Failure of a sag water pipe triggered by aging of the GFRP composite relining

The investigation led to the conclusion that aging had progressed significantly in the centrifuged GFRP pipes. However, this alone would not have precipitated the failure. The partially incomplete backfilling between the composite relining and the original steel pipe had locally increased the stresses in the composite shell considerably due to hoop bending stresses. Hence, only the combination of both effects with a reduction of the composite shell strength due to aging and additional local loads could explain the delayed failure of the pipe after 12 years of operation.