Chapter 11Assembly of Prestressing and Reinforcing Steel

11.1.1 Straightness

11.1.1.1 The wire or strand, as supplied, shall be self-straightening when uncoiled.

11.1.1.2 In the case of high tensile alloy steel bars, any straightening shall be carried out by mechanical means. Bars shall not be bent when their temperature is less than $10^{\circ}\mathrm{C}$.

11.1.1.3 In no case heat shall be applied to facilitate straightening or bending of prestressing steel.

11.1.2 Arrangement and Positioning

11.1.2.1 All prestressing steel shall be carefully and accurately located in the exact positions shown in the design drawings. The permissible tolerance in the location of the prestressing tendon shall be $\pm5$ mm. Curves or bends in prestressing tendon required by the designer shall be gradual and the prestressing tendon shall not be forced around sharp bends exceeding 1 in 6 both in plan and elevation or be formed in any manner which is likely to set up undesirable secondary stresses. The minimum radius of curvature for curved cables shall be specified to ensure that bursting of side cover both perpendicular to the plane of curvature and in the plane of the ducts do not take place.

11.1.2.2 The relative position of prestressing steel in a cable shall be maintained by suitable means such as sufficiently rigid and adequately distributed spacers.

11.1.2.3 The internal cross-sectional area of duct shall generally be 2.5 to 3 times the nominal area of prestressing tendons to ensure free flow of grout.

11.1.2.4 The method of fixing and supporting the steel in the mould or the formwork shall be such that it is not displaced during the placing or compaction of the concrete or during tensioning of the steel.

11.1.2.5 The type of fixtures used for positioning the steel shall be such that it does not give rise to friction greater than that assumed in the design.

11.1.3 Jointing

11.1.3.1 Coupling units and other similar fixtures used in conjunction with the prestressing steel shall have an ultimate tensile strength of not less than the individual strengths of the wires or bars being joined.

11.1.3.2 High tensile wire other than hard-drawn wire may be joined together by suitable means provided the strength of such joints is not less than the individual strengths of the wires being joined. Hard-drawn wire used in prestressed concrete work shall be continuous over the entire length of the tendon.

11.1.3.3 High tensile steel bars may be joined together by means of couplings, provided the strength of the coupling is such that in a test of destruction, the bar shall fail before the coupling.

11.1.3.4 Welding shall not be permitted in prestressing steel.

11.1.4 Cutting

11.1.4.1 All cutting to length and trimming of the ends of wires or strands shall be done by suitable mechanical cutters. Under factory conditions flame cutting may be permitted.

11.1.4.2 Bars shall preferably be ordered to the exact length required. Any trimming required shall be done only after the bar has been tensioned and the grout has set; it shall then be carried out in accordance with 11.1.4.1.

11.1.5 Protection of Prestressing Steel, Sheathing and Anchorages

In all constructions of the post-tensioned type whether prestressing is subsequently bonded or not bonded with concrete, the prestressing steel, sheathing material forming ducts in concrete and anchorages shall be given adequate protection against corrosion at all stages of construction and during service life.

11.1.5.1 Construction with unbonded steel embedded in concrete

For protection of sheathing material forming ducts, the durability requirements shall be followed. Furthermore,

PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5B PRESTRESSED CONCRETE

the internal surface of duct, prestressing steel and anchorage shall be protected by following methods in suitable combination depending upon severity of environment and extent of layers of protection considered desirable:

a) Coating of steel and duct by reactive metals like zinc (Zn) or zinc plus aluminium (Zn + Al) preferably applied by hot dipping method.

b) Use of non-corroding sheath material like high density polyethylene and fibre reinforced plastic.

c) Coating of steel by non-reactive materials like epoxy (factory applied).

d) Covering individual uncoated steel or coated steel strands/wires in sheaths formed by extrusion process in factory using non-corroding material like HDPE or similar. The space between sheathing and duct can be filled with corrosion inhibiting materials like grease and wax.

e) In case of large size prestressing units use of protective and replaceable corrosion inhibition material for grouting the ducts such as grease, wax polyurethane foam, wherein any corrosive trace elements are controlled to remain within acceptable limits. Systems for checking the efficacy of the method during service life and provisions for replacing/making up loss of filler material shall be incorporated.

f) External parts of anchorages and projecting cables should be covered by suitable casing and protected by suitable material and method.

For selection of appropriate method, material specifications and installation process, etc, specialist literature should be referred to.

11.1.5.2 Construction with unbonded steel and anchorages not embedded in concrete (external prestressing elements)

The methods given in 11.1.5.1 shall be applicable for this type of construction also. In addition, the material, strength and environmental resistance of the ducts forming external enclosure shall provide adequate mechanical strength and stability against environmental attack, including solar radiation. The ducts should be strong enough to resist pressure from the grouting operations.

11.1.5.3 Construction with post-tensioned bonded prestressing steel

11.1.6 Nominal Cover to Prestressing Steel

11.1.6.1 Nominal cover

Nominal cover is the design depth of concrete cover. It is the dimension used in design and indicated in drawings.

11.1.6.2 Nominal cover to meet the durability requirements

11.1.6.2.1 Minimum values of the nominal cover which should be provided in post-tensioned work, where cables and large-sized bars are used, the minimum clear cover from sheathing/duct shall be at least 75 mm.

11.1.6.2.2 In pre-tensioned work, the cover of concrete measured from the outside of the prestressing tendon shall be as per Table 16 of Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code.

11.1.6.3 Nominal cover to meet specified period of fire resistance

Minimum values of nominal cover of normal-weight aggregate concrete to be provided to all reinforcement including links to meet specified period of fire resistance shall be as given in Table 16A of Part 6 'Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete' of the Code.

11.1.7 Spacing

11.1.7.1 In the case of single wires used in pre-tension system, the minimum clear spacing shall not be less than the greater of the following:

a) 3 times diameter of wire, and

b) Nominal maximum size of aggregate plus 5 mm.

11.1.7.2 In the case of cables or large bars, the minimum clear spacing (measured between sheathings/ducts, wherever used) shall not be less than the largest of the following:

a) 40 mm,

b) Maximum size of duct or bar, and

c) Nominal maximum size of aggregate plus 5 mm.

11.1.8 Grouped Cables

11.1.8.1 Cables or ducts may be grouped together vertically in groups of not more than two.

11.1.8.2 The minimum clear horizontal spacing between groups of cables or ducts of grouped cables shall be greater of the following:

a) $40\mathrm{mm}$, and

b) Nominal maximum size of aggregate plus 5 mm

The vertical distance between groups shall not be less than 50 mm. In case there is possibility of obstruction of flow of concrete or in case of severe exposure condition, horizontal grouping should be avoided.