The basic concrete column is made of sand and cement mixed with water and poured over a steel skeleton. It’s great for holding things up, but like everything else it’s going to fall apart sooner or later. Nobody wants to be driving on a highway overpass when its columns crumble away, and Professor Shamim Sheikh, a civil engineer at U of T, is making sure that won’t happen.
Sheikh’s specialty is retrofitting concrete columns, either to repair damage or just to make them stronger. But where traditional refits just use more bulky cement and steel, Professor Sheikh uses fancy fabrics to tightly wrap the substandard structures and make them better than new. Columnar corsets, if you will.
Sheikh’s focus has been on the columns used in highway bridges and overpasses, in collaboration with Ontario’s Ministry of Transportation. The biggest problem for Ontario’s highway structures is road salt. As salty water seeps into the porous concrete, it attacks the steel elements inside. Rusting steel causes bulging and cracking and weakens the column.
Another threat to concrete structures is geological disturbance. Earthquakes cause unusually high sideways loads on columns that can make them fail, especially if they are in less than perfect condition. Sheikh’s reinforcement wraps make the columns both stronger and more tolerant of these extraordinary forces.
“Columns are a very important component of these [highway] structures because they support the entire structure,” Sheikh explains. “Column failures are normally associated with complete collapse, and that is what we have to avoid at any cost, since it is associated with great financial cost and with loss of life.”
“In the good old days we would look at an old damaged column and say it needs strengthening, we will make it bigger with the help of steel and concrete.” But this strategy requires a lot of labour and heavy equipment and usually means people have to stop using the structure, so that a highway bridge under repair would need to be closed to traffic. Another flaw is that by using more of the same materials, typical repairs invite the same deterioration to recur. To make matters worse, the traditional methods are somewhat self-defeating, since a bigger column attracts more forces in the structure, so although the column is reinforced it now has to bear larger loads.
Sheikh’s fix-up method uses much less material than does traditional steel-and-cement repair. He uses glass or carbon fibres that are very strong but also very light and easy to work with, so that even though the repair materials might cost a lot, the total cost is lower as the repair takes less time than with traditional methods and often does not require the structure to be shut down during the repair work.
His ideas are beginning to catch on. Near Toronto, Sheikh’s repair method has been used on the 401, 404 and QEW highways, in an oil refinery and an apartment building. The first application was at a highway 401 overpass over Leslie Street in 1994. That repair, like all since, has been completely successful.
But as with any new engineering technique, acceptance is slow. “The most important issue for any new application is: we don’t know enough,” he says. “There is a bit of hesitation, there is a bit of question in people’s minds, that maybe there are things that we don’t know about that could create havoc.”
That is why engineers like Sheikh and his graduate students do a lot of computer analysis and a lot of real world and laboratory testing. In the lab, they build model columns, break them, repair them, and break them again. They have shown that damaged, fibre-reinforced columns are even stronger than new, undamaged columns and that they can deform more without losing strength.
Reliability under deformation is a good thing. “The design philosophy is that after an earthquake, you should be able to walk out of a structure,” Sheikh says. A bridge that has become useless but is at least still standing is much better than one that collapses and kills people.
There are hundreds of thousands of concrete bridges in North America today that need repair, many of them good candidates for fibre reinforcement. The only challenge now is to prove to highways departments that the price is right.
Convinced that the fibre-reinforcement technique is a good one and that public works departments everywhere will soon be exploiting its benefits, Professor Sheikh is also exploring other applications. For example, preliminary results show that the reinforcement fibres are not affected by gamma radiation, making it possible to use them for repairs in nuclear power plants.
Among Sheikh’s other innovations is a special, patented cement that expands as it sets, instead of contracting like most cements. This “expansive cement” has proved useful in conjunction with the new fibre reinforcement methods. By coating a column with expansive cement and then wrapping it in fabric before the cement sets and expands, repair workers can ensure that the fibres fit snugly around the column. Another application could be in the deep underground footings of large buildings. If a concrete footing contracts away from the surrounding earth, the building might shift and settle, causing stresses and cracks in the structure. But footings that expand for a secure fit might prevent the damage that otherwise can ensue as a building settles—and make fibre-reinforcement repairs unnecessary.
