interaction of silica fume and water …docs.trb.org/prp/12-1733.pdfs. marikunte and s. nacer 2...

S. Marikunte and S. Nacer 1

INTERACTION OF SILICA FUME AND WATER CONTENT ON STRENGTH AND

PERMEABILITY OF CONCRETE

by

Shashi S. Marikunte and Samir Nacer

Shashi S. Marikunte, Ph.D., P.E., Assistant Professor of Civil Engineering, School of Science,

Engineering and Technology, The Pennsylvania State University, 777 West Harrisburg Pike,

Middletown, PA 17057, Tel: (717) 948-6132, Fax: (717) 948-6502, E-mail: [email protected]

(Corresponding Author).

Samir Nacer, Project Manager, Thomad Engineering LLC, 4535 W. Russell Road, Suite 12, Las

Vegas, NV 89118, Tel: (702) 388-7755, Fax: (702) 388-7766, E-mail: [email protected].

Date Submitted: November 15, 2011

Word Count: 3,900 + 10 x 250 = 6,400

TRB 2012 Annual Meeting Paper revised from original submittal.


ABSTRACT

Two critical factors for improved concrete performance are matrix (cementitious material) and

water content. Pozzolanic materials such as silica fume and fly ash are now commonly being

added to concrete mixtures to improve performance. This results in complex interaction of

pozzolan type/content and water-to-cementitious material ratio on performance. While a

relatively small number of data points may be sufficient to arrive at general conclusions, a

comprehensive experimental design with sufficient data points and strong statistical tools is

needed for development of accurate models to quantify the effect of individual responses. In this

experimental investigation, cement was partially replaced with pozzolanic admixture silica fume

to achieve improvement in strength and reduced permeability of concrete. Plain Portland cement

matrix was partially replaced with silica fume at 5, 10, and 15% by weight. For each silica fume

replacement, four different water-to-cementitious material ratios (0.35, 0.3, 0.25, and 0.2) were

selected to study their effect and interaction at various levels on compressive strength and

permeability. Statistical regression analysis was then performed on the comprehensive

experimental data generated in this investigation to develop an empirical model for compressive

strength as a function of water and silica fume content. The results indicate that the optimum

percentage of silica fume to achieve maximum compressive strength varies with the water

content. Also, permeability decreases with increased silica fume content and reduced water. This

paper presents the outcome of the comprehensive experimental investigation and statistical

analysis to achieve increased strength and reduced permeability in concrete with silica fume.

Keywords: cementitious material; compressive strength; concrete; empirical model;

permeability; pozzolan; regression analysis; silica fume.

INTRODUCTION

One of the main areas of research on concrete is the improvement of mechanical properties,

permeability, and durability. High-performance concrete is defined by The American Concrete

Institute (ACI) as “concrete meeting special performance and uniformity requirements, which

cannot be achieved routinely using only conventional constituents and normal mixing, placing,

and curing practices.” High-performance concrete is commonly used in bridge structures and

highway pavement. Most high-performance concretes will have supplementary cementitious

materials such as silica fume and fly ash, and a low water-to-cementitious material ratio. The

workability is often an issue with such a mixture design. Concrete that cannot be placed will not

meet the specifications of a high-performance concrete [1 - 2]. Another challenge to coming up

with a high-performance concrete design is to make it economical.

To obtain high-performance, several refinements are made to the selection of mix

ingredients and their proportions. The first approach is to reduce the water-to-cement ratio. The

single most parameter that has significant effect on strength and permeability of concrete is

water-to-cement ratio [3]. A decrease in w/c ratio results in increased strength and reduced

porosity in cement paste and, hence, the concrete becomes more impermeable. However, a

reduction in water content also creates a much dryer mix, and proper placement and compaction

could become difficult. This problem can be overcome through the addition of a high range

water-reducing admixture (superplasticizer). Water-reducers allow for less water to be added



and, at the same time, help achieve proper workability. While water-to-cement ratio has

significant influence on strength and permeability of concrete, one can further achieve improved

performance through matrix modification. Pozzolanic materials such as silica fume, fly ash, and

metakaolin, are now commonly being added to concrete mixtures to improve performance [3 –

8]. Pozzolans play an important role as microfillers and help improve particle-packing density of

cementitious system, rheological properties in fresh state, mechanical properties, and durability.

