SELF-HEALING MATERIAL CONCEPTS AS SOLUTION FOR AGING

INFRASTRUCTURE

Klaas van Breugel, Delft University of Technology, The Netherlands  

37th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 29 - 31 August 2012,

Singapore

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37th Conference on Our World in Concrete & Structures

29-31 August 2012, Singapore

SELF-HEALING MATERIAL CONCEPTS AS SOLUTION FOR AGING INFRASTRUCTURE

Klaas van Breugel

Delft University of Technology Stevinweg 1, 2628 CN Delft, The Netherlands

e-mail: K.vanBreugel@tudelft.nl

Keywords: Aging, Self-healing, Infrastructure, Damage control, Bacterial concrete, Smart

materials, Nano particles, Stewardship

Abstract. The rapidly growing world population and booming economies are two of the major reasons for an increasing demand for buildings and infrastructure. In order to meet these needs large amounts of energy and raw materials are required. In most cases concrete is the main building material for these structures. The question today is how these needs can be accomplished without compromising the ability of future generations to meet their needs (Brundlandt). In this paper first the urgency of this question is explained from the perspective of the building industry. Emphasis is put on the consequences of the lack of quality and related failure costs. This lack of quality results in premature maintenance and repair or even decommissioning and demolishing of structures. But even good quality structures do suffer from aging of the materials from which these structures were built. Given this fact, it is considered a great challenge to design materials with an inherent potential to heal themselves once any kind of deterioration or aging starts. That would extend the service life of concrete structures and, hence, mitigate the pressure on the need of raw materials and energy for new built. But how realistic are self healing concepts? Are they reliable and affordable and is it possible to estimate the potentials savings by using self-healing materials? These are the questions to be addressed in this contribution.

1 INTRODUCTION

Under the heading “Concrete Repairs Its Own Cracks” the New York Times1 wrote in September

1992 about the pioneering work of Prof. Dry on self-healing concrete. Hollow fibres filled with specific chemicals – a healing agent - and distributed throughout the cement matrix would release the healing agent into a crack once this crack meets the fibre and breaks it. This self-healing concrete would improve the strength and durability of the concrete. In the same article research was mentioned on the use of special fibres around steel reinforcing bars. When the rebars would start to corrode the nearby fibres should release anti-corrosive chemicals. The article ends with saying that the research of Dry would be finished within two years, but that it was not known yet when the self-repairing concrete would become commercially available. Since the start of her pioneering work Dry has published many inspiring papers on self-healing concrete

2,3,4. But still, twenty years later, self-healing concrete is in its development stage. Meanwhile

the demand for a material that can repair its own cracks has increased, partly for reasons mentioned by Dry already, i.e. improvement of durability, partly also for new reasons. In the past two decades the

Klaas van Breugel

building industry became faced with a number of serious and unprecedented problems. Problems relating to scarcity of raw materials and energy and emission of hazardous products are among the most frequently mentioned problems. The yearly world-wide demand for concrete has reached 0.5 to 1.0 m

3 per capita. In the UK the building and construction industry is estimated to be responsible for

up to 50% of CO2 production5. The building industry consumes a large amount of energy, while the

production of cement is held responsible for 5 to 8 percent of the world’s CO2 emission. Given the rapid growth of the economies of China and India, this figure is expected to increase rapidly if the technology to produce cement remains unchanged. Another issue concerns the huge maintenance costs for structures built in the past. From 600,000 bridges in the USA one out of four needs to be modernized or repaired

6. About 10% of the bridges are considered structurally deficient and also 10%

is considered functionally obsolete. The total amount of money involved in repair and upgrading is estimated at $ 140,000,000,000 US dollars

7. In The Netherlands 50% of about thousand inspected

bridges, viaducts and tunnels required further detailed inspection of their load bearing structure. In that country one third of the annual budget for large civil engineering works is spent on inspection, monitoring, maintenance, upgrading and repair. In the United Kingdom the costs for repair and maintenance accounts for almost 45% of UK’s activity in construction and building industry

8. Apart

from these direct costs for maintenance and repair of bridges, the indirect costs caused by traffic interruptions have to be considered. These indirect costs are often ten times the direct costs

9. The

costs to the US economy of people spending time in traffic jams are estimated at $63 billion dollars per year

10. There is no doubt: The lack of quality and premature failure our aging infrastructure is a

trillion dollars issue! Both the high financial losses caused by premature failure of concrete structures and the high emissions of greenhouse gases, high energy consumption and increasing scarcity of raw materials are typical sustainability issues. Today the building industry can no longer ignore these issues. Mitigation of the ecological footprint can no longer be considered as something ‘extra’ on top of technological and industrial achievements, but as an inherent component of them. Steps towards a sustainable building industry focus on, among other things

11:

- Minimizing resource consumption - Preference on renewable materials and energy - Closed cycles of non-renewables - Optimize the life span of a structure by using more reliable and easily repairable materials - Reduction of maintenance costs by extending the service life of structures From the materials science point of view the focus is on pro-active management of aging. A possible way to achieve this is by using self-healing materials, suppose they exist! By using self-healing materials the life span of structures can be extended, resulting in reduction of the maintenance costs. The use of scarce resources and energy will be reduced as well, because the moment of new-built can be postponed. Apart from the lower consumption of resources, a longer life span of structures also reduces the need for construction-related transport of materials and people. Knowing that in industrialised countries 30-50% of the traffic is related to building activities, enormous direct and indirect savings can be realised by extending the service life of our infrastructure.

