Nonlinear Ultrasonic Investigation of Concrete with Varying Aggregate Size under Uniaxial Compression Loading and Unloading

Abstract

Nondestructive testing in concrete is complex due to its nonhomogeneous ingredients. The presence of varying sizes of aggregates in concrete results in nonlinear characteristics. This paper aims to compare mortar and normal concrete with varying aggregate sizes. The specimens are damaged gradually with loading and unloading paths. Fast Fourier transform (FFT) of the recorded time domain waveforms is conducted to show the frequency spectra. The amplitude of the second harmonic frequency is used as a reference to the internal damage. The analysis of the nonlinear ultrasonic parameter second harmonic amplitude is used according to each loading branch with two damage levels: total and incremental. Results showed that the magnitude of the total damage produced by mortar at the last loading branch was relatively lower than all of the concrete mixes with different sizes of aggregates. The magnitude of the incremental damage produced by mortar in every step load consistently increased as the load increased. For the normal concrete with different sizes of aggregates, the highest total damage was obtained when mixed with large aggregates. According to a previous study using a finite-element model, increasing the aggregates’ size in concrete increased the crack width. This could have led to the increase in the magnitude of the normalized second harmonic ratio. On the other hand, the incremental damage of concrete with different sizes of aggregates varied per loading branch. These results indicated the complexity of concrete as a heterogeneous material where it grew differently through formation of microcracks that varied from one loading branch to another.