69

3.0 EXPERIMENTAL INVESTIGATIONS

3.1 INTRODUCTION

In the present experimental investigation the following

properties of steel fibre reinforced concrete are studied using OPC

concrete and Metakaolin concrete. The experimental investigations of

these properties are given in sections 3.3 to 3.12

Compressive strength

Splitting tensile strength

Modulus of rupture

Modulus of elasticity

Impact resistance

Residual compressive strength and weight loss at elevated

temperatures

Residual compressive, split tensile strength and modulus of

rupture at different thermal cycle

Durability studies

Flexural behavior of beams

Flexural behavior of slabs

In the present experimental investigation, M20 and M50 grade of

concrete designed as per IS: 10262 - 1982 was used. The details of

the crimped steel fibres and mix proportions of M20 and M50 grade

OPC concrete used in this investigation are given in table 4.1.8, 4.2.3

and 4.2.4 respectively.

70

The following mixes were used to cast the specimens for finding the

above properties given in section 3.1

a) Ordinary Portland Cement Concrete specimens using 53 grade

OPC (ultra tech cement).

b) SFRC specimens using OPC and crimped steel fibres of aspect

ratio 60 and 80 each with volume fraction as 0.5, 1 and 1.50%.

c) Metakaolin concrete specimens (MK 10) with 10% cement

replacement with Metakaolin (binary blending)

d) SFRC-MK specimens with 10% Metakaolin and crimped steel

fibres of aspect ratio 60 and 80 each with volume fractions as 0.5, 1

and 1.50%.

3.2 MATERIALS

3.2.1 Cement:

Ordinary Portland cement available in local market of standard brand

was used in the investigation. Care has been taken to see that the

procurement made from a single batch and is stored in airtight

containers to prevent it is being affected by atmospheric, monsoon

moisture and humidity. The Cement is tested for its various

proportions as per IS 4031-1988. The specific gravity was 3.10 and

fineness was 3200 m2/Kg. The details are given in Table 4.1.1. The

cement confirms to 53 Grade.

71

3.2.2 Coarse Aggregate:

Machine Crushed angular granite metal of maximum size of 20mm

retained on 4.75mm I.S. sieve confirming to I.S. 383-1970 was used in

the present investigation. It is free from impurities such as dust, clay

particles and organic matter etc. The coarse aggregate is also tested

for its various properties. The specific gravity and fineness modulus

are found to be 2.56 and 7.15. The details are tabulated in 4.1.3 and

4.1.4.

3.2.3 Fine Aggregate:

The locally available river sand was used as fine aggregate in the

present investigation. The sand is free from clayey matter, salt and

organic impurities. The sand is tested for its various properties like

Specific Gravity, Fineness modulus, Bulk Density etc in accordance

with IS 2386-1963. Fine aggregate passing through 4.75mm I.S. sieve

and retained on 0.075mm I.S. sieve was used. It confirms to grading

zone – II of I.S. 383-1970. The specific gravity and fineness modulus

are found to be 2.50 and 2.79. These test results are tabulated in

Table 4.1.4. Sieve analysis is carried out and results are shown in

Table 4.1.2.

3.2.4 Super plasticizer:

Conplast SP 430 obtained from Fosroc chemicals (I) Ltd. was used in

this experimental research. It confirm to I.S. 9103-1999, Table 3.7

shows the properties of this super plasticizer. Plate 3.1 shows the

view of super plasticizer SP 430 (FOSROC) container of 5 liters.

72

3.2.5 Crimped steel fibres:

In the present investigation, round crimped steel fibres of 27mm and

36mm length and diameter of 0.45 mm with aspect ratio as 60 and 80

are procured from M/s. Stewols India (P) LTD, Nagpur, Maharashtra

State. The properties of these fibres are given in table 4.1.8. Plate 3.2

shows the view of crimped steel fibres.

3.2.6 Metakaolin

Metakaolin is obtained by calcination of pure or refined kaolin clay at

a temperature between 6500C and 8500C, followed by grinding to

achieve a fineness of 15000 m2/kg (B.E.T).The specific gravity is found

as 2.50. The resulting material has high pozzolanic property. Plate

3.3 shows the view of Metakaolin.

Metakaolin is manufactured from pure raw material to strict

quality standards and not a by-product. Other pozzolanic materials

are currently available, but many are by products, which are available

in chemical composition. They may also contain active components

(such as sulphur compound, alkalis, carbon, reactive silica) which can

undergo delayed reactions within the concrete and cause problems

over long time periods.

