The cement surface of our new concrete pavement and building entrance is coming off. The contractor told us we caused it by melting ice with salt. We do not agree. We used sand and cinders, no salt. What is happening to our concrete?
The surfaces you showed me in photographs and the physical sample you sent demonstrated a classic example of what the American Concrete Institute calls "scaling." I define scaling as the local flaking or disintegration of the surface strata or top thin section of hardened concrete. The contractor you spoke to was correct in saying deicing salt can scale concrete. But, in my opinion, I do not think salt attack is the cause of your particular surface scaling.
I am not going to make you wait for my opinion. I think the scaling is due to freeze-thaw attack of an insufficiently air entrained surface. I further believe that the low air entrainment was caused by poor finishing techniques on the part of the contractor's finishers and not because of a defective concrete mix design. Having said that. you deserve an explanation.
Guidelines given to us by the American Concrete Institute (ACI) (www.aci-int.org) recommend that any exterior concrete that is to be placed in regions of the country with severe climate should be 4,000 p.s.i. and contain 6% +/- 1½ air entrainment to resist damage from freeze-thaw cycling. As seen in the U.S. Climate Zone Map shown to the right, a good portion of the United States lies within the "severe climate range." In this climate range, there are specific guidelines for the placement and finishing of this type of concrete which, when followed, can minimize the probability of surface scaling. The basic guidelines call for screeding, broom finishing, curing and sealing the concrete. All to often, however, these guidelines are compromised by common mistakes in finishing or when curing or sealing steps are omitted.
Our experience tells us that the most common causes of concrete surface scaling are:
- Floating in bleed water during the early placement stages
- Hard troweling of air-entrained concrete
- Aggressive deicing salt attack, especially during the first winter
- Low air content in the concrete mix design
- Concrete maturity issues, i.e., late fall placement subjected to freezing and thawing before the concrete develops sufficient strength
- Improper or insufficient curing
- Insufficient or omission of protective sealer
We suspect that improper finishing of the surface is how your concrete became vulnerable to scaling. In much the same way that air and water can be squeezed out of a damp sponge, protective air entrainment at the surface of the concrete can be squeezed out of the concrete cement paste by poor finishing practices. This is what probably happened to your concrete. It appears that the concrete was given a hard trowel finish. While this is a common practice on interior floors, the nature of hard troweling will remove much of the protective air entrainment at the surface of the concrete.
Air entrainment provides millions of tiny pressure relief voids that accommodate the expansion pressure of freezing water in concrete. Their presence is critical in exterior concrete. Without those millions of tiny voids, the surface of your concrete suffered physical damage, damage that we now know as scaling from freezing and thawing. Deicing salts would have only made this condition worse, if they had been used.
In addition to applying proper finishing techniques, there are product solutions that will minimize the effects of exterior exposure. L&M offers many curing products and sealers, such as Dress & Seal and Lumiseal Plus that effectively protect concrete finding itself in severe climate zones.
One of our more popular treatments is Aquapel. This product provides the best protection against scaling. Aquapel is a penetrating water repellent and salt resistant treatment for new and existing concrete. It provides long-term, invisible protection of concrete from salt attack and freeze-thaw cycling.
Thanks for asking!