10 Questions You Should to Know about Corrugated Metal Pipe

Corrugated metal pipe has been used in drainage applications in the United States since the late ’s.

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For most of the 20th century, the two most frequently installed stormdrain pipe materials were concrete pipe and galvanized corrugated steel pipe.

For instance, there are ~65,000 stormdrain pipes in Gwinnett County’s GIS database with installation dates between and . About 17,000 of the pipes are concrete pipe (~27%) and about 45,000 of the pipes are galvanized or asphalt coated galvanized pipe (~70%).

How have these materials performed over time?

The Problem with Galvanized Corrugated Steel Pipe in Georgia

Every stormdrain pipe material has recommended applications and environmental condition restrictions.

Corrugated metal pipe is recommended for the following environmental conditions according to the Georgia Department of Transportation:

Out of all these corrugated metal pipe products, galvanized steel pipe (zinc coated) is the most sensitive to environmental conditions.

Compare the allowable pH range for galvanized steel pipe (minimum 6.0) vs aluminized type 2 steel pipe (minimum 4.50).

The pH system is a logarithmic scale, so a 1.5 lower pH value translates to an environment that is ~30 times more acidic.

Environmental Conditions in the Atlanta Area

The USDA Natural Resources Conservation Service provides soil survey information online, which allows you to select an area of interest and download the expected soil and water characteristics based on the recorded soil surveys.

Let’s look at the soil conditions in the Atlanta area, using Gwinnett County as an example again.

If you select Gwinnett County as an area of interest and pull up the “Soil Chemical Properties”, then you can select pH values and download a table of the expected soil types and pH values. You can download the information as a PDF report (USDA Expected Soil pH Values – Gwinnett County).

We have summarized the USDA information below.

If you compare the USDA information to the recommended environmental conditions for galvanized steel pipe, then you can see why the material is underperforming in the Atlanta area.

Galvanized CMP should be installed in environments with a pH of 6.0 or higher, but just 1.5% of Gwinnett County falls within that range.

Aluminized CMP is recommended for environments with a pH of 4.5 or higher, so none of the USDA’s expected environmental conditions fall outside of the recommended range for aluminized steel pipe.

Why Does Aluminized Type 2 CMP Perform Better in More Aggressive Environments Compared to Galvanized?

Aluminum and aluminum alloys are very stable when they are exposed to an environment with pH values between 4.0 and 9.0. Included below is a chart from Uhlig’s Corrosion Handbook.

Field measurements of pH may vary, so the industry leaves some safety cushion in the recommended environmental conditions for AT2 CMP, so the recommended lower pH limit is 4.5 instead of 4.0.

How Do We Know That Aluminized CMP will Outperform Galvanized CMP in Georgia?

Let’s start with some visual comparisons of aluminized CMP and galvanized CMP installed at the same time and in the same environments.

The image below is a pipe that was manufactured partially from galvanized steel and partially from aluminized type 2 steel. The pipe was left in service for 30 years and then examined.

The visual difference between the two materials is dramatic.

One of the easiest places to compare the performance of galvanized and aluminized CMP is at a headwall connection. Most precast headwalls are manufactured with a galvanized CMP stubout, so when an aluminized type 2 CMP pipe is connected to the headwall then you can see a direct comparison of the products.

This pipe is located in Barrow County, Georgia. It was 17 years old at the time the picture was taken. The galvanized stubout is beginning to fail at the invert but the aluminized pipeline is performing well.

The red coloration you can see in the pipe invert was easily removed with a brush and the pipe surface still has a silver sheen because of the intact aluminum coating.

The aluminized pipe was measured as part of the NCSPA’s Atlanta area study of AT2 CMP. This pipe still had more than 90% of its aluminum coating remaining when this picture was taken.

The next visual is a video that was published by Contech Engineered Solutions. They examined a 30-year old installation where a 78” diameter aluminized CMP culvert is outperforming the galvanized end section that is connected to it.

What Information is Available on the Real-World Performance of Aluminized Type 2 CMP?

Aluminized type 2 corrugated steel pipe has been around since the early ’s. The product has been thoroughly studied for decades.

NCHRP Report 801 “Proposed Practice for Alternative Bidding of Highway Drainage Systems” states that:

“Currently, there are more well defined methods to estimate the service life of steel culverts than any other type of material” (page 57).

Information Source 1: Minnesota DOT

Minnesota DOT published a report in titled “Improving the Selection and Testing of Culvert Pipe Materials”. They recommend using aluminized type 2 CMP because its service life is 3 to 8 times better than galvanized CMP.

