EP3707110A1

EP3707110A1 - Composition of high tensile strength cement-based mixture with improved rheological properties

Info

Publication number: EP3707110A1
Application number: EP18789408.4A
Authority: EP
Inventors: valeria corinaldesi
Original assignee: Atoms Srl
Current assignee: Atoms Srl
Priority date: 2017-11-10
Filing date: 2018-10-18
Publication date: 2020-09-16
Also published as: MA50550A; WO2019091751A1

Abstract

A composition comprising the following materials (weight proportions): 1000 parts of hydraulic binder, 600-2900 parts of dry granular material with size (D90) smaller than 5.0 mm, 5-130 parts of expansive agent, 5-400 parts of high- modulus (more than 20 GPa) fibers, 1-40 parts of water reducing agent (dry mass), 1-200 parts of colloidal silica (dry mass), 150-550 parts of water, 0-200 parts of filler with size smaller than 0.200 mm, 0-10 parts of shrinkage reducing agent (dry mass). A high-tensile strength hardened mixture can be obtained by means of this composition, thanks to the synergic effect due to the presence of both high-modulus fibers and expansive agent, producing a beneficial self- prestress effect, homogenously diffused. This effect is obtained if both kinetic of expansive reaction and expansion rate are carefully controlled, as a function of the amount of fiber reinforcement used in the mixture. Also the other concrete ingredients such as hydraulic binder and inert granular materials play a fundamental role, indeed, by indirectly influencing both kinetic and rate of expansion. Finally, due to the addition of colloida silica an excellent flowability of the mixture at the fresh state can be achieved (in absence of both bleeding and segregation).

Description

COMPOSITION OF HIGH TENSILE STRENGTH CEMENT-BASED

MIXTURE WITH IMPROVED RHEOLOGICAL PROPERTIES

DESCRIPTION

As already known, one of the problems related to the use of concrete as a building material arises from the fact that concrete presents a very different mechanical behavior, depending on whether the concrete is subject to either compressive or tensile stress.

The difference between the compressive and tensile behavior of concrete can be reduced if the concrete is initially pre-compressed, because when it will be later subjected to tensile stresses, first there will be a phase of material unloading from the previously accumulated pre-stress, and only afterwards this material will undergo traction. In this regard, the technology of prestressed reinforced concrete (using pretensioned bars or cables or strands of steel) has been well known for some time, but its use is limited to large prefabricated structural elements (due to the high costs and complexity of the technology).

The prior art relating to this type of concrete is mentioned below:

A1 ) R. Sahamitmongkol, T. Kishi, Tensile behavior of restrained expansive mortar and concrete, Cement & Concrete Composites, 33 (201 1 ), 131 -141 .

A2) R. Sahamitmongkol, T. Kishi, Tension stiffening effect and bonding characteristics of chemically prestressed concrete under tension, Materials & Structures, 44 (201 1 ), 455-474.

A3) W. Sun, H. Chen, X. Luo, H. Qian, The effect of hybrid fibers and expansive agent on the shrinkage and permeability of high-performance concrete, Cement & Concrete Research, 31 (2001 ), 595-601 .

A4) EP 2069257 A2, Concrete composition with reduced shrinkage, priority date the 20^th September 2006.

A5) M.S. Meddah, M. Suzuki, R. Sato, Influence of a combination of expansive and shrinkage-reducing admixture on autogenous deformation and self-stress of silica fume high-performance concrete, Construction & Building Materials, 25 (201 1 ), 239- 250.

A6) Jung-Ju Park, Doo-YeolYoo, Sung-Wook Kim, Young-Soo Yoon, Drying shrinkage cracking characteristics of ultra-high-performance fibre reinforced concrete with expansive and shrinkage reducing agents, Magazine of Concrete Research, 65(4), 2013, 248-256.

A7) CN 101560082 B, Ultrahigh-strength active powder concrete and preparation method thereof, Google Patents, priority date the 16th April 2008.

A8) Houssam A. Toutanji, Properties of polypropylene fiber reinforced silica fume expansive-cement concrete, Construction and Building Materials, 13 (1999), 171 -177.

(High-strength micro-expansion prestress anchoring grouting material and preparation method thereof), Google Patents, priority date the 2^nd March 201 1 .

