In Place of Failure

May 2, 2007

Most of an agency’s highway budget is used to rehabilitate the existing system. The ultimate goal of every pavement rehabilitation project is to match the correct rehab strategy with the known pavement conditions to achieve the most cost-effective rehab for the desired level of performance.

Most of an agency’s highway budget is used to rehabilitate the existing system. The ultimate goal of every pavement rehabilitation project is to match the correct rehab strategy with the known pavement conditions to achieve the most cost-effective rehab for the desired level of performance.

In the state of Iowa, cold in-place recycling (CIR) is regularly used and considered an effective rehab strategy, but there was very little formal analysis to support this conclusion. In 2003, the Iowa Highway Research Board initiated a study to examine the long-term performance of pavements with CIR. The research was accomplished through the joint efforts of Iowa State University and the University of Iowa with assistance from the Iowa DOT. The results from the joint study are two approaches to examining pavement performance: (1) changes in field distress and (2) variations in CIR material characteristics.

The two methodologies are different, but when combined in a single research study generate important insight on the effect of CIR on pavement performance. The study of changes in field distress is a broad approach to examining pavement performance. The results draw general conclusions about subgrade support as a primary factor, but can only be tied to CIR by association. This approach cannot isolate the value of the CIR layer.

The study of CIR material characteristics focuses exclusively on the CIR layer. The results identify stiffness and air voids of the CIR layer as material characteristics that influence pavement performance. Some of the conclusions contradict generally accepted engineering principles. While this approach does isolate the CIR layer, it compares the material properties to measured pavement condition. That condition may, or may not, be related to the performance of the CIR layer.

The Iowa Highway Research Board study takes an important step toward quantifying the influence of CIR on pavement performance. All of the projects studied included a CIR layer, but there was no particular control of the variables, nor any control sections. The study cannot completely isolate each of the variables that influence pavement performance. Further study of CIR performance will require more control of important variables to better measure the consequences of the CIR layer.

Success in years

CIR is one of the most effective methods to rehabilitate asphalt pavements. However, few extensive long-term performance studies have been conducted to predict its service life. The main objective of this research is to develop a performance model of CIR roads to predict their long-term performance in Iowa.

First, an inventory of CIR roads was created, which included construction information, subgrade and base characteristics and traffic levels. To determine the soil support condition, each selected test section was evaluated using a falling-weight deflectometer (FWD). In consideration of pavement age, level of traffic and subgrade condition, 26 test sections were selected from the inventory of CIR roads and a pavement surface distress survey was conducted on these roads using an automated image collection system (AICS).

Pavement images were analyzed to compute the pavement condition index (PCI) for each test section. The PCI values were plotted against pavement age for each group of pavements categorized by back-calculated subgrade modulus and average annual daily traffic (AADT) as follows:

  • Soil support:
    Good Support >= 5,000 psi
    Poor Support < 5,000 psi
  • Traffic:
    High Traffic >= 800 AADT
    Low Traffic < 800 AADT

On average, CIR roads would last between 21 and 25 years when they are predicted to reach a fair pavement condition as defined by PCI values between 40 and 55, respectively. When the PCI values collected from the 13 sections with poor subgrade support are plotted against age, on average, CIR roads are predicted to last between 18 and 22 years.

When the PCI values collected from the other 13 sections with good subgrade support are plotted against age, on average, CIR roads would last between 26 and 34 years. Based on the result obtained from 26 sections, it can be concluded that having good soil support would increase the service life of CIR pavements by eight to 12 years. The first CIR road constructed in 1986 was rehabilitated in 2005 after reaching a fair condition with a PCI value of 48 in 2004. One section with a high traffic level of 6,200 AADT has performed reasonably well, although rutting started to develop after three years.

Both longitudinal and alligator cracking increased over time whereas transverse cracking did not change much over time. Rutting, patching and edge cracking increased mostly in sections with poor subgrade support.

Overall, it can be concluded that the CIR roads with less than 2,000 AADT in Iowa have performed very well and have not reached the poor condition (PCI=40) when the pavements may be rehabilitated. Some of the test sections are being considered for rehabilitation and this presents a good opportunity to verify the service life and conditions that result in the decision to rehabilitate. It is recommended that these pavement sections should be evaluated again in five years to verify the predicted service lives.

Strength in stiffness

CIR material characteristics were investigated to identify the associations between certain properties and pavement performance. The study included a collection of pavement deflections using FWD, coring of existing pavement, lab testing of CIR materials and eventually the statistical analyses of testing data.

The CIR roads were sampled by coring the outside driving wheel path. Normally about six cores were obtained along the 1,500-ft length of each road.

The CIR portion of the cores was then cut and tested. The indirect tensile test (IDT) was conducted to investigate the moisture sensitivity and the tensile strength.

Figure 1 illustrates how better-than-average-performance roads and worse-than-average-performance roads were determined. The dashed black line in the figure represents the expected PCI. CIR roads with larger-than-expected PCI values (dots above the dashed black line) performed better. The difference between the observed PCI and the expected PCI is defined as relative PCI. Thus a CIR road with a larger positive relative PCI performed better.

Which material properties were associated with roads that performed better than average or worse than average? To answer this question, conceptual statistical analyses were conducted. The results showed that there were no strong associations between the performance and selected important material properties, such as air voids and the stiffness of the CIR layer for all traffic levels combined. However, further statistical analyses proved that such strong associations did exist for some traffic categories.

Modulus of the CIR layer (stiffness)

In a typical flexible pavement structure, material layers are usually arranged in the order of descending load-bearing capacity with the highest load-bearing-capacity material on the top and the lowest load-bearing-capacity material at the bottom. Therefore, the surface course (typically an HMA layer) is the stiffest (as measured by resilient modulus). The underlying layers are less stiff. Serving as the base of the HMA surface course, the CIR layer should not only be stiff enough to provide adequate pavement strength, but also be flexible enough to allow the total pavement structure to deflect under repeated traffic loading.

This study showed that the stiffness of the CIR layer significantly affects performance of high-traffic roads, and that a CIR road with a more viscoelastic CIR layer performs better. This finding agrees with a previous study in that serving as a stress-relieving layer, the relatively less-stiff CIR layer will reduce cracks in the HMA layer.

Air voids (Va)

The results showed that Va was associated with pavement performance for high-traffic roads, but it was not significant for low-traffic roads. Although performance improved within the range of Va in this study (6~12%), it seems that performance would deteriorate if Va increased much beyond those limits.

IDTwet (tensile strength of conditioned wet CIR cores)

IDTwet is associated with the low-traffic-road performance, but not with the high-traffic-road performance.

Cumulative Traffic (product of traffic volume and the age of the road) This factor was affecting performance of high-traffic CIR roads, but not the low-traffic roads.

The study concluded that:

  • The CIR layer acts as a stress-relieving layer. Therefore, within the range of the data analyzed, a lower-stiffness CIR mix with higher air
  • Moisture sensitivity of the CIR mixture may affect performance of low-traffic CIR roads (AADT<800); and
  • As expected, a higher amount of cumulative traffic is associated with lower relative pavement performance in the models for high-traffic roads (AADT>800).

About The Author: Heitzman is a bituminous materials engineer with the Iowa Department of Transportation. Hosin is an associate professor at the University of Iowa. Kim is a research assistant at the University of Iowa. Chen is an assistant professor at Ball State Universi

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