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Surface Moisture or Free Moisture

How do I get my moisture sensor to read surface moisture (free moisture)?

The only water which will react with the cement in your mix design is the total amount of free water. That is the water not bound inside the granules of aggregates. The “bound”, absorbed or internal water is water absorbed into the aggregate granules. When this water is at its maximum but there is no free water, we have what is called Saturated, Surface Dry (SSD) conditions.

This describes the condition where the granules of aggregate are filled to their capacity. Your concrete mix design is always based on your aggregates being in the SSD condition.

A colleague asked me why his aggregate moisture meter reads a (low) moisture reading but when he adds the correct amount of water to the mix, it turns out too dry. And when he adds more water to give the right slump, it exceeds the maximum Water/Cement (w/c) ratio for the mix design. If you have experienced this effect, read on.

The solution

You should always calibrate your moisture sensors to read 0% moisture when the aggregate is at SSD condition. If the aggregate is bone dry, the granules will absorb water, which then becomes bound in the aggregate. That is why my colleague above had to add more water than the mix design called for. This, in turn, will give a false w/c ratio when your batch controller totalizes the water absorbed by the aggregates plus the added water. The reason is that the water absorbed into the aggregate is not “free water” for the purposes of the mix design. The w/c ratio is actually correct, but it comes out incorrect on the batch report.

The only solution to this situation is to spray the coarse aggregate piles to allow them to absorb water. This, in consequence, creates the SSD condition. Most companies in hot areas do this as standard practice. You should not spray your sand pile continuously, however, since it normally has some free water already, from occasional rainfall. If spraying aggregates gives too much free water for a low w/c ratio mix, you should drain each load before you load it into the plant bin. Usually this is not necessary, because you will need to add more water in the mixer.

We have investigated this effect by testing moisture sensors in tropical countries. The hot sun creates a low aggregate moisture. We have found that for sand with moisture lower than 4%, the internal moisture (usually assumed to be 1%) drops to less than ½% when surface moisture is 2 to 3%. The same applies for coarse aggregates. However, the absorbed (internal) moisture in this case is usually much less than 1% when saturated. The actual value depends on how much water the stone material absorbs.


When calibrating your moisture sensors, it is important that you either carry out both the ASTM C 566 and C 128 tests to determine the absorption and hence the SSD moisture or saturate the aggregates before calibration to bring the internal moisture up to SSD conditions. Note that many aggregate and sand suppliers will provide the absorption value on request. During tests at a concrete plant we found an expert in oven-dry testing who had an ingenious way to ensure SSD conditions as he checked the progress of the bake-out. He used a piece of ½” thick plate glass held over the pan for a few seconds. If it misted, it indicated there was still surface or free water. When no mist occurred, he shut off the heater. As a result, he got a very precise SSD condition for weighing the sample and in consequence he did not need to do both the C 566 and C 128 tests. Using his method, we could perform SSD oven-dry tests to an accuracy of 0.2% or better.

The moisture sensor calibration process automatically takes care of the absorbed (internal) water. You should take at least two samples, and preferably more, with both high and low moistures. Then add the oven-dry test results, plus the moisture sensor readings to the calibration table. The computer then finds the best-fit straight line. If you do the oven-dry tests correctly, halting at the SSD condition, the internal moisture will not appear in subsequent readings.

Calibration failure

If your calibration fails, you are probably not selecting a big enough range of moisture during your tests. If the range from driest to wettest is only 2% and your oven-dry tests have a 1/2% error, the slope of the calculated line can be off by a wide margin. Consequently you will get huge errors when the material moisture is outside the range of your samples.

Moisture sensor types

Note that the I.R. type of moisture sensor (Polarmoist) reads only surface moisture, since it measures the light reflected by the free water around the aggregate granules. The TDR type (Sono-Vario) sensors and all regular microwave sensors, such as Scale-Tron’s AquaSense 2280D, measure deeper into the material stream. The Sono-Vario measures deepest and is also less density sensitive. These all measure total moisture, including internal moisture, although the calibration process, if done correctly, eliminates the internal moisture. The Sono-Vario sensor measures aggregates up to 1-1/4″ to within 0.2%, which is impossible with other sensors. For regular microwave sensors, the maximum aggregate size for any real accuracy is 3/8″ and even ¼” aggregates can give +/- 0.5% errors.


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