CASTING BEDS, MOUI.I)S, FORMWOIK AND STAGING. Casting beds shall be properly aligned and levelled.
Moulds and formwork shall be such that the finished concrete surfaces are smooth and even and have true
clean axrises.
Formwork and staging shall be so designed that there will be no undue restraint on the elastic shortening
of the concrete at the time of transfer of prestress.
HANDLING, LIFTING AND STACKING EQUIPMENT. The Contractor shall provide adequate equipment for so
handling, lifting and stacking precast units that they are protected from permanent damage caused by
stresses induced during handling or stacking, or by the use of slings, chains or hooks.
CONSTRUCTION
GENERAL. In addition to complying with the requirements of 5.2-5.7 the Contractor shall comply with the
relevant requirements of Clause 5 of SABS 1200 G.
SAFETY PRECAUTIONS. The Contractor shall take particular precaution during and after tensioning to
protect persons from injury and equipment from damaqe that may be caused by a sudden release of stored
energy in the event of failure of any tendon or equipment.
ROYALTIES. Where applicable, the Contractor shall pay any royalties that may be due on whatever
prestressing system is adopted and he shall indemnify the Employer against any claims that may arise from
any infringement or alleged infringement of patents.
TENDONS
Surface Condition. The Contractor shall ensure that all prestressing tendons and internal and external
surfaces of sheaths are, at the time of incorporation of the tendons and sheaths in the structural unit,
free from pitting, loose millscale, loose rust, paint and oil, grease, soap and other lubricants, and any
other harmful matter. A tendon may be cleaned by wire brushing or by passing the tendon through a
pressure box containing carborundum powder. Solvent solutions for cleaning shall not be used without the
approval of the Engineer.
Any prestressing steel or sheath which, in the opinion of the Engineer, is excessively rusty or otherwise
unsuitable, shall be removed from the Site.
Straightness
a) Wire and strand. Wire and strand shall be in coils of diameter such that the curvature of the
uricoiled wire or strand complies with the applicable requirements for curvature given in BS 5896.
b) Bars. Prestressing bars, as delivered, shall be straight, except that where small adjustments will
achieve straightness, such adjustments may be made by hand on Site under the supervision of the Engineer.
Straightening of bars shall be carried out at ambient temperature. If the ambient temperature is less
than 5OC, any heating required to raise the temperature of the bars above 5OC shall be done by means of
steam or hot water.
Bars that are bent in the threaded part shall be rejected.
Cuttinq
All cutting to length and trimming of ends shall be done by means of
a) a high-speed abrasive cutting wheel, friction saw or any other mechanical method approved by the
Engineer; or
b) an oxy-acetylene cutting flame, excess oxygen being used to ensure a cutting rather than a melting
action and the cutting being so done that neither the flame nor splashes come into contact with the
anchorage or with other tendons.
In the case of post-tensioning systems, cutting shall be done as specified in 5.4.3.1(a), the heating
effect on the tendon being kept to a minimum and being such that the temperature of the tendon adjacent
to the anchorage does not exceed 20OoC.
The tendons or the elements of the tendons, as relevant, shall be cut or released in the order that will
produce the least lateral eccentricity of prestress at any stage.
Positioning of Tendons and Sheaths
Subject to the tolerances permitted in terms of Clause 6 , tendons and sheaths shall be accurately located
and their vertical and horizontal position maintained accurately as shown on the drawings.
The assembly of post-tensioning tendons shall be done on raised platforms or tables that are fully
protected from the weather.
The Contractor shall use separate carriers for so supporting and fixing in position the tendons or the
sheaths, as applicable, that they will not be displaced by heavy or prolonged vibration, pressure of the
wet concrete, workmen or construction traffic. Tendons and sheaths shall not be supported or fixed by
being welded to reinforcing steel. Carriers shall be so fabricated (by means of welding) and so located
that they will not unnecessarily increase friction in tendons when the tendons are being tensioned.
Sheaths shall be handled carefully to avoid their being damaged and in order to ensure that they retain
their cross-section and profile.
Joints in sheaths shall be securely taped to prevent penetration of the sheaths by concrete or laitance.
Ends of sheaths shall be sealed and protected after the stressing and grouting operation.
Tendons shall be clearly marked at each end so that extensions, slips or other movements that occur can
be accurately measured.
Tensioning Procedure
General. The sequence of prestressing shall be such that the permissible stresses in the unit or
structure, as specified in the prestressing force diagram given in the project specification or shown on
the drawings, are not exceeded. The Contractor shall submit for approval a program indicating the
sequence in which prestressing will be carried out, and shall obtain approval before commencing each
tensioning operation.
The Contractor shall, in respect of each tensioning operation, comply with the following requirements:
a) Tensioning shall be carried out by personnel who are suitably qualified and under constant comwtent
supervision.
b) Immediately after concreting has been completed, the sheaths shall be flushed with water and the water
expelled by compressed air.
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