Prestressed structures

Still affect older structures. Tensile stress can damage the protective lilm. and particular care must be taken to minimi/c ingress of CO, and Cl in prestressed structures. 

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. 

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]. 

The problems of hydrogen embrittlement and of gas evolution are usually controlled by limiting the potential of the steel to below the hydrogen evolution potential. However, in acidic pits or crevices it may be possible for the potential to exceed the hydrogen evolution potential without being sensed by measuring electrodes. The cathodic protection of prestressed structures should only be undertaken with great care and input from experienced corrosion experts. An excellent state-of-the-art report on the cathodic protection of prestressed has been published (NACE, 2001). 

Thus it can bee seen that each prestressed structure must be evaluated to determine the feasibility of cathodic protection current reaching the steel that needs protection, whether galvanic cathodic protection can be applied and if not using qualification criteria such as those in NACE 01102 (2002) to determine if the structure is suitable for impressed current cathodic protection. 

Advantages very low maintenance, no power supply, no electronics, no wiring. Installation is simplified with no concerns about short circuits from anode to reinforcement. Can be applied to prestressed structures. 

Due to the problems with prestressing steel, realkalization would probably not be used as the alternative of patch repairing and coating would provide more cost effective solutions. In any case, it would be unusual for carbonation to take place in prestressed structures due to the high cement contents and high cover in such elements. If carbonation had reached prestressing steel in such structures there would probably be concerns about the strength of the concrete. 

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. 

Realkalization has been carried out on prestressed structures, but only on the reinforcing steel. The structure has post-tensioned cables in ducts. These were protected from the current by the ducts and by the fact that they were buried deep in the concrete, well below the reinforcing steel (Miller, 1994). It should be noted that carbonation is a rare problem on prestressed concrete structure.s as they usually use high strength, high cement content, low water/cement ratio concrete mixes that are highly resistant to carbonation. 

They cannot be used on prestressed structures, those with ASR, those with epoxy coating, injections or those with poor electrical continuity. 

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... 

The technology of cathodic protection of reinforced concrete is to elaborate its own protection criteria, other than for steel structures corroding in different environments than concrete. They are still mainly potential criteria, used carefully so as not to lead to overprotection and as a consequence liberation of hydrogen and weakening of the adhesion of concrete to steel. In prestressed structures, the presence of hydrogen can lead to embrittlement. John and Messham (1989) exhaustively describe the following protection criteria of reinforced concrete ... 

Cathodic prevention (CPR) is applied for new reinforced or prestressed structures, which are not contaminated by chlorides, but their contamination is expected during operation (for example, as a result of salt application on streets or the action of a sea atmosphere, etc.). In such conditions, as a prevention, the reinforced concrete structure is subject to a process ensuring CP. Pedeferri... 

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