The microstructure of the transition zone between the aggregate and the cement paste has

direct influence on the strength of concrete and its permeability. Another factor that significantly

influences strength and permeability of concrete is aging, due to densification of matrix. Thus,

performance of concrete is a function of the strength and permeability of cement paste, aging,

aggregate, and aggregate-cement paste interface (transition zone) [9 – 10]. The right combination

of water-to-cementitious material ratio, superplasticizer, and pozzolan can significantly improve

the concrete’s material property requirements into the range of high-performance.

Silica fume is a highly effective pozzolanic material due to its extreme fineness (0.1

microns) and consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). While

small particle size of silica fume is beneficial in particle-packing, it also results in increased

water demand to achieve workability needed for good compaction. It is essential to use the

proper amount of water-reducing admixture to keep the water requirement to a minimum in order

to maximize the benefit of silica fume in concrete. While the beneficial effects of pozzolans are

well known, researchers are yet to arrive at unique conclusions regarding the influence of silica

fume, especially when it comes to the quantifying the optimum amount of replacement needed to

achieve the best performance of the material in different conditions. Different researchers have

reported different replacement levels as optimum for obtaining superior performance. One major

reason for this discrepancy is that compressive strength of concrete is a function of the percent

replacement of cement with silica fume as well as water content [6]. Even though the mechanical

properties (strength) of concrete were observed to improve as a result of incorporating silica

fume by 10 – 15% (replacement of cement by weight), the optimal percent replacement needs

more investigation to achieve high strength with reduced permeability.

In this comprehensive experimental investigation, an effort was made to study the effect

of silica fume, water content, and their interaction, on compressive strength and permeability of

concrete. First, optimization of silica fume content to achieve impermeability was established for

mixtures with a relatively high water-to-binder ratio of 0.45 through comprehensive statistical

analysis. The mixtures were then refined to obtain high-performance concrete. Using statistical

regression analysis, a suitable quadratic empirical model was developed for compressive strength

as a function of water and silica fume content. Rapid chloride permeability test (ASTM C 1202),

used in this research, is an indirect measure of permeability of concrete, and the results have

been observed to correlate well with water permeability [11].

RESEARCH SIGNIFICANCE

The intent of this research is to expand on the previous knowledge of cement replacement with

silica fume and combine it with the benefits of low water-to-cementitious material ratios on

performance of concrete. In this comprehensive experimental investigation, two critical aspects



of concrete performance were investigated, namely: compressive strength, and permeability.

While a relatively small number of data points may be sufficient to arrive at general conclusions,

a comprehensive experimental design with sufficient data points and strong statistical tools is

needed for optimization and development of accurate models to predict the effect of individual

responses. Extensive data collected, statistical analysis performed, and the empirical models

presented in this investigation quantify the effects of silica fume, water content, and their

interaction, on compressive strength and permeability of concrete.

EXPERIMENTAL PROGRAM

Test Series and Mixture Proportions The cementitious materials used in this investigation were ASTM C 150 Type I Portland cement

and silica fume (air-densified microsilica conforming to ASTM C-1240). Local river sand with a

fineness modulus of 2.97 and specific gravity of 2.68 was used as fine aggregate. Crushed

granite with a maximum nominal size of 6.25 mm (0.25 in) and a specific gravity of 2.7 was

used as coarse aggregate.

The mix proportions by weight were: 1: 1.14: 2 (binder: sand: coarse aggregate). Table 1

presents the details of the test series. The selection of the mixture proportions was based on

preliminary investigation and available data on high performance concrete. Sixteen mixes were

selected representing four different silica fume contents and water-to-binder ratios. Partial

replacement with silica fume was varied from 0 to 15%, by weight. Water-to-cementitious

material ratios were varied from 0.2 to 0.35. The amount of superplasticizer was varied for the

mixtures to achieve a workable and uniform concrete. Mixing was done in accordance with

ASTM C 192.

Curing and Sample Preparation

From each concrete mixture, cylinders of 102 mm (4 in.) diameter and 203 mm (8 in.) height

were cast. All the specimens were moist cured in water until the age of testing. Three specimens

of 102 mm (4 in.) diameter and 51 mm (2 in.) thick were cut from each cylinder using a diamond

saw, for rapid chloride permeability testing.