Metakaolin is a high quality pozzolanic material, which is

blended with Portland cement in order to improve the durability of

concrete and mortars. Metakaolin removes chemically reactive

calcium hydroxide from the hardened cement paste. Metakaolin

reduces the porosity of hardened concrete. Metakaolin densifies and

reduces the thickness of the interfacial zone thus improving the

73

adhesion between the hardened cement paste and particles of sand or

aggregate.

Metakaolin manufactured from pure raw material to strict

quality standards. Metakaolin is a high quality pozzolanic material,

which when blended with Portland cement improves the strength and

durability of concrete and mortars. Metakaolin removes chemically

reactive calcium hydroxide from the hardened cement paste. It

reduces the porosity of hardened concrete. Metakaolin densifies and

reduces the thickness of the interfacial zone, thus improving the

adhesion between the hardened cement paste and particulars of sand

or aggregate.

Properties of Metakaolin

Metakaolin grades of calcined clays are reactive allumino silicate

pozzolan formed by calcining very pure hydrous China clay.

Chemically Metakaolin combines with calcium silicate and calcium

processed to remove uncreative impurities producing almost 100

percent reactive material. The particles size of Metakaolin is

significantly smaller than cement particles. IS:456-2000 recommends

use of Metakaolin as mineral admixture.

Metakaolin is a ultra fine pozzolanic which replaces industrial

by-products such as silica fume/micro silica. Commercial use of

Metakaolin has already in several countries worldwide. Metakaolin

removes chemically reactive calcium hydroxide from the hardened

paste. Metakaolin reduces the porosity of hardened concrete.

Metakaolin densifies, reduces the thickness of the interfacial zone,

74

thus improving the adhesion between the hardened cement paste and

particles of sand or aggregate. Blending Metakaolin with Portland

cement improves the properties of concrete and cement products

considerably by:

Increasing compressive and flexural strength

Providing resistance to chemical attack

Reducing permeability substantially preventing Alkali-Silica

Reaction

Reducing efflorescence & Shrinkage

Protecting corrosion

Pozzolanic Reactivity

Metakaolin is a lime-hungry pozzolan that reacts with free

calcium hydroxide to form stable, insoluble, strength-adding,

cementitious compounds. When Metakaolin – HRM(AS2) reacts with

calcium hydroxide (CH), a cement hydration byproducts, a pozzolanic

reaction takes place whereby new cementitous compounds, (C2ASH8)

and (CSH), are formed. These newly formed compounds will

contribute cementitious strength and enhanced durability properties

to the system in place of the otherwise weak and soluble calcium

hydroxide.

Cement Hydration Process

OPC + H2O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> CSH + CH

Pozzolanic Reaction Process

AS2 + CH - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> C2ASH8 + CSH

75

Unlike other commercially available pozzolanic materials,

Metakaolin is a quality controlled manufactured material. It is not a

by-product of unrelated industrial process. Metakaolin has been

engineered and optimized to contain a minimum of impurities and to

react efficiently with cement‟s hydration byproduct-calcium hydroxide.

Table summarizes the relative reactivities of six different pozzolans,

including high reactive Metakaolin-HRM.

Reactivity of Pozzolanic Materials:

Material Pozzolanic Reactivity mg Ca(OH)2 per g

Blast furnace slag 40

Calcined paper waste 300

Micro silica, silica fume 427

Calcined bauxite 534

Pulverized fuel ash 875

High reactivity Metakaolin 1050

3.2.7 Water

Water is the least expensive but most important ingredient of the

concrete. The water, which is used for making concrete should be

clean and free from harmful impurities like oil, alkalis, acids etc. in

general, the water which is fit for drinking should be used for making

concrete.

76

3.2.8 Mix design of M20 grade OPC concrete:

M20 grade of concrete designed as per IS: 10262-1982 is given

in Appendix-A. Trial mixes were cast as given in table 4.2.1 and the

quantity of cement is optimized to a value of 320 kg. The compressive

strength obtained was 30.69 MPa at 28 days and workability obtained

was 0.976 in terms of compacting factor. The mix proportions of this

OPC concrete are given in table 4.2.3. The ratio of the quantities

obtained were cement: Fine aggregate: Coarse aggregate = 1:1.92:2.64

with w/c = 0.55.

