The Steel Fibre Concrete

Published Date: 02 Nov 2017

Acknowledgements

I would like to express my thanks to the people who have helped me most throughout my project. I am grateful to my supervisor Dr. Susana Dessa Aguiar for nonstop support for the project.

This dissertation would not have been possible unless the help and advices from Willie Laing to carry out the laboratory experiments.

It is a great pleasure to thank everyone who helped me write my dissertation successfully.

Introduction:

The concrete is a widely used material since the 19th century. A lot of different structures are made with concrete all around the world because of its amazing properties. This composite material has changed the way of design and build major infrastructures as well as individual houses. Nowadays, it is one of the three main construction materials used for structures with steel and wood. No one of these materials is perfect, each of them has positive and negative aspects. Concrete is cheap and easy to use, but has a low tensile strength and is brittle, for example. Several different technics exist to improve the concrete and balance its weaknesses. Use a better cement to get high compressive strength or chemical products to allow frozen periods are part of them. However, only few technics have an influence on the mechanical weaknesses of concrete. The main solution to balance these weak properties is to use steel bars, the concrete is then called "reinforced concrete". This technic is very efficient and for that reason most of modern structures are made from reinforced concrete and not from concrete. Although steel bars solve some problems, they do not decrease the shrinkage and micro-cracks and are sometimes expensive to use for structures mainly solicited in compression. Another technical solution could be considered in such cases, the fibre concrete. For that reason, since 70’s, engineers are looking for effective fibre concrete solutions. A lot of different methods exist, with various types of fibres. In this dissertation we are going to have a look on the steel fibres which are one of the most commonly used type of fibre. They are added in the concrete mixture and can replace steel bars as well as to be used at the same time. The incorporation of fibres changes the matrix of the concrete and could influence its physical and mechanical properties. The purpose of this dissertation is to test the hypothesis that steel fibres in concrete change the mechanical property called modulus of elasticity. This property is one of the most important in structural engineering. It defines the behaviour (ductility) of the material under stress and is used to carry out calculation of structures made of concrete as well as of steel or wood.

Chapter 1: Literature review

1.1 Steel fibre concrete

1.1.1 What is it?

To understand what is steel fibre concrete also called fibre reinforced concrete (FRC), we have to understand what is an ordinary concrete. An ordinary concrete is a composite construction material made of three main components, water, cement and aggregate. Cement in contact with water become a "glue" and creates a link between the particles (aggregates) forming a strong matrix. Concrete is used for a wide range of applications and sometime specific properties need to be improved. To achieve this goal we add new elements in the mixture. This dissertation will speak about the addition of only one more element, steel fibres. A steel fibre concrete could be described as follow:

A steel fibre concrete is a composite materials made with Portland cement, aggregate and a small quantity of thin and long pieces of steel. It contains short discrete fibres that are uniformly distributed and randomly oriented. Those fibres change the properties of the concrete and transform it in a more complex material.

Advantages

Fibre reinforced concrete (FRC) is used as an ordinary concrete; fibres don’t change completely its properties but improve and modify some of them. Advantages depend on the type of fibre used. Two main groups of fibres bring different benefits. The first group consists of fibres that change the mechanical properties of concrete (compressive strength, flexural strength, tensile strength). The second group of fibres don’t change the mechanical properties but the physical properties of the concrete in which they are incorporated (shrinkage cracking, corrosion, concrete spalling). This dissertation speaks about steel fibres that provide mechanical improvements. The main advantages should be:

-Improve structural strength

-Reduce or delete steel reinforcement requirements

-Improve durability

-Improve abrasion resistance

-Reduce crack width

This listing is not representative of all the benefits that can bring steel fibres. Moreover the steel fibres are not the only type of fibre that can be used to get these improvements. Polymer fibres are very efficient, and could be better than steel regarding several aspects.

Disadvantages

Fibre concrete is more complex than an ordinary concrete. When the complexity increases, the cost increases also because of the time needed to incorporate fibres and the cost of the fibres itself. The fibre reinforced concrete is every times more expensive than an ordinary concrete. Fibres damage the mixing equipment and may cause injuries to labourer. The fibres decrease the workability and chemical admixtures are the only one solution to improve workability if we don’t want to decrease the resistance of the concrete. Add water is not possible, because the ratio water/cement increases quickly and decreases the concrete strength. This is difficult to recycle steel fibre concrete and like for the previous disadvantages, the problem is the cost of the operation. The main disadvantages could be listed as below:

-Complexity

-Workability

-Cost of the fibres (material, time, equipment usury)

-Not recyclable

Fibres don't have any disadvantages about their structural behaviour. But they have a high cost and not only caused by the price of the steel.

Materials (steel, polymer, glass, natural)

Steel fibres are the most used of all fibre materials, but this is not the only one type of fibre. Other types of fibre exist like polymer fibres, natural fibres and glass fibres.

Table 1.1 Types of fibre

Types of fibre

Main advantages for concrete

Steel

Improve strength and ductility, reduce cracks

Polymeric

Reduce cracks propagation and damage from freeze-thaw cycles, not expensive

Natural

Little improvements on a lot of different aspects, environment friendly

Glass

Resistant to deterioration caused by environmental conditions

Each type of fibre has a specific range of applications. The fibre concretes are used for a lot of different purposes, and this large choice of fibres is useful to match as well as possible with the characteristics required.

Shapes and sizes

Different shapes and sizes exist for each type of fibre. Shapes and sizes influence the behaviour of the concrete under loading and the cracks propagation.

The British standard (BS EN 14889-1:2006) classifies steel fibres into five groups, according to the method of manufacture but also according to the shapes. This parameter as well as the size are closely controlled and have an important impact on the concrete. The main geometrical parameters are listed below:

-Shape (specific outer configuration of the fibres)

-Aspect ratio (ratio of length to equivalent diameter of the fibre)

-Equivalent diameter (diameter of a circle with an area equal to the mean cross sectional area of the fibre)

-Length (distance between the outer ends of the fibre)

-Area of the cross section of the fibre

Shapes and sizes are directly linked with the purpose of the fibre concrete made. For example the short fibres are very efficient to reduce micro cracks propagation inside the concrete matrix.

