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Technology Digest 1

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Use of

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Concrete Society Technology D igest 1

Ref CS 133

ISBN 0 946691

82 7

0

Concrete Technology Unit, University of Dundee,

2002

Further copies of this publication, information about o ther Concrete Society publications and mem bership of The

Concrete Society may be obtained from:

The Concrete Society, Century House, Telford Avenue, Crowthorne, B erkshire RG4.5 6YS, UK

Tel: +44(0) 1344-466007, Fax: +44(0) 1344-466008, Email: consoc@ concrete.org.uk

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or

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or

by any means, without the prior permission of the copyright owner. Enquiries should be addressed

to The Con crete Society.

The recomm endations contained herein are intended only as

a

general guide and, before being used in connection

with any report or specification, they should be reviewed with regard to the full circumstances of such use.

Although every care has been taken in the preparation of this report, no liability for negligence or otherwise can

be accepted by T he Concrete Society, the mem bers of its working parties, its servants or agents.

Concrete Society publications are subject to revision from time to time and readers should ensure that they are

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Technology Digest 1

USE

OF FLY ASH

TO BS

EN

450 IN

STRUCTURAL CONCRETE

Ravindra K. Dhir

Michael

J.

McCarthy

Kevin

A.

Paine

Concrete Technology Unit, University

of

Dundee

ENVIRONMENT

TRANSPORT

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Use

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ash to BS EN

450


FOREWORD

This Techno logy Digest has been prepared as part of a technology transfer programme

undertaken at the Concrete Technology Unit (CTU) of the University of Dundee unde r

the Partners in Technology Programme

of

the Department of the Environment,

Transport and the Regions. It has been w ritten by the staffof the CTU.

The project was guided by a steering comm ittee representing the University and all

contributing partners.

STEERING COMMITTEE

University of Dundee

Professor

R K

Dhir, OBE (Chairman)

Dr M J M cCarthy

Dr K A Paine

Departmentof The E nvironment, Transport and the Regions

Dr

S B

Desai, OBE

Industrial Partners

Mr W Armstrong ScotAsh Ltd

Mr C Bennett ScotAsh Ltd

Mr P B rennan

Mr

R

Coombs

Professor T A H arrison

Mr

P

Livesey

Mr F McCarthy

TXU Europe Power Ltd (formerly Eastern Generation Ltd)

National Power plc

Quarry

Products Association

Castle Cement Ltd

Electricity Supply Board, R epublic of Ireland

The S teering Committee

is

grate hl to Dr L

K

A Sear and

Dr

G

R

Woolley for their

comm ents and contributions.

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Use ofjly ash to BS

EN

450 in structura l concrete

3

CONTENTS

1

Introduction

2

B S E N 4 5 0

3 Properties of concrete with BS EN 450

fly

ash

3.1

Fresh concrete properties

3.2

Strength development and a chieving equal strength

3.3 Engineering properties

3.4 Durability

4

Method s for using

BS

EN 450

fly

ash

4.1

Equivalent cement method

4.2 K-value method

4.3

Equivalent concrete performance method

4.4

Treatment with respect to ASR

5

Concrete production

5.1

Storage, handling and use

5.2

Effect of

*10%

fineness variation on strength

5.3

Loss-on-ignition

6

Case studies

6.1 Ratcliffe cooling tower strengthen ing

6.2 High Marnham substation

7

Availability

References

Appendix: Taking accoun t of

fly

ash characteristics in mix d esign

page 5

5

6

6

6

6

7

9

9

9

10

11

11

11

12

12

13

13

14

15

17

19

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ash

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BS EN

450 in

structural concrete

5

1 INTRODUCTION

The European standard BS EN

450 ly ashfor concrete- efinitions requirements and

quality

control

['I

covers a broader range of fly ashes as a cementitious comp onent for

use in concrete than previous UK standards. This Technology Digest brings together

current knowledge of the properties on fly ash to BS EN 450, and offers technical

guidance on how the m aterial should be used in concrete. Factors relating to concrete

production are also covered, and two case studies show how fly ash to BS EN 450 has

been used successfully.

2 BSEN450

The scope of BS EN

450

is fly ash produced by the co mbustion of hard coal, i.e. what

is known as pulverized-fuel ash (PFA) in the UK. Fly ashes produced from burning

other materials are not, at present, w ithin its scope.

The requiremen ts for fly ash to conform to

BS

EN 450 are shown in Table 1.

It should be noted that fly ashes conforming to BS

3892:

Part 1 I fall within the sco pe

of BS EN 450. The m ain difference between these standards

is

in the allowed range of

fineness (measured as percentage retained on a

45

pm sieve). BS

3892:

Part 1 allows fly

ashes of fineness up to only 12.0%, but this has been extended in BS EN 450 to 40 .

To control variability BS EN 450 does not allow the fineness to vary by more than

10 from the declared mean.

