Commercial Construction & Contracting Resources

What is Asphalt Parking Lot Compaction? And Why is It Important?

Posted by Tom Owings on April 19, 2018

measuring asphalt compaction

You’re standing on an asphalt parking lot. Between your feet and God’s earth below is a certain thickness of asphalt. But it wasn’t that thick when it was laid.

The asphalt was properly compacted (if the contractors knew what they were doing) when the lot was first built or later reconstructed. The hot-mix asphalt (HMA) was spread to a certain thickness before it was rolled and allowed to settle.

Why is asphalt compaction important? It increases the strength, durability, and functional lifespan of your asphalt parking lot.

Tips for Proper Compaction: Strengthening and Beautifying Your Asphalt Parking Lot

There’s a formula for proper compaction. But keep in mind, there are several variables that affect the outcome under a variety of conditions.

Here are some rules of thumb.

Placed Thickness vs. Compacted Thickness

Be very mindful of this, because this is where shady contractors will try to sting you. They’ll claim, for instance, the job calls for 4” of asphalt thickness and lay that amount, but when it’s compacted it’ll actually be about 3” thick (and therefore less durable).

Generally, asphalt compacts about 0.25” for each 1” that’s laid. So by that valuable measuring stick:

  • 2” of compacted asphalt requires 2.5” of placed thickness
  • 3” of compacted asphalt requires 3.75”
  • 4” of compacted asphalt requires 5”

An asphalt contractor in Central OH should follow ODOT specs for proper placed thickness.

Consider the Traffic

Will your asphalt parking lot be small and carry limited amounts of traffic, or will it be a behemoth built for thousands of cars daily? You may know this well, or could require a formal traffic study.

Here are some thickness suggestions for light-duty and heavy duty lots (and remember the important role the subbase plays in overall asphalt stability):

  • 3” of compacted asphalt thickness on 6” of aggregate stone for light duty
  • 4” of compacted asphalt thickness on 8” of aggregate stone for heavy duty

But guidelines are just guidelines. There could be other factors that determine an ideal thickness in your particular commercial parking lot situation. It’s best to get on the same page with your contractor to develop the perfect plan.

Compacted Thickness Affects Permeability

Asphalt may appear solid, but it’s actually a porous, permeable surface dotted with tiny holes. On one hand, some holes are good as they allow the asphalt some expansion and flexibility. On the other hand, too many holes allow too much water to seep in and cause damage.

On a micro scale, compaction limits permeability to a level that staves off deterioration and promotes some pliability needed to accommodate expansion and contraction from the damaging freeze-thaw cycle.

Compacted air voids should be between 7% and 3% of the total asphalt volume, depending on:

  • Traffic/purpose
  • Location
  • Asphalt mix
  • Predominant weather patterns

Factor these and other asphalt parking lot variables with your contractor.

Compaction is Key to a Building a Great Commercial Parking Lot

When hot-mix asphalt is first placed, it’s a gooey mix of liquid tar and stone aggregate that eventually hardens and compacts. But it retains a small bit of its pliant and porous nature after rolling and cooling.

An experienced contractor will know how to achieve the appropriate compaction to suit specific needs for your asphalt parking lot. Also remember that compaction is crucial when constructing the dirt subgrade. Everything from the ground up must be settled and solidified to build a strong driving surface that won’t continually drain the company bank account for not-so-routine maintenance.


Ger a FREE Paving Estimate

Topics: Parking Lot, Asphalt Paving, civil construction, Ohio construction

Written by Tom Owings