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Prefacei

ASPHALT PAVING DESIGN GUIDE

Published by theAsphalt Paving Association of Iowa

The Asphalt Paving Association of Iowa (APAI) is an organization of asphalt concreteproducers and their associate members from the paving industry around the state. APAIwas formed in 1955 for the purpose of advancing knowledge in the use of this pavingmaterial and to provide a service to the public and to the users of asphalt.

The APAI is constantly seeking new techniques, product improvements, and designmethods, all of which are made available for the benefit of pavement users. Design and

construction seminars and educational reports and brochures are developed anddisseminated to ensure a high-quality product. A wide variety of technical literature andaudio-visual presentations are available by calling the APAI office.

The ultimate quality of your asphalt paving project is directly related to the experience,skill, and equipment of the contractor doing the work. Behind each contractor is atremendous investment in equipment, highly skilled manpower, and a pride ofworkmanship in building asphalt pavement of the highest quality. Whatever the project,APAI members are your assurance of quality.

APAIs professional staff and member firms are qualified and eager to serve you. They

welcome inquiries about design procedures and cost estimates at any time.

Partners in Quality

Executive Offices3408 Woodland Ave.

Suite 209West Des Moines, IA 50266-6506


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Table of Contents ii

Table of Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiChapter 1 The Asphalt Advantage . . . . . . . . . . 1-1

Chapter 2 Asphalt and Asphalt PavingMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Asphalt Defined . . . . . . . . . . . . . . . . . . . . . . . . 2-1Aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1Asphalt Cement . . . . . . . . . . . . . . . . . . . . . . . . . 2-4Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . . 2-5Cold Mix Asphalt Concrete . . . . . . . . . . . . . . . 2-6

Chapter 3 Design Considerations . . . . . . . . . . 3-1Fundamentals of Design . . . . . . . . . . . . . . . . . . 3-1Traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Traffic Classifications . . . . . . . . . . . . . . . . . 3-2Soil Support Capability . . . . . . . . . . . . . . . . . . . 3-4

Subgrade Classes . . . . . . . . . . . . . . . . . . . . 3-8Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9Design Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11Asphalt Concrete Specifications . . . . . . . . . . . . 3-12Construction of Asphalt Pavements . . . . . . . . 3-15

Construction Equipment . . . . . . . . . . . . . . 3-15Construction Practices . . . . . . . . . . . . . . . . 3-17Pavement Markings . . . . . . . . . . . . . . . . . . 3-18Traffic Control Through Work Areas . . . . 3-19

Testing and Inspection . . . . . . . . . . . . . . . . . . . 3-21

Chapter 4 Thickness Design . . . . . . . . . . . . . . . 4-1General Considerations . . . . . . . . . . . . . . . . . . 4-1Pavement Thickness Design Tables . . . . . . . . . 4-2

Residential Streets . . . . . . . . . . . . . . . . . . . 4-3Collector Streets . . . . . . . . . . . . . . . . . . . . . 4-4Arterial Streets . . . . . . . . . . . . . . . . . . . . . . 4-5Low-Volume Secondary and

Rural Roads . . . . . . . . . . . . . . . . . . . . . 4-6High-Volume Secondary and

Rural Roads . . . . . . . . . . . . . . . . . . . . . 4-7

Chapter 5 Parking Lot Design . . . . . . . . . . . . . 5-1General Considerations . . . . . . . . . . . . . . . . . . 5-1

General Planning . . . . . . . . . . . . . . . . . . . . 5-2

Thickness Design for Parking Lots . . . . . . . . . 5-7Heavy Loaded Areas . . . . . . . . . . . . . . . . . 5-10Industrial Parking Lots . . . . . . . . . . . . . . . 5-10

Asphalt Concrete Curb . . . . . . . . . . . . . . . . . . . 5-11Asphalt Mat-Platform for Building

Construction and Site Paving . . . . . . . . . . 5-13

Chapter 6 Designs for Recreational Uses . . . . 6-1Asphalt Pavements For

Non-Vehicular Use . . . . . . . . . . . . . . . . . . . . 6-1Bikeways, Golf Cart Paths, Recreational

Trails, and Walkways . . . . . . . . . . . . . . . . . 6-2Pavement Thickness . . . . . . . . . . . . . . 6-4

Recreational Areas . . . . . . . . . . . . . . . . . . . . . . 6-5Basketball Courts . . . . . . . . . . . . . . . . . . . . 6-5Pavement Thickness . . . . . . . . . . . . . . . . . . 6-7

Tennis Courts . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8Pavement Thickness . . . . . . . . . . . . . . . . . . 6-10

Asphalt-Rubber Running Tracks . . . . . . . . . . . 6-11Chapter 7 Pavement Management . . . . . . . . . . 7-1

Pavement Management Concepts . . . . . . . . . . 7-1Rating a Road . . . . . . . . . . . . . . . . . . . . . . . 7-2Interpretation of a Condition Rating . . . . 7-2

Pavement Maintainance . . . . . . . . . . . . . . . . . . 7-4Full-depth Asphalt Patching . . . . . . . . . . . 7-5Thin Surface Treatments . . . . . . . . . . . . . . 7-8

Asphalt Concrete Overlays . . . . . . . . . . . . . . . . 7-10Overlay Thickness Calculations . . . . . . . . 7-11

Chapter 8 Rehabilitation . . . . . . . . . . . . . . . . . 8-1Recycling Asphalt Pavements . . . . . . . . . . . . . 8-1Breaking and Seating . . . . . . . . . . . . . . . . . . . . 8-3Rubblizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4Paving Fabrics . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5Sawcut and Sealing Joints . . . . . . . . . . . . . . . . . 8-5

Appendix A Pavement Failure Indentification A-1Identifying and Correcting Pavement

FailuresTypes of Pavement Failures

Appendix B Glossary . . . . . . . . . . . . . . . . . . . . . B-1

Appendix C Conversion Tables . . . . . . . . . . . . . . C-1Table C-1. Approximate Quantities of

Asphalt Concrete perSquare Yard

Table C-2. Gallons of Emulsified AsphaltRequired Per 100 Linear Feet:Various Widths and Rates

Table C-3. Tons of Material Required Per100 Linear Feet for VariousWidths and Pounds PerSquare Yard

Table C-4. Cubic Yards of Material Per100 Linear Feet: VariousWidths and Depths

Appendix D Publications . . . . . . . . . . . . . . . . . D-1National Asphalt Pavement AssociationPublications

The Asphalt Institute PublicationsGovernment Publications

Appendix E Credits and References . . . . . . . . E-1CreditsReferences


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Forwardiii

FORWARD

This Asphalt Paving & Design Guide has been prepared by the members and staff of theAsphalt Paving Association of Iowa to assist you in understanding Asphalt Concretepavement design and construction.

This Design Guide is not intended to circumvent asphalt pavement designs bycompetent design engineers using actual project traffic loading data and known subgradesoil characteristics. However, it will provide the owner, architect, engineer, developer, andgovernment official with basic guidelines to be used in the absence of professionalservices. Readers are cautioned that the information contained in this Design Guide maybe insufficient when used alone, and other resource materials and authorities should be

consulted for specific site design.

The criteria for specific pavement design applications are unpredictably varied. Theexamples contained in the Design Guide are composites of those designs, procedures, andapplications that have proven successful in the state of Iowa. References to authorities andagencies do not constitute their endorsement of this Design Guide. Suggested referencesand authorities should be used by the reader if further clarification is required.

All Asphalt mixes referred to in this Design Guide comply with the Iowa Departmentof Transportations specifications for Asphalt Concrete mixes. These are proven mixes,readily available throughout the state of Iowa from companies experienced in producing

and constructing quality hot mixed Asphalt Concrete pavements.

DESIGN GUIDE FORMAT

The purpose of this Design Guide is to present materials that provide a basic knowledgeof asphalt pavement design without being too technical. Understanding some of the basicproperties of asphalt and aggregates is essential when relating pavement designs tospecific conditions and needs. Thickness design tables and construction details areincluded for various roadway and recreational uses. Pavement management andrehabilitation options are also important considerations.

The Design Guide is presented in a logical format for asphalt paving design,construction, and maintenance processes.


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I.

THE

ASPHALT

ADVANTAGE


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Chapter 1

The Asphalt Advantage

Paving with asphalt concrete allows you to pave faster, more efficiently, more economically, andwith greater serviceability than with any other paving material in the world. That may seem like apretty bold claim, but those who work in paving know its true. Asphalt Concrete has an absoluteadvantage in every paving application. This Design Guide will tell you about those advantages.

SMOOTHNESS

Asphalt will consistently give the drivingpublic the smooth, quiet ride they have cometo expect from this product. Asphalt Concreteis machine-placed, so it has a uniform surfaceunsurpassed by other pavements. Repetitivejoints, noisy surface texture, and blowups areeliminated by this method of construction.These features benefit airport users, too.Asphalt Concrete runways and taxiways meansafer landings and takeoffs, because suchsurfaces are smoother and easier to maintain.

STAGED CONSTRUCTION

A major advantage for Asphalt Concrete isthe potential for staged construction. Theasphalt base course can be placed and usedunder traffic during initial construction. Thispavement can then be overlaid with finalsurface courses. Staged construction improveson-site conditions, removes the aspect ofmuddy soils, and provides a place to store

construction materials and equipment. Thismethod also provides an opportunity todiscover and correct unanticipated problemareas, such as a weak subgrade, poor drainage,or poorly compacted trenches, which can berepaired at minimal cost.

