# Column Lives of Concrete

When you hear about the title “useful lives of concrete,” you might wonder if concrete has a life in the beginning. As we have seen in Figure 1-3-4, the public awareness of the problems of aging social infrastructures has been mounting since the Sasago Tunnel accident, but more than half of the respondents answered that they “did not know” or “have heard of but know little” about them.

This Column introduces to you, by way of an interview of Professor Toshiaki Mizobuchi, at the Faculty of Engineering and Design, Hosei University, the author of “How To Get Prepared Against Collapsing Concrete Crises,” $ ^{*} $ that even concrete, which appears robust, could have its life varied depending on its materials, the method of construction used, the environmental conditions in which it is constructed and so on.

(*Toshiaki Mizobuchi (2013); “How To Get Prepared Against Collapsing Concrete Crises,” PHP, Inc.)

○ You also touch upon the history of concrete in your book.

- The oldest cement material known to date dates back to the one excavated found at the ancient ruins of Iftah, Israel, though it does not have the same composition and process as concrete as we use it today. Concrete excavated from a floor of a large dwelling site has a compressive strength of 15 to 60 N / mm², which is equivalent to or better than the concrete used today. Since Iftah dates back to some 7000 years BC, this concrete can be said to have a life of 9,000 years at least.

I see concrete has such a long history. Does the fact that concrete is found in such ancient runs suggest that it is considerably long-lived?

- Characteristically, concrete is resistant to pushing forces but vulnerable to pulling forces. As a solution to these problems, iron-reinforced concrete, or concrete internally reinforced with iron bars, came to be developed with the coming of the 19th century. A gardener named “Joseph Monier” is recorded to have exhibited iron-reinforced flowerpots at the Paris Exposition of 1867. While the advent of iron-reinforced concrete opened a way for building architectures of unprecedented shapes, it raised the problems of decaying internal iron bars in the meantime, which have trimmed the lives of concrete from several hundred years to several ten years.

The life of concrete is also a parameter of the method of construction used at a construction site. Prewar practice had been to carry hard concrete in a hand cart to construction sites, but in the ages of mass and quick construction following a period of rapid economic growth, the method of pumping concrete to a site needing it after it had been transported from a factory gained popularity. In order for concrete to be pumped under pressure, it must be mixed with lots of water to soften it.

Concrete thus constructed is said to be lived for one hundred years or so under relatively favorable conditions and for 50 years or so under harsher conditions, such as those encountered seaside.

Well, do you mean that the life of concrete is also varied by metrological conditions?

- Factors, such as materials, temperature and humidity, the moisture content and the amounts of chlorides and carbon dioxide present at the installation environment, can also affect the life of concrete. Decaying concrete goes through a number of diverse and complexes processes. As one example, concrete placed in the vicinity of a seaside or concrete kept in contact with a snow melting agent in the winter time could be internally permeated by salt to react with steel reinforcements to corrode them. As another, concrete made of materials, such as andesite, is known to develop alligator cracks as a result of a process called "alkali silica reaction." This phenomenon made a topic as a "concrete crisis" in the 1980s.

Thus, seemingly everlasting concrete decays progressively inside over time for various reasons. Giving proper maintenance to concrete to sustain and care for the functionalities of the concrete structures is important.

Now aging social infrastructures have been