For long span beams, prestressed concrete is used cos PS prevents occurrence of cracking. This is by applying an initial compressive load or ‘PS’ by high-strength steel tendons in a concrete member to enable it to counteract or neutralise the tensile stresses arisen during its service period.

PS-C +

1. PSC section remains uncracked under service load; reduced risk of corrosion to steel and hence improved durability; whole section is uncracked > higher stiffness & less deflection (i.e., improved serviceability) -increase in shear capacity (due to pre-compression)

2. Higher span to depth ratio ; Larger span is possible with PS >  larger column free spacing

in building & bridges.

PSC Limitations

1. $$ for normal structures as special site operation.

2. Loss of PS due to anchorage slip, concrete shrinkage and creep, & steel

relaxation > reduce usable prestressing force.

3. Design & construction more complex reinforced concrete.


Wooden barrels – metal bands tighten under tensile stress > induce a state of initial hoop

compression between the staves allowing them to counteract the hoop tensile pressure

created by filling of liquid in the barrels.

Stack of books – can be held together & moved as a single elongated object from

one place to another if pressed together from both sides hard enough.

Common PSC Terms

Tendon: It comprises of a group of wires or strands, or one or more hot-rolled bars

wound together.

Wire: A single unit made from hot-rolled rod that is cold-drawn through dies to

increase its strength. nominal diameter (5 or 7mm)

Strand: High-strength steel wires wound helically around a centre 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 surrounded concrete.

Bonded tendon: A tendon which is permanently connected to the surrounding

concrete along the full length between its end anchorages. Such connection can be

direct, or through internal grouting of the tendon’s ducting.

Duct: Encapsulating tube-like component, usually constructed from galvanised steel

or polythene, which creates a void space within the concrete and surrounds the

strands, wires or bar of a post-tensioned tendon. 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 a tendon’s force to the surrounding


Dead end anchorage: A “passive” end anchorage of a prestressing tendon which

does not have any jacking operations undertaken at that end.

Live end or stressing end anchorage: A “active end anchorage of a prestressing

tendon which has jacking operations undertaken at that end.

Fully PSC: In full prestressing condition, concrete member would

be crack-free as flexural tensile stresses are not allowed under full working load.

Partially PSC: In partial prestressing condition, tensile stresses

could be permitted and hence partially prestressed concrete could be regarded similar

to conventional reinforced concrete in which cracking is allowed in the flexural

tensile zone

3.5 Basic Methods of PSC

 (1) pretensioning and (2) posttensioning.

(1) 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 will make the steel under tension to go

back to its original position, which will be prevented by the concrete. Concrete will

resist the steel tension as compression; in other words, the steel with tendency to

shrink its length, will transfer 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).

(2) 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 casted, 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 to the duct later to bind the tendon with the duct and

the rest of the beam. 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 important in the 4 x A4 sheets (8 pages).  These pages will be submitted with your exam booklet.

3- If you think that you miss 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 the best for the question and not as punitive tool. Have faith in the system and do your best.