Introduction To Prestressing.pdf

www.civil.eng.usm.my

Demonstrate prestressing.

basic

concepts

in

1.

Explain the principle of prestressing; 2. Describe the prestressing methods; 3. Summarize the prestressing classifications.

• As the load increases, the beam deflects slightly and then fails abruptly. • Under load, the stresses in the beam will be compressive in the top fibers, but tensile in the bottom fibers. • Concrete is strong in compression, but weak in tension. As can be expected, the beam cracks at the bottom and break, even with a relatively small load.

There are two ways of countering the phenomenon of “low tensile strength” in concrete structures: 1. Reinforcement → reinforced concrete (RC) 2. Prestressing → prestressed concrete (PSC)

• Steel is strong in tension. • Steel bars are used as reinforcement. • In RC, concrete is designed to resist compression and to hold bars in position, and steel is used to resist tension. • Tensile strength of concrete is neglected (i.e. zero). • RC beams allows crack under service load.

• To increase the concrete's strength further, a compressive stress (prestressing) is induced into a concrete member before it begins its working life.

• The compressive stress is positioned to be in areas where tensile stresses will develop under working load.

• The initial load or ‘pre-stress force’ is applied to enable the structure to counteract the stresses arising during its service period.

The application of a force to the structure, other than the applied loads, for the purpose of introducing the internal stresses of suitable magnitude and distribution, which assists the performance of the structure.

PSC is a modified form of reinforced concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.

1.

Control or eliminate tensile stresses in the concrete (cracking) at least up to service load levels.

2.

Control or eliminate deflection at some specific load level.

3. Allow the use of high strength steel and concrete.

1.

Improved performance of concrete in “ordinary” design situations (compared to R/C).

2.

Extended range of application of structural concrete (longer spans).

3.

Innovative forms of structures.

Consider a simply supported rectangular beam carrying a uniformly distributed vertical load (Fig. 1.4):

Now, we are aiming to put an initial compression in the beam so that the tension stress (due to external loads) will cancel out. If “the prestress force P” is introduced and applied along the CoG, how much P is required to cancel out the tension stress? The applied prestressing force, P, required to cancel out the tensile stress is: 𝑃 − 9.6 = 0 𝐴 𝑃 = 9.6 × 500 ∗ 200 = 960 kN The stresses developed at the top fiber: 960 + 9.6 = 19.2 NΤmm2 200 × 500

However, the stresses developed in the beam must be limit to a certain value = permissible stress limits, hence, the stresses at the top and bottom must be vary over the full range of permissible stresses for the two extreme loading conditions by introducing an eccentricity, e.

A rectangular concrete beam of cross-section 30 cm deep and 20 cm wide is prestressed by means of 15 wires of 5 mm diameter located 6.5 cm from the bottom of the beam and 3 wires of diameter of 5 mm, 2.5 cm from the top. Assuming the prestress in each wire is 840 N/mm2, calculate the stresses at the extreme fibers of the mid-span section when the beam is supporting its own weight over a span of 6 m. If a uniformly distributed live load of 6kN/m is imposed, evaluate the maximum

working stress in concrete. The density of concrete is 25kN/m3.

1.

Section remains uncracked under service loads • Reduction of steel corrosion • Increase in durability. • Full section is utilized • Higher moment of inertia (higher stiffness) • Less deformations (improved serviceability).

• Increase in shear capacity. • Suitable for use in pressure vessels, liquid retaining structures. • Improved performance (resilience) under dynamic and fatigue loading.

3. 2. High span-to-depth ratios Larger spans possible with prestressing (bridges, buildings with large column-free spaces). Structure

RCS PSC

Span to depth ratio 28:1 45:1

Suitable for precast construction: • The advantages of precast construction are as follows. 1. Rapid construction 2. Better quality control 3. Reduced maintenance 4. Suitable for repetitive construction 5. Multiple use of formwork • Reduction of formwork 6. Availability of standard shapes.

: 1.

Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete.

2. The use of high strength materials is costly. 3. There is additional cost in auxiliary equipment.

4. There is need for quality control and inspection.

The main factors for concrete used in PSC are: 1. Ordinary Portland cement-based concrete is used but strength usually greater than 50 N/mm2;

2. A high early strength is required to enable quicker application of prestress; 3. A larger elastic modulus is needed to reduce the shortening of the member; 4. A mix that reduces creep of the concrete to minimize losses of prestress.

The steel used for pre-stressing has a nominal yield strength of between 1550 to 1800 N/mm2. The different forms of steel may take are: 1. Wires: individually drawn wires of 7 mm diameter; 2. Strands: a collection of wires (usually 7) wound together and thus having a diameter that is different to its area; 3. Tendon: A collection of strands encased in a duct – only used in post-tensioning; 4. Bar: a specially formed bar of high strength steel of greater than 20 mm diameter.

• There are 3 basic types of highstrength steel commonly used as tendons in prestressed concrete construction; 1. Cold-drawn stress-relieved round wire; 2. Stress-relieved strand; 3. High-strength alloy steel bars.

Wires

• Tendon is defined as a wire, strand, cable or bar (or any discrete group of wires, strands or bars) that is intended to be either pretensioned or posttensioned.

• Indented wire: There are circular or elliptical indentations on the surface.

