Potential Benefits of Flyash in Attaining the Workability of Silica Fume Concrete

Dr. Jaspal Singh, Professor, Civil Engg Department, Er. Anil Kumar Nanda, Civil Engg Department, PAU Ludhiana

Introduction

Building industry, is one of the key areas of infrastructure development & for catering to the requirements of building materials, we are dependent on natural resources. As natural resources are depleting day by day, we have to think of alternate measuries. Use of industrial wastes for this purpose is beneficial as by this, not only natural resources are conserved but solution to safe disposal of industrial waste is obtained. Flyash and silica fume are the promising industrial wastes which can be easily harnessed in construction. With the increase in the number of coal-based thermal power plants in India, generation of fly ash has reached enormous proportions. In India, about 100 million tonnes of flyash is accumulated every year which is generated as waste from thermal plants. This is causing enough concern as its disposal involves design and installation of ash ponds covering large areas at each plant site. In spite of concerted efforts on a national scale, only a very small fraction (around 6%) of the fly ash is put to use in India, compared to its utilization to a greater extent in other countries.

Silica fume is also a waste by-product from the silicon metal and ferrosilicon alloy industries. The chief problems in using this material are associated with its extreme fineness and high water requirement when mixed with Portland cement. However, if used with superplasticizers, we can attain good workability of concrete.

Workability of concrete plays a vital role in all construction works, affecting the speed of construction and placing of concrete, which in turn affect the financial aspect of construction project. The use of silica fume reduces the workability of fresh concrete or mortar due to its very high specific surface area; however, it improves many of the properties of hardened concrete or mortar. Earlier workability of concrete was controlled by amount of water added during mixing and setting characteristics were adjusted with the help of admixtures to modify the properties of concrete. Nowadays, Superplasticizers are added to concrete to get highly workable concrete. They are assuming increasing popularity for use in concrete, because of advantages they offer in handling, placing and compaction of concrete. Though use of superplasticizers is very common in developed countries, the superplasticizers are not so common in developing countries like India. It is in this context that effort has been made to study the effect of addition of superplasticizer in addition to fly ash and silica fume on workability of concrete.

Materials

xOrdinary Portland cement (OPC) of 43 grade (Ultratech) confirming to IS: 8112:1989 was being used for making concrete. The relevant cement properties experimentally obtained are given in Table 1.




The coarse aggregate used were a mixture of two locally available crushed stone of 10 mm and 20 mm size in 50:50 proportion. The aggregates were washed to remove dirt, dust and then dried to surface dry condition. Specific gravity and other properties of coarse aggregate are given in Table 2. Then sieve analysis of coarse aggregate was done. Proportioning of coarse aggregate was done and Fineness Modulus was obtained as given in Table 3.


Cumulative percentage weight retained = 674.1

Fitness Modules (F.M.)= Say 6.74

It lies within desirable range 5.5-8.0



Fine aggregates were collected from Chaki River Pathankot. It was coarse sand brown in color. Specific gravity of fine aggregates was experimentally determined as 2.62. Then test on sieve analysis of fine aggregates was performed to get Fineness Modulus.

Fly Ash

Fly ash used in present work was obtained from Guru Hargobind Thermal Plant Lehra Mohabbat, Distt. Bathinda. The fly ash, which was used, falls under class F category. The results of physical properties are given in Table 5.

Silica Fume

The silica fume used was obtained from Orkla India (Pvt) Ltd (Brand name: Elkem Microsilica 920-D), Navi Mumbai. Its chemical composition and other properties are given in Table 6.

Super Plasticizer

The super plasticizer used in the study program was Rheobuild SPI obtained by Basf construction chemicals (India) Pvt. Ltd., Navi Mumbai. It was based on Naphthalene formaldehyde polymer. The physical and chemical properties of super plasticizer, which was obtained from the company, conform to IS-9103-1979 and are given in Table 7.

Mix Design by Indian Standard Recommendations

Present investigation includes design of concrete mix (non-air entrained) for medium strength concrete. The guidelines given in various codes like SP: 23-1982, IS: 10262-1982 and IS: 456-2000 have been adopted for mix design of concrete.