2. SELF-HEALING POTENTIAL OF CONCRETE

2.1 Damage prevention versus damage control

The traditional way to make things better is to make them stronger and stiffer. This philosophy has pushed materials technology in the direction of high strength and ultra-high strength materials. The design philosophy that focuses on preventing damage by using stronger materials has been termed Damage Prevention Paradigm. Improving the strength of a material will certainly increase the load bearing capacity and postpone the moment first damage occurs. However, it is still an incremental improvement of existing technology within the damage prevention paradigm. This is otherwise with the Damage Control Paradigm, where a certain degree of damage is not only considered acceptable, but even supposed to initiate an inherent mechanism of self-repair or self-healing. The two indicated design paradigms, i.e. the Damage Prevention Paradigm and Damage Concrete Paradigm, are applicable for the majority of materials, but not so well for reinforced concrete. Concrete is a brittle materials with microcracks throughout the body of material already prior to application of any load. Moreover, cracks are not considered damage and are acceptable as long as a prevailing crack width criterion is not exceeded. From the materials science point of view, therefore, a design in reinforced concrete is always a design within the Damage Control Paradigm. In a traditional

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concrete design, however, cracks are not considered damage. Microcracks are an inherent feature of ordinary concrete, whereas macrocracks are characteristic for a design in reinforced concrete.

Figure 1: Damage Prevention Paradigm (DPP: Manual repair) (left) and the Damage Control Paradigm (DCP: self-healing) (right).

The fact that in structural (concrete) design cracks are acceptable does not mean that they are desirable. From the safety point of view the presence of cracks is no reason to worry. Cracks, however, and certainly those which form a connected network of cracks, jeopardize the resistance of the concrete against ingress of harmful substances into the concrete. The concrete may then deteriorate fast and the steel reinforcement is no longer adequately protected against corrosion. Cracks may also be undesirable from the functionality and esthetic point of view. So, even though cracks are not problematic from the safety point for view, they are undesirable from the overall performance point of view, more in particular the durability and functionality point of view. It would be good if cracks, if considered unavoidable because of the inherent brittleness of cement-based materials, could be healed by a built-in self-healing mechanism. Damage control should then not be restricted to control of the crackwidth – the common practice in concrete design -, but should also focus on healing of the cracks once they occur. The Damage Prevention Paradigm (DPP) and the Damage Control Paradigm (DCP) are shown schematically in Figure 1. In the DPP a longer service life of structures is achieved by increasing the materials properties, for example increasing the strength and reducing the permeability. Deterioration is postponed and the maintenance free period extended. However, once cracking of the concrete starts, the decay rate is relatively high and manual (expensive) repair is required. In the DCP the material is so designed that small cracks, or other damage mechanisms, are allowed to happen and trigger an autogenous or autonomous self-healing process. The self-healing process will bring the material back to its original performance level.

2.2 Self-healing concrete

2.2.1 Autogenous self-healing and self-sealing

In most of the traditional concrete mixtures 20-30% of the cement is left unhydrated. The amount of unreacted cement is higher the coarser the cement and the lower the water/cement ratio of the mixture. If cracking of the concrete occurs, unreacted cement grains may become exposed to moisture penetrating the crack. In that case the hydration process may start again and hydration products may fill up and heal the crack. This inherent self-healing mechanism is known since long and known as autogenous healing. This autogenous healing of cracks in fractured concrete has been noticed by the French Academy of Science in 1836 already in water retaining structures, culverts and pipes

12. According to Hearn

13

the self-healing phenomenon was studied by Hyde14 at the end of the nineteenth century already. A

more systematic analysis of healing phenomena was executed by Glanville15 and dates back to 1926.

Already at that time a distinction was made between self-sealing and self-healing. Self-sealing was studied systematically by Hearn

12. Out of seven possible mechanisms four were

investigated in more detail, viz. (Figure 2): a) dissolution, deposition and crystallization, b) physical clogging, c) continuing hydration and d) swelling of the cement matrix. From an evaluation of literature data and experiments on 26-years old concrete Hearn concluded that dissolution and deposition is the main mechanism of self-sealing in mature concretes. Continuing hydration was the second important mechanism, beit more important for young concrete than in mature concrete. Hearn emphasized that concrete with the potential of self-sealing has a significantly higher “immune system”

Safety level Safety level

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to the environment than non self-sealing concretes. This improvement of the immune system over time was considered of particular importance in view the structure’s service life.

Figure 2: Self healing/sealing mechanisms: a) dissolution, deposition and crystallization, b) physical clogging, c) continuing hydration and d) swelling of the cement matrix.

