Strategies for maintenance of highway bridges in the US - with the support of nondestructive testing and structural health monitoring

Strategies for maintenance of highway bridges in the U.S. - with the support of nondestructive testing and structural health monitoring. The majority of the more than 600000 inventoried bridges in the U. S. are inspected in 2-year intervals, resulting in more than 300000 bridge inspections per year. These inspections are performed mostly by the highway authorities of the individual states. With increasing traffic loads and aging infrastructure, these inspections are becoming increasingly important. Thus, according to the National Bridge Inventory (NBI), over 70000 bridges (12 %) are considered structurally deficient and over 14 % can no longer meet their functional requirements. Despite this fact, bridge inspections today - 40 years after the introduction of the National Bridge Inspection Standards (NBIS) - are still predominantly visual in nature. The use of modern technologies such as non-destructive testing (NDT) and structural health monitoring (SHM) remains mostly restricted to so-called Landmark Bridges and to the use of simple technologies such as sounding, magnetic particle inspection or concrete cover measurements. This paper presents the evolution of U. S. bridge inspection in the last 40 years and describes the current state of the art. Existing national and local strategies for the application of NDT and SHM in so-called technician and expert systems are discussed by means of case studies. Finally, the current state of research in the U. S., including the development of multi-sensor scanning systems, the methodology of air-coupled ultrasound and the application of ultrasonic piezoelectric and nano sensors for bridge monitoring is presented.