Experimental Procedure

Compressive strength tests were conducted according to ASTM C 39 using a hydraulic testing

machine with a digital display. Displacement rate was maintained constant for testing. Failure

mode for all the specimens was observed to be “columnar,” indicating a true compression failure.

Rapid chloride permeability test (RCPT) was used to measure permeability of concrete

(ASTM C 1202). The goal/purpose of this test is to measure the amount of electrical current

passed through the concrete sample over a fixed period of time. One end of the specimen was in

contact with a 3% by mass sodium chloride (NaCl) solution while the other end was in contact

with 0.3 N sodium hydroxide (NaOH) solution. A potential difference of 60 V dc was applied

across the ends of each of the specimen and maintained for a period of six hours. The current (in

milliamps) was measured over six hours, and the ampere-seconds were calculated by integration

of the curve to obtain the Coulombs. The total charge passed (Coulombs), according to ASTM C

1202, is the measure of electrical conductance and is used to evaluate the permeability of



concrete. Results obtained from RCPT correlate well with the results from conventional

methods. However, RCPT measures electrical conductance, and, as such, the results should be

analyzed carefully, especially for concrete with low permeability. Concrete with silica fume

tends to exhibit lower charges, compared to similar concrete without silica fume [3].

TABLE 1 Test series and mixture proportions: High-performance concrete

Mix

Water-to-

Cementitious

Material Ratio

Silica Fume,

%

Superplasticizer-to-

Cementitious Material

Ratio

Slump,

mm

Air Content,

%

1 0.35 0 0.0025 89 4.5

2 0.35 5 0.0039 152 3.3

3 0.35 10 0.0042 158 3.8

4 0.35 15 0.0058 178 4

5 0.3 0 0.0071 219 -

6 0.3 5 0.0068 152 2.5

7 0.3 10 0.0067 142 2.1

8 0.3 15 0.0078 125 2.1

9 0.25 0 0.0159 191 -

10 0.25 5 0.0112 155 1.9

11 0.25 10 0.0107 178 1.5

12 0.25 15 0.0109 127 2.1

13 0.2 0 0.0221 203 2.8

14 0.2 5 0.0161 163 3.8

14 0.2 10 0.0169 178 2.3

16 0.2 15 0.0166 135 2.2

EXPERIMENTAL RESULTS AND DATA ANALYSIS

The focus of this phase of investigation was to study the effect of silica fume and water content

to achieve high-performance in concrete. Replacement levels of cement with silica fume ranged

from 0 to 15%, based on preliminary investigation. Water-to-cementitious materials ratios

ranged from 0.35 to 0.20, to achieve higher compressive strengths and reduced permeability.

Compressive Strength Compressive strength of concrete modified with silica fume at different water-to-binder ratios is

presented in Table 2. Compressive strength value presented in the table is the average of six test



specimens for any given series and testing condition. Concrete modified with silica fume showed

higher compressive strength than that of plain concrete for all water-to-cementitious material

ratios. Compressive strength increased with the increase in percentage replacement of cement

with silica fume. The use of low water-to-cementitious material ratio led to the highest

compressive strength (128 MPa). At water-to-binder ratio of 0.2 and silica fume content of 5 to

15%, compressive strength of more than 120 MPa was achieved. It was noted that, with the use

of silica fume, high strength could be achieved at relatively low water content. The maximum

strength of plain concrete with a water-to-cementitious material ratio of 0.2 could be achieved

using 5 to 15 percent silica fume with a water-to- cementitious material ratio of 0.25. This means

that concrete of equivalent strength could be made with higher water-to- cementitious material

ratios if silica fume is incorporated. For water-to- cementitious material ratio as low as 0.2, it

becomes extremely difficult to modify the binder content with high percentages of silica fume.

Results showed that, for water-to- cementitious material ratio of 0.2 and silica fume content of

5%, concrete achieved higher compressive strength than concrete modified with 10 and 15

percent silica fume. It could be interpreted that the amount of water is too low for the increased

amount of pozzolan used, which is a very fine powder and requires higher water content for

efficient placing and compaction. However, this can be overcome through the use of improved

high-range water-reducer.

