The temperature of the concrete under the Durgapur Bridge in Chetla had gone up significantly on December 21 when a fire gutted shanties under the bridge, sources in the Kolkata Metropolitan Development Authority (KMDA) said. The heat damaged the concrete.

The soaring temperature caused by the fire decreased the moisture content in the concrete, KMDA sources told The Telegraph. The concrete loosened and fell off in multiple places. If the damaged portions are left unattended, the steel reinforcements inside may get exposed and weaken the structure.

The KMDA plans to start the repairs in a few days. Grouting, a process of injecting concrete mixture in high pressure through a nozzle, will be carried out in the voids or gaps. This will strengthen the structure. The KMDA will also gunite the surface, where a mixture will be sprayed on the surface from where concrete chunks fell off.

“The objective of both grouting and guniting is to increase the bridge’s load-bearing capacity. While grouting works deeper and strengthens the concrete by filling the voids, guniting increases the thickness of the concrete,” said Gokul Mondal, a professor of construction engineering at Jadavpur University.

“Concrete turns acidic if the moisture content decreases significantly. This can lead to corrosion of the embedded steel and weaken the concrete,” Mondal added.

Last Thursday, chief minister Mamata Banerjee raised concerns about the damage to the bridge. “The health of Durgapur Bridge has been damaged because of the fire. We have been forced to impose some restrictions. We have to undertake an extensive repair job. It is going to cost us a lot of money. We sent a team for inspection... which revealed that the flames damaged the bridge,” Mamata said at a news conference at Nabanna, the state secretariat.

The state government has barred the movement of heavy vehicles on the bridge connecting New Alipore and Chetla which is also used by thousands commuting to and from Behala every day.

The fire in the shanties broke out on a Saturday evening and the temperature of the concrete was measured the next afternoon, KMDA sources said.

“We recorded the temperature to be 110 degrees Fahrenheit (43.3 degrees Celsius) on Sunday afternoon. It must have been several degrees higher when the fire was on and even for hours after. The exposure to the heat led to a decrease in moisture content and we have noticed local spalling in multiple points,” said a KMDA official.

Spalling means that concrete fragments have broken off and fallen off the larger concrete structure, said civil engineers.

A visit to the stretch under the bridge — commissioned in 1990 — on Sunday afternoon revealed that an inspection team has marked multiple spots where the concrete broke off or where other interventions will be required. The spots were circled with red lines and white lines.

“The repairs will be started this week. We will also conduct a load test of the bridge to assess its health. We have already tested the concrete’s strength, hardness and compactness,” said a source in the KMDA.

The bridge will be completely shut down for traffic for some time, possibly at night, for the load test.

KMDA sources said a complete health assessment of the bridge was underway and the fire happened in the meantime.

When the team of engineers from the KMDA and external experts went for inspections for the health assessment, they could not inspect the underbelly of the bridge because of encroachment, said a KMDA source.