ASTM C128: Relative Density (Specific Gravity) and Absorption of Fine Aggregate

What the ASTM C128 test measures

ASTM C128 is the fine aggregate counterpart to ASTM C127: relative density, or specific gravity, and absorption of sand and manufactured fines passing the No. 4 sieve. The values feed concrete and asphalt mix design in exactly the same way the coarse aggregate values do.

How the ASTM C128 test works, step by step

The sample is soaked, then dried gradually with warm air while being stirred. The saturated-surface-dry point is found with the cone test: sand is placed in a small metal cone, tamped, and the cone lifted. If the sand slumps slightly, it is SSD. If it holds the cone shape it is still too wet; if it collapsed completely it went past the point.

A known mass of SSD sand is placed in a pycnometer flask, filled with water, agitated to remove air, and weighed. The sand is then oven dried and weighed. The gravimetric procedure gives bulk dry, bulk SSD, and apparent specific gravity and absorption.

What the numbers mean

Natural sands are commonly 2.55 to 2.70 bulk SSD with absorption of 0.5 to 2 percent. Manufactured sands from crushed stone can run higher absorption because of the dust and angular shape. Absorption drives batch water corrections at the plant, and a sand whose absorption changes by half a percent shifts the water-cement ratio enough to see in strength.

What can throw the result off

The cone test is a judgment call and is the largest source of variation between technicians. Angular manufactured sands can be difficult to read because they hold the cone shape from interlock rather than moisture. Air trapped in the pycnometer and temperature effects on the water are the other common problems.

How we help with ASTM C128 testing

Fine aggregate specific gravity and absorption are part of our standard source qualification package with gradation, minus-200, and unit weight, delivered together so a mix design can be finalized from one report.