Self-Healing Concrete

Dear Colleagues,

Concrete has become the most widely used construction material in the world. However, one important issue with concrete is its durability and long-term performance in non-ideal service environment.

Cracks are intrinsic concrete characteristics. However, cracking can endanger durability of the structure, since it eases the ingress of aggressive gasses and liquids. Certainly in case of chloride containing liquids or in case of high CO2 concentrations, there will be a higher risk of reinforcement corrosion, which compromises the long-term durability of the structure. Cracks furthermore drastically affect liquid tightness, which is a major problem in tunnels and underground structures. Current practice requires regular inspection, maintenance and repair, to ensure structural safety and functionality over the service life of the structure. These practices involve large direct and indirect costs, such as economic losses from traffic jams. Additionally, not all structures are easy to access for inspection and repair.

In their search to overcome these problems, researchers have been inspired by nature. Biological systems such as bones, skin or plants have the capacity to detect damage very quickly and repair the damage autonomously. The application of so-called “self-healing” concrete, which will in an autonomous way repair cracks, could reduce the maintenance costs drastically. Therefore, several concepts for self-healing concrete have been fundamentally explored, primarily during the last 10 years, with very promising results.

The intrinsic mechanism of crack healing in cementitious materials, due to hydration of unhydrated binder particles and precipitation of carbonate, is called autogenous healing. Since autogenous healing is more effective when crack widths are restricted, the use of a fiber reinforced engineered cementitious composite has been proposed. On the other hand, as water is always needed for autogenous healing to occur, several researchers investigated the possibility to add superabsorbent polymers (SAPs), also called hydrogels, to provide additional water. Finally, addition of agents, which are able to promote the deposition of crystals inside the crack has been studied. For instance, when certain types of (encapsulated) bacterial spores and nutrients are added into the concrete mix, the activation of the spores when a crack appears and water enters, will initiate the deposition of CaCO3 crystals at the crack faces.

Capsule based self-healing materials sequester a healing agent inside discrete capsules. When the capsules are ruptured by damage, the self-healing mechanism is triggered through the release and reaction of the healing agent in the region of damage. To make the capsule based approach practically applicable, research has been devoted to the development of capsules, which are able to survive the concrete mixing process while they do not influence the final mechanical properties too much. Vascular-based self-healing materials sequester the healing agent in a network of hollow tubes connected to the exterior of the structure.

Techniques to evaluate the self-healing efficiency also form an important research field. This includes regain in mechanical properties by destructive or non-destructive testing, e.g., by transmission of ultrasound waves, acoustic emission or resonance frequency measurements. Regain in air- and liquid-tightness is also very important. Fluid transmission can be registered directly or visualized by radiographic techniques, and related to durability test results. Early modelling work has been undertaken with lattice type models and hydration models, which can simulate self-healing due to the on-going hydration in a crack. Some analytical and numerical models are available to study the distribution of capsules in the concrete.

Prof. Dr. ir. Nele De Belie
Guest Editor

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