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ASTM International · Aggregate · Laboratory test
ASTM C566: Total Evaporable Moisture Content of Aggregate by Drying
Oven- or hot-plate-dried moisture content of stockpile aggregate, used to correct batch water and aggregate masses at the plant so the delivered mix hits its design water-cement ratio.
| Designation | ASTM C566 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM C566 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What the ASTM C566 test measures
ASTM C566 measures the total evaporable moisture content of an aggregate sample by drying it and comparing wet and dry masses. Total moisture includes both the water absorbed into the particles and the free water on their surfaces, and it is the free water that matters at the batch plant.
How the ASTM C566 test works, step by step
A representative sample is weighed wet, dried to constant mass by oven, hot plate, or microwave, and weighed again. Moisture content is the mass lost divided by the dry mass, as a percentage. Subtracting the absorption from ASTM C127 or C128 gives the free moisture, which is the water the plant must deduct from the batch water and add to the aggregate mass to hit design proportions.
Why it matters to your project
Stockpile moisture changes with weather, with the depth in the pile, and with how recently the material was washed. A sand carrying 6 percent free water instead of the 3 percent the plant assumed puts roughly 30 extra pounds of water per cubic yard into the mix, raises the water-cement ratio, and lowers strength. Moisture checks at the start of a pour and after rain are the cheapest strength protection available.
What can throw the result off
Sampling from the dry surface of a pile rather than from inside it, overheating aggregates that contain water of hydration in clay minerals, and not drying to constant mass are the common errors.
Common questions about ASTM C566
How often should stockpile moisture be checked?
At least daily during production and after any rain, more often for sands, which swing the most. Plant moisture probes are calibrated against this test.
Is this the same as soil moisture content?
The principle is the same as ASTM D2216 but the sample sizes, drying temperatures, and reporting are written for aggregate.
How we help with ASTM C566 testing
Our technicians run stockpile moisture at the plant or laboratory as part of concrete quality control, and we correlate the result with plant probe readings so batching corrections are based on measured water rather than assumed.
Scheduling & proposals
Need ASTM C566 testing?
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