chapter 3 design considerations - apai · 1. traffic loading (volume and weight) 2. soil-support...

Design Considerations 3-1

Chapter 3Design 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 and

asphalt)

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 specificdesign 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 pathÕs 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., drivewaysand residential streets) provides the designerwith a background for selecting acceptablevalues.

For the users of this Design Guide, much ofthe design work has been done Ð design chartsare presented for selecting pavement thick-ness. Traditionally, many designers grouppavements according to use and Òuse tablesÓare commonly applied throughout the UnitedStates. Chapter 4 provides design tables byspecific 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, thevolume of traffic and the various types ofvehicles using the facility must be estimatedfor the pavementÕs 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). Thisparameter is defined as the equivalent numberof applications of an 18,000-pound, single-axleload during the pavementÕs 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.

Design Considerations3-2

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


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 entrancesAirports - 60,000 pound maximum gross

weight

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 InstituteÕs publications ThicknessDesign-Asphalt Pavements for Highways and Streets (MS-1) and Asphalt PavementThickness Design (IS-181).


SOIL SUPPORT CAPABILITY

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

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

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

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.

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.


Table 3-1. Classification of Soils and Soil-Aggregate Mixtures (With Suggested Subgroups)

Silt-Clay Materials

General Granular Materials (More than 35% of Total Sample

Classification (35% or Less of Total Sample Passing No. 200) Passing No. 200)

A-7

A-1 A-3 A-2 A-4 A-5 A-6 A-7-5

Group A-7-6

Classification A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7

Sieve Analysis,

percent passing:

No. 10 50 max

No. 40 30 max 50 max 51 min

No. 200 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min

Characteristics of

fraction passing

No. 40:

Liquid Limit 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min

Plasticity Index 6 max NP 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min

Usual Types of

Significant

Constituent Stone Fragments, Fine

Materials Gravel & Sand Sand Silty or Clayey Gravel and Sand Silty Soils Clayey Soils

General Rating

As Subgrade Excellent to Good Fair to Poor

The state of Iowa has been subdivided intosurface soil areas that reflect their engineeringsoil classification. The accompanying figureprovides a general soil classification map ofIowa. In addition, soil bulletins with morecomplete and detailed descriptions of soiltypes are available for each Iowa county. Notethat these maps provide only a generalgrouping of a range of soils. Local spots mayvary considerably.


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 pavementstructure 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.


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.

GoodGood subgrade soils retain a substantial

amount 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 a No. 200 mesh sieve. A soil classified as goodwill have a CBR value of 9 or greater.

ModerateModerate 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.

PoorPoor subgrade soils are those that become

quite soft and plastic when wet. Included arethose soils having appreciable amounts of clayand fine silt (50 percent or more) passing a No. 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 aspoor 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 touse, and the application procedure depend onthe soil classification and the subgrade-support value desired. These should bedetermined through appropriate laboratorytesting.


DRAINAGE

General ConsiderationsHighway 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 issaturated, 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 pavementÕsasphalt layers and dynamic, repeated trafficloading 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 thesystemÕs 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 Ðsurface 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 andfrom 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.

Figure 3-3.

required. Identification of these areas anddetermination of drain locations require thetechnical expertise and insight of an engineer.The choice of drain filter material and thedesign of the drainage system must be givencareful attention by experts. Perforated andslotted pipe usually serve to move the freewater from the trouble spot to a drainage area.

Check Drainage During ConstructionRegardless of the care used in the pre-

liminary investigation, the soil survey, and inthe pavement structureÕs design, it is usuallynot possible to determine from borings theexact elevation of water-bearing strata or therate of flow that will develop. For this reason,it is essential that the engineer reevaluate theconditions and check the need for, and theadequacy of, any subsurface drainage indicat-ed on the plans.

Soil conditions should be observed duringthe grading and subgrade preparation work.Any wet, soft, or spongy areas encountered atgrade should be investigated and provisionsmade for their proper drainage. Even a minorrate of seepage may build up to a largequantity of water over a period of time if ameans of escape is not provided. Such a softspot usually forewarns of a structural failure ata later date-even shortly after traffic has usedthe new facility. After the pavement is in place,corrective measures are costly, create trafficproblems, and can cause poor public relations.

Good design practices will provide cross-slopes both on the surface and in theunderlying pavement courses and subgrade.In this way, water will not accumulate but willflow laterally to the sides.

Subsurface DrainageSubsurface water is free water that

percolates through, or is contained in, the soilbeneath the surface. When it emerges orescapes from the soil, it is referred to asseepage water. The point of emergence iscalled a seepage area or a spring.

Pavement subsurface water usually ispresent as free water that flows under the forceof gravity or as capillary water that movesunder capillary action in the soil.

