Prestressing steels can only be subjected to a noticeable stress in dynamically strained reinforced concrete structures if the concrete is cracked.39 The stress amplitudes of prestressing steel under high dynamic loads (e.g., high traffic load on a bridge) may then amount to >200 N/mm2 in the crack region. In the non-cracked state, the steels will show stress ranges below 100 N/mm2. Cracks may occur in partially prestressed concrete structures. Since such cracks tend to open and to close under the influence of a superimposed fatigue stress, a number of factors must be considered and these are discussed below.
Corrosion fatigue cracking
If corrosion-promoting aqueous media penetrate through a crack in the concrete to a dynamically stressed tendon, corrosion fatigue cracking is possible, although this type of corrosion has not been observed in prestressing steel construction so far. Corrosion fatigue cracking6 occurs because a metallic material under dynamic stress in a corrosive medium (e.g., water or salt solution) will show much more unfavourable fatigue behaviour than a similar material under fatigue loading in air. Whereas steels exhibit a `fatigue limit’ when tested in dry air (i.e., a critical stress amplitude below which failure does not occur even after very large numbers of cycles are applied), this is not maintained in corrosive environments. A decrease in fatigue limit caused by corrosion is more distinct the higher the strength of the steel and the more aggressive the attacking medium. Hence high strength prestressing steels, when simultaneously attacked by, for instance, an aqueous chloride-containing medium, may show very unfavourable fatigue behaviour.
In traffic-carrying bridge structures, it is only the low-frequency stresses that lead to high stress amplitudes, an additional contributor to corrosion fatigue cracking. At lower frequencies, the influence of corrosion will increase and the fatigue limit will consequently drop. Figure 6.11 shows the decrease in corrosion fatigue limit for a cold-drawn prestressing steel wire in an air-water-chloride solution. For frequencies of 0.5 sÿ1 the fatigue limit for stress cycles of 107 is below 100 N/mm2. The problem of corrosion fatigue cracking in cracked com- ponents can be remedied by sufficient concrete cover and limiting the crack width. This keeps pollutants away from the prestressing steel surface.
Fretting corrosion/fretting fatigue
Cracks can occur in concrete due to fatigue loading displacements between the tendon and the injection mortar or the steel duct. In bent tendons, a high radial pressure acts simultaneously on the fretting prestressing steel surface. If air or oxygen reaches the fretting location through the concrete crack, fretting corro- sion can occur.6 Fretting corrosion is described as damage to a metal surface in a manner similar to wear due to oscillating friction under radial pressure with another surface. In the presence of oxygen, oxidation will occur at the reactive surface.
In fatigue-loaded steels subject to simultaneous fretting corrosion stress, fretting fatigue makes fatigue behaviour much worse.40,41 This is attributable to the occurrence of additional tensile stresses in the fretting area. In concrete embedded tendons, subjected to a relative movement and a radial pressure in the concrete crack between prestressing steel and duct or injection mortar respectively, tolerable fatigue limits of about 150 N/mm2 for cycles to fracture of 2 x 10^6 were found.
In prestressed concrete constructions, the anchorages of the tendons also show a reduced fatigue limit as a result of fretting corrosion.42 Under dynamic stress, the fatigue limit of the anchored tendon, depending on the type of anchorage, is reduced to between 80 and 150 N/mm2. For this reason, anchorages are always positioned in areas of least stress changes. In fatigue experiments, the prestressing steels always fracture in the force transmitting area, i.e., at the beginning of the anchorage. Here, the fatigue limit is reduced due to relative movements between the prestressing steel and the anchor body under simultaneous high radial pressures. In prestressed concrete bridges, the coupling joints are particularly problematic. If such joints crack as a result of imposed stresses (e.g., due to non-uniform solar heating and insufficient reinforcement crossing the coupling joint), the tendon couplings will suffer major stress fatigue cycles from traffic loads, which can also lead to fractures in the prestressing steel due to the stress-sensitive couplings.
