Prestressed Concrete Pipe

That sounds like circumferentially reinforced; how is it prestressed? (Longitudinal prestress would be easier; reinforcement in both directions should also be easy.)

Probably it is prestressed with highly tensioned wire wound around the circumference several times, much like a Natgun Water Storage tank is prestressed.

I've used these types of pipe in a few projects where they were highly loaded due to existing building foundation conditions above the pipe that couldn't be modified for several reasons. It's an extremely strong pipe!

I don't see how they could prestress the longitudinal axis on the pipe barrel without expending a ton of $$$$ to do so. Anyhow, the pipe hoop stresses govern in the design of these pressure pipes, and the longitudinal stresses do not come into play....that's why they're wire wound under very high tensile forces.....you don't want to be standing next to the wire winder in the event the wire snap as the whipping wire will cut you in half !!!!!! A very dangerous operation to say the least. I've seen the wire snap on prestressed concrete tanks on 4 separate occasions in the past and it can do a lot of damage to equipment and the concrete tank wall segments. Luckily, no one was hurt or struck. However, the supervising tank crew foreman had been grazed by a whipping wire some 2 years prior and he was hospitalized for months due to the deep gash in his left shoulder that left it almost inoperable and still painful for the poor guy....but he was back in the saddle again because the pay was fantastic and he need to feed his family.

What you describe is post tensioning, not pre-stressing, the wire is wound tightly around the pipe after the concrete has cured. I cannot imagine any way to pre-stress circumferentially.

Pre-stressing longitudinally would not be expensive, prestressed concrete plank is the norm. The wires are stretched through the form, the concrete is placed and cured, bonding to the wires, and then the planks are cut to the required length, releasing the tension so the stress goes into the concrete.

"Pre-stressing longitudinally would not be expensive, prestressed concrete plank is the norm. The wires are stretched through the form, the concrete is placed and cured, bonding to the wires, and then the planks are cut to the required length, releasing the tension so the stress goes into the concrete."

My thinking exactly. But does any prestressed-pipe manufacturer actually do this? If any one did, there would be some interesting, exotic applications for this bi-axially prestressed pipe.

passingtongreen,

Post tensioning and pretensioning are both forms of prestressing. The 'pre' in prestressing means that it is stressed before it is placed in service. The 'pre' in pretensioning means it is stressed before the concrete is placed.

Surely, the circumferential is post tensioned, not prestressed

Yes, the Natgun tank was pretensioned. After each layer of high tensioned wire winding the tank walls were Shotcrete. Once that cures to a specific compressive strength the tank is wire wound again. they do this procedure over and over again until you have anywhere from 6 to 8 layers of wire, all depending on the design hoops stresses along the wall. The Shotcrete has a fair amount of Pozzolan and mid-range plasticizer. The cement amount is very high too per Cubic Yard...somewhere above 600#/CY. I believe the W/C ratio (before the plasticizer is added) is slightly less than 0.40 if I remember correctly.

The concrete pressure pipe is prestressed as well around the steel core (inside face of the pipe) and Shotcrete in the same manner over and over again. If in doubt why don't you call the manufacturer's tech rep., I dealt mainly with Price Brothers as they had several plants nationwide. The actual name of the product is Prestressed Concrete Cylinder Pipe (PCCP).

There are manufacturers in the USA, Canada and worldwide as this type of pipe was first manufactured in 1942. You can Google search for the PCCP and yes, there is even a Youtube video of the manufacturing process!!!

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I hope this answers some questions that you may have had about the product!

I give in. I was aware of this type of construction but I thought it was done in the field. It is a sort of combination because they must place a layer of Shotcrete and let it set before each new layer of cables.

As a follow-on: I only know that Reinforced Concrete Pipe incorporates steel reinforcing in both the longitudinal and transverse axis of the pipe. PCCP is many times more stronger than RCP because RCP should only be used in gravity storm sewers and sanitary sewers where the fluid (mainly water) is NOT under pressure. PCCP is used in the Water Works field as a pressure rate pipe, although it can be used in gravity swer systems as well.

