Striking formwork and props

The question is never really "how many days". It is has the concrete got strong enough to carry what is about to be put on it — and the days are a proxy for that, chosen conservatively so that nobody has to answer the real question on every pour.

Knowing which question the table is standing in for is what tells you when the table is not enough.

BS 8110 Table 6.1, minimum periods

For concrete made with ordinary Portland cement, no admixtures, and a member not loaded when the formwork comes off:

The temperature is the concrete's own surface temperature, not the shade air temperature, and the periods shorten as it rises. Nigerian pours sit above the 16°C column, so that column is the one to work to — as a minimum, not as a target.

Cantilevers are not in the table and should not be read off it. A cantilever has no redundancy: the props are the only thing holding it up until it can hold itself up, and the structure that counterweights it usually has to be complete first. Leave them until the concrete has its full strength, and take the striking sequence from the designer rather than from a rule.

Why the props outlast the forms

They are two different jobs, and conflating them is how slabs end up with a permanent dip in the middle.

• The soffit form shapes the concrete. It can go as soon as the surface is hard enough not to be torn.
• The props carry the weight. They can go only once the member can carry itself — strongly enough and stiffly enough.

The stiffness half is the one that gets forgotten. Young concrete has a lower elastic modulus and creeps far more readily than mature concrete, so a slab propped for a week and then loaded takes a deflection it never recovers. It does not crack, it does not fail any check, and it is visible for the life of the building. The span/depth rules that controlled the design assumed a member loaded at maturity.

The heaviest day of a slab's life is usually week two

Consider what happens the moment the props come out of a 160 mm slab:

The slab is carrying most of its service load, at perhaps two thirds of its strength, days after being cast — and if somebody stacks blocks or sand on it for the next lift, more than its service load. Nothing later in the building's life will be as demanding relative to what the concrete can do at the time.

This is why "the props were only out for a day before we reloaded it" is not a defence. It is also why materials for the next storey should be landed over columns and beams, not in the middle of a young panel.

Back-propping

On any normal programme the next slab is cast long before the one below it is at full strength. The props for the new pour land on the young floor, and they concentrate the entire weight of a fresh slab — wet concrete, forms, workers — onto it.

Back-props are what spread that down through two or three floors so no single young slab takes it all. Two rules make them work:

• Leave them undisturbed, or reinstate them tight but not wedged. A back-prop hammered up jacks the slab and reverses its moments; one left slack does nothing at all.
• Line them up floor to floor. A back-prop that does not sit over the prop below simply loads the slab it stands on.

Striking on strength instead of on the calendar

The defensible way to strike early — and the only way to strike safely when anything is unusual — is to measure:

1. Cast extra cubes from the same load, and cure them beside the member, under the same sun and the same covering. Cubes in a laboratory tank tell you about the mix; cubes on the slab tell you about the slab. See cube tests.
2. Decide what strength you need, by checking the member for the load it will actually carry the day it is struck — its own weight, plus whatever is going on top of it.
3. Crush a cube, and strike when it says so.

Note the order. The strength required comes from a calculation about that member on that day, not from a percentage somebody remembers. A lightly loaded slab may be strikeable well before a heavily loaded transfer beam of the same age and the same concrete.

Striking is also the end of curing

Formwork is the best curing a member gets: it holds water in and keeps the sun off. Take it away at 12 hours and the column starts drying at the surface, which is exactly where the cover concrete that protects the steel lives.

• Keep concrete wet for at least a week after casting for ordinary Portland cement, and start the moment the forms come off, not the next morning.
• Hessian kept damp, plastic sheeting, or ponding on a slab. A hose passed over it twice a day is not curing.
• Hot, dry, windy conditions shorten nothing and lengthen everything — they are the case for keeping forms on longer, not for taking them off sooner.

Concrete that dries out stops hydrating and never gets the strength back later. The 28-day cube in the tank will still pass. The column on site will not be the concrete that cube describes.

A striking order that does not surprise anybody

1. Column and wall sides, and beam side forms — earliest, they carry nothing.
2. Slab soffit, leaving the props standing. The forms come out from between the props rather than the props coming down with the forms.
3. Beam soffit, props still standing.
4. Slab props, then beam props, working from mid-span outwards so the member takes up its own weight gradually instead of dropping onto its supports.
5. Back-props reinstated where anything is going to be cast above.

Next

• Decking a floor
• Cube tests
• Cover, grades and mixes
• Span/depth ratios
• Slab design
• Beam design
• Concrete and block quantities
• Approval drawings
• Lap and anchorage lengths
• All worked examples