Method Statement for Post Tensioning Friction Test
INTRODUCTION General The purpose of this procedure is to determine the actual friction coefficient and wobble coefficient which shall be used in the strand elongation calculation of the post tensioning works. Scope of work To determine friction and wobble coefficient to be used in the elongation calculation, a friction test shall be performed prior to the first stressing operations on site. Definitions
SCC SUMITOMO – CIENCO 6 CONSORTIUM LG Launching Gantry PT Post Tensioning PT duct PT corrugated galvanized steel duct
SPECIFICATIONS According to the Outline Construction Specifications Package -2 : Civil (Elevated and Depot) clause 6.10.5 “Friction”, the actual friction for PT should be investigated and if significantly varies from expected friction, the PT operation should be verified for either stressing force or tendon elongation. Post-tensioning components should follow criteria below: Duct : ID/OD 80⁄87 0.35mm thick following specification EN 523 Strand : OD 15.24mm 7 wire low relaxation strand compliance with ASTM A416 Anchorage : 12T-15 PT system compliance with ETAG 013 requirements Hydraulic stressing jack or load cell should be calibrated within 2% tolerance maximum and within 6 months prior friction test. Precast concrete segment should have 45 MPa characteristic strength (cylindrical) before proceeding with the friction test. THEORY During PT stressing process, friction in the tendon will generate stress loss. This loss shall be taken in account in the elongation calculation using 2 coefficients: and k. Friction coefficient is the result of the expected friction between tendon and duct as the profile of the tendon changes. This loss is related to angular changes in the tendon profile. Below figure shows the trajectory of a tendon within a desired duct profile.
Wobble coefficient k k is used to account for additional friction between strand and duct as a result of unintended duct misalignments. The concept of duct wobble is shown in the sketch below:
Calculation of and k
Friction coefficient (µ) and wobble coefficient (k) are linked to the tendon force applied just before the passive anchorage (FB) and the tendon force applied just after the active anchorage (FA):
e^(-(μ.α+k.x))=F_B/F_A =p_B/(p_A (1-K)^2 ) (equation 1)
With:
F_B=(p_B×S)/(1-K) (daN)
FA = pA × S × (1 – K) (daN)
pA = Recorded pressure at the active side A (bar)
pB = Recorded pressure at the passive side B (bar)
= Total angular change of tendon (rad)
x = Developed length of the tendon (m)
S = Piston area of the jack used, refer to attachment 5 (cm2)
K = Friction losses coefficient inside the jack and the trumplate (%).
K = K1+K2+K3
● K1+K2 = friction loss coefficient in the trumplate and the anchorage,
K1+K2 = 2 to 3% (refer to attachment 3)
Note: The friction loss coefficient K1+K2 changes according to the state of anchorage components (trumplate, head, wedges…). If the surface of the pieces is exempt from oxidation, therefore the lower coefficient shall be used. In the case of oxidation, the highest coefficient has to be chosen.
● K3 = friction loss coefficient in the jack (value given in the calibration certificate of the jack), refer to attachment 4 for example.
To determine both coefficients & k, friction tests shall be performed.
TEST PROCEDURE
The friction test shall be performed prior first span stressing operations.
Test sequence
The PT friction test shall be done after erect segments for the first span with epoxy applied and temporary PT stressed. After 3 pair of tendons 1-1’, 5-5’ & 8-8’ are stressed and transferred 60% of all segments weight to temporary bearings, according to calculation note load transfer UMRTL1-CP2-CSYD-CWS-CAL-00028 and method statement of typical erection UMRTL1-CP2-VDGN-CVS-MST-00041. 2 tests shall be performed. Test 1 (performed on tendon 2-2’) and test 2 (performed on tendon 9-9’) are chosen for the friction test based on following reasons:
Tendon 2-2’ is curve tendon,
Tendon 9-9’ is straight tendon,
Both tendon 2-2’ and 9-9’ shall be stressed at the first stage of the post-tensioning process.
Location of tendon 2-2’ and 9-9’ The sequence of the PT friction test is as below: Tendon 2: Push strands into PT ducts and install anchor heads at the stressing end (active side A) and anchor end (passive side B) without installing wedges Install calibrated jacks at the stressing end and anchor end with stroke about 0mm
Extend the ram of the jack at the passive side B to remove the slack in the strand. Restroke the jack.
Stress tendon at the passive side B to 10% of the design jacking force F0 then turn off pump. The design jacking force F0 is given by the Final Technical Drawing (for 35m straight span – code 1A refer to drawing CP2-CVS-FT-VDGN-22234, F0 = 234400 daN)
Stress tendon at the active side A to 20% of the design jacking force F0.
