ASTM C142: Testing Clay Lumps and Friable Particles in Aggregates

When you think about quality aggregates, you probably imagine strong, clean stone or durable sand. But not every piece of aggregate is solid gold. Sometimes, you get clay lumps or weak, crumbly particles mixed in.

These fragile bits can spell big trouble for concrete strength, durability, and finish. That’s why we have ASTM C142, the standard method for measuring the amount of clay lumps and friable particles in both coarse and fine aggregates.

It’s a simple test but an important one. Knowing how much weak material is in your aggregate helps predict performance and avoid surprises down the road.

Let’s walk through what the test measures, how it’s done, and why it matters in the real world.

What ASTM C142 Measures

Simply put, ASTM C142 identifies the percentage of clay lumps and friable particles in both fine and coarse aggregates.

Clay lumps are clusters of clay that crumble easily after soaking.
Friable particles are any aggregates that break apart with light pressure.

Both are unwelcome in durable concrete. Even a small percentage can:

Cause surface defects like pitting and scaling
Weaken the bond between cement paste and aggregate
Allow water intrusion and freeze-thaw damage

Most project specifications set strict limits (often no more than 1%) to ensure quality.

Equipment Needed

Here’s what you’ll need for the test:

Accurate balance (readable to 0.1% of sample mass)
Flat pans for soaking and spreading
Water container (bucket or wide bowl)
Standard sieves (sizes depend on aggregate type)
Drying oven (110 ± 5°C)
Soft brush or thumb pressure
Clean towels

Preparing the Sample

The test sample must be properly selected and prepared:

Fine Aggregate:
- Only particles retained on a No. 16 sieve (1.18 mm).
- Minimum sample weight: 25 grams.
Coarse Aggregate:
Depending on size, you’ll need larger samples:
- No. 4 to ⅜ inch (4.75–9.5 mm): 1000 grams
- ⅜ to ¾ inch (9.5–19.0 mm): 2000 grams
- ¾ to 1½ inches (19.0–37.5 mm): 3000 grams
- Over 1½ inches (>37.5 mm): 5000 grams

If the material has both fine and coarse fractions, first separate them using a No. 4 sieve (4.75 mm), and treat each fraction independently.

Dry the samples in an oven at 110°C until a constant mass is achieved before proceeding.

Soaking the Sample

Once dried and weighed:

Spread the sample into a thin layer in a container.
Cover with distilled water.
Let it soak for about 24 hours.

This softens any clay lumps, preparing them for separation.

Separating Weak Particles

After soaking:

Roll and squeeze each particle between your thumb and forefinger.
Any particle that crumbles or breaks easily is considered a clay lump or friable particle.
Be careful:
- Don’t apply force: you are checking for natural weakness, not crushing rocks.

It’s a tactile, hands-on step, experience helps you judge correctly.

Wet Sieving the Residue

The next step ensures we separate real clay lumps from normal dust:

Wet sieve the degraded material using the appropriate sieve size:
- Fine aggregate → No. 20 sieve (850 μm)
- Coarse aggregate (depending on size) → No. 8 or No. 4 sieves
Agitate the sieve gently under water to remove smaller particles.
Retain only the sound particles left behind on the sieve.

Drying and Weighing

Oven-dry the separated weak particles at 110°C until constant weight.
Weigh the dried residue to the nearest 0.1%.

You now have everything you need to calculate the result.

Calculations

Use this formula:

Where:

W = Original dry weight of the sample
R = Weight of the retained (sound) particles after sieving

If multiple size fractions are involved (for coarse aggregate), compute a weighted average based on the fraction’s mass contribution.

Note:
If any coarse fraction is less than 5% of the total, you can assume it has the same percentage of clay lumps as the adjacent size fraction. you don’t have to test it separately.

Why This Test Matters

Workability:
High clay content increases water demand, messing up mix proportions.
Strength:
Weak particles reduce compressive and flexural strength.
Durability:
Clay can absorb water, freeze, and expand — causing cracks and surface defects.
Finish Quality:
Surface blemishes like pop-outs often trace back to bad aggregate.

Common Pitfalls

Overhandling during separation: Force can break good particles, skewing results.
Incomplete soaking: Some clay lumps only reveal themselves when fully saturated.
Incorrect sieve use: Always match sieve size to the particle size range.
Inconsistent drying: Moisture left behind changes mass readings and ruins accuracy.