By James Cook, Associate Structural Engineer
“Concrete remains essential to modern construction - but the way we design with it must evolve.”
Concrete gets a bad rap.
Despite its environmental drawbacks, it remains one of the most versatile and effective materials available to structural engineers. It can be cast into virtually any form, combined with other materials and designed to resist compression, tension, shear and torsion efficiently.
Concrete is strong, durable and inherently robust. It offers excellent fire resistance, thermal mass and acoustic performance, and can be constructed quickly and economically.
What makes concrete particularly fascinating, however, is the way it behaves structurally.
When I was a graduate, a colleague advised me to “pick a load path and design for it” and that always stuck with me. In other words, you can choose how it works. Concrete is quite unique in that, when a load is applied, the element will form a system of interconnected struts and ties optimising the properties of the concrete and the reinforcing steel within to transfer the loads to its supports. With a little bit of cracking or yielding (within safe, codified limits) the loads will redistribute until the frame finds itself in a state of equilibrium. This allows the designer to decide on a load path and detail the reinforcement accordingly. Should the building ever behave differently to what was assumed in your analysis, there is generally plenty of redundancy in the system for things to simply ‘work themselves out’. As a designer, I find this quite empowering.
The use-cases for concrete are vast and in lots of situations, it is without question the best material for the job.
With that said, in today’s sustainability-focussed society, concrete is often seen as the enemy and in many ways, it absolutely is. Its environmental credentials are terrible; millions of tonnes of carbon dioxide are released into the atmosphere each year through the production of cement. There is no doubt that the sheer quantities in which we use it globally is having a negative impact on the environment.
However, assuming the human species isn’t suddenly going to stop needing homes, offices, schools, hospitals, train stations, airports, bridges, sports stadiums and so on - can we realistically imagine a world without concrete?
Designing Better
At Price & Myers, every conversation starts with Sustainability. As structural engineers, it is our duty to consider how much ‘stuff’ we are using. Building with concrete might be a necessity but designing efficiently with it is a skill. It requires a deep understanding of how the material behaves, and a commitment to challenging the brief and pushing the limits of what it can do.
Of course I’m not the first person to say this, but the best way to reduce the environmental impact of the buildings we design is simply; to design better.
At Price & Myers, we have been calculating and collecting data via our Embodied Carbon Database for the last six years, to analyse, benchmark and set targets for future projects. It helps us to understand the trends and where best to focus our efforts. If we are going to even get close to the UK NZCBS targets (refer to the recent article written by Ben Golham at Price & Myers in The Structural Engineer ‘Designing to net-zero targets: is our best good enough?’) then we must constantly challenge ourselves to improve workflows, optimise designs and reduce the amount of concrete and reinforcement on every single project.
Optimise the Grid
In structural engineering – regardless of the building type or structural system – the most reliable starting point is to: (a) understand the available materials, including their structural performance and environmental impact; and (b) shape the geometry and form of the design to enable the most appropriate material choice and ensure it works as efficiently as possible.
In concrete framed buildings, in practice this generally means trying to find an arrangement of columns and walls (i.e. ‘the grid’) that is compatible with and complements the function of the space, whilst ensuring the slabs don’t require excessive quantities of reinforcement. Around 40% of the total embodied carbon in concrete frames is known to be attributed to the slabs, so choosing the wrong grid or the wrong slab thickness can have a massive impact on overall quantities.
The reality is that, to function well architecturally, many buildings need tall ceilings and large, column-free spaces, but if you can cleverly arrange your columns (i.e. by squeezing them into party walls or concealing them within cupboards) you might be able to reduce your grid from say 7.5m to 5.5m or potentially shave off 25mm from your slab thickness - and the likely savings in efficiency could be enormous. It is definitely worth having this conversation with the design team at the start of your project to see how much optimisation is achievable without compromising the architecture (tools like PANDA can help you explore those options).
Transfers and Basements
There is often also a desire to change the column grid as you move down the building. Most commonly, this is to accommodate a basement car park or ground floor retail or back-of-house space. This tends to result in transfer structures.
As I’ve alluded to already, concrete is quite a special material; it is capable of a lot. If you want to terminate a column that is supporting fifteen storeys above, to create a 15m clear span over the entrance to your building, then concrete is perfectly capable of achieving this! But at what cost?
