Prestressing Concept, Materials and Prestressing System

A prestressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use. The initial load or ‘prestress’ is applied to enable the structure to counteract the stresses arising during its service period. The prestressing of a structure is not the only instance of prestressing. The concept of prestressing existed before the applications in concrete.

Force-fitting of metal bands on wooden barrels

The metal bands induce a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels.

Force-fitting of metal bands on wooden barrels

Pre-tensioning the spokes in a bicycle wheel The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke.

Spokes in a bicycle wheel

Advantages of Prestressing

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1) Section remains uncracked under service loads

Reduction of steel corrosion Full section is utilised 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.

2) High span-to-depth ratios

Larger spans possible with prestressing (bridges, buildings with large column-free spaces)

3) Suitable for precast construction

Rapid construction Better quality control Reduced maintenance Suitable for repetitive construction Availability of standard shapes.

Limitations of Prestressing

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

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

equipment. There is need for quality control and

inspection.

Types of Prestressing Source of Prestressing Force 1)Hydraulic Prestressing This is the simplest type of prestressing, producing large prestressing forces. The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning.

2)Mechanical Prestressing In this type of prestressing, the devices includes weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear drives and wire-winding machines. This type of prestressing is adopted for mass scale production. 3)Electrical Prestressing In this type of prestressing, the steel wires are electrically heatedand anchored before placing concrete in the molds. This type of prestressing is also known as thermo-electric prestressing.

Internal or External Prestressing Internal Prestressing When the prestressing is achieved by elements located inside the concrete member (commonly, by embedded tendons), it is called internal prestressing. Most of the applications of prestressing are internal prestressing. In the following figure, concrete will be cast around the ducts for placing the tendons

Internal prestressing of a box girder

Pre-tensioning or Post-tensioning Pre-tensioning

The tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through bond over the transmission length near the ends.

Post-tensioning

The tension is applied to the tendons (located in a duct) after hardening of the concrete. The pre-compression is transmitted from steel to concrete by the anchorage device (at the end blocks).

Post-tensioning of a box girder

External Prestressing When the prestressing is achieved by elements located outside the concrete, it is called external prestressing. The tendons can lie outside the member (for example in I-girders or walls) or inside the hollow space of a box girder. This technique is adopted in bridges and strengthening of buildings. In the following figure, the box girder of a bridge is prestressed with tendons that lie outside the concrete.

Linear or Circular Prestressing Linear Prestressing

When the prestressed members are straight or flat, in the direction of prestressing, the prestressing is called linear prestressing. For example, prestressing of beams, piles, poles and slabs. The profile of the prestressing tendon may be curved.

Circular Prestressing

When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing of tanks, silos, pipes and similar structures.

Circularly prestressed containment structure

Full, Limited or Partial Prestressing Full Prestressing When the level of prestressing is such that no tensile stress is allowed in concrete under service loads, it is called Full Prestressing.Limited Prestressing When the level of prestressing is such that the tensile stress under service loads is within the cracking stress of concrete, it is called Limited Prestressing. Partial Prestressing When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing.

Uniaxial, Biaxial or Multiaxial Prestressing Uniaxial Prestressing When the prestressing tendons are parallel to one axis, it is called Uniaxial Prestressing. For example, longitudinal prestressing of beams. Biaxial Prestressing When there are prestressing tendons parallel to two axes, it is called Biaxial Prestressing. The following figure shows the biaxial prestressing of slabs. Multiaxial Prestressing When the prestressing tendons are parallel to more than two axes, it is called Multiaxial Prestressing. For example, prestressing of domes.

Pre-tensioning Systems and Devices

In pretensioning, the tension is applied to the tendons before casting of the concrete.

Stages of Pre-tensioning

The various stages of the pre-tensioning operation are summarized as follows. 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.

Stages of pre-tensioning

Advantages

The relative advantages of pre-tensioning as compared to post-tensioning are as follows. Pre-tensioning is suitable for precast members

produced in bulk. In pre-tensioning large anchorage device is not

present.

Disadvantages of Pre-tensioning

The relative disadvantages are as follows. 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.

Travelling pre-tensioning stress bench Anchoring of strands

Stretching of strands

Pouring of concrete Steam curing chamber

Demoulding of sleeper Storage

Post-tensioning Systems and Devices

In posttensioning, the tension is applied to the tendons after hardening of the concrete.

Stages of Post-tensioning The various stages of the post-tensioning operation are summarized 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 tendons.