A research team at the University of Minnesota presented to MDOT in . The presentation was called “Culvert Pipe Materials and Durability: Making the Right Selection for Minnesota”.

The research team examined the DOT’s culvert inspection database to see how various pipe materials are performing. They recommended replacing galvanized CMP with aluminized CMP because of the better service life. Interestingly, they also state that 18% of the RCP in Minnesota DOT’s database has joint separation issues.

Information Source 2: Gwinnett County’s GIS Database

Gwinnett County has been assessing the condition of their existing stormdrain pipe infrastructure since . Each pipeline was inspected and assigned a condition rating of “Good”, “Fair”, or “Poor”. This condition data was updated in their publicly available GIS database until .

The database includes more than 145,000 pipelines and lists the pipe material, location, estimated date of installation, and pipe condition. The information in the database is summarized in the tables below.

The condition data was gathered internally by Gwinnett County for their own maintenance planning purposes, but the summary tables below were compiled by Southeast Culvert.

Material type acronyms:

• RCP (reinforced concrete pipe)
• BCCMP (asphalt coated galvanized corrugated steel pipe)
• AT2 CSP (aluminized type 2 corrugated steel pipe)
• HDPE (high density polyethylene pipe)
• GALV (galvanized corrugated steel pipe)

When the data is broken down by pipe material type, you can clearly see the performance differences among various material options.

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1. The worst performing pipe material in the database is galvanized CMP.
2. Bituminous coated galvanized CMP is performing significantly better than galvanized.
3. RCP is middle of the pack. It’s doing better than galvanized and BCCMP, but worse than HDPE and aluminized type 2 CMP.
4. HDPE pipe is the second best performing material in the database.
5. Aluminized type 2 CMP is the top-rated material by a significant margin.

The install dates for the database go back to , so let’s look at the statistics for the oldest installations.

Narrowing the time frame to the period between and reduces the number of pipelines to ~65,000. The performance differences between the various material types also becomes more pronounced.

Galvanized CMP is clearly performing worst with ~9% of the galvanized pipes in “poor condition”.

RCP, BCCMP, and HDPE are all in the same ballpark, around 3% in “poor condition.” HDPE is performing the best in that group.

Aluminized type 2 has the most pipes in “good condition” by a large margin (27% vs < 10% for the rest of the materials). It also has the least proportion of pipes in “poor condition” (~1%).

Information Source 3: Cobb County’s GIS Database

Cobb County has a GIS database that is comparable to Gwinnett County. The number of inspected facilities was smaller as of , but the statistics are similar.

The condition data was gathered by Cobb County for their own maintenance purposes, but the summary tables below were compiled by Southeast Culvert.

Their database includes ~ 78,000 pipes categorized by pipe material, location, estimated date of installation, and a pipe condition assessment. They assessed each pipeline and assigned the pipe a condition of “Good”, “Poor”, and “Failing.”

Material Type Acronyms:

• RCP (reinforced concrete pipe)
• BCCMP (asphalt coated galvanized corrugated steel pipe)
• AT2 CSP (aluminized type 2 corrugated steel pipe)
• HDPE (high density polyethylene pipe)
• Uncategorized CMP

Cobb’s statistics show a similar pattern as Gwinnett County.

1. Aluminized Type 2 CMP has the smallest proportion of pipes in failed condition and the largest proportion in good condition.
2. HDPE is the second best performing product.
3. BCCMP / Uncatagorized CMP tie for second worse performing product with ~ 1 percent in failed condition.
4. RCP is the worst performing product in Cobb’s statistics by a large margin.

Let’s look at some of the older pipe in Cobb’s database. When the data is filtered for install dates between and , the number of pipelines drops to ~55,000.

The older pipes show a similar pattern. Aluminized CMP is the best performing product. BCCMP, Uncategorized CMP, and HDPE are middle of the pack, and RCP is the worst performing pipe material.

Information Source 4: Georgia DOT and Oregon DOT

Within the recommended environmental conditions, the expected service life of aluminized type 2 CMP is 75 years minimum for 16 gauge pipe and 100 years minimum for 14 gauge pipe.

Galvanized CMP has an average service life of 40 years in the recommended environmental conditions (which are not commonly found in Georgia).

Because of the performance differences between the products, most DOTs in areas with lower pH environments use aluminized CMP instead of galvanized.

The Georgia Department of Transportation updated their stormdrain standards in and revised their stormdrain pipe selection chart.

They upgraded the use of AT2 CMP to be allowed underneath roads with an expected traffic of 5,000 cars per day. GDOT uses a 75-year min service life guideline for pipe materials underneath a road.