3.2.9 Mix design of M50 grade OPC concrete:

M50 grade of concrete designed as per IS: 10262-1982 is given

in Appendix-A. Trial mixes were cast as given in table 4.2.2, and the

quantity of cement is optimized to a value of 470 kg. The compressive

strength obtained was 61.40 MPa at 28 days and workability obtained

was 0.892 in terms of compacting factor. The mix proportions of this

OPC concrete are given in table 4.2.4. The ratio of the quantities

obtained were cement: Fine aggregate: Coarse aggregate = 1:0.96:3.64

with w/c = 0.33.

3.2.10 Factors affecting the workability and strength of fibre

concrete

Previous investigations13, 15 shows that for a given mix of specific

proportions and water-cement ratio, there is a maximum quantity of

fibre which can be introduced into it without causing balling and

interlocking of the fibres. Increasing the sand content of the mix

makes it possible to increase the fibre content. Increasing the fibre

77

content or the aspect ratio of the fibres will cause a reduction in

workability and / or increased balling of fibres during mixing.

Obviously, improvements in strengths due to fibre addition are

only likely to occur if the mix workable and free from balling. In a

workable, homogeneous mix, improvements in strength can be

ascribed to two major factors: the fibre volume fraction in the mix and

the resistance offered by the fibres to the crack formation and

propagation. For similar materials, the latter is affected by two

variables: fibres having larger aspect ratio lf/df exhibit greater pull-out

strengths and can be considered to be more effective than fibres with

smaller values, crimped fibres will possess higher bond strengths than

similar un crimped ones.

In this study, the influence of these factors is incorporated into

a single parameter called the fibre factor F, given as15:

F = (lf/df) Vf .β

Where

lf/df = The ratio of length of the fibre to its diameter (also called

as aspect ratio of the fibre)

Vf = Volume of fibres per unit volume of concrete (also called

as volume fraction)

β = Bond factor which accounts for different bond

characteristics of the fibres.

Based on large series of tests15 on different types of fibres, the

following relative values are assigned for β.

β = 0.50 for un crimped fibres of circular cross section

78

β = 0.75 for crimped and hooked fibres of circular cross

section

β = 1.0 for indented fibres

For crimped steel fibres of circular cross section used in this

study, β has been assigned a value of 0.75.

3.2.11 Mixing, casting and workability tests

The mixing of SFRC was done in a laboratory PAN mixer

available in the concrete laboratory. The following procedure is

followed for mixing of ingredients. First aggregates were added to the

mixer. Then during mixing 75% water-super plasticizer mixture was

added and mixed for 3 to 5 minutes continuously. Then 75% binder

was added. When Metakaolin is to be used it was first mixed with

cement until a uniform mix is obtained and then fed into the pan

mixer. Fibres were gradually added by sprinkling them into the pan

mixer. Addition of fibres usually takes 5 to 10 minutes. Lastly the

remaining water-super plasticizer mixture and remaining binder was

added one after the other and mixed for 5 minutes. After 5 minutes

rest, concrete was mixed for another 2 minutes to get a homogenous

mix. Plate 3.4 shows the view of the pan mixer used for mixing of

concrete. Plate 3.5 shows the view of mixing of fibres in the pan

mixer.. Plate 3.6 shows the view of dry mixing of fibres in the pan

mixer. Plate 3.7 shows the view of SFRC in fresh state. Plate 3.8 to

3.10 show the view of casting of cubes, cylinders and prisms.

Workability tests such as slump test, compacting factor test and

Vee-Bee test were conducted on all mixes in order to check uniformity

79

in concrete workability. For SFRC and SFRC-MK mixes the super

plasticizer content was suitably adjusted so that workability in terms

of compacting factor is within 0.80 to 0.95. The quantity of super

plasticizer used was 1% of the weight of binder for various mixes to

get workable mixes.

The compaction of concrete was made using a table vibrator. To

eliminate the effect of possible fibre orientation, the cube moulds were

initially half filled, the mix was vibrated and then the remaining half

was filled and the vibration was continued for one minute. No signs of

segregation or air bubbles were observed during mixing or

compaction. After casting, the moulds containing compacted concrete

were covered with a thin plastic sheet in order to prevent the

evaporation of water from the surface of concrete. The specimens were

de-moulded after 24 hours and cured by immersing them in water for

28 days as shown in plate 3.13.

3.3 COMPRESSIVE STRENGTH

3.3.1 Introduction

Concrete is strong in compression but it is weak in tension and

has low strain at fracture. The low tensile strength of concrete is due

to the presence of numerous micro cracks. These micro cracks

further propagate under load and result in poor strength of concrete.

