Epoxy-coated rebar resists chlorides, oxygen, and moisture by providing a barrier around the steel. The coating also serves as an electrical insulator, minimizing the flow of corrosive current. Factory application involves cleaning the steel bars and applying powdered epoxy, then heating and curing at high temperatures. The downside to epoxy coating is the bars can become relatively easily damaged, so they require special handling and storage. During installation, each bar must be inspected for voids, cracks, or thin spots, as any holidays (i.e. discontinuities) in the coating create sites of accelerated, localized corrosion. Bent bars have less corrosion resistance than straight bars, as the coating tends to become damaged in the bend area.
Galvanized rebar applies a zinc coating to the steel reinforcement, sacrificing the zinc to electrochemical action. If the coating is damaged, it can self-heal to some degree, but galvanized rebar only resists corrosion for as long as there is zinc left to sacrifice. Avoiding galvanic coupling between coated reinforcement and uncoated steel is critical—all bars and hardware must be coated with zinc, and cut ends and welds must be coated with zinc-rich primer.
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 corrodes preferentially. In the sacrificial metal’s presence, 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 continues 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.
Ultimately, a belt-and-suspenders approach to controlling corrosion in a new parking structure may be the best strategy. Sound design and construction practices are a must, and, given the expense and difficulty of treating corrosion once it is underway, it is prudent to apply multiple, complementary methods to protect against the deterioration of embedded reinforcement.
Protection of existing structures
Surface treatments are the primary means of protection for existing parking decks, as they are non-destructive and relatively straightforward to apply. Preventative coating use and prompt addressing of cracks and surface defects can avert corrosion and significantly prolong the garage’s life.
Penetrating sealers enter the concrete to form a barrier that prevents water and chloride ions from migrating to the reinforcement. Typically composed of silane and/or siloxane, these treatments are vapor-permeable, allowing trapped moisture to evaporate out of the slab. (Silanes are often considered better for horizontal surfaces that act as wearing surfaces, as they will penetrate deeper due to smaller molecule size. However, in practice, this author finds both silanes and siloxanes need to be reapplied at roughly the same intervals. [However, this varies greatly from garage to garage, based on the amount of traffic experienced.]) They are also relatively inexpensive and quickly applied, with little downtime. However, the treatments require reapplication every five years or so, and do not bridge cracks, which means corrosion can still initiate from water and dissolved salts penetrating through unrepaired openings in the concrete.
Crystalline sealers are a relatively new technology. They react in concrete to grow crystals that seal pores and micro-cracks against water intrusion. As long as moisture remains present, crystals continue to form. The crystalline sealers grow to fill new cracks as they form, reactivating in the presence of moisture to impart self-sealing properties to the concrete.
These sealers may be incorporated into new concrete as an admixture, or applied to existing structures as a surface treatment. However, because parking decks are subjected to constant movement from traversing vehicles, crystalline sealers may not be as effective as they are in other applications, as the recurring movement can break the three-dimensional array formed during the crystallization process.
Traffic-bearing membranes are elastomeric coatings that form a barrier, locking out moisture and chlorides to protect the underlying concrete and reinforcement. Composed primarily of epoxy, methylmethacrylate, or urethane, traffic-bearing membranes are flexible and can bridge cracks, but they are also very expensive and require periodic recoating, as their pliability makes them susceptible to abrasion damage. The lengthy downtime for application, which can take several days, may also be a consideration.
A migrating corrosion inhibitor (MCI) can be applied to the surface of an existing concrete parking structure, as well as incorporated as an admixture into new concrete construction. In theory, a surface-applied MCI is drawn into the pores of the concrete through capillary action, penetrating down to the reinforcing steel level. Through ionic attraction, the MCI adsorbs into the steel, forming a protective coating that displaces chlorides and other corrosive compounds. In practice, however, some studies have shown the MCI may not successfully reach the steel reinforcement in some applications. (For example, see Stephen R. Sharp’s “Evaluation of Two Corrosion Inhibitors Using Two Surface Application Methods for Reinforced Concrete Structures,” published as Virginia Transportation Research Council [VTRC] 05-R16 in December 2004. Visit www.virginiadot.org/vtrc/main/online_reports/pdf/05-r16.pdf.) Preliminary research indicates vacuum and pressure-injection methods may assist in driving MCI through the concrete to the steel.
To determine which surface-applied protective treatment is right for a specific parking garage, the design professional should consider several factors, including:
- facility age;
- level of deterioration;
- chloride content;
- concrete quality;
- exposure; and
Since the type of corrosion protection determines the maintenance demands and future treatment options in years to come, it is worth undertaking an assessment of the parking garage before settling on a product.