Epoxy Coated Rebar - Wastewater Treatment Concrete Tanks 8

[vincentpa]

I am designing concrete tanks for a human wastewater treatment facility. Would you use epoxy coated bars in the concrete for a tank containing human wastewater? ACI 350R-89 states that the concrete does not need to be protected from human wastewater but does not mention anything about the rebar. The previous section states that the rebar is well protected when cover requirements are followed. I am assuming then that epoxy coated rebar is not required. The new ACI 350-01 and -06, ACI 350.2-04 and ACI 350.4-04 do not address wastewater treatment facilities and whether epoxy coated bars are necessary. What do you typically do in human wastewater treatment facilities?

 

[civilperson]

What does the adjective "human" denote compared to normal waste water? Rebar in dense, well-consolidated concrete with 2" cover should work in "waste water" concrete tanks.

 

[vincentpa]

There are many types of "wastewater". I'm denoting wastewater from human activity such as washing, bathroom, eating, etc... as opposed to industrial wastewater or wastewater from other types of environmental clean-up.

 

[JedClampett]

Epoxy coated reinforcing is not worth the premium cost.

 

[Ussuri]

I have never come across epoxy coated reinforcement. Typically (in the UK) the rebar is protected by increasing cover albeit this is at the cost of crack control.

How does the epoxy coating affect the bond with the rebar, do lap and anchorage lengths need to be increased?

 

[SHBH]

How about cathodic protection.

 

[shin25]

I would suggest, add DCI-corrosion inhibitor in the concrete mix, provide 3" clear cover on the contact side and forget about epoxy coated rebars.

Epoxy coated rebars are no good even if you have scratch in the coating.

 

[JKW05]

I've not felt the use of epoxy coated bars to be too beneficial primarliy because the coating can be damaged during rebar delivery, placement, and tying. Unless you have some very deligent installers and inspectors, the coating is probably not field-repaired. I have heard opinions that epoxy-coated bars with damaged coatings may experience more significant deterioration at the area where the coating is lost than if the bar was not coated at all. (or is this just an urban legend?)

I focus more on rebar coverage, concrete mix design, and control of crack sizes.

 

[csd72]

A couple of interesting links came up on google:

http://www.graniterock.com/tn723.html

http://www.nrmca.org/P2P/ci2809bickley.pdf

http://www.civil.tohoku.ac.jp/~concrete/c-indexe.html

http://eng.lacity.org/techdocs/str-man/h-400.pdf

http://www.atypon-link.com/TELF/doi/abs/10.1680/muen.2006.159.1.11?cookieSet=1&journalCode=muen

 

[shin25]

"Epoxy coated rebars are no good even if you have scratch in the coating."- I find this statement not too clear. It should read as-

"Even a small scratch in the coating may render the epoxy coating worthless."

 

[hokie66]

I know that a lot of epoxy coated rebar has been used in bridge decks in the US, with varying success. It has never sounded like a great idea to me, but don't have any experience with it. But what about galvanised bar? With zinc, defects such as scratches do not prevent the sacrificial protection of the steel.

 

[shin25]

In certain occasions I have seen that the galvanizing makes carbon steel (ASTM 615) brittle, in a sense that a straight bar of this steel cannot be bent to a standard bend diameter. This problem, however, can be resolved by using ASTM 706 (which has cap on maximum carbon content). But, usually, the entire process, ASTM 706 and galvanizing is costlier.

 

[csd72]

hokie66,

Galvanising works great with atmospheric or salt corrosion but with many other types of corrosion it is of no help at all.

vincentpa,

It is worth looking into, but I doubt it would help in this situation.

csd

 

[vincentpa]

There is a new product. I believe it is called Zbar. It is a galvanized rebar with a polymer outer layer. I imagine it is expensive though. I haven't priced it.

http://www.gerdauameristeel.com/zbar

We are using plain deformed steel rebar for the WW treatment tanks. The constituents aren't too bad and we are specifying Type II cement with Fly Ash for the sulfates which will be present. I believe the extra cover provided as per ACI 350 and the alkalinity of the concrete provide enough protection for the rebar.

The cost of the epoxy is just too much; not to mention the quality control it takes to touch-up damaged bars. I don't believe we get what we pay for when we use epoxy coated bars because they are not touched-up like they should be and like JKW05 and shin25 write, they corrode faster when they are damaged. In PENNDOT work, all rebar is epoxy coated for use in bridge decks. There is an construction inspector to look at every piece of rebar and touch it up if necessary and they are still not 100% effetive in doing so. Look at PA bridge decks. Horrendous! I agree with JedClampett that epoxy bars are generally not worth the cost.

Cathodic protection is too expensive for the client. They are cheap. I'm not sure if it is necessary here anyway. It seems there are many WW treatment plants in operation a long time without the use of any protection for the reninforcing steel.

Thanks to all for their input. I appreciate it.

BTW. Have you noticed the amount of posts about concrete tanks and ACI 350? That tells me something about the job ACI is doing for Code 350. The recommendation 350.2 and 350.4 aren't the best either. It seems to me that there are too many people doing too many different things and there is no set design methods and no easy to follow prescriptive code references. Have you read ACI 350-06? They have muddied the waters even more. Now we have to calculate the Sanitary Factor based on the service stress. This creates several more steps and much more engineering time. What are they thinking???????

 

[JedClampett]

If it makes you feel any better, ACI 350-89 also required the use of working stress for crack control calculations. It kinda, sorta went away for awhile, but now I understand it's back and badder than ever.