Guideline for the Design and Use of Asphalt Pavements for ...

CHAPTER ONE
ASPHALT AND HOT MIX ASPHALT PAVING MATERIALS
HOT MIX ASPHALT PAVEMENT
Hot Mix Asphalt pavement is known by many different names: asphalt concrete, plant mix, bituminous mix,
bituminous concrete, and many others. With the implementation of Superpave across the United States,
there has been a trend to standardize the terminology and use Hot Mix Asphalt. The terminology that will be
used for asphalt pavement throughout this guideline will be Hot Mix Asphalt (HMA).
Hot Mix Asphalt is a combination of two primary ingredients - aggregates and an asphalt binder. The
aggregates total ninety to ninety five percent of the total mixture by weight. They are mixed with
approximately four to eight percent asphalt binder to form HMA.
The aggregates and asphalt are combined in an efficient manufacturing plant capable of producing Hot Mix
Asphalt paving materials. Plant equipment includes: cold feed bins for storage of graded aggregate; a dryer
for drying and heating aggregates to the required mixing temperature; a pug mill for combining the graded
and heated aggregate with a liquid asphalt cement according to specified mix formulas; and tanks for storing
the liquid asphalt binder. CDOT requires one percent hydrated lime to be added to mixes to help insure the
coating of the aggregate by the asphalt binder.
HMA is transported by truck to the paving site where it is spread to a uniform thickness with a mechanical
paving or finishing machine. After being placed to the desired thickness as specified for each individual lift,
the HMA is compacted to the required density by heavy, self-propelled rollers, producing a smooth, wellcompacted
pavement course.
The paving or finishing machine places the HMA at temperatures above 285° F. Generally, the material is
compacted before the mix temperature falls below 180° F to achieve required density.
There are approximately 12 million tons of HMA placed in Colorado each year. A majority of this amount is
placed on city, county, commercial and residential type projects.
ASPHALT BINDER DEFINED
The black cementing agent known as asphalt binder has been used in road construction for centuries.
Although there are natural deposits of asphalt, or rock asphalt, most of the asphalt used today is produced
by the oil refining industry. Asphalt, a constituent of most petroleum products, is isolated through the refining
process of distillation. The distillation process for obtaining asphalt from heavy crude oil is shown in Figure
1-1.
Asphalt is called a bituminous material because it contains bitumen, a hydrocarbon material soluble in
carbon disulfide. The tar obtained from the destructive distillation of soft coal also contains bitumen. Both
petroleum asphalt and coal tar are referred to as bituminous materials. However, because their properties
differ greatly, petroleum asphalt should not be confused with coal tar. Petroleum asphalt is composed