We're going to supply air-entrained concrete for a job requiring a 4000-psi design strength, a minimum water-cement ratio of 0.50, and a minimum cement content of 600 pounds per cubic yard. The required slump range is 1 to 4 inches. Because we don't have past performance records, trial batches are required, and a testing laboratory has made two sets of trial mixes that we submitted to the engineer for the job. For each submittal, the testing laboratory prepared three concrete mixtures, each with a cement content of 600 pounds per cubic yard. They varied the water content to produce slumps of 2, 4, and 5 1/2 inches, with all mixes having a water-cement ratio less than 0.50. The required average strength of 5200 psi was met by two of these mixtures, but the engineer rejected both submittals. He says to meet the requirements of ACI 318, "Building Code Requirements for Structural Concrete," Section, the trial mixes should have a slump within ?0.75 inches of the allowable maximum and an air content within ?0.5% of the allowable maximum. He also says the lab should be holding the water content nearly constant to produce the target slump and varying the cement content to produce three different water-cement ratios. The testing lab maintains that its procedure is correct. Who=s right? We need to get a submittal approved so we can proceed with the job. A. We think the engineer is right and don't see much sense in making concrete with a 5 1/2-inch slump since it doesn't meet the job requirements. Generally, the water content required to produce a given slump is nearly constant, but for the first batch only enough water should be used to produce the target slump, regardless of the amount assumed in selecting the trial proportions. After measuring air content and unit weight, and calculating yield, the lab personnel can then adjust proportions for subsequent batches to produce the target slump, air content, and minimum cement content. Procedures for doing this are described in ACI 211.1-91, "Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete," Section 6.3.9.