Keith Woffinden and Dr. W. Spencer Guthrie, Civil and Environmental Engineering
Introduction
The traditional construction method for asphalt removal and replacement uses a high-power saw to separate the damaged asphalt from the existing pavement. The remaining material is broken and removed from the site in a time-consuming and laborious process. The saw-cut leaves a smooth, vertical edge on the asphalt surface that can lead to poor mechanical interlock between the old and new pavement materials, a major catalyst for accelerated pavement deterioration.
The Asphalt Zipper is a portable milling machine headquarted in Pleasant Grove, Utah, that mounts easily and securely onto the front of a loader, back hoe or skid-steer with hydraulic clamps. This machine utilizes a steel drum mounted with cutting teeth to grind the asphalt into a fine material that can either be left on site as road base or easily loaded and transported off-site for other uses. The rotating teeth impart a rough scarified edge onto the existing pavement that has the potential to improve the interlock strength of roadway patches without a substantial increase in cost.
If the Asphalt Zipper can significantly increase the bond strength, it could ultimately save millions of dollars in maintenance costs nationally each year. As a result, a team from the Department of Civil and Environmental Engineering, in cooperation with the producers of Asphalt Zipper and with funding from the Office of Research and Creative Activities at BYU, conducted an experiment to determine the difference in bond-strength resulting from the saw-cut and Asphalt Zipper pavement removal methods. This paper reviews the experimental methodology carried out in the test, provides a brief discussion of the results, and outlines the conclusions and publications resulting from the experimentation.
Experimental Methodology
Using the testing yard at Asphalt Zipper’s headquarters, the team made a saw cut some 75 feet in length through a 6-inch layer of asphalt concrete. The Asphalt Zipper Model 480S was then mounted to a loader and used to make a 48-inch-wide cut about 8 feet from the saw-cut edge. All of the asphalt between the saw cut and the outer “zipped” edge was then removed. This configuration ensured that the adjacent asphalt concrete and the underlying base materials at both joint locations were as similar as possible. A tack coat was then sprayed onto both vertical cut faces, and a hot-mix asphalt patch was placed and compacted in the trench by a local paving contractor.
Approximately one month later, researchers removed 25 cores from each patch joint, where each core was centered as closely as possible on the respective joint. A portable 6-in core drill provided by BYU was utilized for the extractions. The cores were then prepared for testing at the BYU Highway Materials Laboratory. Each specimen was trimmed using a masonry saw to create flat, parallel end faces, and the heights, weights, and bond areas of the cores were then measured. These measurements were used to approximate the density of each testing core for statistical analysis of co-variance with the corresponding strength of each specimen.
A specially manufactured testing apparatus was used in an MTS machine to shear each core at a constant strain rate. The load was applied across the joint in the direction parallel to the longitudinal axis of the core. The joint was carefully aligned within a 1-in shear zone provided in the testing apparatus to accommodate variability in joint locations from one core to the next. Each core was loaded to failure, after which the bond strength was calculated for each specimen by dividing the maximum sustained load by the bond area over which the load was applied. At the conclusion of winter, researchers returned to the asphalt testing site and removed an additional 25 cores from each side of the patch. These specimens were utilized to analyze the effects of tension from freeze-thaw cycles on the overall bond strength of each joint. At the lab each core was trimmed, measured, and subjected to the same shear testing as the previous cores.
Test Results
A comparison of the density and bond strength measurements shows that density has a noticeable impact on the overall bond strength for each specimen. This variation in density along the patch needed to be accounted for when analyzing the bond strength data. In order to account for the influence of density on the measured bond strength, a statistical analysis of covariance (ANOCOVA) was performed. An ANOCOVA normalizes the response variable, in this case shear strength, to account for variations in starting conditions, such as density. By applying this normalizing factor, the variations in bond strength can be more precisely measured from the data.
An ANOCOVA was performed to measure the difference in shear strength in the saw-cut and “zipped” joints before and after winter. After removing the effects of density variation, the average bond strength for the Zipped cores was 20.8 percent larger than the average for the saw-cut specimen. The statistical p-value for this difference in means was 0.026, indicating that there is only a 2.6 percent chance that these two joints have the same average bond strength. This suggests that the specimens removed before winter from the Zipped edge are significantly stronger than those from the saw-cut edge.
A similar statistical analysis was performed on the data after winter. In this case, the adjusted average bond strength for the milled cores was 25.1 percent larger than the saw-cut samples. The results of the hypothesis test yielded a p-value of 0.016, which suggests that the strength of the scarified edge after the winter was also significantly higher than the strength of the saw-cut edge.
Another statistical model was created that included the bond strength and density variations for both methods before and after winter in order to determine the influence of time and freeze-thaw cycles on the shear strength. This hypothesis test produced a p-value of 0.0042, indicating that there was a significant loss of strength over the course of one winter between the asphalt interfaces.
Conclusions
This study investigated the influence of scarifications on the shear strength and density of asphalt pavement patch joints. For the cores extracted prior to winter, the scarified edge had an adjusted average shear strength 20.1 percent higher than the saw-cut edge. After the winter the adjusted average shear strength of the milled cores was 25.1 percent higher than the saw-cut cores. Statistical analyses confirm that these represent significant differences in shear strength.
The preliminary results of the this experiment have already been published in the May 2004 edition of Public Works magazine under the title Improving Patch Joint Bond Strength. This article summarizes the results and statistical analyses of the specimens collected before winter. Thanks to the generous funding of ORCA I was named co-author of this article with my faculty mentor Dr. Guthrie. We are now preparing an additional paper for submission to the Transportation Research Board for presentation at their annual national conference held in January.
In April I graduated from BYU, and I will attend Georgetown Law School in the fall. The opportunity to work so closely with Dr. Guthrie and publish research as an undergraduate has helped develop skills that I will use the rest of my life. I appreciate the generous contribution to my education that ORCA has provided.