For the present investigation, it is required to have characteristic compressive strength 40 N /mm². the mean target strength is 49.24N/mm² The compaction factor for the design mix is taken as 0.9. The maximum size of aggregate is 20 mm (angular). Type of exposure is taken as moderate and degree of quality control as very good.

Trial Mixes

The quantity of cement obtained after mix design i.e. 579.375 is much more than the maximum range of cement i.e. 450 kg/m³ as specified in IS 456-2000. So five trial mixes were prepared and average cube strength were obtained after 7 days & 28 days as given in Table 8.

Workability of Concrete

The compaction factor test was performed to see the effect of addition of silica fume and flyash on concrete. The workability of reference and all other concrete mixes as detailed in Table 9 was measured in terms of compaction factor test. It is observed that compaction factor lies between 0.845 to 0.92. Workability of concrete slightly improved with the addition of percentage of flyash to all the percentage of silica fume. In the case of 4 % silica fume and at 0% level of flyash, compaction factor was 0.87. With the addition of 10%, 15% and 20% of flyash, compaction factor improved / increased to 0.887, 0.895 and 0.902 respectively. For 8% of silica fume & at 0% level of flyash, compaction factor was 0.852. With the addition of 10%, 15% & 20% of flyash, compaction factor improved to 0.86, 0.874 & 0.886 respectively. Similarly, for 12% of silica fume and at 0% level of flyash, compaction factor was 0.845. With the addition of 10%, 15%, and 20% of flyash, compaction factor improved to 0.857, 0.869 and 0.882 respectively. The improvement in workability with the addition of flyash to the concrete can be explained on the basis of ball bearing effect of spherical particles of flyash as spherical particles needs less water as compared to other shapes. Probably, another factor contributing to the improvement in workability is increased amount of paste in mix which in turn produces a lubricating effect on ingredients of concrete and helps in achieving a free flowing concrete with closer packing of materials. Conversely, the workability decreased with the addition of percentages of silica fume to all the percentages of flyash. At 0% level of flyash, compaction factors were 0.87, 0.852 & 0.845 with the addition of 4%, 8% & 12% of silica fume respectively. At 10% level of flyash, compaction factors were 0.887, 0.860 & 0.857 with addition 4%, 8% & 12% of silica fume. At 15% level of flyash, compaction factors were 0.895, 0.874 & 0.869 with the addition of 4%, 8%, & 12% of silica fume. Similarly, at 20% level of flyash, compaction factors were 0.902, 0.886 & 0.882 with addition 4%, 8% & 12% of silica fume. The optimum value of compaction factor was at the replacement level of 24% i.e. 20% of flyash & 4% of silica fume by weight of cement. After the optimum level of replacement of flyash & silica fume, if we still add silica fume corresponding to 20% of flyash, the compaction factor starts decreasing. It is due to the fact that surface area is increased due to increased fineness and greater amount of water is required to get a closer packing which results in decrease in workability of concrete mixes at higher replacement levels. The variation of workability with different %ages of flyash and silica fume is as shown in Figure 1.


Effect of various %ages of silica fume and flyash on Workability.

The effect of various %age of silica fume and fly ash on workability was statistically significant at 5% level of significance. The values of critical difference and mean compaction factor are given in Table 10.

Summary

The workability was determined using compaction factor test. The statistical analysis was applied on values/results of workability of concrete. All the values/results were found statistically significant.

From the experimental investigation, the following main conclusions can be drawn:

1. Low water cement ratios like 0.32 can be tried for producing a concrete for commercial purposes but appropriate superplasticizer compatible with the materials are required to be used.
2. Optimum level of replacements of cement by flyash obtained from Guru Hargobind Thermal Plant Lehra Mohabat, Distt. Bhatinda is around 10% for producing medium range of workability concrete.
3. Optimum level of replacements of cement by silica fume is around 4% for producing medium range of workability concrete.
4. The combination of flyash and silica fume is capable of producing a medium range of workability of concrete as partial replacement of cement. The optimum replacement levels of flyash and silica fume are 20% and 4%. This optimum level of combination gave maximum value of compaction factor i.e 0.902
5. As silica fume & superplasticizer are costly materials and it may not be economical to use them. But when these materials are used with flyash (a waste), workability is likely to improve as evident from the investigation carried out by the authors.

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