For other purposes than reduce micro-cracks propagation steel fibres are classified according to their aspect ratio. The aspect ratio is a key factor for quality fibre concrete, manufacturer like Dramix for example, offers three levels of performance based on this property.

Some pictures of fibres showing the wide range of shapes available:

Figure 1.1 Various fibre shapes

Costs

The cost varies depending on the type of fibre (Steel/Glass/…), the global economic environment, the manufacturer and the country were are made the fibres. Glass fibre concrete is more expensive than any other concrete. We could simplify the hierarchy of prices between the different types of fibres as followed:

-Glass (the more expensive)

-Steel

-Natural

-Polymer (the less expensive)

Of course this list is not representative of the variations observable on the market. Moreover considering an international use, some countries could for example use natural fibres rather than steel, because of the resource available (and consequently the price).

1.2 Fresh properties

At this point of the dissertation, only the steel fibres will be considered.

Fresh properties of FCR and ordinary concrete are different because of the adding of fibres, several parameters change. Some of the main parameters are listed below, changes not appear with all of them:

Setting time and Cement hydration

The setting time is the transition process of changing of concrete from plastic state to hardened state.

Concrete derives its strength by the hydration of cement particles. Steel fibre has a low effect on the cement hydration because of steel is not sensitive to water.

Air temperature, ground temperature and weather conditions play major roles in the rate with which cement hydrates.

Steel fibres don’t have any direct impact on the setting time and cement hydration.

Workability

Workability is the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation.

Incorporation of steel fibres in concrete reduces significantly the workability of concrete. This is important to know and understand this effect. If the workability decreases, the FRC cannot be used for every purpose without adding chemical admixtures. Moreover because concrete strength is affected by the presence of voids in the compacted mass, it is significant to achieve a maximum possible density and this requires sufficient workability.

Segregation and Bleeding

Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture.

Bleeding is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete. Steel fibres are light and not particularly subject at segregation. Moreover steel fibres decrease the segregation and bleeding in concrete.

Air entrainment

Air entrainment reduces the density of concrete and consequently reduces the strength. This technical solution allows a higher resistance to freeze-thaw action in hardened concrete. Steel fibres do not decrease significantly the efficiency of this technic.

1.3 Physical and mechanical properties

1.3.1 Density

The density of an ordinary concrete is from 2200 to 2400 kg/m3. The density depends on a lot of parameters; the more important are the density of the aggregate and cement. For example the Portland cement weighs between 830 kg/m3 and 1650 kg/m3 (Portland cement association) but others types of cement can have a density lower or higher. The weight of aggregates is also variable depending on the type of aggregate.

The density of concrete is defined by the addition of the density of its constituent and their proportions. Other parameters influence the final density like compaction or moisture content.

1.3.2 Compressive strength

Compressive strength is the capacity of concrete to withstand axially directed pushing forces. The forces are directed to the centre of the sample tested. Concrete have high compressive strength in opposition with its very low tensile strength. Compressive strength is independent from the size of the sample tested but influence by its geometry. The unit commonly used is MPa (N/mm2). Concrete is widely used because it is cheap but also because it has a high compressive strength, useful in the construction environment. When engineers want to increase this hardened characteristic of concrete, they don't use fibres as primary solution. To create a high performance concrete they use better cement, chemical admixtures, low water/cement ratio etc. Steel fibres do not deeply affect the concrete compressive strength. Some improvement could be observed because FRC has a higher ductility.

1.3.3 Tensile strength

Tensile strength is the capacity of concrete to withstand axially directed pulling forces. Concrete tensile strength is very low; this is for this reason that engineers use reinforced concrete for structural purpose. Steel bars in the concrete provide the tensile strength necessary.

So, why use steel fibres? The first reason is that those steel bars need more time to be placed in the formwork than fibres in the fresh mixture.

The second reason is those steel bars do not provide tensile strength inside the matrix of the concrete. They do not reduce the micro cracks and cracks from the shrinkage for example.

Steel bars and steel fibres work at different scales to provide tensile strength. Concrete is a brittle material and steel bars do not fix this weakness as well as steel fibres, which provide more ductility. For specific purpose, advantages of both methods are combined, to get a high performance reinforced concrete or because steel bars must be used. For example an ordinary bridge deck cannot be made without steel reinforcement, in this case fibres will provide other benefit (shock resistance, shrinkage, abrasion) but not a major tensile strength.

To get an idea of the efficiency of fibres in concrete when we speak about tensile strength, the Quebec concrete association and Canadian cement association speak about an increase of 40% for the tensile strength and 150% for the flexural strength.

Tensile strength in concrete appears most of the time during flexure. High-performance fibre-reinforced cementitious composites are designed to afford an excellent flexural strength to concrete. This type of concrete is not widely used in the industry but allow to understand what steel fibres can do! In fact HPFRCCs possess the unique ability to flex before fracturing solving the structural problem with concrete, its tendency to fail in a brittle manner. (Matsumoto, Takashi and Mihashi, Hirozo, 2007)

1.3.4 Shrinkage

The shrinkage represents changes in volume of concrete, autogenous or induced. Volume change is one of the most unfavourable properties of concrete, which affects the long-term strength and durability. The different types of shrinkage in concrete are:

-Plastic shrinkage

This shrinkage is due to rapid loss of water from surface. Only microfibers are used to prevent it providing sufficient tensile strength inside the concrete to resist cracking creation and propagation. Steel fibres are not used to prevent Plastic shrinkage.

-Drying shrinkage

This shrinkage appears after a long time and represents the loss of free water contained in hardened concrete. The presence of steel fibre is efficient to decrease it. Steel fibres increase the tensile strength of the concrete and reducing the cracks propagation.

-Autogenous shrinkage

This shrinkage is of minor importance and is not applicable in practice to the situations where steel fibre concrete is used.

-Carbonation shrinkage

Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement, causing the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place. The new product is less in volume than the product replaced so shrinkage takes place. Steel fibres cannot reduce the chemical reaction but increase the tensile strength of the concrete reducing the cracks propagation.

Steel fibres change these shrinkages in different amplitudes.

