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

OBJECTIVES OF MIX DESIGN
HMA mixtures should be properly designed to in order to obtain the following desirable mix properties:
! Resistance to Permanent Deformation (Rutting) - The mix should not distort or displace when
subjected to traffic especially when temperatures are elevated.
! Fatigue Resistance - The mix should not crack when subjected to repeated loads over a period of time.
! Resistance to Low Temperature Cracking - The mix should not crack during low or freezing
temperature period of time.
! Durability - The HMA mixture must contain sufficient content of asphalt binder to ensure film coverage of
the aggregate particles.
! Resistance to Moisture Induced Damage - The HMA mixture should be resistant to water causing the
loss of adhesion between the aggregate surface and the asphalt binder. This is a property of the
aggregate even thought some asphalt binders are more susceptible to water than others. This
phenomenon is called stripping.
! Skid Resistance - A mixture designed to provide sufficient resistance to skidding to permit normal
turning and braking movement to occur.
! Workability - The HMA mixture must be capable of being placed and compacted with reasonable effort.
HOT MIX ASPHALT MIXTURE REQUIREMENTS
HMA is composed of aggregates with an asphalt binder and when required, certain anti-stripping additives.
As the result of the recently completed Strategic Highway Research Program (SHRP), now commonly
referred to as “Superpave”, asphalt binders are classified based on their performance at both hot and cold
temperatures. By carefully selecting quality aggregates and the correct Superpave Performance Graded
(PG) asphalt binder, it is possible to produce high quality hot mix asphalt for a wide variety of climatic
conditions in Colorado.
PG graded asphalt binders available in Colorado can be compared to previous viscosity graded binders.
The comparisons were discussed in Chapter 1 and shown in Table 1-3.
When developing a HMA mix design for a project, a good reference to follow is the Colorado Department
of Transportation Standard Specifications, 2005 Edition or the Metropolitan Pavement Engineers
Council’s Pavement Design Standards and Construction Specifications for the particular class and
mixture size required. This will result in uniformity and economy because most Colorado contractors will
have job mixes on several mixtures already prepared for state or local agency use. In the absence of a
previously prepared job mix, the contractor or private testing laboratory should develop a job mix formula for
the desired project, and intended use. The job mix formula should include the following information:
! Project information
! Name of Contractor
! Mix design method and compaction level