Q.: It has been proposed that we try to provide good protection for the rebars in our city bridge decks by covering the decks with concrete toppings that have low permeability. Air-entrained mixes with slumps of 1 inch, made at very low water-cement ratios, have been suggested. We've had some trial mixes made at a water-cement ratio of 0.33 but we can't seem to get any decent amount of air entrained in them and consequently there is virtually no slump. We aren't using fly ash. Could the lack of air be the fault of the air-entraining agent?

A.: We think it's the nature of the beast. This kind of mix requires dosages of air-entraining agent that are up to 10 times as large as the dosages required in mixes made with more conventional amounts of water and cement. We think you should study a recent article by David Whiting, "Air Contents and Air-Void Characteristics in Low-Slump Dense Concretes," published in the ACI Journal, September-October 1985, page 716, which discusses this problem in detail. He studied what are referred to as low-slump dense concretes (LSDC) and found not only that they require more air-entraining agent but that some types of air-entraining agent do not seem to be capable of achieving the needed air content in them at all. Whiting found that retempering can cause dramatic increases in air content. Results are also likely to be different for two cements of different alkali contents. The data obtained in laboratory mixers will probably serve only as a guide to what may be expected in field practice, where the mixing sequence and other conditions may be quite different. Additional comparative data are given in the article.