Self Healing Concrete: A Biological Approach
Henk M Jonkers
Delft University of Technology, Faculty of Civil Engineering and GeoSciences/Microlab, Stevinweg 1,
The Netherlands
E-mail: h.m.jonkers@tudelft.nl
1
An Introduction to Concrete
Concrete can be considered as a kind of artificial rock with properties more or less
similar to certain natural rocks. As it is strong, durable, and relatively cheap, concrete
is, since almost two centuries, the most used construction material worldwide, which
can easily be recognized as it has changed the physiognomy of rural areas. However,
due to the heterogeneity of the composition of its principle components, cement,
water, and a variety of aggregates, the properties of the final product can widely
vary. The structural designer therefore must previously establish which properties are
important for a specific application and must choose the correct composition of the
concrete ingredients in order to ensure that the final product applies to the previously
set standards. Concrete is typically characterized by a high-compressive strength, but
unfortunately also by a rather low-tensile strength. However, through the application
of steel or other material reinforcements, the latter can be compensated for as such
reinforcements can take over tensile forces.
Modern concrete is based on Portland cement, a hydraulic cement patented by
Joseph Aspdin in the early 19th century. Already in Roman times hydraulic cements,
made from burned limestone and volcanic earth, slowly replaced the widely used
non-hydraulic cements, which were based on burned limestone as main ingredient.
When limestone is burned (or “calcined”) at a temperature between 800 and 900◦ C,
a process that drives off bound carbon dioxide (CO2 ), lime (calcium oxide; CaO)
is produced. Lime, when brought into contact with water, reacts to form portlandite
(Ca(OH)2 ) which can further react with CO2 , which in turn forms back into calcite
(CaCO3 ), or limestone, the pre-burning starting material. However, a major drawback of this non-hydraulic cement is that it will not set under water and, moreover,
its reaction products portlandite and limestone are relatively soluble, and thus will
deteriorate rapidly in wet and/or acidic environments. In contrast, portland cement
produces, upon reaction with water, a much harder and insoluble material that will
also set under water. For portland cement production a source of calcium, silicon, aluminum, and iron is needed and therefore usually limestone, clay, some bauxite, and
iron ore are burned in a kiln at temperatures up to 1, 500◦ C. The cement clinker produced is mainly composed of the minerals alite (3CaO.SiO2 ), belite (2CaO.SiO2 ),
aluminate (3CaO.Al2 O3 ), and ferrite (4CaO.Al2 O3 .Fe2 O3 ), which all yield specific hydration products with different characteristics upon reaction with water.
S. van der Zwaag (ed.), Self Healing Materials. An Alternative Approach to 20 Centuries
of Materials Science, 195–204.
c 2007 Springer.
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The contribution of these clinker minerals to the composition of general-purpose
portland cement in weight percentage is typically 50%, 24%, 11%, and 8% respectively. Important characteristics of clinker minerals are reaction rate and contribution
to final strength of the product. For example, of the two calcium silicates, alite is
the most reactive and contributes to early strength, while the slower-reacting belite
contributes more to longer-term strength. Aluminate contributes to early strength as
its hydration reaction is fast but it also generates much heat. The final properties of
cement-based materials can thus vary widely as they strongly depend on the mineral
composition of the cement used and therefore, different types of cement, each suitable for specific applications, are produced. Quantitatively most important hydration
product of general-purpose portland cement is calcium silicate hydrate (C–S–H), an
amorphous mineral somewhat resembling the natural mineral tobermorite. A secondary reaction product is calcium hydroxide (portlandite), which together with the
very soluble sodium and potasium oxides (Na2 O and K2 O) also present in portland
cement, contribute to the high alkalinity of the concrete’s pore fluid (pH ≈ 13). The
high matrix pH is important in structural concrete as it protects the embedded steel
reinforcement from corrosion. The protective oxidized thin layer of Fe3+ oxides
and oxyhydroxides on the reinforcement steel (the passivation film) rapidly degrade
when the matrix pH drops below 9, leading to further oxidation and deterioration of
the concrete structure due to expansion reactions and loss of strength. Corrosion of
the steel reinforcement is in fact one of the major causes limiting the durability, or
lifetime, of concrete structures. For further and more detailed information on general
concrete properties the reader is referred to Reinhardt (1985) and Neville (1996).