The Asphalt Advantage 1-1


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EASE OF CONSTRUCTION

Asphalt Concrete is machine-placed, remov-

ing the need for time-consuming form workand steel reinforcement. Traffic can use thepavement almost immediately no delay isrequired to allow the pavement to cure. Thelack of pavement joints reduces maintenancerequirements. Repair of an asphalt surface isquick and easy, because there is little down-time waiting for a patch to cure.

DURABILITY

Asphalt Concrete is aflexible pavement, withsame bridging action, which allows it to with-stand occasional overloads without seriousdamage. Its resistance to freeze-thaw anddeicing salts allows it to wear better duringwinter. Its lack of repetitive joints removes thepossibility of blowups that plague PortlandCement Concrete during summer. Inch for

inch, asphalt cement concrete performs betterthan Portland Cement Concrete.

ECONOMICAL

The Federal Highway Administration hasshown that a dollar spent on asphalt pave-ments goes 26.9 percent farther than a dollarspent on concrete pavements. Thats because

asphalt is cost-effective. It has a lower first costthan concrete and it lasts longer. Stagedconstruction helps spread out the cost ofplacement. Because asphalt pavement has nojoints to repair and is not affected by freeze-thaw actions, it is much less expensive tomaintain.

The Asphalt Advantage1-2

SAFETY

Asphalt pavements offer high skid resist-ance values. The dark color of asphalt reducesglare, helps melt ice and snow, and provides a

high contrast for lane markings.


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The Asphalt Advantage 1-3

RECYCLABLE

Another major advantage of Asphalt

Concrete is its ability to be completelyrecycled. Not only can the aggregates bereused, but the asphalt cement binder alsoretains its cementing properties and can be re-used in a new mix. Pavements can be recycledboth on site using cold mix or via a hot mixplant. Recycled pavements have been testedand proven in both the laboratory and the fieldto perform at least as well as virgin aggregatemixes. Over 90% of the hot mix asphalt plantsin Iowa are capable of using reclaimed asphalt

pavement (RAP). Asphalt pavements are 100percent recyclable.

VERSATILITY

The versatility and popularity of asphalt is

evident across the state of Iowa and allAmerica factories and schools, office parksand playgrounds, and the overwhelmingmajority of our streets and roads stand as cleartestimony that the advantages of asphalt makeit Americas first choice for paving andrehabilitation.


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II.

ASPHALT

and

ASPHALT PAVING

MATERIALS


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Asphalt Paving Materials 2-1Asphalt Paving Materials 2-1

Chapter 2

Asphalt and AsphaltPaving Materials

ASPHALT DEFINED

The black cementing agent known as asphalthas been used for road construction forcenturies. Although there are natural depositsof asphalt, or rock asphalt, most used today isproduced by the oil refining industry. Asphalt

is a constituent of most petroleums and isisolated through the refining process ofdistillation. (See Figure 2-1.)

Figure 2-1. Petroleum Asphalt Flow Chart forEmulsified and Cutback Asphalts.

Asphalt is called a bituminous materialbecause it contains bitumen, a hydrocarbonmaterial soluble in carbon disulfate. The tarobtained from the destructive distillation ofsoft coal also contains bitumen. Both petrol-eum asphalt and coal tar are referred to asbituminous materials. However, because theirproperties differ greatly, petroleum asphaltshould not be confused with coal tar. Whereaspetroleum asphalt is composed almost entirelyof bitumen, the bitumen content in coal tar isrelatively low. The two materials should betreated as separate entities.

One of the characteristics and advantages ofasphalt as an engineering construction andmaintenance material is its great versatility.Although a semi-solid at ordinary tempera-tures, asphalt may be liquified by applyingheat, dissolving it in solvents, or emulsifyingit. Asphalt is a strong cement that is readilyadhesive and highly waterproof and durable,making it particularly useful in road building.It is also highly resistive to the actions of most

acids, alkalis, and salts.Covering more than 90 percent of the

nations paved highways, Asphalt Concrete isthe most widely used paving material in theUnited States. For versatility, durability, andease of construction, it has no equal.

AGGREGATES

Aggregates (or mineral aggregates) are hard,

inert materials such as sand, gravel, crushedstone, slag, or rock dust. Properly selected andgraded aggregates are mixed with the cement-ing medium asphalt to form pavements.Aggregates are the principal load-supportingcomponents of an Asphalt Concrete pavement.They total 90 to 95 percent of the mixture byweight and 75 to 85 percent by volume.


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Asphalt Paving Materials2-2

ClassificationsAsphalt Concrete paving aggregates are

classified according to source or means ofpreparation. A brief description of the classifi-cations follows.

Pi t or Bank -Run Aggregates

Both gravel and sand are typically pit orbank-run natural aggregates. They usually arescreened to proper size and washed to removedirt before being used for Asphalt Concretepaving purposes.

Processed A ggregat es

When natural pit or bank-run aggregate hasbeen crushed and screened to make it suitablefor Asphalt Concrete pavements, it is consider-ed a processed aggregate. Crushing typicallyimproves the particle shape by making the

rounded particles more angular. Crushingalso improves the size distribution and range.

Crushed stone is also a processed aggregate.It is created when the fragments of bedrockand large stones are crushed so that all particlefaces are fractured. Variation in size of particlesis achieved by screening. Aggregates that havereceived little or no screening are known ascrusher run. These aggregates are generallymore economical than screened aggregatesand can be used in Asphalt Concrete pave-ments in many instances.

In the processing of crushed limestone, therock dust produced is separated from the othercrushed aggregate and may be used as crushedsand or as a mineral filler in Asphalt Concretepavements.


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Asphalt Paving Materials 2-3

2. Cleanliness. Foreign or deleterious sub-stances make some materials unsuitable forpaving mixtures.

3. Toughness. Toughness or hardness is the

ability of the aggregate to resist crushing ordisintegration during mixing, placing, andcompacting; or under traffic loading.

4. Soundness. Although similar to toughness,soundness is the aggregates ability to resistdeterioration caused by natural elementssuch as the weather.

5. Particle shape. The shapes of aggregateparticles influence the asphalt mixtures

overall strength and workability as well asthe density achieved during compaction.When compacted, irregular particles suchas crushed stone tend to lock togetherand resist displacement.

6. Surface texture. Workability and pavementstrength are influenced by surface texture. Arough, sandpapery texture results in ahigher strength than a smooth texture.Although smooth-faced aggregates are easy

to coat with an asphalt film, they aregenerally not as good as rough surfaces. It isharder for the asphalt to grip the smoothsurface.

7. Absorption. The porosity of an aggregatepermits the aggregate to absorb asphalt andform a bond between the particle and theasphalt. A degree of porosity is desired, butaggregates that are highly absorbant aregenerally not used.

8. Stripping. When the asphalt film separatesfrom the aggregate because of the action ofwater, it is called stripping. Aggregatescoated with too much dust also can causepoor bonding which results in stripping.Aggregates readily susceptible to strippingaction usually are not suitable for asphaltpaving mixes unless an anti-stripping agentis used.

Synt het i c Aggregates

Aggregates produced by altering bothphysical and chemical properties of a parentmaterial are called synthetic or artificial

aggregates. Some are produced and processedspecifically for use as aggregates; others arethe byproduct of manufacturing and a finalburning process. Blast furnace slag is anexample of a synthetic aggregate.

Desirable Properties of AggregatesSelection of an aggregate material for use in

an Asphalt Concrete pavement depends on theavailability, cost, and quality of the material,as well as the type of construction for which

it is intended. To determine if an aggregatematerial is suitable for use in asphalt construc-tion, evaluate it in terms of the followingproperties:

1. Size and grading. The maximum size of anaggregate is the smallest sieve throughwhich 100 percent of the material will pass.How the Asphalt Concrete is to be useddetermines not only the maximum aggre-gate size, but also the desired gradation

(distribution of sizes smaller than themaximum).


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ASPHALT CEMENT

Asphalt is produced in a variety of typesand grades ranging from hard-brittle solids tonear water-thin liquids. The semi-solid formknown as asphalt cement is the basic materialused in Asphalt Concrete pavements. Liquidasphalt is produced when asphalt cement is

blended or cut back with petroleum distillatesor emulsified with water and an emulsifyingagent. Liquid asphalt products may be pro-

duced for various uses and applications.

Some of the types and characteristics ofasphalt are noted in the following table.

ASPHALT BINDER TYPE OF CORRIDOR LOCATION TYPE OF MIX

PG 70-22 Heavy Duty Full Depth Asphalt Surface Mixture (sp125 or SMA) and

Class V-VI first underlying lifts

PG 64-22 Remaining lifts

PG 70-22 Asphalt Overlays All Mixtures

PG 64-22 Medium Duty Full Depth All Mixtures

Class III-IV and Overlays

PG 64-22 Light Duty Full Depth All Mixtures

Class I-II and Overlays

Table 2-1. Asphalt Types, Characteristics and General Uses

Percent Flash

Asphalt Types-Percent Penetration Point Applic.

