EXPERIMENTAL STUDY ON SELF-HEALING BACTERIAL CONCRETE

table of Contents
Declaration of originality. i
Turn it in report summary. ii
Acknowledgement. 3
Abbreviations. 6
Abstract. 7
CHAPTER 1: INTRODUCTION.. 8
1.1 Background. 8
1.2 Problem Statement. 10
1.3 Cracks in Concrete. 11
1.4 Objectives. 12
1.5 Justification of Study. 13
1.6 Scope of Work. 13
CHAPTER 2: LITERATURE REVIEW… 14
2.1 Self-healing agents. 14
2.2 Concrete Components. 15
2.3 Bacterial Species. 16
2.4 Importance of the study. 17
2.5 Tests and observations. 18
CHAPTER 3: MATERIALS AND METHODOLOGY. 20
3.1 Materials. 20
3.1.1 Apparatus. 20
3.1.2 Specimens. 23
3.2 Concrete mix design. 25
3.3 Bacterial Ingestion. 25
3.4 Methodology. 26
3.4.1 Obtaining the Bacterial specie. 26
3.4.2 Growth of bacterial culture in the laboratory. 26
3.4.3 Preparing bacteria in beads form.. 26
3.4.4 Casting Concrete Cubes and Cylinders. 27
3.5 Mechanism.. 28
3.6 Laboratory Tests. 29
WORKFLOW… 32
BUDGET. 33
REFERENCES. 34

List of Tables
 
Table 2.1: Emitted energy and CO2 emissions of building materials. 15
Table 2.2: Studies on healing of artificial cracks by different bacteria species. 17
Table 2.3: Mix design proportions for C25 concrete. 25
Table 3.1: Concrete cubes and cylinders to be casted for testing. 27
Table 3.2: Gant Chart showing the entire working schedule. 32
Table 3.3: Table showing approximate quantities and cost for materials. 33
Table 3.4: Approximate cost of equipment and manpower 33
Table 3.5: Approximate costs for laboratory tests. 33
 
List of Figures
Fig 3.1: Culture tube of various sizes. 20
Fig 3.2: Conical/Erlenmeyer flask. 21
Fig 3.3: Glass Pipette. 21
Fig 3.4: Autoclave. 22
Fig 3.5: Petri dish / Cell-culture dish. 22
Fig 3.6: Incubator 23
Fig 3.7: Microscopic image of Bacillus subtilis. 24
Fig 3. 8: Chart showing the Step by Step process of self-healing. 28
 

Abbreviations

ACTM             American society of testing and materials
BS                   British Standards
cm                   Centi-metre
CST                 Compressive Strength Test
Cu.m               Cubic metre
DC                  Direct Voltage
g                      Grams
KES                Kenyan Shillings
Kg                   Kilo-gram
Ln                    Natural Logarithm
m                     Metre
MJ                   Mega-Joules
ml                    Milli-litres
mm                  Milli-metres
N                     Newton
psi                    Pound-force per square inch
RCPT              Rapid Chloride penetration test
SHC                Self-healing concrete
STS                 Split tensile strength
TZS                 Tanzanian Shillings
V                     Volts

Abstract

The formation of cracks in concrete structures due to the penetration of water and other chemicals reduces their durability and strength. The sustainability of steel reinforced concrete is also reduced due to corrosion especially in wet environments. Regular maintenance and repair of structures due to this problem is uneconomical and time consuming. Realizing this, an alternative method was developed by researchers to overcome this problem.
It was observed that the addition of a specific bacteria together with a precursor compound in concrete, during mixing developed a self-healing mechanism during crack formation. Bacteria of the genus “Bacillus Subtilis” satisfying alkaline conditions was used together with calcium lactate (C6H10CaO6) as a precursor compound to form an insoluble precipitate of limestone (CaCO3) which acts as a sealant and makes the concrete watertight.
Promising results were obtained including decrease in permeability and increase in the compressive strength of concrete. Furthermore, research has also shown that addition of bacteria to concrete makes it more eco-friendly due to less emission of CO2 and by avoiding the use of synthetic treatments to repair cracks. Further research and analysis on topic may lead to the development of an eco-friendly and more sustainable concrete particularly suited for wet environments.
Keywords: Bacterial concrete, Self-healing, Bacillus Subtilis, Cracks, Eco-friendly

