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.
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