Acid attack on PCCP mortar coating: Fid-Bau Portal

Acid attack on PCCP mortar coating

Jana, Dipayan / Lewis, Richard A.

During the failure investigation of a 2.59 meter (102 inch) diameter prestressed concrete cylinder pipe (PCCP) sewer force main, petrographic examination of mortar coating samples revealed that the protective qualities of the coating had been environmentally degraded by exposure to acidic soils. While the mechanisms of carbonation of PCCP from atmospheric carbon dioxide in the form of carbonic acid attack are well documented, relatively little information is available on the effects of acidic soils on PCCP mortar coating. Petrographic examinations of mortar coatings exposed to acidic soil is presented along with discussion of the mechanisms involved in the coating deterioration. In-situ examination of the failed and adjacent relatively sound pipes showed varying degrees of coating alteration and the resulting loss of thickness. In places the mortar was discolored with a mottled black appearance. Soils at the site consisted of mixed alluvial deposits of sand and blue-grey clay. Since past PCCP failures have resulted from exposure to sodium montmorillonite clay, clay samples were tested for various properties known to be aggressive to PCCP. Of the laboratory results, the reported pH of 5.0 was considered aggressive to PCCP. Petrographic examinations were done on two coating samples from the failed pipe having nominal thicknesses of 8 mm (5/16-in.) and 19 mm (3/4-in.); and on a 25 mm (1-in.) sample of black discolored coating from the adjacent relatively sound pipe. Severely corroded coating, as thin as 6-mm (1/4-in.), was present among the samples from the failed pipe. Thickness variation between the samples is too great to be the result of faulty manufacturing process indicating that environmental degradation had indeed occurred in service. Based on petrographic examinations the effects of acid attack on PCCP mortar coating are: (a) preferential dissolution of the cement paste from the coating surface, which has exposed the siliceous aggregate particles on the coating of the failed pipe; (b) reddish brown stain of iron compounds on the exposed corroded surface; (c) dissolution of calcium hydroxide component of portland cement hydration by the acidic solution; and, as a result (d) a compositional zonation of the paste from the exposed corroded zone through the zones of carbonation (with reddish brown staining from iron compounds), calcium hydroxide leaching, to the sound interior paste. Water absorption and the volume of permeable voids of coatings are higher in the failed pipe than in the sound pipe. The pH of coating from the failed pipe (6.5) is lower than that from the sound pipe (8.0). All these results are consistent with the high degree of corrosion of the failed pipe due to its exposure to highly acidic soil. Acid attack on PCCP mortar increases the porosity and permeability of the coating and causes the loss of mass and the loss of strength of coating. The initial reaction involves dissolution of calcium hydroxide (CH) when the strength of coating is minimally affected. Subsequent reactions result in the loss of calcium silicate hydrate (C-S-H) component of cement hydration, formation of hydrated amorphous silica and alumina, and resultant loss of strength. Severity of attack depends on the type and concentration of the acid (the pH of the solution) and the amount and rate of flow of solution through and along the coating surface.