A10) W. Huang, Q.Y. Ma, P.B. Cui, Experiment and Analysis of Flexural

Strength for Shrinkage-Compensating Steel Fiber Reinforced Shotcrete,

Advanced Materials Research, 163-167 (2010), 947-51 .

A1 1 ) Wang. A., Deng M., Sun D., Mo L, Wang J., Tang M., Effect of

Combination of Steel Fiber and MgO-type Expansive Agent on Properties of Concrete, J. of Wuhan Univ. of Technology-Mater. Sci. Ed., 26(4) 201 1 , 786-

790.

A12) S.P. Cao, Q.F. Zhou, Y. Peng, G.X. Li, Effects of Expansive Agent and Steel Fiber on the Properties of the Fly Ash Ceramsite Lightweight Aggregate Concrete, Applied Mechanics & Materials, 357-60 (2013), 1332-36.

A13) IT 0001420870, composition of high tensile strength cement-based mixture, priority date is the 2nd December 2013.

A14) RILEM Report 36, State-of-the-Art Report of RILEM Technical Committee TC 201 -TRC Textile Reinforced Concrete', Ed. By W. Brameshuber, RILEM Publications S.A.R.L., 2006.

A15) WO 2015108990 A1 , Addition of colloidal silica to concrete, priority date is the 17^th January 2014. A16) R. Yu, P. Spiesz, H.J.H. Brouwers, Effect of nano-silica on the hydration and microstructure development of Ultra-High Performance Concrete (UHPC) with a low binder amount, Construction and Building Materials, 65 (2014), 140-150.

A17) L.Senff, J.A.Labrincha, V.M. Ferreira, D. Hotza, W.L. Repette,

Effect of nano-silica on rheology and fresh properties of cement pastes and mortars, Construction and Building Materials, 23 (2009), 2487-2491 .

A18) V. Corinaldesi, Combined effect of expansive, shrinkage reducing and hydrophobic admixtures for durable Self Compacting Concrete, Construction and Building Materials, 36(2012), 758-764.

The documents A1 and A2 describe the so-called Chemical Prestressed Reinforced Concrete (CPRC), but this material, despite having been obtained with expansive agent added to the mixture, does not contain fibers inside the cement mixture, and therefore the 'prestress effect' works only thanks to the effect of external constraint offered by the reinforcing metal bars (the classic rods for reinforced concrete). But such concentrated metal reinforcement does not allow to fully exploit the prestress effect, as it involves only a small portion of material, that is the thin layer of expansive concrete that is in the immediate vicinity of the steel bars. In theory, a uniformly distributed fibrous reinforcement on the entire portion of the material in tension would be much more effective.

The study of concretes containing both short reinforcing fibers and expansive agents added to the cement mixture has been very limited till now, and very few scientific works can be found in this regard. For example, in the A3 and A4 documents the focus was only on the hygrometric shrinkage and not on the mechanical performances, due to the fact that the expansive agents have been used only for the purpose of creating expansion inside the mixture for then compensate the subsequent hygrometric shrinkage, and produce the so-called 'shrinkage compensating concrete', and not to increase the concrete mechanical performance.

If not combined with high modulus fibers dispersed in the matrix the expansive agents could not produce any positive effect in terms of tensile and flexural performance, as seen in A5, in which the effect of adding agent expansive (combined with a shrinkage reducing admixture) even produced a slight decrease in: compressive strength, indirect tensile strength and modulus of elasticity. Similarly in the document A6, in which the expansive agent (combined with a shrinkage reducing admixture) has been added to very high performance concretes (UHPFRC, acronym for Ultra High Performance Fiber Reinforced Concrete), and therefore in this case containing metal fibers randomly distributed. But the performance of the UHPFRC was already so high due to the very low water / binder ratio adopted that the joint use of expansive agent and metal fibers was not able to bring significant additional benefit.

Similarly in the document A7, which explicitly states that the sole purpose for which the expanding agent (AEA) was added to the so-called 'ultra high-strength reactive powder concrete' is to counteract the occurrence of cracking, see point [0032] of the A7 description.

Toutanji in A8 showed that the presence of expansive cement and polypropylene fibers with an elastic modulus of 8 GPa did not give rise to any type of synergy. Also in A9 the use of low modulus polypropylene fibers (less than 10 GPa) did not allow any self-prestress effect (the prestress referred to in the document is given by steel cables).

