Additives to water to reduce scaling - are they corrosive to rebar in concrete? 1

[ajk1]

Does anyone know where I can find what additive(s) are put in the water of A/C institutional cooling systems to minimize scaling? I want to check out whether they may be corrosive to rebar in concrete. About 7 years ago we replaced an entire suspended reinforced concrete mechanical room floor due to rebar corrosion/concrete delamiantion from leaking chemicals and spilled chemicals used in water softening for a hospital.

Now we have another building (nothing to do with the hospital) where they are proposing installing a unit to add chemicals to an air conditioning system to minimize lime buildup, and I want to be sure that there is no risk to the floor, or that if there is a risk, then a waterproofing membrane should be installed.

So if anyone can pehaps direct me to where I can fnd what chemicals are used to minimize lime buildup, I should be able to do the leg work from there.

 

[Ron]

AJK1...water softening chemicals are often laden with chlorides. Check the water soluble chloride levels in the concrete. I believe in your area, there is a rapid test for chlorides used for bridge decks. Might be worth a try.

I am attaching a sampling technique I wrote 20+ years ago to check gradient levels of chlorides in concrete. I've used it successfully over the years checking for chloride levels in coastal exposure concrete. It can be used for any chemical testing that requires a pulverized sample of the concrete.

Check the chemistry of the treatment against ACI's durability and chemical attack references.

One other consideration is that there might be an indirect cause of the corrosion such as a reduction in alkalinity of the concrete, causing a loss of the passivity created by an alkaline environment with a pH greater than about 9.5. If the water chemistry has promoted deep carbonation of the concrete, to the level of the rebar, corrosion is inevitable.

 

[BAretired]

If chemicals are to be added in a known area for any purpose, use a waterproofing membrane to protect the concrete.

BA

 

[ajk1]

Ron -- your info that water softening chemicals are loaded with chlorides was something I was not sure of, so this is useful information. Also the test for chlorides. But in the current case, the proposed chemicals are for the purpose of minimizing lime buildup. Are the same type of chemicals used to soften water as to minimize lime buildup in AC systems? I am still trying to find out what the chemcials are that they will be using. Thanks.

BA retired - yes I am considering recommending that they put a membrane on the floor if the dispensing unit is to be placed on a suspended floor - looks like we think alike -- but the people I am dealing with want me to prove that it is necessary. My feeling is that the chemicals are always being changed due to environmental reasons, so even if the chemicals currently being used in the industry are not corrosive, that does not mean the chemicals used next year will also not be corrosive. Thanks.

 

[Ron]

ajk1..."Lime" buildup is caused by carbonate hardness, the most common reason for employing a "water softener". Most softeners are ion exchange systems using sodium chloride for the ion exchange.

 

[TXStructural]

It is unlikely that cooling tower water treatment will intentionally use a chemical which is corrosive to steel. The presence of corrosive materials (like salt) in the feed water can be a factor, as can the tendency of the water to be aerated/oxygenated (or carbonated where CO2 levels in the air are high.) Operators may unknowingly create a problem if they introduce bleach or pool chemicals in an effort to control bacteria and algae, even once, or use bleach to clean surfaces during periodic maintenance, as it can load the concrete surface with chlorides.

 

[ajk1]

Thank you TXStructural - what you say seems logical.