hot mix asphalt pavement design guide

Seventh Edition 1999

HOT MIX ASPHALT

PAVEMENT DESIGN

GUIDE

The Maryland AsphaltAssociation, Inc.

Requirements

for Successful Asphalt

Pavement Construction

1 GOOD PLANNING

2 GOOD DESIGN

3 GOOD CONSTRUCTION

A) Materials

B) Subgrade

C) Workmanship

4 GOOD MAINTENANCE

INTRODUCTION

This guide is provided by the Maryland Asphalt Association as a tool for owners, architects, engineers and developers whenpreparing plans and specifications for pavements to be constructed in Maryland.

The guide is not intended to be used as a substitute for professional asphalt pavement design by competent engineers using specific traffic and soils information. By necessity this manual uses generalizations and simplifications; however, the use of thesedesigns and specifications, together with proper construction controls, will provide users with economic and satisfactory pavements that will provide good performance with relatively little maintenance.

The seventh edition of this guide has been prepared in order to bring designers information on the current Maryland StateHighway Administration Specifications and to incorporate the latest knowledge, techniques and technology that have been developed in asphalt pavement construction over the past several years. This publication is not intended to set standards or regulations. The views of the authors expressed herein do not necessarily reflect the decision making process with regards to theadvice, opinions or merits of certain processes, procedures or equipment

CONTENTS

Hot Mix Asphalt—3Pavement Design—4Specifications for Hot Mix Asphalt—5Hot Mix Asphalt Recycling—7Sidewalks, Golf Cart Paths, Bike Paths—8 Residential Drives, Play Areas—9 Parking Lots, Residential Roads, Storage Areas or Light Duty Road—10 Minor Arterial and Light Industrial Streets—11 Service, Commercial or Industrial Roads—12 Tennis and Multi-Use Courts, Running Tracks—13 Hot Mix Asphalt Resurfacing—14 Equivalencies—15 Other Uses for Hot Mix Asphalt—16 MAA Associate Members—inside Back Cover MAA Hot Mix Asphalt Producers and Contractors—Back Cover

SUPERPAVE

1

Superpave is an emerging new paving system which in the future may entirely replace the existingMarshall design mixes. The transition from Marshall mixes to Superpave is expected to intensify in thenear future. Currently, both Superpave and Marshall mixes are available from most HMA producers.Engineers and architects specifying the Superpave system should consult with industry experts for additional design information. In many cases, the final in-place Superpave product may be different inappearance than a similar graded Marshall mix. Superpave design criteria incorporate such factors as climatic conditions, aggregate properties and expected loadings in establishing the correct mix for thespecific application. The use of different performance grade liquid asphalts, aggregate sizes and shapes aswell as modified compaction methods may be required for Superpave mixes. The Maryland AsphaltAssociation always supports the use of the best available products of the specific applications and willassist with proper HMA selection for your specific site.

Full-depth asphalt pavements are economical to construct and when properly designed, will provide outstandingperformance.

Hot mix asphalt pavements, shown on an interstate highway, provides motorists with a smooth, safe, quiet ride.Similar HMA pavements may be used for virtually every type of paving need. When properly designed, asphaltpavements are economical, durable and versatile.

2

HOT MIX ASPHALT

WHAT IS HOT MIX ASPHALT?

Hot mix asphalt may be referred to by several names. In different sections of the state it may also be referred to asbituminous concrete, hot plant mix, asphalt concrete,blacktop or superpave.

Hot mix asphalt is composed of aggregate bound togetherinto a solid mass by asphalt cement. The aggregates total ninety-three (93) to ninety-seven (97) percent by weight ofthe total mixture and are mixed with three (3) to seven (7) percent asphalt cement. It is manufactured in a central mixing plant where the asphalt and aggregates are heated to atemperature of approximately 300 degrees fahrenheit,properly proportioned and mixed. The completed pavingmixture is hauled by trucks to the mechanical spreader whereit is placed in a smooth layer and compacted by rollers whilestill hot. (Hot mix asphalt may be placed by hand when it isimpractical to use a paver.

