1180.
Pavement Design
1180.1. Introduction preconstruction engineer is responsible for the final
1180.2. Pavement Overview pavement design.
1180.3. Wearing Course
1180.4. Binder Course 1180.2.3 Non-Highway Pavements
1180.5. Base Course Pavement designs for non-roadway projects, such as
1180.6. Subbase/Select Material/Borrow parking areas, paths, and sidewalks, do not require the
1180.7. Gravel Roads use of the AKFPD Manual procedure if the design
1180.8. Glossary equivalent single axel load (ESAL) is below 10,000. If
the ESALs are 10,000 or more, then follow Section
1180.1. Introduction 2.2.4 of the AKFPD Manual. The minimum thickness
of all non-highway hot mix asphalt pavements shall be
Alaska’s road transportation system is vital to the 2 inches.
state’s residents and economy. Pavements must
withstand a variety of traffic and environmental A Type II or III, Class C hot-mix asphalt may be used
conditions and must serve the public in a safe and for parking areas, paths, and sidewalks with low
comfortable manner. In addition, pavements are ESALs. Section 401 of the DOT&PF Standard
expected to perform over extended periods of time. Specifications for Highway Construction
(specifications) addresses Class C asphalt pavement.
This chapter is an overview of the DOT&PF policy
and design philosophy for pavements. Detailed policy 1180.3. Wearing Course
and procedures that govern Alaska’s flexible
pavement design are provided in the Alaska Flexible The wearing course is the top layer of a surfacing
Pavement Design (AKFPD) Manual and its system that is in direct contact with traffic loads. The
companion software. wearing course is designed to:
• Provide resistance to abrasion
The AKFPD Manual is available online at:
• Provide a smooth ride
http://www.dot.state.ak.us/stwddes/desmaterials/mat_
pvmtmgt/pop_flexpaveman.shtml. • Resist plastic deformation
• Resist water permeability
1180.2. Pavement Overview • Resist fatigue
1180.2.1 Pavement Structure • Resist thermal cracking
Pavement structure is a layered system of materials Available surfacing types are listed in section 7.3 of
built on top of a prepared subgrade to protect it from the AKFPD Manual. Following are descriptions of
excessive deformations due to traffic loads. In general, some of the more common types of surfacing
a pavement structure consists of (top to bottom): DOT&PF uses.
wearing course, binder course, base course (stabilized
or unstabilized) and subbase or selected material 1180.3.1 Hot Mix Asphalt (HMA)
(individually or in combination). The principal Hot Mix Asphalt (HMA, or asphalt concrete
function of the layers is to distribute traffic load pavement) is the predominant type of wearing course
stresses within the pavement structure, thus protecting used on DOT&PF roadways. A pavement that
the subgrade from excessive deflection. Layer receives such a surfacing is called a flexible
properties (i.e. density, strength, and stiffness) pavement. HMA is appropriate for highway
normally decrease from the top to the bottom of the pavements, parking lots, pathways, and sidewalks.
system.
HMA pavement structures are designed in accordance
1180.2.2 Highway Pavements with the policies and design procedures in the AKFPD
A pavement design analysis is required for all Manual. Use a minimum thickness of 2 inches for
roadway projects requiring pavement construction, new and overlay HMA layers. The specifications
reconstruction, or rehabilitation. The regional cover HMA in Section 401.
Alaska Highway Preconstruction Manual 1180-1 1180. Pavement Design
Effective January 1, 2005
1180.3.2 Asphalt Surface Treatment (AST) The wearing course and binder course may be
An AST is an asphalt/aggregate application to a road composed of the same material when advantageous;
surface. Usually less than 1 inch thick, asphalt surface however, the binder course often has a different
treatments do not increase the load bearing capacity of asphalt content and gradation or hardness of
pavement structures. They provide friction and aggregate. A binder course is not necessarily required
decrease dust generation. ASTs are appropriate when in a specific pavement design.
unstable embankments are present and/or for low-
traffic roadways. 1180.4.1 Hot Mixed Asphalt (HMA)
Binder courses are usually HMAs, but in the case of
ASTs may be used if any of the following conditions overlays, the original wearing course layer becomes a
are met (refer to Section 2.1 of the AKFPD Manual): binder course layer when topped with a new HMA
• The AADT is less than 1,000 layer.
The specifications cover HMA materials in Section
• Life-cycle cost analysis supports their use
401.
• Unstable foundations underlie more than 60
percent of the project 1180.4.2 Recycled Asphalt Pavement
Recycled asphalt pavement is the process of recycling
• The regional preconstruction engineer has old asphalt into new pavement by in-place cold
approved them mixing process or hot mixing at a plant. To be
Design ASTs using the Asphalt Surface Treatment considered adequate as a binder course, a recycled
Guide, which is available online at: asphalt pavement must have a resilient modulus
greater than 300 ksi.
http://www.dot.state.ak.us/stwddes/research/assets/pdf
/fhwa_ak_rd_01_03.pdf Consult the regional materials engineer for recycled
asphalt pavement special provisions.
