PSI - Issue 67

Gabriele Milone et al. / Procedia Structural Integrity 67 (2025) 90–106 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000 – 000

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2.3.2. Corrosion sensing measurement

During the aforementioned accelerated corrosion test, the smart coatings were used as external sensors to assess the progress of the corrosion in the mortar substrates (Table 3). Via the 2-probe method (Miccoli et al. , 2015), the application of alternate current was implemented daily using a potentiostat PGSTAT204 (Metrohm, Switzerland) to minimize the polarization effect in the system (20 Hz – 300 kHz; U = 0.5 V; 9 points per decade). The bulk resistance, obtained by deconvoluting the impedance spectrum, can be used to define the effective conductivity, defined by equation 3: R 1 bulk ( A L )=σ bulk (3) where R bulk is the resistance value corresponding to the ionic conduction of the interconnected pores in parallel with the electronic conduction through the conductive filler (Wang and Pang, 2019) [Ω], L is the distance between the pair of chosen electrodes or gauge length [m] and A is the cross-section of the coating sensors [m²]. The corrosion measurement of unconfined rebars was obtained from three identical sensing coatings applied perpendicularly to the rebar direction and, therefore, to the crack expansion (Figure 5). The sensors, distanced 40 mm from one another, worked as both strain and damage monitoring devices. The electrical output was given from the Nyquist plot obtained at different corrosion times and the damage was assessed via a portable microscope to measure crack width along the rebar on each surface. Once the corrosion test finished after 24 days of accelerated corrosion, the samples were transversally cut with an electric saw and subsequently manually split open to visualize the oxide production and propagation from the rebar to the surface under study.

Figure 5. Graphical representation of fully corroded reinforced beam and sensor location perpendicular to fracture location.

3. Results and discussion 3.1. Accelerated corrosion

The accelerated corrosion tests, facilitated by the inclusion of 2% NaCl by weight of cement in the mortar mix, were employed to simulate a chloride-contaminated environment for steel rebar corrosion. Initial corrosion assessments via electrochemical measurements revealed that despite achieving moderate to high corrosion levels, as per UNE 112072:2011 standard, the actual rate of corrosion was insufficient to achieve elevated oxides production within a short time. Indeed, with a natural corrosion current of 0.630 ± 0.044 μA/cm², the samples followed an estimated attack penetration rate of ~0.09 μm/day. Therefore, the galvanostatic approach was employed, ensuring a consistent and predictable rate of oxide production (i.e., 4.74 μm/day in agreement with equation 2). To empirically assess the effectiveness of the theoretical attack penetration (equation 2), the diameter of fully corroded rebars was measured via stereoscope. Figure 6 depicts the difference between non-corroded and corroded rebar, treated with a solution of hydrochloric acid and hexamethylenetetramine for steel corrosion prevention, to remove the oxides on the rebars’ surface while limiting any further oxidation of the rebar in the acid. After applying I corr = 100 μA/cm² for 24 days, the theoretical attac k penetration (A.P.) reached ~114 μm. The rebar diameter after