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Cement & Concrete Composites 13 (1991) 203-208 Freeze-Thaw Durability of Air-Entrained CSF Concrete B. B. Sabir Department of Civil Engineering and Building, The Polytechnicof Wales, Pontypridd, Mid-Glamorgan CF37 1DL, UK K. Kouyiali L. G. Mouchel and Partners, West Hall, Parvis Road, Weybridge,Surrey, UK Abstract In recent years considerable attention has been given to research aimed at energy conservation in the cement and concrete industry. This is partly being accomplished by the use of less energy- intensive cementitious materials such as fly ash, slags and pozzolans in concrete. Lately some atten- tion has been given to the use of condensed silica fume (CSF) as a possible partial replacement material for cement. This paper reports the results of a laboratory investigation to determine the strength and freeze and thaw durability of concrete incorporating various amounts of CSF. A total of five concrete mixes were made incorporating 0-12% CSF as partial replacement by mass of cement. All mixes were air entrained and had a constant water to cementitious materials ratio, W/ (C + CSF), of 0.4. CSF improved the compressive strength of 7-days and 28-days concrete. The flexu- ral strength after 35 cycles of freezing and thawing was also increased with increasing CSF content. All the CSF concretes performed satisfactorily when subjected to freezing and thawing, even though the performance was somewhat inferior to that of the reference mix. There were no noticeable differences in the physical appearances of the concrete prisms after the 35 cycles of freezing and thawing. Keywords: Silica fume concrete, freeze-thaw resistance, concrete durability, compressive strength, pulse velocity, air entrainment, durability factor, cement replacement, strength of materials. INTRODUCTION The need for more durable concrete, in particular concrete with improved resistance to freeze-thaw exposure in the presence of salts, has motivated a number of investigations on condensed silica fume (CSF) concrete. These investigations include studies of air pore system characteristics, ice formation and pore structure, freeze-thaw tests with and without de-icing salts, and chemical resistance. Although higher strengths appear to improve the freeze-thaw resistance of concrete, even strength levels of 70 MPa cannot always guarantee adequate performance. Several studies, including that by Sorensen, 1 indicated that air entrainment is necessary to ensure good freeze-thaw resistance of CSF concrete in spite of a possible reduction in compressive strength. This paper reports the results of freeze-thaw tests conducted on air-entrained concrete samples containing varying dosages of CSF. The durability of the concrete prisms exposed to repeated cycles of freezing and thawing was determined from weight, length, resonant frequency and pulse velocity measurements of the test specimens before and after freezing and thawing, by compar- ing these with the corresponding values of the 203 Cement & Concrete Composites 0958-9465/91/$3.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

Freeze-thaw durability of air-entrained CSF concrete

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Page 1: Freeze-thaw durability of air-entrained CSF concrete

Cement & Concrete Composites 13 (1991) 203-208

Freeze-Thaw Durability of Air-Entrained CSF Concrete B. B. Sabir

Department of Civil Engineering and Building, The Polytechnic of Wales, Pontypridd, Mid-Glamorgan CF37 1DL, UK

K. Kouyiali

L. G. Mouchel and Partners, West Hall, Parvis Road, Weybridge, Surrey, UK

Abstract

In recent years considerable attention has been given to research aimed at energy conservation in the cement and concrete industry. This is partly being accomplished by the use of less energy- intensive cementitious materials such as fly ash, slags and pozzolans in concrete. Lately some atten- tion has been given to the use of condensed silica fume (CSF) as a possible partial replacement material for cement. This paper reports the results of a laboratory investigation to determine the strength and freeze and thaw durability of concrete incorporating various amounts of CSF. A total of five concrete mixes were made incorporating 0-12% CSF as partial replacement by mass of cement. All mixes were air entrained and had a constant water to cementitious materials ratio, W/ (C + CSF), of 0.4. CSF improved the compressive strength of 7-days and 28-days concrete. The flexu- ral strength after 35 cycles of freezing and thawing was also increased with increasing CSF content. All the CSF concretes performed satisfactorily when subjected to freezing and thawing, even though the performance was somewhat inferior to that of the reference mix. There were no noticeable differences in the physical appearances of the concrete prisms after the 35 cycles of freezing and thawing.

Keywords: Silica fume concrete, freeze-thaw resistance, concrete durability, compressive

strength, pulse velocity, air entrainment, durability factor, cement replacement, strength of materials.

