cutbacks are highly flammable, safety precautions mustbe carefully followed.A tack coat should be applied only when the surfaceto be tacked is dry; and the atmospheric temperature hasnot been below 35°F for 12 hours immediately beforeapplication.Before applying the tack coat to a surface that issufficiently bonded, ensure that all loose material, dirt,clay, or other objectionable materials are removed fromthe surface to be treated.This operation may beaccomplished with a power broom or blower,supplemented with hand brooms if necessary.Immediately following the preparation of thesurface, the bituminous material should be uniformlyapplied by means of a bituminous (asphalt) distributorat the spraying temperature specified. The amount ofbitumen application, known as rate of application(ROA), for a tack coat should be applied in quantitiesnot less than 0.05 or more than 0.25 gallon per squareyard. The exact quantity varies with the condition of theexisting pavement being tack-coated. Normally, theconstruction project specification denotes the ROA forthe tack coat application; however, when the ROA is notincluded in the project specifications, the NCF uses anROA of .15 for planning and estimating purposes.Following the application of bituminous material,the surface should be allowed to dry until it is in a propercondition of tackiness to receive the surface course;otherwise, the volatile substances may act as a lubricantand prevent bonding with the wearing surface. Clean,dry sand should be spread on all areas that show anexcess of bitumen to blot up and cure the excesseffectively. After excess bitumen is set, any loose sandshould be lightly broomed from the primed surfacebefore the wearing surface is laid.An existing surface that is to be covered by abituminous wearing surface should be barricaded toprevent traffic from carrying dust or mud onto thesurface, either before or after the tack coat is applied.Should it become necessary for traffic to use the surface,one lane may be tack-coated and paved, using the otherlane as a traffic bypass. The bypass lane should beprimed and sanded before it is opened to traffic and itshould be swept and reprimed after the adjacent lane iscompleted. Doing this preserves the base and acts as aThe formula for a tack coat estimate is as follows:16-8WhereROA =L =W =9=WF =Example:rate of application of bitumen in gallonsper square yardlength of treated section in feetwidth of treated surface in feetsquare feet per yard conversion factorWaste Factor of bitumen = 5% at .05 or10% at .10. This will depend on theexperience of the asphalt distributortruck crew.The specification and other data for atack coat application are as follows:L =2 miles= 2 x 5280= 10560 feetW = 12 feetROA = 0.5 gal/sq ydWF = 5% or .05Calculate the number of gallons of bitumennecessary to spray a tack coat.dust palliative (shelter.)
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