For owners or managers who suspect corrosion is already underway and damage is occurring, the first step is to identify the extent of the problem. Unless corrosion is severe enough to force off the outer face of the concrete, reinforcing steel is generally hidden within the concrete slab, making any visual identification of early stages of corrosion difficult or impossible. Instead, the concrete is evaluated through field and laboratory testing to determine whether conditions conducive to corrosion exist within the concrete structure.
Chloride ion content testing identifies the concentration of chlorides in concrete at various depths to evaluate the probability a corrosive environment exists. Dust samples from incremental depths through the concrete slab are extracted and sent to a testing laboratory for analysis.
Half-cell potential testing determines the electrochemical behavior of embedded steel by measuring its electrical potential (i.e. the difference in charge from one area to the next). The greater the electrical potential, the greater the risk corrosion is occurring. Conducted onsite, the test involves removal of concrete cover over reinforcing bar, followed by the connection of exposed steel to an electrode through a voltmeter. Half-cell potential readings can be used to generate an electrical potential map, indicating areas with the greatest and least risk of corrosion.
Loss of steel reinforcement is a concern for areas where corrosion has progressed at an advanced rate. Where reinforcing bar is exposed or where concrete is cracked, delaminated, or spalled, a structural engineer should evaluate the remaining slab’s structural capacity to determine whether corrosion has compromised its loadbearing ability.
Where corrosion-induced spalls have been previously repaired, a characteristic ‘halo effect’ might be observed, with a ring of corrosion staining appearing around the patch site. Patching delaminated and spalled concrete with conventional concrete can lead to an electrochemical reaction at the interface between the existing chloride-contaminated concrete and the new concrete. The large difference in electrical potential between the two, combined with the short distance between anode and cathode, leads to accelerated corrosion. Usually, such patches need to be repaired again in just a year or two.
Instead, spalls should be repaired using patching mortars with integral corrosion-inhibitors to protect against accelerated corrosion at the patch site. Migrating corrosion-inhibitors can also be applied to the concrete surface where testing has revealed chloride contamination and a high probability of corrosion to arrest the electrochemical process before damage becomes pronounced. Severely corroded rebar may need to be supplemented or replaced to restore structural integrity.
Choosing the right strategy
No single approach can guarantee protection against reinforcement corrosion for all parking structures. Determining the best way to prevent and treat the underlying causes of corrosion involves consideration of garage conditions and exposure, concrete quality and construction, environmental contaminants, and other factors specific to the structure and situation. Initial cost and maintenance demands are also important decision criteria.
Often, the most successful strategy involves a multi-component approach, combining preventive treatment with an ongoing program of assessment and repair specifically tailored to the structure to keep corrosion at bay. (Many jurisdictions require annual inspection of parking ramps.) Ultimately, the time and expense required to prevent corrosion and treat early warning signs is far less than that of rehabilitating a garage that has succumbed to corrosion-induced structural failure.
|IMPRESSED CURRENT CATHODIC PROTECTION|
Where galvanic anodes cannot deliver sufficient current to prevent corrosion, impressed current cathodic protection (ICCP) may be used. As with passive cathodic protection, ICCP reverses the electrochemical process of corrosion through the action of an applied electric potential—in this case, the current arises not from the inherent properties of the materials themselves, as it does with galvanic coupling, but from an external power source.
However, care must be taken in designing and installing ICCP systems in parking structures. This is because excessive current density may cause the alkaline concrete to react with acid generated by the anode, leading to concrete damage.
In an ICCP system, it is difficult to provide protection at any significant distance from the anode, since current distribution within concrete is poor. Therefore, anodes must be placed no more than about 0.3 m (1 ft) apart, and the anode material must remain continuous throughout the structure. The ICCP system must take into consideration differing proportion and placement of reinforcement throughout the parking structure to avoid voltage drops from one area to another.
Steven J. Susca, PE, is senior engineer with Hoffmann Architects Inc., an architecture and engineering firm specializing in the rehabilitation of building exteriors. He develops engineering solutions for corrosion and other forms of reinforced concrete deterioration, both for existing parking structures and as preventive treatment for new construction. Susca can be reached via e-mail at firstname.lastname@example.org.