We have compiled some useful reference material for you from the
American Concrete Pipe Associations "Concrete Pipe Insights". Just
click on the links to learn more about the benefits of concrete pipe.
Important to Ignore
Hydraulics: Check the Comparisons
Concrete or HDPE: Strength versus
Handling and Installation
In-Field Product Performance and
Round and Elliptical Pipe Specifications
Durability: Too Important to Ignore
One of the most critical but overlooked factors in project design is
material durability, or service life. Even such fundamental
considerations as a material's ability to perform intended structural
and hydraulic functions become irrelevant if a pipe cannot perform
satisfactorily for an economically acceptable period.
Laboratory and field data have firmly established that concrete pipe is
the recognized leader in service life among buried pipe products.
Studies and historical data prove a product life of 100 years or more
for concrete pipe.
Despite some manufacturers claims, the fact is that no known material is
completely inert to chemical action or immune to physical deterioration.
Therefore, it is necessary to identify that material which offers the
greatest likelihood of long service life.
Contrary to the implications in polyethylene pipe (PE) promotions,
concrete pipe has an excellent service life record. Sulfuric acid attack
from effluents may occur in some sanitary sewers, but not in storm
sewers. Exterior acid attack has never represented a problem for
concrete pipe. There are design options which can extend the life of
concrete pipe in aggressive conditions.
Polyethylene pipe's service life, however, is time dependent, and the
product experiences creep and stress relaxation (deflection). Although
PE is an inert material when not under stress (load), it is susceptible
to attack from some chemicals while under stress. Examples of these
chemicals are strong oxidizing acids, oils, alcohols, and polar reagents
such as detergents. Because of polyethylene pipe's thin walls, slight
scratches or wear from handling and abrasion can be critical. Concrete
pipe, on the other hand, has greater wall thickness than polyethylene
pipe and is very strong and dense, so scratches, gouges and abrasion are
not a factor in the life of concrete pipe.
Polyethylene pipe is flammable, susceptible to ultraviolet degradation,
and is temperature sensitive. None of these conditions affect concrete
pipe. The inherent strength of concrete pipe increases with an increase
in pipe diameter for the same strength classification. Conversely, pipe
stiffness often decreases as pipe diameter increases with most
Hydraulics: Check the Comparisons
Marketing materials published by the smooth-lined polyethylene (SLPE)
pipe industry suggest that SLPE has better hydraulic flow
characteristics than concrete pipe.
Research conducted independently at Utah State University does not
support this claim. In fact, laboratory values for the sizes tested show
a Manning's n value of 0.010 for concrete and 0.010 and greater for SLPE.
The concrete pipe industry promotes its product as having design values
of 0.012 and 0.013. The 20 to 30 percent "design factor" included by the
concrete pipe industry takes into account the differences between
laboratory testing and actual installed conditions.
The plastic industry does not include this design factor in its
promotional materials. Instead, it promotes laboratory values for design
purposes. This is a significant omission, considering that the
laboratory results were obtained utilizing clean water and straight pipe
sections without bends, manholes, debris or other obstructions.
The tests on SLPE indicate that the Manning's n value varied with pipe
diameter, smoothness of the interior liner, and velocity. An
extrapolation of the test values (only 12" through 18" sizes were
tested) results in laboratory values as high as 0.015 for 24" diameter
and 0.019 for 36" diameter SLPE. The smaller than nominal inside
diameters, joints (commercially supplied joints were not used in the
tests), and the installed-deflected-shape of SLPE pipe may also
adversely affect the hydraulic efficiency.
Concrete pipe is also more efficient in inlet control situations due to
its groove end (bell), which results in a lower entrance loss
coefficient. In some cases this allows designers to use smaller diameter
concrete pipe when compared to plastic pipe.
It is therefore important, in any discussion comparing hydraulic
efficiencies of various pipe materials, to ensure that the stated
Manning's n values are design values rather than laboratory values.
Concrete or HDPE:Strength versus Stiffness
The primary difference between concrete pipe and high density
polyethylene pipe (HDPE) is one of structural strength versus pipe
stiffness. These terms are not interchangeable: the differences between
them are significant and technical in nature.
Concrete pipe is a rigid pipe that has significant structural strength.
This is best demonstrated through the traditional method for measuring
pipe strength, the three-edge-bearing test. Easily the most severe
loading to which any pipe will be subjected, the three-edge-bearing test
allows no lateral support for the pipe and applies forces that are
virtually point loads. The load carrying capacity of this rigid pipe
when installed is increased by at least two-fold because of active soil
Furthermore, concrete pipe's structural strength can be adjusted through
several means, most notably by varying the wall thickness, concrete
strength, or the amount and shape of the reinforcing steel.
