Building Against the Rain

About three-quarters of India's rain falls in the four months of the monsoon, and for those months, in most of the country, you cannot reliably place concrete outdoors. On a large dam that is not a delay to absorb. It is a boundary that sets the whole construction calendar, and the engineering is in how you work up to it and restart after it.

The Middle Vaitarna dam, a 102.4 metre roller compacted concrete dam built for Mumbai on the Vaitarna river, is the clearest Indian example. Its concrete was not poured continuously. It was placed only in the dry windows, October to May, between the 2010, 2011 and 2012 monsoons, halting through each wet season and resuming after. Across those windows the team placed roughly 1.2 million cubic metres of roller compacted concrete in thin compacted lifts, an effective placement period of about fifteen months at rates reaching 134,000 cubic metres a month, which puts it among the fastest large RCC dams ever built. The monsoon set the schedule. The engineering was making every dry-season restart sound.

Because the real danger of a monsoon pause is not lost time. It is the joint. Roller compacted concrete lives or dies on the bond between its lifts, and a rain-forced gap lets a lift stiffen into a cold joint, a ready-made seepage plane, unless it is cleaned, roughened and bedded before the next lift goes down. That the discipline held at Middle Vaitarna was proven in the hardest way possible.

The galleries were bone dry during an overtopping during the 2011 monsoon.

Ashtankar and Chore, "Development of Design Mix Roller Compacted Concrete Dam at Middle Vaitarana," Advances in Concrete Construction, 2014.

The half-built dam was overtopped by a live monsoon flood, and the inspection galleries inside it stayed dry. The lift joints, left and treated across three monsoon shutdowns, held out the reservoir.

Where you cannot stage the work across years, you race a single dry season. The Teesta Low Dam IV project in West Bengal built its roller compacted concrete section in one season, 196 days, finishing in May 2015, for a reason its project manager put plainly.

As the rains stop the concrete work, we wanted to finish the job in one season.

Santosh Kumar, Project Manager, quoted in Equipment India.

Then there is the hazard almost nobody schedules for, because it does not look like weather. It is the aggregate. Stockpiles sitting out through the monsoon soak up water, and the sand and stone carry free surface moisture well beyond the saturated-surface-dry point. That surface water is mixing water. Unless the batch water is cut to match it, the effective water-cement ratio climbs, and by Abrams' law strength falls as it does. An unmeasured rise of about 0.05 in the water-cement ratio can cost on the order of ten to fifteen percent of the 28-day strength, quietly, in concrete that looked fine going in. The fix is unglamorous: re-test stockpile moisture through the wet season and correct the batch water for it, using the field method in IS 2386 Part 3. And do not go looking for a special monsoon code, because there is not one. Wet-weather concreting is governed by ordinary IS 456 good practice, by aggregate-moisture control under IS 2386 Part 3, and by the hot-weather code IS 7861 Part 1 only when the monsoon is also hot.

There is one genuine upside, with a catch. Once concrete has set, the monsoon's humidity and rain help cure it, keeping it moist and sparing you the plastic-shrinkage cracking that plagues hot, dry placement. But that is after final set. Rain is a friend to hardened concrete and an enemy to fresh, so the entire discipline comes down to getting the mix placed, compacted and past set before the next shower reaches it.

The lesson: Treat the monsoon as a boundary condition, not a nuisance. Fit the pour programme to the dry window and race each restart. Govern every lift joint so a rain-forced pause becomes a treated construction joint, not a seepage plane. And through the wet season, test your stockpile moisture and correct the batch water for it, every day, because the monsoon's most expensive damage is not the days it costs you. It is the strength it quietly takes out of the concrete you did place, through water you never meant to add.

Read more: Monsoon Concreting in Indian Dam Construction →