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

A sag water pipe failed 12 years after it had been reinforced with a composite relining. The chronological sequence and the failure mode indicated that both the advanced aging of the glass fibre reinforced polymer pipe made in a centrifugal casting process and the increased local loading of the pipe shell due to an incomplete backfilling with injection mortar must have induced the failure. The investigations focussed on a detailed mechanical analysis of the pipe's composite shell in order to assess the progress of aging in the GFRP material which was in some layers reinforced with chopped random short glass fibres and in other layers with silica sand. Structural tests on entire ring segments complemented the experimental study. The results of the mechanical tests indicated a loss of 20% in stiffness and up to 40% in strength after the mentioned 12 years of operation, though only a few reference values of the initial state were available from test certificates of new pipes. The measured strength and stiffness values were used in a nonlinear finite element model to recalculate the strength analysis in the current situation, and to suggest a retrofit procedure. 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.