IMPERMEABILITY REHABILITATION OF CIVIL ENGINEERING STURCTURES

The present invention relates to improvements in or relating to the repair of concrete and other civil engineering structures and in particular to the repair of large concrete structures and to provide impermeability in the structure particularly impermeability to water. In particular the invention provides means for the repair of concrete structures which provides strength as well as impermeability to the structure.

Concrete and other aggregated material such as compacted stone, gravel and clay containing materials referred to further as "aggregate" is used in many instances for the containment of materials particularly for the containment of water. Examples of such uses include dams, water towers, cooling towers such as those employed at power stations, reservoirs, drains and the like. "Aggregate" is also used in many construction areas such as in buildings, bridges, roads, airport runways and the like. As "aggregate" ages it can crack and/or loose otherwise its design impermeability of by material seepage, and voids can form in the "aggregate" structure which can result in leaks I the structure, the cracks and voids can also provide local areas o weakness in the structure.

In addition water can pass into the voids and cracks which can damage the "aggregate" structure particularly if the water freezes and expands as the expansion will enlarge the crack or void in the "aggregate".

This invention is concerned with the plugging of such cracks or voids in either situation above in a manner that retains the strength of the structure and also provides impermeability particularly to water and gas as appropriate. The invention employs a tailored form of microbial biocementation for the plugging of voids or cracks in concrete structures. Appropriately tailored microbial composition in combination with ionic solution and nutrients, according to the invention, is used to plug voids or cracks in the geological structures, while absorbing at least a portion of the CO2 in the carbonate precipitated by the operation.

PCT Publication WO 2006/066326 relates to a method for forming a high strength binder cement in a permeable starting material in which the starting material is combined with effective amounts of a urease micro-organism, urea and calcium ions in an amount to provide a urea hydrolysis rate under standard conditions of 0.5-5.0 m ^M urea hydrolysed

per minute. WO 2006/066326 is concerned with the formation of the cement around the particles of the starting material and hence requires that the starting material be permeable and that the composition of the cement forming material is such that it combines the ability to flow around the particles of the starting material with the timely formation of the cement. WO 2006/0066326 is therefore concerned with the solidification of a porous substrate.

The present invention differs from WO 2006/066326 in that it relates to the formation of a plug of material to provide impervious repair to a aggregate structure and form an integral impermeable structure by biocementation and plugging resulting in the microbial deposition of calcite and/or the microbial formation of a gel without the requirement to have a porous starting material.

The present invention therefore provides a process for the plugging of cracks or voids in aggregate structures comprising the sequential injection of one or more selected carbonatogenic micro-organisms and a nutrient for the micro-organism into the crack or void whereby an impermeable plugging gel in the crack or void is formed and CO2 is absorbed. The CO2 may be provided in any suitable way by for example injection or other means

In a further embodiment the invention provides a process for the plugging of cracks or voids in aggregate structures comprising the sequential injection of one or more selected carbonatogenic micro-organisms and a nutrient containing one or several organic compounds selected from carboxylic acid and salts, amides, esters, anhydrides, carbohydrates, starch, sugars and proteins, into the crack or void whereby an impermeable plug in the crack or void is formed.

In a further embodiment the invention provides a process for the plugging of cracks or voids in aggregate structures comprising the sequential injection into the crack or void of one or more selected carbonatogenic micro-organisms and a nutrient mix containing one or several organic compounds, in the presence of compounds selected from inorganic phosphates, sulfates, and nitrates.

Examples of suitable inorganic compounds are those that are reducible under the prevailing conditions and examples of preferred compounds are given in the tables that follow and the preferred compounds are of metals of Groups 1 to 4 of the periodic table .Alternatively mining waste and soil remediation materials may be used.

In a preferred embodiment calcium and/or magnesium ions are injected into the crack or void in the aggregate concurrently or sequentially to the injection of the carbonatogenic micro-organism and/or the nutrient. In this embodiment the calcium ions can be precipitated perhaps as calcium carbonate to provide additional strength to the biologically developed plugging material. The requirement to add calcium and/or magnesium ions depends upon the nature of the structure that is to be plugged and also the nature of the stream that may be passing through the crack or void. For example with the repair of dams and concrete in general we prefer to add the calcium or magnesium ion. If however the structure is a marine barrier such as a dyke the water itself may contain sufficient ions.

In this preferred embodiment the invention has the benefit that the plugging materials can be tailored to provide the impermeability, CO2 absorption capability and the strength required for the structure that is to be repaired. For example, different grades and compositions of cement may be used for different applications. For example a cement for the production of a thermal electricity plant cooling tower has different requirements from the cement employed in dams and dykes which again is different from the cement used in the production of bridges and roadways and the repair material of the present invention may be tailored accordingly. The nature of the bacteria, the nutrient and the amount of calcium and/or magnesium ions supplied to provide impermeability to the void or crack can therefore be tailored so that the repaired area has the desired strength and impermeability which typically matches that of the undamaged aggregate material.

