Construction of Post Tension Slab System.
Post tension slab system is one of important concrete slab
systems that reduce the percentage of steel reinforcement.
Post-tension is a method of reinforcing
concrete with a high strength steel strands, typically referred as Tendons.MATERIALS.
Strands:
We have several types of strand, but a common type is Strand 12.7 mm/15.2 mm-Grade 270,1860 Mpa
lower relaxation.
G.I Flat Duct:
Strands will insert inside these ducts. The duct shall
sufficiently robust and tough to resist the rigors of the placing and
compaction of the concrete without suffering damage, nor deformation. The duct
are to remain mortar-tight along their entire length and at splice points.
Ducts have several
types and dimension but the most duct used in construction dimension is:
20 mm x 90 mm -Smooth.
20 mm x 70 mm -Smooth.
20 mm x 43 mm -Smooth.
G.I Round Duct.
Live Anchorage:
Live anchorages suitable for traditional internal
post-tension using tendons made up of high-strength steel strands and should
be as per NFA-35-045 or EN- 10138 standard.
Each anchorage device shall be capable of safely
transmitting a force not less that the ultimate strength of the tendon without
over stressing the concrete.
Wedges 13 mm / 15 mm.
Pocket Former.
The figure below show all material will use in post-tension
slab system.
Post-tension slab system Plants & Special Tools:
Stressing Jacks:
Mono-Jack:
Mono-strand tendons:
Are often used in buildings, where each tendon is a single strand, usually
comprising 2 to 5 strands, In deck slabs or similar applications.
Multi-Jack:
Multi-strand post- tension tendons are usually stressed
as an entire group, using very large custom-made jacks. This ensures that all
strands are tensioned together and avoids the risk of trapping an individual strand.
Onion Jack:
Used in tendons less than (30) m to create an onion shape at the end of these strand by spreading and separating the wires.
Grout set (Mixer & Pump):
Used to mix cement, water, and admixtures for grouting to
assure the mixture is not containing lumps or any air bubbles during injection
into the ducts.
INSTALLATION.
Cutting & Preparation of the Strand:
- Cutting list to be prepared from the shop drawing for each tendon considering the stressing lengths are 300 mm for single live end stressing and 600 mm for double live ends.
- The area of cutting should away free pedestrians and vehicles.
- Display the Safety Sign Board that states Work in Progress.
- Arrange the Dispenser, coil, electric connection, grinder, and strip cutter.
- Loading the strand to the strand dispenser.
- Mark the lengths of the strands as per the cutting list, pull and cut the strands to the correct marked length.
- Make the dead end by using the onion jack for the tendons that have single live end.
- The size of the dead end to be 80 mm so that the concrete can bond with the dead end.
- Bundle the strands and tag it with the length, number of strands and tendon number.
- Every Coil is having a tag which states the coil No. the below to be fill form which indicates which coil was installed in which Tendon.
Slab installation:
- Once the slab form-work is ready and has a safe access and safe edges to work, the bottom mesh has been fixed; the material should be lifted in the correct racks or boxes. All item should be shifted by a crane which must be supervised by the Rigger.
- Mark the side shutter with the location of anchors and dead ends.
- Drill and cut the shutter at marking locations.
- Fix the polystyrene blocks and anchor to the side shutter.
- Starting in the long direction, Insert the duct into the anchor
- Keep joining the ducts together always inserting the male into the female until you reach the required length of the tendon.
- The tagged strand should be unrolled, and hand pushed throughout the Tendon, If there is one live end the strand should be pushed from the dead end side first.
- Complete the same for all tendons in one direction.
- Place the anti-bursting steel at the anchor.
- Start the marking of the chair sizes on the ducts as per the approved drawings.
- Align the tendon finally as per the marking on the shutter and start pinning
- (by staple gun).
- Make sure to spread the dead-end wires 10 cm apart and tie them with a steel
- Bar supported by the chairs. Make sure again the anti-bursting before finalizing the dead-end distribution.
- Repeat the same procedure for other tendons and continue till completion of the slab.
- Fix top steel wherever and only when duct profiling has been completed.
