Basic Concept of Prestressing

Historical Background

1872, Jackson, P.H (USA)

Used a tie Rod to construct beams and arches from individual blocks.

1888, Doehring, CEW. (Germany)

Used Metal Wires for Prestressing beams and Slabs.

Historical Background

These attempts were not successful because of loss of prestress with time.

Unavailability of high strength steel to overcome prestress losses due to shrinkage and creep.

1908, stainer, C.R. (USA) Recognized losses due to shrinkage and creep and suggested retightening the rods to overcome lost prestress.

Historical Background

1924, Hewett, W.H. (USA)

Introduced Hoop-stressed horizontal reinforcement around walls of concrete tanks.

1925, Dill, R.H. (USA)

Used High strength unbonded steel rods.

1926, Eugene Freyssinet (France)

Proposed the use of High strength steel wires to overcome prestress losses.

(Father of Prestressed Concrete)

Historical Background

During the seond world war, application of prestress and precast concrete increased rapidly.

T.y. Lin contributed a lot to the art of science of prestressed concrete.

Prestressing Structures

Prestressing Structures

Early attempts

Early attempts

Original Tensile strain = s/ E = 0.5 Fy / E

= 140/ 200,000= 0.0007

Total Loss in strain = Compressive strain due to transfer of prestress + compressive strain due to (creep & shrinkage)

≈ 0.0007

Residual strain is negligible

Solution is to use high strength steel

Advantages of prestressing

1- Considerable reduction in depth of section

Relative to RC Less S.W. (Self Weight)

Longer Spas.

More aesthetic Appeal.

Span/ Depth For Slabs

- Prestressed Slab 45:1

- RC Slab 28:1

Advantages of prestressing 2- Uncracked Concrete under service loads

- Reduction of steel corrosion

- Uncracked Section Higher Moment of Inertia

Less Deformation

3- Suitable for Precast construction

- Rapid Construction. - Better Quality Control.

- Reduced Maintenance. - Suitable for repetitive construction.

- Multiple Use of formwork. - Availability of standard sections

Shapes of Prestressed Elements

Concerns of Prestressing

Need for skilled personnel.

Higher Unit Cost of stronger Materials.

Need for Expensive accessories.

Necessity for close inspection and quality control.

In the case of pretension, a higher initial investment in plant.

Forms of Prestressing Steel

Wires (Single Unit made of steel)

Strands (Two, Three, or Seven wires are wound to form a prestressing strand)

Tendon (A group of strands or wires- Normal Method for applying prestress Force).

Cable ( A group of Tendons)

High Strength Bars.

Forms of Prestressing Steel

Methods of Prestressing

I. Pre-Tension

II. Post-Tension

Pre-Tension:

- Tendons are tensioned before the concrete is placed.

- Usually performed in a casting yard in the following steps:

1. Tendons are placed in a prescribed pattern between two anchorage abutment, Tendons are then tensioned to the required force.

Pretension

Pretension

Pretension

Pretension

Concerns:

- Expensive molds.

- Prepared Plants Anchorage Abutments

Curing

Post-Tension

Post-Tension

Post-Tension

Post-Tension

5- The space in the ducts around the strands may be grouted using:

Pumped Grout Bonded Post-Tensioned

Otherwise Un-bonded Post Tensioned

Post-Tension

Post-Tension

Post-Tension

Advantages of Post-Tension: - Suitable for big cast-in place members.

Concerns:

- Requirement for anchorage devices.

- Stresses are transferred via anchorage devices.

Why Un-bonded?

1) Grouting is eliminated.

2) Ability to de-stress the strands

3) Replaceable.

Post-Tension

Why Bonded?

1) Tendons are less likely to de-stress.

2) Higher Ultimate strength due to bond generated between the strand and the concrete.

3) No Maintenance is needed due to:

Corrosion

Anchorage Ends

Post-Tension

Post-Tension

Duct

Anchorages

Anchorages

Jacking

Jacking

Grouting

Post-Tension

Post-Tension

Post-Tension

Source of Prestressing Force

1) Hydraulic Prestressing (Most Popular).

2) Thermal Prestressing by application of Electric Heat (The wires are anchored before placing the concrete in the molds).

3) Chemical prestressing by means of expansive cement which expands chemically after setting and curing hardening.

Types of Prestressing

Based on Location of Tendons:

1- External Prestressing:

- Tendons Lie outside the member or inside the hollow space or box girder.

- This technique is adopted for bridges and strengthening of existing structures.

2- Internal Prestressing:

- Tendons Lie inside the concrete member ( Concrete of the member is casted around the ducts.

Types of Prestressing

Based on amount of prestressing Force:

1. Full Prestressing: No tensile stress under service loads.

2. Partial Prestressing: Crack width is within allowable limits.

3. Limited Prestressing: Tensile stresses are within cracking strength of the concrete.

Types of Prestressing

Based on the direction of prestressing:

1. Uniaxial: Parallel to one axis. – Example: Beams.

2. Biaxial: Parallel to two axis. Example: Slabs.

3. Multiaxial: Parallel to more than two axis. Example: Domes.

Based on the shape of the member:

1. Linear: Beams & Slabs.

2. Circular: Tanks & Silos.