An experimental investigation was carried out to evaluate the compressive strength of concrete cured at normal curing condition.M20 and M40 grade concrete made with 0%, 20%,40% and 60% replacement of Ordinary Portland cement (OPC) with class F fly ash. Watercementitious material ratios were 0.35 and 0.32. The compressive strength of the above grades were measured and presented in this paper. This study showed that the strength of OPC concrete with various percentage of replacement of class F fly ash increases with increase in time.

Keywords: High volume fly ash; concrete; strength; superplasticizer; curing The production of Portland cement is not only costly and energy-intensive, but it also produces large amounts of carbon emission. The production of every tonne of Portland cement, releases about one tonne of carbon dioxide (CO2) into the atmosphere [7]. Fly ash is a by-product of the combustion of pulverized coal and is collected by mechanical and electrostatic separators from the fuel gases of thermal power plants, where coal is used as a fuel.

Fly ash is commonly used in concrete in replacements,ranging from 0 % to 30 % by weight of the total cementitious material [8]. Large quantities of fly ash are available around the world at low costs and the use of high-volume fly ash (HVFA) seems to offer,the best solution to rising cement demands [11, 12].Partial replacement of cement with supplementary cementing materials (SCMs) reduces greenhouse gas (GHG) emission proportionately and results in a more "green" concrete, through reduced energy consumption (energy required to produce cement) and prevents the depletion of natural resources (raw materials used for manufacturing the cement).

Additional benefits include minimization of waste disposal (land filling these industrial by-products).The use of HVFA in concrete has recently gained popularity as a resource-efficient, durable, costeffective,sustainable option for Ordinary Portland cement (OPC) concrete applications [10].

High Volume Fly Ash Concrete

The current annual worldwide production of coal ash is estimated about 700 million tonne of which atleast 70 % is fly ash. Due to its pozzolanic nature, fly ash is a beneficial mineral admixture for concrete [9]. It influences many properties of concrete in both fresh and hardened concrete. More over utilization of waste materials in cement and concrete industry reduces the environmental problems, utilization also reduces the amount of solid waste, green house gas emissions associated with Portland clinker production and conserves existing natural resources.

Although 25% to 35% fly ash by weight of the cementitious material is considerably higher than 15% to 20%, this is not high enough to classify the mixtures as HVFA concrete according to the definition proposed by Malhotra and Metha [1, 2]. From theoretical considerations and practical experiences the author have determined that with 50% or more cement replacement by fly ash, it is possible to produce sustainable, high performance concrete mixtures that show high workability, high ultimate strength and high durability [3, 4]. In this context, it is planned to investigate the influence of fly ash, with various replacement levels. The details and properties of the various percentage of fly ash with cement,water- binder ratio, workability and strength of the high volume fly ash are emphasized in this paper.

Material used in the Investigation

Cement

The cement used was Ordinary Portland Cement(OPC) and its physical properties are given in Table 1.

Fly Ash

The fly ash (Class F) used was obtained from the Ennore thermal power station in Chennai, India. The chemical properties of the Ennore fly ash are given in Table 2. The specific gravity of the Ennore fly ash was 2.65.

Aggregate

The sand (FA) and coarse aggregate (CA) used were locally available. The gravel was 20 mm maximum nominal size. Specific gravity of fine and coarse aggregate is 2.68 and 3.03.

Superplasticizer

The superplasticizer used was a water reducing agent, suitable for fly ash concrete.

Experimental Program

For determining the influence of fly ash in the concrete, the following parameters were selected for the experimental program and are as follows;Grade of Concrete = M20 and M40 Water / Binder ratio = 0.35 and 0.33 % of replacement of cement with Fly ash = 0, 20, 40 and 60% Compressive Strength (Normal Curing) = 1, 7 and 28 days

Mix Proportions

The Indian standard (IS 10262 - 1982) is followed to design the mix for M20 and M40 grade concrete with the above parameters [5]. In each grade, the control mix (with out fly ash) was designed and further 20, 40 and 60% of the cement was replaced with the fly ash. Finally the following mix designs were arrived, which are given in the following Table 3 and Table 4.

Workability of Concrete

For M20 and M40 grade concrete, the control mixes had higher workability, i.e. the values of the slump were in the range of 60 - 100 mm. But, for 20,40 and 60% replacement of cement with fly ash, the workability was reduced since the mix was highly cohesion less. It may be due to the absorption of more water by the fly ash concrete and hence less water to binder ratio was available for the workability. Hence the workability of control mix, 20, 40 and 60% replacement of cement with fly ash mixes was found by using the slump cone test as per the ASTM standard. The slump values for each mix were tested and the values were in the range of 60mm to 100mm including control mix.The workability of the concrete mixes was compensated or increased [6] by using the water reducing agent in the above concrete mixes. The details of the Superplasticizer dosage are given in the following Table 5 and Table 6. The optimum dosage of the chemical admixture was found out by using the marsh cone method.