The perm itted loss-on-ignition (LOI) for fly ash also differs. BS

3892:

Part

1

has an

upper limit of

7.0

while BS EN

450

is based on an auto-controlled value. The standard

specifies

5.0

as the norm but permits values up to 7.0 on a national basis. The British

Standards Institution have proposed the use of the higher value in the UK. Due to

Table 1. Requirements

o r j l y

ash given in BS EN

450 [ and BS 3892: Part

Property BS EN 450 BS 3892: Part 1

Fineness, maximum

(

retained on 45pm) 40 12.0

Fineness variation

10.0% on mean

value

Loss-on-ignition

( 30)

Particle

density (kg/m3)

Chemical composition:

SO,, maximum ( )

Free CaO, maximum

Yo)

Total CaO, maximum ( )

Chloride,

maximum ( )

Moisture content, maximum ( )

Water requirement,

maximum

( )

Activity

index, minimum

( )

(ii')

Soundness (mm)

5.0 (7.0 on national basis)

auto-controlled

7.0 maximum

150on

mean value

5 2000

3

2

10

(sub-bituminous ashes) 10

0.1 0.1

0.5

95

80

(28

days)

10 0)

10

1.0

or

2.5"'

ii)

75

(28 days),

85

(90

days)

Notes: (i) Soundness

test

required only if free CaO exceeds I .

(ii) Fly ash to be stored and transported dry.

(iii) BS EN 450 uses 25 fly ash content by mass est cam ed out on equal water content basis,

whereas

BS 3892

uses

30

fly ash content by mass, est carried

out

on equal flow basis.

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Useofjly ash to

BS E N 4.50


seasona l variations in LO I, the auto-control value in BS EN

450

will be mo re difficult

to ach ieve than the absolute value in

BS

3892: Part 1.

Other differences from BS 3892: Part 1 are that BS EN 450 requires soundn ess to be

measured only w here the free calcium oxide lim it is not satisfied, and the activity index

(referre d to in

BS

3892

as the ‘strength factor’) should be determ ined on an equal water

content basis. Additionally, BS EN 450 has no water requirement. It should be noted that

with some coarser fly ashes, water reductions for equal workability might not occur.

BS EN 450 is being revised into a ‘harmonised’ form. This means it can be used as the

basis for CE marking. During this process, the activity index is being reviewed, and

considerationgiven to including fly ashes produced by co-combustion of coa l with other

fuels.

3 PROPERTIES OF CONCRETE WITH

BS

EN

450

FLY

-ASH

3.1 Fresh concrete properties

There are many reports on the physical and chemical influences of fly ash on the

properties of fresh and hydrating concrete. In general, fly ash inclusion in concrete

reduces water dem and, improves workability and reduces bleeding and se gregation. The

benefits associated with these effects enable water con tents to be lower and concrete to

be designed w ith reduced w ater/(cement

+

fly ash) ratios for equal workability [31.

Since

BS

EN 450 allows a wide range of fineness to be used, there is a p ossibility that

the beneficial effects of improved w orkability with finer fly ashes w ill reduce as the

coarseness of the fly ash component increases. Investigations into the relationship

between slump and fly ash properties at equal water contents have indicated a strong

correlation with fly ash fineness

41.

However, it has been shown that coarse fly ash

(fineness>30 ) can be used effectively n conjunction with water-reducingadmixtures

to achieve equivalent workability and strength to that of finer fly ash concrete[’]. This

need to include water-reducing admixtures may influence the econo mics of co ncrete

production.

3.2

Strength development and achieving equal strength

Test results have shown that use of coarser fly ashes leads to a reduction in com-

pressive strength for equal watedcement ratio. This effect (shown in F igure

1)

increases

with dec reasing water/(cement

+

fly ash) ratio. Gen erally, a

5

increase in

45

pm sieve

retention will lead to a strength reduction of between 0.4 and 1.5 N / m 2 or typical

cement + fly ash contents.

It has been show n that concretes of equal strength can be p roduced from fly ashes with

finenesses over the full range permitted in BS EN 450. Strength may be controlled by

adjusting the water/(cement

+

fly ash) ratio, changing the relative proportions of

constituent materials or co mbina tions of these.

Use of fly ashes with LOI up to 8.0 has been shown to have only a minor influence on

strength16] nd use of fly ash conform ing to the L OI requirem ents in BS EN 450 should

present no problem s with regard to strength.

3.3

Engineering properties

The effect of using fly ashes of differing fineness on the elastic modulus, creep

coefficient and ultimate shrinkage of co ncrete are compared in T able 2. In general, these

follow the expected behaviour in terms of the effect of the design strength on each

property. However, there are no significant effects of fly ash fineness on any of these

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3.4.2 Carbonation

The effect

of

different fineness and

LOI

of fly ash on carbonation rates appears to have

little influence for equal strength concrete. In stud ies 9-111 he m aximum difference in

depth of carbonation between concrete specimens containing different qualities of fly

ash has been found to be well within experimental variab ility. For practical purposes it

can be assumed that fly ash fineness and LOI have no influence on carbonation rates.

All fly ashes conforming to

BS

EN

450

are therefore suitable for exposure to

environm ents in which carbonation may occur.

Table

3. Comparison of aspects of durabilily for concrete containingfly ashes of digering

finenessLn1.

ly ash

content

=

30% by mass of cement +fly

ash.

Design strength

Fly ash

fineness

(N/mm2) 3.5 -.35.0

Durability property

Chloride diffusion (cm*/s

x

10-9)

i)

35

9.4 10.2

50 4.9 4.3

60 1.2

1.5

Carbonation depth

(mm) ( i i )

25 31.0 32.0

35 17.0 17.5

25 41 45

Sulfate resistance, expansion ( x io- iii) 35 28 28

50

0 0

35

51

48

Freeze-thaw durability factor ( ) (iv) 50 72 65

35 v i 0 98 97

Abrasion (mm) (* vi)

35 1.10 0.92

50

0.79

0.8

.

