Feature

Target Zero

The UK has a target of net zero carbon emissions by 2050, and concrete can help designers meet it. Elaine Toogood explains the key low-carbon decisions.

In June 2019, the government passed legislation setting a target of net zero greenhouse gas emissions by 2050. Designers are already delivering low-carbon solutions using concrete and masonry, so what needs to change for net zero carbon to become a reality? Many are not aware that the UK concrete industry has been working to a sustainable construction strategy since 2008, which includes targets to reduce embodied carbon, alongside performance indicators in a range of other areas, such as environmental management and waste minimisation. This strategy will be relaunched in 2020, to adopt a commitment to a net zero carbon built environment.

CUTTING CARBON NOW

There are many different forms and mixes of concrete, so naturally there are a range of figures for embodied carbon. On average, the carbon footprint of UK concrete has reduced by around a third since 1990 and is both comparable to and lower than other materials – a fact that is often overlooked due to the volume of concrete used.

Aggregate accounts for by far the greatest proportion by volume, and it is both low carbon and locally sourced. The component that contributes the majority of concrete’s carbon footprint is cement, which is used to bind the aggregates and makes up around 10% of its volume. The UK cement industry has been decarbonising faster than the UK economy as a whole and currently contributes 1.5% to UK greenhouse gas emissions. Since 1990, it has reduced absolute emissions by 51%. In 2013, MPA Cement launched an ambitious strategy to achieve an 81% reduction in carbon emissions by 2050, and the industry is now assessing how net zero can be achieved.

Use low-carbon concrete

GGBS, fly ash and limestone lower the carbon footprint of concrete – concrete specification alone can reduce embodied carbon by 50% (see figure 1). Cement and cement replacements are available locally, sourced from the UK and Europe. Designers should also ask concrete manufacturers for environmental product declarations (EPDs) and guidance on the footprint of their proprietary concrete, as a carbon footprint below the industry average can be achieved.

Consider the building’s whole-life carbon

Most definitions of zero carbon focus on reducing a building’s energy use, ie its operational carbon emissions, and this remains an important focus for designers seeking to lower carbon emissions.

“Net zero”, as recently defined by the UK Green Building Council, calls for a consideration of whole-life carbon, which also includes embodied carbon emissions. It is essential that both embodied and operational carbon are considered together, because decisions about one type of emissions can impact on the other. The methodology for measuring whole-life carbon is set out in BS EN 15978:2011 Sustainability of construction works. For further guidance, refer to Whole-Life Carbon and Buildings, published by The Concrete Centre.

Use thermal mass to reduce energy use 

Together with insulation, ventilation and, where necessary, shading, concrete can significantly reduce the amount of energy needed to heat or cool a building. This provides operational carbon savings from day one, accumulating over the life of the building. Concrete does this by acting as a thermal store, capturing free solar energy to reduce heating needs in the winter, and reducing or omitting the need for mechanical cooling by passively regulating internal temperatures.

A design collaboration with A2 Dominion homes predicted that the thermal mass of a concrete and blockwork house would provide an annual reduction of 3% in heating energy required (in kgCO2/m2), as well as a 3.2ºC reduction in peak internal summer temperature. A survey of office buildings showed that those using thermal mass as part of a low-energy servicing strategy had a carbon usage range of 25-50 kgCO2/m2/year compared to 100-175 CO2/m2/year for a typical air-conditioned office. Furthermore, the omission of air-conditioning equipment can save 50% of embodied CO2 per m2 every 15 years, the approximate cycle of replacement of M&E equipment.

Efficient design of the building structure

There are multiple ways in which the structure of a building can be delivered. Optimising the amount of material used can make a significant difference to the embodied carbon, for example through consideration of span and column spacing. Double-curved structures, such as thin-shell structures, can provide extraordinary material efficiency, achieving large spans. The use of permanent void formers in concrete can also reduce the weight of the structure and the volume of material that is needed for the structure and foundations. At the recently completed Royal College of Pathologists headquarters, a ribbed floor slab was designed to lighten the structure, using less concrete while optimising its surface area to gain the benefit of thermal mass. Architect Bennetts Associates reported that this design decision saved around 40% embodied carbon compared to a more conventional flat-slab solution.

Post-tensioning is an efficient way to reduce the depth of a concrete structure. Over ten storeys, incremental savings in floor depth can add up to an entire storey height, compared to a typical steel-framed solution, potentially also removing the embodied carbon of a full storey of perimeter enclosure. Flat soffits also facilitate the installation of services and partitions.

Use concrete to reduce other materials

Doing more with less is a responsible approach to design and construction. Because concrete is naturally fire resistant and it provides good acoustic separation, it does not always need to be treated, coated or additionally insulated. Internal finishes account for around 12-14% of the total embodied carbon associated with office buildings. This can be avoided if the concrete is left exposed or painted – providing a dual benefit by optimising the thermal mass of the structure.

Sevenoaks School Science & Technology Centre, Kent (2018)

This 7,200m2 project includes 30 glass-fronted laboratories and workshops arranged on three floors around a top-lit atrium. The building’s frame is in-situ concrete, in which 50% of the cement was replaced by GGBS. This not only reduces its embodied energy but lightens the colour of the concrete and increases the reflectivity of the exposed soffits, maximising the penetration of natural light into the space. The ribbed roof structure is made from precast concrete in a sawtooth design with 7m spans.

