Understanding Aggregate Grades: A Contractor's Guide

Walk onto any job site and you will hear contractors throwing around numbers and abbreviations: "We need 15 tons of 57 stone," "Spec calls for Grade 1 flex base," "Get me some Class A fill for the backfill." If you are new to the construction industry — or a homeowner trying to understand what your contractor is ordering — these codes can be confusing.

This guide demystifies the most common aggregate grades and classifications used in the United States. You will learn what each number means, how the stones are sized, and exactly which applications call for each grade.

Understanding the Numbering System

The numbers assigned to aggregate grades (like #57, #67, #8) come from the American Society for Testing and Materials (ASTM) standard C33 and the American Association of State Highway and Transportation Officials (AASHTO). The numbers refer to the size of the sieve screens used to sort the stone. Lower numbers generally mean larger stones.

Common Stone Grades Explained

#57 Stone

#57 stone is the most widely used aggregate grade in the construction industry. It is the workhorse — versatile enough for dozens of applications and available from virtually every quarry and supplier in the country. The angular edges interlock under compaction, creating a stable, well-draining surface.

Best uses:

• Driveway surface layers
• French drain fill
• Concrete mix aggregate
• Pipe bedding
• Backfill around foundations

#67 Stone

#67 stone is slightly smaller than #57 and is often used as a transitional layer in multi-layer driveway construction. It nests between the larger base stone and the surface layer, filling voids and adding stability. It is also commonly specified for concrete mix designs and as backfill around drainage structures.

Best uses:

• Driveway middle layer between base and surface
• Concrete aggregate for structural pours
• Drainage backfill behind retaining walls
• Walkway base preparation

#8 Stone (Pea-Sized)

#8 stone is small, angular aggregate about the size of a pea. It is used in asphalt mix, as a top-dressing for flat roofs, and in specialty drainage applications. Do not confuse #8 stone with pea gravel — #8 is angular (crushed), while pea gravel is round (naturally tumbled). The angular edges of #8 interlock better than rounded pea gravel.

Best uses:

• Asphalt mix aggregate
• Flat roof ballast
• Pipe zone bedding for small utilities
• Decorative ground cover in tight spaces

Base and Fill Classifications

Grade 1 Flex Base

Grade 1 flex base is a blended aggregate that meets state Department of Transportation specifications for road construction. In Texas, it meets TxDOT Item 247 standards. "Grade 1" means it has the highest level of plasticity index control — the fines content is carefully managed to ensure proper compaction and load-bearing capacity.

Flex base contains a mix of crushed stone, stone dust, and natural binding fines that compact into a near-solid surface. It is used for road sub-bases, parking lots, construction entrances, and heavy-use driveways. When properly compacted with moisture, it achieves 95 to 100 percent proctor density.

• Road and driveway sub-base construction
• Parking lot base layers
• Construction site stabilization
• Equipment pad foundations

Class A Fill (Select Fill)

Class A fill — also called select fill — is screened, tested fill dirt that meets engineering specifications for structural applications. Unlike standard fill dirt (which may contain unknown material), Class A fill has been tested for particle size distribution, plasticity, and organic content. It is specified by engineers for foundation backfill, structural earth fill, and any application where settlement must be minimized.

• Foundation backfill for commercial structures
• Structural earth fill for embankments
• Utility trench backfill where compaction testing is required
• Any fill application specified by a geotechnical engineer

Quick Reference: Which Grade for Which Job?