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Q. 1 – I see reline projects using CMP inside of CMP – isn't that repeating a material selection mistake?

Not if the new CMP material is selected correctly.  To explain, let me cover two things.  First, the causes of corrosion failures in CMP, and second, the right way to select a CMP product that will work for your reline project.

Two principle causes of early failure in corrugated metal drainage pipe materials are corrosion and abrasion.  Service life can be affected by the corrosive action of the backfill and effluent in contact with the drainage pipe but most commonly by the corrosive and abrasive action of the flow in the invert of the drainage pipe.

Abrasion

The potential for metal loss in the invert of a drainage pipe due to abrasive flows is often overlooked by designers and its effects are often mistaken for corrosion. An estimate for potential abrasion should be made at each pipe location in order to determine the appropriate material and gage.

Medium to high velocities can be tolerated with CMP if the flow is clean water and does not contain abrasive particles.  The FHWA has issued guidance on how to handle abrasion.  It is taken into account in the Contech CMP Design Guide on Durability.

Environment

A design service life analysis should include a check for both the water side and soil side environments to determine which is more critical—or which governs service life.  This involves determination of the water chemistry including pH and Resistivity (in Ohm-cm).  It may also be wise to check for sulfates, chlorides, hardness, and other items as dictated by the possible constituents that could be present.  On the soil side, the in situ soil and the backfill material should be checked to make sure soils that are aggressive toward metallic coatings are not present.

Material

Corrugated metal pipe is available in steel and aluminum.   With steel, there are two types of metallic coatings, galvanized (zinc coated) and Aluminized Steel Type 2 (aluminum coated steel).  Polymeric coated CSP uses a polymer film that is applied over the galvanized sheet and then is converted to CMP in a Contech plant using standard HEL-COR® equipment.

Post Manufacture Applied Coatings / Pavings

Asphalt coatings, paved inverts, and linings can be post applied at some of our CMP plants.  Concrete paved inverts that are placed after installation and backfilling are also a viable way to extend the service life of a CMP structure.

Durability of CMP – What We Know:

Prior to the significant research dollars spent on HDPE in the late 1990s and early 2000s, corrugated metal pipe durability was the most highly studied drainage pipe topic.   As a result, there is vast information on how CMP products perform in various environments over long periods of time.   If the water and soil chemistry is known, and the expected levels of abrasion (due to flow velocities and bed load, which is simply abrasive particles like sand and gravel) is highly predictable, then accurate estimates for the service life of a metal pipe structure can be predicted.  The tables that follow and the information presented in the Design Guide mentioned above is a compilation of research knowledge over many decades by various public agencies and the corrugated metal pipe industry.  

Material Options

Galvanized

Galvanizing was the first coating available to CMP.  This has been the workhorse for a century.  It's the reason for the perceptions of CMP.  It's also been the most highly studied drainage pipe ever.  The result – we know what it can and cannot do.  The zinc coating is a sacrificial coating, meaning zinc oxide gives itself off as electrical current passes through it in order to protect the base steel.

The presence of scaling in hard waters (and non abrasive waters) is a good thing.  It helps interrupt the galvanic cell and that helps keep the zinc in place.  The corrosion rate in soft water can be up to ten times greater than in hard water.  Hard waters are less corrosive because they deposit protective scales on the metallic surface, while softer water does not.  The corrosion rate of zinc increases with the aeration of the water – dissolved oxygen and carbon dioxide both increase the rate of attack.  A 50-year plus service life can be reached in the right conditions, which includes a pH between 6 and 10 and a resistivity between 2,500 and 8,000 ohm-cm.

Aluminized Steel Type 2

Aluminized Steel Type 2 (ALT2) didn't enter the picture on a large scale until the early 1950s when prototype material was placed in service at more than 200 sites in the United States.  The coating process is similar to galvanized CMP only the steel sheet is run through molten aluminum, instead of zinc.  The alloy layer is much harder and thicker than galvanized and offers a barrier protection.  The Morris/Bednar 30 Year Report from 1982 and the AK Steel 43 Year Report include a detailed description of why Aluminized Steel Type 2 resists corrosion.  They include the metallurgical differences compared to galvanized steel. 

Unlike zinc, aluminum does not corrode in the presence of oxygen.  In fact, oxygen is beneficial to aluminum.  Aluminum oxide forms in the presence of oxygen.  The aluminum oxide barrier resists dissolved oxygen, carbon dioxide, and higher velocity waters.  The hard alloy layer is a second layer of defense providing both corrosion resistance and outstanding abrasion/erosion resistance.

