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56 CHAPTER EIGHT CHIP SEAL PERFORMANCE MEASURES INTRODUCTION texture (Abdul-Malak et al. 1993). Basically, the microtexture is determined by the frictional properties of the aggregate, Defining chip seal performance criteria, and how to quantify whereas the macrotexture is determined by the size, shape, and them, is perhaps the most difficult consideration for any pub- spacing of the aggregate particles (Abdul-Malak et al. 1993). lic owner with chip seal projects. Throughout the literature Although there are other acceptable methods for measuring review, an effort was made to identify any particular accep- skid resistance, the most common method for a chip seal tance criteria or performance specifications that would illus- is according to ASTM E274, Standard Test Method for trate chip seal performance measures. The objective of this Skid Resistance of Paved Surfaces Using a Full-Scale Tire chapter is to identify performance specifications and become (Seneviratne and Bergener 1994). This method measures the familiar with their attendant performance measures. Two sliding friction force developed between a tire and roadway separate schools of thought in regard to performance mea- surface and expresses the result as a skid number. The justifi- surement have been identified. Chip seal performance is pri- cation behind using skid numbers as a determinant of chip seal marily either measured quantitatively through engineering performance, and thus also the service life of the chip seal, is principles or rated qualitatively through expert visual assess- that skid numbers drop over time owing to deterioration of the ment. The literature review and survey results discovered lit- pavement's surface texture (Seneviratne and Bergener 1994). tle beyond measuring skid resistance for quantitative chip Most agencies have a specified cycle on which skid resistance seal performance measurements in North America, but quan- is measured as a part of their pavement management system. titative performance measures are widespread in Australia, These data are invaluable to making the decision as to which New Zealand, South Africa, and the United Kingdom. roads to chip seal. Texture Depth ENGINEERING-BASED PERFORMANCE INDICATORS The literature review identified several methods for measur- Quantitative chip seal performance measurement techniques ing a pavement's macrotexture (Abdul-Malak et al. 1993). Of were evaluated by a study at the Pennsylvania Transportation these, the survey results indicated that the only measurement Institute (Roque et al. 1991). Performance measurements with widespread acceptance by the international respondents of rutting and roughness are widely used on other wearing is the sand patch method (ASTM E965). In the Pennsylvania courses such as asphalt pavement. Chip sealed surfaces look study, the mean texture depth (MTD) as obtained by the sand and perform differently from asphalt pavement surfaces; patch method was found to give the best indication of chip therefore, their performance needs to be based on a different seal performance, in addition to being an objective manner of set of visual evaluation methods than for asphalt pavements comparing chip seals on a relative basis (Roque et al. 1991). (Walker 2001). As such, asphalt pavement performance mea- Aggregate retention and resistance to bleeding are both evi- surements are not often applicable to measuring chip seal per- dent by evaluating MTD. The study in Pennsylvania proposed formance, because none of those methods will formally mea- that the rationale for using MTD as the best indication of sure the two most common chip seal distresses, bleeding and performance is that greater macrotexture generally implies raveling. Measuring skid resistance and measuring texture greater skid resistance (Roque et al. 1991). This same study depth are the only two repeatable and objective quantitative found that the MTD, as indicated by macrotexture, decreased methods that may be applicable. with time as a result of both aggregate wear and embedment. Assuming such, chip seal deterioration models can evalu- ate the effects of different variables on expected chip seal Skid Resistance life (Roque et al. 1991). The study in Pennsylvania, which pro- poses that MTD is the best indication of chip seal performance, Skid resistance, an important safety characteristic for all roads, is in agreement with New Zealand and United Kingdom can be used as a performance measure on chip sealed surfaces philosophies in the development of performance specifications (Roque et al. 1991). The skid resistance or friction, which (Design Guide . . . 2002; Notes for the Specifications . . . develops between a vehicle's tires and the surface of the road, 2002). Texture depth appears to be the performance measure is a function of two components, macrotexture and micro- of choice.