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Construction glossary · Sitework

What is aggregate base coursein construction?

Aggregate base in construction is a compacted layer of crushed stone placed under asphalt, concrete pavement, curbs, and sidewalks to spread wheel loads into the subgrade and provide a stable, drainable platform for paving. Specs call the layer aggregate base course (ABC), and estimators take it off as area times depth, carrying square yards (sy) for placement and cubic yards (cy) or tons for material. The pavement section detail calls out the stone class and a compaction requirement, commonly 95 percent of maximum density.

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Updated August 2026 · Reviewed by the Ruh construction team

Measured in sy, cy, tonsTakeoff rule area x depthCompaction factor 20 to 25 percentLives on civil C-sheets

How aggregate base is measured in a pavement section

8 in base
The definition

Aggregate base, explained.

Aggregate base is the structural stone layer of a paving assembly, sitting between the compacted subgrade and the asphalt or concrete surface. It lives on the civil C-sheets: the paving plan shows the limits of each pavement type, and the pavement section details show base depth for each section, with the geotech report recommending the thicknesses. The material is a dense-graded crushed stone whose name changes by state DOT (Class 2 in California, CA-6 in Illinois, ABC in the Carolinas), so estimators price what the local quarry actually sells. The sitework or grading contractor typically owns the scope, sometimes the paving sub. New estimators get three things wrong: they take off only the asphalt footprint and miss base under curb, sidewalk, and dumpster pads; they overlook the detail note extending base beyond the edge of pavement, often 12 inches; and they forget the difference between compacted volume in place and loose volume delivered, which can run 20 to 25 percent.

How it is measured

Aggregate base is measured as area times depth. Pull the area of each pavement section from the paving plan in sf, convert to sy (divide by 9) for the placement and fine grading lines, and compute cy (area times depth in ft, divided by 27) or tons for the material line. Standard duty and heavy duty sections carry different depths, so they are taken off separately. Cost drivers are depth, the specified stone class, haul distance from the quarry, and compaction effort, with proof rolling and density testing sometimes carried as their own lines. Watch the section detail for base extending past the pavement edge and for a geotextile separation layer beneath it.

Base course vs subbase vs surfacing: where each layer sits

A pavement section is a stack, read from the ground up: the subgrade (compacted native soil or fill), an optional subbase of cheaper, coarser stone on thicker sections, the aggregate base course, and the surfacing — asphalt or concrete — on top. The base course is the layer doing the structural work directly under the pavement, which is why it carries the tighter gradation and the 95 percent compaction requirement.

Estimators price each layer as its own line because each has its own material, depth, and placement cost: the subbase and base course come off the same section detail at different depths, and a takeoff that blends them into one stone line cannot be checked against the supplier's quote. Accepted base material is typically dense-graded crushed stone, with many agency specs also allowing crushed recycled concrete at the same gradation.

Layer order is documented at Base course on Wikipedia; placement and compaction requirements in a representative municipal spec: Watertown, SD aggregate base course technical specification.

Class 2 aggregate base (Class II road base)

Class 2 is the workhorse aggregate base designation in California and the naming travels: Class 2 aggregate base, Class II road base, and class 2 road base all describe the same dense-graded crushed stone, most commonly at a 3/4 inch maximum gradation, compacted to 95 percent relative compaction under pavements. Other states run their own names for the equivalent material — CA-6 in Illinois, ABC in the Carolinas — so the estimating rule from the definition above holds: price the class the local quarry actually sells.

The class designation is what ties the takeoff to the spec: the gradation band and compaction requirement come from the agency standard, and the worked parking lot example below prices the same way whichever class the section detail names.

Representative public specs for gradation and compaction: Pueblo, CO Article 6, Aggregate Base Course and Rio Rancho, NM Section 310, Base Course.

Worked example

A worked aggregate base example.

Worked example

Placement area30,000 sf / 9 = 3,333.3 sy, carried as 3,333 sy
Compacted volume30,000 sf x 8/12 ft = 20,000 cf, 20,000 / 27 = about 741 cy in place
Loose order volume741 cy x 1.25 = 926.25, carried as 930 cy
Material cost (illustrative)930 cy x $38 = $35,340
Aggregate base material$35,340
How Ruh handles it

How Ruh handles aggregate base.

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Aggregate base: frequently asked questions

What is the difference between aggregate base and subbase?+

Base is the layer directly under the pavement surface and uses a tighter, higher quality crushed stone gradation. Subbase, when the section includes one, sits between the subgrade and the base and uses a cheaper, coarser material, since its job is bulk support and drainage rather than direct load transfer. Thicker roadway and heavy duty sections often carry both layers; light duty parking sections usually carry base only.

How many tons of aggregate base are in a cubic yard?+

Compacted dense-graded crushed stone is commonly estimated at roughly 1.9 to 2.0 tons per compacted cubic yard, but the real conversion depends on gradation, the rock source, and moisture content. Quarries quote their own conversion factor, so estimators who buy by the ton should use the supplier's number instead of a generic one. A few percent of slip on a thousand-ton order is real money.

Why is the delivered quantity higher than the takeoff volume?+

Three reasons. Loose stone compacts, so filling 741 cy of in-place volume takes more than 741 cy of delivered material, often 20 to 25 percent more. The section detail usually extends the base beyond the pavement edge, adding area the asphalt footprint does not show. And soft subgrade discovered during proof rolling gets undercut and replaced with extra stone, which is why many estimators carry an undercut allowance.

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Related terms

Compactionasphalt takeoffGeotextileFinal gradingwhat is grading in constructionCurb and gutterSoil stabilizationSidewalk

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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