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 of Aggregate ! Description ! Common Grades/Sizes ! Uses in Construction, Roads, and Parking Lots |- | Crushed Stone | Coarse, angular material produced by crushing rocks (e.g., limestone, granite, basalt, trap rock). | #1 (2" to 4"), #2 (1.5" to 2"), #3 (1" to 2"), #5 (1" or less), #57 (3/4" to 1"), #67 (3/4" or less), #8 (3/8" to 1/2") | Base and subbase for roads and parking lots, concrete and asphalt mixes, drainage systems, structural foundations. |- | Gravel | Naturally occurring, rounded rock fragments shaped by erosion, often from riverbeds or pits. | 3/4", 1.5", 3/8" (pea gravel), 1" to 2" (varies by region and source) | Subbase and base layers for roads and parking lots, drainage, driveways, decorative surfacing. |- | Sand | Fine granular material, natural or manufactured, from eroded rock or crushed stone. | Fine (passes 4.75 mm sieve, e.g., concrete sand, masonry sand), Zones 1-4 (per ASTM C33, Zone 4 finest) | Concrete and asphalt production, bedding for pipes, leveling for pavers, drainage layers in roads and parking lots. |- | Recycled Concrete | Crushed material from demolished concrete structures, reprocessed for reuse. | 50 mm to dust (e.g., Type 1 crushed concrete), 3/4" nominal, varies by processing | Subbase for roads and parking lots, backfill, trench fills, over-site fills, cost-effective base material. |- | Slag | Byproduct of iron and steel production, air-cooled or granulated, crushed for use. | Air-cooled (3/4" to 1.5"), granulated (sand-sized), varies by processing | Road bases and surfaces, asphalt mixes, parking lot surfacing, fill for durability and strength. |- | Dense Grade (Quarry Stone) | Compact mix of crushed stone (3/4" or less) and stone dust for tight binding. | 3/4" or less, mixed with fines (varies by region, e.g., Dense Grade, MOT Type 1) | Base and subbase for roads and parking lots, sturdy surface layer, less effective for drainage in wet climates. |- | Ballast (All-in Aggregate) | Mixture of various sizes from pits, rivers, or crushing plants, often unprocessed. | Wide range, e.g., 1.5" to dust, per ASTM C33 or regional specs | Road and parking lot base, repairing cracks, pavement foundations, rail track sub-ballast. |- | Geosynthetic Aggregates | Synthetic materials or composites used to stabilize terrain in civil engineering. | Varies, often custom-sized for project needs | Stabilization of weak soils, drainage layers, reinforcement in road and parking lot construction. |-