WO2025217013A1 - Damage-responsive self-healing concrete - Google Patents

Abstract

The present invention relates to concrete compositions and methods of making a concrete composition having self-healing properties, including a cementitious matrix impregnated with a plurality of biofibers, wherein the biofibers comprise: a polymer fiber core comprising one or more polymers selected from the group consisting of polyester, polyethylene, polypropylene, polyvinyl alcohol, polyamide, aramid, polyacrylonitirile, cellulose, polyurethane, and combinations thereof; a crosslinked hydrogel layer coating on the polymer fiber core, wherein the hydrogel layer comprises endospores, one or more cross-linked anionic polymers, and a (co)polymer shell encapsulating the hydrogel layer, wherein the shell has one or more layers which may be the same or different, and each of the one or more layers is formed with a (co)polymer selected from the group consisting of nitrocellulose (NITR), epoxy resin (ER), polymethyl methacrylate(PMMA), polyvinylidene fluoride (PVDF), cyanoacrylate adhesive (CYA), polystyrene (PS), polylactic acid (PLA), and combinations thereof.

Description

DAMAGE-RESPONSIVE SELF-HEALING CONCRETE STATEMENT OF GOVERNMENT INTEREST This invention was made with government support under grant nos.2029555, CMMI2029555, CMMI-2216175, SCR_022684 from the National Science Foundation. The government has certain rights in the invention. CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No.63/631,885, filed on April 9, 2024, the entire disclosure of which is hereby incorporated by reference as if set forth fully herein. BACKGROUND OF THE INVENTION Fiber-reinforced concrete (FRC) is designed to overcome the inherent brittleness and low tensile strength of traditional concrete, which often results in cracking under stress and affects its durability in structural applications (Abd Elmoaty, Morsy, & Harraz, 2022; Afroughsabet, Biolzi, & Ozbakkaloglu, 2016; Massicotte & Bischoff, 2000; Paul, van Zijl, & Šavija, 2020; Yoo & Banthia, 2016). The demand for more durable materials in infrastructure projects, such as bridges, tunnels, and pavements, has led to the integration of fibers into concrete to enhance its mechanical properties (Makul, 2020a, 2020b; Mohajerani et al., 2019; Navaratnam, Selvaranjan, Jayasooriya, Rajeev, & Sanjayan, 2023; Zamora-Castro et al., 2021). Among these, polymeric fibers like polyvinyl alcohol (PVA), polypropylene, and polyethylene are favored for their resistance to chemicals, durability, and ability to improve the materials’ ductility and toughness (Rajak, Pagar, Kumar, & Pruncu, 2019; Tcherdyntsev, 2021; Várdai et al., 2022; Wang, Chan, Leong, & Zhang, 2020). Fibers used in FRC are classified as macro or micro based on their size, with macro fibers typically measuring between 20 to 60 mm in length and 0.5 to 1.5 mm in diameter, while micro-fibers are generally shorter than 20 mm and thinner than 0.3 mm (Shafei, Kazemian, Dopko, & Najimi, 2021; Sharma, Kumar, Ransinchung, & Kumar, 2013). Macro fibers help manage large cracks and improve the concrete's performance after cracking by enhancing its flexural strength and impact resistance (Kazmi, Munir, Wu, & Patnaikuni, 2018). On the other hand, micro-fibers are distributed throughout the concrete to reduce early-age plastic shrinkage cracking and improve the material’s properties at an early stage (Abolfathi, Omur, & Kabay, 2023; Eikamp, 2020). In general, macro-fibers contribute significantly to energy absorption, while micro-fibers are effective in decreasing brittleness, thereby enhancing the overall durability and performance of FRC (Paul et al., 2020). Fibers play a crucial role in enhancing the mechanical properties of concrete, offering various benefits depending on their composition and characteristics (Kiruthika, 2017; Wu, Lin, & Zhou, 2020). Glass fibers are valued for their high tensile strength and resistance to chemical attack, making them ideal for applications such as concrete repair, architectural panels, and reinforced concrete pipes. Typical amounts of glass fibers range from 0.5% to 1.0% by volume of concrete (Shafei et al., 2021). Carbon fibers are known for their exceptional strength-to-weight ratio and stiffness, and they are utilized in high-performance concrete applications, including prestressed concrete structures and advanced composites (Alam, Maraz, Khan, & Reviews, 2022). Typical amounts of carbon fibers range from 0.1% to 0.5% by volume of concrete (Khalil & Abdulrazaq, 2011). PVA fibers are favored for their superior bonding with the cement matrix and their ability to improve crack control and post-cracking behavior (Wang et al., 2020). Typical amounts of PVA fibers range from 0.1% and 0.3% by volume of concrete, making them suitable for industrial floors, pavements, and precast elements (Balea, Fuente, Monte, Blanco, & Negro, 2021). Polypropylene fibers are commonly used to mitigate plastic shrinkage cracking and enhance toughness, with amounts typically ranging from 0.1% to 0.3% by volume of concrete (Aire, Mendoza, & Davila, 2011; Banthia & Gupta, 2006). Polypropylene fibers are particularly effective in floor slabs, pavements, and tunnel linings (Balea et al., 2021). Polyethylene fibers offer good resistance to chemical degradation and are used to improve impact resistance, with typical amounts ranging from 0.1% to 0.3% by volume of concrete in applications like residential and commercial flooring (Paul et al., 2020). Steel fibers provide high tensile strength and excellent crack-bridging capabilities. The amounts of steel fibers usually range from 0.5% to 2.0% by volume of concrete, making them suitable for industrial floors, bridge decks, and structural components subjected to high stress and dynamic loads (Soufeiani et al., 2016). Each type of fiber contributes distinct advantages to concrete reinforcement, offering tailored solutions for a wide range of applications from infrastructure to specialized construction. Treated polymeric fibers represent a significant innovation in the reinforcement of concrete, enhancing both its performance and application versatility (Mahato, Dutta, & Ray, 2020). These fibers, derived from materials such as polypropylene, PVA, and polyethylene, undergo various surface treatments to improve their integration with the cement matrix and address issues related to fiber adhesion and durability (Adekomaya & Majozi, 2019). Surface treatment techniques are designed to modify the fiber's interface with the concrete, thereby boosting bonding efficiency (Mahato et al., 2020). Methods such as plasma treatment, which etches the fiber surface to increase roughness, and chemical treatments involving coupling agents like silanes or titanates, are employed to enhance the chemical compatibility between fibers and the cement matrix (Latif et al., 2019). Additionally, physical coatings, such as epoxy or latex, can be applied to further improve bonding and protect the fibers from environmental degradation (Marques et al., 2020). Surface functionalization involves modifying the fiber surface to impart specific properties. This can include introducing reactive functional groups or coatings that enhance adhesion or provide additional benefits like increased resistance to environmental factors or self-healing capabilities (Khaneghahi et al., 2023). Examples of functionalization techniques include the application of antimicrobial coatings or hydrophilic/hydrophobic treatments (Kumar & Sharma, 2022). These treatments address common concrete reinforcement challenges, such as improving crack resistance, reducing shrinkage, and increasing load-bearing capacity. SUMMARY OF THE INVENTION Self-healing technologies in concrete offer a promising avenue for addressing the pervasive issue of cracking within concrete structures [1-5]. Microbial-based self-healing, i.e., bio-self-healing, in concrete holds significant promise as a sustainable and effective solution for enhancing the durability of concrete structures [6-7]. By harnessing the natural abilities of certain bacteria to produce calcium carbonate, the present invention provides a proactive means of addressing cracks and fissures that develop in concrete over time [8- 9]. Unlike traditional repair methods, microbial-based self-healing operates autonomously within the concrete matrix, triggered by the presence of moisture and nutrients. Furthermore, the microbial-based self-healing of the present invention aligns with the growing trend towards eco-friendly and bioinspired technologies in the construction industry, making it a compelling option for extending the service-life of concrete infrastructure while minimizing environmental impact [1, 6, 10]. An emerging area of investigation in bio-self-healing research involves leveraging microbially induced calcium carbonate precipitation (MICCP) as a remedial agent for quasi-brittle materials [4]. Calcium carbonate, being inherently compatible with such materials, presents a promising avenue for crack and fracture repair [4]. Through tailored self-healing mechanisms, the deposition of calcium carbonate effectively seals and restores structural integrity by filling cracks [4-5, 8]. The present invention may be described by the following sentences: 1. In a first aspect, the present invention relates to a concrete composition having self-healing properties, including: a cementitious matrix impregnated with a plurality of biofibers, wherein the biofibers comprise: a polymer fiber core comprising one or more polymers selected from the group consisting of polyester, polyethylene, polypropylene, polyvinyl alcohol, polyamide, aramid, polyacrylonitirile, cellulose, polyurethane, and combinations thereof; a crosslinked hydrogel layer coating on the polymer fiber core, wherein the hydrogel layer comprises endospores, one or more cross-linked anionic polymers selected from the group consisting of polysaccharides, hyaluronic acid, colominic acid, polysialic acid, chondroitin, queratane, dextrans, heparin, carrageenan, furcelerans, alginates, agar, glucomannan, gums, pectins, cellulose, starches, sorbitan esters, and combinations thereof and one or more calcium salts, and a (co)polymer shell encapsulating the hydrogel layer, wherein the shell has one or more layers which may be the same or different, and each of the one or more layers is formed with a (co)polymer selected from the group consisting of nitrocellulose (NITR), epoxy resin (ER), polymethyl methacrylate(PMMA), polyvinylidene fluoride (PVDF), cyanoacrylate adhesive (CYA), polystyrene (PS), polylactic acid (PLA), and combinations thereof. 2. The concrete composition of sentence 1, wherein each of the biofibers are randomly distributed throughout the cementitious matrix. 3. The concrete composition of any one of sentences 1 - 2, wherein the concrete is configured to heal one or more microcracks having a width of from about 50 µm to less than 1 mm, or from about 50 µm to 750 µm, or from about 100 µm – 500 µm, or from about 120 µm to about 400 µm, or from about 120 µm to about 250 µm. 4. The concrete composition of any one of sentences 1 – 3, wherein the shell encapsulating the hydrogel layer is a copolymer. 5. The concrete composition of any one of sentences 1 – 4, wherein the shell is a copolymer comprising polystyrene and polylactic acid. 6. The concrete composition of any one of sentences 1 – 5, wherein the plurality of biofibers have an average diameter of from about 0.5 mm to about 2 mm, or from about 0.75 mm to about 1.5 mm, or from about 0.9 mm to about 1.25 mm. 7. The concrete composition of any one of sentences 1 – 6, wherein the hydrogel layer has a water-uptake capacity of from about 1 g/gcore-fiber – 10 g/gcore-fiber, after exposure to an aqueous environment for 30 minutes. 8. The concrete composition of any one of sentences 1 – 7, wherein each of the plurality of biofibers is fully integrated into the cementitious matrix providing internal reinforcement via interfacial bonding with the matrix and biofiber. 9. The concrete composition of any one of sentences 1 – 8, wherein each of the plurality of biofibers is oriented parallel to a loading direction. 10. The concrete composition of any one of sentences 1 – 9, wherein the plurality of biofibers is present in the cementitious matrix in a volumetric amount of from about 0.1% to about 2%, or from about 0.2% to about 1.5%, or from about 0.25% to about 1%, based on a total volume of the curable concrete composition. 11. The concrete composition of any one of sentences 1 – 10, further comprising a polymeric fiber, optionally selected from polyvinyl alcohol. 12. The concrete composition of sentences 11, wherein the polymeric fiber is present in the cementitious matrix in a volumetric amount of from about 0.1% to about 2%, or from about 0.2% to about 1.5%, or from about 0.25% to about 1%, based on a total volume of the curable concrete composition. 13. The concrete composition of any one of sentences 1 – 12, wherein the calcium salts comprise calcium alginate. 14. The concrete composition of any one of sentences 1 - 13, wherein the self-healing concrete has a crack-filling ratio of from about 5% to about 100%, or from about 50% to about 100%, or from about 70% to about 100%, after 28 days, wherein the crack-filling ratio is determined by: ^_{^,^^ ^ %^ = ^,^ ^ × 100} where Rh,t is the crack-filling calcium carbonate precipitation (MICCP) time t, A_h,t is a filled crack area, and A_c,t0 is an initial crack area, as determined by optical microscopic images. 15. The concrete composition of any one of sentences 1 – 14, wherein the endospores are selected from the group consisting of Lysinibacillus sphaericus, cyanobacteria, Synechococcus, and Prochlorococcus and heterotrophs such as Sporosarcina pasteurii (Bacillus sphaericus), B. megaterium, B. subtilis, B.cereus, B. cohnii, B. pseudofirmus, B. alkalinitrilicus, Diaphorobacter nitroreducens, Pseudomonas aeruginosa, Desulfovibrio brasiliensis, and Desulfovibrio vilgaris, B. mucilaginous. 16. In a second aspect, the present invention relates to a method of preparing the self-healing concrete composition of any one of sentences 1 – 15 comprising steps of: a. preparing the plurality of biofibers, wherein each biofiber is prepared by: i. coating the core with a solution comprising one or more anionic polymers and endospores to form a coated core; ii. crosslinking the coated core formed in step a) with a calcium-containing crosslinking agent to form a crosslinked hydrogel layer coated core; iii. encapsulating the crosslinked hydrogel layer coated core with the (co)polymer to form a polymer shell having one or more layers to form the biofiber; iv. preparing a cement paste; and v. integrating the plurality of biofibers into the cement paste. 17. The method of sentence 16, wherein prior to the step of encapsulating the crosslinked hydrogel layer, the hydrogel layer coated core is dried. 18. The method of any one of sentences 16 - 17, wherein the hydrogel layer coated core is dried for a duration of from about 1 hour to about 48 hours, or from about 2 hours to about 36 hours, or from about 5 hours to about 24 hours. