pfa mix design paper

The International Journal of Cement Composites and Lightweight Concrete, Volume 5, Number 4 November 1983

Mix design and properties of concrete made from PFA coarse aggregates and sand R. N. Swamy* and G. H. Lambert¢

© Construction Press 1983

0262-5075/83/05450263/$02.00

*Department of Civil and Structural Engineering, University of Sheffield, Sheffield, England. +AIIott and Lomax, Consulting Engineers, Fairbairn House, Manchester, England.

SYNOPSIS Pulverised fuel ash (PFA) is the principal waste product obtained from coal burning power stations in the generation of electricity. Vast volumes of such PFA anticipated further in the next two decades would create serious disposal and environmental problems in many countries. One effective method of utilising these large volumes of PFA is to manufacture aggregates from them for plain and structural concrete. The paper presents extensive test data on the mix design, strength and elasticity properties of concrete made from PFA coarse aggregates and sand and having 28 day strengths of 20 to 60 N/mm 2. A mix design chart is given together with equations related to strength and elastic modulus. Stress-strain curves for such concrete for structural applications are also reported.

KEYWORDS Fly ash, waste utilization, lightweight aggregates, porosity, water absorption, concrete mix design, lightweight concretes, mechanical properties, elastic properties, strength of materials, failure, stress-strain diagrams, materials testing.

INTRODUCTION It is now being increasingly recognised that with the fastly depleting oil and gas reserves in the world, coal will probably remain as the major form of fuel for the generation of electricity for at least the next few decades. One of the inevitable side-effects of having coal fired power stations is the problem of disposal of the residues from the coal burnt, and this can create serious local as well as national economic and environmental issues. The major waste product obtained from pulverised coal burning power stations is pulverised fuel ash (PFA) - - a fine dust collected in cyclones and electrostatic precipitators from the flue gases of the furnaces.

From an engineering point of view the significant characteristic of these ashes is their high degree of variability - - in colour, in chemical composition, carbon content and in their physical properties such as fineness and density. A wide range of factors influence the chemical and physical characteristics of the ash - - the origin and quality of the coal, the degree of pulverisation, the design of the boiler units, the charging and firing techniques, and finally the process of collection, handling and storage of the ash. The most effective means of removing fly ash of all particle sizes down to 2/~m appears to be through electrostatic precipitators - - small-diameter multicyclones tend to be less efficient compared to the former or a combination of the two.

With the increasing use of coal, the volume of waste PFA is also likely to increase. In the UK alone, by the 1970s, the total PFA produced annually amounted to some 7.5 m tonnes of which about half was being utilised in construction. By 1980, about 12-15 m tonnes of PFA were being produced, with a stockpile of 250 m tonnes; the annual usage, however, has remained static at about 50% of the annual production.

Worldwide production annually of PFA by 1980 is

263

Mix design and properties of concrete made from PFA coarse aggregates and sand Swamy and Lamber~

estimated at about 180-200 m tonnes. Of these less than about 20% was used in concrete related materials such as cement manufacture, concrete and concrete products, and lightweight aggregates. It is estimated that by the turn of this century some 750 m tonnes of PFA will be produced annually world-wide.

A G G R E G A T E S FROM FLY ASH Apart from using fly ash as partial cement replacement and in the manufacture of concrete and concrete blocks, one major area of ash utilisation could be in the form of aggregates for concrete. Current annual production of coarse and fine aggregates from PFA is of the order of 600,000 m 3 in UK and 300,000 m 3 in the USA; this represents the utilisation of only a fraction of the total ash collected in the world. The estimated annual production of aggregates in the UK alone by the 1980s was of the order of 230 m tonnes of which all lightweight aggregates put together amounted to less than 1%. The world-wide annual production of cement by the year 2000 is estimated to be about 1800 m tonnes, which would require aggregates amounting to some 15 000 m tonnes. It can be easily seen that a major utilisation of PFA could be in the form of aggregates, and this is an area that would merit attention in many parts of the world, bearing in mind the rapid dwindling of sources of natural aggregates.

In the UK, aggregates from PFA are produced by the sintering process* [1 --4]. The sintering process produces aggregates the main chemical constituents of which are silica (about 30 to 60%) and alumina (about 15 to 30%). Because of this, fly ash aggregates, like brick, are chemically inert to most substances encountered in building construction, and in particular, are immune to alkali-aggregate reaction.

Like most manufactured aggregates, PFA aggregates are lightweight in nature, and have therefore low bulk density and specific gravity, low particle strength and high porosity [5,6]. The pore structure of aggregates made from sintered pulverised fuel ash, their absorption characteristics and relative density have been studied in detail and reported by the authors [7]. In spite of the low particle strength and high proportion of voids, it has been shown that concrete of structural quality and suitable for prestressing can be easily produced with aggregates manufactured from PFA [5,6].

For a number of reasons, the current tendency is to use the fines obtained from the PFA sintering process in the manufacture of concrete blocks, whilst the coarse aggregates are used with natural fines for structural concrete. There is little systematic information on the mix design and properties of PFA coarse aggregate- natural sand concrete. This paper reports a detailed study of such concrete, and presents a mix design chart for PFA coarse aggregate - - natural sand concrete for 28 day strengths of 20 to 60 N/mm 2. Comprehensive test data are also presented on strength, elasticity and stress-strain behaviour, and where appropriate, regression equations are suggested for prediction of strength and elastic modulus. *Trade name 'Lytag'.

EXPERIMENTAL P R ~ E

Materials. Ordinary portland cement, fly ash coarse aggregates and natural sand fines were used throughout this study. Although several batches of all three materials had to be used during the course of the investigation, the source of each material was constant throughout.