Type/Grade* (Min) Cutback (Min-Max) (Min) Temp. General Uses

SS- 1 57 Water 43 100-200 70- 160 Tack

SS-1 H 57 Water 43 40-90 70- 160 Tack, Slurry Surface Treatment

CSS- 1 57 Water 43 100-250 70- 160 Tack

CSS-1 H 57 Water 43 40-90 Boils 70-160 Tack, Slurry Surface Treatment

RS-1 55 Water 45 100-200 Over 70-140 Bituminous Seal Coat

RS-2 63 Water 37 100-200at

125-185 Bituminous Seal Coat

CRS-1 60 Water 40 100-250180F

125-170 Bituminous Seal Coat

CRS-2 65 Water 35 100-250 125-170 Bituminous Seal Coat

viscosity

RC-70 55 Naphtha 45 70-140 80F Tack

MC-30 55 Kerosene 45 120-250 100F 70-150 Prime

MC-70 55 Kerosene 45 70-140 100F 145-165 Bit. Seal Coat, Tack, Cold Mix, Patch MixMC-250 67 Kerosene 33 250-500 150F 165-200 Bit. Seal Coat, Tack, Cold Mix, Patch Mix

MC-800 75 Kerosene 25 800-1600 150F 175-255 Bit. Seal Coat, Tack, Cold Mix, Patch Mix

MC-3000 80 Kerosene 20 3000-6000 150F 215-290 Bituminous Seal Coat

Note: Flashpoint does not necessarily indicate burning or explosive point. However, care should be exercised when

heating all RC and MC asphalts because the cutback used reacts the same as gasoline. All material used as cold patchshould be mixed at the lowest temperature possible to prevent loss of cutback causing the mixture to harden before use.


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ASPHALT CONCRETE

Asphalt Concrete is known by many differ-

ent names: hot mix asphalt, plant mix,bituminous mix, bituminous concrete, andmany others. It is a combination of twoprimary ingredients - aggregates and asphalt

cement. The aggregates total 90 to 95 percent ofthe total mixture by weight. They are mixedwith 5 to 10 percent asphalt cement to form

Asphalt Concrete.The aggregates and asphalt are combined in

an efficient manufacturing plant capable of


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producing specified materials. Plant equip-ment includes: cold bins for storage of gradedaggregate; a dryer for drying and heatingaggregates to the required mixing tempera-

ture; a pug mill for combining the graded,heated aggregate and liquid asphalt cementaccording to specified mix formulas; and tanksfor storing the liquid asphalt.

Asphalt Concrete is transported by truck tothe paving site where it is spread to a uniformthickness with a mechanical paving or finish-ing machine. Then the material is compactedto the required degree by heavy, self-propelledrollers, producing a smooth, well-compacted

pavement course.The paving or finishing machine places the

Asphalt Concrete at temperatures above 225F. The material should be compacted before themix temperature falls below 175 F to achieveadequate density.

COLD MIX ASPHALT CONCRETE

Cold mix Asphalt Concrete, or cold placed

mixture, is generally a mix made with emulsi-fied or cutback asphalt. Emulsified asphaltsmay be anionic or cationic MS or SS grades.Aggregate material may be anything from adense-graded crushed aggregate to a granularsoil having a relatively high percentage ofdust. At the time of mixing, the aggregate may

either be damp, air-dried, or artificially heatedand dried.

Mixing methods may be performed either in

the roadway, on the side of the roadway, or ina stationary mixing facility. The resultantmixtures usually are spread and compacted atatmospheric temperatures.

Cold mix asphalt may be used for surface,base, or subbase courses if the pavement isproperly designed. Cold mix surface courses

are suitable for light and medium traffic;however, they normally require a seal coat orhot Asphalt Concrete overlay as surface pro-tection. When used in the base or subbase, theymay be suitable for all types of traffic.

Bituminous Treated Aggregate BaseBituminous treated aggregate base is one

type of cold mix Asphalt Concrete. It canconsist of processing gravels; crushed stones;or blends of gravel, sand, and crushed stone

materials each stabilized with a specifiedpercentage of asphalt. Job mix formulas(mentioned in Chapter 3) are not required.These mixtures are placed as a base course andstabilized-shoulder surfacing, although otheruses may be assigned by special design. Alldesigns should provide for a seal coat or sur-face course to provide protection from trafficabrasion and weathering.


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A Grade of emulsion other than FHMS-2h may be used where experience has shown that they give satisfactory performanceB Diluted with water by the manufacturer.

C Diluted with waterD Mixed in prime only.

Table 2-2 acts as a guide to uses of asphalt in cold mixes.

For additional information on asphalt and asphalt paving materials, refer to The AsphaltHandbook. Other references are listed in Appendix D.

Table 2.2. General Uses of Emulsified Asphalt

Type of Construction

Asphalt-aggregate mixtures:

For pavement bases and surfaces: .... .... .... .... XA .... .... .... .... .... .... .... .... ....

Plant mix (hot)

Plant mix (cold) .... .... .... X X .... .... .... .... .... X X .... ....

Open-graded aggregate .... .... .... .... .... X X X .... .... .... .... X X

Dense-graded aggregate .... .... .... .... .... X X X .... .... .... .... X X

Sand

Mixed-in-place:

Open-graded aggregate .... .... .... X X .... .... .... .... .... X X .... ....Dense-graded aggregate .... .... .... .... .... X X X .... .... .... .... X X

Sand .... .... .... .... .... X X X .... .... .... .... X X

Sandy soil .... .... .... .... .... X X X .... .... .... .... X XSlurry seal .... .... .... .... .... X X X .... .... .... .... X X

Asphalt-aggregate applications:

Treatment and seals:

Single surface treatment (Chip Seal) X X .... .... .... .... .... .... X X .... .... .... ....

Multiple surface treatment X X .... .... .... .... .... .... X X .... .... .... ....Sand seal X X X .... .... .... .... .... X X .... .... .... ....

Asphalt applications:

Fog seal .... .... XA .... .... .... XB XB .... .... .... .... XB XB

Prime coat-penetrable surface .... .... .... XC .... .... XC XC .... .... .... .... XC XC

Tack coat .... .... XA .... .... .... XB XB .... .... .... .... XB XB

Dust binder .... .... .... .... .... .... XB XB .... .... .... .... XB XB

Mulch treatment .... .... .... .... .... .... XB XB .... .... .... .... XB XB

Crack filler .... .... .... .... .... .... X X .... .... .... .... X X

Maintenance mix:

Immediate use .... .... .... .... .... X X X .... .... .... .... X X

RS-1

RS-2

MS-1,HFMS-1

MS-2,HFMS-2

MS-2h,HFMS-2h

HFMS-2s

SS-1

SS-1h

CRS-1

CRS-2

CMS-2

CMS-2h

CSS-1

CSS-1h

ASTM D2397AASHTO M140

ASTM D2397AASHTO M140


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III.

DESIGN

CONSIDERATIONS


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Design Considerations 3-1

Chapter 3

Design Considerations

FUNDAMENTALS OF DESIGN

Many types of asphalt pavement structuresexist, along with a number of differentmethods for designing the thickness of eachelement in any pavement. Fundamental toeach design are the following:

1. Traffic loading (volume and weight)2. Soil-support capability (including drainage

considerations)

3. Material specifications (aggregate andasphalt)

Each element is an important variable in thestructural design process. The economic life ofthe final product could depend on the closeattention given to detail when analyzing trafficloadings, soil-support capability, and materialspecifications.

The degree of detail needed in a specific

design situation is related to the type of useintended for the pavement and the sensitivityof each variable. For example, a freewaydesign with large traffic volumes and heavily-loaded trucks requires a careful estimate oftraffic; however, the number of bicycles andthe loading on a bicycle path would not besignificant factors in the paths structuraldesign.

An obviously unstable soil condition (noted,

perhaps, from previous experiences) indicatesthe need for a soil analysis during thethickness design process of almost any type ofpavement. Because drainage and soil-supportvalues are major factors in pavement life, it isimportant to know the quality of the support-ing soil. This is especially true for a facility thatwill require a large construction investment.

On the other hand, a specific traffic study orsoil analysis for a residential street or parking

lot may not be deemed necessary in a certainlocation. For example, a location having a longand successful record of asphalt pavementsconstructed for a specific use (e.g., driveways

and residential streets) provides the designerwith a background for selecting acceptablevalues.

For the users of this Design Guide, much ofthe design work has been done design chartsare presented for selecting pavement thick-ness. Traditionally, many designers grouppavements according to use and use tablesare commonly applied throughout the UnitedStates. Chapter 4 provides design tables by

specific type of facility use.

TRAFFIC

Because the primary function of a pavementis to transmit and distribute wheel loads ofvehicles to the supporting subgrade, informa-tion about the traffic stream is required.Pavement must be designed to serve trafficneeds over a period of years. Therefore, the

volume of traffic and the various types ofvehicles using the facility must be estimatedfor the pavements anticipated life.

A traffic assignment is made based on: (1)historic records of traffic volumes on com-parable types of highways and the anticipatedfunction of the highway under consideration,and (2) the percentage of trucks. The trafficanalysis procedure determines the repetitionsof an equivalent single axle load (ESAL). This

parameter is defined as the equivalent numberof applications of an 18,000-pound, single-axleload during the pavements design life.

