Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44; DOI: 10.3221/IGF-ESIS.18.04 36 However, properties and dimensions of CNTs strongly depend on the deposition parameters and the nature of the synthesis method. In view of a commercial application, the chemical vapor deposition (CVD) technique is the only one that can offer a path towards low-cost and large scale production, albeit it must be highlighted that CVD can produce nanotubes that have non negligible amount of lattice defects along the graphene walls and often these tubes are curled and aggregated to form bundles and ropes (Fig. 3) that have a lower strength and are very difficult to disperse [7]. The amount of defects can play a key role for cement reinforcement application. In fact, defect free CNTs, obtained with a complete graphitization process achieved by heat-treatment at high temperature [8], either in vacuum or inert environment, show outstanding mechanical properties [9], although they are incapable to form proper adhesion with the matrix, causing what it is called sliding [6]. On the other hand, while lattice defects limit mechanical strength, they are reactive spots that can be used to produce functional groups on the outer walls by chemical treatments with acid solutions. These superficial chemical groups, such as carboxylic (-COOH) groups, can originate strong chemical bonds between CNTs and cementitious matrix, thus enhancing the reinforcement efficiency even though to the detriment of graphitization degree [11, 12, 13]. Furthermore, chemical treatments can help to disentangle the bundles, hence facilitating a uniform dispersion at the single tube level, above all in aqueous media such as that for cement composites. Proper dispersion and adequate load transfer are the main challenges in the search for efficient carbon nanotubes reinforced cement composites. Figure 3 : Scanning electron microscopy (SEM) micrograph of a bundle of MWCNTs produced by CVD technique from camphor and ferrocene. Depending on their precise structure, CNTs may be metallic or semiconductors: It is known that the electronic properties of CNTs can be controlled by chemical modifications of outer surface or by the presence, in the surrounding atmosphere or inside poorly degassed nanotubes, of minute quantities of O 2 . In particular, the conductivity type of the CNT can be changed from p-type to n-type by adsorption of O 2 [14]. Normal concrete is not sensitive to applied loads because of its low electrical conductivity. This weakness can be overcome by adding carbon nanotubes, which have a high conductivity (10 2 -10 4 S/cm). Therefore, if concrete is submitted to compressive forces this will increase the number of contact points between the nanotubes and thus, will lead to a decrease in the resistance of the material. This will then confer to concrete a stress-sensitivity that will enable a stress monitoring of the structures through the measurement of the variations of electrical resistivity. This feature of the carbon nanotubes reinforced structures will be used to continuously monitor normal stresses during service life as well as, exceptional events such as seism, shocks and explosions that generate plastic deformations [15]. Experimental results [16] showed that the fabricated self-sensing CNT/cement composites present sensitive and stable responses to repeated compressive loadings and impulsive loadings, and also have remarkable responses to vehicular loadings. These findings indicate that self-sensing CNT/cement composites have great potential for traffic monitoring. CNT S DISPERSION he dispersion of carbon nanotubes in the matrix is complex given the widespread specific surface area of the nanoparticles and due to Van der Waals forces which tend to favor the formation of agglomerates. Different techniques for dispersing said materials in solvents (acetone, ethanol…) in addition to ultrasounds, mechanical T