Determining the remaining life of concrete sewers

Traditionally, the large-diameter outfall sewers installed in South Africa from about the 1950s onwards were made of concrete and designed for a 40-year life. In many cases, these systems have lasted beyond their designed lifespan; in others, they have had to be replaced or are in need of rehabilitation or replacement. By Alaster Goyns, Pr Eng*

The deterioration of these sewers is mainly due to biogenic corrosion and particular problems arise when gradients are inconsistent or sewers are downstream of rising mains. To perform optimally, these pipelines should flow partly full under gravity at reasonable gradients, which ensures effective operation.

Where gradients are too flat, there is insufficient oxygen in the slow-flowing effluent, resulting in hydrogen sulfide (H S) gas being 2 generated. When there are steep gradients, any H S that has been generated is stripped 2 out of the fast-flowing effluent and then biologically converted to sulfuric acid (H SO ). 2 4

This H SO then attacks and corrodes any 2 4 alkaline pipe material, such as concrete or fibre cement. The structural integrity of the pipes is then compromised; if this is not addressed timeously, the sewer collapses.

However, before a sewer collapses, it starts leaking – resulting in cavities forming around the leaks and becoming water paths running adjacent to the sewer. The flow along these paths removes the bedding support around the pipes, further reducing their load-carrying capacity and shortening the time to collapse. As these sewers are frequently placed at depth below the surface, such a collapse of the pipe/soil system invariably results in problems for the services above.

Although many of the outfalls installed in South Africa since the 1950s and 1960s are still in operation – way beyond their planned operating life – some have deteriorated and collapsed. When this happens, the costs are several times that of replacement, even before the unaccounted-for costs to the public are considered. To prevent this from happening, any sewers where this could happen should be replaced or rehabilitated. In urban areas in particular, the use of trenchless techniques is generally the best approach.

Replace or rehabilitate?

Before deciding about replacing or rehabilitating a pipeline, an assessment of its performance and condition should be done. This will establish how well its hydraulic and structural requirements are being met and enable an estimate of its remaining service life. Recently, the great strides made with CCTV and associated inspection systems mean that a complete picture inside sewers, as well as a quantification of internal dimensions, can be provided.

Combining this information with the loading conditions, and an understanding of the corrosion mechanism, provides input for the structural analysis of sewers and an estimate of their remaining life. The extent and severity of the problems can be established and

Corrosion mechanism in sewers

H2SO4 FORMATION

H2S RELEASE

H2S GENERATION

Crown wall

decisions about the appropriate remedial measures taken.

Remaining life is dependent upon the external loads and actual wall thickness, both of which will vary along the length of any sewer.

Over the past two decades, protecting concrete pipes with an inert lining such as polyethylene is frequently done. This is effective, but only economically justifiable when corrosion is predicted for larger sewers (≥1 200 mm diameter). This does not address the issue of how to handle the problem with existing sewers that have already started to deteriorate and will at some point in the future need rehabilitation to avoid their collapse. The problem is to determine when this rehabilitation should take place and what technique should be used.

Long-term test

In 1989, a 65 m long experimental section was installed in Virginia, Free State, as part of a 900 mm diameter sewer where very aggressive conditions were expected. The 21-09-14_040_ID21178_eAZ_Kapstadt_IMIESA_210x148,5_RZgp purpose of this was to compare corrosion rates

of various binder/aggregate combinations. A bypass line was constructed so that the flow could be diverted, and the various pipes physically inspected and estimates of the corrosion losses made. The inspections confirmed that the Portland cement dolomitic aggregate (PC/DOL) pipes performed much better than PC pipes made with siliceous aggregate (SIL), and concrete using calcium aluminate cement (CAC) and SIL performed significantly better than the PC concretes. It was predicted that the control pipes made using PC/SIL would corrode through after 10 years. A physical inspection confirmed that this did in fact happen.

In 2003, the sections of the experimental sewer where the PC/SIL were on the point of collapse were exposed and these pipes, plus short sections of the adjacent pipes, were removed so that the actual wall losses could be measured. There were sections of the PC/SIL pipes where the concrete (84 mm thick) above the flow level had disappeared completely, whereas the PC/DOL pipes had lost about 40% of this and the CAC/SIL pipes only about 25%. The removed pipes were replaced with short sections of various special cementitious materials, which are periodically monitored. The indications are that some of these are at least 20 times more effective than PC/SIL concrete.

