**2. Improvement of mechanical properties and durability**

Improvement of mechanical properties and durability of cementitious materials is mostly obtained by their nanostructure modification, that is, the incorporation of nanomaterials into cement matrix. Nanoparticles possess a high chemical reactivity due to a high surface area and can promote the growth of cement hydration products. Nanomaterials employed in cementitious composites, up to date, are nano-silica, nano-titania, nano-iron oxide, nano-alumina, nanoclay particles, carbon nanotubes, graphene oxide (GO), and graphene nanoplatelets (GNPs) [7].

Nano-silica (nano-SiO<sup>2</sup> ) is proved to enhance the compressive, tensile, and flexural strength of OPC pastes and mortars [8]. The addition of nano-SiO<sup>2</sup> effects in denser cement paste microstructure with improved porosity thus leads to a decreased water penetration and sorptivity [8] and therefore to the reduction of calcium leaching [9]. Nano-alumina (nano-Al<sup>2</sup> O3 ) matches the performance of nano-silica—it leads to a more compacted microstructure of cementitious composites, decreases their porosity, and enhances the compressive strength [10, 11]. It is worth noting that nano-alumina was proved to improve concrete performance at both elevated and low temperatures [10, 11]. The incorporation of nano-titania (nano-TiO<sup>2</sup> ) may lead to the enhancement of compressive and flexural strength as well as to the improvement in the resistance to chloride penetration due to a refined pore structure of the composites [12]. The impact of nano-titania addition on the performance of cementitious composites at an elevated temperature turned out to be comparable to composites incorporating nano-alumina [13].

has been modified several times over the years. Today, it can be defined as *"the application of scientific knowledge to manipulate, control and restructure matter at the atomic and molecular level in the range of 1–100 nm to exploit the size-dependent and structure-dependent properties and phenomena distinct from those at different scales"* [3]. Basically, nanotechnology is based on the statement that we can change any property of any material with reducing at least one dimension

While nanotechnology has attracted attention in many fields of science and technology, including chemistry, electronics, medicine, or biology, its application in civil engineering, up to date, remains limited [1, 5–7]. These days, searching in SCOPUS database for the terms *"nanomaterial"* AND *"civil engineering"* within titles, abstracts and keywords of published papers returns only 18 document results. The RILEM TC 197-NCM Report [5] has highlighted, for the first time in 2004, the potential applications of nanotechnology in construction materials. The e-mail survey carried out among researchers, construction professionals, and large construction companies was the basis of reported information. The report revealed that little awareness of nanotechnology applications in construction is an effect of insufficient information on this subject. Therefore, nanotechnology was perceived as expensive and highly complex, thus discouraging potential customers. However, the results of the survey have shown that nearly 100 research projects carried out by respondents were based on nanotechnology.

Since then, research introducing nanotechnology in civil engineering has followed mainly

This chapter presents a review of the achievements of nanotechnology in Structural Engineering with special emphasis on improved physical parameters of structural materials

Improvement of mechanical properties and durability of cementitious materials is mostly obtained by their nanostructure modification, that is, the incorporation of nanomaterials into cement matrix. Nanoparticles possess a high chemical reactivity due to a high surface area and can promote the growth of cement hydration products. Nanomaterials employed in cementitious composites, up to date, are nano-silica, nano-titania, nano-iron oxide, nano-alumina, nanoclay particles, carbon nanotubes, graphene oxide (GO), and graphene nanoplatelets (GNPs) [7].

and their potential in strengthening repairs and Structural Health Monitoring.

**2. Improvement of mechanical properties and durability**

The potential nanotechnology applications were pointed out as follows:

of this material into the nanoscale [4].

116 New Uses of Micro and Nanomaterials

• understanding nanostructure of materials, • nanostructure modification of materials,

• environment-friendly applications [5].

these abovementioned development paths.

• functional films and coatings, • smart structures and devices,

However, the most studied nanomaterials to be used in cementitious composites are carbonbased nanomaterials. Until 1985, only two allotropic forms of solid carbon had been known, these had been diamond and graphite, which both feature covalently bonded networks [14]. In 1985, the new era of carbon nanomaterials had begun, when fullerenes—molecules composed of 60 carbon atoms, C60—had been discovered [14, 15]. It was less than 6 years later when it turned out that carbon atoms can also form cylindrical tubes. In 1991, Iijima [16] observed first the multi-walled carbon nanotubes (MWCNTs) and then in 1993, Iijima and Ichihashi [17] reported single-walled nanotubes (SWCNTs).

CNTs possess extraordinary electrical, thermal, and mechanical properties, highly relying on their dimensions. The diameters are in the range of 1.4–100 and 0.4–3 nm for MWCNTs and SWCNTs, respectively. Young's modulus for SWCNTs and MWCNTs is equal to ca. 1 and 0.21 TPa, respectively, while the tensile strength for both types of CNTs reaches 500 and 10–63 GPa [15].