The concrete soffits not only form the backdrop to the artificial lighting strategy, they are integral to heating and cooling too, particularly in the naturally ventilated spaces. The thermally massive concrete structure absorbs heat during the day, which is released at night with the aid of a secure natural ventilation system. During the winter, it helps the building to make more efficient use of passive internal heat gains to reduce the load on the heating system. Groundwater-cooled pipes run through the structural slab above the exam space and workshop areas, chilling the slabs and providing low-energy cooling in these high-occupancy, high-gain spaces, augmenting the thermal mass.

The exposed concrete is a key part of the architecture. A “special” finish is typically used where the concrete can be seen, with a plain finish specified in back-of-house areas. Careful setting out of the MDO formwork and tie holes play a key part of the aesthetic: the formwork layouts and reinforcement detailing responded to the practical requirements of the construction sequencing, board dimensions and PERI formwork and falsework system used, while achieving a clean and simple finish. Coordination and detailing of cable sleeves with formwork and rebar allowed wireways to be built into the slabs and walls without compromising the quality of the concrete’s finish.

PROJECT TEAM
Architect Tim Ronalds Architects
Structural engineer Eckersley O’Callaghan
Contractor Gilbert-Ash
Concrete contractor Oliver Connell & Son
Precast concrete contractor Moore Concrete Products

CUTTING FUTURE CARBON TODAY

It is essential that reducing carbon in the short term should not eclipse the need for buildings to continue to be fit for purpose, both now and in the future. Design decisions should consider whole-life carbon and embed energy efficiency, adaptability, durability and resilience to the impacts of climate change into the fabric of a building.

Designing for long life and reuse

Designing for longevity and adaptability maximises the initial investment, not only in carbon but financially too, and is a core principle of design for a circular economy. Concrete is in itself very durable, requiring little or no maintenance over its life, especially in comparison with other structural materials. Internal concrete structures can achieve and exceed a 60-year design life with no additional design or resource requirements.

This is because the mix design and cover recommendations for reinforced or prestressed structural elements in the internal environment of a building are the same for a predicted durability of both 50 years and 100 years, as shown in Annex A, Table A.4 and A.5 of British Standard BS 8500. Choosing concrete therefore provides a material resource for the future, extending the useful life of a structure, with little or no future carbon expenditure required to ensure it is protected from fire, rot or other forms of degradation.

Climate change adaptation

Concrete is inherently resilient and is proven to be a cost-effective solution for climate change adaptation, using readily available local materials and established construction methods. Flooding is a major climate-change risk in the UK – as recent events amply illustrate. Other identified risks include overheating, drought, subsidence and high winds. Enlightened developers are testing their new developments against predicted climate-change conditions for 2030, 2050 and even 2080. Concrete has energy storing properties due to its thermal mass, which can provide cumulative carbon savings over the life of the building and provide for future passive cooling. Through early strategic planning, it is possible to significantly reduce risks with little or no additional financial and carbon expenditure, now or in the future.

Taking account of the CO2 concrete absorbs 

Carbonation refers to the process by which concrete absorbs CO2 from the atmosphere. It is accounted for in structural engineering design, but is only beginning to be acknowledged in carbon calculations. Over the life of concrete, carbonation can absorb around a third of the embodied CO2.

Next-generation cements and low-carbon concrete

Since Concrete Quarterly’s article on low-carbon cements in 2016 (CQ 256) PAS 8820 has been introduced for alkali-activated materials (AAMs) suitable for use in UK concrete products. Successful trial concrete pours for Cemfree have also provided confidence that existing plant can be used to batch, transport and place novel mixes.

Calcium sulfoaluminate cements are now well-established in Europe for specialist applications, although the carbon savings are similar to existing composite cements already widely used in the UK. In the US, Solidia has completed manufacturing trials of concrete pavers formulated with carbon-cured (calcium metasilicate) cement. Celitement (calcium hydrosilicate) in Germany is at the pilot plant production stage with laboratory testing underway.

An alternative to inventing new cement is to save carbon through the more efficient use of cement replacement materials. Recent research has shown that materials such as GGBS, fly ash, calcined clay and powdered limestone can work better in a multi-component cement. Combinations such as cement-GGBS-limestone, cement-fly ash-limestone and cement-calcined clay-limestone potentially enable higher rates of cement replacement. In 2018 MPA initiated a project to develop new low-carbon multi-component cements for UK concrete applications, forming a consortium with Hanson, BRE and Bison Precast.

The research and demonstration are part-funded by the government under the £9.2m Industrial Energy Efficiency Accelerator programme. Good progress is being made, with BRE carrying out validation testing of new concretes in which 65% cement replacement has been achieved. On completion of the technical programme, recommendations will be presented to BSI to support standardisation of low-carbon multi-component cements in BS 8500.

Colum McCague is technical manager at MPA Cement

Energy use at end of life

When concrete gets to the end of its serviceable life, it can be crushed to create aggregate for reuse in construction. This processing requires comparatively little energy, compared to the spikes of carbon associated with end-of-life scenarios for other building materials.

In summary, there are many ways in which designers can make significant carbon reductions using concrete, both now and over the life of a building or structure. Responding to the climate emergency also poses many other important considerations such as climate change adaptation, resource efficiency and the transition to a circular economy, as well as pollution, water use, health and wellbeing, indoor air quality, responsible sourcing and biodiversity. It is essential that in addition to a whole-life approach to carbon, a holistic approach to design for the environment is adopted, to minimise the risk of unintended consequences. We need to act now, but with a full understanding of how today’s design decisions will impact the future.

For more information on low-carbon concrete, go to concretecentre.com/publications

Photos Hélène Binet