Test Program

Then the concrete was placed in the 100mm x 100mm x 100mm cubes and compacted with the high frequency vibrating table. After 24 hours, the cubes were removed from the mould, and were immersed in water continuously. The compressive strength was found at 1, 7 and 28 days and the values are furnished in the Table 7, Table 8, shown in Fig.1 and Fig.2.

Results & Discussion

The following results were arrived from the experimental investigation.For the M20 Grade Concrete;

• the 1 day strength for control mix and 20% replacement is more than 20 N/mm2. Whereas for 40% & 60% replacement the required strength is achieved at 7 days. The strength gain is increasing with increase in time.
• the 60% replacement of cement with fly ash gives 36.60 N/mm2 at 28 days, which is more than the target mean strength.

For the M40 Grade Concrete;

• the 1 day strength for all the mix is less than 40 N/mm2.But the design strength is achieved only on 7 days for the control mix. For 20% replacement, the 7 days strength is nearly to design strength.
• the 20% and 40% replacement of cement with fly ash attains the required strength at 28 days itself,whereas 60% replacement may take longer curing period to achieve the same strength.In general, for the control mix concrete, the gain in strength after 7 days is very less. But for 20%, 40% & 60% replacement of cement with fly ash, the strength gain after 7 days is considerable. From this,it is observed that the gain in strength is increasing with time for the high-volume fly ash (HVFA) concrete.

Conclusion

This paper discusses the benefits of the fly ash in different replacement in concrete along with its structural and workability properties. Even though the concrete is gaining in low early strength with fly ash, it is observed from the experimental investigation that the strength can be achieved by proper reduction in water-binder ratio. This reduction can be obtained by adding proper dosage of superplasticizers. Also properly cured fly ash in concrete products is very homogeneous in micro structure. Hence replacing the cement with fly ash in concrete is one of the best solutions available to the problem of environmental impacts. All these concepts are being put into practice and the large scale replacement of cement in concrete is preferred in future construction material which directly reaps the benefits to ecological and environmental point of view. From this, it is concluded that the gain in strength is increasing with time for the high-volume fly ash (HVFA) concrete. Further detailed study has to be conducted for various curing period and it is under process.

Acknowledgements

The authors convey their sincere thanks to the fly ash mission, Department of Science and Technology, Government of India, India, for sanctioning the grant (FAU/DST/600(9)/2007-08) for this research project and providing financial support for further experimental work. The authors express their heartfelt thanks to Prof. S.K. Kaushik, IIT Roorkee and Prof. K. Ganesh Babu, IIT Madras for the valuable guidance throughout this project.

References

• Dan Ravina and Metha. P. K. (1986), "Properties of fresh concrete containing large amount of fly ash", Cement and Concrete Research, Vol. 16, pp.: 227 - 238.
• Dan Ravina and Metha. P. K. (1988), "Compressive Strength of low cement/high fly ash concrete", Cement and Concrete Research, Vol.18, pp.: 571 - 583.
• W.S. Langley, G.G. Carette, V.M. Malhotra, Structural concrete incorporating high volumes of ASTM class F fly ash, ACI mater J 86 (1989), pp.: 507-514.
• G. Carette, A. Bilodeau, R.L. Chevrier, V.M. Malhotra, Mechanical properties of concrete incorporating high volumes of fly ash from sources in the U.S., ACI Mater J 90 (1993), pp.: 535-544.
• IS: 10262-1982, "Indian Standard Recommended Guidelines for concrete mix design", BIS, New Delhi.
• Ramachandran. V.S. (1995), "Concrete Admixtures hand book,Properties, Science, and technology", Noyes publications,Second Edition.
• Nabil Bouzoubaa and Simon Foo. (Jan 2005), "Use of fly ash and slag in concrete: A Best Practice Guide", MTL 2004-16 (TR-R).
• L.K. Crouch, Ryan Hewitt, Ben Byard., "High Volume Fly Ash Concrete", World of Coal Ash (WOCA), May 7-10, 2007, pp.1-14.
• Mehta, P.K., 'Concrete Technology for Sustainable Development',Concrete International 21 (11) (Nov. 1999) 47-53.
• Malhotra, V.M. and Mehta, P.K., "High-Performance, High-Volume Fly Ash Concrete: Materials, Mixture Proportioning, Properties,Construction Practice, and Case Histories", CANMET (Ottawa,Canada, 2002).
• Mehta, P., Manhoman, D. "Sustainable High-Performance Concrete Structures". Concrete International. July 2006. pp. 37-42.
• Naik, T., Ramme, B., Kraus, R., Siddique, R. "Long-Term Performance of High Volume Fly Ash Concrete Pavements". ACI materials Journal. Vol. 100, No. 2, Mar-Apr. 2003, pp. 150-155.

Dr. A. S. Santhi, Professor,Division of Structural and Geotechnical Engineering,SMBS, VIT University, Vellore, India-632014
Dr. G. Mohan Ganesh, Associate Professor,Division of Structural and Geotechnical Engineering,SMBS, VIT University, Vellore,India-632014
P. Balaji, M.Tech, Student,Division of Structural and Geotechnical Engineering,SMBS, VIT University, Vellore, India-632014