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Efficiency of fly ash in concrete
Cement & Concrete Composites 15 (1993) 223-229
Efficiency of Fly Ash in Concrete K. Ganesh Babu & G. Siva Nageswara Rao Ocean Engineering Centre, Indian Institute of Technology, Madras 600 036, India (Received 22 August 1992; accepted 21 June 1993)
Abstract
w/(c+f)
Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15-75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k ) has been evaluated at all percentages of replacements considering the general efficiency factor (ke) and the percentage efficiency factor (kp). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.
w/(c+ kJ)
Keywords: Fly ash, utilisation, efficiency, concrete construction, compressive strength, cement replacement, water-binder ratio. NOTATIONS C Co
f
k ke
kp w w/c O
Cement content of fly ash concrete Cement content of control concrete Fly ash content Overall efficiency factor of fly ash General efficiency factor of fly ash Percentage efficiency factor of fly ash Water content Water cement ratio of normal concrete
Water cementitious material ratio of fly ash concrete Water cementitious material ratio of fly ash concrete after correction with k e
w/( c+ kff + krf) = w/(c + kf) = W/C o
Aw
Aw l
Aw 2
Water cementitious material ratio of fly ash concrete equivalent to normal concrete after correction with k e and kp Difference between water cementitious materials ratio of fly ash concrete and normal concrete The effect of ke on water cementitious materials ratio The effect of kp on water cementitious materials ratio
INTRODUCTION The use of pozzolans in conjunction with lime for mortars and concretes has been in practice for many centuries. Fly ash as a pozzolanic material in cement concretes has been in vogue for many decades. With the increased use of coal in power production the amount of fly ash available assumed staggering proportions and its disposal particularly with the associated problems of environmental pollution has made it necessary to look for effective utilisation possibilities. Earlier researchers have adopted three different methods for the proportioning of fly ash concrete mixes: (i) a simple replacement method, by an equal mass or volume; (ii) modified replacement method, in which the amount of fly ash replaced is larger than the amount of cement reduced so as to get an equivalent 28 days strength (the additional fly ash component being adjusted in the aggregate pro-
223 Cement & Concrete Composites 0958-9465/94/$7.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain
224
K. Ganesh Babu, G. Siva Nageswara Rao
portions); or (iii) by rational methods, in which the pozzolanic activity of the fly ash is taken into account. The present state of the art regarding the use of fly ash in concrete was reported earlier. 1-3 It can be seen that in spite of the numerous earlier research efforts an exact quantitative understanding of the contribution of fly ash to the strength of the concrete was still elusive. However, it is recognised that the contribution of the fly ash is not a constant determined solely by its physical and chemical characteristics like cementitious compounds, fineness, etc., but can also vary depending on the nature of cement, water cement ratio, etc. It is also not known at present what factors maximise the fly ash contribution. 4 The objective of the present effort is to answer at least some of the questions raised earlier through a systematic evaluation of the available results. In this context, it is important to note that fly ash being a silicious material will impart strength to the concrete through its pozzolanic action, but the pozzolanic reaction being slow fly ash concretes may not be able to attain an equal strength as that of the control within 28 days. Furthermore the ashes of yester years, because of the process of burning coal and the collection procedures of a generally much lower efficiency, resulted in a larger fraction of the coarser particles apart from a larger proportion of unburnt coal (reflected through its loss on ignition). In contrast, the fly ashes available today are resulting from burning powdered/pulverised coals and from the improved collection systems like the electrostatic precipitators of higher efficiency. In view of the above, only the results of the investigations during the past about 10 years were chosen for an evaluation, s-~' All these fly ashes confirm to the minimum characteristics specified by ASTM C 618-89 for use as mineral admixtures in portland cement concrete.~ 7 Early efforts regarding the use of fly ash in concrete were mostly limited to replacements up to a maximum of 35%, and the latest ACI Committee recommendations I also limit the utilisation to the same extent. This may be due to the requirement of additional water for wetting the finer fly ash which in turn increases the water cement ratio resulting in a reduction of the strength. However, investigations in recent years 7,18 have shown that even with low cement contents, high volume fly ash concretes can provide an economical material with strengths reaching 60 MPa. Extensive laboratory investigations indicate that the optimum percentage of fly ash
may be in the range of 55-60%. However, these high strengths are achieved mostly by the utilisation of superplasticisers required for a substantial lowering of the water content and thus water/ cement ratio. The present paper is not considering this aspect of concretes with chemical admixtures but is an effort to evaluate only the effect of fly ash on normal concretes containing ordinary portland cements cured under normal conditions. Information available in literature with fly ash replacements ranging from 15 to 75% has been considered for an evaluation. Furthermore, different researchers have used specimens of varying sizes and shapes and all these have been converted to their equivalents for a cube of 15 cm size through accepted guidelines. ~9 At this juncture, it is important to emphasis once again that the specific variations in the composition of fly ashes used by different investigators has not been considered for evaluation. Also, other parameters like fineness or even the special production and curing procedures have not been included in the present study.
