Method Statement for Post-Tensioning of Concrete Slab

The method statement for post-tensioning of the slab is the systematic procedure of safely performing the work from strand installation, final stressing, grouting operation, and de-shuttering by ensuring that quality control objectives are implemented.

Post-tensioning of concrete slab methodology

Handling and Storage of Materials for Post Tensioning Activity

What is the method statement for post-tensioning of concrete slab?

Pre-stressing strands need to be free of grease and corrosion-enhancing materials.

No flame cutting of the strands is permitted.

Care should be taken in handling the strands in order to avoid mechanical damage to the strands.

Strands should be raised above the ground in order to prevent contamination.

Webbing slings are used for loading and unloading in order to avoid damage to the surface of the strand coils.

All post-tensioning components must be handled and stored in a manner that any contamination, physical damage, or corrosion can be avoided.

Post-Tensioned Strand Materials :

Responsible Personnel Qualifications

Installation, stressing, and grouting of post-tension tendons should be supervised, conducted, and inspected by staff with sufficient training and experience. It is recommended that the project maintain the credentials of the contractor’s employees by either the pre-qualification process or the conditions stated in the standard or project specifications. The main contractor can also employ specialized subcontractors with sufficient pre-qualifications to conduct the job.

Project Engineer

The Project Engineer should be a licensed skilled engineer with five years of experience in slab concrete construction, of which three years were in post-tension concrete. Experience should cover post-tensioning and grouting procedures, plus at least one year in charge of the post-tensioning associated engineering tasks.

Project Superintendent

The Project Superintendent should have at least ten years of relevant experience. Construction experience of three years of post-tension concrete construction.
Experience should cover post-tensioning and grouting activities, with at least one year of length.
Responsible for post-tension associated activities.

Foreman

The foreman is supposed to have at least five years of building experience.
Years in associated post-tensioning operations and a minimum of one year as a foreman responsible for post-tension-associated activities.

Crews for Tendon Stressing & Grouting

Stressing and Grouting shall be carried out by staff accredited in compliance with the Class 1 Bonded PTI.

General Work Procedure

Installation of tendons

Tendons shall be cut to the required length with a minimum of 200 mm for stressing extension using an abrasive wheel grinder. If in any occurrence the stressing length is too short for the stressing jack to grip onto the strand, the method statement for the short pulling gear will be submitted separately for approval.

Tendons shall be monitored to ensure that no damage will occur to the tendons. Any damaged tendons will be replaced.

Only a thin, lightly adhered film or rust will be permitted on tendons. Tendons shall be free from loose rust, paint, mud, or any other deleterious substances.

Before casting the proposed slab the entire deck must be, a high-pressure air hose to free all tendons from debris.

Material for tendons and assembled tendons shall be stored in a weatherproof cover, supported above ground level, and shall be protected from damage and deterioration from exposure or any other causes

Approved design and shop drawings shall be followed for all works.

Relevant shop drawings (showing tendon number/location etc.) in line with the approved consultant/design drawing shall be submitted to the consultant for information.

Ducting shall be laid in the correct positions according to the approved shop drawings. Joints in the duct shall be staggered with a low-point drain. Strands shall either be manually or machined (strand pusher) pushed into the ducting and cut to the required length.

The approved, ducting shall be coupled together using a 150 mm GI duct coupler fitting which must be secured before the duct tape is applied.

Tendons shall be securely fixed at points sufficiently close together (no greater than 601 mm (ref: PTI Manual sec: 4.4.5.4) unless otherwise specified on the drawings) to maintain a smooth tendon profile throughout the length of the tendon.

For slab thicknesses between 200 mm – 610 mm there shall be a tolerance of +9.5 mm or -9.5 mm (vertically) of the position shown on the design (ref: PTI Manual Sec: 4.3.4.1). At peaks and valleys, the position of the tendon shall be adjusted to ensure that the profile is within the specified tolerance.

To support the ducts, set up bar chairs and profiles according to the location given on the drawing. Staple feet of bar chairs to formwork soffit where applicable.

The contractor shall provide 300 mm painted lines on the slab soffit to indicate the tendon locations at high and low points only. These lines shall be painted during the installation of post-tension works to the formwork and will be transferred to the concrete once the formwork is removed.

