20 May, 2010, Kingston, Ontario....... Rust Speeds Parking Garage Collapses!

Did rust cause the collapse of Kingston's Confederation Place Hotel's parking garage? Probably. It's a major factor in almost all concrete structure failures in the Northern Hemisphere. In this particular instance, engineers won't know for sure until inspections are finished, but it was certainly a contributing factor, as it is in almost all structural failure in reinforced concrete structures.

Road salt, water and above freezing temperatures make our underground parking garages the rustiest indoor environments imaginable.

The problem is caused by the melting of the salty slush behind our cars' wheels in winter. The parking garage, protected from ice and wind, is often warm enough to melt salty ice throughout the entire winter. PLOP! It falls onto the concrete decking and the water dribbles away, downhill, of course.

Running downhill takes this salt water into any small (or large) cracks in the concrete. Once water reaches the steel reinforcing rods (rebar) in the concrete, severe rusting begins. Since rust is several times larger than the steel it replaces, the rebar expands and further cracks the concrete. Internal pressures then cause chunks of concrete to split off, like stale icing on a doughnut. This is termed "spalling". Spalling allows even more salty water to spread into the core of the concrete and attack more of the rebar.

It's a vicious circle of corrosion.

Since it's warm enough in the garages to melt the winter's salty slush, it's warm enough to cause rapid rusting. Outside it's colder. Whenever temperatures drop below freezing, corrosion is much slower.

It's these same wet, salty, warm conditions that make springtime so rusty for our cars. Once a few downpours have rinsed off the roads and highways, these extreme conditions largely disappear until next spring. Because covered garages never experience heavy rain, this catastrophic corrosion condition exists inside them almost year round.

The salt that penetrates into the concrete and rebar and drains off a parking garage doesn't just attack in the winter. It's there and is activated for most of the year--every day there's a dew or rain. Roofed or not, the dripping cars moving about a garage distribute the water everywhere. Once it combines with salt, rampant, hidden rust, begins anew. Warmer spring & summer temperatures make it worse. There's little rust around the freezing point in the presence of salt water. With every few degrees rise in temperature above the freezing point, the rate of corrosion increases dramatically.

It's a catastrophe waiting to happen. Much of this corrosion is deep inside the concrete and is invisible until a chunk of cement falls off.  By then, it's extremely difficult to repair in a way that restores the original strength of the structure. Commonly, the cracked concrete is jack hammered away and then the exposed rusted rebar is sandblasted clean. Wet cement is next poured on the shiny rebar. Problem solved? Not exactly.  Now the rebar is thinner than originally called for and the water in the concrete quickly starts secondary corrosion in this de-rusted, thinner steel rebar.

New construction methods use epoxy coatings on the rebar to add some protection from salt and water. This is a help, but it's not perfect, as the coating can crack as it is bent to fit into the forms. It can also be scratched by aggregates and stone in the concrete mix during pouring. Even an unbroken coating will split and crack as the steel in the rebar rusts naturally under any circumstance. No-one is specifying high grade stainless steel for rebar usage. It's more likely an alloy of recycled steel (including steel from your old rusted, re-cycled jalopy.)  It's just a matter of time before corrosion begins on any rebar--shorter times in our northern climate, of course.

Rust is attacking all of Kingston's concrete parking garages. Indeed, corrosion is attacking all the infrastructure in the Rust Belt of North America and Europe. The City of Kingston has fast tracked their parking garage repairs. Let's all hope it's in time and effective.

Interesting article on concrete spalling.
http://activerain.com/blogsview/1391224/spalling-concrete-due-to-rusting-steel