**3. Self-monitoring materials**

dispersion [20, 30] have shown that the most beneficial dispersion is the one with a CNT-to-

CNTs can enhance both the compressive and flexural strength of cementitious composites up to 50 [22] and 87% [24], respectively. The addition of CNTs also improves both the fracture energy and flexural toughness [31]. Young's modulus of cement mortars containing 0.1 wt% of CNTs can be even 100% higher compared to reference samples [24]. According to SEM images, the interaction between cement hydration products and CNTs is observed [32]. CNTs increase the crack bridging capacity of cementitious composites, acting as networks between the crack and the pores [23, 31, 33]. Moreover, nanoindentation investigation indicates that CNTs contribute to a higher growth of strong C–S–H phase [30]. CNTs act as the nanofiller of voids and thus reduce the total pore volume of cement paste [21, 23, 30, 32]. Interestingly, the addition of CNTs decreases the drying shrinkage of composites. Indeed, the authors [34] attributed this behavior to the reduction of micropores. It is worth noting that the influence of CNTs on the microstructure, porosity, and thereby mechanical properties of cementitious composites is highly dependent on the quality of their dispersion within cement matrix as well as on the type of surfactant to be used. Several studies show that the addition of CNTs

may also deteriorate the properties of cementitious nanocomposites [28, 29, 33].

Young's modulus may be estimated at 1 TPa [36].

Over the past decades, graphene—another carbon allotrope, which is a single, planar, twodimensional carbon layer [35]—has attracted considerable attention in science and technology, while its extraordinary properties have been extensively studied by various research groups. Especially, due to its outstanding mechanical [36] and electrical properties [37], graphene has emerged as the most promising nanomaterial for smart structures. Graphene is known to exhibit the intrinsic tensile strength of 130 GPa with a corresponding strain of 0.25, while its

Nevertheless, studies on graphene-cement composites remain, up to date, limited due to the abovementioned perplexing problem of obtaining a uniform dispersion of a nanomaterial within cement matrix. For this reason, over the past years, special attention was paid to one of graphene derivatives, that is, graphene oxide (GO). Graphene oxide is highly dispersible in water [38] and therefore, as was assumed, also in cement mix. However, several studies [39–41] show that calcium ions present in cement paste negatively affect graphene oxide dispersion due to the chemical cross-linking phenomena. To circumvent this problem, different approaches have been persuaded, including the sonication of graphene oxide with polycarboxylate superplasticizer [42, 43] or silica fume [39, 40], which provide surface modification of

nanomaterial and thereby separate graphene oxide nanoplatelets from calcium ions.

However, various cementitious composites incorporating graphene oxide, with or without surface modification, have emerged as materials with improved microstructure, mechanical properties, and durability. With the dosage of 0.03–0.05 wt% of GO, the increase up to 47, 61, and 79% has been reported for compressive [44], flexural, and tensile-splitting strength [45], respectively. The strengthening mechanism of GO in cement matrix is attributed to the chemical reaction between -COOH groups attached on the GO flakes and calcium ions from calcium hydroxide present in cement; thus, a 3D network structure is formed. Moreover, graphene oxide promotes and accelerates the growth of cement hydration products due to the

surfactant ratio of 1:1–1:5.

118 New Uses of Micro and Nanomaterials

Electrical properties of carbon-based materials in structural engineering are drawing attention of scientists for many years, giving hope for smart materials and self-monitoring structures.

One of the first attempts of using carbon-based materials in concrete was made almost three decades ago when cut carbon fibers were mixed with concrete for traffic monitoring and weighting in motion [54]. The results were promising; however, this solution had never been implemented in large scale.

The development of science and technologies during recent years has brought new nanomaterials as graphene or carbon nanotubes with even more interesting properties, also electrical. Former experiences in structural engineering materials but also in other areas of science as medicine or aviation encouraged scientists to return to the concept of self-monitoring materials for smart structures. Clearly, carbon nanotubes are the most studied carbon nanomaterial for self-monitoring applications in concrete.

Typically, various types of sensors are used to evaluate structural health, including optical fibers, strain gauges, and piezoresistive sensors. However, these sensors possess some serious limitations and disadvantages, such as high cost, poor durability, low sensitivity, and insufficient compatibility with concrete and expensive peripheral equipment [6, 55]. The new generation of nanotechnology-based microelectromechanical system (MEMS) sensors has emerged as cheaper, more compact, and easier to install sensors than traditional ones. Nanotechnology/microelectromechanical systems were used, for instance, to measure temperature and internal relative humidity of concrete [56] or to detect cracks in concrete structures [57]. Sensors for detecting the structural integrity of concrete were fabricated as wireless cement-carbon nanotube sensors embedded into concrete beams [57]. These CNTs-cement sensors have emerged as a low-cost small wireless sensor with good sensitivity, significant repeatability, and low hysteresis. Moreover, Lebental et al. [58] have developed well-aligned, ultra-thin, dense carbon nanotube membranes to be used as a vibrating membrane in a capacitive micrometric ultrasonic transducers, which could be used in the durability monitoring of porous materials. Kang et al. [59] have fabricated a long biomimetic artificial neuron sensor, with features such as low cost, simple installation, and low weight. Due to low bandwidth and appropriate strain sensitivity, it can be used for the detection of both small and large strains and cracks in concrete structures, also under dynamic loading.

structures endangered by hazardous loads as, for example, earthquakes [64]. Such products have the potential of creating self-sensing systems in historical structures, giving possibility

Numerous exciting examples of antimicrobial and self-cleaning surfaces as well as energy-

