INSPECTING CONCRETE for REBAR CORROSION 

Corrosion of steel

Reinforcing steel exposed to air oxidizes (rusts) by converting the iron in the steel to iron oxide or iron carbonate (due to the CO2 in the atmosphere).  The oxide formed is a loose material that has a greater volume than the original iron (about 17 times greater) and because it is loose, it flakes off, exposing new steel or iron to corrode.  The process is ongoing until all the exposed iron or steel is consumed.

Rusting of reinforcing steel in concrete is a slightly more complicated matter involving a loss of protection afforded by the concrete.  The loss of protection is two fold; the concrete protects the iron from the environment by covering it.  In addition, the concrete, due to its alkalinity protects the steel from corrosion (It's a little more involved than that).  The problem with corrosion of steel in concrete is that the rust products have a greater volume and cause the concrete to spall (flake off), exposing more steel.  As briefly touched on, for the steel to rust in the first place, the alkaline protection must have been compromised.

the Chemical Process that Protects Steel

Generally speaking, the alkaline environment causes a film to form around the rebar, thereby protecting it from corrosion. The alkalinity protects the rebar by a process called "passivation".  

If the concrete was made using the wrong type of sand/aggregate (mostly chalcedony cherts and similar soft, amorphous silica forms), the alkalinity of the concrete in conjunction with the silica in the aggregate can cause an effect called Alkali Silica Reaction (ASR) or Alkali Aggregate Reaction (AAR).  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 occurred the cover to the reinforcement has failed and corrosion can take place unabated.

The alkalinity levels that provide passivation to steel can be significantly below those that would cause ASR in concrete.

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.  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 creates more surface porosity and that, coupled with the lower pH, will allow more corrosion to occur in the rebar.

Protecting steel
The most significant step you can take to protect rebar in concrete is to have 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.

Testing PH levels

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 reinforcing can begin.

JAMES INSTRUMENTS sells a small CARBO DETECT kit with a small quantity of PHENOLPHTHALEIN.

See the website at: http://www.ndtjames.com/catalog/corrosionTesting/carboDetect.html

Thymolphthalein indicator also works too - dark blue indicator.

1 gallon of 1% phenolphthalein indicator in 95% alchohol cost 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.