The cement used satisfied all the relevant specifications of BS12: [8]. The fly ash coarse aggregates were supplied in five batches and their grading analysis together with fineness modulus are shown in Table 1. The coarse aggregates were sampled and tested in accordance with BS 3681, Part 2, 1973 [9] and BS 3797, Part 2, 1976 [10] respectively. The aggregates satisfied the recommended grading limits [10], and had an average loose dry bulk density of 830 kg/m 3, and a maximum aggregate size of 14 mm.

The sand was sampled and tested according to BS 812, 1975 [111 and BS 882, 1973 [12]. The sand conformed to the grading limits for Zone 2 [12], and its fineness modulus ranged from 2.74-2.92. Its average loose dry bulk density was 1560 kg/m 3.

Table 1 Grading limits for fly ash coarse aggregates from batches 1 to 5.

Percentage passing (by weight)

Sieve 14 mm nominal size graded aggregate Batch Batch Batch Batch Batch (mm) BS 3797 I10i 1 2 3 4 5

20 100 100 100 100 100 100 14 95-100 99 99 99 99 99 10 50- 90 85 83 76 82 88 5 0- 15 9 8 9 8 3

Fineness modulus - - 5.93 5.97 6.05 5.97 5.99

Mixing procedure. Aggregates manufactured from fly ash are necessarily classified as lightweight and have a highly porous structure [7]. A major difference in producing concretes containing such lightweight porous aggregates from those of normal weight aggregates is that the former absorb considerably more water than conventional gravel or crushed rock aggregates. This factor needs to be considered both in designing the mixes and in making concrete containing such aggregates.

In order to have adequate control of the amount of water added to the mix, two pieces of data are necessary, namely, the absorption characteristics of the dry coarse aggregates and the moisture content of the stockpiled aggregate at the time of making the concrete.

The first of these i.e. the percentage absoption by dry weight and the relative density of the fly ash aggregates were determined according to BS 812: Part 2:1975 [11 ]. Samples from several batches were used

264

Mix design and properties of concrete made from PFA coarse aggregates and sand Swamy and Lambert

for both these tests, and the average absorption values after 30 sec, 30 rain and 24 hrs were 9.0%, 9.9% and 13.1% respectively [7]. Based on these data, an allowance of 12% by weight, of dry coarse aggregates, was made for water absorption by the fly ash aggregates. The average relative density of the aggregates based on oven dry, saturated surface dry and apparent conditions [11] were 1.56, 1.76 and 1.96 respectively [7].

The fly ash coarse aggregates were stockpiled in the open air, and used in the moisture condition in which they were found at the time of mixing. To allow for the absorption of the aggregates, the moisture content of several samples of the stockpiled aggregate was determined, immediately prior to use, by means of a 'speedy moisture tester'. The average of these values together with the 12% absorption value of the dry aggregates was then used to adjust the water added at the mixer.

The sand for the concrete was dried and left to cool prior to use so that its moisture content was practically nil for mix design purposes.

The concrete was mixed in a horizontal pan-type mixer. Several different methods of combining the constituents were tried, and the following method was found to produce the most homogeneous mix with the highly absorptive fly ash aggregates. Firstly the coarse aggregates were placed in the pan, and approximately one third of the mixing water added. The aggregates and water were then mixed for approximately one minute to allow the aggregates to absorb water. The cement and sand were then added and mixed for another 30 seconds. The remaining water was then added and the mixing continued for a further 90 seconds.

CONCRETE MIX DESIGN CONTAINING FLY ASH COARSE AGGREGATES In broad terms, the aim of mix design is to produce the most economical concrete mix that will have the required strength and workability characteristics to allow it to be easily placed and fully compacted. There is a wealth of information available on mix design procedures for dense concrete; that on lightweight concrete is not so extensive but nevertheless data are available for different types of aggregates [5,13-16].

The initial trial mixes for the 28 day cube strength of 20-60 N/mm 2 were carried out based on the information supplied by the manufacturers of fly ash aggregates [13] and the experience gained in the Department in previous research [5,6]. The first trial mixes had high effective water contents, namely 180 kg/m 3, which produced very high slump values and made mixes of lower cement contents prone to bleeding. When the time between mixing and placing was extended, as would occur on a site, much of the bleeding was, however, reduced. Based on these trials, an effective water content of 175 kg/m 3 was decided upon for all subsequent mixes used in this test programme.

TEST SERIES 1 Based on the initial trial mix results, concrete mixes,

having a slump of 75-100 mm and 28 day target strengths (dry cured) of 30, 45 and 60 N/mm 2 were designed (Table 2). Apart from investigating the strength characteristics of the concrete, this test series was also designed to examine four other factors: (i)the influence of different batches of aggregates on the target strength, (ii) the influence of internal or external vibration, (iii) the rate of strength development and (iv) the influence of initial moisture content of aggregates on compressive strength.

Table 2 Mix proportions for various concrete strengths

Dry weights per 28 day cubic metre Free Free target (kg) water water/ strength content cement (N/mm 2) O.P.C. Sand Lytag (kg) ratio

30 250 715 715 175 0.70 45 335 645 715 175 0.53 60 485 515 715 175 0.36

Mixes were then cast for each of these target strengths from all five batches of aggregates. All the test specimens were cast in steel moulds, covered with polythene sheet and left in an uncontrolled internal environment for about 24 hours before demoulding. Dry curing was adopted for this series to assess the effect of the extreme condition that is likely to occur on site namely, that of no curing after stripping the formwork.

The results of this series of tests are shown in Tables 3 and 4 and Figures 1 and 2. All the tabulated or plotted values are the average of at least three test specimens. Table 3 shows the average compressive strength for the three mixes cast using different batches of aggregates, and the strength values are seen to be reasonably consistent. The effect of the type of vibration on compressive strength is shown in Figure 1. Test cubes from the same mix were compacted using either a high frequency vibrating table or a 25 mm diameter poker vibrator. The results show no significant effect of the

Table 3 Average compressive strength of air-cured concrete made with various batches of fly ash coarse aggregates.

