PRESTRESS CONCRETE

Post tensioning

Materials PC Strands   Strands 0.50”   Nominal Diameter 12.7mm   Nominal Area 98.71mm²   Nominal Weight 0.775kg/m   Tensile Strength 1860N/mm2   Modulus of Elasticity 195kN/mm   Min Breaking Load of Strand 183.7kN   International Standard Used ASTM A416   Sheathings The sheathings for bonded tendons shall be spirally wound galvanized steel tubing made from galvanized steel strips of 0.3 to 0.4mm thickness. The sheathings shall be mortar tight to prevent entrance of cement grout during concreting and shall be corrugated for better bonding. The sheathings shall be sufficiently strong to retain shapes and to resist damages during construction. The flat sheathings with internal width from 75mm to 90mm have been widely used in UPS Flat anchorage systems.   Design data for friction / elongation calculation   Friction coefficient, u : 0.2 or as per the consultant’s recommendation Wobble factor, k : 0.0017/m Stressing anchorage : Type FA and MA Loss in jack and anchorage : Varies from 0 ~ 2.0% General Work Procedures   · Post Tensioning tendons shall be installed in accordance with the “Approved Drawings issued for Construction”. For Post Tensioning slabs construction · Install bottom slab reinforcement when the slab formwork is ready. · Provide sufficient cover to the bottom slab reinforcement. · Install tendons in accordance with the shop drawings. · Install top slab reinforcement. For Post Tensioning beams construction · Install bottom and side reinforcements and links. No capping links shall be installed at this stage. · Install tendons in accordance with the shop drawings. · Install top reinforcements and capping links of the beam.

Guidelines for Tendon Installation · Tendon alignment shall take precedent over reinforcement. · All dimensions measured shall be based on the formwork level. Therefore, an accurate formwork is necessary. · The tolerances of formwork shall be ±5mm or in accordance with the specifications. · The Flat Anchorages are fixed to the end formwork by support bars to avoid any displacement during concreting. · The Flat Anchorages must be orientated perpendicularly to the cable axis with the grout vent at the top. · If the side formwork is to be erected earlier, the main contractor has to arrange for holes to be provided at the formwork for each anchorage to accommodate protruding strands. · Tolerances for vertical tendon profiles are ±10mm for high and low points. · Tolerances for horizontal tendon profiles are ±50mm (to avoid small M&E openings / pipes or other obstructions). Specialist designer shall be consulted for tolerances more than ±50mm. · Handle ducts with care and no stepping on ducts. · Tendon support bars and bar-chairs are placed at 1m interval and secured with binding wire to avoid movement during concreting. Duct joints are sealed with tape. · Check visually on the ducts’ straightness and do necessary adjustment before concreting. · Grout hoses must be protected from being embedded or removed during concreting. Stressing Stressing shall not commence unless the main contractor has confirmed the concrete of the slab / beam has achieved the minimum cube strength at transfer. Refer to general notes of the project. Preparation for stressing · Removal of the side formwork and anchorage recess former (polyfoam). · Placing of wedge plates and wedges. This operation must be carried out after concreting so that the anchorages are not fouled by grout. · Ensure the protruding strands length is sufficient for stressing. · The theoretical elongation calculations must be approved by the consultant before stressing.

Cutting of Strands commence unless the consultant has approved the stressing result. · Cut the protruding strands (Stressing length) to a minimum of 10mm from the wedge plate. Cutting of strand shall not Patching of Recesses · Remove any sand and debris from the anchorage recess. · Mix non shrink grout or approved cementitious material with water. · Patch the anchorage recess with non shrink grout or approved cementitious material using trowel until it is fully filled.

Grouting Grouting shall not commence unless the consultant has approved the stressing result. Preparation for grouting · Tendons shall be blown with compressed air to ensure no blockage to the tendons. Proposed grout mix · Cement in standard 50kg package (Ordinary Portland Cement) · Water = 0.44 x 50kg = 22 kg = 22 liters · Admixture = 0.4% weight of cement (50 kg) = 200g StoCreat AEG Mixing sequence = water – additive – cement Min mixing time = 3-5 min

Grouting of tendon · Mix the grout as per approved grout mix. · Inject grout from the tendon’s inlet hose. · When grout flows out at the intermediate vents, lock these vents in the direction of the grout flow. · Lock the outlet hose when the grout flows out has the same consistency as the grout being pumped in. · Continue pumping to build up pressure. Lock the inlet hose when a pressure of 3~5 bar is achieved. · Repeat the above steps to the next tendon.

 

Safety Precautions

responsibility of the contractor.

 

Some safety measures taking at prestressing work includes, 

• avoid standing infront of stressing tendons during stressing process

• Use stop boads near stressing end after jack placement, to avoid cable movement in the direction of stressing
• Proper signage to inform educate people
• Use trained staff etc.

other than the normal safety practices in a construction site.