Notes: (i) Two compartment cell. (ii) 4.0% enriched CO,, 20°C, 55

RH 30

weeks). (iii) 6.0g/I MgSO,

1 84 days exposure). (iv) ASTM C666, Procedure A

[ I z 1 ,

(v) Modified BCA method [ I 3 ] (vi) Cured for

seven days wrapped

in

polythene , then air at 20 C,

55 RH

o 28 days. (vii) Air-entrained concrete.

3.4.3 Sulfate resistance

The use o f fly ash in concrete gives adequate resistance to the ettringite form o f su lfate

attack, which is generally considered to reflect the reduction in quantity of reactive

material present and the enhancement in the concrete microstructure

[I4].

Although

different levels of sulfate resistance may be obtained by using fly ash from different

s o~ r c e s [ ' ' ~ ,his is believed to be related to com positional rather than physica l effects of

fly ash['61.

Short-term tests have shown that fly ash concretes of similar strength have comparab le

resistance to sulfate attack, with concretes of strength greater than

50

N/mmz showing

little expansion[']. N ote that, where using fly ash as a com ponent of ce ment o r in com-

bination with a CEM I cem ent for sulfate resistance, BS 8500 I7] the complementary

UK

standard to

BS

EN

206-1 I R 1 ,

recommends a minimum fly ash content of

25%

by mass.

At present no guidance can be given on the use of

BS

EN

450

fly ashes to resist the

thaumasite form of sulfate attack.

3.4.4 Freeze-thaw resistance

The rate of deterioration

of

concrete under freeze-thaw conditions decreases with an

increase in concrete strength. For equal concrete grade there appears to be no influence

of fly ash fineness or LOI on the a bility of concrete specimens to resist freeze-thaw

attack

[6*'o,'91.

Although adequate performance in freeze-thawconditions can be achieved

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450


9

by increasing the comp ressive strength, the 'mo st 'efficient m eans o f enhancin g

performance is through the u se o f air-entraining admixtures (AEA ).

Irrespective of the fly ash fineness, the do sage of AEA required to obta in the necessary

air content is approximately the same. However, for fly ash with high LOI, greater

dosages of AE A m ay be required than for fly ashes with low LOI. U sers of fly ash

should therefore pay particular attention to any variability in L OI and the sub sequent air

content and concrete strength to ensure that concrete freeze-thaw resistance is main-

tained (see Section

5.3).

3.4.5

Abrasion

resistance

There is evidence

[91 to

suggest that, when concrete is properly cured there is a minor

impro vemen t in the abrasion resistance when using fine ash

(3.5

and

13.5

retained

on a 45 pm sieve) over that obtained using c oarser ash (35 retained).

4

METHOD USING

BS

EN

458

FLY

ASH

4.1 Equivalent cement met

The introduction of the equivalent cement method in BS 3892: Part 1 in 1993 has

resulted in

it

becoming the most widely used method in the UK for controlling the

incorporation of fly ash into concrete, and it is used by suppliers to dem onstrate that a

Portland cemendfly ash combination from a defined source has the properties and

proportions required by a cement conforming to the equivalent cement standard. The

basis of the method is that any cemenvfly ash combination that conforms to the equiva-

lent cement stand ard will give adequate performance.

The procedure given in BS

3892:

Part

1 :

199712]

s included in BS

8500

I7]

the comple-

mentary British Standard

to

BS EN

206-

1

Ix1.

It should be noted that testing is carried

out at equal water contents so the rheological improvements brought about by the

inclusion o f fly ash are not recognised. The procedure is applicable to fly ashes con-

forming

to

BS

3892:

Part

1

and BS EN

450

with an LOI not greater than

7 .

Work has

demonstrated [* that the method is applicable

to

fly ashes conforming to BS EN 450, and

its use is recommended.

The equivalence procedure determines the range o f proportions over which the require-

ments of the ceme nt standard are satisfied. The producer is able

to

use any proportion

within this range, thus giving flex ibility

to

optimise the mix design, but proportions of

fly ash shall not exceed

55

of the combination. How ever, the applicable range varies

with the properties of the fly ash and cement.

4.2

K-value method

The k-value approach to using fly ash in concrete was proposed by Smith

[211,

and

assumes that fly ash is 'k' times as effective as an equal m ass of cement in the

developm ent of strength, engineering properties and durability resistance. T he 'effective

cement content' to be used in the calculation of minimum 'cement' content and maxi-

mum water/'cem ent' ratio is therefore calculated as

(c +

kf), where

c

is the actual

cement content an df is the fly ash content.

Any type of cemen t can be used, but the k-value concept is not applied when fly ash is

part of the cement. In this case, the fly ash is, in effect, taken as having a k-value of 1

.O

(equivalent cemen t method Section 4.1)

k-values can be calculated for many aspects of performance but it is usual to use them

for strength. Values of k for streng th between 0.1 and 0.8 [ ' I . 20-231 have been reported,

depending on fly ash fineness, LOI and content in the mix. Users of the k-value

approach usually limit the maximum quantity of fly ash that can be counted as

cementitious and restrict the amount by which the cement content can be reduced. A

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EN 450 in structura1 concrete

value of k

= 0.4

is given in BS EN 206-1 [ I for use with

BS

EN

450

fly ash up to a

maximum fly ash content of 25%, when used in combination with CEM 1-42.5 N

cement. (It is assumed that fly ash above this level acts as filler.) A value of 0.2 is

permitted for use with C EM 1-32.5 N cement.

