Pitting corrosion reduces the cross-sectional area of the prestressing steel
and hence the load-carrying capacity of the structure .
in which [f.sub.pe] and [f.sub.py] are, respectively, the effective and yield stresses in the steel tendons, [[OMEGA].sub.u] is the bond reduction factor which is equal to kl(L/[d.sub.p]), k = 1.5 for one-point loading and 3.0 for third-point and uniform loading, L is the span length, [d.sub.p] is the depth from the extreme compression fiber to the centroid of the prestressing steel
, [E.sub.p] is Young's modulus for the steel tendons, [[epsilon].sub.cu] is the ultimate compressive strain in concrete and is taken equal to 0.003, c is the neutral axis depth, [L.sub.1] is the length of loaded span or sum of loaded spans affected by the same unbonded tendon, and [L.sub.2] is the total length of the unbonded tendons between end anchorages.
The beam cross-section is shown in Figure 1, and the designed strength grade of concrete was C40; the prestressing steel
was [[phi].sup.s] 15 steel strand, the tensile strength of which was [f.sub.ptk] = 1860 N/[mm.sup.2], the inner diameter of corrugated duct was 50 mm, and initial tensile stress [[sigma].sub.con] = 0.75/ptk; bonded tendons were used; the material grade of H-steel (H x B x [t.sub.1] x [t.sub.2] = 100 mm x 100 mm x 6 mm x 8 mm) was Q235, and H-steel was symmetrically arranged in beam cross-section; stirrups are [[phi]12@100 and material was HPB235; the tendon profile is shown in Figure 2.
Where, s 1 and s 2 were the top and bottom fiber stresses, P i was the initial restressing force; A was the cross sectional area; e was the eccentricity of centroid of prestressing steel
with respect to centroid of the gross section; I x was the moment of inertia; C t and C b were the distance from the entroid of gross section to top and bottom fibers, respectively.
These sections cover materials and proportioning of concrete; reinforcement and prestressing steel
; production, placing, finishing, and curing of concrete; formwork performance criteria and construction; treatment of joints; embedded items; repair of surface defects; and finishing of formed and unformed surfaces.
How it works is that post-tensioning tendons - which are prestressing steel
cables inside plastic ducts or sleeves - are positioned in the forms before the concrete is placed.
Concerning SCC phenomena it has to be underlined that in prestressing steel
they evolve through complex and different mechanisms, where anodic metal dissolution and hydrogen embrittlement (HE) are not unique stages [11-12].
On 4 April, the European Commission reduced by 80% the fine levied against steel giant ArcelorMittal for participating in a cartel operated by prestressing steel
The stress-laminated technique consists of a series of timber planks placed side by side between supports of the bridge and compressed transversely with high-strength prestressing steel
With prestressing steel
a key component of construction materials, the Commission is concerned about the inflationary effect of the cartel on a wide range of economic sectors.
The concrete provides perfect protection for the steel, passivating any corrosive reactions, and the prestressing steel
keeps the concrete in the tank walls under constant compression to enhance its water-tightness.