Section 405 of the specifications covers ASTs.
1180.5. Base Course
1180.3.3 Portland Cement Concrete (PCC) The base course is the layer of material placed on top
PCC is rarely used in Alaska pavements. High cost of the subbase or subgrade that supports the wearing
and damage due to foundation settlement preclude its and binder courses. A base course can be stabilized or
use in roadways. non-stabilized.
If a PCC pavement is used, consult the 1993 AASHTO Use of a bound stabilized base is required on all
Guide for Design of Pavement Structures. Consult the roadway construction, reconstruction, and
regional materials engineer for special provisions, rehabilitation projects except for:
specific concrete mix designs, and subgrade
requirements. 1. Projects designed under the Gravel to Pavement
program
1180.3.4 Gravel Surfacing 2. Projects exempted in writing by the regional
See Section 1180.7 for guidance on gravel surfacing. preconstruction engineer. Rationale for an
exemption may include:
1180.4. Binder Course • Projects with a low AADT
The binder course is the bottom layer of pavement • Areas underlain by unstable foundations, such
below the wearing course. The binder course supports as ice-rich permafrost, where settlement
the wearing course. A binder course has all the results in frequent maintenance
properties of a wearing course except that resistance
• Projects for which a stabilized base will not
to abrasion is not essential. The minimum thickness of
a binder course is 3 inches when placed on top of a provide a cost-effective improvement in the
non-stabilized base and 2 inches when placed on top pavement performance, reduced maintenance,
or reduced future rehabilitation costs through
of a stabilized base.
a comprehensive life-cycle cost analysis. The
1180. Pavement Design 1180-2 Alaska Highway Preconstruction Manual
Effective January 1, 2005
period of the life-cycle cost analysis shall be course in the Alaska Renewable Pavement (ARP)
30 years. layering system. Refer to sections 7.4.2 and 7.4.3 of
• Roads designed on behalf of agencies other the AKFPD Manual for a detailed discussion of ATBs
than DOT&PF and the ARP. To be used as a binder course in the
AKFPD method, an ATB must have a resilient
1180.5.1 Stabilized Bases modulus greater than 300 ksi.
Stabilized bases are normally defined as standard base
There is some functional overlap with HMAs, but
course materials containing one or more of the
ATBs have different asphalt oil content, can use softer
following binder additives:
aggregate, and are less restrictive on aggregate
• Asphalt emulsion gradation and in placing and leveling requirements.
• Asphalt cement Therefore, they are more economical than HMAs.
• Foamed asphalt cement Asphalt treated base course is addressed in section
• Lime 306 of the specifications.
• Portland cement Emulsified Asphalt Treated Base (EATB)
• Reclaimed asphalt pavement (RAP) Emulsified asphalt treated base course (EATB) is
addressed in section 307 of the specifications.
Stabilized bases are used to improve long-term
pavement performance, reduce maintenance costs, and When used with emulsified asphalt, crushed asphalt
reduce future rehabilitation costs. While a stabilized base course is also considered a stabilized base.
base has no minimum amount of required binder Crushed asphalt base course is covered in Section 308
additive, it must achieve a resilient modulus of at least of the specifications.
80 ksi.
Reclaimed Asphalt Pavement (RAP)
The minimum thickness of a stabilized base is 3 Reclaimed asphalt pavement (RAP) that contains
inches. In developing flexible pavement designs using greater than 50 percent asphalt concrete pavement or
stabilized bases, refer to general policies GP5, 6, and greater than 3 percent residual asphalt content is
7 in Section 2.1 of the AKFPD Manual. In addition, considered a stabilized base.
use the following:
1. Alaska Soil Stabilization Guide (Report No. 1180.5.2 Non-Stabilized Bases
FHWA-AK-RD-01-6B) Non-stabilized bases comprise materials that do not
2. The mechanistic design method used in the have any binder additive. Crushed aggregate is the
AKFPD computer program most common type of base course. Non-stabilized
base course layers shall be a minimum of 4 inches
3. The definition of stabilized layers as found in thick.
Section 7.4.1 of the AKFPD Manual
The Alaska Soil Stabilization Guide is online at: Aggregate base course is covered in Section 301 of
the specifications. Reclaimed asphalt pavement (RAP)
http://www.dot.state.ak.us/stwddes/research/assets/pdf may be used as base course, or blended with aggregate
/fhwa_ak_rd_01_06b.pdf base course, when produced in accordance with
The resilient modulus, MR, of the stabilized base is section 308 of the specifications.
determined from experience, back calculation, or
testing and is a necessary input variable to run the 1180.6. Subbase/Select Material/Borrow
AKFPD analysis program. The lower pavement structure typically includes
Following are stabilized bases used relatively subbase or selected material, individually or in
frequently in Alaska: combination. Where existing natural material is of
adequate quality, it may serve as the lower portion of
Asphalt Treated Base (ATB) the pavement structure.
