It is not just in Italy that bridges are failing

CONCRETE can last a very long time. The roof of the Pantheon in Rome is the world’s largest unreinforced concrete dome; it was completed in around 125AD by Hadrian, an emperor. But concrete structures can also fail, with tragic consequences. Although it is too early to know the cause of its collapse, something clearly went very wrong with the Morandi bridge in Genoa, which was completed in 1967 and crashed to the ground on August 14th with the loss of at least 38 lives.

In Italy itself fingers are already being pointed: at the operator of the bridge, at the bridge’s designer, at politicians at home and abroad (see Europe section). But the Genoa disaster also carries a warning that stretches well beyond the country’s borders. Concrete, on which the Morandi bridge relied, has become the world’s most widely used building material. The sort reinforced with steel is found in all manner of construction. And unlike the stuff in the dome of the Pantheon, reinforced concrete comes with a problem.

The reinforcing is done by encasing steel bars—or the ties that support bridges, as in the design of the Genoa structure—within the concrete. The difficulty with using metal is that it introduces a potential weakness. Tiny cracks can develop in concrete from battering by the weather, vibrations from traffic, movements in the foundations and other causes. These cracks allow water to creep inside the structure, and once in contact with the metal contained therein, cause it to corrode. That process can dramatically weaken the structure. Bridges are of particular concern because they can be subjected to unusually large stresses and strains, and many span long distances with nothing below them for support.

The risk of deterioration inherent in reinforced concrete can be accelerated by many factors. Shoddy workmanship in the first place does not help. In the case of bridges designed in the 1960s, the loads placed on them have become far higher because traffic volumes are greater, cars are bigger and lorries much heavier. Extreme weather can also play a part, not least because concrete expands when it gets hot and contracts in the cold. Floods are able to undermine foundations. In the normal course of things, most bridges built using reinforced concrete may survive individual events. But if they have been weakened and that vulnerability has not been detected by regular inspections and repaired, for whatever reason, a vital margin of safety has been removed.

Concreting the world

So it is not just in Italy that questions should be asked about monitoring and maintenance regimes. Bridges throughout Europe, America and Asia are all showing signs of deterioration. As long ago as 1999, one study showed that 30% of road bridges surveyed in Europe had some sort of defect, often involving corrosion of their reinforcement. And a report this year found that more than 54,000 out of the 613,000 bridges in America are rated “structurally deficient”. These dodgy bridges are crossed 174m times a day.

In many cases, structures that might have been expected to last a century or so will now probably have to be replaced in half the time because of various forms of structural deterioration. New technologies will help with much closer monitoring; new materials will allow stronger replacements (see Science section), which might turn out to be a cheaper option than restoration. With the world covered in reinforced concrete, this is a problem that spans countries. The failure of the Morandi bridge shows that it must not be ignored.