Slab concrete not curing

[jgailla]

We have a concrete problem and are getting a materials testing company to evaluate the issue.
I would like to get some opinions on what is causing the problem.
The concrete was poured about 4 weeks ago and appears to have not cured yet.
The attached picture shows the area, which is the floor of a very small amphitheatre.
The concrete breaks came back fine, but the surface is not durable. It looks like the very top layer of the concrete has not hydrated properly. The surface can be scratched with a key, but not deeply. There are also several bench pads poured the same day which have the same problem.
The project is in north Florida, and the weather was rainy and hot.
Any ideas? I've seen this a couple of other times, but for some reason I can't remember what causes it.
Thanks.

 

[ishvaaag]

May the cement have been washed in the rain?

 

[conceng]

Without being able to poke at it myself and based on your weather comment and the test cylinder performance, I would say the two most likely causes, or the combined effect of both are;

1. The slab got rained on or blessed and the water was finished into the surface creating an excessively high water/cementitious ratio, weak surface.

2. Proper curing was not done. If the hot weather created rapid drying conditions, the loss of moisture from the surface may starve the surface paste of water required for strength development. Typically you may also see craze cracking or plastic shrinkage cracking.

 

[diazmeyer]

I agree.
However, in a few cases Concrete segregation also produces a weak and scaly top surface layer, which scratches easily revealing a powdery surface underneath which is about 3 mm thick.

 

[RHTPE]

What was the air entrainment?

Was a superplasticizer used and was it dosed at the site? If dosed at the site, how many revolutions and at what speed after dosing to incorporate it into the mix?


Ralph
Structures Consulting
Northeast USA

 

[Ron]

jgailla...it looks like a bleed-water/finish issue, compounded by spraying with a "curing compound". Take a couple of cores and get a petrographic examination done.

The mix design could be a contributor as well. In your area, the ready mix suppliers tend to use a fair amount of slag cement. This retards the hydration. Further, some curing compounds have enough organic material in them that if sprayed on too early, can retard the strength gain of the surface.

 

[jgailla]

Thanks to all for the responses.

RHTPE,
There were no superplasticizers used, as far as I know.

Ron,
When I first looked at it, I thought the concrete was bad, like it had sugar in the mix. This would agree with your comment about the organic material in the curing compound.
As I thought about it, I could also see the finishers getting on it too early before the slab popped.

There are many other concrete areas on the project, and these are the only areas that have problems. The same superintendent was on site for all the pours, and he seems to know what he's doing.

We are getting a petrographic examination. When we get the report I'll post the findings.

 

[ishvaaag]

After looking again the figure, it seems cement was somekind not only washed but "floated up" (la "lechada" in spanish) by the rain so just under the surface there must be less cement than required, resulting in disgregation.

 

[dik]

It looks like overtrowelling which has brought too much bleed water up. Any chance they could have accidentally used a retarder in lieu of a curing compound?

Dik

 

[Ron]

jgailla...not sure who is doing your petrography, but make sure they get a copy of the mix design. dik brings up a good point, even though retarders are not often used in this area, unless it is a water reducing/retarding admixture for hot weather placement (could have been...depending on placement day last month).

Also, in looking at the sawcut joints, one in the background implies a flatness issue, pronounced enough to be visible...you might want to check this.

I noticed also that your slab "lightening" has some relevance to the chalklines. Chalk dust prevent "curing compound" from sticking?

 

[RHTPE]

Just sharing my personal experiences ... A high-range super-plasticizer (or even a mid-range), dosed at the site, has produced a "light weight foam cement paste" that would refuse to integrate with the bulk of the concrete. The amount of this "foam" seemed to be dependent on the intensity of the mixing after adding it to the truck (i.e. extremely fast revolutions would reduce in a higher volume of foam). While striking off the placement, we would skim off the bulk of the foam, but some always seemed to remain and would get finished into portions of the top surface, leaving a less dense surface than areas with little foam.

Another consideration ... Curing compounds and form release agents are often applied using similar sprayers. Any chance that a) the wrong sprayer was grabbed by the person who applied the curing compound, or b) the sprayer used had previously contained a form release agent?

Also - looks like a crack across the slab at the lower left of your picture. Why did that occur?


Ralph
Structures Consulting
Northeast USA

 

[Ron]

jgailla...what happened with this one?

 

[jgailla]

Ron,
I never found out. A local geotech firm was called to look at the slab.
It was obvious that the concrete was unacceptable, we just didn't have a technical explanation of precisely why it was unacceptable.
The contractor was directed to demo the slab and repour. I was a little disappointed, since I would have liked to know.

 

[Ron]

jgailla....thanks. That's unfortunate, since that kind of info needs to be shared so it reduces the likelihood of a repeat of the issues.

Call the Geotech firm and ask them the results....they'll likely tell you.