EFFICIENCY CONCEPTS The efficiency of fly ash is generally defined in terms of its strength characteristics with the control concrete as the reference. However, knowing the improvements in durability due to the additions of fly ash, it is well recognised that other characteristics like durability factors can also be used for such an evaluation, though the exact methodology of the durability test has to be clearly defined. Furthermore, it is possible to define more than one durability factor (sulphate, chloride, freeze-thaw, etc.) and it would be difficult to compare or specify such a factor in codal provisions. Also, it is accepted, in general, that the strength of concrete is a reasonable indicator of the durability for at least the normal concretes without any chemical modifications and the efficiency of fly ash concrete is always defined with respect to the strength of its control. The simple replacement and modified replacement methods were not found to be suitable for a general understanding of the behaviour of fly ash in concrete. The rational methods were expected to take into account the characteristic of fly ash which are known to influence the workability a n d strength characteristic of concrete. Smith 2° was one of the first to propose a factor known as cementing efficiency (k) such that a weight 'f' of
Efficiency offly ash in concrete fly ash would be equivalent to a weight 'kf' of cement. The strength and workability of this concrete with fly ash is comparable to that of the normal concrete with a water cement ratio of [w/ (c + k)9]. Based on the experimental investigations and the results available, the value of the cementing efficiency factor k was assessed to be 0.25. This method was seen to be insensitive to the cement used, curing conditions, etc., apart from the fact that it was not suitable for richer mixes. 21 Later investigations22 have shown that the fly ash efficiency factor was a minimum of 0.3 and this value was also used in the German Concrete Standard DIN 1045. However, a more recent evaluation through studies on concretes containing different cements and fly ashes 23 (fly ash contents up to 28% and with water/cement ratios varying between 0.5 and 0.65) has shown that a value of 0.5 is more appropriate for this efficiency factor. These studies also indicated that with increasing fly ash content the efficiency of the fly ash tends to diminish and that the efficiency of fly ash increases with decreasing water cement ratio. It was also observed that the significant differences in the properties of fly ashes used (particularly fineness) influenced the compressive strengths at 28 days only marginally. It was only at greater ages the effect of the differing pozzolanic reactivities of fly ashes were felt. The efficiency of fly ash also did not show much variation in the range 20-28% replacements adopted. One important contribution of this work was that it defined the consequent reduction in water cementitious materials ratio of fly ash concrete as compared to the water cement ratio of the reference concrete (A w concept) as
={w/(c+ kf)}-{w/(c+ f)} =(w/c)[1/{1 + k(f/c)}- 1/{1 + (f/c)}] The above formulation clearly shows that this reduction depends not only on the size of the efficiency factor (k), but depends additionally on the water cement ratio and more importantly the fly ash content of the concrete mix.
EVALUATION OF EFFICIENCY In principle the evaluation of efficiency was attempted by what was earlier discussed as the rational method. 20 The A w concept explained earlier was used in evaluating the efficiency of fly
100 ~
225
\
A & B - Fly ~ Concrete w.r.t, wl(c,f) C & O - Fly q~h Concrete wr.t. wl(c.ket) N - Control Concrete w.r.t, wit or Fly ash Concrete w.r.t, wl(c,kef,kpl)
\\
so
\ ~
%,
80
~\j~ Low Vcluene Fly ash \ / ~ NormGI Concrete ~r/~/~ - High Volume Fly ~
Concrete
ke
2O
0 0.20
Concrete
~
l
t
1
I
0.30
0.40 w/(c*~)
0.50
0.60
or w / ( c . k e f )
I or
I
0.70 0.80 wl(t°kel*kpf)
I
0.90
1.00
Fig. 1. Conceptual diagram showing the effect of efficiency factors.
ash. Figure 1 shows the relation between the 28 day compressive strengths of concrete and the water cementitious materials ratio for normal and fly ash replaced concretes. As can be seen, lower replacement levels (may be up to 20%) show strengths higher than the control concrete while replacements of higher magnitude result in concretes of lower strengths. ~ and 'B' are the two typical water cementitious material ratios of concretes at the higher and lower percentages of replacement, with that of the control concrete being at 'N' for the same strength. The method now tries to bring the [w/(c+f)] ratios nearer to that of the control concrete by applying the cementitious efficiency of fly ash (k). Now the figure is replotted to check whether a unique value of k can help in bringing both 'X and 'B' to 'N'. This means that the correction (Aw) required can be achieved by the unique cementitious efficiency (k) at all percentages of fly ash replacement. However, this was not possible as the percentage of replacements were far too different and the result was that while the high volume replacements approach the control values due to this correction the low volume replacements resulted in a slightly higher conservative estimate. For this reason the value of k which is generally applicable for all the replacement percentages is from henceforth defined as the general efficiency factor (ke). This means that the points ~ and 'B' now shift to their revised locations 'C' and 'D' due to the application of the general efficiency factor (ke) with the axis as [w/(c+ kJ)]. Thus the original points ~2 and 'B' have been shifted to 'C' and 'D', by a distance of A w~. The revised correction now required (Aw2) was considered to be the effect of the percentage replacement. To counteract this
K. Ganesh Babu, G. Siva Nageswara Rao
226
effect of the percentage replacement an additional factor kp has been evaluated for each percentage of replacement in a very similar way. These two corrections together will bring the points W and 'B' to 'N' so that the water/cement ratio of the control concrete and the water/cementitious material ratio [w/(c+kJ+kvf)] will all be the same for any particular strength.
RESULTS AND DISCUSSIONS
As already stated, the results of different investigators 5-16 were compiled from literature for concretes containing ordinary portland cement with fly ash replacements ranging from 0 to 75%. Table 1 presents the characteristics of fly ash and the different percentages of replacements utilised in the above studies. In general, it was seen that as the replacement percentage and the water cement ratios increase the strength of concrete decreases. However, it was also evident that while concretes above 70 MPa can be produced with replacements up to 25% fly ash, replacement of about 75% can still lead to concretes of 40 MPa through suitable adjustments in water/cement ratio and other concrete constituents. The variation of the 28 day compressive strength with the parameter [w/(c+f)] for all concretes considered with the different percentage of replacement were presented in Fig.2. The curves drawn were the best fits for each of the different
percentages of replacement. It can now be clearly seen that while some of the concretes with replacements up to 25% show strengths higher than the control, while the higher percentage replacements show a reduction corresponding to the percentage replaced and the [ w/(c+ f)]. At this stage, the authors studied the variation of compressive strength with different general efficiency factors (k~) ranging from 0.2 to 0.8. From these studies it was observed that at a general efficiency factor of 0.5 the compressive strengths of fly ash concretes come closest to the control as shown in Fig. 3 (with the A w~ correction applied). This resulted in the concretes containing replacements up to 35% showing strengths slightly higher than the corresponding control and
-~
10o ~"
*..*_*%.* 75 •
\,, k',, oN,.,
~ , ao e
\\
~o~.
o o
"...
" -.
..