Glanville’s first studies on self-healing were followed by studies of Soroker et al

16 and Brandeis

17 of

cracks in bridges and date back to 1926 and 1937. In 1996 Jacobsen et al18 observed self-healing of

concrete specimens exposed to freeze-thaw cycles. Self-healing of leaking cracks was studied extensively by Clear

19 and Edvardsen

20. Otsuki et al

21 suggested that self-healing of microcracks

could have been the reason for densification of the concrete cover, thus reducing the rate of migration of chloride ions into the concrete (Figure 3). Self-healing of microcracks resulting in a decrease of chloride ingress into concrete has also been suggested by Fidjestol et al

22 and Bakker

23. According to

Van Tittelboom et al24 the mix composition, i.e. type of cement, influences the efficiency of the

autogenous self-healing capacity. Reinhardt et al25 found that self-healing of cracks will benefit from

higher temperatures.

Figure 3: Healing of microcracks in concrete cover due to continuing hydration (schematic, after a suggestion of Otsuki et al

21)

In all the afore-mentioned studies self-healing is considered an inherent feature of cement-based materials. This feature makes concrete a material with a lot of ‘forgiveness’. Mankind has taken advantage of this peculiar property of concrete, even though the concrete was never designed deliberately to be a self-healing material. Neither the self-healing process itself, nor the required preconditions for making this process to happen are completely understood today. As a consequence the self-healing capacity of cement-based systems is considered a positive feature of concrete indeed, but too unreliable yet to take into account explicitly in the design of concrete structures. Only a few exceptions are known where designers explicitly count on the occurrence of self-healing of cracks, for example in the design of watertight cellars or reservoirs made of reinforced concrete

26. To

ensure that these structures will behave liquid tight the crack width should not exceed a certain crack width. The acceptable crack width depends on the pressure differential over the concrete wall or slab, the crack width and the stability of the crack. Almost no criteria for the type of concrete itself are given. This in fact demonstrates that the self-healing capacity of ordinary concretes is considered a by-product of the material rather than a feature that could be manipulated by a sophisticated design of the mixture. Preconditions for the occurrence of self-healing in ordinary concretes are, apart from the presence of unhydrated cement and moisture, a limited crack width

27. The smaller the cracks are, the higher the

probability that the cracks will heal. A cement-based product that is designed for a small crack width is ECC (Engineered Cementitious Composites) developed by Li et al

28. The main purpose for designing

ECC was to make a ductile material that is able to make large excursions in the post-cracking phase.

steel

concrete cover Progressive hydration and healing of a crack in concrete cover

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The use of small fibres ensures that on cracking the crack width remains very small, typically 50 µm. Thus it can accommodate large imposed deformations and makes it perfectly suitable as repair material. Because of the small crack width, ECC has a remarkable self-healing capacity, even recovery of the original strength after healing.

2.2.2 Autonomous self-healing of concrete

Whereas autogenous self-healing can be considered an inherent feature of cement-based systems, autonomous self-healing is defined as a purposely designed self-healing mechanism. In a recent PhD thesis Van Tittelboom

29 gave an extensive literature survey of autonomous self-healing.

An often mentioned way to realise autonomous self-healing is by dispersing capsules containing either a cementitious or synthetic healing agent. This encapsulation concept has been proposed by White et al

30 in 2001 for self-healing polymers and, as mentioned in the Introduction already, by

Dry2,3,4

in the early nineties of the past century for self-healing concrete. On cracking the capsules may rupture, while releasing the healing agent into the crack. This is the most common concept, but not often used in concrete yet. In case a cementitious healing agent is used the presence of water is a prerequisite for the self-healing process to happen. The water may penetrate into a crack from external sources. Alternatively water-saturated porous lightweight aggregate particles can be added to the concrete mixture. These particles may release water when a crack occurs and moisture gradients stimulate the flow of water. Van Tittelboom

29 also investigated the use of small glass tubes filled with a self-healing agent. If a

crack passes the brittle glass tube the probability it breaks is almost 100%. The internal diameter of the tubes varied from 1.71 to 3.00 mm. A problem with tubes with this diameter is their vulnerability during the concrete mixing process. Less vulnerable are healing agent containing wood fibres or glass

fibers with a diameter between 30 to 100 µm as used for self-healing polymer composites31. The fact that concrete is a heterogeneous material by definition, makes that adding capsules to the mixture does not significantly change the nature of the material. As long as the capsules are not broken, their role is not much different from that of aggregates, beit that their strength and stiffness is different. The effect of adding capsules on the still uncracked concrete depends on the mechanical properties of the capsules, their size and shape and their amount. The number of capsules needed to ensure a sufficiently high probability of a crack passing a capsule depends on the size and shape of the capsules. In this respect capsules with a large aspect ratio, with cylindrical tubes as the extreme, are more effective than spherical capsules. The most appropriate agent for healing cracks in a cementitious matrix is still a matter of debate. A

polymeric agent is good for filling cracks. However, as a non-cementitious material it may become detached from the crack surface. With elapse of time the polymer may be susceptible to aging which may jeopardise the long-term effectiveness of the healing process. Kishi et al

32 investigated self-

healing of cracks by cementitious recrystallisation of an expansive agent. A cementitious healing agent requires water in order to become effective. In absence of water healing will not occur. How serious this is depends on the required function of the structure and the loading scenario’s the structure has to cope with.

