Characterization of Particle Morphology

rheological properties in the fresh state and mechanical properties in the hardened state of cementitious suspensions (Mehta 1986; Monteiro 2006; Neville and Brooks 1987). The

particle morphology is generally expressed in terms of elongation ratio (i.e., aspect ratio), roundness, sphericity, and surface roughness. Compared to the roundness which describes the degree of sharpness of particle edges/corners, the term “sphericity” is used to characterize the overall form of the granular particle, irrespective of the sharpness of the edges (Alshibli et al. 2014). The latter parameter is a measure of the degree of conformity of particle shape to that of sphere circumscribing the particle.

The term “roundness” was firstly introduced by Wadell (Wadell 1933) and is defined as the ratio of average diameters of curvature in corners to the diameter of inscribing circle. Unlike sphericity, the quantification of roundness is complicated and highly dependent on determination of particle corners (Sun et al. 2014; Zheng and Hryciw 2015). Zheng and Hryciw (Zheng and Hryciw 2015) recommended that the particle corner can be properly identified by fitting the appropriate circle that minimizes the sum of squares of distances from the circle to particle outline pixels. Digital image analysis and computed tomography techniques have been successfully employed to quantitatively characterize the aggregate particle morphology based on two-dimensional (2D) (Al-Rousan et al. 2007; Bessa et al. 2012; Cox and Budhu 2008; Kwan et al. 1999; Shen et al. 2016; Wang et al. 2005; Zheng and Hryciw 2016) or three-dimensional (3D) images (Alshibli et al. 2014; Cepuritis et al. 2017; Fonseca et al. 2012; Garboczi and Bullard 2013; Garbout et al. 2013; Komba et al. 2013; Suh et al. 2017). The schematic illustration of particle morphology characterization using 2D digital image analysis is

shown in Figure 2-7. Roundness is computed by determining the maximum possible

inscribed circle and fitting the possible circles to the edges/corners of particle, as indicated in Figure 2-7(a) and (b). In order to determine the sphericity of particle, the

circumscribing circle diameter should be found, as shown in Figure 2-7(c). The

morphology of the aggregate particle can then be quantified as follows (Shen et al. 2016; Suh et al. 2017; Zheng and Hryciw 2016):

max min 1 D L Elongation ratio W D = = ≥ _2-8 1 1 1 x i i inc D x Roundness D = =

∑

2 / 4 1 / 4 elliptical shape cc area of particle LW Sphericity

area of circumscribed circle D

p p ≈ = ≤ = 2-10 1 1 1 m n ij i j Surface roughness Z mn = = =

∑∑

where L (or Dmax) and W (or Dmin) refer to the length and width of the particle,

respectively. In Eq. (2), x is the number of fitted corner circles, Di is the diameter of the

corner circles, and Dinc refers to the diameter of the largest possible inscribed circle (see

Figure 2-7(a)). In Eq. (3), Dcc refers to the circumscribing circle diameter (see Figure 2-7

(c)). In Eq. (4), m and n are number of pixels in the X- and Y-directions, and Zij

represents surface height at a specific pixel relative to the reference mean plane.

(a)

(b) (c)

Figure 2-7 Schematic representation of particle shape analysis: (a) determination of the largest possible inscribed circle, (b) fitting circles to the rounded corners/edges of particle

to compute roundness, and (c) determination of the circumscribing circle to compute sphericity of particle (adapted from Ref. (Zheng and Hryciw 2016)).

The increase in aggregate volume fraction and sand-to-total aggregate ratio generally increases the water content required to reach a given workability. The use of higher fine aggregate proportion increases the surface area which necessitates a higher paste volume needed to coat the particles. Rounded aggregate typically has a lower degree of interlocking of particles than angular particles, thus enhancing the workability of concrete. In general, angular aggregate with rough surface texture has higher surface area, which can develop stronger interfacial transition zone between aggregate and paste, thus leading to greater mechanical properties. Westerholm et al. (2008) studied the effect of grading and particle shape of sand on rheology of mortars prepared with 0.57 w/c and

a cement content of 635 kg/m3. A total of 14 fine aggregates (0–2 mm) were used in this

research, including 13 crushed sands and a natural sand. The results showed that the rheological properties and water demand of mortars depend significantly on the properties of the fine aggregate. Depending on the shape of fine aggregate, mortars made with crushed sands were shown to have 33% to 133% higher plastic viscosity compared to the mortar prepared with natural sand. Mortar containing elongated sand particles exhibited three times more viscosity than mortar made with spherical sand particles.

Hu and Wang (2011) evaluated the effect of the volume of coarse aggregate on concrete rheology made with w/cm of 0.45. Seven coarse aggregate gradations (four single sizes at 19 mm, 12.5 mm, 9.5 mm, and 4.75 mm and three gradations at G1, G2 and G3) were investigated. Results showed that for a given mortar content, the increase in coarse aggregate volume content from 35% to 41% resulted in 50% to 100% higher yield stress and 40% to 60% higher viscosity. Compared to the single-sized aggregate, continuous graded aggregate can exhibit higher packing density, thus leading to a lower yield stress and plastic viscosity. Aïssoun et al. (2015) investigated the influence of physical characteristics of fine and coarse aggregates, such as fine particle content, shape, texture, and the quantity of elongated particles on the workability and rheological properties of superworkable concrete mixtures made with 0.41 w/cm and a binder content

of 400 kg/m3. The results showed that an increase in the fine content (with diameter

smaller than 315 µm) from 8% to 18% resulted in an increase in plastic viscosity from 15 to 40 Pa.s and reduction in surface settlement from 0.47% to 0.14%. For a given w/cm, the increase in the quantity of fine particles increase the cohesion of the paste, thus

leading to an enhancement in static stability. A summary of current literature concerning the optimization of aggregate skeleton is provided in Table 2-1.

Table 2-1 Summary of investigations on aggregate skeleton optimization.

Reference Methodology Concluding remarks

Shilstone (1990) Coarseness factor chart

water demand ↓ workability ↑

compressive strength ↑ Holland (1990) Percent retained chart

(8-18 distribution) water demand ↓ cement demand ↓ shrinkage ↓ workability ↑ compressive strength ↑ Goltermann et al. (1997) Packing model (modified Toufar)

Model effectively optimizes packing of binary and ternary aggregate blends

Koehler (2007) 0.45 power curve packing density ↑ cement demand ↓ Brouwers and Radix (2005);

Hüsken and Brouwers (2008); Yu et al. (2013);

Wang et al. (2014)

Modified A&A model

water demand ↓ cement demand ↓ shrinkage ↓ Khayat et al. (2000);

Mueller et al. (2014)

Particle packing mix design for designing Eco-SCC

HRWR ↓

cement demand ↓ filling capacity ↑

Aïssoun, Hwang, and Khayat (2015)

Physical characteristics of aggregate on

properties of super workable concrete

volume of granulates ↑ → water demand ↑

packing density ↑ → stability ↑ fine content ↑ → viscosity ↑ fine content ↑ → stability ↑ rounded aggregate ↑ → packing density ↑

rounded aggregate ↑ → stability ↓ flat and elongated particle ↑ → yield stress ↑