COMPOSITION AND METHOD FOR IMPARTING SELF-HEALING IN SELF-COMPACTING CONCRETE USING Bacillus SPECIES

Abstract

The present invention relates to microbially enhanced self-compacting concrete (SCC) composition capable of autonomous self-healing and improved chemical durability. The composition integrates a bacterial culture of Bacillus subtilis and / or Bacillus sphaericus and a nutrient medium containing a carbon source. When microcracks are formed, moisture enters and activates the bacteria. Calcium ions available from the cementitious matrix are metabolized and precipitated to calcium carbonate (CaCO₃). This biomineralization process leads to in-situ crack sealing, enhancing the mechanical performance and extending the service life of the concrete. The SCC composition is designed to achieve M60 grade compressive strength and demonstrates superior retention of strength after exposure to chemically aggressive environments such as 5% hydrochloric acid and 5% sodium sulphate solutions. The invention is suitable for use in structural applications such as marine piles, sewage tunnels, foundations, and basements exposed to corrosive conditions, offering long-term durability and reduced maintenance requirements.

Description

FIELD OF INVENTIONThe present invention relates to the field of construction material and civilengineering-concrete technology, particularly, microbially enhanced selfcompacting concrete (SCC) composition with self-healing properties. The Selfhealing property of the SCC is attained using microbial cultures belonging to thegenus Bacillus. Additionally, the resulting concrete from the microbially enhancedcomposition exhibits characteristic healing of microcracks and demonstrates animproved compressive strength with enhanced durability after exposure to chemicalattack and curing.BACKGROUND OF THE INVENTIONBuildings constructed using conventional concrete are susceptible to crackformation over a period of time due to various environmental and mechanicalstresses. Such cracks may progressively widen and result in water seepage,accumulation of dust and pollutants, degradation of structural integrity, and therebyreducing the lifespan of the concrete.Conventional methods for repairing the cracks are generally manual, timeconsuming and economically inefficient, and struggle to address micro cracks,especially in inaccessible areas. The prior art teachings related to sealing orrepairing the concrete using microbes as repairing agent are discussed, the patentapplication number CN111087192A, teaches about a microbial repair agent to selfheal marine concrete cracks. The Bacillus koshii. bacterium act as repairing agentalong with a regulator calcium lactate, and coral reef calcareous sand. The coralsand is pre‑treated with polyethylene oxide and vinyl methyl ether, making itssurface rougher so it bonds much better with the surrounding cement and reducesharmful gaps where sulfates can enter. When cracks happen, water and air activatethe dormant bacteria on the sand. The bacteria consume calcium lactate andprecipitate calcium carbonate (calcite), filling and sealing the cracks with a dense,stable mineral layer. This both restores compressive strength and improves waterresistance and durability, especially in marine conditions.The patent application number US20210139882A1 teaches a method for preparingmicro capsulated self-healing and sealing material for coal mine ventilationsystems. The ventilation systems in coal mines often develop cracks because ofconstant pressure and natural ground movement deep beneath the surface. Thesecracks can lead to dangerous air leaks, increasing risks like spontaneous combustionand gas exposure. Traditional sealing methods such as organic sprays, polyurethane,or foams often do not bond well with cement. This may weaken structural integrity,release harmful gases, and require manual reapplication, making them unreliableand costly. Therefore, a solution to this challenging problem is addressed by themicrocapsule of sustainable self-healing coal mine ventilation sealing material andpreparation method. Microcapsules that contain spores of urease-producing bacteriasuch as Sporosarcina pasteurii, Bacillus cereus, or Bacillus sphaericus, mixed withnutrients like urea and calcium salts. These spores are encapsulated within a durableinorganic wall made from materials such as water glass, carbide slag, desulfurizedgypsum, and coal ash. The capsule walls