FHWA NDE CENTER RESEARCH & DEVELOPMENT
Bridge Acoustic Emission Local Area Monitoring
System
The rising demand for ensuring the integrity and performance of our nation's
bridges that are usually subjected to severe environmental effects require inspection for evaluation of their structural conditions. A
combination of load and corrosion loss is the main cause of deterioration in steel bridges. For concrete bridges in long-term service,
the degradation caused by corrosion of reinforcement due to chloride ingress and concrete cracking is the major problem. The rate
of deterioration is unpredictable in both materials. Thus, the danger to motorists and the economic consequences that the failure of
a bridge would create is self evident.
For years, it has been proven that a structure under a certain load levels produces acoustic sound. The phenomenon of acoustic
sound generation in structures under stress is termed as acoustic emission (AE). Acoustic emission is simply the stress waves
generated in the materials due to deformation, crack initiation and growth, crack opening and closure, dislocation movement,
twining and phase transformation, fiber breakage and delamination in composite materials. The sources of AE are predominantly
damage-related and AE monitoring leads the investigator to the prediction of material failure. Over the past decades, various
acoustic emission monitoring devices are built for nondestructive testing and evaluation of materials.
The Local Area Monitoring (LAM) is an AE monitoring instrument, developed by Physical Acoustic Corporation (PAC) under joint
cooperative agreement with Federal Highway Administration (FHWA). The FHWA research program has supported the LAM system
design and development over the past years. It is intended to assist in evaluation of bridge infrastructure condition to ensure safety,
to minimize catastrophic structural failure and promote efficient maintenance plan. The FHWA Nondestructive Evaluation (NDE) Center conducted laboratory evaluation and field testing of LAM system.
The LAM system is mainly designed to monitor the AE activities from a known or suspected defects on small areas on the
bridge. Examples of the type of applications suitable for local area monitoring include determining whether a known or suspected
crack is growing, determining if retrofits have halted crack growth, monitoring of a weld or connection point to assess structural
integrity.
LAM, shown in figure 1, is a portable, rugged, and modular 8 channel system for AE monitoring of outdoor structures such as
bridges under harsh environmental conditions.
Figure 1: The LAM System with
8 sensors and holding magnet
It can be easily mounted and operated in the close proximity to the area being monitored. The unit operates on its own internal
battery for typical daily tests and/or AC power. The system can be controlled by direct interface via keyboard, monitor and mouse or
it may be used as a remote monitoring station with a PC computer connected via serial port cable or modems including wired and
wireless modems.
Recently the LAM system was used on the NYDOT prestressed concrete box beam test in Turner-Fairbank Highway
Research Center (TFHRC), shown in figure 2. The deteriorated concrete box beam was repaired initially then it was strengthened
with fiber-reinforced graphite epoxy composite laminates. The fiber-reinforced graphite epoxy composite laminates were supplied
for the test by Mitsubishi Chemical Corporation. The beam was loaded to failure while the LAM system was set up with 8 AE
sensors attached to the beam to monitor AE activities from concrete cracking, delaminations in concrete, delaminations in FRP,
and/or adhesive bond failure at the interfaces.
Figure 2:
AE monitoring of
NYSDOT boxbeam test
For more information, please contact either by e-mail or telephone.
Frank Jalinoos (202) 493-3082
frank.jalinoos@fhwa.dot.gov
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