cracks on new slab?

Hi all

Currently building a home in Adelaide, the slab has been laid since mid October, and I've now noticed there are crack lines showing across the width. Is this normal or cause for concern?

I will approach the builder but would like to know if this is something normal.

Hope you can shed some light.

Not sure …..But I would think better it cracks now than when you have tiles and walls attached to it.

I have what I call the “Fault Line from Hell” in my slab. Started in the loo and has gone straight across the house. Cracking every tile along its way. NICE!!!

But is nothing for me to be concerned about. I think you’ll find its normal, but let’s see what the pros say.

Good luck

Thanks Michelle, for some reason I am expecting the builder to say its completely normal and try to shrug it off.

Now that the crack is there, does anyone know if anything can be done to rectify it, or will it have no effect on the structure in the future?

Heres a photo/

Cracking in a slab is not abnormal & in most cases it's cosmetic & not structural.
All slabs will develop hairline cracking to some extent, with longer slabs more susceptable to larger cracking than shorter slabs.
The size of the top mesh is possibly one of the bigger factors in reducing shrinkage cracks- hence longer slabs, where their is a greater amount of shrinkage, are designed with a heavier mesh.
Having said that there is a difference between slab shrinkage cracking whilst drying & slab failure.
In commercial situations a 'moisture content' test is often done (requested) on the slab prior to laying ceramic tiles to ensure the tiles will 'stick' & also to give an indication that the slab has 'dried out' & shrinkage would have stopped. (no cracked tiles)
This is seldom done in residential work as it is assumed that with the relative smallness of a house slab that it will dry out sufficiently over the 3 or 4 months of construction.
More often than not it's more a case of 'She'll be right mate' & this is not an aspertion on floor tilers but possibly reflects the variance in price between commercial & residential pricing.

I seem to have drifted from your question thomson, to thinking about michelles mention of tiles cracking - if you have any specific questions ask again. It's late in the week & it's been a long week.!

Well I spoke to the builder briefly and as expected they said it is normal due to shrinkage.

Ausdesign thanks for your insight, you were indeed correct about the shrinkage and yes, the slab is quite long and narrow. From my photo , whats you opinion of the cracking? Nothing to be concerned of or should I seek further independant advice. Its a bit concerning for the uninitiated to building to see a new slab with cracks.

NB. Where the cracks appear is appox in the location of where a bedroom will be, so the flooring will be carpeted not tiled.

Thanks for your help

Sorry thompson I won't give an opinion on the crack other than from a laymans point of view it doesn't look bad - I want to enjoy my Christmas.
The slab has only been down for around 2 weeks so I would be surprised if it was nothing more than a shrinkage crack but if your concerned mark the slab across the crack with an angle grinder (for future reference), put a tape measure down & take a close up with your digital. Check it again in a month to see if there is any increase in the width.
Document everything with the date & time including your previous & any future discussions with your builder.
As an aside, all home builders should be in the habit of documenting all conversations & problems or concerns as they arise during the building process. It's probably a bit overboard to do a James Bond with a secret microphone but at minimum have a third party on hand to verify conversations.
If the thing hits the fan it could be invaluable.

Okat thanks for the advice once again.

All slabs cracks. No one has invented a slab yet that doesn't. Cracks of 5mm are acceptable. I know this sounds large but the concrete can't crack anymore than the steel in the slab will allow, hence 5mm is acceptable. The slab is the least of your worries. Your problems will begin when everything else starts to happen. If you have any problems with your builder go to www.shonkybuilders.com. We have access to expert legal advice.

www.shonkybuilders.com

Just came back across this post. My apologies for bringing it up again to those not interested but there should be clarification for any future readers.

Shrinkage cracking in slabs will occur in most cases to a certain extent.
The cause can be from the amount of water in the mix when it is delivered on site, the time taken to 'finish' the job [the dryness of the mix still being worked] & the temperatures [heat] during the initial hours of drying. The main cause is excessive water in the mix.
Shrinkage cracking greater than 1mm should be unacceptable.
Shrinkage cracking will cease when the slab has cured.

Movement cracking is a different kettle of fish & can occur over the lifetime of the home, with the amount & size of cracking in proportion to the type of & amount of clay in the soil.
As the clay takes up water it swells & as it dries out it shrinks back again. This movement is what causes the cracking and is why the higher the reactiveness of the soil the 'stronger' [& more expensive] the design of the foundation is.