TABLE 2 Effect of silica fume and water content on compressive strength of concrete

Water-to-Cementitious

Material Ratio

Compressive Strength, MPa

0% SF 5% SF 10% SF 15% SF

0.35 Mean 52.87 59.86 66.13 69.27

Std. Dev. 2.54 7.39 12.64 15.66

0.30 Mean 65.37 68.90 87.99 96.44

Std. Dev. 14.98 16.78 6.07 6.15

0.25 Mean 81.63 100.15 101.01 107.30

Std. Dev. 2.95 5.72 11.96 7.25

0.20 Mean 103.05 128.39 121.43 123.62

Std. Dev. 3.36 5.99 7.00 4.40

Figure 1 presents the variation of average compressive strength with percent replacement

of cement with silica fume. Average compressive strength values at different water-to-

cementitious material ratios at each silica fume replacement level are presented in Figure 2. The

results show that the optimum silica fume content is not a unique one but may vary with the

water content of the mix. Maximum values of compressive strength were obtained at 15%

replacement level and a water-to-cementitious material ratio of 0.35 through 0.25. However, the

maximum compressive strength at water-cementitious material ratio of 0.2 was obtained at 5%

replacement level (Table 1). This result supports similar findings obtained by other researchers,



which indicated that the optimum silica fume percentage for the maximum strength varies with

the water content [6]. They found that increasing the water content (from 0.26 to 0.42) requires

higher silica fume content to achieve maximum compressive strength (from 15 to 25%).

0

20

40

60

80

100

120

0% 5% 10% 15%

Ave

rag

e C

om

pre

ssiv

e S

tre

ng

th (

MP

a)

Silica Fume Percentage

FIGURE 1 Average compressive strength of high-performance concrete at select silica

fume content.

0

20

40

60

80

100

120

140

0.350.30.250.2

Ave

rag

e C

om

pre

ssiv

e S

tre

ng

th (

MP

a)

Water-to-Cementitious Material Ratio



FIGURE 2 Average compressive strength of high-performance concrete at select water-to-

cementitious material ratio.

The concrete with silica fume contents of 5, 10, and 15% showed 18, 24, and 31% more

strength, respectively, compared to plain concrete. The average compressive strength for

concretes made with 0.3, 0.25, and 0.2 water-to-cementitious material ratios showed an increase

of about 28, 57, and 92%, respectively, when compared with concrete made with 0.35 water-to-

cementitious material ratio. The average compressive strength was also proportional to silica

fume percentage.

Correlation of Compressive Strength with Silica Fume and Water Contents

“Design Expert” software version 7.0.0 was used to analyze the data generated from the

experimental results. All the data points were included in the statistical analysis, except the ones

for which the test was incomplete. In this statistical analysis water-to-cementitious material ratio

and silica fume content were the factors used in a two-factor factorial design to analyze the

response (compressive strength). The model that best fits the data is presented by the following

quadratic equation:

Y = a + b X1+ c X2 + d X22 [1]

Where:

Y is the compressive strength, MPa

X1 is the water to cementitious material ratio

X2 is the silica fume content

a, b, c, and d are regression variables

The values of a, b, c, and d for the proposed regression model are:

a = 184.2214, b = -3.93803, c = 3.149244, d = -0.1162

The Model F-value of 149.06 obtained from analysis of variances (ANOVA) table

implies the model is significant. There is only a 0.01% chance that a "Model F-Value" this large

could occur due to noise. In this case X1, X2, and X22 are significant model terms. Insignificant

terms were eliminated in the analysis process. The "Pred R-Squared" of 0.8300 is in reasonable

agreement with the "Adj R-Squared" of 0.8394.

The final equation in terms of actual factors is:

Compressive Strength = 184.22136 - 3.93803 W/CM+3.14924 SF - 0.11620 SF2 [2]

Where, W/CM and SF are water-to-cementitious material ratio and silica fume

percentages, respectively.

Figures 3 and 4 show the contour and 3-D representation of the compressive strength as

function of both water-to-cementitious material ratio and silica fume to binder ratio. They

represent the way compressive strength develops from low values (bottom right corner) to the

optimum (top left corner) as a result of the interaction of water and silica fume contents.



Interaction of silica fume content and water-to-cementitious material ratios on compressive

strength is represented in Figure 5. The two parallel lines in the figure represent the lower and

the higher values of silica fume percentage (e.g. 0 and 15).

