Bending and Impact Testing of Wood Guardrail Posts Evaluated using Stress Wave Timing Inspection

Abstract

Guardrail systems throughout the U.S.A. often use preservative-treated wood posts. When a vehicle strikes the system, the posts should have sufficient fracture resistance to ensure they perform as intended by rotating in the soil, dissipating crash energy. Preservative chemicals used in wood guardrail posts do not always penetrate deep into the wood; therefore, the outer shell can appear to be in good condition, with a core that is decayed. Stress wave timing (SWT) is a proven technology for evaluating the condition of wood, especially for detecting possible internal decay. This study examines the efficacy of the new SWT prototype to detect internal decay, and to indicate if posts are compromised with respect to flexural strength and impact resistance. SWT proved to be an effective non-destructive testing method for field inspection of wood guardrail posts with 86% accuracy of decay detection. Post-bending stress grade was found not to be a reliable predictor of guardrail post impact integrity. Of 66 posts tested with internal decay, only nine failed to meet the AASHTO M168-07 minimum stress grade in static flexure. However, dynamic impact testing clearly demonstrated that the impact resistance of decayed posts was significantly lower than that of sound posts. Therefore, if SWT is deployed for inspection of wood guardrail posts it is recommended that any posts with stress wave velocity less than a predetermined threshold (such as that reported here) be further inspected using a minimally invasive tool and removed from service to decrease the likelihood of fracture during a collision.