Marking shall also be made on top of the slab since the rebar anchoring requirement for the CHB/CMU wall requires a minimum of 150 mm depth.

If in the opinion of the engineer, tendons are liable to be damaged or deformed during the placing of the concrete, the contractor shall take additional precautions to repair and maintain the position of the shape of the tendons.

Never try to untangle these “knots” directly by hand; use a long enough lever (tube, bar, etc.)

Any defect found in the bulb of strands such as breakage of ply should be replaced with a new bulb.

The contractor’s supervisor shall always appoint a representative to monitor the tendons whilst casting is taking place.

The minimum concrete cover to the outside surface of the tendons shall be specified in the drawings.

The maximum allowable deviation of the ducts in the theoretical lines shown in the drawings is 305 mm in any direction (Horizontally) to allow for MEP openings and other site corrections (ref: PTI Manual Section 4.3.4.3). Horizontal locations are not generally critical, however excessive wobble (i.e unintended curvature must be avoided)

At the point where the duct joins, the direction of the duct shall be precisely parallel to the axis of the anchorage. The straight length of the duct adjacent to the anchorage shall be in accordance with the post-tensioning system. The joints between the duct and the anchor shall be watertight.

It is important to place the beam coils above the formwork brackets. Place additional wood brackets at these positions, if necessary, to distribute the load. When positioning a strand on a concrete slab, ensure that the slab frame has been propelled back, if appropriate, and is not filled with unnecessary material storage. See the consultant who will arrange for the structural engineer’s analysis and validation for this.

Sheathings

The sheathings for bonded tendons are spiral-wrapped galvanized steel tubes of 0.30 mm thick galvanized steel strips.

Sheathings shall be mortar tight to prevent the entrance of the cement groove grout during the concreting process and shall be corrugated for better bonding.

Sheathings shall be strong enough to retain shapes and withstand damage during construction.

Round sheathings with different outer diameters from 68mm to 135mm have been widely used in multiplane anchorage systems.

The sheathings used for the flat anchorage system shall be oval in shape with a width of 80 mm and a depth of 25 mm for accommodating 5 numbers of strands and 54mm in width and a depth of 25mm for accommodating 3 numbers of strands.

Anchorages

Install the slab side formworks and stressing recess along these edge formworks.

All anchorage devices shall be either bolted/tied to the side stressing recess shuttering prior to casting.

Anchor bolts, pipe sleeves, conduits for MEP services, drains, curb and trench angles, and other inserts as shown on the design drawings shall be placed, secured, and protected during the placement of concrete.

Anchor bolts shall be installed as shown on the design drawings.

Anchor bolts shall be positioned and secured within the tolerances as per design drawings.

Each anchorage device shall be set strong perpendicular to the line of the corresponding tendon and shall be positioned securely to prevent movement during concreting.

The placing and securing of the anchorage shall be subject to the approval of the engineer.

The anchorage device shall be cleaned to the satisfaction of the engineer prior to the placement of the concrete.

The anchor recess shall be filled with the approved dry-mixed cement material.

Placing of the Concrete

The pouring of concrete shall be carried out until the reinforcement and post-tensioning components are fully installed.

All concrete shall be fully compacted in the manner specified in the project concrete specification.

Vibrations shall not come into direct tendons. Controlled vibrations shall be done at the proximity of the dead-end and live end of the tendons.

Other trades working on the deck prior to concrete placement must be mindful that the tendons cannot be disrupted or moved to accommodate their work.

All post-tensioned slabs to be poured are to be closely monitored. Concrete pouring will be implemented as per the method statement for in-situ concrete.

Stressing – Method Statement for Post Tensioning

Setup for stressing

The contractor shall remove side shuttering prior to stressing operation.

The anchor recess shall be removed.

The anchor shall be inspected for any concrete slurry.

Any excess concrete slurry shall be removed using a chisel before the wedge block is placed.

All strands shall pass through the wedge block holes, the block shall be seated against the anchor casting.

Wedges shall be installed into wedge blocks firmly using a chisel to secure the wedges.

Strength and Age

Final stress – ( 80% of the ACI Code 18.5.1) the concrete member once it has attained the compressive strength of 36 MPA, the concrete cube report must be submitted and approved by the consultant prior to final stressing.

