Mini Soman1 and Sobha.K2
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Fly ash is a waste product generated by coal burning power plants. It has been generally used for land filling. However in places where regulations preventing atmospheric pollution do not exist, it is billowed into the atmosphere. Fortunately the discovery made several years back that fly ash can be used as a partial replacement for cement in concrete. The present study aims at developing a concrete by replacement of Ordinary Portland Cement (OPC) with 50% Fly Ash by mass. The fresh and hardened properties of High Volume Fly Ash Concrete (HVFAC) with 50% replacement of cement and Ordinary Portland Cement Concrete (OPCC) are studied. The study discloses that high volume of Fly Ash in concrete reduces the water demand and improves the workability. Study also reveals that the OPCC and HVFAC exhibits similar hardened properties. Comparison of flexural response of beams made with OPCC and HVFAC with different percentage of reinforcement are also studied. It is observed that HVFAC beams have shown notable improvement in the deflection , cracking behaviour and load carrying capacity.
|Ordinary Portland Cement; High Volume Fly Ash Concrete; Flexural response.|
|FLY ASH generated by the burning of coal in coal fired power plants was considered till a few years back as mere waste material. This was considered as a material of very low value, useful only for land fill. But its usefulness as pozzolonic additive to cement is an important discovery. Continuous research studies by various engineering research laboratories revealed its varied usefulness as an additive for enhancing the various qualities of concrete including its workability, strength and durability if handled and cared properly. Partial replacement of cement with fly ash in concrete save much of the energy required for production of OPC and also facilitates the economical disposal of millions of tons of fly ash.|
|At present most of the fly ash blended cements commercially produced in India has 18 to 25% fly ash by weight and addition of fly ash to this extent has a beneficial effect on the workability and economy of concrete. It has been found that in order to improve the other qualities of concrete like resistance of sulfate attack and thermal cracking, larger percentage of fly ash is to be used in concrete. Fly ash content greater than 35% can be considered as high volume replacement or high blending. The seven storey structure of 10780m2 office space in Canada was constructed with HVFAC having compressive strength 30-50N/mm2 .|
|Previous studies reveals that the addition of fly ash in concrete resulted in great benefits such as reduction in heat of hydration, minimization of potential alkali aggregate reaction, significant reduction of steel corrosion, improvement in durability of concrete, reduction in cost etc. [1, 9, 11, 12, 13, 14, 15, 16]. In addition, it improves the environment by contributing towards reduction of greenhouse gases . The objective of this investigation is to study the effect of high volume of fly ash in the fresh and hardened properties of concrete and to study the flexural response of HVFAC beams in comparison with OPCC beams.|
|The preliminary experimental investigation consists of test on constituent materials, development of fly ash mixed concrete and determination of fresh and hardened properties of HVFAC and OPCC.|
|A. Test on constituent materials|
|Cement: Cement used in all mixes is OPC (53 grade), which conforms to IS specification . Laboratory tests are conducted on cement to determine its standard consistency, initial setting time, final setting time and compressive strength. The standard consistency of the cement used is 31%.|
|Fine Aggregate: Commercially available M-Sand passing through 4.75mm IS sieve and conforming to zone II of IS 383-1970 is used. All physical properties as per IS recommendation  are determined. Specific gravity and fineness modulus of M-Sand used are 2.45 and 2.53 respectively.|
|Coarse Aggregate: 20mm downgraded aggregate is used for the study. The properties of coarse aggregate conformed to the IS specification [4, 5]. Specific gravity of the coarse aggregate used is 2.78.|
|Water: Potable water is generally considered as being acceptable. Hence clean drinking water available in the college water supply system is used for casting as well as curing of the test specimens.|
|Super plasticizer (SP): Sulphonated Naphthalene Formaldehyde Super plasticizer is used for the study. The dosage used is 0.40% of powder content.Reinforcement: HYSD bars of 12mm, 8mm and 6mm diameters and ultimate strength 785.56N/mm2,630N/mm2 and 610.64N/mm2 respectively are used for the study. The moduli of elasticity of bars are 2x105N/mm2 and the percentage elongations of the bars are in the range of 25-30%.|
