Coarse Aggregate Content
ACI 211.1 sizes coarse aggregate content using a dry-rodded bulk volume fraction, selected from a stable, long-standing reference table by aggregate size and sand fineness — not by an absolute-volume calculation like fine aggregate.
Table 6.3.6: bulk volume by size and fineness modulus
ACI 211.1 Table 6.3.6 gives the dry-rodded bulk volume of coarse aggregate per unit volume of concrete (often written b/b0, the ratio of the coarse aggregate's oven-dry-rodded volume that should be used per unit volume of concrete) as a function of two inputs: the nominal maximum aggregate size (NMAS) and the fineness modulus of the fine aggregate. This is a long-standing, stable published table that has not changed materially across ACI 211.1 editions.
The general shape of the table
Two trends run through Table 6.3.6:
- Larger NMAS, higher bulk volume fraction. Bigger coarse aggregate particles pack more efficiently and a larger fraction of the concrete's volume can be filled with coarse aggregate.
- Higher fineness modulus, lower bulk volume fraction. A coarser sand (higher fineness modulus) already fills more of the fine aggregate's own volume efficiently, so less coarse aggregate volume is needed for a well-proportioned, workable mix at that NMAS.
From volume fraction to mass
The bulk volume fraction read from Table 6.3.6 is multiplied by the coarse aggregate's own dry-rodded unit weight (its mass per unit volume when compacted by the standard dry-rodding test) to convert the fraction into a mass of coarse aggregate per unit volume of concrete. This maps directly to the calculator's dryRoddedCA field (the coarse aggregate's dry-rodded unit weight) and fm field (the fine aggregate's fineness modulus) — together with the selected nominal maximum aggregate size, these are exactly the inputs the table needs.
Frequently asked questions
What is dry-rodded bulk volume?
The volume occupied by coarse aggregate when it is placed and rodded (compacted) in a container by a standard test method, expressed as a fraction of the container's total volume — this is the b/b0 fraction ACI 211.1 Table 6.3.6 gives.
Why does a higher fineness modulus reduce the coarse aggregate fraction?
A higher fineness modulus means a coarser sand, which itself fills more volume efficiently and needs less coarse aggregate volume to produce a well-graded, workable combined aggregate — so Table 6.3.6 reduces the coarse aggregate fraction as fineness modulus increases.
What are the calculator's dryRoddedCA and fm fields?
dryRoddedCA is the coarse aggregate's dry-rodded unit weight (mass per unit volume when dry-rodded), and fm is the fine aggregate's fineness modulus — together with the nominal maximum aggregate size, these are exactly the inputs Table 6.3.6 uses to size the coarse aggregate content.