CTLGroup investigated the premature failure of a coal tar epoxy coating applied on the interior of a concrete sewer pipe. The investigation included field evaluation and sampling, field petrography, and laboratory evaluation of cores and coating samples.
Petrographic study attributed the coating failure to degradation of the coating and subsequent sulfate attack on the underlying concrete substrate. The formation of gypsum (CaSO4 . 2 H2O) crystals on the back of the coating generated pressure sufficient to cause separation of the coating from the concrete. The source of the sulfate was determined to be microbial conversion of sulfides present in the sewage. Literature investigation revealed that the coal tar epoxy coating was not suited to the application.
After determining the cause of the coating failure, CTLGroup developed repair and maintenance recommendations to extend the sewer's service life.