Design
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Q:
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How do I calculate the required class of pipe required for my project? |
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You can either use the ACPA Fill Height Tables to look up the required class of pipe or download Chapter 4 of the Concrete Pipe Design Manual for a thorough explanation of how to calculate the required class of pipe. |
| Q: |
What do the classes of pipe represent? |
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A:
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To make concrete pipe more readily available; rather than produce the pipe to the specific D-load required for every job, precast concrete pipe is often specified in terms of a generalized class system. The classes of pipe represent the minimum D-load capacity a pipe produced to that class must have. The classes are designated in ASTM C 76, or AASHTO M 170. The required D-load capacity per pipe is as follows.
| Class |
0.01 inch Crack D-load (lbs/ft/ft) |
Ultimate D-load (lbs/ft/ft) |
I
II
III
IV
V |
800
1000
1350
2000
3000 |
1200
1500
2000
3000
3750 |
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Q:
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What is a D-Load? |
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A:
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The supporting strength of a pipe loaded under three-edge-bearing test conditions expressed in pounds per linear foot of inside diameter or horizontal span. |
| Q: |
What are standard installations and which type would I use for my project? |
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A:
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The four Standard Installations provide an optimum range of soil-pipe interaction characteristics. As a designer you can choose from a Type 1 Installation that requires high quality backfill material and compaction levels coupled with a lower strength pipe to a Type 4 Installation utilizing a lower strength pipe because it was developed for conditions with little or no control over the fill materials or compaction. A Type 1 installation requires greater soil stiffness from the surrounding soils than the Type 2, 3, and 4 installations, and is thus harder to achieve. Therefore, field verification of soil properties and compaction levels should be performed.
Soil and Minimum Compactions are shown below:
The type of Installation you choose will be based on a combination of factors such as available backfill materials, depth of fill, and required class of pipe. The American Concrete Pipe Association has PipePac software that can be downloaded free of charge by filling out the registration form at the bottom of the page. This software will help you analyze different Installation Types to help you decide on the most economical choice. |
| Q: |
What is the difference in Direct and Indirect Design for RCP? |
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A:
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Indirect Design is the comparison of the structural strength of the pipe found in the Three-Edge Bearing test to the field supporting strength of a buried pipe. More information on Indirect Design can be found in the Standard Installation brochure.
The Direct Design is the design of the pipe in the installed condition. The magnitude and distribution of the loads are determined, and the physical properties necessary to support those loads are calculated. |
| Q: |
When should I use Direct Design vs. Indirect Design for concrete pipe? |
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Indirect Design is the standard method for designing reinforced concrete pipe. It is a simplified method that corresponds to a pipe produced to a performance specification, whereby it is tested at the plant to verify its strength. When a concrete pipe cannot be tested to verify its strength in the plant, than the pipe structure must be designed similar to any other concrete structure using a direct design method that incorporates load factors and process factors into the design. If an engineer comes across a D-load of pipe that cannot be tested in the three-edge bearing test, either because the producer can not apply sufficient load, or the pipe is too large to fit within the three-edge bearing test apparatus, then the engineer might want utilize the direct design method to design the pipe. Small diameter pipe should not be designed using the Direct Design Method due to the fact that the equations for direct design were originally formulated for larger diameters and therefore are overly conservative for the design of small diameter concrete pipe. |
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Q:
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What is the minimum fill height I can bury a concrete pipe? |
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The minimum fill height is a function of both the load being applied at the surface above the pipe, and the strength of the class of pipe provided. Since concrete pipe is a composite of concrete and steel, you can reduce your fill height as low as you like, provided you design the pipe to sustain the applied loads. In some cases where extremely heavy machinery will be traveling over the pipe, you may have to utilize a concrete pipe with strength above a Class V pipe, the highest class of pipe denoted in ASTM C 76.AASHTO M 170. This can be accomplished by working with your local producer. However, in most cases where an AASHTO HL-93 highway load is applied, and the fill height is equal to or greater than 1 foot of cover, a standard Class III pipe or greater will suffice. For standard HL-93 highway loads, the required D-load at fill height increments of 1 foot can be found in the ACPA Fill Height Tables. For other design load considerations, ACPA Design Data #1 “Highway Live Loads on Concrete Pipe”, or the ACPA “Concrete Pipe Design Manual” can be consulted for design assistance. |
| Q: |
What is the difference between a trench and embankment installation? |
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A:
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Concrete Pipe can be installed in either a trench or embankment condition. The type of installation has significant effect on the load carried by the rigid pipe.
- Trench: When concrete pipe is installed in a relatively narrow trench, settlement between the backfill material and the undisturbed soil in which the trench is excavated generates upward frictional forces which effects a load transfer. This load transfer helps support the backfill material within the trench and results in less load on the pipe than the weight of the prism of backfill material over the pipe.
- Embankment: In this condition the soil along the pipe wall will settle more than the soil directly above the pipe. This additional load is accounted for by using a Vertical Arching Factor for the Indirect Design Method.