In this study, the effect of fibres on compressive strength is studied by

casting cubes and testing them in compression.

80

3.3.2 Casting of cubes

For each mix 9 cube specimens of size 100 x 100 x 100mm were

cast in C.I. moulds as shown in plate 3.14a. 3 cubes was tested at 7

days, 3 cubes was tested at 14 days and the remaining 3 cubes were

tested at 28 days of curing. Each compressive strength result is the

average of 3 test results.

3.3.3 Testing of cubes for compressive strength

After 7,14 and 28 days of curing the cubes were removed from

the curing tank, weighed and tested for compressive strength in a

3000 KN digital compression testing machine with the cast face

parallel to the axis of loading at the rate of 140Kg/cm2/minute as per

IS: 516-1959114 as shown in plate 3.14b. The load at which the

specimen fails is recorded. The experimental compressive strength

was obtained by dividing the maximum load applied on the specimen

during the test by its cross sectional area.

3.4 SPLITTING TENSILE STRENGTH

3.4.1 Introduction

Tensile strength of concrete greatly affects the extent and size of

cracking in concrete. It is of great importance while designing liquid

retaining structures, prestressed concrete structures and concrete

pavements. Tensile strength of concrete is very less when compared to

its compressive strength. The determination of tensile strength of

concrete can be classified as direct and indirect methods. The direct

methods suffer from a number of difficulties related to holding the

specimens properly in the testing machine without introducing stress

81

concentration and to the application of uniaxial tensile load which is

free from eccentricity to the specimens. Even a very small eccentricity

of load will induce bending and axial force conditions and the concrete

fails at apparent tensile stress other than the true tensile strength.

Because of the difficulties involved in conduction the direct

tensile test, a number of indirect methods are available to determine

the tensile strength. In these tests, in general a compressive force is

applied to a concrete specimen in such a way that the specimen fails

due to tensile stresses induced in the specimen. The tensile stress at

which failure occurs is the tensile strength of concrete.

The splitting tension test and flexure test (modulus of rupture

test) are some of the indirect tests for finding tensile strength of

concrete.

3.4.2 Splitting Tension Test

The test consists of applying compressive line loads along the

opposite generators of a concrete cylinder placed with its axis

horizontal between the platens of a compression testing machine as

shown in plate 3.15. Due to the applied line loading a fairly uniform

tensile stress is induced over nearly two-third of the loaded diameter.

The magnitude of this splitting tensile stress (acting in a direction

perpendicular to the line of action of applied compression) is given by

σsp = 2P/πdl

Where P = The applied compressive load at failure

d = Diameter of the cylinder

l = Length of the cylinder

82

Due to this tensile stress, the specimen fails finally, by splitting along

the loaded diameter. Immediately under the load, a high compressive

stress is induced. Therefore the load is applied through a packing of

plywood strip, 25 x 4 mm in cross section. As the cylinder splits into

two halves, the test is known as split test.

This test can also performed on cubes by splitting either-

(i). Along a centre line parallel to the edges of a cube by applying

two compressive forces through 15 cm square bars of sufficient

length.

(ii). Along one of the diagonal planes by applying compressive force

along two opposite edges.

In this investigation splitting tension test was performed on cylindrical

specimens of size 150 mm diameter and 300 mm length. This test

was conducted as per IS: 5816 – 1999.

The determination of flexural tensile strength is essential to estimate

the load at which the concrete member may crack. The flexure tensile

strength at failure is called modulus of rupture. This test is described

in the next section.

3.4.3 Casting of cylinders

For each mix three cylinders of size 150 mm in diameter and

300mm in length were cast and cured for 28 days. Each splitting

tensile strength results is the average of 3 test results.

3.4.4 Testing of cylinders for splitting tensile strength

After 28 days of curing the cylinders were removed from the

curing tank, weighed and tested for splitting tensile strength in a

83

3000KN digital compression testing machine as per IS: 5816 – 1999 at

a rate of loading, (1.2 to 2.4) (π/2) l*d, N/min. The maximum load

applied on the specimen was recorded Here l = 300mm and d =

150mm. The experimental splitting tensile stress was calculated

according to the above equation.

