ASTM C127: Specific Gravity and Absorption Test for Coarse Aggregate

Concrete, Tests
ASTM C127

When we think about concrete mix design, it’s easy to jump straight to water-cement ratio, slump, and strength. But one number quietly influences all of it: specific gravity.

Specific gravity tells us how heavy or light the aggregate is compared to water. Absorption tells us how much water the rock soaks up. Together, they affect everything from mix proportions to durability to workability.

And when it comes to coarse aggregate (gravel, crushed stone, and the like) the standard method for figuring this out is ASTM C127.

If you’re serious about getting mixes dialed in, or you’re troubleshooting batch inconsistencies, this is a test you can’t afford to ignore.

What ASTM C127 Actually Measures

ASTM C127 covers two things:

  • Specific Gravity (Bulk, SSD, and Apparent)

  • Absorption Percentage

In simple terms:

  • Bulk Specific Gravity = How dense the aggregate is, considering the natural pores that can fill with water.

  • SSD (Saturated Surface Dry) Specific Gravity = The condition where the pores are filled with water, but no free water is clinging to the surface.

  • Apparent Specific Gravity = Ignoring the pores, just the solid material.

Meanwhile, absorption tells us how much water the aggregate will soak up if it’s dry. That number is crucial because it affects how much effective water ends up in the concrete mix.

Equipment and Tools You’ll Need for ASTM C127

The tools for ASTM C127 are pretty basic, but precision matters:

  • Balance: Sensitive to 0.05% of sample mass

  • Water tank: Big enough to submerge samples without crowding

  • Absorption basket or wire mesh container

  • Large flat drying surface

  • Towels for blotting

  • Oven: Capable of maintaining 110 ± 5°C

  • Scoop, trays, and containers

Keep everything clean. Any dirt, dust, or old paste ruins the accuracy.

Sample Preparation for ASTM C127

Start with about 2000 to 3000 grams of coarse aggregate, depending on particle size.

  1. Wash the sample to remove any dust or coatings.

  2. Place the washed sample in water and let it soak for 24 ± 4 hours at room temperature.

  3. After soaking, carefully drain the water.

📌 The goal here is to get the aggregate to SSD condition — all internal pores full, but no excess water clinging outside.

Achieving SSD Condition

Getting aggregates to SSD is the trickiest part.

  • Spread the soaked aggregate on a towel or cloth.

  • Blot it gently with a second towel, rolling and patting — no rubbing or pressing hard.

  • Keep checking: the surface should lose its glossy wet look, but the pores inside stay filled.

How do you know it’s ready?

Take a few particles and hold them against dry paper. If no water immediately marks the paper but the rock still looks damp, you’re close.

It’s an art as much as a science.

Testing Procedure for ASTM C127

Once you’ve achieved SSD condition, it’s go time.

1. Weigh in Air (SSD Mass)

  • Weigh the SSD aggregate in air. Record this mass as WSSD.

2. Weigh in Water (Submerged Mass)

  • Place the aggregate in a basket submerged in water.

  • Remove air bubbles by shaking or tapping gently.

  • Weigh it underwater. Record as Wsub.

3. Oven Dry and Weigh Again

  • Dry the sample in the oven at 110 ± 5°C.

  • Cool to room temperature in a desiccator.

  • Weigh the oven-dry aggregate. Record as WOD.

Calculations

Now for the math.

Write all results to three decimal places for specific gravity and two decimal places for absorption.

 Why These Numbers Matter

  • Specific Gravity tells us how many kilograms of aggregate fill one cubic meter, critical for absolute volume batching.

  • Absorption adjusts the effective water-cement ratio. If you don’t correct for absorption, dry aggregates can suck water away, leading to low workability or dry mixes.

  • High absorption rates often point to porous, less durable stone, which could mean freeze-thaw issues or long-term strength loss.

Real-World Example

Suppose you’re batching a concrete mix targeting a 0.45 water-cement ratio. If you use dry aggregate and ignore absorption, you accidentally lower the actual w/c ratio — making the concrete too stiff, harder to place, and potentially understrength.

Or, worse, you adjust for it incorrectly and end up with a w/c ratio too high, compromising durability.

This is why most mix designs are based on aggregates assumed at SSD condition and why this test is crucial.

Common Mistakes to Watch For

  • Overdrying during SSD prep — once the surface dries too much, you can’t go backward.

  • Not removing air bubbles in the underwater weighing — tiny bubbles can fake a higher submerged weight.

  • Inconsistent drying in oven — uneven heat ruins the OD mass reading.

  • Dirty sample baskets — particles stuck to wire mesh can throw off weighing.

Be methodical. Rushing this test causes a domino effect of bad concrete properties later.

ref. astm c127

See a Complete List of ASTM Tests for Concrete and Construction Materials