The k-value method is simple to use, but

it

is questionable whether a single k-value of

0.4 is applicable to the full range of fly ashes permitted by

BS

EN 450. The achievement

of sufficient strength to give adequa te performance is therefore uncertain. A hr ther

problem is that k-values based on strength are not necessarily appropriate to many

aspects of durab ility performance, where the relative effectiveness of fly ash co mpared

with cement may be considerably different.

It has also been shown that where the minimum cement content controls the mix design,

the k-value m ethod leads to a significantly higher cement + fly ash content[241han that

obtained using o ther methods. This has significant implications for the co st of con crete

designed using the k-value approach.

Guidance on the methods for measuring the k-value and the problems associated with

this method will be given in a CEN report[251.

4.3 ance

metho

The equivalent concrete performance m ethod,

as

originally conceived, was intended to

permit m ix designs that incorporate non-standard materials, with the aim of produ cing

optimum performance from locally available materials, such as fly ash. The con crete is

specified by performance, usually in terms of d urability, and all concretes that mee t

these requirem ents, irrespective of constituents, are considered equivalent. There is no

requirement to ma tch the performance of a reference concrete. The shortcoming in this

approach is the lack of standardised durability tests on concrete. Therefore, any

performance tests have to be agreed on a project-by-project basis.

As a way forward, an alternative method of app lying the eq uivalent concrete perfor-

mance concep t for fly ash co ncrete is given in

BS

EN 206-1 [ I. This method permits

amendments to the requirements for minimum cement content and maximum

wa terkem ent ratio, if it can be proven that a conc rete made with a pa rticular fly ash and

cement has equivalentperformance to a reference concrete meeting the requirements for

the relevant exposu re condition. The equivalent performance sho uld be judged with

respect to the particular specification for which the concrete is intended, especially

environm ental actions and durability.

The reference concrete against which the fly ash concrete

is

assessed shou ld contain

cement to BS EN 197-11261and have constituents corresponding to the co mbination of

fly ash and cement.

BS EN 206-1 recom mends the following limits on the range of fly ashlcem ent com-

positions:

(i) the total amount of fly ash should be within the limits in BS EN 197-1 for

permitted types of cement

(ii) the sum of cem ent and fly ash should be at least equal to the minimum cement

content in

BS

EN 206-1

(iii) the water/(cement + fly ash) ratio should be no greater than the maximum

waterkem ent ratio in BS EN 206-1.

Tests have shown that equal performance for chloride ingress, abrasion resistance,.

freeze-thaw resistance and ca rbonation can be achieved when the above restrictions are

applied and the fly ash co ncrete has equal 28-day com pressive strength to the reference

concrete. The refore, for BS EN 206- 1 exposure classes XO, XC , XD and XF, no further

durability testing is necessary provided the concrete durability requirements contain a

requirement for compressive strength. This makes the equivalent concrete performance

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Use o jly ash to BS

EN

450 in structural concrete 11

i J

method easy to apply, as long as the effect of fly ash on strength is known, and reliable,

because the strength is checked.

The fineness of fly ash can affect the performance o f concrete and BS EN 450 permits

a greater range than BS

3892:

Part

1.

Because of this, there is a need for a concrete m ix

design method that can take in to account the variations in fineness. Such a mix design

method has been develop ed[61 n which equal strength is achieved for different fly ash

concretes by sim ple adjustmen t to the free water/(cement + fly ash) ratio. T his may be

attained by:

(i) maintaining the existing cement

+

fly ash content and adjusting the fr ee water

content

(ii) maintaining the existing free water content and adjus ting the cemen t + fly ash

content or

(iii) adjusting both the free water and cemen t + fly ash contents.

4.4 Treatment

with

respect

to

ASR

BRE Digest

330 271

offers guidance on minimising the risk of damaging alkali-silica

reaction (ASR) in new construction using different cementitious materials. T his requires

fly ash to conform to

BS

3892: Part 1

,

nd

so

does not cover the w ider range o f fly ash

fineness allowed by BS EN

450.

However, there is nothing in current European

standards or published data to support this restriction. Where the fly ashkement

comb ination is manufactured by inter-grinding the con stituents, the fineness limit is not

required s long as all other properties satisfy BS 3892: Part 1.

Digest 330 states that definitive guidance on fly ash to BS EN 450 cannot be given

because of lack of technical data. Where fly ash is used for purposes other than to

modify the risk of

ASR,

Digest

330

states that

it

should be treated in a man ner similar

to a BS 3892: Part 1 fly ash as if it were a com ponent of a com bination, for the purpose

of its contributions to the recommended maximum combination content, and to mix

alkali contents. Th us the use of BS EN 450 ly ash should be restricted to concrete not

at risk from deterioration due to

ASR,

either from its intended use or the use of non-

reactive aggregates, until data are available to confirm performance levels. However,

when ch ecking a con crete for its resistance

to ASR,

all BS EN

450

ly ashes should be

treated as being equally effective as those conforming to BS

3892:

Part

1.

A research

programme

is

in progress at the University of Dundee to confirm this aspect

of

performance.

5.1 Storage,

handUing and M S ~

Fly ash can be supplied in four forms:

Dry

This

is

how fly ash is supplied for most cement, concrete and specialist grout

applications.

Dry

ash is handled in a similar manner to Portland cement and oth er fine

powders. Storage is in sealed silos with the associated filtration and desiccation

equipment, o r in bags.

