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- Storing Area: Area reserved on site for coils storage.
- Fabrication bay: set up tendon fabrication bay in clean area of site.
Use polythene sheeting if necessary the bay should be long enough to accommodate the longest tendon and the strand dispenser.
- Strands Pulling from dispenser:
From dispenser: pull individual strands from the dispenser to required length.
- This is to include 400mm for stressing length and 1200mm over length for basket dead end label strands in tendon groups for ease of installation.
- Dead end fabrication: Place end of strand in basket rig locating recess and place wedges in bunghole.
Extend ram of basket rig to produce the basket – release ram.
Remove wedges and remove strand from rig.
Check the basket diameter.
- Chairs fabrication: The chairs may be fabricated from mild steel 8mm. The lower part should be painted with epoxy for corrosion protection.
- Slap side anchorage positions on the slab side shutters from the tendons layout drawing.
- Tendons marking: Mark tendon line, chair positions and sizes on slab Shutters from the tendons profile drawing.
- Anchorages fixing: Drill and cut side shutter to accept anchorage fixing bolts and strands protrusions.
- Fix anchorages and pocket formers to side shutter using threaded rods and nuts
- Tendons installation: Lay ducting to line and couple individual lengths. Push strands into ducts.
- Profiling: chair up and tie ducts.
Seal duct to anchorage using dens or duct tape. Seal ducting joints using duct tape. Tie dead end to reinforcement.
- Grout vents: Spacing of grout vents should not exceed 15m.
Install grout vents at the live end and at the dead end ensure that the grout vent height around 300mm above the concrete surface and place a rigid tube or reinforcing bar in the vent to prevent twist during casting
In order to avoid crushing of duct due to designed low space between high points & top reinforcement, additional support to top reinforcement will be provided to avoid sagging of rebar during steel fixing.
- Site Engineer will check cover to all high points of tendon and duct by measurement from the deck. When acceptable sign off on pre-pour check sheet as normal
- Tendons must be laid straight if shown straight though some variation (typically
up to 150mm) is usually allowed to pass obstructions / openings in a slab. ALL
variations in tendon alignment lead to increased friction and LOSS of LOAD
during STRESSING OPERATIONS
- Industry practice is that a tendon height should be +/- 5mm. Therefore, chairs
are typically supplied in graduations of 10mm to allow conformance to
specifications / codes. Accurate placement of chairs will help safeguard
performance of slab during stressing operations.
- Slab Checking
- N (br) of anchorages and tendons.
- Basket dead end: Ensure that at least 1.2m length of strand including the basket dead end measured from the end of the duct.
- Profile and chairs spacing: check all ducts for alignment and level.
- Grout vents spacing should not exceed 15m.
- Bursting reinforcement: check bursting reinforcement behind each live end and dead end.
- Additional rebar: Check all the additional rebar’s required by the designer as shown on the reinforcement drawing.
- Slab opening: Check that no tendon is clashing with the openings and a minimum of 100mm cover is achieved.
- Deck and formwork: check sufficiency and strength and formwork and deck to carry slab weight and other site loads during execution.
- Snagging work: Carry out any snagging works before casting.
- Pouring approval: Get the pouring approval form the designer and submit to the main contractor.
- Suitable vibration around anchorages: Ensure that the concrete around both ends of the tendons is vibrated correctly and compacted, to avoid any problem during stressing.
- Concrete to be cast with care and without damaging the tendons.
- Suitable concrete pouring.
- Side shutters removal: After casting the side shutters must be removed in time to allow installation of bunghole and wedges or bearing block dependent on the system in use and prior to concrete achieving the strength required for stressing.
- Bearing blocks installation: Install wedges to each live end ensure all anchorage blocks/bunghole are tight up against the face of the anchorage.
- Concrete strength: To proceed with the stressing a minimum concrete strength of 25MPa is required unless noted otherwise on the design drawings.
- Stressing: fix the jack to each strand in turn and extend to stressing load in one smooth operation.
Work from central strand and outward symmetrically.
Lock off each strand after stressing.
- Extension: The extension achieved is recorded as the difference between the initial and the final length.
Submit the extension record for approval to consulting engineer prior cutting strands for grouting.
Post-Tensioned Slab Installation For Stressing
-Final Stressing Loads are specified as 75% of the Ultimate Tensile Strength of the 12.7mm Strand.