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1A shows a schematic of cement paste samples reinforced with multiple bioFibers in the targeted reinforced area. Fig.1B shows a schematic of crack creation by flexural loading in the cement paste samples reinforced with bioFibers. Fig.2A shows the same images as figures FIG.1A and 1B, illustrating the sample dimensions and the bioFiber spacing/locations in cement paste samples reinforced with bioFibers. Fig.2B the left image shows example formulations for preparing and curing of the cement matrix reinforced with bioFibers, as well as healing activation agents/dosages with exposure cycles. The right image shows a cross-sectional view of the cement matrix reinforced with the bioFiber. Fig.3A is a schematic demonstrating cracking formation, propagation, and failure in quasi-brittle composites. Fig.3B is a schematic showing the function of the bio-Fiber when subjected to cracking. Fig.3C is a schematic showing the bioFiber manufacturing process. Fig.4A shows a chart illustrating the crack-filling efficiency of bioFiber versus average crack width for 7-, 14-, 21-, and 28-days post MICCP activation. Fig.4B shows crack-filling performance of bioFiber in cement paste over a 28 day period, including one control sample and 5 sample reinforced with bioFiber. Fig.5 shows a photograph of a fully fractured specimen (prior to healing initiation) with BioFiber bridging between the separated parts, qualitatively demonstrating the bridging effect. Fig.6 shows a schematic and photograph of a fractured BioFiber in cement paste sample after 28 days of the healing cycle. Fig.7A shows thermogravimetric analysis (TGA) results for bio-agent, Lysinibneacillus. sphaericus germinated cells. Fig.7B shows thermogravimetric analysis (TGA) results for, urea. Fig.7C shows thermogravimetric analysis (TGA) results for, yeast extract. Fig.7D shows thermogravimetric analysis (TGA) results for, calcium acetate monohydrate. Fig.8A shows thermogravimetric analysis on MICCP precipitates (secondary white residue) collected after 7 days from the crack area in the cement paste samples. Fig.8B shows thermogravimetric analysis on MICCP precipitates (secondary white residue) collected after 14 days from the crack area in the cement paste samples. Fig.8C shows thermogravimetric analysis on MICCP precipitates (secondary white residue) collected after 21 days from the crack area in the cement paste samples. Fig.8D shows thermogravimetric analysis on MICCP precipitates (secondary white residue) collected after 28 days from the crack area in the cement paste samples. Fig.9 shows an SEM image of the bioFiber component indicated in cement paste post cracking, and prior to MICCP initiation. Image a) shows that the bioFiber shell was still adhered to the core-fiber. Image b) shows the bioFiber adherence to the cement matrix. Fig.10 shows an SEM image of the bioFiber prior to MICCP initiation, showing the cement hydration products adhered to the bioFiber shell. Fig.11 shows an SEM image of the bioFiber prior to MICCP initiation. Image a) shows shell adherence to the matrix, and image b) shows the shell interior surface in contact with the inner bioFiber layers after separation. Fig.12 shows an SEM image of the bioFiber precipitation (secondary white residue) collected after 7-day MICCP, from the crack area in the cement paste samples. Fig.13 shows an SEM image of the bioFiber precipitation (secondary white residue) collected after 14-day MICCP, from the crack area in the cement paste samples. Fig.14 shows an SEM image of the bioFiber precipitation (secondary white residue) collected after 21-day MICCP, from the crack area in the cement paste samples. Fig.15 shows an SEM image of the bioFiber precipitation (secondary white residue) collected after 28-day MICCP, from the crack area in the cement paste samples. Fig.16A shows a chart of the XRD results of the qualitative phase identification on collected MICCP powders at day 7. Fig.16B shows a chart of the XRD results of the qualitative phase identification on collected MICCP powders at day 14. Fig.16C shows a chart of the XRD results of the qualitative phase identification on collected MICCP powders at day 21. Fig.16D shows a chart of the XRD results of the qualitative phase identification on collected MICCP powders at day 28. Fig.17A shows a graph of the crack-filling ratio over healing time for 14 and 28 samples, comparing control samples (no bioFiber) with samples reinforced with a limited number of bioFibers. Fig.17B shows an example of the bioFiber-enhanced crack closure in the cement paste matrix for a limited number of bioFibers crossing the crack volume. Fig.18 shows a chart of the thermogravimetric results on collected precipitation (secondary white residue) on a 28-day healed fractured surface, indicating the calcium carbonate content in the collected precipitations. Fig.19 shows an SEM image of the precipitation collected from the fracture surface after day 28 of healing. Fig.20 shows the optical and scanning electron images of polyester and polyvinyl alcohol fibers with the properties listed in the table. Fig.21 shows the sodium alginate in powder and solution forms with different amounts (top left images); shows optical and scanning electron images of polyester and polyvinyl alcohol fibers coated with 8 w/v alginate solution (bottom left images); illustration of ionic crosslinking of sodium alginate with calcium crosslinker (top right image); factors to be considered during hydrogel (alginate) coating (bottom right image). Fig.22 shows the hydrogel thickness when using polyester (PET) versus polyvinyl alcohol (PVA) with different amounts of sodium alginate (Na-Alg) solution. The picture in the middle shows how the thickness is determined based on the core-fiber diameter. Fig.23 shows the swelling capacity of the hydrogel when using polyester (PET) versus polyvinyl alcohol (PVA) as the core-fiber with different amounts of sodium alginate (Na-Alg) solution. The equation in the middle shows how the swelling ratio is calculated. Fig.24 shows the swelling capacity of hydrogel coated on polyester (PET) and polyvinyl alcohol (PVA) core-fiber in solutions with different pH values, ranging from acid to neutral to base. Fig.25 shows examples of shell coating materials on hydrogel coated core-fibers, for the shell materials Nitrocellulose (NITR), Epoxy Resin (ER), Polymethylmethacrylate (PMMA), Polyvinylidene Fluoride (PVDF), Cyanoacrylate Adhesive (CYA), Polystyrene (PS), and Polylactic Acid (PLA). Fig.26A shows average shell coating thicknesses on polyester (PET) core-fiber using polylactic acid: polystyrene (PLA:PS) (1:1wt.%) at varying concentrations of the copolymer in the solvent. Fig.26B shows average shell coating thicknesses on polyvinyl alcohol (PVA) core-fiber using PLA:PS (1:1 wt.%) at varying concentrations of the copolymer in the solvent. Fig.27 shows the fluid ingress resistance of the bioFiber when exposed to synthesized pore solutions having a pH of ~13. The dark areas highlighted in the bottom image show failed coating examples. The dark area shows the exposure of the phenolphthalein pH indicator added to hydrogel exposed to basic solution. Fig.28 shows the casting survivability when using manual mixing versus mechanical shear mixing for polyester (PET) and polyvinyl alcohol (PVA) as core fiber with 8 w/v alginate as hydrogel and polylactic acid: polystyrene (PLA:PS) (1:1wt.%) at varying concentrations of the copolymer in the solvent and number of shell coatings. Fig.29 shows SEM images of the frontal and transverse views of the polyvinyl alcohol (PVA) core-fiber, endospore-laden alginate hydrogel, and outer polylactic acid: polystyrene (PLA:PS) shell. Fig.30 shows the swelling characteristics of the 8 w/v alginate hydrogel coating on polyvinyl alcohol (PVA) core-fiber, over exposure time and increased pH. Fig.31 shows photographs of the self-healing progress for a fractured bioFiber. The bioFiber was prepared with the yeast extract (20 g/L), urea (20 g/L), and calcium acetate (20g/L), visually observed on day 1, day 3, day 6, and day 10. Fig.32 shows SEM images of the fractured bioFiber at day 1 and day 10 from Fig.31. Fig.33 shows SEM images of MICCP precipitations collected from the solution at day 1 and day 10 of the bioFiber activation. Fig.34 shows in the left image the thermogravimetric (TGA) analysis results from the MICCP precipitations collected at different time frame (of bioFiber activation), indicating the increase in calcium carbonate content as time progressed. The right images show the calculated amount of calcium carbonate from TGA results shown on the left, in terms of normalized percentage and weight. Fig.35 shows SEM images and results from X-ray diffraction (XRD) carried out on MICCP precipitate residue to identify the calcium carbonate crystal phases, mainly calcite (C) and vaterite (V). Fig.36 shows the amorphous and crystalline contents in MICCP precipitations collected at different MICP time frames. Fig.37 shows a summary of the properties of candidate bioFiber. Fig.38 is a schematic showing the fiber bridging effect. Fig.39 is a schematic demonstrating how bacteria produce the calcium carbonate healing end- product. Fig.40 is a schematic demonstrating how bacteria produce the calcium carbonate healing end- product. Fig.41 is a schematic showing the ureolysis breakdown accelerated by enzyme(s) produced by bacillus bacteria. Fig.42 shows a schematic for thermal shock endosporulation. Fig.43 shows a breakdown of the components used to induce MICCP. Fig.44 shows an equation to quantify calcium carbonate based on the weight loss recorded in the thermogravimetric results. Fig.45 shows a chart depicting weight loss (TGA) and derivative curve (DTG) in the thermogravimetric test on the bacteria lysinibacillus Sphaericus. Fig.46 shows a chart depicting weight loss (TGA) and derivative curve (DTG) in the thermogravimetric test on the calcium acetate monohydrate. Fig.47 shows a chart depicting weight loss (TGA) and derivative curve (DTG) in the thermogravimetric test on urea. Fig.48 shows a chart depicting weight loss (TGA) and derivative curve (DTG) in the thermogravimetric test on the yeast extract. Fig.49 shows SEM images of the fractured bioFiber before and after casting in concrete. Fig.50 shows more SEM images of the bioFiber and cement paste, highlighting the adherence of hydration product, e.g., portlandite and C-S-H, on the bioFiber outer shell, as an indication of chemical compatibility of the bioFiber shell with the matrix.. Fig.51 shows SEM images of the precipitation powder at 21 days. Fig.52 shows SEM images of the precipitation powder at 28 days. Fig.53 shows the characterization of a healed cracked area using intensity-based image analysis. Fig.54 shows the precipitation kinetics of calcium carbonate over 7 days, 14 days, 21 days, and 28 days. Fig.55 shows the positioning and casting of fiber pullout dogbone specimens of the cement paste matrix. Fig.56 shows a sample placement in a universal testing machine (UTM) using locking grips. Fig.57 shows a schematic of a split tensile test showing stress distribution in the sample cross- section. Fig.58 shows a general profile for force-displacement of a single fiber pullout test. Fig.59 shows the force-slip pullout test results for a PVA fiber. Each line is representative of a replicate. Fig.60 shows the force-slip pullout test results for the bioFiber. Each line is representative of a replicate. Fig.61 shows SEM images of bioFiber/matrix post pullout test, no-to-limited shell/matrix scenario (FM-S1). Fig.62 shows SEM images of bioFiber/matrix post pullout test, moderate shell/matrix scenario (FM-S2). Fig.63 shows SEM images of the surface of PVA core-fibers in image a), and the hydrogel and core-fiber contact surface in image b). Fig.64 shows SEM images of the contact surface of the bioFiber’s outer shell in image a) and the cement paste in image b). Fig.65 shows a graph of the split tensile strength of plain mortar samples (CTRL) and reinforced mortar samples (PVA) and bioFiber (bF) with different volumetric dosage (V%). Fig.66 shows normalized split tensile strength (to plain mortar samples) of reinforced mortars. PVA and bioFiber (bF) with their respective fitted polynomial (Poly) curves. Fig.67 shows a 3D view of XCT scans for cracked bioFiber (1%) reinforced mortar cylinders at 28-days of healing. Cracks are shown in white. Fig.68 shows a sectional view of XCT scans for cracked bioFiber (1%) reinforced mortar cylinders at 28 days of healing. Image a) shows a vertical cross-section (XX) plane. Image b) shows a vertical cross- section (YY plane). Image c) shows a transverse cross-section (ZZ plane). Fig.69 shows a crack-filling ratio (volumetric) in the bioFiber (1 V%) reinforced mortar cylinder. Fig.70 shows an illustration of the crack-filling performance of bioFibers relative to varying crack sizes and volume. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to damage-responsive concrete that can heal damage autonomously. The invention employs the addition of a novel additive to the concrete mix: multifunctional polymeric fiber, referred to interchangeably herein as “biofiber”, “bioFiber”, or “bioFRC”. The biofiber not only provides a bridging effect to increase concrete toughness but also provides autonomous self-healing functionality after the occurrence of micro-cracks in concrete. Microcracks are defined as cracks having a width of less than 1 mm or less than 400 µm. Concrete Composition The concrete composition of the present invention is self-healing. The composition includes a cementitious matrix with a plurality of biofibers. Preferably, each of the biofibers are distributed randomly throughout the cementitious matrix. The biofibers may be present in the cementitious matrix in a volumetric amount of from about 0.1 vol.% to about 2 vol.%, or from about 0.2 vol.% to about 1.5 vol.%, or from about 0.25 vol.% to about 1 vol.