Investigation of different types of fibers to strengthen cement paste, mortar and concrete RALEJS TEPFERS Department of Civil and Environmental Engineering Structural Engineering, Concrete Structures CHALMERS UNIVERSITY OF TECHNOLOGY SE-412 96 Göteborg, Sweden 2008

Report No. 2008:7

REPORT NO. 2008:7

Investigation of different types of fibers to strengthen cement paste, mortar and concrete RALEJS TEPFERS

Department of Civil and Environmental Engineering Structural Engineering, Concrete Structures CHALMERS UNIVERSITY OF TECHNOLOGY SE-412 96 Göteborg, Sweden 2008

Investigation of different types of fibers to strengthen cement paste, mortar and concrete RALEJS TEPFERS © RALEJS TEPFERS, 2008

ISSN 1650-5166 Report no. 2008:7 Department of Civil and Environmental Engineering Structural Engineering, Concrete Structures Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000

Cover: Figure 27.

“Buckypaper” made of carbon nanotubes. http://www.netcomposites.com/news.asp?4955.

Department of Civil and Environmental Engineering Structural Engineering, Concrete Structures CHALMERS UNIVERSITY OF TECHNOLOGY SE-412 96 Göteborg, Sweden 2008

Investigation of different types of fibers to strengthen cement paste, mortar and concrete RALEJS TEPFERS Department of Civil and Environmental Engineering Structural Engineering, Concrete Structures SE-412 96 Chalmers University of Technology

ABSTRACT The development of fiber reinforced concrete is for time being in an interesting promising stage. This report is a kind of state of the art statement on new directions in fiber research. Steel fiber reinforcement has been used to control and reduce crack widths in ordinary reinforced concrete structures. Now a development has started to produce structures of concrete reinforced only with steel fibers in amounts of 100 kg/m3 concrete. Appropriate codes are under development and necessary test methods used, evaluated and discussed. Full scale tests have been performed. Slabs with free span and size 6x6m have been constructed. Savings in extensive work with ordinary reinforcement are obtained. The development is just on its start. The development of nano technique has also influenced the concrete industry. Carbon nanotubes (nanofibers) with 10-100 nm diameter and lengths of µm to some mm up to several cm are now available. The price is very high, but if extensive use can be found for the nanotubes the price will fall dramatically. The nanofibers are extremely strong and stiff, but thanks to their high aspect ration (length/diameter) they are flexible like strings. They have very good electric and heat conductive properties. Concrete with carbon nanotubes can obtain piezoresistivity enabling strain registration. Steady new properties of the nanotubes are found. The research on carbon nanotubes in concrete is just in starting phase. There has to be a follow up of controlling concrete structure on nano level to avoid porosity, disturbing bond of the tiny nanofibers. Therefore the report presents an overview of concrete technology and the necessary provisions to be able to use carbon nanofibers in cementitious matrix. It is essential to investigate the carbon nanotube influence on human health and the recirculation of the fibers and the final deposition parallel with the application research. A new asbestos disaster has to be avoided. Key words: Concrete, Concrete technology, Fibers, Carbon nanotubes, Properties

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Undersökning av olika fiber typers möjligheter för användning som armering för cement-, cementbruks- och betongmatris RALEJS TEPFERS Institutionen Bygg- och miljöteknik, Konstruktionsteknik Betongbyggnad Chalmers tekniska högskola

SAMMANFATTNING Fiberarmerad betong befinner sig för närvarande i en mycket intressant och lovande utvecklingsfas. Föreliggande rapport utgör ett slags kunskapsinsamlande beträffande nya inriktningar inom fiberforskning. Stålfibrer har använts för att kontrollera och reducera sprickbredder i på ordinärt sätt armerade betongkonstruktioner. Nu har en utveckling påbörjats att producera betongkonstruktioner med bara fiberarmering till en mängd av 100 kg/m3 betong. Tillämpliga normer håller på att utvecklas och nödvändiga använda testmetoder utvärderas och diskuteras. Utvecklingen har just börjat. Utvecklingen av nanoteknik har även nått betongindustrin. Kol nanotuber (nanofibrer) med 10-100 nm diameter och längder från µm till några mm, även upp till några cm kan nu fås. Priset är mycket högt, men om omfattande tillämpning kommer att hittas för nanotuberna, så kommer priset att falla dramatiskt. Nanofibrer är mycket starka och styva men tack vara sin extrema längd jämfört med diametern är de böjliga som snören. De har mycket god elektrisk och värme ledningsförmåga. Betong med kol nanotuber kan få piezoelektriska egenskaper som tillåter registrering av töjning. Ständigt uppdagas nya egenskaper hos kol nanotuberna. Forskningen beträffande nanotuber är ännu i startfasen. Det är nödvändigt att följa upp betongteknologin på nanonivå för att kunna undvika porositet som stör de ytterst små nanofibrernas vidhäftning. Därför presenteras betongteknologi översiktligt i rapporten och de nödvändiga åtgärderna ges för att kunna använda kol nanofibrer i cementmatris. Det är nödvändigt att undersöka kol nanotubers inverkan på människors hälsa samt nanofibrernas recirkulation och slutlig deponi parallellt med tillämpande forskning. En ny asbestkatastrof måste undvikas. Nyckelord: Betong, Betongteknologi, Fibrer, Kol nanotuber, Egenskaper

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Contents ABSTRACT

I

SAMMANFATTNING

II

CONTENTS

III

PREFACE

VII

1

BACKGROUND OF PROJEKT

1

2

CONCRETYE TECHNOLOGY – POROSITY IS DECISIVE

3

2.1

Composition of concrete

3

2.2

Casting, hardening hydration

3

2.3

Cement reactions, plasticizing additives

3

2.4

Freezing, frost resistance

4

2.5

Different types of pores in concrete

5

2.6

The concrete can be made stronger

6

3

NANOTECHNOLOGY OF CONCRETE

8

4

FIBERS AND THEIR FUNCTION

11

4.1

Fiber function

11

4.2

Fiber types

13

4.3

Strain sensing in carbon fiber reinforced cement

15

4.4

Mixed fibers – hybrids

15

5

FIBER REINFORCED CONCRETE ALSO WITH HIGH STEEL FIBER VOLUME FRACTIONS

6

17

APPLICATIONS OF FIBER REINFORCED CONCRETES WITH HIGH FIBER VOLUME FRACTION

20

7

POSSIBLE FUTURE DEVELOPMENTS

23

8

NANOFIBERS

24

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III

8.1

Carbon nanotube (CNT)

25

8.1.1

Types of carbon nanotubes and related structures

26

8.1.1.1 Single-walled carbon nanotubes SWCNT

26

8.1.1.2 Multi-walled carbon nanotubes MWCNT

27

8.1.1.3 Vapor-grown carbon filaments VGCF

27

8.1.2

Manipulation of carbon nanotubes

29

8.1.3

Potential use of carbon nanotubes

30

8.1.4 Drawbacks of nanotubes

30

8.1.5

30

8.2 8.3

Nanobud Nanotubes properties List of carbon nanotube suppliers

31 34

9

CEMENTS AND AGGREGATES FOR USE WITH NANOFIBERS

35

9.1

Grinding

35

9.2

Admixtures

35

9.3

Aggregates

36

9.4

Fillers

36

9.5

Fibers

36

9.6

Fabrication

36

9.7

Fabrication techniques

37

9.8

Products and applications

37

9.9

Challenges

37

9.10

Some questions to be clarified

37

9.11

Summary

37

10

BASICS OF HYDRATION

38

11-

CEMENT WITH NANOFIBERS

39

12

SOME POSIBLE USE OF CARBON NANOTUBE PROPERTIES

44

13

NANOFIBER REINFORCED MATRIXES AS TOUGH ARMOR MATERIAL

IV

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CARBON NANOTUBES AND HEALTH EFFECTS

15

CONCLUSIONS CONCERNING CARBON NANOFIBER EFFECTS

46

ON CEMENT PASTE AND CONCRETE

47

16

ACKNOWLEDGEMENT

49

17

REFERENCES

50

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VI

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Preface At Chalmers University of Technology, Department of Civil and Environmental Engineering, Structural Engineering, Concrete Structures projects concerning fiber reinforced concrete are going on performed by Ingemar Löfgren (2005) and Anette Jansson (2008). The projects deals with “Analysis and design methods for fiber reinforced concrete structures” and is financed by the “Thomas Concrete Group (Färdig Betong)”. The project, which is rendered in this report, is meant to support the projects done by Löfgren and Jansson gathering information on ongoing new developments within the fiber technique in concrete technology also taking information from Internet. The aim with the project is to compile possibilities for different fiber technologies to strengthen concrete. With possibilities is meant strength, modulus cooperation with the concrete matrix, durability, workability, recycling abilities and fiber influence on human health. The information is mainly gathered using Internet, but informative visits have been done at Institute of Polymer Mechanics (Professor Vitauts Tamužs), Riga, Latvia and at Luleå Technical University, Division of Polymer Engineering (Professor Janis Varna, Associate Professor Roberts Joffe). A curse “Fiber reinforced Concrete” at “Färdig Betongskolan, Kunskapscenter” in Göteborg has been passed to update knowledge. Also other information sources have been used as contacts with Jānis Ošlejs director of the Company Primekss producing steel fiber reinforced industrial concrete floors and participation in the Mechanics of Composite Materials Conference in Riga 26-30 May 2008. The Riga Technical University RTU (Professor Andrejs Krasnikovs, Dr Videvuds Lapsa and Dr Genadijs Shakhmenko) in Riga, Latvia have also been visited to obtain knowledge. Information has been also gained by personal contacts and participation in seminars as that arranged by the “Tekniska Samfundet i Göteborg” about “Nanotechnology” given by Professor Bengt Kasemo at Chalmers University of Technology. Professor Kent Gylltoft, head of the Department of Civil and Environmental Engineering, Structural Engineering, Concrete Structures, is specially thanked for his interest in this project and that Department resources have been provided for the needs of the project. Special thanks are given Professor Gylltoft for his help finding a sponsor for the project. Ingemar Löfgren and Anette Jansson are thanked for discussions and useful information given for this project. The study presented in this report has been financed by the Åke och Greta Lissheds Foundation, “SEB Enskilda Banken” in Stockholm, which gratefully is here acknowledged. Göteborg, June 2008 Ralejs Tepfers

