granulated CEM II/A-LL, CIIA-L, blastfurnace slag, limestone, CIIA-LL, CEM II/A-S, Portland Cement (CEM I) which can improve performance, but also increase or 6–10% silica fumec CIIA-S, CEM II/A-V, the recycled content and reduce the ECO2 content of the concrete. The use CIIA-V, CEM II/A–D of these secondary materials utilises material which might otherwise be disposed in landfill. IIB-S Portland cement with CEM II/B-S, CIIB-S 21–35% ground granulated blastfurnace slag There is a long track record of using the following cementitious additions with CEM I. The UK average across all concretes is approximately 18% with IIB-V Portland cement with CEM II/B-V, CIIB-V the permitted percentage use of each given in Table 4. 21–35% fly ash
Ground granulated blastfurnace slag (ggbs) IIB+SR Portland cement with CEM II/B-V+SR, 25–35% fly ash CIIB-V+SR Ggbs is a by-product from the manufacture of iron. Molten slag is tapped d, e off from the blast furnace during the production of molten iron. If it is IIIA Portland cement with CEM III/A, CIIIA 36–65% ground granulated cooled rapidly, the granulated material has latent hydraulic properties; i.e. blastfurnace slag when water is added, it reacts very slowly but when placed in the alkaline environment created by CEM I, the reactions are accelerated. The most IIIA+SRe Portland cement with CEM III/A+SRf, f commonly used proportion of ggbs in UK-produced combinations is 50% 36–65% ground granulated CIIIA+SR blastfurnace slag with by mass of total cementitious content. additional requirements that enhance sulfate resistance Fly ash IIIBe, g Portland cement with CEM III/B, CIIIB The majority of fly ash used in the UK is a by-product from the burning of 66–80% ground granulated pulverised coal to generate electricity at power stations. When coal is burnt, blastfurnace slag the resulting fine ash is captured and classified. It has pozzolanic properties e f and therefore does not react when water is added but in the alkaline IIIB+SR Portland cement with CEM III/B+SR , 66–80% ground granulated CIIIB+SRf environment created by CEM I, the pozzalanic reactions are initiated. The blastfurnace slag with most commonly used proportion of fly ash in UK-produced combinations is additional requirements that 25% by mass of total cementitious content. enhance sulfate resistance
Silica fume IVB-V Portland cement with CEM IV/B(V), CIVB 36–55% fly ash Silica fume is a by-product from the manufacture of silicon. It is an extremely fine powder (as fine as smoke) and therefore it is used in Key concrete production in either a densified or slurry form. Due to economic a There are a number of cements and combinations not listed in this table that may be specified for certain specialist applications. See BRE Special considerations, the use of silica fume is generally limited to high strength Digest 1 for the sulfate-resisting characteristics of other cements concretes or concretes in aggressive environmental conditions. The most and combinations. commonly used proportion of silica fume in UK-produced combinations is b The use of these broad designations is sufficient for most applications. 10% by mass of total cementitious content. Where a more limited range of cement or combinations types is required, select from the notations given in BS 8500–2: 2015, Table 1. Limestone fines c When IIA or IIA–D is specified, CEM I and silica fume may be combined in the concrete mixer using the k-value concept; see BS EN 206:2013, Limestone fines can be used as a constituent of cement to produce Cl. 5.2.5.2.3. Portland limestone cement. d Where IIIA is specified, IIIA+SR may be used. e Inclusive of low early strength option (see BS EN 197–1 and the “L” BS 7979: 2016 [12] provides additional information on the specification of classes in BS 8500–2: 2015, Table A.1). limestone fines for use with Portland cement. The most commonly used f “+SR” indicates additional restrictions related to sulfate resistance. See BS 8500–2: 2015, Table 1. proportions of limestone fines in UK-produced combinations is 6-10% by g Where IIIB is specified, IIIB+SR may be used. mass of total cementitious content. SPECIFYING SUSTAINABLE CONCRETE 11
Table 6: Embodied CO of factory-made cements and combinations Designation of cements 2
a b c Table 1 in BS 8500-1:2015 [2] provides details of the cement and Cement Combination Secondary Embodied CO2 Factory made CEM I and Main smc content combination types recommended for UK structures. For most applications cement addition Constituent Low – High, kg and construction scenarios, BS 8500-1:2015 [2] allows considerable combined at (smc) or CO2/tonne specification flexibility in terms of cement or combination type used. concrete plant Addition However, BS 8500 does not provide specific guidance on the relative merits Low – High of cements/combinations in terms of their associated performance and Content % environmental impacts, apart from exposure classes. CEM I Portland 913 The designation CEM refers to materials produced at a cement factory cement as a single powder (e.g, CEM / (C) IIIA, a composite of ggbs and CEM I). CEM II/A-LL or L Within the UK, it is common practice for the concrete producer to purchase Portland 6 - 20 CIIA-LL or L 859 - 745 separate powders and blend them at the mixer to produce the required limestone limestone cement. These are called combinations (designated ‘C’) and are recognised cement to have equivalent performance to factory-made composite cements. CEM II/A-V 6 - 20 Portland fly CIIA-V 858 - 746 Transport distances of additions from the point of production to the point fly ash ash cement of use are similar to that for Portland cement. At ready-mixed concrete plants, producers typically stock Portland cement and either ggbs or fly CEM II/B-V 21 - 35 Portland fly CIIB-V 722 - 615 ash. Limestone fines and silica fume may be available in some ready-mixed fly ash concrete plants, or be made available given sufficient notice, but may not ash cement be available at all locations. CEM II/B-S 21 - 35 Portland slag CIIB-S 735 - 639 ggbs When possible and appropriate, prepare specifications that allow flexibility cement and choice to enable the most appropriate, sustainable and economic CEM III/A additions to be used. 36 - 65 Blastfurnace CIIIA 622 - 363 ggbs cement
