Why Concrete Pipe?
Reasons to Choose Concrete Pipe
Precast Concrete pipe is the strongest pipe available and can be designed and tested to meet any loading condition. Unlike flexible pipe, concrete pipe has the majority of the required strength built into the pipe, and is much less dependent upon the installation of the pipe. Concrete pipe is manufactured per ASTM C-76 and AASHTO M 170 in strength classes ranging from Class I to Class V. These strengths are verified by a 3 Edge Bearing or D-Load test performed in the plant.
In today’s economic environment, designing for long term, sustainable project performance is imperative for the Engineer. Unlike some alternate pipe materials, concrete pipe has a proven track record of performance. Concrete pipe will not rust, burn, tear, buckle or deflect, and is immune to most environmental elements. The United States Army Corps of Engineers has recommended precast concrete pipe for a design life of 70-100 years, and there are numerous examples of installations that have exceeded these parameters.
A dependable product not only has to perform well, it must be completely understood and have the confidence of those who specify and use the product. Understanding how a product performs, being able to anticipate and prevent potential problems, and having experienced installation crews are vital to ensuring a well designed and constructed project. No pipe material on the market is better understood and more frequently used and depended upon more than reinforced concrete pipe. The proven record of use, and the continuous research and development of the concrete pipe industry have instilled confidence in the Engineering and Construction fields for many decades.
All drainage pipe serves one common purpose; to act as a conduit to move a certain amount of storm water from one point to another. This conduit, in conjunction with the surrounding soil, provides the structure the installation must provide. When using reinforced concrete pipe, in addition to the conduit, you get the majority of the structure necessary to support whatever type of load will be generated on top of the conduit. Unlike flexible pipe, where up to 95% of the structure must be painstakingly designed and installed in the field, up to 85% of the design strength of the installation is provided by the concrete pipe.
Concrete pipe will not burn. Thermoplastic pipe is highly flammable. TxDOT issued a directive in 2009 inn response to the Texas wildfires. This required any flammable pipe material to use varying lengths of non-flammable end treatments.
Buried infrastructure is an asset that is financed and maintained by the public or private entities that invest taxpayer or shareholder dollars into the project.
All of the costs of initial design, construction, material cost, inspection, and maintenance make up the total cost or “value” of an asset. Any asset that requires additional investment for repair, maintenance and/or replacement to maintain its’ value over the life of the asset actually adds to the owner’s cost of the asset.
In order to make the proper initial choice of materials, the owner must truly understand the asset, and the value or cost of the asset over the design life. Design, installation, and inspection costs of flexible pipe are greater than comparable concrete pipe costs. The asset value of concrete pipe will not drop over the life of the project.
Nearly every major or mid-size municipality has a concrete pipe manufacturer nearby. This allows for shorter delivery time and greater availability of product. It also means that the local manufacturing facilities are making product designed to local standards. If an issue arises, there is usually engineering and support personnel readily available. These manufacturers also employ a large number of local citizens and contribute to the tax base, thus supporting your local economy.
A least cost analysis is an effective method of evaluating two alternative materials with different service lives or economic equivalence. The factors which affect the traditional analysis are project design life, material life, first cost, interest rate, inflation rate, replacement costs, and residual value. First cost is important to the engineer and owner, but does not reveal the entire cost of the pipeline. Least cost analysis should also consider costs to the traveling public and businesses due to detours and replacement of potential catastrophic failures.
Flexible pipe products may have lower initial costs, but they are not as cost-effective as concrete pipe. Over the long term, flexible pipe has a shorter service life, and requires premium bedding and backfill. Installation procedures have to be precise for the bedding and backfill to take on the required structural characteristics. During and after installation, inspection of flexible pipe systems is critical to performance, and mandrel or laser/video testing is mandatory in many jurisdictions. In general, the true cost (installation, maintenance and replacement) of flexible pipe can be twice that of concrete based on a 50-year or greater service life.
When flexible products are specified correctly, reinforced concrete pipe can compete favorably at the same or lower cost! Concrete pipe is the strongest drainage product available, the most hydraulically efficient, and has great current and future value as an infrastructure asset.
