Why are steel and concrete used?
Steel is a very strong (difficult to break) and stiff (difficult to bend) metal, an alloy of iron and carbon and a few other elements in highly controlled proportions.
Concrete is composed of small stones and sand, bonded together with cement.
The two materials are very different: steel is a homogenous material, concrete is a composite one; concrete’s properties change in the weeks and months after pouring, steel’s generally don’t; steel is formed into standard shapes in the factory when still at high temperature; concrete is cast into bespoke moulds where it is needed.
The raw ingredients for both (iron ore for steel, limestone for concrete) are widely available in large quantities worldwide and both are usually available at low cost.
We can also reinforce concrete with steel, increasing its strength and stiffness, particularly in tension (i.e. a pulling action), whilst the concrete protects the steel from rusting. Steel and concrete allow us to build taller, bigger and faster than we could with previously dominant materials like stone and timber. They are what has allowed us to provide the homes, workspaces and infrastructure that society needs.
The problem with these incredibly useful materials is that producing steel and cement (the active ingredient in concrete) causes about 10% of mankind’s greenhouse gas emissions.
To produce steel, we must heat iron ore to over 1,500C by burning fossil fuels; whilst the chemical reaction that turns limestone into cement inherently releases CO2. Additionally, neither material is renewable – though we can recycle steel, and we can reuse steel and concrete if we design with their reuse in mind.
After a century of innovation, we can produce these materials incredibly efficiently. The processes are already quite close to thermodynamic limits - so we won’t be able to significantly reduce how much carbon it takes to produce a kilogram of either.
However, we can look at how much steel and cement we need to use (hence make) each year – this is where structural engineers come in.
The role of structural engineers
All of the 4,200 million tonnes (Mt) of cement produced globally each year is used in construction, as is about half of steel (c. 900Mt) produced. Structural engineers are responsible for specifying most of this, i.e. where and how they are used.
It is worth stating there is no single ‘right’ material for structures – for every project the most appropriate material should be chosen for the specific requirements, considering not just the structural engineering but the building services, architecture, likely lifecycle, local sources and broader client / user needs.
Timber is often touted as a ‘panacea’ –it is renewable, but there can still be so much carbon released during drying, processing and landfilling that it has a similar environmental impact to steel and concrete.
Modern materials such as fibre-reinforced plastic have their own environmental challenges and would need to be produced at a much larger scale (and build up their associated supply chain) in order to meaningfully displace steel or concrete.
All have some form of drawback – either have inferior technical performance, are equally (or more) polluting to produce, or are uneconomic or unavailable at the scale required.
I hope that a new ‘wonder’ material is discovered that has all the benefits without the drawbacks, but I don’t think we can rely on this happening – instead it will be the difficult task of choosing the right materials for each project, being really efficient in how we design and build, and driving reuse wherever we can.
The good news is that there is huge scope to significantly improve our material efficiency – the challenge is that we will have to move away from current industry practice to find ways of working better together to realise these opportunities.
There are four main strategies a structural engineer can implement today to significantly reduce the material used in buildings and infrastructure:
1. Specify sparingly – aim to design with the minimum amount of material by:
• revising default assumptions, which are often overly conservative
• targeting maximum utilisation – the MEICON survey demonstrated that many engineers deliberately overdesign elements, rather than aiming for 100% utilisation
• decreasing waste by tailoring specifications to their use, rather than ‘standard’ options that cover all bases
2. Adopt alternative methods– for instance off-site manufacture can deliver lower-waste, lower-material products because components surplus to one project can be used on the next. Also, a controlled working environment allows high quality and precision, avoiding waste and enabling savings in design
3. Reuse and refurbish where possible – engineers should propose ambitious changes to an existing structure before ‘knocking down and starting again’; this approach can also often yield time and cost benefits
4. Build in resilience where appropriate – where it is known that, e.g. an infrastructure installation, is likely to have a very long lifespan then it is sensible to anticipate future challenges/changes so that replacement/major overhaul is deferred as long as possible
These new approaches, coupled with technological change, will require different skills to be a ‘competent engineer’ – less production of calculations and more collaboration to deliver innovative solutions.
Ensuring safety is a structural engineer’s first duty, and engineers around the world take it very seriously. A guiding principle is being appropriately conservative with our assumptions – i.e. taking a pessimistic view so that the structure remains safe even in ‘worst case’ scenarios.
We are supported in our tasks by excellent codes of practice which is continually updated based on academic research and industrial experience. A comprehensive and scientific approach to risk is taken when producing this guidance and deciding on safety factors.
The challenge for engineers is not to go unnecessarily beyond the recommended approach and not to layer contingency on contingency to the point of excess.
The evidence is that many projects have much more material than is necessary for safety, primarily due to engineers being over-conservative.
Removing this excess material therefore can safely be done – another key duty (and skill) of the profession is to be efficient with our clients’ (and societies’) resources.
Room for optimism?
I am encouraged by recent conversations within the profession and industry around climate change and the need to be much more efficient in our use of materials.
I do think we are moving in a better direction. However, my fear is that we will not turn these good words and intentions into actions quickly enough to avert the worst consequences of climate change.
This truly is a climate emergency.