Asphalt pavements are constructed of one or more courses ofhot mix asphalt placed directly on the subgrade or on a sub-base.

HOT MIX ASPHALT SPECIFICATIONS

All work involving hot mix asphalt pavements and resurfacingcan be covered by using the Maryland State HighwayAdministration 1993 Standard Specifications for Constructionand Materials in conjunction with the typical sections that follow.

HOT MIX ASPHALT RESURFACING

Resurfacing is the placing of one or more courses of hot mixasphalt over an existing hard surface pavement. This is themeans for salvaging and extending the service life of old deteriorated pavements.

ADVANTAGES OF HOT MIX

1. Versatility

Hot mix asphalt pavements can be designed to handle virtually any traffic loading, soils and materials, and can be used to salvage old pavements as well as to build new ones.Phased construction can easily be incorporated.

2. EconomyHot Mix Asphalt Pavements are:

economical to constructcan be constructed rapidly and are immediately ready for usecan be recycledrequire minimal maintenanceprovide outstanding performance

3. HMA pavements are not affected by ice control chemicals.

4. Building and site esthetics are enhanced.

5. Traffic noise is minimized when HMA pavement is used.

6. Pavement striping is highly visible on the black surface.

7. HMA is a flexible-type pavement.

A modern hot mix asphalt facility provides high production, good quality mixes (including recycledasphalt pavement if desired) while meeting stringentair quality standards.

3

PAVEMENT DESIGNThe design of a hot mix asphalt pavement requires someknowledge of the following:

TRAFFICSUBGRADE SUPPORTDRAINAGE

TRAFFIC

Asphalt pavements must be designed using the proper number and weight of axle loads expected during a givenperiod of time to insure adequate pavement performance. Ofprimary concern is heavy truck traffic. Pavement life can besignificantly affected by truck traffic, therefore, the weight andvolume of the heaviest traffic is a principal factor in pavementdesign. The following traffic assumptions have been made forthe purposes of providing typical pavement sections for thefollowing:TYPICAL PAVEMENT SECTION 2—Residential Drive

Essentially limited to passenger cars.

TYPICAL PAVEMENT SECTION 3—Parking Lots & Residential or Light Duty Streets present and future traffic limited to passenger cars plus normal service trucks.

TYPICAL PAVEMENT SECTION 4—Minor Arterial & Light Industrial Streets present and future traffic limited to a medium duty classification which includes some heavy truck and bus traffic.

TYPICAL PAVEMENT SECTION 5—Principal Arterial,Commercial or Industrial Roads present and future traffic is a heavy duty classification.

DRAINAGEProper drainage is imperative in the design and constructionof hot mix asphalt pavements.

Where high water tables occur or where water may accumulate in low areas, consideration must be given to subsurface drainage. The installation of underdrains and/orinterceptor drains may be required to prevent the accumulation of water beneath the pavement structure.

Good surface drainage is also essential. A minimum slope orcrown of 1.5% per foot is recommended. The roadway shoulder or adjacent ground should be graded so that surfacedrainage runs away from the pavement and does not stand onthe pavement's edge.

On large parking lots, sloped sections, catch basins, and stormsewers may be necessary for proper drainage.

SUBGRADE SOILSA FIRM AND UNYIELDING SUBGRADE IS ESSENTIAL TOGOOD PAVEMENT CONSTRUCTION.For the designs recommended in this manual all soils aredivided into three classes: Good (G), Medium (M) and Poor (P).

California Bearing Ratio (CBR) design values are assigned foreach class.

It is desirable to use laboratory tests to evaluate the load-supporting characteristics of subgrade soils. However, if laboratory test equipment is not available, designs may bemade on the basis of a careful field evaluation by an engineerwho can assign the subgrade soils to one of the following categories:

(G) GOOD SUBGRADE SOILS—Good subgrade soils retain a substantial amount of their load bearing capacity when wet. Included are the clean sands and sandy gravels and soils free of detrimental amounts of plastic materials. A good subgrade will have a CBR value of 10 or higher.

