Purpose of coal-tar-pitch varnish on Manholes Castings

[ropotto]

Many specifications require all castings shall be carefully coated, inside and out, with coal-tar-pitch varnish.

Asphaltic base paints contain organic compounds (VOC), which evaoprate and contribute to air pollution.

In order to comply with US EPA regulations, many casting companies have discontinue the practice of painting their castings.

Does the painting of castings provide a significant added protection to the casting?

 

[bimr]

The specifications are old and obsolete. Coal tar pitch varnishes were invented over 100 years ago and were inexpensive, waterproof, and long lasting.

However, one can't expect a coating to work universally in all conditions of climate, weather, sunlight, erosion, and water.

There is some benefit when the coating is new. However, the coating is sensitive to UV light and will fail over time.

Most covers that I have seen, do not have any coating on them. I don't believe anybody is recoating the manhole covers. It would probably be less expensive to replace the covers.

http://www.wwoa.org/files/published...ha TelzoffWWOA 2010 MH presentation final.pdf

 

[Ron]

If you are talking about the cast iron parts....antiquated spec.
If you are talking about the concrete parts, there are several reasons to coat, but as bimr noted, coal tar pitch is not used much anymore. It was (and is) an excellent coating; however. It also has VOC's that some regulations prohibit.

The coating is to protect the concrete from water intrusion that can affect the rebar. Most sections are thin and there is minimal cover on the rebar, so added protection is good. Another reason is for protection of the concrete and the coarse aggregate. Wastewater, whether sewage or drainage, can contain deleterious materials that cause deterioration of the concrete. Again, added protection is good.

Back to the rebar....codes often require greater cover when concrete is in direct contact with soil. Coating the concrete mitigates this requirement by many interpretations.