-This equates to 146kN for the supplied strand
-Only experienced Supervisors will carry out stressing works. A list of which will be issued to the Structural
-Engineer prior to stressing operations commencing
Procedure – Preparation of Live Ends
- Cast in live ends should be prepared and cleaned as shown above
- Take extreme care not to damage tendons when striking, cleaning or removing formers
- With strands clean the barrels and wedges can be made ready and prepared for stressing
- Barrels should be thoroughly cleaned and greased to ensure sound contact and proper housing of the wedges
- With the assemblies checked and prepared place over the exposed tendon ends
- Again, check tendon for damage and note any obstructions that occur when block is being installed.
- Complete loose fixing of assembly
- Prior to marking of strands for extension measurement (or taking base measurement readings) the wedges and assembly MUST be taped / push securely Main Contractor position. Failure to properly engage the barrels will result in elongations with additional extension as the slack is taken up in the stressing assembly
- With the block pushed back it can be sprayed for reference when measuring elongations using coloured spray.
- Spraying should properly cover each tendon
- Spray both ends for double end stressing
- Any water should be removed prior to spraying and cables dried, especially in live ends
- Failure to remove water will result in elongations which cannot be properly measured
- If water cannot be removed or spraying method fails then the lengths of tendons can be measured and extensions calculated from changes in length
- Measuring from a sprayed datum is preferred.
Procedure- Strand stressing
Strand stressing sequence as specified below should be followed
- The jack calibration should be checked and the machine, gauge and pump checked by the supervisor and
confirmed as in good order
- The marking of tendons should be checked and confirmed as adequate
- Permit to Stress should be completed
- Again the exclusion zone should be confirmed
- With the above checks and balances in place the jack should be engaged at the correct strand and the stressing
- Push the jack home firmly and ensure the ram is in proper contact with the wedges
- Load is applied gradually and evenly to load limit specified
- STRESSING IS CARRIED OUT TO THE SPECIFIED LOAD LIMIT. ELONGATIONS ARE USED ONLY TO CONFIRM LOADS
- With the appropriate load reached the jack is disengaged and the strand held on the barrels and wedges
- Once a tendon set is complete move to next tendon and record elongations
- Where double live ends are recorded stress each end evenly, the first to 50% of the design load. The second to the 100% load
- . Calculate elongations by total changes over the length of the tendon
Confirmation of Elongations and Permits to Cut and Grout Strand
- Once elongations have been measured they will be assessed against the design expected elongation to verify that the correct amount of pre-compression is present in the concrete slab
- Only when all tendons across a slab have been verified will the Permit to Cut and Grout be raised
- Design elongations will not be issued directly to site but will be held by authorised engineers
- Elongation results will be input Main Contractor elongation sheets and variations recorded.
- Strands cutting: Cut strands 20mm clear of wedges using an abrasive wheel cutter.
- Pocket filling: Fill anchorages pockets with non-shrink mortar and allow for curing.
- Ducts testing: Blow air in ducts to ensure clear passage.
Carry out remedial works on vents as necessary.
- Preparation: Set up grout mixer and pump on top of slab and calibrate water-measuring tank.
Stock fresh bagged cement close to mixer Arrange 8Nbr 225 litre drums of suitably chilled potable water close to mixer. ( 5-10C )
- Mixing : Add calibrated volume of water to the mixer , connect compressed air supply and start mixing ( water cement ratio 0.45) Carefully add cement, ensuring that no lumps from in the mixture Mix batch for a minimum of two minutes prior to pumping .
- Pumping: Start pump and discharge grout through injection hose until a good consistency of grout is apparent. Connect grout injection hose to grout inlet and commence pumping whilst keeping the mixer topped up.
- Locking grout vents: seal off grout vents in direction of flow after air has exited and after consistent grout appears.
- Stop pumping: When the mix reaches the opposite end the tendon and good consistent grout is flowing with no trapped air; stop the pump together while locking off the grout exit pipe.
Lock off grout inlet pipe and continue with next tendon repeating the same operation.
- Cement temperature: Check cements temperature (20-30C) and examine for lumps. Discard lumpy bags.
- Water temperature : Check water temperature at regular intervals ( 5-10C)
- Grout temperature: Check grout temperature regularly. Should be below 35C and above 5C.
- Flow ability: Check flow ability using standard flow cone, 3 times per day start of shift mid shift and end of shift.
Also check if grout mix appears visually thick.
- Cubes: Manufacture 3 number cubes during operation at start of shift, mid shift and end shift for strength checking.
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