METHOD STATEMENT FOR PRESTRESSING WORK FOR BRIDGES

 

              Prestressing work for Bridges

1)       Reinforcement work

n  Before placing concrete, reinforcement is fabricated sufficiently and correctly as required

n  Reinforcement shall be tied with accurate type, diameter, length, distance and placed position. The ties and spacer shall be firm as required.

2)       Spiral duct(sheathe) installation

n  The spiral duct is connected to reinforcement of slab.

n  The spiral duct should be installed at correct position horizontally and vertically as specified.

n  The spiral ducts are joined to each other by connection pipes of 200mm long.

n  The connecting points are surrounded 3 times by PVC cohesive tape to prevent cement and water from coming into ducts.

3)       Installation of anchor base

n  Anchor base is joined by bolt M12

n  The anchor shall be installed at the center of the spiral duct

n  The connecting points between anchor head and spiral duct are surrounded 3 times by PVC large-sized cohesive tape.

4)       Form installation

n  The formwork of slab shall be done correctly as designed

n  The slits on the form shall be filled by rubber gasket or soft pieces as waterproof element.

n  Forms shall be located and connected carefully by bolts as designed.

5)       Concrete mixing

n  Concrete materials such as cement, sand, aggregate etc. shall be the same quality as approved by the Engineer.

n  The slab shall be mixed with the approved admixture

n  The materials measurement is taken by automatic electronic scale system.

6)       Placing concrete

n  The concrete will be placed by concrete pump and crane with bucket capacity of 0.5~1.0 m3.

n  Concrete will be placed layer by layer with the depth of 15-30 cm each, without interruption.

n  The concrete is compacted by the vibrators.

n  The curing of slab concrete is done during 7 days (3 shifts a day) after 10 hours from finishing. It is watered 5 times in the daytime and 4 times in the nighttime.

7)       Form demounting

n  Side form of slab will be removed after 3 days not less than 85% of the specified 28 day strength from concreting

n  It is not necessary to remove bottom form. However, it shall be necessary to clear and coat with an anti-cohesive substance after each time of casting and removing slab out of casting base.

8)       Prestressing work

(1)    Equipment for construction

n  Hydraulic oil pump: 4 nos.

n  Hydraulic stressing jack (2100 KN): 4 Nos.

n  Grout mixer: 2 Nos.

n  Grout pump: 4 Nos.

n  Cable cutter: 6 Nos.

n  Cable inserting machine: 2 Nos.

n  Air compressor (9 m3/min): 2 Nos.

n  High pressure water pump: 2 Nos.

(2)    Inserting of cable

n  When the strength of girder concrete reaches 85% of the specified 28 day strength, the cable is inserted into duct to carry out prestressing.

n  The cable is inserted by electric inserting machine with capacity of 7.5 KN, inserting speed of 24 m/min,

n  Each strand is sealed at ends to make it easy for inserting. The cable shall be slipping sufficiently.

n  When projecting cable is cut, 1m of cable shall be left at both side of cable to prestress later.

(3)    Prestressing

n  The tensile force of stressing jack shall be at least 1.1 - .1.2 times stronger tan the tensile force of cable.

n  The stressing jack and oil pump shall be tested before stressing.

n  Each cable is stressed by 2 jacks at both ends of cable.

n  Strestressing jack and hydraulic oil pump shall be completely installed before stressing. Good care of anchorage and cable position shall be taken to avoid twisting.

n  Stress force of cable shall be taken into consideration with inner lost of jack and anchor.

n  The straining is divided under loading steps in correspondence with value indicated on pressure gauge of hydraulic oil pump. There are at least 5 steps.

n  The first loading is about 10%. Design force is normally about 50 bar.

n  The elongation of cable in each loading step shall be recorded.

n  There shall be an interval of 4 – 5 minutes between loading steps to led cable relax.

n  The stroke of jack shall be reset under the principle of stress recover if it is shorter than the elongation at one cable end.

n  When pressure of jack reaches its calculated design, put pump oil into jack. Automatic device of jack is used to close the wedge with its pressure equal or less than 200 bar.

n  After wedging, zero the pressure of hydraulic jack and measure the cable retraction at 2 ends to get the actual elongation.

n  The elongation is the sum of measure elongation at 2 ends excluding the cable reaction in the jack. The elongation obtained shall meet the requirements of design. (That is calculated under the actual elastic modulus of each cable.)

(4)    Check and cut the cable projection length.

n  After getting inspection and approval from the Engineer on prestressing the projecting cable length is cut.

n  Electric plate sawing machine with its function blade is used to cut the projecting cable.

n  After cutting, the rest length is about 25-30 mm far from the wedge.

(5)    Grout pumping sequence

n  The approval of grout shall be given from the Engineer before pumping.

n  The mix design of grout shall be approved by the Engineer

n  The strength of pumped grout shall not be less than 20 Mpa.

n  Grout is pumped into duct by electric grout pump. The guiding pipe system is installed from the pump to the duct of anchor head.

n  The grouting is done uninterruptedly. In case of stopping of pumping in 2 hours or more, grouting shall be re-started after removing grout out of cable duct by water and air compressor. The pumping pressure shall be maintained at 12-15 bar. When the grout comes out at the other end of cable duct, adjust the pumping pressure from 7 – 1.5 bar within 5 minutes. After that, lock valves at 2 ends and stop pumping.

n  All the pumping ducts shall be sealed tightly on concrete surface and such sealing shall be removed after 12 hours from finishing pumping.

 

CHECK LIST FOR PRESTRESSING WORKS

S.No.	Description	OK	NOT OK	NA
1	Tendon profile (Vertical)
2	Tendon profile (Horizontal)
3	Fixing of Anchorages as per drawing
4	Fixing of bursting reinforcement
5	Cut length of strands
6	Gripping length
7	No of strands per tendon
8	Grout vent