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VII

1

Background of project

Fibers have been used since long time to increase the tensile strength of ceramic materials by reinforcing mortar with straw, sisal fibers, hair from cows and other biological materials. In concrete technology fibers, mainly of steel, have been used during last decades to give concrete increased tensile strength and ductility together with ordinary steel reinforcement bars. Cement matrix with asbestos and glass fibers as only additive have been used for fabrication of plates and pipes. Use of asbestos fibers had to be stopped due to health risks for the workers. For glass fibers there are problems with the alkaline surrounding in cement matrix. However this problem has been overcome by using special alkaline resistant glass types. During last years tests and practical applications have been performed with concrete where the ordinary reinforcement has been fully replaced by steel fibers. Such concrete with plasticizing additives is provided with the necessary amount of fibers to take the load. No vibration is necessary, as the concrete has a composition which makes it self compacting and it can be pumped. Comprehensive work with ordinary reinforcement is unnecessary and the manufacturing of the concrete structure can be performed in a more rational and economic way. In the beginning, for reasons of safety introducing the new technique, only industrial floor structures on supporting ground have been built. In many cases the floor slab becomes bearing load due to the settlement of the soil, when its load is transferred to supporting piles of the structure. The bearing of load is accepted for the slab as if something would happen, the slab can fall only some centimeters down on the settled ground. Using the positive load bearing observations, load taking slabs reinforced only with fibers on columns have been built. Membrane bar reinforcement strips in column lines of the slabs have been used for reason of safety. Load tests have been done up to serviceability load and the function has been satisfactory. Carbon fibers can in certain extent transfer electricity and there are a number of types. Carbon fibers can provide the concrete structure electric properties. The electric resistance of the carbon fiber reinforced concrete structure changes when stressed mechanically. The resistance changes can be used to determine mechanical strain as it is done with ordinary resistive strain gauges. However, every structural part used for measurements have to be calibrated on its own right. The usual techniques of developing new materials can be characterized as “top-down” techniques. We start at macro level observing and manipulating materials, successively working down to micro level and further down into atomic scale. The nanotechnology is a “bottom-up” technology starting at atomic level and working up to micro and macro level constructing materials with desired properties. Normally the two ways of working cannot be kept absolutely clear from each other and a mix is often practical to use. There are other types of fibers as carbon, aramid, polypropylene, glass and polyethylene. These are carbon nanotubes. Carbon nanotubes still are very expensive, however, manufacturing techniques develop and prices are expected to fall considerably. Therefore, it seems to be motivated to investigate possibilities of carbon nanotubes for cement based products, which could be manufactured using them. For instance carbon fibers before 40 years were out of question for building structures, but CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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today carbon fiber sheets, strips and reinforcing bars are widely used for reinforcing and strengthening of concrete structures. Also with high material price the economical result can in many cases be cheaper as the work is simplified and facilitated and the structure can become more durable. First, in this report, the concrete technology and its nanotechnology is presented in a summarized way to give background for understanding of the fiber function in concrete, especially the very small fibers, the carbon nanotubes, need practically cementitious matrix without pores to function well. Then, the ordinary fibers and their application possibilities in concrete are discussed. Finally, the research developments using fibers such as carbon nanotubes are presented and its possibilities in cementitious matrixes are discussed.

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Concrete technology – porosity is decisive

Porosity determines the properties of concrete. Porosity is the number of pores in a material for instance pores in certain concrete. Porosity is usually expelled in volume percent. The porosity of concrete has influence on the properties in many aspects. Composition of concrete, casting in practice, maturing and hardening, cement reactions and risks at freezing, durability, all are influenced by porosity. The possibilities to influence the type of porosity are important.

2.1

Composition of concrete

Concrete technology deals in very great extent about the porosity of concrete. Concrete consists of gravel, sand and cement, all particles, and water plus air and eventual additives. The firm substances give the concrete strength. Aggregates are cheap and therefore should fill up the space as much as possible. Therefore the particle size grading should be such that this is possible. The fine cement particles find room in spaces between the aggregate particles. More cement means that the spaces between aggregates are better filled. Consequently, more cement added, stronger concrete. Water and air fills the rest of the spaces.

2.2

Casting, hardening hydration

Water is added in such amount that the concrete becomes possible to cast using vibrations. It usually means that more water is added than the chemical reactions require. The excessive water evaporates from cement glue and leaves pores behind, which weaken the concrete. More excess water, it means higher water/cement ratio, results in weaker concrete. Porous concrete is also more exposed to the environmental influence, because the surface exposed for environmental attack increases. During hardening the concrete should be watered, because the exothermic cement hydration increase the temperature of the concrete, so water evaporates in such an extent that there is not enough water left for the reactions. A wet concrete surface prevents the evaporation. At Water/Cement ratio 0.3 to 0.4 all water is used up in hydration of cement. Concrete with so low Water/Cement ration has a consistency as moist sand and cannot be cast and compacted. Adding plasticizing additive the concrete can be made possible to cast.

2.3

Cement reactions, plasticizing additives

Cement is an ionic material. It means the cement particles have electrical charges. Cement is manufactured by grinding, which result in particles with edges where the positive and negative electrical charges are concentrated, Figure 1. The charges have only short distance influence. When cement particles come into water particle corners with opposite electrical charges orientate against each other and the particles flocculate, which results in stiff spacious structure obstructing flow. Increased amount of water can break up flocks, but then Water/Cement ratio is increased. The alternative is to use plasticizing additive – super plasticizer. When the super CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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plasticizer is added to concrete having a consistency as moist sand and the mixing goes on, the concrete mix suddenly starts to flow and splash in the mixer. The plasticizer chain molecules wind around the cement particles and screen the cement particles electrical charges. In cooperation with water they create particles with equal electrical charge repelling each other. In spite of the repulsion the particles comes closer to each other in comparison with the situation when they were flocculated. More over, the repulsive forces make that the particles can easy slide along each other. Another effect can be obtained by chain molecules fixing themselves on cement particles with their molecule tails protruding from the surface of the particles and obstruct them to come close to each other, however closer than what the flocculation allow for. The result is that the cement particles leave less space between them, which are better field with hydration products. The space can also partly be filled with micro silica particles. Result is that the concrete has fewer pores and we get a stronger and environmentally more resistant concrete.

Figure 1

Flocculated cement particles.

The flow of the fresh concrete is obstructed by aggregates creating arches of stones. The forming of arches can be hindered by increased amount of cement glue. It is not possible to only increase the amount of cement, because this glue will get shrinkage cracks due to contraction shrinkage. In concrete the aggregate hinders the formed cracks to become visible cracks. In stead the amount of cement glue can be completed with lime stone filler, also granite filler has been used. The self-compacting concrete has so much cement glues that creation of aggregate arches is prevented and the concrete can flow freely.

2.4

Freezing, frost resistance

The water in the pores of concrete may freeze and then it expands 9%. The freezing happens successively. From a developing ice crystal the solved salts leave for rest of the water, which gets lower freezing temperature due to increased concentration of solved salts. The increased volume of the ice crystal creates water pressure. The water pressure can be released by water pressing into empty pores and freezing will not damage concrete. Air entraining agent creates small dense positioned pores in cement glue, where the water can be pressed and the pressure released by this. Water molecules have electrical poles and are hold together by hydrogen binds, Figure 2. If an atomic structure has electrical charges water molecules are attracted to it – the 4

CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

surface is wetted, is hydrophilic. The air pores have no electrical charges on its surface due to influence of the air pore forming agent. If the surface has no electrical charges, the water molecules are kept together by hydrogen binding to water drops and the water avoids the surface – the surface is water repellent, hydrophobic. Therefore water does not by itself go into these pores, but is pressed in by active pressure from freezing water. When the pressure is released at thaw, the water does not like to stay in these air pores and returns to the porosity where it was before. Therefore the air entraining created porosity functions also at repeated freezing.

Figure 2

2.5

Water molecules with electrical charges – hydrogen binding.

Different types of pores in concrete

In Figure 3 different types of pores characterized by their diameter are shown along a scaling of length. The scaling goes from right hand side to left and covers diameters from 10mm to 1Å.

Figure 3

Pores in concrete.

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Air pores are closed by vibration. Cement glue gets local pressure increase from the vibrating device. The pressure parts the aggregate particles so the friction is reduced and they can slide along each other. The capillary porosity is reduced using lower Water/Cement relation and plasticizing additives plus micro silica. Gel pores are in principle atom vacancies and the space between atoms is for time being difficult to do something about. Contraction pores appear during hydration, because the volume of the hydration products is less than that of the constituents cement plus water and this is very difficult to do something about. The pores created by the air entraining agent are deliberately created pores to prevent the concrete suffer from freezing.

2.6

The concrete can be made stronger

To day it is possible in production to reach concrete compressive strength up to 150 MPa. In laboratory Aalborg Cement AS, Denmark they have reached 300 MPa and French researchers have reached 800 MPa. In that case they have hardened the concrete at 200oC and under pressure. It can be supposed that they have succeeded to reduce the gel and contraction porosity by pressing atoms with thermo vibrations into the micro pores of the structure. Basically they have applied nanotechnology for achieving this. Summing up, the concrete technology is about to reduce the porosity of the concrete. In this way the concrete becomes both stronger and more environment resistant. People have been clever in concrete technology also in ancient times. Example of ancient times advanced concrete technology is Pantheon in Rome, Figure 4. Here the work was performed with concrete with increasing porosity towards the top of the dome with the aim to reduce the dead load. The porosity increase was obtained by mixing increased amount of light tuff in the concrete. Another example is how water leaking shrinkage cracks in aqueducts in Roman times was avoided. According to Marcus Vitruvius Pollio part of the binder, burned grained limestone, was covered with oil. The idea was that oil prevented hydration for about a month when this prepared binder was added to the inner mortar cladding of the duct. In the alkaline environment of mortar the oil cover of the burned limestone particles was successively deteriorated and the particles could hydrate increasing their volume. As the mortar was bonded to the duct structure, the volume increase created internal pressure in the cladding mortar, hindering water-leaking cracks to appear. This technique prevented the aqueducts to leak water.

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Figure 4

Pantheon concrete dome with diameter 44 m built in Rome 63-12 B.C. The concrete has density, which is reduced toward the top of the dome by using increased aggregates of tuff and pumice stone.

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3

Nanotechnology of concrete

For decades, major development in concrete performance was achieved with application of super-fine particles as fly ash, silica fume, and now, nano-silica and reinforcing fibers as carbon nanotubes. Nanotechnology has changed our abilities to control and develop materials, Sobolev, Ferrada-Gutiérrez (2005). The possibility to observe materials at atomic level and be able to influence material properties gives rise to new expectations of improved materials. Nanoparticles were found to be a very effective additive to polymers and also concrete realized in high-strength performance, durability and self-compacting properties. Better understanding and precise engineering may result in desired stress-strain behavior and “smart” properties as electrical conductivity, temperature-, moisture, strain- sensing abilities. Nanobinders, nano-engineered cement based materials with nano-sized components and particles are on line with next development, which can be defined as “bottom-up”. That is development of materials starting from atomic level. Silicon dioxide, nanodesigned superplasticizers are nano-additives used and now carbon nanotubes are another challenge in application of these fibers in cement composites. According to Sobolev, Ferrada-Gutiérrez (2005) the development of the following concrete-related nanoproducts can be anticipated: • • • • • • •

Well-dispersed nano-particles increase the viscosity of the liquid phase helping to suspend the cement grains and aggregates, improving the segregation resistance and workability of the system; Nano-particles fill the voids between cement grains, resulting in the immobilization of “free” water (“filler” effect); Well-dispersed nano-particles act as centers of crystallization of cement hydrates, therefore accelerating the hydration; Nano-particles favor the formation of small-sized crystals (such as Ca(OH)2 and AFm) and small-sized uniform clusters of C-S-H; Nano-SiO2 participates in the pozzolanic reactions, resulting in the consumption of Ca(OH)2 and formation of extra C-S-H; Nano-particles improve the structure of the aggregates’ contact zone, resulting in a better bond between aggregates and cement paste and also between carbon nanofibers and cement paste; Crack arrest and interlocking effects between the slip planes provided by nanoparticles improve the toughness, shear, tensile and flexural strength of cement based materials.

