Cracked Concrete

7/24/2019 Cracked Concrete

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1 INTRODUCTION

Cracks can occur in concrete structures due to mul-

tiple reasons such as autogenous shrinkage, freeze-thaw reactions, mechanical compressive- and tensileforces. Although micro-cracks do not necessarily re-sult in significant strength loss of concrete, the in-gress of water and other reactive chemicals such aschloride and water may pose a thread to the steel re-inforcement as these strongly enhance its corrosionrate. Thus for durability reasons and potential repaircosts, crack occurrence should be minimized or, al-ternatively, occurring cracks should ideally behealed directly after formation by an autonomousrepair mechanism. Different autonomous repair sys-tems are feasible. One such a self-healing mecha-nism could involve secondary hydration reactions ofstill present but not fully reacted cement particles.Although a high percentage of non-reacted cement

particles within its matrix may result in a concretewith a substantial self-healing capacity, the materialcharacteristics of the initial concrete structure maynot be satisfactorily as it may be more brittle and ini-tially weaker as wanted. Another self-healingmechanism could be based on the addition of a self-healing agent that would make up a part of the con-

crete matrix without or insignificantly affecting itsstructural and mechanical characteristics. In thisstudy the potential of bacteria to act as a self-healingagent in concrete is investigated. Although the ideato use bacteria and integrate them in the concrete

matrix may seem odd at first, it is not from a micro-biological viewpoint. Bacteria naturally occur virtu-ally everywhere on earth, not only on its surface but

also deep within, e.g. in sediment and rock at a depthof more than 1 km (Jorgensen & DHondt 2006).Various species of so-called extremophilic bacteria,i.e. bacteria that love the extreme, are found inhighly desiccated environments such as deserts(Dorn & Oberlander 1981; DeLaTorre et al 2003),

but also inside rocks (Fajardo-Cavazos & Nicholson2006) and even in ultra-basic environments (Peder-sen et al 2004; Sleep et al 2004) which can be con-sidered homologous to the internal concrete envi-ronment. Typical for many desiccation- and/oralkali-resistant bacterial species is their ability toform endospores. These specialized cells are charac-terized by an extremely low metabolic activity, areknown to be able to resist high mechanically- andchemically induced stresses (Sagripanti & Boni-facino 1996) and are viable for periods up to 200years (Schlegel 1993). In some previously publishedstudies the application of bacteria for cleaning ofconcrete surfaces (DeGraef et al 2005) and strengthimprovement of cement-sand mortar (Ghosh et al2005) was reported. Furthermore, in some studiesthe crack-healing potential by mineral-precipitating

bacteria on degraded limestone (Dick et al 2006) andornamental stone surfaces (Rodriguez-Navarro et al2003) as well as on concrete surfaces (Bang et al2001; Ramachandran et al 2001) was investigatedand reported. Although promising results were re-

Self-healing of cracked concrete: A bacterial approach

H.M. Jonkers & E. SchlangenDelft University of Technology, Delft, The Netherlands

ABSTRACT: Crack occurrence in reinforced concrete should be minimized for both durability and economi-cal reasons as crack repair is costly. Autogenous repair, or self-healing, of concrete would save a substantialamount of money, as manual inspection and crack repair could be minimized. Thus, a reliable self-healingmechanism for concrete would not only result in more durable structures, but would also be beneficial for the

global economy. This study exploited the potential to apply calcite-precipitating bacteria as a crack-healingagent in concrete. The potential of different species to precipitate calcite, produce endospores, survive con-crete-production, and heal cracks by sealing them with calcite was investigated. Furthermore, the mechanical

properties of bacterial concrete were tested. ESEM studies showed that alkali-resistant spore-forming bacte-ria embedded in the concrete matrix can precipitate substantial amounts of calcite. The bacterial approachthus seems a highly promising mechanism to mediate self-healing in concrete structures.


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ported, the major drawback of the latter studies wasthat the bacteria and compounds needed for mineral

precipitation could only be applied externally on thesurface of the structures after crack-formation hadoccurred. This methodological necessity was mainlydue to the limited lifetime (hours to a few days) ofthe (urease-based) enzymatic activity and/or viabil-ity of the applied bacterial species. In the present

study the application of alkali-resistant spore-forming bacteria to enhance the self-healing capacityof concrete is investigated. Tensile- and compressivestrength characteristics of reference (no bacteriaadded) and bacterial concrete are quantified. Fur-thermore, the viability of bacteria immobilization inconcrete is quantified and, finally, calcite precipita-tion potential of bacterial concrete is demonstrated

by ESEM analysis.

