Prestressing Steel

In order to obtain high yield strength values, wires can be cold worked, hot rolled, or quenched and tempered (Table 15.2) [3,4]. Cold-worked prestressing steel wires are obtained by drawing wires of steel with a ferritic-perlitic microstructure at room temperature, so that the reduction in the cross section leads to an increase of strength. Prestressing bars with diameter up to 36 mm are manufactured by 

Type Shape, surface Diameter (mm) Anchorage system Strength class European Standard (MPa) Production (ton/y) 

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Prestressed steel in concrete should thus be durable if a dense, impervious and uniform concrete free of chloride surrounds the steel and adequate depth of concrete is given to the steel. 

Consequences of pitting corrosion may be very serious in high-strength prestressing steel, where hydrogen embrittlement can be promoted this is discussed in Chapter 10. 

Failure of prestressing steel is usually induced by atomic hydrogen that penetrates the metal lattice. The conditions required for cracking are a sensitive material, a tensile stress and an environment that produces atomic hydrogen on the steel surface. 

Table IOlI Parameters and criteria for testing susceptibility to stress-corrosion cracking of prestressing steel...

Temporary or permanent protection of prestressed structures may be necessary. The essential elements to be considered are prestressing steel, the ducts containing the tendons, the anchorage system, the protective system over all. 

As far as the prestressing steel is concerned, corrosion prevention has to be applied from the moment the tendons are delivered until they are embedded. The ducts have to be durable and waterproof, filled with protective materials following the guidelines for correct grouting. 

Besides concrete quality, a minimum value of the concrete cover also has to be specified. Eurocode 2 [3] fixes minimum values ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-bearing environments, as shown in Table 11.5. It should be kept in mind that these values are minimum values that should be increased to obtain nominal values by 10 mm, to also take into consideration construction variability. Besides the protection of steel to corrosion, further requirements of minimum cover depth are fixed to ensure adequate transmission of mechanical forces and fire resistance. 

Figure 15.1 Examples of stress-strain curves for reinforcing and prestressing steels [3]...

I 75 Corrosion-resistant Reinforcement Table 15.2 Different types of prestressing steel [3]... 

As far as corrosion behaviour is concerned, prestressing steel needs to be distinguished from reinforcing steel with regard to hydrogen embrittlement, since it only affects the former this has been illustrated in Chapter 10. 

Trials. The effectiveness of chloride extraction depends on characteristics of individual structures, such as the concrete composition, the actual chloride-penetration profile and the depth of cover. So, it may be useful to carry out a trial on an area (about 1 to 10 m ), which must be representative of the structure to be treated and should last at least 4 to 8 weeks. The results of such a trial in terms of the chloride profile before, during and after chloride extraction gives an indication of the duration required and can be used to show that chloride-extraction treatment of the particular structure will be effective under field conditions. Trials are most certainly recommended if prestressed structures are to be treated with chloride extraction. Careful monitoring of the potential of the prestressing steel should be carried out to establish the risk of hydrogen embrittlement. As a safe criterion, the potential should not become more negative than -900 mV SCE, as apphes for cathodic protection [13]. 

Reliable nondestructive methods for providing assurance to the owners that the built structures have met constmction specifications are not available. The main concern is whether the ducts in the posttensioned bridge members have been completely filled with the grouts and whether there is uniform coverage over the prestressing steel. 

The concept of field resources is less familiar, but it may be the key to our inventive problems. By field resources we mean all kinds of fields that can be utilized in our project. For example, in our floor structural system, we could use a stress field, that is, we could use stresses to create a prestressed concrete or even rarely used but feasible prestressed steel structures. Also, we could use an electromagnetic field to transport the structural system to its location in the building or even to keep it in its desired position. We will have also such fields available as the temperature field or the gravity field, which both are easily available and eventually could be used. 

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 steel should not exceed the hydrogen evolution potential, especially for prestressed steel to avoid hydrogen embrittlement. 

The criteria (Section 8.6 of the standard) start with a requirement that no (instant off) potential should exceed a limit of 1,100mV with respect to Ag/AgCl/0.5M KCl for reinforced concrete and —900mV for prestressed concrete. This is aimed at minimizing hydrogen evolution with a wider margin for prestressed steel where the consequences could be more extreme as discussed in Section 7.2.1. 

Extreme caution must be used when applying cathodic protection to prestressing steel or to elements that include prestressing steel. This is for... 

Because if the prestressing steel has corrosion pits in it, those pits are potential stress concentrators and there is a risk of either under protection in the bottom of the pit which could lead to continued corrosion and then to failure of the stressed strands, or of overprotection... 

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