WO2019076449A1

WO2019076449A1 - Method to design a self-healing concrete additionated with a permeable concrete containing biological materials

Info

Publication number: WO2019076449A1
Application number: PCT/EP2017/076603
Authority: WO
Inventors: José Eduardo ALMEYDA CARBAJAL; Homero Ramirez Tovias; Carlos Enrique Castillo Linton; Nestor Isaias Quintero Mora
Original assignee: Cemex Reseach Group Ag
Priority date: 2017-10-18
Filing date: 2017-10-18
Publication date: 2019-04-25
Also published as: MX2020003991A

Abstract

The present invention relates to a healing agent for concrete mix designs or product, method to prepare the same, said agent containing porous micro concrete, bacteria and/or bacteria spores, at least a calcium source and a carbon source, and its use to enable a self-healing of microcracks or cracks that form in a hardened concrete.

Description

METHOD TO DESIGN A SELF-HEALING CONCRETE ADDITIONATED WITH A PERMEABLE CONCRETE CONTAINING BIOLOGICAL MATERIALS

DESCRIPTION FIELD OF THE INVENTION

The present invention relates to a granulated additive for concrete mix designs or product, method to prepare the same, said additive containing porous micro concrete, bacteria and/or bacteria spores, at least a calcium source and a carbon source, in order to enable a self-healing of microcracks or cracks that form in a hardened concrete.

In the description below, the references between square brackets ([ ]) refer to the list of references presented at the end of the text.

BACKGROUND OF THE INVENTION

Concrete represents one of the most resistant materials and used in the construction industry. However, hardened concrete is also very susceptible to cracking due to a number of factors, among which can be mentioned : excess water in the mixture (shrinkage), lack of control joints, freezing / thawing cycles, local high tensile strength, mechanical fatigue. Without adequate treatment, such cracks and microcracks tend to propagate affecting the integrity of the structure, thus favouring harmful external agents like oxygen, water, chloride or other chemicals to penetrate the concrete structure through the cracks networks and start corroding the metal reinforcement (grids, rods, etc.) leading a rapid deterioration of the mechanical resistance of the reinforced concrete.

These failures in concrete structures translate into million dollar losses, citing the United Kingdom as an example, where 4% of its Gross Domestic Product goes to repair and maintenance of concrete structures (Neville, 201 1) [1]. It is for this reason that the self-repair technology originated with the purpose of reducing the costs derived from the continuous maintenance and where alternatives of self-repair have been proposed through physical, chemical and biological methods.

It is known that concrete based materials can show autogenous healing of cracks (without addition of specific healing agents) due to the unreacted cement in the concrete that is exposed to moisture on the crack surface.

Until now, self-healing technologies have focused primarily on physical, chemical and recent biological methods. The physical methods consist mainly of the use of discontinuous fibers (Length: 4-30 mm and diameter: 10 to 100 μιη) of materials such as PVA, polyethylene and polypropylene of high tenacity, reducing cracks smaller than 150 μιη (Patent US 7,572,401) [2]. The repair achieved through the physical processes, could be more due to a process of autogenous repair of the cement than to an effect of the fibers.

The use of blends of rigid steel or copper fibers with flexible polymeric fibers, lignin or polyacrylate has also been reported. The former form a web that avoids the formation of macro- grids, while the flexible fibers permit the generation of bridges between the rigid fibers, avoiding the formation of microcracks by plastic shrinkage and evaporation of the mixing water (Patent CN 102786258) [3]. In general, the use of fibers represents an expensive option and difficult to scale to the production needs of large structures, so its use has not been able to spread in buildings susceptible to damage from cracking.

On the other hand, the chemical methods of self-repair have focused primarily on the use of epoxy resin particles, which, when fractured by the forces that generate the cracks, allow the release of the material contained therein, achieving repair. Some examples which may be mentioned are the technology based on the use of silanes and amines contained in epoxy resin capsules, whereby recoveries of the order of 90% of the integrity of the structures (Patent CN 103396652) [4] or those based in the use of urea- formaldehyde resins with which 98% recovery of flexural strength (Patent US 8,552,092) [5] has been reported.