Water will rise from the underlying soilthrough the subgrade and into an untreatedaggregate pavement course. This free waterwill move readily into an untreated aggregatebase to a low point on the profile. If steepgrades are present, and the subsurface waterflowing in an untreated aggregate base to thelow spot is not intercepted, a hydrostatic headmay result. This lifting force will cause afailure of the pavement structure. Water in thepavement courses also may contribute to thestripping of asphalt films from the aggregateparticles.

SubdrainsWhen water collects in the structural

elements of the pavement, subdrains are


DESIGN TYPES

In general, the design of a new asphaltpavement 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 ofhighway 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 wheretraffic loadings are minimal. The untreatedaggregate base is placed and compacted on the 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.


Figure 3-4 Figure 3-5

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 mixformula 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 factorssuch 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 takenfrom 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.

Aggregate for Type B. Asphalt Concrete

Aggregate for Type B asphalt concrete shallmeet the requirements as specified in Section4126, lowa DOT Standard Specifications.

Gradation: The job mix formula for the mix-ture size specified, when tested by means oflaboratory sieves, shall meet the followingrequirements:

Table 3-3. Gradation of Job Mix: Type BAsphalt Concrete

Percent PassingClass 1 and 2*Mixture Size

SieveSize 1 inch 3/4 inch 1/2 inch 3/8 inch

1-1/2 inch 100

1 inch 92-100 100

3/4 inch 77-92 98-100 100

1/2 inch 60-80 76-95 92-100 100

3/8 inch — 60-88 70-94 98-100

No. 4 34-55 42-70 50-75 63-89

No. 8 20-38 30-56 36-60 44-68

No. 30 7-20 14-32 16-34 20-37

No. 200 2-7 3-7 3-7 3-7

Asphalt Cement Content: 5-7%

AC-5 grade recommended for Type B BaseAC-10 grade recommended for Type B Surface*Class 1 - 30 percent crushed particles (minimum)*Class 2 - no minimum percentage of crushed particles50 - Blow Marshall mix criteria normally specified


Aggregate for Type A Asphalt Concrete

Aggregate for Type A asphalt concrete shallmeet the requirements as specified in Section4127, lowa DOT Standard Specifications.

Gradation: The job mix formula for themixture size specified, when tested by meansof laboratory sieves, shall meet the followingrequirements:

Table 3-4. Gradation of Job Mix: Type AAsphalt Concrete

Percent PassingMixture Size

Sieve Size 1 inch 3/4 inch 1/2 inch 3/8 inch

1 -1/2 inch 100

1 inch 92-100 100

3/4 inch 77-92 98-100 100

1/2 inch 60-80 76-95 92-100 100

3/8 inch — 60-88 70-94 98-100

No. 4 37-58 42-70 50-75 63-89

No. 8 28-44 30-56 36-60 44-68

No. 30 13-27 14-32 16-34 20-37

No. 200 3-7 3-7 3-7 3-7

Asphalt Cement Content: 5-7%

AC-10 grade recommended for Type A Concrete*60 percent crushed particles (minimum)50 - Blow Marshall mix criteria normally specified

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 ConcreteType 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 highvolume 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 thickness and 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 aggregatepercentages, percent asphalt, and gradation aslimited by specified tolerances for eachcontrolling sieve size.

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 Asphalt ConcreteRecycled asphalt concrete mixtures are

composed 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 Asphalt ConcreteType 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.


CONSTRUCTION OF ASPHALT PAVEMENTS

Construction EquipmentIt is the responsibility of the contractor to

provide equipment that will produce results incompliance with the plans and specificationsof the contract. The following section containsinformation on the basic equipment used toproduce and construct Asphalt Concretepavements.

Asphalt Mixing FacilitiesThe mixing facility produces the Asphalt

Concrete mixture placed as pavement. Thefacility should be designed and coordinated toproduce mixtures within specified job mixtolerances.

Asphalt storage tanks must have a device forthe controlled heating of material to tempera-ture requirements as specified. Heating shouldbe accomplished so that no flame will come incontact with the tank. The circulating systemshould be large enough to ensure proper andcontinuous circulation of asphalt betweenstorage tank and mixer during the entireoperating period. While the pump is inoperation, the discharge end of the circulatingpipeline should be kept below the surface ofthe asphalt in the tank.

The facility should have an accurate meansfor feeding the aggregate into the dryer toensure uniform production and a constanttemperature. The facility should contain arotary drum dryer that will continuouslyagitate the aggregates during the heating,drying, and mixing processes.

For batch mixing processes, screens may bepositioned over the hot aggregate storage binsto separate all aggregates to sizes required forproportioning to meet the job mix formulae.Where no such screens are used, proportioningmust be handled as part of the cold-feedsystem.

If drum mixers or continuous mixing plantsare used in the production of mixes, approvedmaterials must be fed into the cold-feed systemin the proper proportions to meet the job mixformulae.

