By Sara S. Tabatabaei and Alfredo Bustamante

A new five-level parking garage is constructed in Arizona and located adjacent to multiple high voltage transmission lines. The contractor first experienced electrical shocks as they started construction of the third level. For obvious safety concerns, construction phasing was altered and an electromagnetic consultant was hired to provide a thorough investigation.

The electromagnetic specialist measured electrostatic and magnetic induced voltage/current and recommended installing a grounding system to resolve the electrical shock issue. The grounding system is designed to establish a low resistance path to ground. When metals are placed in the electromagnetic field, they act like an antenna picking up a voltage related to the magnetic flux, or the rate of change in the magnetic field. If the induced voltage on metal discharges through an electrolyte, such as concrete, the result can be corrosion. To minimize the effects of corrosion, a piece of metal is completely electrically isolated or connected to the grounding network so that the induced current discharges directly to the ground.

Next step after installing a grounding system, was to determine the amount of current that is flowing into the steel reinforcement of the parking structure (both mild steel and post-tensioning steel). A corrosion consultant was hired to assess the potential impacts from induced voltage in the steel reinforcement and metal elements of the building because of the electromagnetic field surrounding the transmission lines.

Field Testing

The corrosion consultant performed four different field testing; grounding conductor measurements, prestress tendon tail voltage to ground, potential gradient survey of each deck, and antenna voltage to ground tests. A summary of each test is presented below:

  1. Grounding Conductor Measurements: This test verified that grounding conductors are attached to the reinforcement and that conductors running from the columns to the ground rods were exposed to obtain the alternating current (AC) current flowing through them. The data collected from this test indicated that ground conductor current measurements vary from 0.55 A to 1.68 A.
  2. Pressure Tail Voltage to Ground: The voltage between the primary slab tendon tails at level 4 and 5 to the ground rod were measured. Pressure tendon tail voltage varied from 0.24 to 24.5 at Level 4 and 5. The current to ground for these two levels varied from 0.05 mA to 0.19 mA.
  3. Potential Gradient Survey of Each Deck: A potential gradient survey was conducted to test the voltage differences between two identical reference electrodes placed in contact with a wetted concrete surface. This test confirms that the grounding system is minimizing the potential differences that occur naturally.
  4. Antenna Voltage to Ground: The antenna test consisted of a reinforcing bar oriented parallel to the transmission lines and was connected through a voltmeter to the ground rod in the building. The collected data indicates that significant voltages can be imparted to metallic objects near transmission lines, and that the voltages drop off significantly as the antenna moved away into the structure.

Disscussion

Stray currents are defined as electrical currents flowing through electrical paths other than the intended paths. Stray currents can arise from railways, cathodic protection (CP) systems, high-voltage power lines, or other resources. Stray currents can deviate from their intended path because they find a lower resistance, parallel and alternative route to flow.

Stray current interference can result in localized corrosion of reinforcing bars when current leaves or, in some cases, enters the steel. Reinforcing steel in concrete is susceptible to stray current because concrete is an electrolyte, thus a conductive medium, that supports the pickup onto and subsequent discharge of stray current from the embedded steel.

Stray current effects do not always result in corrosion of the embedded reinforcing steel. The damage to the reinforcing steel depends on several parameters, such as the type of current (AC or DC); the current strength; the presence of interruptions in the circulation of current; and the chloride content in the concrete.

The risk of corrosion induced by stray currents on steel embedded in sound, alkaline, and chloride free concrete can be extremely low. However, the presence of small amounts of chloride can lead to a noticeable decrease in the charge required to initiate corrosion. In the case of concrete structures contaminated by chloride, stray currents may have very serious consequences, even if the levels are too low to initiate pitting corrosion. Chlorides in concrete reduce its resistivity facilitating the circulation of stray currents1.

Stray AC currents are known to be much less damaging to steel than stray direct currents (DC). The effects of stray AC currents are more complex than the effects of stray DC currents. It has been proposed that by application of an AC current with a density less than 1.9 A/ft2 on bare steel, there is likely no risk of corrosion, between 1.9 and 9.29 A/ft2, corrosion is possible, and at current densities greater than 9.29 A/ft2, corrosion damage is likely2. For the studied parking structure, based on the corrosion consultant’s report, measurements of the grounding conductors current showed a range of 0.3 A to 1.68 A meaning that there was a low risk of corrosion of the steel in this structure.

The electrical resistivity of concrete is an important parameter for corrosion of embedded steel because it can facilitate or hinder the circulation of stray currents. Concrete with high resistivity provides protection for the embedded steel from stray current corrosion because it reduces the current flow3. Waterproofing membranes and coatings on the concrete surface can also provide an electrical barrier to the flow of stray currents.

The tail voltage measurement data indicated relatively small current values ranging from 0.05 mA to 0.19 mA AC. In addition to that, the post-tensioning system at the parking garage is electrically isolated because it is a fully encapsulated system. Therefore, it is unlikely that corrosion of the post-tensioning system will develop from an induction based current.

Recommendations

We recommended to apply a protective top deck traffic coating to the Level-5 slab and to the outermost 18 feet perimeter of parking garage floor slab at all supported levels to avoid any water/chloride intrusion into the slab, early corrosion, and accelerate of corrosion by stray currents. All exposed metals on level-5 should be connected to the grounding system. This parking garage needs an annual periodic inspection of the grounding rods including stray current measurements.

Conclusion

A good grounding system can minimize the effects of stray current and the potential differences that occur naturally. The risk of corrosion induced by stray currents on steel embedded in sound, alkaline, and chloride-free concrete can be extremely low. At low and moderate potentials and in the absence of chloride, steel corrosion caused by stray currents is unlikely. Waterproofing membranes and coatings on the concrete surface provide an electrical barrier to the flow of stray currents.

References

  1. Civil Engineering and Public Works, Corrosion of Prestressing Tendons, A.D. Blackie, Review 66, 74 (1971): pp. 53-55.
  2. 3R International 32, Investigation of Corrosion of Cathodically Protected Steel Subjected to Alternating Currents, G. Heim, Th. Heim, H. Heinzen, W. Schwenk, 5 (1993): pp. 246-249.
  3. Technologic Papers of the Bureau of Standards, Electrolysis in Concrete, No. 18, U.S. Department of Commerce, Government Printing Office, Washington DC (1913). E.B. Rosa, B. McCollum, O.S. Peters.

Sara S. Tabatabaei, Ph.D., P.E., is a Structural Restoration Engineer at Walker Consultants in Houston, TX. She can be reached at stabatabaei@walkerconsultants.com. Alfredo Bustamante, P.E., CDT, is a Vice President and Managing Director of Forensic Restoration and Building Envelope Services at Walker Consultants. He can be reached at ABustamante@walkerconsultants.com.

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