Concrete pipe far out performs plastic or metal. Concrete’s rigidity and mass allow for easy and secure placement in the ditch, without disrupting line or grade. Plus, precast concrete pipe joints are easily assembled, which helps minimize the time needs for installation. When installation time matters, or when the soil poses challenges to installation, precast concrete pipe is quite simply the most logical and responsible option.
Since Concrete pipe is a rigid pipe system that is over 85% dependent on the pipe 7strength and only 15% dependent on the strength derived from the soil envelope, installation is made easy. In many situations, the installation of plastic or metal pipes can take longer than precast concrete pipe. That’s because the structural and hydraulic integrity of flexible pipes rely heavily on how well you prep the surrounding soil at installation, rather than on their own inherent brute strength. Making sure all conditions are right and installing per national specifications can be a costly and time-consuming proposition when installing flexible pipe.
Concrete pipe has an unlimited range of pipe strengths from which to choose, and strength is demonstrated prior to installation. By specifying concrete pipe:
- The designer has more control over pipe strength than any other facet of the project
- There is less reliance on quality installation by the installer
- There is lower embedment material cost
- There is less compaction required
- It is easier to maintain grade and alignment
- There are no excess deflection concerns
- There is a lower life cycle cost of the project
- There is a lower maintenance cost over the design life of the project
- There is a reduced likelihood of failure
- A lower risk for the specifier, designer and owner of the project, and reduced overall liability to the public after the project has been commissioned
Standard Installations is a term for a technology used for precast concrete pipe beddings. Design of the pipe wall – its thickness and amount of reinforcement – is based on the stresses and strains in the pipe. This approach is more precise and can result in pipes that require less material. In addition, the standard installations approach permits greater choice of backfill materials, from granular materials to clay, and needs less compaction of the backfill.
Standard Installations were adopted by the American Society for Civil Engineers (ASCE) as Specification 15-93-Standard Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations. It was adopted later in the 1996 (16th) Edition of the American Association of State Highway and Transportation Officials (AASHTO) Standard Specification for Highway Bridges, Section 17, Soil-Reinforced Concrete Structure Interaction Systems.
Standard Installations provide several benefits when using concrete pipe:
- Provides flexibility to meet design requirements and site conditions
- Allows for narrower excavation limits
- Less expensive backfill materials may be used
- Can reduce the level of compaction
- Increases contractor productivity in installing reinforced concrete pipe
There is a choice of Types of Standard Installations that provide versatility to adapt to field conditions.
- Type 1: Highest Quality installation using select granular soils with high compaction requirements for haunching and bedding.
- Type 2: Allows silty granular soils with less compaction required for haunching and bedding.
- Type 3: Allows use of soils with less stringent compaction requirements for haunching and bedding.
- Type 4: Allows use of onsite native material for haunching and bedding with no compaction required. (6 inches of bedding is required if rock foundation)
The short lengths of concrete pipe make it easier to work with around existing municipal services. Concrete pipe installations using trench boxes do not require special attention when sliding the trench box. Disturbing the bedding and backfill in the process of moving the trench box is referenced by all installation standards and recommendations of manufacturers. Using standard lengths of concrete pipe, line and grade can be checked frequently for accuracy.
Some projects have design elements that are a little more complex or intricate than others. Precast concrete pipe provides solutions for these projects, whether they are open-cut, deep burials, tunnels, trenchless, shallow burials, or with vertical structures or complex alignment changes. Concrete pipe design is simple to do; the math is sound and easily definable.
Precast concrete pipe gives you strength and flexibility to ensure the success of your most demanding applications. Pipes are manufactured with a variety of sizes, shapes, joints and seal options. There is also an array of linings and coatings that can handle the most aggressive environment.
The main attributes of concrete pipe apply to sanitary, storm sewers and culverts. Many attributes also may be applied to box sections used for storm drainage, roadway culverts, tunnels, bridges, and underground detention systems. Concrete pipe and box sections accommodate great volumes of effluent in a tiny footprint.
Concrete pipe produced in the early twenty-first century is a consequence of
- Computer aided design and analysis
- Advanced concrete mix designs
- Automated and computer controlled batching
- Precision fabricated wire reinforcement
- Quality driven manufacturing techniques
- Improved water tight joints
- New installation standards
Precast concrete box sections also have similar advantages to concrete pipe.
