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