Huang et al. in the document A10 studied a cement mixture to produce shotcrete containing both expansive agent and short metallic fibers. They noted some improvement in flexural strength without reaching absolute values above 7 MPa after 28 days of wet curing.

Wang et al. (A1 1 ) studied a cement mixture containing magnesium oxide as expanding agent and short metal fibers. They noticed some improvement in indirect tensile strength (+38%) but without reaching absolute values above 5 MPa (maximum 4.8 MPa) after 28 days of wet curing, compared to a compressive strength after 28 days of approx. 65 MPa (with a ratio between tensile strength and compressive strength equal to 0.07).

Cao et al. (A12) produced high strength lightweight mixture by using both short fibers and expansive agent. They observed a good behavior in terms of mechanical strength development, but without reaching absolute values above 1 1 .2 MPa after 28 days of wet curing, even if the compression resistance after 28 days was equal to 62.7 MPa (with a ratio between bending strength and compressive strength equal to 0.18).

Document A13, in the name of the same applicant, describes the composition of a mixture comprising the following materials (proportions by weight): hydraulic binder comprising cement (1000 parts), and expanding agent (45-180 parts); granular material with dimension (D50) greater than 0.200 mm and dimension (D90) less than 5.0 mm (600-2900 parts); fibers with an elastic modulus greater than 50 GPa (25-400 parts); water (250-550 parts); water reducing admixture (10-100 parts); any kind of filler with particles smaller than 0.200 mm (0-300 parts). By using the composition of the mixture described in A13, concretes can be formed which achieve excellent levels of tensile strength, and hence flexural strength, after a few hours since casting.

This result is obtained thanks to the synergistic effect between the expansive reaction and the fibrous reinforcement homogeneously distributed in the matrix (but this synergy cannot be attributed solely to the presence of expanding agent and fiber reinforcement, but it is promoted by the particular overall composition of the mixture, with an important role also played by other ingredients such as cement and inert). The new material developed in A13, and characterized by high tensile strength, has been called 'autocompressed concrete', also indicated by the acronym SPC (English acronym for Self- Prestressed Concrete). In A13 there is no mention of the rheological properties at the fresh state of those mixtures showing self-prestress effect.

The object of the present invention is to solve the drawbacks of the prior art by providing a cementitious composition which is able to ensure high tensile strength with improved rheological properties with respect to the cement-based compositions of the prior art.

This object is achieved according to the invention, by the cement mixture composition of the independent claim 1 .

Further characteristics of the invention will become clearer from the dependent claims.

The composition of a cementitious mixture according to the invention is expressed by claim 1 . The mixture proportion of the cementitious composite, according to the invention, represents an innovation with respect to that described in document A13, since the mixture according to the invention contains a new ingredient consisting of colloidal silica. This mixture allows an excellent flowability of the mixture to be obtained at the fresh state, in the absence of bleeding and segregation phenomena. In fact it has been studied for optimizing the rheological behavior of the cementitious composite, while maintaining the self- prestress effect, and therefore the same advantages described in the mixture of document A13 in terms of mechanical performance in tension.

Subsequently the quantity of materials is indicated in parts considered as percentage by weight.

The mixture proportion of the cementitious composite, according to the invention, comprises the following materials:

- Cement-based hydraulic binder (1000 parts);

- Dry granular inert material with dimension (D90) less than 5.0 mm (600- 2900 parts);

- Expansive agent (5-130 parts);

- Fiber with elastic modulus greater than 20 GPa (5-400 parts);

- Water reducing admixture (1 -40 parts of dry mass);

- Colloidal silica (1 -200 parts of dry mass);

- Water (150-550 parts).

Optionally the mixture can also include:

- Inorganic filler with particles smaller than 0.200 mm (0-200 parts);

- Shrinkage reducing admixture (0-10 parts of dry mass);

- Viscosity modifying agent;

- Admixture able to act on the degree of hydration of the cement, by shortening or lengthening the setting or hardening times;

- Catalyst of the expansive reaction.

For the accurate definition of the technical terms used in the invention description, also comprising 'superplasticizer admixture', 'shrinkage reducing admixture', 'setting retarder admixture', reference is made to the detailed description in EP2069257A2 (A4). The considerable amplitude of the indicated intervals is related to the large variations which can show the dosage of hydraulic binder (set equal to 1000 parts for simplicity).

