Cracks in slab

[Cavester]

Hi guys,

I'm currently having an extension built and the concrete slab has been down about 6 weeks now. I've only just noticed a few cracks and wanted to check with some of the experts if they felt they were normal or not.

Slab in the back room is approx 12.5 X 5.7m and should be 150mm deep, although I never seen the builder lay it.

Crack goesb across almost the full width. It's currently only about 0.3?mm wide but not noticed it previously.

Is it normal?

Floor still has 100mm pir, ufh and 65mm screed to go on yet before being tiled.

Thanks.

 

[^woody^]

That size of slab should have one or two movement joints.

 

[Cavester]

Thanks woody, it hasn't. Is that likely to cause an issue and is there anything that can be done about it now?

 

[tony1851]

12.5m is large for a single unreinforced slab; all cement-based products shrink to an extent. Concrete can be obtained with polypropylene fibres which reduce cracking; alternatively, anti-crack mesh could have been used.
Maybe leave the floor a little while before tiling.

 

[^woody^]

When a slab cracks, the sections can move independently - up or down in this case.

This movement can cause reflective cracking in the screed above or hollowing and then screed cracking due to that.

If the ground below is sound, and sub base well designed and placed then to a large extent the proposed insulation is thick enough to prevent, or reduce the risk of problems - but the risk remains and will be influenced by several factors.

I can't believe that the designer has specified such a slab and building control approved it. I would expect a good builder to have queried this too.

Anyway, I would get advice from the designer ( and check his insurance), as perhaps a thicker reinforced screed may be advisable, or if the base is considered sound, just extra care with the insulation placement.

Another risk is the DPM splitting if the slab moves too much. Again that is largely down to the quality of the work.

 

[Cavester]

"I can't believe that the designer has specified such a slab and building control approved it. I would expect a good builder to have queried this too."

Tbh, the architect, builder and building control have all been pretty poor with me having to question and chase constantly.