**4. Bridge Monitoring**

AE monitoring on large bridges has a long history of successful outcome [5,9,12,146–148]. Initial emphasis was on steel truss structures as these contain many fatigue-prone joints in difficult to inspect locations. Fatigue damage of bridges has always presented technical challenge and remote monitoring capability with AE has become a viable solution [149]. Well-known fatigue damage in recent years was at the San Francisco–Oakland Bay Bridge, which led to a large-scale AE monitoring of the affected bridge segments following a successful demonstration of AE's capability [150,151]. For this project, 640 sensors of 60-kHz resonance frequency were used to monitor 384 eyebars covering over 6-km distance. The main aim was to detect cracks of 2.5-mm size. By the turn of the 21st century, bridge engineers' interests in acoustic (emission) monitoring of long-span suspension bridges finally reached the level for its practical implementation as shown by Hovhanessian [152]. Initial impetus came from the realization that a protection scheme of cable wire galvanizing (zinc coating) plus red lead (Pb3O4) paste was found inadequate on the Brooklyn Bridge and Williamsburg Bridge after nearly a century of continuous use. After 90–100 years, zinc coating was gone and many broken wires due to corrosion were found [153]. While these broken wires were repaired, more broken wires were also found in many newer bridges. On the Forth Road Bridge in the UK (a main span of 1006 m, opened in 1964), Colford [154] reported 8 out of 11,618 wires were broken at one of test locations in 2004. However, about 90% of the wires inspected exhibited Stage 3 (heavy) and 4 (severe) corrosion (Stage 5 being broken wire, according to NCHRP534 2004 [155]). After rehabilitation of the suspension cables along with the installation of a cable dehumidification system, fortified AE sensors were installed on the Forth Road Bridge in 2006 at 15 stations on each main cable (with about 140 m spacing over 2000 m span between anchorages). AE signals have been remotely monitored according to Hovhanessian [152], who earlier installed a similar AE monitoring system on the Bronx Whitestone Bridge (New York) in 1997 and Ancenis Bridge (France) in 2003. Additionally, M48 Severn Bridge (UK), Humber Bridge

(UK), Quincy Bayview Bridge (cable-stayed, US), Lane Memorial Bay Bridges (US), and a dozen others suspension bridges have AE monitoring systems installed [156].

The suspension-cable dehumidification system was invented for use on the Akashi Kaikyo Bridge (Japan) in 1998 and more than two dozen such systems have been installed globally for new bridges and for rehabilitated ones [156]. While it can keep wire corrosion to a minimum, perhaps eliminating the need for AE, suspension wires used with a conventional protection system will start to experience corrosion problems after 10–20 years. On such bridges, AE monitoring can detect serious deterioration on bridge elements. Hopefully, more AE installation can help reduce the number of structurally deficient bridges in the future.

Section 4 dealt with bridge monitoring using AE. This topic is usually discussed only among project participants or covered in civil engineering circles, but some of the reports provided technical details presented above.
