Table 8-4.-Asphalt Emulsions According to Their Electric ChargeGrades of Asphalt CutbacksWhen a great amount of cutterstock is added to agiven amount of asphalt cement, a very thin liquidresults. Viscosity grade is a measure of the relativeconsistency of an asphalt bitumen after cutterstock isadded to a fixed amount of it. The grade is designatedby a number that corresponds to the lower limit of theviscosity of asphalt cutback as determined by a standardtest. The upper limit of viscosity is defined as twice thelower limit.The viscosity grades of RC, MC, and SC are 70(70-140), 250 (250-500), 800 (800-1,600), and 3,000(3,000-6,000). The numbers in parenthesis are the lowerand upper limits of viscosity, In addition, MC has a grade30(30-60). The grade ranges are 30 (most fluid) to 3,000(least fluid).Uses of Asphalt CutbacksDifferent types and grades of asphalt cutbacks areused to meet various climate conditions for differenttypes of pavement. Asphalt cutbacks are usually usedfor prime/tackcoats and for bituminous surfacetreatments. The prevailing atmospheric temperaturesexisting during construction projects are a major factorin selecting the grade of asphalt cutback. The heaviergrades are preferred for use in warm weather; the lightergrades in cool weather. When the preferred grade of agiven type of asphalt cutback is not available, acomparable grade of another type may be substituted;for example, RC-800 maybe used instead of MC-800,or RC-70 instead of MC-70, without seriously affectingthe finished pavement.Light grades of asphalt cutback maybe produced inthe field by adding solvents to asphalt cutback. Thecomposition of asphalt cutbacks, expressed in percentof total volume, is shown in table 8-3.Asphalt EmulsionsAn asphalt emulsion is a nonflammable liquidsubstance, composed of asphalt cement, water, and anemulsifier mixed together to produce a liquid material.Emulsions are environmentally friendly, have the samebasic uses as cutbacks and are becoming morecommonly used in the field. Asphalt and water will notmix; therefore, a chemical agent called an “emulsifyingagent” must be added. The emulsifying agent keeps theasphalt cement suspended in the water and controlscertain physical properties of the emulsion. Commonemulsifying agents are soaps, animal blood chemicals,and certain specified colloidal clays in dust. Whenemulsion is applied to a surface, the water and asphaltcement break (separate), leaving a thin film of asphaltcement.The speed of separation, referred to as setting rate,is the basis for designating an asphalt emulsion. Thesetting rates are rapid, medium, and slow. The lettersymbols for these rates are RS, MS, and SS,respectively.Asphalt emulsions are classified into two types:those that are negatively charged and those that arepositively charged (table 8-4), Emulsified asphalts maybe of either the anionic electro (negatively) chargedasphalt globules or cationic electro (positive) chargedasphalt globules. The asphalt emulsions are graded onthe basis of viscosity and grouped according to their use.8-21
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