The effect of frequency on the dynamic modulus and damping coefficient of concrete

Abstract

Summary Experiments have been made to determine the resonant frequencies of concrete beams in flexural, longitudinal, and torsional modes of vibration at the fundamental and higher harmonics. The results were used to study the effect of frequency on the dynamic Young's modulus and dynamic shear modulus. It was found that for wet concrete there was no appreciable difference in the dynamic Young's modulus determinedfrom the flexural and longitudinal modes of vibration. The modulus changed by less than 3% as the frequency increasedfrom 70 to 10,000 c/s. The dynamic shear modulus also showed no appreciable change with frequency over the range of 700-10,000 c/s. When the concrete beam was allowed to dry, the Young's modulus calculated from the flexural vibrations was lower than that calculated from longitudinal vibrations. The discrepancy is attributed to moisture differences within the concrete beam. It is suggested that surface drying of concrete beams might account for the differences of modulus previously observed in flexure between beams of different size made of nominally the same concrete. The damping coefficient in the moist concrete did not change appreciably withfrequency and, at a particular age, was approximately the same for all three modes of vibration. It is concluded that damping of the vibrations arises chiefly at the interfacial boundaries within the concrete.