Precast Post Tensioned Beam Installation Procedure

This civil method statement is prepared to state the proposed construction method for prestressing works and shall be, upon approval by the consulting engineer, distributed to all parties concerned so that the prestressing works can be carried out with the same information.

The prestressing systems is STRONG FORCE (M) Multi Strand Bonded System using 15.24mm diameter of strand to ASTM A-416M-80 Grade 270, low relaxation, having a minimum ultimate tensile strength (UTS) of 260.7 kN per strand.

Required minimum initial stressing force in cables is as shown in approved shop drawings.

Tensile strength testing shall be carried out, on strand samples taken from the site.

Sampling and frequency of testing shall be at 50.0 metric-ton interval.

Each tensile test to have 2 samples of 1.2m in length and accompanied with the strand’s coil numbers, and the samples to be properly tagged for easy reference.

Necessary Tools & Equipment

Health & Safety Requirements

All personnel engaged in prestressing works must know, understand and adhere the site safety rules and procedures, to ensure that their work activities are carried out in a safe manner.

To wear protective safety and protective equipment’s during prestressing operations.

All personnel in charge shall attend all safety meetings, comply with all safety measures and shall conduct safety housekeeping of their working areas.

The foreman / supervisor should report all accidents to the Safety Officer and arrange medical treatment for their injured employees, and are required to fill the ‘Notification of Accident’ form.

All the site personnel shall ensure that all equipment used on site are in good working condition prior to actual usage.

The site personnel must not throw any materials indiscriminately around the working area.

All site personnel shall ensure that all electrical equipment used is properly earthed, provided with plug and intrinsically safe to handle.

Sequence of Activities

1. Providing casting yard.

2. Fixing of beams casting beds and reinforcement cage.

3. Fabricating of cables.

4. Fixing of bursting links at both anchorage ends.

5. Installation of cable sheathing supported on profiling bars.

6. Threading of strands, fixing of grout hose and forming dead-end anchorages.

7. Checking of cables profiles.

8. Closing of side formworks

9. Fixing of anchorages casting and PE cone to End formwork (end plate)

10. Closing end formwork (end plate)

11. Concreting.

12. Removal of side and end formworks.

13. Fixing of anchor heads and wedges for stressing.

14. Stressing of post-tensioned beams.

15. Cutting of excess strands (approved by Engineer) and patching with cement mortar on anchor heads only for grouting purposes.

16. Grouting of cables pre stressing ducts.

17. Patching up of remaining recesses at stressing ends

Construction Procedure for PT Bean

Fabrication of Cables

Cutting list shall be made from the approved shop drawings. Cable length to be fabricated including allowance for stressing length, quantity of anchorage and sheathing.

The prestressing cables are 7-wire strands of 15.24mm diameter packed in circular coils.

For the cables, the outer packing materials of the coil of strands shall be removed before fixing the ‘Bripad’. After fixing the ‘Bripad’ the steel strips tying the coils of strand shall be cut and the strand be pulled out from the ‘Bripad’ in reasonable straight manner. ‘Bripad’ to be used where applicable.

The strand shall then be cut according to required length using high speed disc cutter.

Sheathing is made from galvanized steel and shall be made in the diameter as shown in the approved shop drawing and shall be joined with duct couplers.

Fixing of Cables Sheathing

Sheathing will be placed according to its profiles as shown in approved shop drawings after fixing of beam’s steel cage (by others), profile bars shall be fixed at 1.0 meter spacing according to the parabolic profile of the approved shop drawings.

Joint couplers shall be sealed with tape to avoid ingress of concrete grout and foreign matters.

Threading of Strands

Strands is then threaded to all the cable according to approved shop drawings.

Bursting links shall be fixed in position as per shown in approved shop drawings.

Grout hoses shall then be fixed at the inlet and outlet vent of the beams.

Checking of Cables Profiles

All cable profiles shall be checked and adjusted before concreting with a vertical tolerance of ± 10 mm as shown in approved shop drawing.

After obtaining approval, side and end formworks are fixed into place, and anchor casting are fixed to end formwork.

The exposed stressing length of the cables shall be protected by covering them, so as to prevent them from being damaged due to the subsequent works prior to stressing works.

Stressing of Cables

After concreting, the formwork shall be removed after 36 hours.

Anchor heads and wedges shall be fixed to all stressing ends.

When concrete have reached the specified transfer strength, stressing of cables shall be carried out using hydraulic jack. All cables shall be stressed to the required jacking force as shown in approved shop drawings.

The stressing extensions of the cables shall be calculated from the site measured extensions and compared with theoretical calculated values. The tolerance allowable limit shall be ± 6% (BS 5400) part8, clause 5.7.5.4.

Control of stressing operation: Calibration certificates for pressure gauge and stressing jack shall be submitted to the consultant prior to any stressing works.

All stressing length of cables shall be crop-off after getting approval from the engineers.

Criteria and Stressing Procedures

The tendons shall be stressed based on the stressing sequence and required jacking force as shown in the approved shop drawings.

A nominal jacking pressure of 1000psi shall be applied in order to take up the slack in the tendon. The measurement of cable extension shall be done using the measuring tape or steel rule.

The extension measured at 1000psi shall be taken as the datum for the measurement and calculation of the extension of the stressed cables.

Cable extension from 0 to 1000psi shall be extrapolated from the total measured extension from 1000psi to the required final jacking pressure.

Extension of the cables shall be measured to an accuracy within ± 2% or 2mm.

