Raised question

Safety Article January 13, 2011
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The worst of driving conditions calls for the best of roadway markers. With their reflective properties, retroreflective raised pavement markers (RRPMs) have guided many nervous drivers safely to their destination on rainy nights.


The durability and performance of RRPMs are critically important to departments of transportation nationwide. A few years ago, the Texas Department of Transportation (TxDOT) noticed an increased number of RRPM failures resulting from poor retention on pavements, physical damage and loss of retroreflectivity. In some cases, mass failures occurred when an entire section of RRPMs disappeared only weeks after installation.


“All the markers that TxDOT was using met the requirements set by ASTM specifications,” said Yunlong Zhang, a Texas Transportation Institute (TTI) assistant research scientist. “However, RRPM performance varied significantly, and the results from existing testing methods also did not correlate with field performance. We were asked to identify or develop new lab-testing methods that would help us more accurately predict marker performance in the field.”


Over a three-year period, TTI researchers conducted multiple tasks that included lab and field tests, as well as surveying TxDOT districts and RRPM manufacturers to gather information on existing test procedures and marker field performance.


Researchers also monitored four test-deck locations for two years. The decks were chosen based on their traffic condition and pavement type to provide a range of test data under different conditions. One test deck was on the 610 Loop in Houston, a high-volume, concrete roadway. Another was on a low-volume road with a flexible pavement.


“RRPM failures are not only a public safety issue, but also expensive when you have to close the roads for repairs,” said Zhang. “With the results of this research, we were able to recommend that TxDOT emphasize the quality of RRPM installation, since we found it directly relates to performance in the field. And TxDOT is also now able to better predict the life expectancy of these markers for all types of roadways and traffic volumes.”


Swinging pendulum


During the project, the team discovered that many of the failures of RRPMs began with the fracture of the outside shell.


These failures could be caused by something as simple as a stone wedged in a vehicle’s tire tread. Consequently, failure occurred due to the impact of small, hard objects with the surface of the RRPM.


“What we needed was a testing procedure that evaluated the ability of the RRPMs to absorb energy-of-impact-type loading,” said Zhang. “Since there was nothing that existed, we designed and fabricated the pendulum-impact test device.”


The pendulum-impact device allows users to test the durability of the RRPMs outer shell using different weights.


The RRPM is clipped into place, and a weighted arm swings down and impacts the marker. Different weights can be added to the end of the pendulum arm to increase the force exerted on the marker on impact. The marker support is adjustable, so four different impact points can be tested to give a full evaluation. The results of using this test correlate strongly with RRPM field performance.


“We tested six RRPMs with this device using all six weight configurations at each of the four impact positions,” said Zhang. “Using this device to test markers before they are installed will give TxDOT a better idea of the durability and performance they can expect, particularly in high-traffic areas.”


The next step is for TxDOT to test the device across the state and see how it performs in terms of assessing quality control for qualifying RRPM products. Potentially the device could take the place of TxDOT’s more traditional ball-drop test, which involves a more complicated procedure and produces less consistent results. Other states, specifically Ohio and Louisiana, are interested in the device as well.


“The researchers did a great job of modeling the forces on a pavement marker from vehicular impact. This was cutting-edge work,” said Project Director Darren Hazlett of TxDOT’s Construction Division.

About the author: 
The authors are technical writers for the Texas Transportation Institute.
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