Stories from Physics for 11-14 14-16
Concrete is reported to be the most abundant man-made material on the planet. It is considered to be a brittle or quasi-brittle material and, in general, the higher the strength of the concrete the more brittle it will be.
Despite its common association with modern architecture, concrete is a surprisingly ancient material, with some archaeologists claiming lime-based concretes were in use over 20,000 years ago.
The Romans were fans of concrete and used it in many constructions, including the Pantheon’s dome. Researchers have claimed that some forms of Roman concrete are more durable than those commonly used today and, moreover, are continuing to strengthen as they age. Analysis of samples taken from harbour structures shows that Roman concrete contains a material, aluminous tobermorite, that is not found in contemporary concrete and which gives the material additional strength. Aluminous tobermorite is formed through a reaction between concrete and seawater, making the ancient material progressively less and less likely to crack as it ages.
The setting of concrete is not a simple drying process but a chemical reaction (hydration) in which the reaction products gradually fill gaps left between the aggregate material to form a strong matrix. This curing process continues as long as moisture is present and the reaction never reaches completion, meaning that concrete continues to gain strength as it ages. Concrete typically displays some self-healing properties as non-hydrated particles will remain in a sample and cracks can allow water to reach pockets of unreacted material, causing further setting and repair of the crack. However, deliberately leaving non-hydrated particles in concrete is both uneconomical and reduces the strength of the material. Researchers have therefore proposed mixing bacteria into concrete to form a self-healing system. Bacteria can survive within setting concrete and become activated when exposed to water that enters cracks, depositing calcium carbonate into the matrix of the concrete and sealing the damaged area.