The Design of Fly-Ash Concretes (I. a. Smith)

For written discussion* Paper No. 6982

THE DESIGN OF FLY-ASH CONCRETES by

Iain A. Smith B.Sc., A.M.I.C.E. Department of Civil Engineering, University of Glasgow

SYNOPSIS This Paper describes a method for use in the design of concrete mixes to give

a required early strength and degree of workability when incorporating fly-ash. The usefulness of the method is shown by comparison with a large number of tests on fly-ash cement concretes and with analyses of previously published papers from external sources. It is shown that there is no need to accept a loss of early strength when using fly-ash in concrete.

NOTATION The following notation will be used throughout. When the word ‘cement’

is used it will mean cement only. Cement mixed with fly-ash will be termed ‘fly-ash cement’.

W C F A (WC>.

( W C ) ,

N

G K

weight of free water (lb) weight of cement (Ib) weight of fly-ash (Ib) weight of aggregates (lb) effective waterlcement ratio of a concrete with regard to its

strength. In a normal concrete (W/C). is equal to W/C. In a fly-ash cement concrete (W/C) , is the water/cement ratio required in a normal concrete to give it the same strength as the fly-ash cement concrete

water/cement ratio of any orthodox concrete which gives to that concrete a required degree of workability

the aggregate/cement ratio of the above orthodox concrete with waterlcement ratio ( W / O ,

the specific gravity of ash particles The cementing elliciency of an ash relative to cement as measured

by the effect of the ash on the ratio ( W / O . . In its efTect o n this ratio a weight F of ash will be equivalent to a weight KF of cement.

INTRODUCTION A POZZOLAN is a finely divided siliceous material which reacts with lime in the presence of water to give cementitious products. In modern times natural pozzolanic materials have been used for many years in conjunction with Portland-type cements. The cementing action of the pozzolan is believed to be dependent on reaction between it and lime liberated from the cement in its hydration. The residue from modern electricity generating stations, in which

~~

Written discussion should reach the Institution before 15 June, 1967. and will be ~~

published in or after October 1967. Contributions should not exceed 1200 words. 769

770 SMITH ON THE DESIGN OF FLY-ASH CONCRETES

the fuel used is pulverized coal dust, takes the form of extremely small particles which,are generally, though not always, spherical in shape and whose chemical constituents are those of the original clay minerals in the coal. This residue is called pulverized fuel ash or fly-ash. Approximately thirty years ago it was realized that fly-ash possessed properties similar to those of naturally occurring pozzolans and research was instituted to discover whether or not fly-ash actually behaved in the same way.

2. Research since that time has in general followed the methods used by the original investigators who found the strength of a control concrete and compared it with that of a similar concrete which differed from the first only in having a certain quantity of cement replaced by fly-ash. When this is done the result is always a lowering of early strength. This has been repeated so many times that sight has been lost of the fact that it is possible to redesign the fly-ash concrete to recover the loss of strength.

3. The two main factors which affect the strengths of similar normal concretes are the type of cement and the water/cement ratio. If two cements which differ in their rates of hardening are used in two concretes, then one concrete can be made to reach the same strength as the other at a given date by adjusting its water/cement ratio. It is possible to estimate the required alteration in the ratio of water to cement to give a certain change in strength by assuming that the relationship between water/cement ratio and strength has the form of the Abram’s Law curve.

4. It would be reasonable to apply the same method to mixtures of cement and fly-ash if concretes containing differing ratios of these were to reach equal strengths at the same ages. The relative usefulness of a fly-ash could then be judged by its effect on the curve of water/cement ratio against strength.

5. Some confusion exists about the practical use of fly-ash in concrete. This confusion is made evident when it is said that fly-ash can be used in concrete in three different ways-as a cement replacement, as an admixture or as a replacement for sand. These are, in effect, three different ways of looking at the same thing. The cementing effect of fly-ash in a concrete in which 20% of the original cement has been replaced by fly-ash will be the same as in a concrete in which an admixture of fly-ash is 25% of the weight of cement. If the change from cement to fly-ash-cement lowered the workability and the aggregate/cement ratio was reduced to compensate, then it might be argued that fly-ash was replacing sand. The attitude of mind is wrong in each of the three cases. A concrete containing fly-ash should be regarded as a new type of concrete and designed accordingly.

