The Role of Coatings in Manhole Rehabilitation

Today’s sewer collection system is a maintenance challenge for municipal water and wastewater owners, because it is exposed more than in the past to corrosive materials that speed the rate of deterioration.

Factors that have increased corrosive content include less water in the waste stream, longer transport times and slower flows. Also, in some locales industrial waste streams enter the municipal system, contributing fatty acids and other corrosive materials. Other factors complicating sewer maintenance are freeze-thaw cycles, traffic loading, soil movement and erosion caused by cavitation.

Fortunately, high-performance, chemical-resistant coatings are available to protect against deterioration by creating a protective barrier between the substrate and the waste flow. Coatings come in a variety of formulations with different functional characteristics and application requirements. This article explains how corrosion occurs in the sewer collection system — manholes specifically — and how five types of linings work to prevent it.

How Microbial Agents Induce Corrosion

Microbial Induced Corrosion (MIC) is responsible for the deterioration of sewer manholes, most of which today are made of concrete. Older ones were constructed using brick and mortar. Concrete and mortar, which share cement as an ingredient, are highly alkaline, porous substrates. When they are exposed to carbon dioxide and hydrogen sulfide gas carried in the sewage, a complex, multi-phase process of corrosion is set in motion.

In the first phase, sulfur reducing bacteria (SRB) break down sulfates in the waste stream and produce hydrogen sulfide gas (H2S) and carbon dioxide (CO2). In the second phase, these acidic gases reduce the pH of the concrete from 12 to as low as 9. Sulfur oxidizing bacteria (SOB) attach to the surface as sulfates are produced.

Next, in phase three, the SOB Thiobacillus thioxidans consume the H2S and elemental sulfur and discharge sulfuric acid (H2SO4). The pH of the substrate continues to drop, accelerating microbial growth. As the bacteria colonize, more concentrated pockets of H2SO4 are created.

Finally, in phase four, the acid attacking the concrete creates a layer of gypsum (calcium sulfate) that allows the microorganism to reproduce, and more acid is created. Eventually structural failure occurs.

Protecting Waste Stream Infrastructure from MIC

High-performance coatings, including high-performance cements, have been employed for years to protect against the corrosive process caused by MIC. The coatings’ different chemistries determine their functional properties.

Contemporary coatings used in new and rehabilitated structures fall into five generic categories:

Cement Liners: Microsilica Mortars
Cement Liners: Calcium Aluminate Mortars
Epoxy Liners
Epoxy Resin
Fiber Reinforced Epoxies
Epoxy Mortars
Polyurethane and Hybrid Polyurea Liners
Pure Polyurea Liners

Cement Liners: Microsilica Mortars
These cementitious liners use typical Portland cement, which is porous without additives. When the manufacturer adds fumed silica to the cement mixture, the result is a much denser finished product that slows the penetration rate of water and/or chemicals. This class of cementitious liner approximately doubles* the life cycle of the substrate compared to typical Portland-based cements.

Cement Liners: Calcium Aluminate Mortars

Cementitious liners that use calcium aluminate cement rather than Portland cement maintain a higher pH level that is less affected by the corrosive gases in the sewer system. The higher pH level limits the growth of SOBs, thereby prolonging service life approximately four to five times* over that of typical Portland-based cements.

Each of the following three groups of resinous coatings serves as a chemical-resistant protective barrier between the substrate and the corrosive environment. They also should be spark tested (High Voltage Holiday Tested) per NACE International SP0188 to ensure that the coating film is monolithic and free of any holidays or voids.

Epoxy Liners (Epoxy Resin, Fiber Reinforced and Epoxy Mortar Liners)

Epoxy liners have long been favored by owners and specifiers. In addition to their excellent chemical-resistant properties, they are strong and unaffected by wetness/humidity, making them ideal for applying to damp substrates. On the downside, these coatings are rigid, which creates problems if the structure moves beyond the coating’s ability to expand. Epoxy liners are typically bonded directly to the substrate and may require the use of primer. They are spray-applied at dry film thicknesses of 60 to 250 mils.

Polyurethane and Hybrid Polyurea Liners

While they have been slower than epoxy liners in gaining acceptance among specifiers and owners, polyurethane and hybrid polyurea liners are now becoming more widely used. These coatings offer physical toughness and improved elongation over epoxies. A moisture-tolerant primer is required to bond to damp surfaces. This type of resin can be formulated to offer ideal film-build characteristics, allowing the applicator to fill surface voids with the spray-applied liner. Because this eliminates the need for surface repair prior to application, roadway disruption time is shortened.

Polyurethanes and hybrid polyureas can be formulated to have varying performance characteristics, making it difficult to compare them to one another and to epoxy resins especially. The flexible films bond to the substrate with a moisture-tolerant epoxy primer, have crack bridging capabilities and can withstand constant, heavy traffic, as well has minor soil movement and pipe shifting. The rigid films, like epoxies, could crack if the substrate moves more than the film will expand, or they could disbond from the substrate and remain self-supporting. These liners are plural component spray-applied at dry film thicknesses of 80 to 250 mils.

Pure Polyurea Liners

Pure polyurea liners also have been slow to gain acceptance among specifiers and owners. However, they offer physical toughness and improved elongation over other technologies. A moisture-tolerant primer is required to bond to damp surfaces. This type of resinous liner offers an extremely fast dry-to-the-touch time (15 to 45 seconds), which forces the resin to build up around surface imperfections rather than filling them in. For this reason, the substrate needs to be repaired before application to achieve a monolithic liner. The extremely short dry times also necessitate sophisticated application equipment that is hard to maneuver in tight sewer manholes. The coating is difficult to hand-spray without problems and remote centrifugal spray application equipment is available that eliminates problems caused by triggering (off and on) the application gun. This remote centrifugal spray equipment ensures the coating will not be applied “off ratio” due to human error.

Pure polyureas typically have the lowest permeability rating of any of the generic classifications of resinous liners; however, their use in industrial waste streams is problematic because they offer only moderate resistance to concentrated acids. These liners are spray applied at dry film thicknesses of 80 to 250 mils.

A Final Note on Water Infiltration

Regardless of which coating is used, measures must be taken to prevent water infiltration before application. Otherwise, moisture will permeate back through the cementitious liners, causing blisters or other defects in resinous liners, or running into the waste stream between the substrate and a disbonded self-supporting liner. Water infiltration is best stopped by using a reactive polyurethane resin — activated by groundwater leaking into the structure — to create a permanent waterproof barrier.

*Data obtained through a study conducted by Bio-Industrie Heidelburg for Lehigh Cement Company. An exact service life cannot be determined due to the varying corrosive content of individual sewer systems.

Kevin Morris is global market director of water and wastewater at The Sherwin-Williams Co.