Aggregate moisture correction: SSD to wet batch weights

First, put every moisture percentage on the same basis

OD means oven-dry aggregate. SSD means saturated surface-dry: permeable pores contain water, while the outside has no free surface water. Air-dry aggregate lies between OD and SSD; wet aggregate carries water beyond SSD.

Here, absorption and total moisture are both percentages of oven-dry mass. Their difference determines net water relative to SSD. Do not substitute a surface-moisture percentage reported on an SSD basis. These definitions follow ACI E1-16, sections 3.3.2–3.3.5; NRMCA’s public TIP 6 preview illustrates the moisture states.

Four equations from SSD design to batch weights

Let S be SSD aggregate mass, a absorption and t total moisture. Enter a and t as decimals: 2% = 0.02. Apply the first three equations separately to each aggregate.

1. Recover dry solidsD = S ÷ (1 + a)
2. Calculate aggregate to batchB = D × (1 + t)
3. Find net water relative to SSDF = D × (t − a)
4. Balance separately added waterWadd = Wfree − ΣF − Wother

Every mass is in kg. Wother is known mixing water from other sources, counted once. A positive F reduces mixer water; a negative F represents an absorption allowance. The direction of this correction is explained in ACI’s batch-water adjustment guidance.

A complete worked batch: sand, stone and water

This original example uses 400 kg cement and 180 kg target free water, with no SCMs, liquid admixture or other water source. It is one illustrative batch: no concrete grade or 1 m³ yield is assigned.

For sand: 714 ÷ 1.02 = 700 kg dry solids; 700 × 1.05 = 735 kg to weigh; 700 × (0.05 − 0.02) = 21 kg surface water. Repeat for stone: 1,105.5 ÷ 1.005 = 1,100 kg dry solids.

Mixer water = 180 − 21 − 5.5 = 153.5 kg. Check: 153.5 + 21 + 5.5 = 180 kg; w/c = 180 ÷ 400 = 0.450.

The SSD total, 400 + 714 + 1,105.5 + 180, equals the wet ticket total, 400 + 735 + 1,111 + 153.5: both are 2,399.5 kg. This checks mass accounting. Actual yield needs actual batch mass and measured fresh density, as described in NRMCA CIP 8.

What if sand moisture rises from 5% to 8%?

Keep stone, cement and absorption unchanged. Compare an unchanged ticket with one that accounts for the new moisture measurement.

The old ticket delivers 735 ÷ 1.08 = 680.56 kg dry sand. Its free water is 680.56 × (0.08 − 0.02) ≈ 40.83 kg. Including stone and unchanged mixer water gives about 199.83 kg, or w/c ≈ 0.4996.

That is 19.44 kg less dry sand and 19.83 kg extra free water. Simply multiplying the original 700 kg by the moisture change misses the changed solids mass. The corrected ticket restores both: 700 × 1.08 = 756 kg wet sand, and 180 − 42 − 5.5 = 132.5 kg mixer water. These figures do not predict a strength loss or change in yield.

What if aggregate is drier than SSD?

At 1% sand moisture and 2% absorption, the same example needs 707 kg sand. Its net contribution is −7 kg, so the accounting gives 180 − (−7) − 5.5 = 181.5 kg mixer water.

This is an SSD absorption allowance. Standard absorption does not establish how quickly the aggregate will take up water during mixing or before set; see the discussion in ACI E1-16 section 3.3.3. Check material behaviour in trials.

Use the correction in your concrete mix design

1. Start from the project’s design quantities and tested absorption for each stockpile.
2. Record current total moisture, its measurement date and its percentage basis.
3. Update both aggregate weights and separately added water. Account for other water sources once, according to the project’s governing requirements.
4. Review the revised batch ticket and record actual trial measurements separately.

In the Indian lab, use Design parameters → Moisture for corrections and Water lab to explore a frozen-ticket error. In regional worksheets, use Material & project inputs. The article’s clean-number batch is separate from their default examples.

For the denominator and free-water formula, use the water-cement ratio calculator. For a full designed example with moisture adjustments, see M30 concrete mix proportions.