We have used more than 80, 000 tonnes of fly ash without any problem
Concrete mixing has evolved as an art mastered by the few to a precise scientific process that can be replicated anywhere. The key is knowing what to look for during its preparation. Cyrus Pithawalla, Director Engineering, Hiranandani Constructions shares his insights with ICR. Excerpts from the interaction.
Tell us about cement grades suitable for concrete production.
Cement comes in different grades and based on that, its applications vary. In foreign countries, cement is categorised as Class-A, Class-B, Class-C etc (API classifications). That cement cannot be used in India as the climatic conditions here are very different. If we try to use that here, then we will have cracks in structures called shrinkage cracks. Here in India, cement comes in 33, 43, 53, and also 63 grade. Of these 43 and 53 grade are very popular, 43 grade is good for plastering work, and 53 grade should be for making concrete.
There is a big difference in quality and strength of 43 and 53 grade cement. One has to be very careful if both 43 and the 53 grade cement are stocked in the same godown. If workers mistakenly take 43 grade cement instead of 53 grade for mixing in concrete, then the results will be disastrous. Both materials look the same. Mixing fly ash in 53 grade cement serves as a good visual demarcation and helps avoiding such confusion. Now that the cost difference between 43 grade cement and the 53 grade cement has become negligible, it is possible to use 53 grade cement for both plastering as well as for concrete manufacturing. As we go ahead, we will see only 53 grade being used everywhere; 43 grade cement is becoming less available due to its decreasing demand.
What is the role of aggregates and how does one pick the right material?
The basic components of concrete are coarse aggregates (CA), sand and cement. Aggregates are classified as CA-1 and CA-2 based on size. CA-1 is about 15 - 10 mm in diameter while CA-2 is about 25-30 mm in diameter. In India our quarries are supplying 15 mm CA-1 and 30 mm CA-2 aggregates. An ideal concrete mix would have CA1 of 10 mm diameter and CA2 of 20 mm diameter. In concrete, the better the particle packing, the higher the strength. CA-1 fills the gap between the CA-2 aggregates. Sand fills the void between the CA-1 aggregates and then finally, cement fills the voids between the sand particles and holds the entire mixture in place. Another most important ingredient is the water and its quantity is critical to concrete's strength.
Today river sand is being replaced with manufactured sand. How well does that work?
Now river sand is no longer available. The supply has completely stopped. In this scenario, we have to turn to manufactured sand. Unfortunately, very few realise that in several cases, what is being sold as manufactured sand is nothing but crusher dust. There is a huge difference between crusher dust (also called crushed sand) and manufactured sand. The manufactured sand goes through various processes and the product that you get at the end has uniformity in size and good homogeneity while crusher dust is nothing but quarry waste, the fine dust that is generated during the aggregate crushing process. This waste product of the quarry is laden with fine dust and is often sold to unsuspecting buyers. This impacts the quality of concrete and one must avoid buying crusher dust/crushed sand. Also, in the monsoon, crusher dust powder comes along with CA1 as it is stuck to the material due to high moisture in the material. So, extra care must be taken while procuring aggregates in monsoon season.
How open is the industry to using fly ash?
Fly ash so far, has been considered taboo. Some people may have doubts about using it as there are several misunderstandings surrounding it. We have been using fly ash without any problem for a long time. We have used more than 80, 000 tonnes of fly ash without any problem at all.
What care do you take in the selection of fly ash?
You must first understand that fly ash comes as different types. The quality varies depending on the source that it is obtained from and also most importantly, with regards to its fineness. We have identified one such supplier who supplies the fly ash as per our specifications. The company has separate classifiers to collect flyash of different particle size.
In case of fly ash, the quality in terms of the fineness and the consistency in size is very important as it is difficult to make out the variation in fly ash quality on the site where mixing is taking place. And you can`t afford to have one batch of concrete poured differently from another. So fineness and particle size uniformity is very important.
Are there any grades of fly ash that are more suitable for concrete manufacture?
Yes, there are grades of fly ash but very few people are aware of it. Fly ash is a waste product of the thermal power industry and so far, the cost associated is the cost of transporting it. There is as such, no processing that happens once fly ash is generated from a thermal plant. The fly ash is collected from precipitators and separated / classified to obtain a product with uniform mesh size. The first chamber separates the coarse material, the second picks up finer quality fly ash and the third chamber collects the finest material at the end. So the fly ash available is graded as 60 , 63, 80, etc, depending on the mesh size. The finer the fly ash, the better it is. We prefer the one that you get at the end of the classification process as the grade is really good there.
And how much fly ash can we add to concrete?
You can add from 20 - 30 per cent. You can even go up to 40 per cent, depending on the end result you are looking at. According to our experience, 29 per cent is the highest that we can go.
With newer materials and additives available, higher grades of concrete are becoming available. How high can we go?
Concrete strength on a global scale, is measured in terms of N/mm2. So a M30 grade concrete means the compressive strength it acquires after 28 days will be 30 N/mm2. So there is M30, M40, M60, etc. Now note that the concrete strength depends on the water- to- binder ratio or the ratio of water to the fines in it. If you are adding only cement, then it is ratio of water to cement, and if fly ash is added, then you must consider both cement plus fly ash while calculating the ratio. The ratio can be as low as 0.31 at the most. If you try to formulate concrete to say, M100 or anything above, then there is a problem. In such cases, the ratio can drop to ~ 0.22 or lower. At this water binder ratio, autogenous shrinkage will occur in the concrete and micro cracks will start developing in the structure. These micro cracks are not visible to the naked eye but I would certainly worry about such cracks. So the water binder ratio caps the strength limit up to which we can safely go.
Can the addition of polymers help prevent such cracking?
No, polymers are anyway added to concrete in today`s modern process. Despite these polymers, there could be cracks.
How do delays affect the RMC quality and what more can be done at the technical level?
Today, there are plasticisers available that help tweak the slump characteristics and setting properties of concrete. If you are expecting a delay, then you can dose extra before dispatch. However, if the dosage is extra, then the setting time too, is extended. One can always fix things if it is under-dosed. Extra dosage can be added at site. This however, is done manually and there is an element of uncertainty in the process.
Which are the latest materials that excite you?
I am excited about the new age super plasticisers. They help in achieving the target slump. You must understand that the slump requirement will vary from application to application. The column will require concrete with a different slump than that required by the concrete in slab. The third generation super plasticisers available today are highly customisable, and some leading companies are making custom polymers available tailor-made just for your needs. That is very exciting.