Asphalt treated base (ATB) is a stabilized base course Section 304 of the specifications covers subbase.
constructed using a minimum of 4 percent asphalt
cement binder. ATB’s primary use is as a binder
Alaska Highway Preconstruction Manual 1180-3 1180. Pavement Design
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1180.7. Gravel Roads 2. Determine the adequacy of the embankment
strength. Consider in the determination that the
1180.7.1 General existing roadway structure will or will not support
Alaska has several existing major gravel roads and a base and surface course the recommendations
gravel surfacing is appropriate for some new, very from Materials, and Maintenance and Operations.
low-volume roads in rural areas, where dust is not an Supplement as needed by structural support
environmental issue. This section has therefore been readings (i.e. Falling Weight Deflectometer,
included in this “pavement design” section of the California Bearing Ratio, Dynamic Cone
Preconstruction Manual. Penetrometer) or fines content.
Maintenance cost of gravel roads is considerably If the embankment has adequate strength, provide
higher than that for HMA, AST, or PCC surfaced sufficient aggregate surface course to shape cross
roads and is an important consideration when slope and superelevation as a minimum. Aggregate
performing cost-effective or life-cycle cost analysis. surface course is addressed in section 301 of the
specifications.
1180.7.2 Gravel Surface Structure
If the embankment does not have adequate strength,
New Gravel Roads follow the guidance provided in the “New Gravel
The following references provide guidance that will Roads” portion of this subsection.
assist in design of gravel roads:
1180.8. Glossary
• AKFPD Manual
The following is a brief glossary. A comprehensive
• AASHTO Guidelines for Design of Very Low- glossary of asphalt design and construction
Volume Local Roads terminology is found in the AKFPD Manual.
• AASHTO Design of Pavement Structures-Part II AADT: Average Annual Daily Traffic. AADT is a
(Chapter 4 Low-Volume Road Pavement Design) measure of traffic volume.
• Asphalt Institute MS-1 Thickness Design Asphalt Asphalt Concrete: Also referred to as asphalt
Pavements for Highways and Streets. concrete pavement (ACP), hot mix asphalt (HMA),
flexible pavement, and hot bituminous pavement. It is
When future paving of a gravel road is planned, the material most commonly used for surfacing
evaluate the pavement structure in accordance with roadways and airports in Alaska that are subject to
the design guidance appropriate to the anticipated high traffic. ACP is a high-quality, controlled, hot
future pavement type (i.e. AKFPD Manual for HAP). mixture of asphalt cement and graded aggregate,
Consult the regional materials engineer for more thoroughly compacted into a uniform dense mass.
specific gravel roadway design guidance. Aggregate Emulsified Asphalt: A combination of ground
surface courses are addressed in section 301 of the asphalt, emulsifying agents, and water. It cures by
specifications. “breaking,” which is water removal by evaporation or
Consult the regional Maintenance and Operations steaming off. Asphalt emulsions fall into three
(M&O) Section to determine if adequate personnel categories: anionic, cationic, and nonionic. The first
and equipment will be available to maintain the two types are ordinarily used in roadway construction
roadway prior to selecting gravel as surfacing. and maintenance. The anionic (electronegatively
charged) and cationic (electropositively charged)
Existing Gravel Roads classes refer to the electrical charges surrounding the
The roadway surfacing design process for existing asphalt particles. With nonionic emulsions, the asphalt
gravel roads is as follows: particles are neutral. Cationic emulsions are used with
aggregates that are negatively charged. Anionic
1. Evaluate the existing gravel road for previous emulsions are used with positively charged
performance and drainage system adequacy. aggregates. Opposite charges attract. The relative
M&O should participate. setting time of either slow setting (SS), medium
setting (MS), or rapid setting (RS) emulsions further
categorizes emulsified asphalts.
1180. Pavement Design 1180-4 Alaska Highway Preconstruction Manual
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Emulsified Asphalt Treated Base (EATB): A
product of mixing base course material with
emulsified asphalt and sometimes a few percent
Portland cement. It can be mixed on grade by heavy
equipment or by specially made traveling plants. It
can also be produced in a central mixing plant.
Emulsified asphalt treated bases bind up P 200 in base
course material and reduce frost heave and high
moisture content. They also can create an effective
structural support layer so that the otherwise required
thickness of pavement or subbase can be reduced.
ESAL: An acronym for Equivalent Single Axle Load.
An ESAL is the vertical load of a standard 18,000-
pound, dual-tire, single-axle truck. The effect of
pavement performance of any combination of axle
loads is equated to the number of ESALs.
Mix Design: The project-specific combination of
materials to be used in construction of a given
pavement.
Resilient Modulus (MR): An elastic property of
pavements and stabilized bases. The resilient modulus
is defined as the ratio of repeated axial stress over the
recoverable elastic strain.
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