EFFECT OF FLY-ASH ON CONCRETE STRENGTH

6. Before a rational method of fly-ash concrete design could be found it was necessary to discover the effect which the fly-ash would have on the strength of concrete containing it. As strength is linked with water content a first series of tests was directed at discovering the effect of fly-ash on the ‘effective water/cement ratio’ of concrete containing it. This ratio is numeric- ally equal to the water/cement ratio of a cement concrete of the same strength, at the same age, as the fly-ash-cement concrete under consideration.

7. To give some basis for the comparison of test results with known properties of the ashes used, each ash was assumed to have a ‘cementing

SMITH ON THE DESIGN OF FLY-ASH CONCRETES 771

efficiency’ K , such that a weight F of fly-ash would be equivalent to a weight KF of cement. Thus for a concrete containing weights W of water, C of cement and F of fly-ash, the effective waterlcement ratio is

(WIG). = - = -

MATERIALS 8. The cement used in all the tests was Portland cement ranging in quality

from ordinary to rapid hardening. The aggregate used was combined from

CA = 124w C R = 9 6 6 0 C A / C R = 1.28

0 5 0 060 070 080 0.90 EFFECTIVE WATER/CEMENT RATIO (li’/C) I

2 3000 -

U 0.50 0.60 070 080 0.90 1.00

EFFECTIVE WATER/CEMENT RATIO (lV/C),

l 1 . -Strength =2500 .‘.(IY/C), =

0.704

f

f W F 4 0 O O K l Design S[. 4170

2 *--*--* 3ooo - - ~ Actual SI. 2750

W

U

t, Actual st. 2500

L Design X. 2150 2 200n 2 0.50 0.60 070

F4 --+ ---_- ---

3000 @OOVI 2000

3000 4000 U

EFFECTIVE WATER/CEMENT RATIO ( l V / C ) s DESIGN STRENGTH

FIG. 1 : EXAMPLES OF CONSTRUCTIONS, (a) CONSTRUCTION OF CONTROL LINE; (b) ESTIMATION OF EFFECTIVE WATER/CEMENT RATIO FROM CUBE STRENGTH; (c) CON- STRUCTION OF ACTUAL STRENGTHIDESIGN STRENGTH DIAGRAM

772 .WITH ON THE DESIGN OF FLY-ASH CONCRETES

washed sand and separate sizes of 3 in., 4 in., + in. and 4 in. crushed whinstone. The absorption of the combined materials was negligible.

9. The first series of tests on fly-ash concretes used one sample of ash from each of three generating stations. In the second series of tests up to three different samples were tested from each of a total of twenty-four generating stations throughout England and Wales.

FIRST SERIES OF TESTS

10. The tests in this series were intended to show whether or not the factor K would be useful in measuring the effect of the presence of fly-ash on the effective water/cement ratio of a concrete.

11. To calculate the value of K for each ash a control line of strength against water/cement ratio was first plotted from results of tests cm control concretes containing no ash. The abscissae of these lines are effective water/ cement ratios. (The effective water/cement ratio of a normal non-ash concrete is its actual water/cement ratio.) The ordinates of these lines are the Road Research Note No. 4 values1 for strength, each multiplied by a factor which makes the curve the best fit to the points plotted for the control concretes alone. This is shown in Fig. l(a).

12. The strengths reached by concretes containing ash were then used to find the effective water/cement ratios of these concretes against the control curve as shown in Fig. l(b). The factor K was then calculated using the effective water/cement ratio and the mix proportions using the relationship

13. Fig. l(c) shows examples of the construction used to find the positions of the test results in a diagram of actual strength against design strength. To construct this diagram the effective water/cement ratio of each concrete was calculated from ( W/C),= (W/C)[ 1/1+ (KF/C)] using a value of 0.25 for K , and the value of ( WjC), used to find ‘design strength’ on the control line.