|Fly ash: The fly ash collected from HI- TECH Fly Ash (India) Private Limited, Tuticorin is used for the study. IS 456:2000 specifies that ‘Fly ash conforming to Grade I of IS 3812  may be used as part replacement of mixing Portland Cement, provided uniform blending with cement is ensured’. The chemical and physical properties of fly ash provided by the supplier are given in Table I and Table II.|
|The results show that the early strength of fly ash mixed concrete is less compared to control mix but at 28 days the strength of fly ash mixed concrete is similar to that of control mix. The results at 56 and 90 days indicates that there is continuous and significant improvement in strength after 28 days.|
|Splitting tensile strength increased with the increase in fly ash content. This could be attributed to the pozzolanic action due to fly ash. Test results for flexure and modulus of elasticity indicate that addition of fly ash does not affect much on the results compared to control mix.|
|The test programme is designed to study the flexural response of reinforced HVFAC beam and to compare its behaviour with that of similar conventional reinforced concrete beams. Beams of size 100mmx150mm and 1200mm long are cast with different reinforcement percentages. The reinforcement details of the beams are shown in Fig.2. The reinforcements are provided as balanced and under reinforced section based on the provisions of IS code . For each type, 3 beams are cast.The details of beams cast are given in Table VI.|
|Moulds for casting beams are made of waterproof plywood. Concrete is mixed in a concrete mixer in the laboratory. The beams are vibrated with a mechanical vibrator. The formwork of beams are demoulded after 24 hours after casting and cured by ponding. After 28 days, the beams are white washed for easy detection of cracks.|
|A. Test Set Up|
|The specimens are tested in a compression under two point loading with a clear span of 1m as shown in Fig.3.|
|The strength and cracking behaviour reveals that HVFAC beams exhibit more strength and less cracking compared to OPCC beams. From the load deflection plots it can be observed that the deflection of HVFAC beam is less than OPCC beams at all load levels. The area under load deflection plot which is a measure of energy absorption capacity is also higher for HVFAC beams.|
|The cost of cement per cubic meter concrete is taken in to consideration. From the mixture proportion of OPCC and HVFAC used in this investigation, it is evident that in one cubic meter of concrete, the control mix uses 400 kg of cement, while HVFAC uses only 200kg of cement.|
|The material cost for fly ash is negligible since it is an industrial waste and the conveyance charge is taken as Rs.1.00/kg. For a developing country like India the savings in cost will be great step for the infrastructural development. The costs are estimated based on the actual purchase rate and the cost comparison is given in Table IX.|
|From the Table IX it is clear that site mixed OPC with 50% fly ash concrete is 24% cheaper than OPCC. Hence, the fly ash which otherwise creates an environmental nuisance can be used in a productive way contributing to sustainable development.|
|Based on the investigation conducted following conclusions can be arrived at.|
|1. Addition of fly ash improves the workability of concrete.|
|2. Addition of 50% fly ash reduces 7 day strength by about 20% when compared to control mix. But it acquires strength almost equal to that of control mix at 28 days and attained higher strength thereafter.|
|3. All hardened properties are similar for OPCC and HVFAC at 28 days.|
|4. The first crack load and ultimate load are more for HVFAC beams with different reinforcement compared to OPCC beams.|
|5. The load deflection curve shows that the mid span deflection of HVFAC beams are less when compared to OPCC beams at all load levels.|
|6. Cracking behaviour studies indicate that HVFAC beams are more crack free compared to OPCC beams|
|7. Cost comparison reveals that site mixed OPC with 50% fly ash concrete is 24% cheaper than OPC concrete.|
|8. From the studies it is found that the fly ash can be considered as resource material and can be used in a productive way contributing to sustainable development.|
|Authors would wish to acknowledge the provision of facilities by Department of Civil Engineering, College of Engineering Trivandrum. The cooperation and help received from the supplier of fly ash, M/s. HitechFlyash (India), Pvt. Ltd., Tuticorin are greatly acknowledged.|
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