3. BIO-BASED SELF-HEALING

3.1 Mechanisms of bio-based healing

One of the preconditions of self-healing of cracks in concrete is the transport of matter to the crack. In a living organism transport of ingredients takes place via a vascular system. In plants and trees ingredients are transported via a network of pores. A porous material like concrete also has a pore system through which transport processes are possible, but a driving force that makes transport of healing species happening is not inherently present. Temperature, moisture, pressure or concentration gradients are needed to trigger transport of species. But still then it is not easy to transport sufficient matter to the spot, i.e. crack, where healing is needed. Promising in this respect is the concept of bacteria-based healing of cracks

33. The idea is that after cracking mixed-in bacteria on

fresh concrete crack surfaces are activated in the presence of water, and then start to multiply and precipitate minerals, such as calcium carbonate, and close the crack. The healing mechanism is presented schematically in Figure 4.

Figure 4: Scenario of 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 stee

external chemical attack

3.2 Bacteria in high pH

Jonkers33 started his study of bio

potentially act as a self-healing agent in concretethat the bacteria survive the high pH in the concrete of about 12 to 13.microbiological viewpoint the application of bacteria in concretespecialized bacteria, is not odd at al. Although the concrete matrix may seem at first inhospitable for life because of its high alkalinityeven in a very dry environment. Inside rocks, even at a depth in deserts as well as in ultra-desiccation- and/or alkali-resistant bacteria typically form spores, high mechanically- and chemically induced stressesspores are known to be viable for up to 200 years The crack-healing potential has been reported of minerallimestone

42, ornamental stone

43

compounds needed for mineral precipitation were brought into contact with virgin crack surfacesSince the healing agent was added afterwards, this form of healing of cracks can iconsidered as self-healing. For selfmaterial from time zero on. The bacteria to be used as selfbe able to perform long-term effective crack sealing, preferably during the totalThe principle mechanism of bacterial crack healing is that thecatalyst, and transform a precursor compound to acompounds, such as calcium carbonatecement that seals or heals newlybio-cement precursor compound should be integrated in the material matrix. The presence of the matrix-embedded bacteria and precursor compounds should not negatively affect other desired concrete propertiesnature, and these appear related to a specialized group of alkaliinteresting feature of these bacteria is that they are able to form spores, whichspherical thick-walled cells somewhat homologous to plaµm, are shown in Figure 5. They chemical stresses and remain in dry state viable for periods over 50 years.when bacterial spores were directly added to the concrete mixture, their lifetimetwo months. The decrease in lifeto only a few months when embedded in the concrete matrix hydration resulting in matrix porespores. Another concern is whether direct addition of organic bioconcrete mixture will not result in unwanted loss of other concretecement precursor compounds, such as yeast extract, peptone and calcium acetate

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-healing by concrete-immobilized bacteria. Bacteria on fresh crack surfaces become activated due to water ingression, start to multiply and precipitate minerals such as

) which eventually seal the crack and protect the steel reinforcement from further external chemical attack (after Jonkers

33)

started his study of bio-based healing of cracks with a search for healing agent in concrete. A precondition for a successful healing process is

that the bacteria survive the high pH in the concrete of about 12 to 13. It was found that fviewpoint the application of bacteria in concrete, or concrete as a habitat for

acteria, is not odd at al. Although the concrete matrix may seem at first inhospitable for alkalinity, comparable natural systems are known in which bacteria do thrive,

even in a very dry environment. Inside rocks, even at a depth of more than 1 km within the earth crust, -basic environments, active bacteria are found

resistant bacteria typically form spores, i.e. specialized cells able to resist and chemically induced stresses

40. These spores have extremely long lifetimes

viable for up to 200 years41.

healing potential has been reported of mineral-precipitating bacteria on degraded 43 and concrete surfaces

44,45. In these studies the bacteria and

compounds needed for mineral precipitation were brought into contact with virgin crack surfacesSince the healing agent was added afterwards, this form of healing of cracks can i

For self-healing concrete, the bacteria should ideally be embedded in the

The bacteria to be used as self-healing agent in concrete should be fit for the job, i.e. theyterm effective crack sealing, preferably during the total life time

The principle mechanism of bacterial crack healing is that the bacteria themselves act largely as a catalyst, and transform a precursor compound to a suitable filler material. The newly produced

such as calcium carbonate-based mineral precipitates, should than act as a type of bionewly formed cracks. Thus for effective self healing, both bacteria

compound should be integrated in the material matrix. embedded bacteria and precursor compounds should not negatively

properties. Bacteria that can resist concrete matrix incorporation existnature, and these appear related to a specialized group of alkali-resistant spore-nteresting feature of these bacteria is that they are able to form spores, which

walled cells somewhat homologous to plant seeds. These spores, are shown in Figure 5. They are viable but dormant cells and can withstand mechanical and

and remain in dry state viable for periods over 50 years. Jonkers et alres were directly added to the concrete mixture, their lifetime

two months. The decrease in life-time of the bacterial spores from several decades when in dry state embedded in the concrete matrix was attributed to pore-diameter widths typically much smaller than the 1

