Fine Aggregate Closes the Absolute-Volume Balance
In the common ACI PRC-211.1-22 workflow, fine aggregate is not chosen from a fixed “sand recipe.” After the volumes of water, cementitious material, coarse aggregate and air are accounted for, fine aggregate fills the remaining absolute volume.
The calculation sequence
Convert each known ingredient into absolute volume using its mass and specific gravity. Add water, cementitious material, coarse aggregate and air. Subtract that total from 27 ft³ for one cubic yard. The remaining volume is assigned to fine aggregate, whose SSD mass follows from its specific gravity.
Where fineness modulus enters
Fineness modulus characterizes the sand grading and is used earlier in ACI PRC-211.1-22 Table 5.3.6 to select the starting bulk-volume fraction of coarse aggregate. A coarser or finer sand can therefore shift the first-trial coarse/fine balance even though fine aggregate itself is finally calculated by volume closure.
ASTM C33 conformance is more than one FM number
Fineness modulus is useful for proportioning, but it does not replace the complete sieve analysis or aggregate-quality requirements. ASTM C33/C33M establishes grading and quality requirements for concrete aggregates; project specifications can add tighter controls. A sand can share a similar FM with another source while behaving differently because the detailed grading, particle shape and fines differ.
Why the calculated sand mass still needs a trial
SSD versus as-batched sand
The absolute-volume calculation normally uses an SSD basis. Wet stockpile sand contains surface water and weighs more than its SSD design mass; dry sand can absorb water. Convert the design mass to the actual stock condition and adjust batching water before production.