Grey stains on sidewalk spalling

[Kenton H. Shepard, CMI]

The grey stains on this sidewalk were only on the areas that had some surface deterioration and they didn't completely cover those areas. An attempt at repair?

[Doug Edwards, CMI]

Stains appear to have been caused by some kind of liquid that sat on the concrete for some time. The pattern includes some etching around the edges with the dark colorations left. Ive seen where polymer sealers will stain concrete in a similar manner. Just wondering if someone used acid or another medium on the concrete prior to it fully curing. Always like trying figure out these kinds of curiosities.

[Kenton H. Shepard, CMI]

I think you're right , Doug. I'm wondering whether the liquid was an attempt to repair damage caused by problems created during the pour that resulted in early failure of the surface.
I couldn't tell whether it was a residue or a stain caused by a chemical reaction.

[rcooke]

Originally Posted by kshepard

The grey stains on this sidewalk were only on the areas that had some surface deterioration and they didn't completely cover those areas. An attempt at repair?

Could this have been poured in the winter and it got froze before the concrete chemical heat had got started .
Roy Cooke

[Kenton H. Shepard, CMI]

That's one of the possibilities Roy, probably a good one. At least for the spalling. Can't believe I didn't look at the houses next door.

[Marcel R. Cyr]

Kento, I thought you could check this out to try and eliminate possibilities.

http://www.poolhelp.com/Calcium_chloride.html

Hope this helps.

Marcel

[Marcel R. Cyr]

Kenton; A few more info's that might help.


Why does some concrete develop surface deterioration?
Surface deterioration is usually referred to as scaling. Scaling is a local flaking or peeling of the finished, hardened concrete surface, resulting primarily from multiple freeze-thaw cycles. The condition can be aggravated by the presence of deicing materials such as salt. Any product that is intended to melt ice and snow can contribute to scaling. Even products labeled “Safe for Concrete” can contribute to scaling. The probability that scaling may occur can be reduced or eliminated by using a durable concrete mix and following recommended concrete placing, finishing and curing/sealing practices. Ready Mixed Concrete Co. personnel are available to assist you in selecting the concrete mix and curing and sealing products for your project.



What causes discoloration of concrete?
There are different types and causes of concrete discoloration. Dark, shadowy areas may appear if the fresh concrete was placed over alternating wet and dry areas of the subgrade. Darker areas are typically located where the subgrade was too wet or actually muddy. Temporarily covering concrete with poly sheeting or construction blankets can also cause the hardened concrete to be darker in appearance. Another possible cause of dark, mottled areas is the uneven application of curing/sealing products. Curing/sealing products should be applied as uniformly as possible with a nozzle that will produce a fine spray or mist. There can also be differences in coloration between concrete slabs placed on different days. This is usually due to slight variations in temperature and weather conditions from day to day. The potential for discoloration can be greatly reduced by following recommended placing, finishing and curing practices


For more information from the National Ready Mixed Concrete Association.
Links and references to 'Concrete in Practice' publications ('CIPs') are to documents published by the National Ready Mixed Concrete Association, Silver Spring, Maryland, USA. These documents are provided courtesy of the NRMCA.

To order print versions of these CIPs, please e-mail publications@nrmca.org or call (301) 587-1400.

Marcel