Another concern is whether direct addition of organic bio-mineral precursorure will not result in unwanted loss of other concrete properties.

such as yeast extract, peptone and calcium acetate

Activated bacteria

Spores

Calcite formations

immobilized bacteria. Bacteria on fresh crack surfaces become activated due to water ingression, start to multiply and precipitate minerals such as

l reinforcement from further

based healing of cracks with a search for bacteria which could precondition for a successful healing process is

It was found that from a or concrete as a habitat for

acteria, is not odd at al. Although the concrete matrix may seem at first inhospitable for comparable natural systems are known in which bacteria do thrive,

of more than 1 km within the earth crust, basic environments, active bacteria are found

34,35,36,37,38,39. These

specialized cells able to resist extremely long lifetimes:

precipitating bacteria on degraded . In these studies the bacteria and

compounds needed for mineral precipitation were brought into contact with virgin crack surfaces. Since the healing agent was added afterwards, this form of healing of cracks can in fact not be

healing concrete, the bacteria should ideally be embedded in the

healing agent in concrete should be fit for the job, i.e. they should life time of a structure.

bacteria themselves act largely as a le filler material. The newly produced

should than act as a type of bio-formed cracks. Thus for effective self healing, both bacteria and a

embedded bacteria and precursor compounds should not negatively . Bacteria that can resist concrete matrix incorporation exist in

-forming bacteria. An nteresting feature of these bacteria is that they are able to form spores, which are specialized

spores, diameter about 1 are viable but dormant cells and can withstand mechanical and

Jonkers et al47 found that

res were directly added to the concrete mixture, their lifetime was limited to one-bacterial spores from several decades when in dry state

was attributed to continuing cement han the 1-µm sized bacterial

mineral precursor compounds to the properties. Various organic bio-

such as yeast extract, peptone and calcium acetate, were indeed

Activated bacteria

Calcite formations

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Figure 5: ESEM photomicrograph of alkali-resistant spore forming bacterium (Bacillus strain B2-E2-1).

Visible are active vegetative bacteria (rods) and spores (spheres), showing that spore diameter sizes are in the order of one micrometer (Jonkers

46) .

found to result in a dramatic decrease of compressive strength. However, promising results were obtained with calcium lactate. Adding this compound resulted in a 10% increase in compressive strength compared to control specimens

47.

3.3 Encapsulation of bacteria

With the aim to increase the lifetime and associated functionality of bacteria in the concrete, the effect of bacterial spore and simultaneously needed organic bio-mineral precursor compound (calcium lactate), Jonkers

46 tested immobilization of these components in porous expanded clay particles

(Figure 6). It was found that protection of the bacterial spores by immobilization inside porous expanded clay particles before addition to the concrete mixture indeed substantially prolonged their life-time. After 6 months incorporation in concrete no loss of viability of the spores was observed, suggesting that their long-term viability as observed in dried state when not embedded in concrete is maintained.

Figure 6. Self healing admixture composed of expanded clay particles (left) loaded with bacterial spores and organic bio-mineral precursor compound (calcium lactate). When embedded in the

concrete matrix (right) the “loaded” expanded clay particles represent reservoirs containing the two-component healing agent consisting of bacterial spores and a suitable bio-mineral precursor

46.

3.4 Evidence of bacterial self-healing

3.4.1 Materials used and experimental set-up

In order to test the bacterial healing of cracks in concrete test specimens were prepared in which part of the dense aggregate was replaced by similarly sized expanded clay particles loaded with the biochemical self-healing agent (bacterial spores 1.7x105 g

-1 expanded clay particles, corresponding to

5x107 spores dm-3 concrete, plus 5% w/w fraction calcium lactate, corresponding to 15g dm

-3

concrete). The amount of lightweight aggregate represents 50% of the total aggregate volume.

Light weight aggregates with bacterial spores

Light weight aggregates with bio-mineral precursor

Figure 7: Pre-cracking of concrete slab and subsequent permeability testing

Control specimens had a similar aggregate compositionloaded with the bio-chemical agent. The self-healing capacity of prefrom 56 days water cured concrete cylinderstransport through the disks and test. For determination of the permeabilityring and mounted in a custom madewas achieved by a deformation controlled 0.15 mm running completely through the specimen. After crackbacterial concrete specimens were submerged for two weeks in tap water at room temperature. Subsequently, permeability of alpercolation in time during a 24 hours period (Fig

3.4.2 Experimental results - Discussion

Comparison between bacterial and control specimens revealed a significant difference inpermeability. While cracks of all six bacterial specimensmeasurable permeability (percolation of 0 ml waterperfectly healed. The four other controlbetween 0 and 2 ml/h. Microscopic examination of cracks at the control and bacterial specimens precipitation of calciumoccurred. However, in the control specimensmajor parts of the crack unhealedbacterial specimen with mineral precipitation predominant The most obvious reason for massive of the control specimen (Figure 8carbon dioxide are relatively high due to the opposing diffusion gradients of the respective reactantsCalcium hydroxide diffuses away from the crack interior towards the overlying bulk waterdioxide diffuses from the bulk water towards the crack interior wheconcentrations of calcium hydroxide. The process of chemical calciumaccording to the well-known reaction:

