Technical Report Draft #1

Implementing self healing bacterial medium in concrete to heal cracks in monumental buildings.

1. Introduction

This report is prepared to propose to Building Construction Authority (BCA) on the adoption of self-healing concrete to be incorporated into the historical buildings to prolong the longevity of the building structure. This prevents the formation of cracks in monumental and historical buildings.

1.1 Background Information

Preserving our heritage through buildings is essential to our future generations. Some of the heritage and cultures were represented in buildings such as the Old Parliament House, which was built in 1827. These buildings were constructed using mainly concrete consisting 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”, it is stated that concrete has poor resilience against acid. Therefore this will lead to the deterioration of concrete which may result in the formation of cracks. 

According to “Types and Causes of Concrete Deterioration”, it is also stated that 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 shrinkage of concrete due to the volumetric change. 

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, thus,  becomes unsafe for the building occupants and will be required to be restored immediately. 

By Introducing a self healing agent such as bacteria, it aids in healing cracks effectively in structures. The bacteria embedded in this self healing concrete behave in a way such that when triggered by contact with water and air, it produces limestone, and in doing so they repair the crack by filling the air and water voids. From an interview conducted with Professor Fei Jin from Singapore Institute of Technology, he stated that there are two methods to incorporate this self healing bacteria. First method is incorporating the dormant bacteria into the design mix which will be activated when cracks are formed. Second method is incorporating live bacteria into grout for healing purposes as the grout will be injected into cracks to fill up the cracks. 

By incorporating this into 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 self healing concrete costs more than normal concrete, when looking toward the long term, one can expect to save much more on maintenance and refurbishment works. 

The table below shows the comparison of strength between a typical concrete mix used today and a concrete with self healing bacterial admixture.

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

Fig 1 - Comparison of compressive concrete strength (What is Bacterial Concrete or Self Healing Concrete?, 2019).

Ideally, structural concrete in monumental 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 Portland Cement (PC) 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 medium, we aim to preserve structural monuments 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), the adoption of self-healing concrete to be incorporated into the historical buildings to prolong the longevity of that monumental building structurally. 

2.               Proposed Solution

The team proposes a solution to prolong the longevity of monumental buildings by incorporating self healing bacteria into a concrete mix design. There are two types of methods which are the injection of self-healing bacteria into existing concrete structures and recasting of concrete structures using concrete mix containing self-healing bacteria. Before implementing it into high profile monumental buildings, a trial application of the concrete mix to be used in historical shophouses in Chinatown, Singapore. A street in chinatown with shophouses will be selected for inspection of cracks on concrete structures. 2-3 shophouses in the selected street with numerous minor cracks on their concrete structures will be shortlisted for the application of grout injection.

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 Portland Cement (PC), sand, aggregates, self healing bacteria, nutrient (calcium lactate) and water. With a proper mix design of grout injection, cracks with thickness as slim as 0.08mm and as large as 12mm could be filled. Grout injection could be carried out with little disturbance to the surrounding structures. Therefore, it is a preferred method for minor cracks that do not jeopardize the integrity of concrete structures. 

Add a picture of grouting   

2.1.2 Application

The grout injection mix containing self-healing agents and traditional grout injection 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 the period of 2-3 years. (Shall add the parameter we are going to record as results)

The accumulation of the results can be used to determine whether it is feasible and viable for the self-healing concrete to be incorporated in the maintenance of the concrete structures of monumental buildings. 

2.2 Recasting of damaged concrete structure using Self-healing Concrete Mix

A concrete mix containing dormant self-healing agent (bacteria) and nutrients (calcium lactate) shall be used to recast the existing damaged concrete structure. If the similar type of damage occurs the dormant self-healing agent will be activated. The reaction between the bacteria, nutrients and water will produce limestones which will fill the gaps of the cracks. This method is suitable for cracks which are so large that the reinforcements within the concrete are being exposed.    

2.2.2 Application

This method shall be applied to damaged concrete structures which are carrying critical load (eg. Beams and slabs). The damages occurring at these concrete structures can be detrimental to the overall building. Therefore, they shall be fully replaced with a recasted concrete structure. 

3.               Benefits 

3.1 Less Human Intervention

As the self healing bacteria will only be activated when cracks are formed, visual cracks can be seen when the bacteria are all used up. This reduces the maintenance frequency needed for each monumental building. In addition, building management may reduce manpower or even divert their manpower elsewhere. 

3.2 Less Cost of Maintenance 

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

4.               Evaluation

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

The proposed solution of incorporating self healing agent into grout injection and concrete design mix to restore old concrete structures is feasible. From an interview with Dr Fei Jin, he mentioned that there are alot of studies regarding this self healing agent being incorporated in concrete design mix to achieve the self healing properties. He also states that it is possible as there was research that found success in using this self healing agent. 

4.1 Limitation

In order to incorporate this self healing agent, there are several challenges that will surface. 

The social stigma of using bacteria as our main healing agent will not be supported. Bacteria are associated with infection, virus and decomposition of living organisms. In addition, bacteria produce a pungent smell which may lead to additional challenges in future. 

(to be continued)

5.               Methodology and Procedure

5.1 Primary Research

Primary research in the form of an interview was conducted with Dr Fei Jin, who is an Assistant Professor in University of Glasgow Singapore. Dr Fei Jin specializes in Civil Engineering with a Doctor of Philosophy (Ph.D.) from the University of Cambridge.

He mentioned that our two proposed methods were feasible and could be incorporated into concrete mix designs to maintain and conserve historical buildings in Singapore. However, he also mentioned that the initial investment of utilising self-healing agents into concrete structures would be at least twice as high compared to the typical ones being used today. The benefits would rather play out in the long run when the cost of maintaining the concrete structure would be greatly reduced which could lead to every company’s goal of cost saving. 

5.2 Secondary Research

The main source of secondary research Ingenia Inc (2011). The magazine issue published by Ingenia Inc, documented comprehensive information on the topic of self healing agents being incorporated into concrete. It covered the need of these in today’s world, including how these agents specifically work and the advantages and disadvantages of the agent. Other sources of secondary research include online publications and websites. (Not complete)

   

6.               Conclusion

The National Heritage Board states that 73 buildings and structures in Singapore are national monuments.This is a small number of national monuments compared to other countries. Conserving these timeless buildings in Singapore is crucial for our future generations. Most of these buildings were built with concrete which is known to deteriorate over time. The frequency of the maintenance for these buildings is high and costly. With the incorporation of self-healing concrete into the maintenance process of the concrete structures of the historic buildings, we will be able to bring down the maintenance cost in the long run. (Not complete)