Concrete is one of the oldest, most well-known and used material. Romans used it under its primitive shape to creates structures such as bridges and houses. With various characteristics, concrete meets a wide range of Humans’ needs. Over the course of History, it has evolved alongside society and new properties were discovered, for instance in the 18th and 19th centuries with the formalisation of hydraulic cement. One such recent discovery is “self-repairing”, and in particular the self-healing of concrete cracks. Unfortunately, if self-healing properties are known and studied, tests and procedures are not standardised. COST Action “Self-healing As preventive Repair of COncrete Structures” (SARCOS) has created a strong international network of experts and researchers for the search of smart self-healing concrete, with the objective to extend the service-life of new and existing concrete structures.
Improving the service life of concrete by implementing “self-healing” standard methodologies
Prof. Sanchez Moreno, SARCOS’ Action Chair, had been working on methods for protecting reinforced concrete for a few years when she made the COST Action proposal in 2016, and her research was then focusing on considering the preventive repair of concrete, both in new and in existing constructions, by developing new materials, as self-healing concrete. Thus, the cracks can be sealed from the inside or filling it from the outside. “The goal is to extend the service-life of concrete, and at the time we were thinking of ways to bring together the new self-healing approaches with the preventive repair of existing constructions by external repair methods. This was the starting point of SARCOS”, she explains.
Concrete, like many other materials, is subject to time degradation. Water in particular accelerates concrete’s deterioration by infiltrating microscopic already-formed cracks inherent to the structure itself. “Concrete is always going to crack”, she adds, “But micro-cracks are not a great deal. However, if they are allowed to grow, they will compromise the durability of the concrete structure by creating water infiltration or by letting chloride enter to the rebar which will cause corrosion problems”. Indeed, if the crack is small enough, self-healing repairing techniques will enable sealing before deeper problems are created, which will in turn prevent risks of premature collapses of concrete structures.
At first, one of the main difficulties faced by the SARCOS network was the absence of standard and agreed methodologies to compare performance and efficiency of self-healing concrete. Indeed, laboratories use their own methodologies under different testing conditions and with different technologies which result in a variety of approaches that usually are not confronted with one another. Prof. Sanchez Moreno adds: “The idea was to create agreed tests with laboratories experts in these topics to try to define these criteria. We also wanted to include the modelling to simulate and to obtain more significant variables, and to predict the performance and the service-life of concrete”.
The extraordinary properties of concrete
Concrete is made out of cement, sand, aggregates and water. When in contact with water, cement hydrates and becomes hard. Today, there are two main approaches to create self-repair concrete: autogenous healing and autonomous healing. The first is a natural process, intrinsic to the properties of the material itself and mostly caused by the continuous hydration of cement. “The hydrated particles of cement have self-healing properties, but it is a very slow process”, explains Prof. Sanchez Moreno. The second, autonomous healing, is an engineered process that improves self-healing properties of a concrete element. There are three strategies for designing autonomous healing methods:
- Using chemical agents (crystalline admixtures for example) or biological agents like bacteria
- Using encapsulation of water by superabsorbent polymers or porous fibres or using micro-capsules that will break and release healing biological or chemical agents when the crack passes through it. This is the dispersed encapsulation method
- Using localised porous networks and encapsulated vessels containing a chemical or bacterial solution. This is the localised encapsulation method
“In specific conditions, when there are nutrients, a bacteria will produce calcium carbonate which will precipitate in the crack and fill it”, explains Prof. Sanchez Moreno, “For pre-casted concrete, you can add a vascular network in which you can directly inject liquid healing agent that will be distributed through the vascular network to the damage sites”. In unifying testing and establishing guidelines and standards for assessing the effectiveness of these innovative technologies, SARCOS is shaping the future of concrete materials. “Because of its infinite properties, concrete elements that we know and use today are not the ones that we will have in the future”, believes Prof. Sanchez Moreno.
SARCOS’ achievements and future objectives
In the course of its 4 years and a half, SARCOS has created a network of more than 200 researchers (92 of which are women and 147 are young researchers) from 32 countries. 14 meetings, 40 Short Terme Scientific Missions (STSMs), 4 Training Schools were organised, and 6 Inclusiveness Target Countries (ITC) Conference Grants were allocated. Researchers travelled to 17 countries for networking activities and 55% of the Action’ spending was to the benefit of ITC.
“The ongoing tests worked very well because we have been working together with 30 laboratories in the same manner and with the same objective. We are now analysing results and we are looking for ways to continue the activities of the network with a main goal: achieving standardisation and creating guidelines”, concludes Prof. Sanchez Moreno.
Additional information:
Learn more about SARCO’s actions
Further reading:A solid platform for starting a career – COST