19-06-2012, 11:41 AM
SEMINAR ON PRESTRESSED CONCRETE
PRESTRESSED CONCRETE.ppt (Size: 134 KB / Downloads: 364)
INTRODUCTION:
Prestressed concrete is method for overcoming concrete’s natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete.
Pretensioned concrete:
Pretensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension.
The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction.
However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line.
This, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size.
Pretensioned elements may be balcony elements lintels, floor slabs, beams or foundations piles.
Bonded post-tensioned concrete:
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ).
The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.
A set of tendons are fished through the duct and the concrete is poured.
When the tendons have stretched sufficiently, according to the design specifications (see Hooke’s law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete.
The duct is then grouted to protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation.
Unbounded post-tensioned concrete:
: Unbounded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete.
To achieve this each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheating formed in an extrusion process.
The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab.
The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and brust out of the slab if damaged (such as during repair on the slab).