1.4 Modulus of elasticity

1.4.1 General description

Modulus of elasticity is one of the more important properties of concrete. Understand what is the modulus of elasticity is one of the key to understand the behaviour of steel fibres concrete.

What is the modulus of elasticity? The modulus of elasticity (MOE) is a mathematical expression to quantify the elastic behaviour or stiffness of a material. A stiff material has a high MOE and a flexible material has a low MOE. For example, diamond is very stiff so it has a MOE of 1000 GPa, rubber is flexible so it has a MOE of 0.1-1GPa.

A lot of different elastic modulus exist and are used in various engineering field.

The main ones are:

-Young’s modulus (tensile stress over tensile strain or compression stress over compression strain)

-Shear modulus (shear stress over shear strain)

-Bulk modulus (volumetric stress over volumetric strain)

These modulus of elasticity are used in fields like mechanical engineering, materials engineering, chemical engineering or civil engineering.

In this dissertation, only the young’s modulus (E) is going to be studied and more precisely, the compression stress over compression strain value. The SI unit is the Pascal (N/m2) but the practical unit used is the GPa (kN/mm2).

The elastic modulus E-Value is used in the calculation of:

-Deflection – often the controlling factor in slab design


-Moment analysis

-Requirements for prestressed elements

-Column shortening under load


-Stresses due to restrained movements

Modulus of elasticity can be found using laboratory test results or predicted using a specific equation.

Calculation of MOE using laboratory test results

This part is very detailed in the document entitled "Properties of concrete for use in Eurocode 2" written by (P.Bamforth, D.Chisholm, J.Gibbs, T.Harrison).

To find the elastic modulus using compressive strength results, we plot one curve (deformation of the sample under loading) that allow drawing three lines, each line has a proper slope representing a MOE. But these elastic modulus characterise the same material, the same sample. There is not only one E-value because concrete is not a truly elastic material and the relationship between stress and strain is not constant (Fig 1.4).

By consequence, we don’t speak about three different E-value but three different conventions.

The three E-Value conventions used:

-The secant modulus

-The tangent modulus

-The initial tangent modulus

Figure 1.4 The three E-values conventions

The dissertation is going to detail the calculation of the secant modulus.

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Δσ is the force applied on the concrete.

Δεs is the deformation of the concrete when the load increases.

Ec,0 is the modulus of elasticity

The MOE can also be found using the PUNDIT test result. This method is described in the British standard (BS 1881-203:1986). There is an empirical relationship between MOE and pulse velocity (Table 1.4).

Table 1.4 MOE and pulse velocity relationship

1.4.3 Prediction of MOE

Concrete commonly has an elastic modulus in the range of 27-44 GPa. The exact value of the modulus of elasticity depends on the concrete’s uniaxial compressive strength after 28 days (see part 4.2), but can be predicted using formulas. Several different formulas exist and the two main have been developed in America and Europe.

1.4.3.1 Europe

The Eurocode 2 used in Europe gives:

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Ecm is the modulus of elasticity

fcm is the mean concrete strength

This formula gives us an indication of what should be the MOE for each concrete strength class (Table 1.4.1). This table is provided in the British standard (BS EN 1992-1-1:2004, Table 3.1 Page 29)

Table 1.4.1 Strength classes for concrete

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Figure 1.4.1 Evolution of MOE

1.4.3.2 America

In America, the American concrete institute gives the formula:

Ec=4700*sqrt(fc)

Where:

Ec is the modulus of elasticity (MPa)

fc is the mean concrete strength (MPa)

It could be interesting to compare the accuracy of both methods with steel fibre concrete.

Chapter 2: Experimental work

2. Introduction

In this chapter the main aspects of the experimental work will be covered, providing an explanation as clear as possible of the work done in the laboratory (Fig3).

Figure 2.0 Experimental programme carried out in this investigation

2.1 Materials

Different materials are used to make a steel fibre concrete. A steel fibre concrete is more complex than an ordinary concrete, because of the number of components required.

2.1.1 Mineral aggregates

There are two main mineral aggregates, the coarse (4/10mm) and fine (0/4mm) aggregates, both crushed. A sieve test and a density test is done for each type of aggregates. These tests are carried out to do an accurate mix design and check the conformity of the aggregate regarding to the British standard.

Sieve analysis and grading curves are carried out conforming to BS EN 933-1:2012

Density test is carried out conforming to BS EN 1097-6:2000

The coarse aggregate (4/10mm):

Sieve analysis:

The sieve analysis is carried out to assess the particle size distribution. The particle size distribution of a material influences its comportment in the concrete matrix.

The sieve test done for the coarse aggregate gives us the following diagram (fig 2.1):

Figure 2.1 Sieve test results for coarse aggregate

The maximum diameter of the coarse aggregate is 10mm. The table below (Table 2.1) gives us the raw results of the test, used to draw the diagram (Figure 2.1).

Table 2.1 Results of the sieve test

Sieve size (mm)

Mass Retained (g)

Percentage retained (%)

Percent passing (%)

Specific limits

20,00

0,00

0,00

100,00

√

16,00

0,00

0,00

100,00

14,00

0,00

0,00

100,00

√

12,50

0,00

0,00

100,00

10,00

311,20

12,60

87,40

√

8,00

934,68

37,84

49,56

6,30

940,24

38,06

11,50

4,00

275,27

11,14

0,36

√

2,00

1,80

0,07

0,28

√

1,00

0,35

0,01

0,27

0,06

4,80

0,19

0,08

0,01

1,83

0,07

0,00

Density test:

The aggregate density is a key parameter to do the mix design.