Expected development in the following concrete-related nanoproducts can be anticipated: • • • •

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Catalysts for the low-temperature synthesis of clinker and accelerated hydration of conventional cements; Grinding aids for superfine grinding and mechano-chemical activation of cements and aggregates, Dubin J., Tepfers R (1995); Binders reinforced with nano-particles, nano-rods, carbon nano-tubes, nanodampers, nano-nets, or nano-springs; Binders with enhanced/nanoengineered internal bond between the hydration products; CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

• • •

• • • • • • • • •

Binders modified by nano-sized polymer particles, their emulsions or polymeric nano-films; Bio-materials (including those imitating the structure and behavior of mollusk shells); Cement based composites reinforced with new fibers containing carbon nanotubes, as well as with fibers covered by nano-layers (to enhance the bond, corrosion resistance, or introducing the new properties, like electrical conductivity etc.); Next generation of superplasticizers for “total workability control” and supreme water reduction; Cement based materials with supreme tensile and flexural strength, ductility and toughness; Binders with controlled internal moisture supply to avoid/reduce micro-cracking; Cement based materials with engineered nano- and micro- structures exhibiting supreme durability; Eco-binders modified by nanoparticles and produced with substantially reduced volume of Portland cement component (down to 10-15%) or binders based on the alternative systems (MgO, phosphate, geopolymers, gypsum); Self-healing materials and repair technologies utilizing nano-tubes and chemical admixtures; Materials with self-cleaning/air-purifying features based on photocatalyst technology; Materials with controlled electrical conductivity, deformative properties, nonshrinking and low thermal expansion; Smart materials, such as temperature-, moisture-, stress- sensing or responding materials.

Emerging research, as Sobolev, Ferrada-Gutiérrez (2005) suggest, is an incorporation of carbon nanotubes into the cement matrix, which would result in a ductile and energy absorbing concrete. The performance of such concrete can be further enhanced by the addition of polymers and nano-structured materials, such as nano-rods, nanodampers, nano-nets, nano-springs or nano-engineered fibers. Further nano-binder can be proposed as a logical extension of the two concepts: Densified System with Ultra Fine Particles (DSP) and Modified Multi- Component Binder (MMCB) extended to the nano-level. In these systems the densification of binder is achieved with the help of ultra-fine particles: silica fume (SF) dispersed with superplasticizer (SP) in DSP and finely ground mineral additives (FGMA) and SF modified by SP in MMCB; these particles fill the gaps between cement grains. In these systems Portland cement component is used at its “standard” dispersion to provide the integrity of composition. In contrast to DSP and MMCB, the nano-binder can be designed with a nano- dispersed cement component applied to fill the gaps between the particles of mineral additives (including FGMA). In nano-binder, the mineral additives (optionally, finely ground), acting as the main component, would provide the structural stability of the system and the micro- or nano-sized cementitious component (which can also contain the nano-sized particles other than Portland cement) would act as a glue to bind less reactive particles of mineral additives together. Such nano-sized cementitous component can be obtained by the colloidal milling of a conventional (or especially sintered/high C2S) Portland CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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cement clinker (the top-down, from macro to atomic level approach) or by the selfassembly using mechano-chemically induced chemical reactions (the bottom-up approach) Development of nano-binders can lead to more than 50% reduction of the cement consumption, capable to offset the demands for future development and, at the same time, combat global warming. In addition to nano-binders, the mechano-chemistry and nano-catalysts could change the face of modern cement and concrete industry by the great reduction of clinkering temperature and even realizing the possibility of cold sintering of clinker minerals in mechano-chemical reactors, so far Sobolev, Ferrada-Gutiérrez (2005). Even if the spaces between hydrated cement grains are filled with nanoparticles, there will be voids due to smaller volume of hydration products than the reactants cement plus water have. Further there will be atom vacancies in the molecular structure. A possible way of filling these voids is by using pressure and increased temperature during the maturing phase. Increased thermo-vibrations of atoms may move atoms in these voids. By reduced porosity also on nano level, the strength and durability of the material will be enhanced.

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Fibers and their function

4.1 Fiber function Fibers by definition have a length at least 100 times the greatest length across its section and this relation is called aspect ratio. The most effective utilization of a fiber is when the crack crosses the fiber in right angle at midpoint. Then the fiber has maximum anchorage possibility by bond. In weaker concrete the bond will govern the maximum fiber force and at a certain level of increased concrete strength the bond will be enough strong to enable failure of the fiber. However, not always the crack will appear at midpoint fiber. Then the shortest fiber bond length will determine the force fiber can transmit. To improve the bond strength of fibers, these are given different types of surface deformations. Thin fibers may be used without surface deformation relying only on bond of the smooth surface. Thicker fibers need deformations in form of rough surface or wavy (crimped) shape. To improve the anchorage of fibers steel fibers are manufactured with end deformations to ensure about the same anchorage irrespective where the crack along the fiber length will appear. The fiber force in design generally is based on obtainable anchorage capacity. Normally, in to days design, fibers are aimed to hold together cracks, reducing the crack width, providing better bond of ordinary reinforcing bars and ensuring durability, while the main tensile force transfer is provided by ordinary steel reinforcement. If ordinary reinforcement is not there, the fibers have to be able to take over at least the tensile force the concrete transferred at cracking. They should also be able to take care of the design load. Increased amount of fibers will transfer more tensile force. If the elasticity modules of fibers exceed that of concrete considerably, they will also in certain extent contribute to tensile force taking before concrete cracking. Fibers with modulus of elasticity above that of concrete will increase the cracking load depending of the amount of fibers and how much their modulus exceed that of concrete. In a concrete structure with ordinary steel bar reinforcement bending cracks appear and between the cracks the concrete provides the effect of tension stiffening. Between the cracks the bond of concrete to bars loads the concrete with tension resulting in the second generation of bending cracks. If some fibers are added to the concrete, these will transfer some tension through the cracks. The concrete between cracks will be loaded faster in tension resulting in following generations of cracks with closer spacing. Increased amount of fibers will give reason for closer crack spacing ending up with micro cracking of concrete instead of few wide cracks. Fiber amounts, which are able to take over at least the tensile force from concrete when it cracks, can replace ordinary steel reinforcing bars. These fibers will give close crack distances. However in a random mix of fibers in concrete only 1/3 of the fibers will be effective in uniaxial force direction. Moreover the fibers deviating more then 15 degrees from force direction loose radically their efficiency to become zero when the deviation is 90 degrees. A way of determining the fiber efficiency is by tests in different concrete CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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mixes using different amounts of fibers. The testing specimen in form of bent beam (slab strip) should not be too small, because fibers are not perfectly distributed in concrete, see Figure 5. Using bigger specimen the effect in scatter of irregularities in distribution is avoided and a more correct resistance to load is obtained than in small specimens giving rise to big scatter. The size of the four point load bent specimen could correspond to a full size strip of a concrete slab for instance 160mm thick and 500mm wide with a length of 2000mm. Close to the mould walls bottom the fibers will orientate according to these and there will be a “wall effect” on fiber resistance in the specimen. This wall effect is more pronounced in small specimens and therefore they will not really reflect the real fiber resistance in full size concrete structures.

Figur 5.

Fiber random distributions in concrete photographed with X-ray. Courtesy: Koji Otsuka.

The exclusive properties of composites are achieved by mixing concrete and fibers in an appropriate proportion. If we regard a concrete-fiber mix, we have to consider the following. The hybrid fiber mix should be made so that fibers alone are able to carry at cracking of concrete at least 1.1 times the load, which was taken by the concrete together with fibers at rupture of concrete. The shock from failing of concrete will also inactivate by deteriorating bond or rupture probably 1 to 5% of the fibers. Consequently the following formula can be set up for determination of the relation between the area of concrete and area of fibers: 1.1 ⋅ 1.05 (εc ⋅ Ec ⋅ Ac + εc ⋅ Ef ⋅ Af) = εf ⋅ Ef ⋅ Af··keff

(1)

where Ac − area of concrete, Af − area of fibers transverse direction of force, Ec − modulus of concrete, Ef − modulus of fibers, εc − strain at failure of concrete, εf − strain of fibers, keff – fiber efficiency factor. The efficiency factor keff depends on how the fibers are oriented versus the tensile force and their anchorage capacity. Efficiency factor is also influenced by bond conditions of the fibers. Bond by fiber surface deformations or a crimped form is very dependent of how near to the fiber end the concrete crack transverses the fiber. Fibers with end deformations are less sensitive to the place where the concrete crack transverses the fiber. More over the fibers are not perfectly evenly distributed in the concrete mass. The best way to estimate the efficiency factor seems to be by a bending beam test with height corresponding to ordinary concrete slab thickness, as mentioned before. Then the wall effects of fiber distribution might be obtained in a plausible way. Normal fiber reinforced concrete should fail by fiber pull-out at higher load than that of concrete cracking and at a load level based on empirical calculation. 12

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4.2 Fiber types There are very many types of fibers as becomes evident from Table 1, Löfgren (2005). Organic fibers may rotten in moist concrete and have time limited use. Examples of different fibers are shown in Figure 6. This report will mainly be devoted to the possibilities of steel and carbon fibers with special interest in carbon nanotubes. Table 1. Physical properties of selected fibers from Löfgren (2005).

Steel fibers Steel fibers can be used to restrict the concrete cracking and give the ordinary steel reinforcement better bond conditions. However, concrete slabs have been constructed using steel fiber reinforcement solely. And codes are under development for this technique. Different types of metal fiber geometries to enhance bond are shown in Figure 7. Glass fibers Conventional glass fibers do not stand the alkalinity of the concrete and lose strength rather quickly. Better properties have borosilicate glass fibers, E-glass. There are also alkali resistant glass fibers, A-glass (soda-lime-silica glass).

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Figure 6.

Examples of different types of fibers. AB Färdig Betong (2007).

Figure 7.

Examples of typical metal fiber geometries, Löfrgen (2005).

Synthetic fibers Some of the polymers which can be drawn into fibers are: Polyethylene, Polypropylene, Nylon, Polyester, Kevlar, Polyacrylonitrile, Cellulose, Polyurethanes. Polypropylene fibers have low modulus, lower that that of concrete. However, they can be used in hardening concrete in the early stage to prevent and resist plastic shrinkage cracking. They can also be used to let out steam pressure to avoid spalling of concrete, when the concrete structure is exposed to fire. The fibers melt due to fire heat and leave open holes through which the steam in concrete can be released. This is especially important for very dense high strength concretes. Carbon fibers Carbon fibers are the closest to asbestos in a number of properties. Asbestos is not allowed to use due to giving asbestos in the lungs of workers. Carbon fibers do not show this draw back. The carbon fibers can be used as chopped and short carbon fibers in concrete in a form known as carbon fiber cement concrete (CFCC) or carbon fiber reinforced concrete (CFRC). 14

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CFCC has no coarse aggregate and typically contains between 3 to 15 percent by volume chopped and short carbon fiber elements. Two types of carbon fiber are used in CFRC: Pitch-based carbon fiber and polyacrylonitrile-based carbon fiber. Short fibers have strength of 0.5 – 0.85 GPa and modulus of 30 - 40 GPa. Continuous carbon fibers are used in sheets for strengthening and repair of concrete structures and for reinforcing bars and prestressing cables.