Installation using concrete pipe should be considered as a pipe-soil
structure. Because the structural integrity is derived primarily from
the pipe, moderate changes to the soil envelope over time will not
compromise the structural integrity.
HDPE is a flexible pipe which relies upon pipe stiffness, rather than
strength, for its structural integrity. This is reflected by the
parallel plate test, the accepted method for measuring pipe stiffness,
which measures the force required to achieve a given deflection. Under
soil load the pipe deflects, developing passive soil support at the
sides of the pipe. The load carrying capacity of this flexible pipe is
derived almost exclusively from the strength of the embedment soil.
Installation using HDPE should be considered as a soil-pipe system. Such
a system derives its structural integrity primarily from the soil
envelope. Changes to that envelope can-and do-produce deflections that
lead to premature system failure.
Handling & Installation Comparisons
Manufacturers of high density polyethylene pipe (HDPE) claim that the
longer lengths, lighter weights and fewer joints of their products yield
substantial cost savings. This claim, however, is difficult to prove
using actual field data. In some cases, the very attributes of concrete
pipe that competitors cite as disadvantages prove beneficial to the
installation and performance of a system.
LENGTHS Concrete pipe is produced in shorter lengths than most flexible
products. Reasons for this include the mass of the product, shipping
requirements and manufacturing methods. The lengths of concrete may be
an advantage on installations that require a trench box. The length of
concrete pipe allows for a shorter trench box and less open excavation
during installation. Moving the trench box after having bedded and
backfilled the pipe, may result in disturbing the side fill. This could
result in the loss of side support so critical to an HDPE installation.
ASTM D 2321 "Standard Practice for Installation of Thermoplastic Pipe
for Sewers and Other Gravity Flow Applications" states that movable
supports should not be used below the top of plastic pipe unless
approved methods are used for maintaining the integrity of embedment
WEIGHTS Concrete pipe is heavier per foot (or meter) than flexible but,
in most common sizes, even flexible pipe is beyond OSHA limits for
routine manual lifting. Machinery is required to handle both products.
In addition, installation speed is more dependent upon rate of
excavation than pipe placement.
The heavier weight of concrete also is an advantage when flotation is a
concern. It is well documented that the comparative lighter weight of
flexible pipe makes it subject to lateral movement when compacting on
the sides of the pipe. HDPE also tends to lift vertically off the
required grade as material under the haunches is compacted. Select
material is important to a successful installation of flexible pipe.
HDPE installation procedures recommend that the trench width be
established by the designer and ASTM D2321 requires all material in the
haunch area be placed by hand.
JOINTS The increased number of joints is a perceived shortcoming of
concrete pipe while in fact it may be an advantage. Line and grade is
maintained and checked frequently Various joints and gasket designs are
available for most installation conditions from culverts, to storm
sewers, to sanitary sewers.
Concrete pipe can also be designed for jacking, micro-tunneling and
low-head pressure applications. Concrete pipe is available in several
different classes and shapes for each diameter. This gives the designer
considerable versatility. By varying the class of pipe, the type of
bedding or the installation type, a myriad of designs are available.
Cost Analysis of Pipe Envelope (CAPE) is a design aid to help evaluate
the material costs for projects.
OTHER FACTORS With concrete pipe, the majority of the pipe-soil
structure is delivered to the jobsite in the pipe. In some instances, up
to 100% of the pipe-soil structure is provided to the job site, thereby
minimizing potential problems and negative impact of contractor error in
the installation process. HDPE pipe with its low stiffness, comprises 6%
or less of the soil-pipe system at delivery. This places most of the
burden of performance in the field on the installation method, type of
backfill material and adequacy of field inspection. Poor site
conditions, such as weak native soils and groundwater, further aggravate
the problem. A wider trench may be required for HDPE pipe than for
concrete to provide adequate stiffness to support the pipe.
The Insignificance of Cracking
Many, many pieces have been published on the subject of cracking in
reinforced concrete pipe. The purpose of this article is to emphasize
the meaning of cracking. We do not intend to diminish the importance of
cracking, but hope to aid in understanding cracking, consequently, the
following from the CONCRETE PIPE DESIGN MANUAL.
Significance of Cracking
"The occurrence, function and significance of cracks have probably been
the subject of more misunderstanding and unnecessary concern by
engineers than any other phenomena related to reinforced concrete pipe.
