Physical Properties and Constitutive Response

S7-4-6
A STUDY ON PREDICTION AND IMPROVEMENT OF EARLY AGE
STRENGTH OF FLY ASH CONCRETE

Shin-ichiro OKAZAKI
Research Associate, Graduate School of Science and Engineering, Ehime University, Japan.
Isao UJIKE
Professor, Graduate School of Science and Engineering, Ehime University, Japan.
Ava KHATRI
Graduate student, Graduate School of Science and Engineering, Ehime University, Japan.
ABSTRACT:This paper presents a laboratory study on the prediction and improvement of early age strength of fly ash concrete. A
study was carried out for the prediction on early age strength of fly ash concrete with different water powder ratiosand different curing temperatures by the application of the Goral curve, and for the improvement on early age
strength of fly ash concrete. Experimental results have indicated that the Goral curve is applicable for the predictionof early age strength of fly ash concrete in different replacement conditions. And the early age strength improvementof fly ash concrete is possible with the use of rapid hardening Portland cement.
Keywords: fly ash, early age strength, strength growth curve, prediction
1. INTRODUCTION

2. EXPERIMENTAL DETALS

Fly Ash is a by product of thermal power station which
is now a most available supplementary cementing
material world wide. In order to considerably increase
the utilization of fly ash, some problems on concrete
which replaced some of cement by fly ash (here after
we call FA concrete) have to be solved. The
development of FA concrete strength at early age is oneof them. On construction work of concrete structure,
the strength of the concrete at early age after casting of
concrete is the important quality related to construction
process like the determination of removal time of forms
and shores. The strength development at early age of
FA concrete is slower than that of concrete without fly
ash, because the pozzolanic reaction of fly ash is not
active at early age. Therefore, it is necessary to predict
the early age strength of FA concrete more
appropriately and/or to improve that of FA concrete.
Some prediction methods are proposed to evaluate thestrength of concrete [1]. However, there is also no
prediction method of strength of fly ash concretecorresponding to various replacement ratios. This study
investigates the possibility of application of previous
prediction method which combines the equivalent age
based on the maturity and the strength growth curve byGoral curve [2]. In this study, the evaluation of strengthof FA concrete which changed water-powder ratio tothree kinds and replacement ratio of fly ash to fourkinds is examined. Furthermore, the improvement ofearly age strength of FA concrete is investigated using
rapid hardening Portland cement and various
admixtures.

2.1 Materials and mix proportions
Ordinary Portland cement [OPC] (density 3.14 g/cm3)
and rapid hardening Portland cement [RHC] (density
3.16 g/cm3) were used. Crushed sand (density 2.60
g/cm3) and crushed stone (density 2.60 g/cm3) wereused as fine and coarse aggregate, respectively. Fly ash(density 2.33 g/cm3) is a product from Tachibana bay
thermal power station of Sikoku Electric Power Co..
Air-entraining and water-reducing agency andair-entraining agency were used as chemical admixture.
For tests on the improvement of early age strength of
FA concrete, following admixtures were chosen withreference to previous studies [3,4,5].
Admixtures which contributes to strength by thereaction of itself : Blast furnace slag, Silica fumeAdmixtures which promotes the pozzolanic reaction of
fly ash : Slaked lime, Calcium oxide, Calcium
carbonate, Sodium hydroxide
Admixtures which promotes the hydration reaction of
cement : Calcium sulfate, Gypsum, Sodium sulfate,
Magnesium, Lithium carbonate
Concrete specimen was used for the experiment about
the prediction of strength. Mortar specimen was used
for the experiment about the improvement of strength.
The mix proportions of concrete and mortar are shownin Table 1 and Table 2. The value shown in Table 2 isexpressed with the amount per one batch.

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Physical Properties and Constitutive Response

Table1 Mix proportions of concreteTypeof
cementW/(C+F)(%)

F/(C+F)
(%)

s/a
(%)

Water

Cement

0
10
20
30
0
10
20
30
0
10
20
30

45
45
45
45
47
47
47
47
50
50
50
50

170
170
167
165
175
172
172
168
170
165
170
170

425
383
334
289
350
310
272
235
262
229
210
183

Fly
ash
0
42
84
124
0
34
68
101
0
25
52
79

40

20

47

175

350

50
65

20
20

47
47

172
175

275
215

40

OPC

50

65

RHC

Ratio topowder(%)

Unit weight (kg/m3)

Slump
(cm)

Air
(%)

