Using pavement deflections to quantify the benefits of pavement preservation
State and local agencies are constantly facing the challenge of maintaining their road networks with limited resources. Pavements will naturally deteriorate over time due to the effect of traffic loads and the environment, but if appropriate treatments are applied during the early stages of deterioration it is possible to keep good roads good with minimal investments, instead of performing costly rehabilitation treatments later in the pavement’s life when significantly greater effort is needed to improve its condition.
Pavement preservation includes preventive maintenance and minor rehabilitation, as well as some routine maintenance activities. Pavement preservation activities are intended to restore the function of the existing system and extend its service life, not increase its structural capacity or strength. However, by protecting the pavement surface and preventing moisture intrusion, pavement preservation treatments are likely to extend the structural life of the treated pavement.
Little research has been conducted to assess the effect of pavement preservation on structural performance, as the main focus has typically been on functional characteristics (ride quality, surface distress, skid resistance, etc.). Nonetheless, fully identifying all benefits resulting from this practice is important to help agencies make better-informed decisions regarding treatment selection. With that in mind, the National Center for Asphalt Technology (NCAT) at Auburn University started conducting research on full-scale test sections treated with various preservation treatments in 2012. The research effort is funded by state departments of transportation, the Federal Highway Administration (FHWA), and industry, with the objective of quantifying the life-extending and condition-improving benefits of different treatments as a function of traffic, climate, and existing pavement condition.
The scope of work has been expanded over the years to add more test sections subjected to different conditions. Through a partnership established with the Minnesota DOT Road Research Facility (MnROAD) in 2015, the project now oversees a total of 145 test sections located in five different roadways in the states of Alabama and Minnesota. These sections are regularly monitored to evaluate the condition of the pavement, including its structural integrity. Falling weigh deflectometer (FWD) testing is conducted periodically to compare the structural performance of the treated pavements to that of the untreated (control) sections. The data are analyzed using the ELMOD 6 software to compare deflections and layer moduli of treated and untreated sections over time.
The results show that by applying preservation treatments to structurally sound pavements, the pavement structure is capable of maintaining its integrity for a longer time, thus delaying the need for rehabilitation. One example is how the use of thin asphalt overlays has contributed to extend pavement life in low traffic volume test sections. Different types of overlays were placed at a 19-mm thickness, which is not considered to provide a significant structural improvement. Thanks to the analysis of pavement deflections, it was found that the overlays not only provided a functional benefit in terms of ride quality and surface condition, but they also have been able to maintain the sections in sound structural condition throughout the course of the study. Although there was a minimal improvement following construction, the greatest impact was observed through time. While the treated sections reached a condition that would no longer be defined as “sound” after approximately 7 years of service, the thin overlays continue to exhibit a much lower rate of deterioration through 9 years of service and are not predicted to require intervention in the near future.
The findings from this research have contributed to fully assess the benefits of pavement preservation and provide information to help agencies improve their pavement management practices. These lesser-known structural benefits, which can be evaluated through FWD testing and analysis, should also be considered when planning life cycle activities and establishing a proactive approach to pavement management.