protect the bacteria during cement mixingand curing. Once cracks form, these capsules rupture, activating the spores. Thebacteria then convert urea and calcium into calcium carbonate (calcite), filling thecracks and continuously healing the material from within.While prior art such as CN111087192A focuses on marine concrete using pretreated coral sand as bacterial carriers, and US20210139882A1 disclosesmicroencapsulation for underground coal mine ventilation systems, these methodsare either limited to specific environmental applications, rely on complex carriermaterials, or face challenges related to bacterial survival, release control, orcompatibility with self-compacting concrete systems.Accordingly, there exists a need for an improved microencapsulation-based selfhealing solution compatible with Self-Compacting Concrete (SCC), which ensuresbacterial viability and controlled release during crack formation, while offeringenhanced chemical durability and mechanical performance under general civilconstruction conditions.SUMMARY OF THE INVENTIONThe present invention provides a microbially enhanced self-compacting concrete(SCC) composition capable of autonomous self-healing and enhanced chemicaldurability. The invention particularly addresses the prevailing challenge ofmicrocrack formation and degradation in concrete structures caused byenvironmental and mechanical stresses, and proposes a sustainable and autonomoussolution using specific bacterial cultures. The concrete composition integrates aspecially formulated bacterial culture and a nutrient medium into a self-compactingmatrix. Over a period of time and during stress, exposure to adverse conditionsmicrocrack formation becomes a significant issue in concrete structures. Themicrocracks serves as a path for moisture to enter the concrete matrix therebyactivating the microbes to metabolize the nutrient medium. The microbes utilize thesource of calcium from the cementitious matrix and precipitate calcium carbonate(calcite) within the cracks. This results in the effective sealing of cracks anddemonstrates superior performance by maintaining compressive strength andretaining mechanical integrity even after chemical exposure thereby ensuring longterm resistance to deterioration and structural degradation. These demonstratedproperties of the invention prove it to be an ideal solution for a wide range ofstructural applications such as marine piles, basements, sewage tunnels, andfoundations exposed to aggressive environmental conditions, significantlyextending the service life of concrete structures while reducing maintenance needs.BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings, which are incorporated in and constitute a part of thisspecification, show certain aspects of the subject matter disclosed herein and,together with the description, help explain some of the principles associated withthe disclosed implementations. In the drawing,Figure 1 illustrates a flowchart depicting the stepwise preparation, activation, andself-healing mechanism of the microbially active self-compacting concrete (SCC)composition.Figure 2 illustrates crack formation and the progressive reduction in crack width atdifferent stages. The images represent microscopic views of the concrete surfaceobserved at 0th day, 15th day, and 30th day intervals under moist conditions,highlighting the self-healing process facilitated by microbial calcium carbonateprecipitation.DETAILED DESCRIPTIONVarious embodiments of the present invention are described in detail below. Thesedescriptions are provided solely for the purpose of illustrating specificimplementations and are not intended to limit the scope of the invention. A personskilled in the art of civil engineering, material science, or microbiology willappreciate that numerous modifications, substitutions, and variations may be madewithout departing from the spirit and scope of the present disclosure.The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. References to "oneembodiment," "an embodiment," "one aspect," or "some aspects" mean that aparticular feature, structure, or characteristic described in connection with theembodiment or aspect is included in at least one implementation of the invention.The use of such terms in various places in the specification is not necessarilyreferring to the same embodiment or aspect.The invention is capable of other embodiments and of being practiced or carriedout