High Slump Concrete Mix Design 2

8/9/2019 High Slump Concrete Mix Design 2

1/68

i

JOMO KENYATTA UNIVERSITY

OF

AGRICULTURE AND TECHNOLOGY

DEPARTMENT OF CIVIL ENGINEERING

P.O.BOX, 62000 NAIROBI – KENYA TEL 067-52181-3 FAX ((067)52164

PROJECT TITLE:

HIGH SLUMP CONCRETE MIX DESIGN

Project by:

BRENDA YONGO OBILO

(E25-0124/04)

PROJECT SUPERVISOR

MR. MULU

APRIL 2010

This project is submitted in partial fulfillment for the award of a university degree in Civil

Engineering of Jomo Kenyatta University of Agriculture and Technology.


2/68

ii

DECLARATION

I, Obilo Brenda Yongo, do declare that this report is my original work and to the best of

my knowledge has not been submitted for any degree award in any University or

Institution.

Signed_______________ Date ____________

CERTIFICATION

I have read this report and approve it for examination.

Signed_______________ Date_____________


3/68

ii

Acknowledgements

My sincere thanks go to my supervisor Mr. Mulu who assisted me tirelessly throughout

this project, the Civil Engineering staff who guided and assisted me in my lab work and

my colleagues for their support throughout my studies and in accomplishing this research

work.

In addition, I would like to thank my family and friends who stood by me throughout my

studies.


4/68

iii

DEDICATION

I dedicate this work to my family who have always believed in me and supported

me throughout my studies, to Christopher Mutungi for his encouragement and

continuing prayers and mostly to God for His guidance.


5/68

iv

ABSTRACT

High slump or “flowing” concrete mix is an economical mix product that allows maximum

flowability without sacrificing strength by adding water. These high slump, high strength

properties are attained through the use of high range water reducing admixtures (super

plasticizers). It is a highly fluid but workable concrete and is useful for placing in very heavily

reinforced sections, in inaccessible areas, in floor or road slabs and also where very rapid

placing is desired. This paper presents the results of an experimental study whereby a high

slump concrete mix was designed and its properties were tested. The target strength and slump

was 25N/mm2 and 200mm respectively. The slump was attained using W/C ratios of 0.7 and

0.4 .With a W/C ratio of 0.7 no admixture was used and the 200 mm slump was attained

using water only, the compressive and tensile strength after 28 days was 20N/mm2 and

2N/mm2 respectively. This was below the target strength. Despite a lower W/C ratio of 0.4, a

high slump (200mm) concrete mix was attained using a super plasticizer, the compressive

strength and tensile strength after 28 days was 30N/mm2 and 2.95N/mm2 respectively. This

exceeded the target compressive strength of 25N/mm2 after 28 days. The work herein

confirms that a concrete mix can be designed to produce a mix with a high slump and of a

desired high strength without using excess water to increase workability, which consequently

leads to a decrease in strength, but by the use of super plasticizers. Super plasticizers are used

for high strength concretes by decreasing the W/C ratio as a result of reducing the water

content by 12-25%. In this study the water content was reduced by 25% from 1.7kg to

1.313kg.


6/68

v

TABLE OF CONTENTS

CHAPTER ONE ..................................................................................................................... 1

1.0 Introduction: ............................................................................................................. 1

1.1 HIGH SLUMP CONCRETE MIX DESIGN ................................................................... 1

1.3 PROBLEM STATEMENT .......................................................................................... 3

1.2 PROBLEM JUSTIFICATION ..................................................................................... 4

1.4 OVERALL OBJECTIVE.............................................................................................. 4

Specific Objectives. ................................................................................................. 4

1.5 RESEARCH HYPOTHESIS ........................................................................................ 5

CHAPTER TWO .................................................................................................................... 6

2.0 Literature Review: ..................................................................................................... 6

2.1 Introduction .......................................................................................................... 6

2.2 Composition of concrete....................................................................................... 7

Super plasticizers (high-range water-reducing admixtures) ................................... 9

2.3 Concrete Mix Design ........................................................................................... 10

2.4 Problems associated with high slump concrete ................................................. 21

CHAPTER THREE ................................................................................................................ 23

3.0 Research Methodology: .......................................................................................... 23

3.1 Grading of materials for concrete production .................................................... 23

3.2 Fineness Modulus ............................................................................................... 26

3.3 Determination of specific gravity and water absorption of aggregates ............. 27

3.4 Silt content test ................................................................................................... 31

3.5 Concrete mix design ............................................................................................ 32


7/68

vi

3.6 Batching .............................................................................................................. 34

3.7 Mixing of concrete .............................................................................................. 34

3.8 Slump test ........................................................................................................... 34

3.9 Casting of compression test specimen ............................................................... 35

3.10 Concrete placing ............................................................................................... 35

3.11 Curing of the test specimen .............................................................................. 35

3.12 Hard concrete test ............................................................................................ 36

3.13 Tensile strength test ......................................................................................... 38

CHAPTER FOUR ................................................................................................................. 40

4.0 Data Results and discussion .................................................................................... 40

4.1 RESULTS OF SPECIFIC GRAVITY & WATER ABSORPTION TESTS ON FINE

AGGREGATES ............................................................................................................ 40

4.2 RESULTS OF SPECIFIC GRAVITY & WATER ABSORPTION TESTS ON COARSE

AGGREGATES ............................................................................................................ 42

4.3 GRADING RESULTS .............................................................................................. 43

4.4 RESULTS OF SILT CONTENT ................................................................................. 46

4.5 Normal concrete mix design by Department of Environment (DoE) .................. 46

4.6 SLUMP TEST ........................................................................................................ 49

4.7 COMPRESSIVE STRENGTH RESULTS .................................................................... 52

4.8 TENSILE STRENGTH RESULTS .............................................................................. 54

4.9 WATER:CEMENT RATIO ....................................................................................... 55

4.10 Problems associated with the high slump concrete. ........................................ 56

CONCLUSION AND RECOMMENDATIONS ........................................................................ 58

4.11 CONCLUSION ......................................................................................................... 58


8/68

vii

4.12 RECOMMENDATIONS ........................................................................................... 58

Bibliography ...................................................................................................................... 59


9/68

viii

LIST OF FIGURES

Figure 1-SLUMP ................................................................................................................... 1

Figure 2-SLUMP CLASS ........................................................................................................ 2

Figure 3-GRADING SIEVES ................................................................................................. 25

Figure 4-SLUMP TEST ........................................................................................................ 35

Figure 5- FAILURE OF CUBE BY COMPRESSION ................................................................ 37

Figure 6-COMPRESSIVE STRENGTH TEST .......................................................................... 37

Figure 7-CYLINDER BEFORE LOADING ............................................................................... 39

Figure 8- CYLINDER SPLIT AFTER LOADING ....................................................................... 39

Figure 9-FINE AGGREGATE SIEVE ANALYSIS ..................................................................... 44

Figure 10-COARSE AGGREGATE SIEVE ANALYSIS .............................................................. 45

Figure 11-RESULTS OF SLUMP TEST .................................................................................. 49

Figure 12-SLUMP AGAINST POZZOLITH LD 10 ADMIXTURE ............................................. 50

Figure 13-SLUMP AGAINST RHEOBUILD ADMIXTURE ......... Error! Bookmark not defined.

