Multi-span concrete slabs …Expert speaks .

Answer by Patrick Barry:

There’s a ton of ways of doing this that vary between contractors, countries, local codes (eg earthquake considerations) and speed vs expense of construction.

1) The simplest:
Cast the beams and slab of the entire floor at once:

+ Good for checking, last minute alterations and easy to do a concrete boom pump.
– A lot of labour required and not exactly quick, if the formwork’s not sealed properly you might have cracks or honeycombing. It’s not impossible to cast level changes (eg if you want the bathroom floor a step lower) but it can be a pain. If the spacers get displaced during concreting the steel might move out of position vertically (not good, but something that applies to nearly all methods)

Since the formwork is on the bottom (tension for simple slabs), some designs use permeant formwork. The concrete is cast on top and the two make a composite slab together. The trick is to make sure there’s enough shear connectors (often steel studs) to stop them separating under strain.
Some variants use steel shuttering:

Some use precast panels that lock together (often with some small extra rebar used on top to bind them strongly, and tie them into the beams.

Design of these panels can get very complex, with polystytrene blocks included to reduce weight with minimal loss of strength.

3) Beyond that, fully precast slabs can be used, although they typically are finished with a very thin layer of slurry on the top to ensure smoothness.

I’m unaware of any of these slabs types that are truly continuous over beams or supports, so we’re entering the realm of cheating the question. Since the concrete is already at full strength, construction can proceed as fast as the cranes can lift:

And then beyond that, at the current top limits of pre-manufactured slabs, you have finished (tiled!) floors as used here. They’re not concrete though, using steel truss for the reduced weight.

How are multispan concrete slabs constructed?


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