Aggregate moisture content test
How moisture content is defined, what the standard method is, what ranges to expect by material, and how the number feeds back into batch water. This page is reference material — it is not a sales page.
Total evaporable moisture, and why free water is the number that matters
Aggregate holds water in two places, and only one of them affects your mix.
Absorbed moisture
Water held within the pores of the aggregate particle itself, up to the saturated surface-dry (SSD) condition. It is already accounted for in a mix design based on SSD aggregate. It does not become mix water.
Free water
Surface moisture beyond SSD. This water joins the mix water directly and moves your water-to-cement ratio. This is what batch correction is about, and what an instrument is measuring for production purposes.
ASTM C566 in four steps
1. Take a representative sample. This is where most error enters — a scoop off the face of a pile is not the pile.
2. Weigh the wet sample and record the mass.
3. Dry to constant mass. Constant mass, not a fixed time — you are done when further drying stops changing the weight.
4. Weigh dry. Moisture content is the water lost divided by the dry mass, as a percentage.
The method gives total evaporable moisture. To get free water for batch correction, subtract the aggregate's absorption.
Typical moisture ranges by material
| Material | Typical moisture % | Particle size | Note |
|---|---|---|---|
| Concrete sand | 2.0–6.0 | 0–4 mm | The largest single moisture variable in the mix. |
| Coarse aggregate | 0.5–2.0 | 4–32 mm | Lower and steadier than sand, but not zero. |
| Crushed stone | 0.5–2.0 | 4–32 mm | Angular material, abrasive on inline heads. |
| Gravel | 0.5–2.0 | 4–32 mm | Rounded, free-draining, moisture sits low. |
| RAP | 1.0–5.0 | up to 32 mm | Recycled asphalt pavement. Drives burner fuel cost. |
| Foundry return sand | 2.0–7.0 | 0–4 mm | Conductive. Use the coated-rod M1C. |
Aggregate moisture content — frequently asked
What is the moisture content of fine aggregate?
Concrete sand typically runs between 2 and 6 percent total evaporable moisture, depending on weather, stockpile drainage and how recently it was delivered. Sand carries most of the free water in a concrete mix and is the input that varies most.
What is the moisture content of coarse aggregate?
Gravel and crushed stone typically run between 0.5 and 2 percent. Coarse aggregate drains more freely and holds far less surface water than sand, which is why sand dominates batch water correction.
How do you test moisture content of aggregate?
The standard method is ASTM C566: weigh a representative wet sample, dry it to constant mass, and weigh it again. Moisture content is the mass of water lost divided by the dry mass, expressed as a percentage. For production control, a TDR instrument gives the same figure in seconds without destroying the sample.
What is the difference between free water and absorbed moisture?
Absorbed moisture is water held inside the aggregate particles themselves, up to the saturated surface-dry condition. Free water is the surface moisture beyond that point. Free water becomes part of your mix water; absorbed moisture does not. Batch correction is about free water.
How much does aggregate moisture vary during a day?
Enough to matter. Surface drying, rain, drainage through a stockpile and drawing from different parts of a pile all move the number. A full percentage point of change between morning and afternoon is common, and a point of sand moisture is roughly 17 lb of water per cubic yard at typical sand contents.
Want the number in seconds instead of hours?
The oven stays the reference. A TDR instrument is what you run production on between reference tests. Tell us your material and we will confirm the probe.
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