Zach Nikin’s Take on Concrete Deterioration

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Many factors play a role in concrete deterioration. This article will focus on the two headliners as they pertain to my beloved parking garage. Embedded reinforcing steel adds strength concrete, right? Well, it can also weaken the structural integrity by contributing to concrete deterioration. The Midwest experiences both extremes of the weather spectrum. We have frigid winters and balmy summers. The frequency of a structure’s freeze-thaw exposure also contributes to concrete deterioration. My parking garage has fallen prey to both of these factors, but the City of Columbia has a plan.

Ironically, the leading cause of concrete deterioration is the very same thing that adds strength to the structure: its steel bones. For those comic book fans out there, I’ll use Wolverine to illustrate my point. Warning, spoiler alert! Wolverine’s adamantium bones make him nearly indestructible, but they’re also slowly poisoning him. We see the same scenario play out with reinforced concrete. Steel bones increase the concrete’s strength, but when the steel begins to corrode, the increased volume in a confined space stresses the concrete. Over time, this results in cracking and spalling.

Fig. 1 demonstrates how the expansion of rebar due to corrosion can cause cracking in a concrete deck. Fig. 2 is a good example of the resulting spalling. Fig. 3 shows the extent to which the rebar has corroded. Once the rebar was completely exposed to the elements, it’s likely that the corrosion process accelerated. The weakened welded wire reinforcement (WWR) began to break apart. The result was Wolverine-ish spikes. This was obviously concerning to me as a tenant in this garage because I didn’t want to pop a tire, so I reached out to the city.

If you’re from the Midwest, then you know that our winters are like something from Game of Thrones. They’re harsh and they last for years… well, it feels like they do, anyway. Another factor that affects concrete deterioration is freeze-thaw. Fig. 4 shows the frequency of freeze-thaw exposure across the United States. Concrete is a porous building material. Water expands when it freezes. Do you see where I’m going with this?  When moisture is present in the pores of the concrete and the temperature drops below freezing, BOOM! The resulting pressures from the expanding moisture in the pores stress the concrete and cause cracking and crumbling.

Over time, this repetitive process can cause significant damage. One way to mitigate this is by sealing the concrete so that moisture cannot impregnate the pores so easily. When I spoke with the city, they told me that this particular garage hadn’t been sealed since its construction. They also brought to my attention that a majority of the deck damage was located where a vehicle would initiate a turn. Friction from the turning tires likely wore the protective surface down more in these areas, thus making them more susceptible to freeze-thaw conditions. Winter is coming, so it’s a race against time to repair and resurface the decks.

With Columbia being the #1 city in America (source unknown), obviously there was a plan in place to restore my beloved parking garage to its original glory. Walker Consultants, a Chicago-based engineering firm, was contracted by the city to design the specifications and plans for the restoration. Fig. 5 details how the areas are to be repaired. The existing damaged concrete will be removed for a minimum of ¾” clearance around all exposed rebar. This clearance will allow the contractor to clean the corroded reinforcement and apply a corrosion inhibitor. Once all the WWR has been cleaned and coated, the GC will infill new mortar. Fig. 6 is the finished product.

There are examples of concrete deterioration are all around us if you know where to look. Some are very subtle, while others might fall right on top of your head. This one hit close to home for me because I watched it develop slowly over the course of a year. I became involved out of fear for my car tires, but the civil engineer in me became interested in how the city planned to remedy the issue.