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3 CHAPTER 2 Factors Affecting Chip-Seal Performance The performance of a chip seal depends on many factors, application rates. Flushing and bleeding of the existing surface including the condition of the pavement to which the chip often occurs in wheel paths. If this occurs, the emulsion appli- seal is to be applied, pavement geometry, traffic volume and cation rate must be reduced in the wheel paths to prevent type, materials, and construction practices. The following future flushing and bleeding. This can only be accomplished discussion describes these factors and their effects on the per- with variable spray rate distributors or by varying the size of formance of the chip seal. the nozzles in the distributor spray bar. 2.1 Pavement Behavior 2.1.4 Texture and Condition Surface roughness affects the amount of emulsion needed 2.1.1 Deflection of Substrate to hold aggregate chips in place. The texture of the substrate The amount of deflection is an indication of whether fu- pavement should be evaluated using the sand patch test or CT ture fatigue can be expected. If deflection is significant, a chip meter prior to chip sealing to determine whether an adjust- seal may be inappropriate. The maximum level of deflection ment to the design emulsion application rate is appropriate will vary depending on the traffic volume; however, if fatigue and to what level. cracking is already present, chip-seal performance may be reduced. However, a chip seal may reduce moisture infil- 2.1.5 Soft Substrate Surface tration into the subgrade, thus reducing the potential for future fatigue. Therefore, the decision to chip seal over exist- A soft substrate surface can allow chips to be embedded in ing fatigue cracking requires judgment depending on the the surface after trafficking, resulting in possible flushing. The performance expectation of the existing pavement. ball penetration test has been shown to be an effective tool for measuring this potential. 2.1.2 Cracking Severity 2.1.6 Uniformity Chip seals are most effective as a pavement preservation technique before cracks are ranked as high severity (Peshkin The amount of emulsion applied to the substrate may need et al. 2004), defined as a crack width of 3/8 in. Although the chip- to be varied if the substrate surface does not have the same tex- seal binder has the ability to seal cracks greater than this width, ture and compliance along the alignment. Uniformity should as crack width increases, the emulsion residue is less effective be mapped prior to construction to identify locations where at bridging the gap across the crack and sealants should be used emulsion application rates should vary from design. Unifor- to fill these cracks prior to chip sealing. mity can also vary transverse to the centerline, which often occurs when wheel paths are flushed. In this case the emul- sion application rate should be reduced in the wheel paths by 2.1.3 Flushing/Bleeding using a distributor equipped with a variable application spray Chip seals may be applied to remedy friction loss, but pen- bar or placing smaller nozzles in the conventional distributor etration of chips into flushed pavement surfaces may limit spray bar in locations that will affect the wheel paths (Shuler effectiveness unless chips can be retained with lower emulsion 1991, Martin 1989).