Four Steps for Successful Rehabilitation

Correcting manhole condition issues, inside and out, will improve your whole collections system.

Four Steps for Successful Rehabilitation

Structural repair is one of four general steps in the manhole rehabilitation process. Cementitious mortars, which can be troweled, brushed, sprayed or spin-cast, are the most common structural repair materials.

Manholes are critical factors contributing to the accelerated deterioration of collections systems. In addition to significant inflow and infiltration problems, cracked and eroded manholes allow soil, gravel and other abrasives to enter sewer systems and eat away at your infrastructure. 

As the pace of corrosion intensifies, associated costs and risks will continue to rise unless sanitary sewer system owners take action to rehabilitate your manhole infrastructure. Fortunately, you will often be able to repair, rather than replace, these structures by following a four-step process involving surface preparation, stopping inflow and infiltration, making structural repairs, and applying corrosion-resistant coatings and linings. Following these steps can help owners enhance the lives of their sewer systems while also reducing overall life cycle costs.

1. Surface preparation

Properly preparing corroded, contaminated surfaces is critical to ensuring long-term performance of manhole repairs. Surface preparation brings concrete and steel substrates back to a state in which they’ll accept the application of restorative materials. This includes removing deteriorated areas affected by microbial-induced corrosion (MIC). The MIC process breaks down sulfates, creating corrosive gases and acids that lower the concrete’s pH, causing it to deteriorate faster. 

To prepare surfaces effectively, crews must remove any loose and contaminated substrate material, using methods such as pressure washing, grinding and abrasive blasting. To confirm the resulting concrete surface profile (CSP) is appropriate for the application of repair mortars, contractors commonly follow SSPC’s and NACE’s joint standard SSPC-SP12/NACE No. 6, which requires a CSP equal to medium-grit sandpaper. It’s also helpful to follow the ICRI 310.2 standard, which uses rubber replica CSPs for comparison to the prepared surface. 

Following surface prep, crews must check the pH of the concrete to ensure the surface is free of contaminants. If the pH is below 10, contaminants are still present on the substrate, and crews may need to take additional steps.

2. Stopping I&I

As manholes deteriorate, I&I accelerates via leaks, voids and cracks, introducing ground and surface water, soil, and debris into the sewer system. Stopping I&I is critical to ensure long-term system repairs and minimize the unnecessary treatment of excess water.

In extreme cases, the only way to stop I&I is digging around the manhole and waterproofing it from the outside. Trenchless technologies are a more cost-effective approach. They allow crews to drill holes through the concrete and inject a low-viscosity, water-activated polyurethane grout. The injected grout flows around the structure, following the path of the infiltrating water, and cures in place to create a watertight seal between the backside of the structure and the soil. This repair could either be a curtain wall that encapsulates the entire exterior of the manhole to halt multiple leaks or a point injection, which stops a specific area of infiltration. 

Material options to stop I&I vary, but polyurethane grouts have a long life span and remain inert after curing so they aren’t susceptible to MIC. In addition, you can find a range of polyurethane grout properties from very rigid for static operating environments to flexible for dynamic environments subject to traffic loading and/or hydrostatic pressure.

3. Structural repairs

After halting I&I, you need to address any structural issues in the manhole and rebuild the surface to a level plane, restoring the deteriorated concrete closer to its original state. 

The most common structural repair materials are cementitious repair mortars, which combine a cement material with an admixture to enhance its structural integrity. They’re a better choice than stand-alone Portland cement, which is relatively weak for structural repairs and is highly susceptible to MIC. 

Cementitious microsilica repair mortars use Portland cement as a base but also include a fused silica admixture to create a denser substrate. The denser finished repair slows the penetration rate of corrosive materials and moisture. Microsilica repair mortars offer sufficient strength to work as stand-alone liners in areas with mild hydrogen sulfide exposure. However, they are still prone to MIC, so a corrosion-resistant liner is often recommended. 

Cementitious calcium aluminate repair mortars are more robust options that use calcium aluminate cement rather than Portland cement. These mortars offer improved life cycles compared to other materials and maintain a higher pH, which makes them more resistant to MIC. They can be used as stand-alone liners in areas of moderate H2S exposure, but are still susceptible to MIC without a protective liner applied. 

The most appropriate cementitious repair mortar depends on the severity of the substrate deterioration, the service environment, and the coating that will be applied later. The selection of repair mortars is dependent on whether the system will be left as a stand-alone liner or if it will receive a corrosion-resistant resinous liner on top of the repair mortar.

4. Coatings and linings

Once you’ve re-established a manhole’s structural integrity, you’ll want to place a robust protective barrier between the restored substrate and the corrosive waste flow in many cases. 

Semistructural liners come in two styles: freestanding and bonded. A freestanding liner creates problems when it disbonds from the substrate. The high-strength liner may remain in place, but the substrate, hidden from view, may deteriorate as water can flow behind the liner. Corrosion can also continue if the substrate surface was not properly prepared. Continuing deterioration may not be evident until the manhole starts to collapse. In contrast, bonded liners rely on full adhesion to the substrate. If they start to fail, blisters and other indicators will make the need for repair evident, enabling more proactive fixes. 

Epoxy, polyurea, and polyurethane materials all provide robust barriers that protect substrates from MIC, but lining and coating choices should be site specific. Selection factors include the substrate material, the environment and traffic conditions, along with the coating’s chemical resistance and film-build capabilities. 

Epoxies are a popular choice because of their versatility. They are strong and unaffected by moisture, making them ideal for application on damp substrates. However, in areas with heavy traffic loads, rigid epoxies are prone to cracking. 

Polyurethane and polyurea coatings and liners are gaining popularity because of their physical toughness and improved elongation over epoxies. Flexible formulations can bridge cracks and withstand heavy traffic, along with minor soil movement and pipe shifting. Manufacturers offer various formulations to provide an array of desired characteristics. For instance, some products with high film-build characteristics can be spray-applied thick enough to fill surface voids without needing to complete surface repairs, thereby eliminating a rehabilitation step.

Good decisions

Every manhole rehabilitation is different. Some structures will require minimal repairs and have no need for corrosion-resistant coatings and linings, while others will require major overhauls to stop I&I, rebuild structures, and protect the restored structures from corrosion. Most likely, your complete sewer system will require a combination of light and extensive repairs. 

Following the four-step program outlined above and consulting with a reliable coating and lining supplier will help you realize long-term results from any repairs.

Kevin Morris is the water and wastewater market segment director for Sherwin-Williams Protective & Marine Coatings, in addition to holding many accreditations as a coatings inspector and instructor.



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