Which is more flexible?
Although HDPE is a flexible conduit, it’s actually RCP that is more flexible.  How?  Because it can be specifically designed to solve any application with proven, long-term performance.  Why settle for a one-size-fits-all conduit that was not built precisely for your situation?  Do you really believe HDPE will work as effectively as a custom-designed solution that addresses the unique circumstances of application?  RCP can be adapted to perform at the high standards you expect, no matter what your project involves – you simply can’t get more flexible than that.

RCP comes in a variety of shapes and sizes from 12-inch to 12-foot diameters and includes round, arch, elliptical and box shapes to meet the hydraulic capacity and fill height requirements you specify.

RCP is also available in different strength classes that meet national standards using a three-edge bearing test.  This means you have the flexibility to specify stronger pipe for less rigorous installation requirements or a lower strength for a more rigorous installation – all depending on the budget, contractor, soil type, pipe locale, load factors, etc.  It’s completely up to you.

And, RCP can be configured for any alignment (straight, curved or bends) and can also be designed for open cuts, embankments, subaqueous or tunneled applications.  Your design flexibility does not have to be compromised for the pipe’s sake.

Which is faster & easier to install?
You would think that because HDPE is lighter than RCP and has fewer joints, the answer should be HDPE. 

But in actuality, many owners, designers and contractors should be aware that HDPE has very specific installation requirements.  Because HDPE is not rigid, it requires the soil envelope to deliver its strength.  And the amount of strength found in the surrounding soil is tied directly to the soil type and installation procedure.  Without the appropriate soil and precise, verifiable compaction specs, HDPE cannot be relied upon to deliver the performance its manufacturers promise.

Proper installation of HDPE is a much more involved process than typically occurs.  In reality, if done correctly, HDPE is more time intensive and complicated to install than RCP.  So, should HDPE be installed faster?  Absolutely not.

70% or more of the strength of RCP is built into the pipe itself, with the remaining percentage coming from the installation.  Conversely, HDPE has only 5-8% of the strength built in and therefore depends almost entirely on the installation to deliver its strength and hydraulic performance.  With these odds, it’s critical that the special requirements for installation of HDPE be closely followed. 

For HDPE installation, the U.S. Bureau of Reclamation and ASTM require specific trench widths, quality embedment material and precise compaction, each based on the native soil and the HDPE application.  Further, trench protection during HDPE installation can impact the embedment and so must be moved to avoid disrupting the carefully constructed compressive soil environment.

Because most contractors are used to the relative ease of RCP installation, it’s not uncommon for them to assume HDPE can be similarly installed.  Sadly, when this doesn’t happen, HDPE will fail.  It’s just a matter of how quickly.


Which is actually not a pipe?
As I mentioned before, a buried pipe must function as both a structure and a conduit.  If it isn’t a structure, then it won’t have the chance to be a conduit.  And if it isn’t effective as a conduit, then it won’t serve its intended purpose.

That said, a rigid product, like RCP, arrives at the job site as both a structure and a conduit.  However, a flexible product, like HDPE, arrives as a conduit only – the structure is actually created during the installation by the carefully calculated interaction between the conduit and the surrounding soil embedment.  The vertical load is transferred to the side support soil through deflection of the HDPE material. So technically, HDPE isn’t a pipe until its primary structure is created in the field by the lowest bidder.  How comfortable are you with this scenario?

The structural survival of HDPE is dependent on contractors who may not be aware of the critical factors of its installation.  So, the first opportunity for structural problems lies with the installation.  Even small problems with the initial soil envelope surrounding buried HDPE conduit will ultimately negatively impact deflection, performance and the expected life of the pipeline. The second opportunity for HDPE structural failure occurs if anything unexpected happens to the surrounding soil envelope after installation – unusual settling from unforeseen loads or displacement from water runoff, high water tables or flooding, for example.  Without its supporting structure, HDPE will deflect beyond the AASHTO recommended 5%, and issues like sinkholes or complete collapse become possible.

RCP, on the other hand, can help overcome certain installation and soil issues because its structural strength is not affected by these aberrations.  For instance, RCP’s longitudinal rigidity will bridge low spots, whereas HDPE will conform to the trench bottom and develop high and low spots.  And, due to RCP’s weight, water in the trench or water runoff will not shift it out of alignment.

The product you can depend on is the product that’s truly a pipe – RCP.

Which lasts longer?
Forget for a moment that both products claim a 100-year lifespan.  And temporarily ignore the fact that the U.S. Army Corps of Engineers gives RCP a 100-year life and HDPE a 50-year life.  Let’s look at their in situ performance.

