How to cut carbon in reinforcement

Steel contributes around a quarter of a concrete slab’s embodied carbon. Fortunately, there is much designers can do to reduce this often overlooked environmental impact, says Emily Halliwell

As the construction industry seeks solutions for reducing the embodied carbon of concrete buildings, much of the focus has been on the concrete itself – on alternative cements and cement replacements, and on more material-efficient floor systems that use less concrete. But we can’t afford to ignore the reinforcement within the concrete. This contributes approximately 25% of the embodied carbon of a structural floor – a proportion that will become even more significant as the embodied carbon of concrete continues to fall.

This article will look at some of the opportunities to design out carbon from this often overlooked element. As with all carbon assessments, selecting the right data is essential, particularly with large variations in carbon factors due to different steel production routes. Additionally, designers have many options for both refining their designs to minimise material use and for specifying alternatives that can offer savings in reinforcement, concrete or both.

Understanding embodied carbon in reinforcement

When calculating the embodied carbon in a structure, it is important to use appropriate carbon factors from reliable sources. It is recommended to refer to environmental product declarations (EPDs), standardised documents that provide performance data for a given product or material. As there are many EPDs available, selecting the right one is important.

Reinforcement is mainly manufactured either using an electric arc furnace (EAF), which often recycles scrap steel, or a basic oxygen furnace (BOF), which creates new or “virgin” steel. An EAF is powered by electricity whereas a BOF is generally fossil-fuel- fired. As such, reinforcement produced by EAF typically has much lower embodied carbon, and this may fall further as the grid decarbonises.

In Europe, the majority is produced in this way, but this is not necessarily the case elsewhere in the world. So, given the large variations in embodied carbon associated with different production methods, it is vital to understand where reinforcement is sourced from and to obtain EPDs from suppliers and manufacturers.

Where information is not available, or during the early stages of a project, guidance on embodied carbon factors for use in carbon calculations (such as Table 1, above) can be found in the reinforcement appendix to Specifying Sustainable Concrete, published by The Concrete Centre.

Modifying material factors

Embodied carbon is determined not only by the choice of materials in a building, but the quantity. There are many measures we can take to reduce the amount of reinforcement in a concrete structure. To calculate the design strength of a material, the characteristic strength is divided by a material partial factor – so the lower the partial factor, the higher the design strength.

For reinforced concrete structures, the relevant partial factors are set out in Eurocode 2 (EN 1992-1-1). These account for variability in the properties of concrete and reinforcing steel and for geometrical deviations in the structure. By applying a reduced partial factor for reinforcing steel, engineers can reduce the quantity of reinforcement without any impact on performance, lowering both the cost and embodied carbon of the structure.

When calculating the capacity of concrete structures in normal design situations, the recommended partial factor for reinforcing steel, γs, is 1.15 (from EN 1992-1-1 Table 2.1N). However, this is a Nationally Determined Parameter, allowing different countries to specify their own value, and a reliability study using UK CARES reinforcement data has shown that it is feasible to reduce the partial factor for reinforcing steel to 1.05 for bending.

This potentially reduces the area of steel required by 9.5%, although this may also be affected by other demands such as robustness or serviceability. More guidance on using a reducing partial factor can be found in Reducing Carbon and Cost of Reinforcement. 

Saving through reinforcement detailing

Reinforcement detailing is a key process in the design of concrete structures. This involves taking the output from the structural design and turning it into an arrangement of bars that works with the geometry of the structure. There are practical limits on the length of reinforcement bars, both for delivery and installation, and so “laps” (or overlaps) are used to transfer bar forces in locations where it is not possible to use a continuous bar.

Additionally, in some locations reinforcing bars are required to transfer forces directly into the concrete, which means they must be anchored. The required reinforcement length for transferring forces between bars (lap length) or into the concrete (anchorage length) depends on the forces in the bars, as well as geometric parameters such as cover to reinforcement, whether there is a bend in the bar, and the arrangement of adjacent bars.

As the lap and anchorage lengths depend on many factors, conservative assumptions are typically made to simplify the process and avoid having to calculate lengths for each individual bar. This can, however, lead to more steel being used than is required, increasing the associated embodied carbon.

One common assumption is that a bar is carrying the maximum force for a bar of its size, rather than using the actual calculated force. This presents an opportunity to refine the design to reduce lap and anchorage lengths. As an alternative, couplers may be used in place of lap lengths to provide continuity, which can result in material savings, particularly for larger diameter bars. It is important that lap and anchorage lengths are communicated clearly on drawings so that detailers and installers understand the requirements, especially if these vary for different elements.

Avoid over-rationalising

Rationalising reinforcement layouts typically involves grouping elements, such as beams, and only designing for the worst case, such as the maximum load or longest span. This design is then used to develop reinforcement details for the whole group of elements. Rationalising reinforcement layouts is common practice because it simplifies design, checking and installation. However, it can lead to the use of significantly more material than a structural design requires.

There is a balance to be struck between ensuring reinforcement can be installed efficiently and accurately and avoiding unnecessary embodied carbon. A variety of tools are available to designers to assist with this, including in-built carbon calculators in structural modelling software and parametric design tools. Early discussions between the structural engineer and concrete contractor can assist with developing designs that are practical to install while offering embodied carbon savings.

Using alternative reinforcement types

So far we have focused on standard carbon steel reinforcement, but there are alternatives that offer potential carbon savings. One of the main concerns with standard reinforcement is its durability: it can corrode if the cover distance from the surface of the concrete to the surface of the bar is insufficient for the environment the concrete is used in. Alternative reinforcement materials such as stainless steel, fibre-reinforced polymer (FRP) and basalt fibre offer improved resistance to corrosion and may either be used to prolong service life or reduce maintenance, both of which result in carbon savings over the whole life of a structure.

Additionally, there may be scope to reduce concrete cover to the reinforcement. Cover is driven by a number of requirements including fire protection, bond and durability. For aggressive environments where durability often drives the cover requirement, using corrosion-resistant reinforcement may offer an opportunity to reduce cover and therefore the volume of concrete required. For more information, refer to The Concrete Society’s Concrete Advice 64: Cover to stainless steel reinforcement.

Neither are designers limited to using reinforcement in bar form. A wide range of fibres may be used, including steel and synthetic fibres, potentially enhancing the properties of the concrete. This can reduce the requirement for standard reinforcement – for example, in ground-bearing structures where fibres are often used to control cracking – and also offer programme savings by reducing time needed to fix the reinforcement.

Post-tensioning tendons are another alternative that can offer carbon savings compared to a typical reinforced concrete system. While there is limited information on the embodied carbon of post-tensioning systems themselves, they can significantly reduce the amount of steel required, as well as allowing thinner structural elements, reducing the quantity of concrete in a design.

The available information about alternatives to reinforcement has been limited up to now, but this is set to change with the increased focus on embodied carbon and material efficiency. Eurocode 2 is currently under revision, and it is expected that the new version will provide information on stainless steel reinforcement, steel fibre reinforced concrete and embedded FRP reinforcement, which should assist designers considering their use to reduce embodied carbon.

Emily Halliwell is senior structural engineer at The Concrete Centre

Photos: Stefan Gröschel, Lily Maggs, Daniel Hopkinson for Feilden Clegg Bradley Studios, Expedition Engineering, Kier Construction & Skanska