Record pressure on jacks at both side (active and passive) and strand elongation on the Record sheet (refer to attachment 2).
Stress tendon at the active side A to 40% of the design jacking force F0.
Record pressure on jacks at both side (active and passive) and strand elongation on the Record sheet (refer to attachment 2). Stress tendon at the active side A to 60% of the design jacking force F0. Record pressure on jacks at both side (active and passive) and strand elongation on the Record sheet (refer to attachment 2). Stress tendon at the active side A to 80% of the design jacking force F0. Record pressure on jacks at both side (active and passive) and strand elongation on the Record sheet (refer to attachment 2). Release tendon force by release stressing jack.
Tendon 2’, 9 and 9’: Repeat step 1 to 9 as per above. Note: If additional two hydraulic jacks are available, tendon 2 and tendon 2’ or tendon 9 and tendon 9’ can be tested at the same time. The strands are subjected to 80% design jacking force (F0), which is equivalent to 60% of Fu (Fu = ultimate tensile force). This stressing value remains within the elastic range of the strand and therefore, tendons can still be stressed for permanent work after having being used for PT friction test. Determination of & k & k shall be determined based on (equation 1) in section 3.1 above and recorded pressure from the tests, details as below: Step 1: Average the recorded pressure at both side (passive and active) for all stressing steps (20%F0, 40%F0, 60%F0, 80%F0) for test 1 (tendon 2-2’) and test 2 (tendon 9-9’). Step 2: Graph showing the average pressure at the active side A (pA) against the average pressure at the passive side B (pB) for all stressing steps (20%F0, 40%F0, 60%F0, 80%F0) for test 1 and test 2:
Sample of pA against pB graph ● Case 1: If pA against pB graph is closely liner, pA & pB values at final pressure (80%F0) shall be taken to calculate & k. ● Case 2: if pA against pB graph is not closely liner, find the liner equation that best represents the relationship between pA & pB. Using this equation, calculate pB value at final pressure of pA (80%F0). These values of pA & pB shall be considered to calculate & k. Step 3: Determine the value of & x for tendon 2-2’ (1 & x1) and for tendon 9-9’ (2 & x2) based on the tendon profile, refer to Working Drawing CP2-CVS-WD-VDGN-50031.
Determine (rad) & x (m) based on the tendon profile
Step 4: Calculate below equations to determine & k.
{■(e^(-(μ.α_1+k.x_1))=(p_B/(p_A (1-K)^2 ))_1 〖_(from test 1)〗@e^(-(μ.α_2+k.x_2))=(p_B/(p_A (1-K)^2 ))_2 〖_(from test 2)〗) ┤ □(⇒┴calculated ) {■(μ@k) ┤
Safety measures
During stressing process, no one is allowed to stand/ pass behind the tendon.
General:
Communication has to be in good condition between each operator. A bad communication could be the cause of accidents.
Safety measures have to be well known by all personnel working for threading and stressing.
Threading Stage
At the coil dispenser:
“Knots” or “tangles” sometimes occur inside the dispenser. Never try to untangle these “knots” directly by hand; use a sufficiently long lever (tube, bar, etc.).
Ensure to install strand coil into dispenser in correct direction to avoid tendency to form “knots”.
No one must stand close to the strand dispenser during threading.
At the pushing machine:
The operator has to check the uncoiling of the strand, the speed of the threading, the inlet of the duct, and also guide strand into duct.
At the duct outlet:
The strand exit area at the end of a duct must be marked off in a way that prohibits access. The operator’s assistant must stand back from the line of the duct during threading.
The required strand length outside trumplate is 1115 mm at both anchor end and stressing end.
Stressing Stage
All the users of stressing equipment should have been trained for PT operations, in order to know how to use them, and the relative risks.
The high pressure of equipment required a permanent attention of all staff working close to them. The connectors have to be well cleaned, securely screwed, and hoses have to be inspected (no damages, no visible cracks nor defaults).
At the pump:
The operator has to be able to see jack stroke and reading on pressure gauge.
At the jack:
The operator has to stand on one side of the jack, and never pass behind jack.
The back of the jack has to be protected to prevent accident if tendon breaks, or any tensioning problem.
Note: Both ends of tendon (stressing end & anchor end) are restriction areas.
No one is allowed pass behind anchor head during stressing operation.
TEST REPORT
After testing, the report of PT friction test shall be submitted to the Employer’s Representative for approval. The report shall contain the following results:
Prestressing tendon reference numbers.