14. The sources of the ashes, their specific surfaces and carbon contents are shown in Table 1 . This table also shows the average value of ‘K for each ash at seven days old. At the age of 28 days K was found to be virtually unchanged from the seven day value.

TABLE 1 : AVERACE VALUE OF K FOR EACH OF THE THREE ASHES AT SEVEN DAYS IN THE FIRST SERIES

Ash source

Brunswick . . Hackney . , .

Croydon

.surface* Carbon Specific

’ per cent 4300 3-75

5250 9.14

6050 14.76

No. of mixes

0.255 24

0264 , 1 13 I

0.266 1 10

* Sq. cm/gm by an air permeability method. ~~

SMITH O N THE DESIGN OF FL.Y-ASH CONCRETES 773

ANALYSES OF REPORTS IN PREVIOUSLY PUBLISHBD PAPERS

15. As can be seen in Table 1 the first series of tests showed that for three apparently different ashes the efficiency factors K were similar. This was quite unexpected and at first sight was not borne out by previously published work. To find out how much variation could be expected from many different ashes it was decided to analyse such of these earlier reports on the use of fly- ash in concrete as contained sufficient information on mix proportions.2-5

16. Using the data on control concretes, control lines were drawn in the same way as they were in the first series of tests.

17. From mix proportions given in the reports and using a rounded off value of 0.3 for K, the effective water/cement ratios of the fly-ash cement concretes were calculated and plotted against the strengths they attained. The results of these reductions can be seen in Figs 2-5.

18. References 2-5 were analysed in this way to give results on approximately fifty ashes with widely differing properties. The value of 0.3 for K can be seen to group the fly-ash-cement concrete results close or parallel to the control lines. In the cases in Figs 4(a) and 4(b) this is seen to happen even in the concretes in which fly-ash was originally considered to be a replacement for sand.

SECOND SERIES OF TEST5

19. The foregoing tests and analyses show the effect of fly-ash on concrete strength. The tests however were on only three British ashes and did not show the effect which fly-ash would have on concrete workability. For these reasons a second series of tests was carried out which included fly-ashes with a wide range of properties and was intended to test the validity of assumptions made in deriving a practical method for use in the design of fly-ash-cement concretes.

20. The aggregates used in this series were of the same type as those used in the first. Ash samples were received from the generating stations shown in Table 2.

21. Up to three separate samples, from batches obtained at different times, were received from each of these stations. The carbon contents and specific surfaces of most of these ashes were measured. Each ash was used in four concretes which were designed to cover the normal range of structural concretes. Cubes were cast from each mix for testing at the ages of 7 days, 28 days, 56 days, 3 months and 6 months for the first 27 ash samples. For later concretes, cubes for testing at 6 months were not cast. K was found for each ash in the same way as for the ashes in the first series.

22. Cylinders 6 in. diameter and 6 in. long were also cast and tested by splitting at 28 days old to enable an estimate to be made of the effect of the presence of fly ash on the tensile strength of concrete.

DESIGN METHOD FOR FLY-ASH CONCRETE

23. The results of the first series of tests show that a value of 0.25 for K might be a reasonable one on which to base a rule for the design of a fly-ash concrete to reach a desired strength.

24. To allow for the effect of fly-ash on concrete workability it was found to be reasonable to assume that the workabilities of a Ay-ash concrete and a


0.40 0.50 0.60

. EFFECTIVE WATER/CEMENT RATIO ( W / C ) I

FIG. 2: REDUCED RESULTS FROM DAVIS et aL2 TAKING K=0.30. CONCRETE ‘V’ CON-

DEVELOP PQZZOLANIC ACTION TAINED LIMESTONE DUST IN PLACE OF FLY-ASH. NOTE APPARENT FAILURE TO


1 I I

0.40 0 50 0.55

EFFECTIVE WATER/CEMENT RATIO ( W / L ) ~

FIG. 3 : REDUCED RESULTS FROM DAVIS et aL3 TAKING K=0.30

776 .WITH ON THE DESIGN OF FLY-ASH CONCRETES

__ l I 1 I I

(a1 -

500 - f S

r3 H

-

0' -I

Strengths at 7 d

0 I I I I I , I I 1.10 I .20 I .30 l -40 I .so I .60

EFFECTIVE WATEWEMENT RATIO ("/C)$

.+ Pm] 3;

