Aggregates

Aggregates

Aggregates are granular materials such as sand, gravel, crushed stone, and recycled concrete that are used with a binding medium to produce concrete or mortar. They constitute about 60-80% of the volume of concrete.

Classification

Source Classification

• Natural Aggregates: Sand, gravel, crushed rock.
• Artificial Aggregates: Expanded shale, clay, slate, slag, recycled concrete.

Density Classification

• Lightweight: $< 1120 \text{ kg/m}^3$ (pumice, perlite). Used for lightweight concrete.
• Normal Weight: $1520 - 1680 \text{ kg/m}^3$ (sand, gravel, limestone).
• Heavyweight: $> 2080 \text{ kg/m}^3$ (barite, magnetite). Used for radiation shielding.

Key Properties and Testing

Sieve Analysis (Gradation)

Gradation affects the workability and economy of concrete.

• Fineness Modulus (FM): An empirical figure obtained by adding the total percentage of the sample of an aggregate retained on each of a specified series of sieves, and dividing the sum by 100.

$$FM = \frac{\sum \text{Cumulative Percentage Retained}}{100}$$

The standard sieves for fine aggregate are No. 4, 8, 16, 30, 50, and 100.

Specific Gravity and Absorption

• Apparent Specific Gravity: Ratio of the weight in air to the weight of an equal volume of the impermeable portion of aggregate.
• Bulk Specific Gravity (SSD): Includes the volume of permeable pores filled with water.
• Absorption: The increase in weight of saturated surface-dry aggregate over oven-dry aggregate.

$$\text{Absorption (\%)} = \frac{W_{SSD} - W_{OD}}{W_{OD}} \times 100$$

Durability and Soundness

• Abrasion Resistance: Measured by the Los Angeles Abrasion Test.
• Soundness: Resistance to weathering (freeze-thaw cycles), measured by sodium sulfate or magnesium sulfate soundness test.

Shape and Texture

• Round vs. Angular: Angular particles provide better interlock but reduce workability.
• Smooth vs. Rough: Rough texture improves bond with cement paste.