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[Multi] Spore FR - __TOP__ Crack

Other recommendations were derived from knowledge gained during infectious disease investigations in health-care facilities, where successful termination of the outbreak was often the result of multiple interventions, the majority of which cannot be independently and rigorously evaluated. This is especially true for construction situations involving air or water.

[Multi] Spore FR - Crack

Self-healing concrete holds promising benefits to reduce the cost for concrete maintenance and repair as cracks are autonomously repaired without any human intervention. In this study, the application of a carbonate precipitating bacterium Bacillus sphaericus was explored. Regarding the harsh condition in concrete, B. sphaericus spores were first encapsulated into a modified-alginate based hydrogel (AM-H) which was proven to have a good compatibility with the bacteria and concrete regarding the influence on bacterial viability and concrete strength. Experimental results show that the spores were still viable after encapsulation. Encapsulated spores can precipitate a large amount of CaCO3 in/on the hydrogel matrix (around 70% by weight). Encapsulated B. sphaericus spores were added into mortar specimens and bacterial in situ activity was demonstrated by the oxygen consumption on the mimicked crack surface. While specimens with free spores added showed no oxygen consumption. This indicates the efficient protection of the hydrogel for spores in concrete. To conclude, the AM-H encapsulated carbonate precipitating bacteria have great potential to be used for crack self-healing in concrete applications.

FIGURE 3. Ureolytic activity of non-encapsulated spores (S), modified alginate hydrogel (AM-H), and modified alginate hydrogel encapsulated spores (AM-HS; n = 3).

Citation: Wang J, Mignon A, Snoeck D, Wiktor V, Van Vliergerghe S, Boon N and De Belie N (2015) Application of modified-alginate encapsulated carbonate producing bacteria in concrete: a promising strategy for crack self-healing. Front. Microbiol. 6:1088. doi: 10.3389/fmicb.2015.01088

Biological materials possess a variety of artful interfaces whose size and properties are adapted to their hierarchical levels and functional requirements. Bone, nacre and wood exhibit an impressive fracture resistance based mainly on small crystallite size, interface organic adhesives and hierarchical microstructure. Currently, little is known about mechanical concepts in macroscopic biological interfaces like the branch-stem junction with estimated 1014 instances on earth and sizes up to few meters. Here we demonstrate that the crack growth in the upper region of the branch-stem interface of conifer trees proceeds along a narrow predefined region of transversally loaded tracheids, denoted as sacrificial tissue, which fail upon critical bending moments on the branch. The specific arrangement of the tracheids allows disconnecting the overloaded branch from the stem in a controlled way by maintaining the stem integrity. The interface microstructure based on the sharply adjusted cell orientation and cell helical angle secures a zig-zag crack propagation path, mechanical interlock closing after the bending moment is removed, crack gap bridging and self-repairing by resin deposition. The multi-scale synergetic concepts allows for a controllable crack growth between stiff stem and flexible branch, as well as mechanical tree integrity, intact physiological functions and recovery after the cracking.

Internal interfaces in structured biological materials appear at all hierarchical levels and contribute decisively to the overall tissue integrity and function1. In the past, there has been considerable effort to understand specially the role of interfaces in the fracture resistance and deformability of biological materials at the nanoscale2,3. Impressive toughness of hard biological materials, such as bone4,5,6,7, nacre8,9 and enamel10,11 was attributed to the properties of small mineral particles12, soft interface organics13,14 and hierarchical microstructure15,16. These aspects are also responsible for repeated crack deflection, splitting and blunting17 due to the very dense interfacial network18, an optimized size of the hard mineral particles12 and stick-slip phenomena13. In the case of polymer-based biological materials such as wood and coir fibres, recovery of mechanical properties beyond the yield point was attributed to the interface phenomena in the cell wall mediated by crystalline cellulose and amorphous matrix of lignin and hemicellulose19,20. Up to now, functional and mechanical optimization of biological interfaces at the macro-scale has remained mainly unexplored.

