Mechanical properties of aggregates

Mechanical properties show how the aggregate resists loads and wear. Four main properties are:

1. Crushing value (IS 2386 Part 4)

Resistance to crushing under a gradually applied compressive load. A 12.5-10 mm sample of mass $W_1$ in a steel cylinder is loaded to 400 kN in 10 minutes; the fines passing the 2.36 mm sieve ($W_2$) are weighed.

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

Limit: not more than 45% for concrete in general and 30% for wearing surfaces such as runways, roads and pavements (IS 383).

2. Impact value (IS 2386 Part 4)

Toughness under sudden shock. A 12.5-10 mm sample is given 15 blows from a 13.5-14 kg hammer falling 380 mm; the fines passing 2.36 mm as a percentage of the total is the AIV. Limit (IS 383): 45% for concrete other than wearing surfaces and 30% for wearing surfaces.

3. Abrasion value (Los Angeles, IS 2386 Part 4)

Resistance to wear. The sample is rotated with steel balls in a drum for 500 revolutions at 30-33 rpm; the percentage passing the 1.70 mm sieve is the abrasion value. Limit: 30% for wearing surfaces and 50% for others.

4. Hardness / toughness (and strength)

Hardness is the resistance to scratching (Mohs scale); the strength of the parent rock is tested on cores or cubes. Granite is usually 100-250 MPa; the rock strength should not be less than the required concrete strength.

Lower crushing, impact and abrasion values mean a stronger, tougher and more durable aggregate. Weak aggregates cannot make strong concrete, since high-strength concrete fails through the aggregate.