Sand

Gravel

WRA

AEA

747
740
740
738
812
811
809
809
896
901
887
883

924
915
915
912
932
932
932
930
907
911
897
893

0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.35

0.35
0.45
0.45
0.55
0.00
0.25
0.45
0.70
0.00
0.25
0.35
0.37

6.0
9.4
6.0
6.3
8.0
9.3
9.0
9.5
6.0
8.0
12.0
8.0

4.1
4.5
4.1
4.2
5.0
4.9
4.4
5.0
4.0
4.7
4.2
4.0

88

784

870

1.20

0.70

4.5

4.5

69
54

797
818

910
951

0.35
1.00

0.70
0.70

9.4
9.0

4.7
4.9

Table2 Mix proportions of mortar
W/(C+Ad)(%)

F/(C+F)(%)

powder㧦sand

Kinds of admixture

Weight per one batch (kg)
Water

CementFly ashSand

Admixture

1.5
3.0
6.0
(OPC)
1.5
2.1
0.9
6.0
(OPC㧗FA)
1.5
2.1
0.6
6.0
0.30
Blast furnace slag
1.5
1.95
0.9
5.7
0.15*
Silica fume
1.5
2.1
0.8
6.0
0.02
sodium hydroxide
1.5
1.8
0.6
6.0
0.30
Slaked lime
1.5
3.0
0.8
6.0
0.02
Calcium carbonate
1.5
2.7
0.7
6.6
0.06
Lithium carbonate
1.5
2.1
0.9
5.9
0.30
Calcium sulfate
1.5
2.1
0.9
5.7
0.30
Gypsum
Sodium sulfate
1.5
3.0
0.8
6.0
0.26+0.08
36
21
1:1.4
+slaked lime
1.5
2.1
0.8
6.0
0.02
51
28
1:2
Magnesium                          *high-range water-reducing and air-entraining agency
50
50
50
50
51
50
39
43
45
45

0
30
30
30
28
25
21
20
30
30

1:2
1:2
1:2
1:2
1:2
1:2.2
1:1.6
1:1.9
1:1.8
1:1.7

2.2 Specimen and curing
The cylindrical concrete specimens of diameter 10 cm
and length 20 cm and cylindrical mortar specimens ofdiameter 5cm and length 10 cm were prepared. For
concrete specimen, sealed curing was carried out inthree different water temperatures of 10°c, 20°c and30°c. The compressive strength of the specimen wastested at the ages of 2, 5, 7, 10, 14, 21 and 28 days fromcasting. For mortar specimen, curing was done in watertemperature of 20°c. Compressive strength test was
performed at the ages of 3 and 7days from casting.
3. EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Strength growth curve
Based on the previous research, the early age strengthof fly ash concrete was predicted by using the Goral
curve [2].
1244

f

c ,t

a  f c ,28  t mb  tm(1)

where,
fc,t : strength of concrete at age tm
fc, 28 : strength of concrete at the age of 28 days
and at curing temperature of 20°c
tm : equivalent age based on maturity
a,b : constanttm is temperature adjusted age and is calculated byfollowing equation.
.ttm³ T  T dt
.00

Ts  T0

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(2)

2

Compressive strength (N/mm )Physical Properties and Constitutive Response

Constant a and b are related to a type of cement andwater-cement ratio (W/C), and is as follows.

ab  2828(3)

Ordinary Portland cement
b=7.55(W/C)-0.10
Rapid hardening Portland cement
b=5.19(W/C)-0.74
Portland blast-furnace slag cement, B typeb=16.17(W/C)-2.39
Low-heat Portland cement
b=94.31(W/C)-25.27

(4)
(5)
(6)
(7)

Constant b for cement containing fly ash partially has
been not proposed. It has been reported that the
pozzolanic reaction of fly ash is hardly active at earlyage [6]. Because the purpose of this research is theprediction of early age strength of FA concrete, it isconsidered that fly ash is an inactive powder like fineaggregate. Therefore, constant b by Eq.(4) is used forthe prediction of strength in this study.

2Compressive strength (N/mm )3.2 Prediction of early age strength of FA concrete
Fig.1 shows an example of the comparison of measured
strength and strength predicted by substantial
water-cement ratio and water-powder ratios. For
prediction, a measured value is used for concrete
strength at the age of 28 days fc, 28 in Eq.(1). It can beseen from the figure that the measured strength is moreclose to the strength predicted by substantial watercement ratio than that by water-powder ratio. It is
understood that the prediction is more accurate with the30
W/(C+F)=50%

20

F/(C+F)=30%

10

Meas.
Calc.

0
0

10

W/C
W/(C+F)

20

Age (days)

Fig.䋱 Compressive strength of FA concrete(W/F+C=50%,F/F+C=30%)

60
50
40
30

W/(C+F)=40%20

F/(C+F)=30%
Meas.
Calc.

