How to Efficiently Contain Manhole Leaks and Reinforce Structural Integrity
Within our many aging sewer collection systems, manholes are a major point of entry for corrosive materials. As these structures begin to crack and corrode, they leak ground water, introducing soil, sediment, gravel and other abrasives into sewer systems, which can cause increased corrosion and erosion of concrete, steel and linings throughout the sewer system.
Leaks and cracks in manholes should not be allowed to develop and grow, not only because they bleed contaminants into the sewer system, but also because they threaten the structural integrity of this essential feature of sewer treatment systems. While rebuilding or replacing a manhole is a large undertaking, repairs performed from inside the manhole, using a four-step trenchless process, are relatively efficient and cost-effective.
Step 1: Substrate Preparation
For repairs to be effective and long lasting, concrete and steel substrates must be brought to a state in which they’ll accept the application of restorative materials.
Loose and contaminated substrate material must be removed through pressure washing, grinding and/or abrasive blasting. Then, you should verify that the Concrete Surface Profile (CSP) is appropriate for the application of repair mortars, following SSPC’s and NACE’s joint standard SSPC-SP12/NACE No. 6, which requires a CSP equal to medium-grit sandpaper. Similarly, the ICRI 310.2 standard recommends using rubber replica CSPs for comparison to the prepared surface.
Finally, check the pH of the concrete to ensure the surface is free of contaminants. If the pH is below 10, then contaminants are still present on the substrate, and you may need to perform additional actions.
Step 2: Grout Injection
If the manhole is very severely deteriorated, it may be necessary to dig around its exterior and waterproof it from the outside. But in most cases, you can use trenchless technology at considerably less cost, disruption and trouble. The process requires that you drill holes through the concrete wherever there are cracks or leaks. Then, inject a low-viscosity, water-activated polyurethane grout, which flows around the structure, following the path of the infiltrating water. The grout cures in place and forms a watertight seal between the backside of the structure and the soil.
This type of repair can either form a curtainwall that encapsulates the entire exterior of the manhole or a limited barrier that prevents infiltration at particular points.
Among available grout materials, polyurethanes are your best choice. Such grouts have a long lifespan and remain inert after curing so they aren’t susceptible to microbial-induced corrosion (MIC). In polyurethane grout, you can find a range of properties, including some that are very rigid for static operating environments, and some that are flexible for dynamic environments with traffic loading and/or hydrostatic pressure.
Step 3: Structural Rehabilitation
The next step is to address structural issues in the manhole. Usually, you are rehabilitating part or all of the interior surface area, forming a level plane with a cementitious mortar, which is stronger than standalone Portland cement.
Cementitious microsilica repair mortars add fused silica admixture to a base of Portland cement to create a denser substrate. The finished repair slows the penetration rate of corrosive materials and moisture. Microsilica repair mortars can function as standalone liners in areas with only mild hydrogen sulfide (H2S) exposure. However, they are still prone to MIC, so a corrosion-resistant liner is often recommended.
A more robust option are cementitious calcium aluminate repair mortars, which last longer than other materials and maintain a higher pH, making them more resistant to MIC. They can be used as standalone liners in areas of moderate H2S exposure, but are still susceptible to MIC without a protective liner applied.
The right cementitious repair mortar for your manholes depends on the state of the substrate, the service environment and the coating that will be applied later.
Step 4: Applying Corrosion-Resistant Coatings and Linings
Finally, in most cases, you will want to apply a robust protective barrier or liner over all surfaces inside the manhole.
There are two types of semi-structural liners: freestanding and bonded. Freestanding liners may disbond from the substrate, in which case the high-strength liner could remain in place while the substrate, hidden from view, deteriorates. By contrast, bonded liners fully adhere 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. Your lining and coating choices should be site specific, based on the substrate material, environment, traffic conditions and the coating’s chemical-resistance and film-build capabilities.
Here is some preliminary guidance for choosing among liners:
- Epoxies are versatile, strong and unaffected by moisture, making them ideal for damp applications. However, in areas with heavy traffic loads, rigid epoxies are prone to cracking.
- Polyurethane and polyurea coatings and liners are physically tough and offer improved elongation over epoxies. Flexible formulations can bridge cracks and withstand heavy traffic, minor soil movement and minor pipe shifting.
Making the Best Repair Decisions
Every manhole rehabilitation will require a different combination of processes, some involving all four steps outlined above and some only part of them. While using the four-step program, be sure to consult with a reliable coating and lining supplier who can help you identify the relevant steps and materials to use for a long-term solution.
Kevin Morris, global market director – Infrastructure for Sherwin-Williams Protective & Marine Coatings. He has been in the protective and marine water and wastewater industry for the past 17 years and with The Sherwin-Williams Company for 26 years.