Federal Highway Administration Design Manual: Deep Mixing for Embankment and Foundation Support

CHAPTER 9: CONCLUSIONS

The goal of this research project was to develop an integrated monitoring system that, using state-of-the-art sensing capabilities and NDE technologies, was capable of providing information about the conditions of high-strength steel wires inside a main cable of a suspension bridge.

Extensive experimental tests were conducted to test a variety of sensors and NDE technologies that could fit the specifics of field applications in real suspension bridges. The information provided by such a system can be useful in more reliably assessing the remaining service life and the current factor of safety of a main cable. Indirect sensing technologies are useful in monitoring environmental variables that can be related to corrosion and in detecting the onset of corrosion activity, serving as an early warning system. In addition, internal sensors can eventually be used for testing the reliability of a cable dehumidification system, in the case of pressure losses and leaks as well as dead zones along the cable, providing valuable information for estimating the optimum life-cycle cost of the dehumidification system in terms of operation and maintenance cost. The direct sensing system can provide a qualitative assessment of the cable condition along its entire length (e.g., number and location of broken wires) so that a detailed inspection of the main cable can be focused on the most critical segment of the cable.

The proposed sensor network system was successful in providing measurements of parameters such as temperature, relative humidity, and corrosion rate from the interior of a main cable. These parameters are essential for understanding the interior environment of a suspension bridge's main cable. It was the first time that an experiment of such a scale was attempted on a main cable. The construction of an environmental testing chamber permitted the testing of various environments affecting the temperature, relative humidity, and corrosion rate distributions within a full-scale mockup cable and provided a realistic test for the durability and reliability of the selected sensors. A corrosion rate analysis highlighted the clear differences in the dependency of the corrosion process on environmental variables like temperature and relative humidity. As expected, the selected sensors showed that increased levels of relative humidity resulted in increased levels of corrosion activity and that there is a strong linear dependence between the variability of the corrosion rate and temperature variations.

After extensive experimental tests, the selected sensors were integrated in a corrosion monitoring system that was installed on an in-service Manhattan Bridge cable. Field testing was used to determine the real-world functionality of the system and proved to be successful. The field pilot program was essential in validating the laboratory results and showed good correlation with the original assumptions.

The main conclusions of this research project are as follows:

• The selected sensor network system was successful in providing information on the interior environment of a suspension bridge's main cable, helping understanding the conditions in which main cables of suspension bridges operate.
• Tests performed on sensors focused on the accuracy of the measurements as well as on the capability of the sensor to survive in the interior of a suspension bridge's main cable. 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 and of the surrounding environmental chamber allowed researchers to test 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. In addition, the extensive testing of such a specimen allowed researchers to obtain a large amount of information on the various environments affecting the temperature, relative humidity, and corrosion rate distributions within a main cable.
• 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. Through monitoring, it was observed that increased levels of relative humidity resulted in increased levels of corrosion activity and that a strong linear correlation exists between the corrosion rate and the temperature.
• Field testing of the system on the Manhattan Bridge in the City of New York was used to determine the real-world functionality of the system and proved to be successful. The results from the field pilot test were essential in validating 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. Among those, the technology based on the main flux method performed well during the laboratory phase, even on the large cable mockup, and should be further investigated. The technology based on the combination of AE and MS showed great results and promise for small bundles of wires but suffered when dealing with relatively larger cables. It might be used at the anchorage site.
• 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 demonstrated that it is possible to measure corrosion activity inside 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. Because of the continuous advancements in sensor and NDT technologies, it is important to pay attention to any new developments that can help improve such a monitoring system.