Soft Asphalt Driveways & Parking Lots
Dear Customer:
The quality of asphalt pavement is determined by a number of factors. First of all, the Asphalt Hot Mix design that consists of crushed rock, a sand blended with liquid asphalt cement, determines the quality, durability and flexibility of the asphalt pavement. Secondly, the mechanical laydown, handwork and mechanical compaction of the Asphalt Hot Mix is critical for the quality of the finished pavement. Thirdly, the mechanical compaction and the materials of the underlaying subgrade structure are critical to how the Asphalt Hot Mix pavement overlayment will perform.
The subgrade structure provides the stability and support for the asphalt pavement overlayment. If the subgrade is of poor quality containing materials such as silts, sands and/or clays, then they will absorb and retain water. The water will act as a lubricant between the material particles and this will affect the stability and overall strength of the subgrade as well as cause damage to the asphalt pavement surface course during freeze-thaw cycles and heavy loading by vehicles.
The liquid asphalt cement used in Asphalt Hot Mix is a petroleum product and is the "glue" or binder that bonds the aggregate mixture together. All liquid asphalt cements are thermoplastic; that is they become harder (more viscous) as their temperature decreases and softer (less viscous) as their temperature increases. Liquid asphalt cement varies widely in consistency from solid to semi-solid at normal air temperatures.
This characteristic is known as temperature susceptibility, and is one of asphalt’s most valuable assets creating a product that is flexible yet durable over a spectrum of climatic conditions.
Liquid asphalt temperature susceptibility varies among liquid asphalts that have identical viscosity grades but are derived fro different crude oil sources. This is because their individual chemical makeup is different depending on the crude oil source.
The State of Alaska Department of Transportation (DOT) and the Municipality of Anchorage have adopted the new standard superpave asphalt binder classification system. This system is based on performance graded liquid asphalt cements and is referred to as the "PG" system. The PG grading system was developed for varying climates and pavement designs.
The PG designation such as PG 64-28 represents the high and low temperatures at which the liquid asphalt cement or binder would be expected to perform satisfactorily.
The primary liquid asphalt cement used here in Alaska is PG 52-28. For the liquid asphalt cement manufacturers to produce a PG binder greater that PG 52-28 requires that asphalt modifiers be used to change the characteristics of the liquid asphalt cement. This manufacturing process and the addition of modifiers dramatically increases the cost of the liquid asphalt cement and complicate storage issues.
The State of Alaska DOT specifies some of these higher grades of liquid asphalt cement with modifiers for specific airport and highway construction projects.
The PG 52-28 is a relatively soft asphalt binder equal to the old counterpart viscosity grade binder AC-5. Asphalt binders for specific heavy traffic or aircraft use generally use a modified asphalt cement PG 58-28 or a PG 64-28 and are specified in contract documents on a project by project basis.
All of AS&G's standard asphalt mix designs are based on using PG 52-28.
There are also other factors that can affect the “softness” of an Asphalt Hot Mix pavement structure such as aggregate particle size, aggregate blend, aggregate fracture, and liquid asphalt content, etc. However, the liquid asphalt cement is the main contributor to “softness” or “hardness” of an Asphalt Hot Mix pavement structure.
Asphalt Hot Mix designs in Alaska are generally designed with approximately 5.5 to 6.5% liquid asphalt cement by total weight of the Asphalt Hot Mix. Generally, finer mixes like Driveway Mixes have a higher liquid asphalt cement content since there are a greater number of aggregate particles to coat. The glue or liquid asphalt cement combined with smaller finer aggregate particles used in Driveway Mix results in a softer asphalt mix, than say a coarse aggregate Highway Mix with larger crushed aggregate and a lower liquid asphalt content.
The liquid asphalt cement PG 52-28 used in Alaska is known as a “soft” liquid asphalt. PG 52-28 is furnished either by Flent Hill in Fairbanks Alaska or Tesoro Alaska at Nikiski and is refined from Alaskan crude oil. Alaska crude oil has a naturally occurring high paraffin content that also contributes to the softening of the liquid asphalt cement.
As I mentioned before, PG 52-28 is the primary liquid asphalt binder used by the asphalt industry in Alaska unless modified by the State of Alaska Department of Transportation on specific highway projects. Modified asphalt cements are generally two to three times the cost of PG 52-28 and are used in conjunction with coarse aggregate airport and highway mixes for specific projects.
In conclusion, PG 52-28 is a “soft” liquid asphalt cement, which is very sensitive to ambient air temperature changes and temperature changes caused by direct sunlight. When warmer summer temperatures prevail, you will see a dramatic softening of the asphalt pavements. This will cause the asphalt pavement to move, shove, rut and deform when automobile or equipment tires apply weight and pressure.
The best method to avoid some of the rutting and deformation problems are as follows:
1) Do not park or drive in the same spot continuously.
2) Apply cold water to the driveway or parking lot on hot summer days.
3) When parking a vehicle for long periods of time, place something under the
wheels that will disperse the load over a larger area.
The good news is as your asphalt hot mix pavement ages it will oxidize becoming more brittle and ridged. It will also be less susceptible to temperature changes. Of course the down side to this is it will eventually begin to crack and ravel as it hardens.
If you have any questions, or if I can be of any further assistance, please call me.
Sincerely,
ANCHORAGE SAND & GRAVEL CO., INC.
C. Earl Polen
Aggregate Sales Manager
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