Effect of Prestressing

Fig. 3.53 Effect of prestressing on a plate a with small hole and b with large hole. In the former case the shake-down effect is fully developed, in the latter it is not...

Table 13.3 Effect of prestressing of epoxy impregnated carbon textile febric on the flexural properties of cement composite (after Reinhardt et al. [58])...

A more important effect of prestressiag is its effect on the mean stress at the bore of the cylinder when an internal pressure is appHed. It may be seen from Figure 6 that when an initially stress-free cylinder is subjected to an internal pressure, the shear stress at the bore of the cylinder increases from O to A. On the other hand, when a prestressed cylinder of the same dimensions is subjected to the same internal pressure, the shear stress at the bore changes from C to E. Although the range of shear stress is the same ia the two cases (distance OA = CE), the mean shear stress ia the prestressed cylinder, represented by point G, is smaller than that for the initially stress-free cylinder represented by point H. This reduction in the mean shear stress increases the fatigue strength of components subjected to repeated internal pressure. 

Table 5.5 Effect of CaCI2 on corrosion of high tensiie steei wires in normaiiy cured prestressed concrete specimens stored outdoors...

This effect of pre-stressing is due to physical breakdown of some structure of the filler/rubber composite, its exact nature being unimportant as regards testing procedure. It is self-evident, however, that if a rubber during service is subjected to repeated deformations testing, should be carried out after prestressing rather than in the initial state where the result may be different due to the effect of the unstable structure. 

Composite structures Application of two-layer RubCon-conventional concrete structures (with RubCon in the tension region) protects steel reinforcement from the effects of corrosive environments and creates a structure that operates under load without cracks. Research [30] proved that two-layer RubCon-concrete bending structure elements can replace prestressed concrete elements. 

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]. 

Since the maximum voltage that can be generated with zinc anodes is extremely unlikely to generate hydrogen embritdement, galvanic systems have been used to protect prestressed concrete members. They are also used on fusion bonded epoxy coated steel reinforced piles as the effects of electrical discontinuity between bars is unlikely to lead to significant stray current induced corrosion as the currents and potentials are low. 

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... 

Aram, M. R., C. Czaderski and M. MotavaUi (2008b). Effects of gradually anchored prestressed CFRP strips bonded on prestressed concrete beams. Journal of Composites for Construction 12(1) pp. 25-34. 

There are three known side effects of ECE. The first is the acceleration of alkali silica reactivity (ASR), another is reduction in bond at the steel concrete interface. A third issue is hydrogen evolution. Hydrogen evolution is inevitable with ECE and so this process must not be used on steel used for prestressing. 

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