Recorded results from the friction test as per attachment 2.
Graph showing the average pressure at the active side A (pA) against the average pressure at the passive side B (pB) for all stressing steps (20%F0, 40%F0, 60%F0, 80%F0) for test 1 and test 2.
Actual friction coefficient and wobble coefficient k calculated as per section 4.2 above.
Comparison between the expected elongation and the recorded elongation for each tendon. The recorded elongation has to be in the range ±5% of the expected elongation as per AASHTO LRFD Bridge Construction Specifications, section 10.5.3.
Case 1: the actual coefficients and k are within ±5% of the value used in the “Calculation Note of PT Elongation, UMRTL1-CP2-VDGN-CWS-CAL-00044”: Stressing work shall be proceeded according to the “Method Statement of Permanent PT, UMRTL1-CP2-VDGN-CWS-MST-00097” using the elongation values calculated in the Calculation Note of PT Elongation.
Case 2: the actual coefficients and k are not within the range ±5% of the value used in the Calculation Note of PT Elongation: Elongation shall be updated with new coefficients by SCC Technical Department. On site, stressing work will be proceeded using revised elongation values.
RESOURCES TO BE USED
Equipment to be used
Equipment to be deployed for the strand threading
No.
Description Specification Quantity Unit Remarks
1 Strand pusher 50m capacity 1 nos
2 Guide pipe 1 nos
3 H-speed abrasive cutter or Hydraulic cutter 1 nos
4 Crane 4 ton lifting capacity 1 no For coil lifting
5 Strand coil dispenser 1 nos
6 Strand Bullets Plastic type
7 Walkie-Talkie 4 nos
Note: Above stated type & quantity of equipment could be changed if necessary.
Equipment to be deployed for the PT friction test
No.
Description Specification Quantity Remarks
1 Strand Jack CC350 2 nos Freyssinet
2 Hydraulic pump P6M 2 nos Freyssinet
3 Hydraulic hoses & connections 700 MPa capacity 2 nos
4 Pressure gauge 2 nos
5 Measuring tape accuracy in mm 2 nos
6 Spray color bomb
Note: Above stated type & quantity of equipment could be changed if necessary.
Manpowers to be deployed
Manpower to be deployed for the PT friction test
No. Description Detail Quantity
1 General superintendent of the 3 LG of project FVR 1
2 PT supervisor FVR 1
3 PT engineer FVR 1
4 Skilled workers FVR 9
Note: Number of manpower could be changed according to site condition.
Materials to be used
Materials to be used for the PT friction test:
No. Description Sources Remarks
1 Strand Provided by Main Contractor Ø15.2mm, 7 wires, class 2 relaxation strand complies with ASTM A416
2 Anchorage FVR Freyssinet AnC15
Note: The material shall get approval from Employer’s Representative before commencement of PT friction test.
OCCUPATIONAL HEALTH AND SAFETY CONTROL
The Hazard and Risk Assessment (HRA) has been carried out in the form of the Job Safety Analysis (JSA) to include those activities which are specially associated with the PT friction test.
These are reviewed by the HSE Officer to ensure that all the hazards have been identified and adequate control measures are in place.
All personnel shall wear the appropriate PPE at all times.
Job Safety Analysis (JSA) / Hazard analysis & Risk Assessment (HRA) / Safe Work Method Statement (SWMS)
The JSA has been completed by the SCC Technical Department and has been reviewed by the HSE Department. It should also be reviewed by the work crew prior to activities being performed.
The document has identified the hazards associated with the activities involved in the PT friction test and details the measures to be adopted to minimize or eliminate those risks identified.
Refer to attachment 1
Tools and equipment
All tools and equipment for the task shall be in safe condition and fit for its purpose. Where applicable, registration papers, certifications shall be produced and tests certificates will be available for inspection.
At the start of each shift, equipment shall be visually checked for mechanical and structural soundness. All equipment shall be carefully examined and all hoses and couplings checked for the leaks and general condition.
ATTACHMENTS
ATTACHMENT 1: HAZARD AND RISK ASSESSMENT (HRA)
ATTACHMENT 2: RECORD SHEET
ATTACHMENT 3: FRICTION LOSS COEFFICIENT
ATTACHMENT 4: EXAMPLE OF JACK CALIBRATION CHART ATTACHMENT 5: STRESSING JACKS Freyssinet C Range The jack used for the Freyssinet PT system is the CC350. Important: This jack has be used only with Freyssinet C Range and Freyssinet P6M pump. Every 6 months the jack has to be calibrated by external laboratory.
