Prestressed Concrete

◆Prestressed Concrete ◆

Introduction

Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to an external load which is more than the permissible limits of the plain concrete.

Prestressed concrete is basically 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.
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📌Terminology

1. Tendon: A stretched element used in a concrete member of structure to impart prestress to the concrete.
2. Anchorage: A device generally used to enable the tendon to impart and maintain prestress in concrete.
3. Pretensioning: A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by the bond between
steel & concrete.
4. Post-tensioning: A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing .
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📍Materials for prestressing concrete members

1. Cement: The cement used should be any of the following


(a) Ordinary Portland cement conforming to IS269
(b) Portland slag cement conforming to IS455. But the slag content should not be more than 50%.
(c) Rapid hardening Portland cement conforming to IS8041.
(d) High strength ordinary Portland cement conforming to IS8112.

2. Concrete: Prestress concrete requires concrete, which has a high compressive strength reasonably early age with comparatively higher tensile strength than ordinary concrete. The concrete for the members shall be air-entrained concrete composed of Portland cement, fine and coarse aggregates, admixtures and water. The air-entraining feature may be obtained by the use of either air-entraining Portland cement or an approved air-entraining admixture. The entrained air content shall be not less than 4 percent or more than 6 percent.

Minimum cement content of 300 to 360 kg/m3 is prescribed for the durability requirement.

The water content should be as low as possible.

3. Steel:- High tensile steel, tendons, strands or cables

High strength steel should contain:

0.7 to 0.8% carbons,

0.6% manganese,

0.1% silica

Why high grade of concrete & steel?

Higher the grade of concrete higher the bond strength which is vital in pre-tensioned concrete

Also, higher bearing strength which is essential in post-tensioned concrete as well as in pre-tensioned concrete

Creep & shrinkage losses are minimum with high-grade concrete.

Generally, minimum M30 grade concrete is used for post-tensioned & M40 grade concrete is used for pre-tensioned members

The losses in pre-stress members due to various reasons are generally in the range of 250 N/mm2 to 400 N/mm2

If mild steel or deformed steel is used the residual stresses after losses is either zero or negligible.
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📌Advantage of Prestressed Concrete

The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members.

In fully prestressed members the member is free from tensile stresses under working loads, thus the whole of the section is effective.

In prestressed members, dead loads may be counter-balanced by eccentric prestressing.

Prestressed concrete member posses better resistance to shear forces due to effectof compressive stresses presence or eccentric cable profile.

Use of high strength concrete and freedom from cracks, contribute to improving durability under aggressive environmental conditions.

Long span structures are possible so that saving in weight is significant & thus it will be economic.

Factory products are possible.

Prestressed members are tested before use.

Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before the collapse.

Fatigue strength is better due to small variations in prestressing steel, recommended to dynamically loaded structures.
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📌Disadvantages of Prestressed Concrete

The availability of experienced builders is scanty.

Initial equipment cost is very high.

Availability of experienced engineers is scanty.

Prestressed sections are brittle

Prestressed concrete sections are less fire resistant.
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📍Classifications and Types

Prestressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions.

Pre-tensioning: In which the tendons are tensioned before the concrete is placed, tendons are temporarily anchored and tensioned and the prestress is transferred to the concrete after it is hardened.

Post-tensioning: In which the tendon is tensioned after the concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete.
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📍Tensioning Devices

Mechanical devices: The mechanical devices generally used include weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear devices and wire-winding machines.

Hydraulic devices: These are simplest means for producing large prestressing force, extensively used as tensioning devices

Electrical devices: The wires are electrically heated and anchored before placing concrete in the mould. This method is often referred to as thermo-prestressing and used for tensioning of steel wires and deformed bars.

Chemical devices: Expanding cement are used and the degree of expansion is controlled by varying the curing condition. Since the expansive action of cement, while the setting is restrained, it induces tensile forces in tendons and compressive stresses in concrete.
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🔗Prestressing System

Pretensioning system: In the pre-tensioning systems, the tendons are first tensioned between rigid anchor-blocks cast on the ground or in a column or unit –mould types pre-tensioning bed, prior to the casting of concrete in the mould. The tendons comprising individual wires or strands are stretched with constant eccentricity or a variable eccentricity with tendon anchorage at one end and jacks at the other.

2. Post-tensioned system: In post-tensioning, the concrete unit is first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts.

Most of the commercially patented prestressing systems are based on the following principle of anchoring the tendons:

Wedge action producing a frictional grip on the wire.

Direct bearing from the rivet or bolt heads formed at the end of the wire.

Looping the wire around the concrete.
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📌Methods📌:


1. Freyssinet system
2. Gifford-Udall system
3. Magnel blaton system
4. Lee-McCall system

Differences of Prestressed Concrete Over Reinforced Concrete:

In prestressed concrete member steel plays an active role. The stress in steel prevails whether the external load is there or not. But in R.C.C., steel plays a passive role. The stress in steel in R.C.C members depends upon the external loads. i.e., no external load, no stress in steel.

In prestress concrete the stresses in steel is almost constant where as in R.C.C the stress in steel is variable with the lever arm.

Prestress concrete has more shear resistance, where as the shear resistance of R.C.C is less.

In prestress concrete members, deflections are less because the eccentric pre stressing force will induce couple which will cause upward deflections, where as in R.C.C., deflections are more.

In prestress concrete fatigue resistance is more compared to R.C.C. because in R.C.C. stress in steel is external load dependent where as in P.S.C member it is load independent.

Prestress concrete is more durable as high grade of concrete is used which is denser in nature. R.C.C. is less durable.

In prestress, concrete dimensions are less because external stresses are counterbalanced by the internal stress induced by prestressing. Therefore reactions on column & footing are less
as a whole the quantity of concrete is reduced by 30% and steel reduced by about 60 t  70%. R.C.C. is uneconomical for long span because in R.C.C. dimension of sections are large requiring more concrete & steel.