Developing self-healing concrete to improve the durability of structures

Structures made of self-healing concrete have an inherent healing mechanism that becomes active when a crack appears, thus rendering manual crack repair completely obsolete. In order to obtain such automatic crack closure, HEALCON European Project Partners are investigating the use of PU-based polymer precursors, superabsorbent polymers and bacteria. The first stage of this project has already yielded very promising results on laboratory scale.

PU precursors

For the efficient healing of cracked concrete, the use of encapsulated PU-based precursors has already shown great potential. So far, results have shown a good regain in mechanical behavior and liquid-tightness. In proof-of-concept tests, glass tubes were used as encapsulation material. However, to up-scale the technique and make it compatible with conventional concrete production and placing methods, polymeric spherical microcapsules would be more suitable. Therefore, research is ongoing to optimize the encapsulation technique. Moreover, PU-based precursors are adapted to increase the resistance of the healing agents to cyclic loading, so that also the healing of dynamic cracks can be considered.

Superabsorbent polymers

Superabsorbent polymers can absorb large quantities of water. As such, they can help seal cracks and stimulate crack healing if they are incorporated into self-healing concrete. In particular, within the HEALCON project, new synthetic superabsorbent polymers with improved swelling and pH sensitiveness have been developed, as these will seal cracks even more efficiently. Moreover, the HEALCON project aims to investigate other possibilities too, in order to prevent the creation of macropores due to the water absorption by the superabsorbent polymers during concrete mixing.

Bacteria as healing agent

Also bacteria can be used as healing agent. These micro-encapsulated CaCO₃ precipitating bacteria are brought into the concrete in dormant state. When a crack appears and water enters the concrete, these ‘sleeping’ bacteria become active and start to fill the crack with CaCO₃ when nutrients are available. Although good results were obtained with the pure cultures, the elevated cost and the difficulties associated to an axenic production forced the HEALCON partners to use mixed cultures. Further tests performed in mortar specimens will have to determine the real performance of the mixed bacterial community.

Besides ureolytic, also non-ureolytic alkali-resistant spores of Bacillus are examined. These spores have been successfully incorporated into expanded clay particles (i.e. Liapor particles), which serve as healing agent reservoirs for the protection and immobilization of the biogenic healing agent. The impregnation procedure of these particles has been optimized to increase the storage period of the loaded clay particles before use in concrete.

Modelling

It is useful to first simulate the fracturing and self-healing mechanisms of concrete in order to refine lab tests before ultimately up-scaling these mechanisms to an industrial level. In a first stage of the HEALCON project, the bio-based self-healing mechanism is modelled. The Liapor particles are modelled as spheres, and the chemical reactions that take place during healing (i.e. the conversion of the healing agent into CaCO₃) are also considered.

The results show that the ‘healing degree’ (i.e. the ratio of the volume of generated CaCO₃ in the crack to the volume of the crack) is strongly dependent on the width of the crack, while it is almost independent of its depth. The ‘final healing degree’ (i.e. when the whole amount of healing agent has reacted to produce CaCO₃) can reach 50% or even 75%, depending on the crack width and the volume fraction of the Liapor particles.

This model will be refined to consider the effect of the physical properties of the cement paste, Liapor particles, and other factors, and will also include more realistic cracks.

End-user involvement

Before self-healing concrete can be brought onto the market, stakeholders require a proof of efficiency of the healing process. There are obvious advantages to use non-destructive testing techniques prior to destructive ones to prove the efficiency. Non-destructive methods make it possible to assess characteristic parameters like strength and Young’s modulus, but they also allow us to detect visible and invisible defects in structural elements. Within the HEALCON project, several non-destructive testing methods (e.g. acoustic emission analysis and the time-of-flight diffraction technique) have been tested on a laboratory scale. To measure the dynamic modulus of elasticity in the different states (before/after cracking and after healing) resonance frequency measurements and modal analysis have been applied. Later on, these techniques will be applied to construction elements (e.g. large beams) and finally also to constructions in-situ, like bridges or slabs.

Duration: 48 months
Start date: January 2013
Project budget: 5,391,158.80 €
Project reference: 309451
Project funding: 3,997,429.00 €
Contract type: Collaborative project

Consortium

Coordinator: Ghent University (Belgium)

Partners:
Avecom N.V. (Belgium)
Technische Universiteit Delft (The Netherlands)
Acciona Infraestructuras S.A. (Spain)
Technische Universitaet Muenchen (Germany)
Technologie-Transfer-Initiative GmbH (Germany)
Teknologian Tutkimuskeskus VTT (Finland)
Cowi A/S (Denmark)
Teknologisk Institut DTI (Denmark)
INNCEINNMAT, S.L. (Spain)
Fescon OY (Finland)
Devan Micropolis (Portugal)