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SPECIAL CONSIDERATIONS 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 reinforced concrete 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 given 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 its tensile strength, it cracks. Unlike most reinforced concrete structures, reinforced concrete sewer and culvert pipe are 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 may 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 subjected to variations depending upon the zeal and eyesight of the observer. The need soon became obvious for a criterion based on the measurable crack of a specified width. Eventually, the 0.01-inch crack, as measured by a feeler gauge 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. In summary: Reinforced concrete pipe is designed to crack. Cracking under load indicates that the tensile stresses have been transferred to the reinforcing steel. A 0.01 inch wide crack in a pipe does not indicate structural distress and such a pipe will perform successfully in the installed condition. Cracks much wider than 0.01 inch in corrosive environments may 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. Small cracks, in a normally moist atmosphere of a pipe line, will heal autogenously. Jacking Where installations are deep or where surface obstructions are such that it is difficult to install pipe by conventional open excavation and backfill, it is common practice and may be more economical to install concrete pipe by means of jacking or tunneling. A critical factor in jacking and tunneling concrete pipe is consideration of the soil conditions through which the pipe is to be jacked. A thorough investigation and knowledge of soil conditions is necessary to determine the loads on the pipe, type of tunnel boring machinery, jacking equipment, and procedure of jacking. Reinforced concrete pipe to be used for jacking generally falls in the range of 36 in. to 132 in. in diameter and should be of the required D-load class for the overburden earth loading with a minimum concrete compressive strength of 5,000 psi for axial loading. Pipe is designed to carry the D-load as determined by the prescribed procedures in the Design Aids on page 28 of this booklet. The tendency of some engineers is to require that jacking pipe be of a D-load class higher than would normally be needed. Inasmuch as thrust capacity is related to compressive strength of the concrete, increasing the D-load strength does not increase the thrust capacity of the pipe except in those cases where a higher class results in an increase in the minimum concrete compressive strength. The cross section area of the concrete pipe wall is more than adequate to resist pressures encountered in any normal jacking operation. The pipe should have straight outside walls without bell modification. Squareness of ends and spigot shoulders should be maintained within tolerances as prescribed by ASTM standards for precast concrete pipe. Since thrust loads are transmitted through the pipe joints, it is extremely important to maintain uniform distribution of the load around the periphery of the joint. Contractors have used different methods to provide uniform loading from pipe to pipe and jacking frame head. A joint cushioning material such as ½ in. to ¾ in. of plywood, hardboard or similar material is recommended to prevent concrete to concrete contact and reduce the chances of spalling. Supplemental joint reinforcing to withstand shear forces is provided by some manufacturers by means of extra bell and/or extra longitudinal steel. In the jacking procedure, it is important that the direction of jacking be carefully established prior to the start of work. Correct alignment of the pipe guide frame, jacks, and backstop is necessary to prevent altering the directional thrust. If any part of the jacking setup is off line, forces are set up which tend to cause localized stresses or bind the pipe. Backstops in pits must be strong and large enough to distribute the maximum capacity of the jacks against the soil behind the backstops. Alignment can be best achieved by installing guide rails in the bottom of the jacking pit or shaft. In the case of a larger pipe, it is desirable to have such rails carefully set in a concrete slab. The jacks should have a greater capacity than estimated requirements. The number and capacity of jacks used depend primarily on the size and length of pipe to be placed and the soil encountered. A suitable jacking head or frame should be provided to transfer the pressure from the jacks to the concrete pipe. The jacking head will protect the end of the pipe and aid in keeping the pipe on line by distributing the pressure on the pipe joint. Frictional resistance can be decreased by the application of bentonite or other suitable lubricant to the exterior surface of the pipe being jacked. It can be applied through pressure fittings in the pipe cutting shield or wall of pipe, or by pouring it down holes drilled from the top of ground surface to the jacked section. It has been demonstrated that jacking pressures are greatly reduced when using sufficient quantities of bentonite. |Home|
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