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Bridge in Genoa (Italy)

Dear Reader,

I strongly suspect that some of you will be aware that a bridge in Italy has fallen down resulting in a considerable loss of human life. Now some of you might be wondering what caused it to happen.

At this time I do not know for sure exactly what caused the bridge to fall down, there are reports that a lightning bolt hit the bridge shortly before the bridge came crashing down. The bridge was a suspension bridge (cable stayed bridge) which had reinforced concrete elements which go from the tops of the towers to the part of the bridge which the people / cars travel over.

Now years ago in the senior common room at Reading it was explained to me that there are two types of suspension bridge. There is the true suspension bridge where a long cable which has a shape similar to a washing line exists between two towers. From this cable a series of vertical elements are connected. At the bottom end of the vertical elements are connected to the road way on which the people, cars etc travel over. The other design is a cable stayed bridge in which straight elements which are under tension go from the tower to the road.

For the suspension bridge it is impossible to change the main cable (the one with the washing line shape) but for the cable stayed bridge it is possible to replace a cable.

One big problem is tension, now it is important to understand that concrete is very weak in tension but strong in compression. This is why reinforced concrete is so good a material. The strength of a concrete object can be sometimes increased further by stressing the structure by putting the concrete under additional compression. This can be done by having a hole going through the concrete, the concrete is poured and allowed to set. Afterwards a steel rope in the hole is then put under tension, this then subjects the concrete to compression. One way of doing this would be to have a long bolt passing through the hole with large washers on both ends. By putting a nut on this bolt and tightening it up then the bolt will be under tension and the concrete under compression.

post stressed concrete

Another method would be to use prestressing where the rebar is put under tension before the concrete is poured. It is important to note that when carbon steel is placed in concrete made from ordinary portland cement (OPC) that the pore water in the cement is slightly alkaline. This is good for the steel, under these conditions the steel does not corrode quickly. The steel is in a chemical environment in which the corrosion is very slow.

Corrosion or rusting of rebar is a very big problem, the volume occupied by the rust is greater than that of the steel rebar. As a result if rusting occurs then not only does the rebar lose its strength but also it tends to cause concrete to spall off from the structure. This expansion from within tends to make more holes in it.

However there are two main problems, if the concrete becomes contaminated with chloride salts from sea spray or deicing salts used on the road then the rebar can start to corrode more quickly. Also if as a result of cracking carbon dioxide from the air can enter the concrete then the pore water will become less alkaline. The steel will then be exposed to a new chemical environment in which it will corrode more quickly. All concretes will slowly be carbonated by the air, but in the ideal world only the very outer layer will be carbonated.

If the concrete becomes cracked due to vandalism, a changing mechanical load or even due to corrosion then the rate of carbonation tends to increase. This is due to the fact that the air has greater access to the inner part of the concrete object.

The last main thing which we should be aware of is that some corrosion modes such as stress corrosion cracking are worse when the object is placed under tension such as in the cable of the suspension or cable stayed bridge.

What I think that one of the things the investigators should be doing is to check the chemistry of the concrete used in the reinforced concrete cables used to connect the towers to the deck of the bridge. There are two simple chemical tests which can be used.

The first one is to drill a hole into the concrete and apply a solution of phenolphthalein to the surface. This is an acid/base indicator. When it is acidic it is colourless but when it is alkaline it is intense pink. What you then do is to measure how far the colourless reagion extends into the concrete object. This will give an indication of how bad the carbonation effect was at the point where the hole was drilled. As the bridge has fallen down this test can be used without having to worry so much about plugging up the holes made in the test if you were to examine the fallen lumps of concrete.

Another simple test is to grind up a sample of concrete, then to make this into a paste with a known amount of water. If this paste is then examined by either ion chromatography or a chloride test strip (based on the reaction of silver nitrate with sodium chloride to form insoluble silver chloride) then it is possible to determine how much chloride contamination was present in the concrete.

It would be interesting to know if the people responsible for the bridge were using these tests and some other corrosion tests before the bridge fell down. Now I think that the wreckage from the bridge should be subject to these tests and some other tests to try to work out what has happened. I will try to write more about the bridge and steel corrosion when I get the chance.


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