28 day Average compressive strength (N/mm 2) M ix target proportions strength Batch Batch Batch Batch Batch by weight (N/mm 2) 1 2 3 4 5

1:2.85:2.84 30 30.5 32.5 '32.1 31.9 33.0 1:1.94:2.14 40-45 39.6 46 .2 45 .4 46.2 47.0 1:1.06:1.47 60 58.0 - - 59.0 - - 60.0

265

Mix design and properties of concrete made from PFA coarse aggregates and sand Swamy and Lamber1

Table 4 Strength development of air cured concrete made from fly ash aggregates.

Strength as percentage of 28 day strength

Concrete strength N/ram 2 1 day 3 days 7 days 14 days

30 20 45 70 85 45 20 50 75 90

50

E

z 40 t-

c-

g3o -> c:~

~r) , - ~ 20

10

0 0

Figure 1

O

/ O

I

10

Line of equal / strength

S 28 day

? day

I I l I

20 30 L0 50

Compressive Strength ( N/mm 2 )

(Immersion vibrator)

Effect of method of compaction on the 7 and 28 day compressive strength of air cured cubes

60

J~

50

c.Oc~ E

~ z t_ m

E 3C o (D

20

method of compaction on compressive strength provided of course the compaction is thorough.

The strength development for the 30 N/ram 2 and - 45 N/mm 2 strength air cured concrete is shown in Table a / 4, as a percentage of the 28 day strength. The values ~ 8o shown are average values for several batches of ~ concrete made from the five batches of aggregate shown in Table 1. The strength development shown ~ 6o appears to be of the same order as for normal weight concrete. >~ L0

The effect of the initial moisture content of the fly ash coarse aggregates on the 28 day compressive strength of air cured cubes is shown in Figure 2. The ,S 2c results show that there is no significant variation in ~, compressive strength as a result of the initial moisture content of the aggregates. From the tests carried out in ~ 0

o2 this study, it appears that in practice it is better to ensure that the aggregates have some moisture content left in them prior to concreting. This may mean that aggregates kept dry have to be wetted before use.

10 .......

Figure 2

0

0

0

V g V

- - ~ v - - ~ -~ v

V___._V. V V q V V q

0 0 0 0 0 0 0

o o Oo °-e-°-° o- 0 - -

0 o ~ o o o

I I I I I I 2 7 8 9 10 11 1

Initial Aggregate Moisture Content (% by weight}

Effect of initial coarse aggregate moisture content on the 28 day compressive strength of air cured cubes.

WORKABILITY, CEMENT CONTENT AMB DIEI~ iTY The mixes were generally designed for a slump of 75-100 mm, although in practice the slumps were much higher and gave inconsistent results. These variations are thought to be due to the various factors influencing workabil i ty measurements, and in particular, the initial moisture content of the aggregates and the time lag between mixing and testing. The coarse aggregates made from fly ash in UK have a spherical particle shape which helps to increase workabil i ty but has a tendency to give mixes a harsh appearance, which of course is in no way harmful to the resulting concrete.

From the results of Series 1, the relationship between 28 day compressive strength and water- cement ratio, cement content and density were derived and are shown in Figures 3 to 5. The strength-total

Figure 3

o L y t a g - S a n d ( A u t h o r s )

~ . . . ~ A [ [ - L y t a g (13}

" x . o ~ # l r - - - S a n d - G r a v e [ (17) (Bes t f i t c u r v e l

C ~r ~rd ~ - ~ A-'V"'--~____C----~_o__ -

I F _ m I I 1~__ I 04 0 6 0 8 10 12 1L

T o t a l W a t e r / C e m e n t Rat io

Compressive strength - total water cement ratio relationship.

I

16

266


4. z ¢-

>

~a

Q

E o

,m

OD t~

60

50

/,0

30

20

lC

f

x-_, f " Foamed Slag-Sand (1/,) * e ~ ' ~ ----~e7 x Leca-Sand (16)

0 I I I I I

200 300 400 500 600 Cement Content ( kg/m 3 )

Figure 4 Influence of cement content on compressive strength of various lightweight aggregate concretes.

water-cement ratio relationship for the fly ash aggregates - - sand concrete (air cured)is compared with that of concrete containing fly ash coarse and fine aggregates [13] and normal sand and gravel concrete [17]. In general, for a given workability, the total water-cement ratio should decrease when lightweight fines are replaced by sand. In Figure 3, this apparent anomaly in water requirement is due to the fact that the slump of mixes containing fly ash coarse and fine aggregates was only about 40-50 mm compared to 75-100 mm for the fly ash coarse-sand mixes used in this study.

The compressive strength - - cement content relationship for fly ash aggregates is shown in Figure 4 and compared with other lightweight [13,14,16,18] and normal weight aggregates [19]. The cement content for

E 80

t -

~, 6C L-

E 0

2O o

£3

0;

Figure 5

o Lytag-Sand (Authors) A[[- Lytag (13)

/ / o

/ o/ ~ / o

0

/ /

I I I I A I

1500 1600 1?00 1800 1900 2000 Air Dried Density ( kg/m 3)

Relationship between compressive strength and density.

a given strength obviously depends on the type of aggregate used, and fly ash aggregates when used with natural sand appear to compare very well with concrete made from natural aggregates up to a strength of about 45 N/mmL

The differences in the cement contents of the mixes used in this study and those of Balendran [18] are due to the fact that the latter mixes had only a maximum coarse aggregate volume concentration of 50%.

The strength-density relationship is shown in Figure 5. When the concrete contains lightweight coarse aggregates and natural sand fines, the density is increased by about 15% compared to concrete containing both coarse and fine fly ash aggregates.

