**5. CNT and CNF dispersion**

The majority of nanoreinforced composite research has been completed on polymers containing CNT or CNF [6, 19, 20]. One of the main reasons for this is because uniform dispersion is difficult in cement-based materials. Well dispersed CNF results in uniform calcium-silicate-hydrate (CSH) gel formation, which improves the structural and electrical properties of the concrete [21]. CNT and CNF are inherently hydrophobic and are attract‐ ed due to Van der Waals forces, causing the fibers to tend to agglomerate hindering their dispersion in solvants [17, 22-24].

**•** Passing ability – the ability of SCC to pass through congested reinforcement and adhere to

Carbon Nanofiber Concrete for Damage Detection of Infrastructure

http://dx.doi.org/10.5772/57096

129

**•** Stability – the ability of SCC to remain homogenous by resisting segregation, bleeding, and

Gao et al [12] studied SCC containing CNF to see if the same effect was present on the nano scale. In Gao et al's mixing procedure, HRWR, water, and CNF are mixed in a laboratory-grade blender. Simultaneously, fine aggregate, course aggregate, and cement are combined in a centrifugal mixer. The CNF mixture is then slowly added to the mixer to gain a homogenous mix. The fresh concrete was used to create cylinders that were tested in compression. After the test, pieces of the cylinders were observed under a scanning electron microscope (SEM). The SEM showed significant CNF clumping in specimens made of normal CNF concrete and

uniform distribution in SCC containing CNF, as shown in Figs. 2 and 3, respectively.

**Figure 2.** Scanning Electron Microscope Image of CNF Clump in Normal Cement (1670x Magnification)

**Figure 3.** Scanning Electron Microscope Image of Well Dispersed CNF in a Uniform Self-Consolidating Cement (9410x

Magnification)

air popping during transport and placing as well as after placement.

it without application of external energy.

Several solutions have been proposed to solve this issue including dispersing the fibers through milling, ultrasonication, high shear flow, elongational flow, functionalization, in addition to surfactant and chemical dispersement systems [24]. These methods primarily fall into two categories: mechanical and chemical dispersion. The mechanical dispersion methods, such as ultrasonification, while effective in seperating the fibers, can fracture them decreasing their aspect ratio. Chemical methods use surfactants or functionalization to make the fibers less hydrophobic, reducing their tendancy to agglomerate. However, many of the chemicals used can digest the fibers causing the fibers to become less effective. The surfactants also often cause bubbles to form in the composite negatively effecting the strength of the material.

Gao et al [12] proposed a dispersion method specifically used for CNF/CNT dispersion in cement-based materials that eliminates the beforementioned drawbacks. In this method, a high-range water reducer (HRWR) is used to create a self-consolidating concrete (SCC). ACI Committee 237 Self-Consolidating Concrete offers the following definition for SCC [25]:

Self-consolidating concrete (SCC) is highly flowable, non-segregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation.

SCC is a product of technological advancements in the area of under-water concrete technology where the mixtures must ensure high fluidity and high resistance to washout and segregation. Okamura originally advocated SCC in 1986, and the first success with SCC occurred in 1988 [26]. The use of SCC has gained wide acceptance for savings in labor costs, shortened con‐ struction time, a better finish, and an improved work environment [27-30].

Advancement in SCC technology was primarily possible due to the introduction of new chemical admixtures that improved and controlled the SCC rheological properties. Better performing SCC mixes were produced on the advent of melamine, naphthalene, polycarbox‐ ylate, and acrylic based HRWR superplasticizers and viscosity modifying agents (VMA).

Gao et al [12] proposed using SCC because acceptable SCC is not only highly flowable, but it is also highly stable and homogenious on a macro scale. The Prestressed Concrete Institute (PCI) stipulates the following criteria for SCC [26]:

**•** Filling ability – The property that determines how fast SCC flows under its own weight and completely fills intricate spaces with obstacles, such as reinforcement, without losing its stability.