INTRODUCTION

The need for more durable concrete, in particular concrete with improved resistance to freeze-thaw exposure in the presence of salts, has motivated a number of investigations on condensed silica fume (CSF) concrete. These investigations include studies of air pore system characteristics, ice formation and pore structure, freeze-thaw tests with and without de-icing salts, and chemical resistance. Although higher strengths appear to improve the freeze-thaw resistance of concrete, even strength levels of 70 MPa cannot always guarantee adequate performance. Several studies, including that by Sorensen, 1 indicated that air entrainment is necessary to ensure good freeze-thaw resistance of CSF concrete in spite of a possible reduction in compressive strength.

This paper reports the results of freeze-thaw tests conducted on air-entrained concrete samples containing varying dosages of CSF. The durability of the concrete prisms exposed to repeated cycles of freezing and thawing was determined from weight, length, resonant frequency and pulse velocity measurements of the test specimens before and after freezing and thawing, by compar- ing these with the corresponding values of the

203 Cement & Concrete Composites 0958-9465/91/$3.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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204 B. B. Sabir, K. Kouyiali

reference prisms (0% CSF ), and by calculating the durability factors. After the freezing and thawing cycles, the test prisms were broken in flexure.

CONCRETE CONSTITUENTS

Cement The cement used was ordinary Portland cement (OPC) (type 1) commonly used in the UK and complying with BS 12. The physical properties and chemical composition of the cement are given in Table 1.

Fine aggregate Washed natural sea-dredged sand from the Bristol Channel was used throughout the investigation. Sieve analysis showed that the sand complied with grades M and F of BS 882. The results of sieve analysis and the physical properties are shown in Tables 2 and 3 respectively.

Coarse aggregate Ten-miilirnetre single-size crushed limestone from Hobbs quarries in Bridgend was used. The grad-

Table 1. Physical properties and chemical composition of cement (OPC) and condensed silica fume (CSF)

OPC CSF

15 0"15 350-390 15000-20000

3'14 2"20

20"9 92"0 4"5 0"7 2"2 1"2

64-0 0"2 2"3 0"2 0"88 2"0

Physical properties Particle size (/~m) Surface area (m2/kg) Specific gravity

Chemical composition (%) Silicon dioxide (SiO2) Aluminium oxide (AI~O3) Ferric oxide (Fe203) Calcium oxide (CaO) Magnesium oxide (MgO) Sodium oxide (Na20) and

potassium oxide (K20) Loss on ignition 1-0 Insoluble residue 0.5 -- Sulphuric anhydride (SO3) 3.2 --

Table 2. Grading of aggregates

Coarse aggregate Fine aggregate

Table 3. Physical properties of aggregates

Coarse" Hne ~'

Relative density (SSD) 2"65 2.70 Absorption (%) 0.34 2.26

"Crushed limestone. hDredged sand.

ing and physical properties of the coarse aggre- gates are given in Tables 2 and 3 respectively.

Condensed silica fume The silica fume was supplied in a slurry form. The slurry is 50% CSF and 50% water, by weight, with a specific gravity of 1.4. The physical properties and chemical composition of the condensed silica fume are given in Table 1.

Air-entraining agent Cormix AE1 air-entraining agent, a dark brown liquid, was used in all of the concrete mixes. It is a neutralised wood resin with no chloride content and conforms to ASTM designation C 260 and BS 5075: Part II: 1983 for air entraining admix- tures. Its specific gravity is 1.034.

Superplasticizer Cormix SP 1 superplasticizer was used throughout the investigation. This was a dark brown liquid of specific gravity 1.14 at 20°C and with negligible chloride content.

EXPERIMENTAL INVESTIGATION

Mixing and sample preparation The mix proportioning details are given in Table 4. The mixture referred to as the reference con- crete did not contain CSF. The other mixtures were identical to the reference mixture except that they contained 3%, 6%, 9% and 12% CSF

Sieve size % Retained Sieve size % Retained (mm)

10 0-6 5 mm 0.0 5 95.4 2-36 mm 4.0 2.36 98"9 1.18 mm 13'1 2.36 100.0 600 ~m 38.7

300 ~m 88'9 150 ~m 100"0

Table 4. Mix proportions

Mix Cement CSF Slurry Water AE1 SP I no. (kg/m s) (%) (kg/m 3) (kg/m ~) (ml/m ~) (ml/m ~)

1 413"1 0 -- 166"1 330 -- 2 401"1 3 24.3 153"9 320 670 3 388'8 6 48"6 141"8 310 1300 4 376-7 9 72"9 129"6 300 1900 5 364"5 12 97.2 117"5 290 2450

Coarse aggregate -- 1235.3 kg/m 3. Fine aggregate -- 615.6 kg/m ~. Water/cement ratio -- 0.4.