Notice that galvanized (zinc coated) CMP is not allowed inside the roadbed on GDOT projects.

The Oregon Department of Transportation explicitly lists the service life expectations for their stormdrain products in the ODOT hydraulics manual.

Galvanized CMP is listed with a service life between 15 and 40 years depending on the environment.

Aluminized CMP, RCP, and plastic pipe are all listed with a 75-year service life.

Information Source 5: “Durability of Aluminized Type 2 Corrugated Steel Pipe Exposed Throughout the United States” – Report by Elzly Technology Corporation

Aluminized type 2 has been studied on an ongoing basis since the early ’s. More than 125 AT2 CMP pipes were installed in coordination with 20 state and county DOT’s and revisited every 10 to 15 years for an updated performance report.

The most recent durability study was published by Elzly Technology Corporation in and included pipes that have been in service for 60 years.

The conclusions of the study support the 75-year minimum service life for 16 gauge Aluminized Type 2 CMP in culvert applications.

Culvert applications are significantly more aggressive than storm sewer applications because of bedload conditions. Storm sewer applications are considered to have little or no bedload according the FHWA guidelines.

In stormdrain applications, the 75-year minimum service life guideline for AT2 CMP is very conservative, which is why the next study’s results are so dramatic.

Information Source 6: “The Measured Performance of Aluminized Type 2 Steel Pipe in Storm Sewer Applications in the North Georgia Region” – Report by the National Corrugated Steel Pipe Association

In the NCSPA began building a database of aluminized type 2 CMP installations in the Atlanta area. In total, 140 pipes were inspected and measured for aluminum coating loss and the results were charted against the expected wear rates for the material to provide a 75-year service life.

The study locations were widely scattered across north Atlanta. Most of the pipes have been in service for more than 20 years at the time of inspection.

The results of the study show that AT2 CMP installed in stormdrain applications will provide a service life far in excess of 75 years.

The average wear rate of the aluminum coating was 0.84% loss per year. At that rate, there will be substantial aluminum coating on these pipes after 100 years of service.

The average coating wear rate for the pipes that have been installed for more than 30 years was 0.78% loss per year, which shows that the aluminum coating provides consistent protection even after decades of service.

You can find the full report on the NCSPA’s Studies and Reports webpage.

Conclusion

For most people, the distinction between the many types of corrugated metal pipe is blurry. Contractors, engineers, and municipal reviewers often have an opinion on the product as a general category.

A reviewer might say “we allow metal pipe” or “we don’t allow metal pipe” for a project, but the specific type of CMP material makes a big difference on the expected service life of the pipe.

In Georgia conditions, galvanized CMP does not perform very well. The soil and water pH values are generally outside of what is recommended for the product. In other regions of the country with different environmental conditions, the product performs very well and can exceed the 40-year service life expectation

Aluminized type 2 CMP has proven to perform very well in Georgia conditions, which is why it is allowed underneath major roads on Georgia DOT projects and in many Atlanta area municipalities.

If you are specifying a CMP product on a project in Georgia, then mostly likely you should use aluminized CMP.

I am in the process of reclaiming some of my property for horse pastures. There is a creek that runs through it and the previous owner had a 4 foot diameter (?) galvanized corrugated pipe put in the creek bed to create a crossing. Problem is he didn't do it right and the creek washed out the dirt around the pipe. I plan to rebuild it this summer and use it for a tractor crossing. However, I'm not so sure that the pipe is long enough for a safe crossing.

- How long should the pipe be for a safe tractor crossing?
- Is it easy to join 2 sections of pipe together?
- Would it be easier to just buy a longer section of pipe?
- Any idea how much corrugated metal pipe costs?
- Where can I get culvert pipe?
- How much weight can a pipe like that hold if it is surrounded by dirt?

Other areas of the property have a high water table (I don't want to use the term "wetlands"). I was going to try to use field tiles and smaller culverts to make it more usable for me, horses and the tractor. In some areas I may use 18" plastic corrugated pipe.

- Any idea how much weight 18" plastic pipe could hold?

