Like Concrete

A commonly used phrase “like concrete” (or a variation) is used both literally and as a metaphor. Concrete is a composite building material that is made up of a few ingredients, the list of which has changed over time. A An early aspect of cement, a part of concrete, was discovered by accident in 1300 BC when builders in the Mid-East placed a damp thin coating of burned limestone over pounded clay walls. This coating reacted chemically with the gases in the air to make a hard surface protecting the pounded clay wall. While not really concrete, it was the first cement. The Romans are often credited with having discovered concrete. Concrete is what has built civilization and is the most used and recognized building product in the world. Most of our homes contain some concrete—usually the foundation. Many highways are built with concrete. Also, impressive structures have been built with this hard material.

Pantheon Dome, looking up to the inside of the dome
1990 photo by author

The Pantheon in Rome, completed in 125 AD has a 142 foot diameter dome, with a 27 foot wide oculus, that is the largest un-reinforced concrete dome in the world. Interestingly, the dome should have fallen by now due to earthquakes, settling and other movement of the concrete, but the exterior walls contain seven niches which reduce the width of the wall, the weight of the structure, but also have acted as control joints which end up to control the locations of cracks. Not unlike control joints in a highway. However, after the fall of the Roman Empire in the 5th century, techniques for making Roman pozzolan concrete were lost until rediscovered in the fifteenth century. The components would evolve over the following centuries. Portland cement, for example, was invented in 1824. Concrete was not considered a socially suitable material for other than the construction of industrial buildings during the 19th century. In the mid to late 19th century a Frenchman would be the first to add reinforcement rods to the concrete. Interestingly, in 1891 the first concrete street was poured in the US, and it is still in use today. At 8,000psi, its strength is more than twice that of concrete used in residential construction today. In 1898 there were more than 90 formulas for concrete, but by 1900 the formula had become more standardized.  Today we use pre-cast concrete, cast in place concrete and concrete block to build various buildings.  

Concrete Bell Banner of St John's Abbey Church, Collegeville, MN
Designed by Marcel Breuer
2014 photo by Author 

Standardization and testing that followed the move to a consistent recipe allowed some of the most massive structures in the world to be constructed. One can think of the Hoover and Grand Coulee dams in the US which used 3.25 and 12 million cubic yards of concrete, respectively. Concrete would become part of the modernist movement of the 1950’s and 1960’s and would give the world the Sydney Opera House, and St. John’s Abbey Church. Although, our construction techniques (or due to cost) still have been unable to match the grand nature of the Pantheon and its remarkable dome. The Pantheon dome uses volcanic ash with lime, which binds better than our commonly used Portland cement. Portland cement is not very green. Its creation produces 7% of the earth’s carbon dioxide every year. The Roman use of less lime, and use of limestone allowed them to bake the ingredients at a temperature of 900 degrees Celsius, comparted to the baking of Portland cement at 1450 degrees Celsius. Continued study today of Roman structures is turning up knowledge lost with the collapse of that civilization. Perhaps the world will once again be making Roman pozzolan.

The fire resistant properties lead dorm rooms to be made of concrete block
2013 photo by author

While the Romans had a remarkable knowledge of construction materials and techniques, which humanity today is still trying to fully grasp, present time is also throwing out new techniques. We have all seen cracks in our concrete steps, drives, and perhaps our garage or basement floors, and even our foundation. The cracks are related to a serious flaw in concrete; it can crack under tension. Tension is what led to the invention of concrete reinforcement by iron rods or mesh. Cracks can lead to water infiltration which then corrodes the metal reinforcement, which leads to a weakening of the concrete and its potential failure as a structural material. Material engineers would say it jeopardizes “the structure’s mechanical qualities.” That is why I saw an interesting article about methods to create a “self-healing” concrete. Showing the value of living organisms, a recent break-through in material science was made not by an engineer, but by a biologist. The Biologist, Henk Jonkers, who is a professor in the Netherlands, spent three or more years creating a method for concrete to self-heal by use of bacteria. It may not be too long down the concrete road that bio-concrete may be available, provided it can get by the regulators, and more importantly the road and construction lobbies (perhaps the asphalt lobby may be the most opposed). As reported by CNN, Professor Jonkers decided to devise this healing agent for concrete when a colleague posed the issue of the difficulties of repairing and replacing concrete (hence the construction industry may be against). Jonkers realized the possibilities by use of bacteria, but he had to find a bacteria that is long lasting, and will survive in the rough environment of concrete—weather and its alkalinity. Jonkers realized that bacillus bacteria was likely the answer. This bacteria can survive for decades without food or oxygen.

Bacillus Bacteria
Google images

But, the bacteria itself does not heal, it needs to produce a material.  To do that. Jonkers settled on limestone as the material, or more specifically calcium lactate. The calcium and the bacillus bacteria are placed in a biodegradable capsule and added to the concrete mix. When water enters the concrete, the capsule degrades allowing the bacteria to germinate and feed on the calcium lactate and to combine with the carbonates to form limestone which heals the crack. Limestone is the formation of rock from organisms in seas that once covered the world literally eons ago. Today, we have a micro-organism (bacteria) that may help the world to allow concrete to still be used as a building material, but cut down on the amount manufactured as fewer repairs would have to result from its ability to now self-heal. Perhaps, as scientists dig more into the Roman techniques we will find abilities to combine techniques of 2,000 years ago with the science of today that allow for additional improvement in the environmental costs of concrete production. After all, concrete is the work horse material in building construction world wide.  Using the word concrete so much in this post, makes me now desire a Culver's Concrete Shake.  

 

Inside of St. John's Abbey Church, a concrete structure
2014 photo by the author