The cement-based hydraulic binder can be any type of cementitious hydraulic binder commonly available on the market, as for example Portland cement, aluminous cement or supplementary cementitious materials. If you use Portland cement, the hydraulic binder dosage that can vary from 215 to 950 kg per cubic meter of mixture.

The dry granular inert material, as generally known, is sand or gravel, which is ground and sieved with a sieve, so as to obtain dimensions (D90) less than 5.0 mm.

The expansive agent is commonly available on the market. For example, the expansive agent known under the brand Stabilmac^® from BASF Construction Polymers GmbH can be used.

The expansive agent may contain one or more of the following oxides:

CaO, MgO, AI2O3, SO3, Fe203 and S1O2. The expansive agent is preferably ground to a fineness comparable to that of the hydraulic binder, with particles smaller than 0.200 mm.

Fibers have high elastic modulus, higher than 20 GPa; fibers can have any shape or geometry; fibers are coated or uncoated; fibers are treated or untreated with sizing; fibers can have either natural or artificial or recycling origin (coming from the recovery and treatment of industrial waste or byproducts). Fibers can be either randomly disposed or oriented; fibers can be either homogenously dispersed or concentrated in particular zone; fibers can be either single or held together with any technique; fibers can form textiles of any geometry comprising non-woven fabric; fibers can form either 2D-textiles or 3D- textiles, both kinds made with any technique, such as for example the technique described in the document A14.

For example, fibers may include: short metal fibers (length 300 mm, aspect ratio 120, E = 210 GPa) and / or PVA fibers (length 120 mm, aspect ratio 60, E = 25 GPa). The water reducing admixture is commonly available on the market. For example, you can use the water reducing admixture known under the brand Melflux^® from BASF Construction Polymers GmbH.

The water-reducing admixture may be a polycarboxylic compound, comprising a polycarboxylate ether-based superplasticizer (PCE).

With respect to the composition described in A13, the following modifications have been made, which allow a better rheological behavior at the fresh state in terms of ability to flow through the mould (better flowability):

- The content of material with particles smaller than 0.200 mm decreases from a maximum value of 300 parts up to a maximum value of 200 parts, and at the same time the expansive agent content (another substance with particles smaller than 0.200 mm) decreases from 45-180 parts to 5-130 parts. In practice the maximum overall content of particles smaller than 0.200 mm (including the hydraulic binder, 1000 parts) has dropped from 1480 parts to 1330 parts with a decrease of 10%;

- Colloidal silica has been added (1 -200 parts of dry mass);

- The value of fiber elastic modulus has been lowered from 50 GPa to 20 GPa, in order to include high modulus polymeric fibers such as, for example, PVA fibers.

In order to conceive this new formulation it was not possible to draw from the scientific and patent literature. The reason is that, although there are several works which highlight the advantages in terms of development of mechanical resistance linked to the use of colloidal silica in cement-based mixtures (A15 and A16), very few studies have been conducted on the modeling of rheological behavior of cementitious pastes containing colloidal silica to replace inorganic fillers. The only significant document is A17 in which 2.5% by weight of nanosilica has been used on cement-based mixture, but in reality A17 is more focused on the study of the influence of the addition of colloidal silica on the setting time of cement paste, and there isn't comparison with other nanosilica (or silica fume) introduced in the mixture as a powder instead of aqueous suspension. The mixture proportion of the cement-based composite, according to the invention, was based on a series of experimental data never published or disclosed, some of which are given below as an example.

Five concretes were prepared, A, B, C, D and E, with the mixture proportions reported in Table 1 .

Table 1 - Mixture proportions (parts)

It can be seen that mixtures A, B, C and D have a composition included into intervals defined in A1 3, while mixture E contains all the ingredients reported in the claims of the present patent application, according to the claimed dosage ranges; with the exception of the fibers. Fibers, however, can not be introduced into the rheometer, and they simply constitute the carried phase, therefore they have no influence on the rheology of the carrying phase, i.e. the cement-based matrix.

Then the rheological behavior of such mixtures A, B, C, D and E was evaluated, after 30 minutes from the mixing of the ingredients with water, by using the same experimental procedure described in A1 7, therefore referring to the rheological model of Bingham. The results obtained are shown in Table 2.

