SELF HEALING CONCRETE USING BACTERIA
ABSTRACT
Concrete in most structures is designed to crack in order to let embedded steel reinforcement take over
tensile stresses. Crack formation is also a typical phenomenon related to durability. Percolated cracks may
lead to leakage problems or ingress of harmful materials, which can cause deterioration of the concrete
matrix or reinforcement corrosion. Durability can be enhanced by preventing further ingress of water and
other substances. Self-healing is characterized by regaining performance after a defect occurs. Damage
targeted in bacteria-based self-healing concrete particularly relates to increased durability and leakage
prevention and extending service life of concrete structures. This paper introduced a two-component healing
agent to be added to the concrete mixture, consisting of bacteria and a mineral precursor compound. Upon
cracking the system is activated by ingress water. Bacteria convert the mineral precursor compound into the
mineral calcium carbonate, better known as limestone. Precipitation of the limestone on the crack surface
enables sealing and plugging of the cracks, making the matrix less accessible to water and other deleterious
materials.
Cracks in concrete are unpreventable, and it is intrinsic weakness of concrete. Through these cracks, water
and others salts can seep. It initiates corrosion, further reducing the life of the concrete. So, there was a
requirement to develop an intrinsic bio-material, a self-repairing technique which can rectify the cracks and
fissures developed in concrete. Bio-concrete is a material which can successfully rectify cracks in concrete.
This technique is highly desirable because the activity of crack remediation is eco-friendly and natural. The
effect on compressive strength, water absorption and water permeability of cement concrete cubes due to the
mixing of bacteria. It also shows that there is reduction in water absorption and water permeability when
compared to conventional concrete. The bacteria which is going to be introduced in concrete, should have
the property of alkali-resistance and it also should form endospore, so that it can withstand the stresses
produced in concrete while mixing, transporting and placing. Keywords: Concrete crack-healing,
Permeability, Bacteria, Compressive strength.
Keywords: Concrete crack-healing, Permeability, Bacteria, Compressive strength.
INTRODUCTION
The aim of this research project is the development of durability of concrete in which integrated bacteria
promote self-healing of cracks. Traditional concrete does usually show some self-healing capacity what is
due to excess non-hydrated cement particles present in the material matrix. These particles can undergo
secondary hydration by crack ingress water resulting in formation of fresh hydration products which can seal
or heal smaller cracks. However, the integration of excess cement in concrete is unwanted from both an
economical and environmental viewpoint.
In this study durability of concrete is developed by self-healing system in which bacteria converts the
metabolic organic compounds to calcite. The result has been expected that the ingress water channelled
through freshly formed cracks activate present bacteria which through metabolic conversion of organic
mineral-precursor compounds produce copious amounts of calcite. The self-healing capacity of this system
is currently being quantified what should result in an estimate of the materials durability increase.
A self-healing concrete may be beneficial for both economic and environmental reasons. The bacteria-based
concrete proposed here could substantially reduce maintenance, repair and premature structure degradation
what not only saves money but also reduces atmospheric emissions considerably as less cement is needed
for this type of self-healing concrete.
Objectives of the Present Research Work
•
To develop self-healing property in concrete using bacteria.
•
Addition of bacteria and to the mix.
•
To test the self-healing property of concrete.
•
To test the strength characteristics of concrete.
MATERIALS
1. Cement
Cement is a building block fastener, a mixer of substance used underway that built ingadgets and
solidifies and may tie together different materials collectively after setting in order to provide
strength. For this study cement of OPC 53 Grade was used for testing. Cement concrete is one of the
most widely used construction material by mankind and it is the main material used for the
infrastructure development of every country. Cement is a binder, a substance that sets and hardens
independently, and can bind other materials together.
