Pre-Stressing Concrete
We all Know that concrete is usually weak in tension, that's the reason we add reinforcements in the tension zone of a concrete structure. Structures undergo deformations even when reinforcements are added, so in that case Prestressed concrete comes into play. The concept of pre-stressing is simple, i.e when a structure is said to undergo deformation it actually bends in a downward direction so to prevent it from bending downwards it is uplifted during or after casting by compressing the structure with the help of tendons (pre-stressing steel).
Before diving into the topic let's now discuss pre-stressing steel
WIRE
A pre-stressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. There are two types of wire
1) Plain wire: No indentations on the surface.
2) Indented wire: There are circular or elliptical indentations on the surface.
STRANDS
A few wires are spun together in a helical form to form a prestressing strand. The different types of strands are as follows. 1) Two-wire strand, 2) Three-wire strand, 3) Seven-wire strand. The central wire will be larger than the other wires.
TENDONS
A group of strands or wires are placed together to form a prestressing tendon. The tendons are used in post-tensioned members.
CABLES
A group of tendons form a prestressing cable. The cables are used in bridges.
This Prestressing technique is of two types
1) Pre-Tensioning
2) Post-Tensioning
In Pre Tensioning the high tensile steel is stretched between two bulkheads in a prestressing bed or within a self-straining frame called a stress bench. First, moulds are placed in position between the bulkheads or within the self-straining frame. The high tensile steel is threaded through the moulds at required levels with or without eccentricity to the elements to be cast. The steel is then stretched and anchored on the bulkheads or on the booms of the self-straining frames. Afterwards concrete is poured in the mould and allowed to set. When the concrete is well set and has attained required strength, the high tensile steel is released from the bulkheads or the self- straining frames. The steel, not being able to get back to normal position because of the bond established between steel and concrete during setting, compresses the well set concrete structure by bond.
This is preferred or carried out when
The structural elements to be prestressed is small and can easily be transported.
Concrete structure is to be pre-fabricated.
More number of similar prestressed structural elements are to be produced.
Number of external anchorages and the corresponding costs are high.
Labour at site is costly.
In Post Tensioning , the structural elements are cast first with holes left for prestressing elements to run through. The holes are provided with metal sheathing ducts placed in proper position and they are called cable ducts. The prestressing steel elements are threaded through cable ducts later. After the concrete attains the required strength, the prestressing steel elements are pulled against the concrete elements and anchored at the ends by means of special type of patented external anchorage systems. The force is transmitted through the external anchorage systems at the ends of the structural concrete elements.
The structural elements to be prestressed is very heavy.
Number of external anchorages and the corresponding costs are not high.
The product is unique and not repeated many times.
This is done in-situ environments.
Labour at site is not very costly.
Post-Tensioning is of two types
Bonded post-tensioning- Bonded post-tensioning has prestressing tendons permanently bonded to the surrounding concrete by the in situ grouting of their encapsulating ducting following tendon tensioning. This grouting is undertaken for three main purposes: to protect the tendons against corrosion; to permanently "lock-in" the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain structural behaviors of the final concrete structure.
Unbonded post-tensionin- Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of longitudinal movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a corrosion-inhibiting grease, usually lithium based. Anchorages at each end of the tendon transfer the tensioning force to the concrete, and are required to reliably perform this role for the life of the structure.
These techniques have been used in constructing great structures like Capital gate, World Tower, Ocean Heights 2.
Both techniques have their own usage, advantage and disadvantages.
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