Studies of alkali-silica reaction: Part 8. Correlation between mortar-bar expansion and δ values

CEMENT and CONCRETE RESEARCH. Vol. 21, pp. 61-65, 1991. Printed in the USA. 0008-8846/91. $3.00+00. Copyright (c) 1991 Pergmnon Press pie.

STUDIES OF A L K A L I - S I L I C A REACTION: PART 8. CORRELATION BETWEEN M O R T A R - B A R EXPANSION AND ~ VALUES.

H. Strunge, S. Chatterji and A. Damg~rd Jensen Dansk Teknologisk Institut 2630 Taastrup Denmark (Refereed) (Received March 7, 1990)

ABSTRACT Recently a method for the detection of alkali-silica reactivity of sand has been proposed. In this method sand is digested in a mixture of Ca(OH)2 and saturated KCl for 24 hours. The OH ion concentration is determined by titration. The OH ion c o n c e n t r a t i o n for a control mixture without sand is d e t e r m i n e d and d i f f e r e n c e between control and sample OH ion concentration is termed 4. This ~ is a measure of alkali-silica reactivity. The m e t h o d has been tested by six different laboratories. The r e p r o d u c i b i l i t y of the method is fairly high. To determine an acceptance criterion, 16 Danish sand types have been tested and value compared with flint content and expansion. A ~ value of ii0 separates expanding and non-expanding Danish sand types.

Introduction

In a recent paper a simple chemical test method has been proposed for the d e t e c t i o n of alkali-silica reactivity of sand samples (i). This simple method is based on the observation that if a reactive sand is digested in a mixture of excess Ca(OH)2 and a saturated KCI solution at 70°C a complex a l k a l i - l i m e - s i l i c a gel forms along with CaCl 2 according to the following scheme: Reactive silica + xCa(OH)2 + yKCl + aq = y/2 K 2 0 . ( x - y / 2 ) C a O . z S i O 2 . a q + y/2CaCl 2 + aq ............... (i). Freshly formed CaCl 2 will in its turn depress the c o n c e n t r a t i o n of Ca(OH)2 in the solution phase. This reduction of Ca(OH)2 in the reaction mixture is a measure of alkali-silica reaction that has occured during digestion and has been termed 4. This ~ can easily be estimated by filtering the reacting suspension and de61

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H. Storage, et al.

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termining OH ion concentration in the filtrate and comparing with that of a control suspension of Ca(OH)2 in saturated KCI solution (i). The initial determination of A values of a number of sand types suggested that this method may be used not only for the detection of alkali-silica reactivity of aggregates but also for daily quality control of a sand source as well as a selection procedure for safer sand sources. A preliminary report on the reproducibility of this new method has already been published (2). In this paper we publish the inter-laboratory r e p r o d u c i b i l i t y of A value determination as well as the determination of the acceptance criterion for the selection of safe sand sources. Six different laboratories took part in the evaluation of the reproducibility of this method using three different sand types and in the evaluation of the acceptance criterion two laboratories took part and used 16 different sand sources.

Test protocol

i) The inter-laboratory reproducibility test: Each of the six participating laboratories were supplied with a standardized test procedure to follow as well as samples of three sand types. Prior to their dispatch each of the sand types were thoroughly mixed by passing through a sample divider a number of times and finally divided into the required number using the sample divider. Each of the laboratories were asked to report the concentrations of OH ions both of the control and test solutions for each of the three sand types to one of the authors (S.C.), who himself did not take part in any experimental work. The results from different laboratories were then collected and reported (table I). An inter-laboratory discussion of the results led to a somewhat stricter test procedure to be followed in any future work. This revised test procedure is shown in the Appendix. ii) The evaluation of the selection criterion using A value: In this evaluation two laboratories took part. These laboratories were supplied with the revised test procedure and samples of 16 different sand types. These sand types had previously been characterized as regards their reactive flint contents as well as their expansivity by the salt bath method (3). This characterization was carried out in context with a different project not connected with the present investigation. The p a r t i c i p a t i n g laboratories estimated the "A" values for each sand types in duplicates. The results obtained by these two laboratories are plotted in Fig. i. In Fig. 2 the sand characteristics have been plotted against A values obtained by laboratory No. 2. Discussion of the results i) The inter-laboratory reproducibility test: Table 1 shows the mean a values from each of the six laborato-

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ALKALI-SILICAREACTION, OH- CHANGE

63

ties, the overall average and the standard d e v i a t i o n between the laboratories. It can be seen, that when the ~ value is about i00 or higher, the standard deviation is 10% of the overall average. For lower ~ values, the standard deviation is higher. This is due to the d e t e r m i n a t i o n of the ~ value as a small difference between two high values of OH ion concentrations. The standard deviation for the d e t e r m i n a t i o n of the OH ion concentration within one laboratory is about 5%. Table

values

obtained by different

laboratories

Laboratory

Sand type

1

in mgOH/l.

No.