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| Q: |
What is the maximum flow velocity I can design RCP without cavitation? |
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Velocity, by itself, does not create problems for concrete pipe within the ranges normally encountered. At velocities up to 40 feet per second, the severity of velocity-abrasion effects depends upon the characteristics of the bed load. |
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Q:
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Does the American Concrete Pipe Association provide any guidance on designing precast box culverts? |
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Yes. The American Concrete Pipe Association has developed design notes for the design of precast box culverts in accordance with AASHTO. These design notes can be found on our website: www.concretepipe.com, or requested by e-mail from the staff. |
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Q:
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How do I size the culvert required for my project? |
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Selection of the correct value for the coefficient of roughness of a pipe (Manning’s “n”) is essential in evaluating the flow through culverts and sewers. Selection of an excessive n value leads to an uneconomical design due to oversizing of the pipe, while an insufficient value results in an hydraulically inadequate sewer system. More information on calculating the hydraulics in a pipeline can be found in Design Data #11. |
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Q:
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Does Concrete Pipe qualify for LEED credits? |
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A:
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Concrete pipe is suitable for LEED projects and it fits sustainable development. Unlike thermoplastic 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. |
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Q:
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Why is it important to design concrete pipe to the 0.01-inch crack? |
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A:
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Reinforced concrete pipe, like other reinforced concrete structures, is designed to crack. It is well known that while concrete is very strong in compression, its tensile strength is so low that it is considered negligible in design. Therefore, RCP design accommodates the high compressive strength of concrete and the high tensile strength of steel. As load on the pipe increases, and the tensile strength of the concrete is exceeded, cracks will form as the tensile load is transferred to the steel. Typically, the cracks form a V-shape with the largest part of the crack at the surface. The presence of a 0.01-inch crack does not represent failure, but rather an indication that the concrete and reinforcement are working together, as intended. |
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Q:
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What is the difference between service life and design life? |
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The National Cooperative Highway Research Program Synthesis of Highway Practice titled “Durability of Drainage Pipe” defines service life by the number of years of relatively maintenance-free performance. Webster’s Online Dictionary defines design life as the life expectancy of an item to work within its specified parameters. |
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Q:
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What is the service life of concrete pipe? |
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The Army Corps of Engineers recommends a design life of 70-100 years for precast concrete pipe, and there are countless examples of installations that surpass those numbers. This means the expectation for precast concrete’s functional life is at least twice as long as alternate materials. The reasons for this go far beyond concrete’s innate strength. Concrete also won’t burn, rust, tear, buckle, deflect, and it’s immune to the attack of most elements, whether the pipe is buried or exposed. Quality concrete pipe densities typically range from 145-155 pounds per cubic foot. Usually, the higher the density, the greater the durability of the concrete pipe. |
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Manufacturing
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| Q: |
As an owner, how can I be sure that I’m buying quality concrete pipe? |
| A: |
Our QCast program outlines guidelines for quality production and testing of concrete pipe. |
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Installation
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| Q: |
How much of a gap am I allowed between joints? |
| A: |
Joints are manufactured with various geometries and tolerances, and therefore the best way to determine an acceptable joint gap is to contact the manufacturer. |
| Q: |
What’s the tightest radius I can make with RCP?
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| A: |
Joints are manufactured with various geometries and tolerances, and therefore the best way to determine the radius you can turn is to contact the manufacturer.
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| Q: |
Why can’t concrete pipe and thermoplastic pipe and corrugated steel pipe be installed the same way? |
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Concrete pipe is a rigid structure and flexible pipe (thermoplastic and corrugated steel tubes) is no more than a conduit that requires the structure to be built in the field from imported material to hold the conduit in place and support loads. For rigid pipe and flexible conduits to meet the design life of a project, the bedding and backfill is designed differently for both types of infrastructure. |
| Q: |
What needs to be considered when transporting and unloading concrete pipe? |
| A: |
Each shipment of pipe is loaded, blocked and tied down at the plant to avoid damage during transit. However, it is up to the receiver to make certain, damage has not occurred in delivery from the plant to the construction site. ACPA has published a concrete pipe and box installation manual to guide contractors in the onsite handling and installation of concrete pipe. |
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Specification
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| Q: |
Where can I find standards pertaining to the manufacture, installation, and testing of concrete pipe and box culverts? |
| A: |
In most cases the manufacture of concrete pipe and box culverts, and their installation and testing requirements are either governed by ASTM Standards or relatively equivalent AASHTO Standards. To make specifying concrete pipe easier, the American Concrete Pipe Association has compiled all the relevant ASTM standards into one manual, “ACPA’s Annual Book of Selected ASTM Standards”, which can be found on our Resources web page. |
| Q: |
What is the ASTM and AASHTO Standard for Reinforced Concrete Pipe? |
| A: |
- The ASTM Standard for Indirect Design is: ASTM C76 – 11 Standard Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe.
- The ASTM Standard for Direct Design is: ASTM C1417 – 08 Standard Specification for Manufacture of Reinforced Concrete Sewer, Storm Drain, and Culvert Pipe for Direct Design.
- The AASHTO Standard for Indirect Design is: AASHTO M 170-09 Standard Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe.
