Sieve Analysis Test for Water Bound Macadam and Coarse Aggregate

Water bound macadam works by mechanical interlock — big stones wedged together, voids filled with screenings, all locked by rolling and water. That interlock only happens if the stone sizes are right. Too much oversize and the layer will not compact; too many fines and it turns to mush under the roller. The sieve analysis is the test that proves the gradation before a single tonne is spread.

Governing Standards

• ASTM C136 / C136M — “Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates”
• AASHTO T 27 — “Sieve Analysis of Fine and Coarse Aggregates”
• IS 2386 (Part 1) — particle size and shape of aggregates (the Indian/Pakistani-region counterpart)

The method is the same everywhere: a dry sample of known mass is separated through sieves of decreasing opening size, and the mass retained on each sieve gives the particle size distribution.

Why Gradation Matters for WBM

WBM is built in compacted layers of 75–100 mm, and each layer has a specified grading band (see the IRC 19 table below). The reasons are practical:

• Interlock needs a size range, not one size. A proper spread of sizes lets smaller stones key between larger ones.
• Oversize stone bridges over voids instead of compacting, leaving a honeycombed layer.
• Excess fines hold water, soften under traffic, and cause rutting.

Sampling and Sample Size

A gradation result is only as good as the sample. Take representative samples from the stockpile (ASTM D75 / IS 2386 sampling practice — several increments from different points, not just the easy edge), then reduce by quartering (ASTM C702) to the test size and oven-dry the sample before sieving.

Minimum sample masses (ASTM C136) grow with particle size — for example about 5 kg for 19 mm nominal maximum size and 15 kg for 37.5 mm. WBM stone (up to 90 mm) needs large samples; if the full mass is impractical, reduce carefully by quartering and note it on the report — never just grab a bucketful.

Procedure

1. Dry and weigh the reduced sample; record the initial mass.
2. Nest the sieves in decreasing opening size, pan at the bottom. For WBM Grading 2 (63–45 mm), the nest would typically be 90, 63, 53, 45, 22.4 mm plus pan.
3. Pour in the sample and shake — mechanically for about 10 minutes, or by hand with a lateral and vertical motion. Do not overload any sieve; a thick bed of stone blinds the mesh and fakes the result.
4. Weigh the retained mass on each sieve, brushing the mesh clean (a stiff brush for coarse sieves).
5. Check the mass balance: the sum of all retained fractions plus the pan must agree with the initial sample mass within 0.3% (ASTM C136). If it does not, the test is invalid — repeat it.
6. If the aggregate is dusty, wash it first (ASTM C117) so fines stuck to the stone do not report on the wrong sieve.

Worked Example: WBM Grading 2

Sample mass: 10,000 g of crusher-run stone, checked against IRC 19:2005 Grading 2 (63–45 mm):

IS Sieve	Retained (g)	% Retained	Cumulative % retained	% Passing	Spec: % passing
90 mm	0	0.0	0.0	100	100
63 mm	800	8.0	8.0	92	90–100
53 mm	3,500	35.0	43.0	57	25–75
45 mm	4,200	42.0	85.0	15	0–15
22.4 mm	1,300	13.0	98.0	2	0–5
Pan	200	2.0	100.0	—	—
Total	10,000

Result: the sample complies — every sieve’s percent passing falls inside the Grading 2 band (the 45 mm sieve at 15% is right on the limit, so keep an eye on the next delivery).

WBM Grading Requirements (IRC 19:2005, Table 2)

Percent by weight passing, for the three standard gradings:

IS Sieve	Grading 1 (90–45 mm, 100 mm layer)	Grading 2 (63–45 mm, 75 mm layer)	Grading 3 (53–22.4 mm, 75 mm layer)
125 mm	100	—	—
90 mm	90–100	100	—
63 mm	25–60	90–100	100
53 mm	—	25–75	90–100
45 mm	0–15	0–15	65–90
22.4 mm	0–5	0–5	0–10
11.2 mm	—	—	0–5

The pattern is deliberate: coarsest stone at the bottom of the pavement, finer grading toward the top for a tight surface under the bituminous layer.

Practical note: Gradation is only one part of WBM acceptance. The stone must also meet the physical requirements — Los Angeles abrasion max 40% (or aggregate impact value max 30%), combined flakiness and elongation index max 35%, and water absorption max 2% (IS 2386). A perfectly graded weak stone still fails.

Fineness Modulus (FM)

The fineness modulus compresses a whole gradation into one number — useful for comparing deliveries and for concrete mix design:

FM = (Sum of cumulative % retained on the standard sieve series) ÷ 100

The standard series: 150 µm, 300 µm, 600 µm, 1.18 mm, 2.36 mm, 4.75 mm, 9.5 mm, 19 mm, 38 mm, 76 mm.

Worked example — 20 mm nominal coarse aggregate:

Sieve (mm)	38	19	9.5	4.75	2.36	1.18	0.60	0.30	0.15
Cum. % retained	0	5	55	92	98	99	99.5	100	100

Sum = 648.5 → FM = 648.5 ÷ 100 = 6.49

Typical ranges: fine aggregate FM 2.3–3.1; coarse aggregate FM roughly 6.5–8.0. A higher FM means a coarser aggregate. FM is a summary, not a substitute — two very different gradations can share the same FM, so always read the full table too.

Reading the Gradation Curve

Plot percent passing (y-axis, arithmetic) against sieve size (x-axis, log scale) and compare the curve with the specification band:

• Curve inside the band — accept the material.
• Curve above the band (too much passing) — material is too fine; expect poor interlock and a hungry mix.
• Curve below the band (too little passing) — oversize material; expect bridging, poor compaction, and a rough surface.
• A flat middle section (gap grading) — a missing size fraction; the layer may segregate during spreading.

Practical note: One passing test does not approve the quarry forever. Crusher settings drift and stockpiles segregate — re-test each new lot or whenever the stone visibly changes.

Common Mistakes

• Testing a wet or damp sample. Moisture makes fines clump and blinds sieves — always oven-dry first.
• Overloading sieves. More than a single-particle layer on the mesh means incomplete separation; split the sample across two runs if needed.
• Too-short shaking. Hand-shaking for a minute is not a test; follow the standard time and verify with the “no more than 1% passes in one minute of hand sieving” check.
• Grab sampling from the stockpile edge. Edges segregate — coarse rolls to the bottom. Sample from several points and depths.
• Ignoring the 0.3% mass-balance check. Lost material means a wrong gradation, full stop.

Frequently Asked Questions

Sieve analysis or gradation — same thing?
Yes. “Sieve analysis” is the test; “gradation” (or “grading”) is the result — the particle size distribution.

Why are WBM sieves so much bigger than concrete aggregate sieves?
Because WBM is built from 45–90 mm stone for load spreading through interlock, while concrete needs smaller, well-graded aggregate to pack around reinforcement and produce a dense, workable mix.

Do screenings need sieve analysis too?
Yes. Screenings (Type A 13.2 mm / Type B 11.2 mm per MoRTH) have their own grading bands, and the binding material’s plasticity index must stay below 6 (IS 2720 Part 5) — plastic fines in a WBM layer cause exactly the softening the test is meant to prevent.

How often should gradation be tested?
Per the project specification and quality plan — typically each lot or a set tonnage from the crusher, plus a fresh test whenever the source or the visual appearance changes.

Final Thoughts

Sieve analysis is one of the oldest tests in the book and still one of the most decisive: a single table of retained weights tells you whether a thousand tonnes of stone will interlock or ravel. Run it on every lot, read the curve honestly against the band, and reject stone that does not fit — it is far cheaper than rebuilding a failed base.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.