The moisture meter you own or buy will define how you measure your aggregate moisture levels. While most sensors measure bulk moisture, some measure only surface moisture. In either case, though, they can be calibrated to read surface or free moisture. This is the moisture that reacts with the cement in the mix and, therefore, the only moisture you need to worry about.
The free water is the water not bound inside the granules of aggregates. The “bound” or internal water is absorbed into the aggregate granules. When this water is at its maximum but there is no free water, we have the condition called Saturated, Surface Dry (SSD). This describes the condition where the granules of aggregate are filled to their capacity. Your concrete mix design is always based on 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, according to the mix design, 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.
You should always calibrate your moisture sensors to read 0 percent moisture when the aggregate is at SSD condition. If the aggregate is bone dry (no internal moisture), the granules will absorb water from the mix, 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 probably correct, but it comes out incorrect on the batch report.