0,30
FA
U " . I I J 67 • FA AA,,U~ 50 • FA '~,,:.~"~. 35 ~: FA
0.40
,~
0.50
oQooo
0 •
0,80
0,90
FA
~'-'~..~..
0,60
w/(c+O
0.70
.00
Fig. 2. Variation of compressive strength with w/(c + f).
Table 1. Details of the fly ash concretes evaluated in the present investigation
Serial number
Ref.
Year
1
5
2 3
6 7
1981 1982 1986
4 5 6 7 8 9 10 11 12
8 9 10 11 12 13 14 15 16
1989 1989 1984 1986 1986 1986 1986 1989 1990
Fly ash characteristics SiOe + AI,_O~+ Fe,O~
CaO
SO~
88.74 94.40 (a) 87.52 (b) 90"06 (c) 91'35 86"50 91"10 76-20 76"20 75-71 56"03 61.76 72"60 63"03
7.87 2.20 5.18 1"83 2.21 1.71 1.44 15"30 15.30 13-30 25-76 17.32 12.40 25.30
0.41 -0.76 0.57 0.65 0.83 0.16 0.30 0.30 1.16 3.55 3.05 0-80 2.98
Class C 50.0 5.0 6
Class F 70.0 5.0 6
ASTM 618-89 Requirements (Si z + AI203 + Fe203) min % SO3 Max % L1 Max %
L1
% Replacements studied
2'10 4.50 1"36 5"16 5"28 4.34 2'00 0"60 0.60 0-14 0"16 0"36 0-61 0"45
35 25 35 35 35 50, 15 67, 50, 35, 25, 15 75, 67, 50 75, 67, 50 35 75, 50 25 50 65, 35, 25
Efficiency offly ash in concrete
227
1.00
loo
I "4~, k")'~ el', ~ "~.~ \'~,L K~'~
'ao
*.*.lt*.J' 75 S I,I ~ l J 67 ~ ~ 50 • e.O..,l,~¢,35 ~ =-~.~m.~ 25 ~ ~.tx.l~ 15 ~ ..... O~
FA FA
2
28 Days
oolee
0,80
FA
FA FA FA
0,60
FA
u. 0.40 "~ 0.20 4O
8
~ -0.0o(
~ 20
~_-o.2o!
10 20 30 40 Percentage Replacement
~
60 70 "~...4L_.,____e
E~
-0.40
w/(c+k.f)
Fig. 5. Variation of k v with fly ash replacement.
Fig. 3. Variation of strengths after correcting for k~.
100
1.50
| i~
*..*-**J 75 ~ FA LIPIlul 67 • FA
a.
so
==
~,'J ~, ~ ~',~o
o.O.,0,¢e.935 • ~.q.apa 25 • ~.~.A.A 15 ~ a .... o ~
lllll
+ II 1 . 0 0 '
so
v
.i 0.
28 days
~1.25
FA FA FA FA
0.75 o
4C
o.5o
._.e o
"~
2C = o
0 % ...... 61~ . . . . . . . . . .0.40 . . . . . . . . . . . . . . . . .o.so . . . . . . . . . . . . . . . . .o.so . . . . . . . . . . . . . . . . .1.oo . . . . . . . . . . . . . . . . .1.2o .......... w/(a+k,f+kpf)
1.40
1.so
1.8o
2.0o
Fig. 4. Variation of strengths after correcting for ke and kp.
those containing replacements between 50 and 75% showing slightly lower strength with respect to the parameter [w/(c+ kd) ]. Thus it was felt that an average value of 0.5 can be assumed for the general efficiency factor (ke). This also agrees well with the observations of a recent study discussed earlier. 23 This means that instead of using an overall efficiency factor like the earlier researche r s 2°'22'23 the general efficiency factor was kept a constant for all the percentages of replacement. Now, the differences between the water cement ratio including the effect of general efficiency factor and that of control concrete was computed (Aw 2) for the individual mixes at various water cement ratios and the effect of percentage replacement was calculated through an additional percentage efficiency factor (kp). Considering the average of the kp values for the different percentage the variation of compressive strength with parameter w/(c+ kd+ krf ) was presented in Fig. 4. It can be clearly seen that this has resulted in a reasonably close agreement with the control con-
0.25
o o.oo o .......
~'~ ....... h'd ....... L¢6 ....... ~,'d ....... ~'~ ....... Percentage Replacement
~'d ....... V'~ ....... ~o
Fig. 6. Variation of overall efficiency with fly ash replacement.
crete strengths at all the percentages of replacement ranging from 15 to 75%. It was clearly observed during these evaluations that some of the specific parameters like curing conditions, addition of admixtures and in certain cases even the type of aggregates and modifications of their proportions have significant effect on the efficiency of fly ash. The variation of kp with the percentage replacement was presented in Fig. 5 and the variation of the overall efficiency factor k ( -- k e + kp) was presented in Fig. 6. It can now be seen that a replacement of about 10% will increase the cementitious efficiency up to about 1.25 times for the 28 day cube compressive strength and a replacement of around 20% will not alter the compressive strength of control concrete at the corresponding water/cement ratio. Replacements in excess of 20% will reduce the compressive strengths of the concrete continuously and it was seen that a 75% replacement resulted in a reduc-
228
K. Ganesh Babu, (7. Siva Nageswara Rao
tion of efficiency of the fly ash to about 35% (0.35 times). Thus, the proposed method clearly distinguishes the effect of fly ash replacement in terms of the general efficiency factor (ke) and the factor corresponding to the percentage replacement effect (kp). It was also shown that while the general efficiency factor (ke) was a constant (0.5) for the 28 day compressive strength for all percentages of replacements the factor representing the percentage replacements (kp) varies between 0.75 and -0.15 resulting in the overall efficiency factor k varying between 1.25 and 0.35 for replacement percentages of 10 to 75% of cement by an equivalent weight of fly ash respectively.
ke=0.5) will result in conservative values upto about 40% fly ash replacement as reported by earlier researchers. . The results clearly show that the strength of concrete decreases continuously from 100 to 35% at replacements varying from 20 to 75% respectively. . Finally, the method presented using both the general and percentage efficiency factors is able to predict quantitatively the strength variation of concrete at different percentages of fly ash replacements and at different water to ash cementitious materials ratios.