Page 3: Freeze-thaw durability of air-entrained CSF concrete

Freeze-thaw durability of CSF concrete 205

replacement by mass of cement. The water/ cement ratio, based on the total cementitious materials used, i.e. cement (C) plus condensed silica fume (CSF), was kept constant at W/ (C+CSF)= 0.4. A superplasticizer was used to compensate for the loss in workability caused by the incorporation of the CSF. The superplasticizer was added as a percentage by mass of cement. The mixtures contained air-entrainment agent at a dosage of 40 ml for every 50 kg of cement.

To achieve optimum dispersion of the CSF particles, the water, CSF slurry, air-entraining agent and superplasticizer were mixed thoroughly. The coarse aggregate was first placed in the pan mixer followed by the liquid mixture. After a short mixing period the fine aggregate was added and finally after another mixing period the cement was introduced, All the materials were then mixed for a period of 2½ min. Although this procedure may be considered inappropriate from a large-scale production point of view, it was found very help- ful if clusters occurring in the mix as a result of the fast reaction between the cement and CSF were to be avoided. Carette & Malhotra 2 overcame this problem by adding large dosages of superplas- ticizer. These dosages, together with high percen- tages of CSE resulted in the concrete having a gluey consistency. Because of this effect, and the rapid loss of slump, the concrete required inten- sive vibration to achieve proper compaction.

The fresh concrete was subjected to slump and air content tests in accordance with BS 1881: Part 102 1883, BS 1881 and Part 106 1983, respect- ively, and the results are shown in Table 5. From each mix, six 100-mm cubes (three for 7-days and three for 28-days compressive strength tests) and three 100 mm× 100 mm and 500 mm long prisms, for the freeze-thaw tests, were cast into steel moulds. All specimens were left to cure for 24 h and were then demoulded and cured in water at 20°C. The prism specimens were water cured for 28 days before being subjected to the 24-h cycles of freezing and thawing.

Table 5. Properties of fresh concrete

Mix CSF Slump Air content Unit weight no. (%) (mm) (%) (kg/m ~)

1 0 60 6.6 2440 2 3 80 5.6 2430 3 6 70 5"0 2390 4 9 70 - - 2420 5 12 60 4.4 2380

Freeze-thaw chamber The freeze-thaw chamber used in this investiga- tion consists of refrigerating and heating equip- ment, which produces continuous freeze-thaw cycles with chamber temperature ranging between 20°C and -18°C over a period of 24 h. The measured temperatures in the specimens during a cycle changed from - 10°C to 10°C. Each speci- men was kept in its container and was surrounded by water at all times while it was in the freeze-thaw apparatus.

Fundamental transverse frequency Immediately after the specified curing period, the prisms were tested for the fundamental transverse frequency in accordance with BS 1881: Part 5: 1970. The method is based on the principle that the resonant frequency of a concrete beam depends on the velocity of compression waves propagating through it. A moving-coil exciter unit was attached to the middle of one end-face of the specimen. The exciter unit was driven by a variable-frequency oscillator and the oscillations were received by the pick-up unit and amplified by the audio-frequency amplifier. The frequency of excitation was varied until resonance was obtained in the fundamental mode of longitudinal vibration. This frequency was recorded as the natural frequency of the fundamental longitudinal vibration. After every seven freeze-thaw cycles, the prisms were removed from the chamber in a thawed condition and the test was repeated.

Ultrasonic pulse velocity The prism samples were tested for ultrasonic pulse velocity before and after the 35 cycles of freezing and thawing. Pulses of ultrasound with frequencies in the region of 150 kHz were passed through the prisms by means of transmitting and receiving transducers and the time taken was obtained using electronic circuitry.

Durability factors The freeze-thaw tests were conducted in accord- ance with procedure A of ASTM C 666. In this procedure, the specimens are frozen and thawed in water for not less than 2 h or more than 4 h. In this investigation, however, this procedure could not be followed strictly, as a result of the appara- tus being unable to complete a freeze-thaw cycle within the specified time. The durability factor (DF) is determined from the relative dynamic modulus of elasticity using the following

Page 4: Freeze-thaw durability of air-entrained CSF concrete

206 B. B. Sabir, K. Kouyiali

expression:

DF = P N / M

where P is the relative dynamic modulus of elas- ticity at N cycles (as a percentage); N is the number of cycles at which P reaches the specified minimum value for discontinuing the test or the specified number of cycles at which the exposure is to be terminated, whichever is the smaller; and M is the specified number of cycles at which the exposure is to be terminated.

