34234 Reinforced and Prestressed Concrete Strands and Cables

Because of the relatively low stresses, concrete structures are typically less sensitive to fatigue than welded steel and aluminum structures. However, fatigue may govern the design when impact loading is involved, such as pavement, bridge decks, and rail ties. Also, in high-strength concrete the applied stress ranges increase, and so should the concern for fatigue.

According to ACI Committee Report 215R-74 in the Manual of Standard Practice [51], the fatigue strength of plain concrete at 10 million cycles is approximately 55% of the ultimate strength. However, even if failure does not occur, repeated loading may contribute to premature cracking of the concrete, such as inclined cracking in prestressed beams. This cracking could then lead to localized corrosion and fatigue of the reinforcement [52].

The fatigue strength of straight, unwelded reinforcing bars and prestressing strand can be described (in terms of the categories for steel details described earlier) with the Category B S-N curve. The lowest stress range that has been known to cause a fatigue crack in a straight reinforcing bar was 145 MPa, which occurred after more than a million cycles. Mean stress level, yield strength, bar size, geometry, and deformations had minimal effect. ACI Committee 215 suggests that members be designed to limit the stress range in the reinforcing bar to 138 MPa for high levels of mean stress (increases up to 160 MPa as mean stress is reduced).

Fatigue tests show that previously bent bars had only about half the fatigue strength of straight bars and failures have occurred down to 113 MPa [53]. Committee 215 recommends that half of the stress range for straight bars be used, that is, 69 MPa for the worst-case mean stress. Equating this recommendation to the S-N curves for steel details, bent reinforcement may be treated as a Category D detail.

Provided the quality is good, butt welds in straight reinforcing bars do not significantly lower the fatigue strength. However, tack welds reduce the fatigue strength of straight bars about 33%, with failures occurring as low as 138 MPa. Fatigue failures have been reported in welded wire fabric and bar mats [54].

Batchelor et al. [55] tested deck panels with a single stationary concentrated load applied at the center. The contact area represented the assumed contact area of the pneumatic tires of large trucks. The only fatigue tests that did not result in failure at the end of 2.5 and 3 million cycles were performed with a loading of less than the 50% of the estimated ultimate punching strength of the deck (Pu). Other research also supports a fatigue limit of 0.5Pu at 2 to 3 million cycles for a stationary pulsating load [56,57].

However, the behavior and the fatigue limit are different in the case of rolling loads, as on a bridge. Flexural cracking along the reflection of the longitudinal and transverse reinforcement on the bottom surface was detected in bridges in service, and this is different from cracking patterns in tests with stationary pulsating loads [58]. Tests with moving or rolling loads indicate that the fatigue limit was as low as 0.21Pu at 2 to 3 million cycles, comparable to the average flexural cracking load level of about 0.26Pu [58-60]. In some of the tests [56,58,59] it was demonstrated that the transverse cracks from the constructional period and the water penetration during service life decreased the ultimate punching shear and fatigue strengths of the reinforced concrete deck. However, these studies have not established any quantitative interaction between the deterioration from the environmental factors and the repetitive axle load.

Test data show that measured stress ranges in the reinforcement at the location of cracks in a highly deteriorated deck under high axle loads are less than 35 MPa, well below the 138-MPa threshold discussed earlier [61]. Therefore, under service load, fatigue does not appear to be a problem for deck reinforcement. This is consistent with the fact that fatigue of reinforced concrete decks is governed by punching failure of the concrete part of the structure.

If prestressed members are designed with sufficient precompression so that the section remains uncracked, there is not likely to be any problem with fatigue. This is because the entire section is resisting the load ranges, and the stress range in the prestessing strand is minimal. Similarly, for unbonded prestessed members, the stress ranges will be very small. However, there is reason to be concerned about bonded prestressing at cracked sections because the stress range increases locally. The concern for cracked sections is even greater if corrosion is involved. The pitting from corrosive attack can dramatically lower the fatigue strength of the reinforcement [52].

Although the fatigue strength of the prestressing strand in air is about equal to Category B, when the anchorages are tested as well, the fatigue strength of the system is as low as half the fatigue strength of the wire alone (i.e., about Category E). When actual beams are tested, the situation is very complex, but it is clear that much lower fatigue strength can be obtained [62,63]. Committee 215 has recommended the following for prestressed beams:

1. The stress range in prestressed reinforcement, determined from an analysis considering the section to be cracked, shall not exceed 6% of the tensile strength of the reinforcement. (Author's note: this is approximately equivalent to Category C.)

2. Without specific experimental data, the fatigue strength of unbonded reinforcement and its anchorages shall be taken as half of the fatigue strength of the prestressing steel. (Author's note: this is approximately equivalent to Category E.) Lesser values shall be used at anchorages with multiple elements.

The Post-Tensioning Institute (PTI) has issued Recommendations for Stay Cable Design, Testing, and Installation [64]. The PTI recommends that uncoupled bar stay cables are Category B details, while coupled (glued) bar stay cables are Category D. The fatigue strengths of stay cables are verified through fatigue testing. Two types of tests are performed: (1) fatigue testing of the strand and (2) testing of relatively short lengths of the assembled cable with anchorages. The recommended test of the system is 2 million cycles at a stress range (158 MPa) that is 35 MPa greater than the fatigue allowable for Category B at 2 million cycles. This test should pass with less than 2% wire breaks. A subsequent proof test must achieve 95% of the guaranteed ultimate tensile strength of the tendons.

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