Nonlinear dependencies of the degree of corrosive cracking on the process conditions were realized via the synergy of anodic oxidation with mechanical fracture: For example in the absence of ultrasonic loading, parameters such as anodization current and bath temperature, promoting growth of the oxide film via the Arrhenius activation conditions for oxidation, lead to decreased cracked percentile areas. This is apparently due to a concomitant reactive and diffusive mass transport tending to minimize free energy via reduction of the exposed surface, therefore smoothing and closing incipient cracks over time. On the contrary when ultrasound is applied, the same conditions are shown to promote cracking over the short periods of ultrasonic vibration. This suggests that cracks propagate during the tensile part of each ultrasonic cycle, while during the compression part crushing, attrition and dissolution of ruptured oxide films widen the pathways of the corrosive medium to penetrate and promote further cracking. Corroboration of this mechanism behooves time-resolved in-situ TEM or AFM observation of the cracking progress during the process, presently challenged by the high ultrasonic frequencies.
Volume changes in concrete before the hardening of cement-based materials are the main cause of plastic shrinkage strain and cracking [16,17]. Volume loss at the plastic stage is caused by the consolidation of aggregates, bleeding, and evaporation of water. In its plastic state, when undisturbed, the denser solid particles settle and tend to sink down, whilst the lighter-weight materials, such as air and free water, begin to rise to the surface. Air escapes faster, but the escaping water, called bleeding water, escapes slower, and when it reaches the surface, it starts evaporating [18]. When the evaporation rate exceeds the bleeding rate, the concrete surface dries, and at this stage, the possibility of plastic shrinkage cracking increases [19,20,21]. Both environmental conditions and concrete mix composition affect plastic shrinkage, as seen in Figure 1 [22].
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