Figure 14-COMPRESSIVE STRENGTH AGAINST TIME ........................................................ 53

Figure 15-TENSILE STRENGTH AGAINST TIME .................................................................. 55


10/68

1

CHAPTER ONE

1.0 Intro duc t ion:

1.1 HIGH SLUMP CONCRETE MIX DESIGN

INTRODUCTION

One of the basic attributes of any cementitious materials be it mortar or concrete, is its

workability or “consistence”, that is how easy it is to push one way, pull the other way ,

and float to a smooth level . Workability is largely determined by wetness, by how wet is

the mortar or concrete. This is referred to as slump

In essence, the wetter the concrete, the higher the slump. Mortars or concrete with a high

water content are said to have a high slump while those with a low water content have a

low slump. Although slump is often seen as an indication of water content, it is more

legitimately interpreted as a measure of consistence. (Paving expert-Concrete and

Mortar-Slump, 2009)

Figure 1-SLUMP


11/68

2

Slump (consistence) class

Following adoption of the new European Standard for Concrete in 2003 (BS8500),

consistence (workability or slump) is now specified as being of a particular class. There

are five classes, labeled S1 to S5, with each class spanning a range of slump values.

These are shown in the table below;

Class Slump Range(mm) Target Slump(mm)

S1 10-40 20

S2 50-90 70

S3 100-150 130

S4 160-210 180

S5 210-n/a 220

Slump class from BS8500

Figure 2-SLUMP CLASS

When using concrete with high slump values (>150mm, S4 or S5) there is a risk that the

aggregates and cement will settle out or segregate. This is usually countered by the use of


12/68

3

various additives/admixtures to ensure the concrete remains workable and structurally

competent.

The use of admixtures in particular high range water reducers (super plasticizers) results

in a concrete that can be placed with little or no compaction without compromising its

strength by adding excessive water and is not subject to excessive bleeding or

segregation, since it ensures the mix remains cohesive. This type of concrete is known as

high slump concrete.

High slump concrete also known as flowing concrete is a highly fluid but workable concrete

and is useful for placing in very heavily reinforced sections, in inaccessible areas, in floor or

road slabs and also where very rapid placing is desired. It is economical as it eliminates the

need for additional machinery as well as man power, both saving time and money. High slump

concrete leads to a safer working environment as it lowers noise levels caused by vibrating

equipment.

High slump should never be attained through the addition of water.

Therefore high slump concrete mix design is the process of selecting suitable ingredients of

concrete and estimating their proportions with the objective of producing workable, strong

and durable concrete at reasonable cost. In mix design, use is normally made of previous

experience and of several design tables, charts and curves. Final specifications are arrived at

after testing trial mixes.

1.3 PROBLEM STATEMENT

The procedure of high slump mix design entails coming up with trial mixes and consecutive

adjustments. All methods of mix design seem empirical and gives the impression of being non

scientific, but the variability of the properties of the materials used is such that our

calculations are only guesses. However, the better our knowledge of the various properties of

the ingredients of concrete the more accurate our guess can be. Basically, the problem of

designing a concrete mix consists of selecting the correct proportions of cement, fine and

coarse aggregate and water to produce concrete with the specified properties. The mix design


13/68

4

must therefore, take into account those factors that have a major effect on the characteristics of

concrete.

1.2 PROBLEM JUSTIFICATION

Mix design is really more than coming up with the right proportions of each mix constituent; it

is everything that makes the concrete work well for your application. Traditional mixes have

been produced but this can result in a mix that is completely wrong for your application and

could even be inferior concrete. Attaining the required slump with a particular strength, in this

case 25N/mm2 can prove a little bit difficult. Also, high slump concrete has a risk of aggregate

separation, excessive bleeding and the formation of lumps and balls. Thus the study aims to

come up with a mix design which will counter all the above problems and is of the required

slump (200mm) at a minimum strength of 25N/mm2 after 28 days.

1.4 OVERALL OBJECTIVE

To design a concrete mix with the desired slump and strength, that is economical and

workable.

Specific Objectives.

1. To come up with a high slump concrete mix design (Class 25, slump 200mm).

2. To come up with a design that is easily understood and easy to replicate.

3.

To investigate the strength of high slump concrete with and without using admixtures.


14/68

5

1.5 RESEARCH HYPOTHESIS

Concrete mix design enables one to produce a workable, strong, durable and economical

concrete.


15/68

6

CHAPTER TWO

2.0 Literature Review :

2.1 Introduction

High slump or “flowing” concrete mix is an economical mix product that allows

maximum flowability without sacrificing strength by adding water. These high slump,

high strength properties are attained through the use of high range water reducing

admixtures (super plasticizers). High slump concrete provides faster and easier

placement.

Concrete containing a water reducing admixture needs less water to reach a required

slump than untreated concrete. The treated concrete can have a lower water-cement ratio.

This usually indicates that a higher strength concrete can be produced without increasing

the amount of cement.

As stated earlier, following adoption of the new European Standard for Concrete in 2003

(BS8500), consistence (workability or slump) is now specified as being of a particular

class. There are five classes, labeled S1 to S5.

S1 concretes are most likely to be used for kerb and pipe work bedding.

S2 for simple strip footings and cast in situ hard standing slabs.

S3 would be used for trench filled foundations where a high flowability is

required.

S4 and S5 are likely to be used on specialist applications and advice from a

suitably experienced concrete technologist should be sought before specifying

concrete in these classes.

It should be apparent that there are three commonly used slump classes, S1, S2 and S3.


16/68

7

As these slump classes are relatively new to the language of a typical building site,

descriptive names are often used to indicate the approximate consistence of a particular

concrete or mortar mix. These are shown below:

Concrete with S1 is often referred to as semi dry; S2 is probably the most useful and most

commonly specified consistence and is referred to as a moist mix, while S3 would be

known as wet mix . Brick laying mortar is often a S3 consistence, although that used for

laying stone work is usually somewhat stiffer, possibly S2.

2.2 Composition of concrete.

Cement

Portland cement is the most common type of cement in general usage. It is a basic

ingredient of concrete, mortar, and plaster. English engineer Joseph Aspdin patented

Portland cement in 1824; it was named because of its similarity in colour to Portland

limestone, quarried from the English Isle of Portland and used extensively in London

architecture. It consists of a mixture of oxides of calcium, silicon and aluminum. Portland

cement and similar materials are made by heating limestone (a source of calcium) with

clay, and grinding this product (called clinker ) with a source of sulfate (most commonly

gypsum). The manufacturing of Portland cement creates about 5 percent of human CO2

emissions.

Water

Combining water with a cementitious material forms a cement paste by the process of

hydration. The cement paste glues the aggregate together, fills voids within it, and allows

it to flow more easily.

Less water in the cement paste will yield a stronger, more durable concrete; more water

will give an easier-flowing concrete with a higher slump.

