Usually docile and well behaved, it feels like lately many of our city’s concrete beasts have turned vicious over the summer. Bridges are closed, tunnels collapsed, roads are giving away under out wheels… what did we do to deserve this?
In order to understand the finer ponits of taming and looking after the countless concrete beasts that inhabit our urban jungle, I spoke to a structural engineer who works for a local consulting firm. “Theres a lot of stuff falling down for very different reasons,” he says.
Know your concrete
Do not confuse concrete with cement – “that irks me!” says the engineer, “Cement is a powder. It’s just a powder!” When the cement is mixed with water and crushed rocks of various sizes, a chemical reaction takes place which releases heat and bonds everything together. The resulting concrete is tough under compression, but nearly useless under tension. That’s why most strucutres, especially those that are supposed to support heavy loads while hanging in midair, are reinforced with a lattice of steel bars (aka rebar).
The lifespan of concrete
The most commonly used building material in the world (some 20 billion tons are produced annually) has about the same lifespan as a human being. With quality ingredients, good design and regular care, reinforced concrete structures may be lucky to live a hundred years. In Quebec, the best concrete bridges and tunnels are expected to retire by the age of 70 or so. But few of them seem to make it that far without some kind of breakdown. Some were only really designed to stay up for 35 years; others simply did not receive the care and attention they require.
Easy on the salt!
It is pretty much unavoidable for tiny cracks to form in the surface of the concrete over time. Quebec’s annual 60-degree temperature fluctuations don’t help, but the real trouble is the salt used to de-ice the roads. The salt penetrates the tiny cracks and attacks the steel inside, causing it to rust. As the steel rusts, the bonds between the steel and concrete come apart, and the two materials are no longer able to reinforce eachother.
Caring for your concrete
There are plenty of ways to keep concrete strong and healthy for longer: one way is to build the superstructure out of steel, and then coat it with something like cadmium, zinc, or an alloy that will keep the rust at bay. Proper drainage is another thing to watch, and a low-salt diet would almost certainly improve longevity.
But even the healthiest concrete structure needs regular checkups: inspection every 2-3 years is recommended. Look for symptoms like rust appearing on the surface of the concrete. Surgical intervention is usually requried on a 15-year schedule: slice open the structure, put in some new rebar, and then cast a new concrete casing.
If you want to see this in practice, take a stroll under the Turcot interchange. “The Turcot is not falling down, it’s just under constant repair,” the engineer clarifies.
Collapse in the age of concrete
As you can see, concrete is a rather high maintenance creature, one whose adoption requires life-long commitment and care. Now, like a dog turned vicious after years of neglect, our concrete infrastructure is turning against us with potentially deadly results.
After the de la Concorde overapss in Laval collapsed in 2006 (“shoddy construction, followed by very poor inspection”), the MTQ vowed to overhaul the infrastructure inspection system. This took effect in 2009. Next thing you know, the Turcot interchange is under urgent repairs and the Mercier bridge is closed.
“The closer you look at something, the more defects you’re going to find,” points out the engineer, suggesting that the new zeal for inspection may be responsible for what seems like a sudden surge in concrete troubles. “But if the maintenance work isn’t being done afterwards, what’s the point of inspecting?” he adds.
This would seem to be the problem in the Viger tunnel debacle, where a 2008 study by SNC Lavallin that found the tunnel was in critical condition foretold last Sunday’s collapse.
Yet, as this thorough article by Fagstien points out, we may want the safety of our infrastructure to be the government’s number one priority, until other hotter political potatoes like education, health, transit, and taxes also become number one priorities… and all those 2-year-inspections and 15-year renovations don’t come for free. Billions of dollars have been put into upkeeping roads in the past few years, yet we seem to be falling further behind all the time.
Is it possible that, with the sudden proliferation of concrete infrastructure in the past 50 years, we have built beyond our means? What if we simply cannot scrape together the resources required to maintain the structures we’ve come to depend upon – let alone to replace them once a generation?
The impending age of superconcrete?
The top researchers are on it: seven Quebec univeristies have teamed up to create the Research Centre on Concrete Infrastructures. Meanwhile over at MIT, scientists are claiming that tweaking its nano-stucture, they can extend concrete’s lifespan from 100 to 16,000 years.
Just what we need: a Turcot design for the next 16 millenia… can we get a happy medium please?