CHAPTER 1: INTRODUCTION

1.1 Background

In today’s era, concrete is one of the most widely used construction component throughout the world. Just after water, it holds second place in material consumption. Its usage is involved in buildings, pavements, bridges, tunnels, dams, canals, water tanks and the list go on. It is relatively cheap compared to its counter parts and behaves well under compressive stresses but cracks easily under tensile stresses. These tensile stresses are negated by introduction of reinforcement bars. Additionally, cracks provide an un-aesthetical appearance to a structure and more over reduces its durability if not properly maintained. This in turn reduces the structures service life.
Concrete consumption remains at 1.81 Billion tonnes per annum and is estimated to rise by 0.3% year on year-constructech.com. Concrete has a few drawbacks and it gets deteriorated by ingress of moisture, gas, and liquid through cracks. This type of deterioration is highly unacceptable.
Cracking of reinforced concrete is inevitable due to external actions or natural environmental conditions. Micro cracks of less than 0.4mm do not necessarily impact the structural integrity of concrete, however, these micro cracks allow the permeability and passage of corrosive substances which threaten the structure. Also, water retaining structures or high-density concrete for nuclear applications must be considered impermeable and limited to 0.05-0.2mm crack width.
Steel, which is the most common type of reinforcement used in concrete is highly susceptible to corrosion. Various studies show that corrosion in steel results in expansive products, which eventually leads to formation of larger cracks and increased permeability. This can result in catastrophic failures.
One of the most dangerous types of corrosion is rust. A combination of moisture and oxygen, both of which are naturally occurring, deteriorates the steel and results in loss of tensile strength.
It is estimated that around 40-60% of European construction budget is devoted to maintenance and repair of existing structures of which a large percentage is devoted to concrete structures.
Various structures such as underground tanks, tunnels, bridges and high-rise structures undergo excessive maintenance and also require heavy refurbishing due to inaccessibility of a certain element. Technological advancement has implemented various techniques to solve these problems. These include surface waterproofing, epoxy treated bars, stainless steel bars, fibreglass reinforcement and also cathode protection. These methods have all come across technical or economical limitations.
Self-healing concrete has been a hot topic researched and studied in recent years. The ability of concrete to self-heal comes from the basic idea of a human body. A human body when subjected to a cut undergoes self-healing. This is possible due to the production of platelets. Similarly, concrete can be artificially induced with an agent in order to undergo self-healing when subjected to cracks. The self-healing process takes place overtime internally without the need of any external maintenance.
Bacteria of the genus Bacillus have been identified to have favourable properties to be used as a self-healing agent in concrete. The current study is based on introduction of bacterial species in concrete mix.
This bacteria based self-healing agent is believed to remain hibernated within the concrete for up to 200 years. The bacterial-spores start microbial activities when they come in contact with water and oxygen due to the development of cracks in concrete. Recently, the self-healing approaches have been exhibiting promising results in remediating the cracks in the earlier stages of formation of cracks. An extensive long-term study needs to be carried out in order to understand the long-term effects of self-healing concrete.
Presently, the world is focusing on reducing the effects of climate change. It is evident from reputable citations that global temperature is increasing and human activity has led to production of excess carbon dioxide in the atmosphere. Current cement production contributes about 7% to global anthropogenic CO2 emissions”-H.M.Jonkers. The production of limestone in this method prevents the escape of CO2 produced due to hydration of cement when in contact with water.
Current study will also focus on compressive strength behaviour of self-healing concrete compared to that obtained in conventional concrete. Studies have shown the strength of self-healing agent induced concrete is greater than that of conventional concrete.
Self-healing concrete could solve the problem of concrete structures deteriorating well before the end of their service life. Moreover, maintenance cost can be reduced by up to 30-40% in self-healing concrete.

bacteria

1.2 Problem Statement

Concrete, being one of the most widely used construction material, has its specific properties like any other components. From sub structure to super structures, in oceans and dry lands, concrete is widely used.
The behaviour of concrete varies in different areas as well as different parts of structures. When It comes to reinforced concrete, the clear cover plays a big role in designing the elements. Areas which are prone to water ingestion will have a higher cover than areas with low chance of water ingestion.
Similarly, reinforcement bars also play a major role inside the concrete. It provides great resistance to tensile stress whereas concrete is in compressive stress.
The question remains, why the introduction of bacteria species inside concrete?
To answer the above question, let us introduce the concept of corrosion. When the reinforcement is exposed to oxygen and water, a phenomenon known as rusting occurs.
Rusting is known as a major deterioration of reinforcement bars which has the following ill effects:
Due to the above ill effects of corrosion, it is necessary to take preventive measures. It is evident that the corrosion of reinforcement bars occurs due to ingestion of moisture and in the presence of oxygen. Also, the ingestion of moisture occurs through cracks formed in concrete.
 1.3 Cracks in Concrete
When you see a crack in your concrete slab or wall, your first assumption is typically that something has been done wrong–but that’s not always the case. Actually, concrete cracks are very common, some are even inevitable.
These cracks can occur due to poor mix design, poor workmanship, insufficient curing, overloading or flexing of concrete due to lack of expansion joints.
Crack width is an important factor affecting the behaviour and performance of reinforced concrete pavements. Wide cracks can lead to various pavement distresses, including spalling, punchouts, and steel rupture. Factors affecting the cracks are construction season, coarse aggregate type and amount of steel.
Cracks which aren’t repaired will eventually lead to continuous ingestion of moisture and in the long run will cause corrosion of reinforcement bars. This will eventually lead to reduced structural life and heavy maintenance costs.
The average lifespan of a structure is scientifically claimed to be between 50-100 years. In reality, it depends on the exposure conditions, material used, serviceability and many more factors.
To increase the service life of a structure, constant maintenance in required. For some areas, access to repair, refurbish or even to seal cracks is not possible therefore service life of these structures is reduced.
A  good example is a reinforced concrete bridge. It is continuously exposed to water and is susceptible to ingestion of water through cracks which in turn leads to corrosion of the reinforcements. Part of the structure may require repair of cracks but can be impossible to access easily therefore requires large costly methods for repairs. Overall, the cost of maintenance and repair increases tenfold and service life decreases.

