Q.: Before making the final payment on a bridge deck topping we want to be sure it has an adequate air content. Some of the quality assurance measurements of air content made on the job were unreliable, so we took some cores and had some linear traverse measurements made.

The results seem to show that the air content met specifications, but we wonder if we can count on this method? Does it agree with measurements made with an ordinary pressure air meter? We think probably the two methods agree for regular concrete, but this topping was made of low-slump dense concrete.

A.: Based on laboratory studies, in which the mixing is admittedly slightly different from field mixing, the agreement seems to be pretty good for air contents of low-slump dense concrete measured by the two methods. There are data given in the article "Air Contents and Air-Void Characteristics in Low-Slump Dense Concretes," by David Whiting, published in the ACI Journal, September-October 1985, page 716. He reports that some amount of entrapped air voids is to be expected; sometimes up to about 2 percent of the volume of the concrete can be expected. But "if the concrete is properly mixed and consolidated, characteristics of the air-void system, generally regarded as indicative of ultimate durability of the concrete, should be within acceptable limits."