Stressing Precautions

Care shall be taken during the stressing operation to ensure the safety of all personnel engaged with the work and all other persons in the vicinity.

Safety precautions must be well known to all personnel working with threading and stressing.

Jacks shall be secured in such a manner that they will be restrained should they lose their grip on the tendons.

Exclusion zones shall be implemented whilst doing the final stressing operation, no unauthorized persons shall be permitted to stand within these zones.

The operation of jacks, the measurement of the elongation, and associated operations shall be carried out in such a manner that the safety of all is ensured.

The forces produced as the tendons are stressed are strong enough to damage the structure or even cause injuries to people working on the site if the installation and stressing job are not carried out properly.

Protection during stress means ensuring that no one is working near the region where the tendon is stressed.

The high pressure of the equipment required the permanent attention of all workers operating next to them.

A stressing elongation report shall be taken once the final stress of 80% has been completed for all tendons in the corresponding slab.

When, and in the opinion of a supervisor, there is a danger to the general public or to property, special precautions in the form of substantial barriers will be required. These barriers will also be provided when final stressing jobs adjacent to other trades.

The contractor is to conduct a toolbox meeting prior to the start of the stressing operation.

The contractor shall adhere to the project site safety requirements.

Stressing Equipment

Calibration of all equipment must be submitted before the start of work.

Jack and pressure gauge shall be calibrated to the satisfaction of the engineer, and the true force determined from the calibration. The equipment shall be recalibrated at intervals not exceeding 6 months.

Stressing jack calibration shall be submitted to the consultant for their records and approval.

Measurement of the Post-Tensioning Forces

The test shall be conducted to verify the assumed test friction and wobble coefficient.

Once the stressing block is fitted to the anchor casting and the wedges are set, the wedge and strand shall be spray painted before any load has been applied. After the final stress has been completed the contractor will measure from the wedge to the paint line to get an accurate reading for the elongation report.

The tensioning forces applied to any tendon shall be determined through the direct measurement of the force.

The secant modulus determined from the test samples or shown on test certificates shall be used when interpreting the measurement of the elongation. The design shall specify the required values for the elongation and for the force in the tendon at the jack.

The allowance shall be made for anticipated draw-in at the anchorage devices on the release of the jack, for elastic shortening on the member, and for anticipated losses due to the stressing of subsequent tendons.

The tensioning forces required are to be shown on the drawings. Maximum tension in one strand shall not exceed 80% of its ultimate strength.

Allowances should be made for any slippage or draw-in at the anchorage devices on the release of the jacks. The final forces in each tendon shall be within an accuracy of + or -7% of the values stipulated as per the ACI code.

Action to be taken if elongation report is greater or lesser (+/- 7%)

In the event of the strand elongation not achieving the +/- 7% theoretical elongation, the contractor has to demonstrate by using the stressing jack and gauge that the correct force has been applied for the main contractor’s records/approval.

Re-evaluation of elongation measurement, verification of assumption such as draw-in, test friction and wobble coefficient, modulus of elasticity, area of the strand, computation of final elongation, and the whole stressing operation shall be revised and remedied to the satisfaction of the supervision consultant.

General Stressing Key Points of Method Statement for Post-Tensioning of Concrete Slab

All stressing operations take place in the presence of the consultant’s representative. No member shall be stressed until the concrete has attained the strength specified.

The stressing operation shall be supervised by personnel trained and experienced in this type of work. Special care shall be taken to apply the tensioning force smoothly and evenly.

The stressing operation shall be performed in accordance with the best practice applicable to the particular system approved.

The jacks shall be set accurately in the line of the tendons.

Once stressing records have been reviewed by the design team, they will be submitted to the consultant.

Supervision consultant to witness all the stressful activity.

Strand Stressing Records

Complete stressing records shall be kept of pressure gauge readings, and strand extensions for each strand.

The following data where applicable, shall be recorded:

1. Identification number of each gauge and jack

2. Identification of particulars of tendons

3. Elongations of each tendon

4. Design elongation/extension

5. Range of acceptable elongation/extension

6. Final pressure

Final full tensioning shall only be carried out when the concrete has reached a minimum specific characteristic strength. The structural designer shall declare the minimum characteristic strength in his design taking creep and shrinkage into consideration.