Design-Expert® Sof tware

Compressiv e strengthDesign Points134.031

45.1226

X1 = A: w/cX2 = B: SF

20.00 23.75 27.50 31.25 35.00

0.00

3.75

7.50

11.25

15.00Compressive strength

A: w/c

B:

SF

59.7914

73.192786.593999.9952

113.396

44446666665555555555

6666664444666666666666

666666555555555555555

66666666666666666655555

FIGURE 3 Contour of compressive strength model.


Compressiv e strength134.031

45.1226

X1 = A: w/cX2 = B: SF

20.00

23.75

27.50

31.25

35.00 0.00 3.75

7.50 11.25

15.00

45

67.5

90

112.5

135

Co

mp

ress

ive

str

en

gth

A: w/c

B: SF

FIGURE 4 3-D Representation of compressive strength model.




Compressiv e strength

Design Points

B- 0.000B+ 15.000

X1 = A: w/cX2 = B: SF

B: SF

20.00 23.75 27.50 31.25 35.00

Interaction

A: w/c

Com

pres

sive

str

engt

h

42

65.25

88.5

111.75

135

2

222

22

22

2

2

2

3

2

23

2

2

2

2

22

32

3

22

2

22

33

222

222

2

FIGURE 5 Interaction of factors affecting compressive strength of concrete.

The model developed is valid for the water-to-cementitious material ratios and silica

fume contents for the aggregate proportion used in this investigation. Additional test data are

required to refine this model to include the effect of aggregate proportion on compressive

strength of concrete. Maximum amount of silica fume (15% replacement of cement) selected in

this investigation is within acceptable level in construction industry. Typically for bridges, 10 –

15% replacement of cement with silica fume is specified. If the amount of silica fume is

increased beyond 15%, it may be harmful, unless proper precautions are taken to ensure adequate

workability.

Chloride Permeability

Table 3 presents the electrical charges passed through concrete specimens of different water-to-

binder ratios and silica fume contents. Figure 6 presents the change in permeability of silica fume

modified concrete compared to plain concrete. The total charges passed through concrete

specimens reduced as the silica fume content increased. At a water-to-cementitious material ratio

of 0.35, the permeability of concrete reduced by 73, 85, and 90%, respectively, at 5, 10, and 15%

silica fume content. Similar trend was observed with lower water-to-binder ratios. At a water-to-

cementitious material ratio of 0.3 the permeability was reduced by 78, 87, and 87%, respectively,

at 5, 10 and 15% silica fume content.

Figure 7 presents the change in permeability (charges passed in coulombs) due to change

in water-to-cementitious material ratio. For plain concrete, permeability is reduced by 45% when

water-to-cementitious material ratio was reduced from 0.35 to 0.25. A similar trend was



observed for concrete with silica fume. When water-to-cementitious material ratio was reduced

from 0.35 to 0.25 for concrete modified with silica fume, the charges passed were reduced by 63,

54, and 44%, respectively, at 5, 10, and 15% silica fume content. A combination of low water

content (0.25) and 15% silica fume replacement results in lowest permeability (94% reduction in

charges passed).

TABLE 3 Effect of silica fume and water content on permeability of concrete

Water-to-Cementitious

Material Ratio

Charges Passed, Coulombs

0% SF 5% SF 10% SF 15% SF

0.35 Mean 1983 537 303 195

Std. Dev. 395 109 20 7

0.30 Mean 1420 311 180 188

Std. Dev. 639 59 6 35

0.25 Mean 1095 200 140 128

Std. Dev. 36 6 32 15

0

500

1000

1500

2000

2500

0% 2% 4% 6% 8% 10% 12% 14% 16%

Cha

rge

s P

asse

d (

Cou

lom

bs

)

Percentage oF Silica Fume

W/C = 0.35

W/C = 0.3

W/C =0.25

FIGURE 6 Effect of silica fume content on permeability of concrete.



0

500

1000

1500

2000

2500

0.2 0.25 0.3 0.35 0.4

Ch

arg

es P

asse

d (

Cou

lom

bs)

Water-to-Cementitious Material Ratio

0% SF

5% SF

10% SF

15% SF

FIGURE 7 Effect of water content on permeability of high-performance concrete.

