10.Prestressing
[This section requires effective proof reading and altered as deemed necessary. It also requires a critical review in the light of more current practice, codes and specifications.]
10.1General – Hold Point
Hold Point-Submit full details of the prestressing ducting, fittings, equipment and proposed stressing and monitoring procedures for approval.
Provide copies of supporting technical and performance data.
Provide amended drawings and fully detailed supporting computations where the proposed system necessitates any change of details from those shown (e.g. different anchorages, reinforcement, etc). Minor amendments to the shape of the members shown on the Drawings will be considered but the general arrangements, depth, spacing and outward appearance of the members shall remain unaltered unless otherwise approved.
Approval will be given within 2 weeks of receipt of a request if the details are sufficient, acceptable, conforming, and indicate that the techniques are sound.
Provide a supervisor experienced in prestressing to control the application and monitoring of prestress, and do not tension any prestressing steel without that control.
Do not tension any steel if:
there are inadequate safety measures to protect workers and the public from the consequences of tendons that let go whilst under tension,
signs warning of the tensioning operations are not displayed at both ends of the tensioning bed or member,
written approval for the operation has not been received, or,
the Superintendent is not present.
10.2Standards
Conform to the following standards unless specified otherwise:
AS 1310Steel wire for tendons in prestressed concrete
AS 13117-Wire stress-relieved steel strand for tendons in prestressed concrete
AS 1314Prestressing anchorages
AS 1319Safety signs for the occupational environment
AS 1349Bourdon type pressure and vacuum gauges
AS 1478.1Chemical admixture for concrete, mortar and grout – admixture for concrete.
AS 3600Concrete structures
AS 3610Formwork for concrete
AS/NZS 4672
-Part 1 Steel prestressing materials – General requirements
-Part 2 Testing requirements
AS 5100 Bridge Design Set
Part 3:Expanding admixtures for use in concrete, mortar and grout
ASTM C 465Specifications for processing additions for use in the manufacture of portlandhydraulic cement
AUSTROADS AGBT Austroads Guide to Bridge Technology (Set)
Comply with the additional requirements of this Specification.
Comply with the requirements of this Specification if they contradict clauses in the above Standards.
10.3Materials
10.3.1Tendons – Hold Point
Hold Point - Obtain approval for the stressing strand supply.
Use super grade, low relaxation prestressing steel unless otherwise specified.
Do not use prestressing steel that is not accompanied by a test certificate.
Do not use kinked or bent steel.
Do not use any tendons affected by welding, weld splatter, or heat.
Do not use wire or strand that is flame cut within 75 millimetres of where it will be gripped.
In addition to the requirements in the Australian Standards, the maximum projected relaxation loss at 10000 days shall be 5.0% when stressed to 80% of minimum Ultimate Tensile Strength Specified in AS 1311.
10.3.2Prestress Ducting
[Insert this chapter if postensioning is being employed.]
Provide sheathing which:
is strong enough to withstand the normal placing and vibration forces of concreting without suffering any damage or deformation,
has the specified frictional coefficients,
has couplers of sufficient length to allow for tendon extensions.
10.3.3Grout Vents
[Insert this chapter if postensioning is being employed.]
Provide grout vents:
at each anchorage or end (at the highest point in the anchorage),
at each low point, and,
at each high point of the duct.
Use 20 millimetre diameter tubing (at least) having a minimum projection from concrete of one metre.
Provide each with a plug, valve, or similar device capable of withstanding a pressure of 1.0 Mpa without the loss of water, air pressure, or grout.
10.4Equipment – hold point
Hold Point - Provide recent (less than 12 months old) test certificates and calibration curves from an approved laboratory for dynamometers, and sets of stressing equipment (comprising pump, jack, pressure gauge and master gauge).
Calibrate jacks over their full working ranges during both loading and unloading cycles.
Provide concentric scale type pressure gauges of 150 mm minimum nominal size.
Provide gauges that read between 50 and 80 percent of their gauge range at the design force.
