NCAT Update: Benefit Curves for Pavement Preservation
by Mary Robbins, Ph.D., Buzz Powell, Ph.D., P.E., and Jason Nelson
The old adage, “an ounce of prevention is worth a pound of cure,” often is applied to pavement preservation. However, it is difficult to ascertain exactly how much “cure” an agency can get for an “ounce” of pavement preservation.
The ongoing Preservation Group study by the National Center for Asphalt Technology (NCAT) is trying to do just that. By documenting the condition of 25 100-ft. pavement sections prior to treatment, and recording deterioration over time after treatment, life-extending benefit curves will be established for various preservation treatments as a function of not only time and traffic, but also pre-treatment condition.
The test road, Lee Road 159, is a two-lane, half-mile county roadway located in Lee County, Ala., which dead-ends into a quarry and asphalt plant. As a result, the traffic volumes are easy to track and are relatively low, albeit the truck loads are high for a county road.
For this work, Lee Road 159 has been divided into 25 sections which are 100 ft. long and two lanes wide. In the summer and fall of 2012 various pavement preservation treatments and combinations of treatments were placed, including bituminous surface treatments, crack sealing, cape seals, and thin overlays, while leaving two control sections with no treatment applied. For a complete list of treatments, readers can visit www.pavetrack.com.
In the summer of 2012, the research team collected pavement performance data on the county road to characterize roughness and surface distresses prior to placing any preservation treatments. These data will be important in developing life-extending benefit curves, as life extension is defined for this project as the time (traffic) to return to pretreatment condition. To enable complete characterization of the life extension in each section, the two-lane, 100-ft. long sections were further divided into 40 cells each measuring five by ten feet, resulting in 20 cells in each direction, as shown in Fig. 1.
Developing Benefit Curves
To develop the life-extending benefit curves, performance data (IRI, rutting, texture) are collected weekly and video images are captured at least once per month for the purpose of crack mapping. These data are collected for each of the 40 cells in a section. This is important as the 40 cells that make up a section have varying levels of pretreatment condition, as is evident by the cells in Fig. 1.
It is how these 40 cells respond collectively that will define the life-extending benefit curves. To explain the process, focus is placed on the circled cell. After treatment has been placed, the pavement condition, here defined by percent area cracked, returns to 0 percent.
As time and traffic passes, deterioration occurs until eventually the pretreatment cracking returns completely, as shown in Fig. 2. As illustrated in Fig. 3, the time it takes to return to pretreatment condition for this cell is then used as the x-component for the following plot, where the y-axis is the pretreatment condition (percent cracked in this example) and the x-axis is the time to return to pretreatment condition.
Each cell is represented by a singular data point on this plot, and when combined with the other cells in that section, a curve can be fitted to the data. By reversing the axes such that the y-axis is the time (traffic) to return to pretreatment condition and the x-axis is the pretreatment condition (here, the percent area cracked), the life extension of the pavement due to a specified preservation treatment can be determined as a function of the pretreatment condition.
These curves will be developed specific to each preservation treatment (and section). Although Fig. 1 and the above example focus on reflective cracking, an array of pavement condition data can serve as the basis for the life-extending benefit curve.
As Time Goes By
The experiment on Lee Road 159 is now over a year old and as might be expected, individual cells are changing with time and traffic. In some cases, cells have returned to pretreatment condition. However, it should be kept in mind that it is necessary for all 40 cells in a section to return to pretreatment condition to fully develop the life-extending benefit curve. The expectation is that while some sections may reach this end point within the research cycle, there is the potential for others to be so robust that sections will be extended into future research cycles in order to fully return to pretreatment condition.
These figures demonstrate the general framework for developing the life-extending benefit curves. They are meant purely for the explanation of the developed framework and do not reflect conditions in any of the 25 sections. The authors are affiliated with NCAT.