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14 CHAPTER 5 Chip Seal Materials Selection The performance of a chip seal is largely dependent on the greater and where limited time under traffic control is usu- materials used. Therefore, selecting the appropriate aggre- ally desirable. gates and asphalt emulsion and deciding whether to apply a fog seal to the surface play a significant role in the success 5.3 Fog Seal After Chip Seal of the project. The following discussion provides guidance regarding these factors. A fog seal may be applied to any completed chip seal as a means of providing a high color contrast for paint stripes. There are also some preliminary indications that the fog seal 5.1 Chip Gradation provides some additional waterproofing (Shuler 2007). Care should be taken whenever applying a fog seal since pavement The gradation of the chip should be one- or two-sized, friction could be reduced if the fog seal is applied at too high but the maximum size should be selected based on traffic an application rate, the fog seal emulsion has a high residue volume, pavement texture, and the required level of sealing. content, or the fog seal has not broken sufficiently to support Generally, larger aggregate provides greater ability to seal uncontrolled traffic. because of the higher volume of binder required to hold the chips in place, and depending on traffic volume, provides longer life expectancy. However, larger aggregates increase 5.4 EmulsionAggregate Compatibility the chances for vehicle damage, noise, and roughness. There is anecdotal evidence of apparent incompatibility arising from use of anionic or cationic emulsions with siliceous 5.2 Modified or Unmodified or calcareous aggregates, respectively. This incompatibility Emulsion manifests itself with a loss of aggregate from the chip seal. This behavior was not observed during the NCHRP Project 14-17 Modified emulsions usually refer to some sort of elastomeric research, in which 20 combinations of aggregates and emul- polymer or rubber added to the emulsion or to the base asphalt sions were represented. Therefore, unless impractical, anionic binder prior to emulsification. Because modified emulsions emulsions should be pared with positively charged aggregates should offer greater adhesivity and potentially shorter time (i.e., calcareous), and cationic emulsions should be matched required before opening to traffic, they are generally used on with negatively charged aggregates (i.e., siliceous) to avoid pos- higher traffic pavements, where vehicle damage potential is sible incompatibility between the materials.