Nonlinear acoustic testing for concrete materials evaluation

This paper summarizes the results of three nonlinear acoustic tests performed on a series of stress-damaged concrete samples. The three tests were: nonlinear resonant "ultrasound" spectroscopy (NRUS), scaling subtraction method (SSM), and dynamic acousto-elastic testing (DAEQ. The test samples were cut out of a large concrete block emulating common field investigation scenarios, where the quality of an existing structure was examined at different locations. One sample was left intact while the others were pressed to 10, 20, 30, 40, 60, and 70% of the (nominal) ultimate compressive strength of their common concrete mixture. NRUS was performed on all samples, whereas SSM and DAET were conducted on a subset of samples. In addition, the conventional (linear) resonant "ultrasound" spectroscopy and ultrasonic pulse velocity measurements for all samples are reported. The authors present the theoretical background of nonlinear acoustic testing, discuss the principles and outcome of each test, and compare the corresponding results of linear and nonlinear acoustic techniques. The obtained results attest to the high sensitivity of nonlinear acoustic measurements (that is, estimated nonlinear elastic material parameters) to the presence of microcracks. While NRUS, SSM, and DAET clearly differentiate between intact and moderately damaged samples, neither linear wave velocities nor (linear) dynamic elastic moduli can reliably delineate undamaged and damaged concrete.