The effects of truck traffic on a pavement canbe dramatic. Tests have shown that a single-unit, fully loaded, 80,000-pound truck cancause pavement damage equivalent to thatcaused by 6,000 automobiles. This illustrateswhy carefully made estimates of expectedtraffic are critical to proper pavement design.


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Design Considerations3-2

In The Asphalt Institutes Asphalt PavementThickness Design (IS-181), traffic is separatedinto classes. This Design Guide follows the

Institutes traffic class style by breaking trafficinto six classes, I through VI. Each class isdefined by an average daily traffic range, theaverage number of heavy trucks expected onthe pavement during the design period, andthe appropriate type of street or highway.

TRAFFIC CLASSIFICATIONS

Class I

(Very Light) Less than 50 autos per day, lessthan 7,000 heavy trucks expected duringdesign period.

Parking lots, drivewaysLight traffic farm roadsSchool areas and playgroundsSeasonal recreational roadsSidewalks and bicycle pathsGolf cart pathsTennis courts

Class II(Light) Up to 200 autos per day, 7,000 to 15,000trucks expected during the design period.

Residential streetsRural farm roadsParking lots of less than 500 stallsAirports - 7,500 pound maximum gross

weight

Class III(Medium) Up to 700 autos per day, 70,000 to150,000 trucks expected during design period.

Urban minor collector streetsRural minor collector streetsParking lots - more than 500 stallsAirports - 15,000 pound maximum gross

weight.


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Class IV(Medium) Up to 4,500 autos per day, 700,000 to1,500,000 trucks expected during designperiod.

Urban minor arterial and light industrialstreets

Rural major collector and minor arterialhighways

Industrial lots, truck stallsBus driveways and loading zonesAirports - 30,000 pound maximum gross

weight.

Class V(Heavy) Up to 9,500 autos per day, 2,000,000 to4,500,000 trucks expected during designperiod.

Urban freeways, expressways and otherprincipal arterial highwaysRural interstate and other principal arterialhighwaysLocal industrial streetsMajor service drives or entrances

Airports - 60,000 pound maximum grossweight

Class VI(Very Heavy) Unlimited autos, 7,000,000 to15,000,000 trucks expected during designperiod.

Urban interstate highwaysSome industrial roadsAirports - over 60,000 pounds maximum

gross weight

For more information on this subject refer tothe Asphalt Institutes publications ThicknessDesign-Asphalt Pavements for Highwaysand Streets (MS-1) and Asphalt PavementThickness Design (IS-181).


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SOIL SUPPORT CAPABILITY

The ability of the subgrade to support loads

transmitted from the pavement is one of themost important factors in determining pave-ment thickness. The subgrade must serve as aworking platform to support construction

different abilities to provide support. A sandysoil, for example, will support greater loadswithout deformation than a silty clay soil.Thus, for any given traffic volume and weightof vehicles using the roadway, a greaterpavement thickness must be provided on claysoils than on sandy soils.

Figure 3-1. Spread of wheel-load through pavement structure.

equipment and as a foundation for the pave-ment structure that supports and distributestraffic loads. Thus, it is essential to evaluate thestrength of the subgrade before beginning thestructural design of the pavement. Figure 3-1shows the spread of wheel load through thepavement structure and on to the subgrade.

If sufficient pavement thickness is not pro-vided, the applied loads could cause greaterstresses on the subgrade than it can resist. Thismay result in deflection of the pavement andultimately in its failure.

In street and highway construction, thesubgrade provides the foundation for thepavement. Different types of soils have

Soil ClassificationsSoil is classified for road and street construc-

tion in order to predict subgrade performanceon the basis of a few simple tests. The AmericanAssociation of State and Highway Transporta-tion Officials (AASHTO) classification systemfor soils is commonly used as a test forsubgrade-support value.

According to the AASHTO system, soils thathave approximately the same general load-carrying capabilities are grouped in classifica-tions of A-1 through A-7. (See Table 3-1.) Ingeneral the best highway subgrade soils are A-1, and the worst are A-7. The classification isbased on the sieve analysis, plasticity index,and liquid limit of the soil being tested.


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Subgrade StrengthBecause thickness calculations depend on

the strength of the finished subgrade, the soilmust be tested for this information. Tests arebased on bearing capacity related to themoisture and density of the soil. The CaliforniaBearing Ratio (CBR) is one of the most widelyused methods of designing pavementstructure. Once the CBR value is determined,the soil classification can be identified. Or,

when the soil classification is known, a relativeCBR value can also be identified.

The lower the CBR value of a particular soil,the less strength it has to support thepavement. This means that a thicker pavement

structure is needed on a soil with a low CBRrating than on a soil with a high CBR rating.Generally, clays have a CBR classification of 6.Silty loam and sandy loam soils are next withCBR values of 6 to 8. The best soils for roadbuilding purposes are sands and gravelswhose CBR ratings normally exceed 10.

The change in pavement thickness needed tocarry a given traffic load is not directlyproportional to the change in CBR value of thesubgrade soil. For example, a one-unit changein CBR from 5 to 4 requires a greater increasein pavement thickness than does a one-unitCBR change from 10 to 9.

A number of soil classification-strengthsystems are currently in use for roads and air-ports. A correlation chart follows for a generalsoil overview.


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Moderate

Moderate subgrade soils are those that

retain a moderate degree of firmness underadverse moisture conditions. Included aresuch soils as loams, silty sands, and sandgravels containing moderate amounts of claysand fine silts. When this soil becomes acohesive material, it should have a minimumproctor density of 110 pounds per square inch.A soil classified as moderate will have a CBRvalue of 6 to 8.

Poor

Poor subgrade soils are those that becomequite soft and plastic when wet. Included arethose soils having appreciable amounts of clayand fine silt (50 percent or more) passing aNo. 200 sieve. The coarse silts and sandy loamsmay also exhibit poor bearing properties inareas where deep-frost penetration into thesubgrade is encountered for any appreciableperiods of time. This also is true where thewater table rises close to the surface duringcertain periods of the year. A soil classified as

poor will have a CBR value of 3 to 5.

Very poor soils (those with a CBR of 3 orlower) often perform poorly as pavementsubgrades. However, to improve their per-formance, these soils can be stabilized withgranular material or a geotextile. Lime, fly-ash,asphalt cement, portland cement, and combi-nations of cement stabilizers also can be addedto improve the subgrade support. Theselection of a stabilizing agent, the amount to

use, and the application procedure depend onthe soil classification and the subgrade-support value desired. These should bedetermined through appropriate laboratorytesting.

Soil TestingA qualified laboratory can conduct tests to

provide soil classification and subgradestrength information (such as the CBR). Suchtesting is necessary to ensure a properstructural design and is part of all majorprojects. However, such soil testing isrelatively expensive, especially for smallprojects, and may not be available for allprojects.

Subgrade ClassesFor the designs recommended in this

manual, all soils have been divided into threeclasses: good (G), Moderate (M), and poor (P),CBR design values are assigned to thesedifferent subgrade classes.

Good

Good subgrade soils retain a substantialamount of their load-supporting capacitywhen wet. Included are the clean sands, sand-

gravels, and those free of detrimental amountsof plastic materials. Excellent subgrade soilsare relatively unaffected by moisture or frostand contain less than 15 percent passing aNo. 200 mesh sieve. A soil classified as goodwill have a CBR value of 9 or greater.


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DRAINAGE

General Considerations

Highway engineers recognize the impor-tance of good drainage in the design, construc-tion, and maintenance of any pavement.Probably no other single factor plays such animportant role in determining the ability of apavement to provide trouble-free servicethroughout long periods of time.

The accumulation of water in the subgrade,or in an untreated aggregate base course,usually creates problems. When the soil is

saturated, application of dynamic wheel loadsinduces pore pressures and lowers the resist-ance to shear. Some soils have a high volumechange (when water is added), which causesdifferential heaving. The subsequent weaken-ing of the pavement structure causes it to losestability and its capability to support trafficloads.

The combination of water in the pavementsasphalt layers and dynamic, repeated traffic

loading can strip or separate the asphalt filmfrom the aggregate. This reduces the load-carrying capacity of the mixture.

When developing the features of a highwaydrainage system, it is important to consider thesystems principal purposes: (1) to collect anddrain away both surface water and subsurfacewater; (2) to lower the groundwater table, ifnecessary; (3) to intercept water from sur-rounding areas and carry it away from theroadway; and (4) to prevent or retard erosion.

There are two basic categories of drainage surface and subsurface. Surface drainageincludes the disposal of all water present onthe pavement surface, shoulder surface, andthe adjacent ground when sloped toward thepavement. Subsurface drainage deals withwater in the subbase, the surrounding soil, and

in the several pavement courses. Inadequateattention to either of these two drainage condi-tions can lead to premature pavement failure.

Surface DrainageIn surface drainage conditions, the

pavement and shoulders must be crowned orcross-sloped to facilitate the flow of water offof the roadway. Normally, the cross-slopemoves the water to a curbed or inverted-shaped gutter and then off of the pavementinto a storm sewer or flume to a ditch.

On parking areas or playgrounds, the cross-

slope or crown may be inverted toward acenter swale with a grated inlet for drainageinterception.

Shoulders can best be drained if the entireshoulder width has an asphalt-paved surface.If the shoulder is not asphalt, its cross-slopeshould be steeper in order to minimize seepagethrough the aggregate or grass shoulder.

