Corrosion Can Cause Collapse
A suspension bridge hangs from two main cables, which comprise thousands of steel wires. Protecting these from corrosion is paramount because, ultimately, if the cable wires fail, the bridge fails.
Corrosion is caused by a combination of moisture, oxygen and warmth. As there is oxygen in the air and the bridge is warm (at least in the summer), the only recourse is to prevent water from coming into contact with the wires. When the 2500 ft. Angus L. Macdonald suspension bridge in Nova Scotia, Canada, was constructed, the cables were protected by the best system known at the time. The individual wires were galvanized, the bundle of galvanized wires was coated with red lead paste, and the cables were wrapped with more galvanized wires (forming a sort of cocoon), and finally the whole lot was painted.
This traditional system worked well for many years, but recent investigations showed that despite the elaborate precautions, water has penetrated the cocoon on the Macdonald Bridge, which in time will lead to increased corrosion. Fortunately, the corrosion to date is minimal, but it is paramount we do not let it progress.
A Puff of Air
So, is there a better way of protecting the cables? Yes, the new method is “dehumidification,” which is based on the principle that corrosion of steel occurs only if the relative humidity of the air around it is above roughly 40%. In concept, the process of avoiding corrosion thus becomes very simple. The cables are wrapped with a special airtight tape and dry air from an air conditioning unit is pumped through the cables from one end. If the air inside the new airtight cocoon is kept below a relative humidity of 40%, corrosion will not even start.
This project marks the first use of this new method in Canada.
But, the corrosion already exists…
As mentioned, it is already known that there is water in the Macdonald Bridge cables. Fortunately, this does not pose a problem. When dry air is first pumped in at one end, it will be moist when it emerges at the other end, but gradually all the moisture will be extracted, and a sensor at the exhaust end will indicate when this has happened. From that point on, it should take little effort and power to keep the cables dry. If the outer wrap should leak, the exhaust sensor will indicate an increase in humidity, and the leak can be found and repaired.
Of course, the system must be operational and powered virtually all the time, so there is some cost for this. However, the routine painting of the cables is no longer needed, saving time and maintenance costs.
One of the greatest benefits of this method is in the reassurance that corrosion of the cables is not occurring because the exhaust sensors indicate the relative humidity level. Before this system, the only way of checking the condition of the cables was to unwrap them and look. This has two disadvantages; more than a spot check in a few places becomes very costly, so inspectors were able to assess only small portions of the cable, and the cable cannot be rewrapped to its original standard, meaning inspections could actually lead to an increased rate of corrosion. Now, the sensors will complete the inspection work, and engineers can have confidence in the condition of the cables.
COWI was one of the first companies in the world to install cable dehumidification. That was on Storebaelt Bridge in Denmark, the world’s third longest span suspension bridge.