- The AASHTO Standard for Direct Design is: AASHTO Section 12 of the LRFD Code provides provisions to perform a direct design of concrete pipe.
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| Q: |
What is the ASTM and AASHTO Standard for Precast Concrete Box Sections? |
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- The ASTM Standard for the Standard Design Code is: ASTM C 1433 Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers
- The ASTM Standard for the LRFD Design Code is: ASTM C 1577 Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers According to AASHTO LRFD
- The AASHTO Standards are: AASHTO M 259 Precast Reinforced Concrete Box Sections for Culverts, Storm Drains and Sewers, and AASHTO M 273 Precast Reinforced Box Section for Culverts, Storm Drains, and Sewers with less than 2 feet of Cover Subject to Highway Loadings. No AASHTO LRFD Design Code exists. In the mentioned two standards you will find a note that states: “If load–and-resistance factor design is required, then use ASTM C 1577.”
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Inspection
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| Q: |
What is an acceptable crack width in a concrete pipe? |
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For a full explanation on inspection see the ACPA “Post Installation Evaluation and Repair of Installed Reinforced Concrete Pipe”. Longitudinal Cracks–Concrete is strong in compression but weak in tension. Reinforced steel is provided to handle the tensile stresses. Hairline longitudinal crack in the crown or invert indicate that the steel has accepted part of the load. Cracks less than 0.01 inches in width are minor and only need to be noted in the inspection report. Cracks greater than hairline cracks, or those more than 0.01 inch in width but less than 0.1 inches, should be described in the inspection report and noted as possible candidates for maintenance. Longitudinal cracking in excess of 0.1 inch in width may indicate overloading or poor bedding. More information on crack widths can be found in this CP Info. |
| Q: |
What is Autogenous Healing and its relationship to pipe design? |
| A: |
A phenomenon, known as autogenous healing often occurs between two surfaces of cracks in buried pipe. Autogenous healing is the ability of concrete to heal itself in the presence of moisture and air. This explains why the healing occurs in concrete pipe where moist conditions are higher than those of other concrete structures. During this process, calcium carbonate, (a hard white substance), forms when moisture reacts with unhydrated cement powder and regenerates the curing process. This self-healing process creates a monolithic structure. In Ohio, the Department of Transportation developed a post construction inspection standard for installed pipe that requires nothing be done to a pipe with a crack width up to 0.06-inch, due to the autogenous healing that is expected to occur. |
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Concrete Pipe vs. Alternative Materials
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| Q: |
What is the difference between concrete pipe and flexible pipe? |
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- Concrete pipe is a rigid pipe that provides both structure and conduit when it arrives on site. Concrete pipe is a rigid pipe system that is over 85% dependent on the pipe strength and only 15% dependent on the strength derived from the soil envelope. The inherent strength of concrete pipe compensates for construction shortcomings and higher fill heights and trench depths.
- Flexible pipe is at least 95% dependent on soil support and the installation expertise of the contractor. Backfill must be properly engineered and applied to provide structure. Imported fill is usually required for flexible pipe systems.
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| Q: |
How does the hydraulics of concrete pipe compare to the hydraulics of flexible pipe? |
| A: |
American Concrete Pipe Association recommended values of Manning’s “n” :
- 0.012 for a concrete pipe
- 0.012 to 0.024 for corrugated HDPE. It is recommended to use the higher range value to account for corrugation growth. More information on corrugation growth can be found in the University of Texas at Arlington’s study on the performance of HDPE: http://www.uta.edu/ce/aareports2.php
- 0.011 to 0.013 for PVC solid wall
- 0.029 to 0.034 corrugated steel pipe
- 0.016 to 0.018 for spiral rib steel pipe
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| Q: |
Are joints from alternate materials equivalent in performance to concrete pipe joints? |
| A: |
The use of a rubber gasket does not by itself ensure that different joint types are equal. Designers can utilize ASTM Standards to specify for desired performance but in the case of alternate materials, additional guidance may be required. Concrete pipe joints are governed, in national standards, by better, more detailed designs with tighter tolerances and higher test pressures. Additionally, the project owner benefits from the concrete pipe joints’ inherent strength and rigid pipe design to enhance line and grade and assurance against deflection and buckling. |
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Contacting a Concrete Pipe Producer
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| Q: |
How do you locate a concrete pipe producer? |
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Go to www.concretepipe.com – Association tab – Contact a Producer, to find the location of a concrete pipe plant. Producer member lists are complete with geographic locators. |
| Q: |
How can I learn more about concrete pipe, boxes and alternate products? |
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The American Concrete Pipe Association has a network of Regional Engineers who are connected with representatives of state concrete pipe associations and Technical Resource Engineers. These representatives are available to present information to groups on a wide variety of design and quality control topics.
There are also education events throughout the year to share knowledge with people working in the buried infrastructure industry. You find these on our website and under the Education tab. |
| Q: |
How does one arrange for a plant tour? |
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Plant tours are available year round for small parties or large delegations. Contact the American Concrete
Pipe Association, or a local concrete pipe producer to arrange for a tour and information sessions. |
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