How Steel Corrodes
Reinforcing steel bar (rebar) exposed to air oxidizes or rusts through the conversion of 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. Because it is loose, it flakes off, exposing new steel or iron to the elements, along with the potential to corrode. The process is ongoing until all the exposed iron or steel is consumed.
Rusting of reinforcing steel encased in concrete is a slightly more complicated matter involving a loss of protection afforded by the concrete. The concrete protects the iron from the environment by covering it. In addition, the concrete, due to its alkalinity, protects the steel from corrosion. The problem with corrosion of steel embedded in concrete is that the rust products have a greater volume and cause the concrete to spall or flake off, which exposes more steel. In order 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 protects the reinforcement steel by causing a film to form around the rebar, sealing the steel off from the effects of the environment. 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 another result called alkali aggregate reaction (AAR). These processes produce a gel that absorbs water, expands, and causes the concrete to spall. While this does not directly cause the reinforcement to corrode, once it has occurred, the cover of the reinforcement starts to fail, and corrosion begins.
The alkaline levels that cause passivation of steel can be significantly less than those that would cause ASR in concrete. Either ASR or AAR can occur independently 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 a higher, 8-to-9 range, it’s 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.
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 using freshly exposed (freshly chipped or freshly fractured) concrete that is sprayed with a solution of 1% phenolphthalein indicator in 95% alcohol. It will turn bright pink if the pH of the concrete is less than 10, and it will be colorless if the pH is less than 8.
This test can indicate the depth of carbonation which, in turn, indicates the depth of potential corrosion to the embedded reinforcement steel.
A thymolphthalein indicator also works, which uses a dark blue indicator.
A test kit using 1 gallon of 1% phenolphthalein indicator in 95% alcohol is relatively inexpensive. Place a small quantity in a spray bottle, and you’ll be good to go.