Q.: I would like to know how contractors cope with concrete slabs poured in the summer and sealed with a sealer. I've used two different types of sealer but in both cases I got salt scaling the following spring where salt had been thrown around. I know it is hard to cope with salt. I know my problem has not been the concrete because I used a 6-bag mix.

A.: You didn't mention whether or not the concrete was air-entrained. If it didn't contain entrained air, the application of salt combined with the exposure to freezing and thawing would cause the concrete to scale. This would be true regardless of the cement content, sealing, time of pouring or any degree of care exercised.

To produce air-entrained concrete, an air-entraining agent is introduced into the concrete at the time it is batched. As the concrete is mixed, microscopic air bubbles are formed in the cement paste. They provide space for water to expand within the concrete when it freeaes. Without these air bubbles, the expansion of water during freezing causes the surface concrete to loosen and disintegrate.

We are sure you will find that all paving work done by the state or any municipality in your area requires air entrainment. Usually the air content is specified at 4 to 7 percent or 5 to 8 percent. Any ready-mixed concrete producer will be familiar with entrained air. When you order concrete for any outdoor flatwork you will want to make certain that the supplier understands that you want air-entrained concrete.

With adequate air content it is still necessary to use good construction practices. An extremely wet mix, finished too early or too much, may greatly reduce the effectiveness of the air entrainment. Concrete placed late in the year, not cured nor subsequently allowed to dry out, may still be vulnerable to some scaling. A fair guideline is to be sure that the concrete is not exposed to salt and freezing and thawing for at least 30 to 60 days after finishing.

The value of sealers is questionable for concrete properly constructed, cured and dried. Boiled linseed oil has been used to protect pavement placed late in the year with some good results. Many other sealers may have a reverse effect. They serve well as curing compounds but may then serve badly by preventing the concrete from drying out after it has cured. When using boiled linseed oil it is necessary to take the precaution of allowing the concrete to dry out after curing, before applying the linseed oil.

If you use the 6-bag mix that you have been using but with the right amount of entrained air and the construction practices that are outlined here, you really should be able to cope with the sealing problem.