Manufacturing of cementitious composites incorporating carbon-based nanomaterials is an extremely challenging task due to the crucial problem of obtaining a homogeneous dispersion of a nanomaterial within cement matrix. Carbon-based nanomaterials are prone to form aggregates and bundling as an effect of both their high hydrophobicity and strong van der Waals forces [18–20]. Nonuniformly dispersed nanoparticles strongly influence the workability and microstructure of cement composites and hinder the ongoing hydration; thus, it is of significant importance to adopt an appropriate treatment to obtain a sufficient consistency and dispersion of nanomaterial within cement matrix.

Several different attempts to obtain a homogeneous dispersion of CNTs in cement mix were reported, including carboxylation of CNTs [21], that is, special treatment to attach carboxylic acid to their surface or functionalization of CNTs with COOH groups [22, 23]. Nevertheless, the main approach employed to fabricate cement-CNT composites is, clearly, stirring and ultrasonication of aqueous dispersion of CNTs with various types of surfactants, such as polycarboxylic acid-based superplasticizers [23–25], anionic sodium dodecyl sulfate [20, 26], sodium dodecyl benzene sulfonate [27], nonionic polyoxyethylene(23) laurylether [20], Gum Arabic [22, 28], polyacrylic acid polymer [22], and cetyltrimethylammonium bromide [27], to name a few, or solvents, for instance, acetone [29]. It is worth noting that the studies on CNTs dispersion [20, 30] have shown that the most beneficial dispersion is the one with a CNT-tosurfactant ratio of 1:1–1:5.

nucleation effect [46–48]. As a consequence of this 3D network, the microstructure of cement composites is visibly densified with a higher crystal growth and less prominent microcracks. Furthermore, also brittle crystals of ettringite are hardly observed [49]. The addition of GO remarkably refines the porosity, reducing the critical pore size and the volume of macropores [48, 50]. For the reason of reduced porosity, the incorporation of even small amount of GO into cementitious composites leads to a decreased sorptivity [50, 51]. The decrease up to 8 and 44% has been reported for initial and secondary sorptivity, respectively [50]. Therefore, cement-GO composites feature with a tremendously reduced ingress of chlorides. Even the marginal addition of graphene oxide of 0.01 wt% may effect in significant decrease of chloride penetration depth from 26 to 5 mm [51]. Interestingly, the addition of GO and its acceleration effect on cement hydration lead to a higher drying shrinkage at early stages of hydration. Nevertheless, since drying shrinkage depends on the tension of capillary pores, which are highly reduced in composites reinforced with graphene oxide, drying shrinkage after 28 days

Nanomaterials in Structural Engineering http://dx.doi.org/10.5772/intechopen.79995 119

Some attempts [19, 52] of introducing graphene nanoplatelets (GNPs) to improve the barrier properties and enhance the durability of cementitious composites have been reported. In this respect, this low-cost graphene derivative matches the performance of graphene oxide in concrete. The addition of 1.5 wt% of GNPs contributes to pore refinement, reducing the critical pore diameter and the average void size, thereby decreasing the water permeability, chloride diffusion, and chloride migration by 80, 80 and 40%, respectively [52]. It is worth noting that according to various authors, the addition of GNPs does not improve [52] or may even, to

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

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

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

is then reduced [47].

some extent, deteriorate [53] the strength of concrete.

**3. Self-monitoring materials**

implemented in large scale.

for self-monitoring applications in concrete.

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].

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

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 nucleation effect [46–48]. As a consequence of this 3D network, the microstructure of cement composites is visibly densified with a higher crystal growth and less prominent microcracks. Furthermore, also brittle crystals of ettringite are hardly observed [49]. The addition of GO remarkably refines the porosity, reducing the critical pore size and the volume of macropores [48, 50]. For the reason of reduced porosity, the incorporation of even small amount of GO into cementitious composites leads to a decreased sorptivity [50, 51]. The decrease up to 8 and 44% has been reported for initial and secondary sorptivity, respectively [50]. Therefore, cement-GO composites feature with a tremendously reduced ingress of chlorides. Even the marginal addition of graphene oxide of 0.01 wt% may effect in significant decrease of chloride penetration depth from 26 to 5 mm [51]. Interestingly, the addition of GO and its acceleration effect on cement hydration lead to a higher drying shrinkage at early stages of hydration. Nevertheless, since drying shrinkage depends on the tension of capillary pores, which are highly reduced in composites reinforced with graphene oxide, drying shrinkage after 28 days is then reduced [47].

Some attempts [19, 52] of introducing graphene nanoplatelets (GNPs) to improve the barrier properties and enhance the durability of cementitious composites have been reported. In this respect, this low-cost graphene derivative matches the performance of graphene oxide in concrete. The addition of 1.5 wt% of GNPs contributes to pore refinement, reducing the critical pore diameter and the average void size, thereby decreasing the water permeability, chloride diffusion, and chloride migration by 80, 80 and 40%, respectively [52]. It is worth noting that according to various authors, the addition of GNPs does not improve [52] or may even, to some extent, deteriorate [53] the strength of concrete.
