Two well-known classification systems are theAmerican Association of State Highway andTransportation Officials (AASHTO) ClassificationSystem and the Unified Soil Classification System, usedby the Department of Defense.PREPARING ASPHALT FORCONSTRUCTION OPERATIONSPaving grade asphalt (asphalt cement), which atnormal atmospheric temperatures is semisolid andhighly viscous, must be made temporarily fluid(liquefied) for handling during construction operations,such as pumping through pipes, transporting in tanks,spraying through nozzles, and mixing with aggregate.When pavement construction operations are finished,the asphalt cement reverts to its normal condition andfunctions as the cementing (or binding) andwaterproofing agent that makes the pavement stable anddurable.Asphalt cement can be made temporarily fluid(liquefied) for construction operations in three ways:1. By heating the asphalt. After constructionoperations, the hot liquid asphalt cement cools andchanges from a fluid to its normal, semisolid condition.2. By dissolving the asphalt in selected petroleumsolvents. This process is called cutting back; thediluted asphalt is called cutback asphalt. Afterconstruction the solvent evaporates, leaving the asphaltcement in place.NOTE: The use of cutback asphalt in the UnitedStates has declined because of the petroleum shortageand government environmental regulations. It is beingsuperseded by emulsified asphalt, which contains littleor no solvent, and can be used for almost any purposethat cutbacks can.3. By emulsifying the asphalt with an emul-sifying agent and water.While asphalt and waterordinarily do not mix, they can be made to mix bychurning asphalt in a colloid mill. The resultingproduct, called emulsified asphalt, is a fluid and isready for construction operations. During constructionthe water and asphalt separate. The asphalt particlesmerge into a continuous film that cements the aggregateparticles together as the water evaporates. When thewater and asphalt separate, it is said that the emulsionbreaks or sets.A hot-mixed or hot-load paving mixture is the besttype of pavement; the aggregate and binder should beheated to approximately 310°F and laid no colder than16-6250°F. Determining the exact temperature(s) to use willdepend upon the weather and the distance that thematerial is hauled. Some clues that indicate thecondition of the asphalt are as follows:. Overheated asphalt loses some of its bindingqualities. Blue smoke, rising from the spreader hopper,is sometimes an indicator this condition exists.. A generally stiff appearance and impropercoating of aggregate indicates the mix is too cold.l Material laying flat in the bed of the truck with ashiny appearance means the mix is too rich in asphaltcement.. When it is too lean, the mix will look dry and dull.Prime CoatPriming consists of the initial treatment on agranular base before surfacing with a bituminousmaterial or pavement. The purpose of a prime coat is topenetrate the base (about 1/4-inch minimum penetrationis desired), fill most of the voids, promote adhesionbetween the base and the bituminous applications placedon top of it, and waterproof the base. Surfaces must beas clean as possible, and where and conditions exist(dried-out surfaces), a light fog spray with water shouldbe considered before priming actually begins.The priming material may be either a low-viscosity tar, a low-viscosity asphalt, or a dilutedasphalt emulsion. The bituminous materials, used forthe prime coat, should be applied in quantities knownas rate of application (ROA) of not less than 0.2gallon or more than 0.5 gallon per square yard.Normally, the construction project specificationsdenote the ROA for the prime coat application;however, when the ROA is not included in the projectspecifications, the NCF uses an ROA of .3 forplanning purposes. When the base absorbs all of theprime material within 1 to 3 hours or when penetrationis too shallow, the base is underpriced. Underprim-ing may be corrected by applying a second coating ofthe prime material.An overprimed base may fail to cure or set and maycontribute to failure of the pavement or bleed upthrough the asphalt mat. A free film of prime materialremaining on the base after a 45-hour curing periodindicates that the base is overpriced. This conditionmay be corrected by spreading a light, uniform layer ofclean, dry sand over the prime coat to absorb the excessmaterial. Application of the sand is usually followed by
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