Volume of air = (air content) (7 cf/cy) = (0.05) (27 cf/cy) = 1 35 cf/cy
Partial volume (PV)
Absolute volume of FA = 27 cf- PV = 27 cf - 20.47 cf/cy = 6.53 cf/cy
Weight of FA = lb vol of FA x SGFA x 62.4 lb/cf = 6.53 cf/cy x 2.66 x 62.4 lb/cf = 1,083.8 lb/cy
Determine Proportions for First Trial Batch (for 1 cy) by Converting Absolute Volumes to Dry Volumes
Cement Type IA
If needed, mix more trial batches to obtain the desired slump and air content while keeping the w/c ratio constant.
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 methods and does not necessarily imply that one method is better
than another. 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 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 on the maximum size of the aggregates. See Table 2-7. This requirement is
normally met in the design procedure but must be checked to ensure compliance.