There is another approach to self-repair based on the use of mineral mixtures (derived from fly ash, bentonite, alumino silicates and carbonates) capable of interacting with partially hydrated cement, resulting in the formation of a calcium silicate gel Hydrated and hydrated calcium aluminate, which together with ettringite and calcium oxide, is able to fill the cracks or formed cavities (Patent CN 103073212) [6]. These technologies, although attractive, present certain disadvantages, among which we can mention the prohibitive costs, the generation of secondary products that could affect the integrity of the cement as well as its reinforcements, the necessity of generation of fractures with the sufficient force that allow the liberation of the Content of the capsules, poor homogenization and difficulty performing adequately in subsequent cracking events.

As previously mentioned, since many years the use of self-healing technologies in concrete structures turned to a biotechnological perspective. The microbio logically induced calcite precipitation (MICP) is a mechanism on which the environmental conditions and the metabolism of some microorganisms allow the generation of calcium carbonate structures naturally. Procedures and formulations have been developed to provide the occurrence of this phenomena in a controlled manner (Patent CN 1778934) [7], even reaching the generation of high resistances (0.5-5Mpa) from permeable materials such as limestone, gypsum, sandstone, sand, earth, clay, mud sediment, sawdust, cardboard, mortar or softwood (International Application WO 2006/066326) [8]. This technology allowed the production of cement based self-healing materials through the action of microorganisms and their appropriate nutrients, (carbon source) and calcium precursor (calcium lactate), initially from the introduction of spores of alkalophilic bacteria at concentrations of 10⁶-10⁹ U.F.C. per cm³ of cement together with well-defined commercial culture media, reaching a crack repair between 0.4-0.5mm in 30 days (Patent CN 103880370) [9]. However, in this technology, the long term biological viability of the spores/bacteria is low due to the hostile conditions of the cement, which is why its attractive effects at initial ages (up to 1 month) are not maintained in the long term. To solve this problem, the use of porous particles derived from fly-ash or expanded clay (e.g. trademarks Liapor, Argex) has been envisaged, with spheres having a density of between 0.4 - 2.0g/cm³ and a size between 0.4 - 0.8mm for spore/bacteria immobilization (International Application WO 2009/093898) [10].

The pore size of the porous particle differs according to the material with which is impregnated, 0.01 - Ι .Ομιη for the nutrients (yeast extract, peptones, carbohydrates, fatty acids, amino acids, trace elements and a source of calcium such as calcium lactate, calcium acetate or other calcium salt of a carboxylic acid) and from 1.0 - ΙΟΟμιη for bacteria (e.g. Bacillus pseudofirmus and/or Sporosarcina pasteruii).

One of the main drawback of the technology described above is the need to use porous aggregates with weak mechanical resistances to immobilize the bacteria, calcium source and the carbon source. Depending on the volume faction of these porous aggregates in the final concrete, the mechanical properties (resistance in compression, flexion, etc...) will be negatively affected due to the low mechanical resistance of the porous selected aggregates (e.g. expanded clay) as described in International Application WO 2011/126361 [11].

Another important drawback from the prior art is related to the risk that the spores and the bacteria contained in the porous aggregates are leaching into the cement paste when the porous aggregate is mixed in the fresh concrete. This will lead to a depletion of active bacteria and spores since the bacteria/spores trapped into the cement paste will not survive due to the high pH.

BRIEF DESCRIPTION OF THE INVENTION

The present invention aims at providing an industrial cost-effective solution (using bi- products and cheap materials as raw material) that overcomes the drawbacks mentioned here above. More specifically the invention describes the use of a high strength artificial cement based aggregate with controlled porosity to immobilize the bacteria/spores, the calcium and carbon sources. Finally, the present invention provides means related to the manufacturing of the artificial porous aggregate means that reduce the risk of lixiviation of the bacteria/spores when exposed to fresh cement paste in final concrete containing the porous aggregate.