- Better quality control than flexible pipe products
- Ease of installation
- The dangers associated with open trenches are reduced
- Reduced environmental impacts
- Detour time is reduced
- Design time is reduced
- Just-in-time delivery is available from producers’ plants to accommodate small construction sites and tight construction schedules
- Crews familiar with concrete pipe installation procedures can install box sections with minimal training
Concrete pipe offers a variety of joints from soil-tight to pressure. They are not affected by the type of backfill used for the installation. Joint performance must be demonstrated in the plant prior to pipe installation, and joint integrity can be field tested in a variety of ways. With concrete pipe, deflection will not compromise field joint test capability. The cross sectional rigidity of concrete pipe makes joint assembly a simple operation. Rigid joint integrity will minimize the likelihood of embedment intrusion and subsidence of overfill, often referred as infiltration.
Gasketed, leak-resistant RCP joints withstand a minimum hydrostatic internal head of 13 psi equal to 30 feet of water. (ASTM C 443 or C 1628)
Types of concrete pipe joints include:
- O-Ring Gaskets.
- Profile Gaskets.
- Mortar and Mastic Joints.
O Ring gaskets are used on all sanitary and some storm RCP where leak-resistant joints are required. These gaskets may be used in joints following ASTM designations, C 443, C 1628, or C 361 for low-head pressure applications.
Profile gaskets are used on stormwater culverts and RCP storm and sanitary sewers. Pipe is produced with a single offset spigot joint according to ASTM designation C 443 or C 1628.
Mortar or mastic joints are used for storm sewers, culverts, and horizontal elliptical reinforced concrete pipe. Mortar or mastic is applied to the bottom half of the bell end and to the top half of the adjoining spigot.
Mastic and Butyl sealants are applied in accordance with ASTM designation C 990
In some applications, a quality joint may be a wrap applied to the external surface of the joint. These may be specified in accordance with ASTM C 877.
In a low lying or marshy environment, the buoyancy of buried pipelines depends on the mass of the pipe material, the weight of the volume of water displaced by the pipe, the weight of the liquid load carried by the pipe, and the weight of the backfill material. Whenever the water table level is above the invert of the pipeline, the potential for floatation or buoyancy exists. Although the trench for a pipe installation in a marshy area is dewatered, the trench area downstream (after initial backfill) may become saturated. This would lead to a buoyant effect on the pipe. The mass of the concrete pipe typically counteracts this buoyant force. Alternate materials such as thermoplastic pipe and corrugated metal pipe may heave vertically or snake horizontally in wetland conditions. During the backfill operation, the fill may accumulate more on one side of the pipe than the other. The mass of the concrete pipe resists lateral forces, and the structure remains true to line and grade.
The mass of concrete pipe allows for:
- Effective compaction of embedment and backfill
- Prevention of movement during backfilling ensuring adherence to design grade and alignment
- Unlikely movement of structure following installation
- Reduced likelihood of floatation
- Reduced possibility of damage during subsequent construction or maintenance in phased projects
The key to long-term performance and efficiency lies in a material’s ability to retain its original shape and alignment. Precast concrete pipe’s rigidity and mass allow it to greatly outperform flexible pipe systems in this critical area, which in turn helps to improve hydraulic efficiency by minimizing the resistance to water flow that often occurs when the shape or integrity of a flexible pipe is compromised.
The hydraulic capacity (the amount of water a pipe can convey) of all types of pipe depends on the smoothness of the interior pipe wall. The smoother the wall, the greater is the hydraulic capacity of the pipe. Smoothness of pipe is represented by Manning’s Roughness Coefficient commonly called Manning’s “n.” The lower the Manning’s “n” value, the greater is the volume of water that will flow through pipe.
Hydraulic analysis for drainage systems involves the estimation of the design flow rate based on climatological and watershed characteristics. The hydraulic design of a drainage system always includes an economic evaluation. A wide spectrum of flood flows with associated probabilities will occur at the site during its design life. The benefits of constructing a large capacity system to accommodate all of these storm events with no detrimental flooding effects are normally outweighed by the initial construction costs. An economic analysis of the tradeoffs is performed with varying degrees of effort and thoroughness. Risk analysis balances the drainage system cost with the damages associated with inadequate performance. With concrete pipe, there is no risk. With its long service life and hydraulic efficiency, concrete pipe handles the requirements of a system’s hydraulic design.
