Determination of aggregate characteristics using image analysis

the Vermont Agency of Transportation (VAOT)

face of the aggregate. The remaining asphalt on

the aggregate is used as an indicator of the sur-

were characterized using the QMOT image analy-

face texture. This method determines the micro-

sis system. The samples included one sample of

texture of the surface and the absorptive nature of

crushed gravel and two samples of crushed

the aggregate and is still an index to the rough-

stone. The crushed stone was obtained from two

ness of the surface.

different sources. Roughly thirty to forty ran-

Wright (1955) developed a method for quanti-

domly selected aggregates were placed on a light

fying the surface texture of concrete aggregate

table and videotaped. The outlines of aggregates

particles using studies done on 19-mm stones.

from the three sources, as reproduced by the im-

The test aggregates were first embedded in a syn-

age analysis program, are shown in Figures 7 to 9.

thetic resin. Once the resin hardened, the stones

were cut into thin sections (approximately 2.5

102 mm thick). The thin section was placed un-

der a projection microscope and magnified 125

times. The unevenness of the surface was traced

and the total length of the trace was measured.

This length was then compared with an uneven

line drawn as a series of chords (Fig. 6). The

difference between these two lines was defined as

the roughness factor. The reproducibility of this

method was reported to be good (Wright 1955).

However, to obtain a reliable average of rough-

ness of any one aggregate size, it is necessary to

Figure 7. Image of the Pike crushed stone

take many sample measurements.

prior to image analysis.

Determination of aggregate characteristics

using image analysis

The direct measurement methods described in

the previous sections are time consuming and

labor intensive. Thanks to the power of present

computing capabilities, computer-based image

analysis has been developed for analyzing aggre-

gate shape, angularity, and roughness. A feasibil-

ity study was conducted for the Federal Highway

Administration (FHWA) on this technology (Wil-

son et al. 1995). The study was conducted on fine

sands (manufactured versus natural), and the

conclusion was that image analysis was a viable

Figure 8. Image of the crushed stone prior

tool for distinguishing the shape and angularity

to image analysis.

of the particles. Wilson et al. (1995) developed the

Roundness and Shape Indices to quantify the

roundness and angularity of the particles, re-

spectively. The method first involves capturing

images of the aggregates using a high-resolution

video camera. Then an image analysis computer

program is used to identify, separate, and trace

the edges of the aggregates. Based on the traces,

algorithms in the programs are used to determine

the different characteristics of the aggregates.

The Quebec Ministry of Transportation

(QMOT) uses image analysis on a routine basis

for verifying the angularity of its hot mix aggre-

gates. To study the feasibility of this method,

Figure 9. Image of the crushed gravel prior

samples of base course aggregates obtained from

to image analysis.