Download Stray electric current - a little known cause of steel

 Stray electric current - a little known cause of steel
reinforcement corrosion
C Stanley*, EverCrete International (Asia) Ltd, Hong Kong SAR
27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 29 - 30 August 2002,
Singapore
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Stray electric current - a little known cause of steel
reinforcement corrosion
C Stanley*, EverCrete International (Asia) Ltd, Hong Kong SAR
ABSTRACT
Surveys of steel reinforcement corrosion usually concentrate on low cover, high rates of
carbonation and the presence of chlorides in the concrete as the only causes of steel
corrosion in embedded steel reinforcement. However there are many instances where the
severe corrosion of the steel cannot be explained and where the bars have reasonable
cover in a relatively dry environment; where chlorides are not present in the concrete, and
there are no other aggressive agencies present.
The cause may be due to stray electric currents particularly from lift motors and certain
utilities that run on direct current electricity. Such electricity if it comes into contact with
embedded steel can cause rapid deterioration of the metal.
STRAY CURRENTS
The writer was first aware of this problem when studying the rapid deterioration of
reinforcement in a tunnel belonging to one of the utilities in Hong Kong. The tunnel was
completed in 1985 and within five years the steel was severely corroded. Great
expenditure was involved in removing the steel from the tunnel sections and re-casting
the concrete. In addition, other areas including ceiling soffits began to spall off revealing
severe corrosion beneath the surface of a fairly dry concrete which contained no
aggressive agencies that could promote the corrosion. However carbonation and poor
quality concrete were stated to be the cause and stray current was not even considered to
be a factor. Examination of the steel convinced the writer that some unusual problem
must be contributing to the corrosion process but it was several years before he was able
to assess the problem and understand the cause.
85 One day the owner of a flat, who was highly environmentally minded, contacted the
writer and stated that his house was 'magnetic'. He was very concerned 0 the presence of
magnetism and what it would do to his health. The writer re-assured him that magnetism
was all around, and without it to assist navigation, how would aircraft and ships be able
to travel around the Earth. However the flat owner insisted that the magnetism in his flat
was very severe.
A visit confirmed that there was indeed a problem. A TV set was mounted on a table
1.5m above the floor. If the TV was placed near the wall or floor the picture degenerated
into a beautiful rainbow pattern. If a computer was placed near the wall any floppy disks
were wiped of data.
A check of the wall and floor with a compass resulted in the magnetic north being upto
180 0 out of phase. In addition, the strongest magnetic variation was in the vertical
reinforcing bars in the wall.
Normally magnetism can both be created and destroyed.
magnetism is to sweep the area with a powerful AC coil.
The way of removmg
In this instance the floors and walls were swept with a 30 amp coil and the magnetism
was removed. However it returned to full strength within twelve minutes.
When a piece of reinforcement was cut from the wall it was found to be only very
slightly magnetic.
It was therefore decided to put a voltmeter across the cut ends of the reinforcement and a
voltage of 3V D.C. was recorded.
It was also noted that the voltage fluctuated between 1.7V and 3.2V and that the
fluctuations corresponded to changes in emission from a nearby source ofD.C. voltage.
Voltage may leak to earth from a number of sources and for a variety of causes. The
moist ground in this case acted as the electrolyte, and the current entered the pile
foundations, and because of the ambient humidity and the electrical continuity of the
steel, the voltage passed through the building and turned the reinforcement into an
electro-magnet. This generated such a strong magnetic field that a Gaussmeter placed
against the wall registered full-scale deflection.
A steel corrosion problem in some buildings is caused by leakage from D.C. lift motors,
security systems and other sources. The results of D .C. current leakage can have a
devastating effect on steel reinforcement.
For example where potential in corroding steel is measured using the half-cell potential
test, voltages above-200 mV are normally indicative of serious corrosion activity. Even
voltages below this value may be indicative of corrosion activity albeit at a lower level.
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High levels ofD.C. voltage can destroy reinforcement very rapidly.
Normally this cause of corrosion is overlooked during surveys but may be significant.
Although D.C. current can be induced from external sources, it can also develop as a
result of increasing corrosion activity.
Steel can be de-magnetised by using powerful A.C. coils, and another method is to use a
series of earths driven into the ground to earth the system.
Sometimes the voltage cannot be dispersed in which case be prepared for high future
maintenance costs.
In conducting a survey of steel corrosion always be aware that the source of corrosion
may not be obvious, or maybe due to unusual or a combination of causes. Stray current
though seldom considered is a serious cause of steel corrosion.
Stray currents from AC systems are also thought to influence the corrosion of steel
reinforcement but as yet very little research as been carried out on this aspect. However
it is likely that the influence of stray currents makes a very significant contribution to the
corrosion of embedded reinforcement in concrete and this could have very serious
implications in highly stressed sections or where pre-stressed concrete tendons are
involved.
Although modem railway systems are addressing the problem of stray currents and trying
to control the leakage more effectively, there may be a number of other sources of stray
currents leading to steel corrosion that have yet to be identified and understood.
Steel reinforcement corrosion surveys should therefore address the possibility of stray
currents as a possible influencing factor where the cause of steel corrosion is not obvious
or indeed where it could be due to a combination of factors including stray currents.
87 Plate 1
A television set where the picture has broken up as a result of stray currents in the
reinforcement of the concrete wall in the background.
88 Plate 2 Checking the magnetic variation in a wall using a compass. 89 Plate 3
A steel reinforcing bar destroyed by stray current corrosion over a period of 7 years. Part
ofthe bar has been treated with a rust converter to stabalise the corrosion process.
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