AASHTO M43 sorts coarse aggregate for road and bridge construction into 19 numbered size designations, each defined by the percentage of material that must pass a specified series of sieves. The standard’s full title is “Standard Specification for Sizes of Aggregate for Road and Bridge Construction,” and it covers natural gravel, crushed stone, and manufactured products such as crushed air-cooled blast-furnace slag. M43 is a sizing standard only. Whether a given stone is hard enough, clean enough, or chemically stable enough for its intended use is governed by companion specifications like AASHTO M80.
M43 shares its title and sizing framework with ASTM D448, and the two are jointly owned.1Transportation.org. AASHTO Materials Standards with ASTM Equivalencies A project calling for a No. 57 to ASTM D448 and one calling for a No. 57 to AASHTO M43 is asking for the same material. When a concrete standard like ASTM C33 governs the mix, it typically references these same size numbers for the coarse fraction.
How the Size Numbers Work
Each M43 size number identifies a specific band of particle sizes. Lower numbers mean larger stone. Two-digit combinations like 57 or 67 represent blends that span two adjacent size ranges. Across the 19 designations, nominal sizes range from roughly 4 inches down to material passing the No. 8 sieve, about 2.36 mm.2Federal Highway Administration. Strength Characterization of Open-Graded Aggregates for Structural Backfills
The specification is laid out as a table. Sieve sizes run across the top, size numbers down the side, and each cell gives the allowable percentage passing that sieve for that designation. A sample is compliant only if every sieve result falls inside the allowable range. Missing on one sieve is enough to fail the material, even when the sample looks correct visually.
Reading a Gradation: No. 57 Stone
No. 57 stone is the most familiar M43 size on job sites, and its gradation shows how the table works in practice. To meet the specification, a sample must produce these results on a sieve analysis:
- 1.5-inch sieve: 100% passing
- 1-inch sieve: 95 to 100% passing
- 3/4-inch sieve: 90 to 100% passing
- 1/2-inch sieve: 25 to 60% passing
- 3/8-inch sieve: 0 to 15% passing
- No. 4 sieve: 0 to 5% passing
- No. 8 sieve: 0 to 5% passing
Those bands are what separate a compliant, well-graded aggregate from material that only looks right.3Federal Highway Administration. Strength Characterization of Open-Graded Aggregates for Structural Backfills – Chapter 3 Test Program and Results A sample can pass 100% through the 1.5-inch sieve and still fail if 8% slips through the No. 4.
No. 67 stone uses the same structure but a smaller nominal range, roughly 19.0 to 4.75 mm, which is 3/4 inch down to the No. 4 sieve. No. 8, with particles between the 3/8-inch and No. 8 sieves, is finer still. Every step down in nominal size shifts the whole gradation band toward smaller openings.
Common Sizes and Where They Are Used
What you specify depends on what the aggregate has to do. Larger, more uniformly sized stone leaves voids that drain water. Blended sizes pack together tightly for load-bearing strength. A few designations account for most of the coarse aggregate placed on public projects:
- No. 4 (1.5 inch to 3/4 inch): A larger, uniformly sized stone used for railroad ballast, heavy drainage, and erosion control where rapid water flow matters.
- No. 57 (1 inch to No. 4): The workhorse. It shows up as a concrete mixing aggregate, road base, driveway base, and drainage medium. Its particle range balances load-bearing capacity against permeability.
- No. 67 (3/4 inch to No. 4): Slightly finer than No. 57. Common in concrete for structural elements where a tighter void structure or smoother finish is needed, and it handles well during placement.
- No. 8 (3/8 inch to No. 8 sieve): A fine coarse aggregate for asphalt surface courses, walking paths, and pipe bedding.
Open-Graded Sizes for Drainage
When free drainage drives the design, engineers reach for open-graded M43 sizes. An FHWA study characterized 16 designations commonly used in structural backfills and drainage applications, including Nos. 5, 56, 57, 6, 67, 68, 7, 78, 8, 89, 9, and 10.2Federal Highway Administration. Strength Characterization of Open-Graded Aggregates for Structural Backfills Most had less than 3% material passing the No. 200 sieve, which is why they drain so well. The exception was No. 10, which carried about 11% fines and moved water more slowly.
Verifying That a Material Meets Its Size
Compliance with M43 is settled by sieve analysis, run on a sample that actually represents the material in the pile. Both halves matter.
Sampling
AASHTO T 2 sets the procedures for pulling aggregate samples from conveyor belts, transport vehicles, roadways, and stockpiles. The core principle is to collect several increments of roughly equal size from random locations and combine them into one field sample meeting a minimum mass. Stockpiles need extra care because the outer surface holds segregated material where larger particles have rolled to the edges; the sampling face is cut back before any increments are taken.
Sieve Analysis
The primary compliance test is AASHTO T 27. A dried sample runs through a nested stack of sieves, largest opening at the top, smallest at the bottom. The material retained on each sieve is weighed, and the results are converted to cumulative percentage passing. Those numbers are compared against the M43 table for the size number in question. If any sieve result falls outside its band, the material fails.
Related Tests That Support the Result
A few other tests round out the picture used in mix design and acceptance:
- AASHTO T 11 washes fines finer than the No. 200 sieve out of the sample, catching dust and clay that the dry sieve analysis in T 27 can miss.
- AASHTO T 85 measures specific gravity and absorption, values that feed the water-to-cement ratio and strength calculations in concrete mix design.
- AASHTO T 19 measures bulk density, the unit weight of aggregate packed into a standard container, which is used to convert mix designs from weight to volume and estimate yield on site.
Why Compliant Material Can Still Fail on Site
Aggregate that tests perfectly at the plant can fail at the job site if it segregates in the stockpile. Segregation happens when larger particles roll to the outside of a pile and smaller particles settle in the center, creating zones that no longer meet the M43 band. A lot of specification failures start here, and they surprise contractors who assume approved-at-source means approved everywhere.
The common mistake is dumping material into a single tall cone, which almost guarantees segregation. Better practice places loads adjacent to each other in low, flat layers so the pile builds horizontally instead of cascading down a steep slope. When a technician samples a stockpile under AASHTO T 2, the outer edges are avoided and the face is cut back at least two bucket widths before sampling begins, so the segregated exterior does not contaminate the result.
What M43 Does Not Cover
M43 answers one question: is the aggregate the right size? It does not tell you whether the stone is hard enough to survive traffic, sound enough to resist freeze-thaw, or free of reactive minerals that could crack concrete years after placement. Those requirements sit in companion standards, principally AASHTO M80 for aggregate quality, with test methods like AASHTO T 96 for abrasion loss, AASHTO T 104 for soundness, and AASHTO T 303 for alkali-silica reactivity. A material can satisfy M43 in every respect and still be unusable for a given project if it fails those separate limits, so specifications typically call both standards together.