Encapsulation of polymeric healing agents in self-healing concrete: capsule design

The ERA-Net project CAPDESIGN has addressed the development, optimisation and testing of new capsules for applications in self-healing concrete. 40 to 120 million Euro of the maintenance costs for concrete bridges, tunnels and retaining walls in the European Union could be saved by application of a new type of concrete that possesses self-repairing properties during formation of microcracks. For self-healing concrete with polymeric healing agents (e.g. PUR, PMMA), a major bottleneck relies in developing capsules that should both tolerate concrete mixing procedure and break easily under crack formation in cured concrete. The aim of SINTEF's activity in the project was to develop competence in the field of nanocomposite capsules applied for self-healing concrete. SINTEF developed hybrid nanomaterials that were applied in polymer materials in order to tailor-made the properties of composite capsules. Functionalized hybrid polyhedral oligomeric silsesquioxane nanomaterials (POSS) with hydrophobic modification were prepared and used as additives to modify the impact strength as well as the barrier properties of the following capsule polymer materials: Copolyester, PET, PMMA and PS. Hybrid nanomaterials were prepared via a two-step synthesis including a sol-gel process. NMR was used to characterise the POSS materials that were synthesized and to confirm the formation of amide functionality on POSS. The modified polymers were tested for mechanical properties by conducting tensile and impact tests and for barrier properties by measurement of the water vapour/oxygen transmission rates through as-prepared modified films. It was found that the addition of hybrid POSS nanomaterials increased the brittleness of copolyester by lowering its impact strength while maintaining its mechanical strength for all the selected polymers. PET and copolyester were chosen to investigate the barrier properties due to their intrinsic superior barrier properties compared with PMMA and PS. The results showed that water vapour barrier property of the polymer blends was improved with addition of the nanomaterials. Meanwhile the inherent good oxygen barrier of PET was maintained. The polymer compounds were incubated at high pH and no remarkable effect was noticed on the mechanical properties of polymer compounds. The main results show that the nanocomposites produced by mixing copolyester with hybrid POSS are potential candidates for encapsulation of healing agents in self-healing concrete. The projects results were presented in national and international conferences and workshops.

We consider that 40-120 million Euros of the maintenance costs for concrete bridges, tunnels and retaining walls in the European Union could be saved by application of self-healing concrete. For self-healing concrete with polymeric healing agents (e.g. PU R, PMMA), the bottleneck for valorization is however the encapsulation technique since the capsules have to possess multifunctional properties. The capsules with embedded healing agent (i) have to protect the healing agent for a long time, (ii) have to re lease the healing agent when cracking occurs and (iii) should not influence the fresh concrete workability and the early and long term mechanical properties. More important, we are looking for capsules which can easily be mixed in concrete and/or can surv ive the placement technique (e.g. projection). In that way, the concrete production / application process is not too much affected and the processing costs will not rise. The contradictory requirements make it however difficult to find a suitable encapsul ation material: on the one hand, we want no breakage during concrete preparation / application, but on the other hand, we strive for immediate breakage of the capsules when a crack appears. Since no commercial products seem to be appropriate, the challeng ing objective of CAPDESIGN will be to develop, optimize and test new capsules for applications in self-healing concrete. In addition, an innovative and specific placement technique by projection of the capsules in association with concrete will be devel oped. Its main objective is to provide a greater durability of the capsules during the concrete placement. The benefit of capsules which can survive mixing / application is that (i) the cost of self-healing concrete can be reduced (ii) companies can be m ore easily persuaded to produce self-healing concrete (iii) self healing concrete can be valorized.