Surface drainage from the pavement and

from the adjacent land areas must be inter-cepted and disposed of. If a curbed section isprovided, drainage is accumulated in thegutter area and periodically discharged intoeither a pavement inlet or a ditch through aflume. The determination of inlet locationsrequires technical calculations and studies tomaintain a tolerable spread of water on thepavement.

Drainage ditches are constructed along theedges of non-curbed roadway sections. Waterflowing from the pavement and shouldersurfaces moves down the roadway foreslopeinto a rounded ditch area. A backslope leadsfrom the bottom of the ditch up to intercept theadjacent land. The adjacent land is frequentlysloped toward the ditch and can contribute toa sizable portion of the drainage flow.


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Figure 3-4 Figure 3-5

DESIGN TYPES

In general, the design of a new asphalt

pavement structure involves two basicpavement types: (1) full-depth pavements, and(2) pavements with an untreated aggregatebase course.

Full-depth, Asphalt Concrete paving is onein which asphalt mixtures are used for allcourses above the subgrade. Such pavementsare less affected by subgrade moisture and aremore conducive to staged construction. Full-depth asphalt pavement is used in all types of

highway construction and where highvolumes of traffic and trucks are anticipated.

Untreated aggregate base pavements may beused where local aggregates and subsurfacedrainage conditions are suitable and where

traffic loadings are minimal. The untreatedaggregate base is placed and compacted onthe prepared subgrade. In general, an asphaltbinder and surface course are used to completethe pavement structure. Although the initialcost for untreated aggregate base asphaltpavements may be lower than the cost for full-depth, hot mix types, the former type shouldbe used with caution. Moisture in the base maycause pavement failure.


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Table 3-2 Gradation of sample RAP material

Sieve %Passing %PassingSize Rang AVERAGE

3/4" 98-100 1001/2" 94-100 983/8" 84-98 93

No. 4 65-88 77No. 8 51-74 62

No. 16 36-57 49No. 30 28-42 36No. 50 17-30 24No. 100 11-26 18No. 200 9-22 14


ASPHALT CONCRETESPECIFICATIONS

It is recommended that specifications forAsphalt Concrete follow Iowa Department ofTransportation Standard Specifications for theparticular class and mixture size required. Thiswill result in uniformity and economy becausemost APAI-member contractors may have jobmixes on several mixtures already preparedfor state and local agency use. In the absence ofa previously prepared job mix, the contractoror private testing should develop a job mix

formula for the desired project, and intendeduse.

The following gradations are suggestedguidelines for the class and mixture sizespecified. The asphalt cement content is aguide only and may need to be adjusted tomeet local aggregate conditions and intendeduse.

Quality of aggregates (according to factors

such as freeze and thaw, abrasion, plasticityindex, etc.) for the various mixes can beobtained from the Iowa DOT StandardSpecifications in the 3 sections listed in thefollowing tables.

Salvaged and Reclaimed MaterialRecycling of reclaimed asphalt pavement

(RAP) material into new asphalt concrete hasbecome a routine and accepted process for useof the salvaged product. The contractorsubstitutes reclaimed aggregate and binder for

virgin materials at varying ratios from 10-50%by weight. The salvaged material may be taken

from the project or a stockpile provided by thecontractor. Control of the use and quality of therecycled mix shall be through the job mixformula process. Salvaged material may beused in the base, binder, and surface courses oftype A or B mixes for which it qualifies.Historical test reults from milled materialtaken from Iowa DOT projects indicate thatmillings are falling within the following limits.

Salvaged materials, whether previouslyprocesses or not, shall be sized for the intendedmix use. Final gradation of the recycled mixshall meet the requirements for the specifiedmix size and type.


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Standard Mix DescriptionsIt is recommended that designs and

specifications for Asphalt Concrete follows

the Iowa Department of Transportation stand-ards for the specific type and mix designrequired. Instructional Memorandum on FieldInspection manuals published by the CentralOffice of Materials, are available from the DOTstoreroom. Designated mix descriptionsfollow.

Recycled Asphal t Concret e

Recycled asphalt concrete mixtures arecomposed of a combination of virgin gravel,

crushed stone, sand and salvaged/reclaimedasphalt paving (RAP) materials. The combinedaggregates are mixed with asphalt cementthrough a hot mix plant mix process toproduce a recycled mix.

Job mix formulas are required by thespecifications to determine the target percentasphalt binder for a specified mix type andgradation. Recycled materials are routinelyused in base, binder, and surface courses.These mixtures may be designed as type A or Basphalt concrete with the same qualityrequirements, therefore requiring no namechange or designation.

Type B Asphal t Concret e

Type B Asphalt Concrete base, binder,leveling, strengthening, and surface coursemixtures are composed of gravel; crushedstone; or combinations of gravel, stone, andsand, produced from approved sources andformulated to provide service for roadscarrying low to moderate traffic. Theformulation procedure results in a job mixformula for each aggregate combination alongwith a recommended percentage of asphaltcement.

Type B Asphalt Concrete may be placed as abase, binder, or surface course dependingupon mix class and size. Type B specificationsare used in secondary road systems, municipalstreets, parking lots, and other areas. To meetall appropriate requirements, and becauseseveral options are available, care must beexercised in selecting the mix class, liftthickness, and mix size during the variousstages of design and construction.

Job mix formulas are required by thespecifications for all aggregate combinations.The formulas are comprised of the aggregatepercentages, percent asphalt, and gradation aslimited by the specification requirements.

Type A Asphalt Concret e

Type A Asphalt Concrete binder, leveling,strengthening, and surface course mixtures arecomposed of combinations of high-qualitygravel, crushed stone, and sand producedfrom approved sources and formulated forservice on road surfaces carrying a high

volume of traffic; and as surface courses withlower-quality base courses for other uses.Because four mix sizes are available, care mustbe exercised in selecting the lift thicknessand mix sizes during the various stages ofdesign and construction so that appropriaterequirements are met.

Job mix formulas are required by thespecifications for all aggregate combinations.The formulas are comprised of the aggregate

percentages, percent asphalt, and gradation aslimited by specified tolerances for eachcontrolling sieve size.


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The use of surge or storage bins is permittedfor storing asphalt pavement materials.

Haul ing Equipment

Haul trucks are used to bring the AsphaltConcrete from the asphalt mixing facility to thepaving site. Equipment used in haulingbituminous mixtures should be clean and havetight bodies to prevent material loss. Theseunits can be equipped with suitable covers toprotect the mixture in transit duringunfavorable weather conditions.

the depth and crown section specified withoutthe aid of manual adjustment during opera-

tion. Pavers should be capable of spreadingmixes without segregation or tearing andproducing a finished surface of even anduniform texture.

Compact i on Equipment

Compaction equipment is used to compactthe Asphalt Concrete to attain density afterplacement. The compaction equipment shouldbe of the type or types that will produce therequired density and pavement smoothness.

Steel-wheeled rollers are of four types three-wheeled, two-axle tandem, three-axle tandem,and vibratory. These rollers should beequipped with power units. Rollers should bein good working condition and equipped witha reversing clutch. Rollers should haveadjustable scrapers to keep the wheel surfacesclean and an efficient means of keeping themwet to prevent mixes from sticking. Thesesurfaces should have no flat areas, openings,or projections that will mar the surface of the

pavement.Spreadi ng Equi pment

Spreading equipment is used to place theAsphalt Concrete as pavement. Where feasible,the Asphalt Concrete should be placed andspread by a mechanical spreader. Mechanical,self-powered pavers should be capable ofspreading the mix within the specifiedtolerances and true to the line, grade, andcrown indicated on the plans. A motor patrolmay be used for the leveling course.

Pavers should be equipped with efficientsteering devices and should be capable oftraveling both forward and in reverse. Theyshould be equipped with hoppers anddistributing screws that place the mix in frontof screeds. The screed unit should be adjust-able in height and crown and equipped with acontrolled heating device for use whenrequired. The screed must strike off the mix to

Pneumatic-tired rollers should be self-pro-pelled. The rollers should be equipped withpneumatic tires of equal size and diameterthat are capable of exerting average contactpressure.


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The wheels of the roller should be spaced sothat one pass will accomplish one completecoverage equal to the rolling width of themachine. There should be a minimum of 1/4

inch-overlap of the tracking wheels. The rollershould be constructed so that the contactpressure will be uniform for all wheels and thetire pressure of the tires will not vary morethan 5 pounds per square inch. The rollersshould be constructed with enough ballastspace to provide uniform wheel loading asmay be required. The operating weight andtire pressure of the roller may be varied toobtain contact pressures that will result in thedensity.

Cold M i l l ing

Cold milling is the most common pavementscarification method for salvaging material.This method uses a self-propelled millingmachine with a rotating drum containingspecial teeth that cut the pavement to apredetermined depth and reduce the size ofthe salvaged material. Single-pass cuttingwidths of up to 12 feet and depths of 4 incheshave been attained with this type of machine.

The drums are hydraulically controlled andare capable of maintaining road profile anddepth of cut to 1/8 inch. Milled material isusually suitable for hot or cold recycling withlittle additional breakdown.

Construction PracticesPreparat i on of Subgrade

Remove all large rock, debris, and topsoilfrom the area to be paved. All vegetation,including root systems, should be removed. To

prevent future growth, the subgrade should betreated with an approved herbicide. Install alldrainage and utility facilities and thenproperly backfill and compact.