Make provision for the attachment of a master gauge to enable the working gauges to be checked at any time.
10.5Stressing Bed
Display a permanent notice adjacent to the stressing bed showing the maximum tensioning force allowed, the upper limit of height of the force and the Australian Standards used in the design of the bed.
10.6Tensioning Force Required
Stress tendons to the forces shown on the drawings.
Apply sufficient force to the jacks to allow for anticipated slip at the anchorage devices, wedge draw-in, friction losses and elastic shortening of the member.
[Carry out trial stressing operations to establish the frictional resistance offered by the hold-downs and also to confirm that the stated wedge draw-in consistent with the type of jack and operator technique proposed.]
10.7Tensioning Procedure
Accurately align the axis of the jack with the specified line of the tendons.
Apply a nominal initial force to remove slack.
Make allowance for this force when calculating elongations.
Mark both ends of tendons, and at couplers, to measure elongation, slip, and draw-in. Where tendons consist of a number of individual components, mark each component.
Make allowance for any draw-in or movement at the couplingwhere tendons are coupled together.
Stress tendons in the order and to the forces shown on the drawings.
Record the following data for every tendon stressed:
Identification number of the dynamometer, gauge, pump and jack.
Identification particulars of tendon.
Initial jack forces (or pressures) when tendons are marked for measurement of elongation.
Final jack forces (or pressures) and elongations at completion of tensioning.
Elongation remaining after release of jacks.
Elongations obtained of intervals during tensioning, together with corresponding forces (or pressures) if requested by the Superintendent.
10.8transverse stressing and grouting
10.8.1Mortar Joint between Units before Transverse Stressing
Before making the mortar joint, form, with compressible packing, cells at least 75mm wide which enclose each of the cored holes for stressing bars and extend to the unit surface.
During mortar placement, care shall be taken to ensure that the mortar is not forced into these cored holes and that the gaps directly above these holes are maintained. Select packing such that it is easily removed after the mortar has hardened.
Not less than 48 hours before the transverse prestressing force is to be applied, seal the longitudinal joints between the prestressed concrete units with leak proof formwork or seal on the lower surface and the space filled with mortar to form a dense waterproof joint.
Where a compressible seal is used to seal the base of the longitudinal gap and where such seal is pre-attached to one of the units prior to assembly, it shall be thoroughly glued, or otherwise firmly attached so as to prevent dislodgement or rolling-up the gap when deck units are lowered into place.
Before any mortar is placed, check the position of the seal at the base of the joint and, where necessary, be adjusted using a flat, spade-like tool to required depth. The tool shall have a stop attached to prevent pushing the seal further than required.
The required depth to the seal is the depth of the unit less the depth of the approved seal plus any fillet on the unit.
Clear joints of debris prior to placing mortar. The water/cement ratio of the mortar shall be such as to produce a consistency that allows the mortar to be puddled into the joint without being free flowing.
Fill the completed joints with sound mortar throughout, and any porous, honeycombed or defective areas shall be cleaned out and made good to the satisfaction of the Superintendent before transverse prestressing is carried out.
All aspects of the process by which the mortar joints are to be made shall be approved by the Superintendent before work is commenced.
10.8.2Cleaning and Straightening of Stressing Bars
Before inserting transverse stressing bars into the units, remove any grease or protective coating film with a solvent such as petrol. Do not use kerosene or diesel fuel.
Transverse stressing bars which require straightening shall be used only with the approval of the Superintendent. Under no circumstances shall heat be used to straighten a bar.
Straightening shall be done cold, ensuring that no nicking or notching of the bar takes place in the process.
10.8.3Care of Transverse Stressing Bars and Nuts
Bars are supplied with a rolled thread and no further threading by lathe or die is permitted. Minor damage to bar threads may be repaired by careful filing with a saw file or a thread file. More extensive damage can be repaired by using dies supplied by the stressing bar manufacturer. Nuts shall be checked for free running on the thread before commencing stressing. No welding shall be permitted on stressing bars, nuts, anchor plates or couplers.
