Supporting the Load

I confess I love reinforced concrete. It’s one of those frequently overlooked small miracles that are fundamental to modern living. Without it, our city skylines would be unremarkable. Our whole urban environment would be flatter, highway traffic would run slower and bridges would be shorter. Without underground concrete pipes, we wouldn’t have ready access to clean drinking water, or the safe, timely disposal of stormwater or sewage.

Concrete has been around for thousands of years. The Egyptians used a primitive form of it when building the pyramids. In 300 BCE, the Romans used a more advanced concrete in their buildings, several of which still exist today. The Romans even learned to make concrete that would set underwater, and used it in the construction of viaducts and more. Portland cement was developed in the 19th century and is used today as one of the three main ingredients in concrete, the other two being water and sand or similar aggregate.

The discovery of reinforced concrete came about in France in the middle of the 19th century. Its use in building construction caught on quickly, and it wasn’t long before American engineers started using it too. In 1903, in Cincinnati, a skyscraper was built using the newfangled technology of reinforcing concrete with twisted steel bars. It was considered to be a daring, almost foolhardy engineering feat. The 16-story Ingalls Building is still in use today, testimony to the strength of reinforced concrete.

In 1910, a viaduct built by the Delaware, Lackawanna and Western Railroad became the largest reinforced concrete structure in the world. The 1,100-foot Paulinskill Viaduct still stands today, and right now there are plans to restore service for commuters between Lake Hopatcong and Andover, N.J. with the viaduct as a feature.

Things were happening underground, too. The first concrete sewer pipe was installed in 1842 at Mohawk, N.Y. Amazingly, 175 years later, it’s still in use. I wonder what the total lifecycle costs of that system works out to be?

So what makes reinforced concrete so strong? At the most basic level, it’s the compressive strength of concrete matched with the tensile strength of the reinforcing steel that allows reinforced concrete to withstand the stresses of loads.

Integral strength

How about reinforced concrete pipe (RCP), then? Flexible plastic and corrugated metal pipe simply function as a kind of liner that conveys a fluid. The overall structural strength is created by the bedding envelope material that surrounds the flexible pipe. This is built in the field. The RCP pipe is a structure in its own right. The difference may seem trivial, but in fact it’s crucial.

When pipe that isn’t a structure in its own right is installed, it becomes the responsibility of the engineer to select the appropriate drainage product and of the installer to construct the haunch structure — the structural bedding envelope — and the trench soil to create the strength of the overall installation. Engineers must carefully evaluate the correct soil weights, and take steps to ensure that everything complies with the manufacturer installation guides.

When installing RCP, the embedment material is in no way as critical as it is in the case of a flexible pipe such as High Density Polyethylene (HDPE), Polypropylene (PP), Polyvinyl Chloride (PVC), and Corrugated Metal Pipe (CMP). In fact, RCP supports as much as 90% of both the dead and live loads due to its inherent structure manufactured at the plant.

So in the case of a pipe that’s a structure — a reinforced concrete structure like RCP, for example — the majority of the load-carrying capacity of the system is found in the pipe itself. In the case of a flexible pipe, the majority of the strength is found in the structural bedding material, which requires extensive hand labor and costly aggregates to be constructed. As much as 90% of the installation is dependent on the structural bedding envelope that’s installed around the pipe and on its ability to develop reactionary pressures around the pipe, enabling it to support the load.   

How does this impact costs?

If we make the mistake of comparing the pipe material costs of a length of RCP with the pipe material costs of the equivalent length of a flexible pipe (HDPE, PP, PVC, or CMP), then it appears that the RCP is more expensive. However, if we factor in the actual cost of installation, including using different installation procedures and constructing the structural bedding envelope material, then the picture changes. Add to this the lifecycle costs, and it becomes even more clear.

The invention of reinforced concrete changed the way we create our living and working structures, build our roads and bridges, transport our clean water, and remove our sewage and storm water. Many of the buildings, bridges and viaducts that were created more than a century ago are still standing today. Concrete pipes that were installed many decades ago still carry our water and wastewater. These structures are all monuments to the strength and longevity of the technology.

In my world, which is that of underground wet utilities, RCP has a proven track record of being a strong, reliable, long-lasting option. When we factor in all the costs, including design life, it’s also the economical choice.

Next time you look at a skyscraper, or drive over a concrete flyover or bridge, think about the innovation that made it all possible. Next time you open a faucet or flush a toilet, give a thought to the unseen network of concrete pipes that makes it possible for you to take such modern conveniences for granted. 

Until next time,

Branimir Kovac, Vice President, Thompson Pipe Group