CEM III/B Values of embodied CO 66 - 80 2 Blastfurnace CIIIB 381 - 236 ggbs Indicative ECO2 values for the main cementitious constituents of reinforced cement concrete are provided in Table 5. Published by MPA Cement, UK Quality Ash CEM IV/B-V Association and Cementitious Slag Makers Association [13], these figures 36 - 55 Siliceous fly CIVB-V 598 - 413 fly ash are derived using data for the calendar year 2010 and represent ‘cradle- ash cement to-factory-gate’ values as they do not consider transport from place of manufacture to concrete plants. Notes a For CEM I 1% minor additional constituent (mac) and 5% gypsum is assumed. For CEM II, CEM III and CEM IV at the highest proportion of the smc it is assumed that no mac is incorporated and at the lowest Table 5: Embodied CO2 for main constituents of reinforced concrete proportion of smc it is assumed that mac is added at 1% with the appropriate proportions of limestone, fly ash and ggbs. b For Combinations the CO e figure for CEM I is used together with the Material Embodied CO2 2 (kg / tonne) figures for limestone, fly ash and ggbs in the appropriate proportions. c CO2e figures for CEM II, CEM III and CEM IV and their equivalent Portland cement, CEM I 913 combinations are based on the range of smc proportion, where the range is from the minimum to maximum proportion of smc or Ground granulated 67 addition. CO2e can be interpolated for proportions of smc or addition blastfurnace slag (ggbs) Addition between the minimum and maximum, noting that the minimum CO2e or cement is associated with the highest proportion of smc or addition. Fly ash 4 constituent
Limestone 75
Aggregate 5
Reinforcement 427
Corresponding ECO2 values for factory-made composite cements and combination types are presented in Table 6. The ranges presented are clearly a function of both the ECO2 value of the individual materials and their permitted levels of use. The values range from 913kg per tonne (CEM I) to as low as 236kg per tonne (CEM III/B; 80% ggbs content). 12 SPECIFYING SUSTAINABLE CONCRETE
The use of cement additions does affect the total amount of cementitious Admixture use should be considered as an effective way of reducing
binder; yet any increases are typically small. ECO2 reductions for a range of cement/combination content. High range water-reducing admixtures typical concrete designation types are shown in Table 7. (super plasticizers) typically give water reductions of 16% to 30% without loss of consistency or final properties; allowing corresponding reductions in cement/combination content or improved strength for similar cement/ Table 7: Effect of cement type on ECO2 content of designated concretes combination content. (cradle to gate).
It is important to note that ECO2 values for concrete should not be 3 Concrete Concrete ECO2 (kgCO2/m ) considered or specified in isolation. Adopting holistic approaches to type (slump sustainability-related decision-making is always advisable; given the CEM I 30% 50% class) significant impact of cement/combination type and content on a range of concrete fly ash GGBS concrete concrete key concrete properties and benefits.
Blinding, The recent amendment (A2:2019) to BS 8500:2015 has now introduced mass fill, strip GEN1 (S2) 180 130 105 ternary blends with a minimum of 65% Portland Cement clinker with either: footings, mass 6% to 35% limestone and pozzolana, 6% to 35% limestone and fly ash or foundations 6% to 35% limestone and ground granulated blast-furnace slag. Reinforced RC25/30 320 265 200 foundations (S2) **
RC28/35 Ground floors 320 265 190 (S2) * Guidance for specification Structural: in-situ, RC32/40 Minimising ECO2 superstructure, 370 315 235 (S2) ** walls, Recommendation: Specify that concrete should permit a wide basements range of cements/combinations from BS 8500-2: 2015 Table 1 (i.e. Higher strength RC 40/50 ggbs, fly ash, limestone fines or silica fume). 435 355 270 concrete (S2) **
* includes 30 kg/m3 steel reinforcement 3 ** includes 100 kg/m steel reinforcement Total cementitious content and the use of additions
Guidance for the The use of cement additions affects the total amount of cementitious specification of cements binder. Although any increases are typically small, designers should be aware of these differences when assessing relative ECO2 values for concrete. Portland Cement or CEM I is the controlling constituent material in terms of Generally speaking, mass for mass replacement of cement with fly ash the embodied CO2 content of concrete. As such, if ECO2 content is critical for a given structure, close consideration should be given to the concrete’s or ggbs may result in reduced 28-day strengths, particularly at higher CEM I content but within the context of functional design requirements, replacement levels. As such, in order to achieve the specified characteristic construction practice and ultimate fitness for purpose. strength, the total cement/combination type may often be higher for concrete containing additions. When specifying concrete to BS 8500-1:2015 [2], there are several strength classes and cement/combination types permitted for selected For concrete containing 40% fly ash, the total cement/combination content minimum working lives, exposure classes and nominal covers to normal may be around 15% higher than a reference concrete containing CEM I reinforcement. All of these strength classes and cement types should be only. Ggbs concretes typically require cement/combination content considered by the designer. increases up to replacement levels of 50% of 5-10kg/m3; at higher percentages the cementitious content may need to be increased further Giving preference to options with low recommended minimum cement to achieve equivalent 28-day strength. Where practical, the characteristic content, and permitted cement/combination types with the highest levels of strength can be specified to be achieved at a later age. Concrete producers Portland cement replacement, will directly reduce ECO values of concrete. 2 can provide details for specific concrete specifications. However, consideration also needs to be given to savings in concrete and reinforcement through the specification of higher strength concrete as well as potential savings in construction timelines. Concrete surface colour While meeting specified durability requirements, cement/combination and the use of additions contents and types may have a significant impact on associated structural and/or other concrete construction criteria and finishing processes. The surface colour of concrete is dominated by its finest particles, which typically includes cement/combination and sand particles smaller than As well as giving preference to specific cement/combination types at the around 0.06mm. The colour of Portland Cement varies according to the specification stage, consideration may be given to attaching preferred materials from which it is manufactured. The incorporation of additions minimum levels of addition. For cement/combination type IIIA, for example, such as fly ash, ggbs and silica fume also has a major influence. a preferred minimum replacement of cement with ggbs of 50% could be stipulated, but should be discussed with the supplier. SPECIFYING SUSTAINABLE CONCRETE 13
Ggbs is off -white in colour and substantially lighter than Portland Cement. Concretes containing CEM III/B cements are often specifi ed as a more Quick Facts sustainable and economic alternative to white Portland Cement. Fly ash is dark grey in colour, resulting from a combination of iron compounds The cement sector present and carbon residues left after the coal is burned as part of its manufacturing process; the shade depending on the source of coal and the process plant used.
Where aesthetics are critical, the impact of cement/combination type on concrete colour may dominate considerations of local availability and
ECO2 content.