2
Concrete Durability, Deterioration, and Self Healing
Properties
A variety of additives or replacements of cement can be applied in order to improve
the durability of the final concrete product. Also certain industrial waste or recycled
materials can be used to improve the sustainability, or environmental friendliness,
of concrete and some even improve certain properties. The production of cement
is high-energy consuming as raw materials are burned at 1, 500◦ C, a process that
contributes to a significant amount of atmospheric CO2 release worldwide. Thus, for
both economical and environmental reasons, cement production and use should be
minimized. Examples of industrial waste products, which can partly replace and even
improve cement properties, are fly ash, blast furnace slag, and silica fume. Fly ash, a
waste product from coal-burning power plants, is a source of reactive silica and can
substitute 35–75% of cement in the concrete mix. Application of fly ash increases
concrete strength as it reduces the required water/cement ratio and also improves
resistance against chemical attack as it decreases the matrix permeability. Similarly,
silica fume from the silicon industry and blast furnace slag from steel industries can
partially replace cement in the concrete mix, as these are sources of reactive silica and
Self Healing Concrete: A Biological Approach
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both reactive silica and calcium respectively. Other commonly applied additives that
improve or change certain concrete characteristics needed for specific applications
are air-entraining agents to improve freeze/thaw resistance, setting or retarding agents
and plasticizers to enable a lower water/cement ratio to increase concrete strength.
A number of processes negatively affect the durability and result in the unwanted
early deterioration of concrete structures. One major cause that initiates various
mechanisms of concrete deterioration is the process of cracking what dramatically
increases the permeability of concrete. The microstructure of hardened cement paste
is porous as it contains isolated as well as interconnected pores. Specifically the
connected pores determine permeability, as these allow water and chemicals to enter
the concrete matrix. As cracking links both isolated and connected pore systems,
this results in a substantially increased permeability. In most concrete-deterioration
mechanisms permeability plays a major role. Intrusion of sulfate ions into the matrix
may result in ettringite formation, a conversion reaction in which a high-density
phase is transformed into a low-density phase, causing expansion and further cracking of the material. Chloride ions penetrating the matrix through the connected pore
system will destabilize the passivation film of the steel reinforcement and by doing
so accelerate further corrosion. Similarly, in a process called carbonation, CO2 diffusing through the pore system will react with alkaline pore fluid components such
as Ca(OH)2 which will result in a lowering of matrix pH and again depassivation
of the protective film on the steel reinforcement. These examples make clear that
cracking of concrete should be minimized and that a potential healing mechanism
should ideally result in the sealing or plugging of newly formed cracks in order
to minimize increases in matrix permeability. An active self healing mechanism in
concrete should be ideal as it does not need labor-intensive manual checking and
repair what would save an enormous amount of money.
A self healing mechanism or self healing agent in concrete should comply ideally
with all, or at least with some, of the following characteristics:
1. Should be able to seal or plug freshly formed cracks to reduce matrix permeability
2. Must be incorporated in the concrete matrix and able to act autonomously to be
truly “self-healing”
3. Must be compatible with concrete, i.e. its presence should not negatively affect
material characteristics
4. Should have a long-term potential activity, as concrete structures are build to last
typically for at least 50 years
5. Should preferably act as a catalyst and not be consumed in the process to enable
multiple healing events
6. Must not be too expensive to keep the material economically competitive
Different types of potential self healing mechanisms or agents for autonomous concrete repair can be thought of. One series of mechanisms could involve the secondary
formation of minerals which are compatible with the material matrix, i.e. will not
negatively affect but rather increase concrete durability by sealing freshly formed
cracks and so decrease matrix permeability. A chemical agent such as the inclusion
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of still nonreacted cement particles in the concrete matrix is feasible as it complies
with at least some of the listed self healing properties. Besides this, other agents could
work equally well or can contribute to the self healing property of concrete in concert
with the previous one. Next to chemicals one could think of an agent of biological
origin, and in the next part the possible application of bacteria as healing agent will
be considered.
3
The Self Healing Mechanism of Bacterial Concrete
Do bacteria exist which could potentially act as a self healing agent in concrete, and
if so, what would be the healing mechanism? From a microbiological viewpoint the
application of bacteria in concrete, or concrete as a habitat for specialized bacteria,
is not odd at all. Although the concrete matrix may seem at first inhospitable for life,
as it is a very dry and extremely alkaline environment, comparable natural systems
occur in which bacteria thrive. Inside rocks, even at a depth of more than 1 km within
the earth crust, in deserts as well as in ultra-basic environments, active bacteria are
found (Jorgensen and D’Hondt 2006; Fajardo-Cavazos and Nicholson 2006; Dorn
and Oberlander 1981; DelaTorre et al. 2003; Pedersen et al. 2004; Sleep et al. 2004).
These desiccation- and/or alkali-resistant bacteria typically form spores, which are
specialized cells able to resist high mechanically and chemically induced stresses
(Sagripanti and Bonifacino 1996). A low-metabolic activity and extremely long lifetimes also characterize spores, and some species are known to produce spores which
are viable for up to 200 years (Schlegel 1993).
