Unit Weight and Voids in Aggregate

Unit Weight and Voids in Aggregate

Purpose: to determine unit weights of and voids in a sample of coarse, fine or mixed aggregate.

Related Standards: ASTM C29, C127, C128

Definitions:

Air void is a space filled with air.

Unit weight or bulk density is the weight in air of a unit volume of a permeable material (including both permeable and impermeable voids).

Equipment: Balance, 5/8-inch diameter tamping rod (24 in. long), cylindrical metal measure (minimum capacity of ½ ft3 for coarse aggregate of size not larger than 1 ½-in. and of 1/10 ft3 for fine aggregate).

Sample: Aggregate dried to constant weight, preferably in an oven at 110 ± 5 oC

Procedure:

1. Find the empty weight of the metal measure.

2. Fill the measure one-third full with the dry sample.

3. Rod the layer of aggregate with 25 strokes. (Do not allow the rod to strike the bottom of the measure)

4. Fill the measure two-thirds full, level, and rod as in step 3.

5. Fill the measure overflowing and rod as in step 3.

5. Level the surface of the aggregate with a finger and tamping rod such that any slight projection of the larger pieces of the coarse aggregate approximately balances the larger voids in the surface below the top of the measure.

7. Weigh the measure with the aggregate and find the net weight of the aggregate: A.

8. Calculate the unit weight B:

Unit eight or bulk density, B = (A/V) lb/ft3 or kg/m3

where V is the volume of the measure.

9. Calculate the void content or percent void.

Void % = (S x W – B)/(S x W) x 100

where S is the bulk specific gravity (dry basis from the specific gravity of coarse aggregate experiment) and W is the unit weight of water (62.4 pcf or 999 kg/m3)

Report: Calculate unit weight and void for the blended coarse aggregate. Comment on the results.