4.3 Strain sensing in carbon fiber reinforced cement Carbon fibers are able to conduct electricity. The ability of structural material to sense its own strain is attractive for smart structures. Thus the structure can be designed for vibration control, traffic sensing etc. Strain sensing in short carbon fiber (diameter 15 µm, length 5mm, volume fraction 0.5-1.0 %) reinforced cement, as enabled by piezoresistivity, is characterized by the gauge factor, which is defined as the fractional change in electrical resistance per unit strain, Sihai Wen & Chung (2005). Important is to orientate the fibers in the direction of force. Gauge factor is individual for every place of sensing the structure and has always to be calibrated. Under uniaxial compression the gauge factor in both longitudinal and transverse directions decrease in magnitude with increasing specimen size due to a slight decrease in the degree of preferred orientation of the fibers. The same yields for increasing fiber content beyond the percolation threshold, when the fibers touch, resulting in conductive path and decrease in electrical resistivity. Percolation threshold has to be determined and is between 0.5 to 1.0% fiber volume fractions. The fibers have to be very well dispersed in matrix, see Figure 5, where uneven fiber distribution is shown. The specimen shape may influence the measurements. The resistivity is also influenced by micro cracking of the cement matrix as well as by pull-out and push in of fibers. There is also some influence on resistivity by load repetitions. The electrical resistivity of cement mortar changes also during curing.

4.4 Mixed fibers - hybrids Different types and different size of fibers can be used to improve the fiber function in concrete. The bigger fibers will be responsible for the transfer of force in cracks, while the smaller will favor the cracks to be micro cracks instead of few bigger. The combination of carbon nanotubes and conventional polymer based fibers is a future challenge for use as reinforcement in concrete or cement matrix. Incorporation of carbon nanotubes into other types of fiber will increase the strength and influence other properties. However, carbon nanotubes should not be combined with steel fiber or bar reinforcement due to difference in galvanic potential, which will cause corrosion of the steel. Reinforcing fiber blend provided by SIKA-company, consisting of steel fibers and polypropylene fibers, offers an engineered cost effective blend to address plastic shrinkage as well as restrained drying shrinkage in concrete. Figure 8 shows tensile stress strain relation of combined carbon and armid fibers in a composite rod. At strain of about 1% the carbon fibers break and the aramid fibers CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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alone have to take over the load. There has to be enough aramid fibers to be able to carry the load (also considering some aramid fibers fracture due to chock at the breakage of the carbon fibers), which was taken by both fiber types together. This type of relation yields also for fiber reinforced concrete under tension, when concrete cracks and the fibers have to carry the load, see eq.(1).

Figure 8.

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Tensile stress strain relation of combined carbon (C) and aramid (A) fibers, before and after the carbon fibers are broken.

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Fiber reinforced concrete also with high steel fiber volume fractions

An investigation with fiber volume fractions up to 3.75% (for steel fibers 235 to 300 kg/m3) is reported by Balaguru and Najm (2004). The mixing procedure was influencing the optimum fiber content. The mixture was workable and flowable with uniform distribution of fibers. Condensed silica fume and water reducing admixtures were used. The fibers used were 30 x 0.5mm-diameter Bekaert hooked steel fibers and 0.2 mm-equivalent diameter manganese straight steel fibers and 25 x 0.4 mmdiameter straight polypropylene macro fibers. Mixtures were prepared in ready-mix plant. Mixing procedures were as follows. First, sand and coarse aggregates mixed with 2/3 of water for about 1 minute. Cement, silica fume, admixtures and the rest of water were added and mixed for 3 minutes. After arrest period of 3 minutes fibers were added and mixed for 6 minutes. Vibration time at casting was 30 s. Compressive strengths varied from 74 to 100 MPa, flexural strengths from 12 to 24 MPa and splitting tensile strengths from 12.6 to 17 MPa. The straight hooked fibers gave the highest flexural strengths. Increased fiber content results in flexural strength increase. Slurry infiltrated fiber-reinforced concrete (SIFCON) may have fiber volume fractions from 10 to 15% and significant increase in strength and ductility over traditional steel fiber reinforced concretes SFRC. Fabrication of SIFCON requires pre-placement of fibers prior adding a special fine-grained cement slurry matrix. SIFCON is expensive in production. There is also a technique with slurry infiltrated fiber mat called SIMCON for rehabilitation and repair of structures. Knowledge about ultra high-performance steel fiber reinforced concrete (UHPC) has been summed up by Rossi (2008). Water cement ration is typically 0.2. The tensile stress is limited to round than 8.0 MPa and to increase it steel fibers are added. Short fibers 6mm with a volume of 5 and 10% (Type 1) enhance tensile strength but do little to boost ductility. Fibers by volume 2 to 3% and with length between 13 to 20mm (Type 2) enhance tensile strength and ductility. A fiber mix with lengths from 1mm to 20mm and 11% by volume with product name CEMNTECMultiscale® (Type 3) introduced in France, significantly enhance both tensile strength and ductility. The fiber length and dimensions of the mould, as ribs, influence the orientation of fibers, which can be directed more in the tensile stress direction, thereby improving the tensile strength in that direction. The tensile strength can be assessed directly using uniaxial tensile tests or flexural tests. Variations in fiber orientation can give nonrepresentative results. Tensile tests are difficult to make, because when the first crack appears disturbing bending is introduced in the specimen. Notched specimen is not recommended, because the notch gives cause for only one crack while the normal cracking situation is with multiple micro cracks at the start. Also bending of specimen disturbs very much obtained results. Notched bending specimens are not recommended in the strain hardening regime where multiple micro cracks appear. Four point bending test without notch gives representative tensile strength. For full replacement of the traditional reinforcement the fiber reinforced concrete has to have certain ductility. Ductility is related to situation before localizing the final crack. According to Rossi (2008), the ductility index (Id) is given by

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Id = εp / (σp/E);

(2)

Where εp and σp are the strain and stress at the peak on the tensile strain and stress curve and E modulus of elasticity, Figure 9. Ordinary and high performance concretes have Id = 1.0, Type 2 UHPFRC has Id = 1.5 to 3; Type 3 UHPFRC has Id = 1.75 to 30 and steel has Id = 30 to 60.

Figure 9. Determination of the ductility index according to Rossi(2008). The structural scale is conditional. A unit width crack opening in a deep member produces less rotation than in a shallow one – geometrical effect. The special distribution of fibers is a major factor affecting tensile behaviour. Heterogeneity of fiber distribution has strong influence on variability in tensile behaviour. Fatigue has been studied on Type 3 UHPFRC material and indicates the endurance limit to be at 60% of the characteristic tensile strength. Fiber reinforced concretes in general have poor fatigue behaviour related to be very demanding on fiber-matrix bond. Alternating cycles of tensile and compressive loading are believed to be the most harmful for fiber reinforced concrete. Notable is that benefits of increased capacity and ductility associated with fiber reinforcement are greater for structures subjected to impact loading than to static one. According to Rossi (2008) corrosion of fibers is not a great problem. A slight corrosion close to the structures surface does not lead to loss of mechanical properties. For service level cracks autogenous healing, also in corrosive surroundings, seems to be sufficient to prevent corrosion damage. Löfgren (2005) has proposed a wedge splitting tests method for estimation of the efficiency and post cracking behavior of fiber reinforcement in concrete. Using results from this method and testing them on four point beam tests with height 225mm, width 150mm, free span 1800mm and point loads in third span lengths, Jansson (2008) has obtained good agreement with stress-crack openings using FEM analysis. For overall response of a fiber reinforced concrete structure a bi-linear σ-w relationship seems to be sufficient. However, a multi-linear σ-w relationship describes the cracking process slightly better. 18

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Full-scale slab test by Trefilarbed, Bissen S.A. Luxemburg performed 3x3 field fiber reinforced concrete slab tests has been reported by Teutsch & Mandl (2006), Gossla (2006) and Destrée (2004). The square slabs were continuous, 200mm thick on columns with span of 6m. The fibers were Tabix13/50 crimped steel fibers and the amount was 100 kg/m3. In the slab underside between the columns continuous three steel deformed reinforcing 16 mm bars were placed to function as membranes just in case. The bending stress-deflection relations were determined for the fiber reinforced concretes with recommended beam test 150x150x600mm and are shown in figure 10. The scatter in stress-deflection (yields also stress-strain relation) is considerable. However the determined bending strengths in the slab test were about where the mean results for the bending tests are. Small specimens scatter more and the bigger specimen, slab, equalizes the scatter in fiber concentrations by its size. The central slab has reached a plastic deflection at failure of 65mm, the edge slab 120mm and corner slab 260mm. After a week no increase in the deflections was observed. The ductile failure, at a load which was three times that giving the first cracks, was due to fibers being pulled out in the slab yield lines and crack widths more than 10mm registered. A considerable rotation capacity was noted. Great amounts of fibers result in better fiber distribution in concrete than small amounts.

Figure 10.

Stress-deflection diagrams for beam bending tests with 100 kg/m3 Tabix1.3/50 steel fibers in concrete with cylinder strength 30 MPa, according to Gossla (2006).

Recommended beam test 150x150x600mm gives considerable scatter in bending test results. There is an influence of the fiber distribution in concrete and also the wall effect on fiber distribution exists and disturbs. Increased amount of fibers may reduce the scatter in results. The fiber type has also influence on the scatter. Increased size specimen is preferable for results applicable for full size structure.

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6

Applications of fiber reinforced concretes with high fiber volume fraction

The design of fiber reinforced concrete usually is done in cracked stadium II and is based on tensile bending strength after cracking, Teutsch & Mandl (2006). According to the Swiss code, SIA 162/6 (1999) this bending strength can be determined in bending test, square slab test or circular slab test. Applications have been done with fiber reinforced concrete as top concrete on ordinary reinforced form elements for slabs. No additional ordinary reinforcement was added and for continuity over the supports the fibers were used. Slabs with corrugated plate with connections for shear transfer have been erected with fiber reinforced concrete without ordinary reinforcement. For longer spans prestress together with fiber reinforced concrete with fiber amount of 63 kg/m3 has been used in slabs. The fibers increase the shear and punching resistance of the concrete, with 25-30% compared with un-reinforced concrete. Gaimersheim prestressed slab test with fiber concrete, Mandl (2004) has shown that no cracks were observed in serviceability stage and the deflection was 2.5mm compared with the expected 7mm using ordinary reinforcement. The application of fiber reinforced concrete has mostly been limited to industrial floors on ground. Because, the free bearing slab between piles supporting on rock, after the settlement of soil, in case of failure will, only fall some centimeters and not endanger life. In fiber reinforced slabs as anti progressive collapse reinforcement ordinary reinforcing bars are put in the underside of the slabs. In slabs on columns, this reinforcement will be concentrated along the column lines. This reinforcement in case of emergency will act as membrane to hold together the structure. Steel fiber reinforcement can be cost effective, when the ordinary steel bar reinforcement can be deleted in free suspended slabs. According to Destrée (2004) only steel fiber reinforced slabs have been cast in bays with 50m distance between construction joints and the shrinkage has been controlled and exposed by showing a controlled crack pattern. For evaluation of punching shear resistance TABIX crimped fiber reinforced concrete circular slabs with 1500 and 2000mm span and thickness 150 respective 200mm have been tested with a point load in the middle. Fiber contents have varied from 45 to 120 kg/m3 and had different diameters and lengths. Concrete consistency was such that it could be pumped and did not need any vibration. In ultimate stage the cracking has been according to yield line theory, K.W. Johansen (1943). Result is that the smaller size slab with span 1500mm can be adopted as a structural method of testing, because both slabs gave corresponding results. The smaller RILEM TC 162 notched beam test is put under doubt concerning usefulness for estimation of full size fiber reinforced concrete structure function. Steel fiber reinforced concrete slabs without ordinary steel reinforcement on concrete columns have been erected in Daugavpils, Latvia, Ošlejs (2008). The building is called Ditton house and is shown on Figure 11. The concrete slabs were cast with 20

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Trefilarbed crimped steel fibers 1.3/60, figure 12, with amount of 100 kg/m3 concrete. Superplasticizer was used, the concrete with the fibers could be pumped and poker vibration could be deleted. These slabs have been supported along all for sides by beams with ordinary steel reinforcement in lines between columns. The slab thickness was 220mm. The distance between columns was 6m. For the slabs was achieved 50% savings in working time. The casting is shown in Figure 13. The codes, which were used at design, were DIN1045 – 1 July 2001 and DIN1045 September 2005. EN 2006-1. DIN 1055-100 March 2001. Finally the slab was tested up to serviceability load under supervision of Riga Technical University RTU, with load from a water tank filled to 20; 40; 60.1 and 70 cm water (70 cm deep water gives a load of 7000 kN/m2). The deflections were measured and cracks followed up. The mid deflection of slab was 1.9mm for the highest load.