1.4 Objectives

The primary objective of the study is to propose a scientific method and to experimentally prove with promising results a way in which the cracks in concrete are self-healed by introducing a bacterial specimen.
Since the concrete is susceptible to cracks and overtime moisture penetrates through the cracks and corrodes the reinforcement bars, a self-healing process is generated by chemical reactions taking place within the cracks.
To simulate the hypothesis, an example is curated in order to understand the benefit of self-healing process.
When a human body is subjected to a bruise/cut on the knee and leaves blood exposed, immediately the process of self-healing is commenced. This process is initiated because of presence of blood cells and platelets. Now, for instance the platelets did not exist and the bruise was left exposed for a very long time, an infection or a disease is more likely to occur if left untreated and additional medicines and doctor visits is necessary to prevent any further sickness.
Similarly, when it comes to concrete, the cracks are the bruises/cuts. The corrosion or rusting of rebars is the infection or disease and the visits to doctors and medicines is the maintenance and repair costs. The Platelets and blood cells are the self-healing agents
Therefore, in order to avoid heavy maintenance and repair cost, a bacterial specie is introduced inside the concrete which will act as the self-healing agent when the concrete is cracked and exposed to moisture.

1.5 Justification of Study

For many years, researchers have come up with various solutions of maintenance and repair of concrete structures which are prone to cracks and also difficult to access for repair. The sole purpose of these solutions is to reduce costs and increase the service life of the structure.
However, upon determining statistics all over the world, a lot of monetary resource goes into repairing and maintaining such structures. Therefore, this particular study is expected to expose the use of the natural resource readily available to fill in the gaps and solve the problem.
The benefits are not only limited to present population but also to the coming generation as it will also help in reducing the carbon footprint of concrete in the long run by trapping excess CO2.

1.6 Scope of Work

Whilst the study on self-healing concrete using various methods have been researched and documented, the impact of bacteria in different forms introduced in concrete to identify the effects and understand the self-healing of cracks has been poorly understood as of yet. The aim of this study is to analyse and understand long term benefits of using bacteria of a particular specie, Bacillus Subtilis, and conclude on the same.

The scope of the study is limited to addition of bacteria in concrete during mixing. The bacteria will be introduced in two particular forms which are in a solution and in the form of beads. This will expand the scope of the study to identify and document which form produces better results.

The study will involve casting of several concrete cubes and cylinders with different proportions and form of bacteria. Also, a control concrete cube and cylinder sample will be casted to produce a baseline of results. A number of physical tests will be performed on the samples and results will be documented accordingly.

At the end of the study, the results will be analysed, documented and presented in order to identify the effects bacteria based self-healing concrete in terms of strength, economic benefits, self-healing nature and eco-concrete will also be discussed.

CHAPTER 2: LITERATURE REVIEW

2.1 Self-healing agents

A self-healing material is described as a material that is capable of repairing itself back to the original state. The concept of self-healing concrete (SHC) that happens over time (autogenic) has been noticed for over 20 years. It can be observed in many old structures which have remained standing for long periods of time in spite of the fact that they have limited maintenance. This observation concludes that the cracks heal when moisture interacts with non-hydrated cement clinker in the crack. – [1] Alhalabi Z.Sh. et al 2020.
Dr Henk Jonkers, a microbiologist who specialises in the behaviour of bacteria in the environment, has developed self-healing concrete in the laboratory. The first self-healing concrete products (successful research results permitting) are expected to hit the market in twenty years’ time and are expected to increase the lifespan of many civil engineering structures.
Self-healing materials are group of energetic materials that have the structurally combined strength to fix damage created by mechanical way over time. The thought arises from biological methods, which have the capacity to fix after being damage. [8] Rafat Siddique et al 2011.
In another publication, [2] Weing Wang et al suggests that while most healing agents are chemically based, more recently the possible application of bacteria as self-healing agent has also been considered. The principle of microbial self-healing concrete technology is to use the mineralization reaction of microorganisms to form gelling substances. The researchers first discovered in the oil exploitation project that the rock crack can be repaired by microorganisms, and the repairing effect can still be carried out after the microorganisms become dead bodies. After this phenomenon was discovered, the microbial self-healing technology was applied to geotechnical engineering and cultural relics restoration projects, and the application in concrete crack repair engineering was gradually deepened. In a number of published studies by [3][4] Tittleboom et al 2010, Penghui Li et al 2009, the potential of calcite precipitating bacteria for concrete or limestone surface remediation or durability improvement was investigated.
[5] Wiktor et al 2011, added a special new biochemical self-healing agent to the porous expanded clay granules using a two-component matrix which can increase the self-healing capacity of the concrete. This was analysed by [6] H.Jonker et al 2009 , that the use of a mineral precursor compound can increase the calcite precipitation by more than one time. Research was done to observe direct CaCO3 precipitation through metabolic conversion of calcium lactate and indirect formation due to reaction of metabolically produced CO2 molecules with Ca(OH)2 minerals present in the concrete matrix leading to additional CaCO3 precipitation.

