Considering Debonding in Design of Cementitious Liners within Concrete Pipes

Many reinforced concrete pipes currently in service are experiencing unsafe levels of deterioration, and if left unattended may become susceptible to collapse. Rather than proceeding with full replacements, many municipalities opt to rehabilitate damaged concrete pipes using trenchless technologies. For example, the application of cementitious liners can avoid the costly excavations and traffic impediments associated with the installation of new pipes, while restoring the structural integrity and guarding against further deterioration. The design methods previously proposed provide guidance for selecting the required cementitious liner thickness to rehabilitate damaged concrete pipes such that the repaired pipe will remain protected from stormwater ingress and further deterioration. These calculations were based on limiting the tensile strains within the liner at the pipe crown that result from the bending moments induced by earth loading. However, that design approach operates on the assumption of perfect bond between the liner and the host pipe where the effects of debonding were not explored. In the current study, the design method is supplemented by additional calculation procedures addressing the debonded condition. Sample calculations for the debonded condition are presented for the rehabilitation of several pipe geometries experiencing varying levels of deterioration. These calculations indicate the existence of unsafe ranges of liner thickness for the debonded condition. The bounds of these unsafe ranges of liner thickness depend on the curvatures resulting from earth loading and the length of the moment arm within the liner (spanning from the neutral axis to the extreme fiber experiencing tension). The calculation procedures in the current study should be used to verify whether the liner thicknesses suggested from the bonded version of the design method become unsafe if debonding were to occur. The calculations also show the potential benefits of actively facilitating debonding within the liner.