PARKING GARAGE COLLAPSE
About ten years ago, we began to hear a lot of news regarding deteriorated concrete, particularly in bridges and parking garages. Since then we have all seen and experienced the results. Regular users of the Gardiner Expressway in Toronto have grown accustomed to summer traffic tie-ups caused by the annual, April to November concrete rehabilitation of that structure. Other notable local repairs include the Toronto City Hall Parking Garage and the CN Tower. Less noticeable are the many damaged apartment balconies.
The most common cause of concrete deterioration in Canada is well known. The road salt and water of bridges and parking decks reach the steel reinforcing through cracks and by permeating the concrete. Building exteriors are attacked by the salt mist created by heavy traffic. Mist from the Gardiner is reported as the cause for damage well up the CN Tower. With time, the natural alkalinity of the concrete is overcome by increasing amounts of salt which creates a reaction between the salt solution and steel. As a result of the reaction, the reinforcing steel rusts. Rust occupies more space than the original steel and the expansion breaks the concrete apart. For the rusting of the reinforcing steel to occur the following conditions are needed.
a high salt (soluble chloride-ion) concentration in the concrete
moisture in the concrete
suitable temperature, deterioration advances more quickly at higher temperatures.
Concrete deterioration begins out of sight. Regular inspection will reveal the first signs of deterioration; the appearance of small cracks over and parallel to the reinforcing steel locations, and rust staining on the surface of the concrete. (Note that not all cracks in concrete are due to rusting reinforcement - concrete also cracks due to thermal stresses and other causes). Further rusting often breaks the bond between the steel and concrete resulting in delamination. At this point, strength is significantly reduced because the steel no longer transfers the necessary tensile strength to the concrete.
Concrete deterioration from a variety of causes is common throughout the world. Salt in sea water attacks marine structures. Coastal structures become soaked in salt bearing spray. Some deserts with a high salt content sand are very aggressive to concrete. Various types of pollution, particularly sulphur compounds will attack concrete.
Repair of deteriorated concrete is an expensive, time consuming, noisy (jack hammers or high pressure water are normally used), dirty and for some structures such as the Gardiner Expressway, long term, process. Unless all of the concrete contaminated with salt is removed, deterioration will continue once the new concrete is poured. In fact, the deterioration may even speed up! Unfortunately, because of cost and the time a structure must be under repair, total removal is often not an option. At a typical parking garage, only the most deteriorated (i.e. Loose or missing) concrete is replaced every three to five years.
Experience with concrete deterioration has lead to new standards such as CSA S413 and the recognition of the following factors:
Dense impermeable concrete slows the movement of salts towards the steel reinforcing.
A greater thickness of concrete over the steel reinforcing lengthens the time before salts reach the steel.
Epoxy coated reinforcing steel can inhibit rust. Note, however that local rusting can occur more quickly where the epoxy has been damaged.
Good drainage in slabs in conjunction with a waterproof membrane prevents the movement of salts and water into the concrete.
Even with these measures, some concrete deterioration will eventually appear. Especially where cracking has occurred as a result of overloading or thermal movement.
Concrete structures constructed with techniques other than standard reinforcing are also subject to deterioration. Many post tensioned bridges and parking garages have been constructed in the last decade. Steel cables are run through small diameter openings in these structures. The cables are tightened with anchors at either end of a span. The taut cables put the concrete in compression over the length of the span, squashing it if you like. This takes advantage of concrete's high natural strength in compression and avoids stressing its low tensile strength. It has the further advantage of closing any cracks that might otherwise appear. This keeps salt and water infiltration to a minimum.
Unfortunately, the high construction standards needed for this type of construction have not always been maintained. Problems include inadequate concrete cover over anchors and punctured protective sheaths around the cables. Both may lead to rusting and sudden failure. In comparison to the number of reinforcing bars found in typical construction, there are relatively few post-tensioning cables. Thus, each one is critical and a single failure may lead to a large collapse. Because of the hidden construction of the cables and anchors it is nearly impossible to inspect for deterioration without breaking away concrete.
In contrast, sudden failure of normal reinforced concrete is rare. Provided regular inspections are made the condition of the concrete can be observed and ongoing deterioration noted.
Presently concrete restoration contractors in Ontario report little work despite the many acknowledged cases of concrete deterioration. Some government work particularly on bridges is ongoing but very little restoration of Apartment Building Garages is reported. This lack of activity attributed to recent changes in the rent control legislation. Landlords can no longer increase rents to cover repair costs so the work remains undone. This situation will lead to more numerous failures. Mostly falling debris injuring people and damaging property. The possibility of larger slab collapses is also increased, i.e. the Big Owe in Montreal, but previous small failures usually result in minimal preventive work such as the installation of shores or closing of the affected structure.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2