2 " loo0 Concrol

Control

Strcngchr ac 7 d C

090 1.00 1.10 1 .20 1.30 I .40 1.50 1.60 I .70

Control

* D E

EFFECTIVE WATER/CEMENT RATIO ("/C) I

FIG. 4: REDUCED RESULTS FROM FREDERICK, TAKTNG K=0*30, (a) REDUCED RESULTS FROM FREDEXICKS TABLE 114, (b) REDUCED RESULTS FROM FREDEIUCKS TABLE Il l4

normal concrete would be the same if each had the same volume ratio of cement-sized particles to water and the same volume ratio of cement-sized particles plus water to total aggregate. This does not hold true for normal fine powders. That it is approximately true for fly-ash may be due to the spherical nature of the particles.

25. The above assumptions make it possible to design a fly-ash concrete which copies one property of each of two orthodox concretes at one and the same time-one concrete for strength and another for workability. The mix proportions which would be required in the orthodox concretes may be decided either from the designer's personal experience or from curves and tables such as those in Road Research Note No. 4.l

26. The basis of the design method is set out below.

(a) Orthodox concretes 27. A choice must initially be made of the orthodox concretes. The

final fly-ash concrete will be designed to reproduce the strength of one of these and the workability characteristics of the other.

(i) In the first concrete, strength considerations will require a water/ cement ratio of (WIG),.

(ii) In the second concrete, workability considerations will require an aggregate/cement ratio of N when the water/cement ratio is (W/C),. It should be noted here that the value chosen for (W/C), has an effect on the optimum quantity of ash in the fly-ash concrete. The ash quantity decreases with an increasing value of (W/C),.


For this reason (W/C), should be as low as possible. The suggested practical minimum value is 0.4 although the designer is free to choose any value.

(b) Fly-ash concretes

concrete. It will therefore have the same value of ( WjC),. 28. The fly-ash concrete must have the same strength as the first orthodox

(W/C), = i.e. W/C = (W/C),[l+ (KF/C)I . . W

= Actual ratio by weight of water to cement in the By-ash concrete.

29. For workability, the fly-ash concrete is to be designed to have the same

30. In order to have equal volume ratios of water to cement and water to volume ratios as the second orthodox concrete.

cement plus ash in the two concretes, then

where 3. I S is taken as the specific gravity of cement particles and G is the specific gravity of ash particles.

31. Substituting for WlC from equation (1) and rearranging gives the optimum ratio of F/C to be used in the By-ash concrete

32. In order to have equal volume ratios of aggregate to cement mortar and of aggregate to fly-ash cement mortar the aggregate/cement ratio of the fly-ash concrete is given by

A I C = 1 + 3.15( WIC), [1+(3.15F/GC)+(3.15W/C)]

But from equation (2)

i.e. AIC = N - WlC . . . . . . . .

(c) Design procedure 33. The procedure to be followed in the design of a fly-ash concrete is set

out below; weight ratios without subscripts are those pertaining to the fly-ash concrete.

(i) Select (W/C)., the water/cement ratio of an orthodox concrete of

(ii) From a knowledge of the aggregate characteristics and behaviour, the required strength.

77s SMITH ON THE DESIGN OF FLY-ASH CONCRETES

0.60 0.70 0.80

EFFECTIVE WATER/CEMENT RATIO (H'/C!)%

FIG. 5 : (a) REDUCED RESULTS FROM TIMMS AND GRIEB5-'PORTLAND CEMENT A'- TAKING K = 0.30


8000

7000

6000

i -

2

L 5000

f

B -1

I

lr + u

5 4000

U

m

z 2

3000

2000

I e82 I

, e81 \ . A I \

-

-

-

-

- Strengths a t 28 d

- Strengths a t 7 d

I I

I\ '

' '

e A 2

0.60 0.70 0.80

EFFECTIVE WATER/CEMENT RATIO (IV/C)%

FIG. 5 : (b) REDUCED RESULTS FROM TIMMS AND GRIEBS-' PORTLAND CEMENT TAKING K=0.30

B'-


TABLE 2 : AVERAGE VALUES OF K FOR ASH SAMPLES AT AGES TESTED

Ash source

Aherthaw . . Agecroft B . .