The results of stressing generally fall into one of the following cases:

Case 1: Apply the specified prestressing force and check whether extension increased linearly with pressure increased that is extension per 1000psi should be about the same. If measured extension is within +/- 6% of the theoretical extension, proceed with cropping of the cables after approval from the engineer.

Case 2: Apply the specified prestressing force and if extension is between + 6% to + 10% of the calculated extension. Check whether the extension every 1000psi shall be about the same. If yes, proceed with cropping of cables after approval from the consulting engineer.

Case 3: When % difference than is more than +10% check stressing records, whether extension linear with pressure and calculation of total extension and % difference. Inform engineer of the results for further action. Do not crop the cables.

Case 4: Apply the specified prestressing force and if extension is between – 6% to – 10%. Recheck stressing records, whether linear and calculation of total extension and % difference. Prestressing force shall be increased but not exceeding 80% UTS to bring back the negative % within the allowable +/- 6%. Proceed with cropping of the cables upon approval from the Engineer. If result is still out of tolerance, inform SFM Engineer on the next course of action. Do not crop the cables.

Grouting of Cables

Grout shall be mixed according to the approved design mix.

Water shall be poured into the mixer first, followed by admixture additive and the cement.

All material including additive shall be batched by weight except the mixing water, which is batched by volume.

Grout shall be mixed for at least 2 minutes or until a consistent colloidal mix of thick cream is produced.

The grout shall be pumped from the inlet vent. Pumping shall continue until good quality grout is seen oozing out of the outlet vent. The outlet vent is then sealed off.

Grout pressure during grouting in cables shall be of a minimum pressure of 25psi, and maximum of 60psi before sealing off the inlet vent.

Proposed Grout Mix

Cement one bag (50 kg)

Water 0.40 x 50kg = 20kg (5.0 gallon) (22.5 liters) maximum

Admixture – Estogrout PE (or equivalent)

– 225gm (one sachet) for 1 bag of cement (if equivalent, or manufacturer’s recommendation to follow)

Grouting pressure: The grout is pumped to a minimum pressure of 25 psi and maximum of 60 psi before sealing off.

Grout strength: The expected strength of above grout mix is: – 17N/mm² at 7 days.

Grout Cube Test

Six numbers of grout test cube of the size 100mm x 100mm x 100mm shall be made of every batch of grouting process. Grout cubes shall be cured in water after removing from the cube moulds.

Cubes shall be tested for compression test at 7 days and minimum cube strength required is 17 N/mm.

Patching of Anchor Points

Before grouting works can proceed, the anchorage points shall be patched with sand cement mortar to ensure no loses of grout from this point.

Main contractor shall proceed to patch back all stressing box-outs after the grouting operations are completed. (All roughening works pertaining to recesses surfaces shall be carried out only after stressing of tendon complete.

Contingency for Grouting Works

During grouting operation, adequate water shall be made available to facilitate complete removal of the grout in the cables in the event of the breakdown of the grouting operation, either due to mechanical failure or other unseen circumstances.

The equipment used for this operation shall be the same equipment used for the grouting operation.

Water shall be pumped through the grout inlet vents of the cable until clear water is seen flowing out of the outlet grout vents. This operation shall continue until all the remaining grout is flushed out and clear water is seen flushing out of the grout vents.

On complete removal of all grout, the remaining residual water in the cable shall be flushed out with air compressor.

In the event of blockage in the cables; it is necessary to establish the cause of the blockage and have the relevant contractor to take necessary action, to rectify the blockage, before grouting operation can resume.

General Corrective Action Plan

If a few strands in the beam tendon had snapped during stressing, stop the stressing works immediately and examine the strands. One of the remedies shall be to calculate the force lost in the effected tendon and stress the remaining good strands in the tendon to the original UTS. Then stress other tendons to higher forces to compensate for the losses.

If few wires in the tendon had snapped during stressing, recalculate the jacking force and pressure to give the same percentage of UTS base on remaining strands minus the broken strand. Stress balance tendons to higher forces to compensate loss of snapped strand.

If few strands in the tendon had snapped-off after lock-off, check whether the remaining locked off strands have their wedges evenly seated on the anchor block. Record the distance from the stressing marked to wedges at the anchor head for each strand. If strands have not slipped, result is acceptable.

If strands have snapped, detension all strands and examine each strand for any possible damage. Restress the tendon to a force to be determined by engineer. The adjacent tendons may be required to be stressed to higher forces to compensate for the losses in force in the affected tendon.

Calculation of Strands Stressing Pressure

All cables consist of 15.24mm diameter low relaxation 7- wire strands with minimum breaking strength of 260.7kN per strand.

All cables shall be stressed to an initial force 75.0% UTS unless otherwise stated.

Stressing Jack details: (refer sample of calibration certificate)

a) Serial number of stressing Jack: IHS 500 DA 12 CC 53

b) Capacity of stressing Jack: 500TONNE

The stressing pressure of each tendon is computed as below:

Equation of Linear Regression

Load , x = kN

Pressure , y = psi

Equation of Linear Regression

y =2.0564 x + 19.5614

No. of strand per tendon = 19 nos.

% of tendon stressed to = 75%

Breaking Load of strand = 260.7kN

Jacking Force per tendon, = 19 x 260.7 x 0.75

x = 3714.98 kN

Therefore, required Jacking Force,

y = 2.0564(3714.98) + 19.5614 = 76590 psi

Approx.: ≈ 77000 psi

Inspection Checklist for Pre Pour Activities