10.8.4Installation of Stressing Bars
Transverse stressing bars shall be carefully threaded through the holes in the deck units taking care not to damage the threads on the ends of the bars. Nuts, washers and anchor plates shall be assembled as shown on the drawings. Where stressing bars require extending, couplers shall be screwed onto the bar threads for the entire length of socket in the coupler and the joint nipped up firmly.
The actual diameter of the unthreaded portion of the transverse stressing bars shall be measured prior to their insertion in the units to determine their actual cross-sectional area.
10.8.5Transverse Stressing
Each span shall be transversely stressed by applying a transverse stressing force to each of the transverse stressing bars. Apply the transverse stressing force in the presence of the Superintendent.
10.8.6Measurement of Transverse Stressing Force
Tension each transverse stressing bar to a force at lock-off of 350kN in the bar unless shown otherwise on the drawings.
Measure the magnitude of the transverse stressing force by means of a pressure gauge (or gauges) and by the extension of the transverse stressing bar caused by the stressing force. The extension in the bar shall be calculated using the following formula:
Extension = PL / AE
Where:P = total stressing force (kN)
L = length of the transverse stressing bar between far anchorage and the point of attachment to the jack (mm).
A = full area of the transverse stressing bar based on the actual diameter (mm2).
E = Modulus of elasticity of the transverse stressing bar, the value of which shall be taken as 170kN/mm2 unless determined otherwise.
The expected extension for a 29mm diameter bar stressed to 350kN is 3.1mm per metre length of bar.
The extension shall be measured by means of the Vernier scale fixed to the jack. Should there be more than 10% variation between the calculated and measured extension, transverse prestressing shall be discontinued until the reason for such difference has been ascertained.
Care shall be taken to ensure that the ram of the jack does not reach the limit of its travel during the stressing operation.
10.8.7Calibration of Gauges
Provide a current calibration certificate (less than 12 months old) for the jack.
10.8.8Grouting of Transverse Stressing Bars
After stressing, the transverse stressing bars shall be grouted in the cored holes. Pour grout down one or more of the gaps in the mortar above the transverse stressing bars until grout is forced up all the remaining gaps and until it emerges from the drilled hole in the bearing plates on the outside units.
10.9Additional Procedures For Post-Tensioning
Clean out ducts by blowing oil-free compressed air through thembefore placing tendons.
Provide a wooden or steel dolly, and draw it through the sheathing to check for obstructions and conformance with duct tolerance before inserting tendons.
Move tendons 300 millimetres backwards and forwards to ensure that the tendons are free to move before tensioning them.
Remove, check for corrosion, and replace (if necessary) tendons that remained in an ungrouted state for longer than 28 calendar days after their installation within the structure. (This is not necessary if approved steps are taken to protect the tendons from corrosion.)
Do not apply prestress until concrete has cooled to ambient temperature.
Do not trim tendons until they have been grouted.
10.10Tolerance On Pull-In Of Strands
A maximum of 6mm pull-in is allowed.
10.11Suspension Of Tensioning
Suspend tensioning operations if the relationship between measured and computed tendon extension is outside of tolerance.
Use procedures specified or agreed with the Superintendent to investigate and explain the lack of correlation, such as:
providing two jacks and stressing them sequentially,
measuring structural deflections,
carrying out tests on materials, and,
monitoring tendon forces and extensions,
Do not recommence normal stressing operations without approval.
10.12Order Of Applying And Releasing Prestress
Comply with the relative order of stressing and releasing tendons and hold-down devices shown on the drawings.
Wait until the concrete has reached the minimum strength specified on the drawings before applying prestress.
10.13Protection Of Exposed Ends Of Tendons
Cut strands flush with concrete surface.
Abrade the ends of tendons and the concrete surface within 50 mm of the tendons to provide a clean sound surface.
Immediately apply a 1 mm dry film thickness of high build epoxy paint coating over this area.
