Moisture content testing

What does a moisture content test actually measure?

It measures the mass of water in a material as a percentage of the mass of the dry solids, not as a percentage of the wet sample. That distinction matters, because a figure expressed against dry mass means peats and very soft organic clays can legitimately return results above 100 per cent. The result is the starting point for most soils classification work and for any dry density calculation.

How is the test actually carried out in the laboratory?

A representative specimen is weighed wet, dried in a ventilated oven at 105 to 110 degrees Celsius until it reaches constant mass, then weighed again, and the loss in mass is taken as water. Constant mass means less than 0.1 per cent further change after another hour of drying. Sixteen hours normally achieves that for a fine soil, with coarse granular material drying more quickly.

Which standard is moisture content testing usually carried out to?

For soils it is commonly carried out to BS 1377-2 or to BS EN ISO 17892-1, the Eurocode-era equivalent that many current specifications call up. Aggregates are usually tested to BS EN 1097-5. The methods are close in substance but differ in their reporting requirements, and we test and report to whichever your specification names.

How much material do you need for a moisture content test?

It depends on the largest particles present, because the specimen has to be big enough to represent them. Around 30 g is enough for a silt or clay, 100 g for a sand, 500 g where particles reach 10 mm, about 3 kg up to 31.5 mm and around 21 kg up to 63 mm. Send more rather than less if you are unsure of the grading.

Why does the sample have to be sealed and tested promptly?

Because the result only means something if the water is still in the sample when we weigh it. Moisture evaporates through a loose bag or a part-filled tub and is gained if the sample stands in the rain, so the reading drifts away from the condition on site. Use an airtight container filled to the top, keep it out of the sun and get it to the laboratory quickly.

Can moisture content be measured on site instead of in the laboratory?

Yes. Rapid site methods such as carbide meters and microwave drying give an immediate figure for earthworks control, but oven drying to constant mass is the reference method and is what most specifications require for a reported result. Rapid methods are less precise and can mislead in organic or gypsum-bearing soils. Where a site laboratory is running, the quick figures are usually checked against oven-dried results.

What is the result used for once you report it?

Most often to convert a measured bulk density into a dry density, since dry density is bulk density divided by one plus the moisture content expressed as a decimal. That is how a field density result is compared with a laboratory compaction reference. The figure also shows whether fill is near its optimum moisture content for compaction and it accompanies almost every soils classification report.

Do organic soils or gypsum-bearing soils need a different drying temperature?

Yes. At 105 to 110 degrees Celsius organic matter can oxidise and gypsum gives up its water of crystallisation, so both return a moisture content that reads too high. The usual answer is to dry at a reduced temperature, around 50 degrees Celsius, and to state the temperature used on the report. Tell us if you suspect peat, made ground with a high organic content or gypsum in the sample.

Is this test ordered on its own, or with other classification tests?

Usually with others. It sits in almost every soils classification suite, because liquid limit, plastic limit, grading and compaction results are all read against the natural moisture content of the material. It is also run alongside in-situ density testing so that a dry density can be calculated. Ordering it as a single test is perfectly normal where you just need a check on a stockpile or a formation.

How long does the test take, and what makes it slower?

The oven is the controlling step. A fine soil normally needs around sixteen hours to reach constant mass, so a sample arriving one day is typically reported the next working day. Wet, heavy or coarse samples take longer, and organic or gypsum-bearing material dried at a reduced temperature takes considerably longer again. Transit to the laboratory is often the bigger variable. We confirm the expected timing before the work is booked.

Is moisture content testing covered by your UKAS accreditation?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope rather than to every service we provide, and a method can be listed for permanent laboratory work but not for a site laboratory. Ask us to confirm whether the moisture content testing you need is covered before you book.

What do you need from us to quote for moisture content testing?

Send the site postcode, the material type, how many samples you expect, your preferred dates and the specification or testing schedule if you have one. Tell us the largest particle size, because that sets the sample mass and the preparation work involved. Whether you are delivering samples or want them collected, and whether other classification tests are needed on the same material, are the other things that move the price.