PRESTRESSED CONCRETE
Concrete is strong in compression
and poor in tension. With prestressed concrete a precompression
is induced into the member to make full use of its own inherent
compressive strength when loaded. There a
re two types of prestressed concrete.
|
01. |
Pre-tensioned
concrete
In this method, the tendons (or reinforcements) are tensioned
before the concrete is poured. Steam curing is used to accelerate
this process to achieve 24-hour strength in concrete. Stressing
of tendons is done with the help of hydraulic jacks. After
curing the tendons are released or cut and the bond between
the stressed wires and the concrete prevents the tendons
from regaining their original length maintaining the prestresses.
|
|
02. |
Post
tensioned concrete
In post-tensioning the concrete is cast around ducts or
sheeting in which the tendons are to be housed. Stressing
is carried out after the concrete is cured by means of hydraulic
jacks. After stressing, the duct is filled with grout to
prevent corrosion of tendons.
|
In general
it is found that i. up to 6.0m span traditional reinforced concrete
is the most economic method. ii. for spans between 6m - 9m the
two options are compatible iii. over 9m spans prestressed concrete
is economical
Main advantages of prestressing
Makes full use of the inherent compressive strength of concrete.
Makes full use of the special alloy steels used to form the prestressing
tendons.
Eliminates tension cracks thus reducing the risk of corrosion
of steel components.
Reduces sheer stresses
For any given span and loading condition a component with a smaller
cross section can be used thereby giving a reduction in weight.
Individual precast units can be joined together to form a composite
member.
Main disadvantages
High degree of control over materials, design and quality of workmanship
is required.
Special alloy steels are dearer than most traditional steels used
in reinforced concrete.
Extra cost of special equipment required carrying out the prestressing
activities.
Cost of extra safety requirements needed whilst stressing tendons.
