Avoiding false accounting - why correct carbon factors matter
22nd September, 2021

There’s an adage in the world of management consulting that states, ‘if you can’t measure it, you can’t manage it.’ The idea being that you need to know what you’re dealing with before you can make an effective impact on it. The same can be said for managing embodied carbon in the built environment. Accurately calculating the embodied carbon in our structures is vital to tackling the climate emergency, with cement, concrete and steel production estimated to account for about 10-15% of all global carbon emissions. As these calculations become fundamental to our day-to-day lives as structural engineers, it is vital to ensure we understand exactly what we are calculating. One of the biggest sources of confusion surrounds the use of low-carbon alternatives, and the impact that this has on our calculations. Let’s look at the production of two the main materials used in construction.

There are two methods of producing steel, and together they account for an estimated 7% of global greenhouse gas emissions1. Steel however is also indefinitely recyclable and has a recycling rate of over 85%2 .

Virgin steel is produced in blast furnaces, a heavily polluting activity with the production of CO2 fundamental to the chemical process. The waste from this process can be treated and ground to form Ground-granulated Blast Furnace Slag (GGBS). Alternatively, the Electric Arc Furnace (EAF) method uses a high electric current to melt scrap steel down and produce new products. The carbon emissions from this process are a function of the local energy grid only so it can be much cleaner, but this method can only be used to put reclaimed material back into the system.

Despite the high recycling rate of steel, the products that it is used to create often have long lifespans, typically 50-60 years for building materials. Material recycled from the construction industry could have been built at a time when global steel production was only a fraction of its current rate. It is estimated that scrap steel can only supply just over a third of current global demand, and at current predicted rates of growth this ratio will only decrease

The primary source of carbon emissions in concrete is attributed to the production of Ordinary Portland Cement (OPC), which is estimated to account for between 3%3 and 8% of global emission4. OPC is formed by heating Limestone to high temperatures, a process that releases CO2; accounting for up to 60% of the emissions5 in cement production.

It is common practice in the UK to supplement OPC with alternative cementitious materials, known as Cement Replacement Materials (CRMs). The two most common CRMs are GGBS, from the steel production process, and Pulverised Fly Ash (PFA), a by-product from coal-fired power stations. These two waste materials can be used to replace significant amounts of the OPC - up to 70% using GGBS for example. Engineers use increasingly high levels of CRMs to offset the embodied carbon of concrete structures.

As both products are waste materials the production of them is linked to other industries. As coal-fired power stations shut down the sources of PFA become limited, and whilst stockpiles are available this is a very finite resource. UK GGBS production is directly linked to the highly volatile steel industry, and this is likely to reduce further as production of virgin steel declines.

When carrying out embodied carbon calculations engineers can be bombarded with a range of factors to use for these materials, based on varying ranges of recycled contents for steel or CRMs for concrete. The temptation is to always go for the highest possible recycled content, lowering the embodied carbon impact of our designs.

For example, a lot of UK reinforcement is produced using Electric Arc Furnaces, consisting almost entirely of recycled steel with correspondingly low embodied carbon rates. We can clearly see the problem with this approach however, as steel is a globally traded commodity and that on average the recycled content can only be around the 37% figure6. The implication of this is that by utilising higher rates of scrap steel in one product we are essentially removing the ability of other products to use any.

The same argument can be applied to the use of CRMs in concrete. We can specify up to 70% GGBS, but we know that in terms of availability the actual figure available globally is far lower. By increasing our content higher than this average in our building we are essentially removing the ability of someone else’s building to do the same. This may lower the embodied carbon of our individual design, but it does little to help the global situation. It also creates a reliance on these carbon reduction methods which is unsustainable. If the only way to design a SCORS A+ building is with CRM’s, what will the solution be when these products are no longer available?

The other side effect of relying on these materials is that they detract from the real issue, which is the overuse of materials. If during the early stages of designing we assume a lower-than-average embodied carbon factor (ECF) based on certain specification assumptions then we remove the incentive to design leaner, more efficient structures – a process which (along with increased re-use of existing buildings) is really our only way out of the crisis. Engineers assuming 97% recycled content in reinforcement will see little benefit in spending time refining their detailing design.

It is also worth noting that as design engineers, our embodied carbon calculations are estimates, and that problems can occur further down the line due to procurement issues. For example, a groundworks contractor is unlikely to delay a pour if GGBS is unavailable, and the engineer that specified 70% OPC replacement in their design will find their embodied carbon figures rapidly creeping up.

The solution to this is to use centralised carbon figures for the main bulk materials in the early stages of design. Rates for steel needs to be standardised across all products, regardless of recycled content, and CRM content for concrete should also be set at a fixed value. The rates may in fact alter year-on-year but should be academically-led, based on actual material usage. As steel use falls, the recycled content should gradually increase to the point where it becomes circular – where we only use as much as there is generated scrap.

Although Structural Engineers should specify high levels of CRM’s and recycled steel on their projects to make sure that these materials are fully utilised and prompt innovation in new Low-Carbon Options, we should not artificially alter our carbon values from the global averages when making decisions about which material to choose in our scheme designs, and we should report our embodied carbon using these global averages such that the quality of engineering is being judged, rather than an individual projects ability to use up more of a globally limited resource.