**•** Passing ability – the ability of SCC to pass through congested reinforcement and adhere to it without application of external energy.

**5. CNT and CNF dispersion**

128 Advances in Nanofibers

dispersion in solvants [17, 22-24].

consolidation.

stability.

The majority of nanoreinforced composite research has been completed on polymers containing CNT or CNF [6, 19, 20]. One of the main reasons for this is because uniform dispersion is difficult in cement-based materials. Well dispersed CNF results in uniform calcium-silicate-hydrate (CSH) gel formation, which improves the structural and electrical properties of the concrete [21]. CNT and CNF are inherently hydrophobic and are attract‐ ed due to Van der Waals forces, causing the fibers to tend to agglomerate hindering their

Several solutions have been proposed to solve this issue including dispersing the fibers through milling, ultrasonication, high shear flow, elongational flow, functionalization, in addition to surfactant and chemical dispersement systems [24]. These methods primarily fall into two categories: mechanical and chemical dispersion. The mechanical dispersion methods, such as ultrasonification, while effective in seperating the fibers, can fracture them decreasing their aspect ratio. Chemical methods use surfactants or functionalization to make the fibers less hydrophobic, reducing their tendancy to agglomerate. However, many of the chemicals used can digest the fibers causing the fibers to become less effective. The surfactants also often cause bubbles to form in the composite negatively effecting the strength of the material.

Gao et al [12] proposed a dispersion method specifically used for CNF/CNT dispersion in cement-based materials that eliminates the beforementioned drawbacks. In this method, a high-range water reducer (HRWR) is used to create a self-consolidating concrete (SCC). ACI Committee 237 Self-Consolidating Concrete offers the following definition for SCC [25]:

Self-consolidating concrete (SCC) is highly flowable, non-segregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical

SCC is a product of technological advancements in the area of under-water concrete technology where the mixtures must ensure high fluidity and high resistance to washout and segregation. Okamura originally advocated SCC in 1986, and the first success with SCC occurred in 1988 [26]. The use of SCC has gained wide acceptance for savings in labor costs, shortened con‐

Advancement in SCC technology was primarily possible due to the introduction of new chemical admixtures that improved and controlled the SCC rheological properties. Better performing SCC mixes were produced on the advent of melamine, naphthalene, polycarbox‐ ylate, and acrylic based HRWR superplasticizers and viscosity modifying agents (VMA).

Gao et al [12] proposed using SCC because acceptable SCC is not only highly flowable, but it is also highly stable and homogenious on a macro scale. The Prestressed Concrete Institute

**•** Filling ability – The property that determines how fast SCC flows under its own weight and completely fills intricate spaces with obstacles, such as reinforcement, without losing its

struction time, a better finish, and an improved work environment [27-30].

(PCI) stipulates the following criteria for SCC [26]:

**•** Stability – the ability of SCC to remain homogenous by resisting segregation, bleeding, and air popping during transport and placing as well as after placement.

Gao et al [12] studied SCC containing CNF to see if the same effect was present on the nano scale. In Gao et al's mixing procedure, HRWR, water, and CNF are mixed in a laboratory-grade blender. Simultaneously, fine aggregate, course aggregate, and cement are combined in a centrifugal mixer. The CNF mixture is then slowly added to the mixer to gain a homogenous mix. The fresh concrete was used to create cylinders that were tested in compression. After the test, pieces of the cylinders were observed under a scanning electron microscope (SEM). The SEM showed significant CNF clumping in specimens made of normal CNF concrete and uniform distribution in SCC containing CNF, as shown in Figs. 2 and 3, respectively.

**Figure 2.** Scanning Electron Microscope Image of CNF Clump in Normal Cement (1670x Magnification)

**Figure 3.** Scanning Electron Microscope Image of Well Dispersed CNF in a Uniform Self-Consolidating Cement (9410x Magnification)
