Acid Attack - Occurrence

By far, the most common occurrence of acid attack are those found in sanitary sewers and waste water treatment facilities. In most cases, the acidic environment is produced by sulfur reducing bacteria. Some bacteria metabolize sulfates anaerobicaly and produce H₂S as a waste product while others reduce sulfur and oxidize it back into H₂SO_4. The bacteria genus Thiobacillus is the most destructive to concrete, remaining active in concentrations as high as 5% H₂SO₄. (ACI 210R-93, 1996)

Key environmental criteria for these bacteria to produce H₂S are a free water surface, low dissolved oxygen, and a low waste flow velocity. The illustration below depicts the most common deterioration scenario occurring in sewers.

H₂S + H₂O H₂SO₄

_{Hydrogen sulfide + water sulfuric acid}

Acid attack deterioration following this scenario is extremely destructive. Concrete trunk sewers in warm climates may be stripped beyond reinforcement in as little as 10 years. Cases of asbestos coated sewer lines deteriorating in as little as 5 to 10 years are not uncommon. (ACI 210R-93, 1996)

A similar type of scenario involving sulfur reducing bacteria may be found in reservoirs and other natural water source containment structures. Although the mechanism of deterioration is similar the growth environment of the bacteria and hydrogen sulfide production are different.

Sulfate rich runoff caused by agricultural and or industrial activities collect in reservoirs providing the necessary "food" for the sulfate reducing bacteria. An anaerobic environment at the reservoir bottom caused by natural occurring thermalclines provides the necessary environment for the bacteria to metabolize the sulfate rich sediments thus producing hydrogen sulfide. This hydrogen sulfide will normally remain dissolved at the reservoir bottom unless disturbed by outside influence. This outside disturbance caused by water regulating siphons exposes the H₂S rich waters to concrete outlets and sluice gates of the dam. The H₂S rich waters, if allowed to reach low enough velocities will allow H₂S build up and deterioration similar to that experienced in sewer systems will occur. (Thornton, 1978)

B. Carbonic acid attack (Aggressive CO₂ Environment)

Acid attack of concrete can also be caused by exposure to water containing an excessive amount of carbonic acid also referred to as aggressive CO₂ . Free CO₂ dissolved in water takes the form of H₂CO₃ and if not in the presence of adequate amounts of calcium bicarbonate Ca(HCO₃)₂ will contribute to the deterioration of concrete. The destructive reaction of aggressive CO₂ and concrete is a function of the ratio of H₂CO₃ and Ca(HCO₃)₂ to which the concrete is exposed. Both Ca(HCO₃)₂ and H₂CO₃ will react with Ca(OH)₂ in concrete paste forming CaCO₃, as illustrated in reactions 1 and 2 below. If the water, to which concrete is exposed, contains the minimum H₂CO₃ to maintain equilibrium with Ca(HCO₃)₂, calcium carbonate CaCO₃ will form until the pores in the concrete are filled and no damage to concrete will occur. (Terzaghi, 1948)

CH + H₂CO₃ CaCO₃ + 2H₂O      (1)

Calcium Hydroxide+ Calcium bicarbonate Calcium Carbonate+ water

If, however, an excess amount of H₂CO₃ is present, the additional amount of H₂CO₃ will react with CaCO₃ reforming Ca(HCO₃)₂ which is readily removed in solution. This process increases both the porosity and permeability of the cement paste.

Conditions under which this phenomena occurs are difficult to predict but the application of some simple tests will accurately identify the presents of an aggressive CO₂ environment. Areas were volcanic activity is recent "geologically" are particularly susceptible due to the large amounts of dissolved CO₂ in ground water caused by volcanic activity. Large amounts of decaying organic material coupled with pressure due to water depth is characteristic of river, marsh, and reservoir bottom environments and can also contribute to large amounts of dissolved CO₂. The production of dissolved CO₂ in water is also influenced by aquatic plant life growth which may be affected by the introduction of industrial waste. Under normal conditions, however, the percolation of ground waters through calcareous material, common to most subsurface conditions, will provide adequate amounts of Ca(HCO₃)₂ and thus decrease the effects of aggressive CO₂. If however, the Ca(HCO₃)₂ to H₂CO₃ ratio is diluted as is the case in brackish coastal waterways, aggressive CO₂ will be present in solution. (Terzaghi, 1978)