Two basic values are often cited when discussing the coefficient of roughness of a pipe: laboratory test values and design values. The difference between laboratory test values of Manning’s ‘n‘ and accepted design values is significant. Manning’s “n” values were obtained using clean water, smooth joints, no loads, and straight pipe lengths without bends, manholes, debris, or other obstructions. The laboratory results indicate only the differences between smooth wall and rough wall pipes. Rough wall, such as unlined corrugated metal pipe has relatively high “n” values, which are approximately 2.5 to 3 times those of smooth wall pipe.
Smooth wall pipes were found to have “n” values ranging between 0.009 and 0.010, but historically, engineers familiar with concrete pipe and sewers have used 0.012 or 0.013. This design factor of 20 to 30 percent takes into account the differences between laboratory testing and actual installed conditions of various sizes as well as allowing for a factor of safety. The use of such design factors is good engineering practice, and to be consistent for all pipe materials, the applicable Manning’s ‘n” laboratory value should be increased a similar amount to arrive at comparative design values.
Research has concluded that designs using concrete pipe can be downsized by at least one size in most cases when compared to steel, aluminum, and lined corrugated HDPE pipe. For design engineers and owners to select the proper drainage pipe for a specific culvert or sewer application, it is critically important that the applied Manning’s “n” values are design values rather than laboratory values
Using design values, concrete pipe has superior hydraulic characteristics, and engineers understand and posses proper verification of concrete pipe hydraulics.
Grade and alignment are as important as barrel surface characteristics. In addition, inlet and outlet controls impact the hydraulics of a drainage system. The flow of water in the pipe is throttled or limited by the inlet of the pipe. The inlet may have a headwall, flared end, or protruding pipe. This condition exists in most all cross drains, and typical in subdivisions and county roadway crossings. Outlet control occurs when the flow of water through the pipe is controlled by the conditions at the outlet end of the pipe. Outlet control usually does not exist unless the outlet end of the pipe is under water or if the orifice has been damaged and restricted. The outlets of flexible pipe are easily damaged, thereby affecting the hydraulics of the pipeline.
Batching and mixing operations in the industry’s premier plants have been upgraded over the past 10 years. Characteristics of this operation of the pipe production process normally include:
- Computer controlled weighing and proportioning systems
- Computer controlled mixing systems
- Automated recording systems
- Absorption testing
The American Concrete Pipe Association offers an on-going quality assurance program called the “Quality Cast” Plant Certification Program. ( http://www.concrete-pipe.org/qcast.htm)
This 124-point audit-inspection program covers the inspection of materials, finished products and handling/storage procedures, as well as performance testing and quality control documentation. Plants are certified to provide storm sewer and culvert pipe or under a combined sanitary sewer, storm sewer, and culvert pipe program.
Historically, concrete is the most durable and sustainable material for infrastructure and major construction. It continues to function long after a projects life is reached, by maintaining structural integrity, thus reducing the costs associated with repair and replacement.
Precast concrete pipe’s staying power has another benefit; it’s not a passing fad. When concrete pipe is specified, the projects you build today are more likely to be compatible with any future expansions or alterations.
Precast concrete drainage products have a reputation for strength and durability. They will not burn, corrode prematurely, deflect or move off grade to reduce hydraulic performance, or collapse under loads designed into the pipe structure. Comprised of the world’s most commonly used building materials, precast concrete infrastructure is quickly integrated into ecosystems. This is clearly demonstrated by the use of three-sided precast boxes used to accommodate the natural channels of streams at road crossings, and precast concrete pipe for storm sewers and outfalls in valleys and shorelines.
Today, being recognized as a green material or product is growing in importance to many specifiers. Concrete pipe is suitable for LEED projects and it fits sustainable development.
Unlike plastic pipe, concrete is produced with benign, natural materials. Manufacturing of concrete consumes less energy than plastic fabrication. It’s also recyclable and has little if any environmental impact. And, when you use local resources, concrete can also provide lower fuel cost for delivery.