There is a material that has accompanied the human species for centuries and on which numerous civilizations including ours have actually lived: concrete. In particular, reinforced concrete (what we mistakenly commonly call “reinforced concrete”) has become an inevitable presence in our lives: it is what many of the buildings in which we live and work are made, and it is the most used material for construction of bridges, viaducts and numerous other infrastructures. Concrete and steel bars hold our world on its feet, but their union has some side effect that can significantly reduce the life of a building or viaduct. For this reason, researchers have long been working on a more reliable alternative, which extends the life of infrastructure and reduces the risk of collapse.
The biggest problem with reinforced concrete works is carbonation, what is colloquially called “concrete cancer”. As the name suggests, reinforced concrete is made using steel reinforcement, a dense network of metal rods (rods) which is then embedded in the concrete. When this solidifies, you get a material resistant to stress and able to withstand large loads, such as those of a bridge or a skyscraper of tens of floors.
If the concrete is uniform, compact and not very porous, the reinforcement remains protected from external agents and can last for a long time. Things get complicated when small cracks are created in the concrete, which in particular environmental conditions can lead to the formation of rust on the steel. Rust causes the reinforcement to expand, which presses against the concrete around it, causing it to crack. The cracks further expose the reinforcement to external agents, causing accumulations of standing water that can interfere with the solidity of the concrete. In places with severe winters, the water freezes, leading to the risk of further expansion of the cracks.
The so-called cancer of concrete is understandably a circumstance that designers and builders want to avoid as much as possible, also because it can reduce the average life of a structure, well below the 100 years for which buildings are usually designed. The first goal is to avoid corrosion of the metal, but to do so is not that simple.
A little bit of everything has been tried over the years to reduce the risk of getting concrete cancer. For example, it is possible to use cementitious compounds with recipes different from the classic ones, made in such a way as to make them more resistant and to reduce water accumulations. Another solution involves the use of stainless steel armor, which reduces the risk of being left with rust. These solutions are usually effective, but involve a considerable increase in construction costs, especially in the case of large buildings or large infrastructures.
In recent years, some researchers have wondered if the time has not come to change their approach, rethinking the way reinforcements for concrete are made. As the Economist tells us, among the most promising initiatives there is an ongoing research at the Deakin University in Australia, carried out in collaboration with Austeng, an Australian company that has been commissioned to build two pedestrian bridges in Geelong, city not far from Melbourne. The two gangways will be used as prototypes to verify the reliability of a new type of reinforcement, no longer based on steel.
Researchers have created a system that combines concrete and fiberglass and carbon fiber reinforcement, two materials that are lighter but equally strong, and that do not lead to rust problems. According to the designers, a large-scale use of these materials could allow the construction of buildings and infrastructures in reinforced concrete at prices comparable to the current ones, but significantly extending the life of the new constructions, immune to concrete cancer.
The idea is not new in itself: carbon fiber armor has long been considered as an alternative to steel, but it is expensive. For this reason they are used only in particular constructions, with light and resistant shapes, or in hospitals where the presence of metal in the walls could interfere with some diagnostic tools, such as magnetic resonance machines.
The research team wondered if it was possible to use carbon rods only where strictly necessary, making the rest of the armor in glass fiber, a less expensive material. After numerous tests, they have obtained a type of light and resistant armor, to be used in place of traditional steel ones. A first test, carried out in a three-meter long section, gave positive results, and above all allowed to exceed the legal requirements. The pedestrian bridges will have longer sections, of about ten meters, and will be prefabricated.
Prototypes turned out to be stronger and lighter than analogues with steel. The load capacity is 20 percent higher, while the occupied volume is 15 percent lower. Production costs are still a bit higher than those with steel, but the designers say they should be reduced if the new technique becomes widespread. Being less exposed to the risk of concrete cancer, they should also ensure a longer life and require less maintenance, therefore with a lower cost in the long term.
The two pedestrian bridges to be built in Australia will also make it possible to experiment with a different type of concrete, with a lower environmental impact. Concrete is usually made using Portland cement as a binder. It is a cement that has existed for almost two centuries and which has excellent qualities, but requires a lot of energy to be produced, by cooking some of its components in the kiln at very high temperatures. The process involves the production of large quantities of carbon dioxide, which contribute to the greenhouse effect and therefore to global warming.
The Australian project involves the use of an ecological (geopolymer) concrete, with ingredients such as fly ash (coal fine ash) and metallurgical sand, which do not lead to the production of large quantities of carbon dioxide during their construction.
The search for alternative materials by the Deakin researchers is also aimed at seeking further, even cheaper, solutions. Some tests concern basalt, a rather common volcanic rock, and the possibility of melting it, then obtaining fibers that could be tied together to make the armor. It would be a more sustainable alternative to carbon fibers, which are made from petroleum derivatives. This is nothing new and basalt fiber is already used in some areas, but researchers think they can improve its strength and reliability by adding other components.
In ancient times, the Romans distinguished themselves for the use of concrete in their constructions, adopting materials and solutions that have resisted the elements for almost two millennia, reaching up to the present day. The Pantheon in Rome, built between 120 and 124 AD, has a dome with a diameter of 43 meters made with different mixtures of concrete, without a metal armor. Even today, it is the largest hemispherical dome ever made of unreinforced concrete.