MIX DESIGN CHART From the results of test series 1 and further several tests, a mix design chart for concrete containing fly ash aggregates and natural sand has been developed and is shown in Figure 6. The chart is of simple graphical and tabular form and is based on the premise that for a given degree of workability both the coarse aggregate and water contents can be held relatively constant for a wide range of cement contents. Similar charts have been produced by other investigators [5,18,20,21].

The data shown in Figure 6 should provide a simple and adequate basis for initial mix design. Because of the high variability of fly ash not only between those obtained from different power stations but also between ashes from different countries, it is likely that the aggregates derived from them will also have different characteristics. It may therefore be necessary to produce such mix design charts for each brand of aggregates. The data provided in Figure 6 should give a good starting point for such mix design charts.

The cement content shown in Figure 6 is based on

267


Concrete Density Fresh 'Saturated

(kg/m3)at28days. Air Dried 16°C & 50%RH.

Total water / Cement Ratio (By weight ) "Free water/Cement Ratio ( By weight)

Materials

Cement : 0. P. C. Fines : Zone2 sand Coarse: Lytag 12ram

Notes

1. Air cured strength 2. Free water = Total water - 12 %

absorption, by weight of coarse aggregate

3. Effective water content = 175 kg/m 3

4.* Tote[ water to be added allowing 12% for aggregate absorption

(3. E o

o

2035 2015 2010 1980 2050 2040 2030 2025 1935 1875 1855 1.810 1935 1885 1650 1815 0.54 0-78 1-04 1.30 0.36 0.52 0.70 0.88

I I I

-60 ~ ° ~ Stump: ?5-100mm

-50 v ~ ~ o 28Day

' ~ ~ n 3 Day ~ o-.~ -20 ~ " - ~ V ~ v -

-10 ~ A I Day ~ D - - _

"Proportions for one cubic Cement metre of compacted concrete Fines

(kg) Coarse Water *

Figure 6

A -

I I I /,85 335 250 200 515 645 715 770 715 715 715 715 260 260 260 260

Mix design chart for air cured concrete containing fly ash coarse aggregates and natural sand.

strength considerations alone. For low strength mixes of 20 to 30 grade, higher cement contents may be required for durability.

STRENGTH CHARACTERISTICS OF FLY ASH AGGREGATE CONCRETE In order to assess the validity of the mix design chart shown in Figure 6 aswell as to establish the properties of this concrete, tests were carried out to determine compressive strength (100 mm cubes), flexural strength (100 x 100 x 500 mm prisms), tensile splitting strength (100 x 200mm cylinders), elastic properties and stress-strain behaviour in compression. Mix design was carried out according to Figure 6, and three curing regimes were generally used:

1. water at 22°C _ 3°C 2. uncontrolled internal environment 3. constant temperature and humidity conditions: 16°C

_ 0.5°C and 50% _+ 2% RH

TEST RESULTS AND DISCUSSION

Compre=~ve Sll=ength The effects of age and curing conditions on compressive strength are shown in Table 5. The data show that under both water curing and

constant temperature and humidity conditions, there isa continuous increase in strength with age. The maximum increase in strength expressed as a percentage of 28 day strength varied between 10 and 40% at about one year for water curing, and between 1 and 12% after two years under constant temperature and humidity conditions. Under uncontrolled internal environment, there was some loss in strength, ranging up to a maximum of 10%, the loss occurring after about 3 to 6 months. This slow retrogression of strength with time under continued dry curing is associated with shrinkage -- induced micro-cracks, and has also been observed by other investigators [18,22,23]. In some cases the strength loss was recovered, but these data confirm the need for prevention of moisture loss during the early life of a concrete member.

With synthetic lightweight aggregates such as those made from fly ash, the mode of failure depends upon both the cement content and therefore strength. As both these increased, the proportion of fractured aggregates increased under both wet and dry curing. At low strengths, aggregate-matrix bond failure predominated whilst at high strengths, aggregate fractures were predominant (Figure 7 ). This characteristic was observed not only in cubes but also in prisms and cylinders tested for flexural and tensile splitting strength respectively.

268


Table 5 Compressive strength development of fly ash coarse aggregate -sand concrete

Total Cement:Sand: water/ Compressive strength (N/mm 2)

Age Lytag ratio cement (days) (by weight) ratio Water Laboratory C.T.H.R.t

1:3.85:3.58 1.30 11.1 11.4 11.0 1:2.77:2.78": 1.04 14.0 13.3 - - 1:2.60:2.68 1.00 14.5 13,5 12.6 1:1.94:2.14 0.78 22.0 20.5 19.5 1:1.07:1.48 0.54 29.0 31.0 29.0

1:3.85:3.58 1.30 14.8 15.7 14.8 1:2.77:2.78 1.04 20.0 22.2 - - 1:2.60:2.68 1.00 24.0 22.0 19.6 1:1.94:2.14 0.78 30.5 29.5 28.2 1:1.07:1.48 0.54 46.5 46.5 45.0

14

1:3.85:3.58 1.30 17.7 19.3 18.5 1:2.77:2.78 1.04 27.5 28.0 - - 1:2.60:2.68 1.00 28.5 26.0 26.6 1:1.94:2.14 0.78 38.5 36.5 35.0 1:1.07:1.48 0.54 51.5 54.5 55.7

28

1:3.85:3.58 1.30 19.5 22.5 22.0 1:2.77:2.78 1.04 32.0 33.1 - - 1:2.60:2.68 1.00 32.5 31.0 29.9 1:1.94:2.14 0.78 44.0 40.0 38.2 1:1.07:1.48 0.54 56.0 58.0 57.4

91 1:2.77:2.78 1.04 38.6 40.0 1:2.60:2.68 1.00 35.5 36.5 1:1.94:2.14 0.78 45.0 39.5 1:1.07:1.48 0.54 60.0 56.5

B

B

m

182 1:2.77:2.78 1.04 42.6 35.6 1:2.60:2.68 1.00 39.0 41.0 1:1.94:2.14 0.78 48.5 38.0 1:1.07:1.48 0.54 61.5 56.5

m

m

365 1:2.77:2.78 1.04 44.3 36.3 1:2.60:2.68 1.00 40.0 29.5 1:1.94:2.14 0.78 50.5 38.5 1.1.07:1.48 0.54 62.0 56.0

m

m

766 1:2.77:2.78 1.04 44.0 ..... 37.0 ..... - - 1:2.60:2.68 1.00 40.5 31.0 33.4 1:1.94:2.14 0.78 50.0 39.0 38.5 1:1.07:1.48 0.54 61.0 54.0 58.2

"Batch 2 aggregate and cement. "*Tested at 635 days. tConstant temperature and humidity room.

