Organic “High-Build” Spray-in-Place Liners – An Emerging Class of Rehabilitation Methods

By Erez N. Allouche, PhD, P. Eng. and Eric J. Steward, Trenchless Technology Center, Louisiana Tech University | June 2009 Vol. 64 No. 6

Mode of failure
Polyurethane Panels
In each of the 23 panels tested failure occurred in a concentric fashion with respect to the opening walls. Fragments from the panel were collected and pieced together to their original configuration. Visual inspection of the fragments indicated that the material assumed little or no permanent deformation throughout the test (Figure 9). The outer edge of the puncture holes and the fragments of the material have an angular shape. This shape is indicative of a classical punching shear failure, where the failure plane occurs at approximately 45 degree inclination angle to the surface of the panel. Furthermore, the fragments fit together with no sign of permanent deformation suggesting that little, if any, bending occurred prior to failure.

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Fig 9. Puncture hole and fragments - polyurethane

Polyurea Panels
In each of the five tests, the material failed in a concentric fashion with respect to the opening in the test frame. For the thinnest panel, the punctured fragment of panel remained in one piece. Visual inspection of the fragments indicated that the material assumed some permanent deformation as the test progressed (Fig. 10). There was also evidence of bending, as the shape of the fragments appeared bent toward the center of opening. The outer edge of the puncture holes and fragments of the material are mostly smooth, which is indicative of a ‘plug’ shear failure mode.

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Fig 10. Puncture hole and fragments - polyurea

Relationship between failure pressure and panel thickness

Figure 11 provides a visual display of the failure pressure and panel thickness relationship for the polyurethane specimens. The data suggest that the failure pressure is directly related to the thickness of the material. Also, as the opening size was enlarged, failure pressures decreased for panels with the same thicknesses, as would be expected considering the increase in unsupported panel surface area.

A linear regression model was used to develop the following predictive equation that considered the failure pressure, opening size and the thickness of the liner:

Pf = 344.5 – [171 * D] + [(2500 * t] (Eq. 1)