Choosing pipe? Look for strength you can rely on.

When selecting the right pipe material for a specific project, you’re making a decision with repercussions that may require action long after its installation. This is why it’s a good idea to take the broadest possible view, comparing every possible contingency factor that impacts total costs, weighing in potential risk factors and examining every aspect of your choice. Initial sticker price is obviously an important factor, but other variables, such as product longevity, total life-cycle costs, maintenance and overall installation costs are all considerations that should be taken into account. 

Let’s say you’re specifying pipe for a drainage or sanitary project. You’ve already examined performance needs, flow requirements, pipe diameter needs and more. Now it’s time to choose a pipe material. Will you specify a rigid or flexible pipe product? Some choose flexible pipe such as high-density polyethylene (HDPE) or polypropylene (PP) simply because they believe it’s cheaper. But let’s take a step beyond initial sticker price and look at installation costs, labor costs, material costs, liability costs, and so on. 

As you know, reinforced concrete pipe (RCP) is a structure in its own right. At the point of manufacture, it carries up to 90% of the design load (see AASHTO Load Resistance Factor Design). Flexible products only support 10% of this load (see ASTMD D2412), and therefore the pipe bedding/embedment must be designed to provide the remaining 90% of the engineered structural support requirement. In practical terms, a rigid pipe like RCP only needs structural backfill to the springline of the pipe — the maximum width of the pipe — while the embedment (engineered structural bedding) for a flexible pipe must be placed and compacted in layers until it covers the pipe by up to a foot before replacing overfill soil. Thus, in the case of a flexible pipe such as HDPE and PP, not only is there a significant cost incurred importing granular structural bedding/embedment material, but there will also be displacement of surplus soil, which must be exported. With a rigid pipe, such as RCP, the amount of bedding/embedment material is as much as three times less. In fact, in-situ materials, based on meeting proper soil classification, can often be used for structural embedment of RCP up to the springline. This is not the case with flexible pipe such as HDPE and PP.

Structural integrity right off the truck

One of the main advantages of a reinforced concrete structure — whether it’s a joint of RCP or a precast box culvert — is that the product has passed strength load-testing and inspection before it leaves the manufacturing plant. While flexible pipe joints are certainly inspected before leaving the factory, the fact that they can only be expected to carry up to ten percent of any subsequent dead and live loads simply renders load-testing at the plant moot. Added to this, because the structural integrity of the finished pipeline is so dependent on the proper installation itself, the entire burden of structural integrity is placed on the design engineer, contractor and inspector. The ADS Drainage Handbook states:

“Buyer/User is responsible for serviceability of the product in any given application. Seller is not responsible for injury or damage resulting from improper installation, noncompliance with these guidelines for installation of product, or use outside the guidelines set forth herein.”

All pipe manufacturers have testing and inspection procedures in place at the plant level. Quality control is conducted at the factory, and this ensures that the pipe meets structural load design standards. This is good news when 90% of the load-bearing strength is contained within the structure — the RCP pipe — itself. This isn’t the case with flexible pipe, where 90% of the load-bearing strength is in the material surrounding the pipe — the bedding/embedment and soil — and is also dependent on diligent installation in accordance with manufacturer guidelines.

Bearing the responsibility

The use of flexible pipe places the responsibility for the structural integrity of the final product from the pipe manufacturer to the design engineer, contractor and inspector. In fact, less than complete adherence to the relevant flexible pipe installation procedures may, in the event of any type of failure, lead to questions of liability.

ASTM D2321 (Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications) states:

“It is incumbent upon the product manufacturer, specifier or product engineer to verify and assure that the pipe specified for an intended application, when installed according to procedures outlined in this practice, will provide a long-term, satisfactory performance according to criteria established for that application.” 

In fact, the design engineer or authorized representative is mentioned at least 25 times in this specification as requiring approval of different parameters. The fact that some installation operations are delegated to a contractor will not relieve the design engineer from this responsibility, which could even extend to the personal level.

Go to any HDPE or PP pipe manufacturer’s website and you can download the relevant installation guides. Most of them are around thirty pages long. That’s a lot of reading. How confident are you that your installers have read them, much less carefully followed them?

Bearing the load and the cost

With pressure from all sides to contain costs, it may sometimes be tempting to choose a pipe option with a lower initial cost. Flexible pipe such as HDPE and PP may seem to fit the bill, for example for a drainage or sanitary project. In fact, while the initial price tag for RCP may seem to be higher than the equivalent flexible pipe alternative, this will not be the case when you factor in the additional installation labor costs, the cost of importing aggregates, exporting in-situ material, onsite deflection testing and additional inspection. And I’m not even talking here about the possible, if distant, cost of installed liability risk.

For a successful pipeline project now and far into the future, the pipe, bedding/embedment material, backfill and soil must combine to create an overall structure that will last. By selecting a rigid pipe such as RCP, structural integrity in terms of load-bearing capacity is already on the delivery truck. Choosing RCP means a faster, smoother installation and a greater degree of certainty and overall lower life-cycle costs.

Sticker price is one thing, but risk-avoidance and the overall installation cost are surely worth something, too. In a comparison between HDPE, PP and RCP, the rigid option makes sense from both load-bearing and cost-bearing perspectives.

Branimir Kovac, Vice President, Thompson Pipe Group