Alkalinity- protects or promotes embedded steel corrosion?

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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?


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 conjunction 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.

A well established test for loss of alkalinity is a phenolphthalein indicator test - FRESHLY EXPOSED (i.e. freshly chipped or freshly fractured) concrete that is sprayed with a solution of 1% phenolphthalein indicator in 95% alcohol 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.

Thymolphthalein indicator also works too - dark blue indicator.

1 gallon of 1% phenolphthalein indicator in 95% alcohol 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.