2.2 Concrete Components

In civil engineering, reinforced concrete is widely used for construction work. This is associated with a low cost of building and construction materials and also with low maintenance costs. However, both concrete and reinforcement are a huge burden to the environment due to the high energy consumption during production and use.
A study conducted by [7] Dr H. Jonkers presents examples of building materials and the amounts of energy produced by them.
Table 2.

Building Materials Energy (MJ/kg) kg CO2/kg
Aggregate 0.083 0.0048
Concrete (1:1.5:3) 1.11 0.159
Cement mortar (1:3) 1.33 0.208
Steel (General) 20.10 1.37
Bricks (All) 3.0 0.24

[9] Dr. Richard Cooper of Bath’s Department of Biology & Biochemistry says that incorporating bacteria in concrete adds a double layer shield in order to prevent corrosion in steel. Not to mention that it employs oxygen present which would then benefit the process of steel corrosion.
Upon adding the bacteria specie into the concrete matrix, it was evident that the energy released for each component reduced by a certain percentage therefore the bacterial concrete can also be incorporated as green concrete due to the reduction of CO2 emission into the atmosphere.

2.3 Bacterial Species

Largely, bacteria possess?? the ability to generate minerals from the environmental samples. A number of these bacterial species has the capability to biomineralize CaCO3 from different sources as their metabolic activity. [10] Abo el-Enain et al 2013 identified that Urea lytic bacteria can catalyse the conversion of urea to ammonia and CO2.
The high pH content in cement facilitates the conversion of CO2 to bicarbonate and carbonate ions which precipitates with environmental calcium ions to calcium carbonate minerals.
[11] C.C Gavimath et al demonstrated that since the cell wall of the bacteria is negatively charged, the bacteria draw cations from the environment including the Calcium ions to deposit their cell surface.
Ca2+ + O22-    à    O‐Ca2+                                                                                                                          (2.3.1)
The calcium ions subsequently react with carbonate ions leading to the precipitation of CaCO3 at the cell surface that serves as a nucleation site.
O‐Ca2+ + CO32-   à    CaCO3↓ (precipitate)                                                              (2.3.2)
Furthermore, a study by [3]Tittelboom et al 2010 observed certain bacteria of the specie Bacillus Cohnii, Bacillus Pseudofirmus and Bacillus Alkalinitrilicus can induce precipitation of CaCO3 by oxidation of organic compounds such as acetate and lactate. Studies are still going on for the screening of bacteria with high metabolic ability for generation of CaCO3 with high dependence on naturally available resources.
Studies are going on to find suitable entrapment methods/materials for identifying the bacteria with high viability and metabolic activity in concrete structures. Spores are metabolically inactive highly resistant forms of bacteria. These particular types remain trapped in structures and activate only under favourable conditions like cracks leading to seepage of water, air and other compounds. The metabolic activities lead to sealing of cracks preventing further ingress of water. In order to understand the contribution of bacteria in self-healing, artificial cracks of variable width are generated in labs by various approaches. Commonly used bacteria species are the ones that can survive at high pH.
A table showing a list bacteria species studied on self-healing concrete was published on the [12] International conference on advances in construction material and system, Vol 2 – 2018 revealed the following:
Table 2.

Bacteria Specimen Crack Dimension Crack development method Healing (width in mm)
B.Spaericus Cubes 10mm depth
0.3mm width
Split tensile test 1mm
B. Alkaliniticilus Cylinders 1mm width Zinc plated steel bar stretched by tensile loading 0.46mm
B. Pseudofirmus Cubes NA NA NA
B. Pasteris Cubes 2mm width 3-point bending 0.73 mm
B. Subtilis Cubes NA NA NA

2.4 Importance of the study

In the European construction industry, the right choice of building materials is an important factor in achieving sustainable development. The European Union promotes actions aimed at sustainable development. The priority is to reduce the consumption of energy and natural resources as well as to reduce the production of waste and pollution that may be caused by the transport of materials. Principles of sustainable development are being introduced for the entire life cycle of buildings. This may ensure a compromise between economic, as well as environmental and social performance. All the building designs that are being implemented should be functional with regard to increasing the durability, technical and materials performance, and to reducing the life cycle cost of the building. Sustainable building materials are such materials that:

  • Reduce the consumption of resources.
  • Minimise the impact on the environment.
  • Do not pose a threat to human health.

The building materials should be investigated because they play an important role from the moment of conceiving the concept of constructing a building until the end of the building when it is to be dismantled, so that the materials might be recycled. Planners and architects, as well as engineers and builders, are searching for new materials and technologies to be used in new or future structures which will bring benefits such as energy efficiency, water resources and protection, improved air quality indoors, reduced life cycle costs and durability. In order to achieve these effects, it is important to apply the latest developments to various technologies, including the development of material studies and environmentally friendly building materials, and to achieve energy efficiency during the production of such materials. Furthermore, the inclusion of sustainable building materials in construction projects will reduce the environmental impact of building materials. The impact associated with the mining, transporting, processing, manufacturing, as well as installing, reusing and disposing.

