Post-tensioning Prestressed Concrete Elements

  • Document Type: Guidance Note
    Keycode: web only
    Industry: Construction, 
    Category: General, 
    Division Author: Construction & Utilities
    Publication Date: 10 April 2003
    Date First Published: 10 April 2003
    Summary: This guidance note provides information on the post-tensioning of prestressed concrete elements.

PurposeThis Guidance Note provides advice on managing risks to health and safety associated with the post-tensioning of prestressed concrete elements.

It is particularly relevant to:

  • Principal contractors with management and control of construction projects where post-tensioning is intended to take place,
  • Pre-casters of concrete elements intended to be post-tensioned,
  • Post-tensioning contractors, their employees and their employees' elected health and safety representatives.

It sets out recommended precautions to be taken during the manufacture and post-tensioning of concrete elements, whether precast or cast in-situ, and then erected into position.

Background
Prestressed concrete is a composite material which utilises the natural compressive strength of concrete, while overcoming its weakness in tension by pre-compressing the concrete before it is subjected to its full design loadings.

Pre-compressing uses the technique of anchoring tensioned tendons of strands against two faces of the concrete element. The prestressed concrete element can be either pre-tensioned or post-tensioned.

In post-tensioned elements, the tendons are stressed after the concrete has been cast and allowed to reach a specified strength. These tendons are contained in metal sheathing or plastic ducts embedded in the concrete between the element's ends.

The post-tensioning technique can be used for stressing precast concrete elements and for concrete elements cast in-situ.

The stressed tendons may be either bonded or un-bonded. Post-tensioning with bonded tendons is a method whereby the tensioned tendons located in the ducts are grouted with a cement slurry to achieve a bond with the concrete and to control corrosion in the tendons. In Australia, bonded tendons are used almost exclusively.

Producing prestressed concrete elements is a specialised form of construction work requiring highly trained personnel who understand and consistently apply the techniques and necessary precautions set out in the appropriate technical manuals and handbooks.

Nonetheless, minor variations to standardised work methods and procedures may be necessary to suit particular and specific site conditions.

This Guidance Note will assist in identifying likely hazards and their associated risks so that adequate and appropriate control measures can be implemented.

Preplanning and coordination
Preplanning and coordination during the various stages associated with the post-tensioning procedures are essential to ensure that the concrete element is safe and without risks to health at all stages of its construction and during its design life.

Generally, the different stages can be categorised as manufacture, transport, erection, tensioning and grouting.

Preplanning should conclude with the preparation of a Work Method Statement. This should be prepared by the principal contractor, in consultation with all relevant subcontractors and health and safety representatives.

An example of a Work Method Statement is provided in Attachment 1 to this Guidance Note.

Responsibilities
While it is the responsibility of the project design engineer to ensure that the building is designed and detailed so that it can be constructed as intended, the principal contractor, in association with the design engineer, the post-tensioning contractor, the formwork installation contractor, the pre-caster and erector (if applicable), should coordinate the planning of the complete construction sequence, inclusive of stressing.

Any variations to the design specifications must be authorised by the project design engineer and included in the final drawings, issued for construction.

Principal contractors must ensure that their employees and the employees of their subcontractors involved in all of the post-tensioning stages are provided with the necessary information, instruction, training and supervision to enable them to perform their work in a manner that is safe to themselves and other persons. The various subcontractors also share this obligation in relation to their own employees.

A Job Safety Analysis (JSA) should be prepared by the post-tensioning contractor, in consultation with the workers and their elected health and safety representative, to cover all stages of their work. The JSA will assist the principal contractor and the subcontractors to:

  • Identify and manage risks associated with the procurement, materials handling, installation of post-tensioning components, use of equipment and stressing operations.
  • Keep abreast of the current state of knowledge within this specialised field.
  • Identify general and specialist training needs for employees.
  • Ensure the final approved drawings are followed and all health and safety concerns are dealt with.
  • An example of a typical JSA is provided in Attachment 2 to this Guidance Note.

The direct supervision and coordination of the stressing operations is the responsibility of the post-tensioning contractor's nominated supervisor.

A checklist to assist post-tensioning supervisors is provided in Attachment 3 to this Guidance Note.

Specific Issues


Working platforms
The design and construction of working platforms should comply with the relevant Australian Standards. For example, where scaffolds are provided, they should be constructed in accordance with AS/NZS 4576 -- 1995: Guidelines for scaffolding.

To provide a safe working environment, working platforms need adequate working space, appropriate edge protection, and safe access and egress.

They must also be designed and constructed to safely support all expected loads, including impact loads. Factors that will determine the selection of an appropriate working platform include:

  • the type and number of items of stressing equipment that may be in use,
  • the number of people required, or likely to be, on the work platform at any one time, and
  • the likely material storage on the platform.