Tensile Strength The effect of age and curing conditions on flexural strength and tensile splitting strength is shown in Tables 6 and 7. The Tables also show the development of strength as a proportion of 28 day strength and the relationship between tensile strength and compressive strength.

Both tensile strengths show similar trends - - strength generally increasing with age when wet cured, but showing a fall in strength under dry curing. The loss in strength due to moisture loss generally occurs at higher strengths, the loss being higher the greater the strength. The loss of strength is higher with flexural strength, and is invariably fully recovered subsequently by 28 days. The phenomenon of loss of tensile strength due to non-uniform moisture loss, and subsequent recovery when drying becomes more or less uniform is well established in literature [18,19,21,24,25].

Relationship of tensile strength to compressive strength Figures 8 and 9 show the variation of flexural strength and tensile splitting strength respectively with compressive strength for fly ash coarse aggregate-sand concrete. Regression analysis gives the following equations:

fMR = 0.90 f:oo,3 (wet) (r = 0.99) (1) fMR = 1.20 fco ° 26 (dry) (r = 0.65) (2) fsp = 0.30 fcu ° 0, (wet)(r = 0.95) (3) fsp = 0.54 fcu ° 4, (dry) (r = 0.90) (4)

where fMa = modulus of rupture f~p = tensile splitting strength fcu = cube compressive strength,

and r = correlation factor.

In view of the erratic effect of dry curing, it would seem advisable to use separate equations for each curing condition.

Modulus of rupture and tensile splitting strength can be correlated as follows:

fMR = 2.05 fsp o 6, (wet) (r = 0.95) (5) fMR = 1.84 fsp o s2 (dry) (r = 0.59) (6)

Elastic properties Static and dynamic modulus of elasticity as well as Poisson's ratio were determined, the first two according to BS 1881:19 [26]. Four concrete strengths (20-60 N/ram 2) and three curing conditions (as before) were investigated. The static modulus and Poisson's ratio were obtained from 100 × 100 x 300 mm prisms, and the dynamic modulus from 100 x 100 x 500 mm prisms. The tests were carried out at different ages up to 28 days. All the mixes were designed according to Figure 6, and all the test specimens fabricated and cured as before.

Table 8 shows the development of static and dynamic moduli, up to 28 days, and under different curing conditions. Both moduli increase with age and concrete strength; those for wet cured specimens are consistently higher than those for dry cured specimens, by about 5% for static modulus and by about 15% for dynamic modulus.

269


Figure 7 Aggregate fracture in cubes tested at 28 days.

Table 6 Flexural s t rength p roper t ies of f ly ash coarse aggregate - sand concrete.

28 day Percentage of air cured Modulus of rupture (4) (5__.~) 28 day strength

cube (N/mm 2) (3) (3) (%) strength Age (N/mm 2) (days) Water Laboratory C.T.H.R. ~ (%) (%) Water Laboratory Water Laboratory C.T.H.R, ~

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11 ) (12)

Modulus of rupture Compressi~,e strength

at age shown in col. 2 (%)

22.5 1 0.68 0.68 0.68 100 100 22 27 15 15 15 33.5 1 1.09 1.04 1.04 100 100 27 34 19 19 19 47.5 1 1.88 1.88 1.88 100 100 92 56 15 15 15 64.0 1 3.32 3.32 3.32 100 100 65 98 13 13 13

22.5 3 1.72 2.00 - - 116 - - 55 81 15 18 33.5 3 3.32 2.80 - - 121 - - 60 91 14 15 47.5 3 3.16 3.44 - - 109 - - 71 102 12 13 64.0 3 4.80 3.32 - - 69 - - 94 98 11 7

22.5 7 2.00 2.16 - - 108 - - 64 87 14 14 33.5 7 3.20 2.80 - - 88 - - 82 91 14 12 47.5 7 3.92 3.08 - - 79 - - 88 92 11 8 64.0 7 5.00 2.28 - - 46 - - 98 67 10 4

m

m

m

22.5 14 2.66 2.32 - - 87 - - 85 94 15 12 33.5 14 3.32 2.91 - - 88 - - 86 95 13 14 47.5 14 4.40 3.39 - - 77 - - 98 101 11 8 64.0 14 4.93 3.03 - - 61 - - 97 89 9 5

22.5 28 3.12 2.48 2.40 79 77 100 100 16 11 33.5 28 3.88 3.08 2.80 79 72 100 100 13 9 47.5 28 4.48 3.36 3.56 75 79 100 100 11 7 64.0 28 5.08 3.40 2.84 67 56 100 100 9 5

11 9 8 5

*Constant temperature and humidity room

270


Table 7 Tensi le sp l i t t ing strength properties of f ly ash coarse aggregate - sand concrete.

