Alkalinity- protects or promotes embedded steel corrosion? 6

[Kenton54]

Do the alkaline properties of concrete help protect or promote corrosion of steel embedded in concrete? I've read it both ways. Who's right? -Kent

 

[Ussuri]

Generally speaking the alkaline environment protects the reinforcement as it causes a film to form around the rebar protecting it from the environment.

Heres the but, if the concrete was made using the wrong type of sand/aggregate, the alkalinity of the concrete in conjuction with the silica in the aggregate can cause an effect called Alkali Silica Reaction (or Alkali Aggregate Reaction). This results in a gel which absorbs water, expands and causes the concrete to spall. While this does not cause the reinforcement to corrode directly, once it has occured the cover to the reinforcement has failed and corrosion can take place unabated.

We have had quite a problem with older concrete bridges in the UK experiencing this.

 

[dik]

What type of silica is in the aggregate that promotes this type of activity?

Dik

 

[Ron]

Kenton54...the alkalinity protects the rebar by a process called "passivation". The alkalinity levels that provide passivation to steel can be significantly below those that would cause ASR in concrete.

dik...mostly chalcedony cherts and similar soft, amorphous silica forms

 

[Kenton54]

So it sounds like a two-part process. ASR is a reaction between alkaline material and aggregate that causes axpansion and spalling, which may lead to exposure of embedded steel and expansion of iron oxide caused by oxidation of the steel also causes spalling.

 

[zabrab]

As others have said the alkaline conditions from the concrete protect the reinforcement. In fact, that was the origin for Tnemec coatings.

http://www.tnemec.com/history.asp

 

[Ron]

Kenton54...it doesn't have to be a 2-part process....either can occur independent of the other.

All portland cement concrete is highly alkaline. It is only when there are certain forms of silica in the mix that such reactions take place. Similar reactions can take place with carbonate aggregates.

A relatively high alkalinity is necessary to protect the rebar. If the pH of the concrete falls below about 10, the potential for rebar corrosion increases. Further, when the pH is in the 8 to 9 range, it is a good indication that some carbonation is taking place from the surface of the concrete toward the inside. The presence of carbonation allows more surface porosity and that coupled with the lower pH will allow more corrosion to occur in the rebar.

The most significant step you can take to protect rebar in concrete is to have a dense concrete with good cover over the rebar. Several studies have shown that high quality, dense concrete with a bit of extra cover does more to prevent corrosion than extra coatings or additives in the concrete.

 

[Kenton54]

I am interested in inspecting homes in Mexico, many of which are built of concrete. Will testing the PH level of concrete walls in a home built say..5 years ago tell me anything useful?
In other words, if I drill a small hole in the walls, mix the particles from the hole with water and then measure low PH can I assume that there has been corrosion of any steel in the wall?

 

[Ingenuity]

Kenton54

A well established test for loss of alkalinity is a phenopthalien indicator test - FRESHLY EXPOSED (ie freshly chipped or freshly fractured) concrete that is sprayed with a solution of 1% phenolphthalein indicator in 95% alchohol with turn bright pink if pH of concrete is > 10 and colorless if less than 8.

This will indicate the depth of carbonation, whereby the corrosion to embedded reinfrocing can begin.

Thymolphthalein indicator also works too - dark blue indicator.

1 gallon of 1% phenolphthalein indicator in 95% alchohol cost me less than US$50 (a few years back) and has last a few years. Place a small quantity in a spray bottle and you are good to go.

 

[Ron]

Kenton54...good points by Ingenuity...just want to reiterate that it MUST BE FRESHLY EXPOSED BY FRACTURE. Drill tailings will not work.

 

[Kenton54]

I'm trying to figure out the best way to expose material for testing without knocking a chunk out of someone's home. If there is a crawlspace... no problem. In Mexico they're mostly slab-on-grade. Too bad drill tailings won't work. Is there a minimum surface area that should be exposed?

 

[Ron]

Kenton54...no minimum surface area, but you should have some of the surface and down to the anticipated depth of rebar.

There's not much you can do to get around the need for a "chunk" of concrete. Something we often do is take a small diameter core (1-1/2" dia.) about 2 inches long, then turn it on its side like a splitting tensile test, load it to fracture and then you have a nice clean freshly fractured piece with a good gradient from the surface down.

The core hole can be easily repaired with mortar.

 

[Kenton54]

Ingenuity,

Did you mean phenopthalien or phenolphthalein?

-Kent

 

[Ingenuity]

Kenton54,

PHENOLPHTHALEIN is correct.

One manufacturer is RICCA CHEMICAL COMPANY, Arlington Texas.

Also, as Ron mentioned, small diameter cores are a good method to obtain a sample (that you fracture longitudinally, as he states). We use a HILTI hand-held core machine that works well for small diameter samples (1.5" for example), and avoids having to mount the rig with drop-in anchors or use a vacuum base.

HTH

 

[dik]

Ingenuity...
My supply came from a conventional pharmacy... only need a little bit... and it is medically used as a laxitive...

Dik

 

[Ingenuity]

"and it is medically used as a laxitive..."

I will have to remember that next time nature requires!!!

 

[dik]

Have to use a little caution, Ingenuity... a tablespoon can really hurt...

Dik

 

[Kenton54]

Can it be mixed with isopropyl alcohol, also from the pharmacy?

 

[Kenton54]

To experiment with I don't need much phenolphthalein. Sounds like it's available in smaller amounts at pharmacies.
Can anyone simplify this for me a little by providing a recipe? What measure of phenolphthalein to what measure of what type of alcohol to make say... a pint of testing solution?

 

[Ingenuity]

Kenton54,

You can make it simple and go to JAMES INSTRUMENTS and you can purchase a small CARBO DETECT kit with a small quantity of PHENOLPHTHALEIN.

See the website at:

http://www.ndtjames.com/catalog/corrosionTesting/carboDetect.html

The PHENOLPHTHALEIN I use is 1% (w/v) in 95% (v/v) alcohol - not sure of if the alocohol is "isopropyl" or black label Johnnie Walker - I took chemistry classes along time ago.

dik,

I think I will use my PHENOLPHTHALEIN purely for concrete testing - my "system" keeps pretty regular with my balanced diet Probably more info than you needed!