10.14Grouting
10.14.1General
Provide details and request approval for the:
proposed grout mix,
constituents,
equipment for batching, mixing, agitating and pumping it,
the proposed location of grout vents, and,
the type of vents and cutoff valves.
Provide manufacturer's product and technical literature, and specifications.
10.14.2Grout
Use Ordinary Portland cement and water (unless otherwise approved).
Do not use unapproved additives to increase workability and reduce shrinkage and bleeding.
Do not use cement more than one month old.
Do not use additives containing any product liable to damage the steel or the grout itself, such as chlorides, nitrates and sulphites.
Use grout conforming to the following:
water-cement ratio less than 0.45 by mass,
strength equal to or greater than the surrounding concrete but not less than 30 MPa at 28 days,
less than 2 percent bleeding, by volume, three hours after mixing,
less than 4 percent bleeding, by volume, at any time, and,
separated water must be reabsorbed within 24 hours.
Measure bleeding of the proposed grout in the presence of the Superintendent.
Use at least 100 mm height of grout in a 100 mm internal diameter, impervious cylinder.
Apply an airtight cover on the container during the test (to prevent evaporation).
10.14.3Mixing and Pumping Grout
Batch the dry materials by weight.
Measure the amount of water used by a calibrated flowmeter or measuring tank.
Add two-thirds of the cement to the water, then the admixtures, then the remaining third of cement when mixing the grout.
Ensure a continuous supply of grout for long ducts.
Do not mix by hand, or by tumbling action.
Use mixer capable of imparting a high shear action to the grout components when mixing.
Keep the grout continuously agitated after mixing.
Produce a colloidal grout of uniform consistency in a mixing time of less than five minutes.
Fit the inlet side of pumps with a 1.18 mm size sieve. .
Use positive displacement type pumps:
capable of continuous operation with little pressure variation,
which recirculate grout whilst actual grout injection is not in progress,
fitted with a flowmeter,
fitted with a pressure gauge, and,
able to deliver 1.0 MPa pressure.
Obtain the Superintendent's approval of the equipment prior to its use.
Use piping from the grout pump having internal diameters greater than 20 mm and a minimum of bends, valves and changes in diameter.
10.14.4Grouting
Flush ducts immediately prior to grouting until all loose particles are removed and clear water is discharged.
Water test the ducts to 0.5 MPa.
Plug leaks to the satisfaction of the Superintendent.
Simultaneously grout adjacent ducts using multiple pumps if water testing establishes there is leakage between them which cannot be reasonably rectified. Inject grout into ducts blown dry with oil free compressed air in this situation.
Inject grout in water filled ducts having at least one metre of water head against which to grout.
Do not use grout more than 30 minutes after mixing it.
Fill the duct between 6-12 metres per minute.
Progressively close grout vents as clean grout emerges.
Hold the pressure at 0.5 MPa for five minutes when the duct is full.
Flush the grout with water, plug the leaks, and start again, if the pressure cannot be maintained.
Fully grout all ducts in a web during the course of a day; otherwise flush the remaining ducts with clean water, remove unstressed tendons, and blow the ducts dry with oil-free compressed air.
Record the amount of grout injected into each duct, and provide the Superintendent with the records.
Protect the grouted ducts from shock or vibration for 1 day after grouting.
Inspect and top up the grout vents two to three days after grouting and then hermetically seal them.
10.15Payment
Refer to General Conditions of Contract N.P.W.C. Edition 3 (1981) Clause 3.3A.
[Include in all specifications and vary the following payment clauses if deemed necessary.]
Payment for grouting and prestressing is covered by payment clauses in the concrete section.
Payment for grouting of prestressed rock anchors is made in the section of the rock anchor section of the Specification.
DoI Bridgeworks Master – Dec 2012
PRESTRESSING
[If necessary, measure on a tonnage basis (or other unit) to permit payment at different stages in the construction and either put special clauses here or modify the concrete section clauses.]