1 glass funnel

2 Mark

3 Ground section to fit the wide-neck flat bottom flask

4 Wide-neck flat bottom flask

The density test results for the coarse aggregate according to EN 1097-6:2000 Clause 9:

For the small Pyknometer 2L:

Type of density

Result(KN/m3)

ρssd

2,60

ρrd

2,58

ρa

2,60

For the large Pyknometer 5L:

Type of density

Result(KN/m3)

ρssd

2,59

ρrd

2,58

ρa

2,60

Water absorption:

WA24=0.28

The formulas used to calculate the various densities described in the British standard are:

Particle density on an oven-dried basis

Particle density on a saturated and surface-dried basis

Apparent particle density

Water absorption

M1 is the mass of the saturated and surface dried aggregate in the air, in grams;

M2 is the apparent mass in water of the basket containing the sample of saturated

aggregate, in grams;

M3 is the apparent mass in water of the empty basket, in grams;

M4 is the mass of the oven-dried test portion in air, in grams;

ρw is the density of water at the temperature recorded when M2 was determined

Water temperature:20°C ; Water density 0.998

Table 2.2 Various masses measured in the laboratory

Pyknometer

Tray only

M1

M2

M3

M4

5L

2779

1837

8420

7292

1830

2L

2565

1849

4652

3515

1844

The fine aggregate (0/4mm):

Sieve test:

The sieve test done for the fine aggregate gives us the following diagram (Figure 2.2):

Figure 2.2 Sieve test results for fine aggregate

Table 2.3 Results of the sieve test

Sieve size (mm)

Mass Retained (g)

Percentage retained (%)

Percent passing (%)

Specific limits

8,00

0,00

0,00

100,00

√

6,30

0,00

0,00

100,00

4,00

2,67

1,00

99,00

√

2,80

5,65

2,12

96,87

2,00

45,31

17,02

79,85

√

1,00

66,86

25,12

54,73

0,60

44,05

16,55

38,18

0,50

13,73

5,16

33,02

√

0,25

46,40

17,43

15,59

√

0,13

31,76

11,93

3,66

0,06

7,89

2,96

0,69

0,01

1,75

0,66

0,03

Density test:

The density test results for the coarse aggregate according to EN 1097-6:2000 Clause 9:

For the small Pyknometer 2L:

Type of density

Result(KN/m3)

ρssd

2,63

ρrd

2,56

ρa

2,61

For the large Pyknometer 5L:

Type of density

Result(KN/m3)

ρssd

2,63

ρrd

2,61

ρa

2,66

Water absorption:

WA24=0.712

The formulas used to calculate the various densities are the same than for coarse aggregate.

M1 is the mass of the saturated and surface dried aggregate in the air, in grams;

M2 is the apparent mass in water of the basket containing the sample of saturated

aggregate, in grams;

M3 is the apparent mass in water of the empty basket, in grams;

M4 is the mass of the oven-dried test portion in air, in grams;

ρw is the density of water at the temperature recorded when M2 was determined

The density test for this type of aggregate uses the same tools than for the coarse aggregate.

Water temperature:22°C ; Water density 0.997

Table 2.4 Various masses measured in the laboratory

Pyknometer

Tray only

M1

M2

M3

M4

5L

2779

1496

8298

7370

1485

2L

2565

1838

4669

3530

1825

2.1.2 Other materials

Mineral aggregates are not the only materials used to make a steel fibre concrete. Cement, water and of course steel fibres are used.

The cement used is a Portland cement from the company Lafarge, strength class 42,5.

The water is clean and clear and comes from the potable network.

The steel fibres are made by the company "ADFIL construction fibres".

The type of fibre is:

ADTEC SW RANGE, Corrugated steel fibre

The density of the fibres is:

7327 kg/m³

The geometry of the fibre is:

Fibre lengths 30-60mm

Fibre diameter 0.7-2.5mm

2.2 Design of concrete

The concrete described in this dissertation is prepared in a laboratory. In laboratory the quantities and masses used are very accurate to reduce the probability of error or non-conformity. The quantities of each ordinary elements are calculated using the concrete mix design form. The mix design process needs several diagrams and tables to be achieved. The procedure is divided in five stages (fig 2.3) leading to find the fine aggregate content and the coarse aggregate content at stage 5. Previously at Stage 3, the cement content is found using the free water content. The aggregate density and grading are key points at stage 4 and stage 5, see above for more information about that.

Figure 2.3 Flow chart of mix design procedure

The mix design procedure could be described as follow, listing the key values:

-The characteristic strength specified is 30 N/mm2 at 28 days.

-Target mean strength is 30+13=43 N/mm2

-Free water/cement ratio is taken as 0.45 because the initial value 0.59 was too high.

-Free water content is 195 kg/m3

-Cement content is 433 kg/m3

-Relative density of aggregate is 2.61

-Total aggregate content is 1700 kg/m3

-Grading of fine aggregate, percentage passing 0.06mm is 38%

-Proportion of fine aggregate is 51%

This listing shows the importance of some key values. At the end of the mix design form we get the different quantities necessary to obtain the strength that we are expecting and the slump wanted.

Result of the mix design:

The quantities listed below are calculated to provide enough material to make 0.024m3 of concrete (two cylinders, six cubes and three beams).

Table 2.5 Mix design-Ordinary concrete

Component

Cement

Water

Coarse aggregate

Fine aggregate

Quantity (kg)

10.2

4.8

20.1

20

The quantity of steel fibre is calculated separately using the density of steel and the percentage of fibre used.

Design process:

Fibre have to represent 1% of the volume of the concrete

The concrete volume is 0.024 m3


So 1% * 0.024 =0.00024 m3


The weight of fibre is 0.00024*7327=1.758 kg

Quantities of every components composing 0.024m3 of this concrete:

Table 2.6 Mix design-Fibre concrete

Component

Cement

Water

Coarse aggregate

Fine aggregate

Fibre

Quantity (kg)

10.2

4.8

20.1

20

1.758

2.3 Mixing and Casting of concrete

Mix and cast operations are done in a short lapse of time, few minutes to decrease the risk of deterioration of the concrete.

Each components is weighted separately and stored in bags ready to be used one day before the "mix" day. When the mixing process begins, coarse and fine aggregate are loaded into the mixer. As soon as the aggregates are mixed together, we add cement and immediately after water. We wait few minutes before to add steel fibres. Steel fibres are homogenously incorporated inside the concrete mix.

When the concrete is ready to be cast, we proceed the slump test to check if the workability is correct. The result of the slump test depends on the presence of fibre or not. For an ordinary concrete we get 50mm in comparison with the 20mm obtained for the steel fibre mix. For both mixes we use the same quantity of water, exactly 4.83 litres. We observe that steel fibres decrease significantly the workability, 60% less in our case.