Agecroft C . .

Bold . . . . Canington . . Castle Donington

Conah's Quay .

Drakelow. . . Dunston B . . Elland. . . . Ferrybridge' , . Goldington . . Ham's Hall . .

High Marnham . Little Barford B . Littlebrook . . Meaford B . . Rosecote . . . Rye House . . Staythorpe . .

Skelton Grange ,

Stella North . .

Tir John . .

Uskmouth . ,

Unknown . ,

number Sample

1 I 2 3 l 2 3 1 2 1

2 1

2 1

3

2 1

l 2 3

2 1

3 1

2 1

3

2 1

3

2 1

3 1 2 1 2 1 2 1 2 3

2 1

2 1

3 1 2 3

2 1

3

2 1

3

2 1

2 1

7d

0. I78 0.214

0.117 0.335

0.168 0.358

0.259 0.272

0.195 0.326 0.415 0.262 0.320 0.101 n. 174 0.177 0.235

0 12J 0 246

0.635

- .. .

0.290 0.4YS 0.402 0.252 0.318

0. t 93 0.338

0.218 0.038 0.508 0.285 0. I n 9 0.254 0.277 0.331 0.219 0. I70 0.208 n. IIU 0.142 0.318 0.235 0.397 0. I 76 0.229 0. I83 0.135 0.292 0.181

- ....

0.21 1 0.344 0.253 0.264 0.242 0. I79 0.207 0.208 0.247 0.286

- -. .

Average K at age given

28d

0.252 0. I97 0.209 0.287 n. 102 0.233 0.168 0.294 0.140 0.217 0.192 0.143

0 119 0.235

0.203 n. 1417 .

0463 0 157 0.237 0. I32 0.796

0.260 0.257 0.201 0.148 0 372

. . .-

0.280'

0.34 1 0.095 0,108 0.485 0.348 0. I33 0.332 0,406 0.200 0.220 - 04lo7 0,217 0.248 0.297 0.349 0.186 0.307 0.103 0.228 0. I67 0.079 0.404 0.229 0.121 . .-. 0.312 0.287 0.207 0.202

0.2 I7 0.087

0.247 0.165

0.207

56d

0.185 0295

0.226 0.163

- 0.076 0.070

0.162

0.380 0.196 0.307 0.102 0.221

0.199 0.115

0.J28 0,072 0.233 0.070 0.205 0 348

0.297 0494

0,393 0.366

0.321 0.358 0.437 0.47 1 0- l85 O.lJ2 0.235 0.022 0.276 0.289 0.365

0.219 0.268 0.223 0.336 0.419 0,315 0.201 0.333

- 0,073 0.182

0.200 0.363 0. I97 0.165 0.308 0.294 0.268 - 0.304 0.204 0.198 0.305 0.334

3m

0.274 0.303 0.183 0.274 0.045 0.059

0.166 0.296

0.239 0,626

0.125 0 251

-0.019 0.257

0.1 18 0. I55 0.057

0. I16 0.032

0.283 0,306 0.493 0.422

0.395 0.238

0.437 0.495 0.328 0.473 0.163 0.6 I 8 0.236 0464

0.264 O.2R5

0.340 0. I67 0.362 0. I86

0.215 0.398

0.378 0.331 0.318 0.109 0.140 0.139 0. I29 0.406 0.301 0-275 0.427 0.785 0-310 0.142 0.145 0.295 0.2 I0 0.321 0.1 72

6m

For Ferrybridge material retained on a 100 mesh sieve was not counted as ash Each value of K ii the average from four mixes of differing strengths.


decide the aggregatelcement ratio, N , which in a normal concrete of waterlcement ratio (W/C), would give the required degree of workability.