In this work, we have analysed microstructural, mechanical and self-repairing mechanisms at a branch-stem interface of a Norway spruce (Picea abies [L.] Karst). The interface has to support not only basic physiological functions like water and nutrient transport but has to be morphologically and mechanically adapted to static and dynamic loads in order to secure the mechanical safety of the tree and a certain level of damage tolerance21,22. At the cellular level, mechanical properties of trees are primarily dependent on the magnitude of the microfibril angle (MFA), which represents the angle between the direction of the helical windings of cellulose fibrils in the secondary cell wall and the cell longitudinal axis23,24. In previous studies, it was observed that branch tissue is actually embedded in stem collars overgrowing the branch25 where MFA magnitude in the stem envelope was found to be adapted to the environmental and functional requirements21. Consequently there were no pronounced strain concentrations measured at the branch-stem cross-section during branch bending22. MFA distribution in the vicinity of branch-stem junction, local microstructure as well as interface mechanical behaviour have not been studied yet in detail. Since the tree possesses a complex hierarchical microstructure, the interface optimization is expected to take place at multiple length scales.

In Fig. 1, an optical image of an exemplary tree interface during crack propagation documents the presence of a zig-zag crack pattern which was formed in the upper branch region during a bending experiment (cf. supplemental Video 1). The pattern consists of primary cracks which propagate along the interface parallel to the branch axis and secondary cracks which are deflected by about 90 degrees. The unique crack pattern morphology secures high energy consumption during crack growth due to multiple crack deflection (at every annual ring) which results in an increased fracture resistance. At this stage, it is not clear if this toughening mechanism originates from a tissue internal morphology or an intrinsic stress field at the interface. Moreover, after the stress relief, the zig-zag crack shape26,27 allows for a stepwise closing of the crack gap due to the elastic energy stored in the compressively stressed region of the flexible branch.

A radial section through the pith of the stem and branch showing the interface during crack growth initiated by the branch bending moment which results in the formation of a zig-zag crack pattern with primary and secondary cracks. The grey shaded colour of the tissue around the crack (better visible in Supplementary Fig. 1) indicates mainly the out of plane oriented tracheids and the presence of the sacrificial tissue with a relatively small fracture toughness originating from the optimized cell orientation discussed in Fig. 5.

Origins of the specific zig-zag crack pattern (Fig. 1) and the bridging features (Fig. 4b) can be understood from the analysis of the interface microstructure presented in Fig. 5. The optical micrographs show that most of the wood tracheids (depicted as red lines in Fig. 5a) are oriented approximately parallel to the branch or the stem axes, lying in the micrographs plane of Fig. 5a. In the region of the expected crack path, however, the orientation of the tracheids (represented by red circles in Fig. 5a) changes in a very abrupt manner and there is a distinct narrow region with tracheids oriented perpendicular to the micrograph plane. This region denoted as sacrificial tissue is visible in the full resolution in the supplementary Figs 3 and 4.

In the case of branch bending as in Fig. 3b, the tracheids in branch upper region and in the stem are loaded axially whereas the tracheids of the sacrificial tissue are loaded transversely. Since the transverse tensile strength of wood tissue is about 10% of its axial strength28, the crack will propagate along the predefined crack path within the sacrificial tissue which is distributed step-wise along the whole interface from the pith to the bark of the stem (Fig. 5a,6). The region of the sacrificial tissue in Fig. 5a is very well defined within the annual ring structure and its distance from the branch pith increases with the formation of additional annual rings giving origin to the zig-zag pattern (cf. supplementary Fig. 4).

Upon branch bending, the cracking takes place preferably in the region with transversally loaded wood cells of the sacrificial tissue represented by red dots and the crack path arrests here. Due to the finite thickness and shape of the sacrificial tissue, the crack growth is accompanied by a crack branching and repeatable crack zig-zag deflection. Critical bending moments imposed on the flexible branch with large MFA will, therefore, not be transferred on the stem cells (represented by vertical lines) but will result in disconnecting the branch from the relatively brittle stem. Wood rays (green) reinforced with tracheids (red) form tissue bundles responsible for crack bridging. The high concentration of resin ducts (yellow) activated after the cracking secures antimicrobial and hydrophobic protection.

The crack bridging visible in Fig. 4b is mediated by wood rays and tracheids of the sacrificial tissue. The wood rays, visible as dark streaks in Fig. 5a, are oriented approximately perpendicular to the tracheids. During branch bending, the rays are loaded in shear-tension, which results in a formation of characteristic cell bundles (Fig. 4b) that are reinforced with wood tracheids (Fig. 5a).

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