Table 2-2. Maximum permissible w/c ratios for concrete when strength data
from trial batches or field experience is not available.
Variation in Mixtures
The proportions you arrive at in determining mixtures will vary somewhat depending on which method you use.
The variation is due to the empirical nature of the methods and does not necessarily imply that one method is
better than another. You start each method by assuming certain needs or requirements and then proceed to
determine the other variables. Since the methods begin differently and use different procedures, the final
proportions vary slightly. This is to be expected and points out further the necessity of trial mixtures in
determining the final mixture proportions. For variations in a mixture, note that for concrete used in slabs or other
flatwork, there are minimum cement requirements depending upon the maximum size of the aggregates. See
Table 2-3. Minimum cement requirements for concrete used in flatwork.