PSI - Issue 28

Angeliki-Eirini Dimou et al. / Procedia Structural Integrity 28 (2020) 1694–1701 A.-E. Dimou et al. / Structural Integrity Procedia 00 (2019) 000–000

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the voltage through the inner probes allows the measurement of the resistance. The resistance is constantly measured at a rate of 1 Hz for 30 minutes as described by Metaxa (2015). As polarisation phenomena take place in the beginning of the experiment, as noticed by Yang et al. (2018), the electrical resistance was recorded as for a long time, e.g. 30 min. The average electrical resistance was calculated as the average value deriving from the resistance measurements during the last five minutes. The experimental setup can be seen in Fig. 3, where the apparatus depicted is the 34970A Data Logger by Keysight Technologies.

Fig. 3. Experimental setup for the electrical resistance tests.

3. Results and discussion 3.1 Compressive strength

The results of the compressive tests are presented in the diagram of Fig. 4. The compressive strength of the blank paste was found to be 6.1 MPa, while the compressive strength of the MWCNTsCOOH and rGO reinforced pastes was 6.2 MPa and 7.1 MPa, respectively. There is no significant change between the reference paste and the MWCNTsCOOH reinforced paste, since the 0.1 MPa difference is within the standard deviation range. This trend is in line with the findings of Arrechea et al. (2020), who concluded that after 28 days the compressive strength increases by no more than 2 %, in comparison to the reference cement sample. However, the addition of rGO leads to a composite with almost 20 % increased compressive strength. The results are in accordance with similar studies, such as the one by Madbouly et al. (2020). It was found that the compressive strength was increased by almost 27 % and 38 %, when adding 0.04 wt% rGO and 0.05 wt% rGO into cement matrix, respectively. Also, Valizadeh Kiamahalleh et al. (2020) showed that compressive strength can even be increased up to 91 % and that the enhancement of mechanical properties depends on the size of rGO nanoparticles. 3.2 Flexural strength Fig. 5 displays the results of the four-point bending tests, where the flexural strength of the blank paste was equal to 1.6 MPa. The MWCNTsCOOH reinforced paste had 2.5 MPa flexural strength and the rGO reinforced pastes had almost the same flexural strength as the reference paste. Thus, while there is no significant change between the reference paste and the rGO reinforced paste, the addition of MWCNTsCOOH leads to 56 % increase in flexural strength. For example, Gillani et al. (2017) found that 0.05 wt%MWCNTs in the matrix increases the flexural strength by 26 %. Mohsen et al. (2017) also reported that the addition of 0.25 wt% MWCNTs increased the flexural strength of the final composite by 60 %. The incorporation of functionalized carbon nanotubes affected the hydration mechanism of the matrix, specifically the hydration of the calcium silicate that exerts a significant influence on mechanical strength, e.g. by Arrechea et al. (2020). Regarding the incorporation of rGO, the majority of studies reports an increase in flexural strength with addition of GO structures. For instance, Qureshi and Panesar (2020) reported that the incorporation of 0.16 wt% rGO in the cement matrix increases the flexural strength by 60 %. However, Lv et al. (2013) found that the flexural strength increased with increasing GO concentration up to 0.03 wt% and that higher concentrations lead to a decrease in flexural strength.