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have been tabulated and are shown in Tables 2 and 3. The results have been analyzed and the bar charts showing the strength variations have been plotted (fig.1 to 10).

Workability

The workability of concrete is improving with increased percentage of fly ash and in the case of M50 grade concrete the quantity of superplasticizer was decreased. In the case of concrete containing condensed silica fume the workability of concrete is decreasing with increased percentage. However, no segregation was observed. The workability of the mixes was maintained at medium level by judiciously adding superplasticizer.

Compressive Strength

1. From the tables 2 and 3 it is observed that with the increase in the percentage of condensed silica fume the compressive strength of the concrete is increasing at all ages relative to the reference strength. The optimum compressive strength of concrete at 28 days and above is produced with 10 percent replacement of cement by condensed silica fume and the strengths are gradually decreasing beyond 10% CSF.

2. It is observed that the maximum early age compressive strengths of the concrete at the age of 7 days is achieved at 15% and 10% replacement with CSF and is increasing by 5% for M-50 and M-80 grade concretes over the reference concrete.

3. Due to replacement of OPC by fly ash, the strength gets decreased, compared to that of reference mix.

4. With 20% fly ash replacement of OPC the strength values are optimum and they are only marginally less by 2%.

5. There is decrease in strength of M-50 grade concrete with fly ash replacement, as it may be due to lesser availability of cement in the mix and at the same time it is showing an improved gain in strength for M-80 grade concrete.

Table 1: Summary of Mix Proportions Mix w/c

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development

Flexural Strength

1. With the replacement of cement with condensed silica fume by 10% there is a maximum increase in flexural strength of concrete at the age of 28 days, which is more compared to reference mix with 0% CSF. This is true in both the concrete grades considered.

2. With the replacement of cement with condensed silica fume beyond 10%, there is a fall in the flexural strength of the concrete but still it is more than that of control mix.

3. With fly ash replacement, the trend in the flexural strength is same as compressive strength.

Split Tensile Strength

1. With the replacement of cement with condensed silica fume by 10% there is a maximum increase in the split tensile strength of the concrete at the age of 28days and it is (9.82%) more than that of M-50 grade control mix.

2. With the replacement of cement with condensed silica fume beyond 10%, there is a marginal increase in the split tensile strength of the concrete and also with 15%

replacement, but the strength is comparatively less than that of 10% replacement of OPC by fly ash is optimum even for split tensile strength.

Static Modulus of Elasticity

1. The static modulus of elasticity measured based on the IS code procedure has shown 8.8% and 1% increase compared to control concrete at 10% replacement of cement by condensed silica fume for M-50 and M-80 grades respectively.

2. A marginal decrease in static modulus was observed with 20% replacement of OPC by fly ash.

Use of Mineral Admixtures in High Performance Concrete (HPC)

Mineral admixtures like fly ash and condensed silica fume are available as industrial wastes where used as part replacement of OPC in cement composites, helps in several ways. The present investigation is limited to the mechanical properties, where high strength concrete mixes are being used. Replacement of OPC by suitable mineral admixtures not only helps in the flow and strength, but also in durability, which is a very important requirement. Hence, the present study heads to the use of mineral admixtures in HPC which satisfies all the requirements like strength, durability etc.

Fig. 1: Variation of compressive strength of M-50 grade of concrete at various percentages of fly ash 0

Table 2: Test Results of Compressive Strength at Different Ages Mix FA SF 7day

Mechanical Properties of High Strength Concrete Composites with Mineral Admixtures

Fig. 2: Variation of compressive strength of M-50 grade of concrete at various percentages of condensed silica fume

Fig. 3: Variation of compressive strength of M-80 grade of concrete at various percentages of fly ash

Fig. 4: Variation of compressive strength of M-80 grade of concrete at various percentages of condensed silica fume

Fig. 5: Variation of flexural strength of M-50 and M-80 grades of concrete at 28days at various percentages of fly

ash Table 3: Test Results of Split, Flexural Strength and Young’s Modulus Values.

Mix FA SF split tensile

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development

Fig. 6: Variation of flexural strength of M-50 and M-80 grades of concrete at various percentages of condensed silica

fume

Fig. 7: Variation of split tensile strength of M-50 and M-80 grades of concrete at 28days at various percentages of fly

ash

Fig. 8: Variation of split tensile strength of M-50 and M-80 grades of concrete at various percentages of condensed silica

fume

Fig. 9: Variation of young’s modulus of M-50 and M-80 grades of concrete at various percentages of fly ash

Fig. 10: Variation of young’s modulus of M-50 and M-80 grades of concrete at various percentages of condensed silica

fume CONCLUSIONS

Based on the present experimental study, the following conclusions are drawn.

