Ancient Concrete and New Construction
In modern Italy, a unique question has only been answered in the last several years. Modern concrete constructions built at the seaside have degraded and crumbled after only a few years. The salty sea waves and ocean wind had destroyed creations of modern technology while ancient concrete has lasted thousands of years. Why do our modern buildings collapse while ancient concrete islands can withstand the elements for millennia?
How the Romans did it
The ancient Romans started building concrete thousands of years ago. They used various ingredients, including some of which are hard to reproduce in modern labs.
Opus caementicium, or concrete, was used commonly in ancient times. It was made in the same over all method that modern concrete is. It consists of an aggregate and a mortar. The aggregate was made from small rocks, pebbles, and hard pieces, just as it is now. The mortar functioned as the binding agent, keeping everything together as an extremely tough glue. However, a few different construction ingredients and methods keep ancient Roman constructions in good working order while our own fall apart. Opus caementicium used aggregate made from tuff. Tuff is a rock made from volcanic ash, ejected from vents during a volcano’s explosion. There are many types of tuff, but the tuff was not the most important ingredient.
What really keeps the Roman buildings together is how the binder and aggregate interacts. Gypsum and quicklime were used, as well as pozzolana. Pozzolana is a volcanic ash that is resistant to sea salt – more so than modern concrete. The pozzolana was used in conjunction with the binders to create a cement that strengthened and became more resistant over time.
Reactions with the outside world
Pozzolana and quicklime benefits from interacting with salt water over time. A rare crystal, tobermorite, formed over time as these three materials were exposed to one another. As seawater washed between the natural cracks in the concrete, it reacted with materials found in the volcanic rock.
As the seawater flowed in the concrete, it would wash away the volcanic ash. It might seem that washing away part of the ingredients of the concrete would weaken the concrete, but it actually had the opposite effect. As the ash washed away, it allowed new formations. In particular, crystals could form and make interlocking “plates” that strengthen the concrete over time. Phillipsite, naturally found in volcanic rock, and seawater builds up a special type of tobermorite crystals. These crystals are called aluminous tobermorite crystals. Aluminous refers to the element aluminum. Aluminum is a light metal that is known for it’s low weight and tensile strength. These new crystals form over time, filling in tiny fissures and making the building stronger over time.
Thanks to the unique reactions of the Roman concrete with seawater, it has the reputation of being the strongest, longest lasting concrete.
Other elements of construction also aided Roman buildings. The dome-shaped tops of many Roman buildings and their foundations aided longevity as well. The tops of many Roman structures were made of a less-dense, lighter weight concrete. The foundations were made of denser, harder, tougher concrete. The heavy foundations and light tops kept the buildings in place and stable. This type of construction is especially useful in the earthquake-prone Italian peninsula.
What we can learn from ancient concrete
After reading the first half of this article, what we have to tell you now might seem like a surprise. Roman concrete has some amazing properties, but it is not actually better over all than modern concrete. There are quite a few reasons why this is true, and we will take a look at them.
Benefits of Roman concrete
- It gets stronger over time with reaction to sea water.
- It is more natural, and has a lower environmental footprint.
Why it’s not quite as good as we think it may be
To most people, the idea of concrete that strengthens over time seems amazing. However, to any paving contractor or mineralogist, the idea of a concrete strengthening with crystals is pretty normal. Crystals form on all kinds of surfaces. The combination of materials that makes up cement makes the appearance of crystals pretty common.
This article covers the following points, and some of the articles you might find about Roman seawall cement quite well.
There are a few other factors that really do effect how fast deterioration of modern cement occurs.
- Roman cement did not have reinforcing steel. Rebar, or steel beams in the cement changes the chemical structure. Embedded steel actually corrodes the concrete when exposed to seawater. Modern cement has this steel, and thus is more inclined to degrade. Simply not using rebar extends the life of modern cement.
- The southern Italian peninsula has a very temperate weather cycle. Unlike many other places where sea cement building might be compared, there is no freeze/thaw cycle. One of the main reasons for the breakdown of cement is water freezing in it’s small pores.
- Survivor bias is the idea that old things are better than new things. We believe this, because the old things that last are still there, and we don’t see the old things that no longer exist. This goes for buildings, societies, appliances, and vehicles.
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