When it comes to engineering the design of flexible pavement, there’s no “one size fits all”. Each road and environment is unique and must be treated as such. Factors such as traffic conditions, climate, geology, materials, and much more are taken into consideration when it comes to developing the right formulation for each pavement project.
Although each formulation is unique, there are common types of flexible pavement that are utilized for California highways and roadways:
Asphalt Concrete consists of a mixture of asphalt binder and graded aggregates. The aggregates used can range from coarse to very fine particles. Asphalt concrete is commonly referred to as HMA (Hot Mixed Asphalt) because the aggregate and binder are heated to provide sufficient coating and workability. Asphalt Concrete is classified by type depending on the unique conditions of the project.
Caltrans uses the following types of Asphalt Concrete based on the project conditions:
(1) Dense Graded – HMA Type A,
(2) Gap Graded – RHMA-G, and
(3) Open Graded Friction Course (HMA-O, RHMA-O, RHMA-O-HB)
These HMA types are discussed in the Standard Specifications and Standard Special Provisions.
HMA, Type A
Hot Mixed Asphalt concrete, Type A, is classified as a dense-graded mixture. This mixture includes aggregates that are well distributed throughout the entire range of sieves. This is the most commonly specified mixture, as it can be used in the base, intermediate, and surface layers of the pavement structural section. The phrase “Type A” is legacy language from previous versions of the Standard Specifications that refers to the aggregate quality characteristics required to produce these mixtures.
Rubberized Hot Mix Asphalt-Gap Graded (RHMA-G)
Rubberized Hot Mix Asphalt-Gap-graded, or RHMA-G, is a mixture with a high coarse aggregate content with a portion of aggregates missing from the middle of the gradation band – hence the “Gap”. This gap is intended to provide sufficient space in the mixture for rubber modified binder while keeping the stone on stone contact for rut resistance. The base asphalt binder is modified using recycled crumb rubber particles. This produces a mixture that is much more durable, crack-resistant, reduces traffic noise, increases skid resistance, and allows the structural designer to reduce the overall lift thickness providing the agency with more lane miles per ton of mixture.
Public Resources Code section 42703 requires the Department of Transportation to use specific amounts of crumb rubber modifier, or CRM in asphalt pavements. In this case, RHMA-G is used to meet this standard.
The California Department of Resources Recycling and Recovery (CalRecycle) provides technical resources for education and use of Rubberized Asphalt Concrete. Local agencies can benefit from the CalRecycle Rubberized Pavement Grant Program, which is designed to assist local agencies with funding and model specifications for the use of Rubberized Asphalt Concrete to help achieve the mandates set in the Public Resources Code.
Open-Graded Friction Course (HMA-O, RHMA-O, RHMA-O-HB)
Open-Graded Friction Course, or OGFC, is a non-structural wearing course. Open-graded mixtures have an aggregate gradation that is uniformly sized, which produces a mixture that should have between 18-22 percent air voids. Because of this grading design, OGFC mixtures are typically used to improve wet weather skid resistance, reduced water splash and spray, and as a stormwater treatment. This type of flexible pavement also allows for better visibility of pavement markings/markers during wet weather conditions.
Caltrans utilizes three types of non-structural OGFC as asphalt pavement:
(1) Hot Mix Asphalt-Open-graded (HMAO),
(2) Rubberized Hot Mix Asphalt-Open-Graded (RHMA-O), and
(3) Rubberized Hot Mix Asphalt Open-graded-High Binder (RHMA-O-HB)
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