Impure water used to make concrete can cause problems when setting or in causing

premature failure of the structure.

http://en.wikipedia.org/wiki/Portland_cementhttp://en.wikipedia.org/wiki/Mortar_(masonry)http://en.wikipedia.org/wiki/Plasterhttp://en.wikipedia.org/wiki/Joseph_Aspdinhttp://en.wikipedia.org/wiki/Portland_stonehttp://en.wikipedia.org/wiki/Portland_stonehttp://en.wikipedia.org/wiki/Isle_of_Portlandhttp://en.wikipedia.org/wiki/Londonhttp://en.wikipedia.org/wiki/Calcium_oxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Aluminum_oxidehttp://en.wikipedia.org/wiki/Limestonehttp://en.wikipedia.org/wiki/Clinker_(cement)http://en.wikipedia.org/wiki/Clinker_(cement)http://en.wikipedia.org/wiki/Clinker_(cement)http://en.wikipedia.org/wiki/Sulfatehttp://en.wikipedia.org/wiki/Gypsumhttp://en.wikipedia.org/wiki/Concrete#Workabilityhttp://en.wikipedia.org/wiki/Concrete#Workabilityhttp://en.wikipedia.org/wiki/Gypsumhttp://en.wikipedia.org/wiki/Sulfatehttp://en.wikipedia.org/wiki/Clinker_(cement)http://en.wikipedia.org/wiki/Limestonehttp://en.wikipedia.org/wiki/Aluminum_oxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Calcium_oxidehttp://en.wikipedia.org/wiki/Londonhttp://en.wikipedia.org/wiki/Isle_of_Portlandhttp://en.wikipedia.org/wiki/Portland_stonehttp://en.wikipedia.org/wiki/Portland_stonehttp://en.wikipedia.org/wiki/Joseph_Aspdinhttp://en.wikipedia.org/wiki/Plasterhttp://en.wikipedia.org/wiki/Mortar_(masonry)http://en.wikipedia.org/wiki/Portland_cement


17/68


18/68

9

Accelerators speed up the hydration (hardening) of the concrete..

Retarders slow the hydration of concrete, and are used in large or difficult

pours where partial setting before the pour is complete is undesirable.

Air entrainments add and distribute tiny air bubbles in the concrete, which

will reduce damage during freeze-thaw cycles thereby increasing the concrete's

durability..

Pigments can be used to change the color of concrete, for aesthetics.

Corrosion inhibitors are used to minimize the corrosion of steel and steel bars

in concrete.

Bonding agents are used to create a bond between old and new concrete.

Pumping aids improve pumpability, thicken the paste, and reduce dewatering

– the tendency for the water to separate out of the paste.

Plasticizers (water-reducing admixtures) increase the workability of plastic or

"fresh" concrete, allowing it be placed more easily, with less consolidating

effort.

Super plasticizers (high-range water-reducing admixtures)

The use of super plasticizers has become quite a common practice. This class of water

reducers were originally developed in Japan and Germany in the early 1960s.

Chemically, they are sulphonated melamine formaldehyde condensates and sulphonated

naphthalene formaldehyde condensates, the latter being probably the somewhat more

effective of the two in dispersing the cement and generally having also some retarding

properties. At a given water/cement ratio, this dispersing action increases the workability

of concrete, typically by raising the slump from 7-9 inches(175-225mm), the mix

remaining cohesive (The improvement in workability is smaller in high temperatures).

Reduce water content by 12 to 25 percent (Transportation-FHWA, 2010) and can be

added to concrete with a low to normal slump and water-cement ratio to make high slump

flowing concrete of high strength and lower permeability.

http://en.wikipedia.org/wiki/Accelerator_(chemistry)http://en.wikipedia.org/wiki/Accelerator_(chemistry)http://en.wikipedia.org/wiki/Retarderhttp://en.wikipedia.org/wiki/Retarderhttp://en.wikipedia.org/wiki/Air_entrainmenthttp://en.wikipedia.org/wiki/Air_entrainmenthttp://en.wikipedia.org/wiki/Weatheringhttp://en.wikipedia.org/wiki/Pigmenthttp://en.wikipedia.org/wiki/Pigmenthttp://en.wikipedia.org/wiki/Corrosion_inhibitorhttp://en.wikipedia.org/wiki/Corrosion_inhibitorhttp://en.wikipedia.org/wiki/Plasticizerhttp://en.wikipedia.org/wiki/Plasticizerhttp://en.wikipedia.org/wiki/Plasticizerhttp://en.wikipedia.org/wiki/Corrosion_inhibitorhttp://en.wikipedia.org/wiki/Pigmenthttp://en.wikipedia.org/wiki/Weatheringhttp://en.wikipedia.org/wiki/Air_entrainmenthttp://en.wikipedia.org/wiki/Retarderhttp://en.wikipedia.org/wiki/Accelerator_(chemistry)


19/68

10

The main purpose of using super plasticizers is to produce flowing concrete with very

high slump to be used in heavily reinforced structures and in placements where adequate

consolidation by vibration cannot be readily available. The other major application is the

production of high-strength concrete at W/C‟s ranging from 0.3 to 0.4. (Ramachandran,

1984)

Super plasticizers tend to be more stable over a wider range than standard water reducers

and provide more consistent setting times.

Practical considerations

a) Special mixes must be designed for super plasticizers and their use must be

carefully controlled.

b) The effect of super plasticizers lasts only 30 to 60 minutes, depending on the

brand and dosage rate, and is followed by a rapid loss in workability. As a result

of the slump loss, super plasticizers are usually added to concrete at the job site.

c)

They have a relatively high unit cost.

d) Where super plasticizers are used to produce very high workability, the shrinkage

and creep will be increased.

2.3 Concrete Mix Design

The process of selecting suitable ingredients of concrete and estimating their proportions with

the objective of producing workable, strong and durable concrete at reasonable cost is called

mix design.

The proportioning of ingredient of concrete is governed by the required performance of

concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is

not workable, it cannot be properly placed and compacted. The property of workability,

therefore, becomes of vital importance.


20/68

11

In mix design, use is normally made of previous experience and of several design tables,

charts and curves. Final specifications are arrived at after testing trial mixes.

The old fashioned idea in concrete design is that concrete consists of cement, coarse

aggregate, fine aggregate and water, thus the problem of mix design has been seen as how to

select suitable aggregates, and determine their optimum relative proportions and the cement

requirement to produce a given strength at a given slump. Early investigators tended to be

concerned with how to define ideal concrete. These past specifications for concrete prescribed

the proportions of cement, and fine aggregates. Certain traditional mixes were thus produced

but, because of variability of the mix ingredients, concretes having fixed cement-aggregate

proportions and a given workability vary widely in strength. For this reason, minimumcompressive strength was later added but this is restrictive where good quality materials are

available or poor quality materials are the only ones available. In summary, specifying at the

same time strength as well as mix ingredients and their proportions, and also the aggregate

shape and grading, leaves no room for economies in the mix selection, and makes progress in

the production of economic and satisfactory mixes in the basis of the knowledge of the

properties of concrete impossible.

Current consideration in designing for concrete mixes should be:

1. What aggregates are economically available,

2. What properties should the concrete have and

3. What is the most economical way of providing these required properties?

Modern tendency is for specifications to be less restrictive by providing just limiting values,

but sometimes traditional mix proportions are stated for the benefit of the contractor who does

not wish to use a high degree of quality control.


21/68

12

Requirements of concrete mix design

The requirements which form the basis of selection and proportioning of mix ingredientsare:

a) The minimum compressive strength required from structural consideration

b) The adequate workability necessary for full compaction with the compacting

equipment available.

c) Maximum water-cement ratio and/or maximum cement content to give adequate

durability for the particular site conditions

d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in

mass concrete.

Basic concepts

Strength margin

Because of the variability of concrete strengths the mix must be designed to have a

considerably higher mean strength than the strength specified. The method of specifying

concrete by its minimum strength has been replaced in British Standards and codes of

practice such as BS 5328 and BS 8110 by a „characteristic strength‟. The difference

between the specified characteristic strength and the target strength is called the margin.

The margin is based on knowledge of the variability of the concrete strength obtained

from previous production data expressed as a standard deviation or alternatively a

substantial margin is applied until an adequate number of site results are obtained.


22/68

13

Measurement of workability

Two alternative test methods can be used, the slump test which is more appropriate forthe higher workability mixes, and the vebe test which is particularly appropriate for those

mixes which are to be compacted by vibration.