Conditioned

Water can be added to fly ash to facilitate compaction and handling. T he amount o f

water added isdetermine d by the end use. Conditioned fly ash is widely used in aerated

concrete blocks, grout and specialist fill applications.

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13

6

CASE STUDIES

Fly ash conforming to BS EN 450 has had little use in the UK and most work has

involved fly ash to BS 3892: Part 1 , with a tightly controlled fineness. Projects that have

used coarser fly ash were mostly completed before the introduction of BS

3892:

Part

in 1982 or carried out within th e electricity generating industry using ‘run of station’ fly

ash.

6.1 Ratclif fe cooling

tower

strengthening

Because of structural problems occurring in some cooling towers at power stations

belonging to the Cen tral Electricity G enerating Board in the late

1980s,

a strengthening

programm e was initiated. This involved add ing a man tle of concrete to the outside of

the existing shells. Woolley and Cabrera reported this work in 1991 2 ’ ] Data is available

for the work at Ratcliffe power station, Nottingham, between August 1989 and February

1990.

The concrete had a

28-day

characteristic strength of

30

N/mm2 but to enable early

stripping of the formwork, strength o f at least 7 N/mm2 at 24 hours was required. T he

mix design was developed from a plain concrete mix with the fly ash added a s a direct

substitution o f the cement and representing 35 of the cement + fly ash content. The

amoun t of sand was slightly reduced to achieve constant yield.

The mean fineness of the fly ash was 3 1.0 ,varying between 14.4 and 42.9 . This

is outside the variability permitted by BS EN

450 (*to )

but is tending towards the

finer end of the range.

The mean 28-day cube strength was 49.5 N/mm 2 with a standard deviation

of

3.6 N/mm2. This standard deviation is low despite the fly ash being m ore variable than

permitted by BS EN

450.

The plot o f strength against fly ash fineness in Figure

2

show s the scatter of results. A

trend can be seen tow ards lower strength concrete with increasing coarseness o f the fly

ash. From the diagram it

is

estimated that the concrete strength at 28 days varies by

about

4

N/mm2 over a

20

fineness range for the fly ash.

Major defect limit

- 6

0

10

1

20

30

40

FINENESS (

retained On.45 pm sieve)

Maior .defect

limit

k

50 60

Figure

2 .

Scatter of strength results with l y ash fineness for Ratcliffe cooling tower strength-

ening project. The major defect limits relate to the conformity criteria of f

5% in

BS EN

450.

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14

Use ofjly ash to BS EN 450 in structural concrete

The mean

LOI

recorded over the same seven-month period w as

4.4 ,

which gives an

upper limit of

6.1

based on the statistical factors in

BS

EN

450.

Two results were

above

7.0 ,

but below

9.0 .

This means that the fly ash supplied to the contrac t would

not meet the requirements of

BS

EN

450

unless, as proposed for the

UK,

the

7.0

imit

was adopted.

6mz MiqDh

~~Ormnh~oOn

MbSaSl~iOGU

High Marnham power station, near Newark in Nottinghamshire, was constructed

between 1956 and 1962; the 275 kV substation associated with the power station was

constructed in 1957-58. The substation bases were cast in PC concrete, except for a

limited number of control areas which contained fly ash. Unusually for the time, the

work was well-documented

[291

and the concrete was examined

25

years later and the

results published [301

The m aterials used were ordinary Portland cemen t and Trent Valley aggregates with a

maximum size of

38

mm . The fly ash was 'run of station' ash

from

North W ilford power

station in Nottingham. T he properties o f the cement and fly ash are shown in Table

5.