It is this accumulation of knowledge that allows AK Steel to proclaim a 75-year service life for 16 gage in the recommended environment, which is a pH between 5 and 9 and resistivity larger than 1,500 ohm-cm.  It's also why a number of DOTs recognize it to provide 75-year and 100-year service lives.  Notably, the Pennsylvania DOT assigns a 100 year service life to 12 gage aluminized steel type 2.

Aluminum

The aluminum alloy sheet is an aluminum core with an added cladding layer that contains a small amount of zinc.  Service-life expectancy studies performed since the early 1960s have concluded that 16 gage aluminum CMP will last 75 plus years when installed in the appropriate environment, which is a pH that is between 4 and 9 and a resistivity greater than 500 ohm-cm.  Aluminum CMP performs exceptionally well in salt water environments with resistivities as low as 35 ohm-cm, as long as the backfill material is easily drained – the best backfill material for this purpose is clean crushed rock.

Polymer

Polymer has the widest environmental range of corrosion resistant for CMP products.  Contech's polymer protective film bonds to galvanized steel both chemically and physically and features a 10 mil polymer film applied to each side of a galvanized steel coil after the galvanizing process.

CMP coated with a polymer-coating offers chemical resistance to acids, alkalis, and salts, abrasion resistance, weatherability, and a water.  It has been around in its present form since the late 70s.  It's been highly tested and field proven.  It is proven to work best in an environment that includes a pH between 3 and 12 and a resistivity greater than 100 ohm-cm.

Abrasion

Abrasion is a function of flow velocity and bed load.  In the absence of bed load, abrasion is not a factor.  FHWA has provided guidance with abrasion, which is seen in the table below.  They relate it to the type of objects moving through and the velocity of flow.  Level 3 and 4 abrasion, even in a neutral environment, can wreak havoc on the invert, which can greatly reduce service life.  I have included a table below showing how non-metallic coatings may add service life.

Abrasion Level Abrasion Condition Bed Load Flow Velocity (fps)
Level 1 Non – Abrasive None Low
Level 2 Low Abrasive Minor (sand) < 5
Level 3 Moderate Abrasive Moderate
(sand & gravel)
5 - 15
Level 4 Severe Abrasive Heavy
(sand, gravel & rock)
> 15

 

Coating Level 1 & 2 Level 3 Level 4
Asphalt 10 N/R N/R
Asphalt Coated & Paved Invert 30 30 30
Concrete Lined (HCCL) 75 50 N/R
Concrete Paved Invert (3" cover) 80+ 80+ 50

 

So getting back to the original question, which was essentially, "Is relining a CMP with CMP a mistake?". 

The answer is, only if it's designed incorrectly. 

This leads to the next question I get routinely:

Q. 2 – How do I design CMP for a reline project for optimum durability?

It's the same answer I would give for 'How do I design a new CMP direct bury installation for optimum durability?".  Let me explain.  Proper design of culverts and storm sewers requires structural, hydraulic, and durability considerations.  While most designers are comfortable with structural and hydraulic design, the mechanics of evaluating abrasion, corrosion, and water chemistry are not commonly used or readily available.  The materials needed to perform a durability design are not commonly found in most civil engineering handbooks.  The durability and service life of a drainage pipe installation is directly related to the environmental conditions encountered at the site and the type of materials and coatings from which the culvert is fabricated.

Common Methods

The most common approach taken to designing for durability is simply using a product selection guide or usage chart as published by a local or regional governing authority.  Most of the time this is a state department of transportation.  This approach is popular because it is easy and it allows a designer to more easily gain acceptance of his work.  Unfortunately it is not fool proof and usually does not take into consideration any site specific design parameters that could impact the actual service life of a particular structure.

Contech's CMP Design Guide on Durability for Drainage Products

A much smarter approach is to start with any required usage or selection guide, but then cross check the result with the Contech CMP Durability Design Guide linked here.  Collection of site specific data is highly recommended. Specifically, take a water sample at low level flow, which means it hasn't rained for a few days.  Have a lab measure the pH and the Resistivity (inverse of Conductivity).  On a reline job the exterior of the new pipe is encased in grout so the soil side materials do not need to be sampled.

The Contech CMP Design Guide is intended to help a specifier walk through the mechanics of selecting appropriate drainage products to meet defined service life requirements.  The information contained in the Contech guide is a composite of several national guidelines.

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