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Self-protected nitrate reducing culture for intrinsic repair of concrete cracks

View Article: PubMed Central

ABSTRACT

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a NO3− reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5–2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1–0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available NO3− was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing.

No MeSH data available.


SEM micrographs showing the indentation points whilst mechanical testing of the calcite (A) calcite formed in reference specimen (B) calcite formed in microbial specimen (0.5% ACDC). “°” indicates the points analyzed during indentation. Please refer to Supplementary Figure 3 for higher resolution on indentation.
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Figure 7: SEM micrographs showing the indentation points whilst mechanical testing of the calcite (A) calcite formed in reference specimen (B) calcite formed in microbial specimen (0.5% ACDC). “°” indicates the points analyzed during indentation. Please refer to Supplementary Figure 3 for higher resolution on indentation.

Mentions: In addition to visual analysis, mechanical properties of the calcite crystals were quantified through indentation tests (Figure 7). Martens hardness was measured and based on the available data approximate E-modulus values were calculated. The Martens hardness values of 2.1 ± 0.2 GPa and 2.1 ± 0.7 GPa were achieved from the tested CaCO3 minerals in reference and microbial specimens, respectively (Figure 8A). The variation of hardness values for CaCO3 minerals were higher in microbial specimens than the reference specimens, yet they did not significantly differ among the specimens (Figure 8A). Further visual inspection of the indentation points revealed that on some of the minerals nano-cracks occurred under indentation load (See Supplementary Figure 3). These observed cracks were not particular for a certain type of specimen and observed in both cases. Following indentation tests, sampling points and the different layers were visualized. Indentation measurements were also conducted on hydrated cement paste. Martens hardness values of 1.2 ± 0.3 GPa and 1.3 ± 0.7 were obtained for the hydrated cement paste in reference and microbial samples, respectively (Figure 8A).


Self-protected nitrate reducing culture for intrinsic repair of concrete cracks
SEM micrographs showing the indentation points whilst mechanical testing of the calcite (A) calcite formed in reference specimen (B) calcite formed in microbial specimen (0.5% ACDC). “°” indicates the points analyzed during indentation. Please refer to Supplementary Figure 3 for higher resolution on indentation.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4631954&req=5

Figure 7: SEM micrographs showing the indentation points whilst mechanical testing of the calcite (A) calcite formed in reference specimen (B) calcite formed in microbial specimen (0.5% ACDC). “°” indicates the points analyzed during indentation. Please refer to Supplementary Figure 3 for higher resolution on indentation.
Mentions: In addition to visual analysis, mechanical properties of the calcite crystals were quantified through indentation tests (Figure 7). Martens hardness was measured and based on the available data approximate E-modulus values were calculated. The Martens hardness values of 2.1 ± 0.2 GPa and 2.1 ± 0.7 GPa were achieved from the tested CaCO3 minerals in reference and microbial specimens, respectively (Figure 8A). The variation of hardness values for CaCO3 minerals were higher in microbial specimens than the reference specimens, yet they did not significantly differ among the specimens (Figure 8A). Further visual inspection of the indentation points revealed that on some of the minerals nano-cracks occurred under indentation load (See Supplementary Figure 3). These observed cracks were not particular for a certain type of specimen and observed in both cases. Following indentation tests, sampling points and the different layers were visualized. Indentation measurements were also conducted on hydrated cement paste. Martens hardness values of 1.2 ± 0.3 GPa and 1.3 ± 0.7 were obtained for the hydrated cement paste in reference and microbial samples, respectively (Figure 8A).

View Article: PubMed Central

ABSTRACT

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a NO3− reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5–2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1–0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available NO3− was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing.

No MeSH data available.