Recently, it has been shown [6, 65] that some nanoparticles possess tremendous antimicrobial properties and can be used to fabricate antimicrobial materials or coatings. In particular,

per may enhance the photocatalytic activity of nano-titania even under weak UV light [66]. Interestingly, Hochmannova and Vytrasova [67] have presented paints based on aqueous acrylic dispersion with the 5 vol% addition of nano-ZnO, which proved to be a better photo-

was sufficient for nano-ZnO to activate the photocatalytic and microbial processes, deactivating the tremendously wide spectrum of bacteria and fungi. Furthermore, the studies on phenylpropyl type interior wall paints incorporating nano-MgO [68] have revealed that, in contrast

in the absence of light irradiation. In addition, the addition of silver nanoparticles to paints and coating effects in significant antimicrobial properties in case of both Gram-positive and Gramnegative bacteria [6, 69]. In case of carbon nanomaterials, SWCNTs can cause physical cell membrane damage and oxidative stress, impacting also metabolic activity and morphology of *E. coli* bacteria [70]. Grover et al. [71] have prepared laccase-based and chloroperoxidase-based paints, incorporating MWCNTs for biocatalytic coatings. These enzyme-nanotube-based

Apart from antibacterial surfaces, the addition of nanomaterials may also enhance the selfcleaning abilities of construction materials. Self-cleaning surfaces are mainly classified into two categories: hydrophobic and hydrophilic surfaces. As reported by previous studies [72–74], nano silica may be used to fabricate transparent superhydrophobic films and coatings on glass. The

Hydrophobic surfaces were also developed with the use of carbon nanotubes (CNTs). Transparent, conductive, and superhydrophobic films incorporating CNTs were prepared on a glass substrate using, for instance, fluoropolymer-grafted MCWNTs [75] or CNTs produced by plasma-enhanced chemical vapor deposition and functionalized by a 1H,1H-2H,2H perfluorodecyl-trichlorosilane and hexane mixture [76]. Nanoparticles used typically in hydrophilic surfaces are materials with photocatalytic properties. Tan et al. [77] have revealed that trans-

super-amphilicity, which possessed superhydrophilic properties even after 240 days in the

paints exhibited a high bactericidal activity against different evaluated bacteria.

nanoparticles proved to completely damage *Escherichia coli* cells after 1 week under UV

. Moreover, it has been reported that the addition of silver or cop-

and nano-ZnO, nano-MgO possess a sufficient antimicrobial activity

. The normal domestic fluorescent light

Nanomaterials in Structural Engineering http://dx.doi.org/10.5772/intechopen.79995 121

also possesses antireflection properties [72, 73].

nanotube arrays on glass substrate have

thin films due to a higher

films with stable

for high-performance repairs and relatively cheap and invisible monitoring solution.

harvesting applications have also been reported in the last decade [6].

catalytic coatings than the one containing nano-TiO<sup>2</sup>

nanoporous structure made out of nano-SiO<sup>2</sup>

films fabricated by the growth of TiO<sup>2</sup>

exhibited a higher photocatalytic activity than nanoparticulate TiO<sup>2</sup>

surface area. Interestingly, Pan et al. [78] have presented nanofiber-based TiO<sup>2</sup>

**4. Other applications**

irradiation of 1 mW/cm<sup>2</sup>

to paints with nano-TiO<sup>2</sup>

parent TiO<sup>2</sup>

TiO<sup>2</sup>

Interestingly, Nanni et al. [60] have presented self-sensing nanocomposite rods to be applied as both reinforcing elements and sensors in concrete structures. The self-sensing rods are composed of an internal conductive core, that is, glass fibers embedded in epoxy resin with carbon nanoparticles (CNPs) and an external insulting GFRP skin. The nanocomposite rods have proved to be suitable for self-monitoring of concrete beams under a four-point static bending as well as for concrete cure monitoring.

The concept of weighting loads in motion came back then recently with those new materials [61]. The research team conducted tests on compressed blocks of the concrete with carbon nanotubes and registered its performance under static and dynamic loads. The authors registered changes of electrical resistance readings, proving that even micro-strains may be measured by such smart materials. This very recent work demands more calibrating studies; still, it proves high potential of nano-concrete.

Outstanding electrical properties and low cost make graphene nanoplatelets (GNPs) an attractive nanomaterial for use in smart self-sensing concrete. As demonstrated by recent studies [62], the addition of 1.6 wt% of GNPs (a surface area of 192 m<sup>2</sup> /g, a diameter of 6.8 μm, and a thickness of 5.0 nm) decreases more than one order of magnitude the resistivity of tested composites, thus attaining the percolation point, above which GNPs form the continuous conductive network in cement matrix. Interestingly, during the piezoresistive tests under compression, it turned out that no piezoresistive reactions were detected for samples containing 1.6 wt% of GNPs, indicating that conductive network created by tunneling of GNPs is unstable under applied loading. Indeed, the addition of only 6.4 wt% of GNPs has led to a sufficient and stable response of electrical parameters under cyclic loads. Other studies by Lee et al. [63] have revealed that, for GNPs with a surface area of 352 m<sup>2</sup> /g, a diameter of 2.6 μm, and a thickness of 2.6 nm, the percolation threshold was obtained for 3.6 wt% amount of nanomaterial. Tests on samples with different notch depths confirmed the electrically conductive characteristics of manufactured mortar.

A very interesting and novel approach for use in structural engineering is connected with the proposal of Smart Bricks for Structural Health Monitoring of existing, often historical structures endangered by hazardous loads as, for example, earthquakes [64]. Such products have the potential of creating self-sensing systems in historical structures, giving possibility for high-performance repairs and relatively cheap and invisible monitoring solution.
