Concrete Moisture Tests

These meters are tuned and manufactured so that they only indicate the moisture in a shallow region, roughly the top inch of the concrete. The reason is they're putting out an electromagnetic field. That field interacts with steel in the concrete such as reinforcement or wire mesh. If it penetrated, if the field penetrated deeper and it actually hit the steel or the concrete, you'd get a result that looks like the slab is soaking wet because the steel and the concrete acts like an antenna. It will transmit that electromagnetic signal, and the handheld meter will then indicate the floor was soaking wet, even if it isn't.

These meters are tuned so that the fields don't penetrate deep into the concrete where the steel is located. Therefore, they're only measuring what's going on close to the surface.

As you see in this slide, you have no sense of the fact that four or five inches down in the concrete, it may be very wet. The meter is only giving you an indication of what's happening close to the surface and it's designed to that.

So as a survey tool that indicates the surface moisture, these handheld meters are fine. But if you want the true moisture picture of what's happening deeper in the concrete, you have to drill a hole and put a probe down into the slab.