in various ways. While specific details are provided to facilitate thoroughunderstanding, in some instances known methods, procedures, and componentshave not been described in detail so as to avoid unnecessarily obscuring theinvention. The appended claims and their equivalents define the scope of protection.Unless otherwise defined, all technical and scientific terms used herein have thesame meaning as commonly understood by one of ordinary skill in the art. In theevent of any conflict in interpretation, the definitions set forth in this specificationshall prevail.Additional advantages and features of the invention will be set forth in part in thedescription which follows and in part will become apparent to those skilled in theart upon examination of the following, or may be learned by practice of theinventionIn one embodiment, the present invention relates to a novel self-healing and selfcompacting concrete (SCC) composition using biologically active Bacillus species,particularly Bacillus sphaericus and Bacillus subtilis, in combination with specificchemical and mineral admixtures. This invention provides detailed steps for thepreparation, incorporation, and performance testing of the bio-concrete, enabling askilled person to reproduce the invention.In some aspects, the materials used include Ordinary Portland Cement (OPC),aggregates, fly ash, potable water, superplasticizer, viscosity modifying agent(VMA), and Bacillus species. The specific proportions and preparation methods aredescribed to ensure reproducibility.According to one implementation, the bacteria are cultured in Luria-Bertani Broth(LBB) and encapsulated using peptone water and sodium alginate to ensure stabilityand viability within the concrete matrix.In another embodiment, the concrete mix is prepared by combining the bacterialsolution with cement, aggregates, admixtures, and water, followed by casting,curing, and testing for self-healing and strength characteristics.Some aspects of the invention describe the testing protocols including compressivestrength testing, acid and sulphate attack resistance, and microscopic evaluation ofcrack healing.These embodiments illustrate the scope of the invention without limiting it, asvariations in bacterial species, encapsulation methods, or mix proportions may beimplemented by those skilled in the art to achieve similar self-healing effects inconcrete.Further embodiments of the invention include procedures for assessing durability,such as acid ponding tests using hydrochloric acid (HCl) and sulphate resistancetests using sodium sulphate (Na2SO4) solutions. These tests evaluate the long-termintegrity of concrete exposed to aggressive environments. In these embodiments,samples incorporating Bacillus subtilis demonstrated superior resistance to weightloss and retained compressive strength when compared to conventional concrete.In additional embodiments, crack healing performance was assessed over a 30-dayperiod under moist conditions. It was observed that concrete samples treated withBacillus sphaericus and Bacillus subtilis, individually and in combination, showedsignificant reduction in crack width due to microbial calcium carbonateprecipitation. The combined use of both bacterial species provided enhancedhealing efficiency and strength recovery.Some embodiments include microscopic analysis using a 10X magnification lensto measure crack healing progress and confirm the deposition of calcium carbonate.The self-healing capability was effective for cracks up to 0.3 mm in width. Figure2, as referenced above, visually supports these observations by presenting asequence of images demonstrating the reduction of crack width at 0, 15, and 30-dayintervals. In one aspect, the invention enables environmentally sustainableconstruction materials with reduced maintenance requirements. The incorporationof bio-agents in concrete offers a novel method to enhance structural integrity andlongevity without compromising workability or strength.According to further implementations, standard concrete mixing, casting, andcuring procedures can be followed by any skilled person, using the specified bioconcrete formulation. The invention provides a balance between mechanicalperformance, self-healing capacity, and resistance to chemical attack, making itsuitable for a wide range of civil engineering applications.Therefore, this invention offers a reliable, reproducible, and scalable method