TECHBRIEF Corrosion Monitoring Research

of New York City Bridges

FHWA Publication No.: FHWA-HRT-14-024

FHWA Contact: Paul Virmani, HRDI-60, (202) 493-3052, paul.

virmani@dot.gov.

This document is a technical summary of the Federal Highway 

Administration report, Corrosion Monitoring Research of New 

York City Bridges (FHWA-HRT-14-023).

Introduction

Currently, all State and local agencies responsible for the maintenance of suspension bridge cables base their maintenance 

plan mainly on previous experiences and on information from 

limited inspections. Usually, the exterior covering of the cable 

is visually inspected biannually. If deterioration problems are 

suspected and if the maintenance budget allows, then indepth 

inspections are conducted. The cable is then unwrapped at a 

few locations along the cable length and is wedged into its 

center. Next, a visual inspection of the wires’ conditions is 

performed, and in some cases, a few wires are cut and removed 

for laboratory testing.

Unfortunately, current visual inspections do not provide an 

adequate amount or sufficiently reliable data, indicating the 

need for innovative non-destructive testing (NDT) and sensing 

technologies that can provide an immediate, comprehensive, 

and reliable assessment of cable conditions and their evolution 

with time either directly or through measuring-related variables 

(i.e., temperature, humidity, etc.).

Approach

The objective of this research project was twofold: (1) to install 

a network of sensors that can monitor the internal environment 

of main cables and provide information that can be used to 

indirectly assess the cable’s deterioration conditions and their 

evolution over time (indirect sensing method and (2) to explore 

and possibly further develop the most recent and promising

NDT technologies for direct detection of the corrosion 

damage inside main cables of suspension bridges (direct 

sensing method). 

A complete literature review and a series of validation tests 

of the available corrosion monitoring techniques and sensors were conducted. The goal was to find out about the 

state of the art of the currently available sensor technologies, 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

 www.fhwa.dot.gov/research
2

especially corrosion monitoring techniques, 

and to see whether these sensors/technologies could be applied to the monitoring of main 

cables in suspension bridges. The following 

sensors/technologies were considered worthy 

of further investigation:

•	 Linear polarization resistance (LPR) sensors.

•	 Coupled multiple array sensors (CMASs). 

•	 Bimetallic (BM) or galvanic sensors.

•	 Temperature-relative humidity sensors.

•	 Acoustic emission (AE).

•	 Magnetostrictive (MS).

•	 Main magnetic flux method (MMFM).

The main lesson learned from the literature 

review was that no single means of corrosion 

detection (sensors or technologies) is ideal 

or suitable for assessing all possible forms of 

corrosion-induced damage. The key point is 

the peculiarity of the structural element (in this 

case, the cable) under consideration. For example, there are many sensors available on the 

market that can measure temperature and 

relative humidity, but only a few of those can 

be used in a bundle of over 10,000 steel wires 

subjected to strong compaction forces with 

difficult accessibility to the core. Hence, the 

sensor/technology selection must be performed 

as carefully as possible to avoid failure of the 

monitoring system in service.

Indirect Sensing Methods/Technologies

LPR Sensors

LPR sensors provide an estimate of the corrosion rate by measuring the polarization 

resistance of the sensor. The main advantages 

of these sensors are that they are small and 

flexible and can be easily inserted between wires. 

Nonetheless, particular care must be taken to 

protect the electrodes from direct contact with 

metal to prevent shortening the circuit. In this 

study, LPR sensors were extensively tested in a 

series of cyclic accelerated corrosion tests either 

alone or embedded in bundles of bridge wires.

CMASs

CMASs were chosen as a corrosion rate monitoring sensor due to their geometry (a cylinder 

with a similar diameter as a bridge wire so that 

it fits perfectly inside the wire bundle). These 

sensors are made of filaments of either steel or 

zinc, and each filament acts as either an anodic 

or a cathodic area. The filaments are externally 

connected by an instrument that is capable of 

measuring the electron flow and, subsequently, 

converting the flow into a non-uniform corrosion 

rate using Faraday’s Law.

BM or Galvanic Sensors

This type of sensor is basically a galvanic probe. 

For the sensors used in this study, the reference electrode was represented by a filament of 

copper, while the sensing electrode was a filament of either steel or zinc. In the presence of a 

corrosive environment, the current measured in 

the two electrodes is an indication of how much 

the sensing electrode is corroding under the 

polarized condition. 

