Asphalt Emulsion Sprayability and Drain-Out Characteristics in Chip Seals

Streaking and drain-out are two common early chip seal failures related to the viscosity of the asphalt emulsion used. An emulsion with a high viscosity at spraying could result in streaking, while another with a low viscosity after spraying could drain off the road surface under gravity. Some emulsions also tend to show reduction in viscosity during handling; this reduction aggravates the potential for drain-out. The Saybolt Furol viscometer [SFV (ASTM D7496-09)] is currently used to characterize the viscosity of asphalt emulsions. However, the SFV cannot be operated under varying shear rate conditions as experienced by emulsions at different stages. A new test procedure is presented for evaluating sprayability, drain-out, and recovery kinetics of emulsions with a rotational viscometer. The test procedure allows for emulsions to be tested at a higher shear rate to evaluate sprayability and at a low shear rate to evaluate drain-out. Twelve emulsions of different grades and chemistries were tested. Results showed that the proposed test procedure was sensitive to emulsion chemistry, grade, and testing temperature. Sprayability, drain-out, and recovery kinetics were found to be significantly affected by emulsion chemistry and testing temperature. This protocol with variable shear-rate viscosity is recommended for use as a basis for replacing the SFV test as a better performance-related test method. Specification criteria can be easily developed and implemented because rotational viscometers are widely available in the industry, and there is a wealth of experience with various emulsions in the field.