forproducing self-healing concrete that can be applied in infrastructure projects,particularly in environments prone to crack formation, moisture ingress, andchemical exposure. The detailed methodology, material composition, andperformance data described herein enable a skilled artisan to implement theinvention effectively.Detailed Methodology of the Invention:The sequence of steps involved in preparing the microbially active self-healing andself-compacting concrete is schematically illustrated in the flowchart shown in Figure 1.Figure 1Flowchart representing the preparation, activation, and crack-healing mechanismof the microbially active SCC composition.Materials Used:1. Cement: Ordinary Portland Cement (OPC), specific gravity 3.15, initialsetting time 45 minutes, final setting time 360 minutes, consistency 35%.2. Fine Aggregate: <4.75 mm, specific gravity of 2.64, fineness modulus 2.67.3. Coarse Aggregate: 10-12.5 mm size, specific gravity 2.74, finenessmodulus 1.26.4. Water: Potable quality.5. Chemical Admixture: MasterGlenium SKY 8233 superplasticizer per EN934-2.6. Viscosity Modifying Agent (VMA): 0.1% of cement weight.7. Mineral Admixture: Class C Fly Ash, specific gravity 2.15.8. Bacteria:(i) Bacillus sphaericus: Gram-positive, endospore-forming, soilbacterium.(ii) Bacillus subtilis: Gram-positive bacterium with endosporeresilience.Concrete Mix Proportions (per m3):1. Cement: 450-500 kg2. Water: 120-150 kg (w / c ratio 0.28-0.33)3. Fine Aggregate: 600-750 kg4. Coarse Aggregate: 800-1000 kg5. Fly Ash: 120-150 kg6. Superplasticizer: 0.7% of cement weight7. VMA: 0.1% of cement weight8. Bacterial Solution: 8 ml per cube (150x150x150 mm)Bacterial Culture and Preparation:1. Media Preparation:1. Luria Bertani Broth (LBB): 40 g / L2. NaHCO3: 2.12 g / L3. Urea: 10 g / L4. Autoclave the prepared media at 121°C for 15 min, incubate at 30°Cfor 24 hrs to reach 108 CFU / ml.2. Urea-Calcium Broth:1. Urea: 0.25 M2. CaCl2: 0.375 M, pH adjusted to 8 using NaHCO3.3. Microencapsulation:1. 1% Peptone Water: Peptone 10 g / L, NaCl 10 g / L, autoclave at 121Cfor 15 minutes.2. 2% Sodium Alginate: 2 g in 100 ml distilled water, gently stirred.Concrete Mixing and Casting:1. Mix cement, fly ash, fine and coarse aggregates.2. Prepare bacterial solution (pellet + superplasticizer + VMA).3. Combine all components, mix thoroughly.4. Cast in moulds (150x150x150 mm), vibrate and tamp.5. Cure for 28 days in water.Self-Healing Process:1. Bacteria remain dormant until cracks form and moisture / oxygenenter.2. Activation leads to CaCO3 precipitation, sealing cracks.Testing Protocols:1. Compressive Strength: After 28 days curing, load applied at 140kg / cm2 / min. Strength = P / A where, P-load (N) & A-area (mm2).2. Acid Ponding Test:(i) 5% HCl solution for 60 days after 28-day water cure.(ii) Weight loss and strength compared to conventional concrete.3. Sulphate Attack:(i) 5% Na2SO4 solution for 60 days.(ii) Surface dried, weighed, and tested for compressive strength.4. Crack Healing:(i) Monitored at 0th, 15th, 30th days under moist conditions.(ii) Cracks ≤0.3 mm healed effectively. (iii) Microscopic analysis confirms CaCO3 deposition. Results Summary:(i) Bacillus subtilis yielded highest strength (73.1 N / mm2).(ii) Combined use of both species showed enhanced healing.(iii) Bacterial concrete retained strength post acid / sulphateexposure.All the specimens are tested by compression testing machine after 28 days curing.Load should be applied gradually at the rate of 140kg / cm2 per minute till thespecimens fails. Load at the failure divided by area of specimen gives thecompressive strength of concrete. The compressive strength of the concrete wascalculated using the following formula and the results are tabulated:Compressive strength test:Compressive strength = P / A where, P-load (N) & A- area (mm2)TableDurability test:TableSulphate attack test:TableCrack healing on concrete surface:TableFor the purpose of testing and results presented herein, the followingabbreviations are used throughout the specification:SCC - conventional mixBS1 - Bacillus sphaericusBS2 - Bacillus subtilisSS - Bacillus sphaericus and Bacillus subtilisLBB - Luria Bertani brothConclusion: The invention provides an eco-friendly, durable, and self-repairingconcrete suitable for civil structures exposed to cracking and chemical attack. Themethod and composition are reproducible by skilled professionals following theprovided protocol.