Reinforced concrete pipe, like structures in general, are made of
concrete reinforced with steel in such a manner that the high
compressive strength of the concrete is balanced by the high tensile
strength of the steel. In reinforced concrete pipe design, no value is
give to the tensile strength of the concrete. The tensile strength of
the concrete, however, is important since all parts of the pipe are
subject to tensile forces at some time subsequent to manufacture. When
concrete is subjected to tensile forces in excess of it's tensile
strength, it cracks.
Unlike most reinforced concrete structures, reinforced concrete sewer
and culvert pipe is designed to meet a specified cracking load rather
than a specified stress level in the reinforcing steel. This is both
reasonable and conservative since reinforced concrete pipe mat be
pretested in accordance with detailed national specifications.
In the early days of the concrete pipe industry, the first visible crack
observed in a three-edge bearing test was the accepted criterion for
pipe performance. However, the observation of such cracks was subject to
variation depending upon the zeal and eyesight of the observer. The need
soon became obvious for a criterion based on a measurable crack of a
specified width. Eventually the 0.01-inch crack, as measured by a feeler
gage of a specified shape, became the accepted criterion for pipe
The most valid basis for selection of a maximum allowable crack width is
the consideration of exposure and potential corrosion of the reinforcing
steel. If a crack is sufficiently wide to provide access to the steel by
both moisture and oxygen, corrosion will be initiated. Oxygen is
consumed by the oxidation process and in order for corrosion to be
progressive there must be a constant replenishment.
Bending cracks are widest at the surface and get rapidly smaller as they
approach the reinforcing steel. Unless the crack is wide enough to allow
circulation of the moisture and replenishment of oxygen, corrosion is
unlikely. Corrosion is even further inhibited by the alkaline
environment resulting from the cement.
While cracks considerably in excess of 0.01-inch have been observed
after a period of years with absolutely no evidence of corrosion,
0.01-inch is a conservative and universally accepted maximum crack width
for design of reinforced concrete pipe.
Reinforced concrete pipe is designed to crack.
Cracking under load indicates that the tensile stresses have been
transferred to the reinforcing steel.
A crack 0.01-inch wide does note indicate structural distress and is not
Cracks much wider than 0.01-inch should probably be sealed to insure
protection of the reinforcing steel.
An exception to the above occurs with pipe manufactured with greater
than 1 inch cover over the reinforcing steel. In these cases acceptable
crack width should be increased in proportion to the additional concrete
The comment in the fourth paragraph (above), regarding "...eyesight of
the observer" is especially important. Even today there is still
misunderstanding about "hairline" cracks and "0.01-inch" cracks, and
quite understandably. The feeler gauge is an important tool for
determining crack width and depth. Designed with two leaves and a cover,
a feeler gauge has a tapered leaf the thickness of 0.01-inch. The other
square-ended, is marked off in graduations of inches and fractions for
measuring depth of cracks. If you have responsibility for inspecting
concrete pipe, it is essential that you be able to differentiate between
"significant" and "insignificant" cracks.
0.01" Crack Design and Autogenous Healing
The hairline cracks that appear at the obvert and invert of steel
reinforced concrete pipe are often confused with first damage strength.
These cracks are visible evidence that the concrete pipe has deflected,
therefore placing the steel reinforcing into tension as it was designed
to do. The proper design of any reinforced concrete structure requires
the concrete to crack in order for the design to be satisfactory. These
hairline cracks do not provide a source for future corrosion and do not
cause leakage as the do not penetrate the pipe wall. The crack is
V-shaped and is widest at the surface. The crack is not damage. It is
visible evidence that the design is correct. The 0.01-inch crack
criterion is conservative. This is demonstrated by more than 50 years of
experience in the United States and Canada, during which there has never
been a report of deleterious corrosion of reinforcement in a concrete
pipe due to the existence of cracks of a 0.01-inch magnitude. One of the
reasons is that, the concrete pipe seals the crack with calcium
carbonate crystals through a chemical reaction called autogenous
healing. Free lime (calcium hydroxide) in the concrete combines with
carbon dioxide in the presence of moisture to form calcium carbonate
Ca(OH)2 + CO2 = Ca CO3 + H20. This natural repair is impermeable
and very strong.
Evaluation of Cracks
A complete evaluation of the significance of crack widths must consider
the aggressiveness of the pipe environment, the depth of crack
penetration, and the thickness of concrete cover over the reinforcement.
Sources of aggressive chemical attack on concrete pipe are surface
related phenomena in every case except that of chlorides. Furthermore,
in order for destructive reactions to continue, there must be
replenishment of the aggressive solution.