Figure 11.

Ditton market house in Daugavpils, Latvia, Ošlejs (2008).

Figure 12.

Trefilarbed crimped fiber 1.3/60 and to the right Twincone 1.0/54 fiber.

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Figure 13.

Slab casting with fiber reinforced concrete, Ošlejs (2008).

Figur 14.

Load test of slab with water tank load, Ošlejs (2008).

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Possible future development

Steel fibers are not perfectly distributed in concrete. The distribution seems to become more even if the fiber content increases. Start of crack will be initiated where the stress is maximum taking in account the real fiber amount in that place resisting the stress. Where this resistance is lesser the crack will start and try to follow the line of less resistance dictated by amount of fibers and also the stresses. A local weakness in fiber resistance will be more pronounced when the crack is short. In a long crack, the fiber resistance will approach the mean resistance determined with small beam bending tests. If the possible crack is short, the lower bound limit of test curves, Figure 10, will most likely determine the resistance in design. In reality the fibers transverse the cracked surface and its size determine the capacity. Could it be possible in design codes to introduce a factor taking care of the possible cracked surface size to adjust for the resistance? •

Short cracks - moment cracks in smaller beams, shear cracks, punching cracks;

•

Long cracks – Johansen cracks in slabs;

•

Fiber direction and amount transverse the cracked concrete surface and the surface size determine the capacity;

In fiber reinforced concrete the amount of fibers is determined to be able to resist the maximum stress. Thereby a lot of fibers will be in places where they are not needed. It is difficult to adapt the amount of fibers in concrete to the actual stress situation in the slab, because using pump and pumping batch of concrete with certain amount of fibers you do not know, when the concrete will end at casting. Otherwise, if the right amount of fibers could be introduced the concrete in real time at outlet from pump, a future vision could be to design the structure using FEM and provide signals to the concrete casting system for necessary amount of fibers to take care of the actual stresses in the right location in structure. The location could be executed using GPS for positioning of pump outlet casting position horizontally and laser vertically. Introduction of fibers the concrete at the outlet of the concrete pump and there proportioning them according to the actual stress situation needs in structure is a future challenge. A simpler way to cast with two amounts of fibers could be to color the concrete, so it becomes visible that the right concrete comes into the right place. However, the worker casting the concrete has to be very familiar with structural function or the engineer has to supervise the procedure.

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8

Nanofibers

Nano fibers, and with them is understood also carbon nanotubes, have a diameter measured in nm, that is a size close to atomic level. However, the length can be some mm. Even lengths of some centimeters are possible. Such fibers can reinforce a structure on sub-micro level and improve and complete with changed properties the matrix material. The extremely small dimensions make that aware has to be directed to health effects for personnel working with them. The health effects have to be followed up before the technical applications are put into general use. The nano particles are so small that they have many ways how to penetrate the human body. The fear for possible health effects will delay technical development of nano technique, EMPA (2007). The safety of nano products are becoming ever more important and a great deal work is currently in progress in this field. We have to avoid problem like the asbestos disaster. It is known, if a free nano fiber penetrates a human cell, the cell will dye. Under which circumstances there is such an exposure risk for human beings working with nano technology has to be fully understood. According to current research these tiny nano fibers, if embedded in a polymer do not pose any hazard. Probably the same will yield for nano fibers embedded in cement matrix. Also what will be after the utilization period of the materials, when they are destructed, recirculated or put into a deposit, is of importance. Will the nano fibers make a future environmental risk? The table 2 shows the sizes according to SI system (Système International d'Unités) so the nano-size can be better understood. The basis for table is 1 m. The prefixes are used in front of the m. The diameter of the nanotubes has nano-size, nm. The aspect ratio, length to diameter, can be 1000 or even up to 2·106 for carbon nanotubes. This means that the length can be some mm. The fiber lengths are normal distributed.

Table 2

SI-system size classification.

Prefix Factor Symbol Prefix Factor Symbol yottap 1024

Y

yokto 10-24

Y

zetta 1021

Z

zepto 10-21

Z

exa

1018

E

atto

10-18

A

peta

1015

P

femto 10-15

F

tera

1012

T

piko

10-12

P

giga

109

G

nano 10-9

N

mega 106

M

mikro 10-6

µ

103

K

milli 10-3

kilo

M Besides the expected improved mechanical properties of carbon hekto 102 H centi 10-2 C nanofiber reinforced concrete, the “smart” properties as electromagnetic deka 101 Da deci 10-1 D field shielding, self-sensing capabilities, self-control of cracks are of interest. It could be mentioned that Boeing uses for aircraft carbon nanotubes in the paint on composite structural parts. The nanotubes provide electric conductivity and thereby prevent static electricity to load up in the composite structure. 24

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8.1

Carbon nanotube (CNT)

Nanotechnology can be defined as, “The creation, processing, characterization, and utilization of materials, devices, and systems with dimensions on the order of 0.1-100 nm, exhibiting novel and significantly enhanced physical, chemical, and biologic properties, functions, phenomena, and processes due to their nanoscale size”, American Ceramic Society (2004). The information in this following part is partly from Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/Carbon_nanotube. The nanotechnology can be regarded as a bottom-up technology, which means that the start of developing structures is from atomic level. A single-walled carbon nanotube (SWCNT) is a one-atom thick sheet of graphite (with stronger than diamond bounds) rolled up into a seamless cylinder with diameter on the order of a nanometer and carbon caps at either end. This results in a nanostructure where the aspect ratio (length-to-diameter ratio) exceeds 1,000,000. To be qualified as nanotuber, the aspect ratio has to be >100. The fiber length distributions of fibers follow about the Gaussian distribution. Such cylindrical carbon molecules have novel properties that make them potentially useful in many applications materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized. The diameter of a nanotube is in the order of 20-200nm (approximately 1/50,000th of the width of a human hair), while they can be up to several millimeters in length. Nanotubes are categorized as singlewalled nanotubes (SWCNT-s) and multi-walled nanotubes (MWCNT-s). At use as a matrix strengthening material, the nanotubes sticking together must be separated and dispersed throughout the matrix phase. Chemical bond between matrix and nanofibers has to be enhanced in stead of van der Waals forces. The wide-ranging morphology of the carbon nanofibers and their associated properties results in a broad range of scatter for experimental results on processing and characterization of nanofiber composites, Thostenson et. al.(2005). There are also Vapor-Grown Carbon Fibers (VGCF) nanotubes, which are bigger in size and contain more cylindrical layers, Endo (1988). The chemical bonding of nano tubes are similar to those of graphite. This bonding structure, which is stronger than the bonds found in diamond, provides the molecules with their unique strength. Nano tubes naturally align themselves into "ropes" held together by Van der Waals forces. Carbon atoms bonded by its four valence electrons in three dimensions is the hardest structure known, Figure 15. In graphite the carbon atoms come even closer than in diamond forming even stronger bonds. Three of the graphite carbon atoms four valence electrons are used for covalent bonding forming atomic carbon layers, Figure 15, while the fourth one is free between the layers and has properties as the electrons at metal bonding. The distribution of electrons result in electrically charged layers where the free electrons are able to transfer electric charges - current. This bonding is not so strong because each carbon atom can only contribute with “half” electron to each layer. It approaches the strength of van der Waals bonding forces. The result is that the layers are able to slide and therefore graphite is used as grease under heavy loads. CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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In carbon nanotubes the atomic structure is as in graphite. The cylinder covalent bonding is extremely strong, but in between the concentric cylinders van der Waals forces, as free electrons, are holding together the cylinders. Under tensile loading telescopic sliding between the inner and outer cylinder is possible and limits the force taking of the tube.

Figure 15.

Carbon structure in diamond left and in graphite right with electrically loaded layers. The electrical charges in graphite layers are shown. Tepfers (1995).

The major challenges in the research of nanocomposites can be categorized in terms of the structure from nano to micro to macro levels. There is still considerable uncertainty in theoretical modeling and experimental characterization of the nanoscale reinforcement materials, particularly nanotubes. Following issues are also of importance - nanotube dispersion, alignment, high volume and rate in production and cost effectiveness, Thostenson et. al.(2005). Ongoing research reviles unceasing exciting new properties of nanotubes and the future for them is open.

8.1.1

Types of carbon nanotubes and related structures

8.1.1.1 Single-walled carbon nanotubes SWCNT The structure of a SWCNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder, Figure 16, with two limiting configurations, which are armchair and zig-zag, Figure 17. Single-walled nanotubes have a wide-ranging morphology, Thostenson et. al. (2005). Single-walled nanotubes are still extremely expensive to produce.

Figure 16.

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Single-walled nano-tube (SWCNT).

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Figure17.

Armchair structure left and zig-zag structure for carbon nano tubes. Thostenson et. al. (2005)

8.1.1.2 Multi-walled carbon nanotubes MWCNT Multi-walled carbon nanotubes (MCWNT) or nanofibers consist of multiple layers, up to 16 layers, of graphite rolled in on them to form a tube shape. More layers mean less space in the tube center. They are also called nanofibers. Diameter is typically 103 nm, though they can vary greatly. The variability in morphology is more pronounced for MWCNT than for SWCNT, which can be considered as composed of nested SWCNT. There are two models which can be used to describe the structures of multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders with single-walled nanotube (SWCNT) within a larger one. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled up newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.3 Å. The wall thickness increase has linear relationship with nanotube diameter increase. Investigated on very many MWCNTs, the diameter seems to have bimodal distribution. The properties are sensitive to the size of MWCNTs. Thostenson et. al. (2005). 8.1.1.3 Vapor-grown carbon filaments VGCF VCGFs are by far the largest structure of their nature being cylindrically layered graphene structures with diameters of 104 nm. VGCF has diameter of about 10µm, is generally shorter than a few decimeters and has high electric conductivity, Endo (1988). VGCF is much more brittle than the flexible nanotubes. VGCF is made by decomposing hydro carbonic gas (benzene in carrier gas hydrogen) within a furnace. At 1150 to 1300 degrees C (in certain graphitizing processes up to 3000o C), the benzene breaks down and condenses on catalyst iron powder seed (impurity particle serving as nuclei for condensation) carbon as concentric sheets of graphene with other layers on top of what has already formed, Figure 18. The cylindrical shell is a result of the catalyst particle movement leaving behind the carbon tube. This process is called chemical vapor deposition. It is important to keep free from atoms not belonging to process. Temperature and compounds influence the type of nanotube obtained. There are other ways to produce nanotubes including laser evaporation of composite containing carbon and metallic catalyst (Ni, Co, Fe, Y and other), evaporation of an anode rod of the same composition in the DC arc discharge. Large scale production of nanotubes, via both DC discharge and chemical vapor deposition CVD is made possible by recent technological advancements, Ballou & Ford (2005), Figure 19 and 20. CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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Figure 18.

