Structural Rehabilitation of Semi Elliptical Concrete Sewers

This paper discusses the design for structural rehabilitation of man-entry size cast-in-place semi elliptical concrete sewers. Many such sewers were built in the early 1900’s and are still in service, notably in Chicago, Los Angeles and Seattle. Even though some were lined with clay tiles even these are now in need of repair due to severe hydrogen sulfide induced corrosion above normal flow lines of these concrete structures. As these structures were design as arches, the principal load is compression in the wall section and no reinforcing steel is required. The inverts of these sewers are most always very flat (very large radius of curvature). This paper discusses design approaches that address the repair of the plain concrete structure to restore (or increase) its wall section properties, i.e., load capacity. The rehabilitation materials considered are strain compatible with and bond to the concrete structure walls. Thus, composite material design is appropriate using classical transform-section analysis. Also discussed is the often-overlooked (ignored) process of determining the current state of stress of the structure before rehabilitation. This consideration is shown to be important in rehabilitation design and points out the need to accurately determine (as possible and practical) the actual total load on the structure. A design example from a past project is given.