When testing the strength of concrete, the concrete test hammer uses a certain elastic force to transit the impact force of an impact hammer to the surface of concrete, its initial kinetic energy redistributes, a part of energy in the form of plastic deformation or residual deformation is adsorbed by the concrete, and another part of energy which is proportional to the surface hardness is transmitted to the impact hammer, making the hammer resile to a certain height, then the strength of the concrete is derived from the proportional relation between the height of resilience and the concrete strength. With the merits of simple structure, easy correction, maintenance and repair, and portability, the concrete test hammer is widely used in civil engineering and construction industry for testing the strength of concrete. Compared to other nondestructive testers, the concrete test hammer is an economical and practical nondestructive testing instrument. HT-225A is used for testing the strength of various concrete members (slab, beam, column, truss) of normal building strength and bridge.