Temperature-Relative Humidity Sensors

Precon HS2000V humidity sensors were used in 

this study to measure temperature and relative 

humidity inside the main cable with an accuracy

of ±2 percent. This sensor combines capacitivepolymer sensing technology with a novel 

measurement method, eliminating the need for 

temperature correction and calibration by the 

user. It covers a standard temperature range of 

32 to 158 °F with a temperature output range of 

-22 to 212 °F and can operate within a humidity 

range of 0 to 100 percent.

Direct Sensing Methods/Technologies

AE

AE testing is a powerful method used for examining the behavior of materials deforming under 

stress. AE may be defined as a transient elastic 

wave generated by the rapid release of energy 

within a material when the local stresses around 

a discontinuity reach levels close to yielding. 

The high level of stresses can cause cracks 

to grow and fibers to break as well as many 

other modes of active damage in the stressed 

material.

MS

MS technology is based on a phenomenon in 

metals known as “magnetostriction” in which 

the presence of a magnetic field induces small 

changes to the physical dimensions of ferro-
3

magnetic materials and vice versa. When a 

pulse of electrical current is applied to a coil 

around a component of ferromagnetic material, a time-varying magnetic field is applied 

to the component, which in turn generates a 

pulse of elastic waves that propagates in both 

directions along the length of the component. 

Conversely, when the propagating elastic pulse 

reaches a receiving coil, it causes a change in 

the magnetic induction of the material via the 

inverse MS effect and consequently an electric 

voltage that can be amplified, conditioned, and 

processed. When damage occurs in a component, the propagating elastic waves are reflected 

back, and complex wave interference patterns 

arise, which can be analyzed to assess the location and amount of damage.

Combination of AE and MS Technologies

As part of the development of a corrosion condition detection system for suspension cable 

bridges, a study on the potential combination of 

the AE and MS technologies to detect wire breaks 

and consequent reduction of the cable’s cross 

section was performed. The principal idea behind 

the AE-MS is that a baseline measurement of the 

present state of the cable can first be established. 

Then, by interrogating the system periodically, 

any variation of the wave propagation characteristics of the system can be related to a certain 

type of damage. In this type of measurement, the 

MS generates an elastic wave that propagates 

along the cable, and its propagation characteristics are established by analyzing the response 

of the AE sensors to the propagating acoustic wave. If damage appears in the cable, the 

propagation characteristics of the cable change 

in the vicinity of the damaged area and thus the 

characteristics of the acoustic signal propagating 

along the cable. In this way, an array of AE 

passive sensors serves as continuous online 

monitoring detectors of wire brakes or other 

type of damage in combination with MS.

MMFM

MMFM is currently being used for the condition assessment of suspenders, but prior to this 

study, it had never been applied to a main cable 

of a suspension bridge. The fundamental principle of this technology is that when a steel 

wire, rope, or cable is longitudinally saturated 

by a strong magnetic field, the magnetic flux 

in the wire, rope, or cable is proportional to the 

wire’s, rope’s, or cable’s cross-sectional area. 

When a magnetic field moves along the length 

of the cable, then the measured variation of 

the magnetic flux at a given location may be 

correlated to the change in cross-sectional area 

at that location. In this study, this technology 

was successfully tested on the full-scale cable 

mockup. It is recommended to test such a technology on more deteriorated cables.

Full-Scale Cable Mockup

One of the main tasks was to evaluate the 

selected sensors and technologies on a fullscale cable specimen to recreate as closely as 

possible in-service conditions. To accomplish 

this objective, a 20-inch-diameter, 20-ft-long 

cable specimen was fabricated in the Carleton 

Laboratory at Columbia University. The setup 

consisted of a cable specimen made up of 

73 127-wire hexagonal strands for a total of 

more than 9,000 0.196-inch-diameter steel 

wires. Of the 73 hexagonal strands, 7 were 35 ft 

long and were subjected to tension, while the 

remaining 66 strands, which were 20 ft long, 

were unloaded. This cable specimen was placed 

in a loading frame, and the long strands were 

subjected to a load so to induce stresses up 

to 100 ksi. The total length of the experimental 

setup was over 35 ft. An environmental chamber 

was built around the specimen (see figure 1),

which was designed to expose the cable to 

controlled environmental conditions (i.e., 

simulated rain, heating, and cooling) to accelerate corrosion of the cable specimen, thereby 

Figure 1. Cable specimen with environmental 

chamber.
4

assessing the functionality of the sensor 

network. 