VGCF production with indirect method, Endo (1988).

Figure 19.

The formation of VGCF via pryolysis, Ballou & Ford (2005), Endo (?).

Figure 20.

Production of VGCF, Ballou & Ford (2005), Endo (?).

In Figure 21 SWCNT, MWCNT and VGCF structures are shown situated along a diameter scaling.

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Figure 21.

Nanotube (SWCNT), nanofiber (MWCNT) and VGCF, Endo (? ).

8.1.2 Manipulation of carbon nanotubes There is a possibility of joining two nanotubes together to extend in length even if the diameter of the tubes to be joined is different. However, the structure of the graphene must change slightly to accommodate the various differences, Figure 22. Because joining two nanotubes together requires inserting odd sized cells into an otherwise constant pattern, the tube deforms slightly in bending.

Figure 22.

Joining of two nanotubes in length, Ballou & Ford (2005), Lambin (?).

The two prime structures of nanotubes are named zigzag and armchair, but there are also other rarer structures called chial, Figure 23. The structure of the nanotube is defined by number of unit vectors, named m an n. This is more commonly visualized as the number of carbon atoms that is lined up on a particular circumference. The zigzag formation is defined when m = 0. Armchair is defined as m = n. As said, anything else is considered “chial” named after the chial vector, which is defined as (n, m). For any given (n, m), if 2n + m = 3q, where q is an integer, then the nanotube is metallic. Otherwise, it is semi-conducting. Therefore, all armchair structures are metallic. It is theorized that metallic nanotubes have 1000 times the electrical current density of Silver or Copper. While all of these are of the same chemical make up, the CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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same cylindrical graphene sheets, but merely the hexagonal cells are oriented at different angles and different “twists”, Ballou & Ford (2005).

Figure 23.

Chial structures of carbon nanotubes. Ballou & Ford (2005), Dresselhaus (?).

8.1.3 Potential use of carbon nanotubes For potential use of carbon nanotubes can be taken in favor the strength and high modulus, the large surface area, ability for metallic or semi-conducting electricity and the porous structure able to contain fuels and gases. They can be used for reinforcing polymers and cement. Because of its graphite structure the nanotubes can be aligned to increase reinforcement efficiency and create a conductive mat for electric current and heat. There are also other sophisticated physical applications. The strength of commercial Kevlar used in bullet proof vests is 27-33 Joules per gram (J/g) while carbon nanotubes need 600 (J/g) to break, Ballou & Ford (2005).

8.1.4 Drawbacks of nanotubes The nanotube is far from perfect. It is difficult to manipulate and form and to control their size. However this technology is new and its full capabilities are far from complete now, Ballou & Ford (2005).

8.1.5

Nanobud

Carbon nanobuds are a newly discovered material combining two previously discovered allotropes of carbon. In this new material "buds" are covalently bonded to the outer sidewalls of the underlying carbon nanotube. This hybrid material could be useful for increasing the bond of the Nanofibers to the matrix by the nanobuds anchoring in concrete or cement matrix.

Figure 24. 30

Carbon nanobud on nanotube.

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8.2 Nanotubes properties Carbon nanotubes are one of the strongest and stiffest materials known, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent bonds formed between the individual carbon atoms. Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tube undergoes before fracture by releasing strain energy. Carbon nanotubes are not nearly as strong under compression. Because of their hollow structure and high aspect ratio from 1000 to 2.5·106, they tend to undergo buckling when placed under compressive, torsional or bending stress. Large variability exists between property findings analytically and experimentally. SWCNT tensile strength 13 – 53 GPa (determined in bending) and axial modulus of elasticity 0.91(±20%) TPa; diameter 1.3 nm; length 0.5 – 40 µm; specific area 300-600 m2/g. SWCNT sustains bending in large angles; density is 1330-1400 kg/m3, thermal conductivity 6000 W/mK and is stable up to 2800oC in vacuum and up to 750oC in air. SWCNT exhibit important electric properties, current carrying capacity 1 x 109 amp/cm2 (copper has 1 x 106 amp/cm2), that is not shared by the multi-walled carbon nanotube (MCWNT) variants. MWCNT axial tensile strengths have measured yield strengths at 20 to 60 GPa, even up to 150 (±30%) GPa and modulus of elasticity 0.8 -1.3 Tpa and elastic strains at yield above 10%. The fibers have aspect ratio from 1000 up to 2.5 ·106. Diameters are 60 -100 nm; length 0.5 – 40 µm. Specific area is 40-300 m2/g. The modulus approaches that of graphite with large tube diameter and Poissons ratio decrease with tube diameter. The multi layers are hold together by van der Waals forces, which results in that there is some deformability between the layers at force transfer, even pull out is possible. For application with cement these fibers are suited. VGCF nanotubes have diameter about 300 nm and length could reach up to 200mm. The tensile strength and modulus dependence of the tube diameter is shown in Figure 25. This nanofiber type could be the best for application in concrete technology. Tensile strength of straight fibers 2.05 GPa; Crooked fibers 1.09 GPa, break at bends; Modulus of straight fibers 163 GPa and crooked fibers 197 GPa; Fiber diameter 150 nm; Fiber length 10~20 µm; Density 2.0 g/cm3, Bulk density 0.04 g/cm3; Specific surface area 13 m2/g; Thermal conductivity 1200 W/mk; Electrical resistance 1 x 10- 4 Ωcm. The properties are difficult to determine of carbon nanotubes. They are so small that they cannot be tested in a machine. The performed tests are indirect with tubes in a matrix hopefully oriented with length in stress direction. The strength can also be calculated using physical properties of atoms, but imperfections in tube structure can only be estimated. Therefore there is a considerable scatter in stated properties. It can be discussed if it is possible to state stress acting on a tube made of a limited number of atoms. Most likely the stress is stated in macro level in a composite with a certain percent of nanotubes subtracting the stress taken by the matrix resin. The same yields also for other carbon nanotubes property statements.

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Figure 25. Dependens of tensile strength and modulus of elasticity of VGCF on fiber diameter, Endo (1988), Kandani et. al. (1984). Price: Package of 100 g: SWNT US$ 5500; MCWNT diameter < 10 nm US$ 4200 with falling price with rising diameter dawn to for diameter 60-100 nm, US$ 930. (Price information is according to Helix Material Solutions, 2008-01-03). The price is enormous; however it is expected to go down considerably when the production capacity is increased. However, it will be never so low that nano tubes might be used in concrete with ordinary structure dimensions. The possible products will be thin micro concrete sheets and other small dimension products with certain intelligent material properties enabling cooperative technology with other engineering disciplines. Possible use might be with aligned carbon tubes also in very high load bearing elements as suspension bridge cables or prestressing strands.

Figure 26 Carbon nano tubes, probably VGCF-s, (According to: \Nano fiber concrete\Helix Material Solutions 1.htm). Carbon nanotubes have been used to reinforce polymer matrixes enhancing failure strain, toughness and modulus. Nanotubes could also be used to reinforce metals as steel and aluminum and toughening ceramics, (Thostenson et. al. 2005). Extruding or using electric field can align the nanotubes in composite. Under compression fibers in matrix may buckle. There are stress concentrations at ends of fibers and fibers may bridge matrix cracks, de-bond, pull-out and fracture. Creation of chemical bonds, even covalent, between nanotubes and matrix is possible. Interface tailoring also at tube 32

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ends is a technique under development. Addition of nanotubes increases the electric and thermal conductivity of the matrix. Carbon nanotubes are like strings, Figure 26, at contact points hold together by van der Waals forces represented by the fourth valence free electron of the carbon atoms. For dispersion of nanomaterials a separation by breaking van der Waals forces in a solvent is a prerequisite to align them or distribute them evenly in a matrix. However it is difficult to control their alignment in a matrix. A team of nanotechnologists at The University of Texas at Dallas, Baughman & Welch (2008), along with Brazilian collaborators, have discovered that sheets of carbon nanotubes can produce bizarre mechanical properties when stretched or uniformly compressed. These unexpected but highly useful properties could be used for such applications as making composites, artificial muscles, gaskets or sensors. When most materials are pulled in one direction, they get thinner in the other direction, similar to how a rubber band behaves when it is stretched. However, specially designed carbon nanotube sheets, dubbed “buckypaper”, Figure 27, can increase in width when stretched. The buckypaper can also increase in both length and width when uniformly compressed. Ordinary materials contract laterally when stretched - a phenomenon that can be quantified by Poisson’s ratio, which is the ratio of the percent lateral contraction to the percent applied stretch. Possible cause of this behaviour might be that the buckypaper, when compacted erect close van der Waals bonds, which are loosened under tensile stretching resulting in transverse to the load increase in width.

Figure 27.

“Buckypaper” made of carbon nanotubes. http://www.netcomposites.com/news.asp?4955.

SWCNT continuous strands up to 200mm have been made. Electro spinning processes for assembling carbon nanotubes in continuous fibers are under development. VGCF probably could be used to fabricate continuous strands by twisting. Between the twisted fibers force is transferred by van der Waals forces and friction and this means a limitation in force transfer compared to the strength of the nanotubes themselves. Also spider silk has been tried in this context. Ribbons and films are also been tried to produce. A lot of basic research is still necessary. To take exceptional properties observed at the nanoscale and utilize these properties at the macro scale require a fundamental understanding of the properties and their interactions across various length scales. Basic understanding will allow for design of multifunctional material for engineering applications. A material scale up of manufacturing processes is required. Finally high rate in fabrication has to be developed. CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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8.3

List of carbon nanotube suppliers

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Ahwahnee Technology (USA)

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NanoCarbLab (Russia)

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Apex Nanomaterials (USA)

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NanoCraft, Inc. (USA)

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Arry International Group Ltd. (Germany)

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Nanocs (USA)

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Nanocyl S.A. (Belgium)

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Bayer MaterialScience AG (Germany)

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NanoLab (USA)

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BuckyUSA (USA)

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Nanoledge (France)

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CarboLex Inc. (USA)

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NanoNB Corp. (Canada)

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Carbon Designs, Inc. (USA)

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NanoTechLabs, Inc. (USA)

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Carbon Nanotechnologies Inc. (USA)

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Nanothinx S.A. (Greece)

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n-Tec (Norway)

•

Raymor Industries Inc. (Canada)

•

Rosseter Holdings Ltd. (South Cyprus)

•

SES Research (USA)

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carbon NT&F 21.. (Austria)

•

Carbon Solutions, Inc. (USA)

•

Catalytic Materials LLC (USA)

•

Cheap Tubes Inc. (USA)

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Chengdu Organic Chemicals Co., Ltd.(China)

•

Shenzhen Nanotechnologies Co. Ltd. (China)

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Fullerene International Corp. (USA)

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Stanford Materials Corp. (USA)

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HeJi, Inc. (Hong Kong)

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Sun Nanotech Co Ltd. (China)

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Helix Material Solutions (USA)

•

•

Idaho Space Materials, Inc. (USA)

The Australian National University (Australia)

•

•

Iljin Nanotech Co., Ltd. (Korea)

Thomas Swan & Co. Ltd. (England)

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Xintek, Inc. (USA)

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M.E.R. Corp. (USA)

•

•

MicrotechNano (USA)

21st Century NanoTechnologies, Inc (China)

•

MTR Ltd. (USA)

•

•

Nano-C (USA)

Ironbark Composites (Australia)

Here it can be mentioned that some laboratories, when doing tests with carbon nanotubes, have manufactured the nanotubes themselves. 34

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9

Cements and aggregates for use with nanofibers

This chapter is mainly according to: P.N.Balaguru Rutgers University, Ken Chong and Jorn Larsen-Basse National Science Foundation, USA, (?).

