Sieve Analysis of Fine and Coarse Aggregates

Introduction:

The coarse aggregate specific gravity test is used to calculate the specific gravity of a coarse aggregate sample by determining the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It is similar in nature to the fine aggregate specific gravity test,The coarse aggregate specific gravity test measures coarse aggregate weight under three different sample conditions:

Oven-dry (no water in sample).

Saturated surface-dry (SSD, water fills the aggregate pores).

Submerged in water (underwater).

Scope:

Specific Gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. Water, at a temperature of 73.4°F (23°C) has a specific gravity of 1. Specific Gravity is important for several reasons. Some deleterious particles are lighter than the good aggregates. Tracking specific gravity can sometimes indicate a change of material or possible contamination. Differences in specific gravity may be used during production to separate the deleterious particles from the good using a heavy media liquid.

Apparatus:

Sample container, wire basket of No. 6 (3.35 mm) or less mesh wire cloth, with a capacity of 1 to 1 3/4 gal. (4 to 7 L) to contain aggregate with a nominal maximum size of 1 1/2 in. (37.5 mm) or smaller; larger basket for larger aggregates. 5 Water tank, watertight and large enough to completely immerse aggregate and basket, equipped with an overflow valve to keep water at a constant level. Suspended Apparatus, wire used to suspend apparatus with the smallest practical diameter. A hitest fishing leader or other thin wire with utility hook can be used with a small hook attached to the handle of the basket or sample container. Sieves, No. 4 (4.75 mm) or other size as needed

Test Procedure:

1.Obtain a sample of coarse aggregate material retained on the No. 4 (4.75 mm) sieve

2. Prepare the material.

Wash the aggregate retained on the No. 4 (4.75 mm) sieve. This discards small aggregate particles

clinging to the retained large particles.

Dry the material until it maintains a constant mass. This indicates that all the water has left the

sample. Drying should occur in an oven regulated at 230°F (110°C).

Cool the aggregate to a comfortable handling temperature.

Immerse the aggregate in water at room temperature for a period of 15 to 19 hours 

3.  Dry the sample to a saturated suface dry (SSD) condition. Rolling up the aggregate into the towel

and then shaking and rolling the aggregate from side to side 

4. Place the entire sample in a basket and weigh it underwater.

5. Remove the aggregate from the water and dry it until it maintains a constant mass.

6. Cool the aggregate in air at room temperature then determine the mass.

Calculations:

A= 462g, B= 500g, C= 298g.

A = mass of oven-dry sample in air (g)B = mass of SSD sample in air (g)C = mass of SSD sample in water (g)

1. Gsb= A(B−C) = 462

(500−298 ) = 2.287

2. Gbulk SSD= B(B−C) = 500

(500−298 ) = 2.475

3.Gsa= A(A−C) = 462

(462−298 ) = 2.817

Absorption (%)(B−A)A *(100) = 8.23%

Discussion:Certainly, the accuracy of all measurements is important. However, of specific concern is the mass of the SSD sample. The determination of SSD conditions can be difficult. If the sample is actually still wet on the surface then the mass of the SSD sample will be higher than it ought to be, which will cause a lower calculated bulk specific gravity. Conversely, if the sample is beyond SSD and some of the pore water has evaporated (which is more likely), the mass of the SSD sample will be lower than it ought to be, which will cause a higher calculated bulk specific gravity.

References:

1. AASHTO.2. http://www.pavementinteractive.org/