Understanding Prestressed Concrete: Principles and Methods
Prestressed Concrete (PSC)
For long-span beams, prestressed concrete (PSC) is used because prestressing prevents cracking. This is achieved by applying an initial compressive load, or ‘prestress,’ using high-strength steel tendons in a concrete member. This counteracts or neutralizes the tensile stresses that arise during its service life.
PSC +
- PSC sections remain uncracked under service loads, reducing the risk of steel corrosion and improving durability. The uncracked section provides higher stiffness and less deflection, improving serviceability, and increasing shear capacity due to pre-compression.
- Higher span-to-depth ratio: Larger spans are possible with prestressing, allowing for larger column-free spacing in buildings and bridges.
PSC Limitations
- Higher costs for normal structures due to specialized site operations.
- Loss of prestress due to anchorage slip, concrete shrinkage and creep, and steel relaxation, which reduces the usable prestressing force.
- More complex design and construction compared to reinforced concrete.
Examples
Wooden barrels: Metal bands tighten under tensile stress, inducing an initial hoop compression between the staves, counteracting the hoop tensile pressure created by the liquid inside.
Stack of books: Books can be held together and moved as a single elongated object if pressed together hard enough from both sides.
Common PSC Terms
Tendon: A group of wires or strands, or one or more hot-rolled bars wound together.
Wire: A single unit made from a hot-rolled rod that is cold-drawn through dies to increase its strength. Nominal diameter is typically 5 or 7mm.
Strand: High-strength steel wires wound helically around a center wire, usually in a 7-wire arrangement.
Hot-rolled bar: A long, slender structural element, usually made from threaded high-strength steel. A tendon can be made of a single steel bar.
Unbonded tendon: A tendon in which the tensioned element (strand, wire, or bar) is unbonded from and permanently free to move relative to the surrounding concrete.
Bonded tendon: A tendon that is permanently connected to the surrounding concrete along its full length between its end anchorages. This connection can be direct or through internal grouting of the tendon’s ducting.
Duct: An encapsulating tube-like component, usually constructed from galvanized steel or polythene, which creates a void space within the concrete and surrounds the strands, wires, or bar of a post-tensioned tendon. Ducts can be circular or rectangular with round corners.
Anchorage: A rigid component, commonly constructed of cast iron, located at the end of a tendon and which directly transfers the tendon’s force to the surrounding concrete.
Dead end anchorage: A “passive” end anchorage of a prestressing tendon where no jacking operations are undertaken.
Live end or stressing end anchorage: An “active” end anchorage of a prestressing tendon where jacking operations are undertaken.
Fully Prestressed Concrete (PSC): In a fully prestressed condition, the concrete member remains crack-free as flexural tensile stresses are not allowed under full working load.
Partially Prestressed Concrete (PSC): In a partially prestressed condition, tensile stresses are permitted. Partially prestressed concrete is similar to conventional reinforced concrete, where cracking is allowed in the flexural tensile zone.
Basic Methods of PSC
The two basic methods are: (1) pre-tensioning and (2) post-tensioning.
- Pre-tensioning: Tendons are tensioned, one at a time, before the concrete is placed. Each tendon is anchored at the dead end buttress or abutment using a barrel-and-wedge anchorage device and is tensioned at the live (stressing) end with a proper prestressing jack using another anchorage device. After the required tension is attained, concrete is cast into the formwork following the proper compaction procedure. Once the concrete has hardened (achieved its design strength), the tendons are severed. This action causes the steel under tension to return to its original position, which is prevented by the concrete. The concrete resists the steel tension as compression; in other words, the steel, with its tendency to shrink in length, transfers its tension onto the concrete. This method allows for more than one beam to be manufactured in a single operation and is mainly used in factory situations (precast beams).
- Post-tensioning: Tendons are tensioned after the concrete is placed. Initially, steel or plastic ducts are placed at the desired or predetermined profile. After the concrete is cast, properly compacted in the formwork, and hardened to the required strength, the tendons are inserted into the ducts and tensioned using an appropriate prestressing jack by reacting against the concrete. Grout may be injected into the duct later to bind the tendon with the duct and the rest of the beam. This can be done to prestress either a precast beam or a cast-in-place beam.
2- AS3600 tables will not be provided. You can bring whatever you feel is important on 4 x A4 sheets (8 pages). These pages will be submitted with your exam booklet.
3- If you think that you are missing any data in the exam, you can assume a suitable number and state this assumption clearly. The calculations will be marked according to that assumption. Each one may get a different answer, but we will take care and mark appropriately.
The system is tuned to mark what you know best for the question and not as a punitive tool. Have faith in the system and do your best.