National Structural Concrete Specification fifth edition
The latest edition of the concrete frame specification includes detailed guidance on embodied carbon and visual concrete, and an all-new online interface to make it even easier for the whole project team to use. By Katie Puckett
The fifth edition of the National Structural Concrete Specification (NSCS) has just been launched – a comprehensive template covering every aspect of a concrete frame, from loads to reinforcement to finishes.
It’s a significant update on the fourth, which was released back in 2010.
Since then, embodied carbon has become a more urgent priority, many heads have been scratched about exactly what a “plain” finish should look like, and project documentation has moved from paper to the cloud. Just like its predecessors, the fifth edition is free to use and was developed through collaboration between clients, designers, contractors and concrete producers. Among the changes are compulsory environmental product declarations (EPDs), more detailed guidance on visual concrete, and a brand-new online interface.
Paul Toplis, consultant at Price & Myers, was lead editor for the fifth edition, and worked on the fourth and third editions too. “The NSCS was originally developed because people wanted a really useful building specification, and to avoid having lots of different ones, all produced by different practices,” he says.
At tender stage, contractors have limited time to get to grips with a specification and communicate it to their subcontractors, he points out. “Often people forget that the specification is a tool for everybody – that’s why we’ve tried to make the NSCS feel as collaborative as possible.” Toplis remembers the wise words of the previous editor, the late Julian Maw. “He used to say that you have to be careful about writing specifications based on scars of things that have gone wrong in the past. That’s why the NSCS is produced not by specification experts, but by a team of people from across the building industry, and hopefully enough of us to make sure it is balanced and fair to all parties.”
As well as 15 years’ worth of technical updates, the fifth edition reflects industry shifts. The proliferation of logistics warehouses and data centres is recognised in the tolerances section, with new “free movement” or FM clauses that separate the flatness of concrete floors from other surfaces – telehandlers, robots and server cooling equipment all require extremely level floors. Meanwhile, the growing use of offsite construction has been accommodated in the visual concrete specification, with sections on aspects such as lifting points.
One of the most significant areas of continuing evolution is sustainability. Arup associate Tilly Langley says that this is now woven throughout the fifth edition. “The industry knows it needs to change, but we can’t do it overnight, so we’re starting with some general nudges and ways we can help people to benchmark sustainability.”
The NSCS is intended to raise the overall standard, and support smaller consultancies, by making some best practices the default. For example, rather than always specifying design strength at 28 days after casting, 56-day strength is now an option. “That’s great for piles and pile caps and rafts which might not be loaded straight away,”says Langley. “On one of our projects, we saved over 15% of the cement and a lot of material by changing the concrete mix and the structural layout.”
It also requires that any product, component or constituent material within concrete has an EPD. “That means suppliers have to report, and that specifiers might notice that supplier A’s EPD is much better than supplier B’s, and decide to specify that instead.” Compared with 2010, today there is a much wider range of lower carbon concretes, including ternary blends with two different cement replacements, introduced in the most recent update of BS 8500.
The fifth edition is aligned with BS 8500 so it includes these, and will allow the use of new components as they become available, such as calcined clay. There is scope to set an agreed maximum permitted embodied carbon rating, based on industry benchmarks. This is intended to shift the emphasis away from specific ingredients – for example, GGBS – and allow concrete contractors and suppliers to find the best possible solution.
As for the sixth edition, there will never again be a 15-year wait. The advantage of being online is that the specification can be continually evolved and improved, as standards are updated or to add new features. One that is currently under development is a way of highlighting changes to a specification, and labelling them by collaborator. That will save people having to scrutinise two versions of a document to see what’s different, says Toplis, and it will avoid misunderstandings and mistakes.
For example, the contractor might adjust a mix depending on whether it is to be placed by skip or by pump. From the designer’s point of view, the concrete will be almost exactly the same, but it’s a crucial piece of information for the concrete supplier, so they need to know the request came from the contractor, he points out. “It’s not that people don’t try to understand, or they’re difficult, it’s just that sometimes we don’t know what happens along the chain of communication from the top to the bottom of the industry.”
How to use NSCS fifth edition
The NSCS is deliberately simple to use. It follows the same structure as the European standard EN 13670 on the execution of concrete structures, so that the two can be easily compared side by side. Several key format changes to the fifth edition are intended to make it even more accessible.
The fourth edition was in book form, and consisted of standard and project- specific specifications. The fifth is compiled online, displayed as a split screen, with the main specification text in the left-hand column, and the relevant guidance alongside on the right. It can be viewed either as a complete specification containing standard and project-specific clauses, or as a list of project-specific information only.