(iii) Calculate the optimum ratio of fly-ash to cement in the fly-ash concrete

For most ashes the specific gravities lie between 1.9 and 2.3. It is suggested that G be taken as 2.1. Using this value and taking ( WjC), as 0.4 and K as 0.25 the optimum ratio becomes

F/C = ( WlC), - 0.4

0.6 - 0.25( WIC), (iv) Calculate W/C= (W/C).(1+0.25F]C). (v) Calculate for the fly-ash concrete

If (W/C), is taken as 0.4 as suggested in (iii) then

Where N is the aggregatelcement ratio of a normal concrete of water/ cement ratio of 0.4.

(vi) The fly-ash concrete mix proportions then are Water: fly-ash: cement: aggregate = W/C: FIC: 1 : A/C

(d ) Example 34. Design a concrete of medium workability with a mean strength of

3000 Ib/sq. in. at 28 days using ordinary Portland cement, incorporating fly-ash, and using a p in. irregular gravel graded to curve 3 of Road Note No. 4. (In the example Road Research Note No. 4 will be used only to show a com- mon basis. In fact any valid information on the behaviour of the cement and aggregate could be used.)

(i) From Road Note 4, Fig. 1, the specified strength requires a water/ cement ratio of 0.71, i.e. (W/C),=0.71.

(ii) For medium workability with an irregular aggregate of grading curve 3, the table in Road Note 4 shows that in a normal concrete of waterlcement ratio 0.4 the aggregate/cement ratio must be 3 3

i.e. with (WIG), = 0.4; N = 3.5

(iii)

(iv) W/C = (W/C).(1+0.25F/C) = 0.71.1.154 = 0.84

(vi) The mix proportions by weight are then: Water: fly-ash :cement: aggregate = 0.84: 0.735 :'l : 7.35.


TEST RESULTS

35. The above method of mix design was used throughout the second series of tests.

36. As in the first series a control line of water/cement ratio against strength was first obtained for each batch of cement from the results of tests on concretes containing no fly-ash (Fig. l(a)). From these lines the effective water/cement ratio of each fly-ash concrete was obtained and K for the ash calculated from this and the mix proportions (Fig. l(b)).

37. The control curves were also used to determine the design strength of the fly-ash concretes when the value K was assumed to be 0.25. With this value of K the effective water/cement ratio of each concrete was calculated from (W/C), = ( W/C)(l/l + KF/C) and used to obtain the design strength of the concrete from the curve (Fig. l(c)).

38. Examples of these operations are shown in Fig. 1, and the results are shown in Figs GlO.

I

m 8

1 1 -

4 0

I- c 1 Control concrete

L - 8 Fly-arh concrete

ZOO0 3000 4000 5000

DESIGN STRENGTH : LB/SQ. IN.

FIG. 6: ACTUAL STRENGTH/DESIGN STRENGTH OF CONCRETES, AGED 7 DAYS


39. For any one ash the values of K calculated at the ages of 7,28 and 56 days did not differ appreciably. These values of K were therefore averaged for each ash and an attempt made to find a reason for the difference in this quantity from ash to ash. No correlation could be made between the average efficiency of the ashes and a chemical or physical property. Table 3 shows ashes in order of decreasing efficiency together with the measured properties of each ash.

40. The average of all values of K at the respective ages are shown in Table 4.

41. Table 2 shows the value of K obtained from the average of four values at each age. The four values were obtained from four concretes which covered the normal range of concrete strength.

42. Tensile strengths of concretes were measured at 28 days by the splitting

7000

6000 z ;a S

E 2 5000 z + < I u t t,

2 2 4000

<

3000

1oOc

I -v. 1 ;F .* 1 :L .

. 4 Control concrete

Fly-ash concrete

3000 4000 5000 6000 7000 DESIGN STRENGTH : LB/SQ. IN.

FIG. 7: ACTUAL STRENGTHIDESIGN STRENGTH OF CONCRETES, AGED 28 DAYS


I

0 0 .