Grouting

A trial grout mix will be administered prior to starting grout works for the approval of the consultant.

Tendons shall be pressure grouted at a pressure of 0.70 Mpa.

The grouting procedures shall comply with the ACI code.

Tendons shall be pressure-grouted as soon as practical after stressing the member unless otherwise approved by the engineer to confirm that stressing is complete with respect to the force and elongation

No grouting will take place until the consultant has approved the slab to be grouted. The slab must be free of formwork, shuttering, and any loose material.

No cutting of strands shall take place without the approval of the consultant.

Before grouting, post-tensioning anchorages shall be sealed to prevent loss of grout.

Grout vents shall be left at the end of each duct. If a tendon exceeds 25 meters in length an extra grout vent will be installed at the center of the tendon.

Materials for Grouting

Grout for filling tendons shall be composed of cement, water, and admixture to reduce shrinkage and bleeding. The water-cement ratio should be as low as possible and consistent with adequate workability but shall not exceed 0.45 (ref: ACI Code: 18.18.3.3) unless approved by the engineer. The materials and the proportions of the grout mixture shall be approved by the engineer.

Grouting material and admixture specifications are to be submitted to the client for review prior to the operations of the project.

Grout Mix Specification

1. Cement – Ordinary Portland Cement
2. Water – From site supply (Ice added if required)
3. Admixture – Grout (Manufacturer specification in Material Submittal)
4. Grout Mix Ratio
5. Water Cement ratio – 0.45 liter (Per 1 Kg Cement)

Preparation of Grout Mix

Clean the grout pump prior to mixing.

Turn on the grout pump.

Measure water and fill to the specified amount.

Measure and add admixture to the required volume.

Measure Ordinary Portland Cement and fill to the recommended amount.

Mix the grout content for a minimum of 2 minutes.

Properties and Tests for Grout

A mockup of the grout mix shall be performed and witnessed by the consultant for their approval. The following tests will be carried out:

1. Compressive Strength
2. Mixing Ratio
3. Fluidity Test
4. Bleed
5. Expansion

Upon approval, grout cubes must be taken for testing, and the flow cone (fluidity) test, temperature, and mixing ratio must be monitored for each day of grouting operations. If the grout mix temperature exceeds 32°C chilled water (ice) will be added to the mix.

Usually, it is advisable for the temperature of water used while grouting should be less than 18°C.

The grout shall have a minimum compressive strength measured on a 100 mm cube of 27 Mpa at 7 days and about 35 Mpa at 28 days. (ref. ACI Code R 18.18.3)., The test cube shall be cured in a moist atmosphere for the first 24 hours and subsequently in water. All tests are to be carried out by a third-party lab technician.

Mixing and Equipment

Grout shall be mixed in a high-speed mechanical mixer (grout pump) for at least 2 minutes until a uniform colloidal consistency is produced.

Once all the contents have been added to the mix, an adequate grout bowl shall be put in place to prevent injury to the operatives.

Water shall be added to the grout pump first, and then admixture, and finally cement with the grout pump mixing the whole time. Grout shall not be used later than 90 minutes after the addition of cement to the mix and before pouring.

All grouting equipment shall be thoroughly washed with clean water after grouting operations for that day are completed.

Grout Injection Process for Post Tension Slab

The ducts shall be checked for blockages before pumping grout by flushing out with air or other approved means.

Grout shall then be pumped into each tendon in a continuous steady flow until the duct is completely filled and pure grout issues from all vents. The vents shall be progressively closed as required to ensure the complete filling of the duct.

If a blockage occurs pumping may be transferred quickly to the far end of the duct if there are sufficient vents to ensure the duct will be filled with grout.

On completion of grouting and after the grout has hardened sufficiently, any vents of grouting tubes that extend to the surfaces of the concrete shall be cut off and flushed with the concrete surface.

Procedure to be undertaken in the event of a blocked tendon during grouting operations

Locate the whereabouts of the blockage referring to the shop drawing.

Once the blocked tendon has been located, air must be blown into the tendon.

A concrete drill will be used to drill into the tendon.

Once the tendon has been drilled into & the ducting has been pierced more air will be blown into the tendon to ensure that air is flowing through from end to end.

Tendon will then be grouted as per the approved method statement.

