Frequently Asked Questions

Why is it important to design concrete pipe to the 0.01-inch crack?

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 presenceof a 0.01-inch crack does not represent failure, but rather an indication that the concrete and reinforcement are working together, as intended.

Why can't concrete pipe and thermoplastic pipe and corrugated steel pipe be installed the same way?

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 infastructure.

What's the tightest radius I can make with RCP?

Joints are manufactured with various geometries and tolerances.  Please see the 'Design Information', under 'Learn With Us ' and open the Joint Pull and Deflection tables.

What is the service life of concrete pipe?

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.

What is the minimum fill height I can bury a concrete pipe?

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 [u]ACPA Fill Height Tables[/u]. For other design load considerations, [u]ACPA Design Data #1[/u] "Highway Live Loads on Concrete Pipe," or the ACPA [u]"Concrete Pipe Design Manual"[/u] can be consulted for design assistance.

What is the difference in Direct and Indirect Design for RCP?

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. 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.

What is the difference between service life and design life?

The National Cooperative Highway Research Program Synthesis of Highway Practice titled "[i]Durability of Drainage Pipe" [/i]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.

What is the difference between a trench and embankment installation?

Concrete Pipe can be installed in either a trench or embankment condition. The type of installation has a 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.

What is a D-Load?

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.

What do the classes of pipe represent?

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.

Class0.01 inch Crack D-load (lbs/ft/ft)Ultimate D-load (lbs/ft/ft)
I8001200
II10001500
III13502000
IV20003000
V30003750

How much of a gap am I allowed between joints?

Joints are manufactured with various geometries and tolerances, please see our joint deflections tables located in the "Learn With Us" area of this website.

How does one arrange for a plant tour?

Plant tours are available during our production season, for small parties or large delegations. Contact your Cretex Sales Representative for additional information.

How do I calculate the required class of pipe required for my project?

You can either use the fill height tables located in our "Learn With Us" area to look up the required class of pipe, click on our 3EB link to use a pipe class program or you may download Chapter 4 of the ACPA Concrete Pipe Design Manual for a thorough explanation of how to calculate the required class of pipe.

Does Concrete Pipe qualify for LEED credits?

Concrete pipe is suitable for LEED projects and it fits sustainable development. Unlike thermoplastic pipe, concrete pipe 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.