Crushed Miscellaneous Base (CMB): What It Is and Uses

Crushed miscellaneous base, usually shortened to CMB, is a recycled base rock that shows up on plans, quotes and supply yard boards across Southern California. It is made mostly from crushed old asphalt and concrete, processed to meet a public works specification so it can go under pavement, slabs and driveways. If you have seen "CMB" next to "Class 2 base" and wondered which to use, this guide explains what CMB is, how it compares and how to figure how much to order.

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The short answer

What it is: base material made from crushed asphalt concrete and portland cement concrete, sometimes blended with crushed rock.
Spec: Section 200-2.4 of the Greenbook, the Standard Specifications for Public Works Construction that many Southern California cities and counties use.
Closest cousin: Caltrans Class 2 aggregate base, which it is commonly treated as comparable to.
Uses: base under asphalt and concrete pavement, driveways, parking lots, slabs and pads, where the project spec allows it.

What goes into CMB

CMB starts as broken pavement, curbs, sidewalks and slabs from demolition and road work. Recyclers crush and screen it, sometimes blend in crushed rock or aggregate base, and test the result against the spec. The name "miscellaneous" reflects that mixed source. Because it reuses material that would otherwise go to a landfill, many agencies and contractors favor it where it performs as well as virgin base.

A closely related Greenbook material, processed miscellaneous base (PMB, Section 200-2.5), is a lower-grade recycled base that the same comparison lines up with Caltrans Class 3 aggregate base.

CMB vs Class 2 aggregate base

A public works seminar comparing the 2021 Greenbook with the 2023 Caltrans Standard Specifications lists Caltrans Class 2 aggregate base as comparable to Greenbook CMB, with these differences:

R-value: both require a minimum of 78. R-value is a lab measure of how well a base resists deforming under load.
Sand equivalent: CMB has a higher minimum, 35 versus 22. Sand equivalent is a test that flags too much clay-like fine material.
Brick: CMB allows up to 3 percent brick.
Gravel particles: CMB caps gravel particles held on a No. 4 sieve at 15 percent.
Component list: CMB does not list reclaimed lean concrete base or cement-treated base as a component material.

Specs are updated over time, so check the edition named in your contract. For the Caltrans side, see our guide to Class 2 aggregate base.

Where CMB is used

Under asphalt. A compacted CMB layer is a common base for streets, parking lots and driveways in areas that use the Greenbook. See recycled concrete under asphalt for how recycled base performs.
Under concrete slabs, sidewalks and driveways.
Pads and yards where a firm, compacted surface is enough on its own.
Trench backfill and utility work, where the agency allows it.

Pros and cons

Pros: compacts into a firm, stable layer; can be priced below virgin aggregate base where recycled supply is plentiful; keeps demolition material out of landfills.
Cons: quality depends on the recycler's processing and testing; small amounts of brick or other debris can show; color and texture vary between loads. Buy from a supplier that can show test results against the spec.

How much does CMB weigh?

Weight varies with the mix of concrete, asphalt and rock and with moisture. Our crushed stone calculator plans recycled concrete at about 1.3 tons per cubic yard, and our guide to how much crushed concrete weighs covers the loose and compacted range. Mixes with more asphalt run a bit lighter, and compacted, damp base runs heavier. Ask the yard what their CMB weighs per yard and use that for ordering.

How many tons of CMB to order

Area: length × width in feet. A 12 by 50 foot driveway is 600 square feet.
Compacted depth: often 4 to 6 inches under a residential driveway, more under heavier traffic or per the plans.
Cubic yards: square feet × depth in inches ÷ 324. At 6 inches, 600 × 6 ÷ 324 is about 11.1 yards.
Tons: yards × density. At 1.4 tons per yard that is about 15.6 tons; at 1.3 it is about 14.4.
Add about 10 percent for compaction and uneven subgrade.

The asphalt base calculator handles the math if you are building a base for a paved driveway.

Placing and compacting CMB

Prepare and compact the subgrade first, removing soft spots.
Spread CMB in lifts, typically a few inches at a time, and bring it to a damp moisture before compacting.
Compact each lift with a roller or plate compactor until it stops moving under the equipment.
Grade the top to the slope you want for drainage before paving or pouring.

For more on how recycled concrete behaves as a driving surface by itself, see crushed concrete driveway.

Questions people ask

What is crushed miscellaneous base?

Crushed miscellaneous base, or CMB, is a base rock made largely from crushed recycled asphalt concrete and portland cement concrete, sometimes blended with crushed rock. It is defined in Section 200-2.4 of the Greenbook, the Standard Specifications for Public Works Construction used widely in Southern California.

Is CMB the same as Class 2 base?

No, but they are close. A comparison of the 2021 Greenbook and 2023 Caltrans specs lists CMB as comparable to Caltrans Class 2 aggregate base, with the same minimum R-value of 78 and a higher minimum sand equivalent, 35 versus 22.

What is CMB used for?

Base under asphalt and concrete pavement, driveways, parking lots, slabs and walkways, and as a compacted pad, wherever the project spec allows a recycled base.

Can CMB contain brick?

Yes, in small amounts. The comparison of Greenbook and Caltrans specs notes that CMB allows up to 3 percent brick.

How much does a yard of CMB weigh?

It varies with the blend and moisture. Recycled concrete is often planned around 1.3 tons per cubic yard, and compacted base can run higher. Ask the supplier for their figure.

How many tons of CMB do I need?

Square feet times compacted depth in inches, divided by 324, gives cubic yards; multiply by the supplier's tons per yard. A 12 by 50 foot driveway at 6 inches is about 11 cubic yards.

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