Concrete can develop small cracks during drying, temperature change, loading, settlement, or long-term exposure. These cracks may allow water and harmful salts to reach the reinforcement. Therefore, controlling crack width and water entry is important for concrete durability.
Bioconcrete, in this article, means bacterial concrete that uses microbially induced calcium carbonate precipitation (MICP). Selected bacteria and a nutrient source are added to the concrete or repair material. When a crack admits water, the bacteria can create conditions for calcium carbonate (CaCO3) to precipitate and seal part of the crack.
What Is Bacterial Bioconcrete?
Bacterial bioconcrete is a cement-based material that contains microorganisms capable of promoting calcium carbonate precipitation. The complete system normally needs bacteria, a nutrient or carbonate-producing substrate, available calcium ions, and enough moisture for the reaction.
In new concrete, the bacterial spores and nutrients may be added directly, held inside porous lightweight aggregate, or protected in capsules. Similar bacterial systems can also be used in repair mortar or as a surface treatment. Protection is important because fresh and hardened concrete is highly alkaline, and mixing, heat, drying, and limited nutrients can reduce bacterial survival.
| Part | Purpose | Practical point |
|---|---|---|
| Bacterial spores or cells | Change the local chemistry and provide nucleation sites for mineral formation | Alkali-tolerant, often spore-forming strains are studied because concrete may reach approximately pH 13. |
| Nutrient or precursor | Supports the selected metabolic route | Calcium lactate and urea-based systems do not follow the same chemical pathway. |
| Calcium source | Supplies Ca2+ for calcium carbonate | The source may come from the healing agent or the cementitious matrix, depending on the system. |
| Carrier or capsule | Protects the bacteria and can store nutrients | Expanded clay, lightweight aggregate, hydrogels, and other carriers are research options; each can affect concrete properties. |
| Water in the crack | Activates spores, dissolves nutrients, and allows transport | Healing is usually limited when the crack stays dry. |
How Does MICP Seal a Concrete Crack?
- A crack reaches a bacterial carrier or exposes dormant spores and nutrients.
- Water enters the crack and activates the healing system.
- Bacterial activity increases carbonate availability and changes the local chemical conditions.
- Calcium ions combine with carbonate ions and form solid calcium carbonate:
Ca2+ + CO32- -> CaCO3(s)
The crystals grow on crack surfaces and around bacterial cells. This can reduce the crack opening and water flow. The result is mainly crack sealing and durability improvement; it does not automatically return a damaged structural member to its original strength.
Ureolytic and Organic-Nutrient Routes Are Different
In a ureolytic MICP system, a bacterial urease enzyme breaks down urea. A simplified overall reaction is:
CO(NH2)2 + 2H2O -> 2NH4+ + CO32-
The carbonate can then react with calcium to form CaCO3. This route is effective in many laboratory studies, but ammonium or ammonia production is an environmental concern that must be managed.
Other systems use an organic nutrient such as calcium lactate with bacteria selected for that metabolic route. They can also produce calcium carbonate, but they should not be explained by the urea-hydrolysis reaction. Nitrate-reducing and other non-ureolytic routes are also under study.
Which Bacteria Are Used in Bioconcrete?
Researchers commonly examine alkali-tolerant, spore-forming bacteria because spores can remain dormant until suitable moisture and nutrients become available. Examples reported in the literature include:
- Sporosarcina pasteurii, formerly called Bacillus pasteurii, for ureolytic MICP;
- Lysinibacillus sphaericus, formerly called Bacillus sphaericus;
- Bacillus subtilis;
- Bacillus cohnii; and
- Bacillus pseudofirmus.
A long list of bacterial names is not a mix design. The strain, cell or spore concentration, nutrient, carrier, curing condition, and concrete composition must work as one tested system. Biosafety must also be assessed for the selected strain and application.
How Much Crack Width Can Bioconcrete Heal?
There is no single maximum crack width for all bacterial concrete. Published results change with the strain, carrier, healing-agent dosage, crack geometry and age, water exposure, temperature, and the method used to judge healing.
| Evidence | Reported result | Correct interpretation |
|---|---|---|
| Jonkers laboratory study (2011) | Efficient sealing of 0.15 mm cracks with bacteria and organic compounds carried in expanded clay | A result for that tested material and exposure, not a universal limit. |
| Mors and Jonkers demonstrator review (2020) | Laboratory healing up to 0.8 mm depended on healing-agent dosage | Field demonstrators showed no negative construction effects, but little cracking made improved healing difficult to prove. |
| RILEM round-robin evaluation | High scatter among laboratories | Test method and boundary conditions must be stated before results are compared. |
Visible crack closure is only one measure. Water permeability, water absorption, chloride ingress, and recovery of mechanical properties may give different healing percentages. For durability work, reduced water flow can be valuable even when full strength recovery is not demonstrated.
Advantages of Bacterial Bioconcrete
- It can autonomously seal some small cracks when moisture reaches the healing agent.
- Calcium carbonate can reduce water penetration and help slow the entry of aggressive substances.
- It may reduce inspection and repair demand in locations that are difficult to access.
- It can extend service life if the healing system remains viable and the crack stays within the tested range.
- Surface treatment and repair-mortar applications may improve water resistance without replacing the full structural concrete mix.
Limitations and Risks
- Performance depends on moisture, temperature, crack width, bacterial survival, nutrient availability, and carrier distribution.
- High concrete alkalinity and long dormant periods can reduce viable bacteria, especially when they are not protected.
- Capsules or porous carriers occupy space in the mix and may affect workability and mechanical properties.
- Ureolytic systems can release ammonium or ammonia.
- Laboratory studies use different crack and healing tests, so strength or healing percentages should not be transferred directly between projects.
- Cost and long-term field evidence remain barriers to routine use.
- There is no general prescriptive Indian Standard mix-design method for bacterial self-healing concrete.
Where Can Bacterial Bioconcrete Be Useful?
The technology is most attractive where small cracks and water tightness control durability, and where later repair is difficult. Possible applications include water-retaining structures, basements, tunnels, marine or coastal elements, bridge components, irrigation structures, and repair mortars.
For India, the range of hot, humid, coastal, dry, and cold conditions makes project-specific testing important. Temperature and moisture can help or restrict bacterial activity. A proposed system should therefore be tested with the intended concrete, exposure, crack range, and curing condition instead of being selected only from a published strength percentage.
Bioconcrete Compared With Ordinary Concrete
| Point | Bacterial bioconcrete | Ordinary concrete |
|---|---|---|
| Healing agent | Bacteria plus a compatible nutrient/calcium system | No added bacterial healing system |
| Small-crack sealing | Can be enhanced by MICP under suitable conditions | Limited autogenous healing may occur through continued hydration and carbonation |
| Performance basis | Depends on the complete tested biological delivery system | Established concrete mix and durability requirements apply |
| Design status | Emerging; project-specific qualification is required | Covered by established concrete standards and specifications |
| Main practical concern | Viability, activation, environmental conditions, cost, and verification | Crack control, permeability, workmanship, curing, and exposure design |
References
- Jonkers, H.M. (2011), Bacteria-based self-healing concrete, TU Delft Repository.
- Mors, R. and Jonkers, H.M. (2020), Bacteria-based self-healing concrete: evaluation of full scale demonstrator projects.
- De Belie, N. (2016), Application of bacteria in concrete: a critical evaluation of the current status.
- Otiniano et al. (2024), Isolation and selection of spore-forming bacilli with potential for self-healing of concrete.
- Seifan et al. (2019), MICP and Its Potential in Bioconcrete.