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19 Determine Binder Application Rate a comparison of the chip seal design methodologies. To begin, the selection of the binder is a very important decision The previously outlined designed methodologies all deter- and should be made after considering all the factors under mine a basic binder application rate that typically depends on which the chip seal is expected to perform. After all, the pri- the average least dimension (ALD) of the aggregate and type mary purpose of a chip seal is to prevent water intrusion into of chip seal being used. Intuitively, larger-sized aggregates the underlying pavement structure, and the asphalt layer require additional binder to achieve the optimum embedment. formed by the binder is the mechanism that performs this There are different schools of thought with regard to embed- vital function. ment. One approach is to seek to achieve approximately 50% embedment after rolling and thus leave room for traffic to fin- The previously explained design methods are all based on ish the process by further embedding the aggregate after the the assumption that single-course chip seal design requires the newly chip sealed road is opened. This approach strives to use of uniformly graded aggregate spread one stone thick in a avoid bleeding in the wheelpaths by leaving room for the uniform manner. The application rates of all methods appear additional embedment during the chip seal's service life. The to be based on residual binder, and each method has a proce- major disadvantage of this approach is that it leaves the aggre- dure for dealing with adjustments owing to factoring the loss gate that is not on the wheelpaths prone to being dislodged by of binder to absorption by the underlying pavement surface traffic movements across the lane's width. The other school and the aggregate being used. Contemporary design practices of thought is to achieve an embedment of up to 70% during need to determine binder application rates based on surface construction across the entire road width. This approach will characterization, absorption factors, traffic conditions, climate adjust the binder application rate based on the measured or considerations, aggregate selection, and type of chip seal being perceived surface hardness and account for hardness in the constructed. Another important discovery is that all methods design. The latter school of thought means being on guard have a design objective for embedment to be between 50% against aggregate loss, and it may mean leaving the road in a and 70% of the seal's depth. A detailed discussion of formal condition in which it is prone to bleeding if the design calcula- design methods is contained in Appendix C. tions do not exactly match the existing surface. Best practices for chip seal design are difficult to isolate, The design binder application rate is calculated after con- because there appears to be such a large variation in practices sidering a number of correction features or allowances to the from agency to agency. However, the following can be iden- basic binder application rate. Typical adjustments are based tified as meeting this project's definition for best practices: on traffic characteristics, surface texture, aggregate absorp- tion characteristics, and surface hardness. Typically, binder · Chip seals perform best only on roads with low under- application rates are reduced where large traffic volumes are lying surface distress that will benefit from this tech- expected to considerably reorient and embed the aggregate nology. after final rolling. The binder application rate may also be · The international practice is to characterize the under- adjusted depending on the existing surface texture. It is nec- lying road's texture and surface hardness and use that essary to increase the application rate on pocked, porous, or as a basis for developing the subsequent formal chip oxidized surfaces, because such textures will absorb more seal design. U.S. and Canadian agencies obviously binder. In contrast, it is necessary to decrease the binder appli- recognize the need to factor in the underlying surface cation rate on surfaces that exhibit susceptibility to bleeding. into the design, as shown in Figure 10, where the Surface hardness, as measured by the ball penetration test or majority of North American responses indicated a rou- a penetrometer, characterizes the likely depth of aggregate tine use of qualitative characterization in the design embedment into the underlying pavement. process. Thus, the next logical enhancement would be to incorporate international methods to quantitatively CHIP SEAL DESIGN CONCLUSIONS characterize the underlying surface in the chip seal AND BEST PRACTICES design process. · One of those enhancements would be to try using the Unquestionably, all of the design methods can effectively racked-in seal as the corrective measure for bleeding guide inexperienced personnel through the process of chip instead of the North American practice of spreading seal design. The following best practices can be drawn from fine aggregate and sand on the bleeding surface.