Polymer sprays are one important class of products available to address maintenance hole (i.e. MH or manhole) deterioration due to biogenic sulfide corrosion. A variety of products are available on the market, each having different polymer compositions and their own specific guidelines for surface preparation of the deteriorated concrete and choice of liner thickness.
Since the circumferential bending moments that develop around concrete maintenance holes are expected to be small compared to their moment capacity, structural capacity of the concrete MH to resist external earth loads remains more than adequate. Therefore, the primary goal of the polymer spray is to protect the concrete from further deterioration, with a secondary goal often being prevention of ingress of groundwater from the surrounding environment.
A key criterion for consideration is the bond that develops between the polymer spray and the underlying concrete substrate (following cleaning to remove poor quality concrete). Currently, this may be quantified using testing based on ASTM D4541 or ASTM C1583/C1583M-13 based on a pull-off test for a Dolly glued to the surface of the liner material. However, little research has been undertaken to answer the question ‘what bond strength is required?’
Another important consideration relates to the consequences of imperfect construction leading to patches of liner where bond has failed to adequately develop – for example, due to issues with surface preparation of the substrate.
If that patch of debonded material is subjected to external groundwater pressure, questions arise as to whether the liner can support the groundwater pressure behind the liner where it spans across the debonded patch, and what failure modes result if its ability to support those pressures are exceeded.
Alternatively, for a given level of external groundwater pressure, it may be important to establish just how big a patch of debonded material the liner can support (what might be an acceptable zone of debonded material in a field installation, and what would be unacceptable).
Research work is well underway at Queen’s University to answer each of those questions, using facilities in the GeoEngineering Laboratory.
This work is part of a $600,000 Strategic Research project funded by the Natural Sciences and Engineering Research Council of Canada in partnership with the cities of Hamilton, Kingston, and Toronto, and consulting companies Golder Associates and Robinson Consultants. Former doctoral student Jane Peter and current research master’s student Josh Treitz have been working under the supervision of Drs. Ian Moore and Neil Hoult, to develop and use a new testing protocol to examine liner resistance to external groundwater pressure.
The approach involves preparing samples having a circular patch of debonded liner, and with a perforation through the concrete under that patch so that pressurized water can be applied under the zone of debonded liner through the back of the concrete samples.
Specimens of this kind have been prepared on the wall of a new concrete MH segment, as well as on concrete paving stones (to investigate their use as a proxy for the MH segment). Water pressure behind the patch of debonded liner is gradually raised, until the capacity of the liner to hold the water (i.e. liner failure), or the maximum water pressure available in the laboratory is reached. The bond strength for each liner product has also been evaluated using dolly testing. Three different sprayed, polymer liner products have been tested during the project.
The work first showed that it is not necessary to prepare samples using full maintenance hole segments (testing with paving stones is much easier, and results using those paving stones gave the same properties and behaviour as those in a MH segment). Next, it was demonstrated that two different kinds of material failures are possible.
The capacity of two of the liner products was reached when the liner at the boundary of the debonded patch commenced peeling away from the substrate, and this peeling failure progressed to the point where water escaped from the edges of the paving stone or MH segment sample. The third product reached its capacity when tensile bending strains around the periphery of the patch reached the strain limits of the material, and approximately circular cracks developed, releasing the water below.
Equations have been developed relating peeling force to patch diameter or cracking pressure to patch diameter. These equations can be used to estimate the ‘allowable’ patch size as a function of groundwater pressure, or the groundwater pressure that can be resisted for a debonded patch of specific size, provided the peeling force per unit circumference or flexural strain limits of the sprayed polymer material have been established.
All specimens tested to date involved intact concrete, and now that a testing protocol is available to evaluate liner resistance to groundwater pressure acting under patches of debonded liner material, future work can investigate issues like the impact of surface cleaning of degraded concrete and different surface preparation approaches on liner performance.
Note: The author would like to thank M-Con Products Inc. for providing the new maintenance hole segment, Clean Water Works for supplying the Quadex Structure Guard liner, Empipe Solutions for providing the SprayShield Green 2 liner and Liquiforce for supplying the SpectraShield liner.