Q.: We put in a sidewalk for the city in November. By the following summer it had scaled. The city ordered tests made of the air content and found it to be low. Then it accused us of using non-air-entrained concrete. The concrete was air-entrained and it was sealed with a cure-and-seal agent. However, the city salted the sidewalk heavily during the winter in spite of our warnings to them not to do it. Is there a reliable way to measure air content on hardened concrete?

A.: ASTM C 457, "Standard Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete," gives reliable results.

In case the air content turns out to be low, you should know that there are a number of possible causes for the concrete in a sidewalk having less air than specified. The concrete may have been improperly batched or mixed; or it may have lost its original air content by improper handling, by overwetting or by working too early on the job. The surface of flatwork is particularly susceptible to loss of air by overwetting or by working too early.

Be that as it may, any flatwork should be given at least 28 days to dry (after curing) before any deicer is applied. ACI Committee 302 recommends 3 months drying after curing. The Portland Cement Association has recommended that liquid membrane curing agents not be used on flatwork placed in the late fall. They suggest that the flatwork be cured instead with a waterproof covering such as polyethylene.