%, based on a total volume of the curable concrete composition. The concrete composition is configured to heal one or more microcracks having a width of from about less than 1 mm, most effectively cracks with an average width of <250 µm or widths of from about 50 µm to less than 1 mm, or from about 50 µm to 750 µm, or from about 100 µm – 500 µm, or from about 120 µm to about 400 µm, or from about 120 µm to about 250 µm. The concrete composition has a crack-filling ratio of from about 5% to about 100%, or from about 50% to about 100%, or from about 70% to about 100%, after 28 days, wherein the crack-filling ratio is determined by: ^ _{= ^^,^ × 100} where Rh,t is the crack-filling ratio at microbial-induced calcium carbonate precipitation (MICCP) time t, A_h,t is a filled crack area, and A_c,t0 is an initial crack area, as determined by optical microscopic images. BioFiber Each bioFiber consists of three primary components: (i) a polymeric core-fiber, (ii) a sheath composed of endospore-laden alginate hydrogel, and (iii) an outer protective shell layer. The bioFiber is engineered to endow the cementitious matrix with three key functionalities: (i) bio-self-healing, (ii) control over crack growth, and (iii) damage-induced self-activation. These biofibers are demonstrated herein as effective for in filling cracks within a cementitious matrix. Polymeric Core-Fiber The polymeric core-fiber is suitable for providing bore load and absorbing energy. The polymeric core-fiber includes one or more polymers selected from the group consisting of polyester, polyethylene, polypropylene, polyvinyl alcohol, polyamides, aramids, polyacrylonitirile, cellulose, polyurethane, and combinations thereof. Endospore-Laden Hydrogel The endospore-laden hydrogel layer is bio-compatible and coats the polymeric core-fiber. This layer may be formed, for example, with a solution comprising endospores and one or more anionic polymers selected from the group consisting of polysaccharides, hyaluronic acid, colominic acid, polysialic acid, chondroitin, queratane, dextrans, heparin, carrageenan, furcelerans, alginates, agar, glucomannan, gums, pectins, cellulose, starches, sorbitan esters, and combinations thereof. This solution is then crosslinked, using one or more crosslinking agents. Suitable examples of crosslinking agents include a cationic crosslinking agents selected from the group consisting of calcium chloride, calcium acetate, and combinations thereof. Preferably, the hydrogel is suitable for carrying one or more bio-agents. The hydrogel layer includes one or more endospores, optionally selected from Lysinibacillus sphaericus, cyanobacteria, Synechococcus, and Prochlorococcus and heterotrophs such as Sporosarcina pasteurii (Bacillus sphaericus), B. megaterium, B. subtilis, B.cereus, B. cohnii, B. pseudofirmus, B. alkalinitrilicus, Diaphorobacter nitroreducens, Pseudomonas aeruginosa, Desulfovibrio brasiliensis, Desulfovibrio vilgaris, B. mucilaginous. Outer Protective Shell Layer The outer polymeric shell is a (co)polymer shell which encapsulates the hydrogel layer. The polymeric shell is suitable for responding to strain and triggering healing. This shell may have one or more layers which may be the same or different, wherein each of the one or more layers is formed with a (co)polymer selected from the group consisting of nitrocellulose (NITR), epoxy resin (ER), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), cyanoacrylate adhesive (CYA), polystyrene (PS), polylactic acid (PLA), and combinations thereof. The initial component of the bioFiber system utilizes an inner polymeric core-fiber to impart necessary mechanical attributes. This core-fiber integrates load-bearing capabilities and facilitates energy absorption within the bioFiber structure. Additionally, the core-fiber establishes the conditions conducive for the proliferation of endospores embedded within the hydrogel sheath, ensuring optimal functionality of the system. For example, polyvinyl alcohol (PVA) fibers were selected as the core-fibers due to their robust mechanical properties and biocompatibility. In the hydrogel preparation process, for example, sodium alginate undergoes ionic crosslinking with calcium acetate, which provides the necessary divalent cations. A sodium alginate solution with a concentration of 8 w/v and calcium acetate solution with a concentration of 0.259M were employed to form a calcium alginate hydrogel coating on the core-fiber. Lysinibacillus sphaericus endospore may be employed as the endospore with a concentration of 10⁹ cells/ml, as the bio-healing agent encapsulated within the hydrogel sheath. Thermal shock endosporulation is a promising method for producing endospores capable of surviving, germinating, and thriving under harsh environmental conditions. The germination of endospores is induced by the presence of carbon and nutrient sources, i.e., yeast extract (20 g/L). Furthermore, the initiation of urea hydrolysis-driven MICCP can be facilitated by the provision of urea (20 g/L), followed by calcium acetate (20 g/L). The protective shell layer includes a (co)polymer, a suitable example of which may be composed of a polymer blend comprising polystyrene (PS) and polylactic acid (PLA). The working examples were carried out using these components. BioFiber Fabrication The process of creating bioFibers involved several sequential stages: firstly, immersing the core-fiber into a solution comprising one or more anionic polymers, e.g., sodium alginate, and endospores, e.g. Lysinibacillus sphaericus, followed by immediate immersion into a solution of a crosslinking agent, e.g., calcium acetate; secondly, allowing hydrogel mass loss at room temperature for 24 hours; thirdly, promptly immersing the hydrogel-coated core-fibers into a solution of PS and PLA (1 to 1 wt.%) dissolved in chloroform (18 w/v); and finally, drying again at a temperature of 23 ± 1 °C and relative humidity (RH) of 30 ± 5% for 2 hours. The resulting bioFibers had an average diameter of 1.11 mm and a density of 2.13 g/cm³. The hydrogel coating containing endospores exhibited a water-uptake capacity of 4.41 g/gcore-fiber after a 30- minute exposure to distilled water. The performance of the shell coating was assessed through fluid ingress and abrasion resistance survivability tests, with detailed experimental procedures and outcomes reported herein. Specimen Preparation For the cementitious matrix, cement paste samples were prepared with Ordinary Portland Cement (OPC) Type I/II with a water-to-cement ratio of 0.42, based on ASTM C305-20. Chemical and Bogue composition of this cement is summarized in Table 1. The cubic samples were prepared with the dimensions shown in Error! Reference source not found.2A, using a cast mold having dimensions of 100 mm by 100 mm by 10 mm. Integration of BioFiber into the matrix involved placing five BioFibers at 10 mm intervals within the central region of the samples, ensuring a cover thickness of 5 mm. The cement paste samples were demolded 24 hours after casting, then cured in a sealed condition, i.e., a double-sealed plastic bag with a wet towel at an ambient temperature of 23 ± 1 °C for 60 days. Table 1. Chemical and Bogue composition of OPC Type I/II Chemical Composition Item SiO₂ Al₂O₃ Fe₂O₃ CaO SO₃ MgO Na₂O K₂O Total LOI Amount (% wt) 18.94 5.00 4.47 61.36 3.55 2.96 0.36 0.96 97.60 2.09 B_{ogue Composition Item C3S C2S C3A C4AF Amount (% wt) 54 12 5 13} A comprehensive parametric investigation was conducted to explore various material options and key processing parameters influencing the composition and structural properties of the BioFiber [11]. The self-healing process facilitated by BioFiber within quasi-brittle cementitious materials encompasses three sequential stages [13]. Initially, during damage initiation, cracks propagate, allowing ingress of water and oxygen. Subsequently, cracks penetrate the impermeable fiber shell, exposing the bio-agents within the hydrogel to the external environment. Finally, upon exposure to water, oxygen, and nutrients, the germinated bio-agents initiate the production of self-healing end-products, effectively sealing the exposed cracks [14-15]. In the first set of examples, the goals were centered on understanding how BioFibers, when integrated into cement paste, contribute to crack-healing performance. The self-healing process in BioFiber-reinforced cement paste initiates with the ingress of aqueous solution from the environment into the crack volume, followed by the release of endospores from the fractured BioFiber and the diffusion of nutrients, urea and calcium from the matrix into the crack. When MICCP activation occurs, calcium carbonate polymorphs are deposited into the cracked area, effectively filling, and sealing the cracks. The cement paste samples reinforced with BioFibers, were subjected to conditions including a wide range of surface crack widths, to wet/dry cyclic conditions. To test effectiveness, three separate experiments were conducted. The bioFiber-reinforced cement paste samples (BioFRC) were prepared using sequential immersion processes and cracked under controlled flexural conditions to provide a range of widths. They were exposed to wet/dry cycles with bio-agent solutions to activate microbial-induced calcium carbonate precipitation (MICCP) for self-healing and monitored for the development of crack-filling efficiency over time. Experimental analyses include intensity-based image analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Results demonstrated the bioFiber's efficacy in crack filling, with superior performance compared to control samples, especially for moderate crack widths. The prolonged healing duration enhances crack closure, indicating the importance of bacterial activity and environmental conditions. Quantitative data revealed that bioFiber samples achieved peak crack- filling ratios of 99.03±4.17%, 73.93±10.75%, and 77.65±5.43% for crack widths of 120-150 μm, 150-200 μm, and 200-250 μm, respectively, after 28 days of healing cycles. The bioFiber concrete was also suitable for filling larger cracks, achieving crack-filling ratios of 30.94±6.98% and 9.6±1.2% for crack widths of 250- 350 μm and 350-400 μm, respectively. Microstructural analysis reveals the integration of bioFiber into the matrix and the temporal evolution of calcium carbonate polymorphs, highlighting the complex interplay between bacterial activity, environmental factors, and crystal nucleation kinetics. Overall, the study provides quantitative insights into the multifunctional properties of bioFiber and its potential for enhancing the durability and mechanical performance of cement-based composites. Crack creation in the samples was performed in a controlled flexural manner to provide a wide range of crack widths in the bioFiber reinforced zone, as illustrated in Fig.2A. Intensity-based Image Analysis To evaluate the surface crack-healing efficiency, optical microscopic images were obtained from the cracked samples after 7-, 14-, 21-, and 28-day exposures to wet/dry cycles. Intensity analysis was then performed on the acquired images to detect the crack-filling development over time. The crack-filling ratio is defined as: ^_{^,^^%^ = ^^,^ ^ × 100 Eq.1 ^,^^} where R_h,t is the crack-filling ratio at MICCP time t, A_h,,t is the filled crack area (white area), and A_c,t0 is the initial crack area (dark area) in the optical microscopic images. Thermogravimetric Analysis (TGA) To quantify the amount of precipitated calcium carbonate, thermogravimetric analysis (TGA) was performed on the solid residue from sacrificial samples after each healing cycle, i.e., 7, 14, 21, and 28 days. After precipitate collection, the particles were exposed to a temperature of 105 °C for 1 hour to terminate the MICCP process via removal of remining moisture in the samples and further obtain the residue. Then, the solid residue was ground to collect the particles with sizes less than 75 μm. An amount of 20-30 mg of the samples was deposited at a high-temperature platinum pan, and TGA tests were conducted at 30-900 °C with a ramp rate of 10 °C/min. Scanning Electron Microscopy (SEM) Apreo2S-ThermoFisher scanning electron microscopy (SEM) using secondary electrons was performed to determine morphology, surface features and textures on healing end-products, matrix, and bioFiber prior/post healing. SEM was performed via detectors, in this case an Everhart-Thornley Secondary Electron Detector (ETD) at 2 kV high-voltage, 50 pA current, and high vacuum pressure (2-9×10^-3). For crystal structure analysis on precipitates, SEM was performed on the powder samples collected for TGA. For the morphology analysis on the bioFiber/matrix samples, the SEM specimens were collected prior to and post cracking/healing phases. X-ray Diffraction (XRD) X-ray diffraction (XRD) was carried out on the healing precipitates residue to identify the calcium carbonate crystal phases. Benchtop X-ray diffractometer Rigaku-MiniFlex 600 equipped with Cu Kα (λ=1.387 Å) was used at Bragg angle 2θ in the range of 20° to 80° with a step of 0.02°. The XRD data were then analyzed using Rietveld refinement open-source software (Profex). 1. Results and Discussions BioFiber Crack-filling Efficiency Example 1 In this Example, the performance of BioFiber was evaluated for filling cracks with an average width of 120 µm-450 μm. The assessment of crack-healing capacity was conducted using an optical-based approach for quantification. The filling ratio was determined by analyzing images obtained from five replicate samples, results are shown in Figure 4A. An exemplification of the crack-filling process is depicted in Error! Reference source not found.B, which serves as a visual representation of the phenomenon under investigation. This figure indicates the crack closure process over 14- and 28-days exposure to healing cycles. This illustration demonstrates the performance of the BioFiber for self-healing concrete applications. The comprehensive analysis of crack-filling performance in bioFiber concrete unveils notable trends across various crack widths and healing durations, substantiating the efficacy of the self-healing mechanism. The examination commenced with crack widths spanning from 120 to 150 μm, where bioFiber samples exhibited a progressive enhancement in crack filling. At 28 days, the crack-filling ratio peaked at 99.03±4.17%, indicating a substantial reduction in crack width compared to the control specimens. In comparison to control specimens, the application of bioFiber consistently exhibited superior crack-filling capabilities throughout the 28-day healing cycles. As crack widths expanded to 150-200 μm and 200-250 μm, the bioFiber concrete sustained its efficacy, achieving peak crack-filling ratios of 73.93±10.75% and 77.65±5.43%, respectively, at the conclusion of the 28-day healing cycle. These findings underscore the robustness of bioFiber in addressing cracks of moderate width, with standard deviations reflecting consistent performance across the healing period. A slight attenuation in crack-filling efficacy was observed for wider cracks, yet bioFiber samples still exhibited appreciable improvements. Crack widths of 250-350 μm and 350- 400 μm demonstrated peak crack-filling ratios of 30.94±6.98% and 9.6±1.2%, respectively, at 28 days. The presence of bioFiber substantially contributed to crack closure, showcasing its versatility across a spectrum of crack widths. Moreover, the influence of healing time on crack-filling efficacy is evident across all crack widths evaluated. Extended healing durations consistently resulted in enhanced crack closure, as indicated by the progressive increase in crack-filling ratios over the 28-day healing period. This underscores the importance of prolonged healing durations in maximizing the self-healing potential of bioFiber concrete. Additionally, the necessity for prolonged healing durations can be attributed to the biological processes inherent in the bioFiber treatment. The microorganism utilized in bioFiber, being an endospore, requires an initial period of 24-36 hours to undergo the germination phase, followed by the outgrowth phase, where it increases the number of cells and initiates the production of urease, an enzyme crucial for urea hydrolysis. Subsequently, the onset of urea-driven MICCP occurs, contributing to the self-healing process. This intricate biological pathway underscores the importance of allowing sufficient time for the microorganism to activate and initiate the self-healing mechanism, ultimately leading to enhanced crack-filling efficacy over extended healing durations. Example 2 A second set of examples were carried out in a similar manner as in Example 1. Controlled induction of cracks was initiated within the samples and was meticulously carried out, targeting a maximum crack width of less than 150 µm. Subsequent to the crack induction phase, the samples underwent a series of wet/dry cycles over a period of 28 days. This involved immersing the samples for one hour in a solution containing urea, yeast extract, and calcium acetate, each at a concentration of 20 g/L, followed by 23 hours of exposure to dry conditions at room temperature. The crack healing/filling process persisted for a duration of 28 days. To quantify the effectiveness of crack filling, intensity analysis was performed on images captured at 0, 14, and 28 days. The crack-filling ratio was determined by comparing the filled crack area to the initial crack area. Furthermore, precipitate was collected for characterization through thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). TGA tests were performed under nitrogen gas (flow rate of 25 mL/min) over a temperature range of 23-900 ⁰C at a ramp rate of 10 ⁰C/min. SEM imaging was performed using an Everhart–Thornley detector (ETD) at 10 mm working distance and a voltage of 2 kV. The performance of the BioFiber was evaluated for filling cracks with an average width of 129 µm. The filling ratio was determined by analyzing images obtained from five replicate samples. The results are shown in Figure 17A. Following 14 days of exposure to healing cycles, the crack-filling ratio demonstrated a noteworthy increase to 58.71 ± 8.1%, indicating substantial progress in crack closure. Impressively, this ratio further improved to 93.7 ± 3.3% after 28 days, highlighting the sustained effectiveness of BioFiber for filling cracks over time. These findings underscore the significant contribution of BioFiber in enhancing the healing process of cement paste, thus presenting a promising solution for addressing cracks and enhancing the overall durability and resilience of concrete structures. Material characterization tests revealed the formation of calcium carbonate crystals, including combinations of calcite and vaterite, in the healed samples. An exemplification of the crack-filling process is depicted in Error! Reference source not found., which serves as a visual representation of the phenomenon. This figure shows the crack closure process after 14- and 28-days exposure to healing cycles. bioFiber Bridging Effect The fiber bridging effect in fiber reinforced concrete is a mechanism that enhances the structural integrity of cementitious materials. By spanning across cracked regions, fibers effectively hold separated parts of the cement paste together, mitigating crack propagation and improving mechanical resilience. This phenomenon underscores the importance of fibers in reinforcing concrete structures and minimizing the detrimental effects of cracking. The fiber bridging effect in fiber reinforced concrete not only enhances structural integrity but also limits crack width by controlling crack growth. By effectively spanning across cracked regions, fibers restrain crack propagation, thereby facilitating the concrete's self-healing ability and ensuring long-term durability. The fiber bridging phenomenon seen in Error! Reference source not found., showcases the ability of fibers to maintain structural integrity by spanning across cracked regions, effectively holding separated parts of the cement paste sample together. This visual evidence demonstrates the crucial role of fibers in mitigating crack propagation and enhancing the mechanical resilience of cementitious materials. The incorporation of bioFiber is visually depicted in Error! Reference source not found.. The sample has been deliberately cut open to reveal distinct features within the matrix. Calcium carbonate precipitation, evidenced by its white residue, is prominently observed. This precipitation, a result of the bio-self-healing via activated bioFiber, underscores the bioFiber’s ability to facilitate self-healing mechanisms through deeper cracked areas. Additionally, the core-fiber, composed of polyvinyl alcohol (PVA), provides structural reinforcement within the fractured regions. Surrounding the core-fiber, the remaining shell layer is observed, adhering to the cement paste matrix in white. This configuration illustrates the integration of bioFiber into the cementitious material, showcasing its ability to enhance the mechanical properties concrete structures through crack mitigation and self-healing capabilities. Precipitate Composition Analysis To elucidate the kinetics of bioFiber healing, it is useful to identify the primary compound of interest: calcium acetate. In this study, the crack volume contained germinated cells, urea, yeast extract, and calcium acetate during the MICCP process. Bio-agents were introduced to cracks by immersing samples in a solution comprising urea for hydrolysis, yeast extract for cell germination and outgrowth (of which the cells were provided through bioFiber hydrogel), and calcium acetate as an external calcium source, culminating in calcium carbonate formation. In general, the precipitates comprised organic remnants from dead cells and calcium carbonate. However, the MICCP residues, particularly in early stages, also contained unhydrolyzed urea, excess yeast, and calcium acetate. TGA was employed to characterize these components before quantifying the calcium carbonate content, which is typically evident in the TGA results within the temperature range of 600-800 °C. Error! Reference source not found. illustrates weight loss and its derivative (DTG) curves, with quantified data presented in Table 2. For Lysinibacillus sphaericus (LS) germinated cells, the TGA results indicated a major weight loss in the temperature range of 150-600 °C. Within this temperature range, organic compounds such as proteins, lipids, carbohydrates, and other biomolecules that constitute the bacterial biomass started to undergo thermal degradation. These organic materials break down into volatile gases and various organic fragments. Proteins, which are major constituents of bacterial cells, undergo denaturation and decomposition at elevated temperatures. This results in the release of amino acids and other volatile compounds, contributing to the observed weight loss. Carbohydrates, such as polysaccharides and glycogen, found in bacterial cell walls and energy storage granules, may undergo pyrolysis at high temperatures, releasing volatile gases. As a result, the decomposition of these organic constituents led to a decrease in the overall weight, i.e., 66.88 % for germinated LS cells. Table 2. Quantitative thermogravimetric analysis results for bio-agents L.S Urea Yeast Extract Calcium Acetate Peak # WL^* (%) T^** (°C) WL (%) T (°C) WL (%) T (°C) WL (%) T (°C) 1 66.88 150-600 68.45 125-280 10.66 30-150 5.33 150-240 2 2.98 770-900 29.48 280-400 63.97 145-600 34.39 400-520 3 5.79 400-480 1.82 620-760 27.08 660-790 4 4.94 760-900 * WL: Weight Loss ** T: Temperature During TGA conducted under a nitrogen purge gas, the thermal decomposition of urea exhibited a series of intricate steps in the temperature range of 150-500 °C. Initially, at temperatures below 100 °C, dehydration transpires, leading to the expulsion of water molecules from the urea specimen, which was almost zero in the samples confirming no moisture contamination. Subsequent temperature elevation triggered urea breakdown, yielding ammonia (NH3) and isocyanic acid (HNCO) as primary decomposition products, typically commencing around 150-180 °C. Unlike in oxygen-rich environments, where combustion reactions prevail, the absence of oxygen prevents the direct formation of carbon dioxide (CO2) and carbon monoxide (CO) from urea's carbon component. Instead, any residual carbonaceous matter may undergo further decomposition, potentially resulting in the generation of gases such as methane (CH4), hydrogen (H2), and nitrogen oxides (NO_x). The TGA results for yeast extract show that it typically experiences thermal decomposition. The specific decomposition process and temperature ranges can vary depending on the composition and purity of the yeast extract. However, in general, the breakdown of yeast extract involves the decomposition of organic components such as proteins, carbohydrates, and lipids. Since the yeast extract used herein is considered an undefined nutrient medium, identifying the exact decomposition products without additional experiments such as XRF (X-ray fluorescence spectroscopy) can be challenging. Nevertheless, TGA data indicated that the decomposition temperature for yeast extract fell within the range of 145-600 °C with a weight loss of 63.97 %, similar to that observed for bacteria. Importantly, this temperature range does not overlap with the weight loss associated with the decomposition of calcium carbonate. In addition, the yeast extract weight loss in the temperature range of less than 100 °C can be attributed to the 10.66% moisture content presented in the yeast extract powder. The thermal decomposition behavior of calcium acetate monohydrate (Ca(CH₃COO)₂.H₂O) was investigated via TGA. The initial weight loss, totaling 5.33%, occurred within the temperature range of 120- 240 °C, corresponding to the removal of water (H₂O) in two stages. This process involves the sequential elimination of half a mole of water in each step from the monohydrate form, resulting in the formation of calcium carbonate (CaCO₃). The calculated molar loss of water (xH₂O) was found to be 2.3%. Subsequently, a weight loss of 34.39% was observed in the temperature range of 400-550 °C, attributed to the breakdown of calcium acetate into acetone ((CH₃)₂CO), along with the residual presence of calcium carbonate. In the final decomposition stage, occurring within the temperature range of 660-790 °C, calcium carbonate decomposed into calcium oxide (CaO), resulting in a weight loss of 27.08%. The presence of residual acetate in the crack volume may lead to an increased calcium carbonate content precipitated via MICCP. Calcium Carbonate Precipitation Kinetics The material characterization tests conducted on the precipitates collected from the samples after 28 days of healing provide valuable insights into the composition and properties of the healed material. Error! Reference source not found.A–8D and 18 illustrate the TGA results, depicting weight loss (TGA) and derivative curve (DTG) corresponding to temperature changes. The TGA results reveal distinct weight loss events occurring in different temperature ranges. Initially, a weight loss is observed at temperatures below 100-105°C, attributed to the evaporation of residual moisture present in the samples. Subsequently, multiple peaks are observed in the temperature range of 150-500°C, indicative of complex decomposition processes. These peaks are believed to originate from two primary components: firstly, the residue of the cement paste samples, which may remain following the collection of precipitates from cracked samples, and secondly, the organic content representing biomass in the samples. The residue of hydrated cement in the collected particles resulted in the small trace of calcium-silicates-hydrates and portlandite in the TGA results. Typically, organic materials exhibit weight loss within the temperature range of 200-500°C. Furthermore, a notable weight loss is observed in the temperature range of 600-800°C, corresponding to the decomposition of calcium carbonate present in the healed material. These findings provide an indication of the presence of calcium carbonate as a bioFiber self-healing end-product in cementitious systems. In terms of calcium carbonate content quantification, beginning with an initial content of 15.22% at 7 days, the calcium carbonate content exhibits a notable rise to 54.64% at 14 days. However, a slight decrease is observed at 21 days, with the content recorded at 45.83%, followed by a resurgence to 54.88% at 28 days. This trend suggests a dynamic process of calcium carbonate precipitation influenced by the activity of bacteria over time. The initial increase in calcium carbonate content indicates an active phase of precipitation, possibly facilitated by bacterial metabolic processes. Subsequently, a slight reduction at 21 days may be attributed to factors such as variations in environmental conditions within the cement paste matrix or possible residue of acetate in the system. However, the subsequent rise in content at 28 days suggests a continuation or resurgence of precipitation processes, possibly indicating prolonged bacterial activity or favorable environmental conditions conducive to calcium carbonate precipitation. These findings underscore the complex interplay between bacterial activity, environmental factors, and calcium carbonate precipitation kinetics in MICCP within cementitious materials. Microstructural Analysis BioFiber Interface with Matrix post Cracking prior to Healing SEM images in Error! Reference source not found. depicting the presence of bioFiber components in cement paste post-cracking provide valuable insights into the interaction between bioFiber and the cementitious matrix. In the observed images, two distinct phenomena are evident: (a) the bioFiber shell remains adhered to the core-fiber, and (b) bioFiber is shown adhering to the surrounding matrix. The adhesion between the bioFiber shell and core-fiber indicates robust fiber-fiber bonding, suggesting that the bioFiber composite maintains its structural integrity even after experiencing cracking within the cement paste. Additionally, the adherence of bioFiber to the surrounding matrix underscores its compatibility and integration within the cementitious material, for enhancing the overall mechanical properties and crack resistance of the composite. These observations highlight the role of bioFiber as a reinforcing agent in cement- based materials, offering both internal reinforcement through fiber-fiber bonding and improved interfacial bonding with the matrix, thereby contributing to the overall durability and performance of the composite. Error! Reference source not found. illustrates bioFiber prior to self-healing activation, revealing the presence of cement hydration products adhered to the bioFiber shell. This SEM image provides valuable visual evidence of the interaction between bioFiber and the cementitious matrix during the early stages of cement hydration. The cement hydration products appeared to coat the surface of the bioFiber shell, indicating the incorporation of bioFiber into the evolving cementitious microstructure. This adherence involves interactions between bioFiber and cement hydration products, which may illustrate the overall performance of the bioFiber-reinforced cement paste. Error! Reference source not found. highlights two key observations, the adherence of the bioFiber shell to the surrounding matrix, and the morphology of interior surface of the shell in contact with inner bioFiber layers after separation. SEM image (a) reveals the bioFiber shell firmly adhering to the matrix, indicating effective integration of bioFiber into the cementitious material. This interaction provides reinforcement of the matrix by bioFiber, enhancing the fiber bridging effect. In SEM image (b), the separated bioFiber shell exposes the interior surface, revealing contact with inner bioFiber layers. This shows strong bonding between bioFiber layers within the shell structure, indicative of robust layer-layer interactions. BioFiber Precipitation The microstructural analysis of MICCP precipitates offers insights into the temporal evolution of calcium carbonate polymorphs over a 28-day duration. In the initial stages, specifically at 7 days, vaterite emerges as the predominant polymorph, with concurrent partial calcite formations. The prevalence of vaterite in the early stages may be attributed to its metastable nature, facilitating its rapid nucleation and growth kinetics under the prevailing environmental conditions. Furthermore, vaterite's relatively higher solubility compared to calcite may favor its precipitation in the initial stages of MICCP. As the MICCP progresses, a notable transition from vaterite to calcite polymorphs becomes evident, indicative of recrystallization processes occurring between the two phases. At 14 days, hexagonal vaterite plates and spherical vaterite crystals dominate the SEM images, alongside the presence of dead cells