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 crack(s) 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 therefore 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
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 judgment 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.

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