10
0
0

10

20

30

Age (days)

Fig.2 Compressive strength of FA concrete(W/F+C=40%,F/F+C=30%)2-10 °C)Ts : reference temperature (usually taken to be
20 °C)

Compressive strength (N/mm )where,
T : Curing temperature
T0 : datum temperature (usually taken to be

30
20
W/(C+F)=65%

10F/(C+F)=30%
Meas.
Calc.

0
0

10

20

30

Age (days)

Fig.3 Compressive strength of FA concrete(W/F+C=40%,F/F+C=30%)
substantial water cement ratio. Hence the prediction is
performed by using the substantial water cement ratio
and the constants b of ordinary Portland cement.
The strength prediction of FA concrete is done withdifferent fly ash replacement ratios and water powder
ratios 40%, 50% and 65% respectively at curing
temperature 20ºc. Fig.2 and Fig.3 show an example ofcomparison of a measured value and a predicted value.
As shown in Fig.2 and Fig.3, the measured values andthe prediction values are very close to each other.
Fig.4 shows the comparison of measured strength andpredicted strength for all mixing. RMS and SD in figure
are root mean squared error and standard deviation,
respectively. RMS and SD are calculated by following
equations.
RMS

30
SD

¦

prediction  measurement2

measured‫ޓ‬ number§ prediction  measurement ·¸measurement¹
measured  value¦ ¨©

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(8)

2

(9)

1245

2
2

RMS=1.32N/mm
SD=12.8%50
40
30

FA concrete
W/(C+F)

20
10
0
0

40%
50%
65%

F/(C+F)=0%

W/(C+F)=50%

10F/(C+F)=20%
Meas.
Calc.

40
30
20

W/(C+F)=50%

10F/(C+F)=20%
Meas.
Calc.

10

20

Age (days)

30

0
0

10

20

Age (days)

30

Fig.6 compressive strength of FA
concrete cured at 30ºc
temperatures can be evaluated using Eq.(1) within acertain amount of accuracy.
3.3 Improvements of early age strength of FA
concrete(1) Use of rapid hardening Portland cement
The cause of reduction in early age strength of FA
concrete is the decrease of cement content byreplacement of fly ash. Therefore, as one of the
improving methods, it is considered that the hydration
reaction of cement itself is made more active. In thisstudy, rapid hardening Portland cement (RHC) is used
to replace the ordinary Portland cement (OPC). Fig.7
and Fig.8 show the compressive strength of FA concrete
using RHC. As shown in figures, the strength of FA
concrete at early age is improved almost to the samelevel as the strength of concrete using OPC without fly
ash by using RHC.
Furthermore, in Fig.7 and Fig.8, the predicted
compressive strength of FA concrete using RHC isshown. The predicted values for FA concrete using
RHC is calculated by Eq.(1) and Eq.(5). In this case,substantial water-cement ratio is also substituted for
Water-cement ratio in Eq.(5). The early age strength of22compressive strength (N/mm )Curing㧦10͠

Fig.5 compressive strength of FA
concrete cured at 10ºc
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20

Measurement‫(ޓ‬N/mm )Fig.5 and Fig.6 show the prediction strength andmeasured strength of FA concrete at curing temperatureof 10ºc and 30ºc. For prediction in Figs.5 and 6,
strength at the age of 28 days of concrete cured attemperature of 20ºc is used for fc, 28 in Eq.(1). In thecase of curing temperature of 30ºc, it can be seen thatthe prediction strength is close to the measured strength.
And, in the case of curing temperature of 10ºc, a
predicted value overestimates a measured value a little.
The early age strength of FA concrete cured at different

0
0

30

2

From figure, it can be understood that there is accuracy
in prediction of early age strength of FA concrete in all
mixings. The values of RMS and SD in Fig.4 arecalculated from the data about FA concrete. RMS andSD for strength of concrete without fly ash are 1.85
N/mm2 and 15.0%. RMS and SD for FA concrete aresmaller than those for concrete without fly ash. In thisstudy, as substantial water-cement ratio of mixture isused for the prediction of concrete strength, it can
predict irrespective of replacement ratio of fly ash.

Curing㧦30͠

40

10 20 30 40 50 60 70

Fig.4 Comparison measured and predicted
compressive strength of FA concrete

50

50Compressive strength (N/mm )602

Prediction (N/mm )70

Compressive strength (N/mm )Physical Properties and Constitutive Response

70
60

OPC:F/(C+F)=0%50

OPC:F/(C+F)=20%40
30

W/(C+F)=40%

20
10
0
0

RHC:F/(C+F)=20%
Meas. Calc.

1020

Age (days)

30

Fig.7 compressive strength of FA concrete
using RHC (W/(C+F)=40%)

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decreases by about 15ºc by replacement of fly ash.
However, the maximum temperature of mortar using
RHC with fly ash is high compared with that of mortar
using OPC with fly ash. Consequently, temperature
behavior of mortar using RHC with fly ash is almost
the same as that of mortar using OPC without fly ash.