ACKNOWLEDGEMENTS CONCLUSIONS This study, reporting the efficiency of fly ashes in concrete containing ordinary portland cements and cured under normal conditions, had led to the following general conclusions. 1. It was seen that a unique value of the overall efficiency factor (k) will not be able to predict the strength behaviour of fly ash concretes at different percentages of replacement. However, in keeping with the recent studies reported, a general efficiency factor (ke) was defined for evaluating the 28 day cube compressive strength. Based on the results available from different investigators in recent years this was estimated to be 0.5 for fly ash in concrete at all percentages of replacement. 2. To evaluate exactly the overall efficiency of fly ash (k) the authors proposed further a percentage efficiency factor (kp) through which the general efficiency factor can be modified to achieve the total efficiency at any percentage of replacement. The values of the percentage efficiency factor were evaluated and presented for the various percentages of replacement. 3. The overall efficiency of fly ash at different percentages of replacements evaluating has shown that the maximum efficiency in terms of compressive strength can be achieved at around 10% replacement and at 20% replacement the strength is not very much affected. 4. It may also be said that estimating the strength by using an efficiency factor (k or
The authors gratefully acknowledge Professor P. Schiessl, Director, Institut fiir Bauforschung, RWTH, Aachen for the information made available and the detail discussions which helped in narrowing the study to specific aspects. In particular, it was his views that made us look for the pozzolanic efficiency at the different percentages of replacement resulting in the present general and percentage efficiency factors.
REFERENCES 1. ACI Committee 226, Use of fly ash in concrete. ACI Mater J., Sep-Oet (1987) 381-409. 2. Berry, E.E. & Malhotra, V.M., Fly ash in concrete, supplementary cementing materials for concrete, CANMET report, SP 86-8E, 1987, pp. 37-163. 3. Wesche, K. (ed.), Fly ash in concrete, state of the art report, RILEM TC 67-FAB, 1990. 4. Popovics, S., What do we know about the contribution of fly ash to the strength of concrete?, in Proc. Second Int. Conf on Fly Ash, Silica Fume, Slag and Natural Pozzolanas in Concrete, Vol.I, ACI SP-91, 1986, pp. 313-31. 5. Ravina, D., Efficient utilization of coarse and fine fly ash in precast concrete by incorporating thermal curing. A CI J., Sep-Oct ( 1981 ) 194-200. 6. Mehta, P.K. & Gjorv, O.E., Properties of portland cement concrete containing fly ash and condensed sifica-fume. Cement Concr. Res., 12 (1982) 587-95. 7. Cabrera, J.G., et aL, Evaluation of the properties of British pulverised fuel ashes and their influence on the strength of concrete, in Proc. Second Int. Conf. on Fly Ash, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 115-44. 8. Thomas, M.D.A., Matthews, J.D. & Haynes, C.A., The effect of curing on the strength and permeability of PFA concrete, in Proc. Third Int. Conf. on, Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete Vol. I, ACI SP-114, 1989, pp. 191-217. 9. Gopalan, M.K. & Haque, M.N., Mix design for optimal strength development of fly ash concrete. Cement Concr. Res., 19 (1989) 634-41.
Efficiency of fly ash in concrete
10. Haque, M.N., Langan, B.W. & Ward, M.A., High fly ash concretes. ACIJ., Jan-Feb (1984) 54-60. 11. Haque, M.N., Gopalan, M.K., Joshi, R.C. & Ward, M.A., Strength development of inadequately cured high fly ash content and structural concretes. Cement Concr. Res., 16 (1986) 363-72. 12. Hooton, R.D., Properties of a high-alkali lignite fly ash in concrete, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 333-46. 13. Tse E.W., Lee, D.Y. & Klaiber EW., Fatigue behaviour of concrete containing fly ash, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 273-90. 14. Gebler, H.S. & Klieger. P., Effect of fly ash on physical properties of concrete, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 1-50. 15. Nasser, K.W. & Lai, P.S.H., Creep and shrinkage of concrete containing 50% fly ash and superplasticizer, in Proc. Third CANMET/ACI Int. Conf. on Superplasticizers and Other Chemical Admixtures in Concrete, (Supplementary papers) 1989, pp. 86-105. 16. Naik, T.R. & Ramme, B.W., Effects of high-lime fly ash content on water demand, time of set and compressive
229
strength of concrete. A CI J., Nov-Dec ( 1990) 619-26. 17. ASTM C 618-89, Standard specifications for fly ash and raw or calcined natural pozzolana for use as a mineral admixture in portland cement concrete, Annual Book of ASTM Standards, Vol. 04.02, 1989, pp. 296-8. 18. Langley, W.S., Carette, G.G. & Malhotra, V.M., Structural concrete incorporating high volumes of ASTM Class F fly ash. ACIJ., Sep-Oct (1989) 507-14. 19. Neville, A.M., Properties of Concrete, 3rd Edition. ELBS and Pitman Publishing, 1981, pp. 557-63. 20. Smith, I.A., The design of fly ash concretes, in Proc., Inst. of Civil Engineers, London, 36(1967)769-90. 21. Munday, G.L.J., Ong, T.L. & Dhir, R.K., Mix proportioning of concrete with PFA: A critical review, in Proc. First Int. Conf. on Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, Vol. I, ACI SP-79, Detroit, 1983, pp. 267-88. 22. Wesche, K., Schubert, E & Weber, J.W., Zur Festigkeit und Dauerhaftigkeit von Beton bei Zusatz yon Steinkohlenflugasche, (Strength and durability of concrete with addition of fly ash), Betonwerk und Fertigteil-Technik, 50 (1984) 367-74. 23. Schiessl, P. & Hardtl, R., Efficiency of fly ash in concrete evaluation of ibac test results. Technical Report of Institut fiir Bauforschung, RWTH, Aachen, 1991.
