Aggregate Size and Grade Reference

The size rating for gravel used by some states (specifically those in rural United States) particularly limestone-based rock gravel aggregate, is guided by specifications from the state Department of Transportation. The gradation and size of gravel for granular surfacing and shoulders, which are common applications for rural county roads, are outlined in specific sections the state DOT Standard Specifications and related Materials Instructional Memoranda (I.M.).

Iowa is used a reference here as it tends to represent in general states in the rural Midwest that utilize this type of limestone aggregate.

Granular Surfacing and Granular Shoulder Aggregate:

The Iowa DOT specifies a uniform mixture of fine and coarse particles, which can include crushed stone, gravel, or a combination, often with sand. For rural roads, the focus is often on Class A Crushed Stone or Class C Gravel.

Class A Crushed Stone: As per Section 4120.04 of the Iowa DOT Standard Specifications (revised 10/15/2024), this material must meet specific gradation requirements. While exact size ranges can vary slightly based on contract documents, a common gradation for Class A crushed stone requires:

• 100% passing the 1-inch sieve.
• Other gradations may be specified, but the material is typically crushed limestone or similar stone, with a focus on durability and compaction properties.

Class C Gravel: According to Section 4120.03, this gravel must have:

• No less than 5% and no more than 12% passing the No. 200 sieve (fine material).
• The coarser fraction includes a range of sizes, with 100% typically passing the 1-inch sieve, ensuring a mix of sizes for stability and drainage.

Gradation Numbers: The Iowa DOT uses gradation tables to define acceptable size distributions. For example, Gradation No. 10 from the Aggregate Gradation Table (Article 4109.02) is often referenced for granular shoulders:

• 30% to 50% crushed stone or recycled materials, with 8% to 16% passing the No. 200 sieve.
• 100% passing the 1.5-inch sieve, with a mix of smaller sizes down to fines for compaction.

For other applications, such as driveway surfacing, Section 2315 references similar materials, with 100% passing the 1-inch sieve for "other aggregate" options.

Practical Size Range:

In practice, limestone gravel for rural Iowa roads typically includes a range of sizes, with the largest particles not exceeding 1 to 1.5 inches in nominal size. This ensures a compactable surface that can handle traffic loads while allowing for drainage.

The presence of fines (material passing the No. 200 sieve) is controlled to balance dust control and surface stability without compromising drainage.

Iowa DOT and County Practices

Iowa DOT Oversight: The Iowa DOT’s Standard Specifications and Materials I.M. documents (e.g., I.M. 409, revised 10/20/20) govern aggregate quality and gradation for state and county projects. Counties often adopt these standards for consistency, especially for farm-to-market roads and local access roads.

County Customization: Individual counties may adjust gradations slightly based on local needs, traffic levels, and available materials, as noted in the Iowa County Roads website. However, they typically align with Iowa DOT gradations like No. 10 or similar for granular surfacing.

Limestone Prevalence: Limestone is widely used due to its abundance in Iowa, with quarries like those operated by River Products Company and Douds Stone, LLC supplying crushed limestone meeting Iowa DOT specs for roadstone and base materials.

The size rating for limestone gravel used by Iowa counties and the Iowa DOT for rural roads generally involves a mix of sizes, with 100% of the material passing the 1-inch or 1.5-inch sieve, depending on the specific gradation (e.g., Gradation No. 10 or Class A/C specifications). The mix includes coarse particles (up to 1-1.5 inches) and finer material (down to 5-16% passing the No. 200 sieve) to ensure a stable, compactable surface. Exact sizes may vary slightly based on local contract documents or county preferences.

Type	Description	Common Grades/Sizes	Primary Uses
Crushed Stone	Coarse, angular rock fragments from quarries, typically limestone, granite, or basalt, crushed to specific sizes.	#1 (2"–4"), #2 (1.5"–2"), #57 (0.75"–1"), #67 (0.75" or smaller), #8 (0.375"–0.5")	Road base, subbase, concrete mixes, asphalt pavement, parking lot foundations
Gravel	Naturally occurring, rounded rock fragments from riverbeds or deposits, shaped by erosion.	3/8", 3/4", 1.5", pea gravel (3/8"), seal, hydraulic base	Road subbase, drainage, parking lot surfaces, decorative applications
Sand	Fine, granular material from natural deposits or crushed rock, variable composition.	Concrete sand, masonry sand, sand 4, sand 5, washed sand	Concrete and asphalt mixes, leveling base for parking lots, drainage layers
Recycled Concrete	Crushed material from demolished concrete structures, processed to desired sizes.	50mm down to dust, Type 1 crushed concrete	Road subbase, parking lot base, trench fills, backfills
Slag	Byproduct of iron/steel production, air-cooled or water-cooled, crushed for use.	Air-cooled slag, granulated slag (sand-sized)	Road bases, surfaces, parking lot bases, asphalt mixes
Dense Grade	Compact mix of crushed stone (3/4" or less) and stone dust for tight binding.	3/4" or smaller, mixed with fines	Road base, parking lot surfaces, driveways, semi-solid foundation
Type 1 MOT	Crushed stone or concrete, graded from 50mm to dust, high load-bearing capacity.	50mm to dust	Subbase for roads, parking lots, under blocks or asphalt
Ballast	Single-size crushed stone or all-in aggregate from pits or rivers, mixed sizes.	Various sizes, often 1"–2"	Road base, parking lot foundations, railway ballast, crack repairs
Fine Aggregate	Small particles passing 4.75mm sieve, subdivided by fineness.	Zones 1–4 (Zone 4 finest), <4.75mm	Concrete, mortar, base for parking lots, asphalt mixes
Coarse Aggregate	Larger particles, crushed or uncrushed, graded by sieve size.	#1 to #8, 3/8" to 4" range	Road base, concrete mixes, parking lot foundations, structural layers