Aggregate Moisture Correction
A mix design is proportioned on paper assuming aggregate in one specific moisture state. Real aggregate stock is almost never in that state, and the difference has to be corrected for at the batch plant to keep the true w/cm on target.
Three moisture states, one aggregate
Aggregate moisture is described in three related states:
- Oven-dry — all moisture removed, including what the aggregate's internal pores can absorb.
- SSD (saturated surface dry) — internal pores fully saturated with water, but no free water film on the particle surfaces. Mix designs are proportioned assuming this state, because SSD aggregate neither adds water to nor absorbs water from the mix.
- Actual stock moisture — the real, as-received condition of the aggregate at the plant, which can be drier than SSD (and will absorb water from the mix) or wetter than SSD (and will contribute free water to the mix), depending on stockpile conditions, weather, and handling.
Absorption vs free water
Absorption is the amount of water an aggregate's own internal pores can hold at the SSD condition, measured as a percentage of the oven-dry mass. Because SSD is the design basis, absorbed water doesn't change the batch's effective mixing water — it's already accounted for. What matters for the batch water calculation is free water: moisture on the aggregate beyond the SSD condition, sitting as a surface film rather than absorbed internally. Free water in wet stock adds directly to the water in the mix; a moisture deficit below SSD in dry stock means the aggregate will pull some mixing water out of the batch by absorption instead.
Keeping w/cm on target
Because the w/cm is calculated from the water that actually ends up in the mix, not the water that was intended, free water carried in by wet aggregate has to be subtracted from the batch water added at the plant, and a moisture deficit in dry aggregate has to be added back — in both cases so that the total effective water matches the design value and the true w/cm stays where it was designed. Skipping this correction is one of the most common ways an on-paper mix design ends up with a different actual w/cm in the truck; see Common Mix Design Mistakes.
Where this shows up in the calculator
This is a real differentiator of this calculator versus many other regions' tools, which often don't expose moisture correction as directly. The calculator's caMoisture and faMoisture fields take the coarse and fine aggregate's actual stock moisture content, and caAbs and faAbs take their absorption values. Together, these let the calculation determine the free water each aggregate is carrying (or the deficit it will absorb) and adjust both the added batch water and the as-batched aggregate mass so the final mix matches the design w/cm.
Frequently asked questions
What does SSD mean?
Saturated surface dry — the aggregate condition where its internal pores are fully saturated with water but there is no free water film on the particle surfaces. Mix designs are proportioned assuming aggregate is in the SSD condition, since that's the state that neither adds nor absorbs mixing water.
Why does absorption matter separately from surface moisture?
Absorption is water the aggregate's own pores can hold internally, at the SSD condition, and it does not participate in the fresh concrete's water-cementitious ratio. Only moisture beyond the SSD condition — free surface water — adds to or subtracts from the batch's effective mixing water.
How does this map to the calculator's fields?
The calculator's caMoisture and faMoisture fields are the coarse and fine aggregate's actual stock moisture content, and caAbs and faAbs are their absorption values. Together these let the calculator work out the free water each aggregate is carrying and adjust batch water and aggregate mass accordingly.