1. Fly ash improves the workability of concrete and condensed silica fume (CSF) in higher percentages decrease the workability of concrete there by requiring higher dosages of superplasticizer.

2. Addition of fly ash decreases the compressive strength and 20% fly ash can be taken as optimum from strength considerations. Strength of fly ash concrete improves with age. This is true in all the strengths considered in the present investigation.

3. CSF improves the compressive strength of concrete.

Ten percent CSF used as replacement to OPC is optimum. Beyond this, the strength gradually reduces.

This is true at all ages. The split tensile and flexural strengths also follow same trend.

0.00 2.00 4.00 6.00 8.00 10.00

0 5 10 15

%Condensed silica fume M50 grade M80 Grade

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

0 20 30 40

split tensile strength in MPa

%fly ash

0.00 1.00 2.00 3.00 4.00 5.00 6.00

0 5 10 15

%Condensed silica fume

0.00 10.00 20.00 30.00 40.00 50.00 60.00

0 20 30 40

Static young's modulus in GPa

%fly ash

0.00 10.00 20.00 30.00 40.00 50.00 60.00

0 %Condensed silica fume 5 10 15

Mechanical Properties of High Strength Concrete Composites with Mineral Admixtures 4. Even the values of Young’s modulus follow the same

trend as the strengths. CSF contributes towards higher modulus.

5. 20% fly ash is optimum from workability and strength considerations where as 5 to 10% CSF in the matrix generates optimum mechanical properties.

6. Replacement of OPC by certain percentages of CSF is necessary to produce HPC.

ACKNOWLEDGEMENTS

The authors thank the Management and Principal, Vasavi College of Engineering, Hyderabad. Our special thanks to Prof & Head Dr. B.Sridhar, for his constant encouragement and help.

REFERENCES

[1] N. Krishnaraju, “Design of Concrete Mix”- CBS publishers -1985

[2] Grutzek, M.W., Atkinson,S., and Roy, D.M.(1983).Mechanism of hydration of condensed silica fume in calcium hydroxide solutions. ACI Special publications, 79-33, 643-664.

[3] Malhotra, V.M., 1980, “Strength and durability characteristics of concrete incorporating a pelletized blast furnace slag fly ash, condensed silica fume, slag and other mineral by-products in concrete”, SP-79. V.2’American Concrete Institute, Detroit pp. 891-922.

[4] P.K. Mehta &J.M.M.Paulo, “Concrete Microstructure Properties and Materials” – McGraw Hill Publishers 1997.

[5] Mazloom, M., Ramezanianpour, A., & Brooks, J.

(2004).Effect of silica fume on mechanical properties of high-strength concrete. Cement and Concrete Composites, 26(4), 347-357.

[6] A.M.Neville, “Properties of Concrete” English Language book society – 1988.

[7] Yogendran, V., Langan, B., Haque, M., & Ward, M.

(1987). Silica fume in high-strength concrete. ACI Materials Journal, 84(2).

[8] IS: 2386 (Part III), Methods of Test for Aggregates for Concrete: Specific Gravity, Density, Absorption and Bulking.

[9] IS: 2386 (Part IV)-1963, Methods of Test for Aggregates for Concrete: Mechanical properties.

[10] IS: 516-1963, Method of Test for Strength of Concrete, 1.

IS 1344 – 1968, “Indian Standard Specifications for Pozzolonas”- Bureau of Indian Standards.

[11] IS 4031 – 1988, “Indian Standard Methods of physical tests for hydraulic cement” – First revision, Bureau of Indian Standards.

[12] IS 383 – 1970, “Indian standard specifications for coarse and fine aggregate for the Natural sources for concrete” – 2^nd revision- Bureau of Indian standards.

[13] IS 7869 (Part II), “Indian standard specifications for Admixtures in concrete” Bureau of Indian standards.

[14] IS 456 – 2000, “Plain and reinforced concrete Indian standard Specifications” Bureau of Indian standards.

[15] Thomas, M. D. A., Hopkins, D. S., Girn, G., Munro, R., &

Muhl, E. (2002, June).The use of high-volume fly ash in concrete. In Proceedings, 7^th International Gypsum and Fly Ash Science and Technology Conference, Toronto.

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development (ACEID-2014), Vasavi College of Engineering, Hyderabad, A.P. 6 - 7 February, 2014. pp.148-152.