Cracks normally do not penetrate the wall of a reinforced concrete pipe.
Ordinarily, when cracks occur, the penetration is to the depth of the
reinforcement, and the maximum penetration would be to the neutral axis
of the pipe wall. The geometric shape of crack is triangular, with the
maximum width at the surface and tapering to zero. Thus, the depth of
penetration of any given width of surface crack is controlled by and
related to the thickness of the cover over the reinforcement. The
0.01-inch crack criterion has historically been related to the standard
one-inch cover provided over the reinforcement in concrete pipe.
Specifying a limitation of surface crack width of 0.01-inches in
concrete pipe, even in aggressive exposure conditions, is unnecessarily
conservative. From a durability standpoint, surface cracks up to
0.02-inches in width which do not completely penetrate the pipe wall,
and with a minimum of one inch cover over the reinforcement, should be
acceptable in an aggressive environment. Pipe with such cracks will have
the same durability performance characteristics as an un-cracked pipe.
Consideration should be given to sealing cracks wider than 0.02-inches,
particularly under condition of sever exposure.
Cracking aka Crazing
Concrete pipe which has lain in the pipe yard for a considerable time
will sometimes develop a multitude of checks - referred to in the
concrete industry as crazing. Many reasons are advanced for crazing
which occurs in practically all concrete structures exposed to the
weather. Crazing, like beauty, is only "skin deep" and has no effect on
the strength or value of the pipe or other concrete structure, except
where beauty or appearance is an essential requirement. There are
several reasons for pipe cracking in the yard. Pipe may have been
stacked too high - especially pipe partially cured. Minor thermal
cracking may occur in the crown of the top row of pipe stack due to
temperature differences between pipe exposed to sunlight and that which
is shaded by rows above it. Too rapid rising, or extremely rapid
cooling, of concrete temperatures during curing can result in micro
cracking of the pipe surface. Rapid loss of moisture from concrete may
cause shrinkage cracks and low strengths.
Cracking After Installation
In very few isolated cases, cracks have appeared in newly installed
pipe. The appearance of a crack, or cracks in an installed pipe can
reasonably be assumed to be due to trench, site handling of from loading
during the backfilling process. Since autogenous healing of fine cracks
will take place with time in the normally moist atmosphere of a pipe
line, the occurrence of fine cracks in a pipe at the time of
installation are not a cause for concern, unless severe.
Sever cracking, over 0.02-inch (0.508mm), or slabbing of the concrete
cover over the reinforcing should be investigated as to cause. Poor
bedding under the pipe, under-designed pipe (wrong strength), excessive
loading from construction equipment are some causes of pipe
overstressing. Once a cause is determined, a decision as to repair or
replacement can be made.
The principles of concrete pipe design are basically the same as for
reinforced concrete structural building members. Reinforced concrete is
a composite structure and specifically designed to utilize the best
features of both the concrete and the reinforcement. The concrete is
designed for the compressive force and the reinforcement for the tensile
force. Unless the concrete cracks, the reinforcement is not being
utilized to its design capacity.
As more tensile forces are carried by the reinforcement, hairline cracks
become visible, but these occur at loads well below the design loading
of the reinforced concrete member. Hairline cracks are not an indication
of danger, distress, or loss of structural integrity. If ultimate
strength is exceeded, the concrete pipe will deflect, mobilizing passive
soil pressures and therefor continue to perform structurally as a four
Some engineers object to cracking in reinforced concrete pipe based upon
the erroneous belief that a crack is an indication of loss of structural
integrity. Generally, reinforced concrete pipe is designed to withstand
a specified load at 0.01-inch crack in the three-edge bearing test.
If a reinforced concrete pipe develops a 0.01-inch crack after
installation, it has not failed, nor is it in danger of imminent
collapse. The crack is an indication that the pipe and reinforcement are
performing as intended.
The structural considerations of cracks in reinforced concrete pipe were
studied in Texas and California. Some conclusions of the Texas study
The load-strain and load-deflection curves indicate that the reinforcing
steel becomes structurally effective only after the concrete cracks and
thus enables the pipe to sustain greater loads than those which produce
A reinforced concrete pipe will continue to have structural integrity
when loaded beyond the loading required to produce a 0.01-inch crack.The
California study concluded:
The presence of a 0.01-inch crack in reinforced concrete pipe in the
installed condition does not constitute failure of the pipe. In fact,
cracks substantially larger than 0.01-inch did not significantly affect
the structural integrity of the pipe.
