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1、Reference is made to your letter D110479/4/L/620/3935 dated 20 August 2011, by which you asked a response to the COMMENTS ON THE CONTRACTORS METHODS STATEMENT ON PRESTRESSING SYSTEM FOR BLUE NILE RIVER BRIDGE. We have the following response.1、Calculation for Stressing/Extension(friction factor betwe
2、en the tendon and duct)=0.17.(influence factor on frication caused by local deviation of duck)=0.0016.The angle of curvature of the tendons and the distance of every section is shown in the drawing from G1000（07/73）（1/4）to G1000（07/73）（4/4）.2、Concrete casting2.1The concrete used for the superstructu
3、re box girder is C50 and the CONCH（Chinese cement）OPC52.5 cement will be used. The slump of the concrete is 145mm205mm, the cement water ratio is 0.46, the quantity of the cement used in per cubic concrete is 460KG and the quantity of the SIKAment V100 additive is 1.2L per 100KG cement. The test res
4、ults of the compressive strength of the concrete is shown in following：3 days：35.11Mpa5 days：43.35Mpa7 days：52.6Mpa28 days：66.4MpaThe slump of the test concrete is the following：0min. 210mm30min. 160mm1h. 110mm1.5h. 95mm2h. 72mm According to the test result, the 85% of the designed strength of the c
5、oncrete will be achieved in 5days. Therefore, the planed stretching of the tendons will be started when the concrete has 5 days age and the 85% of the designed strength of the concrete is achieved. 2.2The sequence of concrete casting:The concrete pump is used to delivering the concrete of the box gi
6、rder and according to the cooperation of the workers and equipment, the concrete casting quantity will be 15cubic meters per hour.2.2.1Every segment of the box girder will be casted in sequence according to the drawing from G1001(71/73)(1/3) to G1001(71/73)(3/3).a. Segment 0 and 1 will be casted on
7、the supporting truss in two phases. The overall height of the box girder is 7 meters and the first phase will cast 4 meters and the second phase will cast the remaining 3 meters. The segment 2 also will be casted on the supporting truss and the whole segment will be casted at one time. b. The suspen
8、ded crane will be used to casting segment 3 to segment 16 and every segment will be casted at one time. There will be 2 sets of suspended cranes for every pier and 4 sets for the whole bridge. c. The scaffolding will be used to casting segment 17 and 18 and every segment will be casted at one time.
9、2.2.2 The concrete casting procedure is shown in the following: aThe concrete will be casted from the lower side to the higher side,bThe bottom slab will be casted at first then the web slab and top slab, cThe concrete cannot be casted at the same time at the both ends of the two segments and the up
10、stream and downstream of the web slab of the same segment during casting. The concrete will be casted at every section during casting. Therefore, the differentials of the concrete casted in the both ends of the two segments is not more than 13.5 cubic meters and the elevation of the top surface of t
11、he concrete of the upstream and downstream of the web slab is not more than 1meter. d. The 1.1KW spud vibrator will be used to jolt ramming the horizontal level layers of concrete and the thickness of every layer is not more than 50cm and the horizontal distance of every insertion point of the vibra
12、tor is not more than 30cm and the vibrating spear will insert into the under layer for 5-10cm when jolt ramming the upper layer. Every suspended crane will be equipped with 3 sets of vibrators during concrete casting. The diameter of the vibrating spear for 2 sets is 5cm and for 1 set is 3cm which i
13、s used for jolt ramming the chamfer of the body of the box girder. e. Removal of formwork: considering that the end of every segment will be dealt with daubing so that it is good for the connection of the two segments. The end formwork will be removed and the daubing will be started 24 hours after t
14、he completing of concrete casting. The internal formwork will be removed 48hours after the completing of concrete casting and the external formwork will be removed after the completing of the stretching of the prestressed tendons of the segment. 2.2.3There will be 6 technicians and 6 ordinary worker
15、s at every segment and the construction schedule is as the following:a. The suspended crane pushed on station 2days.b. The bottom slab and web slab reinforcement and tendons installation and internal formwork erection 5days.c. The top slab reinforcement and prestressing duct installation and end mou
16、ld erection 3days.d. Concrete casting 1day.e. Tendon and anchorage installation and the time for the concrete strength meets the requirement of stretching 5days.f. Stretching and grouting 1day.3、The protection of the concrete and the prestressing duct3.1 Concrete protection The top slab will be cove