In my 32 years as a Civil Engineer I have never encountered "post-tensioned" pipe of any flavor, and of those 32 years I was with the USACE 8 years as anEngineerng Officer.

Maybe somewhere in the world some engineer has design and had built such a weird pipe, but I' have never come across any reference to such....possibly poste-tensioning has been employed in case-in-place pipes or in tunnel linings, such as in dam turbine penstocks or bypass overflow tunnels, but not to my knowledge has it ever been used in any civil water works projects big or small......maybe I'm wrong, but in the same token I think I may be 100% correct on this one.

It's all well and fine until someone tries to drill a hole in it.

The means of applying circumferential "prestress" in the manufacture of prestressed concrete pipe has been a helical wrapping of the pipe cylinder while the wire is under substantial tension. As others have noted, it is obviously also possible to insert longitudinally stressed tendons or wires into all manner of products including panels and beams etc., with one very visible example being quite long-span interstate highway and other bridge beams, that I believe have such unseen tendons embedded mostly in their tensile-loaded bottom side. Prestressed concrete cylinder pipe does have some longitudinally oriented steel, that being the steel cylinder that serves as a waterproofing membrane, and also as a form for either a centrifugally spun or statically cast concrete core. However, that cylinder is normally very thin (in past years in some cases for even quite large sizes very similar to thin "sheet metal"), e.g. compared to the wall thickness of ductile iron or even most common steel pipes.

The distinct profit motive for manufacturers of prestressed pipe relative e.g. to lesser strength but solid wall steel and ductile iron pipes has been that only a very light weight (in e.g. pounds per linear feet of pipe) of very high strength steel wires is at least in theory counted on/provided as a bare minimum to contain the pressures. Concrete is on a relative weight basis also a quite cheap construction material. While the pipe LOOKS quite formidable, in the thickness of the walls and ring stiffness etc. that is typically furnished, one thing that perhaps should be noted is that the small very high strength and highly stressed wires of prestressed concrete pipe, that are the heart of the pipe's ability to maintain the core in a state of "prestress" as well as principal means to contain pressures, have been purposefully for whatever reasons (some pipe weight or O.D. size reduction/savings?) located radially quite close to the outside of the concrete coating. In other words, there has been lesser cover afforded these very small, and also very highly stressed, high carbon steel wires than has been provided for some much less stressed and less critical reinforcement of many other traditional reinforced concrete structures (and in some less critical services) for many years.

The outside of this concrete coating is of course what is directly exposed in the pipe's life cycle to any handling, shipping, and storage exposures etc. of the incidentally also quite heavy pipes, and eventually also this surface is of course what is directly exposed to the ground, groundwater, or whatever else in claimed perpetuity of buried service. Couple all this with very low or no safety factors vs cracking of this non-ductile brittle mortar coating when the pipe is exposed to internal service pressures (that are inexorably attempting to expand large pipes), and/or some inevitable but not often examined in detail axial or beam loads that are attempting to crack same circumferentially, and that the small and very highly stressed carbon steel wires are more vulnerable to breakage with even minor corrosion weakening/notching than lesser strained and more ductile steels, in aggregate all of these factors have created at least an expensive and disruptive headache for all those involved when one of these big pipelines lets loose in often a quite explosive or even self-excavating failure (that you would find in a quick web search with a few key words is incidentally also not necessarily easy nor quick to fix).

While many experts (some with deep roots in the industry itself - "What a country!"), have claimed that (for a not insignificant after-market fee) they can subsequently divine the condition and future serviceability of these unseen and embedded wires, in the most extreme such failures, often without many years service, have reportedly even been deadly (e.g. http://www.jasonconsult.com/projectbriefs/pressure_pipe_rehab.html ).