Table 9. Results of particle index tests for various aggregates

3 represent aggregates passing the 19-mm and re-

between 10 and 50 strokes. It was found to be de-

tained on the 12.7-mm sieve; passing the 12.7-mm

pendent on the shape, angularity, and texture of

the aggregates. Huang (1962) found that this "zero

and retained on the 9.5-mm sieve; passing the 9.5-

stroke void ratio" became smaller as the aggregate

mm and retained on the No. 4 (4.75-mm) sieve, re-

became more spherical, rounded, and smooth. He

spectively. Shapes, a, b, and c represent bulky,

also found that this was more pronounced when

elongated, and flat particles, respectively. Gravel

the mold was rhombohedron shaped.

(ii) was visually classified as gravel with smooth

The particle index test initially was developed

surfaces and rounded corners. Gravel (iii) was

for three aggregate sizes: passing the 19-mm and

classified as gravel with rough surfaces and sharp

retained on the 12.7-mm sieve, passing the 12.7-

corners and edges. Crushed gravel (p) was made

mm and retained on the 9.5-mm sieve, and pass-

up of gravel aggregates as received from the plant.

ing the 9.5-mm and retained on the No. 4 (4.75-

No screening between crushed and uncrushed

mm) sieve. For each size, the test involves tamp-

gravel was made. Crushed gravel (pp) was made

ing the uniform-sized aggregate into a mold in

up of those that were sorted to have at least one

three equal layers using a standard tamping rod

crushed face. The shapes, angularity, and rough-

with 10 strokes per layer. The tamping rod is

ness classifications were based on visual observa-

raised to a height of 50 mm from the top of the

tions by several operators and were not based on

aggregate surface. At the end of the third layer,

any measurements (Huang 1962). The results indi-

material is added to make sure that the aggregate

cated that the test method is capable of distin-

surface is flush with the rim of the molds. The test

guishing the difference between smooth, rounded

is repeated using 50 strokes. The percentage of

and rough, angular aggregates by the increasing

voids in the aggregate is calculated using the fol-

particle index from gravel to crushed stone. It also

lowing equation:

appears that this index is capable of distinguish-

ing different levels of crushed gravel. A modified



W 

version of this test has been standardized by

Vn = 1 - n  ⋅ 100

 S * v

ASTM. The test is designated as ASTM D 3398-93

(1996), Standard Test Method for Index of Aggregate

where Vn = percentage of voids at n strokes per

Particle Shape and Texture (1996), and it covers both

layer

coarse and fine aggregates. For the coarse portion,

Wn = net weight of aggregate in the mold at

the maximum size has been increased to 38 mm.

n strokes per layer (g)

Depending on the size of the aggregates, different

s = bulk density of the aggregate (g/cc)

size molds are used, as shown in Table 10. The

v = volume of mold (cc).

mold as prescribed by this standard is cylindrical

in shape. Although the effect of this test method

The particle index (Ia) is calculated using the for-

was more pronounced in a rhombohedron-shaped

mula

mold, it could also be seen to a lesser degree with a

circular mold.

Ia = 1.25V10 0.25V50 32

The problem with this test method, with respect

to coarse aggregates, is in the determination of the

where V10 = percentage of voids in aggregates at

volume of voids. Initially, the voids were calculat-

10 strokes per layer

ed based on the volume of the mold. This required

V50 = percentage of voids in aggregates at

making certain that the aggregates were filled to

50 strokes per layer.

the rim of the mold and the aggregates' surfaces

were flush with the rim. Because of this it was

Typical particle index results reported by

found that the results were operator dependent.

Huang (1962) are shown in Table 9. Sizes 1, 2, and

Table 9. Results of particle index tests for various aggregates. (After Huang 1962.)

Size 1

Size 2

Size 3

Sample

Shape a

Shape b

Shape c Shape a

Shape b

Shape c Shape a

Shape b Shape c

Gravel (ii)

7.5

8.7

9.8

7.1

9.1

9.4

7.5

8.5

Gravel (iii)

8.8

10.0

13.2

9.0

10.2

14.0

9.1

10.4

12.2

Crushed gravel (p)

9.5

11.7

13.6

Crushed gravel (pp)

11.3

14.6

14.8

Crushed stone

13.4

16.0

16.9

12.4

15.9

17.4

12.6

14.9

16.4