Concrete finish

[FinC]

Concrete that was poured and vibrated with a vibrating poker on a dry Friday had by Saturday shown bad patches where concrete surface seemed to have peeled very badly. The surface of the footpath was covered in a light dust which had brushed off the surface of the concrete. I am trying to determine if this laitance is the fault of the concrete supplier or the concrete finishing crew. The same crew had poured the previous sections of footpath with no problems with concrete from the same supplier. I have requested batch results from the supplier but am almost certain they will come back with no fault to the concrete supplier.
Has anyone any practical advice on the causes of laitance as I need to determine the source of the problem.
Thanks

 

[BigH]

My take on this would be the finishing crew. Perhaps, being Friday, they didn't stick around long enough to remove the bit of laitance that would come a bit later in the curing process (say within the hour or so). I had this so many times in India - you explain the need to remove the laitance but hardly ever was done - and you get the same peeling/dusting.

 

[Rjeffery]

could be that the surface dried out before it could hydrate... or the finishing crew used a lot of water before the concrete set up. both could [may] cause dusting and failure of the surface.

 

[Jason9999]

Occasionally a concrete finishing crew will "powder" the surface of a slab that is setting up slow or the aggregate is not pushed down enough. They want a creamy surface for the final finish normally this is worked to the surface with a magnesium trowel then they run a steel trowel over the surface for the finish. They can cheat and throw dry cement on the surface and work it in a little. Much less shoulder work. Yeilds a nice ceamy easy to finish surface. That surface is usually weaker and prone to pealing and spalling. Just an idea.

Jason

 

[FinC]

It turned out that the crew didn't stay as late as they originally said and that it did in fact rain that evening which caused the damage to the surface. Thanks for your input everyone.