PSI - Issue 42

E. Tziviloglou et al. / Procedia Structural Integrity 42 (2022) 1700–1707 Tziviloglou et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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superior fracture toughness of mixtures M-0.05 and M-0.10 can be attributed to the density enhancement, accompanied by inhibition of crack propagation, originating from the microstructure and porosity refinement, caused by an effective dispersion of the nanoparticles in the cementitious matrix. On the other hand, a less efficient dispersion that might occur in higher concentrations of nanoparticles can weaken the flexural response of the cementitious nanocomposites. In fact, an inadequate GnP dispersion may lead to the formation of agglomerations that in turn can create gaps between the clusters of the nanoparticles and the cementitious matrix, resulting in an inferior mechanical response.

1,4

Fracture toughness of pastes with different xGnP content

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0,5 )

1,2

0,9 Fracture toughness-K IC (MPa·m 1,0 1,1

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0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40

xGnP content (% wt.)

Fig. 3. Average fracture toughness K Ic values of the investigated pastes with different GnP content.

3.2. Electrochemical impedance 3.2.1. Nyquist plots and electrical resistivity

Exploiting the data acquired by the EIS scans, Nyquist plots of the negative imaginary versus the real part of impedance over the corresponding frequency range were constructed, as shown in Fig. 4. In each plot, the test frequency decreases from 0.391 MHz to 1 Hz, as the real part of impedance increases along the horizontal axis. It is evident in Fig. 4 that every Nyquist plot comprises of two arcs: the lower-frequency arc (dashed line) and the higher frequency arc (continuous line). The lower-frequency arc corresponds to the electrochemical response of the steel electrodes used for the measurements, while the higher-frequency arc to the properties of the cementitious composites. The point of the Nyquist plot, where the two arcs meet (cusp point) represents the electrical resistance of the cementitious matrix according to McCarter et al. (1988). As derived from Fig. 4, the cusp point for the reference mixture is located further to the right of the graph, while for the GnPs-modified mixtures the cusp point is shifted to the left on the graph. This implies that the electrical resistance of the reference mixture exhibits the highest value, while mixtures with GnPs show a noticeable decrease in their resistance. The electrical resistivity for each specimen was calculated using the cusp point value obtained from the corresponding Nyquist plot. In particular, the electrical resistivity of each specimen was calculated according to Eq.3: = ∙ , (3) where ρ is the resistivity, R is the electrical resistance as indicated by the cusp point of the Nyquist plot, A is the cross section of the specimen, and l is the distance between the two steel electrodes (approximately 30 mm).