Average ± S%

1

2

3

4

5

6

Nym~lle

191

168

188

151

157

193

175 + 10%

PC sand

108

112

132

102

130

114

116 -+ 10%

CN sand

25

27

50

33

41

40

36 + 26%

ii) The evaluation

of the selection criterion

using ~ value:

Fig. 1 shows the & values obtained by two different laboratories on 16 different Danish sand types. It can be seen that the correlation between the laboratories is high, with a c o r r e l a t i o n coefficient of 0.95.

/ //

o

L A B l = I/..6 +0.89 -4LA B 7"[

/

CORR. = 0.95

/ / / o

/

/

~

o

/ /

/.0

80

120 LAB. Z

160

200

2/,,0

.' [OH-]

Fig. i. C o r r e l a t i o n of ~ values obtained by two laboratories.

In Fig. 2 the sand characteristics i.e. % porous flint, and o/oo expansion at 8 and 20 weeks are plotted against the ~ values for the 16 sand types m e a s u r e d at the laboratory No. 2. In Danish sand types porous flint is the main reactive component; an acceptance criterion of m a x i m u m 2% by volume of porous flint content in sand is prevalent in Denmark. However, porous flint grains are seldom free of associated c a l c i u m carbonate particles and as such the d e t e r m i n a t i o n of porous flint content by the petrographic point counting technique always involves certain u n c e r t a i n t y as re-

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H. Storage, ct al.

gards the true reactive

silica content. From Fig. 2 it can be seen that the flint content is weakly correlated with both expansion and A values. A flint content of 2% does not distingish clearly between expansive and non-expansive sand types. This is not surprising in view of the a b o v e - m e n t i o n e d uncertainty of reactive silica contents; two sand types of nominally same 2% porous flint content may have very different reactive silica contents and have the weak correlation with both expansion and A values.

II, l s

o Oo o 0 • 5

¢~

: u~

I

20 WEEKS

~,-

Fig. 2 also shows a strong correlation between A values and expansion; the correlation improves with longer storage of mortar prisms in salt bath i.e. from 8 weeks to 20 weeks. This point is important as long-term expansion is more relevant to the life-time of a structure than the shortterm expansion.

3-

=I t"

0

• ee

. e

io I

7 6

¢5

"

0

>I

!

o

o

o

g3

o

~o

~o

0 i

200

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240

a [OH-I

Fig. 2 also shows that a A value about 100 - 120 is able to distinguish between sand types having expanded after 8 and 20 weeks by the salt bath method and those which have not expanded.

Fig. 2. Relationships among A values, flint contents and expansions. Considering the standard deviation of 10% between different laboratories for the determination of the A value, a selection criterion for the A value = ii0 will make a safe distinction between expansive and non-expansive sand types.

Conclusions

From these test results the following conclusions

might be made:

The reproducibility of the method for determining the A values is very high. The standard deviation is 10% for different laboratories and 5% within the same laboratory. For Danish sand types, a selection criterion of A value of about 110 distinguishes between expanding and non-expanding sand types.

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If similiar results could be obtained for sand of other origins, then this technique might be a quick and simple m e t h o d for detection of expanding aggregates outside Denmark as well. References (i) S. Chatterji and A. DamgArd Jensen - Cement and Concrete Res. i_88, 654-656, 1988. (2) S. Chatterji - Proc. 8th Intern. Conf. A l k a l i - a g g r e g a t e reaction. Kyoto. 1989. P. 295-299. (3) S. Chatterji - Cement and Concrete Res. 8, 647-50,1978.

Appendix

The sand, 0 - 4 mm, is dried at I05°C. Two samples, 100 g each, are w e i g h e d out, using a sample divider. The exact weights are registered. In two conical bottles (with ground glass joints and plastic stoppers) KCl-solution is weighed out, so that KClsol./sand weight ratio is 2.0. Two lots of Ca(OH)2 , 4.4 - 4.6 g each, are weighed out and poured into the KCl-solution, and the bottles are stoppered. Suspensions and sand are kept at 70°C ± l°C for 12 hours. The sand is poured into the suspensions, stoppers replaced and the bottles are shaken carefully. Bottles are replaced at 70°C enclosure. Note the time! Every two hours, 4 times totally, the bottles are shaken carefully. After 24 hours (max. 25 hours) at 70°C ± I°C, the bottles are cooled in a w a t e r - b a t h at 20°C ± I°C. Every 20 minutes, the bottles are shaken carefully, for at least 3 hours (max. 4 hours). Temperature may be controlled with a thermometer. Suspensions are filtrated under suction. Together with the samples a control experiment is performed to determine the solubility of Ca(OH)2 in KCl-solution. It is important that the apparaturs is dry, and that the filtrates are kept out of contact with the air to avoid/minimize carbonation in the suspension. 20.00 ml of filtrate is pipetted out in a i00 ml bottle, indicater is added, and the solution titrated with 0.025 N HCI.