(M) MEDIUM SUBGRADE SOILS—These retain a moderate degree of firmness under adverse moisture conditions.Included are such soils as loams, silty sands and sandy gravels containing moderate amounts of clay and fine silt.A medium classification will have a CBR value of 6 through 9.

(P) POOR SUBGRADE SOILS—These soils become quite soft and plastic when wet. Included are those soils havingappreciable amounts of clay and fine silt. The coarser silts and sandy loams also may exhibit poor bearing properties in areas where frost penetration in to the subgrade is a factor. A poor classification will have a CBR value of 3 through 5. Soils with a CBR value of less than 3 should be stabilized or removed and replaced using suitable material.A stable subgrade is always required.

California Bearing Ratio (CBR) is a test used for evaluatingbases, subgrades and subbases for pavement thickness design.The correlation chart below may be used when one of thesystem values indicated is known.The Asphalt Institute's "Soil Manual for Design of AsphaltPavement Structures" Series No. 10 (MS 10), describes in detailthe commonly used soil evaluation systems and test proce-dures. Field evaluation of the soil involves visual inspectionand simple field tests.

TABLE ICorrelation Chart Soil Classification—Strength

4

SPECIFICATIONS FOR

HOT MIX ASPHALT

MARYLAND HOT MIX ASPHALT COMPOSITIONS

There are a wide variety of hot mix asphalt compositions usedin Maryland that vary from fine to coarse in size and gradation. Furthermore, Maryland hot mix asphalt mixturesutilize different aggregates or combinations of aggregates, theprincipal types being crushed stone, crushed slag, sand, graveland bank run gravel. Good economics dictate the use of localaggregates.

The most widely used hot mix asphalt specifications inMaryland are those contained in the State HighwayAdministration Standard Specifications for Construction andMaterials dated October 1993 including revisions and additions thereto.

The Maryland Asphalt Association recommends the use ofthese specifications.

A copy of the State Highway Administration StandardSpecifications for Construction and Materials can be obtainedfrom the Administration located at P.O. Box 717, Baltimore,Maryland 21203.

ASPHALT PAVING MIXTURES

The pavement design requires the proper hot mix asphaltpaving mixtures for the base and surface pavements.

It is important that a Job Mix Formula for the paving mixture be established for and approved by the engineer.A reputable hot mix asphalt producer, providing standardmixes, can readily furnish the engineer with a written Job Mix Formula.

The asphalt mixtures referred to in this manual are MarylandState Highway Administration Job Mix Designs which are readily available throughout Maryland and have proventhrough extensive use to have all of the desirable characteristics of good asphalt pavements.

BASE DESIGNATIONSMarshall Mixes SuperpaveBC (Base—Coarse) 25 MM

Generally a minimum lift thickness of 2 l/2" is recommended.Aggregate sizes of the mix design should be checked prior to use in thinner lifts.

BF (Base=Fine) 19 MMGenerally a minimum lift thickness of 2" is recommended.Aggregate sizes of the mix design should be checked priorto use in thinner lifts.

SURFACE DESIGNATIONSMarshall Mixes SuperpaveSC (Surface—Coarse) 12.5 MM

This surface mix has a high stability designed for use on roadways with:Average Daily Traffic(ADT)—greater than 5,000 Recommended minimum lift thickness 2".

SC (Surface—Coarse) Type A & B 19 MM These surface mixes have a high stability designed for highvolume roadways:

ADT—greater than 15,000or

Heavy truck traffic or

Highways with 55 mph speed limit SC Type B uses a polymer modified asphalt binder.Recommended minimum lift thickness 2”.

SF (Surface—Fine) 9.5 MMThis surface mix is recommended for all situations that do not meet the SC criteria.Recommended minimum lift thickness 1".

SNThis surface mix was previously specified by Maryland State Highway Administration; however, it has been replaced by the SC mix referred to above. The SN mix is still occasionally produced; however, we do not recommend this mix. In order to obtain high stability requirements, the SC mix has fewer fine aggregates; therefore, resulting in a coarse surface.

