Design Requirements for Prestressed Concrete

Prestressed Concrete (PSC) Works

• The design of the end block and bearings should permit periodic inspection and servicing of the bearings.
• Provision should be made to cater for an additional prestressing force of 15% of the design prestressing force, for easy installation of prestressing steel at a later date.
• Admixtures/plasticizers of approved type only should be used, and the minimum grade of concrete for PSC work shall be M/35.

Tensioning of Prestressing Steel

• The stress induced in the tendons shall be determined by measurement of elongation and also independently by direct measurement of force using a pressure gauge or other means.
• The two values shall be comparable to each other and the theoretical values within a tolerance of 5%.
• Calculations for elongations and gauge readings must include appropriate allowances for friction, strand wire slippage, and other factors as applicable.
• Breakage of wires in any one prestressed concrete member shall not exceed 2.5% during tensioning.

Preparation and Installation of Cables

• Prior to stressing of strands, bottom forms should be kept clean, and accuracy of alignment ensured.
• Form surfaces to be in contact with concrete must be treated with an effective release agent.
• Special care must be exercised to prevent contamination of strands from release agents, grease, or other coatings.
• Cables shall not be left unstressed in ducts for long duration, and hence threading of cables in ducts shall be done just prior to stressing.

Post-Tensioning Systems

• Post-tensioning systems shall be installed in accordance with the manufacturer's directions and proven procedures.
• Manufacturers' recommendations regarding end block details and special arrangements in anchorage zones applicable to their particular system should be observed.

Ducts and Anchorage

• Ferrous metal is recommended for duct material, and Aluminium should not be used.
• Metal ducts must be such that destructive galvanic action on duct and tendon will not occur.
• As the alignment and position of ducts within the member are critical, short kinks and wobbles shall be avoided.
• The trajectory of ducts shall not depart from the curve of straight lines shown in the drawing by more than 1 in 240.
• The cable position shall not deviate by more than 5 mm from the designed trajectory vertically.

Grouting and Anchorage

• The stressed cables shall be grouted immediately after the prestressing operation for the girder is completed.
• Anchorage devices for all post-tensioning systems must be aligned with the direction of the axis of tendons at the point of attachment.
• Concrete surface, against which the anchorage devices bear, must be normal to this line of direction.

Concrete and Curing

• The method of curing shall be as per IRS Concrete Bridge Code.
• The appropriate value of minimum cement for different exposure conditions and maximum cement content for RCC and PSC works, as well as the water-cement ratio, shall be as per the provisions given in the IRS Concrete Bridge Code.
• Equipment, material, and the proportions of the mix to be used shall be submitted to and approved by the engineer before the work is started.

Compaction of Concrete

• All concrete shall be compacted by vibration.
• Generally, internal vibration shall be used on all sections that are sufficiently large to admit them.
• Vibrators shall be distributed so that the concrete becomes uniformly dense and plastic mass.
• Vibrators shall be used for compaction only and not for moving concrete horizontally along the form.

Protection to Prestressing Steel

• All prestressing steel shall be free of deleterious materials such as grease, oil, wax, dirt, paint, loose rust, or other similar contaminants that would reduce bond between steel and concrete.
• Prestressing steel shall not be contaminated with form release agents used on forms or beds.
• High-strength steel is to be stored under cover to prevent corrosion.

Safety and Quality Control

• Good safety practices should be established and each employee complies with the same.
• Quality control is essential to ensure the required standard of quality for prestressed members.
• The most important factors to be ensured in this connection are:
  • Testing and inspection of materials
  • Clear and complete detailed working drawings
  • Accurate stressing procedures
  • Proper control of dimensions and tolerances
  • Proper location of anchors
  • Proper proportioning and adequate mixing of concrete
  • Proper handling, placing, and consolidation of concrete
  • Proper curing
  • Proper handling, storing, transporting, and erection of members
  • Thorough documentation

Form Work Requirements

• Form work requirements shall be as per IRS Concrete Bridge Code, including stripping time.
• In the case of PSC works, support shall not be removed till sufficient prestress has been imparted to the member.

Curing of Concrete

• Special attention shall be given to curing of concrete to ensure maximum durability and minimize cracking.
• The method of curing shall be as per IRS Concrete Bridge Code.

Concrete Mixing and Transportation

• The equipment, material, and proportions of the mix to be used shall be submitted to and approved by the engineer before the work is started.
• No segregation shall occur while transporting concrete from the mixer to the form work, and necessary precautions shall be taken to ensure this.
• Concrete shall be deposited as nearly as practicable in its final position without rehandling, and compacted before setting commences.

Compaction of Concrete

• All concrete shall be compacted by vibration, generally using internal vibration on all sections that are sufficiently large to admit them.
• Mechanical vibrators complying with IS:2505, IS:2506, IS:2514, and IS:4656 shall be used for compacting concrete.

Work on Single Span

• The construction of a single span arch bridge involves providing stagings or temporary support underneath and putting up the arch above.
• Back filling of abutments must be ensured before taking up the construction of the arch.

Work on Multiple Spans

• Work can be done simultaneously on a number of spans using more than one set of forms.
• Due care shall be taken to ensure that the horizontal thrust on the pier/abutment is not such that they give way.

Supporting Arrangement for Arches

• Over dry beds of streams, stagings can be constructed from the bed itself.
• Supporting the staging columns on the bed by giving a suitable timber support to spread the load and to check the stagings at various stages.
• If the work has to be carried out in flowing water, the staging will have to be supported over shallow thin piles driven into the sand bed.

Launching Methods

• End launching methods involve assembling the girder on the approach bank and traversing it over the opening it has to span.
• Different techniques are adopted in different situations, such as launching with rail cluster method, launching with the help of a BFR, and launching with the help of a launching pad.

Launching with Rail Cluster Method

• This method involves pulling two rails across the openings on either side and inserting wooden blocks in the form of distance pieces at intervals to prevent tilting.

Launching with the Help of a BFR

• This method involves using a pair of 600 x 210mm joists to balance the beam about a central pivot.
• The launching beam has a tackle at its balancing end, with a 3 sheaved pulley fixed on the launching beam and 2 sheaved pulley at the point where the load is taken.

Launching with the Help of a Launching Pad

• The launching pad consists of a combination of grillage beams mounted on 4 half sets of dip lorries, forming a kentledge.
• A loading trolley in the form of an 'A' frame with 4 wheels is provided in the center.

Erection with the Help of Gantry Girders

• Gantry girders are built up using mostly released steel, and kept at 3.66 m centers.
• Steel cribs are made on the abutments and first pier, and the gantry girders are moved in place using a pair of gallow trollies.

Launching of Girders by Using Service Span

• A service span can be adopted for launching of girders, and consists of a Warren truss with verticals and cross.
• The service span has a gantry fitted with rails over which 2 dip lorries run.

This quiz covers the essential design requirements for Prestressed Concrete (PSC) work, including provisions for bearing inspection, additional prestressing forces, and concrete grade. It also touches on tensioning methods and stress induction in tendons.