Table 5. Mean values fo r OPC an dfl y ash properties used in High

Marnham project.

~~~~ ~

Property

QPC

Fly ash

SiO, ( )

A1,0, ( )

Fe203

CaO ( )

20.8 46.7

7.2 27.8

3.3

9.5

61.7

.,

7.1

MgO ( )

2.3 3.8

Na,O ( )

0.9

0.9

K,O

( )

0.5 0.6

so,

( )

1.7 1.24

Loss-on-ignition( )

1.6 2.5

Specific surface area [Blaine] cm2/g) 2250 3260

Fineness( retained on 90

pm

sieve) 4.6 12.3

Soundness (mm)

3.1

Particle densitv (g/cm31 2.03

Th e cement was relatively coarse and had a high alkali content. Th e specific surface at

2250

cm 2/g was at the low er limit for Portland cement of the time, and would now be

considered as a controlled fineness cement. The alkali content was

1.22

sodium

equivalent (Na,O), which would be considered a high-alkali cemen t according to BRE

Digest

330[*'].

.

The fly ash was unusual in its chemistry: the calcium, magnesium and sulfate levels

were higher than are typically found, although they w ere within the lim its set in

BS

EN

450;

the alkalis were low

(1.26

sodium equivalent).

The specific surface value indicates that the fly ash was finer than the cement, the

opposite to what was sug gested by the

90

pm sieve retention. The fineness of fly ash is

currently expressed as the percentage retained on a

45

pm sieve, but a mean retention

on a 90 pm sieve of 12.3 s equivalent to a retention of around 25 on a 45 pm sieve.

This is above th e limit in BS 3892: Part 1 , but within the limits of BS EN 450. There are

no data for the variability in fineness of the fly ash, nor any indication of its control.

Details of the mixes are shown in Table 6. The mix design was based on Road N ote 4 [3'1,

with

a

minimum 28-day compressive strength of 2 N/mm 2. Road N ote 4 requires the

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Use ofj ly ash to BS EN 450 in structural concrete 15

minimum strength to be

60

of the mean,

so

the target mean strength was

35

Nlmm2.

The fly ash was added as a direct replacement of cement to represent 20% of the cement

+

fly ash content by mass. Beca use the density of fly ash

is

lower than that of cement,

the volum e of fines increases. However, n o correction was applied, nor was any adjust-

ment made to the water content, and hence a more workable mix was accepted .

Table

6

Concrete

mix

design used at High Marnham.

M aterial DroDertv Con trol mix Flv ash mix

Cement (kg/m3) 280

220

Fly ash (kg/m3)

Fine aggregate (kg/m3)

600 595

55

Coarse aggregate (kg/m3)

1390 1375

Water (kg/m3)

145 145

Waterlcement ratio

0.52 0.52

Nominal slump (mm) 25

40

The strengths are summarised in Table

7.

The compressive strength, expressed as

equivalent cube strength , used a facto r of 1.15 for all cores, whether they w ere obtained

from horizontal or vertical drilling. Th is would give conserv ative results overall.

The streng ths of the plain and fly ash con crete are similar after 25 years, although the

fly ash concrete had a marginally greater strength gain from on e year to 25 years.

The plain concrete was slightly more porous than the fly ash concrete, and had more

pores of 5 pm and above in diameter.

There wa s no evidence of carbonation o f the fly ash concrete, even close to the surface,

whereas carbo nation was observed up to 15 mm depth in the plain concrete. All bases

were set in the ground with upper surface s exposed

to

the atmosphere.

Although the alkali content of the concrete was high, no evidence of alkali-silica

reaction (ASR) was found[291.

This case study indicates that even with a simplistic mix design, durab le concrete could

be m ade using a relatively coarse fly ash.

Table 7. Summary of 2.5-year strengths at High Marnham[3n1.

Mean equivalent cube Indirect tensile

compressive strength strength

(N/mm2)

(N/mm2)

Control mix

66.5 4.10

Fly ash mix

69.0 4.20

7 AVAl lABlLlTY

The annua l UK production of coal fly ash is around 7 million tonnes, which is produced

by 18 coal-fired power stations. The geographical distribution of stations enables fly ash

to be supplied to all major cities and industrial centres.

Fly ash

is

produced

24

hours each day, throughout the year, with production varying

only with electricity demand. Most stations have the capability

to

offer stockpiled

conditioned fly ash or lagoon ash

to

the market alongside dry ash, ensuring adequate

supply throughout the year.

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16

Use ofj ly

ash

to BS EN 450

in

structural concrete

For a num ber of reasons there has been a move away from coal-fired generation of

electricity since privatisation of the industry. However, it is now accepted that there

is

a role for coal as a fuel in electricity generation for the foreseeable future, due to the

need to maintain a balanced approach to energy supplies.

There is an increasing emphasis placed by G overnment and the m arket on sustainable

developmen ts and waste minimisation. These objectives are met by the utilisation of

industrial by-products, like fly ash and reclaimedrecycled materials, which are of

particular benefit where there

is

a proven track record

of

use in the construction

industry.

Further information about availability of fly ash can be obtained from:

United Kingdom Q uality Ash A ssociation (UKQAA)

Regent House, Bath Avenue,

Wolverhampton, West Midlands WV1 4EG

Tel: +44 (0)1902 576586, Fax:

+44

(0)1902 576596

[email protected];www.ukqaa.org.uk

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Use

ofj ly ash

to BS

EN 450


17

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

BRITISH STANDA RDS INSTITUTION, BS EN 450: 1995, Fly ash fo r con-

