what is post tensioning[2]

8/8/2019 What is Post Tensioning[2]

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WHAT IS POST TENSIONING


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The principle idea of Prestressed concrete is to introduce compression in theTension zone or reinforced concrete in order to reduce deflection and cracking


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PRESTRESSED CONCRETE

PRE-TENSIONING

( usually pre-cast )

POST-TENSIONING

(cast insitu )

BONDED SYSTEMUNBONDED SYSTEM


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Post tensioning construction procedure1- post tensioning ducts are placed in the slab according to the design profile

2- high strength strands inserted inside

3- Dead ends are made by special machine at one end of the strands

4- live ends anchors are placed at the other ends of the strands

5- minimum reinforcement are placed according to the design

6- concrete are poured in normal way of reinforced concrete slabs

7- after 12 hours initial stress of 20% are done in order to reduce the plastic cracks

8- full stress of the tendons are done after 3 days when the concrete strength reachat least 25 N/mm2

9- The tendons are grouted with special grout in the bonded system

10- The anchors places are closed with a grout

11- The scaffolding can be removed for further construction


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BONDED AND UNBONDED SYSTEMS

Bonded systemFlat duct usually galvanized steel 20m By 70mm is used

Multi strands are inserted inside ( 3 to 5 strands)The duct is grouted with non-shrinking grout after full tensioning of the strands

One end is dead end and the other is live end unless the tendon is too long

Un-bonded systemRound Sheathing ducts (usually plastic) are used

Single strand is inserted in the ductThe duct if full grease for corrosion protection

Two live ends anchors are used


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Deference between bonded and un-bonded system

Bonded system:1- The ducts and strands are integrated with the concrete in similar way of ordinary

Reinforcement2- The failure of anchorages and local cut of the strands will not resulted in total

failure of the post tensioning forces3- Need no maintenance during the useful life of the structure4- demolition can be done as in ordinary reinforced concrete5- less flexible to deflect tendons horizontally around opening

Un-bonded system :

1- more flexibility in placing tendons in the plan2- anchorage failure or strands cuts will result in total loss of the prestressing forces3- Needs maintenance during the life of the structure4- needs two live ends for all tendons5- demolition need special care and safety as it can be explosive


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Advantages of post tensioning system

1- Construction proceed like ordinary cast in place reinforced concrete

2- Less time is required for the placement of the PT and conventional reinforcement

3- Faster construction can be achieved because scaffoldings can be removed after3 days

4- It more economic than reinforced concrete


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5- Less deflection and cracks under service loads


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6- less slabs thickness is required which resulted in saving of the floor heightsand loads on the columns and footings in addition to giving more flexibility

in AC design


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Live ends of tendons ( bonded system )


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Dead end of tendon ( bonded system )


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Post tensioning machines


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The Grouting pump

Dead End Machine ( onion machine )

PT Strands


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Actual Construction photos ( UAE )

PT Circular beam at Al Wahda mall ( Abu Dhabi )


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Dead End (3 strand)

Live End


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Tendons and reinforcement layout (Ajman site)


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Post tensioning operations


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Post tensioning slabs types

Almost all types ofSlabs can be usedIn post tensioningsystems


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Comparison study of RC and PT slabs systems

The study based on 300 Dh. For one cubic meter of concrete and 2.5 Dh. For

One kilogram of reinforcement including fixing , the price of the post tensioning aretakenFrom the actual construction practice in Dubai

A- 6 meters span slab

1- R.C. System :-Slab thickness is 25 cm-Concrete cost 75 Dh./m2- Reinforcement ( 140 kg/m3) 87.5Dh/m2-Total cost 162.5 Dh/m22- PT System :-Slab Thickness is 20 Cm , reinforcement content is 40 kg/m3- Concrete cost 60 Dh./m2

-Steel cost (40kg/m3) 20 Dh./m2-PT cost 45 Dh./m2-Total cost 125 Dh./m2

-Saving 37.5 Dh./m2


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B- 8 meters span solid slab

1- RC system 30 cm thick-Concrete cost 90 Dh/m2-Steel cost (150kg/m3) 112.5 Dh/m2-Total cost 202.5 Dh/m2

2- PT system 22cm thick-Concrete cost 66 Dh/m2-Steel cost ( 45kG/m3) 25 Dh/m2-PT cost 46 Dh/m2

- Total cost 137 Dh/m2

-Saving 65.5 Dh./m2


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C- 10 meters solid slab

1- RC slab system 40cm thick- Concrete cost 120 Dh./m2

-Steel cost (170 kg/m2) 170 Dh/m2- Total cost 290 Dh./m2

2- PT system 28 cm thick-Concrete cost 84 Dh./m2

-Steel cost (50 kg/m3 ) 35 Dh./m2-PT cost 52 Dh./m2-Total cost 171 Dh./m2

Total saving 119 Dh./m2

P t t i i d i


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Post tensioning design

The post tensioning design mainly done by sophisticated structural softwareUsing either equivalent frame method or finite element method , In both caseAn accurate results are obtained and the concern code requirement are satisfiedThe tendons and steel are accurately located and the stresses and the deflections

Are calculated

3D model by computer


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Tendon location and profiles

Are done by the software

Tendons can be visualized in3D for checking


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Deflection can be visualized graphically

Stress can be displayed graphically

For accurate design


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Code requirement can be checkedAutomatically by the software

Reinforcement locations andAmount can be drawn by thesoftware


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Thank YouThank You

what is post tensioning[2]

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