ASTM C128: Specific Gravity and Absorption Test for Fine Aggregate

In concrete mix design, sand might not get the same attention as cement or gravel — but it quietly influences almost everything. From water demand to workability to strength, fine aggregate properties matter. A lot.

If you’re serious about getting concrete right, you need to know how much your sand absorbs water, and how heavy it really is relative to water.

That’s where ASTM C128 comes in the standard method for measuring the specific gravity and absorption of fine aggregates.

This test is a little trickier than its coarse aggregate cousin (ASTM C127), mostly because sand is lightweight and hard to handle without making mistakes. But once you know the steps, it’s pretty straightforward.

What ASTM C128 Measures

ASTM C128 gives you:

Bulk Specific Gravity (SSD condition)
Bulk Specific Gravity (Oven-Dry condition)
Apparent Specific Gravity
Absorption (%)

In simpler terms:

Specific gravity tells you how heavy sand is compared to water.
Absorption tells you how much water the sand will soak up when dry.

Both numbers are essential when designing concrete based on absolute volume methods, and they control how much free water is actually available for cement hydration.

Equipment You’ll Need

Running ASTM C128 requires a few specific tools:

Pycnometer (a specialized vessel — either a flask, jar, or metal container with a tight-fitting conical cap and hole for pouring water)

Cone mold (small frustum cone, 40 mm base diameter, 90 mm top diameter, 75 mm height)
Tamper (a small rod or tamping tool)
Towels for blotting
Oven (capable of 110 ± 5°C)
Scale (accurate to 0.1 g)
Scoop and containers

You’ll also need clean water at about 22 to 32°C (normal room temperature).

Preparing the Sample

You’ll need about 1000 grams of sand for the test.

Start by:

Washing the sample to remove dust or coatings.
Oven-drying the sand completely at 110°C.
Letting it cool to room temperature before beginning.

You want dry, clean sand — no moisture hiding inside.

Bringing Sand to SSD Condition

Getting sand to Saturated Surface Dry (SSD) condition is the most delicate step in the whole test.

Here’s the method:

Spread the oven-dried sand out and gently sprinkle water onto it while mixing.
As you mix, keep checking moisture content using the cone test.

Cone Test for SSD:

Lightly fill the cone mold with damp sand.
Tamp it lightly 25 times with the small tamper.
Remove the cone vertically.

Result you want:

If the sand slumps slightly, like a soft jelly mold — it’s SSD.
If the cone collapses, it’s too wet.
If the sand holds a rigid shape with cracks, it’s still too dry.

📌 It’s part science, part instinct — getting SSD takes practice.

⚖️ Weighing and Testing for ASTM C128

Once you achieve SSD condition:

Weigh the SSD Sample (W₁):
Place exactly 500 ± 10 g of SSD sand on the balance.
Add to Pycnometer and Fill with Water:
Transfer the SSD sand carefully into the pycnometer.
Fill the pycnometer with water until full — no trapped air.
Weigh the Pycnometer + Sand + Water (W₂):
Wipe the outside dry and record the total mass.
Dry the Sand Completely:
Remove the sand from the pycnometer and oven-dry it again at 110°C until constant weight.
Weigh the Oven-Dry Sand (W₃):
After cooling, record the mass.
Weigh the Full Pycnometer of Water (W₄):
Fill the pycnometer with just clean water (no sand) and weigh it.

📐 Calculations for ASTM C128

Here’s how you find everything:

📏 Make sure to round specific gravity to three decimal places and absorption to two.

Why These Numbers Matter

Concrete isn’t mixed by mass. it’s proportioned by volume. And since specific gravity ties mass to volume, you need it to correctly batch mixes.

More importantly:

High absorption sands can steal mix water, lowering workability.
Low absorption sands behave differently in hot weather.
Specific gravity helps calculate yield, batch weights, and adjustments for moisture content in the field.

Without this data, you’re basically guessing.

Common Pitfalls to Watch For

Trapped air inside the pycnometer skews results, always agitate gently when filling.
Over-dried or under-dried sand during SSD prep causes major errors.
Poor cone testing technique leads to wrong SSD judgment.
Dirty or wet pycnometer when weighing. it must be clean and dry outside.

This is a precision test. Rushing through it almost guarantees unreliable results.

Summary

Fine aggregate might seem like a background player in concrete, but it controls everything from water demand to paste volume to final surface quality.

ASTM C128 isn’t flashy. It’s not breaking cylinders or vibrating slump cones. But it quietly feeds the numbers that decide whether your concrete mix hits the mark or misses.

Know your sand. Know your mix. That’s how good concrete gets built.