Table 2 - Rheological parameters measured after 30 minutes since mixing with water A (^*) B (^*) C D D O E

Yield stress (Pa) -5 25 15 20 16

Plastic viscosity (Pa-s) 0.02 2.55 2.20 1 .80 0.02

Spread, ref EN 1015-3 (mm) 280 130 170 150 260

It can be noticed that mixture A shows a negative value of the yield stress. Therefore, the cohesion of this mixture, corresponding to a rotation speed equal to zero, is negative; as a consequence: the mixture has internal segregation, as confirmed by the relative mortar.

The mixtures B, C and D show an adequate value of yield stress

(absence of segregation), but the value of the plastic viscosity is very high with a consequent poor flow ability of the mixture (spread value abundantly less than 200 mm) .

The mixture E is the only one able to combine the requirements of high flowability and absence of segregation.

An example of mixtures containing also fibers is reported below with six compared mixtures.

Mixtures shown in the example illustrate the invention without limiting it. Materials contained in the mixtures are: Portland cement type CEM I 52.5 R, silica sand (with a particle size of less than 1 .0 mm), short metal fibers (length 300 mm, aspect ratio 120, E = 210 GPa), PVA fibers (length 120 mm, aspect ratio 60, E = 25 GPa), carboxylic-based superplasticizer (in powder), expansive agent with calcium oxide content higher than 50%, white silica fume, colloidal silica (30% dry mass).

The mixture proportions are reported in Table 3.

Table 3 - Mixture proportions (dosages in kg referred to 1 m³)

Ref-FRC SPC-1 0 SPC-20 SPC-30 New A New B

Water/cement 0.44 0.44 0.50 0.36 0.44 0.44

Water 200 200 225 200 197 197

Cement 450 450 450 450 450 450

Sand 1250 1210 1 190 1240 1240 1240

Expansive agent - 40 40 40 20 20

Superplasticizer 4.5 5.0 5.0 6.0 6.0 6.0 Steel fibers 100 100 150 150 150 -

PVA fibers - - - - - 26

Limestone filler 100 100 100 50 - -

Silica fume - - - 50 - -

Colloidal silica - - - - 3 3

(^*) composition within intervals described in A13

Rheological and mechanical performances of these mixtures are reported in Table 4.

Table 4 - Rheological and mechanical performance

(^*) composition within intervals described in A13

The values of spread obtained have been measured after 15, 30 and 45 minutes from ingredient's mixing, according to the procedure described in the EN 1015-3 standard.

The values of mechanical strength obtained have been measured after 1 , 7 and 28 days of wet curing at 20°C, on three 40 by 40 by 160 mm specimes for each curing time, according to the procedure described in the EN 1015-1 1 standard. The 'SPC-1 ' mixture described in A13 presents an evident synergic effect between fiber and expansive agent with an increase in the bending mechanical strength after 1 day of curing equal to +135% (more than double) with respect to the corresponding mixture prepared with the only fibers without expansive agent.

The mixture 'SPC-1 ' is borderline for being defined as Self Prestressed Concrete (SPC), in the sense that it has performances close to the minimum limits as defined in A13 to be SPC, which are:

- Bending strength after 1 day: higher than 10 MPa;

- Bending strength after 28 days: higher than 15 MPa;

- Ratio between bending and compressive strength: higher than

0.20).

However, the mixture 'SPC-1 ', as the mixture 'SPC-2', presents mixture segration.

With the mixture 'SPC-3' it was possible to avoid bleeding and segregation, but the fluidity of the mixture was poor, not allowing to achieve the same performance obtainable in terms of flowability by the mixtures 'New-A' and 'New-B'. In these two last mixtures colloidal silica was used, by allowing to reduce the dosage of expansive agent (-20 kg/m³) and filler (-100 kg/m³) and thus by reducing the viscosity of the system in the absence of bleeding and segregation.

The mixtures 'New-A' and 'New-B' comply with the criteria defined in A13 to be called Self-Prestressed Concrete (SPC), but they present the additional requirements of both fluidity and internal cohesion, i.e. an improved rheological behavior. 'New-A' and 'New-B' mixture proportions are not included within the range of the mixture claimed in A13, but they are included within the composition range claimed in the present patent.