3.5 MODULUS OF RUPTURE

3.5.1. Introduction

The determination of flexural tensile strength is essential to

estimate the load at which the concrete members may crack. As it is

difficult to determine the tensile strength of concrete by conducting a

direct tension test, it is computed by flexure testing. The flexural

tensile strength at failure is called modulus of rupture. The knowledge

of modulus of rupture is useful in the design of pavement slabs,

airfield runways, finding deflections and crack widths as flexural

tension is critical in these cases.

3.5.2 Modulus of rupture test

The modulus of rupture is determined by testing standard test

specimens (Prisms) of size 100 mm x 100 mm x 500 mm over a span

of 400 mm, under symmetrical two-point loading according to IS: 516-

1959. The modulus of rupture is determined from the moment at

failure as:

σr = (M * y) / I

Where σr = Modulus of rupture

M = bending moment at failure

Y = distance of extreme fibre from neutral axis

84

I = Moment of inertia of the section

Thus the computation of σr assumes a linear behaviour of the

material up to failure.

3.5.3 Casting of prisms

For each mix three beams (prisms) of size 100 x 100 x 500 mm

were cast, de-moulded after 24 hours and then cured for 28 days.

Plate 3.16 shows the view of casting of prisms.

3.5.4 Testing of prisms for modulus of rupture

After 28 days of curing the prisms were taken out from the

curing tank, weighed and tested for modulus of rupture under two

point loading in a flexure testing machine according to IS: 516 –

1959114. The maximum load „P‟ and the distance of the crack from the

nearer support „a‟ measured on the centre line of the tensile face of the

specimen are recorded. The modulus of rupture was calculated

according to the clause 8.4 of IS: 516 – 1959114 as given below.

Case (i) If „a‟ is less than 133mm but greater than 110mm for 100mm

specimens,

σr = P*l/bd2

Case (ii) If „a‟ is less than 133mm but greater than 110mm for 100mm

specimens,

σr = 3P*a/bd2

Where σr = Modulus of rupture

A = Distance of the crack from the near support, measured along

the centre line on the tensile face of the specimen

L = length of the span = 400mm for 100mm specimens

85

b = width of the cross section = 100mm

d = height of the cross section = 100mm

Case (iii) If „a‟ is less than 110mm for 100mm specimens the results of

the test are discarded.

3.6 MODULUS OF ELASTICITY

3.6.1 Introduction

The modulus of elasticity of concrete is largely controlled by the

volume and modulus of the aggregate. The modulus of elasticity of

concrete is a constant, defined as the ratio of axial stress to axial

strain within the elastic limit (i.e. when the load does not exceed 1/3

of the ultimate load) under uniaxial loading. The modulus of elasticity

of concrete is designated in various ways as:

(a) Initial tangent modulus, defined as the slope of the straight line

drawn at the origin of the stress-strain curve.

(b) Tangent modulus, defined as the slope of the tangent drawn at

any point on the stress-strain curve.

(c) Secant modulus, defined as the slope of the line joining any

point on the stress-strain curve to the origin.

(d) Modulus is the slope of the line joining any two points on

stress-strain curve.

The modulus of elasticity most commonly used is the static modulus.

The modulus of elasticity in this investigation was determined under

load control mechanism by subjecting a concrete cylinder of 150mm

diameter and 300mm height to stress in uniaxial compression in a

digital compression testing machine of 3000 KN capacity and

86

measuring the deformations on a dial gauge using a longitudinal

compressometer. The modulus of elasticity determined in this

investigation is the initial tangent modulus.

3.6.1.1 Factors affecting the modulus of elasticity of concrete

Strength of concrete – higher strength shows higher modulus of

elasticity.

Volume and modulus of aggregate.

Wetness of concrete – wet concrete shows higher modulus of

elasticity than dry concrete.

Age of concrete – modulus of elasticity increases with age of

concrete.

Richness of the mix- richer mixes shows higher modulus of

elasticity.

3.6.2 Procedure for finding Modulus of Elasticity of concrete

After 28 days of curing, the specimens were taken out from the

curing tank and for modulus of elasticity while the specimens were

still in the wet condition using a longitudinal compressometer in a

digital CTM of 3000 KN capacity. The specimens were tested as per

clause 9 of IS: 516 – 1959114.

The compressometer consists of two circular frames for

clamping to the concrete specimen by means of five tightening screws.

The two circular frames are held in position by means of two spacers.

Spacer screws are provided to fix the spacers to the frame. The centre

distance of the tightening screws of the bottom and top frame is

87

200mm. A dial gauge of 0.002 x 10mm gives the deformation over a

gauge length of 200mm.