Free water

The total water in a concrete mix consists of the water absorbed by the aggregate to bring

it to a saturated surface dry condition, and the free water available for the hydration of the

cement and for the workability of the fresh concrete.

In practice aggregates are often wet and they contain both absorbed water and free

surface water so that the water added at the mixer is less than the free water required. The

workability of concrete depends to a large extent on free-water content; if the same total

water content were used with dry aggregates having different absorptions then the

concrete would have different workabilities.

Similarly the strength of concrete is better related to the free-water/cement ratio since on

this basis the strength of the concrete does not depend on the absorption characteristics of

the aggregates.

Types of aggregates

Early mix design methods used in the UK classified the shape of aggregate as rounded,

irregular or angular. There is in sufficient difference between the behavior of rounded and

irregular aggregates in concrete to justify the use of separate classifications for these two

shapes of aggregates. There are however significant differences between these

aggregates, both of which are usually rough in texture and invariably produced by a

crushing process.


23/68

14

Two of the characteristics of aggregate particles that affect the properties of concrete are

particle shape and surface texture. Particle shape affects the workability of the concrete,

and the surface texture mainly affects the bond between the matrix and the aggregates

particles and thus the strength of the concrete. Generally, crushed aggregates consist of

rather angular particles having a rough surface texture resulting in a concrete of lower

workability but higher strength compared with a similar mix made with uncrushed

aggregates.

The type of aggregate becomes of greater importance for concrete having a high specified

strength. If the specified strength at 28 days is 50N/mm2 or more it may become

necessary to use crushed aggregates than uncrushed gravel. The higher the specifiedstrength the more critical the selection of the source of the aggregates.

Aggregate grading

Early methods of mix design used, specified grading curves for the combined fine and

coarse aggregates. These required the use of fine aggregates having a restricted range ofgrading compared with the limits specified in BS 882. Fine aggregates having such

restricted grading are not easily available in most parts of the country.

Fine aggregates should comply with the C, M or F grading requirements of BS

882:1983, but these limits overlap and are too wide for mix design purposes. The method

for deriving suitable fines content takes into account the many relevant factors i.e. the

type and maximum size of coarse aggregate, the grading of the fine aggregate

characterized by the percentage passing the 600 micrometer test sieve, and the cement

content and workability of the concrete.


24/68

15

Mix parameters

It is the general custom to specify by a system of proportions or ratios, e.g. 1:2:4 (beingthe proportions of cement: fine aggregate: coarse aggregate) either by weight or by

volume, or as cement/aggregate ratio or water/cement ratio and fine aggregate/coarse

aggregate ratio usually by weight.

Such systems have certain merits in terms of simplicity of expression. However, they are

not so convenient when discussing the effect of mix parameters on the characteristic of

the concrete, nor do they adequately describe the quantity of cement required to cast a

given volume of concrete.

The most fundamental way to specify mix parameters is in terms of the absolute volumes

of different materials required in a concrete mix. A more practical method, based on

similar principles, is to refer to the weights of materials in a unit volume of fully

compacted concrete.

In order to use this approach, knowledge is required of the expected density of the fresh

concrete. This depends primarily on the relative density of the aggregate and the water

content of the mix.

Durability

A durable concrete is one which gives a satisfactory performance during an adequate life

in a given environment; this includes providing protection of the steel against corrosion

in reinforced concrete and prestressed concrete. There are some durability problems

associated with the constituent materials, and others due to the effect of hostile

environments.

A major factor in providing durable concrete is the production of a dense, impermeable

concrete, having adequate cement content and low free water/cement ratio, which is fully


25/68


26/68

17

Types of Mixes

Nominal Mixes

In the past the specifications for concrete prescribed the proportions of cement, fine and

coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate

strength are termed nominal mixes. These offer simplicity and under normal

circumstances, have a margin of strength above that specified. However, due to the

variability of mix ingredients the nominal concrete for a given workability varies widely

in strength.

Standard mixes

The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength

and may result in under- or over-rich mixes. For this reason, the minimum compressive

strength has been included in many specifications. These mixes are termed standard

mixes.

Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mix

proportions are determined by the producer of concrete, except that the minimum cement

content can be laid down. This is most rational approach to the selection of mix

proportions with specific materials in mind possessing more or less unique

characteristics. The approach results in the production of concrete with the appropriate

properties most economically. However, the designed mix does not serve as a guide since

this does not guarantee the correct mix proportions for the prescribed performance.

For the concrete with undemanding performance, nominal or standard mixes (prescribed

in the codes by quantities of dry ingredients per cubic meter and by slump) may be used

only for very small jobs, when the 28-day strength of concrete does not exceed 30

N/mm2. No control testing is necessary reliance being placed on the masses of the

ingredients.


27/68

18

Factors affecting the choice of mix proportions

The various factors affecting the mix design are:

Compressive strength

It is one of the most important properties of concrete and influences many other

describable properties of the hardened concrete. The mean compressive strength required

at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix.

The other factor affecting the strength of concrete at a given age and cured at a prescribed

temperature is the degree of compaction. According to Abraham‟s law the strength of

fully compacted concrete is inversely proportional to the water-cement ratio.

In summary factors affecting the concrete compressive strength include;

a) Water/cement ratio

b) Type of cement- determines the rate of gain of strength

c) Aggregate characteristics; strength, grading, surface texture, maximum size

affect the strength of concrete.

d) Moisture conditions during curing; prolonged moist curing leads to higher

strengths

e)

Temperature conditions during curing

f)

Age of concrete

g)

Rate of loading; standard cylinder test is carried out at a loading rate of 35 psi

and the maximum load is reached in 1.5 minutes to 2 minutes. For lower rates,

strength is reduced to about 75% of standard test. For higher rates, strength is

increased to about 115%.


28/68

19

Workability

This may be defined as the amount of useful work necessary to produce full compactionof concrete. Workability implies the ease with which a concrete mix can be handled from

the mixer to its finally compacted shape. The provision of adequate workability is critical

to enable the transportation, placing and compaction of the concrete with the available

equipment. It has been proposed that the workability should be defined by at least 3

separate properties:

a)

Compactabilty or the ease with which the concrete can be compacted. A fully

compacted mix contains minimal voids and hence will produce higher strength

concrete of less permeability.

b) Mobility or the ease with which concrete can flow into moulds around steel and

be remoulded.

c) Stability or the ability of concrete to remain a stable coherent homogeneous mass

during handling and vibration without the constituents segregating.

Durability

The durability of concrete is its resistance to the aggressive environmental conditions.

High strength concrete is generally more durable than low strength concrete. In the

situations when the high strength is not necessary but the conditions of exposure are such

that high durability is vital, the durability requirement will determine the water-cement

ratio to be used.


29/68

20

Maximum nominal size of aggregate

In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase

in maximum size of the aggregate. However, the compressive strength tends to increase

with the decrease in size of aggregate.

Grading and type of aggregate

The grading of aggregate influences the mix proportions for a specified workability and

water-cement ratio. Coarser the grading leaner will be mix which can be used. Very lean

mix is not desirable since it does not contain enough finer material to make the concrete

cohesive.

The type of aggregate influences strongly the aggregate-cement ratio for the desired

workability and stipulated water cement ratio. An important feature of a satisfactory

aggregate is the uniformity of the grading which can be achieved by mixing different size

fractions.

Quality Control

The degree of control can be estimated statistically by the variations in test results. The

variation in strength results from the variations in the properties of the mix ingredients

and lack of control of accuracy in batching, mixing, placing, curing and testing. The

lower the difference between the mean and minimum strengths of the mix lower will be

the cement-content required. The factor controlling this difference is termed as quality

control.


