The Sand You Have to Make

For as long as there was a choice, river sand was the good material and crushed sand was the fallback, the thing you reached for when you could not get the real thing. The metro being built under the city of Surat did not have the choice, and in being forced off river sand it found the ranking was backwards.

Gujarat's river sand came in the wrong gradings. One source, from the bed of the Orsang, was too coarse; another, from the Narmada, was too fine, and even blended the two could not reliably hold the particle-size distribution that the high-performance tunnel concrete demanded, load to load. The project's engineers put the problem plainly.

At present river sand is not available in metropolitan cities of India like Mumbai region of Maharashtra and Gujarat are also facing a shortage of consistent quality of river sand.

Sonule, Tayade and Naik, "New Generation Fine Aggregate for Using in High Performance Concrete," IJCIET (2025), the Surat Metro QA/QC team's own paper.

So they switched to crushed rock sand, made on a vertical-shaft impact crusher for a cubical grain and washed to strip the excess stone dust, and ran it head to head against the river-sand mix. By the executing team's own reported figures, the M50 tunnel-segment concrete made with the washed manufactured sand reached about 68 megapascals at 28 days against about 61 for the river-sand control, both above the 50 the job required. The durability numbers were the ones that mattered. The chloride permeability of the manufactured-sand concrete came in around 698 coulombs against about 1,200 for the river sand, on a specification that allowed no more than 1,000. The river-sand concrete failed the limit the crushed sand passed comfortably. These are one project team's published numbers, not an independent trial, and they should be read that way. But they point exactly where the mechanism predicts.

That a city had to reach for crushed sand at all is the larger story. The scale of sand use is hard to picture. The United Nations Environment Programme, in its 2019 report on sand, found that sand and gravel are the second largest resource extracted and traded by volume in the world, after water, at an estimated forty to fifty billion tonnes a year and rising. Most of it is for construction, and the rivers cannot refill at that rate. The damage from taking it is not abstract to a dam engineer.

A decrease in bed load or channel shortening can cause downstream erosion including bank erosion and the undercutting or undermining of engineering structures such as bridges, side protection walls and structures for water supply.

UN Environment Programme, Sand and Sustainability (2019).

India has been at the sharp end of it. Unregulated river-sand mining lowered riverbeds, undercut bridges, and pulled down water tables, and the courts moved. In 2012, the Supreme Court, in the Deepak Kumar judgment, ruled that even the smallest sand lease could no longer be worked without environmental clearance, a decision that reshaped sand mining across the country and was followed by state restrictions, monsoon-season bans, and the environment ministry's sand-mining guidelines. Legal, consistent river sand became harder to get.

And often the river sand that can be got is the wrong material to begin with. In the Himalaya especially, where many of the country's dams are built, river sand carries mica, the flat flaky mineral that glints like tiny mirrors in the grains. Mica flakes are weak and they trap water, and in concrete they cut strength and durability; the published record includes a documented case from eastern Nepal where micaceous sand degraded the concrete, and laboratory work has measured roughly a 15 percent loss of 28-day strength at a mica content of 5 percent. Crushed hard rock carries none of it. It is part of why remote mountain dam sites have so often crushed their own aggregate rather than rely on the nearest riverbed.

Why manufactured sand can match or beat river sand comes down to one number engineers used to read wrong. Crushed sand carries more fine powder than river sand, the dust thrown off by crushing, and for years that counted against it. The Indian standard for aggregates, IS 383, was revised in 2016 to draw the distinction that matters. Natural sand is allowed only 3 percent of material finer than the 75 micron sieve, because in a river that fine fraction is plastic silt and clay that coats the grains and starves the cement bond. Crushed stone sand is allowed up to 15 percent, because its fines are inert rock powder, not clay. The same revision gave manufactured sand a formal definition for the first time.

Fine aggregate manufactured from other than natural sources, by processing materials, using thermal or other processes such as separation, washing, crushing and scrubbing.

IS 383:2016, Clause 3.1.4.

The lesson: The river sand that Indian construction was built on is running out, and it was never the ideal material to start with. Crushed rock sand, once dismissed as the poor substitute, is now the specified equal or better, on two conditions. The fines have to be washed and held to what IS 383 allows for crushed material, not confused with the tighter limit that exists because natural silt is plastic and crusher dust is not. And the angular grains have to be well shaped and carried by a water-reducing admixture, or the mix loses the workability the roundness of river sand used to give it for free. Where the choice has been taken away, the substitute has turned out to be the upgrade.

Read more: Manufactured Sand for Dam Concrete: Properties, Performance, and IS 383 Compliance →