In a number of recent studies the potential for application of bacteria in concrete
technology was recognized and reported on, e.g. for cleaning of concrete surfaces
(DeGraef et al. 2005) as well as for the improvement of mortar compressive strength
(Ghosh et al. [53]). Moreover, bacterial treatment of degraded limestone, ornamental stone, and concrete structures for durability improvement has been the specific
topic of a number of recent studies (Bang et al. 2001; Ramachandran et al. 2001;
Rodriguez-Navarro et al. 2003; De Muynck et al. 2005; Dick et al. 2006). Due to
bacterially controlled precipitation of dense calcium carbonate layers, crack-sealing,
as well as significant decreases in permeability of concrete surfaces were observed
in these studies. In these remediation and repair studies the bacteria and compounds
needed for mineral precipitation were brought into contact with the structures’ surface after setting or crack formation had occurred, and were not initially integrated
as healing agents in the material’s matrix. The mechanism of bacterially mediated
calcite precipitation in those studies was primarily based on the enzymatic hydrolysis of urea. In this urease-mediated process the reaction of urea (CO(NH2 )2 ) and
water yields CO2 and ammonia (NH3 ). Due to the high pK value of the NH3 /NH+
4
system (about 9.2) the reaction results in a pH increase and concomitant shift in the
−
carbonate equilibrium (CO2 to HCO−
3 and CO32 ) which results in the precipitation of calcium carbonate (CaCO3 ) when sufficient calcium ions (Ca2+ ) are present.
Self Healing Concrete: A Biological Approach
199
Fig. 1 Scenario of crack-healing by concrete-immobilized bacteria. Bacteria on fresh crack surfaces
become activated due to water ingression, start to multiply and precipitate minerals such as calcite
(CaCO3 ), which eventually seal the crack and protect the steel reinforcement from further external
chemical attack
Whether this system can be integrated in the concrete matrix in order to act as truly
self healing agent or that its application is tailored for remediation purposes remains
to be investigated as both enzyme (urease) activity and substrate (urea) may not have
a long-term (years) stability when immobilized in the concrete matrix. One drawback of the urease-based system from an environmental viewpoint is the excessive
(equivalent) production of ammonium to carbonate ions. Thus ideally, for bacterially
based self healing concrete, both the bacteria and environmental friendly biomineral
precursor compounds should be embedded in the material. The immobilized bacteria should immediately start to precipitate minerals and seal cracks upon concrete
cracking and water entrance. Such a scenario is schematically depicted in Figure 1.
4
Experimental Evidence for Bacterially Controlled Self Healing
in Concrete
In a number of published studies the potential of calcite precipitating bacteria for
concrete or limestone surface remediation or durability improvement was investigated (Bang et al. 2001; Ramachandran et al. 2001; Rodriguez-Navarro et al. 2003;
De Muynck et al. 2005; Dick et al. 2006). However, as the bacteria and mineral
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H.M. Jonkers
precursor compounds were not initially part of the material matrix but rather externally applied, the remediation mechanism in those studies cannot be truly defined as
self healing. Therefore, in order to investigate the potential of autonomous bacterially
mediated self healing in concrete, a series of experiments were performed. Firstly, a
number of potentially suitable bacterial species were selected. Four species of alkalitolerant (alkaliphilic) spore-forming bacteria of the genus Bacillus were obtained
from the German Collection of Microorganisms and Cell Cultures (DSMZ), Braunschweig, Germany. These bacteria were cultivated and subsequently immobilized
in concrete and cement stone (cement plus water in a weight ratio of 2:1 without
aggregate addition) to test compatibility with concrete and bacterial mineral production potential respectively. As was listed above (paragraph 2), the ideal self healing
agent should comply with certain characteristics, and one of them is that its presence
should not negatively affect the material characteristics. To test this, a dense culture
of Sporosarcina pasteurii was washed twice in tap water and the number of bacteria
in the resulting cell suspension quantified by microscopic counting before addition
to the concrete mix make up water. Two parallel series of nine concrete bars (with
and without bacteria) of dimensions 16 × 4 × 4 cm were prepared and triplicate bars
of both series were subsequently tested for flexural tensile and compressive strength
after 3, 7, and 28 days curing. Table 1 shows the composition of the concrete mix and
Figure 2 depicts the strength development of both types of concrete in time.
The results of the concrete compatibility test show that the addition of bacteria to
a final concentration of 109 cm−3 does not affect strength characteristics. Moreover,
incubation of cement stone pieces in a medium to which yeast extract and peptone
(3 and 5 g L−1 respectively) was added as a bacterial food source revealed that on
the surface of bacteria-embedded specimen, but not on control specimen, copious
amounts of calcite-like crystals were formed (Figure 3). From the latter experiment
it can therefore be concluded that suitable bacteria, in this case alkali-resistant sporeforming bacteria, embedded in the concretes’ cement paste are able to produce minerals when an appropriate food source is available.
