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Page Contents
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Introduction
The materials used for constructing for the transportation of water have evolved over a very long period of time from the most basic ditches and stone channels in the years BC through hollowed out wooden logs to labour intensively constructed large scale brick sewers in Victorian times to the most modern of composite and manmade materials we see today.
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This page shows some of the more common materials seen today by CCTV surveyors during pipe inspections.
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Material Classes
Drainage materials are generally classed into two types, with two additional sub types:
Modular - sewer conduits that are constructed from small building blocks without any discrete joints, like brick sewers.
Piped - sewers and drains that are constructed from discrete jointed manufactured pipes:
Rigid pipes - pipes that have no capacity to bend when stressed under load, they just break.
Flexible Pipes - pipes that can absorb a degree of deformation without any loss of structural integrity as a result of their design and manufacturing process.
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Modular Sewer Construction
Modular sewers, usually referred to as ‘Brick’ or ‘Masonry’ are constructed using building blocks and mortar and are always rigid, but there is a distinct difference between the materials ‘Brick’ and ‘Masonry’ although the construction methods are basically the same:
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Where there is no option in a material list for tiled constructions, we would use ‘Other’ with a comment. Here, the sewer is constructed from manufactured interlocking clay tile segments creating a smooth internal surface that is hard wearing. If you look in the distance, you can see the design of the tiles where the sewer has failed and there were often two layers of interlocking tile skins in the construction.
Piped Sewer Construction
As written previously, piped sewers are usually sub divided into rigid and flexible groups where there are advantages and disadvantages of both:
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Lack of flexibility, so any local ground movement cannot be absorbed by the pipe joint, and the pipe simply breaks and cracks either side of the joint which in itself is a large immovable object under the ground.
Disintegration and loss of seal - when these pipes were laid maybe 100 years ago, society did not have the cleaning products that we have nowadays, most of which are advertised on our TVs as being able to effortlessly remove limescale from our kitchen worktops and bathroom basins. This feature of these products, which we just tip away down the plughole also dissolve away the lime in the mortar joints in the sewers, leading to joint failures and leakage both in and out of the sewer.
Flexible and Rigid Pipes
As mentioned, pipes can be sub divided into flexible and rigid materials. Generally speaking, it is easier to define which pipes are flexible materials, and therefor by deduction, if it is not flexible, it must be rigid.
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As far as the metals are concerned, there are two distinct properties of metals that come into play here - is the material malleable or brittle? These are terms used to describe the properties of the material, and materials like steel are malleable, which means that they will bend under load, even if it is very hard to actually do it, so it is a flexible material. On the other hand, cast iron is rigid. It does not bend, it just breaks, in the same way that concrete and clay do, but again, it is extremely difficult to actually do this in real life.
Clay Pipes (Rigid)
Clay pipes are always rigid have been around for a very long time in sewer construction due to their hard wearing and smooth surface properties. Most clay pipes nowadays are plain ended with plastic jointing collars and neoprene seals where as older clay pipes were usually socket and spigot pipes with tarred yarn and lime mortar joints.
Clay pipes are usually brown or deep orange in colour, but the colour can vary considerably since clay is a natural material dug from the ground. It is not uncommon to see the component pipes in a sewer change colour as you pass along with the CCTV inspection, but there has been no change in material. It is simply most likely that the next pipe came from a different batch of pipes at the manufacturing plant and was fired in a different kiln etc.
These pipe materials are often referred to around the world as VC, or Vitrified Clay, but this is often a misused term, along with the commonly used ‘Salt Glazed Clay’.
The word ‘Vitrified’ means glasslike, so by definition, ‘Vitrified Clay’ has been glazed and has a shiny surface. Not all clay pipes are glazed, in fact most modern clay pipes are not glazed, so the word ‘Vitrified’ should be used carefully. Additionally, ‘Salt Glazed’ clay pipes and vitrified clay pipes are not the same thing. 'Salt Glazed pipes are extremely rare but very attractive to look at. These are very old pipes where salt was used in the glazing process to give a white surface to the pipe.
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Vitrified Clay Products
Notice in all of the three pictures above that the clay has a shiny surface - this is vitrified clay. The last image is not even a sewer pipe at all, it is actually a roof tile, but the material is the same as is used in drains, and the photo nicely shows the glazed finish.
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Non Vitrified Clay Pipes
These are the modern version of clay pipes that have plain ends, plastic collars and are not vitrified. The pipe material has no glazing at all and is just bare clay. Still very hard wearing and great for sewers. Notice the pipes in the 2nd picture that have perforations (or holes) in the sides. These are often seen in surface water drainage systems where the holes are either used to allow water to enter the pipe from the surrounding ground, or are used to allow flooded water from the pipe to soak away into the surrounding ground.
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Salt Glazed Clay
Often seen in drains around luxury town houses in the old parts of cities, where the drainage system is extremely old, and and often still working just great. See the white clay work of the drainage channels in this chamber, and even the tiles making up the benching (thanks to Sam Bean for this photo).
Common structural problems with clay pipes usually revolve around cracks, fractures, joints and breaks due to the very nature of the joints and the material itself. If you try to deform a clay pipe by even 5%, then it will not bend, it will fracture and break.
Concrete Pipes (Rigid)
Concrete is usually used for larger diameter sewers from 300mm dia upwards to almost any conceivable size. Concrete pipes can be:
Almost any shape.
Segmental (most common), so they are discrete jointed pipes, usually socket & spigot joints with neoprene seals.
Reinforced - except where there is evidence of failure inside a concrete pipe, there is usually no way of knowing if the pipe is reinforced or not. Where the concrete material has steel reinforcement bars set into its wall, water can get through the fabric of the concrete, attack the steel reinforcement and cause it to rust, which then in turn makes it expand and break away the internal surface of the pipe revealing damage and a loss of structural integrity.
Cast in Place (or in situ) - quite rare, but this is where the sewer has not been constructed from pre-fabricated pipe sections but has been poured in place by the construction team.
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Prefabricated Concrete Pipes
The above images show various concrete pipes. All are the same colour and the same surface texture, not as smooth as clay pipes. All have socket and spigot joints and these are also available in perforated versions. Note that there is no way of knowing from looking at any of these pipes as to whether they are reinforced or not.
Concrete as a material is highly susceptible to Hydrogen Sulphide attack in sewers which is a noxious gas created by the decomposition of sewage. It damages the surface of concrete pipes causing the surface skin to blow off and disintegrate and exposing the reinforcement where it exists. For this reason, concrete pipes are most often (but not always) found in surface water drainage systems.
Concrete sewers also suffer the same structural issues as described for clay pipes above.
Some common mistakes made when inspecting concrete pipes:
Mis-identifying Asbestos Cement as Concrete:
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Asbestos Cement Pipes
Asbestos as largely banned in most countries in the world nowadays, but still exists in our sewer networks from years gone by. Determining the difference between Asbestos Cement and Concrete is subtle in a CCTV inspection, but suffice it to say that the colour is lighter, almost white and the surface may appear to be almost fluffy.
Mis identifying Spun Cast Iron as Concrete:
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Spun Cast Iron pipe is cast iron pipe that has a cement lining attached to the inside at the point of manufacture. These types of pipe are often used under buildings because the offer the superb load bearing properties of cast iron, while removing the common problem with cast iron pipes where the metal rusts and decays and becomes rough over time. Being able to spot this on a CCTV truck is often almost impossible from inside the pipe, unless you can see the end of the pipe in the manhole, where the structure is clear.
In a CCTV inspection report, this pipe should be labelled as cast iron with a spray coated lining inserted during manufacture.
Cast Iron Pipes (Rigid)
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