If the workability is congruent, we cast the concrete. In our case, the first ordinary mix had a workability too high. This is for this reason that we had to correct the water quantity. Our first water quantity was 5 litres for 0.024m3 of concrete. After the first slump test we observed that the water quantity was really too high. We decided to correct weight of water used, decreasing it from 5 kilos to 4.83 kilos. That correction changed the water/cement ratio, the previous one was 0.49 and the new one is 0.47.

With the new water/cement ratio the slump test result is correct according to the British standard.

The next step is to cast, this is done on a vibrating table to compact the concrete, layer by layer. The concrete is compacted to decrease the air quantity and increase its density and quality. The compaction time must be kept short to avoid bleeding. The concrete is poured in the moulds compacted and next, a damp cloth is placed over the top surface of the mould. The cloth is used to prevent the concrete loosing moisture too quickly while drying.

The last step is to de-mould the concrete, we do it 48 hours after the mix and cast operations. The specimens are carried in a curing room under controlled temperature (20%) and moisture content (64%).

2.4 Concrete test procedure

The steel fibre concrete is tested exactly in the same way than an ordinary concrete. Their properties are very similar and the results obtained can be compared.

In this part, the tests of hardened properties only are described. The fresh property (slump test) is described in the previous part (2.4 Mixing and casting of concrete) to simplify the reading using a chronological layout for the report. In fact, the fresh property test is done between the mix and cast of the concrete in comparison with its hardened properties, tested at least 28 days later.

For each specimen geometry, the tests are different. The specimens used to model the modulus of elasticity are the cylinders. Their theoretical dimensions are 30 centimetres length and 15 centimetres diameter. In the reality the length varies between 29.5 and 30.5 cm, the diameter is more constant and often very close to the theoretical value.

We have done three different mixes, the first one was a control mix to test our mix design, the second one was carried out to get reference values and the third and last mix was made of steel fibre concrete. Each mix is made of two cylinders and several cubes and beams, but only the cylinders are used to get the modulus of elasticity.

2.4.1 Ultrasonic waves

The quality of the concrete is tested for each specimen using ultrasonic waves speeds. This is a non-destructive method used to test the quality of the concrete. If the sound is very fast through the specimen, it means that the concrete is dense with a small amount of air bubbles. If the concrete is dense, we conclude that the mixing and cast of concrete is a success. The tool used is a PUNDIT (Portable Ultrasonic Nondestructive Digital Indicating Tester). This type of tool is used for a large range of other purposes like:

-Evaluating the uniformity of concrete within a structural member

-Locating internal voids and cracks

-Estimating severity of deterioration

-Estimating depth of fire damaged concrete

-Evaluating effectiveness of crack repairs

-Identifying anomalous regions for invasive sampling with drilled cores

-Estimating early-age strength (with correlation)

There are several methods of propagating and receiving ultrasonic pulses, we use the method called direct transmission (fig 2.4).

Figure 2.4 Direct transmission

The measures are done according to the British standard (BS 1881-203:1986).

The wave speeds are calculated using the length of the specimen and the time measured.

V=L/T

V is the pulse velocity (km/s)

L is the path length (unit: km)

T is the time taken by the pulse to traverse that length (unit: μs)

Table 2.7 Classification of the quality of concrete on basis of pulse velocity:

Longitudinal pulse velocity (km/s)

Quality of concrete

>4.5

Excellent

3.5-4.5

Good

3.0-3.5

Doubtful

2.0-3.0

Poor

<2.0

Very Doubtful

The velocity in steel is up to twice the velocity in plain concrete so the presence of steel fibre could influence the results of the measurements.

The other factors which influence the pulse velocity measurements are:

-Moisture content (Chemical and physical effects, the differences between a properly cured cube and a structural element made from the same concrete may be significant). For this dissertation the moisture content is under control and should not influence the accuracy of the measurement.

-Temperature of the concrete (No significant impact if the concrete temperature is between 10°C and 30°C). For this dissertation, the temperature of the concrete is slightly under 20°C.

-Path length ( It should be long enough not to be influence significantly influenced by the heterogeneous nature of concrete). For this dissertation the maximum size of aggregate is 20mm, the BS gives us a minimum path length of 10cm. The path length of the cylinder is around 30cm and for the cube this is 10cm.

2.4.2 Relative density test

The relative density test is carried out weighting the specimen in air and next in water. Both weights allow us to calculate the relative density of the concrete. In the same manner than for the PUNDIT test, if the concrete has a high density we qualify it of good quality concrete. This test is carried out after the ultrasonic waves speed test and before the compression test. We use a scales able to give us the weight in air as well as in water (fig 2.5).

Figure 2.5 Scales used to carry out the relative density calculation

The relative density is obtained using the formula:

2.4.3 Compression test

The compression test is the ultimate test carried out on the specimens. Six cylinders have been crushed and two of them was equipped with two strain gauges. The strain gauges and the compressive stress allow us to calculate the modulus of elasticity.

All the test procedure is done according to the British standard (BS EN 12390-3:2009).

General procedure for the compression test:

We use a machine (fig 2.6) designed to apply a very high load on the specimen until the concrete fails.

Process for an ordinary test:

-We clean the testing machine and centre the specimen to apply an uniform load.

-The load is applied at a constant rate (0,6 ± 0,2 MPa/s) until failure.

-The machine is controlled by a computer that record the maximum load indicated in kN.

Process for a compression and strain test:

Two strain gauges are positioned on the specimen (right and left side at equivalent distance from the end faces of the specimen) and connected to a computer that record the strain values during the compression test (fig 2.7) and then provides us a file listing the deformation of the specimen.

-We clean the testing machine and centre the specimen to apply an uniform load.

-We proceed three preloading cycles at a constant rate until reach the specified value (σa = fcm / 3) according to the European standard (EN 12390-13).

-After three cycles are completed we increase the stress until failure occurs.

Figure 2.6 Testing machine Figure 2.7 Testing machine and strain recording computer

2.6 Summary

The experimental work described in this chapter allow us to obtain a lot of values and measures that will be used to study the modulus of elasticity of a steel fibre concrete. The results of this work is presented in the next chapter (Chapter 3: Results) and will be discussed in the Chapter 4. All the work done previously is done according to the European standard and British standard.

