SECTION 5B (A) GENERAL
1 SCOPE
1.1 This Code (Part 6/Subsection 5B) deals with the general structural use of prestressed concrete. It covers both work carried out on site and the manufacture of precast prestressed concrete units.
1.2 Special requirements of structures such as pipes and poles covered in respective standards have not been covered in this Subsection; these standards shall be used in conjunction with this Subsection. This Subsection does not cover specific provisions regarding bridges.
2 TERMINOLOGY
For the purpose of this Subsection, the following definitions shall apply, and for other terms those given in the accepted standard [6-5B(1)] shall apply.
2.1 Anchorage Device — In post-tensioning, the hardware used for transferring the post-tensioning force from the tendon to the concrete in anchorage zone.
2.2 Bonded Member — A prestressed concrete member in which tendons are bonded to the concrete either directly or through grouting.
2.3 Bonded Post-Tensioning — Post-tensioned construction in which the annular spaces around the tendons are grouted after stressing, thereby bonding the tendon to the concrete section.
2.4 Cable — A group of wires or bars or strands or rods.
2.5 Characteristic Load — Load which has 95 percent probability of not being exceeded during the life of the structure (see 20.2).
2.6 Characteristic Strength — Strength of material below which not more than 5 percent of the test results are expected to fall (see 20.1).
2.7 Column or Strut — A compression member, the effective length of which exceeds three times the least lateral dimension.
2.8 Creep — Time dependent deformation due to sustained load.
2.9 Creep Coefficient — The ratio of creep strain to elastic strain in concrete.
2.10 Final Prestress — The stress which exists after substantially all losses have occurred.
2.11 Final Tension — The tension in the prestressing tendon corresponding to the state of the final prestress.
2.12 Initial Prestress — The prestressing force in the tendon at transfer.
2.13 Initial Tension — The maximum stress induced in the prestressing tendon at the time of the stressing operation.
2.14 Post-Tensioning — A method of prestressing concrete in which prestressing steel is tensioned against the hardened concrete.
2.15 Prestressed Concrete — Concrete in which permanent internal stresses are deliberately introduced, usually by tensioned steel, to counteract to the desired degree the stresses caused in the member during service.
2.16 Pre-Tensioning — A method of prestressing concrete in which the tendons are tensioned before concreting.
2.17 Relaxation — Time dependent decrease in steel stress at constant strain.
2.18 Sheathing — A material encasing a prestressing tendon to prevent bonding the tendon with the surrounding concrete during placement of concrete and to provide protection against corrosion.
2.19 Short Column — A column, the effective length of which does not exceed 12 times the least lateral dimension.
2.20 Slender Column — A column, the effective length of which exceeds 12 times the least lateral dimension.
2.21 Shrinkage Loss — The loss of stress in the prestressing steel resulting from the shrinkage of the concrete.
2.22 Stress at Transfer — The stress in both the prestressing tendon and the concrete when the prestressing tendon is released from the prestressing mechanism.
2.23 Tendon — A steel element, such as a wire, cable, bar, rod or strand, or a bundle of such elements used to impart prestress to concrete when the element is tensioned.
2.24 Transfer — The act of transferring the stress in prestressing tendons from the jacks or pre-tensioning bed to the concrete member.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5B PRESTRESSED CONCRETE
2.25 Transmission Length — The distance required at the end of a pre-tensioned tendon for developing the maximum tendon stress by bond.
3 SYMBOLS
For the purpose of this Subsection, the following letter symbols shall have the meaning indicated against each; where other symbols are used, they are explained at the appropriate place:
A = Area B = Breadth of beam = Breadth of web or rib D = Overall depth of beam DL = Dead load d = Effective depth of beam = Effective depth of beam in shear = Short term modulus of elasticity of concrete EL = Earthquake load = Modulus of elasticity of steel e = Eccentricity F = Characteristic load = Bursting tensile force = Design load f = Characteristic strength of material = Cube strength of concrete at transfer = Characteristic compressive strength of concrete = Compressive stress at centroidal axis due to prestress or average intensity of effective prestress in concrete = Modulus of rupture of concrete (flexural tensile strength) = Design strength = Characteristic strength of prestressing steel = Maximum prestress after losses = Maximum initial prestress = Ultimate tensile stress in the tendons = Maximum principal tensile stress = Characteristic strength of reinforcement IL = Imposed load M = Bending moment m = Modular ratio P = Load due to prestressing effect s = Spacing of stirrups T = Torsional moment V = Shear force = Ultimate shear resistance of concrete
Ultimate shear resistance of a section uncracked in flexure
Ultimate shear resistance of a section cracked in flexure
Wind load
Depth of neutral axis
Partial safety factor for load
Partial safety factor for material
Percentage reduction in moment
Shear stress in concrete
Diameter of tendon or bar
Section 5B (B) Materials, Workmanship, Inspection and Testing
4 MATERIALS
4.1 Cement
The cement used shall be any of the following conforming to accepted standards [6-5B(2)] and the type selected should be appropriate for the intended use:
a) Ordinary Portland cement,
b) Rapid-hardening Portland cement,
c) Portland slag cement,
d) Portland pozzolana cement (fly ash based),
e) Portland pozzolana cement (calcined clay based),
f) Hydrophobic cement,
g) Low heat Portland cement, and
h) Sulphate resisting Portland cement.
