prestessed slabs

mark2003

Hi there,
We are trying to decide on what type of concrete slabs to use on the first floor.Has any one used the pretressed slabs.they say these slabs do not need a structual screed on top,the insulation can go straight on top.the only concern i heard about was that they curve on top.has any one ideas.
Thanks

muffler

To give you a bit of reading to start off with there was some discussion in this thread

Mellor

mark2003 wrote: »

Hi there,
We are trying to decide on what type of concrete slabs to use on the first floor.Has any one used the pretressed slabs.they say these slabs do not need a structual screed on top,the insulation can go straight on top.the only concern i heard about was that they curve on top.has any one ideas.
Thanks

The curve is caused by the pre-stressing. These will straightenout under the load of walls, screed and furniture.

brendansmith

This curving business is abit worrying alright, what about post-tensioned?

Mellor

It really isn't a worry. As I said, when laid, the slab is flat.
The curve is there becuse of the tension, which is negated by the self weight.

What is post-tensioned?

brendansmith

I would have thought that as a first floor slab that if its curving one way or the other that you are in trouble seeing as how the whole theory of prestressing the slab in a factory is to use the compressive strength of concrete against itself in a lateral position due to the fact that the steel is constantly trying to return to its original state, and if its bending its not doing what its supposed to do.

Post tensioning is where the slab is cast with lubrucated steel bars in and when the slab is set they attach plates and tension the bars to give the same effect as prestressing.

Mellor

brendansmith wrote: »

I would have thought that as a first floor slab that if its curving one way or the other that you are in trouble seeing as how the whole theory of prestressing the slab in a factory is to use the compressive strength of concrete against itself in a lateral position due to the fact that the steel is constantly trying to return to its original state, and if its bending its not doing what its supposed to do.

Post tensioning is where the slab is cast with lubrucated steel bars in and when the slab is set they attach plates and tension the bars to give the same effect as prestressing.

I think you are misunderstanding the idea of the slab bending. Its a tiny amount. This is completely normal.

I'm not overly familiar with post-tensioned, but its is a form of pre-stressed (the more common form is correctly called pre-tensioned). And would likely bend too if the stressed load was high enough.

A regular slab will already have some stressed in it due to the self weight, basically a downward load.
A pre-stressed slab is stressed due to the cables. These stresses are opposite to the stressed weight. And negate the self load (even surpass it). Bascially an upward force. Bare in mind this force is upwards because the tension cables are in the lower section of the slab. Essentially pulling the the two lower corners. This creates a slight camber or curve. However, if the slab were inverted after casting, the cables now direct the load downwards, so that the load is acting in the same direction as the self weight, and this huge load causes the slab to break. It basically rips its self assunder.

However, this camber is very small. In a domestic situation. It stress load is quite low and could be negated by the self load. Which would result in a flat slab. If it was a higher load due to bigger span or higher dead loads, the there would be a camber until these dead load are in place. These would be the screed and internal concrete walls. Once in place the slab would be flat.