T-

m . . 7000 - 2

d t,

U

1 Control concrete

Fly-ash concrete

3000 e .

I I 1 I

4000 5000 6000 7000 8000 DESIGN STRENGTH : LE/SQ. IN.

FIG. 8 (above): ACTUAL STRENGTH/DESIGN STRENGTH OF CONCRETES, AGED 56 DAYS

FIG. 9 (top right): ACTUAL STRENGTH/DESIGN STRENGTH OF CONCRETES, AGED 3 MONTHS

FIG. 10 (right) : ACTUAL STRENGTHIDESIGN STRENGTH OF CONCRETES, AGED 6 MONTHS


I I I I -I

9000 -

-. m _1

e ' e

1 Control concrete

0 Fly-ash concrete

I 1 I 1 l I

4000 5000 6000 7000 8000 9000


7000

d t

4000 5000 6000 7000 B000 9000 10000


786 SMITH ON T H E DESIGN OF FLY-ASH CONCRETES

1 . . . . . . . . . . . . . . . l . . . . . . . . . . . . . .


TABLE 4: AVERAGE VALUE OF K FOR ALL SAMPLES AT ALL AGES TESTED

Age I l-day 0.268 0.352 0.256 0.23 1

K 1 0.225

6-rnths 3-mths 56-day 28-day

cylinder test. Tensile strength was then plotted against 28 day cube strength. The results are shown in Fig. 11 where it is apparent that the addition of fly- ash to a concrete does not affect the ratio of tensile to compressive strength.

CONCLUSION 43. The intention of the above investigations was to produce a rational

method of mix design by which trial mixes of fly-ash concretes could be produced with an accuracy equivalent to the accuracy obtained when applying Road Note No. 4 to the design of orthodox concretes. The design method does achieve this. 44. No figures of workability are given in the results of the tests. The

fly-ash-cement concretes did have ' placeabilities' approximately equal to those of their orthodox counterparts. The addition of fly-ash, however, does normally increase the cohesion of the fresh concrete and this change is made

700 t Tensile S t . = 10 %Comprerrive

0

y-. b e

/ P e 1 Control concrete

Fly-ash concrete 1 2000 3000 4000 5000 6000 7000

CUBE STRENGTH AT 28 d : LB/SQ. IN.

FIG. 11 :, TENSILE STRENGTH COMPAHED WITH COMPRESSIVE STRENGTH


apparent in the normal tests of concrete workability in such a way as to give misleading figures.

45. The mcthod of mix design given here was found to give the required values of strength and placeability to the fly-ash concretes. In particular, strength of fly-ash concretes is shown to depend only on the relative proportions o f ash, ccment and water. That this result was not influenced by any particular factor in the testing of the specimens can be seen by reference to

TABLE 5 A : COSTS AND SAVINGS PER CUBIC YARD OF CONCRETE WITH AGGREGATE AT 20.9 PER TON

28d strength

3000

Concrete Costs/cu. yd concrete type sldlings

\Cement/ Ash 1 Agg.

OPC+ PFA OPC only 27.0 nil 28.6

RHPCCPFA 1 ;;:! 1 1 Savinglcu. yd concrete Total

slrillings* saving* Cement1 Ash I Agg. I +4.2 -2.57 +2.9 +4.53 +4.9 - 1 -2.95 - 1 - 1 +2.9 f4.85 -

6000 OPC only

RHPC + PFA OPC+ PFA

40.7 nil 27.0

405 1 1-31 1 25.7 40.3 0.77 25.7 + O s 4 -077 + l e 3 +093

+0*2 - 1 - 1 - 1 - -1.31 + l a 3 +0.19

All savings relative to non-ash concrete with OPC.

