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Re: Reinforcement failure due to concrete expansion

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Ozturk,

        Salt can, indeed, be the cause of localized failure in concrete structures.  The most common examples I know of are car parking structures, and to a lesser extent bridges, in Canada and some of the northern states in U.S.A.  The most common cause is the use of common salt, NaCl, to remove or prevent ice in winter.  Exposure to ocean water spray, as in your case, would probably have the same effect but at a slower rate.  The economic rational in using salt is that the corrosion would be less costly than the accidents which would result if salt was not used.

        The general mechanism of the failure is as follows:

1.) Chloride ions from the salt penetrate the concrete in solution, changing the background pH from "basic" to "acidic"

2.) This change in pH allows (in fact, encourages) the steel reinforcing to rust.

3.) the progressing rust not only corrodes away the steel causes the steel to "grow" like the root of a tree placing the concrete in tension which causes the concrete to crack and fall off.  The remaining concrete itself can remain in excellent condition; there is usually nothing whatever wrong with the concrete.

        The generally accepted methods of preventing this are the following.

1.) Use a more dense, less permeable concrete mix design.  This reduces the migration of both water and oxygen which are required to cause corrosion of the steel.  This also gives you a higher strength concrete; but strength is not the main purpose.

2.) Use epoxy coated reinforcing steel.  The epoxy barrier largely prevents corrosion of the steel.

3.) Use more concrete cover.  Use 3" (75 mm) instead of 2"(50 mm).  With the steel further from the concrete surface the steel has less exposure to both water and oxygen

4.) Use a surface membrane to prevent penetration of liquid into the concrete.  For parking structures this may be in a form similar to roofing material with suitable wear resistance; for bridges it may be in the form of asphalt pavement.

        I hope this will be helpful to you.

Regards,

H. Daryl Richardson

Ozturk OZGUR wrote:

Hi, Recently I've read a short article about the failure of the main reinforcement in the concrete bridge decks (I guess similar problems may occur at the other RC structures).
Up to now, there have been many researches and articles about the effects of the salt which causes chemical reaction and causes cracks in the concrete, however, I haven't heard of the complete failure of the reinforcement in the concrete due to the expansion of the concrete because of the alkali-metal reaction. I also heard that there is a bridge which was completed in Oregon, North Bennd city last year and Stainless Steel reinforcement had been used. Was it to avoid of that reinforcement failure problem? I would appreciate if anyone knows and shares detailed information, idea or shows any source about the expansion of the concrete due to the alkali-metal reaction which causes the serious failure in the reinforcement. Recently I'm involved in a bridge deck design which will be built near a sea side where salinity is rather high. I would like to take into account of the effects of this matter. Thanks for any comment,,, Ozturk (Civil Engr.)
Bridge Design Dept.
Tokyo