Several bacterial biomineralisation processes are known and any of these may be employed in the process of the present invention. Three possible pathways are described in the publication Bacterial Roles in the Precipitation of Carbonate Minerals by Castanier et al 2000 Microbiol Sediments which describes various metabolic pathways of bacterial calcium carbonate formation as autotrophic pathways, and heterotrophic

pathways and it also describes the relationship between bacteria minerals and the environmental conditions. The following may be derived from this publication.

Autotrophic Pathways In autotrophy, three metabolic pathways are involved: non-methylotrophic methanogenesis anoxygenic photosynthesis and oxygenic photosynthesis. All three pathways use CO2 as carbon source to interact with microbial organisms to produce organic matter. Thus, they induce CO2 depletion of the medium or of the immediate environment of the bacteria. When calcium ions are present in the medium, such a depletion favours calcium-carbonate precipitation as depicted in Figure 1.

Heterotrophic Pathways

Heterotrophic pathways are those which cannot synthesize organic compounds directly from CO2. Organic compounds is their starting point. Hence in heterotrophy two bacterial processes may occur, one, autotrophic, providing the organic source for the second one, which is heterotrophic sometimes concurrently. Alternatively, a nonbacterial source of organic compound can be provided to the sole heterotrophic pathway.

Passive Precipitation

Passive precipitation or passive carbonatogenesis operates by producing carbonate and bicarbonate ions and inducing various chemical modifications in the medium that lead to the precipitation of calcium carbonate. Two metabolic cycles can be involved: the nitrogen cycle and the sulphur cycle.

In the nitrogen cycle, passive bacterial precipitation follows three different pathways: (i) the ammonification of amino-acids in aerobiosis (i.e. in the presence of gaseous or dissolved oxygen), in the presence of organic matter and calcium); (ii) the dissimilatory reduction of nitrate (in anaerobiosis (i.e. in the absence of oxygen) or microaerophily (i.e. in the presence of very low amounts of oxygen), in the presence of organic matter, calcium and nitrate); and (iii) the degradation of urea or uric acid (in aerobiosis, in the presence of organic matter, calcium, and urea or uric acid). Both urea and uric acid result from eukaryotic activity, notably that of vertebrates. These three pathways induce production of carbonate and bicarbonate ions and, as a metabolic end-product,

ammonia, which induces pH increase as depicted in Figure 2. When the H+ concentration decreases, the carbonate-bicarbonate equilibria are shifted towards the production of CO3 2- ions.

If calcium ions are present, calcium-carbonate precipitation occurs. If Ca2+ (and/or divalent cations) are lacking in the medium, carbonate and bicarbonate ions accumulate, and the pH increase and bacterial activity may favour zeolite formation.

In the sulphur cycle, bacteria use a single metabolic pathway: the dissimilatory reduction of sulphate which is depicted in Figure 3. The environment must be anoxic, and rich in organic matter, calcium and sulphate. Using this pathway, bacteria produce carbonate, bicarbonate ions and hydrogen sulphide. If calcium ions are present, the precipitation of

Ca-carbonates depends on the hydrogen sulphide behaviour. If the hydrogen sulphide degasses, this induces pH increase and, calcium bicarbonate precipitation. On the other hand, hydrogen sulphide may be used by other bacteria. If anoxygenogenic sulphide phototrophic bacteria are involved, the hydrogen sulphide is oxidised into sulphur which forms intra-cellular or extra-cellular deposits. Hydrogen sulphide up-take induces pH increase favouring calcium-carbonate precipitation. If autotrophic sulphide-oxidising aerobic bacteria are involved, they produce sulphate ions. Together with hydrogen ions from water this gives sulphuric acid, the pH decreases and no solid Ca-carbonate appears. If hydrogen sulphide is neither used by bacteria nor discharged, pH decreases and Ca-carbonates cannot precipitate.

Active Precipitation Active precipitation or active carbonatogenesis is another means for the bacterial production of carbonate ions and is independent of the other previously mentioned metabolic pathways. The carbonate particles are produced by ionic exchanges through the cell membrane by activation of calcium and/or magnesium ionic pumps or channels, probably coupled with carbonate ion production. Numerous bacterial groups are able to operate such processes. Carbonatogenesis appears to be the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter. After a phase of latency, there is an exponential increase of bacterial strengths together with the accumulation of metabolic end products. These induce an accumulation of carbonate and hydrogenocarbonate ions in the medium and, by different ways, a pH increase that

favours carbonate precipitation. This phase ends into a steady state when the majority of the initial enrichment is consumed. Particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state. In most cases, the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth and shape modifications of initially produced particles.

