ARTICLES
Is flyash blended concrete beneficial

Use of flyash admixtured cements and the flyash itself as admixture to the cement concrete to be used in the reinforced cement concrete (RCC) structures, was not permitted till recently by the CPWD and other governmental organisations. It was considered that such use leads to increased corrosion of reinforced steel, thus shortening the lifespan of such buildings.

However, with the advancement of technology and research in the field of concrete and flyash utilisation, a change came, and in year 2000 the BIS permitted the use of flyash as mineral admixture from the consideration of “Durability criterion” (IS 456-2000). Still the CPWD stuck to the stand taken earlier.

Earlier this year, discussions started between the Department of Science & Technology and the CPWD on the topic and the CPWD softened its stand. A circular (CDO/SE/RR/FLY ASH (Main)/ 387) was issued on May 12, 2004 permitting use of flyash as “part replacement of OPC.

However, the permission is restricted to such structures which use ready-mixed concrete obtained from RMC plants and grade of concrete not below Grade 30. In case of the prestressed concrete structures, such use was not allowed in the case of bridges and flyovers. Other restraints were also imposed which related to the mode to be used for obtaining flyash from the power plants.

Flyash is to be obtained in dry form i.e. not from the ash ponds but from the ESP’s (Electro-Static Precipitators) and properly stored. But when the concrete is obtained from RMPs this aspect is ensured automatically. It may be mentioned here that barely 7 per cent of concrete of the total quantity used is supplied by RMPs.

So, over 93 per cent of the concrete users are left uncovered by the above “permission”. Also the most commonly used grade of concrete is M20.

Another restriction imposed is that the use is for concrete used in the superstructure (limited to 30 cm below ground level) and not in the foundation or basements.

The flyash to be used should conform to the requirements of IS 3812-1981, relating to Grade I flyash, which fixes the “Lime Reactivity Strength” to 60 kg / cm2.

It will not be possible for a common builder to obtain “flyash” in dry form, because no such facility is available at any of the thermal plants located in Punjab.

The Gujarat Ambuja Cement Ltd (GALL) is utilising eight lakh tonnes of flyash annually at Ropar. The utilisation by its plant located at Bathinda is not known. However, the GACL has captured over 12 per cent of the market share of PPC (Puzzolana Portland Cement) by its two units located in Punjab, but had hardly cared to make arrangements for supply of flyash to a common user at Ropar or Bathinda. The only alternative available to the common user is to utilise PPC if he wants to make his house more durable and in that direction hardly any demonstration structures have been put by GACL.

The specifications need be further relaxed so that the benefit is available to the common user and the cement manufactures should undertake “demonstration trials” to show that by using PPC in the RCC structures, the obtained structure would be more “durable” than if only OPC had been used.
 


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