Devastating flooding in Mexico urged the government to refurbish the sewer system in the capital city. Located 80 meters underground, the system was inspected by a team of engineers using boats. Pedro Sanchez discusses the methods involved to replace the pipe’s protective coating and the equipment used.
In October 2007, a catastrophic flood hit the city of Villahermosa in Mexico’s Tabasco State, causing a state of emergency that placed many lives at risk. The flood had many repercussions in the capital, Mexico City, where 20 million people were living with the potential for a similar flood. The city’s aging sewer system had not been maintained for 35 years due to time and cost considerations. This situation was not only a municipal concern in Mexico City, but also a geographical one. For many years, this city has had to fight against continuous flooding due to the nature of its location.
Founded on land dredged from an old lake, the necessity of draining water out of the city forced authorities to construct a large scale system to channel the flow of water outside city boundaries. After the devastating experience in Villahermosa, the government acknowledged that they could no longer ignore Mexico City’s deteriorating sewer system and decided to inspect the city’s main sewer line to assess the situation.
"Sistema de Aguas de la Ciudad de México," the public office in charge of the city’s sewer system, arranged for a team of engineers to inspect the 35-year-old sewer. Due to its large size, 196,000ft (approx) long and 21.3ft diameter, the personnel carrying out the inspection used a boat to go through the pipe. The inspection involved evaluating the condition of the tunnel and the damage suffered by the concrete walls. Once the water in the tunnel was drained, the engineers were able to use an all-terrain car named "unimog". Based on the engineers’ findings, the authorities decided to coat the concrete pipe.
According to the engineers’ report, the upper dome of the sewer had been attacked by acidic vapors generated by the decomposition of organic matter in the water. The decomposition of sewer water produces sulfuric acid, high amounts of methane, and hydrochloric acid, all of which accelerate the rate of chemical attack on the pipe. Along with a pH decrease, a consequent risk of passivation is driven on the reinforcing steel. This effect is described as the deterioration of the protective film covering the steel in the concrete. Furthermore, the transportation of all sorts of debris caused abrasion on the concrete walls, leading to cracks and exposed steel supports which could eventually compromise the structural integrity, if not promptly treated.
The Mexican authorities, in conjunction with the Engineering Department of Mexico University, the office in charge of technical support for this project, agreed to use the Belzona 5811 (Immersion Grade) to protect 129,166.9 ft2 of concrete wall.
This is a two-component, high performance epoxy barrier coating designed to protect metallic and non-metallic surfaces against attack from aqueous solutions. One feature of the product is its capability of being spray applied under the "wet" conditions of the substrate and the environment. The Belzona 5811 (Immersion Grade) is suitable to operate under immersion conditions up to 122°F and is recommended to be applied in two coats of 10 mils each, for a total dry film thickness of 20 mils. The product consists of a base and a solidifier which are mixed in proportions of three parts of base per one part of solidifier. For this specific application, the immersion grade used was presented in two colors: beige and grey.
Equipment, personnel and surface separation
The equipment used for this application was a Graco pump equipped with a SuperCat plural component proportioner, which mixed the Belzona product as it was being used. The base and solidifier of the Belzona 5811 (Immersion Grade) were separately placed into two hoppers located at both sides of the pump. Each hopper was equipped with a silicon rubber heating band to increase the temperature of both the base and solidifier up to 149°F.
A customised cart was designed to transport the spray equipment, product, crew, and scaffolding equipment, which was erected on the cart for the spraying personnel to reach the top of the pipe. Since the main floor was not level, the cart was constructed with wheels that rolled along the walls of the pipe.
All personnel in charge of the application were required to wear safety harnesses and Personal Protective Equipment (PPE) including respirators, goggles, suits, and gloves. The applicators were trained to perform in confined spaces. Gas detectors were used to constantly monitor the air inside of the pipe. Fans located at the surface were also used to ensure that fresh air was constantly recirculated into the pipe.
In order to access the pipe, located 70 to 80 meters underground, the crew members took an elevator. A safety meeting was conducted every morning before accessing the tunnel. Each applicator was required to report to a safety supervisor whenever accessing or exiting the tunnel.
Surface preparation was performed by a qualified company contracted for this purpose. The surface was pressure washed using 5,000 psi high-pressure water jetting to clean the substrate and achieve a minimum 3 mil surface profile as per NACE No. 5/SSPC-SP 1210. Environmental parameters such as temperature of the surface, dew point, and relative humidity were measured and logged on a daily basis.
The application was carried out by local company, Ivresse International.
"Sistema de Aguas de la Ciudad de México" required a 28-day interval for the application to be completed. The total area of deteriorated concrete to be coated was 129,166.9 ft2. The job was carried out during the dry season when the level of the water in the system was the lowest.
For this application the tunnel was strategically divided into several sections of 262.5ft each. The first day’s application comprised spraying a 10 mil layer of Belzona 5811 (Immersion Grade) throughout a 262.5ft section of the tunnel. The following day a second 10 mil layer was applied over the same section coated the previous day, and a 10 mil layer was coated over the subsequent 262.5 ft section of the tunnel. The same procedure was repeated until completion of the job. In all cases, the first coat was grey and the second, beige. The aim of this process was to ease the application of the second coat and avoid "pinholes and misses". The entire job lasted 30 days and post-application inspection was carried out by a qualified third party to ensure proper application standards.
The product and process used has proven itself in extending the life of the sewer system in Ciudad, Mexico. Not only did this product prolong the operational capability of this system, but it also provided resistance against the attack of acids. It is recommended that this unique drainage system should be regularly inspected and maintained to avoid higher maintenance costs.Author’s note: Pedro Sanchez is a technical service engineer for Belzona Inc. For further information please visit: www.belzona.com.