<正>In this study,two bacteria-based self-healing systems were developed for the proof-of-concept and approach to a realistic self-healing.A self-healing system with glass capillaries and silica sol gel carried bacterial cells was first built.The bio-CaCO_3 formed in-situ(in silica gel) after glass capillaries breakage preliminarily showed the feasibility of this system.The investigation on the self healing efficiency demonstrated that the water permeability was decreased by about two orders of magnitude due to self-healing.However,practical application of this system was limited by the use of the un-mixable and expensive glass capillaries.A second self-healing system therefore was built in order to approach a realistic self-healing,by using hydrogel encapsulated bacteria.Great superiority in healing efficiency was obtained in this system.A maximum crack width of 0.5 mm could be healed within 7 days in the specimens of the bacterial series;while the maximum crack width can be healed in other series was in the range of 0.2～0.3 mm.Water permeability was greatly decreased(68%) in the bacterial series. Originality Two items in the development of a microbial based strategy for self-healing concrete are of utmost importance.One is the microbial source;the other is a suitable bacterial carrier.In previous research,it was found that Bacillus sphaericus was the most suitable strain due to its spore-forming property and alkali-resistant nature.Therefore, this strain was also used in this study.Due to the harsh environment in concrete, encapsulation of bacteria is preferable before incorporation into the matrix. The bacteria carrier should on the one hand protect bacteria during the mixing and hardening stage;on the other hand not inhibit bacterial carbonate productivity upon cracking. Silica gel used in this research can be formed in situ in the cracks and embed bacteria inside to protect them from the high pH in the surroundings.Moreover,it is also a kind of filling material to block the cracks.However,silica gel has low water retention capacity.Therefore,it easily becomes dry in normal humidity conditions,which is not good for long term bacterial activity.Besides,dry silica gel is really fragile and prone to cracking,resulting in further permeability problems in concrete.Compared with silica gel,hydrogel has the distinct advantages of high water absorption and retention capacity,which greatly benefits for long term bacterial activity,and hence an enhanced healing efficiency.Furthermore,some hydrogels have high moisture uptake capacity, which is really promising to achieve a realistic self-healing for practical application.
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