The facility must have the means to obtainthe required percentage of asphalt in the mixwithin the tolerance specified. This can beaccomplished by weighing, metering, ormeasuring volumetrically. Steam jacketing orother insulation should maintain the specifiedtemperature of asphalt in pipelines, meters,buckets, spray bars, flow lines, and othercontainers.

A thermometer ranging from 200¡F to 400¡Fshould be placed in the asphalt feed line ortank.

The facility should have a dust collector, amixer cover, and whatever additional housingnecessary to ensure proper dust control.

Adequate and safe stairways to the mixerplatform and guarded ladders to other units ofthe facility should be provided. All gears,pulleys, chains, sprockets, and other danger-ous moving parts should be well guarded andprotected. A platform for sampling andinspection of the mix should be located nearthe facility.


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

Hauling EquipmentHaul trucks are used to bring the Asphalt

Concrete 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.

Compaction EquipmentCompaction equipment is used to compact

the 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 Ð 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 thepavement.Spreading Equipment

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 diameter that are capable of exerting average contactpressure.

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 1/4inch-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 MillingCold milling is the most common pavement

scarification 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 PracticesPreparation of Subgrade

Remove all large rock, debris, and topsoilfrom the area to be paved. All vegetation,including root systems, should be removed. Toprevent 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 areasmust be removed and replaced with suitablematerial and compacted or stablized using ageotextile. The use of Asphalt Concrete base orcourse granular material is recommended.

Constructing Asphalt Concrete BaseThe Asphalt Concrete base may consist of

one 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 4inches 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.

Untreated Aggregate BaseThe crushed aggregate base course may

consist of one or more layers placed directly onthe prepared subgrade. The material must bespread and compacted to the requiredthickness, grades, and dimensions indicated inthe 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.

Binder and Surface CoursesThe upper lifts of the pavement may consist

of one or more courses of Asphalt Concreteplaced on the previously constructed AsphaltConcrete 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.


Tack CoatA tack or bond coat of CSS-1, SS-1, MC-70 or

an approved alternate should be appliedbetween 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 UniformTraffic 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 passingzones. White lines are used to separate trafficlanes flowing in the same direction or to markthe outside edge of pavements.

Minimum GradeIt is recommended that the minimum

pavement 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 thedriverÕs attention from the roadway. Anasphalt pavement clearly has an advantage inproviding highly visible, attention-attractingmarkings Ð 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 normalline is 4 to 6 inches wide.


Figure 3-6

It is beyond the scope of this Design Guideto present these standards in more detail. Theuser should refer to these standards whenplacing pavement markings. All city andcounty engineering offices and most othertransportation engineering organizations havea copy of the MUTCD.

Traffic Control Through Work AreasThe control of traffic through work areas in a

safe and expeditious manner, while maintain-ing good public relations, is an essential part ofhighway construction and maintenanceoperations. In todayÕs litigious society, efficient

traffic control may mean the differencebetween no liability and a large financialaward should an accident occur. No agency,owner, or construction company is immunefrom alleged responsibility for an accident.

Because of the multitude of construction andmaintenance applications, it is impossible tolist the standards for signs, barricades, andmarkings in this publication. For more detailedinformation on a specific application, refer toPart VI in the MUTCD and to the Iowa DOT.

Flaggers may be neededfor safety. It is importantthat they be properlydressed and instructed inflagging standards.Flagging procedures areset forth in Part Xl of theMUTCD, and the lowaDOT providessupplemental informationand training booklets.

Figure 3-7. Use of handdevices by flagger.


Barricades are portabledevices used to controltraffic by closing,restricting, or delineatingconstruction areas. TheMUTCD specifies the typeof barricade to use for aspecific situation anddescribes barricadecharacteristics. Thediagonal stripes slopedownward either left orright from the upper tolower panel and should“slope downward in thedirection toward whichtraffic must turn indetouring.”

When a road or a site normally used by traffic is closed, it should be barricaded and signed inaccordance with the MUTCD.

Figure 3-8. Channelizing devices.


Aggregate Weighing-Job Mix Formulas Compaction Testing-Nuclear Gauge

Hot Box Sample Smoothness Testing-California Profilograph

TESTING AND INSPECTION

Inspection and testing of the production andplacement of Asphalt Concrete Ð or of anymaterial Ð is necessary to ensure a qualityproduct. Plant and field inspections include:(1) the preparation of the aggregate; (2) theAsphalt Concrete plant setup and operation;(3) the control of the Asphalt Concrete mixture;(4) the delivery and placement; and (5) thefinal finishing and compaction.

It is beyond the scope of this Design Guideto do more than emphasize the importance ofa quality testing and inspection program. TheIowa DOT publishes manuals on asphalt plantinspection and field testing that are availablethrough the DOT Storeroom. Training courseson this subject are offered by APAI and otheragencies.


chapter 3 design considerations - apai · 1. traffic loading (volume and weight) 2. soil-support...

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