Efficiency of Fly Ash in Concrete K. Ganesh Babu & G. Siva Nageswara Rao Ocean Engineering Centre, Indian Institute of Technology, Madras 600 036, India (Received 22 August 1992; accepted 21 June 1993)
Abstract
w/(c+f)
Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15-75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k ) has been evaluated at all percentages of replacements considering the general efficiency factor (ke) and the percentage efficiency factor (kp). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.
w/(c+ kJ)
Keywords: Fly ash, utilisation, efficiency, concrete construction, compressive strength, cement replacement, water-binder ratio. NOTATIONS C Co
f
k ke
kp w w/c O
Cement content of fly ash concrete Cement content of control concrete Fly ash content Overall efficiency factor of fly ash General efficiency factor of fly ash Percentage efficiency factor of fly ash Water content Water cement ratio of normal concrete
Water cementitious material ratio of fly ash concrete Water cementitious material ratio of fly ash concrete after correction with k e
w/( c+ kff + krf) = w/(c + kf) = W/C o
Aw
Aw l
Aw 2
Water cementitious material ratio of fly ash concrete equivalent to normal concrete after correction with k e and kp Difference between water cementitious materials ratio of fly ash concrete and normal concrete The effect of ke on water cementitious materials ratio The effect of kp on water cementitious materials ratio
INTRODUCTION The use of pozzolans in conjunction with lime for mortars and concretes has been in practice for many centuries. Fly ash as a pozzolanic material in cement concretes has been in vogue for many decades. With the increased use of coal in power production the amount of fly ash available assumed staggering proportions and its disposal particularly with the associated problems of environmental pollution has made it necessary to look for effective utilisation possibilities. Earlier researchers have adopted three different methods for the proportioning of fly ash concrete mixes: (i) a simple replacement method, by an equal mass or volume; (ii) modified replacement method, in which the amount of fly ash replaced is larger than the amount of cement reduced so as to get an equivalent 28 days strength (the additional fly ash component being adjusted in the aggregate pro-
223 Cement & Concrete Composites 0958-9465/94/$7.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain
224
K. Ganesh Babu, G. Siva Nageswara Rao
portions); or (iii) by rational methods, in which the pozzolanic activity of the fly ash is taken into account. The present state of the art regarding the use of fly ash in concrete was reported earlier. 1-3 It can be seen that in spite of the numerous earlier research efforts an exact quantitative understanding of the contribution of fly ash to the strength of the concrete was still elusive. However, it is recognised that the contribution of the fly ash is not a constant determined solely by its physical and chemical characteristics like cementitious compounds, fineness, etc., but can also vary depending on the nature of cement, water cement ratio, etc. It is also not known at present what factors maximise the fly ash contribution. 4 The objective of the present effort is to answer at least some of the questions raised earlier through a systematic evaluation of the available results. In this context, it is important to note that fly ash being a silicious material will impart strength to the concrete through its pozzolanic action, but the pozzolanic reaction being slow fly ash concretes may not be able to attain an equal strength as that of the control within 28 days. Furthermore the ashes of yester years, because of the process of burning coal and the collection procedures of a generally much lower efficiency, resulted in a larger fraction of the coarser particles apart from a larger proportion of unburnt coal (reflected through its loss on ignition). In contrast, the fly ashes available today are resulting from burning powdered/pulverised coals and from the improved collection systems like the electrostatic precipitators of higher efficiency. In view of the above, only the results of the investigations during the past about 10 years were chosen for an evaluation, s-~' All these fly ashes confirm to the minimum characteristics specified by ASTM C 618-89 for use as mineral admixtures in portland cement concrete.~ 7 Early efforts regarding the use of fly ash in concrete were mostly limited to replacements up to a maximum of 35%, and the latest ACI Committee recommendations I also limit the utilisation to the same extent. This may be due to the requirement of additional water for wetting the finer fly ash which in turn increases the water cement ratio resulting in a reduction of the strength. However, investigations in recent years 7,18 have shown that even with low cement contents, high volume fly ash concretes can provide an economical material with strengths reaching 60 MPa. Extensive laboratory investigations indicate that the optimum percentage of fly ash
may be in the range of 55-60%. However, these high strengths are achieved mostly by the utilisation of superplasticisers required for a substantial lowering of the water content and thus water/ cement ratio. The present paper is not considering this aspect of concretes with chemical admixtures but is an effort to evaluate only the effect of fly ash on normal concretes containing ordinary portland cements cured under normal conditions. Information available in literature with fly ash replacements ranging from 15 to 75% has been considered for an evaluation. Furthermore, different researchers have used specimens of varying sizes and shapes and all these have been converted to their equivalents for a cube of 15 cm size through accepted guidelines. ~9 At this juncture, it is important to emphasis once again that the specific variations in the composition of fly ashes used by different investigators has not been considered for evaluation. Also, other parameters like fineness or even the special production and curing procedures have not been included in the present study.
