HMA Design (Surface)

CEE 320Steve Muench

04/18/23 1

The surface course is the layer in contact with traffic loads and normally contains the highest quality materials. It provides characteristics such as friction, smoothness, noise control, rut and shoving resistance and drainage.

Surface HMA

This top structural layer of material is sometimes subdivided into two layers:

Wearing Course. This is the layer in direct contact with traffic loads. It is meant to take the brunt of traffic wear and can be removed and replaced as it becomes worn. A properly designed (and funded) preservation program should be able to identify pavement surface distress while it is still confined to the wearing course. This way, the wearing course can be rehabilitated before distress propagates into the underlying intermediate/binder course.

Intermediate/Binder Course. This layer provides the bulk of the HMA structure. Its chief purpose is to distribute load.

HMA Types

The three most common types of HMA pavement are:

Dense-graded HMA. Flexible pavement information in this Guide is generally concerned with dense-graded HMA. Dense-graded HMA is a versatile, all-around mix making it the most common and well-understood mix type in the U.S.

Stone matrix asphalt (SMA). SMA, although relatively new in the U.S., has been used in Europe as a material for years to support heavy traffic loads and resist studded tire wear. Also known as Gap Graded

Open-graded HMA. This includes both open-graded friction course (OGFC) and asphalt treated permeable materials. Open-graded mixes are typically used as wearing courses (OGFC) or underlying drainage layers because of the special advantages offered by their porosity.

Dense-Graded HMA

    

Stone Matrix Asphalt (SMA)

Open-Graded Mixes

0.45 Power Gradation

0.45 Power Gradation Curve Using 19.0-mm (0.75-inch) Maximum Aggregate Size

Calculations for a 0.45 Power Gradation Curve Using 19.0-mm (0.75-inch) Maximum Aggregate Size

           

  

0.45 Power Gradation Curve for Different Maximum Aggregate Sizes

Typical Aggregate Gradations and Permeabilities

           

  

FHWA gradation graph showing representative gradations

19 mm0.075 mm 4.75 mm

No.200 No.4 ¾-in

0.45 power curve

Gap Graded

Open Graded

Dense Graded

Uniform Graded

Restricted Zone

Sieve Size

% Passing

Hot Mix Asphalt (Dense-Graded)

Hot Mix Asphalt (Open-Graded)

Hot Mix Asphalt (Gap-Graded)

HMA MIX DESIGN

Texas Gyratory Compactor (TGC). For dense-graded hot mix asphalt

Superpave Gyratory Compactor (SGC).

Marshalls Method

HMA MIX DESIGN, Performance Concerns

Resistance to Permanent Deformation Resistance to Fatigue and Reflective

Cracking Resistance to Low Temperature (Thermal)

Cracking Durability Resistance to Moisture Damage (Stripping) Workability Skid Resistance

Marshall Mix Design Procedure

The Marshall mix design method consists of 6 basic steps:

Aggregate selection. Asphalt binder selection. Sample preparation (including compaction). Stability determination using the

Density and voids calculations. Optimum asphalt binder content selection.

Marshall Mix Design Procedure

The Marshall Stability and Flow Test

Marshall Mix Design Procedure

.

Mix Criteria

Light Traffic(< 104 ESALs)

Medium Traffic(104 – 106 ESALs)

Heavy Traffic(> 106 ESALs)

Min.

Max.

Min. Max.Min.

Max.

Compaction (number of blows on each end of the sample)

35 50 75

Stability (minimum)2224 N(500 lbs.)

3336 N(750 lbs.)

6672 N(1500 lbs.)

Flow (0.25 mm (0.01 inch)) 8 20 8 18 8 16

Percent Air Voids 3 5 3 5 3 5

Typical Marshall Design Criteria

Bad Mix

blue smoke - blue smoke indicate that the mix is too hot.

stiff appearance mix slumped in truck. lean, dull appearance - this indicates that the

mix has insufficient asphalt. rising steam - too much moisture. segregation. contamination.