Futuristic Concrete Heals Itself with Built-In Bacteria

Introduction

Concrete is one of the most durable building materials. It is widely used for construction and in different types of buildings and roads as it provides better fire resistance than wooden construction and it is relatively inexpensive compared to other building materials. The biggest downside of building with concrete is its tendency to develop internal damage over time, which can greatly reduce the structure’s service life, increase maintenance costs and raise many environmental issues.

The concrete

Concrete is composed of coarse aggregate, such as gravel and crushed rocks (limestone or granite), along with fine elements, such as sand, bound together with a fluid cement. It starts as a fluid mixture, which is easily poured and molded into shape and through a chemical reaction with water, cement and the other ingredients gradually harden and form a sturdy and durable material. To add strength to the structure, reinforcing materials such as steel bars, are often imbedded in it.

The earliest recorded usage of concrete goes back thousands of years ago, but it was used only at a small-scale until the Ancient Egyptian and Roman civilizations. In fact, the word concrete comes from the Latin word “concretus”, meaning compact or condensed. The Romans used concrete extensively for over seven centuries, which helped them revolutionize architecture previously limited by to stone and brick material. Scientific tests have shown that Roman concrete had as much strength as modern Portland-cement concrete, and many ancient structures surviving to the present day undoubtedly prove that as well. Since Roman times, two important changes have been made to concrete: today it is a homogenous mix, which can be easily poured and molded, while in Roman times it was much coarser material that had to be layered by hand; second, Roman concrete depended purely on its own strength, while modern concrete is reinforced and thus more durable.

However, reinforcement of the concrete does not resolve all the issues with its durability. Generally, all materials degrade over time due to wear and tear or environmental conditions. The elasticity of concrete is relatively constant at low stress levels but decreases as the stress level increases. It has a very low coefficient of thermal expansion, meaning it does not expand much as temperatures increase, and tends to shrink as it matures. When subjected to long-duration forces, concrete develops internal damage, which shows as cracks at microscopic levels. The micro-scale cracks have been shown to change the thermal, electrical, and acoustical properties of the materials, and their propagation can lead to eventual material failure.

Nature-inspired solutions

Microscopic cracks are hard–often impossible–to detect through manual inspection, unless they have developed into macroscopic flaws in the structure. When there are cracks in concrete, water and gasses come through them, dissolving the material and corroding the steel bar reinforcement. Because of that, it is important to control crack width and to repair it as soon as possible, before the whole structure collapses. The conventional solution is to manually repair damage once it is discovered through inspection, but in addition to being costly, this method also does not restore the structure’s original integrity.

As always, nature provides us with the best solutions. It often builds with materials which can repair themselves–plants and animals have the ability to seal and heal their own wounds by mechanisms from within the organism. Inspired by these properties, scientists have been looking for a solution to concrete weaknesses. The idea was to develop a self-healing material, with a built-in ability to automatically repair damage to itself without any external diagnosis of the problem or any other human intervention.

Henk Jonkers, a microbiologist at Deft University of Technology in the Netherlands, has designed a new type of concrete that is able to fix its own cracks–it heals itself using the power of amazing bacillus bacteria. These bacteria can survive without food, water, or oxygen for years in the alkaline environment of the concrete. They are placed inside biodegradable plastic capsules and mixed in with the wet concrete, where they can remain dormant for decades. When the micro cracks start to develop and water eventually seeps in, they are activated and start producing limestone sealant to close up the cracks. Other researchers in the U.K. are working on a very similar concrete project with the idea of using bacteria as glue or plaster rather than building it into the concrete itself. However, Jonker’s solution is the only one that has been tested in the field, with a self-healing lifeguard station that is already beginning to show signs of self-repair.

The conclusion

In an era of growing environmental protection awareness, self-repairing materials are a revolutionary new idea that we should all look forward to. Self-repairing technology will lower the repairing costs and make the concrete more durable, and the property to instantly correct damage caused by normal usage will also make the concrete building more sustainable. While we often forget that we are a part of the nature that surrounds us, this is a unique way to redeem ourselves and tie nature and our building environment together.