Prestressing techniques

A similar construction type is winding of a corrugated band in a helix on the core shell to form a high-pressure vessel. The additional advantage of the corrugation is the capability to transmit longitudinal forces from one layer to another, which allows 

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From Figure (13.44) it can be seen that the stress falls off rapidly across the wall and that the material in the outer part of the wall is not being used effectively. The material can be used more efficiently by prestressing the wall. This will give a more uniform stress distribution under pressure. Several different prestressing techniques are used the principal methods are described briefly in the following sections. 

The maximum elastic stress range that can be achieved through prestressing techniques is equivalent to two times the yield condition, since the bore of the vessel can only be prestressed to the compressive yield strength value. 

Answer The pressure is too high for welded construction and a standard HSLA steel with a yield strength of900 MPa is best suited for this application. The ratio of P/Sy is 0.67 and from the diagram a diameter ratio of 2.7 is required at the minimum. The operating pressure is above the yield condition so that prestressing techniques have to be used. 

Figure 12.9 Prestressing techniques. Residual tangential stresses in vessel wall, induced by different manufacture processes (a) autofrettage (monoblock) (b) shrink design, two layers (c) multilayer, wire/plate winding (ds shrink diameter dc core diameter).

For the radial direction, the temperature difference over a thick cylindrical wall causes an increase in tangential stress in the bore when the cylinder is heated and the opposite when it is cooled. Since these thermal stresses are only secondary, which means that in case of reaching yield level they would limit themselves, they do not affect the static strength of the cylinder. Their main impact is on fatigue life of the heat exchanger since thermal stresses create additional fatigue load cycles and since they may affect the residual stresses that were intentionally introduced with prestressing techniques. 

In one such prestressing technique, high-strength steel wires are positioned inside the empty molds and stretched with a high tensile force, which is maintained constant. After the concrete has been placed and allowed to harden, the tension is released. As the wires contract, they put the structure in a state of compression because the stress is transmitted to the concrete via the concrete-wire bond that is formed. 

Other workers have published improved procedures for inspecting both reinforced concrete and prestressed concrete structures with regard to determination of the embedded steel components [110]. A prototype ultrasonic procedure was developed to determine the condition of prestressed and pretensioned tendons in concrete. The application of electrochemical surface-mounted systems for estimating the rate of corrosion of reinforcing steel and other embedded steel components in large concrete structures was described using this technique. 

Prestressing steel is loaded typically to around than 80% of its ultimate tensile strength. Therefore modest section loss, particularly due to pits, can lead to crack initiation and rapid, catastrophic failure. There are apocryphal tales of prestressing rods shooting out of buildings due to corrosion induced failures. The following test techniques are used in these situations ... 

The NACE standard was the first to be produced (1990) and has been revised since the original. Its scope states that it excludes prestressed concrete. There is a separate NACE report on cathodic protection of prestress NACE 01102 (2000). This gives valuable information on applying CP to prestressing but there has been no pressure to convert this to a standard. A list of NACE recommended practice documents, test methods and reports for all electrochemical techniques is given at the end of this chapter. 

For carbonation the usual options are patch repairing usually with a suitable anticarbonation coating applied afterwards (Section 6.3.1) and realkalization (Section 7.11). Realkalization is very difficult in the presence of prestressing. The latest evidence is that ASR is not a significant problem. Realkalization (and all electrochemical techniques) becomes less cost effective if there are large number of unconnected rebars. Realkalization is most... 

Chloride removal cannot be applied to prestressed structures due to the risk of hydrogen embrittlement. The use of lithium-based electrolytes suggests that ASR can be controlled. As stated earlier for impressed current cathodic protection, there must be electrical continuity within the reinforcement network for any of the electrochemical techniques to be applied. We do not know how long the treatment process will last but a range of 5-20 years is likely, depending upon conditions. 

If strips or fabrics are applied in a prestressed state, the use of special techniques is necessary. Different products for anchorages using steel or FRP anchors in combinations with special prestressing devices are available on the market and/or under development see, e.g., Kim et al. (2008). 

Conventional techniques for dealing with such situations involve the construction of additional supports, prestressing or the enlargement of structural members. In the latter case this may involve the bonding of new concrete to old. An alternative solution is to use mild steel plates bonded by an adhesive to the external surface of the concrete member in question. These two techniques will now be discussed in turn. 

Therefore the designer must allow for the various techniques for accomplishing the compressive prestress while allowing the flexibility of the supplier to apply their specialized technology. 

The epoxy should be kept in compression. The coil is pretensioned to minimize the amount of tensile stress the epoxy sees. (This technique is analogous to the technique used to make prestressed concrete.)... 

For a wire wound PCRV, it has been calculated that If all the prestress were removed except for the wire windings a little either side of the equator, vertical tensile stresses large enough to cause cracking would develop around the equator level. This technique therefore would have very limited application and Is not thought to be feasible for a helically prestressed vessel. 

A relatively new repair technique for concrete structures, including prestressed structures, consists of externally bonding flexible sheets of FRP composites to the concrete surface. Depending on the type of application, the function of the externally bonded reinforcement... 

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