Transfer beams tend to be very large, heavily reinforced elements designed for stiffness and strength to ensure the building above doesn’t experience too much deflection or, crucially, structural failure. The embodied carbon associated with transfer elements is huge. Transfer slabs are typically thicker and working a lot harder than your typical floorplate and crammed full of reinforcing steel and punching shear links.
Of course architecture is important, as is the revenue-earning potential of a building. Perhaps the transfer is unavoidable - but is there still scope to improve things?
If we again consider reducing the main column grid from say 7.5m to 5.5m, the forces arising in the transferring column could be around 35% lower. You might also explore the option of reducing the 15m clear span over the entrance to say 11m. Both measures would greatly reduce the demand on the transfer beam, allowing you to make it smaller with less rebar in it - and the architect still gets their column-free entrance.
Again, it’s worth having these conversations during concept design when the opportunity to influence the scheme is at its greatest.
Are there any transfers that can be designed out altogether?
(And do you really need that basement!?)
Material Technologies
Concrete has been around for millennia, but we are still discovering new ways of using it. In recent years, the industry has seen major research and development into the replacement of cement with alternatives such as limestone fines and calcined clays. BS 8500-1 now permits the combination of these materials (and others) in a huge variety of ways, offering greater scope to improve the sustainability of concrete mixes without relying solely on just a few supplementary materials (such as GGBS, which we know cannot meet its demand worldwide).
At Price & Myers, we are following a new approach: rather than prescribing a specific cement combination, we are instead specifying the durability, performance and environmental requirements for the mix and allowing the supplier to propose the best combination of constituent materials to satisfy them. After all, they are the experts and will have a better idea of current economic and market factors.
We are seeing limestone fines used extensively now on many of our projects, and some initial use of calcined clays around the UK - a material which is already widely used in other regions worldwide. We envisage these developments will continue in years to come.
Cement and concrete suppliers are also offering new innovative technologies including carbon capture/entrainment, recycling of aggregates, and the re-use of old concrete sections. Designers have quite an array of techniques at their disposal now to help reduce the impact of the concrete on their projects.
What else can we do?
Understanding the Detail
This one is really important.
It is the role of the engineer to coordinate with other consultants and suppliers to ensure the details are integrated into the structural design. One of the benefits of reinforced concrete is that, in its wet form, you can cast things into it, form penetrations or shape it to suit. To enjoy the maximum benefit from this (and to avoid costly post-fixing or remedial works on site) it is crucial that we work closely with our collaborators to understand their requirements. We need to think holistically about the details as, quite often, we can achieve significant improvements to buildability, performance or cost-effectiveness at these interfaces by carefully thinking about the detail.
So - coordinate those SVPs, think about how the masonry support is going to work, and ensure your balcony stubs aren’t clashing with your rebar! Can the detail be simplified?
Active Involvement During Construction
It is absolutely crucial that we as designers ensure our information is complete, thoroughly checked, and code compliant. However, once we hand over our drawings and specifications to the contractor, plenty can still go wrong.
You might argue that our job is ‘done’ by this point - the design is complete and it’s now over to the contractor to build it correctly. Personally I’m of the opinion that the engineer is well-placed to spot errors, identify poor workmanship, explain key principles to the workforce and generally be on hand to oversee the construction as it progresses. It takes time and effort (you have to leave the comfort of your office and put your hard hat and boots on!) but your client and the frame contractor will certainly appreciate your support - and you are likely to learn plenty in return.
Therefore, I would encourage engineers of all levels (but particularly grads and those at the start of their careers) to go to site regularly, inspect the works and talk to the subcontractors. You can add real value to the project and you will become a much better designer through doing so.
Final Remarks
We have established that concrete is an excellent material. That’s not to suggest we should build everything out of it (this would be a terrible idea!).
In many cases, traditional building materials like masonry or timber can offer much better value in terms of embodied carbon, buildability and cost. Simple, lightweight structures can generally achieve their function without needing much concrete at all. There will be certain instances where hot-rolled steel is much more efficient, for example for really long spans or open-plan office grids where concrete starts to become inefficient, or for single-storey extensions or retrofit projects where concrete would be too heavy and too difficult to integrate with the existing structure. Other materials should always be evaluated at concept stage and the most environmentally-friendly, buildable and cost-effective option recommended.
With that said, for larger buildings and for many of our clients, the speed and ease of construction, contractor familiarity, performance, ease of achieving Building Regulations compliance and of course cost, are still the biggest drivers - and more often than not, concrete still comes out on top.
So concrete isn’t going away, but it is changing, and we as designers should be changing with it.
