Volume 41, Issue 2 (March 2013)
Calibration of Seal Coat Application Rate Design
This paper presents seal coat performance evaluations of various emulsion and aggregate application rates using three different evaluation methods: International Roughness Index (IRI), friction test, and visual evaluation. Then, considering the seal coat failure criteria, correction factors for seal coat application equipment are introduced. This study confirms the lack of relevance between seal coat application and IRI values because of the thickness of a seal coat. In addition, friction improvements caused by seal coat applications were confirmed within the range of seal coat rates applied. Overall, IRI, friction, and visual inspection did not reveal distinct differences in seal coat performance in terms of application rates in the testing range. Accordingly, seal coats with rates based on the McLeod method showed acceptable performance. The aggregate application rate should be high enough to protect the seal coat from immediate failure during construction but low enough to avoid unacceptable levels of accumulated fines content. Furthermore, immediate failure occurring locally during construction because of incorrect application rate (e.g., insufficient aggregate rate) can cause total failure of a seal coat road because of a chain reaction. However, discrepancies between designed rates and applied rates were observed in the study even after the seal coat equipment was calibrated prior to operation. The equipment factor for aggregate can compensate for rate discrepancies between the target and actual application rates. It was found that the designed emulsion application rate does not need to be corrected for the emulsion distributor because the emulsion rate discrepancy has an insignificant impact on a seal coat’s performance.