History
Although concrete was used for a
long time as a construction material, it was not comparable with steel, stone
wood at all, because despite its high pressure resistance, it acts weakly when facing
tensile stress. As a result, it was only used as a masonry material. Later, engineers applied a
combination of reinforcing steel and concrete which resists tension and tensile
pressure in order to get over this problem. They applied bar to reinforce
concrete in early stages. However, they applied pre-stressed technology later
in order to change concrete as one of the most efficient construction material.
The idea of pre-stressing was presented in first decade of 20thcentury
and a variety of research was done during 1930 to 1940. This method has been
applied since 1955 in various construction fields.
Theory of PreStressing
Pre-Stressing
refers to imposing permanent compression force before imposing service loads in
order to reduce and/ or remove tensile stresses.
Locating
pre-stressing steel and anchoring it in both sides of the member creates
constant (compression stress) in concrete section. Alongside the bending
members, both the lower fiber (at the middle of the span) and upper fiber (on
the support) have tensile stress. In pre-stressed sections, the distribution of
tensile stress is controllable by replacing the cable position.
Prestreesing Methods
Pre-Stressing
is done in 2 ways, pre tensioning and post tensioning. It means stressing the
cables can be done before or after casting concrete. In pre tensioning method,
at first cables stressed on prefabricated formworks between two ends. After
casting concrete and obtaining the required compressive strength, they are cut
and the existing force is transferred to concrete by pressure. This method is
used in concrete pieces producing in factories and plants, and the products are
usually prefabricated. In
post tensioning method, cables are located in a structural element (inside
duct). Then concreting and following its achieving to required compressive
strength, it is stressed and anchored.
This method can be applied in producing prefabricated and cast in situ pieces.
Cables are placed inside post tensioning pieces as bonded or unbonded systems.
Bonded System
Here,
several cables are located in one metal duct and all of them are ended to an
anchor. The total series of these ducts are installed on the formwork and
stressed after concrete casting and reaching required strength. To make a link
between cable and concrete, grout is injected inside duct and consequently
cable and duct are adequately stuck together. Thread like pattern on the duct
is designed to create an appropriate bond with concrete. So, a reliable bond is
established between cable and concrete, too. Bonded method is mostly used when
high pre-stressing force is needed. Other items are among the usages of this
system:
- pre-stressed bridges
- pre-stressed beams for large span
- Transfer Plates
- pre-stressed foundations
Unbonded System
In
this method, stress force is transferred to concrete through ending anchors and
cable has no link with concrete alongside it. Unlike bonded method, here, each
cable has plastic coating and is held directly by an anchor. Since there is no
need for bonding between cable and concrete, grout injection is omitted in this
system. Unbonded system is mostly used when concrete members have small
thicknesses and it isn’t possible to place bigger ending anchors used for
anchoring several cable strands. The following items are among the usages of
this system:
- Building Slab
- Slab on Ground
