Hardness test of aggregate

Hardness of aggregate is its resistance to wear (abrasion and attrition) caused by the rubbing action of vehicle wheels and by the movement of the particles themselves. It is measured by the Los Angeles abrasion test (IS:2386 Part IV; ASTM C131), and also by the Deval abrasion test.

Los Angeles abrasion test - apparatus

• Los Angeles machine: hollow steel cylinder 700 mm diameter and 500 mm long, closed at both ends, with a shelf fixed inside, rotating about the horizontal axis.
• Abrasive charge of steel balls (about 48 mm diameter, 390-445 g each; 6 to 12 balls).
• IS sieve of 1.7 mm and a balance.

        axis (30-33 rpm)
     .-----------------.
    /   o o  <- balls    \
   |  aggregate  | shelf  |
    \               ____ /
     '-----------------'
         cylinder 700 x 500 mm

Procedure

1. Take oven-dried aggregate of 5 kg or 10 kg, according to the grading (A to G).
2. Place it with the specified number of balls in the cylinder; close the cover.
3. Rotate at 30 to 33 rpm for 500 revolutions (gradings A-D) or 1000 (E-G).
4. Remove the material, separate the steel balls, and sieve on a 1.7 mm IS sieve; wash and oven-dry the retained portion and weigh it.

$$\text{LAAV} = \frac{W_1 - W_2}{W_1}\times 100\%$$

where $W_1$ = original weight and $W_2$ = weight retained on 1.7 mm sieve.

Significance

• It indicates the ability of the aggregate to withstand the wear from traffic, and the crushing and impact in rolling.
• Good aggregate has a low value: below 30% for the wearing course, up to 35-40% for the base, 50% for the lower layers (IRC/NRS 2070). A value below 20% is considered hard.
• It is a simple, reliable test used in aggregate source selection.