Technical report

Implementing Self-Healing Bacteria Agents in Concrete to Heal Cracks in Historical Buildings

Introduction

This report has been developed in response to the request for proposals on developing solutions for engineering problems.

1.1 Background Information
Preserving our heritage through buildings is essential for our future generations. Buildings such as the Old Parliament House, which was built in 1827, are rich in the culture and heritage of Singapore. These buildings were constructed mainly with concrete, which consists of Ordinary Portland Cement (OPC), which is known to deteriorate over a long period of time. The deterioration of the concrete in Singapore is mainly due to the shrinkage caused by high temperatures and the acidity of rain. According to Hu, Balasubramanian, and Wu (2003), Singapore’s rainwater is “typically acidic” due to impurities from the atmosphere such as carbon dioxide. According to “Types and Causes of Concrete Deterioration” (n.d), concrete has poor resilience against acid. This will lead to the deterioration of concrete which may result in the formation of cracks.

According to Portland Cement Association (n.d.), the formation of cracks in concrete is imminent due to the “drying shrinkage” phenomenon. This phenomenon is due to the evaporation of moisture in the concrete over time. This results in the shrinkage of concrete due to the volumetric change in the material.

Due to these cracks, water could seep in and as a result, corrode the steel reinforcements inside the concrete. According to Arnold (2011), steel-reinforced bars are added into concrete structures as it improves the concrete strength. The corrosion of these reinforcements could be detrimental to the concrete structure as the overall weight-bearing capacity of the concrete structure would be greatly reduced. Under such circumstances, the concrete structure becomes unsafe for the building occupants and will be required to be restored immediately.

By introducing a self-healing agent consisting of bacteria, it aids in healing cracks in structures effectively. The bacteria embedded in this self-healing agent behave in a way such that when triggered by contact with water and air, it produces limestone, and in doing so it repairs the crack by filling the air and water voids. From an interview conducted with Professor Fei Jin from the Singapore Institute of Technology (SIT), he informed us that the usage of self-healing agents, especially with the use of bacteria, is popular among current-day researchers. Both methods of incorporating dormant and live self-healing bacteria into the concrete mix design have been supported by professor Fei Jin. Dormant bacteria will be activated when cracks are formed while live bacteria will be injected into cracks for healing purposes.

By incorporating self-healing bacteria agents into the concrete mix design of existing buildings, we will ultimately be able to heal concrete without any external actions, increase the structural strength, and also reduce the corrosion of the inner steel reinforcements. Even though it costs more to use concrete consisting of self-healing bacteria agents, in the long run, one can expect to save much more on overall maintenance costs and refurbishment works.

The table below shows the comparison of strength between a typical concrete mix used today and a concrete mix with self-healing bacteria agent as an additional admixture.

Table 1. Comparison of compressive concrete strength

S/N	Days	Normal Concrete Strength (N/mm^2)	Self-Healing Concrete Strength (N/mm^2)
1	7	20.35	26.90
2	28	30.50	37.97

Ideally, structural concrete in historical buildings should have self-healing properties so the formation of cracks can be alleviated. Without such self-healing mechanisms, these cracks could lead to a structural failure, thus reducing concrete longevity of the structure.

1.2 Problem Statement
Traditional concrete such as OPC does not contain self-healing properties and tends to deteriorate over a certain time span, which makes it susceptible to cracks. With the introduction of bacteria as a self-healing agent, we aim to preserve the historical structures thus prolonging the longevity and maintaining its structural integrity.

1.3 Purpose Statement

The purpose of this report is to propose to Building Construction Authority (BCA) and National Heritage Board on the adoption of self-healing concrete to be incorporated into the national historical buildings to prolong the longevity of the building structure.

Proposed Solution

The team proposes a solution to incorporate self-healing bacteria agents into concrete mix designs to prolong the longevity of historical buildings. There are two types of methods which are the injection of self-healing bacteria agents into existing concrete structures and recasting of concrete structures using concrete mix designs consisting of self-healing bacteria agents.

Before the concrete mixture is implemented into high profile historical buildings, a trial application of our proposed concrete mix design will be implemented on historical shophouses in Chinatown, located at Upper Cross Street. Inspections will be carried out to locate any apparent cracks in the structure. Upon identifying such cracks, it will be shortlisted for either grout injection or recasting depending on the severity of the cracks.

2.1 Grout Injection

Grout, by definition, refers to a composite material used to fill in voids and openings in the surface. The proposed grout consists of a mixture of OPC, sand, aggregates, water and the self-healing agent consisting of bacteria and nutrients such as calcium lactate. With a proper mix design of the proposed grout, cracks with thickness from a range of 0.08mm to 12mm could be filled. Grout injection could be carried out with little disturbances to the surrounding structures. Therefore, it is a preferred method for minor cracks that do not jeopardize the structural integrity of concrete structures.

