2. Reinforced concrete:
• Concrete is strong in compression weak in tension.
• Steel in strong in tension
• Reinforced concrete uses concrete to resist compression
and to hold bars in position and uses steel to resist tension.
• Tensile strength of concrete is neglected (i.e. zero )
• R.C beams allows crack under service load.
3. Pre-stressed Concrete
• What is Pre-stressed Concrete?:
stresses.
— In 1904, Freyssinet
attempted to introduce permanent
acting forces in conc. to resist
elastic forces under loads and was
named
“Pre stressing”.
— Internal stresses are induced to counteract extern
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5. Principle of pre-stressingt
• Pre-stressing is a method in which compression force is
applied to the reinforced concrete section.
• The effect of pre stressing is to reduce the tensile stress in
the section to the point till the tensile stress is below the
cracking stress. Thus the concrete does not crack.
• It is then possible to treat concrete as a elastic material.
• The concrete can be visualized to have two compressive force
i.Internal pre-stressing
force. ii . External forces (d.I ,
I.I etc )
• These two forces must counteract
each other.
6. Components of Pre-stressing:
Concrete —M30 to M60 grade
Reinforcement-steel-High tensile strength steel
wires called as cables or tendons.
Hard drawn wires of dia 1.5 mm to 8 mm used
as single wire or cable. OR
High steel alloy bars 9.5 mm to 32 mm
7. Pre-stressed Concrete: Methods
• There are two basic methods of applying pre-stress to
a concrete member
—Pre-tensioning — most often used in factory
situations
—Post-tensioning — site use
8. Types of pre-stressingt
I . Pre-tensioning
In Pre-tension, the tendons are tensioned against some
abutments before the concrete is place. After the concrete
hardened, the tension force is released. The tendon tries to
shrink back to the initial length but the concrete resists it
through the bond between them, thus, compression force is
induced in concrete. Pretension is usually done with precast
members
Tendon ancho age
Compregsion Tensio»
10. II . Post tensioning
• In Post tension, the tendons are tensioned after the
concrete has hardened. Commonly, metal or plastic
ducts are placed inside the concrete before casting.
After the concrete hardened and had enough
strength, the tendon was placed inside the duct,
stressed, and anchored against concrete. Grout may
be injected into the duct later. This can be done either
as precast or cast-in-place.
13. Advantagest
•Take full advantages of high strength concrete
and high strength steel
•Need less materials
•SmaIIer and lighter structure
•No cracks
•Use the entire section to resist the load
•Better corrosion resistance
•Good for water tanks and nuclear plant
•Very effective for deflection control
•Better shear resistance
14. Disadvantages compared to RC:
• Need higher quality
materials
• More complex technically
• More expensive
• Harder to re-cycle