SMA (Stone Matrix Asphalt) This is a high stability mix designed for specific conditionssuch as high volume Interstate Highways.

SR (Surface Rehabilitations)This mix is designed primarily for thin lift leveling or correcting minor deviations. It is a fine mix that can be placed in lifts as thin as 1/2". It should not be placed in lifts exceeding 3/4" and should not be specified as a final wearing course.

SPECIFICATIONSMaryland State Highway Administration StandardSpecifications for Construction and Materials, dated

5

October 1993, including revisions and additions thereto,contain the following sections for Hot Mix Asphalt Pavementand Asphalt Cements and Hot Mix Asphalt:

Section 504 —Hot Mix Asphalt PavementSection 505 —Hot Mix Asphalt PatchesSection 508 —Milling Existing Hot Mix Asphalt

PavementSection 901 —AggregatesSection 904 —Asphalt Cements and Hot Mix

AsphaltSection 915 —Production PlantsSection 915.02 —Hot Mix Asphalt Plants

TACK COATA tack coat of asphalt (usually emulsified asphalt) is applied toensure bond between the existing surface and the asphaltoverlay. It should be applied in an even, thin coat at a rate ofapplication from .01 to .05 gal/yd2. Excessive tack coat cancause slippage or can flush to the surface.

SUBGRADE SPECIFICATIONS

Topsoil, large rocks and other types of low quality, unsuitablesoil shall be removed and replaced. The subgrade must beproperly shaped to the desired sections and elevation andshall be compacted so that it is firm, hard and unyielding. Afirm and unyielding subgrade is essential to good pavementconstruction. It shall be compacted to 95% of dry weight density as determined by MSHTO Designation T- 18O, MethodC or D as applicable. (Note:There are areas in Marylandwhere, due to the type of soil, it is not possible to attain 95%of dry weight density using the T- 180 Method. In these areasthe compaction requirements shall be 95% of the dry weightdensity as determined by AASHTO Designation T-99 Method.)To prevent growth of weeds, the subgrade should be treatedwith an approved herbicide. (See NAPA Publication IS-51)

ENGINEERING SERVICESThe Maryland Asphalt Association welcomes the opportunityto work with architects, consulting engineers, public agencyofficials, owners, and industry engineers in the preparation ofpavement specifications, construction details and constructioncontrols toward the end of quality asphalt pavement installations. The Association has professional engineering services available for this purpose. The Associations addressand telephone number appear on page 1 and on the backcover. There are three other sources where information and guidance on asphalt pavement construction may be obtained:

National Asphalt Pavement Association5100 Forbes BoulevardLanham, Maryland 20706-4413(301) 731-4748 FAX(301) 731-4621

The Asphalt InstitutePO. Box3372 E. Harrisburg StreetDillsburg, Pennsylvania 17019Carlos Rosenberger, District Engineer(717) 432-5965

The Asphalt InstitutePO. Box 14052Lexington, Kentucky 40512-4052(606) 288-4960

ESTIMATING QUANTITIESFor a cursory estimate of the quantity of material required,use—1 ton of hot mix asphalt covers 17 square yards, l" thick.

6

Hot mix asphalt recycling is a proven technology. It is costeffective, meets all specifications and the quality of the recycled asphalt pavement has been proven to be equal toconventional mixes. When using reclaimed asphalt pavement(RAP) in HMA, the virgin aggregate and asphalt are reduced,therefore significant savings can be realized. In addition theuse of rap\reclaimed asphalt materials aids the environment.Recycling may include the use of glass, plastic, asphalt shingles, and paper.

The use of reclaimed material is less expensive than using all virgin aggregate and asphalt. Cold milling existing asphalt pavements allow desired section and grade to be established. It can also be used to maintain adesired height of curb when overlaying existing roadways. Existing pavements may be removed by the coldmilling process.