There are many other sustainability benefi ts gained by using concrete as a fi nish. Although visual concrete may have a small cost premium compared to a standard concrete, considerable savings are made when comparing the cost including other materials that only provide the fi nish. Visual concrete also encourages the exposure of the concrete surface; increasing operational energy savings in buildings from the eff ect of thermal mass.
Precast visual concrete can be specifi ed in collaboration with your precast concrete manufacturer.
Coloured concrete can also be produced by adding coloured pigments to the concrete (see Admixtures, page 16).
Overall, 44% of the UK cement industry’s fuel requirement Guidance for specifi cation in 2017 was met by alternative fuels.
Colour ¢ The UK produces 95% of its Portland cement and cementitious Recommendation: When aesthetics are critical, specify the cement/ additions requirement. combination to ensure colour consistency. ¢ The cement industry is a net consumer of waste, using waste as a fuel source and by-products from other industries as cementitious additions. ¢ Waste-derived fuels used by the cement industry include solvents, meat and bone meal, sewage sludge, paper and plastics. Overall, 44% of the UK cement industry’s fuel requirement in 2017 was met by alternative fuels [14]. ¢ In 2017, carbon dioxide emissions per tonne of production from all cement manufacturing sites were 25% lower than 1998 [14].
For more information visit The MPA Cement Sustainable Development Report can be downloaded at www.mineralproducts.org/sustainability
UK Quality Ash Association (www.ukqaa.org.uk)
Cementitious Slag Makers Association (www.ukcsma.co.uk)
Silica Fume Association (www.silicafume.org) 14 SPECIFYING SUSTAINABLE CONCRETE
When early strength is important, some compromise on the level Early strength development of cement replacement may be needed. In precast factories, rate of For a given value of 28-day strength, concrete containing additions such as production and turnaround of mould may be important. For in-situ fly ash and ggbs will exhibit lower relative early age strengths than those concrete, under normal circumstances, the striking times for concretes containing Portland Cement only. This is because concrete’s early strength containing up to 50% ggbs do not increase sufficiently to significantly is dependent, primarily, on its Portland Cement content. The table below affect the construction programme. However, concretes with higher levels provides information on strength gain of different concretes. of ggbs will not always achieve sufficient strength after one day to allow removal of vertical formwork, particularly at lower temperatures, lower cementitious contents and in thinner sections. Generally, high Table 8: Strength gain of different concretes (> 50%) ggbs levels are not appropriate for soffit applications and thin sections; particularly during winter months unless the slower Concrete Strength* at 7 days Strength* gain from strength gain and prolonged striking times can be accommodated 28 to 90 days in the programme.
CEM I concrete 80% 5-10% Water reducing and accelerating admixtures can be added to enhance 30% fly ash concrete 50-60% 10-20% early strength (see Admixtures). 50% ggbs concrete To limit any impact on programming, established methods for more 50% fly ash concrete 40-50% 15-30% accurately determining in-situ early age concrete strengths and/or 70% ggbs concrete formwork striking times are available [15, 16, 17]. These include the use of * Strength as a percentage of 28-day strength maturity methods using site-specific or predicted input data; testing of These figures are based on standard cure at 20oC. site-cured or temperature-matched test cubes; and penetration, pull-out or break-off tests.
In terms of maturity methods, for example, it is understood that concrete Figure 2: Influence of embodied CO2 on early strength strength is a function of time between casting and testing and the temperature at which concrete specimens are stored. For a particular 40 concrete, therefore, it is possible to develop a time-temperature % ggbs (35 to 90% by mass) concrete 35 relationship to predict maturity and strength. On-site temperature history can be measured using thermocouples or predicted using established 1-day strength 30 models which account for variables such as cement/combination type 3-day strength 25 and content, section size, ambient conditions and formwork materials. Test a cubes, match cured at the same temperature as the element poured, can 20 add relevant data to decisions about striking and load transfer times. 15 Specialist contractors are able to erect in-situ concrete structures, such as Strength MP 10 framed buildings, conventionally (to programme and budget) using low Data set is from 18 concrete 5 mixes with 28-day strengths ECO2 concrete mixes. Indeed, using the established assessment techniques ranging from 15 to 70 MPa described above, innovative UK construction teams are presently erecting 0 050 100 150 200 250 300 350 400 high rise structures year-round using average to high Portland Cement 3 replacement levels. Further details may be sourced from CONSTRUCT and Embodied CO2 (kg/m ) British Ready Mixed Concrete Association members.
Clearly, this relationship introduces a potential conflict between demands
for achieving low concrete ECO2 values (driven, most likely, by architects, consulting engineers or clients) and the achievement of adequate Guidance for specification early strengths to satisfy programming requirements, such as timely formwork removal (driven, most likely, by contractors). Specifications Strength should, therefore, be written to allow flexibility and compromise between Recommendation: Do not over-specify strength. conflicting concrete attributes. It may be beneficial to involve the contractor at the earliest stage of specification production to assist in Recommendation: Consider the possibility of strength conformity optimising concrete specifications. at 56 days rather than the conventional 28 days. SPECIFYING SUSTAINABLE CONCRETE 15
Water
BS EN 1008: 2002 [18] gives guidance on the use of water recovered from processes in the concrete industry. This includes water which was used to clean the inside of static mixers, mixing drums of truck mixers/agitators and concrete pumps; process water from sawing, grinding and water blasting of hardened concrete; and water extracted from fresh concrete during concrete production.
Limitations on use of recovered water include additional mass of solid material (which must be less than 1.0% by mass of the total mass of Performance report aggregates present in the concrete) and any impacts on the chemical and physical concrete properties such as setting time and strength. Specification guidance
Recovered or combined (mixture of recovered and from other sources) water may be used for both un-reinforced and reinforced (including pre-stressed) concrete, and its use should be allowed for at the specification stage.
If its use is proposed, its influence should always be taken into account as there may be special requirements for the production of concrete; for example, air-entrained concrete or concrete exposed to aggressive environments. As recovered water generally contains varying concentrations of very fine particles (typically less than 0.25mm), its use in visual or architectural concrete should also be assessed in detail.