The subgrade must be properly shaped tomeet true lines and elevations and compactedto not less than 95 percent of maximumlaboratory density. The surface of the com-pacted subgrade should not vary more than3/4 inch from the established grade.

Areas showing pronounced deflectionunder construction traffic indicate instabilityin the subgrade. If the situation is not correctedby reworking and additional rolling, the areas

must be removed and replaced with suitablematerial and compacted or stablized using ageotextile. The use of Asphalt Concrete base orcourse granular material is recommended.

Const ructi ng Asphalt Concret e Base

The Asphalt Concrete base may consist ofone or more courses placed on a prepared sub-grade. It must have a total compactedthickness as indicated on the plans or as speci-fied. In general, a base with total thickness of 4

inches or less should be placed in one lift. Abase with a total thickness of more than 4inches may be placed in two or more lifts withthe bottom lift having a minimum of 3 inches.

Unt reat ed Aggregate Base

The crushed aggregate base course mayconsist of one or more layers placed directly onthe prepared subgrade. The material must bespread and compacted to the requiredthickness, grades, and dimensions indicated in

the plans or as specified. The minimumcompacted thickness of each lift should be noless than two times the size of the largestaggregate particle, or 4 inches, whichever isgreater. The maximum compacted lift thick-ness should be 6 inches.

Bi nder and Sur face Cour ses

The upper lifts of the pavement may consistof one or more courses of Asphalt Concreteplaced on the previously constructed Asphalt

Concrete base. In general, the top or wearingcourse must not be constructed to a depthgreater than 3 inches. Where a thicknessgreater than 3 inches is indicated, it should beplaced in two courses consisting of a binderand a surface or wearing course. The mini-mum lift thickness must be 1 inch, but thisthickness should never be less than two timesthe maximum particle size.


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Figure 3-6

Tack Coat

A tack or bond coat of CSS-1, SS-1, MC-70 oran approved alternate should be applied

between each course at an undiluted rate of0.02 to 0.05 gallons per square yard. Thesurface must be cleaned of all dust, dirt, orother loose material before the bond coat isapplied. If emulsion is used, it should bediluted with equal parts of water or asspecified in the proposal.

All pavement markings on public highwaysmust comply with the Manual on Uniform

Traffic Control Devices (MUTCD). Standardsfor color, materials, width, shape, and conceptare set forth in the MUTCD.

The most frequently used pavement mark-ings are longitudinal markings. The basicconcept is to use yellow lines to delineate theseparation of traffic flows in opposing direc-tions or to mark the left edge of pavement ofdivided highways and one-way roads. Solidyellow lines are also used to mark no passing

zones. White lines are used to separate trafficlanes flowing in the same direction or to markthe outside edge of pavements.

M inimum Grade

It is recommended that the minimumpavement grade be not less than 2 percent(approximately 1/4 inch per foot) to ensureproper surface drainage.

Pavement MarkingsPavement markings have an important

function in traffic control. They convey certainregulations and warnings in a clearly under-

standable manner without diverting thedrivers attention from the roadway. Anasphalt pavement clearly has an advantage inproviding highly visible, attention-attractingmarkings even under adverse weatherconditions. White- and yellow-painted mark-ings or thermo markings stand out on theblack background.

The patterns and width of longitudinal linesvary with use. A broken line is formed bysegments and gaps, usually in the ratio of 1:3.On rural highways, a recommended standardis 10-foot segments and 30-foot gaps. A normal

line is 4 to 6 inches wide.


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III.

DESIGN

CONSIDERATIONS


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Thickness Design 4-1

Chapter 4

Thickness Design

GENERAL CONSIDERATIONS

Several procedures can be used to calculatethe thickness of the proposed asphaltpavement. All are based on the volume andweight of the traffic that will use the facilityand on the load-supporting capability of theunderlying soil.

The AASHTO Road Test and other studieshave indicated that heavy-vehicle wheel loadscause much greater damage to roads than dolight loads. Thus, where large volumes oftraffic with heavily loaded trucks are antici-pated, an in-depth analysis of the pavementthickness is important. Because all of thehigher functional classifications have thepotential for heavy loadings, a traffic analysisis an important part of the preparation forthickness computations. Similarly, a know-ledge of the load-bearing capability of the soilis an important aspect of the structural design

process. The lack of a soil study with appro-priate corrective action could significantlyshorten the life of a poorly drained pavement.

All of the design procedures available for astructural thickness analysis cannot beincluded here. Additional information isincluded in The Asphalt Institutes ThicknessDesign Manual (MS 1) and their Simplifiedand Abridged Version published in Informa-tion Series No. 18 (IS-181). Another reference isThe AASHTO Guide for Design of PavementStructures, 1986. These guides are based onmechanistic/empirical design models, andthey use Nomographs to attain pavementthickness. Several computer programs fordesigning pavements (including AsphaltInstitute and AASHTO programs) are alsoavailable. The APAI or your contractor canhelp you with design questions.


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Thickness Design4-2

PAVEMENT THICKNESS DESIGNTABLES

Future traffic assignments can be rathernebulous and are subject to many externalinfluences. Some areas see no growth over adesign period. Therefore, common practice isto group categories of traffic into classes.Similarly, it has been found that, based on alocal knowledge, soil-supporting values can begrouped into classifications of poor, moderate,and good. These classifications (see Chapter 3)provide an opportunity to use pavement

thickness design tables rather than moredetailed formula procedures. These tableshave been prepared by experts in the industryto simplify the process for engineers, tech-nicians, and architects who prepare apavement design.

Design Procedure

Tables 4-1 through 4-5 can be used directly toselect design thicknesses from the design inputfactors. In order to use the tables, appropriatetraffic and subgrade classes must be selected asfollows.

Traffic

The design procedure separates traffic intosix classes (I through VI). Each class is definedby the number of autos per day, the averagedaily number of heavy trucks expected on thefacility during the design period, and the typeof street or highway. Traffic classifications arepresented in Chapter 3. The pavementthicknesses given in the tables of this and thefollowing chapter are based on the averagedaily traffic (ADT) values over a 20-yeardesign period. Heavy trucks are described astwo-axle, six-tire vehicles or larger.

Soils

It is desirable to have laboratory tests on thesubgrade soil. However, if tests are not

available, a design may be based on carefulfield examinations by an engineer. Soils may

be classified as good, moderate, or poor or bya CBR value. Soil classifications are presentedin Chapter 3. If a soil CBR value lies betweenthose given in the classifications, the lowerclassification is used.

Design Steps

The following steps can be used to deter-mine a pavement thickness.

1. From the known average daily traffic,

determine the total number of trucksover the design period. Using thisinformation, select the trafficclassifications (Class I through VI) fromChapter 3.

2. Select a subgrade class (good, moderate,or poor) from Chapter 3 using soil datafrom the project. If no soil information isknown, use the poor classification for thesubgrade.

3. Select a design thickness from Tables 4-1through 4-5 using the selected trafficclass and subgrade class.

Desi gn Example

A collector street is estimated to carry 500vehicles and 20 trucks a day. Traffic classIII is selected using Chapter 3.

No soil data is known, so the engineer

selects the poor soil classification.

The total design thickness selected fromTable 4-2 is 7-1/2 inches. The base courseis 6 inches, and the surface course is1-1/2 inches.


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Thickness Design 4-7

High-Volume Secondary and Rural RoadsHigh-volume rural roads consist of arterial

roads and the highway system. They providethe highest operation speeds and highest level

of traffic service. These roads serve as themajor corridors of traffic and frequently havemultiple lanes.

These roads frequently carry large trafficvolumes and heavy truck traffic. The informa-tion contained within this guide shouldaugment local guidelines in assuring properplanning and design of high-volume roads. Thevalues found here are only applicable to lowtruck volumes. Design of Asphalt Concrete

pavements for trucking highways requiresconsiderable expertise and detailed analysis.

Table 4-5. Thickness Design: High Volume Secondary and Rural Roads

Thickness in InchesDesign Criteria* Asphalt Concrete

Traffic Class Subgrade(ADT) Class CBR Base Surface Total

III Good 9 4.0 1.5 5.5(201-700 ADT) Moderate 6 5.0 1.5 6.5

Poor 3 6.0 1.5 7.5

IV Good 9 5.5 2.0 7.5

(1,501-4,500 ADT) Moderate 6 6.5 2.0 8.5

Poor 3 7.5 2.0 9.5

V Good 9 7.5 2.5 10.0(6,001-9,500 ADT) Moderate 6 8.0 3.0 11.0

Poor 3 9.0 3.0 12.0

VI Good 9

(9,501 & Above ADT) Moderate 6

Poor 3

*See chapter 3 for traffic and soil class details

Special design considerationneeded. Refer to a morecomplete design procedure.


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Parking Lot Design 5-1

Chapter 5

Parking Lot Design

GENERAL CONSIDERATIONS

The parking lot is the first - and the last -part of a building complex to be viewed by theuser. It is the gateway through which allcustomers, visitors, and employees pass. Thisfirst impression is very important to theoverall feeling and atmosphere conveyed tothe user.

