Dr. S.M.Gupta, Department of Civil Engineering National Institute of Technology, Kurukshetra.
The use of silica fume as a mineral admixture for the production of high strength concrete and high durable concrete is gaining importance in recent years. The objective of the present experimentation is to study the effect of silica fume as additive on the strength and durability characteristics of concrete obtained using locally available material. Concrete mix for M20 gradeis designed which serves as basic control mix. Silica fume concrete mixes are obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. The compressive strength development and durability against acidic and alkaline attack is studied.

Introduction

The present trend in concrete technology is to increase the strength and durability of concrete to meet the demands of the modern world. These factors can be achieved in concrete by adding various blending materials with cement or separately to concrete. The materials suitable for blending are flyash, blast furnace slag, silica fume, etc. Silica fume concrete (SFC) is emerging as one of the new generation construction material. It can be considered as high strength concrete or high performance concrete

The use of pozzolanic admixtures like condensed silica fume, because of its finely divided state and very high percentage of amorphous silica, proved to be the most useful if not essential for the development of very high strength concretes and/or concretes of very high durability. It is recommended that for applications in concrete silica fume should conform to certain minimum specifications such as silicon dioxide content of not less than 85%, spherical shape with a number of primary agglomerates with particles of size ranging from 0.01 to 0.3 microns (average of 0.1 to 0.2 microns), amorphous structure and a very low content of unburnt carbon.

Silica fume is known to improve both mechanical characteristics and durability characteristics of concrete since, both the chemical and physical effects are significant. Physical effect of silica fume in concrete is that of a filler, which, because of its fineness, can fit into spaces between cement grains in the same way that sand fills the spaces between particles of coarse aggregate and cement grains fill the spaces between sand grains. As for chemical reactions of silica fume, because of high surface area and high content of amorphous silica, this highly active pozzolan reacts more quickly than ordinary pozzolans.

Experiments have revealed that silica fume in concrete essentially eliminates pores between 500 to 0.5 micron sizes and reduces the size of pores in the 50 to 500 micron range. Physical and chemical mechanisms made the silica fume more effective in reducing pore size.

Experimental Programme

An experimental program was carried out to find out the strength and durability characteristic of concrete containing silica fume as an additive. Concrete mix for M20 grade was designed, which served as basic control mix. Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement.

Materials Used

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
Ordinary Portland cement was used throughout the Experimentation. Silica fume used in the experimentation was obtained from FOSROC Chemicals (India) Limited. The physical and chemical properties of OPC and silica fume (SF) are given in Table 1. Locally available aggregates were used. Coarse aggregates crushed from igneous basalt rock of 20mm and down size having specific gravity of 2.74 and conforming to IS 383-1970 were used. For Fine aggregate local sand having specific gravity of 2.56 and conforming to grading zone I of IS: 383-1970 was used. Superplasticizer based on sulphonated naphthalene formaldehyde was used to impart additional desired properties to the silica fume concrete. The dosage of super plasticizer was 0.7% by weight of cement. Ordinary potable water was used for mixing of the ingredients.

Concrete Mixes

Mix design for M20 grade of concrete was carried out using the guidelines prescribed by IS: 10262- 1982. The designed concrete mix for M20 served as basic control mix (CM). Silica fume concrete mixes were obtained by adding silica fume to basic control mix in percentages varying from 0 to 16% at an increment of 2% by weight of cement. (viz SFC2, SFC4, SFC6, SFC8, SFC10, SFC12, SFC14, SFC16). The Basic control Concrete mix proportion obtained was 1 part cement: 1.62 parts of fine aggregate: 3.28 parts of coarse aggregate with water–cement ratio of 0.5 and 0.7% of Superplasticizer.

Batching, Mixing, and Curing

The concrete ingredients viz. cement, sand and coarse aggregate were weighed according to proportion 1:1.62:3.28 and are dry mixed on a platform. To this the calculated quantity of silica fume was added and dry mixed thoroughly. The required quantity of water was added to the dry mix and homogenously mixed. The calculated amount of superplasticizer was now added to the mix and then mixed thoroughly. The homogeneous concrete mix was placed layer by layer in moulds kept on the vibrating table. The specimens are given the required compaction both manually and through table vibrator. After through compaction the specimens were finished smooth. After 24 hours of casting, the specimen were demoulded and transferred to curing tank where in they were immersed in water for the desired period of curing

Tests Conducted

The tests were conducted both on Fresh and Hardened concrete. The tests on fresh concrete was the workability test conducted through Slump test, Compaction factor test; Table 2 and Vee-bee consistometer test. The strength and durability tests conducted on hardened concrete are briefed here:

Compressive Strength Test

The compressive strength test was carried out on cube specimens of dimensions 150 ´ 150 ´ 150 mm. The compressive strength test specimens were cured and tested for 3-days, 7-days, 28-days, and 60-days in compressive testing machine. Three specimens were used for each test.