HDPE has only been in use for storm water management since the early 1970s.  Concrete, on the other hand, has been used for drainage since the Roman Empire and has a long, proven history of durability and effectiveness. 

One of the primary reasons for RCP’s much longer lifespan is its inherent, built-in strength. With HDPE, only 5-8% of the strength is manufactured into the pipe and more than 92% depends on the resistant forces of the carefully specified bedding and backfill compaction in the haunch area.  Not so with RCP.  In fact, different strength classes can be specified to address even extreme challenges of the application.  RCP is simply a more reliable conduit since it’s not vulnerable to the variances in installation that reduce the effectiveness of HDPE.

Not only is RCP inherently strong, but it actually gets even stronger with time.  This ongoing hardening occurs in the concrete mix due to a chemical reaction called hydration. Portland cement and water harden and bind the aggregates into a rocklike mass.  The surface is completed first, but these reactions persist at the interior of the concrete for years, continuing to cure, harden and increase the pipe strength.

Hanson Pipe & Precast reported that in 2009, the City of Melbourne, Florida, unearthed a section of concrete pipe that had been buried in 1959.  The City was delighted to find the 50-year-old pipe in excellent condition, even after examining coupon samples.

So, it’s important to recognize that RCP is more durable – it has the weight and strength to withstand the most challenging applications.  Whether it’s storms and flooding or heavy loads, you can count on RCP to perform the way you intended … for a century.  When comparing the life span of pipe alternatives, HDPE simply cannot compete with RCP.

Which is more likely to cause a pothole in the road?
Because it deflects and because it’s been proven to degrade over time, the answer is HDPE.  The life of HDPE is almost totally dependent on the conduit’s interaction with the soil envelope – in fact, more than 92% of its strength comes from the quality of the embedment.  So, at any time, if the surrounding soil infiltrates the joints and creates a pocket or uneven areas of counter force, then HDPE deflects beyond the AASHTO recommended maximum of 5%.  And this deflection, in turn, creates greater soil infiltration and larger pockets in the embedment, which causes the earth beneath the roadway to slowly sink – or suddenly collapse – when vehicles pass over.

While movement of the soil envelope can certainly occur with any pipe, a pothole with RCP is far less likely simply because of the concrete’s incredible mass, rigidity and built-in strength.  Even with infiltration, RCP won’t deflect and won’t shift in the ground.  This stability means safer roadways free from drainage pipe problems that can cause dangerous accidents.

The scary thing is that potholes can be the precursor to far more dangerous sinkholes.  In Elmore County, Alabama, a 24-inch diameter HDPE pipe collapsed under a street in a new subdivision, causing a sinkhole.  The HDPE pipe, which had been installed two years earlier, was replaced with RCP.

In Corning, New York, failure of a 60-inch diameter HDPE line at the Youth Sports Complex has caused three sinkholes in the past three years.  The pipe had only been installed for two years before the first sinkhole appeared.  Specimens of the pipe failed the stress crack resistance test and the carbon black content requirement.  HDPE was chosen for the project to gain an initial savings of $3,000 over RCP.  Repairs have cost approximately $70,000 and the entire line will be replaced with RCP.

And sinkholes can hit anywhere – in a relatively new subdivision in Pelham, Alabama, 42 inch and 48 inch diameter HDPE pipe collapsed in the backyard between houses.  The HDPE pipe, which had roughly three to seven feet of cover, was replaced with RCP.

So, although RCP may be seem more cumbersome to install, once it’s in the ground, you won’t have to worry about it – and pay for it – later.  

Which can truly weather severe storms?
Managing storm water drainage can be a real challenge.  The goal is to handle everyday weather as well as times of extreme rainfall.

Severe flooding pushes a pipe’s hydraulic capacity to its maximum.  It’s during these periods that RCP’s mass becomes a tremendous benefit because RCP is far less likely to be pushed downstream.  Conversely, from the moment HDPE is installed in the ground, it is fighting to float to the surface.  That’s because the corrugated exterior of HDPE actually traps “rings” of air around the barrel of the interior lining.  Being less dense than water, HDPE will always want to float to the top of the soil’s water table – and heavy rainfall only exacerbates this tendency. 

Extreme storms cause enough damage with high winds and flooding, but RCP can help prevent bridges and roadways from being compromised by the damaging forces of excess water.  RCP is the reliable choice for all the residents relying on you.

Many would claim that one of the biggest benefits of HDPE is its light weight, making it cheaper to transport and easier to move around for installation.  All true.  But if you really think about it, light weight is not what is needed once a pipe material is in the ground.  Once HDPE touches the bottom of the trench, its light weight becomes a liability. 

