Concrete Pipe Resources 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.

Durability: Too Important to Ignore

Hydraulics: Check the Comparisons

Concrete or HDPE: Strength versus Stiffness

Handling and Installation Comparisons

In-Field Product Performance and Expectations

Technical Specifications

Round and Elliptical Pipe Specifications

Box Culverts

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 polyethylene products.

 

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 pressure.

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 material.

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 performance.

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 harmful.
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 cover.
Feeler Gauges

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 crystals.

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.

Structural Considerations
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 hinged arch.

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 were:

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 hairline cracks.

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.

Durability Considerations
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.

Conclusion
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 decades.

Concrete Pipe Resources
USES
Storm Drains, Culverts, Storm Sewer and Sanitary Sewer Pipe.

SPECIFICATIONS
ASTM C76 - Standard Specification for Reinforced Concrete Culverts, Storm Drains and Sewer Pipe.
ASTM C443 - Standard Specification for Joints for Circular Concrete Storm Sewer and Culvert Pipe,
Using Rubber Gaskets.
ASTM C655 - Standard Specification for Reinforced Concrete D-Load Culvert, Storm Drain and Sewer Pipe.
AASHTO M170 - Standard Specification for Reinforced Concrete D-Load Culvert, Storm Drain and Sewer Pipe.
ASTM C507 - Standard Specification for Reinforced Elliptical Culvert, Storm Drain and Sewer Pipe.
  ASTM C361

SIZES
Round Pipe - 12" thru 120"
Elliptical Pipe - 23"x14" thru 68"X43"

JOINTS
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.

DESIGN STRENGTHS
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.

PIPE STYLES
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 types.

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!

USES
Bridge replacement, culverts, storm sewers, storm drains, utility vaults and storm water retention/detention systems.

SPECIFICATIONS
Box culverts can be designed to meet HS20, Interstate or E80 Cooper loads.
ASTM C1433 specifications have been combined to form the new precast box specification.
AASHTO M259 - Precast Reinforced Box Sections for Culverts, Storm Drains and Sewers.
AASHTO 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 criteria.

SIZES
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.

ADVANTAGES
Precast boxes are very versatile and offer many advantages which include:
Minimum Traffic Delays
Assured Quality Control
High Early Compressive Strengths
Faster, Easier and Safer than Cast-In-Place Boxes
Quick Project Completion
Ease of Installation - 6 foot laying lengths with a tongue / groove joint - culvert type installation
Easy Conversion to Metric Design
Readily Available
Easily converted to "Detention Systems"

DESIGN SOFTWARE
Steel areas and reduced wall/slab thickness is calculated by using the FHWA "BOXCAR" software program.