17、red with sackcloth and the whole segment will be maintained with water for 7days after the completing of concrete casting.3.2 Prestressing ducts protection The prestressing ducts will be stored at warehouse or with cover to prevent rain water or sunlight. The prestressing ducts will be erected accor
18、ding to the coordinate in the drawing and12 steel bar will be used to form shape along the duct to prevent the displacement of the duct during concrete casting. The shape steel bar will be fixed on the steel of the box girder at an interval of 50cm. The ducts in the box girder section will be sealed
19、 with plastic tape to prevent miscellaneous articles to falling into the ducts prior to bundling the tendons. 4、Stretching of the prestressed tendons The individual strand will be tensioned for the vertical prestressed tendons and 2 sets of 25T jacks will be used. The longitudinal prestressed tendon
20、s will be tensioned from both ends and 4 sets of 250T jack and 4 sets of 500T jacks will be used. The demarcation of jacks will be completed and the demarcation document will be submitted 15 days prior to use. The alternate tensioning from the root of the box girder to the end direction upstream and
21、 downstream will be used for vertical prestresed tendons. According to the required tendons length of tensioning of every segment, the short longitudinal prestressed tendons will be tensioned at first, then the long tendons, and alternate tensioning from outer side of the box girder to the internal
22、side for the same length of tendons will be used. The stretching will be operated by experienced workers and a safety area for tensioning will be marked and the person who does not operate the stretching cannot enter the safety area. A steel meter will be used to measure the extension of tendons. A
23、marker pen will be used to mark a sign at the end of the tendons prior to tensioning. The measurement of the extension of tendons is the distance from the sign to the anchor plate. The accuracy of the extension is 2% or 2mm and the most accurate one shall prevail. The accuracy of the retraction is 2
24、mm.4.1 Procedure for tensioning:Initial tensioning（tensioning force P0 is 0.2 times of designed tensioning force P）extension measurement0tensioning to the designed tonnage P sustained load 2 minutes extension measurement1oil scavenge extension measurement2The error of the measured and the theoretica
25、l extensions of single tendons is not more than 6% through the comparison of the measured and theoretical extensions. The average variation between the measured and theoretical extensions of all the tendons in a structural member is less than 3%.Checking and deciding whether 2-1 is more than 7mm. St
26、rand slip will occur if 2-1 is more than 7mm. The reason should be checked and methods should be taken to solve the problem then tensioning can be continued. Then check the end sign of the tendons whether it is a plane. The signal tendon slip occurs if the plane has changed and measures must be take
27、n to deal with it. 4.2Tensioning problems and solutionThe wire breaks and strand slip of every tendons is not more than a thread and the sum of wire breaks of every cross section is not more than 1% of the total steel wire of the section. In no circumstances the maximum tensioning force of the tendo
28、ns is more than 80% of the standard strength and the repeated tensioning is more than 3 times. 4.2.1 The oil gauge of the jack cannot return to zero: replace it with a backup oil gauge or re-demarcate the jack.4.2.2 Strand slip: Using YDC240Q 25T jack and unloading bearer. The jack will be used for
29、tensioning the slipped tendons and the wedge will be taken out and replaced with a new one and tensioning the tendons to the controlled stressing force. The tendons will be taken out if the slipped strand has been damaged and tensioning it again after replacing the damaged tendons. 4.2.3 Wire breaks
30、a. Increasing the controlled tensioning force of other tendons as compensation, but the maximum tensioning force is not more than 80% of the standard strength. b. Replacementc. The backup tendons will be used in special case.4.2.4 The measured extension is more than the allowable error of the theore
31、tical extension4.2.4.1Too small elongation at jacking end under full loadToo small elongation may occur due to a kink close to the stressing anchor; the jack may reach full load, but the elongation will be very small. When this happens, the required elongation may possibly be achieved by stressing t
32、he tendon from the other end. However, this will not be feasible if low elongation is due to duct misalignment over the whole length of the tendon.4.2.4.2 Low elongation for whole tendonWhen low elongation is due to duct misalignments occur over the whole length of the tendon, stressing from the oth
33、er end may not be enough to attain elongation. Consideration may be given to lubricating the tendon with water soluble oil or with graphite powder. This can reduce friction and result in better elongations. After a tendon has been successfully stressed, water soluble oil should be thoroughly removed
