Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44; DOI: 10.3221/IGF-ESIS.18.04 43 Saez de Ibarra et al. [10], after addition of MWCNTs or SWCNTs to cement paste with gum Arabic, observed an higher change in the samples with SWNTs, as a result of their superior mechanical properties when properly dispersed and of the lower quantity of SWNTs and gum arabic in the mix, indicating that the mix design is closer to an optimum value. S ENSING PROPERTIES s such carbon nanotubes can be used to decrease the electrical resistivity of cement composites by the formation of a well meshwork, while, COOH-MWCNTs have a stronger effect on enhancing pressure-sensitive properties because they were covered by C-S-H and no meshwork was formed [27]. This latter microscopic characteristic leads to the fact that the contact points and distance among nanotubes noticeably changes with the variation of a compressive force, as illustrated by Fig. 16, where the relationship of resistivity along stress for untreated CNTs (PCNT) was 10%, and the fractional change of resistivity for treated CNTs (SPCNT) was about 14%. Similar results have been reported by Gao et al on carbon nanofibers (CNF) [31]. Coppola et al [15] have also shown that in the absence of an external applied force, the increase of nanotubes content in the cement paste leads to a decrease of the electrical resistance and that for 10 wt% of carbon nanotubes in the cement paste the sample becomes a good conductor. Moreover, they observed that capillary pores, when containing water, are the preferential paths for current, thus leading to a minor variation of the resistance under load because of a minor effect due to the carbon nanotubes. Figure 16 : Fractional change in resistivity in function of time under cyclic compressive loading (0-15 kN) on 40×40×160 mm 3 prisms (PCNT: untreated CNTs, SPCNT: treated CNTs, 0.5 wt%/ cement) [27]. C ONCLUSIONS he present paper reviews the current state of the art of carbon nanotubes cement-based composites. The potential of nanotechnology to improve the performance of concrete and to lead to the development of novel, sustainable, advanced cement-based composites with unique mechanical and electrical properties is really promising. However, current challenges need to be solved before the full potential of nanotechnology can be exploited in concrete: in particular, proper dispersion and functionalization of MWCNTs, concrete processing, handling issues of CNTs, scale-up and materials cost. R EFERENCES [1] O. Breuer, U. Sundararaj, Polymer Composites, 25 (6) (2004) 630. [2] J. Makar, J. Margeson, J. Luh, In: 3 rd International Conference on Construction Materials: Performance, Innovations and Structural Implications, Vancouver, B.C., (2005) 1. A T