The Kolkata Metropolitan Development Authority (KMDA) has started repairs on the Durgapur bridge, which connects New Alipore and Chetla and whose underbelly was damaged in a fire that broke out last month in the shanties under the structure.

The KMDA, the custodian of the bridge whose official name is Derozio Setu, plans to complete the repairs by the end of this week. A series of tests will be conducted to check the efficacy of the repairs, an official said.

The temperature of the concrete under the bridge had gone up significantly on December 21, when a fire gutted shanties under the structure. It reduced the moisture content of the concrete making it brittle on the surface, KMDA sources said.

“We are undertaking grouting, guniting and other necessary repairs. We plan to end the repairs by this week,” said a source in the KMDA.

Grouting is a process during which a concrete mixture is injected through a nozzle in high pressure into a void or gap. This helps strengthen the structure. During guniting, a concrete mixture is sprayed on the surface from where chunks have fallen.

The KMDA tested the structure after the fire and learnt about the deterioration in the stability of the concrete. All the tests will be repeated once the repairs are over.

“The first round of tests was done to assess the damage. The second round will show whether the repairs have been effective and went according to the plan,” said a KMDA official.

The construction of Derozio Setu started in August 1986 and was over by December 1988.

Last Thursday, chief minister Mamata Banerjee had raised concerns about the damage to the bridge.

“The fire has affected the health of the Durgapur bridge. 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 had said.

The state government has barred the movement of heavy vehicles on the bridge, which is used by thousands of commuters.

The reduction in moisture content raised the acidic content of the concrete, which could lead to corrosion of the steel embedded in the concrete and weaken the structure.

The decline in the strength of the concrete led to spalling, a condition marked by concrete falling off the surface even under slight pressure.

The KMDA will seek police’s permission to close the bridge to traffic for a night to conduct a load test once the repairs are over, an official said.