The Comprehensive Guide to Asphalt Millings (RAP)
Whether you're paving a long rural driveway, creating a large parking area, or building a sturdy base, asphalt millings offer a highly durable and cost-effective alternative to traditional gravel or expensive hot mix asphalt. This guide covers everything you need to know about using crushed recycled asphalt.
What Are Asphalt Millings?
Asphalt millings, technically referred to as Recycled Asphalt Pavement (RAP), are generated when road milling machines grind up the surface layer of old roads and parking lots during resurfacing operations. Instead of sending this material to a landfill, it is collected, sometimes crushed and screened, and repurposed.
The resulting material looks similar to dark gravel, but it retains a crucial component: bitumen. This residual tar binder gives millings unique properties that standard stone or gravel lack. When exposed to heat and pressure, the tar begins to melt and bind the aggregate back together, creating a semi-solid surface that minimizes dust and resists washouts far better than loose gravel.
Asphalt Millings vs. New Asphalt vs. Gravel
How do millings stack up against the alternatives? Here is a quick reference comparing the most common driveway and lot materials.
| Feature | Asphalt Millings | Hot Mix Asphalt | Crushed Gravel |
|---|---|---|---|
| Cost per Ton | $10 – $30 | $80 – $150 | $15 – $40 |
| Durability | High | Very High | Medium (Requires maintenance) |
| Dust Level | Low | None | High |
| Washout Risk | Low | None | High |
| Aesthetics | Rustic, dark grey | Smooth, jet black | Natural stone colors |
The Compaction Factor Explained
The single most common mistake DIYers and inexperienced contractors make with millings is failing to account for compaction shrinkage. Because millings are chunky and irregular, a pile of loose millings contains a significant volume of air. When you spread this material and run a heavy vibratory roller over it, it compresses dramatically.
Example: The 20% Rule
On average, loose millings will shrink by about 20% to 25% under heavy compaction.
If you want a final, compacted driveway depth of 4 inches, you cannot just spread 4 inches of loose material. You must calculate the loose depth required:
- Formula: Final Depth ÷ (1 - Compaction Rate) = Loose Depth
- Calculation: 4 inches ÷ (1 - 0.20) = 5 inches
You need to spread 5 inches of loose millings to achieve a 4-inch solid surface. Our calculator automatically handles this conversion to ensure you order enough tons.
Best Uses for Asphalt Millings
Because of their low cost and excellent binding properties, millings are ideal for specific applications:
- Long Rural Driveways: Far cheaper than paving miles of hot mix, and much less dusty than gravel.
- RV and Boat Parking Pads: Provides a firm, weed-resistant base for heavy vehicles.
- Farm Roads and Equipment Lots: Handles heavy agricultural machinery well without turning to deep mud.
- Sub-base Material: An excellent, highly stable base layer beneath poured concrete or new hot mix asphalt.
When NOT to Use Millings
Avoid using asphalt millings if you require a perfectly smooth, refined surface (like for skateboarding or basketball courts), if your municipality restricts RAP usage due to environmental runoff concerns in sensitive wetlands, or if the area receives zero direct sunlight, as the heat of the sun is required to help the tar bind the millings together.
How to Install Asphalt Millings: A 5-Step Guide
Proper installation is the difference between a driveway that lasts for decades and one that falls apart after the first winter.
- Site Preparation: Grade the area properly to ensure water drains away from the center. Remove soft topsoil and vegetation.
- Lay a Solid Base: While millings can be placed on dirt, laying down a geotextile fabric and a solid aggregate base significantly improves longevity and prevents sinking.
- Spread the Millings: Using a skid steer or tractor, spread the millings evenly. Remember to spread them about 20-25% deeper than your desired final thickness.
- Crown the Surface: Grade the millings so the center is slightly higher than the edges (creating a "crown"). This ensures water runs off the sides rather than pooling in the middle.
- Heavy Compaction: This is the most critical step. Use a heavy vibratory roller (at least 3-5 tons) while the weather is hot. The combination of heat, pressure, and vibration locks the pieces together and activates the bitumen binder.
Frequently Asked Questions
What are asphalt millings?
Asphalt millings, also known as Recycled Asphalt Pavement (RAP), are crushed pieces of old asphalt that have been removed from roads or parking lots during resurfacing projects. They are repurposed as an eco-friendly and inexpensive paving material.
How much do asphalt millings weigh?
Compacted asphalt millings typically weigh around 130 pounds per cubic foot. This translates to roughly 3,500 pounds (1.75 tons) per cubic yard. Loose millings weigh less per volume due to air gaps.
Do asphalt millings harden in the sun?
Yes. Asphalt millings contain remnant bitumen (tar) which softens in the heat of the sun. When properly rolled and exposed to hot summer sunlight, the pieces bind together and harden significantly over time, creating a semi-solid crust.
How thick should asphalt millings be installed?
For standard residential driveways, a final compacted depth of 4 to 5 inches is recommended. Due to a 20% compaction rate, you should spread the loose millings roughly 5 to 6 inches deep before rolling.
Are asphalt millings good for driveways?
Yes, they are an excellent, cost-effective alternative to gravel or hot mix asphalt, especially for long driveways. They produce far less dust than gravel, resist washing out in heavy rain, and harden well over time.
Can you use a compactor on asphalt millings?
Not only can you, but you absolutely must. Heavy compaction using a vibratory dirt roller is crucial for asphalt millings to bind together properly. A simple hand tamper or driving over it with a truck is not sufficient for a lasting result.