Advantages of Post-tensioning

The relative advantages of post-tensioning as compared to pre-tensioning are as follows:1)Post-tensioning is suitable for heavy cast-in-place members. 2)The waiting period in the casting bed is less. 3)The transfer of prestress is independent of transmission length.

Devices The essential devices for post-tensioning are as follows. 1) Casting bed 2) Mould/Shuttering 3) Ducts 4) Anchoring devices 5) Jacks 6) Couplers (optional) 7) Grouting equipment (optional).

PRESTRESS LOSSES

Concrete

Concrete is a composite material composed of gravels or crushed stones (coarse aggregate), sand (fine aggregate) and hydrated cement (binder). It is expected that the student of this course is familiar with the basics of concrete technology.

Concrete

Aggregate The coarse aggregate are granular materials obtained from rocks and crushed stones. They may be also obtained from synthetic material like slag, shale, fly ash and clay for use in light-weight concrete. The sand obtained from river beds or quarries is used as fine aggregate. The fine aggregate along with the hydrated cement paste fill the space between the coarse aggregate.

The nominal maximum coarse aggregate size is limited by the lowest of the following quantities. 1) 1/4 times the minimum thickness of the member 2) Spacing between the tendons/strands minus 5 mm 3) 40 mm.

Cement

In present day concrete, cement is a mixture of lime stone and clay heated in a kiln to 1400 – 1600 ºC.

Water

Water used for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to concrete and steel.

Admixtures The admixtures can be broadly divided into two types: chemical admixtures and mineral admixtures. The common chemical admixtures are as follows. 1) Air-entraining admixtures 2) Water reducing admixtures 3) Set retarding admixtures 4) Set accelerating admixtures 5) Water reducing and set retarding admixtures 6) Water reducing and set accelerating admixtures.

The common mineral admixtures are as follows. 1) Fly ash 2) Ground granulated blast-furnace slag 3) Silica fumes 4) Rice husk ash 5) Metakoline

Properties of Hardened Concrete

1) High strength 2) Durability 3) Stiffness 4) Minimum shrinkage and creep

High strength The maximum grade of concrete is 60 MPa. The minimum grades of concrete for prestressed applications are as follows. 1)30 MPa for post-tensioned members 2)40 MPa for pre-tensioned members.

Stiffness of Concrete

The stiffness of concrete is required to estimate the deflection of members. The stiffness is given by the modulus of elasticity.

Durability of Concrete

The durability of concrete is of vital importance regarding the life cycle cost of a structure. The life cycle cost includes not only the initial cost of the materials and labour, but also the cost of maintenance and repair.

Creep of Concrete Creep of concrete is defined as the increase in deformation with time under constant load. Due to the creep of concrete, the prestress in the tendon is reduced with time. Hence, the study of creep is important in prestressed concrete to calculate the loss in prestress. The creep occurs due to two causes. 1. Rearrangement of hydrated cement paste (especially the layered products) 2. Expulsion of water from voids under load

Shrinkage of Concrete Shrinkage of concrete is defined as the contraction due to loss of moisture. The study of shrinkage is also important in prestressed concrete to calculate the loss in prestress. The shrinkage occurs due to two causes. 1. Loss of water from voids 2. Reduction of volume during carbonation

Grout Grout is a mixture of water, cement and optional materials like sand, water-reducing admixtures, expansion agent and pozzolans. The water-to-cement ratio is around 0.5. Fine sand is used to avoid segregation.

The desirable properties of grout are as follows. 1) Fluidity 2) Minimum bleeding and segregation 3) Low shrinkage 4) Adequate strength after hardening 5) No detrimental compounds 6) Durable.

Prestressing Steel The development of prestressed concrete was influenced by the invention of high strength steel. It is an alloy of iron, carbon, manganese and optional materials. In addition to prestressing steel, conventional non-prestressed reinforcement is used for flexural capacity (optional), shear capacity, temperature and shrinkage requirements.

Wires A prestressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different types of wires are as follows. 1) Plain wire: No indentations on the surface. 2) Indented wire: There are circular or elliptical indentations on the surface.

Strands A few wires are spun together in a helical form to form a prestressing strand. The different types of strands are as follows. 1) Two-wire strand: Two wires are spun together to form the strand. 2) Three-wire strand: Three wires are spun together to form the strand. 3) Seven-wire strand: In this type of strand, six wires are spun around a central wire. The central wire is larger than the other wires.

Tendons A group of strands or wires are placed together to form a prestressing tendon. The tendons are used in post-tensioned members.

Cables A group of tendons form a prestressing cable. The cables are used in bridges

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 following sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm.