Concrete can last forever

Check out this photo of a concrete bridge built in 1926 that has been submerged behind a dam since 1952. (Details about the ghost town and the maintenance work being done on the earthen dam used to supply water to the Santa Clara area are in this article from the Mercury News in San Jose, California.)

My dad made two observations about this bridge:

1. It is very similar to bridges my grandfather designed when they were building the first highways in Michigan in the early 20s
2. Concrete in fresh water simply cures and gets stronger. This bridge not only looks like new (having been washed by an artificial lake every day), it is stronger than new.

Concrete is made from Portland cement, sand and gravel, and water along with various chemicals and other additions to suit a specific purpose. The water and cement form a hydrate that glues together the rocky "aggregate" material. Concrete actually gets stronger over a period of decades before slowly weakening. (There are many examples of concrete Roman structures that are 1800 years old.) It loves being in fresh water, but anything that likes to remove water (particularly salt) will attack it and weaken it prematurely. Concrete bridges along the gulf, like ones up north where salt is used to de-ice roads, have a hard life. This bridge has had an easy one.

Ran into one of my former students a year or two ago, and he was finishing up his degree in Civil Engineering with a advanced course in concrete. If civil engineering interests you, get a start by reading what Wiki has to say about concrete. That appears to be a high quality, very detailed article including information about newly developed types of concrete that are still in the research stage.

Video of fatal NC bridge collapse

The collapse starts at about 2:39 into this video. For details, read the news article from StarNews Online.


You need to be looking at the very top of the picture. The pre-stressed concrete girder that fell is the second from the left. The left-most girder was being placed by the crane at the time. It's too bad that the camera is looking down at the crane at this moment, because a change in the shadows between the girders makes it seem like the failure started at the top plate of the girder. If you look closely around 2:39 and 2:40, it would appear that some material breaks off and falls before the entire girder falls.

It would have helped a lot to see the entire girder during the failure, but forensic work will probably identify the point of failure and its cause. Did the new girder bump the previous one, or was there a manufacturing or design flaw? We will find out in a few months.