2 METHODS

2.1 Cultivation of alkali-resistant spore-formingbacteria

Four strains of alkaliphilic spore-forming bacteriawere purchased from DSMZ (German Collection ofMicroorganisms and Cell Cultures), Braunschweig,Germany: Sporosarcina pasteuriiDSM 33;Bacilluscohnii DSM 6307; Bacillus halodurans DSM 497andBacillus pseudofirmusDSM 8715 and cultivatedaccording to the suppliers recommendations (me-dium DSMZ-2 for S.pasteuriiand DSMZ-31 for the

others).Endospore-forming potential was determined inmineral medium. This medium contained per liter ofMilli-Q ultra pure water: 0.2g NH4Cl, 0.02gKH2PO4, 0.225g CaCl2, 0.2g KCl, 0.2gMgCl2.6H2O, 1 ml per liter trace elements solutionSL12B, 0.1g yeast extract, 6.45g citric acid triso-dium salt and 8.4g sodium bicarbonate. The pH ofthis medium was 9.2. Aerobic batch cultures wereincubated in 2-l Erlenmeyer flasks on a shaker tableat 150 rpm. Growth was monitored by microscopyand cell numbers and percentage of sporulating cellswere quantified by microscopy using a Burger-Turkcounting chamber.

2.2 Preparation and strength characteristics ofbacterial concrete

Concrete bars with and without (control) added bac-teria were prepared for tensile- and compressivestrength determination. The aim of these tests was tocheck whether the strength of the concrete was notnegatively affected by the bacteria. Firstly, for the

preparation of bacterial concrete, a dense culture ofS.pasteurii was obtained after growth in mediumDSMZ-2. Total cell number was quantified by mi-croscopy using a Burger-Turk counting chamber.Subsequently, cells were washed twice by centrifu-

gation (20 min x 10000g) and resuspension of thecell pellet in tap water. Washed cells were finally re-suspended in a 20-ml aliquot of tap water. This cellsuspension was applied as part of the needed waterfor concrete bar preparation.

Concrete bars for tensile- and compressivestrength determination were prepared as follows.Two sets (bacterial concrete and control concrete

without bacteria) of nine bars each (bar dimensions16 x 4 x 4 cm) were made using ordinary portlandcement (ENCI CEMI 32.5R), a water-cement ratioof 0.5 and aggregate composition (sand and gravel)as listed in Table 1. The bars were initially cured for24 hours in plastic foil-sealed molds at room tem-

perature, subsequently uncased and further cured intap water-filled separate plastic containers at roomtemperature. Subsets of three bars each were testedfor flexural tensile- and compressive strength after 3,7 and 28 days curing following the procedure ac-cording to EN 196-1 Standard Norm.

Table 1. Cement, water and aggregate composition neededfor the production of 9 concrete bars of dimensions 16 x4 x 4 cm used for tensile- and compressive strength char-acterization of bacterial- and control (no bacteria added)concrete. For bacterial concrete, the 20-ml cell suspension

was part of the total water volume needed.

Compound / Aggregate size (mm): Weight (g):

Cement (ENCI CEM I 32.5)

Water

Aggregate size fraction:

4 - 8

2 - 4

1 - 2

0.5 - 1

0.25 - 0.5

0.125 - 0.25

1170

585

1685

1133

848

848

730

396

2.3

Viability of concrete-immobilized spores

The viability (ability to germinate) of spores of the

alkaliphilic bacterial species B.cohnii, B.haloduransand B.pseudofirmus immobilized in cement stonewas determined. Cultures of the respective specieswere firstly grown in mineral medium (see above).These cultures were washed twice by centrifugation(20 min x 10000g) and resuspension of the cell pel-let in tap water after the number of spores formedwas quantified by microscopy. Obtained spore sus-

pensions were divided in two parts, one part wasstored in a fridge at 4C and served as non-concreteimmobilized control for determination of spore vi-

ability during storage (see below), and one part wasused for cement stone sample preparation. For thelatter, the spore suspension was used as part of themake up water, and bacterial and control (no bacte-rial spores added) cement stone specimen were pre-


