A probe for measuring the hardness of concrete includes a body adapted to receive a retainer such that the combined body and retainer can be held in an aperture extending through a wall of a mould for the hardenable material, with the body extending from the inside of the wall. A sensor circuit includes detector elements supported by the body and disposed when in use in the concrete, and circuit terminals which face the exterior of the mould wall when in use, and are accessible through the retainer for connection to terminals of an external instrument. Preferably, the detector elements are conductive portions of a printed circuit board forming a resonator. The instrument can operate a switch in the probe which interconnects the terminals to indicate a good connection and measures resonator impedance at multiple frequencies.