Method Statement of Utracon Post-Tensioning Systems

The Post-Tensioning systems commonly used in building and bridge construction are grouped into two principal categories. These are the Bonded and unbonded systems.

General :

Utracon Post Tensioning Systems are developed and maintained by Utracon Structural Systems Pte Ltd Singapore serviced and distributed to a worldwide network of subsidiaries owned by the Utracon Corporation, Singapore.

Our systems comply with international specifications and recommendations eg: ASTM, AASHTO, BS, EUROCODE, DIN, AUSTRIAN CODE, SIA, FIP, EOTA, IS CODE, etc. The quality of UPS products and services is in full compliance with the above.

This method statement has been developed to provide guidance for all personnel involved in the project in using UPS systems which covers the installation, stressing and grouting operations.

Materials :

PT Strands :

Strands	0.50 inches	0.60 inches
Nominal Diameter	12.7mm	15.24mm
Nominal Area	100mm2	140mm2
Nominal Weight	0.775kg/m	1.102kg/m
Tensile Strength	1860N/mm2	1860N/mm2
Modulus of elasticity	195kN/mm2	195kN/mm2
Min breaking load of strand	183.7kN	260.7kN

Sheathings :

The sheathings for bonded tendons shall be spirally wound galvanized steel tubing made from galvanized steel strips of 0.3 mm thickness.

The sheathings shall be mortar tight to prevent entrance of cement grout during concreting and shall be corrugated for better bonding.

The sheathings shall be sufficiently strong to retain shapes and to resist damages during construction.

The round sheathings with various outer diameters from 68mm to 135mm have been widely used in UPS Multiplane anchorage systems.

The sheathings used flat anchorage system shall be oval in shape with the dimension of 80 mm in width and 25 mm depth for accommodating 5nos of strand and 54 mm in width, 25mm depth for accommodating 3nos of strand.

Anchorages

The types of anchorage used are Flat Anchorage (FA), Multiplane Anchorage (MA), Coupler, Plate Anchorage and Bond Head (Dead End) Anchorage. The various types of anchorage are available to serve the different requirement of the projects. The purpose of the anchorage is transferring the tendon force to the concrete structure after the strands are stressed.

Anchorages used in this project shall be of flat anchorage (FA) & (MA) Types as the structural member intend to use is slab as well as beams.

Design data for friction / elongation calculation

Friction of coefficient, u	0.2 Or as per the consultant´s recommendation.
Wobble factor k	0.0017/m
Draw in of wedge	6mm
Stressing anchorage	Type FA, MA and Coupler
Loss in jack	Varies from 0 ~ 2.0%

Handling and storage of materials

• The pre-stressing strands shall be free of grease and corrosion enhancing materials.
• No flame cutting of strands is allowed
• Care should be taken in the handling of strands in order to avoid mechanical damage to the strands.
• Strands should be raised above ground to avoid contamination.
• Webbing slings shall be used for loading and unloading in order to avoid mechanical damage to the strand coils.
• All Post Tensioning components must be handled and stored such that any contamination, mechanically damage or corrosion can be avoided.

General Work Procedures

• Post Tensioning tendons shall be installed in accordance with the "Approved Drawings issued for Construction"
• Install bottom slab reinforcement when the slab formwork is ready.
• Provide sufficient cover to the bottom slab reinforcement
• Install tendons in accordance with the shopdrawings
• Install top slab reinforcement
• Under the Tendon, identification mark shall be provided with paint before concreting and after de shuttering tendon position shall be marked in the ceiling for identification
• Install bottom and side reinforcements and links. No capping links shall be installed at this stage.
• Install tendons in accordance with the shopdrawings.
• Install top reinforcements and capping links of the beam.

Guidelines for Tendon Installation

• Tendon alignment shall take precedent over reinforcement.
• All dimensions measured shall be based on the formwork level. Therefore, an accurate formwork is necessary.
• The tolerances of formwork shall be 5mm or in accordance with the specifications
• The Multiplane Anchorages are fixed to the end formwork by support bars to avoid any displacement during concreting.
• The Multiplane Anchorages must be orientated perpendicularly to the cable axis and with the grout vent at the top.
• If the side formwork is to be erected earlier, the main contractor has to arrange for holes to be provided at the formwork for each anchorage to accommodate protruding strands.
• Tolerances for vertical tendon profiles are 5 mm for high and low points.
• Tolerances for horizontal tendon profiles are 100mm (to avoid small M&E openings / pipes or other obstructions). Specialist designer shall be consulted for tolerances more than 150mm.
• Handle ducts with care and no stepping on ducts.
• Tendon support bars and bar-chairs are placed at 1m interval and secured with binding wire to avoid movement during concreting.
• Duct joints are sealed with tape.
• Check visually on the ducts’ straightness and do necessary adjustment before concreting.
• Grout hoses must be protected from being embedded or removed during concreting.
• Any defect found in the bulb of strands such as breakage of ply should be replaced with new bulb.

Stressing :

Stressing shall not commence unless the main contractor has confirmed the concrete of the slab / beam has achieved the minimum cube strength at transfer.

Compressive Strength required for commencing stressing operation shall be 70% of the concrete grade used or minimum of 25 N/mm2.