Developers want their new facilities to beattractive, well designed, and functional.Though many hours are spent on producingaesthetically pleasing building designs, thesame design consideration for the parking area

is often overlooked. Pavements in parkingareas that are initially under-designed canexperience excessive maintenance problemsand a shortened service life.

When properly designed and constructed,parking areas can be an attractive part of thefacility that is also safe, and most important,

usable to the maximum degree. In addition,parking areas should be designed for lowmaintenance costs and easy modification forchanges in use patterns.


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Parking Lot Design5-2

Rules have been developed for optimizingparking area space. Among them are thefollowing:

1. Use rectangular areas where possible.2. Make the long sides of the parking areas

parallel.3. Design so that parking stalls are located

along the lots perimeter.4. Use traffic lanes that serve two rows of

stalls.

Table 5-1. Recommended Parking Requirements

Land Use Spaces/Unit

Residential

Single-Family 2.0/Dwelling

Multifamily

Efficiency 1.0/Dwelling

1 -2 Bedroom 1.5/Dwelling

Larger 2.0/Dwelling

Hospital 1.2/Bed

Auditorium/Theater/Stadium 0.3/Seat

Restaurant 0.3/Seat

Industrial 0.6/Employee

Church 0.3/Seat

College/University 0.5/Student

Retail 4.0/1000 GFA

Office 3.3/1000 GFA

Shopping Center 5.5/1000 GLA

Hotels/Motel 1.0/Room

0.5/Employee

Senior High Schools 0.2/Student

1.0/Staff

Other Schools 1.0/Classroom

GFA, sq. ft. of gross floor area

GLA, sq. ft. of gross leasable area

The information in this chapter will providea general guide to proper parking area design,construction, and facility layout. Minimumpavement thickness designs are given forvarious size parking lots, heavily-loaded areas,and industrial parking lots. In addition, thischapter gives comparable designs for both fulldepth asphalt pavements and asphalt overuntreated aggregate base.

General PlanningIn developing the parking area plan, several

important details should be considered. Firstand foremost in the mind of the developer maybe providing the maximum parking capacityin the available space while ensuring conveni-ence and safety.

If the locality does not have a zoningordinance identifying specific requirements foroff-street parking, the general recommenda-

tions in Table 5-1 may be useful.


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Figure 5-1. Parking lot angles

Special attention should be given to the flowof traffic in and out of the lot as well as circu-lating routes inside the lot. Keep entrances faraway from busy street intersections and fromlines of vehicles stopped at a signal or stopsign. Be sure that the entering vehicles canmove into the lot on an internal aisle, therebyavoiding entering congestion caused by in-volvement with turning vehicles. A pedestriantraffic-flow study is important to provide in-formation about both safety and convenience.

spaces for a given area but is the onlyacceptable angle for a herringbone parking lotpattern.

The 90 parking angle provides the mostparking spaces for a given area. The highdegree of difficulty for entering and leavingthese parking stalls makes this type of parkingmore suited to all-day parking, such asemployee parking. This angle is generally notpreferred for in and out lots such as those offast food restaurants and banks.

Parking AngleThe most popular angles for parking stalls

are 60, 45, and 90 . The most common anglefor parking is the 60 angle because of the easeof operation it provides. This angle permitsreasonable traffic lane widths and eases entry

and exit of the parking stall.

Where lot size restricts the dimensionsavailable for aisles and stalls, a 45 angle maybe used. The smaller change of directionrequired to enter and back-out of the stallspace permits use of narrower aisles. The 45angle reduces the total number of parking

Parking Space DimensionsTypical parking stall dimensions vary with

the angle at which the stall is arranged inrelation to the aisle. Stall widths (measuredperpendicular to the vehicle when parked)range from 8-1/2 to 9-1/2 feet. The minimumwidth for public use parking spaces is 9 feet by19 feet. Recommended stall dimensions forcompacts and similar-sized vehicles are 7-1/2

feet by 15 feet. If a number of such spaces areto be provided, they should be groupedtogether in a prime area to promote their use.Stall widths for parking lots where shoppersgenerally have large packages, such assupermarkets and other similar parkingfacilities, should be 9-1/2 feet or even 10 feetwide.


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The use of Asphalt Concrete base (comparedto use of untreated aggregate base) will greatlyreduce the potential for problems related towater strength and stability.

Subgrade Preparat i ons

All underground utilities should be protect-ed or relocated before grading. All topsoilshould be removed. Low-quality soil may beimproved by adding granular materials, lime,asphalt, or other mixtures. Laboratory tests arerecommended to evaluate the load-supportingcharacteristics of the subgrade soil. However,designs are frequently selected after carefulfield evaluations based on experience andknowledge of local soil conditions.

The area to be paved should have all rock,debris, and vegetation removed. The areashould be treated with a soil sterilant to inhibitfuture flora growth. Grading and compaction

of the area should be completed so as toeliminate yielding or pumping of the soil.

The subgrade should be compacted to auniform density of 95 percent of the maximumdensity. This should be determined inaccordance with Standard Proctor density(Test Method 103). The compaction require-ment may substitute a specified number ofdiskings and roller coverages of each lift.When finished, the graded subgrade shouldnot deviate from the required grade and crosssection by more than 1/2 inch in 10 feet.

Pri me Coat

An application of a low-viscosity liquidasphalt may be required over untreatedaggregate base before placing the AsphaltConcrete surface course. A prime coat and itsbenefits differ with each application, and itsuse often can be eliminated. Discussrequirements with the paving contractor.


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Asphal t Base Const ructi on

The asphalt base course material should beplaced directly on the prepared subgrade inone or more lifts. It should be spread andcompacted to the thickness indicated on theplans. Compaction of this asphalt base is oneof the most important construction operationscontributing to the proper performance of thecompleted pavement. This is why it is soimportant to have a properly prepared andunyielding subgrade against which tocompact. The asphalt base material should

meet the specifications for the mix typespecified.

Unt reat ed Aggregate Base Constr ucti on

The untreated aggregate base course shouldconsist of one or more layers placed directly onthe prepared subgrade. It should be spreadand compacted to the uniform thickness anddensity as required on the plans. The minimumthickness of untreated aggregate is 4 inches. Theaggregate material should be of a type approv-

ed and suitable for this kind of application.

It should be noted that an untreatedaggregate base is sensitive to water in thesubgrade. The pavement failures associatedwith water in the subgrade are accelerated ifan untreated base allows water to enter thepavement structure.

Tack CoatBefore placing successive pavement layers,the previous course should be cleaned and atack coat of diluted emulsified asphalt shouldbe applied if needed. The tack coat may beeliminated if the previous course is freshlyplaced and thoroughly clean.

Asphal t Concret e Surf ace Course

Material for the surface course should be anAsphalt Concrete mix placed in one or more

lifts to the true lines and grade as shown on theplans. The plant mix material should conformto specifications for Asphalt Concrete.

The asphalt surface should not vary fromestablished grade by more than 1/4 inch in 10feet when measured in any direction. Anyirregularities in the surface of the pavementcourse should be corrected directly behind thepaver. As soon as the material can becompacted without displacement, rolling and

compaction should start and should continueuntil the surface is thoroughly compacted andall roller marks disappear.


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Table 5-3. Thickness Chart: Parking Lots with Less Than 50 Spaces

A. For Asphalt Concrete Base Pavements

Thickness in Inches

Design Criteria* Asphalt ConcreteTraffic Class Subgrade

(Spaces) Class CBR Base Surface Total

I Good 9 3.0 1.0 4.0

(


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Table 5-4. Thickness Chart: Parking Lots with More Than 50 Spaces

A. For Asphalt Concrete Base PavementsThickness in Inches

Design Criteria* Hot Mix Asphalt

Traffic Class Subgrade(Spaces) Class CBR Base Surface Total

II Good 9 3.0 1.0 4.0

(50-500 spaces) Moderate 6 3.5 1.5 5.0

Poor 3 4.5 1.5 6.0

III Good 9 3.5 1.5 5.0

(500 & Above spaces) Moderate 6 4.5 1.5 6.0

Poor 3 5.5 1.5 7.0

B. For Untreated Aggregate Base Pavements

Design Criteria* Thickness in Inches

Untreated AsphaltTraffic Class Subgrade Aggregate Concrete

(Spaces) Class CBR Base Surface Total

II Good 9 4.0 3.0 7.0

(50-500 spaces) Moderate 6 6.0 3.5 9.5

Poor 3 8.0 3.5 11.5

III Good 9 6.0 3.0 9.0

(500 & Above spaces) Moderate 6 8.0 3.5 11.5

Poor 3 8.0. 4.0 12.0

*See chapter 3 for traffic and soil class details


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PLANNED STAGE CONSTRUCTION

Planned stage construction is a means ofproviding fully adequate pavements with theeffective use of funds, materials, and energy.As defined, it is the construction of an AsphaltConcrete parking lot or roadway in two ormore stages, separated by a predeterminedinterval of time. In many situations, buildingpavements by stages makes good economicalsense. It is a technique long used by city andhighway engineers.

Stage Construction is not maintenance. It isthe placement of a minimum depth ofpavement during initial construction, and afinal surface course placed at a planned futuredate. Asphalt Concrete lends itself to this kindof construction.