encapsulated within calcium carbonate matrices. This stage signifies a transitional phase where vaterite undergoes recrystallization processes, transforming into calcite polymorphs. The formation of calcite crystals with layered structures and larger dimensions observed at 21 days corroborates this transition, marking the predominance of calcite polymorphs in the later stages of MICCP. By day 28, calcite continues to prevail, with further increases in crystal size, accompanied by persistent observations of dead cells within the calcium carbonate matrices. The observed agglomeration of particles throughout all observed time points suggests inter-particle/layer bonding between calcium carbonate polymorphs, indicative of ongoing processes of crystalline growth and interfacial interactions within the MICCP system. The SEM image shown in Error! Reference source not found. reveals the presence of precipitates, predominantly composed of calcite crystals. These calcite crystals exhibit distinctive morphologies and distribution patterns. The SEM image shows calcite crystals with sizes of 5-20 µm surrounded by spherical- shaped particles which are believed to be the combination of amorphous calcium carbonate and vaterite. The SEM images and the TGA results demonstrated the presence of calcium carbonate precipitated in the crack volume in the cement paste samples. Crystal Phase Identification The XRD analysis conducted on calcium carbonate crystals precipitated via MICCP within cementitious materials provided insights into the phase evolution of these crystals over time. The results, as shown in Error! Reference source not found.A – 16D, reveal a dynamic transformation of calcium carbonate polymorphs as the samples age. Initially, at 7 days, vaterite emerges as the dominant polymorph, indicating the rapid nucleation and formation of metastable calcium carbonate phases facilitated by bacterial activity. Subsequently, at 14 days, a shift towards recrystallization is observed, transforming vaterite into more stable crystalline forms. At 21 days and 28 days, calcite emerges as the predominant phase, indicating the progressive maturation and transformation of calcium carbonate crystals towards more thermodynamically stable structures. These findings are further corroborated by scanning electron microscopy (SEM) analysis, which provided visual confirmation of the presence and morphology of calcium carbonate crystals within the cementitious matrix. Examples 1 and 2 demonstrated the self-healing mechanism and crack-filling performance of multifunctional bacteria-laden fibers (bioFiber) within a cementitious matrix. Through experimental analyses including intensity-based image analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD), the following conclusions can be drawn: ^ Crack-Filling Performance: BioFiber demonstrated superior crack-filling capabilities compared to control samples, especially for moderate crack widths (120-250 μm). The crack-filling ratio peaked at 99.03±4.17% for crack widths of 120-150 μm after 28 days of healing cycles. The bioFiber still contributed to filling large crack widths, achieving peak crack-filling ratios of 30.94±6.98% for crack widths of 250-350 μm at 28 days. ^ Healing Duration: Extended healing durations consistently resulted in enhanced crack closure, indicating the importance of prolonged healing durations in maximizing the self-healing potential of bioFiber concrete. The biological processes of the bioFiber treatment require sufficient time for activation, germination, and production of urease for MICCP, ultimately leading to enhanced crack-filling efficacy over extended healing durations. ^ Fiber Bridging Effect: BioFiber exhibited a fiber bridging effect, spanning across cracked regions and effectively holding separated parts of the cement paste together. This phenomenon not only enhanced structural integrity but also limited crack width, controlling crack growth and facilitating the concrete's self- healing ability. ^ Precipitation Composition Analysis: Thermogravimetric analysis provided insights into the decomposition processes of bio-agents, with calcium acetate playing a role in calcium carbonate precipitation. The analysis also revealed the temporal evolution of calcium carbonate polymorphs, with vaterite initially emerging as the dominant phase subsequently transitioning to calcite over time. ^ Microstructural Analysis: SEM and XRD analyses showed the integration of bioFiber into the cementitious matrix, showcasing its ability to enhance mechanical properties and crack resistance. Additionally, microstructural analysis provided insights into the dynamic processes governing the temporal evolution of calcium carbonate polymorphs and the phase transformation towards more stable structures. Example 3 Crack-Bridging Performance In Example 3, the crack-bridging performance of the BioFiber and its impact on the mechanical properties of the bioFiber-reinforced composites was tested. Single-fiber pullout tests were conducted to quantify the bond strength between the bioFiber and the cementitious matrix. Also, split tensile tests were conducted on fiber-reinforced mortar samples with varying compositions. Additionally, X-ray Computed Tomography (XCT) was employed to quantify the crack-filling ratio of bioFiber in mortar samples, with a particular focus on deep and large cracks. Cement paste and fiber-reinforced mortar samples were prepared for this study. The cement paste samples were fabricated for dogbone-shaped pullout tests, utilizing cement with the composition outlined in Table 2 and a water-to-cement ratio of 0.42. For the dogbone samples, a single fiber (PVA was used as the control and bioFiber) was placed in the center of the samples. Both types of fibers had a length of 30 mm. The fibers were held in place in the middle of the sample by a thin plastic sheet, which also served as a separator, creating two parts within the sample after the casting/demolding stage (shown in Error! Reference source not found.). The top part of the sample was designated as a section to which a direct tensile load was applied. The samples were cast in silicon molds that were created in-house based on the dimension shown in Figure 55. For the reinforced mortar samples, cylindrical molds measuring 50 mm in diameter and 100 mm in length were used. The mortar had a water-to-cement ratio of 0.42 and an aggregate-to-cement ratio of 1.33, each by weight. The fine aggregate had a density of 2.63 g/cm³ and an absorption capacity of 0.87%, while the cement had a density of 3.05 g/cm³ and a Blaine fineness of 409.2 m²/kg. Fibers were incorporated at volumetric percentages of 0.25, 0.5, 0.75, and 1%, based on the total volume of the samples, for both PVA and bioFiber. The fibers had a length of 30 mm, with the PVA fiber having an average diameter of 0.62 mm and the bioFiber having an average diameter of 1.07 mm. The density of the PVA fiber was 2.12 g/cm³, while that of the bioFiber was 1.3 g/cm³. The PVA fiber, which was also employed as the core fiber of the bioFiber, had an elastic modulus of 23 GPa and a tensile strength of 800 MPa. All samples, both cement paste and mortar, were demolded after 24 hours and then cured in a moist environment at 23°C for 60 days. The curing was achieved by placing the samples in double-sealed bags to maintain the required moisture conditions. Cracks in the cylindrical samples were induced by applying approximately 80-90% of the average maximum load recorded in the split tensile test. Once cracks were created, the samples were sealed at the sides and bottom, leaving the top open for exposure to the bio-agent solution (urea, yeast extract, and calcium acetate as described in (Khaneghahi et al., 2023; Khaneghahi et al., 2024; Rahmaninezhad, Houshmand, Sadighi, Ahmari, et al., 2024; Rahmaninezhad, Houshmand, Sadighi, Kamireddi, et al., 2024)), allowing one-directional diffusion. The healing process commenced by adding the bio-agent solution until it overflowed, thereby filling the entire crack volume. This procedure was repeated daily for 28 days, with the bio-agent solution being added once each day. Table 2. Chemical composition of the cement Chemical Composition Item SiO2 Al2O3 Fe2O3 CaO SO3 MgO Na2O K2O Mn2O3 CaCO3 TiO2 LOI Amount (% wt) 18.4 4.36 2.96 61.2 2.73 2.76 0.11 0.57 0.11 3.48 0.27 2.09 Fiber Pull-out Test The interfacial bond between fibers and the cementitious matrix was assessed through single-fiber pullout tests, adopted based on ASTM D7913/7913M-14, conducted on dogbone-shaped specimens under quasi-static conditions (Farooq & Banthia, 2019, 2021; Farooq, Bhutta, Borges, Zanotti, & Banthia, 2018; Graybeal, 2006; Yoo, Shin, & Banthia, 2021). The displacement-controlled loading rate was set at 0.08 mm/s, and a 5 kN load cell was utilized for collecting force-displacement data (Farooq & Banthia, 2019). The positioning of the sample in the Universal Testing Machine (UTM) is illustrated in Figure 56. The dimensions of each fiber were measured prior to casting, and the bond stress was computed based on these measured dimensions. A uniform bond model was employed for bond strength and energy analysis (Farooq & Banthia, 2019). According to this model, the interfacial bond stress at a specific fiber slip was calculated by distributing the load measured at that slip across the area of the fiber embedded in the matrix, which was determined by multiplying the fiber’s perimeter by the embedded length (Farooq & Banthia, 2019, 2021; Qi, Wu, Ma, & Wang, 2018). The parameters obtained from the pullout tests included the peak interfacial bond load and slippage, which were used to calculate the maximum bond strength (τmax), pullout energy (Ep), and equivalent bond strength (τ_eq). The equations for these calculations are provided below. ^ _{^^^^ = ^^^ ^ ∙ ^ Eq.2 ^^} where P_max is the fiber (15 mm for all samples), Ps is the load at each slippage s, and Lbf is the total length of the fiber (30 mm for all samples). Split Tensile Test The Split Tensile Test (ASTM C496/C496M) measured the tensile strength of fiber-reinforced mortar by applying a diametral compressive load to a cylindrical specimen (C496/C496M-11, 2011). In this test, a cylindrical specimen, i.e., 50 mm in diameter and 100 mm in length, was placed horizontally between the loading platens of a testing machine. Wooden plates were placed on the top and bottom surfaces of the sample to ensure uniform load distribution and to prevent localized crushing of the specimen. A compressive load was applied along the length of the cylinder until failure occurred, resulting in the specimen splitting along its diameter due to tension (as shown in Figure 57). The tensile strength was calculated using the equation: 2_{^ *^ =} where σt represents the tensile strength, P is the applied load at failure, D is the diameter of the specimen, and L is the length of the specimen. Scanning Electron Microscopy (SEM) SEM with backscatter and secondary electrons was conducted using an Apreo2S-ThermoFisher system to analyze the morphology, surface features, and texture of the fiber/matrix specimen. The SEM analysis was carried out with an Everhart-Thornley Secondary Electron (ETD) detector, at a high-voltage of 2 kV, a current of 50 pA, and under high vacuum conditions (2-9×10⁻³). Morphological analysis of the bioFiber/matrix samples was performed on specimens collected both before and after the mechanical testing. 3D X-ray Microscope (XRM) The Xradia 620 Versa 3D X-ray microscope (XRM) was utilized for non-destructive 3D X-ray imaging (CT scan) to achieve high aspect ratio tomography with submicron resolution. This advanced imaging technique enabled detailed visualization and analysis of internal structures without physically altering the samples. For the CT scan, a voltage of 140 kV, a power of 10 W, and a current of 71.5 µA were used, with a High Z filter employed to enhance contrast and reduce artifacts in the dense materials such as the concrete specimens. The CT scans were performed on cylindrical samples cracked under ~80-90% of the maximum split tensile test to assess the depth of cracks and measure crack-filling capability, with evaluations conducted over 7, 14, 21, and 28 days. Furthermore, the X-CT was used on dogbone pullout specimens after testing to analyze and determine the failure modes, specifically identifying whether failure occurred due to debonding between the bioFiber shell and matrix, the shell and hydrogel sheath, or between the hydrogel layer and core-fiber. Quantitative analysis of the XRM reconstructed images was performed using scientific image processing software (named Dragonfly), to obtain detailed insights into material structures for the specimens after mechanical testing and the healing process. BioFiber and Matrix Interfacial Bonding Properties Error! Reference source not found.58 illustrates that during the final stage of a single pull-out test, three distinct regimes manifest. These regimes include constant friction, slip-softening, and slip-hardening. Constant friction occurs when the resistance force remains steady as the fiber is pulled out, indicating a uniform interfacial bond between the fiber and matrix. Slip-softening is characterized by a gradual decrease in resistance, suggesting a weakening of the bond, possibly due to micro-cracking or interface degradation. Conversely, slip-hardening is marked by an increase in resistance, which may indicate the engagement of an additional mechanical interlock or the contribution of frictional forces that enhance the bond as the fiber is drawn out. The force-slip results for PVA fibers (8 replicates) and bioFiber samples (9 replicates) are presented in Error! Reference source not found.and Error! Reference source not found., respectively. Although 20 replicates were initially prepared for each fiber type to ensure repeatability in the pullout tests, approximately half of the samples were excluded from the analysis. This exclusion was primarily due to issues such as fiber misalignment, where the fibers were not oriented parallel to the loading direction as intended, as well as a few samples that failed prematurely. For the PVA fibers (Error! Reference source not found.), which were used as control and remained untreated, the predominant behavior observed was slip-softening, with a few instances of constant friction. The slip-softening is likely due to progressive debonding at the fiber-matrix interface, potentially exacerbated by micro-cracking within the cement matrix. The smooth surface of the PVA fibers likely contributes to this behavior, allowing the fibers to slide out with reduced resistance following the initial peak load. Following the slip-softening phase, the tests occasionally reached a regime of constant friction, where the force required to continue fiber pullout stabilized. This steady force reflects a balance between the frictional forces at the interface and the resistance provided by the matrix. Due to the high tensile strength of the PVA fibers, no fiber failures in tension were observed during the tests. For the bioFiber samples (Error! Reference source not found.), slip-hardening was the predominant behavior observed (in 67% of the samples), while slip-softening was noted in the remaining 33%. This behavior was likely due to the complex interfacial interactions within the bioFiber structure, including the interfaces between the inner hydrogel layer and the outer shell, the shell and the matrix, and the hydrogel and the core fiber. Typically, load transfer was expected to