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pared. Number of spores added to cement stone asdetermined by microscopic counting was 109 cm-3.Ordinary portland cement (ENCI CEMI 32.5R) anda water-cement ratio of 0.5 was used for the prepara-tion of cement stone disks (4 cm diameter, 1 cmheight), cast in plastic vials closed with a plastic lid.After 24 hours curing at room temperature, diskswere further cured in tap water at room temperature.

Disks were removed from the plastic vial molds afterten days curing, chipped to pieces with a chisel andfurther crushed to powder using a robust pharmaceu-tical stone mortar. Powdered cement stone (1.84 grepresenting 1 cm3, containing 109spores) was sub-sequently slurried and diluted 10-fold by addition of9 volumes sterile mineral medium. In parallel, theendospore-containing cell suspensions which werekept in a fridge at 4C were diluted to a spore den-sity of 109ml-1and also 10-fold diluted by additionof 9 ml sterile mineral medium. Cement stone slur-ries and original spore suspensions were further ho-

mogenized by three cycles of vigorous mixing at2500 rpm and 20 seconds ultrasonic treatment in aBranson 1210, 47 KHz, 80 Watts Ultrasonic bath.

Number of viable spores in cement stone slurriesand spore suspensions was estimated according tothe Most Probable Number (MPN) dilution tech-nique. For this procedure, 8 x 12 wells sterile micro-titer plates were filled with mineral medium, 180 l

per well. Four consecutive wells of the first rowwere inoculated with 20-l slurry or control cellsuspension aliquots, and these were subsequently se-

rially diluted in ten-fold dilution steps up to the 1011

dilution level, leaving the last (12th) row as non-inoculated control to check for medium contamina-tion. Thus, each cement stone slurry and correspond-ing control cell suspension was serially diluted infour parallel series. During the following incubation

period at room temperature, growth occurred in thelower but not in the higher dilution levels due to di-lution-to-extinction of the viable cells present in thesamples. Growth could easily be determined visuallydue to increased turbidity of positive wells duringthe following 2-weeks incubation period. Viablenumber of cells in cement stone slurries and theircorresponding spore suspensions was calculatedfrom the number of positive wells using the MPNcomputer program of Clarke and Owens (1983).

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Calcite precipitation potential of bacterialconcrete

Chips of 10 days cured cement stone samples (seeunder 2.3) were incubated in rich medium (yeast-extract and peptone based medium) after pasteuriz-ing for 30 min at 70C. Pasteurization of bacterialand control cement stone chips before incubationwas done to inactivate bacteria that potentially cameinto contact with the cement stone samples during

curing period or non-sterile handling of the cementstone samples and chips after curing. As bacterialendospores are not killed by the pasteurization pro-cedure, this treatment ensured that potential differ-ences between control and bacterial concrete sam-

ples after incubation were mediated by addedbacteria and not by accidentally introduced contami-nants. Rich medium contained 5 g peptone, 3 gram

yeast extract and 8.4g sodium bicarbonate and had apH of 8.6. Individual chips were incubated aerobi-cally in 100-ml medium aliquots on a shaker table at100 rpm at 25C for 12 days. Chips were rinsed withtap water after incubated and stored wet in closed

plastic vials until ESEM analysis, what was donewithin two days after incubation without any furthertreatment. Chips were mounted on a 1-cm2 metalsupport and kept in place with adhesive tape and ob-served with a Philips XL30 Series EnvironmentalScanning Electron Microscope.

3 RESULTS

3.1 Cultivation of alkali-resistant spore-formingbacteria

Three out of four strains produced copious spores inmineral medium, except for S.pasteurii, which didnot grow in this medium. Spore production was con-siderably less in rich yeast extract- and peptone-containing medium. Percentage of cells with en-dospores was quantified by microscopic counting,

and amounted to 75, 50 and 25% for B.cohnii,B.haloduransandB.pseudofirmusrespectively.