The combined view could be helpful for newcomers to the industry, says Toplis. “You can see the context reallyeasily, and you’ve got the guidance as well. That’s a very powerful tool to bring you the best thinking on any particular topic as you’re entering the information for your project.”
But it’s also important for those receiving the specification to be able to quickly see what’s different from the norm: “Otherwise, there’s a danger that people might miss it. If you don’t understand at tender, that doesn’t help further down the line when you’re trying to build a team where everybody is pulling together to produce the best building for the client, not watching their own backs.”
Missing information will also stand out much more clearly online: “On screen, it will say “type text here”, or it will show a blank on the PDF export. It might be that it doesn’t matter and the answer is ‘normal’, ‘none’ or ‘not applicable’, but quite possibly it may matter and somebody has just forgotten about it.”
The specification is both free to use, and to store on CONSTRUCT’s cloud. Designers create an account on the CONSTRUCT website (nscs.construct. org.uk), and click “Create New Project”. They will then be taken to an online template, to which they can invite collaborators, or export as a PDF.
Specifying visual concrete using NSCS
When is a plain finish not a plain finish? That’s one of the perennial questions that compilers of the fifth edition of the National Structural Concrete Specification have set out to resolve once and for all.
“There’s quite a lot of ambiguity in visual concrete,” explains Paul McNamara, pre-construction manager at concrete contractor Getjar. “It’s like describing something as ‘grey’ – it doesn’t tell you very much.” At the heart of the issue is that an architect’s perspective of visual concrete may be very different to the specialist contractor or concrete producer who is responsible for achieving it.
Every aspect of the build will affect the desired finish, from the mix to the formwork to the vibration technique. It is also very hard to describe in words, which has made previous specifications prone to misinterpretation. Colour is a common area of disagreement: “The spec might say ‘uniform colour’ but what is the uniform colour in concrete? It’s never all one colour. Plywood and board layouts are another one. It might just say ‘regular and even’, but that could mean many different things to different people without further discussion.”
The task for McNamara’s visual concrete working group, consisting of architects, engineers and contractors with experience of delivering visual concrete, was to come up with agreed definitions – essentially, to rehearse all possible arguments so that future teams using the NSCS wouldn’t have to. “We spent many sessions debating what’s inherent – like natural variations in colour – and what’s avoidable, like big blobs of dark grey concrete where the pump has been washed out in the middle of a pour.” Some blowholes are inherent; honeycombing is a blemish.
The result is an optional visual concrete appendix, in the form of an interactive, step-by-step guidance document with prompts for every element of visual concrete that needs to be considered. Specifiers are encouraged to include visual examples and drawings, and contractors to provide samples, benchmarks and mock-ups.
One of the biggest topics was how to distinguish between a “plain” finish and a “special” one, the two classifications for visual concrete. Plain means a good quality, consistent finish, free of major blemishes – but it is a standard specification, not a customisable one. As soon as there are any specific requirements, that becomes a special.
“Architects are reluctant to ask for a special, as they think it’ll be more expensive. So they say, ‘we just want a plain finish, but we’ll give you the plywood layout’. But that’s then not a plain, it’s a special. The problem is that if you start by calling it a plain, the contractor may stop listening and miss the fact that you want them to deliver it to a formwork layout.”
The visual concrete specification is now presented in tabular form, allowing the architect to be more prescriptive, within the limits of each class. Planning is key to achieving good visual concrete, says McNamara. That includes post-finishing, which should be embraced as a fact of life – many architects have horror stories of poor “making good” so they demand concrete be “as struck”. “But post-finishing is nothing to be scared of when it’s done well. It’s an essential tool in producing good visual concrete, and not necessarily because the contractor’s done a bad job.
It might get damaged during the build process, or in two years’ time when the building is in use. Remedial works can be done very well, but they should be talked about and planned for.” Architects also need to be aware of the additional costs and time incurred by overspecifying – is a plain finish really necessary in a back-of-house plant room, or would an “ordinary”, non-visual finish do the job? “Or maybe you’re asking for 1mm steps between panels instead of the standard 3mm step, which is very difficult to achieve. The plain spec is there for a reason – it’s achievable.”
Ultimately, the fifth edition of the NSCS is intended to level the playing field, says McNamara. “We’ve tried through many, many workshops to bring as much definition as possible, so that everybody is very clear on what is expected. That way, the client knows what they’re paying for, the architect is clear about what they want and what they will receive, and the contractor should be able to price, programme and deliver it correctly.”
Please visit our examples of visual concrete projects with plain and special finishes.
Photo Hufton + Crow, Andy Matthews, Rob Parrish, Mark Harrington / AKT II, Jack Hobhouse, Nick Kane, Rob Parrish