occur from the matrix to the outer shell, and under ideal bonding conditions, from the outer shell to the hydrogel, and finally from the hydrogel to the core fiber. The slip-hardening behavior could be explained by the rougher surface texture of the bioFiber outer shell compared to the smoother surface of the PVA monofilament. The rougher surface of the bioFiber facilitated mechanical interlocking and a jamming effect between the matrix and the shell during slippage, leading to a slight-to-moderate increase in force as slippage progressed. The jamming effect could lead to resistance of further fiber movement, thereby contributing to the slip-hardening response. Additionally, the bioFiber manufacturing process, which involved immersion deposition, resulted in shell accumulation at the fiber ends, acting similarly to an end hook and further reinforcing the slip-hardening behavior. Moreover, the diameter variation in bioFiber was greater than that of PVA fibers, making interlocking, jamming, and slip- hardening more likely to occur in the bioFiber samples. The greater variability in bioFiber manufacturing also contributed to greater variation in the pullout test results compared to the more consistent PVA fibers. The peak bond strength, bond energy, and equivalent bond strength for the pullout tests were calculated using Equations 1 through 3, as detailed in Table . It was observed that PVA fibers demonstrated a higher peak bond strength, averaging 2.49 MPa, compared to bioFiber, which exhibited an average strength of 1.15 MPa—46% lower than PVA. To account for post-peak behavior, bond energy was evaluated, reflected by the total energy required to pull the fiber out of the matrix, including both the initial debonding process and frictional sliding. A higher bond energy was indicative of a more effective bond, which was critical for enhancing the mechanical performance of fiber-reinforced composites. Bond energy was directly related to the toughness of the composite material, providing insight into its ability to absorb and dissipate energy, particularly under conditions that induced cracking or deformation. The bond energy for PVA fibers was calculated to be 600.20 N·mm, while for bioFiber it was 415.03 N·mm, representing a 31% decrease. This highlighted the significance of considering both energy and force-slip behavior in the analysis. The bond energy of bioFibers was 31% lower than that of PVA, whereas the bond strength was 54% lower. In terms of equivalent bond strength, which accounted for bond energy, PVA fibers had a value of 72.75 kPa, while bioFiber had a value of 60.31 kPa, indicating that bioFibers achieved 83% of the performance of PVA fibers. Despite these differences, the behavior of bioFibers was found to be comparable to that of commercial fibers such as PVA, demonstrating their potential as viable alternatives in fiber-reinforced composites. Table 3. The calculated bond strength, energy, and equivalent bond strength based on force-slip curves. Max Force Peak Bond Strength Pullout Energy Equivalent Bond Strength (N) (MPa) (N.mm) (kPa) Sample CTRL^* bF^* CTRL bF CTRL bF CTRL bF 1 84.22 51.09 2.93 1.02 220.70 364.28 23.29 63.38 2 57.41 36.09 2.00 0.72 458.90 94.74 71.06 23.33 3 91.39 49.00 3.18 0.97 975.06 434.24 94.83 78.78 4 78.75 111.92 2.74 2.22 806.66 1015.37 91.05 80.64 5 37.43 50.73 1.30 1.01 188.91 652.57 44.86 114.34 6 65.54 46.77 2.28 0.93 480.20 199.09 65.12 37.84 7 86.30 92.20 3.00 1.83 751.48 574.10 77.40 55.35 8 71.46 51.06 2.49 1.01 919.69 166.65 114.39 29.01 9 - 34.63 - 0.69 - 234.21 - 60.12 x^*̄ 71.56 58.17 2.49 1.15 600.20 415.03 72.75 60.31 s_x ^* 17.82 26.12 0.62 0.52 306.05 293.40 28.94 28.64 ^* x:̄ Average value; sx: Standard Deviation; CTRL: PVA Fiber; bF: bioFiber To investigate the failure modes in the bioFiber pullout test, electron microscopy was employed. SEM imaging was performed under two scenarios: one with minimal to no debonding between the matrix and shell (denoted as FM-S1), and another with moderate debonding between the shell and matrix (denoted as FM-S2). In the FM-S1 scenario, depicted in Error! Reference source not found.61, complete debonding between the shell and hydrogel was observed. Multiple cracks were noted in the hydrogel, indicating that the hydrogel layer had been partially destroyed between the shell and the core fiber, which left the core fiber exposed. The shell remained adhered to the cement paste, with almost no microcrack formation observed in the paste, consistent with the slip-softening behavior noted for the samples. In contrast, FM-S2, shown in Error! Reference source not found.62, revealed debonding between the shell and hydrogel, along with partial debonding between the shell and matrix. This scenario exhibited microcracks in both the cement paste and the shell, as well as near-complete destruction of the hydrogel layer. The partial debonding behavior between the shell and matrix was reflected in the slip-hardening effect observed in the force-slip curve, which was attributed to a jamming or interlocking effect. The viscoelastic nature of the solid hydrogel may have contributed to the observed debonding. The hydrogel's viscoelastic properties could cause it to deform and flow under stress, leading to debonding when subjected to prolonged loading or high strains. This viscoelastic behavior likely contributed to the observed partial destruction and debonding between the hydrogel and surrounding components. Surface roughness variations, as illustrated in Error! Reference source not found.63, showed that the core fiber and hydrogel had smooth surfaces, while the shell and matrix exhibited more surface roughness (Error! Reference source not found.64). The increased surface roughness of the shell and matrix likely contributed to the observed interlocking effect, which led to slip-hardening behavior. The rougher surfaces facilitated mechanical interlocking between the shell and matrix, causing increased resistance during slippage. This behavior, combined with the hydrogel's viscoelastic properties, resulted in complex failure mechanisms involving debonding between the shell and hydrogel, as well as between the hydrogel and core fiber. Similar behavior was noted in control samples, where PVA fibers exhibited analogous interactions with the matrix. bioFiber Reinforced Mortar Mechanical Properties To assess the mechanical properties of reinforced composites with varying fiber dosages, split tensile tests were conducted on plain mortar samples (as control) and on samples reinforced with PVA and bioFiber at volumetric dosages of 0.25, 0.5, 0.75, and 1%. The results, presented in Error! Reference source not found., show that the control samples had a split tensile strength of 3.87±0.1 MPa. For the reinforced mortar samples, the addition of fibers (either PVA or bioFiber) generally led to an increase in split tensile strength. When comparing PVA and bioFiber, PVA reinforcement provided higher tensile strength than bioFiber. This difference can be attributed to two main factors: first, PVA exhibited a slightly higher equivalent bond strength compared to bioFiber, and second, the higher density of bioFiber. The density of bioFiber is 2.12 g/cm³, whereas PVA has a density of 1.30 g/cm³. Consequently, for the same volumetric dosage, a greater number of PVA fibers are incorporated into the matrix compared to bioFiber. This greater number of fibers in the PVA samples likely results in a more uniform distribution within the matrix, thereby enhancing the mechanical properties of PVA-reinforced mortar. In terms of dosage effects, an increasing trend in split tensile strength was observed from 0.25% to 0.75% fiber dosages for PVA, with a slight decrease at 1%. This decrease at higher dosages may be attributed to fiber clamping and agglomeration. Conversely, bioFiber samples showed a continuous increase in tensile strength with increasing dosage, although fiber clumping might occur at dosages exceeding 1%. This trend may also be related to the generally lower number of bioFibers compared to PVA fibers. In all tested samples, three replicates were evaluated, and the coefficient of variation (CoV) for the results was consistently below 6%. The tensile strength capacity of cylinder mortar was enhanced by the addition of both bioFibers and PVA fibers. To illustrate this enhancement, the normalized tensile strength was plotted against varying fiber dosages, as shown in Error! Reference source not found.. To determine the optimum fiber dosage for maximum tensile strength enhancement, a polynomial regression analysis was performed on the data. The regression results indicated that the R² values, which measure the proportion of variance explained by the model, for both bioFiber and PVA fibers were less than 0.99, demonstrating a high degree of accuracy in the regression model. Optimization calculations revealed that a 0.85% dosage of PVA fibers resulted in a 19.82% increase in tensile strength, while a 1.19% dosage of bioFiber led to a 16.81% increase. To account for fiber dispersion, it was considered that a greater number of macro fibers could potentially achieve better spatial distribution within a unit volume of the matrix. Given the optimal dosages, a greater mass of PVA fibers was required for the 0.85% dosage compared to the 1.19% bioFiber dosage. Consequently, despite the optimal dosage, the higher quantity of PVA fibers added to the matrix may contribute to improved spatial dispersion compared to bioFiber. bioFiber Deep Crack-Filling Performance To evaluate the self-healing efficiency of bioFiber-reinforced mortar, the crack-filling ratio was measured using XCT scans. After subjecting the mortar cylinder samples to controlled cracking, XCT scans were conducted at intervals of 7, 14, 21, and 28 days. These scans allowed for the precise quantification of both solid and crack volumes within the samples over time. By comparing the initial crack volume at day 0 with subsequent measurements, the reduction in crack volume was determined, indicating the extent of crack filling due to the self-healing process. The crack-filling ratio, expressed as a percentage, was then calculated to quantify the volumetric recovery of the material, providing insights into the time-dependent self-healing behavior of the bioFiber-reinforced mortar. The 3D images of the mortar cylinder samples at 28 days are presented in Error! Reference source not found., illustrating the progress of crack healing over time. Additionally, Figure 68 depicts a representative sectional view of the middle portion of the sample, highlighting the distribution of bioFibers, the extent of cracking, and the observed precipitation within the cracks. The crack-filling ratio, i.e., the initial volume of cracks getting filled, is shown over the span of four weeks of healing in Figure 69. The results indicated that up to 21.97% of crack volume, with 10.25% initial crack volume, was filled with bioFiber precipitation after 28 days of healing. To evaluate the crack-filling performance of bioFiber based on crack size and depth, three types of cracks were examined (Figure 70): a surface crack with a depth of less than 10 mm and an opening of less than 1 mm, a deep crack with a depth of 10-40 mm and a small opening of less than 1 mm, and a deep wide crack with a depth exceeding 40 mm and an opening greater than 10 mm. Several factors influenced the effectiveness of bioFiber crack-filling, including the number of bioFibers activated adjacent to the crack. A higher number of activated bioFibers led to a greater initial concentration of released bacteria, which, in turn, increased calcium carbonate precipitation within the crack volume. The availability of water, essential as a medium for the biochemical healing reactions, and air, necessary for bacterial growth, also played roles in the healing process. Limited access to water or air could hinder these reactions, thereby reducing the amount of precipitate formed. This limitation often arose due to the manner in which precipitates, primarily calcium carbonate precipitates, were deposited within the crack. When the precipitate bridged the sides of the crack, it could seal the top crack volume, restricting access to water and air in the deeper parts of the crack. Precipitation typically began at the top surface of the crack due to greater air access. In surface cracks with small openings, this led to effective crack-filling without significant concerns about sealing effects, as these shallow cracks maintained access to both air and water. In contrast, deep cracks with large openings allowed healing material deposition to start at the crack edges. Due to the width of these cracks, the precipitation could not easily bridge across, ensuring that the entire crack volume remained accessible to water and air, though the crack-filling ratio was limited by the amount of bio-agent available and the number of fractured bioFibers within the crack volume. For deep cracks with small openings, precipitation often bridged the crack edges, limiting access to the lower areas, which restricted healing to the upper portions of the cracks. XCT scans confirmed that in such cases, more healing occurred at the top of the sample, with minimal healing beneath the sealed area. These observations suggest that bioFiber is most effective in filling surface cracks, where a higher filling rate is achieved compared to deeper cracks. Consequently, the reinforced matrix should be designed to limit the formation of deep cracks, favoring the development of shallow microcracks to maximize the benefits of bioFiber self-healing. These examples were conducted to evaluate the performance of bioFiber for concrete reinforcement and crack filling, by comparing the mechanical properties and interfacial bonding with conventional PVA fibers. The focus was on evaluating bioFiber effectiveness in crack bridging for mitigating crack propagation, mechanical reinforcement, and self-healing capabilities. ^ Interfacial Bonding Properties: bioFiber demonstrated slip-hardening behavior in 67% of samples, attributed to its rough outer shell and complex interfacial interactions. Despite a lower peak bond strength and bond energy compared to PVA fibers, bioFiber achieved about 83% of the equivalent bond strength of PVA, highlighting its potential for applications requiring enhanced mechanical interlock. ^ Mechanical Reinforcement: Both PVA and bioFiber fibers improved the split tensile strength of mortar, with PVA fibers outperforming bioFiber due to better spatial dispersion and higher bond strength. Optimal amounts for tensile strength enhancement were 0.85% by volume of the sample for PVA and 1.19% by volume of the sample for bioFiber. ^ Self-Healing Performance: BioFiber effectively filled surface cracks through self-healing mechanisms, as evidenced by XCT scans. The performance was limited in deeper cracks, suggesting that concrete designs should aim to foster shallow microcracks to maximize bioFiber's self-healing benefits. Overall, the present invention provides a viable alternative or supplement to traditional fibers, combining mechanical reinforcement with self-healing capabilities. The results from the pullout tests indicated that the primary failure mode in the bioFiber was slip- hardening, in contrast to the slip-softening observed with polyvinyl alcohol fibers. These failure modes were confirmed through visual observation and microstructural analysis using Scanning Electron Microscopy (SEM). 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Claims