Water extraction and BES 6001 Much of the data in this publication has only been made possible due to the formation of the UK Concrete Industry Sustainable Water extraction is an important aspect of responsible sourcing certification Construction Strategy. The commitment to a comprehensive to BES 6001 [4] (see page 4). To achieve a primary level of performance the industry strategy and report has required coordination and further organisation must establish a policy and metrics for water extraction in development of sector and company processes. Previous industry terms of reducing mains water use and the efficient and effective use of reports and company performance reports are available at ‘controlled groundwater’. Controlled groundwater is defined as all water www.sustainableconcrete.org.uk. abstracted from boreholes and other surface water features which need an abstraction license known as a ‘Full License’ in the Water Act 2003. To The concrete industry is monitoring both its mains and groundwater achieve a higher performance rating in BES 6001 the organisation must consumption, with the aim of achieving reductions in water use. demonstrate external verification of the reported data on water extraction. An example of a water-saving industry initiative is wash-water admixtures. The MPA Water Strategy was published in 2017 Wash-water admixtures following review of the processes used in the concrete supply chain. This is based on three main principles: Specialist admixtures are available that reduce the waste produced at a 1. Minimising water consumption. ready-mixed concrete plant. At the end of a working day, concrete trucks 2. Prioritising use of the most sustainable water sources available. need to be cleaned to prevent the build up of hardened concrete in the 3. Protecting the environment through good water stewardship. mixer drum. Traditionally, large quantities of water have been added to the For more information see www.mineralproducts.org mixer, which has then been spun and the detritus dumped in a settlement pit. An alternative treatment involves incorporating a wash-water stabilising admixture into the drum overnight. The admixture stops the hydration of the main phase of the Portland cement even after initial hydration has started.
The following day, the wash-water residue is incorporated into the first delivery of the day. The addition of significant volumes of cementitious material activates the hydration reactions. Alternatively a special activator can be added to the wash-water. 16 SPECIFYING SUSTAINABLE CONCRETE
Admixtures
Admixtures are defined in EN 934-2 [19] as ‘material added during the mixing process of concrete in a quantity not more than 5% by mass of the cement content of the concrete, to modify the properties of the mix in the fresh and / or hardened state’. In both cases there are potential benefits for the sustainability of concrete.
Modifications to the fresh concrete can significantly improve the handling and compaction of both site placed and precast concrete production, Improved durability performance allowing more efficient and lower energy processes. A good example of how admixtures can enhance sustainability relates to the requirements for concrete used in different exposure conditions and In the hardened state admixtures can significantly improve the durability particularly for resistance to freeze-thaw. of the concrete to a range of aggressive environments, extending the maintenance free service life. The improvements that admixtures can bring When specifying concrete to BS EN 206 [1] and BS 8500 Parts 1 and 2: can contribute specifically to a reduction in the ECO2 of concrete and more 2015, consideration needs to be given to the environmental conditions widely enhance the sustainability credentials of concrete. the concrete will be exposed to. The five main exposure classes defined in BS 8500 are listed below. Each class has a number of sub-categories Typical dosage rates for admixtures are shown in Table 9. In certain depending upon the severity of exposure. specialist applications such as very high strength concrete, these dosages may be exceeded. Despite this relatively small dosage, the modifications ¢¢ XC Exposure class for risk of corrosion induced by carbonation
to concrete properties achievable by admixtures can reduce the ECO2 ¢¢ XD Exposure class for risk of corrosion induced by chlorides other than of concrete, mainly through the more effective use of the cementitious from sea water component, while maintaining and even enhancing the properties of ¢¢ XS Exposure class for risk of corrosion induced by chlorides from the concrete. sea water
The Cement Admixtures Association (CAA), www.admixtures.org.uk, ¢¢ XF Exposure classes for freeze/thaw attack estimates that current admixture use already saves about 700,000 tonnes ¢¢ DC Exposure classes for chemical attack
of ECO2 per annum and this could be significantly increased by further mix optimisation. [20] Depending upon the exposure condition and the cover, BS 8500 gives recommendations for a minimum cement content, maximum water- cement ratio and possibly required strength to give the desired design life. Table 9: Typical UK use and dosage rates for admixtures [21] The use of water-reducing or super-plasticizing admixtures enables a given Admixture Type to EN 934-2 Proportion of Average strength and/or water cement ratio to be achieved with lower cement total admixture dosage % content (subject to achieving the minimum cement content). Thus, the sales % by weight of correct use of admixtures can allow concrete to meet the requirements cement for an exposure class at a lower cement content, reducing the ECO2 while Superplasticizers 45 0.70* enhancing long-term performance.
Normal Plasticizers 34 0.45 Resistance to freeze-thaw Accelerating 2 1.65
Retarding 2 0.45 When concrete is exposed to significant freeze-thaw cycles, it should be specified in accordance with the guidance set out in BS 8500-1 Table A.9 Air Entraining (AEA) 4 0.20 to resist XF exposures. To achieve this, either a minimum quantity of air is entrained using an air-entraining admixture or a minimum strength class All other concrete admixtures 13 – is specified. Notes: *Dosage based on 40% solution, some super-plasticizers will be sold at The most severe form of freeze-thaw exposure is when there is also the greater dilution with a correspondingly higher dose. possibility of high water saturation; typically, horizontal surfaces. Under these conditions, freeze-thaw resisting aggregates are required and there are limitations on the type of cement which should be used. Cement with more than 35% fly ash should not be used and, when de-icing agent is Guidance for specification used, no more than 55% ggbs should be added to minimise surface scaling.
Admixtures XF3 exposure is when concrete is exposed to significant freeze-thaw cycles and high water saturation but where de-icing agents are unlikely to Recommendation: The use of admixtures by the concrete producer be used. For a maximum aggregate size of 20 mm, the requirements are should be permitted in the specification. shown in Table 10. SPECIFYING SUSTAINABLE CONCRETE 17
Table 10: Designated concrete for freeze-thaw exposure XF3 Extending design life Exposure Min. Max. w/c Min Min. air Alternative through use of admixtures class Strength ratio cement content designated class content, concrete Structures with concrete using BS 8500 are generally specified to have a 50- 3 kg/m or 100-year life, although EN 1990 [22] defines five working life categories XF3 C25/30 0.60 280 4.5 PAV1 ranging from 10 to 100 years.
