A simple probe system that permits non-destructive, on-site test for the compressive strength of concrete may be the solution to the problems of traditional testing methods. The modern probe technique makes it possible to test floor or paving slabs, ceilings, side walls and curved surfaces. Results are immediately available, even when the probe is used under water. The probe technique involves use of a pistol-like driving device powered by a precisely loaded explosive charge, as well as the metal probe itself, and related measuring instruments. The driving device fires the probe into the surface of the concrete to be tested. The probe is specifically designed to crack aggregate particles and to compress the concrete being tested. When discharged into the concrete, the probe penetrates deeply and compresses a measurable zone. The zone is correlated to pounds per square inch. The probe value used in this system is precisely the measure of compressibility of a localized area of in-place concrete. In effect, the depth of the probe's penetration into the surface represents a specific measurement of the compressive strength of the concrete. The results compare within narrow limits of accuracy to properly molded cylinders loaded to failure in a compression press.