9.1 Grinding Electron microscope picture of hydrated cement structure of ordinary concrete is shown in Figure 28. It is obvious that the structure has a lot of pores. Carbon nanofibers on the cement grain surfaces cannot be effective in so porous structure. It is clear, that the tiny nanofibers cannot have satisfactory bond to the porous matrix. To reduce the porosity the cement specific area is to be increased at least four times by grinding. Micro silica has to be added and superplasticizer used to make the hydrated structure denser, so the nanofibers can have good bond conditions. The possible strength the grinded cement will provide after hydration will not be influenced by the grinding. The grinding has to be done in dry atmosphere. The increased risk of hydration of the micro cement particles during storage due to moisture in air has to be avoided by storing dry. There should not be contact with other reactive medium. Agglomeration of particles has to be avoided.

Figure 28.

Hydrated cement of ordinary concrete enlarged by electron microscope.

Still Portland cement has not completely explored potential. Better understanding of the complex structure of cement at the nano level will apparently result in new types of concrete with added nanotubes, which are stronger and more durable, with desired stress-strain relation and possibly with the range of “smart” properties such as electrical conductivity, temperature-, moisture-, stress- sensing abilities. Nanoengineered cements obviously will be the next break through development using nano silica and reinforced with carbon nano tubes, Sobolev & Ferrada Gutiérrez (2005).

9.2 Admixtures The influence of chemical additives, pozzolans, water reducers, nano-silica fume and nano glass particles have to be investigated. However according to Sobolev & Ferrada Gutiérrez (2005), among new nano-engineered polymers, there are highly efficient superplasticizers for concrete and high strength fibers with exceptional energy absorbing capacity. Nano particles, such as silicon dioxide (silica), were found CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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to be a very effective additive to concrete, a development realized in highperformance and self-compacting concrete with improved workability and strength.

9.3 Aggregates As aggregate ground sand, on nano or micro level could be acceptable. Titanium oxide and zinc oxide can also be used.

9.4 Fillers For reduction of shrinkage, caused by too much cement, fillers larger/smaller than cement particles could be used. The larger particles should be used for filling up more of the volume. How the specific surface area is influenced by the particle size is shown in Figure 29.

Figure 29.

Particle-size and specific surface-area scale related to concrete materials, Sobolev & Ferrada Gutiérrez (2005).

9.5 Fibers Possible fibers are carbon nanotubes, carbon whiskers, short carbon fibers (7 microns), fiber tows, fiber fabrics, silicon carbide whiskers, alkali resistant glass fibers, metallic fibers, ceramic fibers for high temperature applications, polymeric fibers and flexible membranes.

9.6 Fabrication Ordinary concrete casting may not be feasible. Extrusion could be used as well as pulltrusion. 36

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9.7 Fabrication Techniques Vacuum bagging can be used for fabrication. Also curing under pressure eventually with vibrations and high temperature is possible and might give rise to very high compressive strength for the concrete.

9.8 Products and applications As earlier mentioned, it is not possible, due to costs, to cast concretes with ordinary structure dimensions. What could be manufactured are micro meter thick sheets, bars, tubes, laminates, coatings to reduce corrosion, ingress of harmful chemicals or to change electrical properties and use for crack filling in existing structures. Nano carbon tube reinforced concretes could be used as sensors thanks to electrical conductivity and resistance changes, when mechanical stress alters. Possible use is also as fillers in sleeves for cables in bridges and prestressing tendons and laminates to protect against terrorism thanks to its tough response.

9.9 Challenges Heat of hydration and setting has to be studied and put under control with specially developed organic and inorganic additives. The grinding methods of nano size cement particles have to be developed.

9.10 Some questions to be clarified For ordinary concrete the water-cement ratio governs properties. Is the same relevant for cement with nano particles? Will the strength and strain capacity remain same as for ordinary concrete? What about strength, modulus, strain, hardness, thermal conductivity and electrical properties? Will it be possible to dry process the cementfiller-fiber mix and cure using steam impregnation?

9.11 Summary Large amount of funds and effort are being utilized to develop nano technology. Even though cement and concrete may constitute only a small part of this overall effort, it could pay enormous dividends in the areas of technological breakthroughs and economic benefits. Current efforts are focused on understanding cement particle hydration, nano size silica and super plasticizer additions and sensors. Unique opportunity exists for the development of nano-cement that can lead to major long-standing contributions.

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10 Basics of Hydration Three major solid components of hydrated cement paste are: Calcium Silicate Hydrate (CSH), Calcium Hydroxide crystals (CH or portlandite) and Calcium Sulfoaluminates (CS or ettringite). CSH occupies about 50 to 60 percent of the volume where as CH and CS occupies 20 to 25 percent and 15 to 20 percent respectively. Following factors have importance for hydration:

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•

The size of CSH sheet is less than 2 nm and the space between the sheets vary from 0.5 to 2.5 nm. Aggregation of poorly crystalline CSH particles could occupy 1 to 100 nm. Inter-particle spacing within an aggregation vary from 0.5 to 3 nm.

•

CH products are typically large with a width of about 1000 nm.

•

CS has needle type structure and is unstable.

•

Size of capillary voids range from 10 to 1000 nm. However in well hydrated paste with a low water-cement ratio the pore size is typically less than 100 nm.

•

C3A generates the most heat and C2S generates the least amount of heat.

•

Heat of hydration has two peaks, one occurs during the dissolution stage and the second occurs during the formation of compounds

•

Aluminates hydrate much faster than silicates. Silicates, which make up about 75 percent of cement plays a dominant role on strength development.

•

Of the two mechanisms of hydration through-solution hydration is more suitable for nano cements. In this mechanism, complete dissolution of anhydrous compounds to their ionic constituents and eventual precipitation of hydrates are assumed to take place.

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11 Cement with nanofibers Multi wall carbon nanofibers in form of nanotubes MWCNTs have potential for use as reinforcement in cement matrix. The fibers have modulus of elasticity of about 1 TPa and measured yield strengths at 20 to 60 GPa and elastic strains at yield above 10%. The fibers have aspect ratio from 1000 up to 2.5 ·106. The very fine fibers can be very well distributed and therefore can interrupt crack growing at an early stage. It is believed that addition of carbon nanofibers will give the cement matrix greater strength and especially toughness. According to Makar J.M. Beaudoin J.J. (2003), there are two problems to all carbon nanotechnology, CNT, composite applications. First, there is the question of dispersing the fibers into the matrix material. Dispersion is much more complex than simply mixing powder of nanotubes into the liquid material. Fibers adhere due to van der Waals forces, which makes an even distribution of fibers in matrix difficult to obtain. Use of surfactants in combination with sonication, where the acoustic energy breaks up the bonds and enables the surfactant molecules to come in between the tube contact points, is a method. Surfactant ensures that the fibers stay dispersed. The second problem is achieving suitable CNT-matrix bonding to avoid pull-out. Research is going on for overcoming this problem. Ordinary Portland cement hold grains of order 5 to 30 µm. However, smaller particles are also present. Nanofibers are of the same size as the nanostructure materials used to modify the cement and concrete behavior. Carbon nanotube bundles distributed on un-hydrated cement powder are shown in Figure 30. Hydrated cement is a brittle material with excellent compressive and low tensile strengths. Improved mechanical performance by acting as reinforcement is one of the benefits expected to be obtained by introducing nanofibers in the cement system. There are possibilities to improve the nanofiber bonding to cement matrix by functionalizing chemical reaction. However due to extremely small size, the nanofiber will be in contact with different minerals in the hydrated cement and thereby have different bond conditions. The bond of the different minerals to the nanotube has to be studied. For macro fibers this is not so important, because of much bigger size the contact surface can be regarded to have the different mineral mean bond properties. Treatment of nanotubes with acids can make the tube surface rough by damaging carbon bonds. However the carbon tube strength will be reduced.

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Figure 30.

Carbon nanotube bundles distributed on un-hydrated cement powder, Makar J., Margeson J., Luth J. (2005).

One route to disperse nanofibers in cement matrix is to mix nanofibers in a surfactant mixed with water or another solvent. It has been shown that small amounts of nanofibers can be dispersed by sonication in water containing 5% superplasticizer. However it is not clear if this is possible for nanofiber concentrations of 2-10%, which is necessary to achieve for good mechanical performance. Another route is to disperse nanofibers by sonication in ethanol described as follows. Nanofibers stick together and therefore cannot be mixed into concrete just by adding them. At first the fibers must be separated and dispersed. This is done by mixing them in ethanol under vibration with ultrasound, according to Makar J., Margeson J., Luth J. (2005). They produced a mix of CNT/(Portland cement) of 0.02 by weight. This was done by adding cement to the beaker containing CNT-ethanol under continuous sonication. After four hours the sonication was stopped and the ethanol was allowed to desiccate under hand stirring. The resulting cement cake was broken apart and ground. Cement particles covered with nanofibers were obtained and by thermal analysis was controlled that cement had not hydrated. Samples of 0.8, 0.5 and 0.4 water/cement ratios were prepared also adding superplasticizer. Maturing took place at room temperature and 100% relative humidity. Measuring of properties was done after 1, 2 and 3 days and 1 to 4 weeks. Wickers hardness tests can be done on small samples. These test results can be correlated to elastic modulus and the compressive strength. Measurements showed that CNT seem to accelerate the hardening process at beginning of hydration. However, after 3 weeks the properties were same as for control specimens. By breaking up specimens to induce cracks it was observed that the fibers were bridging the cracks and showed ordinary reinforcement performance, Figure 31 and 32, but it is not for sure that they have good bond. The fibers are able to elongate 10%, but it seems not to be enough for their observed extension over the crack. Contributions must have come also from fiber slip and this means some kind of bond rupture. If the bond is mainly by friction then the nanofiber contribution can be by making the composite tougher, but not especially stronger. The ultimate properties of cement-CNT composite after these tests still remained unknown.

Figure 31.

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Nanofibers bridging cracks. Makar J., Margeson J., Luth J. (2005).