Cement is generally graded according to a few sets of specifications defined IS 650:1996. which
differentiate the various grades of cements. The most important factor considered for cement grading
is the average compressing strength of the material in a given particular amount of time over a
particular area in a particular composition of the cement mortar mixture. There was an increase in
compressive strength of the cement with the use of the bacteria Bacillus pasteurii. The ingredients
are mixed in the proportion of about two parts of calcareous materials to one part of argillaceous
materials and then crushed and ground in ball mills in a dry state. The dry powder or the wet slurry is
then burnt in a rotary kiln at a temperature between 14000 C to 15000 C. The clinker obtained from
the kiln is first cooled and then passed on to ball mills where gypsum is added and it is ground to the
requisite fineness according to the class of product.
2. Fine aggregate
The fine aggregate which were used in this study were free from clay and found to be hard and
durable. The fine aggregates are those which passes through a 4.75mm is sieve example like sand,
silica etc. normally sand in concrete matrix as fine aggregate. Depending upon the grains size and
particles size distribution of fine aggregates IS: 383 has divided the fine aggregate into four zones.
The fine aggregates which used should contains sharp, angular and coarse grains, chemically inert,
free from clay, organic matter and should be well graded. The fitness modulus is a number, which
gives the mean size of particles present in the taken sample of fine aggregates number, which gives
the mean size of particles present in the taken sample of the aggregates.
3. Coarse Aggregates
The coarse aggregate used in this present study have passed through 20mm IS-sieve and retaining on
12.5mm IS sieve. Example like namely crushed, uncrushed and partially crushed jelly is used as
coarse aggregates in concrete mix. Several investigators recommend that the maximum nominal size
of course aggregates are restricted to 10mm, as fibres also in effect, as aggregates.
4. Water
The water whose pH value more than 6 is considered, generally ordinary tap water supplied from
water supply network is considered, to be potable water. The water which is used for mixing and
curing of concrete should be clean and free from injurious such as amounts of acids, salts, organic
matter or other substances that may be deleterious to concrete or steel. Water is an important
ingredient of concrete in the chemical reaction with cement. It helps to form the strength giving
cement gel. In the present work a good quality of potable water is used throughout the study.
5. Bacteria
Bacteria are relatively simple, single celled organisms. In suspension state, concrete mix is added
with bacteria. Concrete being extremely alkaline in nature, the bacteria added should fit in some
special norms. The added bacteria should be able to withstand the harsh environmental conditions of
concrete. Concrete is a dry material and the pH value of cement and water when mixed is up to 13
which makes it confrontational as most of the organisms cannot survive in an environment having
pH value higher than 10.
6. Selection of Bacteria
The bacterial family Bacillaceae plays a vital role. There are various types of Bacillus can be used in
concrete in which B.subtilis, B.pasteurii, B.cohnii, B.licheniformisare widely used. As per the
availability in India the bacteria namely B.subtilis (MCC 2511) and B.cohnii (MCC 2819) are used.
This is formally known as Hay bacillus or grass bacillus and it is a gram positive. B.subtilis is a rod
shaped bacteria and forms a tough, protective endo-spore, allowing it to tolerate extreme
environmental conditions.
Compressive Strength of Self-Healing Concrete
It is well known that concrete is weak in tension and strong in compression. As a result, the compressive
strength of concrete is often considered the most important property of concrete, and is the most common
measure used to evaluate the quality of hardened concrete. The Compressive strength can be defined as “the
measured maximum resistance of a concrete specimen to axial loading”, and is determined by computing the
maximum stress that the specimen carries after being subjected to uniaxial compressive force.
One of the important properties of concrete is its strength in compression. The strength in compression has a
definite relationship with all the other properties of concrete, i.e., improvement in compressive strength,
hence the importance of the test. The height of the test piece in relation to its lateral dimensions greatly
influences the results. The more slender the test piece, lower will be crushing strength. The ratio of the
minimum dimension of the specimen to maximum size of aggregate size should be at least 4:1.
There are two types of standard test pieces cubes and cylinders. Of the various strengths of concrete, the
determination of compressive strength has received a large amount of attention because the concrete is
primarily meant to withstand compressive stress. Cubes and cylinders are the two types of compression test
specimens used to determine the compressive strength. The specimens are cast, cured and tested as per
standards prescribed for such tests. When cylinders are used, they have to be suitably capped before the test,
an operation not required when other types of specimens are tested.
