Does Chlorine attack concrete? 1

[JAE]

I have an existing concrete floor (beam and slab) that supports some hot-tubs - built above the floor with grout and tile. The tubs are leaking and the client wants an investigation into the condition of the floor, and the structural integrity of the floor.

First, I can take care of visually inspecting the concrete for signs of reinforcing deterioration and concrete spalling.

I know that chlorides attack concrete and destroy the cement matrix, but does chlorine do anything? I doubt that it is as bad as chlorides as bridge decks are severely hurt by the salt on roadways, but I've seen old swimming pool decks, covered with chlorinated water for years that doesn't seem to be affected by it.

Just curious as to what tests (material) could be done on core samples to verify concrete integrity vs. the chlorine.

 

[letrab]

My understanding has always been that chlorides are compounds of chlorine and that the chlorides reduce the passivity of the concrete leading to corrosion of the rebar.
Chlorine attack, indoors on a suspended slab could therefore be a problem. BRE Information Sheets 12/77 and 13/77 provide tests on drilling powder that can be used to determine the depth of penetration of chlorides.
I have never thought about pool surrounds until I read your mail.
Research done on mass concrete in the 1920's indicates marginal decrease of concrete strength when seawater is used as mixing water, significant increase in efflorescence and mottling though. Chlorides are therefore not detrimental to the structural properties of concrete.
Are pool decks reinforced? Could it be that the chlorine used in swimming pools are unstable and get oxidised too quicky by sunlight to penetrate the concrete?

 

[RiBeneke]

JAE

I wondered about the chlorine issue - the swimming pool example does not fit with my limited knowledge. I would have expected chlorine to produce hydrochloric acid that would dissolve concrete and corrode steel.

Some websites on swimming pools (

http://www.poolinfo.com

and

http://www.caromal.co.uk)

gave the answer. The solution in a swimming pool is a complex buffered solution. It contains chlorine, but the other buffer chemicals (should) keep it in a slightly alkaline state with pH of about 7.5 . In this state the concrete is not attacked.

I think the reference quoted by letrab which indicates that chlorides are not a problem, would only be applicable to unreinforced concrete.

In the case you are investigating, I think that if your core tests include testing residual alkalinity and carbonation depths of the concrete, you can base your remedial work on this data.

Regards

 

[JAE]

So if I simply test for the pH level that can give me a clue as to whether the chlorine in the water is capable of attacking the concrete?

I can also simply ask the manager what his chemical recipe is for the hot tubs. Would the knowledge of these chemicals lead me to a conclusion about the alkalinity and damage potential of the water?

 

[RiBeneke]

It would seem to be the case, after reading the pool experts comments on what causes 'concrete to rot'.

If you want expert advice on concrete durability and newly developed testing methods, you could contact Prof Mark Alexander (mark@eng.uct.ac.za) at University of Cape Town.

 

[Ron]

JAE,
Chlorine is not generally a problem for the concrete matrix. It is only when it forms weak acids (hydrochloric or hypochlorous acid) that matrix attack takes place. Chlorines and chlorides are obvious problems for metal appurtenances and reinforcement.

There is some debate as to the potential for chlorine presence to somehow assist in the formation of carbonation, which does weaken the affected areas by matrix deterioration. My opinion is that a reduction in pH of the concrete due to the presence of very weak acidic reaction is the culprit.

 

[JAE]

Thanks for the responses.

Ron - haven't heard from you in a while - good to get a response from you as I always respect your concrete knowledge!

 

[Ron]

JAE...thanks. I've been slammed for the last few months. Good to be so, but doesn't give me any time for reflective thought that the forums promote!!