The relative dynamic modulus of elasticity P is calculated from

P,, = ( n l / n ) 2 x 100

where P is the relative dynamic modulus of elas- ticity after c cycles of freezing and thawing (as a percentage); nl is the fundamental transverse frequency after c cycles of freezing and thawing; and n is the fundamental transverse frequency before freezing and thawing cycles.

The above calculation for the relative dynamic modulus of elasticity is based on the assumption that the weight and dimensions of the specimen remain constant throughout the test. This assump- tion is clearly not true and the P values thus deter- mined can be considered adequate only for purposes of comparison between concretes of dif- ferent formulations.

RESULTS AND DISCUSSION

Compressive and flexurai strength It has been known, since the early 1950s, that the strength of concrete could be enhanced by the incorporation of CSF in the mixture. Many in- vestigations have shown that the addition of small amounts of CSF results in increasing compressive strength, whereas larger amounts (more than 5% by mass of cement) very often result in decreasing strengths as a result of increased water demand. The use of suitable water-reducing agents, how- ever, allows the production of concrete with greater CSF contents at a fixed slump without an increase in the water content. In this way, the cement paste is made denser and increase in strength is obtained with increasing CSF content. Yamato et al. 3 found that for superplasticized con- crete maintained at W/(C+CSF) of 0-4 some increase in compressive strength was obtained regardless of the CSF content. The results for the 7-days and 28-days compressive strength

Table 6. Compressive and flexural strengths

Mix CSF Compressive strength Flexural strength no. (%) (MPa) (MPa after 35 cycles)

7 days 28 days

1 0 40'1 61"1 6"90 2 3 41"2 64'3 7"05 3 6 43-1 71"6 7"22 4 9 44-9 74"8 7'35 5 12 48"0 79-2 7'50

obtained in the present work, shown in Table 6, confirm these findings. These results also show that whereas the 7-days strength for the reference concrete is approximately two-thirds of the 28- days strength, this ratio is somewhat lower for the concretes incorporating CSE This suggests that there is a delay in strength gain in CSF concrete and confirms the belief that CSF does not react in concrete before 3-7 days. This could lead to con- fusion in the control of compressive strength of concrete because the ratio of the 7-days and 28- days strength is no longer in the range of 65%-70% expected for standard concretes con- taining 100% ordinary Portland cement.

The flexural strength development of CSF con- crete is somewhat similar to that for compressive strength. Carette & Malhotra 2 found that for CSF replacements of 5%-15% by mass of cement, the 28-days flexural strengths were higher than the corresponding strengths of the reference concrete. The highest flexural strength was obtained for 5% CSF replacement. The results for the flexural strengths obtained in the present study, after 35 cycles of freezing and thawing, are presented in Table 6. These show that increasing CSF contents (0-12% by cement mass) lead to consistent increase in strength. It must be pointed out that the results of Ref. 2 were for concrete mixtures with W/(C + CSF ) = 0.65 and direct comparison with the results obtained here could not be made.

Freeze-thaw durability To date, only limited information has been published on the durability of CSF concrete. In 1952, concrete prisms containing 15% silica fume dust (as replacement of Type 1 Norwegian cement) were placed in Blindtarmen tunnel in downtown Oslo, as part of a research programme aimed at finding a concrete composition capable of with- standing the attack of the very aggressive under- ground water that collected in the tunnel. These concrete prisms were continuously examined, and in 1972 the prisms containing CSF were found to

Page 5: Freeze-thaw durability of air-entrained CSF concrete

Freeze-thaw durabifity of CSF concrete 207

be in a good condition when compared with those made with Type 1 cement, which were badly damaged.

The durability of CSF concrete under freeze-thaw conditions has not been clearly established and the very few available data are contradictory. Nevertheless, the oldest structure built with CSF concrete in Norway is now 20 years old and appears to be in a good condition. Also the SKW Canada sidewalk built in 1980 in Becancour, Quebec, has survived 10 Canadian winters and numerous de-icing salt applications. 4 The high compressive strength of CSF concrete and its low permeability are the factors that may be responsible for its improved durability when compared with standard concrete.

