In Italy at mid august we celebrate the so called “ Ferragosto”, that sounds a bit like the end of summer. In the city everything is closed, all the families are on the beach and kids play with water balloons.
Two years ago, on the 14th of August, a bridge named ponte Morandi fell causing the death of 43 people. It was inaugurated in 1967 and after hours of heavy, incessant rain it just fell down.
The material it was made of was reinforced prestressed concrete ( the translation from italian is a bit tricky because we call it “cemento armato pre-COMPRESSO”). Obviously during the sixties traffic in Genova was way less, and the long term resistance of this material to tensile strenght was not well known.
Let’s start making a bit of order among different materials: the main concrete binder is portland cement made of calcium ( from calcareous rocks) , silicon ( silica clay), iron and aluminium.
When concrete is in contact with water, as it occurs in water pipelines , the material might dissolve. Indeed the cement structure obtained after cement solidification is porous. The cement dissolution leads to an increase of porosity in the material: as a consequence the strenght of concrete decreases and in the long term this can comprimise the safety of structures.
There are several ways of improving the resistance to leaching ( and to compression as well) of concrete, like the addition of some materials able to react with calcium: for instance adding a small percentage of fly ash ( usually 25 %) in the mixture can improve a lot the resistance to leaching.
Regards what happened in Genova 2 years ago, we must say that the pre-stressed concrete was used for the bridge decking ( the main part) and for the cable stays as well.
When you are an engineering student in Italy, many professors mention you a place in Rome called Ponte Milvio: that bridge is made of mortar and it has been there for more than 2000 years.
Why? Well, the stones from which the bridge is made of are placed to form a nice arch, so they are subjected only to compression ( yes, the roman empire is very well known for his warriors, but we need more movies about their engineers ).
Ponte Milvio
Concrete has a great resistance to compression but not to tensile strenght, a behaviour which is opposite to the one of steel and iron. That’s why there are many buildings where concrete horizontal beams are reinforced with iron or steel cable: moreover this prevents iron oxidation as well because the concrete actually wraps the iron cables.
But what is pre stressed concrete?
In this kind of concrete the iron cables undergo a strong tensile stress in advance in order to increase their tension resistance. The problem with pre stressed concrete is that, when it breaks, it breaks all of a sudden while classic concrete usually shows some signs of degradation in advance.
Indeed the prestressed cables are way more immersed in concrete rather than the steel reinforcing bars and this makes monitoring more difficult.
Generally, concrete reinforced with iron or inox steel is a very adaptive material which makes possible to realize several structures. However it’s not as longlasting as stone.
Moreover when ferrous materials are wrapped in concrete, the humidity can higly damage them, especially if the structure is close to groundwater or brackish water ( like in Genova).
Usually the cable stays are made of steel ( you might have in mind the amazing Brooklyn bridge) which guarantees a great resistance to tensile stresses. The problem of cement, even if it is pre-stressed, is that it does not guarantee a long term resistance to tension.
The main problem of the bridge in Genova was that wrapping the cables with cement made very difficult to know the actual usury of the reinforcement. Moreover, structures which undergo very high loads require super frequent inspections.
Now a new, safe bridge is ready in Genova: some dehumidification systems have been installed to limit the effects of corrosion due to brackish water. The main innovation is related to the surveillance system, which will rely on some robots that will make a sort of radiography of the entire structure, to report any anomaly. This system was realized thanks to a collaboration between Istituto Italiano di Tecnologia and Gruppo Camozzi.
The monitoring of the state of wear is fundamental to prevent damages: this is true for buildings, bridges and hydraulic pipelines as well. The most popoular control techniques are NDT ( non destructive testing) like for instance ultrasound techniques.
Acquaint B.V. ( from The Netherlands, Leeuwarden )makes use of ultrasound techniques: if you know the propagation velocity of high frequency acoustic waves in the material ( concrete, steel, cast iron) it's possible to quantify the residual wall thickness by comparing the front wall and the back wall signal. This technique might be more known in the field of metallurgy and it can really make the difference, even when it comes to pressure pipes monitoring for corroded pieces detection. Thanks to these inspections it's possible to locate the critical point ( or section) of the pipeline in order to fix it before it's too late. Of course the inspection data must be combined with an accurate mapping.
Regards bridges and viaducts, the monitoring is a bit more complicated due to the variations of wind intensity and vibrations but the idea is the same: preventing failure. Concerning metallic reinforcements in bridges, it’s possible to control the electric potential variations to spot oxidation as well.
Unfortunately nothing can bring back the people who have lost their lives during that accident in 2018, but hopefully the combination of “ old style “ monitoring methods ( resistivity tests, electric potential) with ultrasonic techniques and innovative automatization will help us to prevent events like that. And there’s another thing: it’s always good to investigate some new materials in the lab, but we can’t expect them to support loads 100 times bigger than the ones they have been tested for. Infrastructures and civil engineering are part of a business too, but the safety of people worths way more than money and patents.
Ponte Morandi in Genova before the collapse
The collapsed bridge in 2018
Giulia Ioselli
References
Materiali cementizi e ceramici, Diamanti
Leaching of alkali from concrete in contact with waterways, Setunge et al.
River Engineering, P.Salandin
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