C40/50 0.45 340 – RC40/50XF The Design Manual for Roads and Bridges [23], published by the Department for Transport calls for 60- or 120-year design life. This is achieved through a combination of specifying cover (where corrosion of steel reinforcement From Table 10, it is evidently easier to call up PAV1 or RC40/50XF than set is a risk) and concrete quality (specified through maximum water cement out the limiting values of a designed concrete. ratio and minimum cement content and possibly strength). Admixtures can be used to achieve these durability requirements in a more sustainable way. A freeze-thaw resisting aggregate will be a reasonably strong aggregate When the concrete is exposed to a particularly aggressive environment or 3 and coupled with a minimum cement content of 280kg/m , plus addition guaranteed long-term performance is critical, specialist admixtures can be of a water-reducing agent, could give a concrete that achieves around utilised. Admixtures falling within this category include corrosion inhibitors, 2 45 N/mm at 28 days, in the absence of an air-entraining admixture. waterproofers and shrinkage reducers. However, introduction of entrained air affects strength and each 1% entrained air reduces 28-day strength by about 5% and to ensure a In recent years the production of very high strength concrete has become minimum air content of 4.5%, as required for a PAV1 concrete, the average more common. Admixtures are almost essential for producing high value will be about 5.5%. At 5.5% air, 280 kg/m3 may only achieve strength concrete which enables a reduction in the thickness of sections 35 N/mm2; to safely achieve the required C25/30 strength class, the of concrete elements, which in turn makes more efficient use of material 3 cement content may need to be 300 - 340 kg/m . resources and allows reduced ECO2.
Even with a reasonable quality aggregate and a water reducing admixture or high range water reducing admixture, it is likely that the cement content required to achieve C40/50 concrete will be in excess of 340 kg/m3 and Quick Facts may be as much as 380 kg/m3. Admixtures Thus, air-entrained concrete will normally have lower cement content than a non-entrained concrete to meet the recommendations for freeze-thaw resistance, and therefore a lower ECO2 content. However, if C40/50 is required anyway to meet structural requirements, then all the cement will be usefully employed. Construction site benefits
In addition to the benefits to design and the embodied impacts of concrete the use of admixture can make site processes more efficient. For example: ¢¢ The durability, sustainability and environmental profile of concrete can all be enhanced by admixture use. ¢¢ Accelerating admixtures can reduce curing times and allow for earlier removal of formwork and faster strength development. The need for ¢¢ Admixtures provide enhanced concrete quality and deliver cost accelerated curing by heating in precast concrete production can also benefits to both the producer and the user. be reduced or avoided altogether. ¢¢ A range of technical guidance is available including: ¢¢ Pumping aids reduce the energy for hydraulic pumps where site ¢¢ Normal water reducing/plasticizing admixtures procedures are enhanced by concrete pumping. ¢¢ High range water reducing/super-plasticizing admixtures ¢ ¢ Anti-washout admixtures prevent potential environmental hazards ¢¢ Retarding A more recent innovation facilitated by newer admixtures has been the ¢¢ Accelerating development of self compacting concrete (SCC). The production of this ¢¢ Air-entraining type of high workability concrete allows the placing of complex shapes and ¢¢ Water resisting (waterproofing) heavily reinforced components without the need for significant vibration ¢¢ Corrosion inhibiting techniques, thus saving energy and reducing some of the more physical and labour-intensive operations on site. ¢¢ Polymer dispersion admixtures ¢¢ Pumping aids ¢¢ Self-compacting concrete ¢¢ Precast, semi-dry concrete ¢¢ Shrinkage reducing admixtures ¢¢ Anti-washout / underwater admixtures ¢¢ Truck washwater admixtures
Further guidance on the use of admixtures is available from the Cement Admixtures Association (www.admixtures.org.uk). 18 SPECIFYING SUSTAINABLE CONCRETE
Reinforcement
Concrete on its own performs well in compression but not in tension. Steel reinforcement is used to deliver tensile capacity where it is needed. Hence reinforced concrete uses different materials very efficiently. This minimisation of material use is often taken for granted but is a major contributor to sustainability.
About half of all concrete cast in Britain is reinforced. Steel reinforcement The EAF process normally uses approximately 98% scrap metal as the raw should comply with BS 4449: 2005 [24] or BS 4483: 2005 [25] and be cut and material. An EAF furnace generally produces 0.5 to 1.0 million tonnes per bent in accordance with BS 8666: 2005 [26]. Efficient use of reinforcing steel annum, making it ideally suited to smaller-scale steel making operations is dependent on good structural design and on the material’s chemical typically used for the manufacture of reinforcing steel. EAF production sites composition, mechanical properties and rib geometry, as well as accurate typically include specialised rolling mills producing long products such as cutting, bending and fixing. reinforcing bar.
The embodied energy values of reinforcing steel are based on the energy The majority of reinforcing steel used in the UK is produced by the used to melt scrap metal and reform it. Although all steel manufacture is EAF process. an energy-intensive process, the energy needed to produce one tonne of reinforcing steel is as low as one third of that needed to make one tonne of structural steel from iron ore. Equally, reinforcing steel itself can Guidance for specification be recovered, recycled and re-used at the end of a building or structure’s of reinforcement steel service life.
The ECO2 of reinforcing steel is shown in Table 5 (page 11). Steel contents of reinforced concrete will vary and this will influence the ECO2. At a value of Manufacturing of 110 kg of reinforcement per cubic metre of concrete (considered typical for reinforcement steel the UK), the reinforcement will add 15 kg of ECO2 per tonne of concrete as illustrated in Table 11 [27]. There are two common steelmaking processes used for steel in the UK market. These are Basic Oxygen Steelmaking (BOS) and Electric Arc Furnace The ECO2 for the cementitious content in this example is based on the UK (EAF) steelmaking. The BOS route is the most widely-used steelmaking weighted average value of 720 kg CO2 T plus an allowance for transport. If a process worldwide and involves the smelting of iron ore, coal and other cement/combination utilising fly ash or ggbs at a normal UK addition rate of raw materials in a two-stage process. The EAF production process involves 30% or 50% respectively was used, a lower ECO2 would be achieved for both passing an electric charge through scrap metal, melting it; thus enabling examples, but the differential due to reinforcement would stay the same. recycling into new products. In order to guarantee material is produced in conformation with British Standards, it is recommended that all steel reinforcement should be obtained from companies holding a valid CARES (Certification Authority for Reinforcing Steels) certificate of product approval.
