PSI - Issue 13

Zoi S. Metaxa et al. / Procedia Structural Integrity 13 (2018) 2011–2016 Z.S. Metaxa and S.K. Kourkoulis / Structural Integrity Procedia 00 (2018) 000 – 000

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manufactured using a water-cooled saw. Preliminary mechanical tests have shown that when notched specimens are used the results are much more consistent compared to the results of the typical three-point bending test with specimens without a notch. Three specimens (at least) were tested per each experimental batch. An MTS loading frame (capacity 10 kN) was used. A clip gauge (Fig. 1) measured the Crack Mouth Opening Displacement (CMOD). The experiments were quasi-static under displacement control conditions, at a constant rate of 0.001 mm/s. The load, crosshead dis placement and CMOD data were recorded and stored during the test.

3. Results and discussion 3.1. Effect of dispersing agent concentration

It is widely recognized that one of the crucial factors affecting the nanomaterials’ homogeneous distribution is the dispersing agent’s concentration. According to Ragstogi et al. (2008) and Yu et al. (2007) there exists an optimum dispersing agent amount that results to a uniform nanomaterials distribution. This optimum concentration depends both on the dispersing agent and the nanomaterials nature. Commonly, suspensions containing dispersing agent amounts lower or higher than the optimum one demonstrate poorer dispersion (Rastogi et al. (2008)). For cementitious materials made from Type I cement, with carbon nanomaterials having a fiber shape (CNTs and CNFs), the optimum dispersing agent to nanomaterials’ ratio is close to 4.0 (Konsta -Gdoutos et al. (2010) and Metaxa et al. (2013)). Type II cement paste nanocomposites reinforced with 0.1 wt.% GNPs, having a water to cement ratio of 0.3, treated with eight different superplasticizer concentrations (0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.00 % by weight of cement) were produced. Fig. 2a demonstrates the electrical resistivity of the nanocomposites. The results are compared with the reference cement paste having no superplasticizer and GnPs (superplasticizer + GnPs = 0.0). Compared to the re ference cement paste, which has insulator characteristics, all the nanocomposites exhibit lower resistivity. Adding the GnPs, even at such low concentration and in some cases at a non-uniform distribution, creates a conductive network in the matrix that aids the electrical current to pass through its body resulting at the observed reduction of the electrical resistivity. This conductive network is improved when the GnPs are homogeneously dispersed resulting to measure ment of lower resistivity. Taking that into account, the results of the electrical resistivity can be associated with the GnPs dispersion state, i.e., the lower the electrical resistivity, the more uniform the GnPs dispersion into the ce mentitious matrix (Metaxa (2015)). The nanocomposites dispersed at a concentration of 0.7 wt% of cement demon strate the lowest resistivity. The results support the theory that there exists an optimum dispersing agent concentration cause the nanocomposites dispersed with slightly lower or higher amounts demonstrate much higher resistivity. To further test the aforesaid theory and also study the effect of dispersing agent concentration on the mechanical performance of the nanocomposites three-point bending tests on pre-notched samples were conducted. The normalized flexural strength values of the nanocomposites reinforced with GnPs treated with different superplasticizer concentra-

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Cement paste (CEM II, w/c=0.3) + 0,1wt% GnPs 8 μm

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Electrical resistivity (MOhm*cm)

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 0,0 Reference-no GnPs Normalized flexural strength (-)

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 0,0 Reference cement paste-no GnPs Superplasticizer concentration (% by weight of cement)

Superplasticizer concentration (% by weight of cement) Fig. 2. Effect superplasticizer concentration of type II/GnP cementitious nanocomposites on the (a) electrical resistivity and (b) flexural strength normalized over the respective value of the reference samples. (a) (b)