Chapter 3: Results

3. Introduction

The results presented in this chapter come from the experimental work carried out in laboratory and described in the previous chapter.

3.1 PUNDIT

We use the PUNDIT test to estimate de density of the concrete and so its quality. If we want to do a good assessment of the MOE we have to keep as low as possible the number of variable. The table below (fig 3.1) gives a summary of the test done on cylinders as well as cubes. Larger is the amount of value more accurate will be our concrete quality assessment. A good concrete allows high speed ultrasonic waves, see Table 3.1.

Shape

Type of mix

Number

Speed(μs)

Length(cm)

Speed(km/s)

Quality

Cube

Control mix

1

22,7

10

4,41

Good

Cube

Control mix

2

22,8

10

4,39

Good

Cube

Control mix

3

23,6

10

4,24

Good

Cube

Control mix

4

22,9

10

4,37

Good

Cube

Control mix

5

22,8

10

4,39

Good

Cube

Control mix

6

23,5

10,1

4,30

Good

Cube

Fibre mix

7

23,2

10

4,31

Good

Cube

Fibre mix

8

23,2

10

4,31

Good

Cube

Fibre mix

9

23

10

4,35

Good

Cube

Fibre mix

10

23,4

10

4,27

Good

Cube

Fibre mix

11

23,3

10,1

4,33

Good

Cube

Fibre mix

12

22,9

10

4,37

Good

Cylinder

Control mix

13

68,5

30,3

4,42

Good

Cylinder

Fibre mix

14

69,1

30,3

4,38

Good

Cylinder

Control mix

15

67,5

30

4,44

Good

Cylinder

Fibre mix

16

67,3

29,6

4,40

Good

Figure 3.1 PUNDIT test results

Longitudinal pulse velocity (km/s)

Quality of concrete

>4.5

Excellent

3.5-4.5

Good

3.0-3.5

Doubtful

2.0-3.0

Poor

<2.0

Very Doubtful

Table 3.1 Link between speed and quality

Average velocity, according to the type of mix (fig 3.1):

Table 3.2 Average velocity

Type of mix

Speed(km/s)

Control mix

4,37

Fibre mix

4,34

Calculation of modulus of elasticity from PUNDIT test results:

Table 3.3 Modulus of elasticity from pulse velocity

Using the average pulse velocity table 3.2 and the table 3.4 from the British standards we can compute the MOE (table 3.4).

Table 3.4 MOE from PUNDIT test

Type of mix

Average speed

MOE(GPa)

Control mix

4.37

26.25

Fibre mix

4.34

25.5

3.2 Relative density test

The relative density test is done like the PUNDIT test, to check the density and so the quality of the concrete.

The various density obtained are listed in the table 3.5.

Shape

Type of mix

Number

Ma(g)

Mw(g)

Density

Cube

Control mix

1

2246

1206

2,16

Cube

Control mix

2

2233

1182

2,12

Cube

Control mix

3

2262

1206

2,14

Cube

Control mix

4

2249

1202

2,15

Cube

Control mix

5

2254

1203

2,14

Cube

Control mix

6

2387

1282

2,16

Cube

Fibre mix

7

2339

1283

2,21

Cube

Fibre mix

8

2344

1288

2,22

Cube

Fibre mix

9

2250

1228

2,20

Cube

Fibre mix

10

2306

1258

2,20

Cube

Fibre mix

11

2402

1309

2,20

Cube

Fibre mix

12

2328

1275

2,21

Cylinder

Control mix

13

12655

7167

2,31

Cylinder

Fibre mix

14

12873

7396

2,35

Cylinder

Control mix

15

12524

No value

No value

Cylinder

Fibre mix

16

12162

No value

No value

Table 3.5 Relative density test results

There are no values for the cylinders 15 and 16, because of the gauges on their sides when we carried out this test.

From the table 3.5 above we get the average density for both types of mix (table 3.6).

Table 3.6 Average density

Type of mix

Results(kN/m3)

Control mix

2,17

Fibre mix

2,23

3.3 Compression test

The compression test result is divided in two parts, the first part uses the raw values of compression to predict the modulus of elasticity, and the second part uses the strain gauges records to build stress/strain curves. At this stage of the results presentation, only cylinders are considered to carry out the calculations.

3.3.1 Predicted MOE

The predicted MOE is calculated using the max loading recorded in laboratory for each cylinder.

We use the European formula to get the MOE values in the table 3.7 below.

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Honours project:Dissertation:Ressources:MOE prediction.png

Ecm is the modulus of elasticity (MOE).

Table 3.7 MOE from European formula

Type of mix

Number

Speed

Length

(cm)

Diameter

(cm)

Area

(mm2)

Load

(kN)

Stress

(MPa)

Fcm

(MPa)

MOE

(GPa)

Control mix

13

68,5

30,3

15,1

17908

508,26

28,4

36,4

32,41

Fibre mix

14

69,1

30,3

15,1

17908

522,07

29,2

37,2

32,62

Control mix

15

67,5

30

15

17671

502

28,4

36,4

32,42

Fibre mix

16

67,3

29,6

15

17671

482

27,3

35,3

32,11

We use the American formula to get the MOE in the table 3.8.

Table 3.8 MOE from American formula

Type of mix

Number

Speed

Length

(cm)

Diameter

(cm)

Area

(mm2)

Load

(kN)

Stress

(MPa)

Fcm

(MPa)

MOE

(GPa)

Control mix

13

68,5

30,3

15,1

17908

508,26

28,4

36,4

28,35

Fibre mix

14

69,1

30,3

15,1

17908

522,07

29,2

37,2

28,65

Control mix

15

67,5

30

15

17671

502

28,4

36,4

28,36

Fibre mix

16

67,3

29,6

15

17671

482

27,3

35,3

27,91

3.3.2 Stress-Strain curves

The stress-Strain curves give us enough information to carried out the calculation of the MOE. Three cycles of loading/unloading are done, before to do a fourth and last loading until failure occurs.