Table 11: Indicative ECO2 for C28/35 concrete; unreinforced and reinforced Constituents of product
Embodied CO2 for the product UK concrete products Cementitious Water Aggregate Rebar Content (kg/m3) (kg/m3) (kg/m3) (kg/m3) 3 2 (kg CO2/m ) (kg CO /t) C28/35 unreinforced 300 165 1915 0 225 95
C28/35 reinforced 300 165 1915 110 270 110 SPECIFYING SUSTAINABLE CONCRETE 19
Sustainability accreditation of reinforcement steel Quick Facts
Sustainability credentials can be demonstrated by specifying Reinforcement reinforcement accredited to the Eco-Reinforcement or CARES sustainability certification scheme.
CARES is an independent, not-for-profit product certification body, which provides confidence to the users, purchasers and specifiers of constructional steels through a regime of regulation, testing and inspection. It operates for the benefit of the construction industry offering certification schemes for companies that produce materials, components or offer services, primarily to the reinforced concrete industry. Clients can specify CARES approved companies and products with confidence that they will comply with the relevant product or system standards and without the need for verification testing by the purchaser or contractor.
CARES has developed The CARES Sustainable Reinforcing Steel scheme that quantifies the environmental impact of the reinforcing steel supply chain. CARES has worked with important stakeholders with a view to providing a stern test of compliance as well as a level playing field for demonstrating the sustainability of reinforcing steel and other steel construction products. The CARES scheme takes into account specific environmental and social impacts and provides recognition for reinforcing steel producers and processors embracing genuine sustainability. The CARES scheme has been established to comply with BS 8902 [28], which provides a framework for the management, development, content and operation of sector certification schemes for responsible sourcing and supply of construction products. It will enable the CARES approved reinforcing steel supply chain to demonstrate the responsible sourcing of construction products and its commitment to sustainable development. This scheme meets a number of private and public sector sustainability initiatives and may only be awarded to firms which already hold a valid CARES Certificate of Approval. For more information see www.ukcares.com
Eco-Reinforcement is a third-party certification scheme developed by the ¢¢ The combination of reinforcement and concrete utilises reinforcing steel industry to comply with BRE BES 6001:2008 – Framework tensile and compressive qualities respectively: an efficient Standard for the Responsible Sourcing of Construction Products. sustainable solution. ¢¢ UK-produced reinforcement uses UK scrap steel. Eco-Reinforcement provides a means for construction clients, specifiers and contractors to purchase reinforcing steel from a supply chain which ¢¢ UK-produced reinforcement and the majority of imported is pro-actively addressing issues of sustainability. The Eco-Reinforcement reinforcement uses the low-energy EAF process. scheme assesses against a number of different organisational, supply chain, For more information visit: UK CARES (www.ukcares.com) and environmental and social criteria; with some defined as compulsory and British Association of Reinforcement (www.uk-bar.org). others voluntary or ‘tradeable’. Certificates are awarded on a ‘Pass’, ‘Good’, ‘Very Good’ and ‘Excellent’ scale, based on the number of points awarded for different performance levels. All Eco-Reinforcement companies are required to print information such as transport-related CO2 emissions from scrap-yard to site on their delivery notes.
The scheme intends to develop further and provide more extensive environmental impact information. All companies supplying Eco- Reinforcement will be certified to BS EN ISO 14001 and will operate an auditable H&S management system. All Eco-Reinforcement is manufactured through the EAF process, from recycled scrap metal. For more information see www.eco-reinforcement.org. 20 SPECIFYING SUSTAINABLE CONCRETE
Specification of Concrete to BS 8500
In the UK concrete is specified in accordance with the European Standard for Concrete, EN 206[1], and the UK complementary Standard BS 8500[2]. BS 8500 is published in two parts: BS 8500-1, Method of specifying and guidance for the specifier, and BS 8500-2 Specification for constituent materials and concrete.
BS 8500 sets out five standard ways to specify concrete. These are: some applications. There is no requirement to demonstrate the strength designed concrete, designated concrete, prescribed concrete, standardized of ST concrete but BS 8500 Part 1:2015+A1:2019 Annex A provides some prescribed concrete and proprietary concretes. The term ‘concrete mix’ is indicative values for the strength class that may be assumed for structural sometimes used to refer to a prescribed concrete but otherwise it is always design, e.g. the highest grade of standardized prescribed concrete is ST5, a ‘concrete’ that is specified. where this may be assumed to be at strength class not greater than C20/25. To ensure this recommendation is safe for the indeterminant range of Designed concretes are for the informed specifier, where the designer materials and site supervision the prescribed cement content is very high, considers all the requirements for the hardened concrete such as and the use of ST concretes should be deprecated where sustainability is strength and durability to derive the necessary strength class and considered important. other properties such as cement type, minimum cement content and maximum water/cement ratio. Normally the designer will assess the Proprietary concretes are developed by the concrete producer and are exposure conditions and consider the recommendations set out in BS marketed on the basis of their enhanced fresh or hardened properties. 8500: Part 1:2015+A2:2019 Annex A to determine the concrete properties The producer will normally guarantee the performance of these products and minimum cover to reinforcement required to achieve the structural and provide test certificates. Proprietary concretes may be covered by performance and service life. Due to the flexibility of designed concretes third-party product conformity certification. Proprietary concretes are often they are suitable for specifying concrete to minimise embodied carbon by used for high performance applications where the sustainability benefit is specifying low carbon cements or combinations, and to specify the use of in the reduction in the total material volume used rather than the value per recycled or secondary aggregate where their use is considered beneficial. volume. For commercial reasons the producer may not disclose the exact The designer’s specification is passed to the contractor where the concrete composition and is not required to do so by the concrete Standards, EN 206 specification is completed with the addition of requirements for fresh and BS 8500. concrete properties, such as consistence. For more information on specifying to BS 8500, refer to How to Design Designated concretes are types of designed concretes, where for a Concrete Structures to Eurocode 2, published by The Concrete Centre. limited range of housing and other applications the specifier only calls up the required designation, e.g. FND2 is a concrete suitable for use in unreinforced concrete in ground assessed as ‘DC-2’, Design Chemical Class 2. Similarly, RC28/35 is a designated concrete of C28/35 strength Additional aids to specification class suitable for use in an internal suspended floor. Designated concretes are quality-assured designed concretes that conform to a specification National Structural Concrete detailed in BS 8500-2. These concretes have been selected to be fit for their Specification (NSCS) - Edition 4: 2010 intended end use and they can only be supplied by ready-mixed concrete Covering three sections - Standard Specification, Project Specification and producers who have third-party product conformity certification. A QSRMC Guidance - this document assists the project team in the specification of or BSI Kitemark logo on the delivery ticket provides this confirmation. structural concrete and concrete finishes. The fourth edition recognises Purchasers can be confident that the concrete will be delivered as specified the latest standard for responsible sourcing, BES 6001 [4] and, in the final and ordered, and as the concrete is optimised by the producer it will be the section, provides guidance on materials for sustainable construction. most sustainable. National Building Specification (NBS) Designated concrete is the preferred method of specifying concrete when working to the NHBC Standards. Guidance entitled ‘Concretes for Housing NBS encompasses a range of services provided by RIBA, many via - Designated concrete’ is available from the British Ready-Mixed Concrete subscription, to assist in the creation of accurate and up-to-date Association (BRMCA). Designated concretes are not available for concrete specifications for building projects. to resist the risk of corrosion by the ingress of chloride where the specifier should specify a designed concrete.