Even in those areas where cracks were as wide as 0.20-inch have
occurred, structural integrity has been maintained.
Another concern relative to cracking is based on the belief that the
crack may provide a path for moisture to reach the reinforcement and
introduce corrosion. Such durability concerns were also investigated in
the TExas and California studies. Both studies indicated corrosion was
not a problem. A main conclusion of the Texas report was:
There is little or no probability of deterioration of either the
reinforcing steel or of the concrete surfaces exposed by a hairline
crack, even when sulfuric acid is present. Editors note: ( as a
part of the investigation, specimens were immersed in laboratory
solutions of sulfuric acid.)
Three-Edge Bearing Test (T.E.B.)
Of great comfort for those who specify, buy, and use concrete pipe is
the extensive testing performed on the products prior to acceptance and
installation. Illustrated below is a three-edge bearing test underway.
Cracked pipe may be found at the manufacturing plant or on a
project, and should not be ignored. Understanding and judgement based
upon sound investigation over many years need to be brought into play.
In the vast majority of cases the pipe can be approved with confidence
that it will serve the purpose for which it was made for many, many
Concrete Pipe Resources
Storm Drains, Culverts, Storm Sewer and Sanitary Sewer Pipe.
- Standard Specification for Reinforced Concrete Culverts, Storm Drains
and Sewer Pipe.
- Standard Specification for Joints for Circular Concrete Storm Sewer
and Culvert Pipe,
Using Rubber Gaskets.
- Standard Specification for Reinforced Concrete D-Load Culvert, Storm
Drain and Sewer Pipe.
M170 - Standard Specification for Reinforced Concrete D-Load Culvert,
Storm Drain and Sewer Pipe.
- Standard Specification for Reinforced Elliptical Culvert, Storm Drain
and Sewer Pipe.
Round Pipe - 12" thru 120"
Elliptical Pipe - 23"x14" thru 68"X43"
Full range of choices to fit project requirements from "soil-tight" to
"water-tight" seals designed to meet or exceed applicable ASTM and
AASHTO standards to bell and spigot, o-ring and straight
wall joint designs.
Strength classes available from II, III, IV, V and "special design" to
meet project load requirements. Ask about the "3EB" software program
which will determine the class of concrete needed based upon your
specific site conditions.
Newly designed "straight
wall" pipe is available in 12" thru 72" diameters. This "no bell" design
makes pipe installation simple and easy. Contractors no longer need to
modify their bedding material to accommodate pipe bells. With
"straight-wall" pipe you maintain ultimate contact with the pipe bedding
throughout the line.
DURABILITY AND INSTALLATION STRENGTHS
Concrete pipe continues to be the choice when durability, structural
integrity, 100 year design life, constant hydraulic efficiency, and low
maintenance are considered. The brute strength of concrete pipe also
plays an important role in the bedding and backfill requirements.
Because concrete pipe also acts as a structure, it is less dependent on
the soil placed around it. Consequently, AASHTO Sections 17 and 27 were
re-written in 1995 highlighting four new Nationally Approved Standard
Installations which outline proper concrete pipe installation. These
Standard Installations replace the 1920's Marston and Spangler bedding
Concrete Pipe won't rust, buckle, split,
deflect, deteriorate, burn, pollute, waste energy or lose it's hydraulic
capacity. Concrete Pipe - It stays in shape!
Bridge replacement, culverts, storm sewers, storm drains, utility vaults
and storm water retention/detention systems.
culverts can be designed to meet HS20, Interstate or E80 Cooper loads.
ASTM C1433 specifications have been combined to form the new precast
M259 - Precast Reinforced Box Sections for Culverts, Storm Drains and
M273 - Precast Reinforced Box Sections for Culverts, Storm Drains and
Sewers with less than 2 feet of cover, subjected to Highway Loads.
Our precast boxes can also be specifically designed using the "BOXCAR"
FHWA software program. This software allows you to alter the wall
thickness and steel reinforcing to obtain the most cost effective
precast box design while still meeting the moment, shear and thrust
We offer a wide variety of sizes with inside dimensions ranging from 4'
x 3' thru 12' x 8'. Other sizes may be obtained by special request.
Precast boxes are very versatile and offer many advantages which
Early Compressive Strengths
Easier and Safer than Cast-In-Place Boxes
Installation - 6 foot laying lengths with a tongue / groove joint -
culvert type installation
Conversion to Metric Design
converted to "Detention Systems"
Steel areas and reduced wall/slab thickness is calculated by using the
FHWA "BOXCAR" software program.