2.5 Tests and observations

Researchers conducted various tests to prove the enhanced functionality of bacterial concrete as compared to that of conventional concrete. Tests such as split tensile strength, compression test, gas permeability, water permeability and stiffness test were conducted on bacterial induced concrete as well as conventional concrete.
Promising results have been observed in the tests involving various types of bacteria species. 90% of researchers proved that the addition of bacterial concrete not only has an advantage of durability but also its strength had increased.
Compressive strength of the concrete is the capacity of the structure to resist the load acting on them. A study by [13] Jonkers et al 2011 showed by the adding of bacteria to the concrete it improves the compressive strength of concrete as compared to conventional concrete. The compressive strength of concrete was improved by 14.92% by adding Bacillus Sphaericus as compared to the conventional concrete. Also, another study by [14] Renee Mors et al found that B. Pasteuri improved the compressive strength of concrete by 30.76% in 3 days, 46.15% in 7 days and 32.21% in 28 as compared to conventional concrete.

  1. Jonkers also studied the tensile strength behaviour of bacterial concrete using Bacillus Sphaericus and the results showed an average of 15% increase in tensile strength compared to conventional concrete. The tensile strength is a vital characteristic when it comes to reinforced concrete. The tensile forces are taken by reinforcement bars since concrete is strong in compression and weak in tension.

Water permeability is another important factor to be considered in self-healing concrete. [15] Tittelboom et al 2011, while studying self-healing concrete using silica gel infused bacteria, observed the water permeability behaviour through experiment by creating artificial cracks in concrete cylinders. Once cracked, the cylinders were immersed in a curing chamber for 3 days and later dried and prepared for the water permeability test.
The whole setup was kept watertight so that the specimen remains in a saturated state throughout the process of measurement. The time for water level to decrease from h0 to hf  was recorded and with the help of Darcy’s equation, the coefficient of permeability of the specimen was calculated using the following equation:
K=at/ATln( h0 / hf)                                                                                                         (2.5.1)
Where k coefficient of water permeability (m/s), a is the cross-section area of the glass tube (m2), A is the cross-section area of the cylinder (m2), T is the thickness of the cylinder (m); t is the time of water falling from h0 to hf (s), h0 and hf are the initial and final water levels (cm).
After the experiment, the results showed that the bacterial concrete had limited water permeability and crack width was decreased from 0.5mm to 0.15mm.

CHAPTER 3: MATERIALS AND METHODOLOGY

3.1 Materials

Throughout the experiment process of the study, different materials will be used which include apparatus, chemicals, solvents, laboratory testing equipment etc.
Some of the material is readily available naturally whereas some will be available in the laboratory and some will have to be procured externally to facilitate the study.
A brief detail of the materials which will be used is described below.

 3.1.1 Apparatus

The proposed apparatus in this study are listed down as follows:

  1. Culture tubes
  2. Autoclave

An autoclave is a machine that provides a physical method of sterilization by killing bacteria, viruses, and even spores present in the material put inside of the vessel using steam under pressure.
Autoclave sterilizes the materials by heating them up to a particular temperature for a specific period of time.
In this study, the autoclave is used to sterilize the nutrient broth mixture before adding the bacterial culture into it.
Fig 3.4: Autoclave

  1. Petri dish

A Petri dish or a cell-culture dish is a shallow transparent lidded dish that biologists use to hold growth medium in which cells can be cultured.
In this study, cells of bacterial specie will be culture and grown in the petri dish.

Fig 3.5: Petri dish / Cell-culture dish

  1. Incubator

An incubator is a device used to grow and maintain microbiological cultures or cell cultures. The incubator maintains optimal temperature, humidity and other conditions such as the CO2 and O2 content of the atmosphere inside. Incubators are essential for much experimental work in cell biology, microbiology and molecular biology and are used to culture both bacterial and eukaryotic cells.
In this study, the nutrient broth mixture once sterilized and mixed with bacterial culture will be placed inside the incubator for 24 hours to grow in a controlled environment.
                                      Fig 3.6: Incubator

3.1.2 Specimens

The specimens, chemicals and others ingredients which will be used for the study are:

  1. Nutrient broth

Nutrient Broth is a basic media composed of a simple peptone and a beef extract. Peptone contributes organic nitrogen in the form of amino acids and long-chained fatty acids. Beef Extract provides additional vitamins, carbohydrates, salts and other organic nitrogen compounds.
[16] To prepare the Nutrient broth solution, dissolve 8 grams in 800-900ml of distilled H2O stirring gently with heating until completely solubilized. Adjust pH of the medium to the desired level. Add additional water to bring the solution to 1L. Dispense into appropriate containers, loosen caps and autoclave for 15 minutes at 121ºC (15psi).