Removal of badly deteriorated pavements, surface irregularities and maintaining curb reveal, etc. can be economically accomplished with minimal disruption to trafficusing the cold milling or carbide grinding process. The RAPobtained through these processes can be recycled in a costeffective rehabilitation process. (Maryland State HighwayAdministration Standard Specifications for Construction andMaterials—Section 508—Milling Existing Hot Mix AsphaltPavement)

HOT MIX ASPHALT RECYCLING

7

SIDEWALKS, GOLF CART PATHS,

BIKE PATHS

SUBGRADESubgrade soils must be evaluated to determine the load-supporting characteristics. Refer to page 4.The subgrade must be properly shaped to the desired section and elevation and shall be compacted. Refer to page 6.

DRAINAGEGood subsurface and surface drainage is essential to quality hot mix asphalt pavements. Refer to page 4.

HOT MIX ASPHALT CONSTRUCTION

A single course of 9.5 mm mixture is recommended due tothe confined areas generally involved where constructingSidewalk, Golf Cart Paths or Bike Paths except where poorsubgrade is encountered. Hot mix asphalt shall be composedof, mixed and constructed in accordance with Maryland StateHighway Administration Standard Specifications forConstruction and Materials, October 1993 including revisionsand additions thereto.

CONSTRUCTION DETAILS

SubgradeClass

Good Medium

Poor

Subgrade

Single Course

CompactedThickness

MixtureDesignationConvential

Superpave

3”

4”

SF Band

SC Band

9.5 MM

12.5 MM

Subgrade

Single Course

8

* All 12.5 MM and SC Surface must be placed by a paver.

RESIDENTIAL DRIVES,

PLAY AREAS

PAVEMENT WIDTH Residential driveways for a single automobile, the pavement should be a minimum 10' in width.The minimum radius for a right angle where the drivewaymeets the street is 6'.

SUBGRADESubgrade soils must be evaluated to determine the load-supporting characteristics. Refer to page 4.Top soil must be removed and subgrade properly shaped tothe desired section and elevation and shall be compacted.Refer to page 6. Treat the subgrade with an approved herbicide to inhibit future weed growth.

AGGREGATE BASE CONSTRUCTIONIn certain situations graded aggregate base construction maybe desirable. See "Pavement Base Course Equivalency" page15 for recommended thickness.

DRAINAGEGood subsurface and surface drainage is essential to qualityhot mix asphalt pavements. Refer to page 4.

TACK COATRefer to page 6.


Hot mix asphalt shall be composed of, mixed and constructed in accordance with Maryland State HighwayAdministration Standard Specifications for Construction andMaterials, October 1993 including revisions and additionsthereto.

The hot mix asphalt base may be placed in a single course tothe required thickness. Small mechanical pavers are available for this type of construction. When ever possible,hand placement should be avoided. However, where accessto the work area is limited, hand placement may be the onlyfeasible method.

SUGGESTIONS FOR OBTAINING A QUALITY RESIDENTIAL DRIVEWAY PAVEMENTSelect contractors known for quality work and established reputations in the community.Insist on a written contract or agreement. Specify materials and workmanship in accordance with Maryland State HighwayAdministration Specifications. Specify a minimum compacted thickness.


SubgradeClass

Good Medium

Poor

Base Course

Subgrade CBR 6-10 or Greater

Surface Course

CompactedThickness


1 1⁄2”

2 1⁄2”

1 1⁄2”

Base Course

Subgrade CBR 3-5

Surface Course

3 1⁄2”

SF Band

BF Band

SF Band

BF Band

9.5 MM

19 MM

9.5 MM

25 MM

Superpave

9

PARKING LOTS, RESIDENTIAL

OR LIGHT DUTY STREETS,

STORAGE AREAS

SUBGRADE

Subgrade soils must be evaluated to determine the load-supporting characteristics. Refer to page 4.

The subgrade must be properly shaped to the desired section and elevation and shall be compacted. Refer to page 6.

DRAINAGE

Good subsurface and surface drainage is essential to quality hot mix asphalt pavements. Refer to page 4.

