Table 13-4.-Penetration Grades and AP Numbers of Asphalt Cementbituminous cement are to hold the aggregate particlestogether and to seal the surface, which then resists thepenetration of water.Bituminous cements are available in several formssuitable for different procedures of mixing orapplication under wide variations in temperature. Someasphalts and tars are solid or semisolid at roomtemperature. Other grades are relatively viscous (thick)liquids at room temperatures, and all become liquid athigher temperatures. Mixing bitumens with petroleumsolvents or water produces cutbacks or emulsionsthatbecome liquid at atmospheric temperatures. Such liquidasphalts and tars are used for cold mixes or applied assprays in building pavements.ASPHALTS.— Asphalts may be natural or manu-factured and they maybe solid, semisolid or liquid inconsistency. Natural asphalts occur in lakes (as lakeasphalt), pits, or rock structures (as rock asphalts).Asphalt cement is one of the by-products from therefining of crude petroleum.Generally, the military engineer depends upon themanufactured asphalts that are obtained when crudepetroleum is refined for the purpose of separating thevarious fractions (fig. 13-26). The crude oil vapors areseparated into gasoline, kerosene, and fuel oils, and theresidue is asphalt cement and lubricating oils. Thelonger the process and the higher the temperatures, theharder the residue becomes because of the increased lossin volatiles.Asphalt cement is commercially available indifferent standard ranges of consistency (grades). Theranges for the penetration grade are based onmeasurements by the penetration test in which therelative hardness of asphalt cement is determined by thedistance that a standard needle, under a standardloading, will penetrate a sample in a given time underknown temperature conditions. The asphalt petroleum(AP) number is a number from 00 to 7 that is assignedto these penetration ranges. Table 13-4 lists the rangespresently recognized along with relative consistenciescorresponding to those ranges.Asphalt cement is also graded on the basis ofviscosity, using special testing equipment (not in theNaval Construction Force Table of Allowance) tomeasure the time that a given amount of liquid asphaltmaterial will flow through a tube of standard dimensionsunder rigidly controlled temperature and pressureconditions. Multiplying that measured time by acalibration factor for the equipment gives a numericaldesignation called kinematic viscosity, measured instokes (square centimeters per second) or centistokes(stokes 100).The viscosity grades of asphalt cement are availablein two series. One series includes grades AC-2.5, AC-5,AC-10, AC-20, and AC-40. The other series includesgrades AR-1000, AR-2000, AR4000, AR-8000, andAR-16000. Normally, but not always, the lowerviscosity-graded asphalts correlate with the softerasphalts having higher penetration values, and thehigher viscosity-graded asphalts correlate with thelower penetration grades.Since all asphalt cements are solid or semisolid atroom temperature (77°F), they must be converted in oneof three ways to a fluid state before they can be pumpedor sprayed through pipes or nozzles and be mixed withaggregate. One way to liquify an asphalt cement is toheat it. Then, when it cools, it becomes a semisolidcementing material. The other methods to liquifyasphalt cement are dissolution (producing cutbackasphalt) and emulsification (producing emulsifiedasphalt).CUTBACKS.– When asphalt cement is dissolvedin volatile petroleum solvents (called cutterstock orflux oils), the resulting liquid is known as cutbackasphalt. The idea behind cutback asphalt is that upon13-36
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