Post-tensioned structures

Little, B. Staehle, R. (2001). Fungal influenced corrosion in post-tension structures. The Electrochemical Society Interface, Winter 2001, 44-8. 

U. Ntirnberger, Special Problems in Post-tensioned Structures , COST 509 Workshop Corrosion and Protection of Metals in Concrete ,... 

When a high early strength is required, such as for post-tensioned structures or pre-tensioned precast elements, Portland cement with a strength class of 52.5 may be used. Blended cements are usually not suitable because of the slow rate of hydration, with the exception of Portland-silica fume cement and special (fast) slag cements. 

In the present discussion, reinforced concrete and prestressed concrete corrosion control methods are combined. Although prestressed concrete bridges have special concerns such as anchorage in both posttensioned and pretensioned structures and ducts for post-tensioned structures, the general corrosion control methods are applicable to both prestressed and conventional reinforced bridges. 

Particular problems arise when the corrosion problem is the black rust described in Chapter 2 and prestressed, post-tensioned structures where corrosion is difficult to detect as the tendons are enclosed in ducts. Tendon failure can be catastrophic as they are loaded to 50% or more of their ultimate tensile strength and modest section loss leads to failure under load. The particular problems of assessing prestressed concrete structures are addressed in Chapter 4. 

The L S re.search will be watched with interest, It has already been noted that a post-tensioned bridge collapsed in Wales a few years ago. The bridge had no waterproofing on the deck, the ducts were poorly grouted and salt got into the joint.s between segments and corroded the steel. The UK DoT has no idea how many bridges are in a comparable condition in the UK, Further failures are therefore possible. As cathodic protection cannot protect the cables inside ducts on post-tensioned structures, it has limited applicability for protecting such structures,... 

The latest Concrete Society (1995) report on durable bonded post-tensioned concrete bridges will lead to improved design and construction of post-tensioned structures. However, improved repair and assessment methods for the existing structures are needed. 

Ayats, A. Gnagi, B. Elsener, Electrical isolation as Enhanced Protection for Post-Tensioning Tendons in Concrete Structures , Proc. Int. Jib Congress Osaka (2002), Vol. 6 Session 8 pp. 169 176, Japan Prestressed Concrete Engineering Association (2002). 

Concrete is presently used as the primary structural material for land-based storage of LNG facilities for storing some 3 million barrels have been constructed. The concrete is either pre- or post-tensioned in most applications. Both internal and external insulation is used. In some cases, the concrete is in contact with the LNG. The initial skepticism as to the practicality of using concrete as a structural material at LNG temperatures has currently given way to optimism, as practical experience has been gained. 

It can provide the thickness of concrete slabs up to an accuracy of 3% and can determine the location and extent of flaws such as cracks, delaminations, voids, honeycombing and debonding in plain, reinforced and post-tensioned concrete structures It can also locate voids in the subgrade beneath slabs and pavements. For masonry it can determine thickness and locate cracks, voids and other defects where the brick or block units are bonded together with mortar. Impact-echo is not adversely affected by the presence of steel reinforcing bars. 

Price (1966) contended that the majority of joints are post-compressional structures, formed as a result of the dissipation of residual stress after folding has occurred. Some spatially restricted small joints associated with folds, such as radial tension joints, are probably initiated during folding. Such dissipation of the residual stresses occurs in the immediate neighbourhood of a joint plane so that a very large number of joints need to form in order to dissipate the stresses throughout a large volume. 

Structural collapses of reinforced concrete structures due to corrosion are rare. The author knows of two multistory parking structures in North America which collapsed due to deicing salt induced corrosion. A post-tensioned concrete bridge collapsed in Wales due to deicing salt attack on the strands (Woodward and Willicams, 1988), and so did one in Belgium. Concrete damage would usually have to be well advanced before a reinforced concrete structure is at risk. 

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. 

However, it may be feasible to design a system, possibly of interlocked, post-tensioned, prestressed concrete blocks, which would perform tlie structural duties required and maintain adequate biological shielding, but would not require cutting during demolition. As with the PCRV planes of v akness proposals, lack of fit due to tolerances would need to be considered. 

The containment is a prestressed reinforced concrete structure in the shape of a cylinder with a torispherical dome and a flat foundation mat. The cylindrical portion of the containment is prestressed by a post tensioning system consisting of horizontal tendons and vertical tendons. The dome is prestressed with crossed tendons that are anchored at the dome stiffening ring girder. The foundation mat is conventional reinforced concrete. 

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