Long-term strength of high fly ash concretes

CEMENT and CONCRETE RESEARCH. Vol. 20, pp. 193-196, 1990. Printed in the USA. 0008-8846/90. $3.00+00. Copyright (c) 1990 Pergamon Press plc.

LONG-TERM STRENGTH OF HIGH FLY ASH CONCRETES

Torben C. Hansen Building Materials Laboratory Building 118 Technical University of Denmark 2800 Lyngby Denmark (Communicated by D.M. Roy) (Received April 19, 1989)

ABSTRACT Five f l y ash concretes were found to gain considerable strength beyond 28 days when standard cured in water at 20°C. However, strength development came to an almost complete stop after 3½ years, at what time the f l y ash activity factors with respect to concrete compressive strength ranged from 0.86 to 0.93. Introduction It was studied for how long time portland cement concretes with high contents of fly ash had to be cured in water at 20°C before they reached their ultimate strength. The study was originally designed and carried out by Narud (1) for a different purpose. The main investigation was discontinued years ago, but a few remaining specimens were inadvertently kept in the curing tank and tested after 181 weeks and 362 weeks of standard curing. Because of the long test period involved, the results may be of interest to a larger audience in spite of the fact that only a few specimens were tested in each series. Experimental and results The cement was somewhat similar in composition and properties to an ASTM type I ordinary portland cement. Approximate composition of the cement is shown in Table 1. The cement was ground to a fineness of 2800 cm2/g and the density was 3100 kg/m 3. Chemical composition of the fly ash is shown in Table 2. Loss on ignition was 3.20 percent. 25.8 weight percent of the material was retained on the 45 IJm sieve and the density was 2180 kg/m ~. Concretes were produced with a partly siliceous, partly calcareous sand and gravel of glacial origin. Concrete compositions are presented in Table 3. All concretes were produced with a slump of approximately 60 ram. Twenty 100 mm by 200 mm cylindrical test specimens were cast from each series of concrete. The specimens were cured and stored in water at 20°C. 193

194

Vol. 20, No. 2 T.C. Hansen

Five specimens of each series were tested for compressive s t r e n g t h after 1, 2 and 16 weeks. Two specimens of each series were tested after 181 weeks and three specimens were tested after 362 weeks. The results are presented in Table 4 and Figure 1.

Clinker component

Weight percent

Chemical component

Weight percent

C3S

55

SiO2

44.9

C2S

20

AL203

36.6

C~A

10

Fe203

5.7

C.AF

9

CaO

9.8

Na20 aekv

0.6

MgO

2.1

CaSO•

4

SO3

0.64

f l y ash

none

Na20 eqv

0.65

Table 1.

Series

Approximate clinker composition of cement

Table 2.

Chemical composition of f l y ash

Mix proportions, in kg per cubic meter Cement I Fly ash I Sand < 4mm

Free water

no.

Gravel > 4 am

0

163

181

0

736

1280

I

154

167

146

570

1322

2

154

125

146

604

1331

3

154

107

146

619

1334

4

154

94

146

633

1333

5

154

83

146

644

1333

Table 3. Concrete mix proportions

Concrete. compressive strenclth, Series no.

2 weeks

in MPa after:

4 weeks

16 weeks

181 weeks

I

362 weeks

13.4

14.8

20.8

23.8

26.3

19.0

25.4

39.4

55. I

56.1

12.0

17.1

29.2

46.4

45.9

8.8

13.4

25.2

39.8

42.2

6.9

10.5

22.1

36.0

37.9

5.8

8.4

20.6

34.2

33.9

Table 4. Concrete compressive s t r e n g t h as a function of curing time in water at 20°C.

Vol. 20, No. 2

195

STRENGTH, LONG-TERM, HIGH FLY ASH, CONCRETE

I MPo

6O ~ERIES NO

~

so

~

40

~ ~

~ ~

SER,ESNO SERIES NO ER,ES O

N w

30

u

SERIES NO

Y

g 20 10 0

F i g u r e 1.

2

'

I

,

'

8

16

32

64

r 181

i 3162

,

128 256 5t2 102/. LOG CURING TIME. IN WEEKS

Concrete compressive s t r e n g t h as a f u n c t i o n of log. c u r i n g time in water at 20°C.

2 weeks 22.0

cx-value in Equation (1) at concrete age: 4 weeks I 16 weeks I 181 weeks ] 24.2 34.2 39.0

362 weeks 43.0

Table 5. (x-values at d i f f e r e n t ages of concrete, from Series 0 and Equation 1.

Fly ash a c t i v i t y factor k at concrete age: 2 weeks 4 weeks J 16 weeks 181 weeks

Series n o . I

361 weeks

B

1 2 3 4 5 Table 6.

O. 29 0.25 0.22 O. 22 O. 24

0.49 0.42 0.38 0.34 O. 33

0.60 0.57 0.57 0.57 0.59

0.87 0.93 0.87 0.86 0.88

0.76 0.80 0.83 0.81 0.77

Fly ash a c t i v i t y f a c t o r s a f t e r d i f f e r e n t periods of c u r i n g .

Discussion If we assume with Smith (2) and Bolomey [3) t h a t the compressive s t r e n g t h of f l y ash concretes can be e x p r e s s e d as a f u n c t i o n of the ratio between the water c o n t e n t and the sum of the cement and f l y ash contents in concrete, as well as a f u n c t i o n of an a c t i v i t y f a c t o r k f o r the f l y ash, we a r r i v e at Equation (1)

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Vol. 20, No. 2 T.C. Hansen

S =a where

S = C = W= F = (~ = k =

C+kF l, ~"

0.~0)

Equation(l)

concrete compressive strength after standard curing for a given period of time. in MPa. cement content, in kg per cubic meter. water content, in k9 per cubic meter. fly ash content in kg per cubic meter. constant, the value of which depends on the length of standard curing of concrete. fly ash activity factor = the cementing efficiency of ash relative to cement = the ratio by weight between the contribution of a certain quantity of fly ash to the strength of concrete after a given period of curing, and the contribution of the same quantity of cement.

The values of ~ which are shown in Table 5 for different periods of curing are calculated by inserting in Equation 1 values of C and W for Series 0 (F = 0) from Table 3 and appropriate values of S from Table 4. The values of k. which are presented in Table 6. are calculated for different periods of curing by inserting in Equation 1 appropriate values of from Table 5. values of C. F and W from Table 3. and values of S from Table 4 It will be seen from Table 6 that the fly ash activity factors k increased from 0,22-0.29 after two weeks of standard curing of concretes in water at 20°C to 0.86-0.93 after 181 weeks. The fly ash activity factors then declined to 0.76-0.83 after 362 weeks. Conclusion The main conclusion of the study, which can be drawn on the basis of results presented in Table 4 and Figure 1, is that high fly ash concretes continue to gain considerable strength beyond 28 days when standard cured in water at 20°C. However, in this study the concrete strength development came to an almost complete stop after 3½ years, it appears that investigators, planning to study long-term behaviour of f l y ash concretes, may not have to wait for decades before the hydration process stops. In this investigation hydration of fly ash concretes appears to have stopped within a 3-5 year period. References

Ill 121 131

H. Narud. Low Strength Concretes with High Fly Ash Contents Technical Report no. 109. Laboratory for Building Materials. Technical University of Denmark. Lyngby. 1982. (In Danish). I.A. Smith. Proc. Inst. Civ. Engr. 36. pp. 769-790.

1967.

J. Bolomey. Bulletin Technique de la Suisse Romande. 1927.

16. 22 and 24.