Hairline Cracked Concrete Upheaval after 4-6 days 1

[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?

 

[1503-44]

Too much water in the mix. It shrinks when it dries out.
Or reinforcement issues.
What is your reinforcement arrangement?
Where are your shrinkage control joints?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[onealmond1]

#4 bar 16" on center, 4" slump, control joints every 10 feet.

 

[Compositepro]

Something is pushing pushing up in those spots. Probably air or water compressed by rain water flooding the granular fill.

 

[1503-44]

4" slump may be on the too dry side.
I assumed the volcanos were from excess water. Were they dry, like what air bubbles might leave if they rose and burst?
It was not covered with wet burlap or the surface kept wet before it rained, right?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleWheels]

It is likely one of the earlier responses has nailed the actual cause but, for my own amusement, here are some alternative guesses.

Generally sulphate attack takes much longer, so unlikely.

Does the granular base contain cobbles within the well-graded aggregates. They can cause hard spots if the granular base is not well compacted and settles.

Was the concrete poured in windy, dry conditions? If so, you may have plastic shrinkage cracks. Crack width can then be widened by insufficient moisture while curing (differential drying).

 

[onealmond1]

About two hours after we did our last broom finish, we wet the concrete and covered it with vinyl. The next day, or 24 hours after, we took off the vinyl and stripped forms. No cracks formed until 3-4 days later, around the time it rained.

Can't really give a good answer about the air bubbles

Concrete was poured in a relatively humid environment, no wind

I don't think it is a settling issue or a curling issue due to the size of the cracks and the distribution.

 

[Compositepro]

Littlewheels' brainstorming made me think of another possibility. When granular materials get really wet or saturated they tend to compact. The cracks do not appear to have had water in them. There could have been compaction and settlement due to water in the areas where there are no cracks.

 

[jayrod12]

How did they support the reinforcing during the pouring?

 

[onealmond1]

We used dobie blocks

Compositepro, could you elaborate your point about compaction and settlement, I don't quite understand

 

[Compositepro]

Water added to dry sand or gravel will lubricate and agitate angular particles so that they can reposition into a closer packed structure, just as vibratory compaction will. Rain water flooding under your poured slab from the edges will flow in channels. There will be dry spots and wet spots. If the wet areas settle the effect will be the same as if the dry spots expanded and lifted the slab. Voids under slabs are not uncommon, but they usually take a long time to form, so the concrete has already developed full strength. Your concrete obviously was not very strong when it cracked. And cure shrinkage would not lift the concrete.

 

[1503-44]

Subgrade: "12" engineered well-graded granular based". My original assumption was that there was some control that occurred during subgrade placement, but other comments made me think further along that line. Your description is actually quite subjective and in engineering terms means little. It seems like you should have mentioned what % Procter density was achieved. That would go a long way in determining if the fault laid with subgrade settlement, or a problem with the concrete mixture.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[onealmond1]

Thanks copositepro. So the cracks occurred only a couple days after placing, so are you saying the voids formed quickly? Or may have been already there. I would also like to point out that some of the sand we mixed into the concrete was wet and may not have been fully mixed into the cement and water.

The subgrade details are as follows: 64% gravel, 35% Sand, and 1% fines, PI is 17, MDD is 2218 kg/m3, OMC is 9%, we achieved 95% compaction using a vibratory roller.

 

[1503-44]

some of the sand we mixed into the concrete was wet and may not have been fully mixed into the cement and water.

Would have been very helpful if you mentioned that before.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[onealmond1]

But it was also wet in the other placements tha we have made, are there were no cracks because of it

 

[1503-44]

It's all in the mix. It must be uniform. You cannot have any unmixed regions. Concrete is only as good as the weakest spot.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[hokie66]

The 'upheaval' has the appearance of the slab being supported there and allowed to settle elsewhere. Could those points be the 'dobie block' locations, and the cracking be plastic settlement caused?

 

[swazimatt]

either upheaval in the centre or subsidence at the edges. You have measured the "volcano" as being higher than the rest of the slab, are you able to confirm the edges of the slab are still at the design level or have subsided?
does this slab have slabs next to it? If not the post about self compacting sands is quite plausible as the side soils would have become saturated first by the rain and subside/compact more than the centre

 

[1503-44]

There was no control of the mix.
Rip it out and replace.
End of story.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."