In the bacteria-modified specimen two additional reactions are supposed to explain the efficiency of the healing process. The selfthe active metabolic conversion of calcium lactate Schematically the reaction is:

Lactate + O

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cracking of concrete slab and subsequent permeability testing

Control specimens had a similar aggregate composition, but these expanded clay particles were not chemical agent.

pre-cracked concrete disks (10 cm diameter, 1.5 cm thickness)from 56 days water cured concrete cylinders, was tested by measuring the evolution of water transport through the disks and by taking light microscopic images before and after

ination of the permeability pre-cracked the concrete disks were glued in an aluminium custom made permeability setup. Crack formation in concrete specimen

controlled splitting test (Figure 7, left). The generated 0.15 mm running completely through the specimen. After cracking both sets (6 of each) of control and

were submerged for two weeks in tap water at room temperature. permeability of all cracked specimens was quantified by recording of tap water

percolation in time during a 24 hours period (Figure 7, right).

Discussion

Comparison between bacterial and control specimens revealed a significant difference inpermeability. While cracks of all six bacterial specimens were completely sealedmeasurable permeability (percolation of 0 ml water / h), only 2 out of six control specimens appearedperfectly healed. The four other control specimens featured permeability (water percolation) values

Microscopic examination of cracks at the water-exposed side of the slab revealed that in both control and bacterial specimens precipitation of calcium carbonate-based mineral precipitates

control specimens precipitation largely occurred near the crack rim leaving major parts of the crack unhealed, whereas efficient and complete healing of cracks occurred in

precipitation predominantly within the crack (Figurereason for massive white precipitation of calcium carbonate

ure 8A) is that concentration of both reactants calcium hydroxide and relatively high due to the opposing diffusion gradients of the respective reactants

Calcium hydroxide diffuses away from the crack interior towards the overlying bulk waterdioxide diffuses from the bulk water towards the crack interior where it is concentrations of calcium hydroxide. The process of chemical calcium carbonate reaction from dissolved calcium hydroxide occurs

reaction:

CO2 + Ca(OH)2 → CaCO3 + H2O

modified specimen two additional reactions are supposed to explain the efficiency The self-healing process in bacterial concrete is much more efficient due to

the active metabolic conversion of calcium lactate (Ca(C3H5O2)2) by the

Lactate + O2 → acetate + CaCO3 + CO2

bacteria

cracking of concrete slab and subsequent permeability testing47

clay particles were not

(10 cm diameter, 1.5 cm thickness), sawn tested by measuring the evolution of water

by taking light microscopic images before and after the permeability were glued in an aluminium

permeability setup. Crack formation in concrete specimen disks The generated crack width was

both sets (6 of each) of control and were submerged for two weeks in tap water at room temperature. l cracked specimens was quantified by recording of tap water

Comparison between bacterial and control specimens revealed a significant difference in completely sealed, resulting in no six control specimens appeared

ured permeability (water percolation) values

side of the slab revealed that in both based mineral precipitates

precipitation largely occurred near the crack rim leaving efficient and complete healing of cracks occurred in

ly within the crack (Figure 8). precipitation of calcium carbonate near the crack rim

is that concentration of both reactants calcium hydroxide and relatively high due to the opposing diffusion gradients of the respective reactants

47.

Calcium hydroxide diffuses away from the crack interior towards the overlying bulk water while carbon scavenged by high

carbonate reaction from dissolved calcium hydroxide occurs

(1)

modified specimen two additional reactions are supposed to explain the efficiency is much more efficient due to by the present bacteria.

(2)

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Figure 8. Light microscopic images of pre-cracked control (A) and bacterial (B) concrete specimen before (left) and after (right) healing. Healing after 2 weeks submersion in water. Efficient crack

healing occurred in all six bacterial and two out of six control specimens47.

In formula form:

Ca(C3H5O2)2 + 7O2 → CaCO3 + 5CO2 + 5H2O (3)

Eq. (2) shows that in this reaction also carbon dioxide is formed. This is another source of CO2; it can react with the calcium hydroxide according to (Eq. 1) resulting in additional carbonate-based precipitation. The overall conclusion of Jonkers

46,47 studies is that the proposed two component bio-chemical

healing agent, composed of bacterial spores and a suitable organic bio-cement precursor compound, is a promising bio-based and thus sustainable alternative to strictly chemical or cement-based healing agents. Before practical application becomes feasible, however, further optimization of the proposed system is needed..

