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.
Case study: Network Rail
Network Rail has worked with suppliers to reduce the embodied carbon of precast concrete planks used to refurbish station platforms by 64%.
The greatest contribution came from substituting 80% of the Portland cement in the mix with GGBS, but the team also made a number of improvements to the reinforcement.
This included reducing the partial factor for steel reinforcement from 1.15 to 1.05, and developing the arrangement to increase utilisation and optimise the layout.
Where non-corrodible reinforcement is required, stainless steel was replaced by basalt-fibre reinforcement. Load testing also demonstrated that in some of the precast units, loose bar or mesh reinforcement could be replaced by fibre reinforcement.
Expedition Engineering was innovation partner on the 18-month project, working with structural engineer Studio One, concrete consultant AMCRETE UK, contractor G-Tech Copers and precast supplier Anderton Concrete.
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.
Case study: Laing O’Rourke
Laing O’Rourke has trialled the use of basalt fibre reinforced polymer (BFRP) in two structural systems as part of its Decarbonising Precast Concrete Manufacturing project.
Megaplank (below) is a one-way spanning precast concrete slab. Two units were produced, reinforced with BFRP meshes, using geopolymer concrete and AACM concrete. The use of BFRP saved 67% of the reinforcement’s embodied carbon, and 22% across the overall unit.
Meanwhile, the Arup Vault prototype is a lightweight compression shell-based reinforced- concrete floor system, including a perimeter tension beam ring. Two tie-beam units were manufactured using geopolymer concrete and BFRP cages (above). The use of BFRP saved an estimated 72% of the reinforcement’s embodied carbon, and 45% overall. Further testing of the beam units is planned.
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.
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.
Case study: National Highways
Skanska has used basalt fibre reinforcement as part of a trial of low-carbon concrete on the National Highways M42 Junction 6 improvement scheme. Working with Tarmac and Basalt Technologies, it cast four different reinforced concrete slabs as part of a temporary haul road and monitored their performance and durability.
Two of the slabs contained steel reinforcement and two contained basalt fibre with a polymer binding, which is non-corrodible and between four and five times lighter than steel. Each reinforcement material was combined with both conventional concrete and a low-carbon alkali activated cementitious material (AACM).
Results show that the basalt fibre solution reduced carbon by more than 50%. It proved equally resilient when compared to conventional reinforced concrete using steel.
Skanska is now working with National Highways and High Speed 2 on the next phase, which will trial the low-carbon combination on a permanent road. The ultimate aim is to roll out the solution across the UK’s strategic road network.
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