The top and bottom frames are assembled by keeping the

spacers in position. The pivot rod is kept on the screws and adjusted.

The specimen (cylinder) after removing from water is placed on a level

surface. The compressometer is placed centrally on the specimen so

that the tightening screws of the bottom and top frame are at an equal

distance from the two ends of the concrete cylinder. The tightening

screws are tightened so that the compressometer is held on the

specimen. The two spacers are removed by unscrewing the spacer

screws before applying the load on the specimen. Plate 3.17 shows

the view of the compressometer fitted on to the concrete cylinder.

The specimen with the compressometer is placed centrally on

the platen of the Digital compression testing machine. The load was

applied continuously, at a rate of 140Kg/cm2/min (4.12 KN/sec) until

an average stress of (C + 0.5) N/mm2 is reached, where C is one-third

of the average compressive strength of the cubes calculated to the

nearest 0.5 N/mm2 . The load was then reduced gradually to an

average stress of 0.15 N/mm2 and dial gauge readings were taken.

The load was then applied a second time at the same rate until an

average stress of (C + 0.15) N/mm2 was reached and dial gauge

readings were taken. The load was gradually reduced and dial gauge

readings were taken. The load gradually reduced to 0.15 N/mm2 and

dial gauge readings were taken.

88

The load was applied a third time and dial gauge readings taken

at ten approximate equal increments of stress up to an average stress

of (C + 0.15) N/mm2. If the overall strains observed on the second

and third readings differ by more than 5%, the loading cycle has to be

repeated until the strain between consecutive readings at (C + 0.15)

N/mm2 does not exceed 5%.

The strains are calculated and plotted against the stress and a

curve is plotted through these points. The slope of this curve gives the

modulus of elasticity expressed to the nearest 100 N/mm2.

3.6.3 Casting of cylinders

For each mix three cylinders of size 150mm diameter and

300mm long were cast in cast iron moulds, de-moulded after 24 hours

and cured for 28 days.

3.6.4 Testing of cylinders for modulus of elasticity

After 28 days of curing the cylinders were taken out from the

curing tank, weighed and tested for modulus of elasticity using a

compressometer in a digital compression testing machine of 3000KN

capacity according to IS: 516 – 1959114. The load applied and the

deformations up to 1/3 of the compressive strength are recorded. The

modulus of elasticity was calculated according to the clause 9 of IS:

516 – 1959114.

89

3.7 IMPACT RESISTANCE

3.7.1 Introduction

Concrete structures are often subjected to short duration

(dynamic) loads. Loads originate from sources such as impact from

missiles and projectiles, earthquakes, wind gusts and machine

vibrations. Due to a relatively low tensile strength and fracture

energy, impact resistance of concrete that exhibits improved impact

resistance than conventional concrete. Fibre reinforced concrete

(FRC) has emerged as a viable structural material for use in such

applications.

One of the significant aspects of randomly distributed fibres in

cement composites is their ability to slow down the propagation of

tensile cracks, thereby improving the post cracking behavior, flexural

toughness and ductility of the cement composite. These

characteristics have rendered the fibre reinforced concrete, especially

steel fibre reinforced concrete (SFRC), as one of the suitable materials

for the construction of structures which are subjected to impact and

suddenly applied loads.

The improvements in the impact resistance of SFRC come

primarily from the large amount of energy absorbed in de-bonding,

stretching and pulling out the fibres which occur after the concrete

has cracked. This improvement in the impact resistance is measured

generally by using different types of impact tests. They are:

Weighted pendulum charpy-type impact test

Drop-weight Impact test

90

Constant-strain rate test

Projectile impact test

Split-Hopkinson bar test

Explosive test

Instrumented pendulum impact test

Blast and projectile impact tests are generally used to evaluate the

impact resistance of structural members. Charpy impact test is

employed for metals, even though some researchers have tried for

concrete as well. Modified charpy impact test set up simplest among

all the above tests and results in quantitative estimate of the impact

resistance of SFRC has been employed in this investigation.

3.7.2 Casting of prism specimens

For each mix three prism specimens of size 100x100x500mm

were cast and cured for 28 days and then tested for impact resistance.

Plate 3.16 shows the view of casting of prisms for impact test.

3.7.3 Testing for impact resistance

After 28 days of curing the prisms were removed from curing

tank, and when the surface was dry they are kept ready for testing.

