In the fifth in a series of features showcasing experiences with building materials, Himanshu M Vakharia, Vice President (Contracts & Procurement), New Consolidated Construction Company Ltd (Jasdanwalla Group), tells us about manufactured sand as an alternative to river sand.
For a long time, the rough surface texture, flaky and elongated particle shape and often inadequate size gradation of fine aggregate - a by product of the crushing and screening process - led to its poor performance in concrete, coupled with the fact that workable concrete requires large volumes of water; this lead many concrete manufacturers to shy away from using crushed sand.
However, in recent years, a change - driven by the sometimes acute need for a product that can match the properties of natural sand in concrete - has tilted the balance once again.
Himanshu M Vakharia, Vice President (Contracts & Procurement), New Consolidated Construction Company Ltd (Jasdanwalla Group) discusses the merits of manufactured sand vis-à-vis river sand, in conversation with Shriyal Sethumadhavan.
Stepping in
In the present day, manufactured sand, otherwise called crushed sand or fine aggregate, is replacing natural sand and gravel in most countries. This includes the Indian construction industry, and most engineers will agree that this will not change dramatically in the foreseeable future. In the last 15 years, the availability of good quality, natural sand has been decreasing, and with a few local exceptions, this seems to be a global trend. Environmental concerns are also being raised against the uncontrolled extraction of natural sand.
For particle sizes upwards of 4 mm, crushed aggregate can replace natural sand in most construction applications, unless there are specific requirements for a very rounded shape and smooth surface. This is probably why there has been a natural shift towards crushed aggregates in most markets over the past three decades. Considerable research and development has been initiated on a national basis, both by manufacturers' associations and by individual companies. In general, the findings point towards the possibilities of replacing natural sand with crushed fine aggregate, with additional positive effects on concrete.
The manufacturing process
Basically, manufactured sand is a kind of aggregate where the fineness modulus is minimal. With sand being a binding material, the general criterion required is 4.75 micron to 4 micron. Aggregate is of two kinds - coarse aggregate (bigger size stones) and fine aggregate (sand). Basically, when mixed with concrete, the coarse aggregate varies between 10 mm, 20 mm and 40 mm. Hence, manufactured sand is mixed with cement to fill the gaps between the bigger stones.
Due to climatic conditions and erosion, there are a lot of impurities and size variations in river sand. However, when manufacturing sand, a vertical shaft impact (VSI) crusher crushes materials like basalt stones, or any other stone or rock, into the required size. The velocity and centrifugal action of this machine gives the particles a 100 per cent uniform shape. When used in concrete, there are certain gradations for that particular sand, which can be achieved and controlled through the VSI machine. Hence, the process basically involves crushing the sand and conducting a final screening for size gradations, after which a particular level of sand is taken out.
Coming into play
Manufactured sand has been available in India since the last 14 to 15 years. In areas like Pune and Bengaluru, this material has been in use since the last 10 years. But, it has come into play in Mumbai practically in the last two to three years. In 2002, the former State Bureau of Building Materials Industry proposed and implemented a national standard, GB/T14648 'building sand' to determine the definition of artificial sand, its technical requirements and testing methods, which indicated that manufactured sand was being used in mainstream construction. After having faced a number of problems with river sand as far as quality and supply was concerned - also with certain political issues involved - the availability of sand became a problem. Eventually, we began importing river sand from Gujarat and other places. But, this is more expensive, and we turned to manufactured sand.
Initially, when I began using manufactured sand, there was resistance from the construction industry. However, one has to give in eventually because constructing an RCC-frame structured building is impossible without sand. So, after conducting extensive research, which led to positive results, the consultants were convinced that manufactured sand is the best substitute. Also, when we used manufactured sand for the first time in Mumbai, there were certain issues pertaining to the quality during crushing. However, with the passage of time, all this has been improved with efficient technology and machines.
Overcoming challenges
The motivation to use manufactured sand has mainly come from three factors - availability, quality and price. When compared to river sand, one can save almost 40 per cent using manufactured sand. Also, in Mumbai, the requirement is 15,000 trucks per day, while the availability is 400-500 trucks. Hence, it was very difficult to manage the supply of sand, and it further hampered the progress of work.
During concreting, the proportion of fine aggregate varies from 0.4 to 0.5 cu m, per cu m of concrete. Considering this, we tested the physical and chemical properties of manufactured sand and ultimately, the consumption co-efficient for the concrete. All of this was retested by a third party. We took cube moulds of the final product, and the material was taken through a testing procedure of seven, 14, 28 and sometimes 56 days. Also, we vetted our analysis and mix design through Veermata Jijabai Technical Institute (VJTI) and Sardar Patel (SP) College
of Engineering.
After all this, today, we use manufactured sand in nearly 90 per cent of our projects. Some of the projects are Geometric Software, KPIT Cummins, Shreya Biotech, and Gera - a residential complex at Kharadi. In Mumbai, we have used this material in Nirlon Knowledge City at Goregaon, the Hindustan Unilever corporate office at Andheri and for all our residential projects like Nirmal Lifestyle and Acme Ozone. We are also constructing some industrial buildings in Bhiwandi, and one of our main projects is the Narsee Monjee Institute of Management at Vile Parle. All these are being constructed with manufactured sand.
Quality assurance
Generally, when you develop any building, there is a cycle time for the slab, which varies from 10 to 15 days. With river sand, we faced instances where our slab would get held up for 20 to 25 days due to non-availability of sand. However, with manufactured sand, we are now able to achieve the slab cycles in 12 to 15 days. So, completing our projects well within the set timeline is the biggest advantage. Also, for top quality, the sole parameter to be considered is the density of the concrete, which is generally 2,400 kg per cu m. By adding manufactured sand, we not only get the required strength but at times, have an additional gain of 5 to 10 per cent over and above the required standard.
Technical requirements have to be certified and need to be up to the mark as per the engineering norms. Nowadays, technology is so advanced that you have a computerised plant, which, based on the design, provides the exact proportion of sand required. Hence, you know the exact quantity, avoiding the use of less or excess material.
The difference lies in...
With excellent quality and low percentage of pin and chip products, manufactured sand is suitable for high standard construction needs. Because its production line is steady, it offers a more stable quality than natural sand, which differs with the geographical location. Also, while the amount of clay and powder can be controlled, there is no need for washing and screening before use, further reducing labour cost.
Unlike river sand, manufactured sand contains no organic impurities and strengthens the concrete even more with the same quantity of cement. In river sand, while irregular sizing results in wastage in the form of singles to the tune of 10 to 15 per cent, manufactured sand is
already sieved in the required size (below 4.7 mm), hence avoiding wastage.
Current scenario
Particularly in Mumbai now, almost 80 to 90 per cent projects are being developed with manufactured sand. However, the usage of manufactured sand is limited to concrete as certain properties do not qualify it for plastering work. Also, concrete grades start from
M 10 to M 50 or M 60, and nowadays, grades up to M 80 are being designed. Manufactured sand can be best used for higher grades of M 45 or M 50. But over and above that, one would have to opt for micro silica.
Companies like Ultra Tech, Lafarge, and ACC, which are basically into manufacturing concrete (ready-mix concrete), have conducted a thorough analysis, and have used various gradations in order to control quality and sizing. In terms of affordability, concrete manufacturing requires around 50 per cent of sand (river or manufactured). If the raw material price is lesser than that of river sand, the final product cost will be cheaper. Also, the demand for and availability of manufactured sand in India is excellent. Earlier, from only four to five plants across the country, we now have around 100 to 1,000 plants today. Hence, there is huge scope for the use of this material in the near future.
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