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Improving patch joint bond strength: a rough surface texture introduced by one innovative alternative appears to have clear advantages over the saw-cut method of asphalt removal.

Traditionally, asphalt concrete that must be removed because of deterioration or utility work is saw-cut and excavated with a backhoe. But the process of breaking the asphalt into manageable pieces for hauling can be time-consuming and laborious. Also, the smoothly cut vertical faces of the original pavement then offer minimal mechanical interlock with the patch material.

Increased mechanical interlock between patch materials and original pavement can improve patch performance by reducing joint faulting. The extent to which patch bond strength can be improved by the introduction of a rough surface texture, such as that developed by Pleasant Grove, Utah-based Asphalt Zipper, has not been previously investigated. Therefore, in cooperation with the firm, researchers in the Department of Civil and Environmental Engineering at Brigham Young University (BYU) in Provo this spring conducted an experiment to compare the bond strengths of saw-cut and "zipped" patch joints.


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. The patch material was an AC-20 with a maximum aggregate size of 1/2 inch. The asphalt cement content was 5.5% by weight of total mix.


A 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-inch-diameter core drill was used for the extractions. The cores were then prepared for testing at the BYU Highway Materials Laboratory where each specimen was trimmed using a masonry saw to create flat, parallel end faces. The heights, weights, and bond areas of the cores were then measured.


Researchers sheared each core at a constant strain rate of 0.05 inch per minute with a testing apparatus specially manufactured for use in an MTS machine. The load was applied across the joint in the direction parallel to the longitudinal axis of the core. Joints were carefully aligned within a 1-inch shear zone provided in the testing apparatus to accommodate variability in joint locations.

Each core was then loaded to failure at room temperature, then the bond strength was calculated for each specimen by dividing the maximum sustained load by the bond area over which the load was applied.


Of the original cores, 20 Asphalt Zipper cores and 21 saw-cut cores were analyzed. The others were either damaged during laboratory preparation or not correctly centered on the joint. The average bond strength and standard deviation for the Asphalt Zipper cores was 10,086 psi and 3223 psi, respectively, while the saw-cut cores measured 8426 psi and 2986 psi, respectively. Therefore, according to this research, the strength of the Asphalt Zipper cores was 19.7% higher than that of the saw-cut cores.

In empirical research, replications are necessary to improve the accuracy of the average sample response by reducing its variation from the "true" value, or population mean. The population mean in this research would be determined by coring 100% of a given joint and computing the average bond strength from all of the specimens. While cost and other constraints typically prohibit such extensive analyses, information about populations can be inferred from sample data. The more samples, the more reliable the average sample response.

To determine whether this study reliably represented the population means, a two-sample "t-test" was performed, wherein the saw-cut and Zipper cores were each considered samples of separate populations. The t-test allows comparison of two population means while controlling the probability of making a Type I error.

A Type I error is committed upon rejection of a true null hypothesis in favor of a false alternative, where the null hypothesis is the postulation that the population means are equal and the alternative is the conjecture that one mean is larger than the other. The probability of occurrence for a Type I error is denoted by the symbol a, which is selected by the researcher as the tolerable level of error for the given experiment.

The value of a is compared to the level of significance, or p-value, computed from the sample data in the t-test, where the p-value represents the probability of observing a sample outcome more contradictory to the null hypothesis than the observed sample result. When the p-value is less than or equal to a, the null hypothesis can be rejected, leading to acceptance of the alternative hypothesis. However, when the p-value is greater than a, one must conclude that insufficient evidence exists to reject the null hypothesis.

The null hypothesis in this research was that the average population bond strengths of the saw-cut and "zipped" joints were equal, and the alternative hypothesis was that the Zipper joint had a higher average population bond strength than that of the saw-cut joint.

Analyses were conducted using a standard error rate of 0.05. At this level, only a 5% chance existed for falsely claiming that the two joint treatments were significantly different. After the data were checked to ensure compliance with statistical test requirements, the t-test was performed using a pooled standard deviation and yielded a p-value of 0.047.

Because the p-value is less than the selected value of a, one can conclude that sufficient sample evidence exists to reject the null hypothesis and accept the alternative. Therefore, in asphalt similar to that used in this experiment, the scarification imparted to vertical cut faces by the Asphalt Zipper can reasonably be expected to improve patch bond strength better than the saw-cut method of asphalt removal.

By W. Spencer Guthrie, Ph.D., and Keith Woffinden

--Guthrie is an assistant professor in the Department of Civil and Environmental Engineering at Brigham Young University. Woffinden is an undergraduate student and research assistant.
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Title Annotation:Asphalt Maintenance
Author:Guthrie, W. Spencer; Woffinden, Keith
Publication:Public Works
Date:May 15, 2004
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