A total of 72 sensors were installed in the 

cross section of the cable. Figure 2 provides a 

schematic representation of the locations where 

sensors that successfully performed during the 

tests were placed. Sensors were placed along 

three diameters inclined at a 60-degree angle 

with respect to each other. Such distributions 

allow for measurements that are distributed 

along the radial direction in order to have a 

three-dimensional distribution of the variations 

of the various parameters (e.g., temperature, 

humidity, etc.). The sensors were hard wired, 

and the electrical wires were run out of the 

center of the cable at two vertical locations.

After the cable mockup was built, a series of 

cyclic corrosion tests was performed to test 

the sensors in order to design the monitoring 

system for main cables of suspension bridges. 

These tests consisted of subjecting the cable 

specimen to cycles with varying durations of 

rain, heat, and cooling. Each experiment lasted 

for days. At the end of each test, data were 

analyzed, and changes of the test conditions 

were made if necessary. The total duration of 

this experimental phase lasted about 1 year.

Results from Accelerated Cyclic Tests

The data collected by the sensors during the 

entire environmental campaign were analyzed 

to assess the capability and reliability of the 

sensor monitoring system.

Temperature and Relative Humidity Sensors

The temperature measurements recorded by 

the Precon HS2000V sensors provided an accurate and reliable picture of the temperature 

distribution inside the cross section of the 

cable. The largest temperature increases and 

fluctuations were recorded in the outer level of 

the cable’s upper section because of the proximity of the sensors to the heat sources. The 

temperature variations within the cable’s cross 

section diminished moving further away from 

the heat source.

The interpretation of the relative humidity 

data was more complex because general trends 

were not as identifiable for the relative humidity data. This is because water can penetrate 

inside the cable from many different locations, 

spread between the wires, and reach the sensors 

at different times. However, it was found that 

relative humidity levels varied significantly 

across the mockup cable cross section, especially in the outer layers. Sensors located closer 

to the cable surface recorded elevated relative 

humidity levels, while those closer to the cable 

center recorded substantially lower levels.

Corrosion Rate Sensors

Some of the LPR sensors recorded corrosion 

rates above noise values, with a strong correlation with the corresponding temperature 

measurements. In fact, corrosion rate values 

recorded by LPR sensors appeared to follow 

distinct cyclic trends associated with temperature, while initial rate levels were affected 

by relative humidity. Similarly, the CMAS 

sensors identified cyclic fluctuations in corrosion 

rate readings as well as the fluctuations of the 

surrounding environmental variables.

The other sensors considered in this testing 

phase did not show the same sensitivity as those 

of the LPR and CMAS sensors. The recorded 

measurements were consistently below the 

noise threshold which could be due to either 

a reduced sensitivity of the sensor or to a lack 

of corrosion activity in the proximity of the 

sensor itself.

Analysis of Wire and Sensor Conditions

In an effort to directly determine the effects 

of cyclic corrosion testing, the cable mockup 

specimen was unloaded and disassembled. 

Figure 2. Schematic locations of sensors in cable 

mockup cross section.
5

Once the wrapping was removed, a general 

inspection of the cable was performed. It 

appeared that the cable was heavily covered 

by a white powder, which was a mix of salt 

residue and zinc oxide. In a couple of spots along 

the length of the cable, small pockets of water 

were found. Heavy corrosion appeared where 

the wires exited the cable. The neoprene strap 

that served as a protection for the electric wires 

kept the wires in a wet environment for a long 

time (stage 4 uniform corrosion). Areas of high 

ferrous corrosion occurring underneath the 

stainless steel straps were easily identifiable in 

the south central location, whereas the north 

side of the cable seemed to have experienced 

little to no ferrous corrosion under the stainless 

steel straps. With regard to the conditions of 

the wires underneath the cable bands, they 

experienced various amounts of corrosion.

In the interior of the cable, the majority of the 

wires showed substantial salt deposits accompanied by deterioration of the zinc coating. 

Areas of salt deposition and zinc coating 

corrosion did not show any particular pattern 

and seemed to follow the potential path of 

the water that penetrated inside the wrapping. 

