Load Carrying Capacity, Cracking and Jacking

Supporting strength and the load factor

The supporting strength of an installed pipe is not the number produced in the laboratory. It is the three-edge bearing strength established under ASTM C 497 multiplied by a load factor that reflects the bedding and installation condition, with a factor of safety applied. That is the whole basis of the design sequence: determine earth and live load, select a bedding, determine the load factor, apply the factor of safety, select the pipe strength.

The practical consequence is that bedding is a strength decision, not a construction detail. Improving the bedding class can be a more economical route to the required supporting strength than moving up a pipe class, and degrading it in the field — most commonly by failing to fill the haunches — silently removes capacity the design assumed was there. Design data on bedding factors for trench and embankment installations, and on the standard installations used with the indirect design method, is listed under design aids.

The significance of cracking

The occurrence, function and significance of cracks have probably caused more misunderstanding and unnecessary concern among engineers than any other phenomenon related to reinforced concrete pipe. The confusion is understandable and the correction is straightforward.

Reinforced concrete pipe, like reinforced concrete structures generally, is made of concrete reinforced with steel so 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 given to the tensile strength of the concrete. That tensile strength is nonetheless important, because all parts of the pipe are subject to tensile forces at some time after manufacture — and when concrete is subjected to tension in excess of its tensile strength, it cracks.

So: reinforced concrete pipe is designed to crack. Cracking under load is the indication that tensile stress has been transferred to the reinforcing steel, which is what the steel is there for. This is why the acceptance criterion in the three-edge bearing test is a crack of defined width rather than collapse.

Why 0.01 in.

The concern behind crack width is exposure and potential corrosion of the reinforcing steel. If a crack is wide enough to give both moisture and oxygen access to the steel, corrosion can initiate; and because oxygen is consumed by oxidation, progressive corrosion requires continuous replenishment of it.

Bending cracks are widest at the surface and narrow rapidly toward the reinforcement. Unless a crack is wide enough to permit circulation of moisture and replenishment of oxygen at the steel, corrosion is unlikely — and it is further inhibited by the alkaline environment the cement provides. Cracks considerably wider than 0.01 in. have been observed after years of service with no evidence of corrosion whatever. The 0.01 in. figure is a conservative and universally accepted maximum design crack width, not a threshold of damage.

In summary

• Reinforced concrete pipe is designed to crack; cracking under load indicates tensile stress has transferred to the steel.
• A 0.01 in. crack does not indicate structural distress, and such a pipe will perform successfully in the installed condition.
• Cracks much wider than 0.01 in. in corrosive environments may be sealed to ensure protection of the reinforcement.
• Where pipe is manufactured with more than 1 in. of cover over the steel, the acceptable crack width should be increased in proportion to the additional cover.
• Small cracks in the normally moist atmosphere of a pipeline heal autogenously — unhydrated cement continues to hydrate at the crack face and closes it.

Jacking

Where installations are deep, or where surface obstructions make conventional open excavation and backfill difficult, it is common practice and may be more economical to install concrete pipe by jacking or tunnelling.

The critical factor is the soil through which the pipe is to be jacked. Thorough investigation and knowledge of the soil conditions is necessary to determine the loads on the pipe, the type of tunnel boring machinery, the jacking equipment and the jacking procedure. Nothing about a jacked installation is recoverable once it is under way, which is why the ground investigation carries more weight here than in open-cut work.

Reinforced concrete pipe used for jacking generally falls in the 36 in. to 132 in. diameter range, should be of the D-load class required for the overburden earth loading, and should have a minimum concrete compressive strength of 5,000 psi for axial loading. Pipe is designed to carry the D-load determined by the procedures set out in the design aids.

A common and expensive misconception

The tendency among some engineers is to require jacking pipe of a D-load class higher than would otherwise be needed, on the reasoning that jacking is severe. This does not do what it is intended to do. Thrust capacity is a function of the compressive strength of the concrete, not of the D-load class. Increasing the D-load class does not increase thrust capacity except in those cases where a higher class also brings an increase in the minimum concrete compressive strength. The cross-sectional area of the pipe wall is more than adequate to resist the pressures encountered in any normal jacking operation.

What does matter is the geometry of the ends. The pipe should have straight outside walls without bell modification, and squareness of ends and spigot shoulders should be maintained within the tolerances prescribed by the ASTM standards for precast concrete pipe. An out-of-square end concentrates the jacking thrust on part of the annulus instead of distributing it, and that is how jacked pipe is actually damaged.

Structural design practice generally is published by ASCE; the detailed design manuals and data sheets referenced throughout this page are catalogued by the American Concrete Pipe Association.