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Figure 32. Carbon nanofibers filling the cracks in a cement composite, http://eurserveur.insa-lyon.fr/LesCours/physique/approphys. From Figure 32 it is obvious that a certain deformation is needed to stretch the curved nanotubes over the crack to make them active in taking tension. It is also visible that the carbon nanotubes lay curved everywhere and to be effective they have to be stretched, which gives a certain deformation. Further the bond of carbon nanotubes to matrix have to be good and therefore the matrix should not be porous and it is not for sure that it is so. Time influencing factors are change in fiber surface properties causing change in bond strength, change in local microstructure and morphology and weakening of fiber. The bond can be influenced by well-known concrete deteriorating processes as water dissolution leakage through material with increase of porosity and decrease in strength. Efficiency of fiber adhesion determines the load. Nanofibers have high contact surface with matrix, greater than 15m2/g. The weathering conditions for the composite are of importance as temperatures and moistures influence on bond. The influence of porosity obtained by additives as silica fume superplasticizers, degree of cement grinding and maturing conditions have to be studied. Nanofiber preparation for addition to the mix may also be of fundamental importance. If the nanofiber reinforced products are to contain certain substances the influence of them on the composite has to be stated. Obtained results by Borwankar A.D., Sanchez F. (2006), indicate that carbon nanofibers have minimal effect on strength of cementitious matrix. However, they got nanofibers clumped in void spaces and thereby ineffective. They have not mentioned the percentage of nanofibers in the mix. There has to be enough nanofibers to take over at least the tensile force released at cracking of matrix. In force taking are less than 1/3 of the fibers active, because in a random fiber mix there is only one third of them in direction of force. Further the crack may transverse the fiber in such a way that one end of the fiber does not have sufficient bond. This reduces the nanofiber capacity additionally. If the nanofibers are unable to take the force at matrix cracking, there will not be any strength increase of the composite. CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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It may be so that the carbon nano tubes are so strong that the available anchorage length is not enough to provide the necessary force to fracture the carbon nano tube fiber in tension. Bond, influenced by matrix porosity, will be the governing failure cause. According Borwankar A.D., Sanchez F. (2006), the durability of the cementitious matrix was improved by the nanofibers reducing mass loss during decalcification. Carbon nanofibers seem to be ineffective in strain sensing by piezoresistivity due to their tendency of clumping in cement matrix. They have to be lined up to be useful. Geng Ying et.al. (2007) performed tests with Portland cement mixed with carbon nanofibers. There were with H2SO4 and HNO3 mixture treated fibers and not treated fibers in the investigation. The treated fibers got somewhat rough surface giving better bond of the cement reaction products C-S-H. The treated fibers increased the cement paste compressive strength somewhat in comparison with the not treated fibers and also the flexural strength. Nanofiber reinforced specimens compared with unreinforced control specimens showed strong reinforced effect on flexural strength, compressive strength as well as failure strain. The electric conductivity was reduced for the treated fibers; because the carbon fibers did not have direct contact with each other due to cement past sticking to the treated surfaces and being in between breaking the contact. The carbon nanotubes can be treated with strong acids as H2SO4 and HNO3 to make the tube surface rough. Some carbon connections are broken by the acids and carbon atom chains curl of from the tube surface and make it appropriate to bind cement hydration products. Corresponding effect is achieved using electric corona treatment of macro polypropylene fibers, at production of so called Krenit fibers, Figure 33. The cement hydration products insulate the nanotubes electrically from each other and increase the electric resistivity of the composite. The treatment damages the tubes and therefore their tensile strength decreases. However it does not matter so much, because the original tube tensile strength is so high that it cannot fully be used.

Figure 33.

Krenit electric corona treated polypropylene fibers with curling off strings appropriate for binding to matrix.

Yakovlev et. al. (2006) investigated foamed light weight non-autoclaved Portland cement based concrete where the pore walls were reinforced by carbon nanotubes. The idea was to find a substitute reinforcing material for asbestos not having negative 42

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influence on health. The nanotubes were made according a Russian patent. The density of the foam concrete was 350 kg/m3 and amount of nanofiber reinforcement was 0.05% based on initial mass. Tests were done on cubes 100mm. Addition of carbon nanotubes meant a slight decrease in density, increase in compressive strength with 70% and 25% reduction in heat conductivity. Comparative tests at higher concrete densities showed that addition of nanotubes allows for increase in strength by the same percent. The addition of carbon nanotubes ensures the absence of pore wall percolation, which occurs generally in common foam concrete. Several theoretical approaches have been developed, Bafekrpour & Salehi (2008). However all of them lack precision due to the needed in data, as bond of fiber to the matrix, describing the material property badly. For instance bond is assumed to be perfect, which is not at all the case for the carbon nanotubes. Also the carbon nanotubes have defects and this fact has to be accounted for.

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12 Some possible use of carbon nanotube properties Tests are going on to include carbon nanotubes in the polymeric structure of different polymers and thereby obtaining new materials with improved properties. Including carbon nanotubes in the wind shield glass of cars it could be possible by using the electric conductivity and resistance of the nanotubes to heat the wind shield glass and melt ice without loosing visibility through the glass. If the piezoresistivity of the carbon nanotubes can be used to register deformations in a structure, this arrangement has always to be calibrated. Eventual threshold or percolation levels have to be established to estimate limitations in use. However, this use seems to be limited to fabrication of strain gauges of carbon nanotubes in cement matrix to be glued on or cast in the structure to be measured. There is a risk that the resistivity changes due to breakage of some nanotubes or connections in its structure may change properties. There is also a possibility to develop smart structures, which adapt its form to requirements of load or something else by using carbon nanotube electric properties. Carbon nanotubes, when electrically connected in matrix, have magnetic shielding properties. It is possible to line up carbon nanotubes by electric field or by letting them grow from a paper sheet in one direction and then move these directed fibers to place of application, for instance joining laminates in structure. By lining up fibers very great improvement in strength could be obtained. However these operations are very difficult to perform. There are ideas to use nanotubes for cracking observation in materials. Every new crack means reduction in electric conductivity through the material. However, the electric resistivity increase for each new crack has to be calibrated and then the possible application could be established.

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13 Nano fiber reinforced matrixes as tough armor material According to Net Composite News, ApNano Materials: The NanoArmor products will be based on ApNano’s proprietary nano spheres and nano tubes, which ApNano say are excellent shock absorbing materials and among the most impact resistant substances known in the world today, with up to twice the strength of today’s best impact resistant protective armor materials. According to ApNano, NanoArmor will provide multi-hit protection as well as enhanced ballistic and blast resistance. It will enable the development of special trauma layers behind the armor, reducing the level of blunt force trauma injuries. "Laboratory experiments conducted by Nobel Laureate Professor Sir Harold Kroto and his colleagues have demonstrated that ApNano’s nanotubes are strong enough to withstand a pressure of 21 GPa – the equivalent of 210 tons per square centimeter”.

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14 Carbon nanotubes and health effects As mentioned already in the introduction, the health aspects of carbon nanotubes on living bodies have to be investigated and stated parallel to the technical development efforts. Further the recycling and final deposition of the carbon nanofibers from end used structures has to be solved without creating environmental problems. The concrete engineers making research with nanotubes are end users. They do not develop the carbon nanofibers. However, there must be straight connection between the end users and the developers of carbon nanotubes, so the requirements emerging at application can be met by the producers. All researchers in the line from production to application and those working with health and environmental effects have to cooperate for a possible successful outcome of the new technique.

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15 Conclusions concerning carbon nanofiber effects on cement paste and concrete Successfully mimicking nature, the bottom-up construction processes is one of the most promising directions, Sobolev & Ferrada Gutiérrez (2005). Nanotechnology was first introduced by the famous lecture of Nobel Laureate Richard P. Feynman, “There´s Plenty of Room at the Bottom” given in 1959 at the California Institute of Technology. This technique has also reached the concrete technology. Nanotechnology deals with production an application on the range from few nanometers to submicron dimensions. It also deals with integration of the resulting nanostructures into larger systems. To the contemporary concrete technology now the bottom-up technology starts to come into use. Usually there is a mix at development of techniques and materials of bottom-up and top-down techniques. Nature has controlled the nanotechnology, but now man start to understand and to manipulate and apply this technique. The carbon nanotubes stand for promising developments. The strong fibers with their extremely small diameter and their high aspect ration seem well suited as reinforcement material for matrixes. Single walled nanotubes (SWNT) seem to be less useful for concretes as multi wall nanotubes (MWNT), which could be regarded as SWNTs concentrically combined in a single entity. Vapor-grown carbon filaments VGCFs, which are bigger in size, seem to be the most suitable for use in concrete. Concrete technology is one of very many possible applications for the carbon nanotubes and is just at the starting point in research. Nanotubes can influence the strength, toughness, electric and thermal conductance and other properties of concrete. A co-operation in research with other disciplines has to be opened up. Not only concrete with nanofibers but also the concrete gel structure influenced by superplasticizers is studied on nano level. To make nanofibers efficient, the bond of them to matrix has to be assured. The bond is disturbed by porosity and influenced by different minerals in matrix and therefore much interest has to be devoted towards developing a concrete material with as low porosity as possible. Precise engineering of an extremely complex structure of cement-based materials will result in new generation of concrete, stronger, more durable possessing a range of newly introduced properties as electrical conductivity as well as temperature-, moisture- and stresssensing abilities, Sobolev & Ferrada Gutiérrez (2005). The obtained results with carbon nanotubes bridging matrix cracks indicate that there is bond loss between the nanotube and matrix a considerable distance counted from the crack. The cause for it is most probable porous structure of the matrix and also that the tube surface is smooth. In the pores there is no bond and the remaining contact surface is unable to provide the necessary strength for the very strong nanotubes in tension. Further the nanotubes probably are not straight and at cracking are stretched giving a certain deformation. The result is that micro cracks become more open then if there had been perfect bond. It is observed by researchers that the nanotubes increase the matrix toughness, but less the strength. The toughness is provided by nanotubes bond sliding with breaking friction. To obtain strength increase for the matrix it seems to be necessary to make the matrix compact also on nano level. Carbon nanotubes driven by van der Waals forces tend to gather in matrix pores, which should be avoided. A uniform dispersal of nanotubes in matrix is CHALMERS, Civil and Environmental Engineering, Structural Engineering, Report no. 2008:7

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desirable. It is important to separate the nanotubes adding ethanol and treating them with ultrasound before mixing them in matrix material. The research with nanotubes must be combined with concrete technology research also on nano level. Here, grinding of aggregates and cement and additives as superplasticizer become of great importance to combine and control. Also the casting technology using pressure and elevated temperature can be of interest. MWCNTs and VGCFs have telescopic carbon tube structure. The cylindrical shells within the tube are joined and hold by van der Waals forces. Under tension the tubes tend to slide telescopic thereby not allowing development of full carbon tube strength. Research for time being with carbon nanotubes cannot guarantee success, but there is always a possibility for a break through for application and when it comes the production will increase and the price for carbon nanotubes will fall dramatically. Finally and not less important is to carry on the technological development parallel to the investigations of the carbon nanotubes effects on health and environment also after serviceability period, when the structures have been demolished. The research is still fundamental and no one can tell the outcome. Expectations in one direction may result in progress in quite different field of application. What can be the future use of carbon nanotubes in connection with cement and concrete is still an open question, but research to clarify mechanical properties and health effects have to be performed parallel to avoid an eventual human health disaster.

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Acknowledgement

The investigation is supported by Åke and Greta Lissheds Foundation, SEB Bank, SE106 40 Stockholm, which here is acknowledged, greatly appreciated and thanked for. Chalmers University of Technology, Department of Civil and Environmental Engineering, Structural Engineering, Concrete Structures under the leadership of Professor Kent Gylltoft has enabled me as emeritus professor to have room and computer as well as the Department facilities to perform this work. I forward my sincere thanks to him for the provided possibilities. I also wish to thank my colleagues at the Department for the friendly atmosphere. .

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