Other combinations of Portland cement with mineral admixtures (see 4.2) of quality conforming to relevant Indian Standards laid down may also be used in the manufacture of concrete provided that there are satisfactory data on their suitability, such as performance test on concrete containing them. Rate of slow gain of strength with Portland slag cement and Portland pozzolana cement should be taken into account while deciding the de-shuttering time, curing period and the time of prestressing (see also 10 and 13).
4.1.1 Low heat Portland cement conforming to the accepted standard [6-5B(3)] shall be used with adequate precautions with regard to removal of formwork, etc.
4.1.2 High alumina cement conforming to the accepted standard or supersulphated cement conforming to the accepted standard may be used only under special circumstances with the prior approval of the engineer-in-charge. Specialist literature may be consulted for guidance regarding the use of these types of cements.
4.1.3 The attention of the engineer-in-charge and users of cement is drawn to the fact that quality of various cements mentioned in 4.1 is to be determined on the basis of its conformity to the performance characteristics given in the respective Indian Standard specification for that cement. Any trade-mark or any trade name indicating any special features not covered in the standard or any qualification or other special performance characteristics sometimes claimed/indicated on the bags or containers or in advertisement alongside the ‘Statutory Quality Marking’ or otherwise have no relation whatsoever with the characteristic guaranteed by the ‘Statutory Quality Marking’ as relevant to that cement. Consumers are, therefore, advised to go by the characteristics as given in the corresponding Indian Standard specification or seek specialist advice to avoid any problem in concrete making and construction.
4.2 Mineral Admixtures
Mineral admixtures used shall conform to 4.2 of Part 6 'Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete' of the Code. When mineral admixtures are used, rate of slow gain in strength should be taken into account while deciding the de-shuttering time, curing period and the time of prestressing (see also 10 and 13).
4.3 Aggregates
All aggregate shall comply with the requirements of accepted standard [6-5B(6)].
4.3.1 Heavy weight aggregates may also be used provided the engineer-in-charge is satisfied with the data on the properties of concrete made with them.
NOTE — Some of the provisions of the standard would require modification when these aggregates are used. Specialist literature may be consulted for guidance.
4.3.2 Size of Aggregates
The nominal maximum size of coarse aggregate should be as large as possible within the limits specified but in no case greater than one-fourth of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all prestressing tendons and reinforcements thoroughly and fill the corners of the form. For most work, 20 mm aggregate is suitable.
The nominal maximum size of the aggregate shall be less than the spacing between the tendons, sheathings, ducts or untensioned steel, where provided.
4.4 Water
The requirements of water used for mixing and curing shall conform to the requirements given in 4.4 of
Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code. However, use of sea water is prohibited.
4.5 Chemical Admixtures
Chemical admixtures shall be used according to 4.5 of Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code.
4.6 Prestressed and Untensioned Steel
4.6.1 Prestressing Steel
4.6.1.1 The prestressing steel shall be any one of the following conforming to accepted standards [6-5B(7)]:
a) Plain hard-drawn steel wire (cold-drawn stress relieved wire),
b) Indented wire,
c) High tensile steel bar,
d) Uncoated stress relieved strand, and
e) Uncoated stress relieved low relaxation seven ply strand.
4.6.1.1.1 All prestressing steel shall be free from welded joints and splits, harmful scratches, surface flaws; rough, jagged and imperfect edges and other defects likely to impair its use in prestressed concrete.
4.6.1.2 Mechanical properties
Data in respect of modulus of elasticity, relaxation loss at 1 000 h at mean temperature of C, minimum ultimate tensile strength, stress-strain curve, etc, shall necessarily be obtained from manufacturer. Prestressing steel shall be subjected to acceptance tests prior to actual use on the works. The modulus of elasticity value, as per acceptance tests, shall conform to the values assumed in the design which shall be within a range not more than 2.5 percent between the maximum and the minimum.
4.6.1.2.1 For design purposes, the minimum requirements given in the relevant Indian Standards may be adopted.
4.6.2 Untensioned Steel
4.6.2.1 The reinforcement used as untensioned steel shall be any of the following conforming to accepted standards [6-5B(8)]:
a) Mild steel and medium tensile steel bars.
b) High strength deformed steel bars.
c) Hard-drawn steel wire fabric.
4.6.2.2 All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil mud or any other substances which may destroy or reduce bond. Sand blasting or other treatment is recommended to clean reinforcement.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5B PRESTRESSED CONCRETE
Special precautions like coating of reinforcement may be required for reinforced concrete elements in exceptional cases and for rehabilitation of structures. Specialist literature may be referred to in such cases. Reduction in design bond strength of coated bars shall be looked into.
4.6.2.3 The modulus of elasticity of steel shall be taken as . The characteristic yield strength of different steel shall be assumed as the minimum yield stress/0.2 percent proof stress specified in the relevant Indian Standard.
4.7 Storage of Materials
Storage of materials shall be as described in good practice [6-5B(9)].