Furthermore, the invention does not aim achieving 100% healing of the cracks (all cracks fully filled with precipitation of calcite from the healing agent) but to achieve a level of healing that will inhibit external water to reach the metallic reinforcement in the concrete avoiding their corrosion and degradation.

The invention concerns the preparation of a porous micro concrete to produce dry micro concrete porous aggregates (carrier), to be impregnated (immobilized) with biological compounds, for use as a healing agent in partial substitution of the conventional dense aggregates, in the production of a self-healing concrete.

The micro concrete is produced using cement as a main binder with a controlled porosity using air entraining agents in the micro concrete mix design to optimize the mechanical strength of the dry micro concrete porous aggregates (carrier) while maintaining a level of porosity suitable to immobilize the bacteria/spores, calcium and carbon sources during the impregnation process.

The impregnation material comprises a carbon source, a calcium source and bacteria.

Advantageously molasses (a waste product from sugar production out of sugar cane) is used as the carbon source, representing the nutrient source for the growing and propagation of bacteria.

Advantageously calcium chloride was used as a calcium source. Advantageously the bacteria used is Bacillus pseudofirmus.

The impregnated porous micro concrete aggregates (healing agent) are substituted to the normal dense aggregates for various concrete mix designs in order to reduce or avoid the mechanical resistance drop of the concrete due to the presence of porous aggregates while enabling a high healing rate capability of the final concrete containing the impregnated micro concrete porous aggregates (healing agent).

DEFINITIONS

C.F.U - Colony- forming unit is a unit used to estimate a number of viable bacteria or fungal cells in a sample.

Healing agent - by healing agent we mean the dry porous micro concrete aggregates impregnated with the bacteria/spores, the calcium source and the carbon source. Carrier - by carrier we mean the dry porous micro concrete aggregates not yet impregnated with the bacteria/spores, the calcium source and the carbon source.

Impregnation: by impregnation we mean to saturate with a solution the micro concrete aggregates.

Final concrete: by final concrete we mean the concrete that contains a binder, dense aggregates, porous aggregates and water.

Substitution rate: by substitution rate we mean the percentage dense aggregates that is replaced by the micro concrete porous aggregates.

Healing rate: by healing rate we mean the percentage of the total length of a crack that is fully filled with precipitation of calcite.

EXAMPLES

EXAMPLE 1: PREPARATON OF A HEALING AGENT ACCORDING TO THE PRESENT INVENTION A) Preparation of the micro concrete based porous aggregates (carrier)

1) Mix design of the micro concrete.

The porous micro concrete production was realised using, for lm³ :

• Cement Type I, II, III, IV, V, IL, IS, IP according to ASTM CI 50 with dosages between 280kg/m³ to 350Kg/m³

• Fine aggregates typically 0.5mm to 1.0mm with dosages between 580kg/m³ and 640Kg/m³.

• Water to cement ratio located between 0.3% and 0.6 in weight % of cement

• Air entrainer (type cationic) with dry solid content located between 0.03% and 0.12% weight % of cement.

· Optionally, an alkali salt added with dosages located between 0.1%> and 0.4%> in weight %> of cement.

• Optionally, pozolanic additions (pulverized fly ash, slag, natural pozolans, etc..) can be added to the mix design.

Table 1 shows one typical composition with and without alkali salt, one typical mix design to produce the micro concrete to produce the porous aggregate. Element Micro concrete 1 Micro concrete 2

Ordinary Portland Cement 300 Kg/m³ 300 Kg/m³

Fine aggregates (0.55-0.84 mm) 587 Kg/m³ 587 Kg/m³

H20 121 L/m³ 121 L/m³

Air Entrainer 240 g/m³ 240 g/m³

NaCl additions 600 g/m³ O g/m³

Tflftfe 7: Typical concrete mix design for lm³ of porous micro concrete to produce porous aggregates (carrier)

Micro concrete cylindrical 30cmxl5cm samples size were prepared and cured for 28 days at 22±2°C and under 95% to 100% relative humidity.

2) Mechanical properties, densities of the micro concrete

The micro concrete prepared without air entrainer has a density typically located between 2,65ton/m³ and 2.73ton/m³.