3.2 Strength characteristics of bacterial concrete

The 20-ml washed cell suspension of the S.pasteuriiculture used for the making of bacterial concrete

bars contained 3.48*1012cells, what resulted in a fi-nal density of 1.14*109 cells cm-3 concrete. As theaverage volume of an S.pasteurii cell equals about2.5 m3, the total cell volume amounts to 0.3% of

the bacterial concrete volume. Tensile- and com-pressive strength tests after 3, 7 and 28 days curingrevealed no significant difference between control-and bacterial concrete (Fig 1).

3.3 Viability of cement stone-immobilized spores

The number of viable spores in cement stone sam-ples after 10 days curing as well as original sporesuspensions, both with a spore density of 109 cm-3,were estimated (Table 2). Results revealed thatabout one percent of the spores in the spore suspen-sions (107ml-1) could be retrieved as viable (Table2). The number of viable spores in the correspondingcement stone samples appeared significantly lower,i.e. between 105 and 106 cm-3. Compared to spore


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suspensions, estimated viable spores in cement stoneslurries amounted to 1.9, 7.0 and 2.0% for

B.halodurans, B.pseudofirmus and B.cohnii respec-tively. Number of viable bacteria in control cementstone samples (no bacterial spores added) and tapwater used for concrete sample preparation were be-low detection limit (


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strates (yeast extract and peptone), bacterial en-dospores germinate and start to produce CO2due tometabolic turnover of growth substrates. CO2, whatcan locally reach high concentrations due to rapidmetabolic conversion of organic compounds, willchemically react with Ca(OH)2 produced from C2Sand C3S hydration reactions. The Ca(OH)2that leaksout of the concretes pore system reacts with CO2

and precipitates as calcite or any other calcium car-bonate based mineral. The calcite-like crystals foundon the surface of bacterial- but not on the surface ofcontrol cement stone samples support this hypothe-sis.

A: Control

B: + Bacteria

Figure 2. Concrete samples incubated in yeast extract- and pep-

tone-containing medium. A: Control (concrete with no bacteria

added) and B: Concrete containing 109 cm

-3B.pseudofirmus

endospores. The inset in Figure 2B (5000x magnification)

shows a close up of the massive calcite-like crystals formed on

the concrete surface.

The experiments done in this study show that al-kaliphilic endospore-forming bacteria integrated inthe concrete matrix can actively precipitate calciumcarbonate minerals. Water, needed for the activation

of endospores, can enter the concrete structurethrough freshly formed cracks. Furthermore, formineral precipitation, active cells need an organicsubstrate that can metabolically be converted to in-organic carbon what can subsequently precipitatewith free calcium to calcium carbonate. Free cal-cium is usually present in the concrete matrix, butorganic carbon is not. In the present experiments or-

ganic carbon was applied externally as a part of theincubation medium, while ideally it should also bepart of the concrete matrix.In that case only external water is needed to activatethe concrete-immobilized bacetria which can thenconvert organic carbon present in the concrete ma-trix to calcium carbonate and by doing so sealfreshly formed cracks. We currently investigatewhich specific kind of organic compounds are suit-able to include in the concrete matrix. This is cer-tainly not trivial as such compounds should be asuitable food source for bacteria as well as be com-

patible with concrete. Certain classes of organiccompounds are less- or not suitable at all, e.g. com-

pounds such as carbohydrate derivatives that areknown to inhibit the setting of concrete even at lowconcentrations. We furthermore presently investigatethe long-term viability and potential possibilities toincrease the viability of concrete immobilized en-dospores to ensure long-lasting bacterially enhancedself-healing. Other ongoing investigations addressthe possible decrease in concrete permeability andthe change of mechanical characteristics of healed

cracked concrete due to bacterial calcite precipita-tion.To conclude we can state that the bacterial approachhas potential to contribute to the self-healing capac-ity of concrete. We have shown that bacteria incor-

porated in high numbers (109 cm-3) do not affectconcrete strength, that a substantial number of added

bacteria remain viable and, moreover, that these vi-able bacteria can precipitate calcium carbonateneeded to seal or heal freshly formed cracks.

5 ACKNOWLEDGEMENTS

Financial support from the Delft Center for Materi-als (DCMat: www.dcmat.tudelft.nl) for this study isgratefully acknowledged.

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Cracked Concrete