28 day Percentage of air cured Tensile splitt ing strength (4) (5) 28 day strength

cube (N/mm 2) (3) (3) (%) strength (N/mm ~)

Tensile splitt ing stren£1th Compressive strength

at age shown in col. 2 (%) Age

(days) Water LaboratoH C.T.H.R.;: (%) (%) Water Laboratow Water Laboratow C.T.H.R. ~

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

22.5 1 0.51 0.51 0.51 100 100 24 21 11 11 11 33.5 1 0.55 0.55 0.55 100 100 21 22 10 10 10 47.5 1 1.24 1.24 1.24 100 100 35 41 10 10 10 64.0 1 2.16 2.16 2.16 100 100 47 63 9 9 9

22.5 3 1.02 1.11 - - 109 - - 48 46 9 10 33.5 3 1.14 1.44 - - 126 - - 43 58 9 10 47.5 3 2.17 2.06 - - 95 - - 61 69 10 9 64.0 3 3.15 3.34 - - 106 - - 69 97 7 7

m

m

22.5 7 1.43 1.37 - - 96 - - 67 57 10 9 33.5 7 1.69 1.95 - - 115 - - 64 79 10 9 47.5 7 3.09 2.46 - - 80 - - 87 82 9 7 64.0 7 4.01 2.96 - - 74 - - 88 86 8 5

m

22.5 14 1.77 1.93 - - 109 - - 83 81 10 10 33.5 14 2.92 2.36 - - 81 - - 110 96 10 9 47.5 14 3.32 2.39 - - 72 - - 93 80 8 6 64.0 14 4.38 3.03 - - 69 - - 96 88 8 5

m

m

22.5 28 2.13 2.39 2.16 112 101 100 100 11 11 10 33.5 28 2.65 2.47 2.58 93 97 100 100 9 7 8 47.5 28 3.57 2.99 3.02 84 85 100 100 8 6 6 64.0 28 4.58 3.44 3.15 75 69 100 100 8 5 5

Constant temperature and humidity room

¢ '4

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1

Figure 8

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v Water cunng

and humidity

lb 3'0 s'0 6b 7b Compressive strength ( N/ram 2 I

28 day relationship between flexural strength and compressive strength.

- I c~ Z, E 4.

Z

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Equation (3)

~ g o "

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/ / o UncontrotLed internal.

/ / / ~ ControlLed temperature and humidity

I I I ~) I I I

10 20 30 A 50 50 70 Compressive strength (N/ram 2)

Figure 9 Tensile splitting -compressive strength relationship at 28 days,

271

Mix design and propertles of concrete made from PFA coarse aggregates and sand S vv,~_~m~, r~l~ L ~fr}:~;,.~ !

Table 8 Development of elastic moduli with age

28 day compressive static modulus at '3 rd cube strength strength Dynamic modulus

Age Wet Dry Water Laboratory C.T.H.R. ~ Water Laboratory C.T.H.R. (days) (N/mm 2) (N/mm 2) (kN/mm 2) (kN/mm ~) (kN/mm 2) (kN/mm 2) (kN/mm 2) (kN/mm 2)

14

28

19.5 22.5 . . . . . 11.0 11.0 10.5 29.5 33.5 . . . . 12.5 12.5 13.0 45.5 47.5 - - - - - - 16.0 16.5 16.0 59.0 64.0 . . . . 19.5 20.0 19.5

19.5 22.5 12.0 11.5 12.5 17.0 16.0 15.5 29.5 33.5 14.0 14.0 14.0 18.0 18.0 18.0 45.5 47.5 15.0 15.5 15.0 21.5 21.5 21.0 59.0 64.0 t 7.5 18.0 18.0 25.0 24.5 24.5

19.5 22.5 12.5 12.5 13.0 20.0 17.5 17.5 29.5 33.5 15.5 15.5 15.5 21.5 20.5 20.0 45.5 47.5 18.0 17.5 17.0 24.0 23.5 23.0 59.0 64.0 20.0 19.5 19.0 26.5 25.5 25.5

t9.5 22.5 14.5 14.0 15.0 21.5 18.0 18.5 29.5 33.5 17.0 16.5 16.5 23.0 21.5 21.0 45.5 47.5 19.0 18.5 19.0 25.5 24.0 23.5 59.0 64.0 21.0 20.0 20.5 27.5 26.0 26.0

19.5 22.5 15.5 14.0 16.0 22.5 18.0 18.5 29.5 33.5 18.0 17.5 17.5 24.5 21.5 21.5 45.5 47.5 20.0 19.0 19.5 26.0 24.0 24.0 59.0 64.0 22.0 21.5 21.5 28.5 26.0 26.0

Constant t empera tu re and humid i ty room

The fol lowing regression equat ions were obtained from an analysis of the data in Table 8, relating elastic moduli and cube strength.

Static: Es= 5.82 fcu o 32 (wet and dry) (r : 0.95) (7) Dynamic: ED= 13.34fcu°2°(wet ) ( r - 0 . 9 9 ) (8)

ED -- 6.57 fcu o 33 (dry) (r : 0.99) (9)

If all other available data on fly ash aggregate - sand concrete are considered [18,19], a single equation of suff icient accuracy can be developed as fol lows:

Static: Es = 6.84 foe° 28(wet and dry)(r - 0.79) (10) Dynamic: ED = 9.92 fcu ° 24 (wet and dry) (r = 0.77 ) (11)

It is wor th noting that equations (7) to (11) are independent of density. Al though many formulae currently being used relating elastic modulus to densi ty show good correlation wi th exper imental results, it is considered simpler to relate elastic modulus to compressive strength for a given type of l ightweight aggregate.

A comparison of the static modulus obtained in this study for fly ash coarse aggregate - sand concrete is made in Figure 10 wi th those recommended by CP l10 [27] for dense and l ightweight concrete as wel l as wi th

other available data [18, 19]. The results show that the values predicted by equation [7] for fly ash coarse aggregate - sand concrete is about 60% of that of dense concrete given in CP110 127 I. The Code values appear to over-est imate the values obtained in this study by some 10% on average. The use of natural sand as fine aggregate appears to enhance the elastic modulus by about 20 -25% compared to those when both the coarse and fines are derived from fly ash.

