Tack coat bonds old pavement to new pavement
Posted by Jesse Willoughby on Thu, May 07, 2009 @ 05:04 AM
The Standard Specifications call for STE-1 EmulsifiedAsphalt to be used for tack coats. On some contracts the special provisions maycall for a heavy grade of cutback asphalt instead.
A distributor truck applies the tack. Proper operationof the distributor truck is the key to a good tack job.
Application rates vary and will be set forth in thecontract specify 0.04-0.10 gals/sq.yd.. The tack coat should give a uniformcoat without excess. The inspector may adjust the application rate if thecoverage is too heavy or too light. As a general rule, a small amount of theexisting pavement should show through the tack coat. Too much tack can causeslippage between old and new pavements or bleeding.
Care must be taken to prevent spray overlap or missedareas at longitudinal joints between shots of tack.
Missed spots can be tacked with the hand sprayer, butthe result will be better if the application is done right in the first place.
Â
At transverse joints, building paper should be placedover the end of the old shot of tack and the new shot should begin on the paper.
The tack should be applied the same day the surface ispaved and must be in good condition when the paving machine reaches it. It isthe Contractor’s responsibility to protect the tacked surface from damage untilthe pavement is placed.
Tack should be allowed to break before paving begins.When it breaks it will change from chocolate brown to black and from gooey (itwill stick to your fingers) to tacky (it will feel sticky but will not stick toyour fingers). Paving before the tack breaks results in the equipment pickingthe tack up off the road, which defeats the benefit of the tack.
Weather limitations given in the contract should bestrictly adhered to. Rain can wash unbroken emulsion off the grade, ruining thetack and creating a serious pollution problem. It can cause a serious publicrelations problem with the traveling public, if this oil is splashed on theircars. And it can cause extremely hazardous driving conditions. Tacking is neverallowed in rainy weather. Tack is normally paid for by weight. Asphalt emulsion(STE-1) is diluted with an equal amount of water prior to application. CSS-1 ispaid for before it is diluted.
The distributor truck is weighed before and after theapplication and during the mixing process to determine the amount of tack thatwas placed.
Curbs, manholes, and other surfaces on which asphaltconcrete will be placed or abutted must be tacked by hand prior to paving.Surfaces of curbs, etc. that will not have pavement placed on them mustbe protected from over spray from the distributor.