3.3.2.1 Control mix

First loading:

Values from the laboratory test:

Table 3.9

Load

Stress(MPa)

Average strain(Microstrain)

0

0

0

50

2,83

165,5

70

3,96

197,5

90

5,09

236

110

6,23

278

130

7,36

321

150

8,49

362

170

9,62

400

180

10,19

422,5

Stress-Strain curve of the first loading:

The extremities of the curve above allow us to calculate the initial secant modulus Ec,0.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Second loading:

Values from the laboratory test:

Table 3.10

Load

Stress(MPa)

Average strain(Microstrain)

0

0

4

50

2,83

169

70

3,96

203,5

90

5,09

237,5

110

6,23

283

130

7,36

316,5

150

8,49

359,5

170

9,62

404,5

180

10,19

428,5

Stress-Strain curve of the second loading:

The extremities of the curve above combined with values from the next loading allow us to calculate the stabilized secant modulus Ec,s.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Third loading:

Values from the laboratory test:

Table 3.11

Load

Stress(MPa)

Average strain(Microstrain)

0

0

8,5

50

2,83

167,5

70

3,96

209

90

5,09

250

110

6,23

288,5

130

7,36

329

150

8,49

365

170

9,62

405,5

180

10,19

426,5

Stress-Strain curve of the third loading:

The extremities of the curve above combined with values from the previous loading allow us to calculate the stabilized secant modulus Ec,s.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Calculation of the initial secant modulus:

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Trimester 2:Honours project:Dissertation:Ressources:Initial secant modulus.png

Initial secant modulus

Unit (GPa)

Ec,0

28,63

Calculation of the stabilized secant modulus:

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Trimester 2:Honours project:Dissertation:Ressources:Stabilized secant modulus.png

Stabilized secant modulus

Unit (GPa)

Ec,s

28,57

Δσ is the difference between the applied stress

ε s is the corresponding strain difference measured at the first loading

σra is the real stress corresponding to the nominal value σa

σrb is the real stress corresponding to the nominal value σb

εa,1 is the average strain at σa at first cycle

εb,0 is the average strain at σb before first cycle

Loading until failure:

Values from the laboratory test:

Table 3.12

Load

Stress(MPa)

Average strain(Microstrain)

50

2,83

8

75

4,24

171,5

100

5,66

210

125

7,07

264

150

8,49

312

175

9,90

361

200

11,32

407

225

12,73

457

250

14,15

510,5

275

15,56

574

300

16,98

633,5

325

18,39

703

350

19,81

779,5

375

21,22

865

400

22,64

964

425

24,05

1079,5

450

25,47

1222

475

26,88

1400

500

28,30

1604

525

29,71

2204

Stress-Strain curve of the ultimate loading:

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

From this last loading we can get another value of the modulus of elasticity for this cylinder:

Ecm=(22.64-4.24)/(964-171.5)*10^-6=23.21 GPa

3.3.2.2 Fibre mix

First loading:

Values from the laboratory test:

Table 3.13

Load

Stress(MPa)

Average strain(Microstrain)

0

0

0

50

2,83

125,5

70

3,96

161,5

90

5,09

203

110

6,23

238,5

130

7,36

280

150

8,49

315,5

170

9,62

359

180

10,19

382

Stress-Strain curve of the first loading:

The extremities of the curve above allow us to calculate the initial secant modulus Ec,0.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Second loading:

Values from the laboratory test:

Table 3.14

Load

Stress(MPa)

Average strain(Microstrain)

0

0

10,5

50

2,83

155,5

70

3,96

190,5

90

5,09

226

110

6,23

267

130

7,36

305

150

8,49

344

170

9,62

387

180

10,19

409

Stress-Strain curve of the second loading:

The extremities of the curve above combined with values from the next loading allow us to calculate the stabilized secant modulus Ec,s.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Third loading:

Values from the laboratory test:

Table 3.15

Load

Stress(MPa)

Average strain(Microstrain)

0

0

12,5

50

2,83

156,5

70

3,96

190

90

5,09

227,5

110

6,23

274,5

130

7,36

307,5

150

8,49

346,5

170

9,62

386,5

180

10,19

408

Stress-Strain curve of the third loading:

The extremities of the curve above combined with values from the previous loading allow us to calculate the stabilized secant modulus Ec,s.

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

Calculation of the initial secant modulus:

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Trimester 2:Honours project:Dissertation:Ressources:Initial secant modulus.png

Initial secant modulus

Unit (MPa)

Ec,0

28,68

Calculation of the stabilized secant modulus:

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Trimester 2:Honours project:Dissertation:Ressources:Stabilized secant modulus.png

Stabilized secant modulus

Unit (MPa)

Ec,s

29,14

Δσ is the difference between the applied stress

ε s is the corresponding strain difference measured after three cycles

σra is the real stress corresponding to the nominal value σa

σrb is the real stress corresponding to the nominal value σb

εa,3 is the average strain at σa at third cycle

εb,2 is the average strain at σb after second cycle

Loading until failure:

Values from the laboratory test:

Table 3.16

Load

Stress(MPa)

Average strain(Microstrain)

50

2,83

15

75

4,24

158,5

100

5,66

202,5

125

7,07

249

150

8,49

299,5

175

9,90

347,5

200

11,32

395,5

225

12,73

447,5

250

14,15

502

275

15,56

552

300

16,98

619,5

325

18,39

694,5

350

19,81

785

375

21,22

873

400

22,64

977,5

425

24,05

1105,5

450

25,47

1263

475

26,88

1487

500

28,30

1967

Stress-Strain curve of the ultimate loading:

The unit of the Strain axis is (Microstrain).

The Stress unit is (MPa), calculated using the load applied on the cylinder and its diameter.

From this last loading we can get another value of the modulus of elasticity for this cylinder:

Ecm=(22.64-4.24)/(977.5-158.5)*10^-6=22.47 GPa

3.4 Summary

Every modulus of elasticity obtained from theoretical and practical technics are listed in the table 3.17 below.

