By: Prithvi "Ken" Kandhal, P.E.
Chip seal disadvantages for HMA overlays Some highway agencies apply a chip seal (or seal coat) before placing a dense-graded hot-mix asphalt (HMA) overlay. Quite often the HMA placement follows immediately behind the chip seal operation. The seal usually consists of spraying an emulsified asphalt on the existing or milled HMA surface, spreading 1¦2 in. (12.5 mm) maximum size aggregate, and compacting. Applying chip seals below HMA overlays prevents reflection cracking in the new overlay, and the infiltration of water from above.
Studies have shown that chip seals consisting of 1¦2-in. aggregate do not prevent reflection cracking but may delay it by about a year. Only large aggregate-1¦2-in. or more-chip seals show some promise in minimizing cracking.
The chip seal is likely to prevent infiltration of water from above into the pavement system below except through the reflection cracks after a year or so. However, if the subsurface drainage system of the pavement is inadequate, the seal will act as a barrier to the moisture vapor rising from the subbase or subgrade upwards. This results in the accumulation of moisture and saturation of the HMA course underlying the chip seal, leading to stripping in that course. Properly designed HMA courses are generally permeable to moisture vapor allowing it to escape but almost impermeable to free water. Therefore, there is no need to use a waterproofing layer below a dense-graded HMA course.
Besides creating a stripping potential in the underlying HMA course, the chip seal may have the following additional disadvantages: It may induce rutting in the overlying HMA course when the aggregate particles in the chip seal realign to the most stable orientation under the action of the traffic; the chip seal may trap surface water coming from above through reflection cracks, and thus induce stripping in both underlying and overlying courses-such a phenomenon has been observed when dense-graded HMA overlays are placed on existing open-graded friction courses; the adhesion of the new HMA overlay to the dusty surface of the chips of the seal may not be adequate; it also is questionable whether the HMA pavement system acts as a monolithic structure when the dense-graded courses are interspersed with weak seal layers.
In view of this discussion it is important to consider the potential disadvantages of using a chip seal below a dense-graded HMA course before its widespread routine use.
About The Author: Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.
