4.2 Sieve Analysis & Gradation (AASHTO T27 / T11)
Key Takeaways
- AASHTO T27 (equivalent to ASTM C136) is the standard sieve analysis test used to determine the particle-size distribution of fine and coarse aggregate.
- AASHTO T11 (ASTM C117) measures material finer than the No. 200 (75 micrometer) sieve by washing, and is normally run together with T27.
- The No. 4 sieve, with a 4.75 mm opening, is the standard dividing line between coarse aggregate and fine aggregate.
- Well-graded aggregate contains a continuous range of particle sizes that interlock with minimal voids, while uniformly graded material is dominated by a single particle size.
- Gap-graded aggregate is intentionally missing a mid-size fraction and is used in specialty applications such as certain HMA surface mixes, not general structural fill.
Why Gradation Controls Everything Downstream
Gradation — the distribution of particle sizes in an aggregate sample — is arguably the single most inspected property in highway construction, because it drives compactability, strength, permeability, and workability in everything from embankment fill to HMA surface mixes. The standard test is AASHTO T27 (equivalent to ASTM C136), Sieve Analysis of Fine and Coarse Aggregates, almost always run together with AASHTO T11 (ASTM C117), Materials Finer Than 75 μm (No. 200) Sieve by Washing, because dry sieving alone cannot accurately separate the finest silt- and clay-sized particles that cling to larger particles.
The Standard Sieve Set
Sieves are identified either by opening size (in millimeters or inches) or by a mesh "sieve number." A typical highway aggregate sieve stack, largest opening on top, includes:
| Sieve | Opening |
|---|---|
| 2 in | 50.0 mm |
| 1 in | 25.0 mm |
| 3/4 in | 19.0 mm |
| 3/8 in | 9.5 mm |
| No. 4 | 4.75 mm |
| No. 8 | 2.36 mm |
| No. 30 | 0.600 mm |
| No. 50 | 0.300 mm |
| No. 100 | 0.150 mm |
| No. 200 | 0.075 mm |
The No. 4 sieve is the standard boundary between coarse aggregate (retained on No. 4) and fine aggregate (passing No. 4) — a distinction inspectors use constantly when reading specs and mix designs.
Running the Test
The general T27/T11 procedure:
- Obtain a representative sample by quartering or splitting per AASHTO T248, and dry to a constant mass.
- Wash the sample over a No. 200 sieve (T11) to remove fines coating the larger particles, then oven-dry again.
- Nest the sieves in descending order and mechanically shake the dried sample through the stack for a specified time.
- Weigh the material retained on each sieve.
- Compute cumulative percent retained and percent passing at each sieve size.
Worked Gradation Calculation
Assume a 2,000 g fine aggregate sample. After washing and sieving, 220 g is retained on the No. 4, and an additional 640 g is retained on the No. 8. Cumulative mass retained on the No. 8 = 220 g + 640 g = 860 g. Percent retained on the No. 8 = 860 ÷ 2,000 × 100 = 43%. Percent passing the No. 8 = 100% − 43% = 57%. This percent-passing value is the number checked directly against the job-mix or specification gradation band for that sieve.
Reading the Gradation Curve
Plotting percent passing (y-axis) against sieve size (x-axis, log scale) produces the gradation curve, which inspectors and lab techs compare against the specification's upper and lower control limits — a "gradation band." Three basic shapes matter:
- Well-graded (dense-graded) — a smooth, continuous curve with material present across the full size range; particles of different sizes fill the voids left by larger particles, producing high density and interlock. Most base, subbase, and dense-graded HMA specs target this shape.
- Uniformly graded (open-graded/single-sized) — a steep curve concentrated in a narrow size range; more void space, higher permeability, but less interlock and stability (e.g., some drainage aggregate, chip-seal cover stone).
- Gap-graded — a curve that is nearly flat through a mid-size range because that size fraction is intentionally minimized; used in specialty surface mixes and architectural applications, but not appropriate for general structural fill unless the mix design specifically calls for it.
Acceptance and Rejection
A sample outside the specified gradation band on any one sieve does not automatically fail the load or stockpile — many specs allow blending stockpiles, adjusting a plant's cold-feed proportions, or accepting a single "check point" deviation within a tolerance. But a pattern of out-of-band results, or a badly gap-graded material presented as dense-graded, is documented and typically triggers rejection, a stop-work notice on that source, or a required blend before further placement. The inspector's gradation report — sieve, percent passing, spec band, pass/fail — becomes part of the materials acceptance record referenced later in project documentation (Chapter 10).
Sampling and Stockpile Segregation
A gradation test is only as good as the sample that fed it. Aggregate stockpiles naturally segregate as they are built: coarse particles have more momentum and roll further down the slope of the pile, while fines stay concentrated near the loading point, so a sample grabbed from one spot on the face of a pile does not represent the pile as a whole. Correct sampling per AASHTO T2/T248 pulls multiple increments from various locations and depths within the pile (or from the moving belt/discharge stream, which avoids segregation entirely) and composites them into one representative sample before it is split down to test size. An inspector who accepts a single grab sample from the easiest-to-reach part of a stockpile — rather than a properly composited sample — risks passing material that would fail if sampled correctly, and risks rejecting good material sampled from an unrepresentative fines-rich pocket.
Which statement correctly distinguishes coarse aggregate from fine aggregate under standard highway sieve analysis (AASHTO T27)?
A 1,500 g aggregate sample is sieved per AASHTO T27. The cumulative mass retained on the No. 8 sieve is 900 g. What percent of the sample is retained on the No. 8 sieve (and larger)?