Prescribed concretes allow the informed designer to specify concrete by prescribing the composition. This method is rarely used but is useful where a particular ratio of constituents is required for exposed aggregate concrete finishes.
Standardized prescribed concretes are intended for small building sites where concrete is either mixed by hand or in a small, less than 150 litres, concrete mixer. They are designated ST and are accepted by NHBC for SPECIFYING SUSTAINABLE CONCRETE 21
Specification examples
Designated concrete example Optimising strength class
For a building with external reinforced vertical elements exposed to rain A reduction in concrete strength class will typically offer immediate
(exposure class XC3/4 to BS 8500) with an intended working life of at least savings in terms of ECO2 (reduced cement/combination content) unless 50 years, a range of concretes are appropriate depending on the minimum limited by minimum cement content specifications. As an example, a cover to the reinforcement. These are shown in Table 12. reduction in strength class from C70/85 to C32/40 may reduce concrete’s cement/combination content by around 150kg per m3 of concrete, with
corresponding ECO2 reductions of around 185 kg of CO2. Table 12: Designated concretes for exposure class XC3/4 and minimum 50 years For a typical concrete column scenario (applied load of 24000kN and moment of 500kNm), however, the higher strength class would afford Minimum Strength Min Max w/c Designated element size reductions of around 30% (from around 900 x 900mm to 750 cover (mm) class cement ratio concrete x 750mm) and corresponding reinforcement content reductions of about content a third. In addition, there may be potential reductions in foundation size. (kg/m3) The more slender, high strength structural solution may offer potential 20 C40/50 340 0.45 RC40/50 economic, environmental and social benefits to the design team, contractor and client alike that offset the higher ECO per m3 of concrete. 25 C30/37 300 0.55 RC30/37 2
30 C28/35 280 0.60 RC28/35 Table 13 illustrates an example case in which three options are considered compared with a base option. For this building, there is a small net ≥35 C25/30 260 0.65 RC25/30 reduction of ECO2 by using higher strength concrete for columns (see option 3 compared with option 2 and option 1 compared with base An allowance for deviations (generally 10 mm for in situ work) is added to minimum cover to give nominal covers to reinforcement. option). The opposite is true for slabs (see option 2 compared with option 1). The effect of increasing concrete strength and reducing section size on
the floor area available to let and the total ECO2 of a structure is considered In practice, for reasonable quality aggregate, RC30/37, RC28/35 and in more detail in Concrete Structures 9 [29]. RC25/30 should be achievable at the minimum cement content with the use of water reducing or high range water reducing admixtures. This Table 13: Comparison of ECO2 for different construction options applies to all cements incorporating not more than 20% silica fume or Base Option 1 Option 2 Option 3 limestone, 35% fly ash or 65% ggbs. At higher levels, an extra cementitious Option content above the minimum should be expected. Concrete class C32/40 C32/40 C50/60 C50/60 Even with reasonable quality aggregates and high performing admixtures, for slab an extra cementitious content is likely to be required for RC40/50 concrete Concrete class for C32/40 C50/60 C50/60 C60/75 to achieve the required strength. vertical elements
Volume of slab 2,110 2,110 1,841 1,841 concrete (m3)
Volume of concrete 1,112 956 956 885 in verticals (m3)
Change in nett 0% 1.22% 1.22% 1.78% lettable area
Tonnes ECO2 1,369 1,346 1,492 1,477 Variation from 100% 98% 109% 108% base option 22 SPECIFYING SUSTAINABLE CONCRETE
References
1. BS EN 206:2013+A1:2016, Concrete. Specifi cation, performance, 24. BS 4449:2005 + A3:2016, Steel for the reinforcement of concrete – production and conformity, BSI, 2016. Weldable reinforcing steel – Bar, coil and decoiled product specifi cation, 2. 2a) BS 8500-1:2015 +A2:2019, Concrete - Complementary British BSI, 2005. Standard to BS EN 206, Part 1: Method of specifying and guidance for the 25. BS 4483:2005, Steel fabric for the reinforcement of concrete – specifi er, BSI, 2016. Specifi cation, BSI, 2005. 2b) BS 8500-2:2015+ A2:2019, Concrete - Complementary British 26. BS 8666:2005, Scheduling, dimensioning, bending and cutting of steel Standard to BS EN 206, Part 2: Specifi cation for constituent materials reinforcement for concrete – Specifi cation, BSI, 2005. and concrete, BSI, 2016. 27. Sheet C1 – Embodied CO2 of concrete and reinforced concrete, MPA on 3. Concrete Industry Sustainability Report, The Concrete Centre on behalf behalf of the Sustainable Concrete Forum, www.sustainableconcrete.org.uk. of the Sustainable Concrete Forum, 2018. 28. BS 8902:2009 Responsible sourcing sector certifi cation schemes for All reports can be downloaded from www.sustainableconcrete.org.uk. construction products – Specifi cation, BSI, 2009. 4. BES 6001, BRE Environmental & Sustainability Standard Framework 29. Concrete Structures 9, The Concrete Centre, 2009. Standard for the Responsible Sourcing of Construction Products – Issue 3.1, BRE, 2016. 5. Ecoreinforcement. www.eco-reinforcement.org 6. CARES www.ukcares.com 7. Concrete and BREEAM, MPA The Concrete Centre, 2015. Everyman Theatre, Liverpool, utilised 8. Guideline to BES 6001, Concrete Industry Guidance Document to ground-granulated blast-furnace slag support BES 6001, Issue 1, MPA on behalf of the Sustainable Concrete (GGBS) as a cement replacement and was Forum, December 2008. Download from www.sustainableconcrete.org.uk. winner of 2014 RIBA Stirling Prize. Image courtesy of Philip Vile. 9. BS EN 12620:2002+A1:2008, Aggregates for concrete, BSI, 2002. 10. Survey of Arisings and Use of Alternatives to Primary Aggregates in England 2005, Construction and Waste, DCLG, 2007. 11. Methodology for environmental profi les of construction products, section 7.2.1, BRE, 2007. 12. BS 7979, Specifi cation for limestone fi nes for use with Portland cement, BSI, 2016.
