Static and dynamic assessment of the influence of asphalt layers on the structural response of steel guardrail posts

Abstract

The practice of installing asphalt layers (usually called “mow strips”) around guardrail posts is common in many states where vegetation growth around and adjacent to the guardrail system is problematic. Recent studies have indicated that these layers can produce excessive ground-level restraint on the post, which may result in unsatisfactory performance of the guardrail system. However, the influence of critical design parameters related to the asphalt mow strip on the structural behavior of guardrail posts has not been evaluated. This study presents the results of a comprehensive structural performance assessment on a guardrail subcomponent system including a standard steel post installed through an asphalt mow strip. A total of 19 static tests were conducted to provide a first-stage evaluation of mow strip design alternatives, and a total of 14 dynamic tests were performed to provide a more comprehensive evaluation of the dynamic behavior of these design alternatives. Specifically, a dynamic test protocol using a high-speed hydraulic actuator was proposed as an alternative test method. The reduction of the thickness/width of the asphalt behind the post (rear distance) as well as the installation of pre-cuts resulted in less ground-level restraint than a mow strip with a conventional leave-out. The variability in asphalt strength as a function of temperature and age was evaluated by uniaxial compression tests on asphalt cylindrical specimens. A simplified semi-empirical analysis model was constructed and calibrated using experimental data to predict the expected structural performance of posts installed with a specific mow strip design condition. Design guidance for mow strips was developed based on the analysis results and the service conditions including the critical design temperature and the target asphalt age.