Relation between static and dynamic moduli The dynamic modulus test is easier to carry out compared to the static test (equipment being available), and the results more consistent. It is therefore often more convenient to est imate the static modulus from the dynamic modulus. Taking into consideration all the data shown in Table 8 as wel l as those of Balendran [18; we t curing], the fol lowing regression equation is suggested:

Es = 0.93 ED - 2.56 (r : 0.93) (12)

Poisson's Ratio The values of Poisson's ratio obtained in this study are shown in Table 9. These values do not appear to have any consistent relationship to compres-

272


3 8

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Figure 10

• Dense Concrete CP 110 • Lightweight Concrete CP 110 (D c=1875 kg/m 3 I o Lytag- sand (Authors)(Wet .Dry) v At[-Lytag (19}(Air) + Lyteg-sond (19)

Lytag or Tac[ite-sand (18) (Wet]

/ /

/

/0 / /

/ /

/ /e

/ /

/ /e /

/ /

/ /

/

AI -~ . -o Equation 7 o ~......~ . ~ ~ o " + ~

/ "O/ V ~ V

i JO i I ° i i 10 2 30 4 50 60

Compressive Strength (N/mm 2}

Compar ison of static m o d u l u s of elasticity with publ ished data.

I

70

sive strength. In general, the static Poisson's ratio for fly ash coarse aggregate -sand concrete lies between O. 16 and 0.21 i.e. 0.185 - 0.025.

Stress-strain behaviour A knowledge of the complete stress-strain behaviour of concrete is necessary for limit state design. This characteristic was obtained in this study for four different strengths from 75 x 75 x 300 mm prisms. The test specimens were water cured for 28 days, and the results presented are the average of three tests.

In order to obtain the descending part of the stress-strain curve, the constant rate loading testing machine was transformed into a constant rate of straining device by using the machine to deform an elastic member, the deformation of which was utilised to control the deformation of the test specimen. This technique has been used to obtain the complete stress-strain curve in compression, tension, modulus of rupture and splitting strength [28]. In this study a steel tube was used in parallel with the test specimen, similar to the technique adopted by Wang e t al. [29], the steel having a linear elastic stress-strain characteristic.

Figure 11 shows the test arrangement and instrumentation. The steel tube was calibrated before and after each test; from a knowledge of the total load, and the strains in the steel tube, the concrete stress-strain behaviour was evaluated.

The results of the tests are shown in Figure 12. Each curve is the average of three specimens. The maximum stress attained in the tests was sensibly constant at 84 -+ 2% of the cube strength.

Table 9 Static Poisson's ratio values at var ious ages.

28 day compressive strength (N/mm =) Static Poisson's ratio

Age (days) Water Laboratory Water Laboratory CTHR

14

28

19.5 22.5 0.101 0.116 0.109 29.5 33.5 0.074 0.111 0.117 45.5 47.5 0.119 0.122 0.116 59.0 64.0 0.145 0.161 0.169

19.5 22.5 0.117 0.152 0.142 29.5 33.5 0.100 0.133 0.147 45.5 47.5 0.109 0.152 0.139 59.0 64.0 0.142 0.196 0.193

19.5 22.5 0.146 0.157 0.152 29.5 33.5 0.140 0.162 0.197 45.5 47.5 0.142 0.157 0.190 59,0 64.0 0.148 0.207 0.200

19.5 22.5 0.210 0.181 0.212 29.5 33.5 0.159 0.168 0.199 45.5 47.5 0.157 0.161 0.192 59.0 64.0 0.181 0.180 0.202

m•[ Machine Head 2 [ ,///. / / / / , \ / / , , / / , , , / / / / //~. - - ~ - 1 5 m m

O.5mm H ] ~o:o '6~ I Plastic "J ~' o~8:

Padding ~ 25 or 4Omm

H .0','0. T .~ o. o, I

.:"~ ~:~ rJ/, Strain Oauge E

Mitd steel tube

Specimen

75x75x300 mm ~:~;, ..o:&. IJ F~eo ~.~.~.' rJ

mm--~ i.~.~.::. ~'.o:a. rl '1 / / / / " e / / 77 ~ " ~ - [ ' 1 5 m m

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Figure 11 Test a r r a n g e m e n t for stress-strain curves.

273

Mix design and properties of concrete made from PFA coarse aggregates and sand. Swamy and Lambert

4/-

/.0

36

32

~ 28 E

~ 20

16

12

8

0 0

Key

28 Day woter fcu cured strength (N/ram 2)

• 195 a 295 . /.55 o 59"0 81% fcu

83% fcu

83%

I I I I

1000 2000 3000 4000 Strain (m/m * 10 6)

I

5000

Figure 12 Stress-strain behaviour of fly ash coarse aggregate - sand concrete.

The ascending portions of the stress-strain curves shown in Figure 12 compared favourably with the tests on elastic moduli, and the calculated elastic moduli ranged from 14.0 kN/mm 2 to 22.0 kN/mm 2 for concrete strengths of 20-60 N/mm 2. These values compare well with those given in Table 8. The strain at maximum stress for the strengths shown in Figure 12 varied from 2250 to 3250 microstrains, compared to 2500-3500 microstrains reported for concrete containing coarse and fine fly ash aggregates (al l Lytag concrete) of 25-30 N/mm 2 strength [13]. Tests reported on Ameri- can lightweight concretes showed strains at maximum stress varying from 2750 to 3750 microstrains for strengths of 20--55 N/mm ~.

The strain capacity of the specimens tested and reported here is considered to be low by about 10% due partly to the capping material used and partly to the end effects of the specimen. The maximum compressive strains obtained from tests in flexure of reinforced concrete beams were higher and these will be reported in due course.