TACK COAT

Refer to page 6.

HOT MIX ASPHALT CONSTRUCTIONHot mix asphalt shall be composed of, mixed and constructed in accordance with Maryland State HighwayAdministration Standard Specifications for

Construction and Materials, October 1993 including revisions and additions thereto.

The hot mix asphalt base may be placed in a single course. Atotal thickness of up to 6" of base mix may be placed in onelift. If several courses are desired or if job specificationsrequire several courses, it is recommended that the first basecourse be 3" minimum depth.


SubgradeClass

Medium

Poor

Base Course

Subgrade CBR 10 or Greater

Surface Course

CompactedThickness


Superpave

1 1⁄2”

3 1⁄2”

2”

Base Course

Subgrade CBR 3-5

Surface Course 2”

5”

Base Course

Subgrade CBR 6-9

Surface Course

4”

Good SF Band

BF Band

SF Band

BF Band

SF Band

BC Band

9.5 MM

19 MM

9.5 MM

19 MM

9.5 MM

25 MM

10

MINOR ARTERIAL AND LIGHT

INDUSTRIAL STREETS

SUBGRADE


The subgrade must be properly shaped to the desired sectionand elevation and shall be compacted. Refer to page 6.

DRAINAGE

Good subsurface and surface drainage is essential to qualityhot mix asphalt pavements. Refer to page 4.

TACK COAT

Refer to page 6.


Hot mix asphalt shall be composed of, mixed and constructedin accordance with Maryland State Highway AdministrationStandard Specifications for Construction and Materials,October 1993 including revisions and additions thereto.

The hot mix asphalt base may be placed in a single course.Atotal thickness of up to 6" of base mix may be placed in onelift. If several courses are desired or if job specifications require several courses, it is recommended thatthe first base course be 3" minimum depth.


SubgradeClass

Medium

Poor

Base Course


Surface Course

CompactedThickness


Superpave

2”

4”

2”

Base Course

Subgrade CBR 3-5

Surface Course 2”

6 1⁄2”

Base Course

Subgrade CBR 6-10

Surface Course

5”

Good SC Band

BF Band

SC Band

BC/BF Band

SC Band

BC/BF Band

12.5 MM

19 MM

12.5 MM

25 MM

12.5 MM

25 MM

11

PRINCIPAL ARTERIAL,

COMMERCIAL OR

INDUSTRIAL ROADS

SUBGRADE

Subgrade soils must be evaluated to determine theload-supporting characteristics. Refer to page 4.


DRAINAGE

Good subsurface and surface drainage is essential to quality hot mix asphalt pavements. Refer to page 4.

TACK COAT

Refer to page 6.


Hot mix asphalt shall be composed of, mixed and constructed in accordance with Maryland State HighwayAdministration Standard Specifications for Construction andMaterials, October 1993 including revisions and additionsthereto.

The hot mix asphalt base may be placed in a single course. Atotal thickness of up to 6" of base mix may be placed in onelift. If several courses are desired or if job specificationsrequire several courses, it is recommended that the firstcourse be 4" minimum depth.


SubgradeClass

Medium

Poor

Base Course


Surface Course

CompactedThickness


Superpave

2”

5”

2”

Base Course

Subgrade CBR 3-5

Surface Course 2”

8”

Base Course

Subgrade CBR 6-9

Surface Course

6”

Good SC Band

BF Band

SC Band

BF/BC Band

SC Band

BC Band

12.5 MM

25 MM

12.5 MM

25 MM

12.5 MM

25 MM

12

TENNIS & MULTI-USE COURTS,

RUNNING TRACKS

SUBGRADE



TACK COAT

Refer to page 6.


Hot mix asphalt shall be composed of, mixed and constructedin accordance with Maryland State Highway AdministrationStandard Specifications for Construction and Materials,October 1993 including revisions and additions thereto.

A special mix other than SF or 9.5 mm may be used for the surface course for running tracks.There are severalresilient hot mixes available which combine rubber aggregate with regular aggregate plus a high asphalt con-tent. Specifications and technical assistance are available from the firms that furnish the special aggregate.