FYI - my tractor weighs 4,000 pounds and I have a backhoe, FEL and ballasted tires. I guessing the whole thing could weigh over 6,000 pounds. I do a lot of "low cost," meaning whatever we can scrounge up, culvert work for the hunt club where I have a backhoe membership. All seat-of-the-pants, no formal training stuff, so any answers I might offer are strictly caveat emptor, and worth what you paid for them.
"Any idea how much weight 18" plastic pipe could hold?"
None. Same for metal. They're only to keep the dirt out of the hole. Deep enough and they'll carry any load, if they don't have dirt on top they won't stand one crossing. Depending on the fill you're using 12 to 18 inches from the road surface to the top of the pipe will carry anything short of a loaded dump truck.
Four-foot diameter pipe? Needs to be deeper. I'm sure there are engineers who would have a table that shows how much deeper for how much greater diameter. Or a formula that says something like,"One inch of depth for each two inches of diameter." The idea is that the dirt spreads the load around the pipe rather than having the weight press down onto it.
"How long should the pipe be for a safe tractor crossing?"
Well, what really matters is how wide the road bed is. If you can approach straight on, three feet wider than the wheels of your widest trailer would be a decent rule of thumb. If the pipe is to be eighteen inches below the surface it should be at least three feet longer than the road is wide. Here again, that depends. What you want is to be able to have the slope from the road to the top of the pipe gentle enough that the dirt doesn't fall away. If it's sand you need longer pipe, if you're going to build a concrete wall on each end six inches overhang would be plenty. /forums/images/graemlins/wink.gif
"Is it easy to join 2 sections of pipe together?"
No. At least, used metal pipe would be hard to do. If you're going to have much or even frequent flowing water you want a good seal. Any openings the water will find and eat away the dirt. First thing you know you've got sink holes between the sections.
"Would it be easier to just buy a longer section of pipe?"
Yes, but...
"Any idea how much corrugated metal pipe costs?"
Like I said, we mostly scrounge. Have old stuff donated. Best we can do with what we have to work with.
"Where can I get culvert pipe?"
I know the 8 to 24 inch plastic pipe is available from Lowes, etc. Not too pricey.
Like most home projects, there is the prime trade-off. The better you do it, the longer before you have to re-do it.
Good luck, Wm
http://www.wimmark.com/culvert_maintenance.htm You didn't say how long your 4 ft diamater pipe is. Depending on how far below grade it is and how much height you want to build up on the pipe, will dictate how wide your road across it will be.

To stop it from eroding again, you will need to put something on the upriver side to stop erosion. Rocks work if you have them, concrete sacks stacked on top of each other with rebar through them also work really good.

Once a culvert is packed down on all sides, it will be able to easily handle anything you own. My backhoe weighs 14,000 pounds and crosses my culverts all the time with no problems. I've run a 40,000 pound RV's over them also and just the other day a neighbor crossed a few moving a moble home off his land through mine. I dont' know what that weighs, but it wasn't a problem either.

Joining the culvets requires a sleeve that bolts over the two ends. Sleeves for that large a culvert would be fairly pricey, but I have no idea of how much.

A 12 inch plastic culvert 20 feet long goes for a little over $100 in East Texas. Lowes has them along with most farm supply stores.

Each size you go up just about doubles the amount of water you can handle. I use 12 inch for just about everything. 18 inch would be a waste of money for me, but you may need it. I'd ask around to see what others are using in your area before buying something so big. I should have said that I plan on putting dirt on top of the pipes. Sounds like if I have at last 24 inches on top I should be fine.

I think the existing pipe is about 5 or 6 feet wide at the top. The downstream side is cut at an angle so that the bottom of the pipe lying in the creek bed is twice as long. So, if my tractor is about 5-1/2 feet wide it sounds like I should have a pipe that is about 12 feet wide on top.

I have heard of using bags of unopened concrete (I think here at TBN) with rebar driven through them. That sounds like an excellent idea. I was also going to use some pieces of chain link fence sandwiched in between rocks or riprap to reinforce the upstream side of the pipe.

I would love to have a bridge over the creek but I am afraid that would be too much money and time. Can't give specific #s on what a 18 inch plastic pipe will hold, but from personnel experience it's alot. I replaced a 2 ft galvanized pipe in front of my house that was smashed during construction of it from the heavy trucks/equipment. The pipe was to big for the ditch as alot of it was exposed so the ground couldn't help support it. I replaced it with a 18 inch plastic pipe with a smooth inner lining to help with water flow, also think it helps with the pipe strength. Only has about 6 inches of dirt/gravel on top of it and a tri axle dump with full load of rock went over it with no problems. Also the road in front of the house as two 18 inch plastic pipes and they are also shallow at only 6-8 inches of earth and tar/gravel road above them and they handle the semi's, Large tractors, and everything else that uses the road with no problem. The pipe comes in twenty foot lengths which is plenty wide for a crossing and I think I paid around $75 dollars for it. Lowes has them. Got mine from a local hardware farm and country store. Don't even look at the old steel pipes. Even the DOT has switched to plastic.