By Joe Blake
With increasing traffic volume and shrinking budget, state agencies are exploring ways to better preserve existing roads. Increasingly, microsurfacing is one of the preservation tools used by state agencies to extend the life of asphalt pavement.
However, the current microsurfacing specifications in U.S. do not seem to differentiate between the various climate conditions encountered and thus limit its potential for successful applications. According to “A-143 Guideline for Microsurfacing – ISSA (International Slurry Seal Association)”, the current specification stipulates a 40-90 dmm penetration range and a softening point of 57°C after the base is modified using 3 percent polymer solid.
This essentially makes base asphalts like PG 64-22 more economically favorable. Although a PG 64-22 may be a suitable material in the southern U.S., when it is used in northern U.S., the asphalt is too brittle and is not flexible enough to delay the progress of the crack developments.
Some state agencies like Minnesota, Wyoming and Montana have investigated using softer binders in order to avoid damage from colder weather distresses such as thermal cracking or fatigue cracking. Expanding the penetration range or specifying softer PG binder would enable more flexibility for cooler climates. Furthermore, changing to a performance grade (PG) binder would address the need for a better specification which would suit the climatic conditions of different regions.
The Minnesota Department of Transportation (MnDOT) performed a study on the use of microsurfacing in colder climates. The research team used a softer grade of binder, PG 48-34. They performed pre- and post-construction evaluations of cracking, rutting and smoothness. The results were positive. While transverse cracking had made its way through the microsurface, reflective cracking from longitudinal cracks and patched areas was minor. The microsurfacing achieved a 20 percent decrease in rutting.
Based on the results of the study, MnDOT has changed their specifications for microsurfacing to include CQS-1P with a penetration range of 90-150 to get a softer base binder such as PG 58-28. Both South Dakota and Montana are evaluating the performance of microsurfacing using a similar specification in 2016. Wyoming DOT has specified some projects using PG 58-28 as the base asphalt and is evaluating the performance.
To better understand the relationships between asphalt properties and the field performances of microsurfacing, new rheological tests on asphalt residues have been employed by agencies and researchers. Utah Department of Transportation (UDOT) implemented the Multiple Stress Creep Recovery (MSCR) test for microsurfacing to ensure the flexibility of the material and create a more stable product. Compared to the penetration criteria outlined in ISSA A-143, the MSCR criteria is a better indicator of how asphalt will perform in the field.
Another important performance indicator to evaluate is fatigue resistance. The Linear Amplitude Sweep Test (LAS), developed by UW-Madison, measures fatigue resistance in asphalt.(4) Tests show that adding polymer to an emulsion commonly used for microsurfacing improves fatigue resistance. Some polymers performed better than others because of the better durability at low temperatures. Different climates call for different polymers and different asphalt bases. The adoption of the LAS test to the current specification is another way to ensure that the microsurfacing emulsion resists fatigue failure and proper performance in different climates.
Current ISSA specifications could be improved with performance guided tests on the emulsion residue. Without them, agencies sometimes struggle to correctly select the proper base grade asphalt suited for their specific climate. Additionally, changing to a PG based system versus penetration based would allow agencies to properly select the necessary binder grade based on climate conditions. Employing the MSCR and the LAS with the option of a softer PG binder would enable agencies to make informed decisions about the materials used for microsurfacing.
Blake is quality and safety leader at Idaho Asphalt Supply in Idaho Falls. This article is Mr. Blake’s view and not necessarily the position of the Asphalt Institute.
• International Slurry Surfacing Association, “ISSA A-143, Recommended Performance Guidelines For Micro-Surfacing”, Annapolis, MD 21401, 2010
• Johnson, E., Wood, T., and Olson, R.,”Flexible Slurry– Microsurfacing System for Overlay Preparation: Construction and Seasonal Monitoring at Minnesota Road Research Project”, Transportation Research Record, Vol. 1989: 321-326.
• UDOT, 2016 Standard Specifications, “Section 02735M Microsurfacing
• AASHTO TP101, “Method of Test For Estimating Fatigue Resistance Of Asphalt Binders
Several new standards were released in 2016 that may address many microsurfacing concerns.
“AASHTO M 316-16 Standard Specification for Polymer-Modified Emulsified Asphalt”
“AASHTO PP 83-16 Provisional Standard Practice for Micro Surfacing Design”
“AASHTO MP 28-16 Standard Specification for Materials for Micro Surfacing”