Just Laid Concrete Slab - Now it has cracks

[RogGriffin]

Hi,
This is the first time I have laid such a big concrete slab (4mx5.7mx0.15m) and also the first time I have used ready mixed concrete.
The Concrete we had delivered was mixed with Fibres (mix C30), so as to stop cracks, but cracks are still occurring.
The concrete has now been curing for 6 hours.
Is there anything I can do to reduce/stop new cracks occurring, or is this just part of the drying process ?
What is the best way to fill such cracks ?
Many Thanks

 

[noseall]

are you sure the "cracks" you're seeing aren't just a bit of surface shrinkage?

 

[RogGriffin]

Thanks for the reply, but how can I tell the difference between cracks and surface shrinkage ?

 

[noseall]

if concrete is laid on a hot day it is advisable to spray a mist of water over or lay wet hessian sacking until the concrete has cured.
is the concrete indoor or out ?
is it float finish or tamped?
are you laying a screed on top?

 

[RogGriffin]

HI,
The concrete was laid outside. Temperature today has been about 10-12 centigrade.
It is tamped finish.
No screed is planned to be put on top.
It is intended as a garage base for a sectional concrete garage.

I have now covered the concrete with a tarpoulin (sp ?) , but sparyed a mist of water over the top befor elaying the tarp down.

Thanks in advance for any advice on this !
Cheers

 

[noseall]

if it were a trowel finish, then you can still "work the fat" hours after the concrete has been laid. in fact, armed with a power float, you have to wait several hours to be able to use the machine on the concrete surface.
but im afraid if its tamped then you're stuck with how it dries.
you could always apply a tough grade floor levelling compound later on.

 

[RogGriffin]

OK, Thanks,

Could I use a sand/cement mix to fill the cracks in a few days time ?

If so what sort of mix would you recommend ?

Thanks again

 

[noseall]

to be honest, if they're just shrinkage cracks and are superficial, i wouldn't worry too mutch, particularly as the slab will be covered anyway.
i cant understand why you tamped the concrete instead of floating it as this will be your finished surface? its a pig to sweep a floor thats been tamped!

 

[JohnD]

I was wondering, if the mix was a bit wet, could it be water and cement "fat" lying on the surface that has cracked? If so it will sadly be dusty for ever unless you put some kind of coating or screed on it later.

If it was too big to float I think I would have given it a brushed finish.

Try to keep it damp for at least a week, preferably two. it really will gain strength in all that time.

 

[Static]

How wide are the cracks?

 

[RogGriffin]

The widest part of the cracks is about 1/8th of an inch.

 

[noseall]

how far down do the cracks permeate?

 

[RogGriffin]

Not sure, but probably no more than 1 inch.

 

[noseall]

the cracks are in no way going to affect the structural integrity of a 150mm thick slab.
if you're worried about the cosmetic appearance then mix up some sharp sand and cement and brush it in.
if it were me though id leave it well alone!

 

[RogGriffin]

OK, Thanks. That has put my mind at rest...

Thanks to everyone for your replies !

Cheers