crete. Definitions requiremen ts and quality control. 20pp.

BRITISH STA NDARD S INSTITUTION, BS 3892: Part 1: 1997,Pulverized-jiuel

ash. Specijication o r pulverized-fuel ash o r use with Portland cement. 22pp.

ELL IS, M.S.,

Compositional characterisation of

UK

PFA fo r use in concrete

University of Dundee Internal Report, 1984.

DHIR, R.K., HUBBAR D, F.H., MUN DAY , J.G.L. and JONES , M.R. Charac-

teristics of low -lime fly ashes significant to their use in concrete, Fly ash silica

firme and naturalpozzolans in

concrete. V.M. M alhotra (Ed.) American Concrete

Institute S P 91-33, 1986, pp.693-721.

CRIPW ELL, B. Research and developm ent: a review of PFA in the literature,

Concrete

Vol. 26, No. 3, MayIJune 1992, pp.21-26.

DHIR, R.K., M cCAR THY , M.J. and MAG EE, B.J. Impact of BS EN 450 PFA

on con crete construction in the UK, Construction and Building Materials Vol.

BRITISH STAN DAR DS INSTITUTION. BS 1881: Part 121: 1983. Testing

concrete. Method fo r determination of static m odulus in compression. 8pp.

THO MA S, M.D.A. Marine performance of PFA concrete,

Magazine

of

Concrete

Research Vol. 43, No . 156, 1991 , pp.171-185.

DHIR, R.K., McCA RTH Y, M.J. and M AGE E, B.J. Use of PFA to EN 450 in

structural concrete

Report CTU I 196, Concrete T echnology Unit, U niversity of

Dundee, 1996.

LEW AND OW SKI, R. Effect of different fly ash quan tities and qualities on the

properties of concrete, Betonwerk und Fertigteil-Technik Vol. 49, 1983. Part 1

:

No 1, January, pp.11-15; Part 2: No. 2, February , pp.105-111; Part 3:

No.

3,

March, pp.152-158.

SCHIESS L, P. and HAR DTL, R. Eflc ienc y offry ash in concrete: Evaluation of

ibac test results

Document N42, Institut E r Bauforschung, RWTH , Aachen.

1991.

12, NO. 1, 1998 , pp.59-74.

AMER ICAN SOCIETY FOR TESTING AND M ATERIALS. ASTM C666-97.

Standard test method fo r resistance of concrete to ra pid fiee zing and thawing.

Philadelphia. 6pp.

LAWRENCE, C.D., Sulphate attack on concrete, Magazine of Concrete

Research Vol. 42, No . 153, 1990 , pp.249-264.

DHIR, R.K., HEW LETT , P.C. and CHA N, Y.N. Near surface characteristics of

concrete: abrasion resistance.

Materials and Structures.

Vol. 24, No, 140, 1991.

MEH TA, P.K., E ffect of fly ash co mpo sition on sulfate resistance of cement,

Journal of the American Concrete Institute Proceedings Vol. 83, No. 6, 1986,

pp.994-1000.

TIKA LSK Y, P.J. and CAR RASQ UILLO , R.L. Influence of fly ash on the sulfate

resistance of concrete,

ACZM aterials Journal

Vol. 89, No. 1, 1992 , pp.69-75.

BRITISH STANDA RDS INSTITUTION, BS 8500, Concrete. Complementary

British Standard to BS EN

206-1. Part 1: 2001.

Method of specihing and

guidance fo r the specijier.

50pp. Part 2: 2002.

Specification fo r constituent

materials and concrete. 38pp.

pp. 12 1-128.

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20/24

18

Use

offly

ash to BS EN 450 in structura1 concrete

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

BRITISH STANDARDS INSTITUTION, BS EN 206- 1 2000,

Concrete. Speci-

fication performance p roduction and conformity.

74pp.

DHIR, R.K., McCARTHY, M.J. and PAINE, K.A. Technology transfer pro-

gramme

for

the use

of

PFA to EN

450

in structural concrete: Freezehhaw

resistance using CEN /TC

51

test method

Document No. EN 450/2 2, Concrete

Techno logy Unit, University of Dundee. 199 9.

DHIR, R.K., McCARTHY, M.J. and PAINE, K.A. Technology transfer

programme fo r the use of PFA to EN 450 in structural concrete Report No.

CTU/ 1400, Concrete Technology Unit, University of Dund ee. 2000.

SMITH, I.A. The design of fly ash concrete,

Proceedings of the Institution

of

Civil Engineers

Vol. 36, 1967 , pp.769-790.

WESCHE, K., SCHUBERT, P. and W EBER, J.W. Strength and durability of

concrete with coal fly-ash as an additive,

Betonwerk und Fertigteil-Technik

INTRON, Institute for Material and Environmental Research BV,

Fly ash as

addition to concrete CUR Report 144, AA Balkema Publishers, 19 92,9 9pp.

HARRISON, T.A. The effect of the k-value for fly ash on concrete mix pro-

portions, Proceedings of XIth European Ready Mixed C oncrete Congress June

1995, Istanbul. Turkey Ready Mixed . Concrete Association, Istanbul.

pp.409-4 18.

CEN, K-value orp ow der coalfly ash CEN Draft Committee Document, 1998.

BRITISH STANDARDS INSTITUTION, BS EN 197- 1 2000,

Cement. Compo-

sition speci3cations and conformity criteria

for

common cem ents.

5Opp.

BUILDING RESEARCH ESTABLISHMENT, BRE Digest 330: Part 2,

Alkali-

silica reaction in concrete: Detailed guidance o r new con struction

1999.

WO OLLEY , G.R. and CABRE RA, J.G. Early-age in-situ strength developm ent

of fly ash concrete in thin shells,

Blended cements in construction

ed. Swamy,

R.N., Elsevier Applied Sc ience , Barking , 1991 . pp. 166-1 78.

HOWELL, L.H.

Report o f pulverised fuel ash as a partial replacement o r

cement in normal works concrete

Central Electricity Generating Board, East

Midlands Division, 1958, I, 36 and

11,

37.

CABRERA, J.G. and WOOLLEY, G.R. A study of twenty-five year old

pulverised fuel ash concrete used in foundation structures,

Proceedings of

Institution of C ivil Engineer s

Part

2

Vol. 7 9, March 1985, pp.149-165.

ROAD RESEARCH LABORATORY, Design of concrete mixes HMSO,

London, 1950, Road Note 4,2 nd edition.

BUILDING RESEARCH ESTABLISHMENT. Design

of

normal concrete mixes.

BRE Report 33 1. Garston, 1997.

Vol. 50, NO. 6, 1984 . pp.367-374.

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21/24

Use ofjly ash to BS

EN

450

in

structural concrete

19

APPENDIX

Taking account of f ly ash characterist ics in mix design