On completion of grouting and after the grout has hardened sufficiently, any vents of grouting tubes that extend to the surfaces of the concrete shall be cut off flush with the concrete surface.

Removal of Formworks

The slab bottom formworks are to be removed only after the completion of the final post-tensioning stressing. However, side formworks and the stressing recess formworks shall be stripped off before the stressing operation.

Back propping details are to be stated in general details design on drawing.

References for pour strips pour sequences, construction joints, and removal of formwork shall be referred to the general notes on the approved shop drawings.

De-shuttering at the pour strip area can be carried out after the concrete completion and upon the achievement of the required compressive strength.

A pour strip, also known as a closure strip, is a section of concrete slab left exposed to control shrinkage and elastic shortening; it may also be used to provide access to stress for post-tension tendons.

Method Statement for Post Tensioning – Quality Control

The contractor shall prepare work procedures for the preparation of the post-tensioning units on-site and, a detailed inspection plan covering all requirements hold points, shall be prepared for production inspection and testing. Quality records shall be on standard inspection forms.

The contractor’s quality control shall include but not be limited to checking, testing, and keeping records of the following:

Check on correct positioning, fastening, and water-tight sealing of ducts, anchorages, and couplers. Records from post-tensioning stating equipment, force, and elongation for each tendon

• Calibration results for tensioning equipment
• Pre-pour check record to be submitted t e client or to .1 sting.
• Stressing record, stressing result, and grouting certificate to be submitted after completion of the work.

Quality Control Specification Documents and Forms

The following documents are to be used to control the quality of the entire project in the execution of the method statement for post-tensioning of concrete slab activity:

• Pre-pour inspection record
• Stress/stressing result
• Grouting
• Coil log
• Daily Report
• Weekly report

Health and Safety Documents

Safety Manual
Safety Risk Assessment

Method Statement for Post Tensioning – Quality Control Inspection and Test Plan (ITP)

Quality Control Inspection and Test Plan (ITP) to be submitted for review & approval by the consultant.

Takeaways

Brief History of Pre-stressing Technology

The first application of post-tensioning is believed to have been conceptualized by Eugene Freyssinet in 1933 for the establishment of a maritime terminal in France and the technology was introduced in the United States in the 1950s.

Post Tensioning Concept

Designing the shape of the PT layout and specifying the correct system is a vital step in the process and requires sound engineering consideration in order to maximize the benefits for all project stakeholders.

Work on the application of the post-tensioning system may begin after any formwork or bottom reinforcement has been installed. Typically, for a bonded system, the ducting will be arranged first, then the strands will be installed with the help of a strand-pushing machine and the end anchorages will be installed. However, the off-site prefabrication of post-tension tendons may also be of great benefit to certain construction projects.

The concrete placement is next. When the concrete has reached its minimum strength, the strands are anchored in a common anchorage device and stressed with a large, multi-strand jack or mono-strand stress tool. When the jack reaches the correct load, the tendon is released.

The tendon then retracts slightly, activating the anchoring system and creating a tight lock on the tendon. The anchor retains the force applied to the tendon and transfers it to the surrounding concrete.

The duct is then filled with cementitious grout that protects the strand from corrosion and bonds the tendon to the concrete surrounding the duct.

Industry Resources:
The one particular institute for post tensioning field which has helpful industry resources can be found here:

The Post Tensioning Institute provides architects and civil engineers with a variety of online resources.
The Concrete Reinforcing Steel Institute offers a comprehensive collection of publications, including its Ready Reference Reinforcing Steel Resource Guide.

For Publication:
EB705th PCA Notes to ACI 318-05 Building Code Specifications for Structural Concrete with Design Applications, EB705
The ninth version of this standard PCA resource reflects the improvements to the code implemented in Building Code Requirements for Structural Concrete, ACI 318-05. These notes will help users to apply the provisions of the code relating to the design and building of concrete structures.

F.A.Q.

Q: What is post-tensioning?

A: Post-tensioning is essentially a method of producing prestressed concrete and other structural elements. The term pre-stressing is used to describe the method of applying internal forces (or stress) to a concrete element during the construction process in order to counter the external loads imposed when the structure is placed into use. These internal forces are applied by tensioning high-strength steel, which can be applied after the concrete has been poured.

