Purpose of Asphaltic Bitumastic Coating on Manholes 1

[EngWade]

Quick question for some of you experienced engineers out there.

What is the purpose of asphalt bitumastic coatings on manholes? In my experience, I've noticed that this coating is only specified on sanitary sewer manholes, and typically not on storm sewer manholes. This leads me to believe that the reason for this is because the coating adds an extra level of seal protection from having septic substances mixing with ground water. Does anyone have any ideas regarding this? I practice primarily in NY.

Thanks in advance.

 

[cvg]

sanitary manholes are exposed to hydrogen sulfide which dissolves in water and produces sulfuric acid. This attacks the concrete and steel reinforcement. Coatings are intended to protect the structure from corrosion.

 

[Ron]

agree with cvg. Further, wastewater contains a variety of other sulphates which will attack the concrete directly. The bituminous coating, whether asphaltic or coal tar, is to protect the structure...really has nothing to do with groundwater contamination, though I suppose that could be a side effect. Please note also that "Bitumastic" is a proprietary name (Koppers Corporation).

 

[EngWade]

Thanks cvg and Ron. But to follow up, the sealant is specified only on the outside of the manhole (in our typical details). Why would that be? I always understood that (in most cases) if you provided 3" of clear separation from the outside of a concrete surface to the surface of rebar, that this distance was sufficient to prevent water penetration from reaching the rebar, thereby preventing any rebar corrosion. So speaking of the manholes again, they definitely do not have this clear distance (because the manhole walls are only 6" thick, themselves). So why is the coating only specified on the sanitary sewer manhole, and not also the storm sewer manhole, as both would surely have rebar in their walls?

Thanks again.

 

[bimr]

Exterior coatings may be used on all or portions of the barrel, cone, and chimney sections of manholes. These coatings are unique in that they require less of a bond than interior coatings, due to the fact that the backfilled soil holds them in place and any groundwater presses the
coating onto the manhole. These coatings come in different types: spray-on cementitious, epoxy, or polyurethane; shrink-wrap plastic; and/or adhesive rubber tapes.

The exterior coatings are mainly for water proofing although ther may be some conditions where the soil is corrosive. Another water proofing consideration is that the concrete may crack and the exterior coating is considered more flexible than the concrete which should allow the coating to seal the crack somewhat.

Bitumastic is also used with concrete plugs to seal the lift holes on precast elements.

Some utilities also do not use coatings on the exterior of the manholes.

In addition, coatings such as coal-tar epoxies have a history of poor performance where hydrogen sulfide attack can occur. PVC or HDPE liners are more effective.

 

[CarlB]

Some Utility companies experience infiltration of groundwater into sanitary sewer system which affects the function of the Wastewater treatment Plant. So in typical fashion they require all manholes be coated and have joints sealed, regardless if there is an infiltration potential. They may also see it as additional protection against wastewater leaking from the manhole.

 

[cvg]

3 inches of cover over steel is not a magic number, minimum cover is often less than this, especially with precast concrete elements such as manhole shafts. Standard manhole shafts around here are only 4 inches thick. Not all manholes even have steel reinforcing, a lot of the older ones are brick and mortar.

Waterproofing and joint sealing is usually only required on sanitary manholes and generally on the interior only. This is preferable because the hydrogen sulfide is on the inside of the manhole and the inside can also be inspected and re-coated if necessary. Waterproofing the outside with bituminous coating to prevent infiltration of groundwater would be minimally effective at best.

 

[bimr]

The sulfate ion present is typical municipal wastewater may be anaerobic ally reduced in sewers to sulfide ion. The sulfide ion then combines with the hydrogen ion to form sulfuric acid. The sulfuric acid then will dissolve and erode away the concrete structure from the inside of the manhole.

Corrosion experts have moved away from specifying interior bitumastic linings because of the historically poor performance. The best option for corrosion protection where you know hydrogen sulfide to be present is internal linings made from HDPE, PVC, FRP, etc.

Concrete is not a water proof material and concrete may crack. That is one of the reasons that the bitumastic is specified on the manhole exterior. This is especially true in areas with high groundwater. A similar asphalt-based coating is used on foundation walls of houses for the same reason, waterproofing.

In addition, if you are in a marine environment, seawater submergence conditions, or estuarine areas, the effect of elevated sulfate and chloride concentrations should be carefully considered. High sulfate concentrations will spall concrete, while chlorides will penetrate through the concrete and induce accelerated corrosion of reinforced steel.

Bitumastic is also used for spot sealing repairs on the exterior of manholes.

 

[Ron]

For code purposes, the cover over rebar is required to be greatest when the concrete is exposed directly to ground contact. To provide a bituminous coating will allow one to, theoretically, define the concrete as separated from the ground and thus keep the cover requirement down. This saves concrete thickness, thus money.

Coating a manhole on the inside is to protect the concrete from direct attack by sulphates in the wastewater and to reduce the incidence of permeation of the concrete to the rebar levels. Since the rebar is closer to the outside, this is less of an issue than the attack on the concrete matrix itself.

When you use asphaltic coatings, remember that the concrete must be primed before the asphalt mastic or coating. If you are using a coal tar based material (Bitumastic for instance) you do not need to prime the concrete before application.

 

[cvg]

I'm not sure what "code" covers sewers and manholes concrete cover over steel reinforcement. However, the "standards" for most reinforced concrete pipe such as ASTM and AASHTO do not require bituminous coating on thin precast concrete pipe. This is routinely installed underground without any waterproofing. Thickness of the concrete coating is often approaching one inch for small diameter pipes. ASTM C76 requires 3/4" minimum cover over the steel, however with permissible variations, it may be as small as 1/2" cover.

 

[bimr]

Corp of Engineers C-200, C200a
AWWA C-210-92 for exterior
SSPC-Paint 16

Most specifications are require manhole waterproofing not protection of steel reinforcement. Here are examples:

3.2.7 Manholes – Bitumastic Coatings
A. The entire exterior surface of all masonry and concrete (whether precast or cast-in-place) structures associated with sewerage systems, such as: manholes, grease traps, holding tanks, tight tanks, septic tanks, aeration tanks, pump stations, valve pits, etc., shall receive two coats of
waterproofing such as Carboline Bitumastic 300M as manufactured by SOMAY Products, Inc., Miami, FL; Sonnoshield HLM 5000 as manufactured by Sonneborn, Shakopee, MN at a minimum thickness of 7 mils per coat and a total thickness of 14 mils; however, in no case shall the
thickness per coat be less than that recommended by the manufacturer.


Bitumastic is a century-old trade name that is often associated with long-term protection from water penetration. Typically made from coal tars and blends of resins such as epoxy, these products have been used as tank linings, to coat the interior and exterior of buried pipe, and to protect equipment subjected to water immersion.

http://www.farwestcorrosion.com/ccp/protect/carbolin/bitumastic300M.htm

E. Waterproofing:

1. Apply coal-tar coating to exterior walls on all manholes from base to finish grade:

a. Carboline - Bitumastic 300M.

b. International - Intertuf 100.

c. Tnemec - HB Tnemecol 46-465.

2. Apply coating in two coats to minimum 0.3 mm (12-mil) dry-film thickness per coat.


19. Exterior waterproof coating: Manholes shall receive a two coat application of Koppers Super Service Black waterproof bitumastic coating or approved equal.

3. Manholes shall have a water proof Bituminous Coating on the exterior & interior. Koppers bitumastic black solution 300M, or Proco EP-214.

N. The exterior surfaces of all Precast Concrete Manhole Barrel Sections shall be waterproofed. Waterproof coating shall be two coats bitomastic material or