The compressive strength and the density of the micro concrete samples was measured after 28 curing days using a conventional compressive press for concrete and mortars. The air content was determined using the norm ASTM CI 85 - 15a Standard Test Method for Air Content of Hydraulic Cement Mortar, table 2 presents properties of the hardened micro concrete prepared according to table 1. Typically, the porous micro concrete according to the invention had densities located between 1.99 and 2.30 ton/m³ corresponding to air contents of respectively 25 volume % and 15 volume %. Property Micro concrete 1 Micro concrete 2

Compressive Strength (MPa) 19.5 18.5

Density (ton/m³) 2.2 2.2

Air content (Vol %) 17.8 18.0

Table 2: Example properties of the hardened porous micro concrete

Porous micro concrete samples were prepared with different air content from 15 volume % to 25 volume %, corresponding to air entrainer dosages (dry solid content) of 0.03 to 0.12 weight % of cement.

Air content clay based aggregates was located between 35% and 45%, average 40%> in volume.

The size (diameter) of the air voids (pores) in the porous micro concrete was measured by microscopy (scanning electronic microscope measuring 5 to 10 pores in 10 fields from 500 to 1000 magnifications with an electronic microscope.). For each sample, at least 10 different zones or fields were selected and 5 to 10 pores were measured using magnifications of 500x to lOOOx. The measured voids/pores sizes were located between 15μιη and 150μιη with an average at 90 μιη.

From the experimental results (not shown), the air void dimensions (average diameter) located between 20 and 150 microns were regarded together with an air void volume % located between 15 volume % to 25 volume % to be an optimum colonia retention of at least about 6xl0⁷ C.F.U./g (5.89xl0⁷ C.F.U./g) of micro concrete. Air content volume below 15 volume % in the micro concrete will not enable to have sufficient colonia retention to heal the crack in the concrete, whereas micro concrete porous aggregates with more than 25 volume % air content will have low mechanical resistances thus affecting the mechanical resistances of the final concrete.

The porous micro concrete to produce the porous aggregates according to the invention advantageously optimizes both the colonia retention and the mechanical resistance of the final concrete containing the porous aggregates.

The alkali salt was selected typically from the list of, for example, NaCl, KC1, Na₂SiC"3, NaOH, etc.... Advantageously the addition of an alkali salt to the mix design of the micro concrete to produce porous aggregates will reduce the setting time of the micro concrete, increase the hydration of the cement in the micro concrete minimizing the amount of unreacted cement in the porous aggregates.

Dosages of the alkali salt bellow 0.1 weight % of the cement will have a lower effect on the acceleration of the hydration whereas dosages above 0.4 weight % of the cement will not further increase the acceleration of the hydration and can deteriorate the steel reinforcement in the final concrete.

3) Producing the porous aggregates (carrier) from the micro concrete.

The samples of micro concrete were crushed typically using a jaw crusher and sieved, in order to reach aggregates particles with size located between 0.5mm and 12mm, specifically between 0.8mm and 10mm, more specifically between 1mm and 8mm.

The density of the aggregates was assumed as the same of the micro concrete due to the size of the porous that are much smaller than the size of the aggregates.

It can be understood that the crushing process of the porous micro concrete will expose poorly hydrated cement (unreacted cement).

B) Preparation, selection of the substances entering into the porous aggregates

1) Bacteria cultivation and preparation

Bacteria Bacillus pseudofirmus were cultivated using the following procedure:

Step 1 - The raw molasses was diluted with water to form a solution with dry solid content of molasses located between 0.05 and 3weight %, specifically between 0.08 and 2.5 weight %, more specifically between 0.1 and 2 weight %.

Step 2 - The pH was adjusted using a basic solution of sodium hydroxide 4N to be in a range of 8 to 12, specifically between 8.5 and 11, more specifically between 9 and 10.5 measured with a potentiometer. This is the optimum pH for the bacteria growth according to known literature.

Step 3 - The removal of the solid particles was made by using the Number 1 filter Whatman or by centrifugation on a Sorvall Legend X1R centrifuge.

