ASPHALT PLUG JOINTS: CHARACTERIZATION AND SPECIFICATIONS

Abstract

The asphaltic plug joints (APJs) are a type of bridge expansion joint that have been promoted as simple general purpose expansion devices for bridges with less than 50 mm (2 in.) total motion. APJs have been developed by trial and error and used when little engineering research existed outlining their complex behavior. To help designers to better understand this behavior and be able to properly apply them, the University of Wyoming has developed engineering-based design guidelines and material specifications. With the material relaxing by viscous flow at about the same rate as the bridge places motion demands on the joint, minimal loads and stresses are created. With these small loads almost no limit on the allowable motion exists from a structural perspective. However, functional limitations exist from a volumetric perspective and the need to maintain a smooth and serviceable transition from the pavement onto the bridge deck. This volumetric limit is the basis for the bridge-motion limits that are proposed. Two material characteristics fully define the structural applicability of an APJ location/application. These are the material time constant for the load to relax to 75% of its initial load and the material glass-transition temperature. Both of these qualifying characteristics may be obtained using a UW-modified AASHTO TSRST test. This test was used in this research and is proposed for future use. It needs to be further refined and qualified. While the relaxation allows for the material to function from a structural perspective, it can be detrimental to roadway roughness. The design tradeoff is that a soft viscous material allows bridge motion but also allows the material to flow out of the wheel paths creating rutting. Another problem is that the asphalt binder is highly modified and will lose ductility as it ages, resulting in structural failures. These problems can be mitigated and some concepts are included in this report. In summary, APJs are good for some applications but should not be considered maintenance-free, long-term solutions. Periodical replacement will be required.