2.1.1 Application of grout injection. The proposed grout mix containing self-healing agents and traditional grout mix will be prepared according to the sizes of the cracks. The mixes will be applied to separate concrete structures with minor cracks in the shophouse. After the application of the grout injection into the cracks, the results between the mixes will be monitored and compared over a period of 3 to 5 years.

The feasibility and viability of the self-healing concrete in the concrete structures of the historical buildings will be evaluated from the results collected over the years.

2.2 Recasting Damaged Concrete Structural Elements using Self-healing Concrete Mix Design

A concrete mix design containing dormant self-healing agents, bacteria, and nutrients such as calcium lactate, will be used to recast the existing damaged concrete structure. If a similar type of damage resurfaces, the dormant self-healing bacteria agent will be activated. The reaction between the bacteria, nutrients, and water will then produce limestones that will fill the cracks. This method is suitable for large cracks whereby the steel reinforcements within the concrete are being exposed. An example of this process can be seen in the images below.

Fig 1. Test sample of self-healing concrete that has just been cracked (CNN, 2016).	Fig 2. Limestone filling the crack (CNN, 2016).

2.2.1 Application of recasting damaged concrete structural elements. This method will be applied to damaged concrete structures which are carrying a critical load such as beams and slabs. The damages occurring at these concrete structures can be detrimental to the overall building. Therefore, these structural elements must be fully replaced by recasting such elements to assure they satisfy the full structural integrity of the building. 

During the duration of our proposed trial, critical structures will be identified and recast using our proposed concrete mix which consists of self-healing bacteria agents. Similar to grout injections, the concrete will be monitored over a span of 3 to 5 years. The feasibility and viability of the self-healing concrete in the concrete structures of the historical buildings will be evaluated from the results collected over the years.  

2.3 Summary of Proposed Solution

 The table below shows a summary of the two proposed methods.

Table 2. Summary of Proposed Solution

Type of Application	Grout Injection	Recasting of Damaged Concrete
Crack Diameter	0.08mm to 12mm	> 40mm and/or Depending on the severity of cracks whereby steel reinforcements can be visible
Example of Cracks	Fig 3. Minor cracks due to aging of concrete.	Fig 4. Structural cracks from a building built in the year 1900.
Concrete Mixture	Ordinary Portland Cement (OPC) + fine & coarse aggregates + self-healing bacteria agent (bacillus + calcium lactate) + water

Benefits

The injection of self-healing bacteria into existing concrete structures and recasting of concrete structures using a concrete mix containing self-healing bacteria agents will produce long term benefits such as less human intervention, less cost of maintenance and an increase in sustainability.

3.1 Less Human Intervention

The self-healing bacteria agent will only be activated when cracks are detected and cracks can only be visible to the naked eye when all self-healing bacteria agents are used and depleted. Due to such characteristics, the maintenance frequency needed for each historical building can be greatly reduced. With fewer cracks, lesser manpower will be needed for maintenance and inspections. Thereby allowing the building management department to divert their available manpower elsewhere accordingly.

According to Ng J. S. (2017), chief executive from EM Services claims that half of its 1,500 staff are facility managers. With the implementation of our proposed solutions, facility managers can be reduced where the excess manpower can be diverted to other major or crucial departments such as the project planning department.

3.2 Less Cost of Maintenance

With lesser human intervention, it will mean that there will be a great reduction in maintenance needed which leads to a decrease in manpower and materials required. This leads to a lower maintenance cost as the frequency will be lower.

3.3 Increase in Sustainability

Due to its small size, having a sustainable construction in Singapore is crucial especially with the limited natural resources the country has. Nearly all construction materials are being imported which include cement, aggregates, and water. With the implementation of our proposed solutions, material imports can be significantly reduced.

As Singapore progresses towards having a sustainable construction industry, our proposed solutions can be mentioned as one of the green technologies. By optimizing the use of materials, concrete structures will have a longer lifespan. The introduction of the self-healing bacteria agents enables the concrete structure to repair itself thus resulting in lesser human intervention and maintenance cost required.

3.4 Key Stakeholders Action Plan

Potential key stakeholders and action plans required are summed up in the table below.

Table 3. Action Plan for Respective Key Stakeholders

Key stakeholders	Roles and Responsibilities	Action Plan
Build Construction Authority (BCA)	Role: Regulators Responsibility: Oversee all the construction works in Singapore.	Ensure the building and facility management adopt the technology correctly.
National Heritage Board	Role: Owner Responsibility: Preserve and protect Singapore’s historical buildings.	Support the idea of self-healing bacteria agents in concrete mix to be incorporated in historical buildings.
Building / Facility Management	Role: Client Responsibility: Maintain the operation of the facilities.	Promotes the usage of self-healing bacteria agents in concrete mix via grouting injection or recasting of concrete to its maintenance contractor.
General public	Role: Consumers Responsibility: Rights to comfortability, security, and safety.	Ensure the concrete is safe and secure for public use.