What is Claimed is: 1. A concrete composition having self-healing properties, comprising: a cementitious matrix impregnated with a plurality of biofibers, wherein the biofibers comprise: a polymer fiber core comprising one or more polymers selected from the group consisting of polyester, polyethylene, polypropylene, polyvinyl alcohol, polyamide, aramid, polyacrylonitirile, cellulose, polyurethane, and combinations thereof; a crosslinked hydrogel layer coating on the polymer fiber core, wherein the hydrogel layer comprises endospores, one or more cross-linked anionic polymers selected from the group consisting of polysaccharides, hyaluronic acid, colominic acid, polysialic acid, chondroitin, queratane, dextrans, heparin, carrageenan, furcelerans, alginates, agar, glucomannan, gums, pectins, cellulose, starches, sorbitan esters, and combinations thereof and one or more calcium salts, and a (co)polymer shell encapsulating the hydrogel layer, wherein the shell has one or more layers which may be the same or different, and each of the one or more layers is formed with a (co)polymer selected from the group consisting of nitrocellulose (NITR), epoxy resin (ER), polymethyl methacrylate(PMMA), polyvinylidene fluoride (PVDF), cyanoacrylate adhesive (CYA), polystyrene (PS), polylactic acid (PLA), and combinations thereof.

2. The concrete composition of claim 1, wherein each of the biofibers are randomly distributed throughout the cementitious matrix.

