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Infrastructure rehabilitation: Sarnia Road bridge project

The Sarnia Road Bridge (B112), located within the eThekwini municipality between the suburbs of Bellair and Hillary, is a vehicular bridge that runs over the N2 and carries traffic along the M10 route.

Sarnia Road Bridge (B112), located within the eThekwini municipality between the suburbs of Bellair and Hillary.

A familiar sight for many motorists, the bridge incurred significant damage in 2017, especially to the underside of the structure, when a vehicle accident took place on the N2 Northbound carriageway, resulting in a tanker truck catching alight directly beneath the bridge.

The South African Roads Agency Limited (SANRAL) put the project out to tender in 2022, with the scope of work as N002-250-202/3-The rehabilitation of B112, Sarnia remedial.

Local construction company, Afrostructures (Pty) Ltd, which specialises in structural concrete, building, and development, emerged victorious after a competitive bidding process. Located in KwaZulu-Natal, with a grade 9CE and a level 1 BBBEE rating, the industry leader secured the contract in April 2022. Naidoo Consulting Engineers was contracted to oversee all engineering works and supervision of the project, while specialist company, Structural Maintenance Works (Pty) Ltd was to handle the fire damage repairs, spalling, and coating of the bridge. Under the careful consideration of this team of experts, the project was underway.

Sarnia Road Bridge before repairs.

 Refurbishment Process

Step 1: High-Pressure Cleaning (350-bar pressure)

A high-pressure clean at 350-bar pressure was conducted, to remove debris and compromised concrete, and prepare for restoration.

Step 2: Sounding Survey

A sounding survey was undertaken to assess concrete integrity. This precision test streamlines the process and ensures that only compromised sections are addressed.

Step 3: Exposing Sound Undamaged Concrete

All fire-damaged concrete was broken out, to expose undamaged sections. This selective approach minimises reconstruction needs, which in turn reduces cost and environmental impact.

Step 4: Rebar Preparation and Substrate Dampening

The exposed rebar was meticulously cleaned, and the substrate pre-dampened for a strong bond, ensuring subsequent layers would adhere securely.

Step 5: Application of Structural Repair Mortars

Following substrate preparation, Sika MonoTop®-1010 ZA was brush applied to both the rebar and the substrate as both a protective coating for reinforcing steel, and a bonding agent. This cement-based slurry contains recycled materials, which leads to a reduced carbon footprint. As a bonding primer, it offers reinforcement corrosion protection. Once the primer had cured, Sika MonoTop®-412 NFG – an R4 repair mortar with a corrosion inhibitor – was applied using a wet-spray method.

Side view of bridge after repairs.

Due to concerns about bridge flexing under live loads, it was imperative that this phase be completed in a single day. This was made possible by dividing the work into sections, each spanning approximately 2 metres across the width of the bridge (+-15m long).

Step 6: Spalling Repairs

It was found that small sections of the bridge required spalling repairs. Due to chloride ingress, the substrate preparation process mirrored the one above, with a minor adjustment of Sika MonoTop®-412 NFG being trowel applied to affected areas. Where more extensive repairs were required, a shutter-and-pour method was employed, using Sikacrete®-214; a cementitious, high, early-strength structural micro-concrete.

Step 7: Addressing Concrete Blistering

There was evidence of concrete blistering in the centre of the bridge, where the heat of the fire had been less intense. This was addressed using the same preparatory steps, after which Sika MonoTop®-3020 ZA – an R3 cementitious pore filler and levelling mortar with a reduced carbon footprint – was meticulously trowelled on, in a 1-5mm thick layer.

Protective Coating Systems

Step 1: Corrosion Protection

Sika® FerroGard®-903 Plus was applied via low-pressure spray method to the entire bridge soffit. This coating delays corrosion and reduces the future corrosion rate.

Step 2: Chloride and Carbonation Protection

The abutments, piers, and sides of the bridge received the Sikagard®-550 W Elastic system. This comprehensive system consists of one coat of Sikagard®-550 Elastic Primer and two coats of Sikagard®-550 Elastic, a crack-bridging protective coating for concrete which enhances durability and protects the structure against chloride and carbonation ingress.

Step 3: Protection against Graffiti

To deter vandalism and protect the aesthetics of the bridge, Sikagard®-850 AG, an anti-graffiti coating, is applied. This final touch ensures that the lower 3 meters of the two abutments and the pier on Palm Drive remain in pristine condition and any graffiti applied will easily wash off with water.

This project would not have been possible without the wealth of knowledge and expertise of the contractors involved. SIKA was honoured to be able to contribute to the successful rehabilitation of the Sarnia Road Bridge (B112). The completion of this project contributes to the safety and functionality of the bridge and the overall transportation infrastructure in the area and, perhaps even more importantly, it demonstrates that even in the realm of rehabilitation, there’s room for innovation and improvement. Throughout the tender period, the new COTO draft specifications (October 2020) were adhered to, reflecting a firm commitment to the latest industry standards and best practices.

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