Here is the Video Showing Step by Step Stressing Related to the Method Statement for Post Tensioning of Slab

Q: Is post tensioning a recent invention?

A: No, it’s been around for a long time now. In particular, pre-stressed concrete has been the most commonly used construction material for today’s bridges and a growing number of residential, industrial, commercial, and public buildings development.
Perhaps as soon as reinforced concrete had been created, at the end of the 19th century, the engineers discovered that their efficiency could be enhanced if the bars could be held in tension – and the concrete could be kept in compression. It was not until much later, though, that the use of post-tensioning for structures became popular.

Q: What are the types of post tensioning?

A: The Post-Tensioning systems commonly used in building and bridge construction are classified into two major categories. These are the bonded and unbonded systems. However, they come in a number of varieties and cover a wide range of applications.

There are four main types:

Internal bonded tendons – when one or two strands are placed into a metal or plastic duct that is embedded in the concrete. By pouring the duct with a special grout, the tendon is ‘bonded’ to the underlying concrete. Internal bonded tendons are positioned until the concrete is poured and locked into the concrete. This technique is often used for bridges and heavily loaded beams in buildings – flat internal sheathing systems are also an excellent choice for thin slabs.

Internal unbonded tendons – where the prestressing steel is not actually bonded to the concrete that covers it, except for its anchorages. They are used in slabs and slabs-on-grade for buildings and parking structures, but even more in infrastructure projects.

External unbonded tendons – these are mounted on the exterior surface of concrete structures. This method of post-tensioning provides access for maintenance and replacement, which is also the ideal option for bridge upgrades and refurbishments.

Ground anchors – are used to support the sides of excavations, hillsides, and tunnel walls. They are also used for resisting uplift for towers or seismic strengthening.

Q: How is post tensioning installed?

A: The PT is installed by a specialist post-tensioning team. The specialist whose staff has undertaken the appropriate training and is accredited by a major international organizational network.

Post-tension systems can only be designed and installed by PT Specialized Firms. Designing the structure of the PT system and specifying the correct framework is a critical stage in the process and needs sound engineering consideration in order to optimize the gains for all project stakeholders.

Q: What is the benefit of post-tensioning technique?

A: The concrete is strong in compression while weak in tension application. Steel is stable under tension forces, so the combination of the two elements results in the formation of very strong concrete components. Post-tensioning can help to produce innovative concrete components that are thinner, longer, and tougher than ever before.

Many of today’s highly-performance concrete structures, including many iconic bridges and buildings, employ some kind of pre-stressing. Parking garages, high-rise residential towers, and many other types of facilities also use post-tensioning techniques.

Post-tensioned concrete is a concept that is increasingly common in today’s construction industry. This method of concrete reinforcement allows the builder to take advantage of the tremendous benefits of pre-stressed concrete while at the same time maintaining the flexibility offered by the cast-in-place method of constructing concrete structures.

Post-tension concrete construction occupies a very important role, the reason for this lies in its decisive technological and economic advantages and, in certain cases, the combination of lightweight panels relative to cmu block walls decreases the loading on structural components such as pt slabs, resulting in less thickness leading to less weight.

The use of post-tensioning encourages a more environmentally sustainable approach to construction. First of all, post-tension systems have fewer construction materials – steel construction, concrete – and thus lower greenhouse emissions in terms of construction and transportation. The actual construction process can also be environmentally friendly – for example, the building of an incredibly long PT bridge span does not require temporary intermediate supports, thereby mitigating the environmental effect below.

Q: What is the difference between Post-tensioning and Pre-tensioning?

A: Post Tensioning
Post-tensioning is the application of the force of compression to the concrete at some point in time after casting. When the concrete has achieved strength, the process of pre-stress is induced by tensioning the steel tendons, which pass through the ducts embedded into the concrete, and by locking the stressed tendons with mechanical anchors. The tendons are then usually grouted in place.

Pre-tensioning
Pre-tensioned concrete is mostly always built in a precast plant. Pre-tensioned prestressed concrete is cast on a preformed casting bed. The bonded wire tendons are tensioned before the hardening of the concrete. After the concrete hardens to approximately 75% of the specified compressive strength, the tendons are released and the axial compressive load is then transferred to the cross-section of the member.