Questions from the Field is a selection of questions the NPCA Technical Services Department receives from calls, emails and comments on blogs or magazine articles on If you have a technical question that needs an answer, contact us by calling (800) 366-7731 or visit

Clinton writes:

Do you have a chart showing wall thickness for reinforced concrete pipe by size and class?

Technical Services Department answers:

ASTM C76, the manufacturing and purchasing standard for reinforced concrete pipe, provides a variety of tables illustrating rcp characteristics based on class and wall thickness.

Examples of RCP wall thickness rules to consider:

“A” Wall – the wall thickness in inches is the diameter in inches converted to diameter in feet. For example, a 48-inch diameter pipe is 4 feet. “A” wall thickness is 4 inches.

“B” wall – the wall thickness is an “A” wall plus 1 inch. For example, a 60-inch diameter pipe is 5 feet. “A” wall thickness is 5 inches meaning the “B” wall thickness is 6 inches.

“C” Wall – the wall thickness is an “A” wall plus 1.75 inches. For example, a 96-inch diameter pipe is 8 feet. “A” wall thickness is 8 inches meaning the “C” wall thickness is 9.75 inches.

Other points to consider:

Manufacturers provide options for the wall thickness of the pipe to be furnished, while designers require the strength (pipe class and D-load).

The D-load strength of a pipe or corresponding pipe class (III, IV, V) can be obtained by any of the wall thicknesses furnished. For example, a 60-inch class IV “B” wall is not necessarily weaker than a 60-inch class IV “C” wall. Both meet the required class IV strength. However, the thinner “B” wall will require more steel reinforcement than the “C” wall.

For projects using rubber connectors for pipe and manhole junctions, the precast pipe manufacturer needs to clarify what pipe wall is being proposed, so the appropriate rubber boot is used.

“A” wall pipe forms, if owned, are only used for large-diameter pipe or manholes, such as 96 inches and above. “B” wall forms are the most common wall thickness used for RCP. Many companies do have the equipment to produce “C” wall, but will analyze the costs involved. It’s always good to verify what is available within your project region.

Another great reference chart is available in the American Concrete Pipe Association’s Concrete Pipe Design Manual, Chapter 5, “Supplemental Data.”

reinforced concrete pipe

Mohammad writes:

For concrete cylinders and cubes, should a seven-day break be taken from the date the sample was made or the next day? For example, If I make a sample on the 10th, do I break on the 16th or 17th?

Technical Services Department answers:

Testing is done after seven 24-hour periods from the date you cast the cylinder or cube. Therefore, in your example, testing needs to be completed on the 17th since the test specimen was fabricated on the 10th. ASTM C39 and C39M, Section 7.3, “Procedure,” provide allowable tolerances to the time to conduct the test. For seven days, the standard states +/- 6 hours or 3.6%.

It should be noted if your sample is made from concrete cores cut from cast concrete, all the required date information of the sample is stated within ASTM C42 and C42M, Section 7.10, “Cores for Compressive Strength.” However, for a 14-day old core, it is implied testing is completed from when the concrete was cast and not from the date the sample was taken.

Guillermo writes:

I would like to know if committees addressed design method questions for special considerations such as high live-load conditions (railroad or airport), deep-bury reducer cones, and load and resistance factor design analysis. I need to design a 48-inch manhole and cone for an airport aircraft.

Technical Services Department answers:

The straightforward answer is no product committee has specifically reviewed those design issues. However, for special loading conditions on flattops, the governing code may include portions of ACI 318, “Building Code Requirements for Structural Concrete” and/or ASTM C478, “Standard Specification for Circular Precast Reinforced Concrete Manhole Sections.” ASTM C478 does make reference to ACI design codes for flattops and base sections. For department of transportation work, they defer to the AASHTO Bridge Design Manual that uses LRFD methods.

Your question regarding manhole reducer cone analysis is being considered within subcommittee ASTM C13.06 on Manholes and Specials. It first came to NPCA when a member needed to design a reducer cone for a railroad project. The NPCA Manhole Product Committee consulted Gary K. Munkelt & Associates for design and engineering advice. Munkelt said the reducer cone behaves as an arch section depending on the angle or cut. He analyzes the cones as compression sections. More information on the topic is available in the Precast Inc. 2014 March-April article, “Speaking in Cone.”

This may be adequate for shallow cone reducers which are not shallow or short with respect to diameter reduction. These cones develop an extreme slope angle in excess of 45 degrees (as measured off vertical). There are few instances reported, but not confirmed, of cracking within deep bury reducer cones, so it is suggested to treat them like a flat slab analysis.

Regarding your specific question on designing a 48-inch manhole with aircraft loading, the cone is shallow and has a standard size. It would be logical to analyze it as a compression section. However, if it is on a vertical wall, the compressive strength of the manhole section and wall thickness will be adequate. For the angled cone section, the resultant force on the arch shape will also be adequate.