30/68


31/68

22

Bleeding

Bleeding, known also as water gain, is a form of segregation in which some of the waterin the mix tends to rise to the surface of freshly placed concrete by capillary action. This

is caused by the inability of the solid constituents of the mix to hold all of the mixing

water when they settle downwards. Bleeding can be expressed quantitatively as the total

settlement per unit height of concrete.

As a result of bleeding the top of every lift may become too wet and if the water is

trapped by superimposed concrete, porous, weak, and non durable concrete will result. If

the bleeding water is remixed during finishing of the top surface a weak wearing surface

will be formed. This can be avoided by delaying the finishing operations until the

bleeding water has evaporated, and also by the use of wood floats and avoidance of

overworking the surface. On the other hand if evaporation of water from the surface of

the concrete is faster than the bleeding rate plastic shrinkage cracking may result.

Bleeding need not necessarily be harmful. If it is undisturbed (and the water evaporates)

the effective W/C ratio may be lowered with a resulting increase in strength. On the other

hand, if the rising water carries with it a significant amount of the finer cement particles a

layer of laitance will be formed. If this is at the top of a slab a porous surface will result,with a permanently dusty surface. At the top of a lift a plane of weakness would form and

the bond with the next lift would be inadequate. For this reason, laitance should always

be removed by brushing and washing.

Bleeding depends largely on the properties of cement; increased alkali content and

fineness of cement decreases the tendency to bleed. Addition of pozzolanas and air

entraining agent may also decrease bleeding.

The bleeding capacity and the rate of bleeding can be determined experimentally usingthe test of ASTM Standard C 232-71


32/68

23

CHAPTER THREE

3.0 Research Methodo logy :

Experimental study design was employed. A high slump concrete mix was designed and the

process entailed selecting suitable ingredients of concrete and estimating their proportions to

yield the best mix. Design tables, charts and curves were used in the process. Final

specifications were arrived at after testing trial mixes.

The process involved performing tests on the materials to come up with the design, tests on

the fresh concrete to determine its workability and on the hard concrete to determine its

compressive and tensile strength.

The following were done during the study:-

1. Grading of materials according to BS 882 and other associated codes.

2. Carrying out of the specific gravity tests and water absorption tests for the fine

and coarse aggregates3.

Carrying out the bulk density tests.

4. Determining the silt content of the fine aggregates that will be used.

5. Coming up with a high slump mix design and making trial mixes.

6.

Establishing the properties of green concrete.

7. Establishing the properties of hardened concrete from casting concrete cubes and

cylinders.

3.1 Grading of materials for concrete production

In order to design and produce a concrete mix, it is important that the grading of the

constituents be done. This is done on coarse and fine aggregates to establish whether the

particular particle distribution of a batch is good for concrete production. This then

enables the materials engineer to choose the source of his materials (quarry and river).


33/68

24

Grading for ordinary material was done using the British standards (BS 882: 1992

specification for aggregates from natural sources for concrete).

The code gives the sieves and envelopes (bounds) or limits required for coarse and fine

aggregates.

Coarse aggregate

Coarse aggregate is defined as aggregate mainly retained on a 5.0 mm BS 410 test sieve

and containing no more finer material than is permitted for the various sizes in this

specification (CL 2.2).

Coarse aggregate may be described as gravel (uncrushed, crushed or partially crushed) as

defined in 2.2.1, or as crushed rock as defined in CL2.2.2, or as blended coarse aggregate

as defined in CL2.2.3.

When determined in accordance with BS 812-103.1 using test sieves of the sizes given in

Table 3, complying with BS 410, full tolerance, the grading of the coarse aggregate were

within the appropriate limits given in Table 3. The material used was 20 mm and below.

Fine aggregates

When determined in accordance with BS 812-103.1, using test sieves of the sizes given in

Table 4 complying with BS 410, full tolerance, the grading of the sand complies with the

overall limits given in Table 4. Additionally, not more than one in ten consecutive

samples shall have a grading outside the limits for any one of the grading C, M or F,

given in Table 4 (CL 5.2.1).

The method of grading for both fines and coarse aggregates is described:-


34/68

25

Figure 3-GRADING SIEVES

Objective

To determine the particle size distribution of aggregates by sieving.

Apparatus

I. Balance accurate to 0.5% of mass of test sample.

II. Test sieves as listed a below

III.

Oven capable of maintaining constant temperature to within 5%

IV.

Mechanism of shaking sieves.

V. Chart for recoding results.

VI. Sieve sizes

VII. Coarse aggregates:


35/68

26

VIII.

Fine aggregates:

Procedure

Dry the test samples to a constant mass by oven drying at not more than 105+50 C

Take an approximate sample from the original sample by riffling.

Make sure the sieves are dry and clean before using them.

Weigh out the required sample

Stand the sieve of the largest mesh size in the tray and put the weighed sample on to the

sieve.

Shake the sieve horizontally with a jerking motion in all directions for at least 2 minutes

and until no more than a trace of a sample passes. Ensure that all material passing falls

into the tray.

Weigh any material retained on the sieve.

Tabulate the results in the table provided and calculate the cumulative weight passing

each sieve as a percentage of the total sample to the nearest whole number.

Plot the grading curve for the sample in the grading chart and comment on the curve

obtained.

3.2 Fineness Modulus

A single factor from the sieve analysis is used that is the fineness modulus. It is the sum

of cumulative percentages retained on the sieves divide by 100. Usually the fineness

modulus is calculated for fine aggregate rather than coarse aggregate. Typical values

range from 2.3 and 3.0 a higher value indicating a coarser grading. The usefulness of the

fineness modulus lies in detecting slight variations in the aggregate from the same source.


36/68


37/68

28

Method for fine aggregates (5mm and below)

Objective

To determine the specific gravity and the water absorption values of aggregates

Apparatus

i. A balance

ii. A drying oven

iii.

A pycnometer bottle

iv. Sample containers

v. Stirring rod

A sample of about 500g is used for aggregates less than 5mm.The sample shall be

thoroughly washed to remove all material finer than 0.075mm test sieve as follows:-

Place the test sample in the tray and add enough water to cover it. Agitate vigorously and

immediately pour the wash water over the sieve which has previously been wetted on

both sides. Repeat the operation until the wash water is clear. Return all material retained

on the sieve to the washed sample.

Procedure

Transfer the washed sample to the tray and add further water to ensure that the sample is

completely immersed. Ensure that the sample is completely immersed.

Keep the sample immersed in water for 24 hours. Place the aggregate in the pycnometer

and fill it with water.

Screw the cone in to place and eliminate any entrapped air by rotating it onsides.


38/68

29

Dry the bottle on the outside and weigh it as (A).

Empty the sample in to the tray, refill the pycnometer with water to the same level as before, dry it on the outside and weigh it as (B).

Carefully drain water from the sample by decantation through a 0.075mm sieve and

retain any material retained to the sample.

Expose the aggregate to a gentle current of warm air to evaporate surface moisture and

stir it at frequent intervals to ensure uniform drying until no free surface moisture can be

seen. Then weigh the saturated and surface dry sample (C).

Place the sample in the tray and dry it in an oven at a temperature of 104 0 – 1050 C for 24

hours. Cool it in a dessicator and weigh it as (D).

Calculations:

i. Specific gravity on an oven dried basis

=

ii. Specific gravity on a saturated and surface dried basis

=

iii. Apparent specific gravity

=

iv. Water absorption (% of dry mass)

=


39/68


40/68

31

Results

Calculate the results as follows:-

(i) Specific gravity on saturated- surface dry basis

=

(ii) Absolute dry specific gravity

=

(iii) Water absorption (% of dry weight)

=

3.4 Silt content test

Apparatus

(i) A beam balance.