Claims

1. A microbially active self-healing and self-compacting concrete (SCC) composition, comprising: (i) a cementitious binder comprising Ordinary Portland Cement (OPC), fly ash, or combinations thereof; (ii) fine aggregate and coarse aggregate; (iii) potable water for mixing, containing a bacterial culture in suspension or encapsulated form, said culture comprising viable spores or cells of at least one Bacillus species; (iv) a nutrient medium comprising a calcium ion source selected from calcium lactate or calcium acetate, and a carbon source selected from urea or glucose; and (v) one or more chemical admixtures selected from high-range water-reducing agents and viscosity-modifying agents, wherein the superplasticizer is present in an amount ranging from 0.6% to 1.2% by weight of the cementitious binder, and the viscosity-modifying agent is present at approximately 0.1% by weight of the cementitious binder; wherein the composition exhibits: (i) self-compacting behavior in its fresh state, eliminating the need for mechanical vibration; (ii) microbial viability after curing and activation in the presence of moisture and oxygen upon microcrack formation; (iii) autonomous in-situ precipitation of calcium carbonate (CaCO3) at crack sites, enabling self-healing of cracks; (iv) a 28-day compressive strength in the range of 62 N / mm2 to 73 N / mm2; (v) retention of compressive strength in the range of 51.6 N / mm2 to 60.7 N / mm2 after 60 days of immersion in 5% hydrochloric acid; (vi) retention of compressive strength in the range of 58.7 N / mm2 to 65.3 N / mm2 after 60 days of immersion in 5% sodium sulphate solution; and (vii) a crack healing capability from the range of 0.1 mm 0.4 mm width within 30 days under moist curing conditions.

2. The composition as claimed in claim 1, wherein the bacterial culture is in the form of pellets prepared by encapsulating viable spores or cells using 1% peptone water and 2% sodium alginate, to preserve microbial viability during mixing and curing of the concrete.

3. The composition as claimed in claim 1, wherein the bacterial species is selected from Bacillus subtilis, Bacillus sphaericus, or a combination thereof.

4. The composition as claimed in claim 1, wherein the use of Bacillus subtilis results in a 28-day compressive strength of 73.1 N / mm2 under standard curing conditions.

5. The composition as claimed in claim 1, wherein the combination of Bacillus subtilis and Bacillus sphaericus results in a 28-day compressive strength of 68.5 N / mm2 and enables complete healing of surface cracks of 0.24 mm width within 30 days.

6. The composition as claimed in claim 1, wherein the bacterial culture is present at a concentration of at least 108 colony-forming units (CFU) per millilitre.

7. A process for preparing the composition as claimed in any one of claims 1 to 6, the process comprising: (i) preparing a dry mix of the cementitious binder, fine aggregate, and coarse aggregate; (ii) preparing an aqueous solution comprising the bacterial culture and nutrient medium and adding the pelleted bacterial culture obtained by encapsulation using 1% peptone water and 2% sodium alginate; (iii) mixing the dry and wet components along with the chemical admixtures to obtain a fresh SCC mix; (iv) casting the mix into moulds without mechanical vibration; and (v) curing the concrete under moist conditions to enable microbial activation and calcium carbonate precipitation at crack sites, as illustrated in Figure 1.

8. The process as claimed in claim 7, wherein the pH of the nutrient medium is adjusted to approximately 8 using sodium bicarbonate (NaHCO3).

9. Use of the composition as claimed in any of the preceding claims, in constructing structures exposed to chemically aggressive environments, selected from acidic industrial environments, marine piles, sewage tunnels, basements, and foundations, where autonomous self-healing and chemical durability are required.

10. The composition as claimed in claim 1, wherein the concrete retains a compressive strength of at least 60.7 N / mm2 after immersion in a 5% hydrochloric acid solution for 60 days.

11. The composition as claimed in claim 1, wherein the concrete retains a compressive strength of at least 62.2 N / mm2 after immersion in a 5% sodium sulphate solution for 60 days.