Crack sealing is a cost-effective, preventive maintenance method that helps extend the service life of pavements by preventing liquid and solid materials from penetrating the pavement system. Depending on the temperature of the sealing material at the time it is applied, sealants are classified as either “cold-pour” or “hot-pour.” When properly selected and installed, sealants have the ability to extend the service life of pavements from three to five years.
However, current standards and specifications used for selecting crack sealants fail to measure the fundamental properties of sealants; vary even among states with the same environmental conditions; poorly characterize the rheological properties of bituminous-based crack sealants; and fail to predict sealant performance in the field.
In order to develop performance-based guidelines for selecting hot-poured bituminous crack sealants, a pooled-fund study, led by the Illinois Center for Transportation (ICT) of the University of Illinois at Urbana-Champaign (UIUC) and the National Research Council of Canada, was initiated in 2002. The research team proposed several new fundamental tests that resulted in the development of a “Sealant Grade” (SG) system for selecting hot-poured crack sealant based on environmental conditions. The new tests, aimed at evaluating aged sealants, allowed measurement of the rheological and mechanical properties of hot-poured bituminous-based crack sealants over a wide range of service temperatures. Preliminary thresholds were identified to ensure desirable field performance, but were determined based on limited field data. This gave rise to the need for a comprehensive field study to validate and fine-tune the identified threshold values.
Building on the comprehensive tests and procedures developed in Phase I, and based on the rheological properties of crack sealants, Phase II of the study aimed to comprehensively validate previously developed laboratory tests; determine thresholds using a diverse array of field performance data; and implement crack sealant guidelines for sealants installation and best practices.
ICT research team, led by Professor Imad Al-Qadi as Principal Investigator and research assistant professor Hasan Ozer as co-Principal Investigator, modified and developed new laboratory tests, overseeing the installation of various types of sealants and conducting annual field inspection of crack sealant performance in seven sections located in different environmental regions in North America. The researchers applied “rout and seal” treatments using various reservoir geometries and “clean and seal” treatments and compared the two sealing techniques.
Field performance evaluation was followed by a validation using the developed laboratory test methods. Performance data such as visual distress identification, crack opening displacement, and temperature measurements were collected for laboratory evaluation. “The new tests and performance grade system will allow the selection of proper sealants for the proper locations based on the fundamental properties of sealants,” says Al-Qadi.
The research work resulted in eight new standards of test methods and specifications. The tests, published as American Association of State Transportation Officials (AASHTO) specifications, include AASHTO TP 85 Apparent Viscosity of Hot-Poured Bituminous Crack Sealant, AASHTO TP86 Accelerated Aging of Sealants and Fillers Using a Vacuum Oven, AASHTO TP87 Measuring Low-Temperature Creep Stiffness of Crack Sealant using Bending Beam Rheometer, AASHTO TP88 Low-Temperature Tensile Property of Bituminous Sealants by Direct Tension Test, AASHTO TP89 Measuring Adhesion of Hot-Poured Crack Sealant Using Direct Adhesion Tester, AASHTO TP90 Measuring Interfacial Fracture Energy of Hot-Poured Crack Sealant Using a Blister Test, and the recently submitted Dynamic Shear Rheometer (DSR) test method for determining high-temperature grade and tracking resistance. The performance-graded sealant materials and grading system have also become an AASHTO provisional specification (AASHTO MP 25).
The study was concluded with a life-cycle cost analysis of crack sealing treatments. “Based on the life-cycle cost analysis (LCCA), sealing pays off—even if the benefit is an incremental lifetime extension of a pavement for less than a year,” says Professor Ozer. “The cost-effectiveness of this approach increases with right selection of materials and proper application.”
The consortium sponsoring this study consisted of 11 U.S. state departments of transportation, 13 Canadian transportation agencies, and industry in Phase I and eight states in Phase II. The U.S. contribution was made through pooled-fund research projects TPF-5(045) and TPF-5 (225), led by the Virginia Department of Transportation (VDOT)/ Virginia Transportation Research Council (VTRC).