- Once the top steel is completed, the grout vents should be fixed in the anchor
- Holes and the dead ends.
- If tendons are longer than 30 m fix intermediate vents by puncturing the duct
- And inserting the vent.
- Make sure the Pan Boxes have been fixed in the required alignment and height.
- Make sure the additional rebar is fixed around the pan box as per the details.
- Fill up the installation check list prior to handover the slab installation undertaking.
CONCRETING:
- The concreting operation for a post tensioned floor is very similar to that for a reinforced floor, the contractor must be aware of the following:
- Ducts can be easily damaged during concreting; this would cause problems in the stressing and grouting operations.
- Anchorages and tendons must not get displaced during concreting; Vibrations should not be allowed to contact the tendons.
- The concrete behind and around the anchorages is well compacted.
STRESSING:
Stressing of the tendons is the central and most important operations
of the post tensioning process.
Prior starting the Stressing, the following needs to be checked.
- Make sure the required quantity of required sizes of Stressing blocks and wedges are available at the site.
- Make sure the required equipment is on site i.e. stressing jack, Stressing pump, Hydraulic hoses, and deviator, Gauges, Stressing safety barrier and Stressing boxes are available.
- Make sure the jack and gauge’s calibration Is valid.
- Check the slab especially the stressing end for honeycomb.
Stressing Preparation:
- Anchor bolts, side shutter, Pan boxes and polystyrene need to be removed once the concrete has normally set no longer than 24 hours.
- Start the loading of the block and wedges.
- If double end stressing is required, check both ends of the tendon has been properly loaded prior to starting the stressing of that tendon.
- Make sure the safety stressing sign is available.
Initial Stressing (Slabs):
- Check and confirm that concrete has achieved the required minimum compressive strength of 10 MPa (cubic).
- Calculate the jacking pressure equivalent to 25% of final jacking pressure as per the calibration report.
- Mark the tail lengths at any specific distance e.g. 200 mm with the help of any template (wooden/steel sheet cut in the specific shape or a plastic pipe.
- Reach to the required pressure, adjust the valves on return position.
- Use the deviator for the pan box stressing in such a way that the jack is free from all sides during the stressing.
- Repeat the same procedure and complete all the tendons.
- Nobody should be behind the stressing jack or in front of the passive end (in case of double end tendons) during stressing.
Final Stressing:
- Check and confirm the concrete has achieved the minimum required the compressive strength of 30 MPa (Cubic) for Slabs & 40 MPa for beams.
- Calculate the final jacking force according to the calibration force.
- Repeat the same stressing procedure and extend the jacking force to the final jacking force for the entire slab.
Measurement of Elongation:
- On completion of the final stressing report should be prepared and issued to the engineer for approval.
- The report should contain the theoretical elongation, provided by the designer, measured elongation as well as the percentage of variation.
- Measure and record the elongations which are the difference of the initial mark position(200 mm) and the final mark position.
- According to ACI -318 the allowed discrepancy between theoretical and measured elongation is ±7%.
- If the measured elongation are below the tolerance, another 5% of Stressing might be applied, or de-tension and stress again.
- On the engineer approval, cut back the exposed strands to cover the external concrete face.
- The recesses making up should be covered by a dry pack mixture:
- ➢3 parts sand.
- ➢1-part cement.
- ➢1-part water.
- Pan boxes should be filled with the concrete of same grade as of the slab.
GROUTING:
The durability of post-tensioned
floors depends mainly on the success of the grouting operation.
- After 24 hours of cutting and patching, prepare all necessary materials and equipment including the safety sign.
- The grout consists of water, cement, and plasticize as per the below mix
- design:
- ➢100 kgs Ordinary Portland Cement (OPC).
- ➢38 liters Chilled water.
- ➢6 kgs Additive.
- Connect grout hose to near tube and start pumping.
- Allow good quality grout to come from end tube before bending tube over and closing off using tie wire.
- STOP PUMPING, and close off near tube.
- Repeat steps for all tendons until deck/pour complete.
- Wash and clean down mixer and flush hoses until clean.