Plate 3.19 shows the view of testing method for impact

resistance. The pendulum is made to hit repeatedly and the number

of blows required to cause the first visible crack on the top surface of

the specimen are recorded as the first crack strength. The loading

with the drop pendulum is continued till the prisms fails by opening of

the cracks in the specimen sufficiently so that the specimen

completely fails. The number of blows that causes this condition is

91

recorded as the failure strength. With fibre reinforced concrete

specimens, the prism was held together by fibres as shown in plate

3.20 where as in plain concrete prisms, there was a brittle failure as

shown in plate 3.21.

3.8. DURABILITY STUDIES

3.8.1 Introduction

A durability concrete is one that performs satisfactorily in the

working environment during its anticipated exposure conditions

during service. Inadequate durability manifests itself by deterioration

which can be due to external factors or to internal causes within the

concrete itself. The various actions can be physical, chemical or

mechanical. Mechanical damage is caused by impact, abrasion,

erosion or cavitation. The chemical causes of deterioration include

the alkali-silica and alkali-carbonate reactions. External chemical

attack occurs mainly through the action of aggressive ions, such as

chlorides, sulphates, or of carbon dioxide, as well as many natural or

industrial liquids and gases. The damaging actions can be of various

kinds and can be direct or indirect.

Physical causes of deterioration include the effects of high

temperature or of the difference in thermal expansion of aggregate and

of the hardened cement paste. An important cause of damage is

alternating freezing and thawing of concrete and the associated action

of de-icing salts. It is observed that the physical and chemical

processes of deterioration can act in a synergetic manner.

92

3.8.2 Experimental investigation

In this experimental investigation the external chemical attack

due to exposure in 5% HCL and 5% H2SO4 was studied by immersing

100 x 100 x 100mm size concrete cubes in the above three liquids for

a period of 120 days, beyond 28 days of water curing. Plastic tubs

were used to immerse the cubes in the acid solutions. Care is taken

to maintain a minimum of 40mm distance between the cubes placed

in tubs containing acid solutions.

The weight and compressive strength of the cubes were found at

various ages of 30, 60, 90 and 120 days of exposure in the above

solutions. After every 30 days the cubes were removed from the tubs,

brushed with a soft nylon brush and rinsed in tap water to remove

loose surface material and placed in fresh acid solutions to study the

properties up to 120 days of exposure.

Plate 3.22 shows the view of specimens, after normal curing.

Plate 3.23 shows the view of specimens, after exposure to acid attack

in 5% HCI.

Plate 3.24 shows the view of specimens, after exposure to acid attack

in 5% H2SO4.

The loss in weight and loss in compressive strength are

calculated as follows

Loss in weight % = (W1 – W2) * 100

W1 Where

W1 = Weight of concrete cube specimen before immersion in

acid.

93

W2 = Weight of concrete cube specimen after immersion in acid.

Loss in compressive strength % = (σ1 – σ2) * 100

σ1

Where

σ1 = Compressive strength of concrete cube before immersion

in acid

σ2 = Compressive strength of concrete cube after immersion in

acid

The mixes studied for finding loss in compressive strength and

loss in weight is given in Table 3.8.0. At the end of each period of 30,

60, 90 and 120 days, the specimens were removed from the chemical

solutions cleaned with water and weighed. Similarly the specimens

were tested for compressive strength. The loss in weight and loss in

strength were calculated using the above equations.

For determining the resistance of concrete specimens to

aggressive environment like acid attack, the durability factors are

used based on relative compressive strength. The relative strength are

found at the end period of water curing i.e., at 28 days.

The acid durability factor (ADF) is given by

ADF = Sr * N

M

Where Sr = Relative strength at N days in %

N = Number of days at which the durability factor is

calculated

M = Number of days at which the exposure to acids is to be

terminated

94

Acid attack test was terminated at 120 days, so M is 120 in this case.

Table 3.8.0

S.No.

Type of

concrete

mix

Mix ID Percentage

of fibres

Aspect

ratio

Cement

%

MK

%

1 Plain

concrete

OPC

Concrete 0 0 100 0

2 MK

concrete 0 0 90 10

3

SFRC

AR - 60 1.50 60 100 0

4 AR - 80 1.50 80 100 0

5

SFRC-MK

AR - 60 1.50 60 90 10

6 AR - 80 1.50 80 90 10

95

3.9 EFFECT OF THERMAL CYCLES ON VARIOUS MIXES

OF M20 AND M50 GRADES

The present investigation is to study the effect of thermal cycles

on the compressive strength, split tensile strength and modulus of

rupture of various mixes of M20 and M50 grade specimens subjected

to various thermal cycles at a temperature of 500C and 1000C. This

was planned to be carried out through an experimental programme on

concrete cubes for compressive strength, cylinders for split tensile

strength, and prisms for modulus of rupture. The test specimens

were demoulded after 24 hours of air cooling and kept in water curing

for 28 days.

