Abstract
The aim of this work was to provide further confirmation of the possible use of non-linear ultrasonic techniques for detecting the cracking due to corrosion of steel reinforcements in concrete. To this end accelerated steel corrosion tests have been conducted on model reinforced cement mortar specimens, while monitoring the appearance and width evolution of visible surface cracks, and performing non-linear ultrasonic measurements based on the phenomena of harmonic distortion and intermodulation. A new parameter, based on the difference between the amplitude of the fundamental frequency and the sum of the amplitudes of all the first-order and second-order intermodulation products, has been proposed in this work. The results confirm that the appearance of visible surface micro-cracks are preceded and accompanied by the observation of strong non-linear features in the received signal. Furthermore, the new parameter proposed in this work is as efficient as the relative non-linearity parameters, classically used in harmonic distortion non-linear ultrasonic studies, for detecting the non-linear features associated with the critical events of the cracking of cement mortar due to embedded steel corrosion. A hypothesis has been developed considering the possible effect of the filling of the void space by liquid containing rust products after the formation of new cracks or the enlargement of its width. This filling process, which might be particularly enhanced by net convective transport of liquid, would explain the evolution of the values of all the parameters used for putting in evidence the non-linear elastic features after the critical events of the cracking process.
Highlights
Corrosion of steel rebar in reinforced concrete or cement mortar constructive elements is a problem of major concern in both the exploitation and maintenance of building and civil engineering structures (American Concrete Institute 1996)
Where x is expressed in μm, Icorr is the constant anodic current density expressed in μA/cm2, and t is the time elapsed since the beginning of the accelerated corrosion test, in days
The presented results suggest that the new parameter DIFA proposed in this work is as efficient as the relative parameters Bn and Bpn for detecting the strong non-linear features associated with the critical events of the cracking of cement mortar damage due to embedded steel corrosion
Summary
Ultrasonic techniques are one of the most extended approaches for the detection of cracks and defects in a wide range of materials, including cementitious materials, due to its non-destructive character and straight applicability to on-site studies (Blitz and Simpson 1995) In this context, there are passive techniques for damage monitoring in concrete, based on acoustic emission phenomena (Ohtsu 2010; Zaki et al 2015); and active ones that imply the generation and propagation of ultrasonic waves throughout the material under analysis. Many research efforts have been made on the development of these non-linear ultrasonic (NLU) techniques so as to increase the detection features of the tests Some of these techniques are based on changes in the resonance frequency (spectroscopy) (Van den Abeele et al 2000), on the generation of higher order harmonics (Shah and Ribakov 2009b), or on the emergence of intermodulation products (Burrascano et al 2019). A new simple and easy to implement method, termed as the Scaling Subtraction Method (SSM), has been proposed for enhancing the capabilities of detection of the non-linear elastic response of a system (Scalerandi et al 2008; Antonaci et al 2010; Antonaci et al 2013)
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