Durability Test Resistance Against Acid Attack

For acid attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after 24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sulphuric acid (H2SO4) solution for 60-days. The pH value of the acidic media was at 0.3. The pH value was periodically checked and maintained at 0.3. After 60-days of immersing in acid solution, the specimens are taken out and were washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Resistance Against Alkaline Attack

For alkaline attack test concrete cube of size 150 ´ 150 ´ 150 mm are prepared for various percentages of silica fume addition. The specimen are cast and cured in mould for 24 hours, after 24 hours, all the specimen are demoulded and kept in curing tank for 7-days. After 7-days all specimens are kept in atmosphere for 2-days for constant weight, subsequently, the specimens are weighed and immersed in 5% sodium sulphate (Na2SO4) solution for 60-days. The pH value of the alkaline media was at 12.0. The pH value was periodically checked and maintained at 12.0. After 60- days of immersing in alkaline solution, the specimens are taken out and are washed in running water and kept in atmosphere for 2-day for constant weight. Subsequently, the specimens are weighed and loss in weight and hence the percentage loss of weight was calculated.

Results and Discussions

Workability Test Results
Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
The result of workability of concrete as measured from slump, compaction factor and, Vee-bee degree are shown in Table 2. According to these results, workability of concrete decreases as the silica fume content in concrete increases from to 16%. No wide variations in the slump and compaction factor values for the mixes containing increased amount of silica fume were observed. The silica fume concrete did not show tendencies for seggregation and bleeding. This is due to the fact that as the percentage of silica fume increases the water available in the system decreases thus affecting the workability. As compared to control mix (CM), the mix containing 16% silica fume (SFC16) has a slump reduction of 28% and compaction factor reduction of 5.26%. The effect of silica fume content on the workability with regard to slump of concrete is shown in Figure 1.

Compressive Strength Test Results

The compressive strength of concrete containing silica fume given in Table 3 shows an increasing trend as the percentage of silica fume increases, from 0 to 16%. This is true for 3-days, 7- days, 28-days, and 60-days compressive strength. The strength activity index for 3-days, 7-days, 28-days, and 60-days for 16% of silica fume is 1.65, 1.33, 1.49 and 1.41 respectively. The effect of silica fume content on the Compressive strength of concretes is shown in Figure 2.

Resistance Against Acid Attack

Table 4 shows the change in weight of control mix and silica fume mix when immersed in 5% sodium Sulphric acid (H2SO4) solution. Under a very low pH (0.3 pH) of 5% - H2SO4 Solution, all hydrated products, hydrated silicate and aluminate phases and calcium hydroxide, can easily be decomposed. The control mix was markedly affected by 5% - H2SO4 solution with a significant weight loss. On the other hand, the progress of deterioration in silica fume concrete immersed in 5% - H2SO4 solution varied widely depending on the percentage of silica fume. SFCl6 mix was found to be most effective in preventing the Sulphuric acid attack. It appears that in the Sulphuric acid attack, the early decomposition of calcium hydroxide and subsequent formation of layer amount of gypsum are attributed to the progressive deterioration accompanied by the scaling and softening of the matrix. The percentage weight loss, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65

The effects of silica fume content on the acidic media durability shown in Figure 3.

Resistance Against Alkaline Attack

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing

Experimental Investigation on the Strength and Durability Characteristics of Concrete Containing
Table 4 shows the change in weight of control mix and silica fume concrete when immersed in % sodium sulphate (Na2SO4) solution. The pH value of 5% sodium sulphate (Na2SO4) solution was found to be 12. The percentage weight loss, which is an indication of durability, decreases as the percentage of silica fume in concrete increases.

The weight loss index for SFC16 is 0.00 while for control mix it is 1.00. This may be due to the fact that the silica fume, which also acts as a filler material, increases the density of concrete by filling the voids. The voids, which are very compactly filled up by the silica fume, do not allow the alkaline media to penetrate into concrete mass and also reduced content of calcium hydroxide in die silica fume concrete due to pozzolanic reaction. Thus the percentage weight loss will be less as the percentage of silica fume in concrete increases. The effect of silica fume content on alkaline media of concretes is shown in Figure 4.