One thermal cycle constitute a heating period of 8 hours and

subsequent cooling (in air room temperature) period of 16 hours. The

standard specimens after curing period were placed in electric ovens

at 500C and 1000C for 0, 28, 90 and 180 thermal cycles. The

specimens were removed from oven and then allowed to cool in air for

2 hours after specified time. Then the specimens were tested for

compressive strength, split tensile strength and modulus of rupture.

Plate 3.25 show the view of specimens placed in electric oven.

The details are tabulated in 4.5.1 to 4.5.6.

96

3.10 TEMPERATURE EFFECTS ON VARIOUS MIXES OF

M20 AND M50 GRADES

The present investigation is to study the temperature effects on

compressive strength, pulse velocity and percentage weight loss of

various mixes of M20 and M50 grade specimens at room temperature,

when subjected to elevated temperatures of 2000C, 4000C and 6000C

at different time intervals of 4 hours, 8 hours and 12 hours and

allowed to cool for a duration of 24 hours. The test specimens were

demoulded after 24 hours of air cooling and kept for water curing for

28 days. The standard specimens after curing period were placed in

electric furnace at requisite temperatures of 2000C, 4000C and 6000C

at different time intervals of 4 hours, 8 hours and 12 hours. After the

specimens were removed from the furnace the specimens were allowed

to cool in air for 2 hours. Then the specimens were tested for

compressive strength, percentage loss of weight and pulse velocity.

Later the specimens were cooled to room temperature for duration of

24 hours and then tested in compression.

The details are tabulated in tables 4.6.1.1 to 4.6.1.14 and 4.6.2.1 to

4.6.3.2.

Plate 3.26 show the view of specimens placed in electric furnace for

exposure to elevated temperatures.

Plates 3.27 and 3.28 show the view of specimens after exposure to

elevated temperatures.

97

3.11 FLEXURAL BEHAVIOUR OF SFRC AND SFRC-MK

BEAMS

To study the suitability of the SFRC and SFRC-MK beams

investigations were carried out for ultimate load and load deflection

characteristics.

Steel fibre reinforced concrete and steel fibre reinforced

Metakaolin concrete beams by varying the percentage of steel fibres

from 0 % to 1.5% and of size 1200 x 150 x 100 mm were cast with

reinforcement of 2 nos of 12mm diameter HYSD bars as tension

reinforcement and 6 mm diameter stirrups spacing at 150 mm c/c as

shear reinforcement. To hold stirrups in position two hanger rods of 2

nos of 10mm diameter HYSD bars at top were used. These specimens

were cured in water for 28 days and tested for ultimate load,

deflections and failure characteristics under one third point loading.

The test setup is shown in plate 3.29. Plate 3.30 shows the view of

beam under testing. Plates 3.31 to 3.33 show the view of crack pattern

in the beams after failure. The results are tabulated in Table 4.9.1 to

4.9.4.

3.12 FLEXURAL BEHAVIOUR OF SFRC AND SFRC-MK

SLABS

To study the suitability of the SFRC and SFRC-MK slabs

investigations were carried out on ultimate load and load deflection

characteristics.

98

Steel fibre reinforced concrete and steel fibre reinforced

Metakaolin concrete by varying the percentage of steel fibres from 0 %

to 1.5% of size 1400 x 1200 x 100 mm were cast with reinforcement of

8mm diameter HYSD bars with a spacing of 200mm c/c on either side

of the slabs by varying the percentage of crimped steel fibres from 0%

to 1.5%. These specimens were cured in water for 28 days and tested

for ultimate load, deflections and failure characteristics under one

third point loading. The test setup is shown in plate 3.34. Plate 3.35

shows the view of noting of central deflection for slab under loading.

Plate 3.36 shows the view of highlighting the crack pattern of slabs

after testing. Plates 3.37 to 3.40 show the view of crack pattern of the

slabs after testing. The results are tabulated in Tables 4.10.1 and

4.10.2.