Study of the Stress Waves in the Plunger of a Rebound Hammer at the Time of Impact


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Title: Study of the Stress Waves in the Plunger of a Rebound Hammer at the Time of Impact

Author(s): Toyoki Akashi and Syouji Amasaki

Publication: Special Publication

Volume: 82


Appears on pages(s): 17-34

Keywords: amplitude; calibrating; concretes; hardness; impact; impact hammer tests; nondestructive tests; stress waves.

Date: 9/1/1984

The nature of the stress waves found in the Schmidt Hammer after impact during the testing of concrete were examined experimentally. Using a specially designed plunger, the authors have been shown that the impact of the hammer mass produces a large compressive wav e and a large reflected stress waved at r the centre of the plunger. The ratio,or/or of the amplitudes of these waves and the time T between their appearance was found to depend upon the surface hardness of cured concrete. The rebound number was found to be approximately proportional to the ratio of the two stresses and was not significantly affected by the moisture conditions of the concrete. The magnitude of the first stress wave at the centre of the plunger is almost constant and is approximately 80 percent of the value calculated by Smith’s numerical solution, which does not consider the efficiency of the impact of the hammer. The authors have concluded that the principal of operation of the N-type Schmidt test hammer may be more complex than is assumed when consideration is given only to the simple problem of applying Newton’s laws to impacting bodies. It may involve considerable components of longitudinal wave transmission. It is further concluded that, to correctly measure the rebound number of hardened concrete, the Schmidt hammer can be calibrated by testing a material with a constant hardness and measuring the resulting impact stress wave. By observing the behaviour of the impact stress in the plunger the surface hardness of concrete canbe measured with higher accuracy.