The original pipes have now been exposed to these aggressive conditions for 32 years. The results have provided valuable quantitative information on the corrosion rates of various cementitious materials for sewers and can be used for predicting what will happen in existing sewers. It has also provided the criteria for

Wall at any section

Invert wall

Outside diameter Outside to invert

Physical dimensions to check

CAPE TOWN

EFFICIENT SEWAGE SYSTEM

State-of-the-art tunnelling technology from Herrenknecht was applied to upgrade the sewage system (Cape Flats 3 Bulk Sewer – Phase 2) in Cape Town. The new, sustainable tunnel infrastructure has been constructed with minimal disruption to the local community. www.herrenknecht.com

Corroded RCP (front) and FC (back) pipes Mushroom-shaped pipe PC/SIL pipe wall gone in 10 years

evaluating the performance of CAC-based concretes for the mortar relining of sewers.

Residual strength

The critical issue for the utility owner is the sewer’s remaining life before it needs rehabilitation or replacement, and then the most suitable method for doing this. A secondary issue is how effectively and efficiently the sewer will perform during this remaining life.

The strengths at three stages in a sewer’s life need to be determined, namely: the initial, when installed to meet the design specification; the residual, at time of investigation; and the minimum required to take the actual loads imposed on the pipes. Although the strengths for a specified pipe class, as determined at the time of installation, will be constant, the minimum initial strengths needed along the sewer will depend on the actual loading.

The residual strength along the sewer length may also vary as the corrosion losses over time may differ due to the changes in the hydraulic conditions along the sewer. The minimum strength needed will also vary along the sewer length, depending on the actual loading conditions and corrosion losses.

Assuming that the soil around the pipes remains intact, it will consolidate over time. The vertical loads and moments generated on the pipes will then probably be less than those originally designed for, as the lateral pressures – if the bedding support has remained intact – will have increased due to consolidation and countered these loads.

On the other hand, if some of the material surrounding the pipeline has infiltrated into it through leaking joints, cavities will have formed around the sewer, resulting in a loss of bedding support. The load-carrying capacity of the pipe/soil system will then deteriorate, and collapses can be expected.

Choice of rehabilitation or replacement technique

Once the rehabilitation and replacement priorities are determined, decisions about the most appropriate technique to use need to be made. Although the utility owner will invariably want to choose the rehabilitation method based on price, this should not be done if it compromises the pipeline’s longterm functionality or structural integrity.

The most economical solution would generally be slip lining. However, as it is not a tight or close-fit liner, it may not provide the capacity needed. In addition, there will be a gap between the liner and host pipe – and this will have to be grouted to prevent a water path developing next to the sewer and a loss of soil support if joints on the host pipe have been leaking.

When the internal cross-sectional profile of a sewer is not circular due to corrosion, and has sharp longitudinal deviations from this, as with a mushroom-shaped pipe with longitudinal sills, a tight or close-fit liner would form a sharp bend over the sills and could become overstressed.

Under such conditions, instead of a curedin-place or a fold-and-form liner matching the sewer profile, which will be excessively stressed where it bends over such sills, a spirally wound liner could be a better option. This liner can be installed so that a circular profile is maintained.

When the sewer capacity is inadequate due to population densification in urban areas, pipe bursting or reaming is an effective way of increasing the pipeline diameter and capacity with minimal surface disruption.

As there will be less space for digging trenches in future urban landscapes, many new sewers and water supply pipelines will be installed using trenchless techniques such as microtunnelling and directional drilling.

As there will be less space for digging trenches in future urban landscapes, many new sewers and water supply pipelines will be installed using trenchless techniques such as microtunnelling and directional drilling."

Leverage your assets

However, there are still many ageing sewers below congested city and urban areas that have been designed for historic population densities, and their condition is unknown.

It is these ‘holes through the soil’ that are the utility owner’s real assets: the pipelines are merely liners that should enable them to perform effectively and efficiently on a sustainable basis. Many of these are now in need of rehabilitation or replacement. However, before any decisions about this are made, it is essential that their hydraulic performance and structural integrity are assessed by using the techniques that are now available. The benefit of doing this is that the asset owner is informed of which sections of sewer need attention and how to prioritise this work. *Alaster Goyns is the founder of Pipeline Installation and Professional Engineering Services CC.