TABLE 5B: COSTS AND SAVINGS PER CUBIC YARD OF CONCRETE WITH AGGREGATES AT 10s PER TON

28d strength

3000

Concrete type

OPC only OPC+ PFA RHPC + PFA

Cost/cu. yd concrete

--- Ash I Agg. Cement

27.0 nil 14.3 22.8 2.57 12.85 22.1

~ 2,95 12.85

~ shillings Savinglcu. yd concrete Total

sldlings* savmg*

6000 1 OPC+ PFA ~ 5; 1 :::5 1 +F40 1 -;77 1+&65 1 +F28 OPC only

RHPC+ PFA 40.5 12.85 +0.20 -1.31 +0.65 -046

All savings relative to non-ash concrete with OPC.

SMITH ON THE DESIGN OF FLY-ASH CONCRETES 789 Figs 2-5 which were plotted from data given in reports from other sources. It should be noted here that Frederick’s report4 is the original source of the belief in sand replacement and yet the efficiency method can be applied equally well to those concretes in which fly-ash was considered as replacing sand as in those where it replaced cement.

46. The design method given here can be used to obtain any normal desired degree of strength and workability. The cementing efficiency method is also suitable for evaluating the usefulness of any pozzolan or admixture intended for use in concrete.

41. No evidence has yet been produced to show a relationship between any chemical or physical property of a fly-ash and its cementing efficiency when used in concrete. Some ashes are better than others but the results suggest that a value of 0.25 for K would be suitable for use in preliminary design.

48. Methods of fly-ash concrete mix design which are at present in use rely on the adjustment of a known concrete mix. These adjustments usually consist of a certain proportional replacement of cement by fly-ash with or without other variations in water or aggregate content. It is unlikely that a fly-ash concrete prepared in this way will have volume relationships similar to those of a normal concrete and the design method is not particularly versatile. In any case an investigation of the mix proportions of these concretes will show the ash to have the same efficiency as in a more rationally designed concrete. What may at f i t appear to be a more economic concrete must only have been arrived at by a decrease in some quality of the fresh or finished material.

49. There are two main reasons for the use of fly-ash in concrete. One is that fly-ash allows the design of concrete to be extended to lower strength’ concretes which would otherwise segregate. The other reason is that the use of fly-ash results in a lowered cost of materials in the finished concrete. Tables 5a and 5b show the savings which can be expected per cubic yard of finished fly-ash concrete relntivc to normal concrete of equal strength and equal, medium, workability. These Tables have been constructed using the data of Road Research Note No. 4 on a 4 in. irregular aggrcgate to grading curve No. 3. The specific gravities of the materials have been taken as 3.15, 2.1 and 2.5 for cement, fly-ash and aggregate respectively. The costs taken for these materials are per ton OPC, 130s; RHPC, 150s; and fly-ash, 20s. There will be an increased saving in cost when the cement is of better quality than that shown in Fig. 1 of Road Research Note No. 4.

ACKNOWLEDGEMENTS 50. The work described in this Paper was carried out in the Department of

Civil Engineering at Glasgow University under Professor W. T. Marshall, Ph.D., M.I.C.E., M.1.Struct.E.

51. The tests were sponsored by the Central Electricity Generating Board to whom the Author is indebted for permission to publish this Paper.

52. The Author’s thanks are also due to the Generating Station Super- intendents who arranged the deliveries of fly-ash samples and Messrs J. Colernan and J. Thomson of the concrete laboratory staff at Glasgow University for their assistance throughout the series.


REFERENCES 1. ROAD RESEARCH LABORATORY. Design of concrete mixes. Road Research

2. DAVIS R. E. et al. Properties of cements and concretes containing fly-ash.

3. DAVIS R. E. et al. Weathering resistance of concretes containing fly-ash cements.

4. FREDERICK H. A. Application of fly-ash for lean concrete mixes. Proc. Am.

5. TIMMS A. G. and GMEB W. E. Use of fly-ash in concrete. Proc. Am. Soc. Test.

Road Note No. 4, H.M.S.O., London.

J. Am. Concr. Inst., 1937.33 (May-June) 577-612.

J. Am. Concr. Inst., 1941, 37 (Jan.) 281-296.

Soc. Test. Mat., 1944, 44, 810-820.

Mat., 1956,56, 1139-1160.

The Design of Fly-Ash Concretes (I. a. Smith)

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