34、 by flushing. Flushing water should be thoroughly drained and blown from the ducts. Graphite powder may remain in the duct and is, therefore, generally preferred by many Contractors.4.2.4.3 Elongation greater than toleranceAn elongation can be more than expected. This may be because of less friction
35、 than anticipated or because of slip of strands and wedges that went unnoticed. The wedges should be examined at both ends. It is for this reason that marks should always be made on strand tails at both ends the tendon. If there is no wedge slip and tendons persistently give an elongation greater th
36、an expected, the stressing calculations should be examined and appropriate adjustments made.4.2.3.4 Low stressing forceIt would be very unusual to not to be able to stress a tendon to a required jacking force; more often a problem is revealed by lack of elongation, not force. If force cannot be atta
37、ined, the system should be checked. The possibility of increasing the jacking force may be considered.However, it should be checked by calculations using a higher wobble and friction coefficient to make sure that the stress in the tendon after anchor set does not exceed allowable stresses.4.2.4.5 Ov
38、erall Tolerance on a Group of Tendons If none of the above lead to a satisfactory solution, it is possible to consider a problematic tendon as part of a whole tendon group for example, one tendon out of perhaps sixteen to twenty in a cantilever, or similar. A tolerance for the whole group should be
39、given in specifications or project special provisions. If all other tendons have a good stressing record, one poorly stressed tendon ought not to influence the group tolerance too adversely.To make up for a loss of force in one tendon a compensating increase in force in other tendons may be consider
40、ed, if there is sufficient reserve holes in the wedge plates to accommodate4.2.5 Breaking WedgesSometimes wedges break. This causes the loss of the whole strand. It falls under the category of slipped strands and should be treated as such. When a few wedges break on the same tendon, all wedges shoul
41、d be considered potentially defective. The whole batch of wedges should be examined and, if necessary, replaced.Very often wedges show radical cracks in their visible ends after seating. Experience shows that this is usually a localized cracking of the annular lip containing the retainer ring. Provi
42、ding the strand has not slipped and providing this type of crack does not extend into the barrel of the wedge, then it is not of any major concern.Repeated slippage problems and large cracks in the gripping nose of wedges are cause for concern and should be remedied.5、Grouting5.1 Cement mortarThe ba
43、gged OPC52.5 or OPC52.5 cement, SIKAment V100 water reducing agent and Intraplast-Z Expanding grout admixture will be used for grouting and the 28days strength is not less than 40 Mpa. The cement water ratio is not more than 0.45. The bleeding is not more than 2% of the mentioned and the maximum ble
44、eding is not more than 4% 3 hours after the cement mortar has been mixed. The bleeding will be absorbed again 24 hours later. 5.2 Craftmanship for grouting5.2.1 Ordinary grouting craftmanship for vertical ducts.5.2.2 Vacuum grouting craftmanship for longitudinal ducts.5.2.3 The grout will be pressur
45、ed from the lowest position of the duct and a vent port will be arranged in the highest position.5.2.4 An inlet will be arranged at the bottom of the duct and the vent port and an outlet will be arranged at the top anchor plate. 5.2.5 The lower side of the anchor plate of the longitudinal duct of th
46、e top slab is inlet and the top side of anchor plate is vent port and outlet. 5.2.6 The lower side of the anchor plate of the longitudinal duct of the web slab is inlet and the top side of anchor plate is outlet and the vent port will be arranged at 1/2 of the duct.5.2.7 The inlet will be arranged i
47、n the lowest position of the longitudinal duct of the bottom slab and the vent port and outlet will be arranged at the anchor plate at the both ends.5.2.8 For normal operations grout should be injected at a pressure of less than 0.52 MPa at the inlet. Pumping pressures should not exceed 1MPa.5.3 Gro
48、uting Problems and Solutions5.3.1 Interruption of Grout FlowIf there is a breakdown, then use the available standby equipment. Standby equipment should be periodically checked to make sure it is in working order. Standby equipment may be a second set of production grouting equipment in operation nea
49、rby. In any event, standby equipment should be mobilized as soon as possible.Standby equipment should be brought into operation within 15 to 30 minutes or else grout may begin to solidify and it will be too difficult to mobilize the grout, especially on long tendons.If standby equipment cannot be brought into operation, then the g