Abstract
Polymeric capsules can have an advantage over glass capsules used up to now as proof-of-concept carriers in self-healing concrete. They allow easier processing and afford the possibility to fine tune their mechanical properties. Out of the multiple requirements for capsules used in this context, the capability of rupturing when crossed by a crack in concrete of a typical size is one of the most relevant, as without it no healing agent is released into the crack. This study assessed the fitness of five types of polymeric capsules to fulfill this requirement by using a numerical model to screen the best performing ones and verifying their fitness with experimental methods. Capsules made of a specific type of poly(methyl methacrylate) (PMMA) were considered fit for the intended application, rupturing at average crack sizes of 69 and 128 μm, respectively for a wall thickness of ~0.3 and ~0.7 mm. Thicker walls were considered unfit, as they ruptured for crack sizes much higher than 100 μm. Other types of PMMA used and polylactic acid were equally unfit for the same reason. There was overall good fitting between model output and experimental results and an elongation at break of 1.5% is recommended regarding polymers for this application.
Highlights
An increasing amount of research on encapsulation of healing agents for self-healing concrete has been published during the last decade
The results show that polyethyleneglycol monomethylether (PEG) did not leach out of the PMMA_1-PEG material, or it did not leach out results show that PEG did not leach out of the PMMA_1-PEG material, or it did not leach out enough for capsules of this material to behave as capsules made out of PMMA_1, instead rupturing enough for capsules of this material to behave as capsules made out of PMMA_1, instead rupturing for crack sizes more than twice as large
The numerical model identified a wide range of performances for the polymeric capsules and was confirmed to be a useful tool for preliminary screening of materials to be used for encapsulation
Summary
An increasing amount of research on encapsulation of healing agents for self-healing concrete has been published during the last decade. Other than being able to effectively release the healing agent after the onset of damage, capsules need to meet other more basic, but challenging requirements They have to resist the mechanical stresses experienced during placing of concrete, in case of pre-placement of the capsules in the formwork, or during the mixing process, if added to fresh concrete during mixing. Other researchers still are focusing on the complex mechanism of bonding between capsule and matrix [19] and on the input necessary to develop models that simulate the release and dispersion of healing agent once the capsules are ruptured [20]. Numerical modelling is used to simulate breakage of tubular capsules made of different polymers and with various wall thicknesses This makes it possible to reduce the number of experiments and quickly reject materials that are not suitable, regarding their capability to rupture when crossed by cracks in concrete
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