What Precast Concrete Actually Buys

The floor arrives finished

The thing that changes a programme is not the speed of the frame going up. It is that each floor is a working platform the day it lands. Hollowcore units are craned in, grouted, and the trades follow the crane up the building instead of waiting for the whole frame. On a four storey block that means the first floor apartments are being first fixed while the top floor is still being erected, which is a fortnight or more that simply does not exist in a wet trade programme.

That compounds with the weather, which in this country is the real schedule risk. A precast erection sequence has a handful of days that genuinely stop for wind, and very little else. In situ concrete has pours that stop for rain, curing that slows in the cold, and a formwork cycle that has to be resourced whether the weather cooperates or not.

Repetition is what makes it cheap

The saving is in repetition rather than in the material. A student scheme or an apartment block is the same room a few hundred times, and crosswall precast puts the structure on the party walls that were going to be there anyway for acoustics and fire. One panel type per condition, one connection per junction, the detail resolved once and then not touched. No downstand in the bedroom, no column in the corner of the living room, and no chasing a wet trade around a repeated layout.

Where a building has no repetition, that logic reverses. A one off house with a different wall arrangement on each level is a set of bespoke panels, each one drawn, cast and craned as a special. At that point in situ or steel is usually cheaper and always more forgiving of the changes that arrive halfway through.

The catch is that decisions move earlier

This is the part worth reading twice. Precast moves every decision to the front of the job. Once a panel is cast it is a finished object: the opening for the service riser is where it is, the socket box is where it is, and the connection is where it is. A change on site becomes a cut, a patch or a new unit on a lorry, and none of those are cheap.

So the building services have to be coordinated before the first unit is cast, not after. That is the single largest source of grief on precast jobs, and it is also why we model the frame with the services in it rather than drawing them separately. A clash found on screen is free. The same clash found on site is a core drill through a structural panel and a phone call about whether it is still a structural panel.

How to decide

Ask two questions before anything else. Is the building repetitive, and can the design be frozen early enough to cast to it. Two yeses and precast will almost certainly be quicker and often cheaper. One no and it is worth pricing both. Two nos and you should be building it another way, and any engineer who tells you otherwise is selling you something.

What this costs

Not a fee in this case, but the frame itself, and the comparison is where most of the arguments happen. A rate per square metre for precast set against a rate for in situ will usually flatter the in situ, because it prices the concrete and not the programme. What belongs in the comparison is the crane time, the preliminaries for the weeks the two options differ by, the follower trades standing on a dry floor instead of waiting for one, and the haulage from whichever yard is casting it.

What moves the precast number is repetition, how early the design can be frozen and the distance from the yard. What moves the in situ number is weather, labour and how long the site stays open. Price them against the same programme rather than the same square metre and the answer is usually not close.

We design in precast, in situ concrete, steel and the hybrids that most buildings actually end up being, so we have no stake in which of them your job turns out to want.