In the present work, the durability of concrete prisms exposed to repeated cycles of freezing and thawing was determined from weight, length, resonant frequency and pulse velocity measurements of the test specimens before and after the freezing and thawing cycles. These

Table 7. Summary of test results after 35 cycles of freezing and thawing

Mix CSF Percentage change Durabifity no. (%) factor

Weight Length Pulse velocity

1 0 -0.51 0.05 -0 .99 99.7 2 3 -0 .41 0.05 -1.11 99.5 3 6 -0-35 0.07 -0 .90 98.9 4 9 -0 .31 0-08 -0 .84 98.0 5 12 -0 .28 0-12 -0.71 96.7

Each result is the average of three tests.

measurements were compared with the cor- responding values obtained for the reference prisms. The test data shown in Table 7 demon- strate that all prisms, i.e. the reference concrete and the concretes incorporating CSF, performed excellently in the freezing and thawing tests. The durability factors throughout the test were greater than 96% (see Fig. 1), and little expansion was taking place. Although concretes incorporating

D u r a b i l i t y Factor (%) 101

1 0 0

gg

9 8

9 7

g 8 0

0% CSF

3% CSF

e% CSF

9% C8F

12% CSF

I

5

Fig. I.

I I I I I I I

10 15 2 0 25 30 38 4 0

N u m b e r of F r e e z i n g and T h a w i n g C y c l e e

Durability factors for varying CSF contents.

Page 6: Freeze-thaw durability of air-entrained CSF concrete

208 B. B. Sabir, K. Kouyiali

CSF gave somewhat lower durability factors than the reference concrete, the general level of dur- ability remained high. One factor contributing to the satisfactory performance is probably the pre- sence of a good air void system/This could not, however, be ascertained as the air void para- meters of the hardened concrete were not deter- mined.

Carette & Malhotra 2 found that air-entrained concretes without and with 5% CSF showed much lower air void spacing factors than those containing 10%-30% CSF, although all the con- cretes had approximately the same air content. The poor performance of concretes incorporating 20% and 30% CSF could not be explained in terms of the high values of the spacing factors alone, because concretes containing only 10% and 15% CSF also had high spacing factors. The poor performance of concretes with high CSF contents may be due to the very dense cement matrix, which, in turn, adversely affects the movement of water. Generally, all the concretes that had W/ (C + CSF)= 0.4, air entrainment of 4%-5% and cement replacements by CSF of up to 15% per- formed satisfactorily when subjected to repeated freeze-thaw cycles. These results are in agree- ment with the results reported in this paper.

CONCLUSIONS

The incorporation of condensed silica fume in concrete by up to 12% improved the compressive strength at 7 days and 28 days. Also, the develop- ment of the flexural strength followed a similar

pattern to that of the compressive strength. All the test prisms performed well when they were sub- jected to 35 24-h cycles of freezing and thawing, even though the CSF specimens were somewhat inferior in performance when compared with the reference prisms. Using a fixed dosage of air- entraining agent, the resulting volume of air decreased appreciably with increasing CSF con- tent.

REFERENCES

1. Sorensen, E. V., Freezing and thawing resistance of con- densed silica fume (microsilica) concrete exposed to de- icing chemicals. Proc. First Int. Conf. Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, Montebello, Canada, 31 July-5 August 1983, ed. V. M. Malhotra. A CI Special Publication SP- 79, pp. 709-18.

2. Carette, G. G. & Malhotra, V. M., Mechanical properties, durability and drying shrinkage of Portland cement con- crete incorporating silica fume. Cement, Concrete and Aggregates, CCA GDP, 5 ( 1 ) (1983) 3-13.

3. Yamato, T., Emoto, Y. & Soeda, M., Strength and freezing and thawing resistance of concrete incorporating con- densed silica fume. Proc. Second Int. Conf. Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Madrid, April 1986, ed. V. M. Malhotra. AC1 Special Publication SP-91, pp. 1095-118.

4. Regourd, M., Mortureux, B., Aitcin, P. C. & Pinsonneault, P., Microstructure of field concretes containing silica fume. Fourth Int. Conf. on Cement Microscopy, Inter- national Cement Microscopy Association, Las Vegas, NV, 1983, pp. 249-60.

5. Bilodeau, A. & Carette, G. G., Resistance of condensed silica fume concrete to the combined action of freezing and thawing cycling and de-icing salts. Proc. Third Int. Conf. Use of Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Trondheim, June 1989, ed. V. M. Malhotra. A C1 Special Publication SP- 114, pp. 945-70.