13. Factsheet 18: Embodied CO2e of UK cement, additions and cementitious material, MPA, 2015. 14. Sustainable Development Report 2018, MPA Cement, 2018. 15. John Reddy, A Decision Making Tool for the Striking of Formwork to GGBS Concretes (a project report submitted for the award of diploma in Advanced Concrete Technology), The Institute of Concrete Technology, 2007. 16. CA Clear, Formwork striking times of ground granulated blastfurnace slag concrete: test and site results, Proceedings, Institution of Civil Engineers, Structures & Buildings, 1994, 104, Nov. 441-448. 17. TA Harrison, Formwork striking times – criteria, prediction and methods of assessment, CIRIA Report 136, 1995. 18. BS EN 1008: 2002, Mixing water for concrete – Specifi cation for sampling, testing and assessing the suitability of water, including water recovered from processes in the concrete industry, as mixing water for concrete, BSI, 2002. 19. BS EN 934-2: 2009 + A1:2012, Admixtures for Concrete Mortar and Grout - Part 2: Concrete admixtures – Defi nitions, requirements, conformity, marking and labelling, BSI, 2012. 20. AES 14 – Admixture Environment And Sustainability Information, CAA, 2016. 21. A Minson & I Berrie, Admixtures and Sustainable Concrete, The Structural Engineer, Institution of Structural Engineers, Jan 2013. 22. EN 1990 + A1:2005, Eurocode. Basis of structural design, BSI, 2002. 23. The Design Manual for Roads and Bridges, Highways England, 2018. SPECIFYING SUSTAINABLE CONCRETE 23
Resources from The Concrete Centre Download from: www.concretecentre.com/publications
Material Effi ciency Concrete Floor Solutions for
Concrete floor solutions for Material Efficiency Passive and Active Cooling Minimising the production of waste is an important passive and active cooling factor in material resource effi ciency. The concrete This updated guide details all the various systems industry is a net user of waste, diverting signifi cant currently available, from the simple passive amounts of waste from potential land fi ll and approach to the more sophisticated active systems. Design guidance for doing more with less, using concrete and masonry Design options for low energy buildings reducing depletion of natural resources. Designers Case studies are included to highlight recent can use concrete and masonry in ways that reduce projects featuring the various systems covered. This waste during the construction and operation of guide focuses on issues including cooling capacity, buildings thereby achieving material effi ciency. system control, visual appearance and buildability/ spans etc. and aims to assist designers select the Publication date: 2018 best fl oor option to meet specifi c project needs. Ref: TCC/05/21 Publication date: 2017
Ref: TCC/05/26
Visual Concrete Whole-life Carbon and Buildings
The desire to optimise concrete’s benefi ts and Concrete-frame construction, of course, provides a express the visual aesthetic of the structure requires durable structure, which is a fundamental aspect of high quality and detailed consideration of formwork, whole-life performance. But less understood is its
workmanship, curing and concrete mix design. compelling whole-life CO2 performance, resulting These requirements should be understood by from several attributes largely unique to concrete architects, structural engineers and other members buildings. This guide sets out the ways in which
of the team to ensure a successful outcome. This these attributes can be used to minimise CO2 guide aims to enable designers to realise their emissions. aspirations using concrete. Publication date: 2016 Publication date: 2015 Ref: TCC/05/30 Ref: TCC/05/29
Thermal Mass Explained Concrete and Fire Safety
Concrete and masonry are building materials that Concrete and Fire Safety Building owners, occupiers and insurance steadily absorb any heat that comes into contact with companies are increasingly asking for more than the surfaces. The heat is conducted inwardly, and stored minimum fi re resistance as specifi ed by the Building until the surface is exposed to cooler conditions and Regulations. Of particular concern is the need for
Guidance on the use of concrete and masonry its temperature begins to drop. When this occurs, any for fire resistant and efficient structures a burning building to provide a level of structural heat begins to migrate back to the cooler surface integrity that goes beyond the means of escape and be released. In this way, heat moves in a wave times required by the Building Regulations. It is a like motion, alternately being absorbed and released concern that is also shared by fi re fi ghters. This report in response to the variation in day and night-time explains how concrete and masonry construction can conditions. Exploiting thermal mass on a year-round minimise the impact of fi re upon a building. basis is not diffi cult, but does require consideration at the outset of the design process when requirements Publication date: 2019 for the building form, fabric and orientation are being established. This guide explains what thermal mass is, Ref: TCC/03/43 and how best to utilise it.
Publication date: 2019
Ref: TCC/05/11 The Concrete Centre provides material, For more information and downloads, visit: design and construction guidance. Our aim www.concretecentre.com/publications is to enable all those involved in the design, www.concretecentre.com/events use and performance of concrete to realise www.concretecentre.com/cq the potential of the material. Subscribe to our email updates: The Concrete Centre provides design www.concretecentre.com/register guidance, seminars, courses, online resources and industry research to the Follow us on Twitter: design community. @concretecentre @thisisconcrete The Concrete Centre is part of the Mineral Products Association, the trade association for the aggregates, asphalt, cement, concrete, dimension stone, lime, mortar and silica sand industries. www.mineralproducts.org www.concretecentre.com
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Ref. TCC/05/24 ISBN 978-1-908257-01-7 First published 2011, this version February 2019 © MPA The Concrete Centre 2019
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