4. SMART NANOPARTICLES FOR MITIGATING RISK OF CORROSION

In 2012 the 4th international conference on Nanotechnology in Construction was held on Crete,

Greece. Obviously there is a great interest of the building sector in this topic. Since healing processes, by definition, start at the smallest conceivable scale, we may presume that nanotechnology can play a significant role in the design of self-healing concrete. Meanwhile promising results have been reported about the use of admixed nano-particles for modifying the microstructure and hence the mechanical properties and permeability of cement-based materials

48,49,50. Densification

of the microstructure reduces the permeability and the ingress of hazardous substances into the concrete, thus increasing the concrete’s durability. Koleva et al

51,52,55 and Hu et al

53,54 observed

significant microstructural changes after adding micelles to plain mortar. The micelles she used were prepared from poly-ethylene oxide di-block polystyrene (PEO-b-PS) (Figure 9). Even with a low concentration of micelles, 0.025 % by weight of dry cement, in a mortar with cement-to-sand ratio 1:3 and water-to-cement ratio 0.5, the porosity of both the bulk matrix and a steel-matrix interfacial transition zone (ITZ) decreased significantly

53. The coefficient of water

Klaas van Breugel

Figure 9: Formation of frozen core-shell micelles from PEO113-b-PS70 di-block copolymer in aqueous

media (Koleva et al55)

Figure 10: ESEM Micrographs (left) of hybrid aggregates as received at pH 11.9 (a), at pH 9 (b) and at pH 3. EDX analysis for C and Ca in the indicated locations (right)

58

Element [wt. %]*

Figure 10a) pH = 11.8 C (Carbon)

Ca (Calcium)

Core, spot 1 Shell, spot 2 Bulk, spot 3

33.11 29.20 12.22

43.21 17.22 3,34

*Subtracted background; Stand. Dev.: C: 0.01 – 0.03 Ca: 0.05 – 0.09

Figure 10b) pH = 9 C(Carbon) Ca (Calcium)

Core, spot 1 Shell, spot 2 Bulk, spot 3

19.10 18.21 3.41

14.71 24.60 7.76

*Subtracted background; Stand. Dev.: C: 0.01 – 0.02 Ca: 0.03 – 0.07

Figure 10c) pH = 3 C(Carbon) Ca

(Calcium)

Core, spot 1 4.2 8.2

*Subtracted background; Stand. Dev.: C: 0.01 Ca: 0.04

50 nm

1111

3333

Klaas van Breugel

permeability was 3 orders of magnitude lower for the micelles-containing specimen compared to the micelles-free mortar

56,57.

The observed improvement of the microstructure at the steel-matrix ITZ is expected to reduce the risk of rebar corrosion, by far the most frequent cause of premature deterioration of reinforced concrete. A next step to even further mitigate the risk of rebar corrosion could be the use of smart nano particles, which have the potential to react to changes of the chemical environment by a sort of self-healing mechanism. Koleva and her co-workers

58,59 invested the effect adding PEO113-b-PS780

vesicles, which are particles similar to micelles but carrying an “active” compound, on the rate of corrosion of steel bars places in a simulated pore solution. The active compound was CaO. The hypothesis is that in the event of an aggressive external influence, i.e. carbon dioxide penetrating the material thus carbonating the matrix, or Cl

- penetration followed by localized corrosion on the steel

surface, the “charged” vesicles will participate in a self-healing mechanism by releasing the core material. The released core material, i.e. CaO, will restore the alkalinity in the bulk matrix and repair the passive layer on the steel surface. How the vesicles react on a drop of the pH is visualised in Figure 10, where ESEM pictures are presented of the hybrid nano-particles at a pH 11.8, 9.0 and 3.0, respectively. As can be observed from Figure 10, the morphology of the particles changes with decreasing pH from 11.8 to 3. These changes go along with a decrease of the Ca content in the “core” and an increase in the “shell” around the core. At the same time the polymers vanish (dissolve), whereas the calcium-containing compounds are only detected in the bulk. Koleva et al

58 found that, in line with what was expected, the surface of a steel rebar placed in pore

solution with and without loaded vesicles exhibited a significant difference, even for a very low concentration of ”charged” particles (4.9 10

-4 g/l). When NaCl was added to the solution, as corrosion

accelerator, the steel in vesicles-containing pore solution exhibited again superior performance compared to the vesicle-free solution. ESEM-observation of the product layers on the steel surface after 7 days of conditioning revealed a more homogeneous and compact protective layer on the steel surface of the specimens conditioned in vesicle-containing solution. When treated in cement extract only, a much higher heterogeneity of the surface layer was observed, which will make the steel more susceptible to corrosion.

5. ECONOMIC CONSIDERATIONS

The production of self-healing materials will most often exceed the costs of traditional materials. What justifies the extra initial costs of self healing materials is the reduction of the costs for inspection, maintenance and repair and a longer service live. Schematically this is shown in Figures 11 and 12. In Figure 11a,b the performance and costs for a low quality (curve A) and a high quality (curve B) structure are compared, both designed according to the Damage Prevention Paradigm (DPP). Figure 12a,b shows similar curves for a system designed according to the Damage Control Paradigm (DCP). Although schematic, a comparison of the costs of a system designed according to the DPP and the DCP illustrate that, depending on the required life time of the structure, higher initial costs will finally pay off

60. If the indirect costs of repair work – not considered in Figure 11b and 12b – would have

been taken into account as well, higher initial costs are almost always justified. Similar conclusions were drawn by Wolfseher