The time taken to install a pipe is only a fraction of its intended design life.  After it’s in the ground, RCP’s greater mass actually benefits the owner since it’s much harder to move off the installed line and grade as designed by the engineer.  RCP stays in place and does its job with little to no maintenance for the life of the pipe.

Because HDPE is so light, it offers little resistance to any excessive backfilling compaction during construction – or after construction, to any soil movements along its trench.  At any time, if you excavate too closely to installed HDPE, it will move toward the path of least resistance; in this case, the removed material along side it.  Not so with RCP.  It’s possible to excavate closely to installed RCP without compromising line and grade.  As a result, RCP can accommodate much narrower trench widths, which take up less valuable construction right-of-way.

For the owner who truly wants the performance of a 100-year pipe, RCP is well worth its weight.

Which gets stronger over time?
RCP is not only inherently strong, but also gets even stronger with time.  This ongoing hardening occurs in the concrete mix due to a chemical reaction called hydration. Portland cement and water harden and bind the aggregates into a rocklike mass.  The surface is completed first, but these reactions persist at the interior of the concrete for years, continuing to cure, harden and increase the pipe strength.

RCP can be manufactured to meet virtually any strength class needed and is plant tested to meet these demanding performance criteria with three-edge bearing tests.  These strength tests prove RCP meets or exceeds ASTM quality standards and allow the designer to know the specific load carrying capacity of the pipe.

The point is that RCP arrives at the job site with a quantifiable baseline strength & performance that actually improves during the life of the pipe.  HDPE can’t.

Did you know that RCP is designed to crack?  RCP design combines the high compressive strength of concrete with the high tensile strength of steel.  So as the load on the pipe increases and the tensile strength of the concrete is exceeded, cracks form as the tensile load is transferred to the steel.  The presence of these minimal cracks is an indication that the concrete and reinforcement are working together, as intended.

Concrete has the ability to heal its own cracks.  This phenomenon is known as autogenous healing.  Calcium carbonate crystals form when moisture reacts with unhydrated cement powder and regenerates the curing process.  This is proof again that over time, RCP gets stronger and stronger.  Once HDPE experiences deflection beyond the AASHTO recommended maximum 5%, it only continues to fail.  Which would you choose?

Which is a greater risk to the design engineer?
Clearly HDPE.  Because the engineer must rely almost completely on imprecise installation for the pipe to perform as designed. 

A rigid product, like RCP, arrives at the job site as both a structure and a conduit.  However, a flexible product, like HDPE, arrives as a conduit only – the structure is actually created during the installation by the carefully calculated interaction between the conduit and the surrounding soil embedment.  Sadly, most contractors are unaware that HDPE cannot be installed like RCP.  So, without following the specific ASTM requirements for HDPE trench width, bedding, fill soil type, compaction and cover heights, plastic will not deliver as promised.  Without a thorough understanding of these requirements, the engineer puts his or her reputation in the hands of the lowest bidder.

In late 2009 after four years of analysis, the City of Fort Worth, Texas, agreed to allow limited usage of HDPE for certain applications – if, and only if, the contractor is trained and certified in HDPE installation.  City officials are carefully inspecting each step of installation and monitoring the performance of these applications to determine the product’s true long-term effectiveness.  Why go through all this trouble if HDPE was naturally on par with RCP?

In 2002, the American Concrete Pipe Association commissioned Wiss, Janney, Elstner Associates, Inc. in a study of “Condition Investigations of HDPE In Service in the United States (Six States).”  The report indicates that of the 39 HDPE pipes tested (installed between 1989 to 1999) the percent experiencing greater than the 5% maximum allowable deflection ranged from more than 90% in Ohio to more than 40% in Utah.

A similar study was conducted by Ali Abolmaali, Ph.D., P.E. the Director of UT Arlington Center for Structural Engineering Research in December 2007.  This report tested the structural performance of 22 HDPE pipelines throughout Texas and discovered that 100% of the lines suffered from one or more failures including:  cracking/fracture, excessive deformation, joint displacement, inverse curvature, corrugation growth and buckling.

In a lawsuit against an HDPE manufacturer by Ridge Line, Inc. in West Virginia, a 48” pipe was found to be collapsing.  Dr. Ernest Selig, Professor Emeritus, University of Massachusetts, was quoted in his assessment of the failed pipe, “Both ductile tearing and slow crack growth brittle cracking were prominent, accompanied by folding over of the damaged portions of the pipe wall.”  What’s more, the HDPE manufacturer itself stated, “the key to any flexible conduit…is the installation, which is a specific, fact by fact, case by case issue.”

So, if you assume that HDPE is a viable substitute for RCP, you are taking your chances.