Step 4 - The sterilization of the molasse solution was done by autoclaving using a known sterilization process at a temperature of 121°C for 30 minutes at 1.03 bar.

Step 5- The inoculation of the molasses solution was done with 100 μΙ_, of Bacillus pseudofirmus at 10⁷-10⁹ C.F.U. per mL of molasses solution. Step 6 - As required for Bacillus pseudofirmus bacteria, the culture of step 5 was agitated at 200rpm at 30°C for 24h.

Step 7 - The solution passed then through a centrifugation process for lOmin at 6000rpm between 4°C and 10°C to gather the bacterial pellet (cellular mass). Step 8 - Using a standard culture broth 0.025% v/v MgS0₄.7H₂0, 0.1% KC1, ImM Ca (N0₂)3, 0.0 lmM, MnCl₂.4H₂0 and O. lmM FeS0₄.7H₂0 as referenced on known literature, a 100% sporulation in 72 hours incubation was achieved at 30°C.

Step 9 - The broth was centrifuged for lOmin at 6000rpm between 4°C and 10°C to gather the bacterial pellet (cellular mass). Step 10 - The bacterial pellet was washed with distillate water, and bacteria containing solution having a bacteria concentration of 10⁹ - 10 ¹¹ C.F.U per ml is stored at 4°C.

2) The Ca source

The calcium source was selected from inorganic calcium salts such as calcium chloride. A water solution with calcium source concentration between 0.1M and 0.3M, more specifically between 0.15M and 0.25Molar was prepared

3) The C-source

The organic carbon source was selected from raw molasses that were diluted with water to form a solution with dry solid content of molasses located between 0.08 and 2.5 weight %, more specifically between 0.1 and 2.0 weight %.

C) Impregnation of the micro concrete aggregates (carrier)

The micro concrete aggregates were impregnated with the calcium source solution, carbon source solution and the solution containing the bacteria in two separate steps:

Step 1: The micro concrete was impregnated with a solution A containing the carbon source solution and the calcium source solution obtained by mixing the carbon source solution and the calcium source solution described here above. In solution A, the concentration of molasses (dry solid content) was located between 0.08 and 2.5 weight %, more specifically between 0.1 and 2.0 weight % and the concentration of CaCl₂ was located between 0.15M and 0.25M. Typically, the impregnation required 0.3 - 1.0 volume of solution A in liters per weight of porous micro concrete aggregates (carrier) in kg. The impregnation took place at 50°C for 30 minutes and then the impregnated micro concrete aggregates were dried at 30°C until no loss of weight is verified.

Step 2: The material of step 3.1 was encapsulated in a cartridge- like column, the bacteria/spores solution B (with concentration of 10⁹ - 10¹¹ C.F.U per ml) was then poured slowly for impregnation using 0.5 - 1.0 volume in litres of the solution B per weight of porous micro concrete aggregates (carrier) in kg. The material was then incubated for 12 to 24 hours and then the bottom of the column was opened to drain by gravity the excess of the bacteria or their spores solution, the process was repeated until the expected retention of at least 6xl0⁷ C.F.U./gram of porous micro concrete was reached.

The impregnated porous micro concrete (healing agent) material was then dried at 30°C until no loss of weight is verified.

According to a bacterial viability test, the porous micro concrete impregnated with bacteria/spores solution (healing agent) had an original concentration of 6.5xl0⁷ C.F.U./gram of porous micro concrete. After 6 months, the same material showed a concentration of l .OxlO⁷ C.F.U./gram of porous micro concrete, according to the same viability test. Therefore, it is recommended to use the impregnated micro concrete porous aggregates (healing agent) within some days to a couple weeks, maximum one month.

It is known from prior art that bacteria/spores are very sensitive to the pH.

The hydration of cement typically results in a pH rise between 12.5 to more than 13, this pH level will affect the survival rate of the bacteria/spores and therefore reduce the healing capability of the final concrete related.

