Correlating brick compressive strength to its fundamental transverse natural frequency

Several ASTM standards use the principle of sonic resonance and resonant frequencies to determine or estimate mechanical properties such as Young’s modulus, shear modulus, Poisson’s ratio of refractory materials and concrete specimens. However, such studies do not exist which relate the fundamental natural frequency corresponding to transverse flexural vibration of bricks to its compressive strength. This paper reports the results of a study conducted to find a correlation between the frequency of elastic waves of vibrating brick prisms and the soundness of brick material, as natural frequencies are directly related to modulus of elasticity of the material. Twelve bricks were first tested for their natural frequency and then tested destructively to evaluate their compressive strength. Brick specimens were set in the fundamental mode of transverse vibration and an IEPE accelerometer was used to measure the acceleration time history. A simple Fast Fourier Transform (FFT) of recorded accelerometer signal time history gives the resonant frequency of the brick specimen. The bricks tested in this study ranged from 15–30 MPa (21 MPa on average), which is typical of bricks found in northern India. The modulus of elasticity was found to vary between 5000-7500 MPa (with an average of 6100 MPa) which is about 150-500 times compressive strength and about 300 times compressive strength on average. The experimental dataset so developed can be evaluated for acceptability for a given target strength and bricks can be classified into categories such as very good, good, fair or poor. Therefore, an objective assessment of brick quality is possible by measuring fundamental transverse natural frequency of bricks using accelerometer sensors. This assessment procedure can be extended to smartphones, utilizing its on-board sensor capabilities.

In P.B. Dillon & F.S. Fonseca (Eds.), Proceedings of Thirteenth North American Masonry Conference. Paper presented at the 13th North American Masonry Conference, Salt Lake City, Utah (pp. 746-755). Longmont, CO: The Masonry Society.