3. The concrete composition of any one of claims 1 - 2, wherein the concrete is configured to heal one or more microcracks having a width of from about 50 µm to less than 1 mm, or from about 50 µm to 750 µm, or from about 100 µm – 500 µm, or from about 120 µm to about 400 µm, or from about 120 µm to about 250 µm.

4. The concrete composition of any one of claims 1 – 3, wherein the shell encapsulating the hydrogel layer is a copolymer.

5. The concrete composition of any one of claims 1 – 4, wherein the shell is a copolymer comprising polystyrene and polylactic acid.

6. The concrete composition of any one of claims 1 – 5, wherein the plurality of biofibers have an average diameter of from about 0.5 mm to about 2 mm, or from about 0.75 mm to about 1.5 mm, or from about 0.9 mm to about 1.25 mm.

7. The concrete composition of any one of claims 1 – 6, wherein the hydrogel layer has a water-uptake capacity of from about 1 g/g_core-fiber – 10 g/g_core-fiber, after exposure to an aqueous environment for 30 minutes.

8. The concrete composition of any one of claims 1 – 7, wherein each of the plurality of biofibers is fully integrated into the cementitious matrix providing internal reinforcement via interfacial bonding with the matrix and biofiber.

9. The concrete composition of any one of claims 1 – 8, wherein each of the plurality of biofibers is oriented parallel to a loading direction.

10. The concrete composition of any one of claims 1 – 9, wherein the plurality of biofibers is present in the cementitious matrix in a volumetric amount of from about 0.1% to about 2%, or from about 0.2% to about 1.5%, or from about 0.25% to about 1%, based on a total volume of the curable concrete composition.

11. The concrete composition of any one of claims 1 – 10, further comprising a polymeric fiber, optionally selected from polyvinyl alcohol.

12. The concrete composition of claim 11, wherein the polymeric fiber is present in the cementitious matrix in a volumetric amount of from about 0.1% to about 2%, or from about 0.2% to about 1.5%, or from about 0.25% to about 1%, based on a total volume of the curable concrete composition.

13. The concrete composition of any one of claims 1 – 12, wherein the calcium salts comprise calcium alginate.

14. The concrete composition of any one of claims 1 - 13, wherein the self-healing concrete has a crack-filling ratio of from about 5% to about 100%, or from about 50% to about 100%, or from about 70% to about 100%, after 28 days, wherein the crack-filling ratio is determined by: ^_{^,^^%^ ^ = ^,^ ^ × 100 ^,^^} where R_h,t is the crack-filling ratio at microbial-induced calcium carbonate precipitation (MICCP) time t, Ah,t is a filled crack area, and Ac,t0 is an initial crack area, as determined by optical microscopic images.

15. The concrete composition of any one of claims 1 – 14, wherein the endospores are selected from the group consisting of Lysinibacillus sphaericus, cyanobacteria, Synechococcus, and Prochlorococcus and heterotrophs such as Sporosarcina pasteurii (Bacillus sphaericus), B. megaterium, B. subtilis, B.cereus, B. cohnii, B. pseudofirmus, B. alkalinitrilicus, Diaphorobacter nitroreducens, Pseudomonas aeruginosa, Desulfovibrio brasiliensis, and Desulfovibrio vilgaris, B. mucilaginous.

16. A method of preparing the self-healing concrete composition of any one of claims 1 – 15 comprising steps of: a. preparing the plurality of biofibers, wherein each biofiber is prepared by: vi. coating the core with a solution comprising one or more anionic polymers and endospores to form a coated core; vii. crosslinking the coated core formed in step a) with a calcium-containing crosslinking agent to form a crosslinked hydrogel layer coated core; viii. encapsulating the crosslinked hydrogel layer coated core with the (co)polymer to form a polymer shell having one or more layers to form the biofiber; iv. preparing a cement paste; and v. integrating the plurality of biofibers into the cement paste.

17. The method of claim 16, wherein prior to the step of encapsulating the crosslinked hydrogel layer, the hydrogel layer coated core is dried.

18. The method of any one of claims 16 - 17, wherein the hydrogel layer coated core is dried for a duration of from about 1 hour to about 48 hours, or from about 2 hours to about 36 hours, or from about 5 hours to about 24 hours.