One of the most important aspects of a successful asphalt paving project is the thickness of the asphalt itself. It doesn't matter how great the base is, if the asphalt is laid at an insufficient thickness the parking lot likely won't last much longer than 10 years. There are many standards for thickness depending on what the expected use of the paved surface is.
The thickness of the asphalt can either be stated as pre-comapction or post-compaction, it can be hard to know which is being used on a quote. Most engineering plans will call for a specified post-compaction thickness, if the contract is written to provide work based on an engineered specification then most likey the quoted thicknesses will be in post-compaction to match the specifications provided. However, more often than not when a specification is not provided, the contractor will provide the quote in pre-compaction thicknesses, this is done for a variety of reasons:
- Traditionally, asphalt was laid by hand and the only time thickness can be easily checked and modified is before compression.
- It allows for a larger number to be written on the quote, if all of the competition is writing 2.5 inches in pre-compaction but you are specifying a post-compaction thickness of 2 inches, most clients will assume less asphalt is being offered, when in actuality the quotes are offering the same amount of materials.
- Specifying pre-compaction laid thickness leaves more wiggle room in the post-compaction thickness. Due to varying temperatures, aspahlt grades and compaction levels, the final thickness can vary.
A good rule of thumb for asphalt thickness specification on a quotation or contract is that if compression is mentioned after the thickness, than they thickness given is likely pre-compaction. Where as if compression is mentioned in the same line or before the thickness of the aspahlt than likely post-compactions thicknesses are given. Of course it never hurts to ask!
Residential / Smaller light duty parking lots
Most often for residential driveways and smaller light duty parking lots we will use pre-compaction thicknesses, which for 99% of all driveways and about 75% of light duty parking lots is 2.5 inches, or once compressed sufficenlty the thickness will be approximately 2 inches. A proper 2 inch compacted layer of aspahlt on a good base can easily last for 20+ years, we often see driveways we paved 30 years prior still in excellent shape. Our gallery features photos of various older driveways and parking lots.
All else being equal the big determining factor for longevity of the standard 2 inches of aspahlt is that heavy vehicles like garbage trucks, buses, or other 5+ ton vehicles are very rarely driven over or parked in the parking lot. Essentially if you don't need an upgraded license to drive the vehicle then 2 inches compacted of aspahlt will be enough for the driveway to last a very long time.
Most residential projects are completed with a single layer of aspahlt, this is due to the cost/benefit relationship. While more aspahlt can be used if the driveway or parking lot is never going to see weight above what it was designed for, the additional thickness of aspahlt will do nothing at all for lifespan.
With residential work insufficent thickness actually laid is the main reason the paved surface doesn't last more than 10 years.
Commercial / Parking lots
Often, but not always, larger parking lots (greater than 10,000 sqft) are quoted post-compaction. This is generally due to the more sofisticated client and specifications provided. Most often larger parking lots are paved in 2 layers, this is because there is a maximum thickness of aspahlt than can be properly compressed with a certain size of roller, it also allows different grades of aspahlt to be used depending on the function of the layer.
There are 3 most often seen thickness specifications:
- 40mm HL3-HL4 Base Course with a 40mm HL3F-HL3 Surface Course - for very light duty use (cars and small trucks only)
- 50mm HL4-HL8 Base Course with a 40mm HL3 Surface Course - for larger lots which may see some heavy traffic. (or have heavy traffic sections like firelanes)
- 50mm HL8 Base Course with a 50mm HL3 Surface Course or 60mm HL8 Base Course with a 40mm HL3 Surface Course - for heavy trafic areas of larger parking lots.
You do not need to have an engineering firm tell you what thicknesses of aspahlt you should request on your quote, O'Brien Paving Inc. are very experienced with all things aspahlt and would be happy to consult with you and help you make the best choices in regards to your project.