Surface scaling in very small areas on concrete slab 2

[sitepro]

Recently we poured approximately 12,000 sf of slab. We used standard 3000 psi plant mix with 2% calcium added in pellet form (looks like styrofoam balls) in bags at the site. Therye was no monitoring of the temperature of the mix nor of the mix time for the calcium. The next morning when we cut control joints there was some places where the concrete flaked under the wheels of the saw ( approx 40 LF). Approximately two weeks later we noticed that there were several spots on the slab that were pitted about a quater of an inch square and flaked out to about two inches square. The problem areas accounted for about 1/10th of the slab or less.

Quality control from the concrete company had the suppliers engineer visit the site. His opinion was that the slab had frozen and that this was surface scaling and had nothing to do with the mix.

My argument was that I believed that some of the calcium balls had not totally disolved and that the pits were caused by heat from the calcium after placement. It was three days after pouring that the slab could have frozen and four days until the first rain.

Is it reasonably possible that the calcium not being completely dissolved could have caused this.
Thanks for your time
sitepro

 

[Ron]

It is possible that the calcium chloride pellet did not completely dissolve and could contribute to this problem; however, your problem is more likely to have been caused by a finishing technique error. Check some of the small spalls or scales and see if there is a residue from the calcium chloride pellets at the apex of the spall. This would be indicative of lack of diffusion of the pellets.

If they did contribute to the problem, it was not caused by heat generation from the calcium chloride but more likely a physical separation that imbibed more water than other locations which got trapped during the finishing.

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[JAE]

So, Ron, you think they finished (trowelled) the slab prior to bleed water coming up? Trapping moisture just below the surface and causing spalls with a huge w/c ratio?

 

[dirtsqueezer]

sitepro,

I think these are all good theories, including yours.

From this point, given the flimsy explanation of the concrete company, and the lack of proper inspection (otherwise you'd have temperature records), there are two options I see:

I'm guessing you are more worried about the structural aspect? You could always go back and test strength using an impact hammer. My testing company has cleared up a few issues similiar to this when the concrete company's explanation does not wash. That, and it would also be be perfectly reasonable to core the affected areas and test for strength. I would just mention it to the engineer. What he says usually goes. Either way, it's definitely better to worring about it now than paying for it later.

Best of luck!

 

[Ron]

JAE...you got it. That's the likely culprit.

 

[clayR]

JAE and Ron,
Is it possible that the evaporation rate exceeded the available water as a result of the calcium chloride?
Calcium Chloride will reduce bleed by decreasing the initail set, at least this has been my experience with cement pastes....your thoughts?

 

[dirtdoctor]

Two points

1)"pitted about 1/4 of an inch" - every supplier around these parts has a disclaimer about calcium stating that if not properly mixed, calcium 'balls' will develop. These are commonly seen on the surface as shallow pits. As a general rule, for every 1% of calcium used, mix the truck for 3 minutes (obviously this has a lot to do with truck blades, mixing speed, size of load, etc.). And, if your finishers insist on using calcium as an accelerator, please specify the flakes, as they disperse much better than those pellets.

2)"concrete flaked off under the wheels of the saw" - can you say de-lamination. As JAE and Ron stated, the finishers got on it too early and trapped moisture AND AIR just below the surface. Weight of the saw wheels just got to be too much for thin layer of hardened paste at surface.

I'll bet you a large sum of cash that the mix was wet, high in air content (it's cheaper to produce an air entrained mix than a normal one), and burned out too soon. Sounds like a typical residential slab finisher to me - how did he/she get away with this on a 12000 sf slab??

 

[Ron]

clayR...evaporation exceeding bleed rate would typically result in plastic shrinkage cracking...short, discontinuous cracks usually occurring within a couple of hours of placement. sitepro's description did not include this premise.

dirtdoctor...I agree with your comments. That's what happens when we get residential finishers on commercial projects.