86 Lecture Note – 17 PRESTRESS CONCRETE Code: IS1343: 1980 Introduction: Definition of Prestress: 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 external load which is more than the permissible limits of the plain concrete. The precompression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression. Basic Concept 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. 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 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. Materials for prestress 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.
87 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 The steel used in prestress shall be any one of the following:(a) Plain hard-drawn steel wire conforming to IS1785 (Part-I & Part-III) (b) Cold drawn indented wire conforming to IS6003 (c) High tensile steel wire bar conforming to IS2090 (d) Uncoated stress relived strand conforming to IS6006 High strength steel contains: 0.7 to 0.8% carbons, 0.6% manganese, 0.1% silica Durability, Fire Resistance & Cover Requirements For P.S.C Members:According to IS: 1343-1980 20 mm cover for pretensioned members 30 mm or size of the cable which ever is bigger for post tensioned members. If the prestress members are exposed to an aggressive environment, these covers are increased by another 10 mm. Necessity of high grade of concrete & steel: Higher the grade of concrete higher the bond strength which is vital in pretensioned concrete, Also higher bearing strength which is vital in post-tensioned concrete. Further 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 pretensioned members.
88 The losses in prestress 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. Hence high tensile steel wires are used which varies from 1600 to 2000 N/mm2. Advantage of Prestressed Concrete 1. The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members. 2. In fully prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. 3. In prestressed members, dead loads may be counter-balanced by eccentric prestressing. 4. Prestressed concrete member posses better resistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 6. Long span structures are possible so that saving in weight is significant & thus it will be economic. 7. Factory products are possible. 8. Prestressed members are tested before use. 9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse. 10. Fatigue strength is better due to small variations in prestressing steel, recommended to dynamically loaded structures. Disadvantages of Prestressed Concrete 1. The availability of experienced builders is scanty. 2. Initial equipment cost is very high. 3. Availability of experienced engineers is scanty. 4. Prestressed sections are brittle 5. Prestressed concrete sections are less fire resistant. Classifications and Types Prestressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions. 1. 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.
89 2. Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete. The various methods by which pre-compression are imparted to concrete are classified as follows: 1. Generation of compressive force between the structural elements and its abutments using flat jack. 2. Development of hoop compression in cylindrically shaped structures by circumferential wire binding. 3. Use of longitudinally tensioned steel embedded in concrete or housed in ducts. 4. Use of principle of distortion of a statically indeterminate structure either by displacement or by rotation of one part relative to the remainder. 5. Use of deflected structural steel sections embedded in concrete until the hardening of the latter. 6. Development of limited tension in steel and compression in concrete by using expanding cements. The most widely used method for prestressing of structural concrete elements is longitudinal tensioning of steel by different tensioning devices. Prestressing by the application of direct forces between abutments is generally used for arches and pavements, while flat jacks are invariably used to impart the desired forces. Tensioning Devices The various types devices used for tensioning steel are grouped under four principal categories, viz. 1. 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. These devices are employed mainly for prestressing structural concrete components produced on a mass scale in factory. 2. Hydraulic devices: These are simplest means for producing large prestressing force, extensively used as tensioning devices. 3. 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. 4. Chemical devices: Expanding cements are used and the degree of expansion is controlled by varying the curing condition. Since the expansive action of cement