Hairline Cracked Concrete Upheaval after 4-6 days. 2

[onealmond1]

We placed a 100' x 10' x 6" thick reinforced concrete pad over a 12" engineered well-graded granular based. After 4-6 days, we began noticing some hairline cracks in a radial direction. The cracks were no more than 3 inches across, and every single one of them measured 1/16" to 1/8" higher than the elevation of the pad (they kind of looked like mini volcanos). The average temperature throughout the week was around 85 deg F, no fluctuations more than 10 degrees (coastline of Africa). It did rain right before we began seeing cracks, but the pad was placed on a dry subgrade. I have also attached a picture for reference. This is the 5th concrete pad placed utilizing the same method of placement, vibing, and finishing.

What do you think could be the cause of this?

 

[phamENG]

Looks like it could be a large blister.

 

[lexpatrie]

I'd be tempted to consider this due to a high piece of large aggregate, but I'd like to see a more consistent pattern of those cracks across the surface of the concrete.

Beyond that, what can be ruled out?
Is the cast concrete being appropriately covered/cured with the burlap being wetted or similar measures? High temperature placement issues? Wasn't covered and hot and windy so top of slab lost a lot of moisture quickly?

I believe D cracking shows up much later, and it's too early for a sulfate reaction and doesn't look like ASR, either. Is the top surface critical for impermeability or is it generally in compression? Is it monolithic in the 100' direction or are there some cast/control/joints that way? It doesn't really "smack" of curling, either.

Some sort of random inclusion in the aggregate/mix? It doesn't look like something from an animal or vandalism.

 

[onealmond1]

The cracks were consistent, about 15 total mini volcano looking crack, but only on one side of the pad.

The cast concrete was wet and covered with vinyl for 24 hours after placement. It was however very hot and somewhat windy at the time and did dry very quickly.

Definitely not sulfate attack. Control joints are every 10 feet. No signs of curling

It is entirely possible that our aggregate was not clean or did not mix properly; however, we have placed 4 pads before this (same method), and this is the first time this has happened.

I don't believe it is a blister because it looks like the cracks run a lot deeper than if there was water trapped below the surface.

 

[rapt]

I was thinking plastic shrinkage cracking from the pattern, but have never see the upheaval like that. Was an extra thin layer added during the finishing to get the correct surface level?

 

[onealmond1]

Not a full extra layer, but what it could be is some low spots had concrete thrown on late to finish elevation.

 

[TigerGuy]

Refer to thread194-520512 in Civil Engineering.

You're not supposed to post the same thread in different forums.