Lixiviation tests with water done with micro concrete porous aggregates (carrier) prepared respectively with micro concrete 1 and 2 showed that the pH increase was respectively 11.6 and 13.3. The high pH value measured for micro concrete porous aggregates (carrier) prepared with micro concrete 2 was related to the amount of unreacted cement exposed during the crushing process to produce the aggregates, whereas the presence of the alkali salt in micro concrete 1 reduced the amount of unreacted cement in the micro concrete porous aggregates (carrier). Therefore, the presence of an alkali salt in the porous micro concrete (micro concrete 1, table 1) will reduce the risk of bacteria/spores depletion during impregnation by maintaining a pH that is below 12 to ensure a maximum bacteria/spores survival rate.

EXAMPLE 2: PREPARATION OF A FINAL CONCRETE CONTAINING THE HEALING AGENT Final concrete samples were prepared mixing the constituents below typically for 4 to 5 minutes, using a conventional concrete mixer, following representative final concrete samples were prepared as described in Table 3.

Constituents of the final concrete samples:

· Portland cement type I

• Normal dense gravel within size range 9.0 - 25.0 mm

• Normal dense sand within size range 0.25 - 4.5 mm

• Expanded clay within size range 1 mm -10mm replacing part of the dense aggregates (sand + gravel).

· Micro concrete porous aggregates within size range 1.2 - 8 mm replacing part of the dense aggregates (sand + gravel)

o Micro concrete 2 (table 1) porous aggregates not impregnated

o Micro concrete 1 (table 1) porous aggregates not impregnated

o Micro concrete 2 (table 1) porous aggregates impregnated according to the procedure described above

o Micro concrete 1 (table 1) porous aggregates impregnated according to the procedure described above

• Water

According to the invention and with respect to the reference mix part of the dense aggregates was replaced by porous aggregates. In Mix 1 the porous aggregates were expanded clay, in Mix2 the porous aggregates were non- impregnated micro concrete 2 (table 1) porous aggregates (carrier), in Mix 3 the porous aggregates were non-impregnated micro concrete 1 (table 1) porous aggregates (carrier), in Mix 4 the porous aggregates were impregnated micro concrete 2 (table 1) porous aggregates (healing agent), in Mix 5 the porous aggregates were impregnated micro concrete 1 (table 1) porous aggregates (healing agent).

According to the invention the replacement percentage (substitution rate) of dense aggregates by micro concrete porous impregnated aggregates (healing agent) was located between 0.5 volume % and 10 volume %.

Some examples of final concrete mix designs are presented in the Table 3. In the examples, the substitution rate of the coarse and sand aggregates (with respect to the Reference Mix) by porous micro concrete aggregates in Mixl, Mix 2, Mix 3, Mix 4 and Mix 5 was maintained constant at 1.4 volume %.

Table 3: Mix designs of the various concrete prepared, dosages in Kg or volume per m3 of final concrete

For each mix, samples with dimensions 25x15x15cm were prepared and cured for 28 days at 22±2°C and under 95% to 100% relative humidity, to perform healing tests and cube samples were prepared to measure the standard mechanical resistance in compression respectively at 7 and 28 days.

EXAMPLE 3: TESTING HEALING PROPERTIES OF THE FINAL CONCRETE

The final cured concrete samples prepared according to the mix designs examples of Table 3 were pre-cracked using a hydraulic press at a velocity of 0.35 N/mm2*sec until cracks were visible (naked eye). In average 15 surface cracks having width between 200μιη to 2mm and length between 5 to 10cm per sample were selected and marked. Pre-cracked samples were stored for 100 days at 22±2°C and under 95% to 100% relative humidity.

The healing rate of each sample after 100 days was calculated by averaging the repair percentage (coverage reached of the total crack) of 10 cracks (length: 1.5-2 cm and wide: 100- 700μιη) in a sample.

Table 4: Healing rate at 100 days for the final concrete prepared with the mix designs of table 3.

At 7 and 28 curing days, sets of 3 cubes of each mix design were tested in compressive strength using a standard press tests. Table 5 presents the average results for the different final concrete prepared with the mix designs described on table 3.