The platform should be large enough to enable the operators to remain clear of anchorages during stressing, and it should be at a height which eliminates or minimises risk of injury from over-reaching or awkward postures.

Formwork
In the case of cast in-situ concrete constructions, working platforms may be built integrally with the formwork.

Design and documentation for formwork should comply with the requirements of AS 3610 -- 1995: Formwork for concrete.

There are "fast shuttering" formwork systems designed to allow removal of forms within two to three days, leaving slabs to continue their curing supported on props at up to 3 metre spacings.

Such systems may not be suitable for some post-tensioned slabs, which are usually lightly reinforced and might not have developed adequate strength to be self-supporting when forms have been removed, particularly where these slabs are likely to receive early additional loads from materials and equipment.

Formwork and propping systems should be designed by a structural engineer experienced in formwork design.

Assembled formwork and propping systems should be checked by a competent person for compliance with the formwork design drawings and documented proof of such on-site checks should be readily available.

Provisions for stressingAnchorages for stressing should be set out and tendon spacings marked on the ends. Since the wedging forces at anchorages are high, anti-burst provisions, such as special reinforcement, need to be installed and secured into position.
[Note: "Anti-burst reinforcement" is a small steel reinforcing cage located at an anchorage.]

Prestressing ducts should be laid in accordance with the specified profile and adequately secured. Inadequate cover to duct tubes can result in concrete blow-outs during grouting operations.

Prestressing safety considerationsStressing operations and associated preparations for stressing involve a variety of tasks which, if appropriate precautions are not taken, could endanger the health and safety of workers carrying them out and/or those in the vicinity.

  • The area where preparations and stressing are intended to take place must be fully barricaded with solid panels and signage prominently posted to keep unauthorised personnel clear of this area.
  • All personnel involved in the tasks should wear the personal protective equipment identified in the JSA. This will generally include safety goggles, gloves, sturdy protective footwear and safety helmets.
  • All personnel involved in the tasks should have adequate training in identifying the hazards of stressing and their associated risks.

Uncoiling, Cutting and Placing Strands

  • Coils are very heavy, typically weighing 3 to 4 tonnes, and therefore the structural adequacy of the area where coils are to be placed must be verified. Manual handling issues associated with handling the coils should be controlled in accordance with the recommendations given in WorkSafe Victoria's Code of Practice for Manual Handling.
  • Ensure that the coils are restrained with uncut restraining straps when placing them onto the strand frame. Uncontrolled release of the coil can result in whip-back with sufficiently high force to cause serious injury.
  • Strands should not be cut by heat-type cutting equipment such as oxy-acetylene or LP gas torches, as this may compromise their load-bearing capacity under tension.
  • When assembling tendons, thoroughly inspect each individual wire or strand for obvious flaws.

Pushing Strands into Ducts

  • All strands for each of the tendons should be pushed into place in accordance with the drawings, making sure all personnel are kept clear of the direct line of ducting to prevent injury from strands exiting from the other end of the duct.
  • Once the specified number of strands is in place, ensure that a "dead end" is created for each strand by securing them at the end of the element.
(Note: "Dead end" is the end of the tendon which is anchored in the concrete beam or slab whilst stressing takes place from the opposite (live) end. Dead ends may be of a "swaged", "onion" or "H" type.)

In strand set-ups where ends will protrude above the face of the concrete element and may create a hazard, they should be boxed or barricaded to prevent injury.

Concrete Pour
Concreting may be placed with either pumps or kibbles.

When concreting is being pumped, "chairs" or other means to support concrete pump lines above the reinforcement and tendons should be in place and well secured.

Where kibbles are used to place concrete, avoid dropping concrete in one place as tendons could be displaced. Concrete should always be allowed to flow in a controlled manner.

During concrete pouring:

  • Ensure that the ducts and strands are not damaged during the pour. All damage should be promptly notified to the contractor's supervisor for repair. However, concrete around the anchorages needs adequate vibration to ensure a safe and sound seating for the anchorage.
  • Concrete test cylinders should be taken at agreed intervals for storing and curing on site under conditions similar to those applying to the element being poured.

Barricades
Before final stressing commences, adequate barricades should be erected at all live ends of tendons being stressed. In the case of double live end tendons, barricades are needed at both ends, even if stressing is only occurring from one end.

An adequate barricade is one that has been designed to restrain and reduce impact from strands or jacks if strands snap or release under tension or if the system fails in some other way. They are generally constructed from timber and two separated layers of 17 mm form ply sheets and are placed behind jacks during final stressing and de-tensioning.

Typical examples of barricades are shown in Figures 1 and 2, below.
Figure 1: An example of a portable impact absorbing barricade suitable for placing behind stressing jack

Figure 1: An example of a portable impact absorbing barricade suitable for placing behind stressing jack.

