by Steven J. Susca, PE
Parking garages are an integral part of our nation’s infrastructure. Although they are subject to more deterioration than other building types, their maintenance is typically not considered of primary importance to building owners or managers, who often are compelled to prioritize high-profile façade issues or roof leaks above a patch or two of unsightly concrete. Still, deferred maintenance eventually means costly repairs. 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, which are 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 due to cracks, flaws, thin coverage, or poor concrete consolidation 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.
Fortunately, there are preventative measures building owners, managers, and designers can take to protect against the onset of this corrosion. For garages already exhibiting signs of corrosion, various treatment options can stop the cycle of damage and restore structural integrity. Good design and construction practices are central to the prevention of reinforcement corrosion, as are products and materials that help prevent corrosive elements from reaching embedded steel. Creating favorable conditions that can overcome corrosion-inducing electrochemical reactions can also be very helpful.
By identifying early warning signs of reinforcement corrosion and responding promptly, building owners and managers can avoid or mitigate the expensive and time-consuming repairs that typically result from unchecked parking structure deterioration. (The rate at which deterioration progresses will accelerate over time.)
How corrosion works
When steel is exposed to the acidic environment created by dissolved chloride salts and water, the effect is that of a giant battery. As oxygen diffuses into the concrete, it reacts with water to form hydroxide ions at the steel’s surface, creating the cathode (i.e. negative pole of the battery).
To maintain electrical neutrality, an anode forms through an oxidation reaction where positively charged iron ions migrate away from the rebar, leaving electrons behind and forming a pit in the steel. The iron ions travel toward the cathode by way of an electrolyte solution in the concrete pore structure (usually composed of dissolved chloride salts in water). The remaining negatively charged electrons then travel along the bar toward the cathode where the aforementioned hydroxide ions are formed through a reduction reaction.
Under the acidic conditions of the salt solution, the ferrous ions initially lost from the steel recombine with the hydroxide ions at the cathode to form hydrated ferric oxide (i.e. rust), which is deposited at the interface between the steel and the surrounding concrete. Without the presence of the electrolyte solution in the concrete, this reaction cannot occur.
As the steel corrodes, it expands up to eight times its original volume. Expansive forces cause extreme pressures within the concrete, which eventually are relieved when the concrete cracks. In turn, these cracks admit more water and chloride salts, accelerating the corrosion process, which leads to more cracking, then more corrosion, in a progressive cycle of damage. Over time, the cumulative process reduces the cross-sectional area of the reinforcing steel, compromising the parking structure’s structural integrity.