Which has better hydraulic efficiency?
When determining the appropriate pipe type for the installation, designers look at rainfall history, intensity and runoff, as well as existing infrastructure and buildings, drainage system layout and ground conditions – and tie all this back to the Manning’s n value of the pipe. The goal is to choose the best pipe product to balance the risk and cost of underperformance in extreme circumstances with the overall initial cost of the drainage system.

While this analysis can be complicated, the answer in almost every circumstance is simple – RCP.

Why?  Because this coefficient is based on RCP’s smooth interior wall, is maintained for the life of the pipe – not so with HDPE.  That’s because the inner wall of HDPE frequently experiences corrugation growth where a deformation occurs in the liner creating waviness that makes the interior of HDPE undulated, similar to the inside of a corrugated metal pipe.  Thus, the Manning’s n factor is negatively impacted – and will continue to be impacted during the life of HDPE.
The varying shapes of RCP (round, elliptical, arch and box) allow the designer to address specific drainage situations like greater capacity needed at shallow depths of flow, higher flushing values needed at low flows, height or width restrictions, embankment installations and more.  That means that even in unusual or challenging circumstances, RCP can provide the hydraulic efficiency and structure that HDPE can’t.

Research shows that drainage designs utilizing RCP can be downsized by at least one size in most cases when compared to steel, aluminum and HDPE.  It’s important to utilize Manning’s n values that demonstrate actual efficiency in the field versus the very different Manning’s n values obtained in pristine laboratory conditions.  After all, how often does Mother Nature follow the rules?

Which is safer for the environment?
Overwhelmingly, concrete.  Why?  Because concrete is made primarily of local, natural, non-toxic elements from the earth – sand, gravel, crushed stone – so it’s inert.  Concrete is not flammable, will not react with other materials and does not release chemicals into its surroundings.

HDPE, on the other hand, has been proven to leach VOCs – volatile organic compounds – into the water, according to a study released by the Princeton Institute of Science and Technology of Materials.  These VOCs are known to have public health and environmental implications by affecting developmental processes in humans, other mammals, aquatic life and wildlife. 

HDPE is a petroleum-based plastic that emits powerful chemical leachates – many of which are known carcinogens – such as benzene, arsenic, formaldehyde, carbon dioxide, CFCs and HCFCs, cyanide, lead, mercury, and the aforementioned VOCs.  You can get more details in a report prepared by the Plastics Division of the American Chemistry Council.

Hanson Pipe & Precast recently conducted a comprehensive analysis of its RCP manufacturing process and determined that the production of concrete results in an impressive 81% fewer carbon dioxide emissions than equivalent quantities of HDPE – a significant benefit to the environment.  And this conservative figure reflects not only the carbon footprint of Hanson Pipe & Precast's manufacturing facilities, but also took into account the carbon emissions produced during the extraction and production of the raw materials used.

Which costs less?
If the question were ‘which pipe material initially costs less,’ the answer might be HDPE.  But over the life of the pipe, it’s RCP that actually costs less.

Like the U.S. Army Corps of Engineers, local and state governments are increasingly using a Least Cost Analysis (LCA) in their material selection process.  This reflects the lowest lump sum of money that would have to be set aside at the start of a project to cover all expenditures during the entire life cycle of the project and therefore includes both interest and inflation.  Because RCP has a demonstrated service life of 100+ years, because it is less expensive to install properly and because it has minimal, if any, maintenance costs, RCP compares more favorably than HDPE in LCAs.

One critical cost factor that is often misrepresented in LCAs is the installation cost of HDPE.  Many contractors don’t realize that the performance of HDPE almost completely depends on the soil envelope.  If the specific trench width, embedment material, compaction levels and inspection procedures are not followed, then the pipe will fail long before its expected service life.  If the costs of this extra installation care are not accounted for, then the LCA for HDPE is inaccurate – and the true outcome will be even more costly with replacement expenses when the pipe fails.

Typically, if the property is going to be re-sold, the owner is more interested in the possible initial capital savings of HDPE, but if the owner wants a long-term, cost-effective solution, RCP provides the better value.

And, with more and more focus on green building and sustainability, RCP is one of the most energy efficient and eco-friendly drainage products available. Made of primarily local, natural, non-toxic elements from the earth – sand, gravel, crushed stone – RCP is not flammable, will not react with other materials and does not release chemicals into its surroundings.  Plus, RCP essentially has no life cycle end – not only does it last longer, but it can be recycled and re-purposed indefinitely.  Thus, replacement pipe using depleting resources is needed far less frequently. There’s practically no waste, and contribution to landfills is dramatically reduced.

The bottom line?  Lower initial pipe costs rarely result in the smartest, most economical product.