In addition, in the 'New-B' mixture, PVA fibers have been used with elastic modulus equal to 25 GPa, value not included in A13, in which the minimum value of elastic modulus indicated was 50 GPa. On the other hand, this value of 25 MPa is included in the claims of the present patent application, in which the minimum value of elastic modulus indicated is 20 GPa.

Claims

1 . Mixture composition, wherein the parts are indicated by weight, comprising the following materials:

- Cement-based hydraulic binder (1000 parts);

- Dry granular inert materials with size (D90) smaller than 5.0 mm (600- 2900 parts);

- Expansive agent (5-130 parts);

- Fiber with elastic modulus higher than 20 GPa (5-400 parts);

- Water reducing agent (1 -40 parts of dry mass);

- Colloidal silica (1 -200 parts of dry mass); and

- Water (150-550 parts).

2. Composition according to claim 1 wherein

such cement-based hydraulic binder comprises one or more of the followings components: Portland cement, aluminous cement and supplementary cementitious materials; and

such dry granular inert material comprises sand.

3. Composition according to claim 1 or 2, wherein such expansive agent comprises one or more of the following oxides: CaO, MgO, AI2O3, SO3, Fe203

4. Composition according to anyone of the previous claims, wherein such fibers comprise short metallic fibers (lenght 300 mm, aspect ratio 120,

E=210 GPa) and/or PVA fibers (lenght 120 mm, aspect ratio 60, E=25 GPa).

5. Composition according to any of the previous claims, wherein such water reducing agent comprises a polycarboxylate ether-based superplasticizer (PCE).

6. Composition according to anyone of the previous claims, including in addition inorganic filler with particle size smaller than 0.200 mm (0-200 parts).

7. Composition according to anyone of the previous claims, including in addition shrinkage reducing admixture (0-10 parts of dry mass).

8. Composition according to anyone of the previous claims, including in addition viscosity modifying agent.

9. Composition according to anyone of the previous claims, including in addition a chemical admixture able to change the degree of cement hydration, by shortening or extending times for setting or hardening.

10. Composition according to anyone of the previous claims, including in addition a catalyst of the expansion reaction.

1 1 . Composition according to anyone of the previous claims, wherein the fibers can be either randomly disposed or oriented; the fibers can be either homogenously dispersed or concentrated in particular zone; the fibers can be either single or held together with any technique; the fibers can form textiles of any geometry comprising non-woven fabric; the fibers can form either 2D- textiles or 3D-textiles, both kinds made with any technique; the fibers can be a mixture of different kind of fibers, comprising a mixture of short fibers and textiles.

12. Process for preparing a mixing having a composition according to anyone of the previous claims, such process comprising:

- mixing a substantially dry composition containing: cement-based hydraulic binder, inert granular materials, and expansive agent;

- mixing of said substantially dry composition, with: water and colloidal silica in order to obtain an acqueous suspension;

- addition of a water reducing admixture in any moment, before mixing in water, if the water reducing admixture in powder, or after mixing with water, if the water reducing admixture in acquoeus solution;

- addition of fibers with elastic modulus higher than 20 GPa, in any moment, before or after mixing with water.

13. Process according to claim 12, wherein to said substantially dry composition are mixed also one or more of the following materials in powder: inorganic filler with particle size smaller than 0.200 mm; shrinkage reducing admixture; viscosity modifying agent; chemical admixture able to change the degree of cement hydration; and catalyst of the expansion reaction;

14. Process according to claim 12 or 13, wherein to such substantially dry composition are mixed also one or more of the following materials in acqueous solution: shrinkage reducing admixture; viscosity modifying agent; chemical admixture able to change the degree of cement hydration; catalyst of the expansion reaction.

15. An object, obtained by the hardening of a mixture composition according to anyone of the claims 1 to 1 1 , by using the process according to anyone of the claims 12 to 14, said object having the following features:

- spread at the fresh state after 30 minutes since mixing with water: greater than 250 mm;

- absence of both bleeding and segregation;

- compressive strength after 1 day of curing: from 30 to 80 MPa;

- flexural strength after 1 day of curing: from 5 to 20 MPa;

- compressive strength after 28 days of curing: from 60 to 180 MPa;

- flexural strength after 28 days of curing: from 15 to 45 MPa;

- absence of autogenous shrinkage;

- hydraulic shrinkage in standard conditions (RH = 50%, temperature = 20°C) after 180 days: from 0 to 400 μιτι/m.