The hot mix asphalt may be placed with a mechanical paveror by hand, if the contractor elects. In either case, for tenniscourt construction, the finished course must not vary fromline, grade or cross-section more than l/8" in 10' when measured in any direction. For running tracks, the finishedcourse must not vary more than 1/4" in 10'.

Proprietary surfacing is available for tennis courts and running tracks. Most members of The Maryland AsphaltAssociation are familiar with the various materials and canprovide information on them or it may be necessary to contact the manufacturer directly.

DRAINAGE

The finished tennis court surface should slope l" in 10', 0.83%,on a true plane from end to end, corner to corner, or side toside. Side to side is preferable for a playing surface and forconstruction. The surface should not slope to or away fromthe net in two directions.


SubgradeClass

Good Medium Leveling Course

Base Course

Surface Course


CompactedThickness


1”

1”

3”

SF Band

SF Band

BF Band

Super Pave

9.5 MM

9.5 MM

19 MM

Poor Leveling

Base Course

Surface Course

Subgrade CBR 3-5

1”

1”

3”

SF Band

SF Band

BF Band

9.5 MM

9.5 MM

19 MM

13

HOT MIXASPHALT

RESURFACING

Asphalt overlays are the most common, practical andeconomical way to salvage, strengthen and modernizeold pavements.

Properly designed, constructed and maintained hotmix asphalt pavement resurfacings offer the followingadvantages:

1. Strengthen existing pavement structurally2. Reduce maintenance appreciably3. Extend service life appreciably4.Allows use of roadway while improvements are in progress5 Provides a better pavement than original6. Provides safer pavements7. Improve ride quality and appearance8. Considerable savings in energy products

RESURFACING CONSIDERATIONS

The key to successful resurfacing is careful evaluation of theexisting pavement and the selection of an adequate pavement design. The following information should be considered: 1. Physical condition including surface conditionand structural adequacy. 2.Type of construction of existingpavement 3. Drainage of existing pavement 4. load and trafficuse (weight and volume of heaviest traffic)

The thickness of the overlay is designed to improve a lower-than-average pavement condition, but not to provide the extrastrength needed for localized, structurally weak areas. Shouldthe overlay thickness be based on the weakest condition inthe section, it would be overdesigned for the rest of the sec-tion, and thus, needlessly costly. The proper procedure,therefore, is to correct the weaker areas. Removal of structurally weak areas and patching with full-depth hot mix

asphalt to ensure strength equal to or exceeding the surrounding pavement is an economical means of preparingfor overlays.

A minimum thickness of 41⁄2" of hot mix asphalt is recommended for rigid pavement overlays.

FABRIC

Fabrics are available for use with HMA overlays. When placedbetween the original pavement and the HMA overlay orbetween two courses of the HMA overlay, it becomes an integral part of the roadway section. When properly used,fabrics reduce water infiltration and reflective cracking.There are several proprietary fabrics available which shouldbe installed in accordance with the manufacturer's instructions.

TACK COAT

A tack coat of asphalt (usually emulsified asphalt) is applied toensure bond between the existing surface and the asphaltoverlay. It should be applied in an even, thin coat at a rate ofapplication from .01 to .05 gal/yd2. Excessive tack coat cancause slippage or can flush to the surface.


Base Course

Leveling (when required)

Surface Course

Old Pavement

AverageCompactedThickness


1 1⁄2”

Design Thickness

1”= (variable)

SF Band

BC, BF Band

SF Band

Superpave

9.5 MM

19.5MM, 25MM

9.5 MM*

Leveling Course

Surface Course

Old Pavement

1 1⁄2”

1”= (variable)

SF Band

SF Band

9.5 MM

9.5 MM

* If Wedge and Level course exceeds 3” you may consider going to SC or12.5 mm W/C course.

14


Hot mix asphalt shall be composed of, mixed and constructed in accordance with Maryland State Highway AdministrationStandard Specifications for Construction and Materials, October 1993 including revisions and additions thereto.