C O N C L U S I O N S The paper shows that one effective method of utilising the large volumes of waste PFA currently in stockpile and likely to occur in the future is to manufacture aggregates from them. In the tests reported here, coarse aggregates manufactured by sintering pulverised fuel ash (trade name Lytag) are used with natural fines. The mix design of such concrete is discussed and a mix design chart is presented for strengths of 20 to 60 N/mm 2 at 28 days. The paper reports extensive test data on compressive

strength, tensile strength, elasticity and stress strain behaviour. Equations are presented where appropriate relating strength and elasticity.

A C K N O W L E D G E M E N T The tests reported in the paper form part of a larger investigation based on a CASE project between the University of Sheffield and Lytag Ltd. The authors wish to express their thanks to the Science and Engineering Council for the CASE Award and to Lytag Ltd. for their support of the project.

R E F E R E N C E S 1. Kinniburgh, W., 'Lightweight aggregate from pulver-

ised fuel ash', Concrete i and Constructional Engineering, Vol. 51, No. 12, December 1956, pp. 571-4.

2. Hobbs, C., 'Building materials from putverised fuel ash', British Chemical Engineering, Vol. 4, No. 4, April 1959, pp. 212-6.

3. Orchard, D. F., 'Concrete technology- Volume 1 - Properties of materials', 4th Edition, Applied Science Publishers, Barking, Essex, 1979, pp. 487.

4. Short, A. and Kinniburgh, W., 'Lightweight concrete', Applied Science Publishers Ltd., Barking, Essex, Revised edition, 1968, pp. 368.

5. Swamy, R. N., 'Prestressed lightweight concrete,, Chapter 5 in Developments in Prestressed Concrete - 1, Edited by F. Sawko, Applied Science Publishers Ltd., Barking, Essex, 1978, pp. 149-91.

6. Swamy, R. N., Sittampalam, K., Theodorakopoulos, D., Ajibade, A. O. and Winata, R., 'Use of lightweight

274


aggregate concrete for structural applications', Advances in Concrete Slab Technology, Edited by R. K. Dhir and J. G. L. Munday, Pergamon Press, Oxford, 1980, pp. 40-8.

7. Swamy, R. N. and Lambert, G. H., 'The microstruc- ture of Lytag aggregate', International Journal of Cement Composities and Lightweight Concrete, Vol. 3 No. 4, November 1981, pp. 273-82.

8. British Standards Institution, Portland Cement (ordinary and rapid hardening), BS12: Part 2: 1971, British Standards Institution, London.

9. British Standards Institution, 'Methods of sampling and testing of lightweight aggregates for concrete', BS3681: Part 2: 1973, British Standards Institution, London.

10. British Standards Institution, 'Specification for lightweight aggregates for concrete', BS3797: Part 2:1976, British Standards Institution, London, 1976.

11. British Standards Institution, 'Methods of sampling and testing of mineral aggregates, sands and fillers', BS812: 1975, British Standards Institution, London.

12. British Standards Institution, 'Aggregates from natural sources for concrete (including granolithic)', BS882, 1201: Part 2: 1973, British Standards Institution, London.

13. Lytag Ltd., Private communication. 14. Owens, P. L., 'Lightweight concrete - - Develop-

ment of mixes suitable for structural applications', Proceedings, Symposium on Advances in Concrete, University of Birmingham, 1971.

15. American Concrete Institute, ACI Standard 211, 2-81, 'Standard practice for selecting proportions for structural lightweight concrete', ACI 211.2-81, American Concrete Institute, Detroit, 1981.

16. Lodon, F. D., 'Concrete mix design', Applied Science Publishers Ltd., Barking, Essex, 1982, pp. 198.

17. Teychenne, D. C., 'Lightweight aggregates: their properties and uses in concrete in the UK', Proceedings, First International Congress on Light- weight Concrete, Vol. 1, May 1968, Cement and Concrete Association, London, 1968, pp. 23-37.

18. Balendran, R. V., Private communication. 19. Teychenne, D. C., 'Structural concrete made with

lightweight aggregates', Concrete, Vol. 1, No. 4, April 1967, pp. 111-24.

20. Owens, P. L., 'Mix design charts for lightweight aggregate concrete', Proceedings, First International Congress on Lightweight Concrete, Vol. 2, May 1968, Cement and Concrete Association, London, 1970, pp. 24-25.

21. Bandyopadhyay, A. K., 'Material properties and behaviour of high early strength lightweight aggregate (Solite) concrete', Ph.D. Thesis, Univer- sity of Sheffield, 1974, pp. 376.

22. Brooks, J. J. and Neville, A. M., 'A comparison of creep, elasticity and strength of concrete in tension and compression', Magazine of Concrete Research, Vol. 29, No. 100, September 1977, pp. 131-41.

23. Price, W. H., 'Factors influencing concrete strength', Journal American Concrete Institute, Proceedings, Vol. 47, No. 6, February 1951, pp. 417-32.

24. Hanson, J. A., 'Tensile strength and diagonal tension resistance of structural lightweight concrete', Journal, American Concrete Institute, Proceedings Vol. 58, No. 1, July 1961, pp. 1-39.

25. Grieb, W. E. and Warner, G., 'Comparison of splitting I tensile strength of concrete with flexural and

compressive strengths', Proceedings, American Society for Testing and Materials, Vol. 62, 1962, pp. 972-95.

26. British Standards Institution, 'Methods of testing concrete', BS1881: Parts 1 to 6: 1970, British Standards Institution, London.

27. British Standards Institution, 'The structural use of concrete, Part 1, design, materials and workman- ship', CP.110: 1972, British Standards Institution, London, pp. 154.

28. Swamy, R. N., 'The inelastic deformation of concrete', Highway Research Board, No. 324, January 1971, pp. 13-40.

29. Wang, P. T., Shah, S. P. and Naaman, A. E., 'Stress-strain curves of normal and lightweight concrete in compression', Journal, American Con- crete Institute, Proceedings, Vol. 75, No. 11, November 1978, pp. 603-11.

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pfa mix design paper

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