OPC:F/(C+F)=0%

OPC:F/(C+F)=20%(2) Use of admixtures
Mortar test specimens were prepared using different
admixtures with the mixing ratios as shown in Table2
and compressive strength was measured at 3 and 7 days
from casting. The compressive strength test results are
as shown in Fig.10. The admixtures chosen in this study
is referring to previous studies and is divided roughly
into three as follows. 1) Increase of admixture strength
helps to increase the strength of mortar, 2) Make thepozzolanic reaction of fly ash more active, 3) Promote
the hydration of cement at early ages.

W/(C+F)=65%
RHC:F/(C+F)=20%
Meas. Calc.

30

Fig.8 compressive strength of FA concrete
using RHC (W/(C+F)=65%)

However, in most of the cases, the early age strength
could not be increased, and it is decreased oppositely.
As shown in Fig.10, with the addition of lithium
carbonate and blast furnace slag, the early age strength
is increased by little at age of three days in comparison
to that of fly ash mortar, but at age of seven days it
could not be increased. With the addition of
Na2SO4+CaO, the early age strength is increased inboth days in comparison to that of fly ash but it is notsatisfactory in comparison to strength of mortar using
ordinary Portland cement. As one of the reasons why
early age strength is not improved by use of admixtures,a difference of the experimental condition between thisstudy and previous studies is considered. This meansthat admixtures must be used, after checking thevalidity of the admixture in consideration of mix
proportion, materials, construction condition and so on.

FA concrete using RHC can be also predicted in good
accuracy like that using OPC.

50
40
30
20
10
0

10

20

30

40

50

60

elapsed time‫(ޓ‬hours)

70

Fig.9 Temperature changes of FA mortar
and mortar without FA

10
Magnesium

RHC㧗FA
OPC
OPC㧗FA

Sodium sulfate㧗lime

60

20

GypsumRHC

3 7
age(days)

30

Calcium sulfate

Temperature (͠)

70

W/(C+F)=50%

40

Lithium carbonate

The highest temperature of mortar using RHC

2

FA mortar using RHC [W/(C+F)=50%, F/(C+F)=20%]
Mortar using RHC [W/(C+F)=50%, F/(C+F)=0%]
FA mortar using OPC [W/(C+F)=50%, F/(C+F)=20%]
Mortar using OPC [W/(C+F)=50%, F/(C+F)=0%]

Compressive strength (N/mm )As mentioned above, early age strength of FA concrete
is improved by use of RHC. The merit of utilization of
fly ash has control of the temperature rise of concreteby hydration heat of cement. However, use of RHC
affects the temperature rise of concrete. Fig.9 shows
temperature change of mortar. The temperature wasmeasured at center of mortar specimen with
15x15x15cm and mortar specimen is enclosed withthermal insulation with thickness of 20cm. Since theperfect heat insulation state was unreproduceable on
this condition, hydration heat of following mortar
mixtures was measured on the same conditions, in oderto evaluate relatively.

Slaked lime20

Age (days)

Calcium carbonate

10

Sodium hydroxide

00Silica fume10

F/(C+F)=30%

20

Blast furnace slag

30

F/(C+F)= 0%

2

Compressive strength (N/mm )Physical Properties and Constitutive Response

Fig.10 Compressive strength of mortar
with admixtures

4th International Conference on Construction Materials:
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Physical Properties and Constitutive Response

4. CONCLUSIONSIn this study, to promote the utilization of fly ash as theconcrete mixture, the investigation on the strength
prediction and the strength improvement of FA concreteat early age are carried out. The results obtained by thisresearch are summarized as follows:

Structures and Pavement,Vol.63㧘No.1㧘pp.52-65㧘
2007.1(1) Early age strength of FA concrete with ordinary
Portland cement can be predicted with different
water powder ratios by the strength growth curve
combined Goral curve and maturity.
(2) The early age strength of FA mortar could not be
increased with the addition of admixtures.
(3) With the addition of sodium + lime, the early age
strength of FA mortar can be increased by a little,
but not much satisfactory as compared to concrete
without fly ash.
(4) The early age strength of FA concrete can be
improved by use of rapid hardening Portland
cement and its prediction is also able to be
performed in good accuracy.
(5) Although strength of FA concrete can be improvedby use of rapid hardening Portland cement, a heatgeneration characteristic becomes comparable asthat of ordinary Portland cement without fly ash.
ACKNOWLEDGEMENT
This research was performed as a part of activity“Subcommittee on research of effective utilization of
fly ash “ (Chairman : Professor Ko-ichi Maekawa),
Concrete committee, JSCE. The support from thesubcommittee is gratefully acknowledged.
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