With regard to the sensor conditions, almost all 

of the sensors appeared to have survived the 

cable compaction. Few corrosion rate sensors 

suffered extensive damage. 

Field Installation on a Cable on a 

Suspension Bridge

The last phase of the project called for the installation of the developed integrated system of 

sensors and technologies on a real cable of one 

suspension bridge in New York City. In agreement with the New York City Department of 

Transportation, cable D of the Manhattan Bridge 

was chosen. Built in 1912, this suspension bridge 

has 4 main cables, each made of 37 strands 

of 256 wires each for a total of 9,472 wires 

bundled in a cable with a 21-inch diameter.

The developed sensor system was installed on 

two 19.7-ft panels of cable D on the Brooklyn side 

span between panel points 13 and 15. The main 

cable was wedged at 4 groove positions along 

the circumference, and 38 sensors (19 per panel) 

were installed. The sensors were wired to a data 

acquisition system located at roadway level with 

wireless modem, which in turn transferred the 

data to a secured server. Once the sensors were 

installed, the cable grooves were closed, and the 

cable rewrapped with neoprene wrapping. The 

collection of data lasted for almost 11 months, 

after which all the sensors and the data acquisition system were removed. 

From the data analysis of the various sensor 

measurements, it appeared that all the sensors 

performed well and provided unique and valuable information. An example is provided in 

figure 3, where the distribution of the maximum 

and minimum daily temperature and relative 

humidity over the cable’s cross section are presented for different seasons.

Conclusions

The main conclusions of this research project 

include the following:

•	 The selected sensor network system was 

successful in providing information on 

the interior environment of a suspension 

bridge’s main cable, helping researchers 

understand the conditions in which main 

cables of suspension bridges operate. 

•	 The sensors selected in this study showed 

good reliability in all the testing phases, 

from the stand-alone sensor test in a small 

corrosion chamber to the insertion into a 

real main cable of a bridge.

•	 The construction of a cable mockup as 

well as the surrounding environmental 

Figure 3. Distribution of temperature and relative 

humidity throughout the cable cross section during 

winter, spring, and summer. 
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Researchers—This study was performed by Columbia University, Parsons Transportation Group, and 

Physical Acoustics Corporation. 

Distribution—The unpublished report covered in this TechBrief is being distributed through the 

National Technical Information Service at www.ntis.gov.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.

gov/research.

Key Words—Suspension bridge cable, Corrosion, Corrosion sensor, Acoustic emission, Main flux 

method, Magnetostriction, Remote monitoring.

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability for the 

use of the information contained in this document. The U.S. Government does not endorse products or 

manufacturers. Trademarks or manufacturers’ names appear in this TechBrief only because they are 

considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides high-quality 

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December 2013 FHWA-HRT-14-024

HRDi-60/12-13(600)E

chamber allowed for the testing of the 

sensor system in conditions as close as 

possible to the in-service conditions, 

providing insight into the performance of 

the various types of sensors. 

•	 A detailed analysis of the data recorded 

during the laboratory phase highlighted 

the distinct differences in the dependency 

of the corrosion rate with environmental 

variables like temperature and relative 

humidity. Increased levels of relative 

humidity corresponded to increased levels 

of corrosion activity. There was a linear 

correlation between the corrosion rate and 

the temperature.

•	 Field testing of the developed monitoring 

system on the Manhattan Bridge in New 

York City was used to determine the 

functionality of the system. This proved to 

be successful, as the results from the field 

pilot test validated the laboratory results 

and showed good correlation with the 

original assumptions.

•	 During the field test, the sensor monitoring 

system was successful in detecting the 

onset of corrosion at one location caused 

by a damaged protective wrapping. This 

showed that this system could also work 

as an early warning system for the safety of 

the cable.

•	 Some of the technologies for a direct 

sensing system showed great potential for 

future applications.

•	 The system can be commercially used 

on any suspension bridge cables and can 

be adapted with some modification to 

stay cables and possibly post tensioning 

tendons.

In conclusion, this study demonstrates that it 

is possible to measure corrosion activity inside 

the main cables of suspension bridges. The 

information provided by such a system can be 

used to make more reliable estimations of the 

safety factor and remaining service life of such 

important structural elements as well as to help 

bridge engineers in conducting more efficient 

and cost effective inspections.