Table 3.17 Summary of every MOE got in this document

Cylinder

number

Types

PUNDIT

Test

(GPa)

Initial secant modulus

(GPa)

Stabilized secant modulus

(GPa)

MOE ultimate loading

(GPa)

Predicted MOE Europe

(GPa)

Predicted MOE

America

(GPa)

13

Control

26.25

No Gauges

No Gauges

No Gauges

32.41

28,35

14

Fibre

25.5

No Gauges

No Gauges

No Gauges

32.62

28,65

15

Control

26.25

28.63

28.57

23.21

32.42

28,36

16

Fibre

25.5

28.68

29.14

22.47

32.11

27,91

The unit of the various MOE in the table is GPa. Cylinders number 13 and 14 are not equipped with gauges, by consequence the modulus of elasticity obtained using this tool are not available.

The MOE obtained from laboratory work without using the formula below

Macintosh HD:Users:alex:Documents: Professionel:Genie civil:BEng:Fourth year:Honours project:Dissertation:Ressources:MOE prediction.png

are called "real" in contradiction with the MOE called "predicted" computed using that formula.

The table 3.18 below provides the average MOE value for each type of mix.

Table 3.18 Summary of the average MOE

Type of mix

MOE Predicted

MOE Real

Variation

Control mix

30.39 GPa

26.58 GPa

14.3%

Fibre mix

30.32 GPa

26.26 GPa

15.5%

Chapter 4: Discussion

4. Introduction

This chapter try to analyse and bring out some important ideas to understand the laboratory test results. Obviously the number of parameters involved in the results we got is huge and we cannot mention them all.

4.1 Influence of fibres

The tables 3.17 is a summary of the various MOE obtained, and allow us to easily observe the influence of fibres. From the table 3.17 we build the table 4.1 which shows variations between the MOE of control mix and fibre mix.

Table 4.1 Variation between Control and Fibre mixes

Cylinder

number

Types

PUNDIT

Test

(GPa)

Initial secant modulus

(GPa)

Stabilized secant modulus

(GPa)

MOE ultimate loading

(GPa)

Predicted MOE Europe

(GPa)

Predicted MOE

America

(GPa)

15

Control

26.25

28.63

28.57

23.21

32.42

28,36

16

Fibre

25.5

28.68

29.14

22.47

32.11

27,91

Variation

-2.94%

+0.02%

+2%

-3.3%

-0.9%

-1.61%

According the variations observed in the table above, we can conclude, that fibres don’t have any significant influence on the modulus of elasticity of concrete.

Fibres decrease slightly the values of;

The PUNDIT and Ultimate loading MOE

Fibres increase slightly the values of;

The initial and stabilized secant modulus of elasticity

Predicted MOE are not considered because the formula used to predicted them are not influenced by fibres

On of the aims of this dissertation was to study the influence of fibres on the modulus of elasticity of concrete. The table 4.1 gives a first answer, the modulus of elasticity is not significantly influenced by fibres. All the variations observed above, positive as well as negative are too small to be considered as significant. Reading these results, that is important to keep in mind that only two cylinders have been tested with gauges and only one type and ratio of fibres have been tested. That results may be different with other kind of fibres, with different shapes. The results could also change if we increase the amount of fibres used, from 1% to 3% of the volume for example.

4.2 Predicted MOE

There is no specific formula to predict the MOE of fibre concrete. The formula used to calculate the predicted MOE are the same for ordinary concrete and fibre concrete.

It is interesting to observe variations of accuracy between the European and American formulas, knowing that both are not designed to work with fibre concrete.

The tables below are used as a foundation to build the discussion about prediction of MOE. We consider only two cylinders, those equipped with gauges.

Table 4.3 Comparison of European and American "predicted" MOE

Cylinder

MOE

Average "real"

MOE

Europe "predicted"

MOE

America "predicted"

14

25.5 GPa

32.62 GPa

28.65 GPa

16

26.45 GPa

32.11 GPa

27.91 GPa

Table 4.4 Comparison of "real" and "predicted" MOE using formula from Eurocode 2

Cylinder

MOE

Average "real"

MOE

Europe "predicted"

Variation

14

25.5 GPa

32.62 GPa

27.92%

16

26.45 GPa

32.11 GPa

21.39%

Table 4.5 Comparison of MOE using formula from the American institute

Cylinder

MOE

Average "real"

MOE

America "predicted"

Variation

14

25.5 GPa

28.65 GPa

12.35%

16

26.45 GPa

27.91 GPa

5.52%

From the tables above, we get important information.

The formula from Eurocode 2 is less accurate than the formula from the American concrete institute. And this observation is done for both types of concrete (ordinary and fibre). Using the European formula we get a variation from 28% (ordinary) to 21% (fibre) between the reality and the prediction. Using the American formula we get a variation from 12% (ordinary) to 5.5% (fibre) between the reality and the prediction.

When we consider only the cylinders made from fibre concrete, the American formula is four times more accurate than the European formula. When we consider only the cylinders made from ordinary concrete, the American formula is at least two times more accurate.

First conclusion, predict MOE using European formula is less accurate than predict MOE using American formula.

Second conclusion, fibres don’t change the accuracy of the predicted MOE if we use the formula from Eurocode 2. This is logical because we saw in the previous part that fibres didn’t influence MOE.

Third conclusion, the MOE of the fibre concrete is predicted with a excellent accuracy by the American formula for an ordinary concrete but yet better for a fibre concrete. Fibres reduce the difference between "real" and "predicted" MOE if we use the formula from the American concrete institute.

These results were not expected, they are in fact the opposite of what we were expecting.

The prediction of the modulus of elasticity is linked to the influence of fibres. Therefore these results could be explain by the small amount of cylinders tested or be caused by the small amount of fibre, 1% of the volume is not the maximum ratio for a fibre concrete. It could be interesting to increase this amount of fibre in a further project to study its influence on the predicted MOE.

4.3 Other properties

4.3.1 Quality of concrete

Average velocity, according to the type of mix (table 4.6):

Table 4.6 Average velocity

Type of mix

Speed(km/s)

Control mix

4,37

Fibre mix

4,34

The concrete used to carry out the laboratory work is a good quality concrete. Both results are below 4.5 km/s and above 3.5 km/s showing that fibres do not decrease the quality even if they influence it. The PUNDIT