Facility Management Contractors are often tasked with maintenance of parking structures. Made of concrete and steel, these multi-level hubs provide visitors and their vehicles with shelter from the elements and often provide access to housing or office space. However, protecting the structure itself from the constant attack of environmental stressors and wear-and-tear comes with its own set of challenges.
Vehicles regularly entering parking garages leave water, oil and dirt behind that can corrode the structure’s concrete and steel support system.
One of the greatest issues related to the deterioration of parking structures is the corrosion of embedded reinforcement. Structural concrete used in parking structures is strengthened by means of steel reinforcement bars, or “rebar,” which is embedded into the concrete to improve resistance to tensile and compressive stresses. Ordinarily, the surrounding concrete protects this embedded steel from the corrosive effects of water and dissolved salts in the environment. However, breaches in the concrete, whether due to cracks, flaws, thin coverage, or poor concrete composition, can allow steel reinforcement to come into prolonged contact with corrosive elements. As the steel corrodes, it expands, leading to further damage to the concrete, greater water infiltration, and additional corrosion in a self-perpetuating cycle of deterioration. If not arrested early on, the progressive nature of the cracking and corrosion can eventually lead to an unsafe structure and can cause costly repair.
There are several ways contractors can retrofit concrete parking structures to ward off the effects of chloride-induced corrosion.
One of the effective ways to stop corrosion is the use of a cathodic protection system.
Corrosion is the electrochemical process of reinforcing steel losing electrons and decomposing to iron oxide. Reinforcing steel that loses electrons acts as an anode. One way to stop further loss of electrons, and t h e re f o re stop corrosion, is to reverse the current flow and turn the steel into a cathode.
Passive cathodic protection controls steel corrosion by connecting the reinforcing bar to a sacrificial anode, a metal that is more active than steel and so will corrode preferentially. In the presence of the sacrificial metal, the steel surface becomes polarized to a more negative potential, until the driving force for the oxidation of the steel is removed. The galvanic anode will continue to corrode until it is consumed by the electrochemical reaction and must be replaced. Galvanized rebar is one example of passive cathodic protection, where the zinc coating acts as the sacrificial anode. Other commonly used galvanic anodes include magnesium and aluminum-based alloys.
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. Care must be taken in designing and installing ICCP systems in parking structures, however; 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 a foot 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, so as to avoid voltage drops from one area to another.
Choosing the Right Strategy
Different approaches nowadays may or may not 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, one which combines preventive treatment with an ongoing program of assessment and repair to keep corrosion at bay. Ultimately, the time and expense required to prevent corrosion and treat early warning signs is far less than that of rehabilitating a garage that succumbs to corrosion induced structural failure.