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Dr. hammida
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• What is Prestressing?
Prestressing is a method of reinforcing concrete,
counteracting applied loads.
• Pre-Tensioning Steel tendons are stressed prior to concrete placement,
usually at a precast plant remote from the construction
site. • Post-Tensioning
Steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction
Prestressed Concrete
In post-tensioning systems
the ducts for the tendons (or strands) are placed along with the
reinforcement before the casting of concrete. The tendons are placed in the ducts
after the casting of concrete. The duct prevents contact between concrete and the
tendons during the tensioning operation.
Unlike pre-tensioning the tendons are pulled with the reaction acting against the hardened
concrete.
if the ducts are filled with grout
then it is known as bonded post-tensioning. The grout is a neat cement paste or a sand-cement mortar containing suitable admixture. The
grouting operation isdiscussed later in the section.
In unbonded post-tensioning,
as the name suggests, the ducts are never grouted and the tendon is held in tension solely by the end anchorages.
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The various stages of the post-tensioning operation are summarised as follows.
1) Casting of concrete.
2) Placement of the tendons.
3) Placement of the anchorage block and jack.
4) Applying tension to the tendons.
5) Seating of the wedges.
6) Cutting of the tendon
The relative advantages of post-tensioning as compared to pre-tensioning are as
follows.
• Post-tensioning is suitable for heavy cast-in-place members.
• The waiting period in the casting bed is less.
• The transfer of prestress is independent of transmission length.
Grouting Grouting can be defined as the filling of duct, with a material
that provides an anti corrosive alkaline environment to the prestressing steel and alsoa strong bond between
the tendon and the surrounding grout.
The major part of grout comprises of water and cement, with a water-to -cement ratio of about 0.5, together with some water-reducing admixtures, expansion agent and
Pozzolans
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Loss of Prestressفقدان االجهاد
المسلط على حديد التسليح مسبقة االجهاد يقل بشكل مؤثر مع مرور الوقت ان االجهاد -
-:االجهاداتوفيما يلي اهم العوامل التي تؤثر على فقدان
.%3وهو بحدود elastic shortening of cocncrete انضغاط الخرسانه المرن .1
%7-6بحدود creep of cocncrete & shrinkage انكماش وزحف الخرسانه .2
%3-2بحدود relaxation of creep in trndons ارتخاء وزحف حديد التسليح .3
post tenstion في الشد الالحقاالحتكاك بين حديد التسليح واالنابيب المستخدمة .4
5. tension friction along the duct used in post
حديد التسليح داخلد دات على طول حديد التسليح واسلوبمقدار الفقدان في االجها ويعتم -
.فقدان باالجهاد لالسباب اعاله% 22-51االنابيب بشكل مستقيم او منحني وبشكل عام يمكن قبول بنسبة
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Dr. hammida