(ii) 75 micron sieve

Procedure

1. Take a sample of sand and oven dry it to a constant weight for 24 hours at 1050C.

2. Weigh the sample W1.

3. Wash the sample through a 75 micron sieve until the water becomes clear.

4. Decant the water and add the retained silt with the sample.

5.

Oven dry the sample to a constant weight for 24 hours at 1050C.

6. Weigh the sample W2.

The silt content is determined from:

W1-W2 x 100%

W2


41/68

32

3.5 Concrete mix design

Concrete mix design was carried out to determine the proportions of constituents of

concrete that met the desired strength and other properties. This was done according toaccepted standards and specifications.

Mix design enables in choosing of a mix that will be recommended in the casting of

precast element for testing.

It entailed coming up with adequate water/ cement ratio that would gave adequate

compressive strength.

The mix design was according to the Department of the Environment (DOE).

The procedure is as follows:

Selection of target water/cement ratio

The standard deviation to be adopted in determining the target strength should be that

obtained from line A from the graph showing the relationship between standard deviation

and characteristic strength.

The margin can then be derived from

M=k x s

Where M = the margin

k = a value appropriate to the percentage defectives permitted below the

characteristic strength

s = the standard deviation

The target mean strength is determined through fm = fc + M

Where fm = the target mean strength

fc = the specified characteristic strength

M = the margin


42/68

33

Using this value the water/cement ratio is obtained from the graph showing the

relationship between compressive strength and free water/cement ratio.

Selection of free-water content

Stage 2 consists simply of determining the free water content depending upon the type

and maximum size of the aggregate to give a concrete of the specified slump or vebe

time.

Determination of cement content

The cement content is determined from:

Cement content = free water content

free water/cement ratio

The resulting value should be checked against any maximum or minimum value that may

be specified. If the calculated cement content is below a specified minimum, this

minimum value must be adopted and a modified free water/cement ratio calculated.

Determination of total aggregate content

Stage 4 requires an estimate of the density of the fully compacted concrete which is

obtained depending upon the free water content and the relative density of the combined

aggregate in the saturated surface dry condition (SSD).

Total aggregate content = D – C - W

(Saturated and surface dry)

Where D = the wet density of concrete (kg/m3)

C = the cement content (kg/m3)

W = the free water content (kg/m3)

Selection of fine and coarse aggregate contents

Fine aggregate content = total aggregate content x proportion of finesCoarse aggregate =

total aggregate content – fine aggregate content


43/68

34

3.6 Batching

Batching involves proportioning the material or the constituents of concrete to produce

the concrete. The batches are according to the mix design results. These proportions are

then reduced to a volume corresponding to the amount of concrete required. The size of

the mix was arranged so that there was a small surplus when all the compression test

samples were made.

3.7 Mixing of concrete

After mix design, the trial mixes were done and their properties as fresh concrete

established. The mixing was done by hand using a pan. The interior surfaces of the pan

should be cleaned and then wetted a bit. The ingredients were added in a definite order so

that the total quantity of one particular material or grading is not added all at once.

Mixing was continuous to ensure that all material forms a homogeneous mix.

3.8 Slump test

This is a well established test that is carried out in the form of a frustum of a cone having

an upper diameter of 100 mm, and a lower diameter of 200 mm and a height of 300 mm.

the mould is placed in a smooth, horizontal, vibration free and non -absorbent surface and

is filled in three equal layers with the concrete to be tested, each layer being tamped 25

times with a standard tamping rod. The top layer is struck off level with the mould and

the cone is immediately lifted and amount of by which concrete slumps is measured. It is

important that the cone is lifted truly vertical. The slump is measured using a steel rule.

The inside of the mould should be free from superfluous moisture.


44/68

35

Figure 4-SLUMP TEST

3.9 Casting of compression test specimen

The mixes were used to cast cubes for testing. This was done according to BS 1881 part

108 -1983. Eight cubes were cast for each mix. The cubes were then crushed to determine

the strength development at different ages. Concrete cubes made from 100 x 100 x 100

mm moulds were used.

3.10 Concrete placing

Concrete was placed in layers and compaction carried out using a poker vibrator. During

compaction at all times, effort was made to maintain uniformity so that the final structure

was monolithic and uniform. Prior to placing, the moulds for cubes and cylinders were

screwed to place and oiled on the inside surface to facilitate cubes removal.

3.11 Curing of the test specimen

This was done according to the British practice (BS 1881 Part 111). The test specimens

were cured 24 hours after casting. This was done at a constant temperature of about 20 –

220

C and relative humidity of about 90%.


45/68

36

3.12 Hard concrete test

Compressive strength determination

The test cubes and cylinders were crushed using a universal test machine complying with

BS 1881 part 115 – 1986 specifications.

The testing procedure is as described in BS EN 12390-3: 2003:-

Procedure

The test cube is removed from the curing tank and the excess moisture from the surface

of the specimen wiped and weighed before placing it on the testing machine.

All testing machine bearing surfaces are wiped clean and any loose grit or other

extraneous material removed from the surfaces of the specimen that will be in contact

with the platens. The cube specimens are placed in a way that the load is applied

perpendicularly to the direction of casting. The specimen is centered with respect to the

lower platen to an accuracy of ± 1 % of the designated size of cube. A constant rate of

loading within the range 0.2 MPa/s (N/mm2 _ s) to 1.0 MPa/s (N/mm2 _ s) is selected.

The load to the specimen is applied without shock and is increased continuously, at the

selected constant rate ± 10 %, until no greater load can be sustained. This load isrecorded.

The crushing was done as follows:-

3 cubes for 7th day strength



The specimen was as shown in the figure


46/68

37

Figure 5- FAILURE OF CUBE BY COMPRESSION

Figure 6-COMPRESSIVE STRENGTH TEST


47/68

38

3.13 Tensile strength test

This test is of considerable importance in resisting cracking due to changes in moisture

content or temperature. A split test is carried out on a cylinder to determine the horizontal

tensile stress. The cylinder is placed with its axis horizontal between the patens of a

universal testing machine with the load being increased gradually until failure. The water

content is 10% of weight of dry materials. A vibrator is used to thoroughly mix the

mortar after which the cylinders are demoulded after 24 hours and further cured in water

until tested in a wet surface condition. Cylinders measuring 200mm height and 100mm

diameter were used.

Procedure

Oil was applied in the interior surfaces of the moulds to prevent the mortar from sticking

to the surfaces. The specimens were then cast in cylindrical moulds. The moulds were

filled to overflowing and after filling excess mortar were removed by a sawing motion

using a steel rule. The surface was then finished smooth by means of trowel. Each layer

of mortar was compacted by not less than 35 strokes of 25mm square steel punner.

The moulds were then stored undisturbed for 24hrs in a laboratory at temperatures of 18

to 200c (64 and 68℉) and a relative humidity of not less than 90%.The moulds were then

stripped and the cylinders further cured in 19 to 210 c water. The standard moulds were

placed under the universal testing machine, one at time, and tested at 7, 14, and 28days.