61 from an evaluation of repair costs versus initial costs for high quality

concrete structures. In Figure 12a the performance of an ideal self-healing material has been proposed. In reality the self-healing and self-repairing potential of a material will be limited. This means that it is not realistic to expect that the use of self-healing materials will make inspections, monitoring, maintenance and repair completely superfluous. However, the building sector can already benefit from incremental improvements of the self-healing capacity of a material. If the maintenance-free period can be extended and the moment of repair can be postponed, high savings are conceivable already. In this respect it is important to realise the huge scale at which concrete is being used. Because of this scale effect minor improvements of the materials performance can already result in huge savings of repair and maintenance costs. Moreover, situations are conceivable where degradation of a structure should be avoided at all costs because of extremely high consequences in case of failure (for example leakage of radioactive waste). In those cases the use of a robust self-healing material could be the only solution. If the use of a self-healing material is the only realistic solution, the extra costs of the material will be no limiting factor at all.

Klaas van Breugel

6 CHALLENGES AND PROSPECTS

6.1 From “static solids” to “dynamic systems”

Designing self-healing materials requires another way of thinking. The occurrence of some form of damage is not prevented at all cost (the traditional Damage Prevention Paradigm), but is used to initiate, on purpose, a mechanism or process of healing. As indicated in paragraph 2 in this paper concrete is already an inherently self-healing material because of the presence of unhydrated cement grains. In the presence of water, transported via cracks to these unhydrated grains, hydration products may heal the cracks. This so called autogenous self-healing capacity of concrete, although present in most concretes mixtures, cannot prevent our concrete infrastructure from aging and degradation beyond acceptable limits. If healing is accomplished by incorporating additional healing agents, the resulting healing process is called autonomous self-healing. After a brief overview of possible self-healing concepts, this paper has focused first on autonomous healing of microcracks with appropriate bacteria. It was explained how bacterial activity is triggered by the occurrence of microcracks and the presence of oxygen. In the presented example bacteria and food were encapsulated in lightweight aggregate particles. Precipitation of calcium carbonate in microcracks restores or even improves the density/tightness of the concrete, thus increasing the resistance against ingress of aggressive substances, like chloride ions. Another autonomous self-healing concept discussed in more detail in this paper concerned the use of smart nanoparticles, i.e. CaO-charged vesicles, for increasing the resistance against corrosion of steel rebars. The self-healing process was triggered by a decrease of the pH of the pore water, on which the vesicles started to dissolve while releasing CaO to the bulk. The pH in the pore solution increases, while at the same time the passive layer on the steel surface remained intact, c.q. was restored. These examples illustrate the need for another way of thinking about materials performance and materials design. Self-healing materials are no “static solids”, but “dynamic systems”, able to respond to external loads with a damage-healing process.

cost

A

B

time

Fig. 11b

Figure 11:. Performance (a) and costs (b) with elapse of time for low (A) and high quality (B) structures. Direct costs of repair included. Interest and inflation not considered

60.

cost

Fig. 12b

Figure 12: Performance (a) and cost (b) of a structure made with self-healing material (concrete) with elapse of time. Interest and inflation not considered

60.

Required performance

performance

Fig. 12a.

A

B

performance

Required performance

time

Fig. 11a

1st repair 2nd repair

Klaas van Breugel

6.2 Autogenous self-healing and sustainability

The inherent self-healing capacity of concrete is supposed to increase with higher cement content of the mixture. From that point of view high strength concrete with high cement content is potentially more prone to self-healing then concrete with low cement content. From the sustainability point of view, however, mixtures are desired with cement content as low as possible

61. Using low cement

content is positive in view of reduction of the cement-related CO2 emission, but it reduces the concrete’s autogenous self-healing capacity. If mixtures with low cement content are required for sustainability reasons, self-healing should, therefore, be realised preferably through autonomous self-healing.

6.3 Self-healing materials in view of environmental stewardship

According to Long63 the infrastructure in industrialized countries accounts for at least 50% of our

national wealth. From that he inferred that the performance and quality of our infrastructure are of fundamental importance to urban sustainability and the well-being of our environment. Extending the service life of our infrastructure will certainly contribute to mitigation of the ecological footprint. Engineers should be aware of this when designing infrastructural works and when making choices for concrete mixtures. The development and use of self-healing materials are most challenging options to accomplish the need for durable infrastructure. In view of the large impact of the building industry on the environment, promoting self-healing materials can be considered as a matter of environmental stewardship. Since concrete is, volume wise, the most often used building material, enormous savings are achievable, even if we make small improvements in the quality and durability of our infrastructure. On top of that it is worthwhile to know that investing in self-healing materials in view of reduction of maintenance costs finally pays off

60,61.

Acknowledgements

The author would like to thank his colleagues Dr. E. Schlangen, Dr. V. Wiktor, Dr. D.A. Koleva, Dr. J. Hu, Dr. H.M. Jonkers and Dr. O. Copuroglu, who are all active in research on self-healing materials and kindly provided me with valuable input. The financial support of the Delft Centre for Materials of the TU Delft and the Ministry of Economic Affairs (IOP-program Self-healing materials) of The Netherlands is gratefully acknowledged. REFERENCES

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