Improved piezoresistive sensitivity and stability of CNT/cement mortar composites with low water–binder ratio
Introduction
Application of carbon nanotubes (CNTs) in cement-based composites for various purposes has been increasingly attracting attention [21], [25], [26], [27], [28]. One of the main purposes of incorporating CNTs is to provide electrical properties to cement composites. Conventionally, low-priced carbon materials (i.e. coke powder, graphite, and carbon black) and/or metallic inclusions (i.e. steel fibers and shavings) have been utilized in cement composites for this purpose [10], [11], [12], [13], [14], [29]. Conductive cement composites incorporating these materials have been applied as anti-corrosion earth connectors for electrical-shock protection and electric-heating pavement materials for de-icing roads [11], [12].
The conventional conductive materials, however, have some disadvantages. In the case of the metallic inclusions, such as steel fibers and shavings, corrosion of these materials leads to degrade the durability of cement composites. In the case of low-priced carbon materials, the required content is too high to provide sufficient electrical conductivity for cement composites. For example, the content of carbon black in a cement composite to obtain electrical resistivity lower than 100 Ω cm was more than 20% by weight of cement [13]. Similarly, the content of graphite powder in a cement composite to obtain electrical resistivity lower than 100 Ω cm was more than 40 vol.%, i.e. about 60–80% by weight of cement [15]. These high contents of the conventional carbon materials degrade the mechanical properties and durability of cement composites [13], [23].
On the contrary, it was found that a small amount of CNTs (i.e. less than 1 wt.% by weight of cement) could provide cement composites with a similar level of electrical conductivity as obtained with high contents of carbon black or graphite [13], [23]. Moreover, when small amounts of CNTs were incorporated, the mechanical properties of cement composites were not degraded or were even improved [23].
By the addition of CNTs, electrical conductivity was not only provided to cement composites, but piezoresistivity was also introduced at the same time [16], [17]. Piezoresistivity is an effect of electrical resistance change induced by deformation of materials [6]. A number of studies on the piezoresistivity of cement composites containing conventional carbon materials have been reported ([9], [13], [6], [8]; Wen and Chung, 2009). However, studies on the piezoresistivity of CNT/cement composites are on a preliminary level, and most have only dealt with the feasibility of the composites [7], [4], [7], [1], [3]. Few in-depth studies on the factors affecting the piezoresistivity of composites, such as the mix proportion of the cement matrix or the moisture content, have been reported to date.
In previous studies by the authors, the effects of mix proportion on mechanical and electrical properties of CNT/cement composites were investigated [21], [22], [2], [23]. It was found that the dispersion of CNTs in the cement matrix was enhanced with lower water–binder ratio (W/B) and inclusion of some silica fume, without a sonication process [23]. This led to enhanced effects of CNTs on mechanical and electrical properties of cement composites [22], [23]. In particular, for a cement paste composite containing 0.3% of CNTs, the electrical resistance was drastically decreased from 108 to 104 Ω (reaching a level of 1/10,000) when the W/B of the composites was decreased from 0.25 to 0.20 [23].
The objective of this study was to investigate the piezoresistive sensitivity and stability of CNT/cement mortar composites with low water–binder ratio. The effect of absorbed water on the piezoresistivity of the composites was also investigated, since it strongly affects the electrical properties of the composites [5]. Cement mortar composites with three different W/B values, 0.4, 0.5, and 0.6, were designed, and for each mixture, CNTs in amounts of 0.1%, 0.3%, and 0.5% by weight of cement were applied. Specimens with two different moisture contents, saturated-surface dry (S.S.D.) and oven dry (O.D.) conditions, were prepared for all experimental studies. The initial electrical resistance of the CNT/mortar composites was measured prior to loading. Also, the changes in the electrical resistance of the specimens induced by external cyclic loading were measured to investigate their piezoresistive sensitivity and stability.
Section snippets
Preparation of specimens
Type I Portland cement (C) and silica fume (SF), which is a proprietary product of Elkem Inc. (EMS-970 D), were used as binder materials. Crushed sand (A) having a specific gravity of 2.60 in the S.S.D. condition was used as a fine aggregate. A poly-carboxylic acid-based super-plasticizer (SP), a proprietary product of BASF Pozzolith Ltd. (GLENIUM 8008), was added to enhance the workability of the fresh mortar. Multi-walled carbon nanotubes, a proprietary product of Carbon Nano-material
Initial electrical resistance prior to loading
Fig. 3 shows the initial electrical resistance of the composites prior to loading. The experimental results indicate that the effect of moisture content on the resistance of the composites was decreased as the W/B of the composites was decreased. It should be noted that the error bars in the figure denote the upper and lower values of the experimental results. As shown in the figure, for the composites with a high W/B of 0.6, the resistance was not significantly affected by CNT content, was
Piezoresistivity under cyclic loading
In the present work, the following three factors were evaluated to investigate the piezoresistivity of the CNT/cement composites: (1) stability of piezoresistivity under cyclic loading, (2) range of electrical resistance change by loading, and (3) time-based sensitivity of piezoresistivity.
Concluding remarks
In the present study, the sensitivity and stability of piezoresistive CNT/cement mortar composites with low W/B were investigated. Cement mortar composites with different W/B water–binder ratios and CNT contents were designed. Specimens with two different moisture contents, saturated-surface dry (S.S.D.) and oven dry (O.D.) conditions, were prepared for all experimental studies. The initial electrical resistance of the CNT/mortar composites was measured prior to loading. To investigate the
Acknowledgments
This research was sponsored by a Grant from the National Research Foundation of Korea (NRF) (2013028443) funded by the Korean government and by Chosun University in South Korea. The authors would like to thank Mr. I.W. Nam at KAIST for his help in preparation for the experiment and in writing the manuscript.
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