Water reducers or GGBFS and cracking

[magnumv8]

We recently poured a 13000 sq. ft slab on grade during a very hot period of summer. We have always used a pure portland mix in our concrete and we were not told that the mix was being changed to include a ground granulated blast furnace slag and fly ash additive. we also used a sealer called ashford formula which was applied 24 hours after the concrete was placed. ( I mention this becuase the manufacter reccomended that we not cover the slab withj poly as it would interfere with his installation) All expansion joints were saw cut at about 18' to 20' squares / 1-2" deep. The concrete mix was supposed to be 4000 psi. and was placed at about a 5" slump. the reinforcment was a wwf. The floor has cracked severely in a spider web pattern.
My question is this: is it a normal practice to use these additives and if so shouldnt we have been made aware that changes were being made to allow for a 6 day wet cure as reccomended.

 

[ishvaaag]

Definitely, only if lenient specification and inspection is in place the concrete providers can and use to take such liberties without worrting. Where the specifications clearly indicate what can an can not be made respect the concrete mix and concrete placement and cure, it is far less frequent that they risk to introduce any change without making the authorizing/inspecting party aware. So check what the specifications were, and if no allowance was made for changes without previous consent, proceed as you think is correct.

 

[RiBeneke]

I think that to omit any curing method for 24 hours in hot weather is a poor recommendation for concrete slabs. Perhaps your sealer supplier should reconsider his recommendation and try to accomodate some curing method.

 

[steve111]

If a concrete mix is specified it must be supplied as specified. Testing of concrete delivered to site should also be specified and a requirement to verify. Fly ash is often substituted for cement to reduce heat of hydration re temperature shrinkage but should only be done in consultation with engineer of record. WWM only is effective as plastic/temperature reinforcing if it ends up in the middle of the slab, this requires constant inspection during the pour. Control Joints should be cut as soon as workers can access the floor slab and need to at least 1/4 of the slab depth, water cement ratio should be as low as possible to faciliate placement, 5" slump seems high. Plastic shrinkage/crazing cracking generally a result of rapid hydration, accelerated set, surface evapouration rate exceeding bleed water rise rate, and then tensile stresses in the surface layer as hardened surface cools... etc. Lots of things can go wrong and will with out constant inspection and involvement.

 

[kjde]

The time at which the cracks developed is more important. If it happened whwen the concrete was plastic, i.e. within 6 hours, it was plastic shrinkage cracking and was due to the high evaporation. You should have gone for a delayed refinishing and early wetting with a wet hessian cloth or covering with a plastic sheet.
If it was after the concrete hardened, your sealer did not work and the concrete lost so muc of water that it had a drying shrinkage.
Normally web type pattern is noticed in plastic shrinkage cracks and structurall y there is no problem. You can seal the cracks with a crack filler and stop worrying.
Next time take precautions of delayed finishing and early curing.

 

[Jim6758]

It is entirely appropriate to use fly ash in hot weather but you should have been advised. Hopefully the finishers were advised, but the cracking pattern you got leads me to believe that they were not. What was the time between the batching and the discharge of the concrete? In really hot weather, the old 90 minute rule doesn't work. The slower setting generally experienced with GGBF doesn't apply in temperatures greater than 85 Degrees. Twenty four hours is way to long to wait to apply sealer in hot weather and you don't have long to saw either.