Major Damage to Concrete and Reinforcing Steel by N204 and Concentrated Nitric Acid


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Title: Major Damage to Concrete and Reinforcing Steel by N204 and Concentrated Nitric Acid

Author(s): Oswin Keifer, Jr.

Publication: Special Publication

Volume: 100


Appears on pages(s): 1549-1574

Keywords: acids; beams (supports); chemical attack; reinforced concrete; reinforcing steels; nitric acid; nitrogen dioxide; walls; General

Date: 4/1/1987

Describes the effect on concrete and reinforcing steel in a structure that was subjected to attack by pure nitrogen tetroxide (N2O4), subsequently converted to nitric acid by dilution with water. Although N2O4 without any combined water is relatively inert in its reaction with steel, with the addition of only a little water it becomes fuming nitric acid and with the total dilution encountered here, the end product was a strong nitric acid with a concentration of about 18 percent. During the first 24 hours and during the initial application of water, the material was fuming violently. As the water was added, the temperature rose to an estimated 400 F. After the diluted liquid was pumped out of the structure and wash down had taken place, the bottom of the structure was inspected. Initial visual observation indicated quite significant damage to concrete and reinforcing bars, plus major damage to attached steel accessories, all in the area below the final liquid level reached after dilution with the water. Further investigation by an inspection team using light hand tools (rock hammers, chisels, etc.) showed that the damage to the concrete, while widespread, was generally shallow with only isolated areas being damaged to any great depth. A few areas showed narrow zones of deterioration to a depth of 12 in. However, it was found that there was major damage to reinforcing steel that was quite extensive and deep with major areas showing complete disappearance of considerable lengths of No. 14 and No. 18 reinforcing bars. Surprisingly, in many of these areas, the remaining concrete showed perfect impressions of the missing reinforcing bars with no significant damage to the concrete surface making up the impression. Very little evidence of rust was present and it is assumed that, rather than ferric oxide (common rust) being formed, soluble iron oxide and soluble iron nitrate were formed and washed away. Repair procedures chosen consisted of use of jackhammer crews to remove all suspect concrete and, particularly, all concrete necessary to replace damaged reinforcing steel; splicing in new reinforcing bars by welding to replace portions of bars missing or damaged; and replacement of concrete with preplaced aggregate concrete.