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.

Photo Hufton + Crow, Andy Matthews, Rob Parrish, Mark Harrington / AKT II, Jack Hobhouse, Nick Kane, Rob Parrish

Above
At Moss House, University College Birmingham, Glenn Howells Architects and concrete contractor Thames Formwork spent about three months testing the specification with different suppliers and different formwork. The chosen mix was a 55% GGBS concrete, light in tone, cast against a laminate-faced board

Above
A “special” finish was specified for the structural trees at the Marshall Building, London School of Economics. A mock-up helped Grafton Architects and concrete contractor Getjar to refine the specification, improving the finish by changing the release agent. “Plain” soffits were specified on the upper floors

Above
The Salvation Army HQ in south London by Tatehindle. The exposed surfaces were specified to be “C – fine smooth” under BS 8110, the relevant standard at the time, equivalent to NSCS “special”

Above
Pigmented concrete, such as the mix used on the precast facades of 333 Kingsland Road by Henley Halebrown, is always classified as a “special” finish. The growing use of precast elements is reflected in the NSCS, with new sections on visual aspects such as lifting points

Above
A “special” finish was specified for the waffle beams and slabs above the council chamber at Tower Hamlets Town Hall. Concrete supplier London Concrete produced a number of trial panels in order to benchmark the colour and finish, working closely with architect Allford Hall Monaghan Morris