By Eri Vokshi, PE and Leonardo Paim, PhD
By Eri Vokshi, PE and Leonardo Paim, PhD
Opened in 1985, the Carajás Railroad ranks as the most efficient railway in Brazil. The 554-mile (892-km) long rail links the world’s largest open-air iron ore mine in Carajás, to the port of Ponta da Madeira, in São Luís, transporting 120 million tons of cargo and 350 thousand passengers each year.
A section of the line passes over the Tocantins River via Ponte Rio Tocantins, a mile-long (1.67 km) bridge in Maraba PA. The bridge is supported by 147 reinforced concrete underwater piles with steel jackets that are maintained by a global mining company. A few years ago, mining production increases led to the need to extend the life and capacity of the pilings that had suffered from extensive corrosion on the steel jackets. Though the original steel jackets were designed to be sacrificial, the increase in capacity requirement meant the steel jackets would need to perform as structural elements to support the bridge. A coating system was required to prevent further corrosion and extend their field life. The piles are 60 in OD (1.5 m OD) and approximately 20 ft (4.5m) long.
Two alternative solutions were considered before selecting ClockSpring|NRI’s Syntho-Shield™ to protect the steel jackets from corrosion.
One of the options evaluated was epoxy putties, which have been used for years as a steel coating for underwater applications. These systems are adhesion critical, requiring hydro blasting to ensure a good bond, which means achieving a good surface preparation underwater can be at times difficult, with flash rust slowing down the installation process or causing premature disbondment on newly installed product.
Installation also requires considerable mobilization, which includes a boat, a generator, a hydro-blaster, and a team of divers. It is important to note that the quality of installation depends on the experience of the diver. Since the application is all done using the diver’s hands, the thickness of the epoxy cannot be controlled well. Time is also a consideration. The application time for the Ponte Rio Tocantins project for one piling with 10 divers was estimated to be one day, which meant the project would take nearly five months to complete using this solution.
Pile jacketing is another common method for repairing underwater piles. A solution considered for this project involved rubber jackets with a petrolatum tape primer. This method of repair was quickly eliminated. Jackets are size specific, and the cost, long lead-times (up to 6 months) and logistics challenges in transporting the jackets to the remote area made this solution impractical.
A composite solution emerged as the most viable for this installation, and the decision was made to use the ClockSpring|NRI Syntho-Shield system. Syntho-Shield comprises three components: a Petrolatum primer/tape system undercoating for long-term corrosion protection, Syntho-Glass™ XT for mechanical protection, and Syntho-Glass™ SPF as a final mechanical and UV protection barrier. The tape is designed to provide long-term corrosion protection as well as mechanical and impact strength to the piles.
One of the benefits of the Syntho-Shield system is that it can be applied to marginally prepared surfaces. The requirement for surface preparation is removal of loose scale, rust, and other foreign matter in accordance with SSPC SP2 “Hand Tool Cleaning” or high-pressure water washing.
The mining company elected to use this system for several reasons – simple surface preparation requirements, ease and speed of installation, quality control delivered by the uniform repair, low material wastage, and non-toxicity to divers.
The general contractor for this project, subcontracted the diving companies that carried out the installation. Using three five-person crews working five days/week, the project was completed in 135 days. Above-water installation performed by a four-member crew achieved approximately 40m2/day per pile. For submerged applications, a team consisting of a supervisor and one reserve diver and three active divers achieved 100m2/day installation productivity per pile. Installation productivity varied depending on the repair length and the ease of accessibility of the piles.
The Syntho-Glass system was applied in 5-ft (1.5-m) sections to accommodate the large size of the piling and the installation limit imposed by the curing time. The application process included the following steps:
Steps 3 and 4 were required to prevent the petrolatum tape and XT resin from bleeding into the UV coating system, which would have made the UV coating layers look damaged and discolored overtime.
At the termination of each completed 5-ft (1.5-m) section, layers of the petrolatum tape and the composite layers were staggered with at least 2- to 4-in (5- to 10-cm) gap between to allow subsequent sections to tie in properly on the piling. Compression film was used to cover any exposed petrolatum tape sections on piling areas that were not fully wrapped by the end of a workday.
The Ponte Rio Tocantins project presented various challenges, including high current conditions, difficult access to the pilings, and poor visibility underwater. Using the Syntho-Shield system, the project was executed successfully, leaving the client and contractor very pleased with ease of application, installation productivity, cost, and little mobilization required. The success of this project has led to the mining company’s decision to use the same product to repair a damaged pier at another location.