EFFICIENCY CONCEPTS The efficiency of fly ash is generally defined in terms of its strength characteristics with the control concrete as the reference. However, knowing the improvements in durability due to the additions of fly ash, it is well recognised that other characteristics like durability factors can also be used for such an evaluation, though the exact methodology of the durability test has to be clearly defined. Furthermore, it is possible to define more than one durability factor (sulphate, chloride, freeze-thaw, etc.) and it would be difficult to compare or specify such a factor in codal provisions. Also, it is accepted, in general, that the strength of concrete is a reasonable indicator of the durability for at least the normal concretes without any chemical modifications and the efficiency of fly ash concrete is always defined with respect to the strength of its control. The simple replacement and modified replacement methods were not found to be suitable for a general understanding of the behaviour of fly ash in concrete. The rational methods were expected to take into account the characteristic of fly ash which are known to influence the workability a n d strength characteristic of concrete. Smith 2° was one of the first to propose a factor known as cementing efficiency (k) such that a weight 'f' of
Efficiency offly ash in concrete fly ash would be equivalent to a weight 'kf' of cement. The strength and workability of this concrete with fly ash is comparable to that of the normal concrete with a water cement ratio of [w/ (c + k)9]. Based on the experimental investigations and the results available, the value of the cementing efficiency factor k was assessed to be 0.25. This method was seen to be insensitive to the cement used, curing conditions, etc., apart from the fact that it was not suitable for richer mixes. 21 Later investigations22 have shown that the fly ash efficiency factor was a minimum of 0.3 and this value was also used in the German Concrete Standard DIN 1045. However, a more recent evaluation through studies on concretes containing different cements and fly ashes 23 (fly ash contents up to 28% and with water/cement ratios varying between 0.5 and 0.65) has shown that a value of 0.5 is more appropriate for this efficiency factor. These studies also indicated that with increasing fly ash content the efficiency of the fly ash tends to diminish and that the efficiency of fly ash increases with decreasing water cement ratio. It was also observed that the significant differences in the properties of fly ashes used (particularly fineness) influenced the compressive strengths at 28 days only marginally. It was only at greater ages the effect of the differing pozzolanic reactivities of fly ashes were felt. The efficiency of fly ash also did not show much variation in the range 20-28% replacements adopted. One important contribution of this work was that it defined the consequent reduction in water cementitious materials ratio of fly ash concrete as compared to the water cement ratio of the reference concrete (A w concept) as
={w/(c+ kf)}-{w/(c+ f)} =(w/c)[1/{1 + k(f/c)}- 1/{1 + (f/c)}] The above formulation clearly shows that this reduction depends not only on the size of the efficiency factor (k), but depends additionally on the water cement ratio and more importantly the fly ash content of the concrete mix.
EVALUATION OF EFFICIENCY In principle the evaluation of efficiency was attempted by what was earlier discussed as the rational method. 20 The A w concept explained earlier was used in evaluating the efficiency of fly
100 ~
225
\
A & B - Fly ~ Concrete w.r.t, wl(c,f) C & O - Fly q~h Concrete wr.t. wl(c.ket) N - Control Concrete w.r.t, wit or Fly ash Concrete w.r.t, wl(c,kef,kpl)
\\
so
\ ~
%,
80
~\j~ Low Vcluene Fly ash \ / ~ NormGI Concrete ~r/~/~ - High Volume Fly ~
Concrete
ke
2O
0 0.20
Concrete
~
l
t
1
I
0.30
0.40 w/(c*~)
0.50
0.60
or w / ( c . k e f )
I or
I
0.70 0.80 wl(t°kel*kpf)
I
0.90
1.00
Fig. 1. Conceptual diagram showing the effect of efficiency factors.
ash. Figure 1 shows the relation between the 28 day compressive strengths of concrete and the water cementitious materials ratio for normal and fly ash replaced concretes. As can be seen, lower replacement levels (may be up to 20%) show strengths higher than the control concrete while replacements of higher magnitude result in concretes of lower strengths. ~ and 'B' are the two typical water cementitious material ratios of concretes at the higher and lower percentages of replacement, with that of the control concrete being at 'N' for the same strength. The method now tries to bring the [w/(c+f)] ratios nearer to that of the control concrete by applying the cementitious efficiency of fly ash (k). Now the figure is replotted to check whether a unique value of k can help in bringing both 'X and 'B' to 'N'. This means that the correction (Aw) required can be achieved by the unique cementitious efficiency (k) at all percentages of fly ash replacement. However, this was not possible as the percentage of replacements were far too different and the result was that while the high volume replacements approach the control values due to this correction the low volume replacements resulted in a slightly higher conservative estimate. For this reason the value of k which is generally applicable for all the replacement percentages is from henceforth defined as the general efficiency factor (ke). This means that the points ~ and 'B' now shift to their revised locations 'C' and 'D' due to the application of the general efficiency factor (ke) with the axis as [w/(c+ kJ)]. Thus the original points ~2 and 'B' have been shifted to 'C' and 'D', by a distance of A w~. The revised correction now required (Aw2) was considered to be the effect of the percentage replacement. To counteract this
K. Ganesh Babu, G. Siva Nageswara Rao
226
effect of the percentage replacement an additional factor kp has been evaluated for each percentage of replacement in a very similar way. These two corrections together will bring the points W and 'B' to 'N' so that the water/cement ratio of the control concrete and the water/cementitious material ratio [w/(c+kJ+kvf)] will all be the same for any particular strength.
RESULTS AND DISCUSSIONS
As already stated, the results of different investigators 5-16 were compiled from literature for concretes containing ordinary portland cement with fly ash replacements ranging from 0 to 75%. Table 1 presents the characteristics of fly ash and the different percentages of replacements utilised in the above studies. In general, it was seen that as the replacement percentage and the water cement ratios increase the strength of concrete decreases. However, it was also evident that while concretes above 70 MPa can be produced with replacements up to 25% fly ash, replacement of about 75% can still lead to concretes of 40 MPa through suitable adjustments in water/cement ratio and other concrete constituents. The variation of the 28 day compressive strength with the parameter [w/(c+f)] for all concretes considered with the different percentage of replacement were presented in Fig.2. The curves drawn were the best fits for each of the different
percentages of replacement. It can now be clearly seen that while some of the concretes with replacements up to 25% show strengths higher than the control, while the higher percentage replacements show a reduction corresponding to the percentage replaced and the [ w/(c+ f)]. At this stage, the authors studied the variation of compressive strength with different general efficiency factors (k~) ranging from 0.2 to 0.8. From these studies it was observed that at a general efficiency factor of 0.5 the compressive strengths of fly ash concretes come closest to the control as shown in Fig. 3 (with the A w~ correction applied). This resulted in the concretes containing replacements up to 35% showing strengths slightly higher than the corresponding control and
-~
10o ~"
*..*_*%.* 75 •
\,, k',, oN,.,
~ , ao e
\\
~o~.
o o
"...
" -.
..
0,30
FA
U " . I I J 67 • FA AA,,U~ 50 • FA '~,,:.~"~. 35 ~: FA
0.40
,~
0.50
oQooo
0 •
0,80
0,90
FA
~'-'~..~..
