Structural Retrofitting of Chitpore Bridge

The Chitpore Bridge was repaired using anti-corrosive treatment, rebuilding of concrete section with micro concrete and polymer, followed with strengthening by using SRM C-wraps and SRM C- Laminates of carbon fiber. After successful completion of repairing and strengthening work, the bridge was open for the public.
Dr M.V. Joshi, PhD IIT Bombay, CEO of SSSPL; Vivek S V, M-Tech IIT Kharagpur; and J. D. Bhanushali, MSc in Civil Engineering, IIT Tirupati

The Chitpore Bridge has contributed to the economic growth of Kolkata city from as early as 1843. It was constructed under the leadership of John Thomson, who was the superintendent of canals and the agent for iron suspension bridges. The bridge spans 99 feet and has a roadway which is 24 meters wide. It was popularly referred to as the Bagbazar Bridge and is one of the earliest examples of the bowstring bridge (Figure 1).


KMDA conducted structural inspection of Chitpore Bridge
Deterioration is a common problem in bridges and Chitpore Bridge is no exception.

The Kolkata Metropolitan Development Authority KMDA, conducted structural inspections of several bridges in Kolkata and found that Chitpore Bridge falls under the category of “vulnerable bridges and flyovers” and therefore needed immediate repair and restoration to avert any major mishap.

Non-Destructive Tests Performed on Chitpore Bridge
In August 2019, LASA was awarded consultancy services for Chitpore Bridge. The condition of the bridge was surveyed and non-destructive tests were performed. The seemingly healthy bridge revealed active corrosion in selected areas. The NDT test revealed the following:

• In the vertical hangers, steel reinforcement had lost its diameter at several locations
• Spalled portions and discontinuous rebars were visible in hanger columns, under the deck slab and on cross beams (Figure 2)
• The structural strength of sectional members was compromised by corrosion
• Pitting corrosion of the embedded steel in the vertical hangers and in the beams supporting the deck

Adding to its already distressed condition, was the nearby Tala Bridge, which was closed down, sending substantial traffic to Chitpore Bridge. To prevent a catastrophe, an immediate strengthening of the structural system was necessary. It had to be done fast with due diligence. That’s when Sanrachana came to the rescue.

Repair and Retrofitting of Chitpore Bridge Executed by Sanrachana
Sanrachana is one of the pioneers in the area of structural retrofitting and protection in India and has decades of experience in executing bridge repairs across the country. It has served clients on over 2000 commercial properties, industrial facilities, energy plants, and public infrastructure, and many other projects of varying size and scale.

Overview of work executed:

• Sandblasting, anti-corrosion treatment
• Providing additional rebars, micro-concreting
• Strengthening of hangers, deck & cross beams with SRM carbon laminates & SRM C-wraps
• Post-tensioning 4 Mac-alloy bars with 5 tons each, to counteract tension on each hanger.

The team started repair work with sandblasting in Chitpore to get clarity on the underlying distressed concrete (Figure 3). Delamination and spalling were detected and removed with hammer and chisel. The exposed rebars of the hangers were treated for corrosion. The section was rebuilt with high-strength micro concrete. The underside of the bridge was made accessible with platforms supported by a structural steel section.


The strengthening of hangers, decks & cross beams was done with SRM C-Lam carbon laminates and SRM C-wraps as shown in Figure 4.


The base of the hangers was armored with a 1.2m cover block. Structural steel sections were used on top of the hangers above the RCC arch. Connecting these, high-strength MC Alloy threaded bars were installed. The tensioning mechanism was to be operated from the top of the hangers.


Mechanism of post-tensioning
Upon tightening the top nuts of the McAlloy bars, tension force gets developed in the bar, because of which a reaction force of compression is generated on the RCC hangers. Post tensioning in this manner keeps the concrete under compression while the McAlloy bars handle the tension. Typically, while post tensioning, PT force is applied using hydraulic jacks. The advantage of using McAlloy bars is that PT force can be applied without such machinery. The trained workforce of Sanrachana is seen applying PT force manually by using spanners (Figure 6). With the application of protective coating, the work was completed. All the contract criteria were met by Sanrachana.

References

1. American Concrete Institute, Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures (ACI 440.2R-2017), ACI, Farmington Hills, MI, 2017.
2. getbengal.com - (https://www.getbengal.com/details/chitpur-bridge-the-bridge-that-protected-kolkata-from-maratha-bargis).