As an example, the owner of a newdepartment store with a 350-car parking lot,for financial reasons, decides to stage constructthe 6-1/2 full-depth asphalt parking lot. Stage

1 is constructed at the time the store is built. Atotal depth of 5 of asphalt concrete is placed.Stage 2, consisting of the final surface course of1-1/2, will be placed at a set time in thefuture. The truck loading zone and dumpstersite are paved the full depth during initialconstruction.

Stage construction has the advantage ofproviding a thoroughly adequate, all-weatherpavement for the initial development of anarea. Any damage to the Stage 1 pavementcaused by traffic, settlements, or utility tearupscan be repaired prior to placement of the finalsurface. With a proper asphalt tack coat, whereneeded, the Stage 2 pavement bonds to the oldsurface and becomes an integral part of theentire pavement structure.

ASPHALT CONCRETE CURB

Asphalt curbs have become increasinglypopular as accessories to paving because theyare: (1) economical and easy to construct; (2)can be built much faster than other types; (3)are not affected by ice- and snow-meltingchemicals; and (4) can be laid on an existingpavement using a slip form paver.

Many parking facilities have some form ofcurbing around the perimeter for bothfunctional and aesthetic reasons. The curbs

control drainage, delineate the pavement edge,prevent vehicular encroachment on adjacentareas, and enhance the parking lot.

Curb MixtureThe method of mixing the Asphalt Concrete

and the composition of the mixture must

conform with IDOT Specification 2303, 2304,or an approved commercial mix. The bitumencontent should be modified as necessary toproduce a suitable mixture for AsphaltConcrete curb construction. Curb mixes thatare proportioned using the mixture sizes of3/8- or 1/2-inch have proven to be mostsatisfactory and are recommended for curbconstruction in Iowa.


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Figure 5-3. Figure 5-4. Typical curb sections

The addition of 10 to 25 pounds ofpowdered asphalt per ton of mix will producean exceptionally tough and durable curb. The

asphalt cement used in the mix should bereduced on a pound-for-pound basis whenpowdered asphalt is added to the mixture.The temperature of the mixture at the time ofmixing and laying should range from 250 Fto a maximum of 300 F.

Curb ConstructionBefore curb construction begins, the place-

ment area must be cleaned thoroughly. A tackcoat must be applied to the pavement surfaceat a maximum rate of 0.10 gallons per squareyard.

The Asphalt Concrete curb must be laid trueto the specified line, profile, and cross section

with an approved self-propelled curb-layingmachine. The mixture must be fed to thehopper of the machine directly from the truckwith a chute or conveyor, or it should beshoveled by hand into the hopper.

Asphalt Concrete curbs should be backedwith earth fill or by constructing a double lineof curb and filling the median with compactedasphalt mix.

The following illustrates two basic types ofsystems Asphalt Concrete curbs andPortland Cement Concrete curb and gutter.


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ASPHALT MAT-PLATFORM FOR BUILDING CONSTRUCTION AND SITE PAVING

Site paving is the recommended first step inmany types of building construction projects.It offers several advantages as a working mator platform before building construction beginsfor shopping centers, schools, manufacturingconcerns, warehouses, and similar facilities.

In this technique, an Asphalt Concrete basecourse is constructed on a prepared subgradeover the entire area that will become parkingareas, service roadways, and buildings. Whenbuilding construction is completed, a finalAsphalt Concrete surface course is placed onthe asphalt base.

AdvantagesPaving a building site before construction is

completed has several benefits. These includethe following:

1. It ensures constant accessibility andprovides a firm platform upon whichpeople and machines can operate efficiently,speeding construction.

2. It provides a dry, mud-free area forconstruction offices, materials storage, andworker parking, eliminating dust controlexpenditures.

3. It eliminates the need for costly selectmaterialthe asphalt subfloor ensures afloor slab that is dry and waterproof.

4. Steel-erection costs can be reduced becausea smooth, unyielding surface results ingreater mobility for cranes and hoists.


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5. The engineer can set nails in the asphaltpavement as vertical- and horizontal-controlpoints, effectively avoiding the risk of lossor disturbance of this necessary survey work.

6. Excavation for footings and foundationsand trenching for grade beams can beaccomplished without regard for theasphalt base.

Construction PracticesSubgrade Preparat i on

All vegetation (including root systems),rocks, debris, and topsoil should be removedfrom the area to be paved. To prevent futuregrowth, the subgrade should be treated withan approved soil sterilant. Install drainage andutility facilities; backfill and compact. Adjust-ments in utilities or underground facilities canbe readily accomplished through the asphaltbase should changes occur.

The subgrade must be properly shaped tomeet true lines and elevations. It must becompacted to not less than 95 percent ofmaximum laboratory density. The surface ofthe compacted subgrade must not deviate bymore than 3/4 inch from the established grade.A minimum slope of about 2 percent or 1/4inch per foot should be maintained to provideadequate drainage of surface water from thefinished pavement.

Areas that show pronounced deflection underconstruction traffic indicate instability in thesubgrade. If reworking and additional rollingdo not correct the situation, the area soil must beremoved, replaced with suitable material, andcompacted. The use of asphalt-treated base orcoarse granular material is recommended.

Base-Pl at form Const ruct i on

Asphalt Concrete Base Material must beplaced on the prepared subgrade. A base of 4inches or less in depth should be placed in onelift. A base of a total thickness of more than 4inches may be placed in two or more lifts with

the bottom lift being a minimum of 3 inches.The material must be spread and compacted tothe required thickness and density as specifiedand in the grades and dimensions shown on

the plans.

The surface of the base must not deviatemore than 1/2 inch when measured with a 10-foot straight edge.

Surf ace Course Constr ucti on

After building construction is essentiallycompleted, and all building materials andoffices have been removed from the previouslypaved base, preparation for placement of the

final surface course of Asphalt Concrete canbegin. Should building operations or winterweather delay placement of the final surface,the Asphalt Concrete base will adequatelyserve traffic needs during the interim.

Preparation for the surface course requiresthorough cleaning and sometimes washing ofthe asphalt base to remove tracked-on dirt andforeign particles. After cleaning, any crackedor broken areas in the base should be removed,replaced with bituminous mix, and thoroughlycompacted. All manholes, valve boxes, andother pavement fixtures should be brought tofinished grade.

The hot mix asphalt surface course consistsof one or more layers placed on the previouslyconstructed Asphalt Concrete base course. Thematerial must be spread and compacted to therequired thickness and in the grades anddimensions shown on the plans.

The finished surface must not deviate morethan 1/4 inch when measured with a 10-footstraight edge.

Tack Coat

Before placing the surface course, the basecourse should be cleaned thoroughly. Ifneeded, a tack coat of diluted emulsifiedasphalt may be applied for bonding.


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VI.

DESIGNS FOR

RECREATIONAL

USES


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Designs for Recreational Use 6-1

Chapter 6

Designs for RecreationalUses

ASPHALT PAVEMENTS FOR NON-VEHICULAR USE

In addition to highways, streets, andparking lots that carry autos and trucks, manyother applications for asphalt pavements exist.Sidewalks, bicycle and golf cart paths, play-

ground areas, tennis courts, and site paving aresome common applications.

Because of the unique nature of theseasphalt pavement applications, a more detail-

ed approach to their design is presented here. Inmany cases, the primary design considerationis a pavement structure capable of supportingoccasional maintenance and emergency

vehicles and resisting freeze/thaw cycles.Therefore, a minimum thickness to accom-modate these loads may be the basis of thethickness design.


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Designs for Recreational Use6-2

BIKEWAYS, GOLF CART PATHS,RECREATIONAL TRAILS, ANDWALKWAYS

It is desirable to blend this type of pathwayinto the contours of the existing ground topreserve aesthetics and to reduce the impacton the natural environment. Surface drainageshould flow away from these pathways wher-ever possible.

Because of the variety of designs and appli-cations, individual pathway widths are notlisted here. For bikeway and golf cart paths in

particular, the size and availability of conven-tional road construction and maintenanceequipment may determine width. Generally, aminimum width of 8 feet is recommended; a12-foot width may be more cost effective. As a

safety measure, additional widening on sharpcurves is recommended.

Recreation trails and walkways are usuallypaved to an 8-foot width to accommodateconstruction and maintenance operations andto provide access for emergency vehicles. Itmay be desirable to pave a walkway in anurban environment only 4-feet wide (or widerif significant numbers of pedestrians are

present). These pavements usually are notdesigned to withstand repeated loads frommaintenance or emergency vehicles, but anoccasional heavy-load application can be madewithout damage.


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Designs for Recreational Use 6-3

Construction PracticesDrainage

It is very important to keep water away fromthe subgrade soil. If the soil becomes saturated,it will lose strength and stability, making theoverlying pavement structure susceptible tobreakup under imposed loads. Both surfaceand subsurface drainage must be considered.All drainage must be carefully designed andshould be installed as early in the constructionprocess as practical.

Bicycle and golf cart paths should have a

minimum slope of 2 percent or 1/4 inch perfoot. They should be constructed in such a waythat water will not collect at the pavementedge. Areas of very high natural permeabilitymay require an underdrain system to carrywater away from the pavement structure.

Subgrade Preparat i on

Because the subgrade must serve both as theworking platform to support constructionequipment and as the foundation for the

pavement structure, it is vital to ensure that thesubgrade is properly compacted and graded.All underground utilities should be protectedor relocated before grading. All drainagestructures sh

asphalt composite

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