0,60
w/(c+O
0.70
.00
Fig. 2. Variation of compressive strength with w/(c + f).
Table 1. Details of the fly ash concretes evaluated in the present investigation
Serial number
Ref.
Year
1
5
2 3
6 7
1981 1982 1986
4 5 6 7 8 9 10 11 12
8 9 10 11 12 13 14 15 16
1989 1989 1984 1986 1986 1986 1986 1989 1990
Fly ash characteristics SiOe + AI,_O~+ Fe,O~
CaO
SO~
88.74 94.40 (a) 87.52 (b) 90"06 (c) 91'35 86"50 91"10 76-20 76"20 75-71 56"03 61.76 72"60 63"03
7.87 2.20 5.18 1"83 2.21 1.71 1.44 15"30 15.30 13-30 25-76 17.32 12.40 25.30
0.41 -0.76 0.57 0.65 0.83 0.16 0.30 0.30 1.16 3.55 3.05 0-80 2.98
Class C 50.0 5.0 6
Class F 70.0 5.0 6
ASTM 618-89 Requirements (Si z + AI203 + Fe203) min % SO3 Max % L1 Max %
L1
% Replacements studied
2'10 4.50 1"36 5"16 5"28 4.34 2'00 0"60 0.60 0-14 0"16 0"36 0-61 0"45
35 25 35 35 35 50, 15 67, 50, 35, 25, 15 75, 67, 50 75, 67, 50 35 75, 50 25 50 65, 35, 25
Efficiency offly ash in concrete
227
1.00
loo
I "4~, k")'~ el', ~ "~.~ \'~,L K~'~
'ao
*.*.lt*.J' 75 S I,I ~ l J 67 ~ ~ 50 • e.O..,l,~¢,35 ~ =-~.~m.~ 25 ~ ~.tx.l~ 15 ~ ..... O~
FA FA
2
28 Days
oolee
0,80
FA
FA FA FA
0,60
FA
u. 0.40 "~ 0.20 4O
8
~ -0.0o(
~ 20
~_-o.2o!
10 20 30 40 Percentage Replacement
~
60 70 "~...4L_.,____e
E~
-0.40
w/(c+k.f)
Fig. 5. Variation of k v with fly ash replacement.
Fig. 3. Variation of strengths after correcting for k~.
100
1.50
| i~
*..*-**J 75 ~ FA LIPIlul 67 • FA
a.
so
==
~,'J ~, ~ ~',~o
o.O.,0,¢e.935 • ~.q.apa 25 • ~.~.A.A 15 ~ a .... o ~
lllll
+ II 1 . 0 0 '
so
v
.i 0.
28 days
~1.25
FA FA FA FA
0.75 o
4C
o.5o
._.e o
"~
2C = o
0 % ...... 61~ . . . . . . . . . .0.40 . . . . . . . . . . . . . . . . .o.so . . . . . . . . . . . . . . . . .o.so . . . . . . . . . . . . . . . . .1.oo . . . . . . . . . . . . . . . . .1.2o .......... w/(a+k,f+kpf)
1.40
1.so
1.8o
2.0o
Fig. 4. Variation of strengths after correcting for ke and kp.
those containing replacements between 50 and 75% showing slightly lower strength with respect to the parameter [w/(c+ kd) ]. Thus it was felt that an average value of 0.5 can be assumed for the general efficiency factor (ke). This also agrees well with the observations of a recent study discussed earlier. 23 This means that instead of using an overall efficiency factor like the earlier researche r s 2°'22'23 the general efficiency factor was kept a constant for all the percentages of replacement. Now, the differences between the water cement ratio including the effect of general efficiency factor and that of control concrete was computed (Aw 2) for the individual mixes at various water cement ratios and the effect of percentage replacement was calculated through an additional percentage efficiency factor (kp). Considering the average of the kp values for the different percentage the variation of compressive strength with parameter w/(c+ kd+ krf ) was presented in Fig. 4. It can be clearly seen that this has resulted in a reasonably close agreement with the control con-
0.25
o o.oo o .......
~'~ ....... h'd ....... L¢6 ....... ~,'d ....... ~'~ ....... Percentage Replacement
~'d ....... V'~ ....... ~o
Fig. 6. Variation of overall efficiency with fly ash replacement.
crete strengths at all the percentages of replacement ranging from 15 to 75%. It was clearly observed during these evaluations that some of the specific parameters like curing conditions, addition of admixtures and in certain cases even the type of aggregates and modifications of their proportions have significant effect on the efficiency of fly ash. The variation of kp with the percentage replacement was presented in Fig. 5 and the variation of the overall efficiency factor k ( -- k e + kp) was presented in Fig. 6. It can now be seen that a replacement of about 10% will increase the cementitious efficiency up to about 1.25 times for the 28 day cube compressive strength and a replacement of around 20% will not alter the compressive strength of control concrete at the corresponding water/cement ratio. Replacements in excess of 20% will reduce the compressive strengths of the concrete continuously and it was seen that a 75% replacement resulted in a reduc-
228
K. Ganesh Babu, (7. Siva Nageswara Rao
tion of efficiency of the fly ash to about 35% (0.35 times). Thus, the proposed method clearly distinguishes the effect of fly ash replacement in terms of the general efficiency factor (ke) and the factor corresponding to the percentage replacement effect (kp). It was also shown that while the general efficiency factor (ke) was a constant (0.5) for the 28 day compressive strength for all percentages of replacements the factor representing the percentage replacements (kp) varies between 0.75 and -0.15 resulting in the overall efficiency factor k varying between 1.25 and 0.35 for replacement percentages of 10 to 75% of cement by an equivalent weight of fly ash respectively.
ke=0.5) will result in conservative values upto about 40% fly ash replacement as reported by earlier researchers. . The results clearly show that the strength of concrete decreases continuously from 100 to 35% at replacements varying from 20 to 75% respectively. . Finally, the method presented using both the general and percentage efficiency factors is able to predict quantitatively the strength variation of concrete at different percentages of fly ash replacements and at different water to ash cementitious materials ratios.
