With present time thinking many seem to think we are most advanced age, and that we have most all the answers. After all, in our world today there more experts in more fields than at anytime in human history. Yet, for all of the learning, certain items can still be wrong. It happens in the health field, it happens with food (remember trans fats?), and it even occurs with concrete. Concrete is a common construction material since it was first used by the Nabataen's in Syria and Jordan over 6500 years ago. (We best know the Nabataen's for having built Petra in the early fourth century.) As a construction material concrete rose to popularity with the Romans who were the first to use it on a large scale--the Pantheon, the port of Ostia, just two of their large projects. Roman use dates back to perhaps 600 BC. It turns out that all our present time engineering of concrete it is not as good as what the Romans developed over 2,000 years ago. What engineers and scientists thought was sloppy mixing of concrete was in fact something much different. Recent research indicates that the material that was thought to be of sloppy mixing, was actually part of the Roman process of having created self-healing concrete.
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| Crack in Concrete Driveway |
We have all been on concrete roads with cracks, particularly in northern climate zones with freezing weather. Water that makes its way into the concrete freezes and expands and then cracks the concrete. Perhaps 30 years ago WIDOT attempted to place clear stone, rather than gravel under concrete roads with a drainage system in an attempt to get the water to move out. Two examples of which I am aware of this use are Highway 51 near McFarland, WI, and Highway 151 north of Sun Prairie toward Columbus. I am not sure that met with success. I think the clear stone, rather than a gravel base allowed other soft spots and cracks to develop.
Cracks in one area lead to more cracks as water continues to intrude into the mass. Concrete has very good compressive strength but has poor tensile strength. Concrete is made of small "stones" or grains which means it will always have some microscopic cracks in its formation. Thus, when tensile forces are applied to concrete, these cracks become elongated and eventually the concrete breaks apart. Hence, the need for reinforcement to improve the tensile strength. The reinforcement is usually what we know as rebar, but for some situations like, sidewalks fiberglass strands are used. If concrete is not too thick it may not need much, if any, reinforcement.
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| Interior of part of Pantheon Dome |
Ironically, the study of Roman concrete by MIT, Harvard, and universities in Italy and Switzerland, was published in the journal Science Advances. In this case, science was advanced by looking at the advanced methods of Roman concrete, processing and construction methods over 2000 years old. For many decades it has been thought the strength and durability of Roman concrete was related to the use of "pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time." (MIT News)
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| Part of the Exterior of the Pantheon |
However, it turns out the durability of Roman concrete comes from something called lime clasts, which present day genius' thought was the result of sloppy mixing practices, or poor quality material. Also crucial was the material used and the processing--hot mixing of the concrete. The Roman process also sets and cures much more quickly than our current concrete. It turns out those Roman engineers knew quite a bit more about concrete than our present day engineers. But, it gets much better, and let me quote MIT news so I don't get the substance wrong:
During the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide a critical self-healing functionality. As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and therefore automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.
Hence, the ingenious Romans developed a self-healing concrete and being over 2,000 years old makes modern day concrete look cracked up (pun intended). The Roman leaders liked their public work projects, regardless of debt, to keep the masses happy. Public works projects to please the masses became more and more prevalent during the dictatorship era, from Julius Caesar becoming the anointed Emperor, on to the end of the empire when it was overthrown by the Visogoths, a Germanic tribe of barbarians. Some of the Roman Emperors may have taken depravity to new heights, but their engineers knew what they were doing.
The end of the Roman Empire led to a decrease in learning and opportunities. The barbarian invaders apparently did not much care for concrete or the unique methods the Romans used to make this durable material, and it has now taken 1547 years for why Roman concrete was so good to be made known. I wonder if the road building lobby will adjust to this process? There is already much debate about asphalt or concrete and the state tries to please both powerful lobbies. It could give concrete an advantage, but it may mean they are not rebuilding roads as often. The healing concrete is an amazing invention, and a very good reason why modern people should not think they do every thing better and know more than those in the long past.
Reference:
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
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