Despite the fact that concrete modified with silica fume requires more water to achieve

decent workability, the results obtained show that the increase in silica fume content did not lead

to higher permeability. This could be attributed to the use of high-range water-reducing

admixture (superplasticizer), which provided the needed workability for the desired water-to-

cementitious material ratio. This reduction could be primarily attributed to the increased density

of the matrix in the presence of silica fume. Addition of silica fume reduces the pores in the paste

as well as the permeability at the interface between cement paste and aggregate. As a result,

concrete becomes almost impermeable.

SUMMARY AND CONCLUSIONS

Based on the data generated and analysis performed, the following conclusions were made on the

effect of silica fume content and water on two critical aspects of concrete performance, namely

compressive strength and permeability.

The use of pozzolan silica fume in concrete reduces the permeability of concrete. The

permeability of concrete reduces with an increase in silica fume replacement.

The compressive strength of concrete increases with increased silica fume content for

most water-to-cementitious material ratios. A combination of low water-to-cementitious

material ratio and presence of silica fume provides the highest compressive strength.

However, at a very low water-to- cementitious material ratio of 0.2, concrete becomes

sensitive to increased silica fume content.

Optimum percentage of silica fume to achieve maximum compressive strength varies

with the water content. On compressive strength, the nonlinear quadratic regression



model presented in this paper provides the effect and interaction of these parameters at

various levels.

Permeability decreases with increased silica fume content and reduced water. A

combination of low water content (0.25) and 15% silica fume replacement results in

lowest permeability (94% reduction in charges passed).

ACKNOWLEDGEMENT

This research was carried out at the Materials Testing Research Laboratory, Southern Illinois

University – Carbondale. Financial support for this research was provided by the Materials

Technology Center at SIUC through a grant from Federal Highway Administration. Material

support provided by Elkem Materials Inc., Sika Corporation, and Cresset Corporation, is greatly

appreciated.

REFERENCES

1. Chang, P. K. An Approach to Optimizing Mix Design for Properties of High-

Performance Concrete, Cement and Concrete Research, Vol. 34, 2004, pp. 623-629.

2. Larrard, F., and Sedran, T. Mixture-proportioning of High-Performance Concrete,

Cement and Concrete Research, Vol. 32, 2002, pp. 1699-1704.

3. Mindess, S., Young, J. F., and Darwin, D. C. Concrete, 2nd

edition, Prentice Hall, New

Jersey, 2002.

4. Shah, S. P., and Ahmad, S. H. High Performance Concretes and Applications, Edward

Arnold, London, 1994.

5. Mohammed, T. U., Yamaji, T., and Hamada, H. Chloride diffusion, Microstructure, and

Mineralogy of concrete after 15 Years of Exposure in tidal Environment, ACI Materials

Journal, Vol. 99, No. 3, 2002, pp. 256-263.

6. Bhanja, S., and Sengupta, B. Optimum Silica Fume Content and its Mode of Action on

Concrete, ACI Materials Journal, Vol. 100, No. 5, 2003, pp. 407 – 412.

7. Bleszynsky, R., Hooton, R. D., Thomas, M. D. A., and Rogers, C. A. Durability of

Ternary Blend Concrete with Silica Fume and Blast-Furnace Slag, ACI Materials

Journal, Vol. 99, No. 5, 2002, pp. 499-507.

8. Kosmatka, S. H., Kerkhoff, B., and Panarese, W. C. Design and Control of Concrete

Mixtures, 14th

Edition, Portland Cement Association (PCA), 2002.

9. Regourd, M., Mortueux, B., Aitcin, P. C., and Pinsonneault, P. Microstructure of Field

Concretes Containing Silica fume, Proceedings, 4th

International Symposium on Cement

Microscopy, Nevada, USA, 1983, pp. 249-260.

10. Scrivener K. L., Bentur, A., and Pratt, P. L. Quantitative Characterization of the

Transition Zone in High Strength Concretes, Advances in Cement Research, Vol. 1, No.

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11. Whiting, D. Rapid Determination of the Chloride Permeability of Concrete, Report No.

FHWA/RD-81/119, Federal Highway Administration, Office of Research &

Development, Washington, D. C., 1981

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Scientists, 6th

Edition, Prentice Hall, 1996.


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