Asphalt Paving Materials 2-1Asphalt Paving Materials 2-1
Chapter 2Asphalt and Asphalt Paving Materials
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. Asphaltis a constituent of most petroleums and isisolated through the refining process ofdistillation. (See Figure 2-1.)
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 mostacids, alkalis, and salts.
Covering more than 90 percent of thenations paved highways, Asphalt Concrete isthe most widely used paving material in theUnited States. For versatility, durability, andease of construction, it has no equal.
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.
Figure 2-1. Petroleum Asphalt Flow Chart forEmulsified and Cutback Asphalts.
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ClassificationsAsphalt Concrete paving aggregates are
classified according to source or means ofpreparation. A brief description of the classifi-cations follows.
Pit or Bank-Run AggregatesBoth gravel and sand are typically pit or
bank-run natural aggregates. They usually arescreened to proper size and washed to removedirt before being used for Asphalt Concretepaving purposes.
Processed AggregatesWhen natural pit or bank-run aggregate has
been 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. Crushing alsoimproves 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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Synthetic AggregatesAggregates produced by altering both
physical and chemical properties of a parentmaterial are called synthetic or artificialaggregates. 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).
2. Cleanliness. Foreign or deleterious sub-stances make some materials unsuitable forpaving mixtures.
3. Toughness. Toughness or hardness is theability 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 mixturesoverall 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 easyto 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 asphalt
ASPHALT BINDER TYPE OF CORRIDOR LOCATION TYPE OF MIX
PG 70-22 Heavy Duty Full Depth Asphalt Surface Mixture (sp125 or SMA) andClass 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 MixturesClass III-IV and Overlays
PG 64-22 Light Duty Full Depth All MixturesClass I-II and Overlays
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paving mixes unless an anti-stripping agentis used.
ASPHALT CEMENTAsphalt is produced in a variety of types
and 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.
Table 2-1. Asphalt Types, Characteristics and General Uses
Percent FlashAsphalt 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
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 Mix
MC-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 whenheating 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 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 formAsphalt Concrete.
The aggregates and asphalt are combined inan 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