Tests [see Section 4.31 have shown that a simple approach to applying the equivalent

concrete performance method with respect to chloride ingress, abrasion, freeze-thaw

resistance and carbonation is the achievement of equal 28-day compressive strength to

a su itable reference concrete. Thus, for

EN

206- 1 exposure classes XO, XC, XD and XF,

no hr th er durability testing is necessary provided the concrete durability requirements

include a m inimum cube strength.

Research on the effects of fly ash fineness [see Section 3.23 has dem onstrated that fly

ash with properties across the range of

BS

EN

450 requirem ents may influence concrete

cube strength.

A

method developed at the University of D un de eh ak es into account the

effects of fly ash characteristics on cube strength by simp le adjustment to the free

water/(cement

+

fly ash) ratio. T his a straightforward approach which is e asy to apply

in practice; the method is descr ibed below.

The adjustment in the w ater/(cement

+

fly ash) ratio to account for variations in fly ash

fineness over a range of typical concrete strengths is shown in Figu re

A-

1 for a fly ash

content of 30% by mass. The required adjustm ent in the water/(cement

+

fly ash) ratio

increases with cube strength, because of the increasingly significant effect of fly ash

fineness on cube strength as the cemen t

+

fly ash content increases[ .

The following two examples show the selection of w ater/(cement

+

fly ash) ratio for

two

fly ashes, fly ash

A

with fineness

5

(retained on 45pm sieve) and fly ash

B

with

fineness 35%.

70

60

50

40

30

20

10

0

10

I I

15 20 25

30

35 40 45

50

55 60 65

28-day

cube

strength, Nlrnrn'

-

-

w/(c+9

ratio cu

with fineness

of 5%

(45 pm sieve retention)

0.30 0.35

0.40

0.45

0.50 0.55

0 60 0.65 0.70 0 75

w/(c+f) ratio

Figure

A-1. Relationship between watedcement + fl y ash ratio (w/(c+fl)

and 28-day cube strength or jl y ash conforming with BS EN 450.

FIy ash content: 30% by mass.

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22/24

20

Use ofjly

ash

to BS EN

450


Examples

Requirement to m eet the exposure XC 1 for carbonation using

30

fly ash as cement

comp onent by mass (i.e. minimum streng th 25 N/mm2,maximum w/c

=

0.65, minimum

cement = 260 kg/m3), where a reference C25 PC concrete with a target strength o f

35

N/m m2 has been show n to perform adequately.

Fly

ash

A

Fly ash fineness “h etained on 45pm sieve) 5 (1)

Figure A-l(a ). Water/(cement

+

fly ash) ratio

0.49 (2)

Variation of fineness from 5 [(1)

- 5 ] /5

0 (3)

ratio per

5%

fineness variation 5.2

x

1 0 - ~ (4)

Target cube streng th of reference concrete 35 N/mm2

Figure A-l(b). Correction to water/(cement

+

fly ash)

Water/(cement

+

fly ash) ratio

(2)

-

(3)

x

(4)]

Therefore, assuming water content = 165 l/m3with a plasticizer in the mix, possible m ix

proportions are given in Table A-1. The aggregate proportions are calculated using

normal mix design methods, e.g. BRE Design of normal concrete mixes [321, in which

aggregate proportions are calculated by estimating) he wet density of concrete, and

proportioning the percentage o f fine aggregate to the required slump.

Table A-1

0.49

Design w/(c+f) Concrete mix proportions

(kg/m3)

strength Free PC Fly ash Aggregate

(N/mmz) water Fine

lOmm

20mm

35.0 0.49 165

235

100

j

680 405

810

Fly ash B

Fly ash fineness

(

retained o n 45pm sieve)

I ’

35 (1)

Figure A-l(a). Water/(cement

+

fly ash) ratio 0.49 (2)

Variation of fineness from

5 [(1)-5115 6 (3)

ratio per

5%

fineness variation

5.2

x

10-3

(4)

Target cube streng th of reference concrete

35 N/mm2

Figure A- 1 b). Correction to water/(cement + fly ash)

Water/(cement

+

fly ash) ratio

(2) - (3)

x

(4)]

Correspo nding mix proportions for fly ash B would be a s given in Table A-2. Aggregate

proportion s have minor adjustmen ts to maintain yield.

Note that the use of the coarser fly ash

B

requires a w/(c+f) ratio

0.03

lower than that

for the finer fly ash A. For the same free water content, this means an increase of 25

kg/m3 in the total cemen t

+

fly\ash content. Alternatively, the lower w /(c+f) ratio co uld

have been achieved by reducing the water content, or altering both the water content and

cement + fly ash content.

Table A-2

0.46

Design w/(c+f) Concrete mix proportions (kgjm’)

strength Free PC Fly ash Aggregate

(N/mm2)

water Fine lOmm 20mm

35.0 0.46 165 250 110 680 395 790

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The Concrete Society Technology Digest

C . E W D ~ V M W B r n r n r n ~ ” W1

Ravindra K. Dhir Michael J McCarthy Kevin

A.

Paine

Concrete Technology Unit University

of

Dundee

The European standard BS EN 450 Fly ash for concrete - definitions, requirements

and quality control covers a broader range of fly ashes as a cementitious component

for use in concrete than previous UK standards. This Technology Digest brings

together current knowledge of the properties on fly ash to BS EN 450, and offers

technical guidance on how the material should be used in concrete. Factors relating

to concrete production are also covered, and two case studies show how fly ash to

BS EN 450 has been used successfully.

This Technology Digest has been prepared as part of a technology transfer

programme undertaken at the Concrete Technology Unit of the University of Dundee

under the Partners in Technology Programme of the Department of the Environment,

Transport and the Regions.

The project was guided by a steering committee representing the University and all

contributing partners.

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Documents

Use of Pfa to Bs en 450