Underground Infrastructure - November 2023 - 34

How to Extend Longevity of
Underground Concrete Structures
By NASSCO Member John Sierzega, Sherwin-Williams Protective & Marine
From precast manholes to cast-in-place lift stations
and pump stations, concrete structures are found
everywhere throughout underground sewer
infrastructure. Installed below grade and left to
serve their critical functions, many such assets are
at the point of requiring rehabilitation and need
attention before they deteriorate further and require
full replacement.
Asset owners typically coat these structures from
the outset to improve their lifecycle and resist
attacks from hydrogen sulfide (H2S) and other
corrosive substances found in sewer systems.
These coatings include epoxies, polyureas, and
other polymeric compounds. However, coatings will
deteriorate over time, wearing away and exposing
the concrete substrate to direct corrosive attacks.
Paying attention to the critical details outlined below
when coating underground concrete structures will
result in the greatest potential for increased service
life.
Prep Surfaces Properly
Surface preparation is the foundation for any
successful concrete coating project. The goal is to
prepare the surface to an acceptable level for the
rehabilitation materials to adhere to the concrete
and protect the structure. The process is not
complicated. Contractors simply need to remove
old coatings and any loose concrete, while ensuring
a sufficient surface profile remains to enable the
rehabilitation materials to adhere. This can be done
with a pressure washer, grinder, or sand blaster
and is usually done quickly. Still, there are some
important considerations to follow.
Making termination points for coatings is one step
that is often missed. Terminations allow for an
impermeable ending edge of a coating system to
ensure water and corrosive substances will have
difficulty getting behind the system. They are often
used when water will flow through the structure and
when the coating system won't be monolithic
throughout the structure. Creating a proper
termination point involves grinding or cutting in a ¼inch
x ¼-inch keyway around the area where the
system will be terminated and then applying the
material inside the keyway. This keyway and the
additional coating materials anchor the system
deeper into the substrate so moisture or chemicals
will have to penetrate a full ¼ inch deep at the
perimeter before getting behind the coating system.
Such termination points are especially important if
back flushing will be done with a vac truck.
Jump on Joints
Joints in precast structures can be an area of
concern for concrete structure deterioration. Such
joints are typically packed with a cementitious
product during construction. But if movement has
occurred, these areas may be cracked, which can
lead to inflow and infiltration, erosion, and corrosion.
A different design detail may be needed for such
areas. Applicators could add a keyway on each side
of the joint and then honor the joint by filling it with a
flexible chemical-resistant caulk such as a
polysulfide sealant. They would first apply the
rehabilitation system, or polymeric coating, followed
by insertion of the flexible sealant. This option allows
the joint to move without cracking, ensuring the
area remains resistant to chemical attack. This
approach to joint rehabilitation is often used on lift
stations that are covered with a precast lid, as
thermal cycling happens at the top of the structure,
increasing the likelihood that a joint may crack in
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