Table 5: Compressive strength of the different samples From table 5 it can be seen from the comparison between the reference Mix and Mix 1 , that replacing 1.4 volume % of dense aggregates with expanded clay aggregates resulted in a mechanical resistance decrease of about 18% at both 7 and 28 days.

From table 5 it can be seen from the comparison between the reference Mix and Mix 2, respectively Mix 3, that replacing 1.4 volume % of dense aggregates with micro concrete porous aggregates did not affect negatively the mechanical resistance measured at 7 or 28 days.

Similarly, table 5 shows that the replacement of 1.4 volume % of dense aggregates with micro concrete porous impregnated aggregates with or without Alkali salt additions did not affect negatively the mechanical resistance measured at 7 or 28 days.

From table 4 taking the reference Mix as a reference at 100 days (healing rate 36.5%) it can be seen that replacement of 1.4 volume % of dense aggregates with micro concrete porous impregnated aggregates without alkali salt additions (Micro concrete 2) resulted in a healing rate increase of 40% (Mix 4).

From table 4 taking the reference Mix as a reference at 100 days (healing rate 36.5%) it can be seen that replacement of 1.4 volume % of dense aggregates with micro concrete porous impregnated aggregates with Alkali salt additions (Micro concrete 1) resulted in a healing rate increase of 96% (Mix 5).

It can be generally admitted that the healing rate was strongly related to the bacteria/spores C.F.U active in the final hardened concrete. The comparison between the healing rate of the final concrete prepared respectively with Mix 4 and 5 showed that the presence of the alkali salt in the mix design to prepare the porous micro concrete aggregates positively impacted the survival rate of the bacteria/spores.

In concrete samples prepared with the mix design 5, the alkali salt will be present in the porous micro concrete aggregates and, when exposed to the cement paste of the final concrete during the mixing processs, the alkali salt will accelerate the setting of the cement paste of the final concrete at the surface of the porous aggregates. As a result the bacteria/spores located inside the porous aggregates will be protected from the high rise of pH due to cement hydration and leaching of the bacteria/spores into the concrete matrix where they would not survive will be strongly reduced.

Additional tests were performed based on the Mix design 5 by increasing the healing agent dosages (substitution) to 5 volume %, 10 volume %, 15 volume %, 20 volume % and 30 volume % of the total aggregates of the final concrete. It was observed that a mechanical resistance decrease in compression of the final concrete only occured for healing agent dosages (substitution) above 20 volume % of the total aggregates of the final concrete.

The healing rate, on the other hand, remained constant at 69% to 72% for dosages of the healing agent of 5 volume %, 10 volume %, 15 volume % and 20 volume % and 30 volume % of the total aggregates of the final concrete.

From an industrialisation (manufacturing capability, storage capacity, separate silo for the healing agent) and cost (manufacturing cost of the healing agent is very high compared to normal aggregates) point of view, it is preferable to use a low dosage of the healing agent while ensuring the maximum healing capability.

The healing agent according to the invention thus provides important advantages with respect to the prior art. According to the invention high healing rates, above 70%, can be achieved with low dosages of the healing agent (0.5 to 10 volume %, preferably 0.5 to 8 volume %, even more preferably 0.5 to 5 volume % of the total volume dense aggregates) and the mechanical properties of the final concrete containing the healing agent are not negatively affected by the presence of said healing agent.

Also, another advantage according to invention relates to the fact that the healing agent contains the bacteria/spores, the calcium source and the carbon source, whereas in the prior art the healing agent consisted in at least two components that needed to be added jointly to the final concrete.

Another advantage of the healing agent according to the invention is that it can advantageously be produced industrially at low manufacturing costs using available cheap resources and requiring low dosages to provide self-healing capabilities to any concrete.

