Nonlinear Acoustic Spectroscopy and Frequency Sweep Ultrasonics: Case on Thermal Damage Assessment in Mortar

Abstract

An exhaustive study on thermal damage of Portland cement-based materials is addressed. Damage carried out at different temperatures on concrete between 40 and $$525\,^{\circ }\hbox {C}$$ were assessed by means of microstructural, physical and nondestructive tests. Microstructural analysis (thermogravimetry and scanning electron microscopy) showed the principal changes of the Portland cement hydrated products for the different analysed temperatures. Compressive and flexural strengths remained constant or even increased at a low heating temperature range, while the mass loss increases. Dilatometry analysis revealed important information about deformation incompatibilities between the paste and the aggregate. These results have been correlated with nondestructive tests: nonlinear impact resonance acoustic spectroscopy (NIRAS) and ultrasonic measures. The dynamic modulus and ultrasonic pulse velocity have closely predicted the linear stiffness decay of the specimens. However, hysteretic parameter from NIRAS analysis exhibited a different trend from stiffness-related parameters, keeping constant until $$250\,^{\circ }\hbox {C}$$ and suffering a huge increasing for 400 and $$525\,^{\circ }\hbox {C}$$ . Ultrasonic attenuation computed with a broadband ultrasonic signal (chirp) revealed interesting information about scattering components inside the material, and is sensitive to interfacial transition zone between aggregate and paste in a large range of frequencies. The correlation between microstructural, mechanical and nondestructive techniques were carried out successfully. Nonlinear vibration and ultrasonic attenuation are non-conventional parameters that gave specific information about a complex damage process, such as a thermal attack in highly heterogeneous materials (e.g. Portland cement composites).