Are reinforced concrete pipe joints infiltration proof?

By Lui Sammy Wong

Concrete sanitary sewage networks are part of key infrastructure assets in which larger flow volumes are collected from smaller flexible pipes. Pipelines are made of segmental units and joints are unavoidable. Performance of reinforced concrete pipe (RCP) joints has been challenged by the owners for decades. To a certain extent, the RCP industry is losing the competitive edge to emergent materials.

The Weibull distribution, the “bathtub” curve, is the best illustration of the performance of a pipeline, where the failure rate during the early life of a structure and near the end of a structure are usually substantially higher than during its service lifespan (Luko and Neubauer, 2020). This is the reason why CCTV inspection is especially important prior to a pipeline being assumed by a municipality.

Observing leaky joints during inspection is not uncommon and, in some cases, is too frequent and very costly to repair. Contractors rely on their experience to manage infiltration risks. Installation quality can only be revealed after the pipeline is backfilled and put into service. Poor pipe joints were claimed in a recent report to be the main reasons for infiltration (Norton Engineering Inc., 2017).

Infiltration resulting from RCP joint performance is caused by three main reasons: inadequate material specification requirements in dealing with infiltration; lack of technical knowledge of RCP joint performance; and a disconnected link between installation quality and the joint performance.

Existing RCP specifications do not deal with infiltration at all. RCP carries a gravity flow, so internal hydrostatic pressure is trivial, in comparison to the external pressure when groundwater is present. The hydrostatic test required by CSA A257 requires RCP manufacturers to test three pipes horizontally for an internal pressure reaching 105 kPa, 90 kPa and 35 kPa for 10 minutes, under straight, deflected and offset alignments respectively.

Internal pressure is not nearly equivalent to external pressure (Figure 1) and 10 minutes is too short a time to allow the viscoelastic rubber material to settle in the annular space. In addition, the test is mandatory only for pipe sizes up to and including 900 mm. Canadian standards, in terms of hydrostatic testing, are more stringent than anywhere else in the world (Wong and Nehdi, 2018). However, the methods of testing do not reflect external pressure conditions.

Gasket suppliers also evaluate the sealing pressure of rubber gaskets by plotting their load-deformation behaviours. However, not only is the test being conducted under ideal conditions, it also ignores the frictional effects of the hydrostatic pressure that is perpendicular to the applied loads. The results, in fact, are far from reality.

Research conducted by Western University, and sponsored by Con Cast Pipe, quantifies RCP joint performance for infiltration (Wong et al., 2020). Tests show that existing designs are capable of handling over 600 kPa of pressure externally. Figure 2 shows the test results of two different gasket materials using 10 minutes of sustained external pressure and different joint gaps. These tests demonstrate that the main impact factor is the joint gap. By opening the joint from 6 mm to 13 mm, pressure capacity is reduced by half.

The capacity is also influenced by gasket materials, duration and alignment. There are large variations in the test results, and the research also shows that gasket materials require a longer time to settle under sustained pressurized conditions. During these tests a water supply connecting cylinder was used to allow the technician to monitor leaks. Air pressure was introduced into the top of the cylinder during the tests and the water was pushed into the annular space between the pipe samples. If a leak occurs, or the gasket moves, the water level drops, indicating an increase in the annular space. Figure 3 shows water level reduction over 20 hours of sustained external pressure in the cylinder.

A reduction of water level, without observing leaks in the system, indicates that gasket movement is causing an increase in the annular space. Taking the worst test results and the worst testing conditions (large gap), the performance of the existing joint for infiltration still generally exceeds the 105 kPa threshold. Research shows that external pressure tends to push the gasket inward. With the existing design, the gasket can be pushed out of the annular space of the joint when the gap is excessive. Figure 4 shows observations of failures in the infiltration test and in the field. This indicates that joint gaps definitely separate a functional sewer from a failed one.

How can joint gaps be controlled in the field? Contractors often use joint gaps to adjust construction imperfections in alignment. Some contractors will ask for the maximum allowable gap, which by definition is 13 mm, because the existing standard requires a 13 mm gap in the hydrostatic test under deflected alignment.

From a technical standpoint, without considering the site conditions and blindly using the 13 mm maximum gap allowance will lead to costly leak repairs. No technical guidance exists for pipe installations. Existing field inspections are not required until after the completion of the pipeline, which is too late to make easy leak repairs.

In some cases leak-free inspections do not guarantee water tightness, because these tests rely on external groundwater conditions at the time of the tests. Based on a survey, close to 70% of municipalities in Canada do not conduct these tests. (Norton Engineering Inc., 2017).

Infiltration tests in a laboratory were reported by Fenner (1990), and Moore et al., (2015). The testing methods introduced by Wong and Nedhi (2018) were designed to allow manufacturers to conduct tests in a factory environment. These are done using a vertical setup focusing on joint performance for infiltration (Figure 5).

This test had been conducted over 100 times during a research project, with pipe sizes ranging between 600 mm and 1200 mm. The largest pipe size tested using the same method was 1800 mm. It is easy to set up and the footprint of the equipment is far less than the existing internal test. It is also safe to operate because of the limited amount of water required to fill the annular space. This allows the manufacturer to test to a much higher pressure, i.e., 685 kPa. In the case of 1800 mm pipe, it was set up on a set of bunking wood because the size of the testing base frame was exceeded. However, the testing method was identical to those of smaller sizes. Unlike the conventional test, the method is not limited by pipe size.

The apparatus includes a water cylinder, which allows monitoring and quantifying the leaks during the test. Spacer pads can be used for the test with various joint gaps. In 2019, ASTM C497 included a standard test for infiltration that the owner can specify. The RCP industry is now under pressure to adopt the test as a part of routine evaluation for infiltration.

Testing the joint only for infiltration is not sufficient. As discussed earlier, part of the issue occurs in the field. Linking test results to field conditions is key. Performance charts revealing the infiltration capacities under various conditions allows the designer to set boundaries for the installation.

Figure 2 shows a simplified performance chart for 600 mm RCP. If the pipe is subjected to 10 m of hydraulic head after it is put in service, the joints are required to withstand 100 kPa of external sustained pressure.

A safety factor can be introduced to calculate maximum allowable joint gaps during installation. It has to account for all variations and unforeseen circumstances encountered in the field. In this example, using a safety factor of four, joints are required to withstand 400 kPa. 9.5 mm will be the maximum allowable value in the field by the contractor to control the joint gap when Gasket B is being used.

The contractor would be required to meet this limit in order to reduce the risk of infiltration. If an excessive gap is unavoidable, immediate remediation such as patching and external protection would be required to reduce the cost of repairs and the risk of infiltration. These can all be built into the construction specifications.

Conclusions

Reinforced concrete pipe is proven to be durable. RCP joints have the potential to offer outstanding joint performance for infiltration. However, solutions should be implemented at the early stages of the supply chain by manufacturers, and by designers who are required to consider groundwater conditions when specifying joint performance. Contractors also need technical guidance on joint gaps and the risk of infiltration.

Lui Sammy Wong is a PhD candidate in civil engineering at Western University and is with L.S. Wong and Associates Inc. Email: info@wongengineering.ca. Article references are available upon request.

Read the full article in ES&E Magazine’s June/July issue below.