A final advantage is that the healing agent can be stored for times up to one month without losing its efficiency thus simplifying the logistics related costs. Listing of references

1) Neville, "Properties of concrete" 5^th Edition, Pearson Education Limited, Essex, 201 1

2) Patent US7,572,401

3) Patent CN 102786258

4) Patent CN 103396652

5) Patent US 8,552,092

6) Patent CN 103073212

7) Patent CN 1778934

8) International Application WO 2006/066326

9) Patent CN 103880370

10) International Application WO 2009/093898

11) International Application WO 2011/126361

Claims

1. A granular dry crack healing agent comprising (i) a dry micro concrete based porous aggregates with an air content located between 15 and 25 volume percent, (ii) an inorganic calcium source, (iii) an organic carbon source, and (iv) bacteria and/or bacteria spores, wherein (ii) to (iv) are located into the pores of (i), and

wherein (i) is obtained from crushing and sieving a cured micro concrete obtained from a concrete mix design having the following characteristics :

between 250kg and 400kg of cement per cubic meter of micro concrete;

- fine aggregates;

- water;

at least one air entrainer with a dry solid content located between 0.03 and 0.12 weight % of the cement;

optionally at least one alkali salt; and

- optionally pozolanic additions.

2. The granular dry crack healing agent according to claim 1, characterized in that said healing agent has a particle size located from 0.5mm to 12mm.

3. The granular dry crack healing agent according to claim 1 or 2, wherein the alkali salt has a dosage located between 0.1 and 0.4 weight % of cement.

4. The granular dry crack healing agent according to any one of claims 1 to 3, wherein the alkali salt is selected from NaCl, KC1, Na₂SI0₃, NaOH.

5. The granular dry crack healing agent according to any one of claims 1 to 4, wherein the organic carbon source is molasses.

6. The granular dry crack healing agent according to any one of claims 1 to 4, wherein the inorganic calcium source is CaCl₂.

7. Use of a granular dry crack healing agent according to any one of claims 1 to 6 in a concrete mix design to enable self-healing of microcracks or cracks that form in the hardened concrete.

8. A self-healing concrete mix design comprising a granular dry crack healing agent as defined in any one of claims 1 to 6, in a dosage located between 0.5 to 10 volume % with respect to the volume of sand, fine and coarse aggregates of said concrete.

9. A self-healing concrete mix design according to claim 8, wherein the dosage of granular dry crack healing agent is located between 0.5 to 8 volume % with respect to the volume of sand, fine and coarse aggregates of said concrete.

10. A self-healing concrete mix design according to claim 9, wherein the dosage of granular dry crack healing agent is located between 0.5 to 5 volume % with respect to the volume of sand, fine and coarse aggregates of said concrete.

11. A method of preparing a granular dry crack healing agent comprising the steps of: a) preparing a concrete mix design having the characteristics as defined in claim 1 ; b) curing said micro concrete mix design obtained in step a);

c) crushing and sieving said cured micro concrete mix design obtained in step b) to obtain dry micro concrete based porous aggregates;

d) impregnating said dry micro concrete based porous aggregates obtained in step c) with a solution A containing a carbon source and a calcium source;

e) drying said impregnated micro concrete based porous aggregates obtained in step d) until no weight loss is measured;

f) impregnating said dry impregnated micro concrete based porous aggregates obtained in step e) with a solution B containing bacteria and/or bacteria spores;

g) incubating said impregnated micro concrete based porous aggregates obtained in step f) for 12 to 24 hours;

h) drying said impregnated micro concrete based porous aggregates obtained in step g) until no weight loss is measured.

12. The method according to claim 11, wherein a porous micro concrete with an air content located between 15 and 25 volume percent is obtained in step a).

13. The method according to claim 11 or 12, wherein said dry micro concrete based porous aggregates obtained in step c) have a size range located between 0.5mm and 12mm.

14. The method according to any one of claims 11 to 13, wherein said solution B in step f) has a concentration of 10⁹ to 10¹¹ C.F.U. per ml.

15. The method according to claim 14, wherein 0.5 to 1.0 volume in litters of solution B is used by weight of porous micro concrete aggregates in kg.

16. The method according to any one of claims 11 to 15, wherein steps f) and g) are repeated until obtaining a retention of at least 6x10⁷ C.F.U. per gram of impregnated micro concrete based porous aggregates.

17. The method according to any one of claim 1 to 16, wherein a step g') of measuring the C.F.U. per gram of impregnated micro concrete based porous aggregates is added after the step g) and before the step h).