An example of a fixed bed impact absorbing barrier in a restricted positionFigure 2: An example of a fixed bed impact absorbing barrier in a restricted position.

 The ply sheets if struck by a strand or anchorage failure would be pierced, preventing any rebound hazards for operators.

Stressing operationsBefore stressing operations
Prior to commencing stressing operations, the post-tensioning supervisor should verify that:

  • Concrete around the anchorages has been examined. The principal contractor should be notified if the concrete is of poor quality.
  • All concrete test cylinders have achieved the specified strength.
  • The grips in the jacks on the stressing equipment are clean and free from dirt or grit and in good condition.
  • The stressing equipment, i.e. jacks and their gauges, has appropriate service records and up-to-date calibration certificates. All jacks should have a durable tag securely attached which clearly shows the following information:
    • Final stressing pressure
    • Diameter and grade of the strand for which the jack is being used
    • Jack number
    • Corresponding gauge number
    • Date calibration expires
  • The operator of the stressing equipment has documented evidence of appropriate training.
  • A "NO GO" area of at least 2 metres radius is in place around the anchorages at the dead and live ends, with barricades behind the line of jacks and "Stressing in Progress. Keep Clear" signage prominently displayed at all appropriate locations

During stressing operations

  • Tendons should be stressed in order from the furthest to the closest reachable to ensure that no person is standing in direct line of the jack or previously stressed strands.
  • Ensure stress is applied gradually and evenly to tendons.
  • Ensure that the specified initial and final stressing levels are not exceeded.

After stressing operations

When stressing operations are completed:

  • Gain the design engineer's approval prior to cutting off excess tendons.
  • Seal anchorage recesses following approval and prior to grouting the ducts.
  • Do not perform tasks requiring impact, such as hammering, drilling or coring in the vicinity until the grouting of the ducts has been completed.

Grouting
Build-up of excessive pressure during grouting can result in "blow-outs" of the concrete, which could injure personnel in the vicinity.

To prevent blow-outs:

  • Ducts should be blown through to ensure there are no blockages.
  • Avoid non-continuous grouting to ensure no blockages or voids are in the tubes.
  • Monitor the gauge of the equipment throughout grouting to ensure that excessive pressure does not develop.
  • Retain barricades used during stressing operations and also barricade at a lower level if formwork has already been removed.

Acts and Regulations

Acts and regulations are available from Information Victoria on 1300 366 356 or order online at www.bookshop.vic.gov.au.

View the legislation at Victorian Law Today at www.legislation.vic.gov.au.

Standards Australia

AS/NZS 4576 -- 1995: Guidelines for Scaffolding
AS 3610 -- 1995: Formwork for concrete

Copies of standards can be obtained by contacting Standards Australia on 1300 654 646 or by visiting the web site at www.standards.com.au.

Further information


WorkSafe VictoriaCode of Practice for Manual Handling

Copies of publications including Codes of Practice can be obtained by contacting WorkSafe Victoria on 03 9641 1333, by going to the WorkSafe Victoria web site at www.workcover.vic.gov.au or your local WorkSafe Victoria office. Or contact our Advisory Service on 9641 1444 or toll free 1800 136 089.

AcknowledgementsWorkSafe Victoria acknowledges that this Guidance Note is based, in part, on the recommendations given in the WorkCover NSW Code of Practice, Mono-strand Post-tensioning of Concrete Buildings, first published in 1993. The illustrations used in this Guidance Note are also taken from it.

WorkSafe also acknowledges the assistance of the CFMEU's Health & Safety Representatives' Working Group and the post-tensioning contractor, VSL, in the development of this Guidance Note.

Special Note on Codes of Practice: Codes of Practice made under the Occupational Health and Safety Act 1985 provide practical guidance to people who have duties or obligations under Victoria's OHS laws. The Occupational Health and Safety Act 2004 allows the Minister for Workcover to make Compliance Codes which will provide greater certainty about what constitutes compliance with the OHS laws.

Codes of Practice will continue to be a practical guide for those who have OHS duties and WorkSafe will continue to regard those who comply with the topics covered in the Codes of Practice as complying with OHS laws. WorkSafe will progressively review all Codes of Practice and replace them with guidance material and in appropriate cases, with Compliance Codes.

Note: This guidance material has been prepared using the best information available to WorkSafe Victoria. Any information about legislative obligations or responsibilities included in this material is only applicable to the circumstances described in the material. You should always check the legislation referred to in this material and make your own judgment about what action you may need to take to ensure you have complied with the law. Accordingly, the Victorian WorkCover Authority extends no warranties as to the suitability of the information for your specific circumstances.