Q.: We are replacing the deck of a 15-year-old parking garage one section at a time, using expansive cement concrete. The water-cement ratio is 0.40 in a mix containing 750 pounds of cement per cubic yard, and we are meeting the early strength requirement of 2500 psi in 3 days. However, the concrete finishers want us to increase the water-cement ratio to 0.45 because they are having trouble finishing the concrete. Would this be too high a water-cement ratio for this concrete?
A.: The crucial requirement in ACI 223-77 (Revised 1982), "Recommended Practice for the Use of Shrinkage-Compensating Concrete," appears to be what is given in Section 2.5.7, "Durability." This section states that "Recommendations of ACI Committee 201.2R should be followed. Before being exposed to extended freezing in a severe exposure, it is desirable that concrete attain a specified compressive strength of 4000 psi.... A period of drying following curing is advisable. For moderate exposure conditions, a specified strength of 3000 psi...should be attained." Table 4.1.2 of ACI 223-77 (Revised 1982) indicates that a water-cement ratio of 0.50 to 0.53 is a good first approximation in making trial mixes of expansive cement concrete for a 28-day compressive strength of 4000 psi. On the basis of such information we see no reason why the water-cement ratio couldn't be increased to 0.45, which appears to be low enough to achieve the 4000-psi 28-day strength.