Preparation for stressing

• Removal of the side formwork and anchorage blackouts (polyfoam).
• Main contractor / Client shall provide the working platform(Min 750mm wide from the face perimeter beam) to commence the preparation & stressing operation.
• Placing of wedge plates and wedges. This operation must be carried out after concreting so that the anchorages are not fouled by grout.
• Ensure the protruding strands length is sufficient for stressing.
• Protruding length if strand required for stressing shall be 350 − 400 mm for FA & 750 to 900 mm for MA Anchorages.
• The theoretical elongation calculations must be approved by the consultant before stressing.
• Elongation shall be checked after 24 hrs of final stressing and to be compared with earlier elongation and also to confirm that there is no slippage of strands.

Stressing of Multiplane Tendons

• The multistrand jack shall be secured and hoisted to the anchorage´s level by chain blocks or crane.
• All strands shall be inserted through the jack´s guide pipes.
• Connect jack to hydraulic pump.
• At the beginning of stressing the strands are locked in the jack pulling head.
• The pressure readings and the measured elongation are recorded in the stressing report.
• Stress initially to 10 Mpa to remove slack in the strands
• Proceed stressing and record the strands´ elongation for every incremental of 10MPa pressure.
• When the jack has reached the end of its stroke or the desired force has been obtained, the pressure in the jack is released and the strands will locked uniformly by the wedges in the wedge plate.
• The jack piston returns.
• Stressing is continued in as many stages as are necessary to obtain the required forces.

Stressing of Flat Tendons

• The jack shall be carried by hand to the stressing anchorage.
• Connect jack to hydraulic pump with hydraulic hoses
• Strand shall be inserted to the jack
• 25% of the stressing force shall be applied to the tendon to remove slack in the strands
• Spray mark shall be made to the strands
• Proceed with the stressing to the full force.
• Measure the distance of the spray mark to the wedge plate. This will reflect the strand’s elongation from 25% to 100% force. ( assume X mm )
• Add the wedge draw-in of 6mm to the X.
• Effective strand’s elongation from 25% to 100% stressing force, Y
• The total elongation (from 0% to 100% force) shall be calculated by extrapolation of the value Y.
• Compare the full elongation (actual elongation) against the theoretical value

A sample of the Stressing Report and interpretation of stressing report are shown in Appendix A.4.

De-Shuttering at pour strip area can be carried out after completion of concreting in that area and after achievement of required compressive strength

Unbalance load from the floor under construction to be distributed with calculated spacing of props.

Stressing of Flat Tendons

Grouting shall not commence unless the consultant has approved the stressing result.

Preparation for grouting

• Cut the protruding strands (stressing length) to a minimum of 10mm from the wedge plate.
• The anchorage blackouts shall be filled with the approved cementitious material.
• The grouting shall be continued after a minimum period of 24 hours of drying of the patch up.
• Prior to grouting, the ducts shall be thoroughly cleaned by oil free compressed air.
• Spray mark shall be made to the strands

Proposed grout mix

• Cement in standard 50kg package (Ordinary Portland cement)
• Water = 0.45 x 50kg = 22.5 kg = 22.5 litres
• Admixture = 0.2% weight of cement ( 50 kg ) = 100 ml
  (Conplast P211M from Fosroc)
  Mixing sequence = water – additive – cement
  Min mixing time = 3-4 min

Tests for grouting

• Viscosity
• Compressive strength.

Viscosity test

• Viscosity test will be executed by means of a flow cone
• Flow time is measured with a stop watch
• It is the measurement of time that the grout takes to completely flow out of the cone.
• It is carried out after the 3-4 min minimum mixing time.
• The flow time should be between 10 to 25 seconds.
  

Compressive strength test

• 6 test cubes of 100 x 100 x 100 mm shall be prepared per grouting session.
• After 24 hours, remove cubes from mould and store in the water storage tank.
• Compressive strength at 7 and 28 days for 3 cubes each shall be tested.
• According to specification ,average compressive strength of 3 cubes shall be:
  7 days : minimum of 17 N/mm2 28 days : minimum of 30 N/mm2

Grouting of tendon

• Mix the grout as per approved grout mix.
• Grout mix shall be prepared in a W/C ratio of 0.45 with workability additive of 0.2 % of weight of cement.
• Inject grout from the tendon’s inlet hose.
• Lock the outlet hose when consistent grout flows out from it.
• When grout flows out at the intermediate vents, lock these vents in the direction of the grout flow.
• Lock the outlet hose when the grout flows out has the same consistency as the grout being pumped in.
• Continue pumping to build up pressure. Lock the inlet hose when a pressure of 3~5 bar is achieved.
• Repeat the above steps to the next tendon.
• Electrically operated helical screw type with water reservoir & mixing tank.

Precautions to be taken during grouting

• Pressure should be duly controlled so as not to cause segregation of grout.
• Required pressure to be maintained shall be 3 − 5 bars.
• Excessive mixing (especially at high temperatures) can stiffen the grout already in the mixing drum.
• In case of interruption (more than 45 minutes) the grout shall be flushed out of the tendon using water and compressed.