The process of the present invention can employ any of these pathways and tailor the production of the impermeable concrete repair according to the pathway that is best suited to the repair requirements according to the nature of the aggregate structure that is to be repaired. The repair materials used in the process of the present invention can also be tailored to have the desired setting time to enable the repair to be made in the optimum manner. For example, the micro-organism and the nutrient are selected to ensure the impermeable repair is performed and they may be selected so that a gel is developed rapidly or slowly as is required and the gel formed may have sufficient strength to provide the desired impermeability to the concrete structure. Furthermore the source and the quantity of calcium and/or magnesium ions, when used, may be selected to enable a control on the rate of precipitation of the carbonate and the quantity of carbonate material that is precipitated.

The repair materials of the present invention may be delivered to the crack or void in any suitable manner which can depend upon the size of the crack or void to be repaired. The materials may be injected into the crack or void by means of a gunning mechanism. The injection means may also be associated with an inspection mechanism that may be used for regular inspection of structures such as those that are used for the inspection of cooling towers, dams, bridges and the like. The nature of the repair materials can also be tailored to effect the repair at the temperature at which the concrete structure is operating. For example where the structure is a cooling tower requiring the concrete to be at elevated temperature the ingredients of the repair material can be tailored to effect the repair at the elevated operating temperature. In this way downtime to enable repair can be significantly reduced or eliminated.

In some instances the natural microbial biocementation process may be sufficient to effect the repair and create a gel that provides the desired impermeability. In other instances it may be desirable to increase the cementitions content of the repair material

to increase the strength of the repair. This may be accomplished by the provision of a solution of calcium and/or magnesium ions as well as the microbial and nutrient materials. In this instance it is preferred that the microbial and/or nutrient materials contain nitrogen. In this way the natural processes will liberate alkaline materials leading to the precipitation of calcium and/or magnesium carbonates. For example, this may be accomplished by the presence of amino-acids, urea and/or uric acid whereby the production of ammonium ions increase the pH which stimulates the initial precipitation of carbonates which in turn provides the nucleation sites for further precipitation. The calcium and/or magnesium ions can be provided from solutions of calcium or magnesium salts such as chlorides, sulphates and nitrates and mixtures thereof may be used as may be desired to provide the carbonate mix appropriate for the cementatious material that is being repaired.

The invention therefore provides an effective, environmentally friendly and simple method for the repair of concrete structures, particularly aged concrete structures. It may be employed with any particular structure but it is particularly useful with dams, cooling towers, buildings, pipelines, roadways, runways, bridges and the like.

Examples of microbes and nutrients that may be used to produce the gels and biocement according to the present invention are summarized in the following table. The combination of microbe and nutrient is selected according to the required mineralizing and filming to provide the desired impermeable layer in the damaged concrete structure.

Vibrio strains produce magnesium calcite, whereas H. eurihalina produces spherical bioliths of 50 μm of magnesium calcite, aragonite and monohydrocalcite in

For the Halomonas, variable proportions, and Marinomonas cultures, 1% the Halomonas & yeast extract (Difco) ; 0.5 % Marinomonas produce

Moderately halophilic proteose-peptone no.3 calcite, magnesium calcite, bacteria like Vibrio, (Difco); 0.1% glucose;

Variety of aragonite, dolomite, Halomonas supplemented with a

Calcium monohydrocalcite, eurihalina, 14 other balanced mixture of sea and hydromagnesite and

Magnesium species of salts to final concentrations struvite in variable carbonates Halomonas and 2 of of 2.5 %, 7.5 % and 20 % proportions. Aragonite, Marinomonas, and (w/v). The medium is

(see magnesium calcite, vaterite, also amended with 0.4 % comments) monohydrocalcite, struvite,

Chromohalobacter calcium acetate and the pH kutnahorite and huntite can marismortui. adjusted to 7.2 with 1 M be produced by KOH. To obtain solid Chromohalobacter media, 20 g/l "Bacto-Agar" marismortui. These species (Difco) can be added. are adapted to high salinities, and hence, may be used in saline aquifers and any coastal concrete structures or dykes which need to be consolidated and made impervious.

First developing competency to degrade a surfactant by growing the bacteria with the surfactant present as the sole organic nutrient; then starving the

To prevent seepage of resulting competent water from water-retaining bacteria until the cells reach structures, will benefit from a diameter less than about

Enterobacter, the bacteria based 0.4 μm. "Starvation" is

Film Serratia, Bacillus, precipitation of calcium achieved by placement of forming Klebsiella, or carbonate. Bacillus firmus the bacteria in a carbon-

Pseudomonas and Bacillus sphaericus and free environment, such as a Bacillus amyloliquefaciens phosphate buffer salts can be triggered to the solution (PBS), for at least same use. two weeks. Nutrient solution provided to "revive" the microbes is trisodium citrate, and will allow the digestion of the surfactant before plugging.