  1. Bacterial specimen.

[17] Bacillus subtilis known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants and humans. A member of the genus Bacillus, B. subtilis is rod-shaped, and can form a tough, protective endospore, allowing it to tolerate extreme environmental conditions. B. subtilis has historically been classified as an obligate aerobe. This particular specie is considered the best studied Gram-positive bacterium and a model organism to study bacterial chromosome replication and cell differentiation. It is naturally available and it is also being used at an industrial level therefore also proven to be economical.
Fig 3.7: Microscopic image of Bacillus subtilis

  1. Sodium Alginate and Calcium Chloride solution.

Sodium alginate and calcium chloride mixture will be used to make the bacterial culture in gel beads.
The mixture allows the bacterial culture to remain in a capsulated gel like bead in which the culture is liquid from the inside and gelled from the outside.

  1. Silica Gel

Silica gel is used as a desiccant in the concrete mix. Its main purpose is to expedite the process of self-healing at an early stage since it has the ability to absorb moisture and change its property into liquid form immediately.

3.2 Concrete mix design

The concrete mix design which will be used for the study is prepared at Nyanza Road Works laboratory in Dar es Salaam by myself under the guidance of Chief Materials Engineer Mr. Zinabu Kebede.
The following components was used to establish the mix design:

  1. Cement grade 42.5 N – Twiga OPC cement
  2. Coarse aggregate – Obtained from Msata Nyanza Quarry
  • Fine aggregate – Obtained from Bagamoyo river
  1. Potable water – Natural ground water

Table 2.3: Mix design proportions for C25 concrete
[18][19][20] The mix design was prepared as per BS absolute volume method (BS EN 206-1 and its complementary standards BS 8500 parts 1 & 2)

3.3 Bacterial Ingestion

The bacterial specimen obtained from natural fertile soil will be converted into two forms in order to be ingested into the concrete while mixing.

  1. In solution form

Bacterial mix at 10% and 20% diluted in 1000ml of water will be??? prepared and added directly into the mix while casting the concrete cubes and cylinders.
2. In beads form
The bacterial solution will be??? alginated in an agar mix and formed into beads.
The beads were again added in proportions of 10% and 20% into the mix while casting the concrete cubes and cylinders

3.4 Methodology

3.4.1 Obtaining the Bacterial specie

The bacterial specie used in the study is Bacillus subtilis. This particular specie is found on top layers of soil. The top layers of vegetative soil are proven to be an ideal habitat of B. subtilis. A location which meets the habituative requirements of the specie will be identified in the vicinity and a handful of fertile soil will be sampled in sampling bags and taken to laboratory for further processing.

3.4.2 Growth of bacterial culture in the laboratory

Nutrient broth powder which is a mixture of peptone and beef extract is obtained from local manufacturers.
[16] Dissolve 8 grams in 800-900ml of distilled water stirring gently with heating until completely dissolved. Add additional water to bring the solution to1000mL. Dispense into appropriate containers, loosen caps and autoclave for 15 minutes at 121ºC (15psi).
Once the nutrient broth solution is obtained, a handful of bacterial culture is added into it and incubated for 24 hours at 37ºC. The bacterial solution is therefore prepared.
Similarly, a number of such bacterial solutions are prepared and kept aside.

3.4.3 Preparing bacteria in beads form

1000mL of 5% sodium alginate and 2000mL of 6% calcium chloride is prepared and autoclaved at 121ºC for 20 mins. The calcium chloride solution is refrigerated at 10ºC.
1000mL of bacterial solution is added to 1000mL of sodium alginate solution and mixed well in a conical flask. Using a pipette, add the sodium alginate-bacterial solution drop by drop inside the chilled calcium chloride solution and beads will be formed. Refrigerate the end product for 12 hours and remove any excess calcium chloride solution.

3.4.4 Casting Concrete Cubes and Cylinders

As per the concrete mix design prepared, test and control specimens will be casted in form of cubes and cylinders. Cube moulds of 15cm X 15cm X 15cm and cylinder moulds of diameter 15cm and height 30cm will be used for the study.
Bacterial solution and beads will be added to the test specimens in different proportions pertaining to the required % in each sample.
The number of test and control specimens which will be casted during the study has been tabulated below. A total of 90 cubes and cylinders will be casted throughout the study.
Table 3.Concrete cubes and cylinders to be casted for testing

Test Bacteria % Form Mould type No of Moulds Test day
CST/Crack width 0 NA Cube 3 7
CST/Crack width 0 NA Cube 3 14
CST/Crack width 0 NA Cube 3 28
STS/water permeability 0 NA Cylinder 3 7
STS/water permeability 0 NA Cylinder 3 14
STS/water permeability 0 NA Cylinder 3 28
CST/Crack width 10 Solution Cube 3 7
CST/Crack width 10 Solution Cube 3 14
CST/Crack width 10 Solution Cube 3 28
STS/water permeability 10 Solution Cylinder 3 7
STS/water permeability 10 Solution Cylinder 3 14
STS/water permeability 10 Solution Cylinder 3 28
CST/Crack width 10 Beads Cube 3 7
CST/Crack width 10 Beads Cube 3 14
CST/Crack width 10 Beads Cube 3 28
STS/water permeability 10 Beads Cylinder 3 7
STS/water permeability 10 Beads Cylinder 3 14
STS/water permeability 10 Beads Cylinder 3 28
CST/Crack width 20 Solution Cube 3 7
CST/Crack width 20 Solution Cube 3 14
CST/Crack width 20 Solution Cube 3 28
STS/water permeability 20 Solution Cylinder 3 7
STS/water permeability 20 Solution Cylinder 3 14
STS/water permeability 20 Solution Cylinder 3 28
CST/Crack width 20 Beads Cube 3 7
CST/Crack width 20 Beads Cube 3 14
CST/Crack width 20 Beads Cube 3 28
STS/water permeability 20 Beads Cylinder 3 7
STS/water permeability 20 Beads Cylinder 3 14
STS/water permeability 20 Beads Cylinder 3 28