Calculation

Tensile Strength of a concrete cylinder

=F/A

Where F=Tensile load on cylinder

A=Area of cylinder=πDL

D=Diameter

L=Length of specimen


48/68

39

Figure 7-CYLINDER BEFORE LOADING

Figure 8- CYLINDER SPLIT AFTER LOADING


49/68

40

CHAPTER FOUR

4.0 Data Resul ts and d iscu ssio n

4.1 RESULTS OF SPECIFIC GRAVITY & WATER ABSORPTIONTESTS ON FINE AGGREGATES

A summary of computations of specific gravity and water absorption tests on fine

aggregates are shown below;

Ordinary Sand

Sample A Sample

B

Av.

Weight of jar + sample + water 1706 1734.5 1720.25

Weight of jar +water 1417 1417 1417

Weight of saturated surface dry

Sample

460 505.5 482.75

Weight of oven dried sample 457.5 503.5 480.5


50/68

41

Specific gravity on an

oven dried basis

=

2.68 2.68 2.68

Specific gravity on a

saturated and surface

dried basis =

2.69 2.69 2.69

Apparent specific

gravity

=

2.71 2.71 2.71

Water absorption

(% of dry mass) 2.71

Table 1


51/68

42

4.2 RESULTS OF SPECIFIC GRAVITY & WATER ABSORPTIONTESTS ON COARSE AGGREGATES

A summary of computations of specific gravity and water absorption tests on coarseaggregates are shown below;

Granite coarse aggregates

A B Av.

Weight of wire basket (a) 420 417 418.5

Weight of wire basket +

aggregate (b)

1015 1020 1017.5

Weight of aggregate in

water (a+b) (Ww)

595 603 599

Weight of saturated

surface dry sample (Ws)

983.5 1003 993.25

Weight of oven dried

sample (Wd)

963 984 973.5

Specific gravity on

saturated surface dry basis

=

2.53 2.51 2.52

Absolute dry specific

gravity

=

2.36 2.46 2.41

Water absorption (% of

dry weight) =

2.1 1.9 2.0


52/68

43

4.3 GRADING RESULTS

The main purpose of grading is to determine whether or not a particular grading is

suitable to produce a good mix. In the first instance, grading is of importance only in so

far as it affects workability, because strength is independent of the grading. However,

high strength requires a maximum compaction with a reasonable amount of work, which

can only be achieved with a sufficiently workable mix. In fact, there are no ideal grading

requirements because of the main influencing factors on workability; the surface area of

the aggregate which determines the amount of water necessary to wet all the solids, the

relative volume occupied by the aggregate, the tendency to segregate and the amount of

fines in the mix.

Fine Aggregate Sieve Analysis Results

The results obtained, from sieve analysis of fine aggregates are shown below. From the

table and graphs, it can be observed that, natural fine aggregates particle distribution is

reasonably uniform and it is in agreement with BS grading requirement.

Sieve sizes

(mm)

Wt.

retained(g)

Wt.

passing (g)

% retained Cumulative

% retained

Cumulative

% passing

5.0 40.5 1496.00 2.64 2.64 97.36

2.0 47.0 1449.00 3.06 5.69 94.31

1.2 210.0 1239.00 13.67 19.36 80.64

0.6 419.5 819.50 27.30 46.66 53.34

0.3 537.0 282.50 34.95 81.61 18.39

0.2 215.5 67.00 14.03 95.64 4.36

0.1 67.0 0.00 4.36 100.00 0.00

Sample weight 1537g

Table3


53/68

44

Figure 9-FINE AGGREGATE SIEVE ANALYSIS

FINENESS MODULUS = 2.52

Typical values of the fineness modulus range from 2.3 and 3.0 a higher value indicating

a coarser grading.The value obtained was 2.52 indicating uniformity. The usefulness of

the fineness modulus lies in detecting slight variations in the aggregate from the same

source.

Coarse Aggregate Sieve Analysis Results

The results obtained, from sieve analysis of coarse aggregates are shown below. From the

table and graphs, it can be observed that, coarse aggregates particle distribution is

reasonably uniform and it is in agreement with BS grading requirement.


54/68

45

Sieve sizes (mm) Wt. retained

(g)

Wt. passing

(g)

% retained Cumulative %

retained

Cumulative %

passing

50 0 5399.5 0.00 0.00 100.00

38.1 0 5399.5 0.00 0.00 100.00

20 1184 4215.5 21.93 21.93 78.07

15 984 3231.5 18.22 40.15 59.85

10 1698 1533.5 31.45 71.60 28.40

5 636.5 897.0 11.79 83.39 16.61

2.36 42 855.0 0.78 84.17 15.83


55/68

46

4.4 RESULTS OF SILT CONTENT

A summary of computation of silt content is shown below;

WEIGHT OF OVEN DRY SAMPLE W1 305

WEIGHT OF SAMPLE + SILT W2 289.5

W1-W2 x 100%

W2

5.35%

Silt content of sand

Table 5

The silt content test is important as silt in high degrees affects the overall strength of the

mix. The result of 5.35% silt in the sample obtained is acceptable.

4.5 Normal concrete mix design by Department of Environment(DoE)

The mix developed was as shown;

Stage Item Reference or

calculations

values

1. 1.1 Characteristic

strength

specified 25 N/mm2 at 28

days

Proportion defective 10%

1.2 standard deviation Fig 3 N/mm2

or no Data 8 N/mm

2

1.3 Margin C1

or

(k = 1.28 ) 1.28 x 8

= 10.24 N/mm2


56/68

47

specified

1.4 target mean strength C2 & Para8.1 25 + 10.24= 35.24 N/mm2

1.4.1 air content %

1.5 cement type specified OPC/ SRPC/ RHPC

1.6 Aggregate type:

Coarse

Aggregate type:

fine

Crushed/ uncrushed

Crushed/ uncrushed

1.7 Free- water/Cement

ratio

Table 2, Fig

4

0.63 Use the lower

value l

1.8 Maximum free

water/ cement ratio

Specified

2 2.1 Slump or V B time Specified Slump 180 mm or V B

s

2.2 Maximum aggregate

size

Specified 20 mm

2.3 Free- water content Table 3 &

Para 8.2

225

Kg/m3

3 3.1 Cement content C3 225 / 0.63

= 357 Kg/m3

3.2 Maximum cement

content

specified Kg/m3

3.3 Minimum cement

content

specified Kg/m3

3.4 Modified free-

water/ cement ratio


57/68

48

Kg/m3

4 4.1 Relative density ofaggregate (SSD)2.52

Known/ Assumed

4.2 Concrete Density Fig 5 Para

8.3

2400 Kg/m3

4.3 Total aggregate

content

C4 2400 _- 225 -

357 = 1818 Kg/m3

5 5.1 Grading of fine

aggregates

Percentage

passing 600µ

53.34%

5.2 proportion of fine

aggregate

Fig 6 45%

5.3 Fine aggregate

content

C5 1 818 X 0.47

= 855 Kg/m3

5.4 Coarse aggregatecontent

1818 - 855= 963 Kg/m3

Quantities Per m3 to nearest 5kg

Cement 360kg Water 225kg Fine aggregates 855kg Coarse aggregate 965kg

Per trial mix of 0.001m3

Cement 0.36 kg Water 0.225kg Fine aggregates 0.855kg Coarse aggregate

0.965kg

Table 6


58/68

49

4.6 SLUMP TEST

The slump test was a key test in the project as a high slump of 200mm was to be

achieved. This was done initially by using water and subsequently using two types of

admixtures i.e. Pozzolith LD 10 and Rheobuild, both super plasticizers. The graphs below

show how the workability increased, in both cases, as the admixtures were added until a

slump of 200mm was obtained.

