Epoxy Coating: A Thin Film That Protects the Concrete Surface

The epoxy systems used in these applications are surface coatings that harden when the resin and hardener components are mixed. Solvent-free types may be preferred; however, the continuity of the coating depends not only on the type of material but also on surface preparation, coat thickness and curing conditions.

The coating does not replace the concrete; it separates the concrete’s surface from the environment. In other words, the load-bearing function still belongs to the concrete body, and the protective function to the coating.

The need for a coating is determined by assessing together the characteristics of the wastewater, the risk of biogenic corrosion, abrasion and maintenance conditions. A properly selected system can contribute in the following areas:

• Chemical resistance: The coating is selected to suit the type, concentration and temperature of the chemicals with which it will be in contact.
• Abrasion resistance: It slows the abrasion of the surface by sand and solid particles in the flow.
• Smoother surface: Low roughness can contribute to hydraulic performance; capacity is assessed together with the cross-section, gradient, joints and operating conditions.
• Ease of maintenance: The result is a surface that is easier to clean; operating processes become more controlled.
• Applicability to complex geometry: Besides pipes, it can also be used on the internal surfaces of manholes, chambers and structures. This is a significant advantage where a permanent liner cannot be fitted.

One of the important mechanisms that can be encountered in sewer systems is biogenic sulphuric acid corrosion. When environmental conditions are favourable, the process can develop as follows:

• Under conditions in which the oxygen in the wastewater is depleted, sulphate-reducing bacteria produce hydrogen sulphide (H₂S) gas. This is the source of the familiar rotten-egg smell of sewers.
• H₂S passes from the liquid into the gas phase and dissolves in the water film on the moist surfaces in the upper part of the pipe.
• Under suitable oxygen and moisture conditions, sulphur compounds are oxidised through the action of microorganisms, which can lead to the formation of sulphuric acid (H₂SO₄).
• Acid attack can cause deterioration of the cement paste and loss of surface material. Where damage is advanced, the reinforcement of reinforced elements may become exposed.

Biogenic sulphuric acid attack can be seen particularly on moist surfaces above the water level and at the crown of the pipe. This does not mean that no damage will occur at the invert; flow-induced abrasion and the direct chemical effects of the discharge are assessed separately.

An epoxy coating that has suitable chemical resistance and whose continuity is ensured limits contact between the aggressive environment and the concrete. The protection achieved depends on product selection, application quality and keeping the coating intact during service.

• Sewer lines and inspection manholes
• Wastewater treatment plant structures, pumping stations
• Industrial wastewater lines and discharge structures exposed to chemical attack
• Collector sewers and junctions where high H₂S levels are expected

The extent of the coating is determined according to the effects expected along the line. If different solutions are to be used in pipes, manholes and transition zones, continuity of protection between them is addressed in the design. The suitability of full or partial surface protection is assessed for each specific project.

The performance of epoxy depends on the material properties and the application conditions. Surface preparation, appropriate moisture, coat thickness, curing time and ambient conditions are checked in accordance with the technical instructions for the selected product. Factory application can make these checks easier; where suitable conditions are provided, site applications can also be considered.

In applications that will be in contact with drinking water, the suitability of the epoxy to be used for this purpose must be documented; this is a matter to be assessed specifically for each project.

On request, epoxy coating can be considered for our precast concrete and reinforced concrete products. The appropriate system is determined by examining together the surface to be coated, the product geometry and the chemical characteristics of the environment.