Evaluation

In this section, the feasibility and challenges of the proposed solution will be evaluated and discussed.

The proposed solution of incorporating self-healing bacteria agents into grout injection and concrete design mix to restore old concrete structures is feasible. From an interview with Professor Fei Jin, he informed of multiple studies on incorporating self-healing agents in concrete mix to achieve the self-healing properties. He mentioned the feasibility of the proposed solutions as successful research with regards to the incorporation of self-healing agents into concrete designs can be found.

4.1 Limitations

There are a few limitations to using bacteria as a self-healing agent such as the social stigma of bacteria, alkalinity of concrete, high initial cost and pungent smell produced by bacteria.

4.1.1 Social stigma of bacteria. The social stigma of society towards bacteria are more towards the negative aspects. As bacteria are frequently associated with diseases such as pneumonia, decompositions of living organisms, etc, the society is currently not receptive to the idea of using bacteria as a main healing agent. This obstacle can be overcome by educating the society on the advantages of non-pathogenic bacteria and how our proposed solution benefits Singapore.

4.1.2 Alkalinity of concrete. Most of the bacteria do not thrive in high alkaline conditions such as concrete as it hinders its livelihood. There is a need to cultivate a type of bacteria strain which is capable of surviving in such a high alkaline environment.

Bacteria such as bacillus are proven to have a high survival rate in alkaline places. These bacteria will be used as the main healing agent in the proposed solution. However, in-depth research will be conducted to widen the list of such bacteria to avoid shortage in the future.

4.1.3 High initial cost. The cultivation of bacteria for large scale quantities is not available in Singapore as there are no demands currently. In addition, the adoption of green technology has a higher initial cost due to the lack of demand. Industrial players are still skeptical about investing in the production of bacteria as they do not foresee any market or demand in the near future. To combat this issue, business meetings and presentations are needed to attract potential industry players. Successful results from the proposed trial will ensure industry players on the foreseeable demand and market.

According to Wong L. (2019), S$20 million of the national fund was set aside for developers and building owners to introduce green and technological innovations. Industry players may request funds to cover the initial cost of the cultivation of bacteria.

4.1.4 Pungent smell produced by bacteria. As bacteria is a form of a living organism, it will feed on nutrients which will produce waste together with its pungent smell. The society, inclusive of the National Heritage Board, may raise concerns about the smell as it could deter patrons from entering historical buildings or any structure consisting of the bacteria.

Bacteria will only be activated when cracks are formed as it is in contact with water due to seepage. The quantity during the healing period is small-scaled and a strong smell will thus not be produced.  As the National Heritage Board are concerned regarding the decrease in visitors, data from the proposed trial can be presented to tackle this issue. Programs or interesting facts can be displayed to educate and indirectly attract support from the public.

Methodology and Procedure

In this section, the method of research and resources that we have utilized will be explained.

5.1 Primary Research

Primary research in the form of an hour-long interview was conducted with Professor Fei Jin, who is an Assistant Professor at the University of Glasgow, Singapore, in order to determine if he found our proposed solution feasible in Singapore and what his thoughts on it were. He mentioned that our proposed solution had not been implemented anywhere in the world as of yet, with the exception of a structural element which was done for a research purpose. He also mentioned that the team could consider applying the proposed solution in small scale construction.

5.2 Secondary Research

The main source of secondary research is Ingenia Inc (2011). The magazine issue published by Ingenia Inc, documented comprehensive information on the topic of self-healing bacteria agents being incorporated into concrete. It covered the need for it in today’s world, including how these agents specifically work and the benefits and limitations of the agent. Other sources of secondary research include online publications and websites which support this technical report.  

Conclusion

According to the Urban Redevelopment Authority (URA), there are 75 historical buildings in Singapore. It is a small number as compared to other countries. Thus, making it important to conserve these timeless buildings for the future generations as these buildings are rich in culture and history.

The implementation of self-healing bacteria agents into concrete mix design enables buildings, especially historical structures, to prolong the deterioration period. Two applications of the mix will be made available for different purposes. The proposed trial in an identified street of shophouses at Chinatown, Singapore, stimulates in gathering large scale live construction data for future improvements and implementations.  By tackling limitations on the social stigma of bacteria, the alkalinity of concrete and high initial cost, the implementation of self-healing bacteria agents in concrete mix designs provide the long term benefits in the reduction of frequency and cost in maintenance that benefits all stakeholders. As Singapore is progressing toward automation from self-driven trains to self-check-out systems, the team believes the idea of implementing self-healing bacteria agents in concrete mix to heal cracks will soon be accepted by the country. 

References

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