Acid Attack - Proactive Avoidance

The complete avoidance of acid attack deterioration may not always be possible due to the nature of particular environments especially those found in the transmission and treatment of waste water. A structures life span , however, may be significantly enhanced by applying some simple measures to decrease the long term effects of acid attack. Engineers should strive to design both the concrete mix and the structure to decrease the affects of acid attack .

Understanding the basic chemistry of concrete and it’s relation to the basic mechanism of acid attack will produce some basic approaches to designing a more acid resistant concrete mix. Calcium hydroxide, Ca(OH)₂, formed as a product of cement hydration is the "decisive point" of the acid attack mechanism. Therefore, the reduction of Ca(OH)₂ found in the cement paste is the principal consideration of the concrete mix design. This reduction of Ca(OH)₂ can be accomplished in a number of ways. Including pozzolans such as fly ash or blast furnace slag, in the mix design is most likely the best option to achieve an overall reduction in Ca(OH)₂ in the hydrated cement paste. Pozzolans react with Ca(OH)₂ in the cement to form more calcium silicate hydrate allowing less Ca(OH)₂ to be available for dissolution by acids. For the same reason, the overall reduction of the cement content in the mix design is another option. Constraints on the concrete’s workability, however, may not allow for a low cement content mix design. The production of low permeability concrete may also be helpful in less aggressive acid environments. Maximizing the life of the concrete in an aggressive acid environment should combine all three of the aforementioned measures.

Structural design considerations are more dependent upon the environment in which the structure is to perform. In the case of high bacterial acid attack environments found in sewage transmission and treatment structures the following measures should be considered. (ACI 210R-93, 1996)

• Flow rates (greater than 2 ft/sec) that preclude the build-up of destructive bacteria on structural surfaces.
• Ventilation, particularly at the ends of collector lines, to prevent the build up of destructive H₂S gas.
• Decrease turbulence particularly at manholes and junction structures. (decreases the amount of H₂S released from solution)
• Aeration of sewage to prevent anaerobic bacterial growth environment

Structural considerations for dams and other impoundment structures revolve more around the reservoir and corresponding watershed characteristics. Common design considerations include the following: (Thornton, 1978)

• Ventilation of siphon structures to prevent the build up of H₂S
• Siphons should be designed not to take water directly from the reservoir bottom which is characteristically rich in dissolved H₂S.

Of particular importance in the prevention of deterioration due to carbonic acid attack is the early identification of an aggressive CO₂ environment. The most common and most successful means of precluding this type of deterioration in closed systems such as water treatment and storage facilities is simply adding Ca(HCO₃)₂ to the water. This may be accomplished by treating water with a crushed marble medium. Other methods of prevention include concrete sealants and epoxies. (ACI 210R-93, 1996)