EQUIVALENCIES

Consideration may be given to a pavement section using agraded aggregate base course with a hot mix asphalt binderand surface course. Maryland has several excellent aggregatesources that may make locally available aggregate an economic alternative. All graded aggregate base should meetMaryland State Highway Administration Specifications forConstruction and Maintenance, October 1993 including revisions and additions thereto.

Paving materials have different abilities to sustain the loads towhich they are subjected. Relative strength values of the various materials used in asphalt pavements have been evaluated so that it is possible to develop designs with different components to give comparable pavement performance. The AASHTO Design Committee from road testfindings established a list of coefficients of pavement components for surface, base and subbase courses. The use ofthese coefficients permits equating materials into inches ofone component to inches of another in the asphalt pavementstructures.

The criteria established in the AASHTO Guide for Design ofPavement Structures,The Asphalt Institute's "Thickness DesignManual" MST,The National Asphalt Paving Association's "A

Guide to Thickness Equivalencies for Design of AsphaltPavements" IS20, and The Maryland Asphalt Association's experience with millions of square yards of pavements constructed in Maryland were considered and utilized in partto develop the pavement designs and material equivalenciesused in this manual.

Asphalt pavements must retain a minimum thickness of hotmix asphalt surfacing (including binder courses) when usedon granular bases. Minimum surface thicknesses should beincreased with increased weight and volume of traffic andwhen used on poor subgrades. For example, usage similar toTypical Pavement Section 5 should have a minimum thicknessof 4" hot mix asphalt pavement on a granular base.

PAVEMENT BASE COURSE EQUIVALENCY

ONE INCH HOT MIX ASPHALT BASE COURSE EQUALS 21⁄2''CRUSHER RUN OR GRADED AGGREGATE BASE. ONE INCHOF HOT MIX SAND ASPHALT BASE COURSE EQUALS 2"CRUSHER RUN OR GRADED AGGREGATE BASE.


Old Pavement

Surface Course

AverageCompactedThickness

MixtureDesignation Superpave

2”

11⁄2”

SC Band 12.5 MM

SF Band 9.5MM

Old Pavement

Surface Course

15

OTHER USES FOR

HOT MIX ASPHALT

There are a wide variety of uses for hot mix asphalt mixesand/or pavements that are not listed or shown elsewhere inthis guide. The same basic principle used for constructingsuccessful pavements indicated previously in this guidewould apply to installing pavements for other uses. Somepavement or mix modifications might be required, but gener-ally, such are of a minor nature. Some of the other uses forthe product follow:

AIRFIELDSRunwaysTaxiwaysHolding AreasHelioports

AUTOMOBILE RACE TRACKS

DRAINAGE FACILITIESDitches & GuttersCurbsBermsSlope Protection

EROSION CONTROLJetties & GroinsGrout for Jetties

FARMBarnyardsFeed lotsStable FloorsSilage Storage

HYDRAULICSReservoir LiningsPond LiningsDam FacesCanal LiningsSewage Lagoons

INDUSTRIAL-COMMERCIALDock SurfacesBuilding FloorsMaterial Storage Areas

Hot MixAsphalt is used extensively for runway and taxiwaypaving.The full-depth asphalt pavement runways and taxiwayat BWIAirport have provided excellent service with low main-tenance cost since their construction in the 1950s.

Sandy Hook Go-Cart Speedway Road Course.

16

NAPA publications are available for specialized mix design and placement techniques.

PATCHINGMaintenance For All Types of Pavements

RAILROADSHMA Ballast UnderlaymentFull Depth Hot Mix Asphalt

RECREATIONBoat RampsFootball FieldsSkate board and roller blade facilities

MARYLAND ASPHALT ASSOCIATION, INC.

2408-G Pepper Mill Drive

Glen Burnie, Maryland 21061

410-761-2160 • Fax 410-761-0339

E-mail: [email protected]

hot mix asphalt pavement design guide

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