Figure 11-RESULTS OF SLUMP TEST


59/68

50

SLUMP (mm) POZZOLITH

ADMIXTURE (ml)

1 0

4 52

27.5 104

123 156

204.5 290

Table 7

Figure 12-SLUMP AGAINST POZZOLITH LD 10 ADMIXTURE


60/68


61/68

52

This was achieved by a W/C ratio of only 0.4. To obtain a slump of 200mm by using

water only without the use of admixtures the W/C ratio was 0.7.

Super plasticizers are used for high strength concretes by decreasing the W/C ratio as a

result of reducing the water content by 12-25%. In this case the water content was

reduced by 25% (from 1.7kg to 1.313kg).

4.7 COMPRESSIVE STRENGTH RESULTS

For a W/C ratio of 0.7 to obtain a slump of 200mm the compressive strength results for 7

day,14 day and 28 day are as shown below;

DAYS STRENGTH

7 12 N/mm2

14 16 N/mm2

28 20 N/mm2

Table 9

The main objective of this experimental study was to attain class 25 concrete with a

slump of 200mm.

On the basis of experimental results obtained, it can be seen that the target strength was

not attained after 28 days of curing the cubes. The strength increased steadily from

12N/mm2 on the 7th day to 20N/mm2 on the 28th day but this was only 80% of the desired

25N/mm2

strength. Thus it is clear, that to achieve a high strength concrete with a highslump, the use of water alone is not an option as it compromises the strength.

For a W/C ratio of 0.4 and by using a super plasticizer to obtain a 200mm slump, the

compressive strength results are as shown;


62/68

53

DAYS STRENGTH

7 20 N/mm

2

14 22 N/mm2

28 30 N/mm2

Table 10

It can be seen that after only 7days high strengths could be achieved i.e. 20N/mm2 and

after 28 days of curing the strength achieved was 30N/mm2. This is 120% of the desired

25N/mm

2

strength. This was achieved by a much lower W/C ratio of 0.4 compared to 0.7without the use of admixture.

The graph following shows the relationship of compressive strength against the number

of days it took to attain that strength for the control cubes and the cubes with admixture.

Figure 14-COMPRESSIVE STRENGTH AGAINST TIME

From the graph it can be seen that the rate of increase of the compressive strength of

cubes with super plasticizers is at a much faster rate as opposed to cubes without the

admixture.


63/68

54

4.8 TENSILE STRENGTH RESULTS

This test is of considerable importance in resisting cracking due to changes in moisture

content or temperature.

For a W/C ratio of 0.7 to obtain a slump of 200mm the tensile strength results for 7

day,14 day and 28 day are as shown below;

DAYS STRENGTH

7 1 N/mm2

14 1.6 N/mm2

28 2 N/mm2

Table 11

The tensile strength increased from 1N/mm2 on the 7

th day to 2N/mm

2 on the 28

th day.

For a W/C ratio of 0.4 and by using a super plasticizer to obtain a 200mm slump, the

tensile strength results are as shown below;

DAYS STRENGTH

7 2 N/mm2

14 2.2 N/mm2

28 2.95 N/mm2

Table 12

The concrete cylinders cast using a super plasticizer exhibited higher tensile strengths

than those cast without using a super plasticizer.

A graph is illustrated below showing the relationship of the tensile strength against the

number of days it took to attain that strength for the control cylinders and the cylinders

with admixture;


64/68

55

Figure 15-TENSILE STRENGTH AGAINST TIME

The rate of increase of the tensile strength of cylinders cast using a super plasticizer is

higher than that of the control cylinders. The maximum strength attained by the controlcylinders was 2N/mm2 whereas it was 2.95 N/mm2 for the cylinders with admixture.

4.9 WATER:CEMENT RATIO

From the results tabulated above it is clear that the W/C ratio affects both the

compressive and tensile strengths of concrete. Very high W/C ratios produce low

strengths in concrete as observed from the experimental data and vice versa.

The cubes and cylinders with a W/C ratio of 0.7 exhibited lower compressive and

tensile strengths i.e. 20N/mm2 and 2N/mm2 respectively after 28 days.

The cubes and cylinders with a W/C ratio of 0.4 exhibited higher compressive and tensile

strengths i.e 30N/mm2 and 2.95N/mm2 respectively after 28 days.


65/68

56

4.10 Problems associated with the high slump concrete.

Segregation

In this study two types of superplasticizers were uased to come up with trial mixes.

Initially Pozzolith LD 10 was used.

The trial mix that was developed using this super plasticizer experienced segregation i.e the

separation of the constituents of a heterogeneous mixture so that their distribution is no longer

uniform, this was manifested by the separation of grout (cement plus water) from the mix

particularly common in wet mixes.

The cubes took almost 48 hours to set and once they were placed in the curing tank, all thecubes segregated to chunks of concrete.

This could have been as a result of;

1. Over dosage which can result in;

Retardation of initial and final set

Slight increase on air entrainment

Increase in workability

2. Improper handling of the concrete i.e. over vibrating and excessive rehandling.

The mix required little or no vibration at all and over vibration could be the cause of

segregation. Since the same sample was being excessively rehandled by being used over and

over again to determine the slump, this could have consequently lead to segregation.

A good picture of cohesion of the mix is obtained by the flow test.


66/68


67/68

58

CONCLUSION AND RECOMMENDATIONS

4.11 CONCLUSION

A concrete mix was designed with the desired slump and strength, which is economical

and workable.

From the experimental data,it is clear that in order to obtain a high slump concrete

of 200mm and of class 25, a super plasticizer must be used rather than a higher

water content which would detract from the strength of the concrete.

The super plasticizer used reduced the water content by 25%. Super plasticizers

can be added to concrete with a low to normal slump and water-cement ratio tomake high slump flowing concrete.

The resulting concrete, flowing concrete, can be placed with little or no

compaction and is not subject to excessive bleeding or segregation.

Special mixes must be designed for super plasticizers and their use must be

carefully controlled as an over dosage results in segregation and improvement in

workability is smaller in high temperature.

The high slump concrete mix design was; Cement, Fine aggregate, Coarse aggregate in

the ratio of 10:24:27 respectively with a W/C ratio of 0.4. It had the best performancein terms of strength and workability considerations. The normal dosage of a super

plasticizer is between 750ml and 2500ml per 100 kg of cementitious material.

4.12 RECOMMENDATIONS

A good picture of cohesion of a high slump mix is obtained by the flow test. This

should be done prior to placing of high slump concrete to determine whether a

particular mix is experiencing excessive segregation.

To design a concrete mix with a desired high slump and of high strength, that is

economical and workable the use of a super plasticizer should be adopted.


68/68

BibliographyA.M., Neville. (1995). Properties of Concrete. Dorling Kindersley (India) Pvt. Ltd.

D C Teychenne, R. E. (1988). Design of normal concrete mixes. Department of the

Environment.

Construction Advantages-High Slump Concrete. (2009). Retrieved from

http://www.durisolbuild.com/cons_advhsc.shtml.

Paving expert-Concrete and Mortar-Slump. (2009). Retrieved from

http://www.pavingexpert.com/conc_slump.htm.

Ramachandran, V. a. (1984). Super plasticizers. In Concrete admixtures

handbook:Properties ,Science and technology . Park Ridge,N .j.: Noyes Publications.

Transportation-FHWA, U. S. (2010, January 1). Super plasticizers. Retrieved from

http://www.fhwa.dot.gov/..suprplz.htm.

Wikipedia. (2008). Portland Cement . Retrieved from

http://en.wikipedia.org/wiki/Portland cement.

Wikipedia. (2008). Concrete. Retrieved from http://en.wikipedia.org/wiki/Concrete.

Documents

High Slump Concrete Mix Design 2