ACKNOWLEDGEMENTS CONCLUSIONS This study, reporting the efficiency of fly ashes in concrete containing ordinary portland cements and cured under normal conditions, had led to the following general conclusions. 1. It was seen that a unique value of the overall efficiency factor (k) will not be able to predict the strength behaviour of fly ash concretes at different percentages of replacement. However, in keeping with the recent studies reported, a general efficiency factor (ke) was defined for evaluating the 28 day cube compressive strength. Based on the results available from different investigators in recent years this was estimated to be 0.5 for fly ash in concrete at all percentages of replacement. 2. To evaluate exactly the overall efficiency of fly ash (k) the authors proposed further a percentage efficiency factor (kp) through which the general efficiency factor can be modified to achieve the total efficiency at any percentage of replacement. The values of the percentage efficiency factor were evaluated and presented for the various percentages of replacement. 3. The overall efficiency of fly ash at different percentages of replacements evaluating has shown that the maximum efficiency in terms of compressive strength can be achieved at around 10% replacement and at 20% replacement the strength is not very much affected. 4. It may also be said that estimating the strength by using an efficiency factor (k or
The authors gratefully acknowledge Professor P. Schiessl, Director, Institut fiir Bauforschung, RWTH, Aachen for the information made available and the detail discussions which helped in narrowing the study to specific aspects. In particular, it was his views that made us look for the pozzolanic efficiency at the different percentages of replacement resulting in the present general and percentage efficiency factors.
REFERENCES 1. ACI Committee 226, Use of fly ash in concrete. ACI Mater J., Sep-Oet (1987) 381-409. 2. Berry, E.E. & Malhotra, V.M., Fly ash in concrete, supplementary cementing materials for concrete, CANMET report, SP 86-8E, 1987, pp. 37-163. 3. Wesche, K. (ed.), Fly ash in concrete, state of the art report, RILEM TC 67-FAB, 1990. 4. Popovics, S., What do we know about the contribution of fly ash to the strength of concrete?, in Proc. Second Int. Conf on Fly Ash, Silica Fume, Slag and Natural Pozzolanas in Concrete, Vol.I, ACI SP-91, 1986, pp. 313-31. 5. Ravina, D., Efficient utilization of coarse and fine fly ash in precast concrete by incorporating thermal curing. A CI J., Sep-Oct ( 1981 ) 194-200. 6. Mehta, P.K. & Gjorv, O.E., Properties of portland cement concrete containing fly ash and condensed sifica-fume. Cement Concr. Res., 12 (1982) 587-95. 7. Cabrera, J.G., et aL, Evaluation of the properties of British pulverised fuel ashes and their influence on the strength of concrete, in Proc. Second Int. Conf. on Fly Ash, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 115-44. 8. Thomas, M.D.A., Matthews, J.D. & Haynes, C.A., The effect of curing on the strength and permeability of PFA concrete, in Proc. Third Int. Conf. on, Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete Vol. I, ACI SP-114, 1989, pp. 191-217. 9. Gopalan, M.K. & Haque, M.N., Mix design for optimal strength development of fly ash concrete. Cement Concr. Res., 19 (1989) 634-41.
Efficiency of fly ash in concrete
10. Haque, M.N., Langan, B.W. & Ward, M.A., High fly ash concretes. ACIJ., Jan-Feb (1984) 54-60. 11. Haque, M.N., Gopalan, M.K., Joshi, R.C. & Ward, M.A., Strength development of inadequately cured high fly ash content and structural concretes. Cement Concr. Res., 16 (1986) 363-72. 12. Hooton, R.D., Properties of a high-alkali lignite fly ash in concrete, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 333-46. 13. Tse E.W., Lee, D.Y. & Klaiber EW., Fatigue behaviour of concrete containing fly ash, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 273-90. 14. Gebler, H.S. & Klieger. P., Effect of fly ash on physical properties of concrete, in Proc. Second Int. Conf. on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Vol. I, ACI SP-91, 1986, pp. 1-50. 15. Nasser, K.W. & Lai, P.S.H., Creep and shrinkage of concrete containing 50% fly ash and superplasticizer, in Proc. Third CANMET/ACI Int. Conf. on Superplasticizers and Other Chemical Admixtures in Concrete, (Supplementary papers) 1989, pp. 86-105. 16. Naik, T.R. & Ramme, B.W., Effects of high-lime fly ash content on water demand, time of set and compressive
229
strength of concrete. A CI J., Nov-Dec ( 1990) 619-26. 17. ASTM C 618-89, Standard specifications for fly ash and raw or calcined natural pozzolana for use as a mineral admixture in portland cement concrete, Annual Book of ASTM Standards, Vol. 04.02, 1989, pp. 296-8. 18. Langley, W.S., Carette, G.G. & Malhotra, V.M., Structural concrete incorporating high volumes of ASTM Class F fly ash. ACIJ., Sep-Oct (1989) 507-14. 19. Neville, A.M., Properties of Concrete, 3rd Edition. ELBS and Pitman Publishing, 1981, pp. 557-63. 20. Smith, I.A., The design of fly ash concretes, in Proc., Inst. of Civil Engineers, London, 36(1967)769-90. 21. Munday, G.L.J., Ong, T.L. & Dhir, R.K., Mix proportioning of concrete with PFA: A critical review, in Proc. First Int. Conf. on Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, Vol. I, ACI SP-79, Detroit, 1983, pp. 267-88. 22. Wesche, K., Schubert, E & Weber, J.W., Zur Festigkeit und Dauerhaftigkeit von Beton bei Zusatz yon Steinkohlenflugasche, (Strength and durability of concrete with addition of fly ash), Betonwerk und Fertigteil-Technik, 50 (1984) 367-74. 23. Schiessl, P. & Hardtl, R., Efficiency of fly ash in concrete evaluation of ibac test results. Technical Report of Institut fiir Bauforschung, RWTH, Aachen, 1991.