• Wires are cold-drawn solid steel elements, circular in cross-section, with diameter usually in the range of 2.5 – 12.5 mm. The different types of wires are as follows. • Plain wire: No indentations on the surface.

Strands

a) Plain wire

b) Indented wire

• The typical characteristic tensile strength fpk for wires is in the range of 1570 – 1860 MPa. Wires are sometimes indented or crimped to improve bond characteristics.

• A few wires are spun together in a helical form to form a prestressing strand. The different types of strands are as follows. • 2-wire strand: Two wires are spun together to form the strand. • 3-wire strand: Three wires are spun together to form the strand. • 7-wire strand: In this type of strand, six wires are spun around a central wire. The central wire is larger than the other wires. • 19-wire strand: 19 wires are spun together to form the strand.

Strands

• Stress-relieved strand is the most commonly used prestressing steel. • 7-wire strand is widely used in both pretensioned and post-tensioned applications. • 19-wire strand consists of 2 layers of 9 wires or 2 layers of 6 and 12 wires spirally wound around a central wire. • The 19-wire strand is used in posttensioned application, and not recommended for pretensioned applications due to relative low surface area to volume ratio.

Tendon

Cables

• A group of strands are placed together to form a prestressing tendon. The tendons are used in post-tensioned members. The following figure shows the cross section of a typical tendon. The strands are placed in a duct which may be filled with grout after the posttensioning operation is completed.

• Cables consist of a group of tendons formed by multiwire strands woven together as shown in Fig 1.9 below.

Bars • A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire. • Bars are available in the range of diameter 20 – 50 mm with typical characteristic minimum breaking stresses in the range of 1030 – 1230 MPa.

Example of Prestressing Contractors in Malaysia: • Freyssinet PSC (M) Sdn Bhd; • VSL Engineers (M) Sdn Bhd; • BBR Construction System (M) Sdn Bhd.

• The high-strength steel tendons are pulled (pretensioned) between two end abutments prior to the casting of concrete. • The abutments are fixed at the ends of a pre-stressing bed. • Once the concrete attains the desired strength for pre-stressing, the tendons are cut loose from the abutments.

• The pre-stress is transferred to the concrete from the tendons, due to the bond between them. • During the transfer of pre-stress, the member undergoes elastic shortening. • If the tendons are located eccentrically, the member is likely to bend and deflect

Stages of pretensioning

1.

Anchoring of tendons against the end abutments

2. Placing of jacks 3. Applying tension to the tendons 4. Casting of concrete 5. Cutting of the tendons

• The essential devices for pretensioning are as follows : • Prestressing bed; • End abutments; • Shuttering/mould; • Jack; • Anchoring device; • Harping device (optional).

• An extension of the previous system is the Hoyer system. This system is generally used for mass production. The end abutments are kept sufficient distance apart, and several members are cast in a single line. The shuttering is provided at the sides and between the members. This system is also called the Long Line Method.

• The jacks are used to apply tension to the tendons. Hydraulic jacks are commonly used. These jacks work on oil pressure generated by a pump. • The principle behind the design of jacks is Pascal’s law. • The load applied by a jack is measured by the pressure reading from a gauge attached to the oil inflow or by a separate load cell.

A double acting hydraulic jack with a load cell.

Anchoring devices are often made on the wedge and friction principle.

In pre-tensioned members, the tendons are to be held in tension during the casting and hardening of concrete. Here simple and cheap quick-release grips are generally adopted.

Examples of anchoring devices

Advantages • Pre-tensioning is suitable for precast members produced in bulk. • In pre-tensioning large anchorage device is not present.

Disadvantages • A prestressing bed is required for the pre-tensioning operation. • There is a waiting period in the prestressing bed, before the concrete attains sufficient strength. • There should be good bond between concrete and steel over the transmission length.

• 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.

• In unbonded post-tensioning, the ducts are never grouted and the tendon is held in tension solely by the end anchorages.

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 tendons.

• Casting bed • Mould/Shuttering • Ducts • Anchoring devices • Jacks • Couplers (optional)

• Grouting equipment (optional).

Casting Bed, Mould and Ducts

Anchoring Devices

Couplers

• In post-tensioned members the anchoring devices transfer the prestress to the concrete.

• The couplers are used to connect strands or bars. They are located at the junction of the members, for example at or near columns in posttensioned slabs, on piers in posttensioned bridge decks.

• The devices are based on the following principles of anchoring the tendons. • Wedge action • Direct bearing • Looping the wires

Grouting • Grouting can be defined as the filling of duct, with a material that provides an anticorrosive alkaline environment to the prestressing steel and also a 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.

Source of prestressing force:

External or internal prestressing:

• This classification is based on the method by which the prestressing force is generated.

• This classification is based on the location of the prestressing tendon with respect to the concrete section.

• There are three sources of prestressing force: Mechanical, hydraulic and electrical.

External prestressing of a box girder Internal prestressing of a box girder

Pre-tensioning or post-tensioning

• This is the most important classification and is based on the sequence of casting the concrete and applying tension to the tendons. Linear or circular prestressing • This classification is based on the shape of the member prestressed.

Circularly prestressed containment structure

Full, limited or partial prestressing

• Based on the amount of prestressing force, three types of prestressing are defined. Uniaxial, biaxial or multi-axial prestressing • As the names suggest, the classification is based on the directions of prestressing a member.

Biaxial prestressing of a slab