Compressive strength of Silica Fume Concrete after 60-days Immersion in Acidic Media and Alkaline Media

Table 5 shows test result of 60-days compressive strength of silica fume concrete, when exposed to two different media viz Acidic and Alkaline media the strength activity index shows an increasing trend as the silica fume increases from to 0 16%.

The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media. The effects of silica fume content on the compressive strength after 60-days immersion in Acidic media and Alkaline media of concretes is shown in Figure 5.

Conclusions

These studies have lead to the following conclusions:
  1. The workability of concrete as measured from slump, compaction factor and Vee-bee degree decreases as percentage of silica fume in concrete increases. As compared to the control mix, SFC16 has a slump reduction of 28% and compaction factor reduction of 5.26%. Thus the workability of concrete decreases as the percentages of silica fume in concrete increases.
  2. The compressive strength of concrete shows an increasing trend as the silica fume content increases, from 0 to 16%. This increasing trend is evident for 3-days, 7-days, 28-days, and 60- days compressive strength. The strength activity index for SFC16 is 1.65, 1.33, 1.49, and 1.41 at 3- days, 7-days, 28-days, and 60- days respectively. Thus silica fume acts as a pozzolanic material, hence the compressive strength of concrete increases as the percentage of silica fume increases.
  3. Resistance against acidic attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in acidic media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.65. Thus since silica fume acts as a filler material and fills up the voids of concrete, the durability of concrete in acidic media increases as the percentage of silica fume in concrete increases.
  4. Resistance against alkaline attack of silica fume concrete increases as the silica fume content increases from 0 to 16%. The percentage weight loss, which is an indication of durability in alkaline media, decreases as the percentage of silica fume in concrete increases. The weight loss index for SFC16 is 0.00 while for control mix it is 1.00.
  5. The 60-days compressive strength of silica fume concrete, when exposed to two different media viz. Acidic and Alkaline media shows an increasing trend as the silica fume increases from 0 to 16%. The strength activity index for SFC16 is 1.92 for acidic and 1.42 for alkaline as compared to the control mix in the respective media.
References
  1. ACI Committee 226R (March- Apr. 1981), “Silica Fume in Concrete,” ACI Material Journal, 84, 3, pp. 158–166.
  2. Conferences/Seminars/ Workshops.
  3. Cook, J.E., “Research and Application of High Strength Concrete, 10.000 psi Concrete,” Concrete International, Oct., 1989, pp. 67-75.
  4. Duval R. and E.H. Kadri (1998), “Influence of Silica Fume on the Workability and the Compressive Strength of High Performance Concrete,” Cement and Concrete Research, 28, 4, pp.533-547.
  5. Gupta, S.M., “Experimental Studies on the Behavior of High Strength Concrete,” Ph.D. Thesis, 2001, K.U.Kurukshetra.
  6. I.S. : 10262–1962, “Indian Standard Recommended Guidelines for concrete mix design,” BIS, New Delhi.
  7. I.S.: 383-1970 (1990), “Specification for coarse and Fine Aggregate from Natural source for concrete,” Bureau of Indian Standards, New Delhi.
  8. Leming M.L., “Properties of High Strength Concrete: An Investigation of Characteristics High Strength Concrete Using Materials in North Caroling Research Report FHWA/ NC/88-006,” Department of Civil Engineering, North Carolina State University, Raleigh, N.C., July, 1988.
  9. Mehta, P.K., and Gjorv, O.E. “Properties of Portland Cement Concrete Containing Silica Fume,” Cement and Concrete Research, V. 12, No. 5, Sept.1982, pp. 587-595.
  10. Moreno, J., “The State–of–the–Art of High Strength Concrete in Chicago, 225W. Wacka Drive. Concrete International, Jan., 1990, pp. 35-39.
  11. Neville A.M. (2000), “Properties of Concrete,” Fourth and Final Edition - Pearson Education Asia Ltd.
  12. Ojho, R. N., “Use of Flyash and Condensed Silica in Making Concrete,” IE (I), Journal V. 77, November, 1996, pp. 170-173.
  13. Rachel J. Detwiler and P. Kumar Mehta (Nov.-Dec. 1989), “Chemical and Physical Effect of Silica Fume on the Mechanical Behavior of Concrete,” ACI Materials Journal, 86, 6, pp. 609-614.
  14. Sellevold, E.J. and Nilsen, T. Supp1ementary Cementing Materials for Concrete, Ed. By V.M. Malhotra. CANMET, SP 86-8E, pp. 167-246/1987.

NBMCW June 2007