The cubes were prepared for concrete mix of M30 grade with and without addition of microorganisms (B.
Subtilis). The size of the cubes was 100mm x 100mm x100mm. Cubes were tested for compressive strength
at 14 days and 28 days. The test being conducted using the compressive strength testing machine (CTM).
The material required for preparing are weighted according to proportion. The fineness and setting tests are
taken to check the quality of cement. Then, the uniform mix of coarse and fine aggregate is prepared and
mixed properly. Then, cement is added to the dry mixture and accordingly required quantity of water is
added with specific interval. The dose of bacteria is added as per no. of cubes. While adding bacteria the
measuring cylinder is first sterilized then dose is added in it and measured. The mixtures is prepared and
poured in cubes with proper vibrations and care. And cubes were kept for hardening and opened after 24
hours and kept for curing for corresponding days. The compressive strength taken on UTM and results are as
follows according to dose and days of curing.
Self-healing concrete should be able to heal or seal by filler material formation, freshly formed in cracks to
inhibit ingress of water and other chemicals which would cause preliminary degradation of the material
matrix or embedded reinforcement .In this study we investigated the crack repair ability of concrete in which
bacteria was incorporated as self-healing agent. The integrated bacterium applied in this study is Bacillus
subtilus, alkaliphilic species of genus bacillus. When this bacterium integrated with concrete enhances the
properties of concrete as stated. The compressive strength of concrete at 7 days, 14 days, 28 days for
optimum cell concentration of 10^5 cells per ml of mixing water were given in Table 1. It is observed that
with the addition of bacteria the compressive strength of concrete showed significant increase by 14.92% at
28 days. The Loss in Weight and Loss in Compressive Strength at different ages were given in Table 2.
With the addition of bacteria, it is observed that there is less percentage of loss in weight and compressive
strength.
M30 Design Check Test
Strength
(N/mm2) at 7
Days
Conventional
Concrete
AVG
30.33
32.30
33.19
36.95
Concrete + 5%
Bacteria
41.67
41.06
42.72
45.42
OBSERVATIONS AND CONCLUSIONS
The goal of this paper is to introduce bacteria-based self-healing concrete, currently being developed in our
laboratory. On the lab-scale a fully functional system exists. To the concrete mixture a healing agent is
added, consisting of two components immobilized in expanded clay particles. Due to bacterial activity a
calcium carbonate layer is deposited on the crack surface, sealing and blocking entrance to deteriorating
substances. Further research and development is needed in order to make the material ready for application
in practice. Since potential advantages are mainly anticipated in reduction of costs for maintenance and
repair and service life extension of concrete structures, the self-healing material needs to be cost efficient
and durable.
Conclusions
From the experiments reported herein, the following conclusions may be made:
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Bacillus Subtilis is proved to be safe and cost effective.
The compressive strength of concrete is maximum with the addition of Bacillus subtilis 10^5
cells/ml. So, bacteria with a 10^5 cells/ml is used in the investigation.
The addition of Bacillus subtilis bacteria improves the hydrated structure of cement mortar as
bacterial deposition and precipitation of calcite minerals in cement matrix which improves adhesive
properties by pore filling of bacteria itself.
The results showed that there is a significant improvement in the load carrying capacity of the
Bacterial Concrete, then compared to the Conventional concrete.
Bacterial Concrete has served its purpose by the enhancement of compressive strength.
Self-healing of concrete yielded by using natural method and is environmentally friendly.
Improving the lifespan of concrete structure which indirectly improvise the sustainability of
environment.
Effectiveness of crack repairing and regaining of lost strength is also depend on microorganism
embedded in concrete matrix.
Mechanical properties also improve by incorporating bacteria as long term compressive and tensile
strength increases depending on bacteria embedded.
Durability properties of concrete improve by incorporating bacteria as concrete perform better
against water permeability, chloride ingression and water absorption.
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