3.5 Mechanism

Once the concrete cubes have been casted, the following mechanism takes place.
Fig 3.
The bacteria in the concrete initially is in a dormant state. Once cracks are formed in the concrete, exposure to atmospheric moisture activates the dormant bacteria.
Ingress of water or other chemicals activates the bacteria leading to the formation of spores. Dense layers of calcium carbonate are produced by bacterial conversion of an incorporated mineral precursor compound (Calcium lactate) where the bacteria only act as a catalyst. The favouring reaction is as follows:

3.6 Concrete Tests

The following tests will be performed on the concrete cubes and cylinders to determine the effects of adding bacteria in the concrete mix.
All tests will be performed at the Civil engineering laboratory at the University of Dar es Salaam Main campus.

  1. Compressive cube strength test.

The concrete cubes casted will be tested for compressive strength after 7 and 28 days respectively. The test will be conducted as per [21] BS 1881: Part 116: 1983.
A compressive testing machine will be used to perform the test.
The concrete cubes will be marked for this study as follows:

Concrete cube identification no.
Date of casting
Age of Specimen
Type and % Bacteria added
Date of crushing
  1. Split tensile strength test

The tensile strength of concrete is one of the basic and important properties which greatly affect the extent and size of cracking in structures.
Concrete cylinders will be tested for its tensile strength indirectly using the split tensile strength test method as per [22] BS 1881: Part 117: 1983.
The apparatus used for the method is split cylinder testing machine.
The cylinder samples will be marked for testing as follows:

Cylinder identification no.
Date of Casting
Age of Specimen
Type and % Bacteria added
Date of Testing
  1. Water permeability test/ Rapid chloride penetration test

Rapid Chloride Permeability Test Equipment (RCPT) is used to evaluate the resistance of a concrete sample to the penetration of chloride ions. Method used will be based on ASTM C 1202 – 97 / AASHTO T 277-831.
The test is performed by placing a 100 mm diameter concrete cylinder into the sample cells that contain 3.0 % salt solution NaCl and 0.3 N sodium hydroxide solution. A voltage of 60 V DC is maintained across the ends of the sample throughout the test and the charge that passes through the sample is recorded. Based on the charge, a qualitative rating can be made of concrete’s permeability.
Deleterious materials like water, CO2, SO2 & Cl which permeates through the pores of the concrete and reacts with the reinforcement forms rust which increases the volume of the reinforcement and damages the structure.
The specimens will be marked for testing as follows:

Cylinder identification no.
Date of casting
Age of Specimen
Type and % Bacteria added
Date of testing
  1. Crack Width

In this particular test, concrete cubes of control specimens and test specimens will be subjected to minor surface cracking after 7 days of curing using external force.
The samples will then be placed in a curing tank for another 14 days and later removed and kept under normal atmospheric conditions.
In intervals of 7, 14 and 28 days, the cracks will be monitored and recorded for any changes among the test samples.
The main aim of this experimental observation is to determine extent of self-healing in the test specimens with different bacterial compositions.
The aim of the above exercise is listed below:

  1. To observe the behaviour of self-healing concrete vs conventional concrete by observing cracks and crack widt
  2. To observe the strength characteristic of bacterial concrete vs conventional concrete at 7 days and 28 days of casting.
  3. To perform permeability tests on the test specimens and determine water permeability in self-healing concrete and conventional concrete.
  4. To observe healing of cracks in the test specimens,
  5. Long term effects such as CO2 emissions and green concrete.

WORKFLOW

The workflow will include:

  • Procurements – obtaining all necessary materials, equipment and resources needed for the study
  • Growth of bacteria culture and preparing the bacterial solution and beads.
  • Casting of concrete cubes and cylinders in the laboratory.
  • Performing the necessary tests on the specimens.
  • Analysing and reporting the results obtained.
  • Documentation and presentation of thesis report.