Table 1 Cement, water, and aggregate composition needed for the production of nine concrete bars
of dimensions 16 × 4 × 4 cm. The washed cell suspension used for bacterial concrete was part of
totally needed makeup water
Compound
Cement (ENCI CEMI 32.5)
Water
Aggregate Size Fraction (mm):
4–8
2–4
1–2
0.5–1
0.25–0.5
0.125–0.25
Weight (g)
1, 170
585
1, 685
1, 133
848
848
730
396
Self Healing Concrete: A Biological Approach
201
Fig. 2 Flexural tensile (a) and compressive (b) strength testing after 3, 7, and 28 days curing
revealed no significant difference between control and bacterial concrete. The latter contained
1.14 × 109 S. pasteurii cells per cubic centimeter of concrete
The mechanism of bacterial mineral production here is likely metabolically mediated. As the bacteria metabolize the available organic carbon sources (yeast extract
and peptone) under alkaline conditions, carbonate ions are produced which precipitate with access toward calcium ions present in the concrete matrix. The produced
carbonate ions can locally reach high concentrations at the bacterially active “hot
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Fig. 3 Cement stone samples, which were cured for 10 days and subsequently further incubated in
yeast extract- and peptone-containing medium. (A) Control (cement stone without added bacteria)
and (B) cement stone containing 109 cm−3 spores of B.pseudofirmus. The inset in Figure 2B (×5000
magnification) shows a close up of the massive calcite-like crystals formed on the specimen surface
Table 2 Flexural tensile- and compressive-strength characteristics of control and organic carbonamended concrete bars after a 28 days curing period
Type of Concrete
Control
Na-aspartate
Na-glutamate
Na-polyacrylate
Na-citrate
Na-gluconate
Na-ascorbate
Tensile Strength
(MPa)
7.78 ± 0.38
7.33 ± 0.37
7.16 ± 0.19
6.42 ± 0.47
3.48 ± 1.72
0
0
Compressive Strength
(MPa)
31.92 ± 1.98
33.69 ± 1.89
28.52 ± 3.56
20.53 ± 4.50
12.68 ± 1.82
0
0
spots” and here calcium carbonate (calcite) crystals form. For autonomous self healing, however, all compounds needed for the healing reaction should ideally be incorporated in the material matrix. As in the previous experiment bacterial food sources
were part of the medium but not of the concrete matrix, an additional experiment
was done to investigate the compatibility of concrete and various organic compounds.
The compounds chosen for this test, all suitable food sources for the applied bacteria,
were added to concrete in a concentration of 0.5% of cement weight (see Table 1 for
composition of the concrete mix). After 28 days curing, some compounds appeared
to be better compatible with concrete then others (Table 2).
No significant difference was found in flexural tensile and compressive strength
between control and amino acid (aspartic acid and glutamic acid)-containing concrete bars. Concrete to which polyacrylic acid and citric acid was added suffered significant strength loss, while gluconate- and ascorbic acid-amended concrete did not
develop any strength at all. Specific organic compounds such as amino acids appear
thus suitable candidates to act as self healing agent in concert with suitable bacteria.
Self Healing Concrete: A Biological Approach
5
203
Conclusions and Future Perspectives
Some previous studies reported on the successful application of bacteria for cleaning of concrete surfaces as well as concrete-, limestone-, and ornamental-stone crack
repair (DeGraef et al. 2005; Dick et al. 2006; Rodriguez-Navarro et al. 2003; Bang
et al. 2001; Ramachandran et al. 2001). As the bacteria in these studies were brought
into contact with the material only after damage had occurred, these examples cannot be considered as truly, autonomous, self healing mechanisms. The experiments
presented in this study, focused on the healing potential of concrete-immobilized
bacteria, i.e. bacteria that are part of the concrete matrix. The results of the experiments show that immobilized bacteria mediate the precipitation of minerals and,
moreover, the bacteria and certain classes of needed food sources do not negatively
affect concrete strength characteristics. It can therefore be concluded that bacterially
controlled crack-healing in concrete by mineral precipitation is potentially feasible.
The concept, however, needs further developments on some areas. It should still be
clarified whether bacterial mineral precipitation effectively seals cracks, i.e. significantly reduces the permeability of cracked concrete in order to protect the embedded reinforcement from corrosion and thus increases the durability of the material.
Furthermore, bacterial species must be selected which, when part of the concrete
matrix, remain viable for at least the expected lifetime of the construction. If so, the
bacterial approach can successfully compete with other (abiotic) self healing mechanisms as such bacteria comply with all the listed characteristics of the most ideal self
healing agent.
Acknowledgements Arjan Thijssen is acknowledged for help with ESEM analysis and for providing
ESEM photographs. Financial support from the Delft Centre for Materials (DCMat: www.dcmat.tudelft.nl)
for this work is also gratefully acknowledged
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