Main contractor
A board-marked elevation cast in four lifts
Sawn board grain on an in-situ wall, four lifts from one liner, and a release agent that had to be tested before anything was poured.
Read the jobExample project
A reveal and a service rebate, cast into an in-situ wall.
Two details that look trivial on a drawing. A chamfered reveal that has to strip without taking the arris with it, and a rebate that has to line up bay after bay.
- Route
- Cast inserts and strips
- Details
- Reveal and service rebate
- Pours
- Around 60
- Insert area
- 3 m² total
The brief
Two small details on a long wall.
A civils contractor was casting a long in-situ wall in bays. The detail called for a chamfered reveal at each bay junction and a rebate along one run to take services later. Both details were drawn in two lines on a section and neither of them was trivial to cast.
They had already tried timber fillets. The chamfers were coming out with spalled and ragged arrises, and the rebate was drifting relative to the bay above.
The difficulty
The arris fails at the moment the formwork comes off.
A chamfer on green concrete is a thin wedge of material with an unforgiving geometry, and everything that goes wrong with it goes wrong during stripping. A rigid fillet grips, so the arris comes away with the fillet or spalls along its length. So it looked ragged on their first bays, and it is not a workmanship problem.
An elastomeric insert solves the release and introduces a new problem. The insert is clamped inside formwork and loaded by the head of concrete. Too soft and it compresses, so the reveal loses its line and the chamfer varies down the pour. Too hard and it behaves like the timber fillet it replaced. The hardness has to be chosen for that specific loading rather than picked off a chart.
The rebate had a separate and duller failure. It was being set out by measurement in each bay, and small errors accumulated down a long wall, so by the time the services came through the rebate was not where the drawing said it was.
How we did it
Hardness for the load, and location built into the insert.
01
Hardness chosen against the head of concrete, not off a chart
We set the insert hardness for the pour height and the clamping arrangement they were actually using, at the hard end of what still releases a green arris. The trade-off is deliberate and it is written into the specification, and the reasoning is set out on the materials page.
02
The insert locates itself off the formwork
The rebate insert was made with a locating feature that registers against the formwork panel rather than against a mark on the last pour. Set-out then comes off the panel, which is jigged, instead of accumulating down the wall.
03
Section changed so the strike pulls away from the arris
A small change to the insert section means the insert deflects away from the concrete as the formwork comes off, so the chamfer is released rather than dragged. This is the change that stopped the spalling.
04
Trialled on one bay, then repeated
The first bay was treated as the trial and looked at properly before the inserts were made for the rest of the run. One bay is a cheap place to find out that a hardness is wrong.
The numbers
Small details, sixty pours, and a very small number per square metre.
Same rate, same division as the rest of this site.
The inserts and strips came to about 3 square metres of moulding face across the run. At an indicative £350 per square metre that is £1,050. Across around sixty pours of wall, casting roughly 180 square metres of detailed face, that is about £5.83 per square metre, being £350 divided by sixty.
A number that small is not the argument for the tooling. The argument is that a spalled chamfer on an exposed civils wall is either accepted, patched or cut out, and all three of those cost more than the inserts on a wall of this length. The tooling is cheap. Making good is not.
All of the arithmetic below runs at the same indicative rate of £350 per square metre of liner, from published figures from PERI UK and RECKLI. It is a market figure carried here so the division can be checked, and the liner calculator will run it on your own rate instead.
Every money figure on this page is indicative. It is taken either from the illustrative two year duty cycle used across all five TCI sites or from published third-party pricing, and it is shown so the arithmetic can be checked. None of it is a quotation, and all of it must be replaced with TCI figures before launch.
Where it got to
Where the job ended up.
- Insert hardness set for the pour height and clamping actually in use
- Insert section changed so it deflects away from the arris on strike
- Rebate located off the formwork panel instead of measured bay by bay
- About £5.83 per square metre of detailed face across sixty pours on the indicative rate
The pages behind this job.
The services and the reference material this project drew on.
More jobs
Other work of this kind.
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Chamfers coming off ragged?
Send a photograph of the arris and tell us the pour height. Those two things usually separate a fillet problem from a strike problem.