Table 3.2: Gant Chart showing the entire working schedule

APPROX TIMELINE
MAY 2021 JUNE 2021 JULY 2021
W1 W2 W3 W4 W1 W2 W3 W4 W1 W2 W3 W4
Procurement of materials
Growth of Bacteria
Casting of cubes and Cylinders
Observations and conducting tests
Analysis and interpretation of results
Documentation and presentation of thesis report

BUDGET

S/n Item Unit Rate (TZS) Quantity Amount (TZS)
1 Nutrient Broth Powder Grams 50 1,000 50,000
2 Sodium Alginate Litres 5,000 5 25,000
3 Calcium Chloride Litres 5,000 5 25,000
4 Calcium lactate Grams 50 500 25,000
5 Silica Gel Grams 50 500 25,000
6 Cement Bags 15,500 10 155,000
7 Coarse aggregate Cu.m 85,000 1 85,000
8 Fine Aggregate Cu.m 23,000 1 25,000
Total 415,000

Table 3.3: Approximate quantities and cost for materials

S/n Item Unit Rate (TZS) Quantity Amount (TZS)
1 Vehicle Days 40,000 10 400,000
2 Driver Days 12,000 10 120,000
3 Lab Technicians Days 17,000 5 85,000
4 Labourers – 2 No’s Days 8,000 10 80,000
Total 685,000

Table 3.4: Approximate cost of equipment and manpower

S/n Item Unit Rate (TZS) Quantity Amount (TZS)
1 Compressive Strength No.s 5,500 20 110,000
2 Split tensile test No.s 5,500 20 110,000
3 Water permeability test No.s 10,000 20 200,000
Total 420,000

Table 3.5: Approximate costs for laboratory tests
Sub-Total approximate cost = TZS 1,520,000
Add 10% contingency cost = TZS 152,000
Grand total = 1,672,000 TZS (78,000 KES)                                      (1 KES = 21.5 TZS)

REFERENCES

[1] Alhalabi Z.Sh, Dopudja D. Self-healing concrete definition, mechanism and application in different types of construction – International Research Journal ▪ No. 05 (59) ▪ Part 1 ▪ May 2020.
[2] Wei Wang, Tieyi Zhong, Xiaoxue Wang and Zhenyu He. Research Status of Self-healing Concrete 2018 International Conference on Civil, Architecture and Disaster Prevention, Beijing, China.
[3] Tittelboom, K. V., Belie, N. D., Muynck, W. D., & Verstraete, W. (2010). Use of bacteria to repair cracks in concrete. Cement & Concrete Research, 40(1), 157-166.
[4] Penghui Li, Nan Dong, Fengqi Chen. (2009) Experimental study on self-healing properties of XYPEX blending agent: Seminar on anti-seepage and anti-freeze technology of irrigation districts and hydraulic structures nationwide. Dalian, Liaoning, China.
[5] Wiktor, V., & Jonkers, H. M. (2011). Quantification of crack-healing in novel bacteria-based self-healing concrete. Cement & Concrete Composites, 33(7), 763-770.
[6] Jonkers, H. M., & Schlangen, E. (2009). A two component bacteria-based self-healing concrete. Ecological Engineering, 35(6), 215-220
[7] Jonkers, H. M., Thijssen, A., Muyzer, G., Copuroglu, O., & Schlangen, E. (2010). Application of bacteria as self-healing agent for the development of sustainable concrete. Ecological Engineering, 36(2), 230-235.
[8] Rafat Siddique, Navneet Kaur Chahal, (2011) “Effect of ureolytic bacteria on concrete properties”, Construction and Building Materials 25 (2011) 3791–3801.
[9] Micro-capsules and bacteria to be used in self-healing concrete // University of Baath [Electronic resource] URL:http://www.bath.ac.uk/news/2014/12/03/micro-capsules-and-bacteria-to-be-used-in-self-healing-concrete/ – 2014.
[10] Abo-El-Enein, Ali, Fatma Talkhan, Abdel-Gawwad, “Application of microbial biocementation to improve the physico-mechanica properties of cement mortar”, Housing and Building National Research Center (2013).
[11] Potential application of Bacteria to improve the strength of cement concrete. C. C. Gavimath*, B. M. Mali1, V. R. Hooli2, J. D. Mallpur3, A. B. Patil4, D. P. Gaddi5, C.R.Ternikar6 and B.E.ravishankera7.
[12] International conference on advances in construction material and system, Vol 2 – 2018
[13] Jonkers, H., ‘Bacteria-based self-healing concrete’, HERON 56 (1) (2011) 1-12.
[14] Renee Mors, H Jonkers (2007) Bacteria based self-healing concrete and introduction.
[15] Use of silica gel or polyurethane immobilized bacteria for self-healing concrete Jianyun Wanga, b, Kim Van Tittelboom a, Nele De Belie a, Willy Verstraete b, available on (14 July 2011)
[16] https://www.neogen.com/solutions/microbiology/nutrient-broth/
[17] https://en.wikipedia.org/wiki/Bacillus_subtilis
[18] BS EN 206:2000 Concrete. Specification, performance, production and conformity
[19] BS 8500 Part 1 Complementary British Standard to BS EN 206. Method of specifying and guidance for the specifier.
[20] BS 8500 Part 2 Complementary British Standard to BS EN 206. Specification for constituent materials and concrete.
[21] BS 1881: Part 116: 1983, “Method for Determination of Compressive Strength of Concrete Cubes,” British Standard Institution.
[22] BS 1881: Part 117: 1983, “Method for Determination of Tensile Splitting Strength,” British Standard Institution.
[23] ASTM C 1202 – 97 / AASHTO T 277-831 Method: Rapid chloride permeability test- American Society of testing and materials.
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