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Frequently asked post-tensioning questions are listed below, You may reach us through contact form for quotation and technical queries.
Post-Tensioning
Post-Tensioning
Post-tensioning is an innovative technique for reinforcing concrete. It involves placing high-strength steel tendons (cables inside plastic or galvanized iron ducts) and anchorages within the formwork before the concrete pour. Once the concrete reaches sufficient strength but before service loads are applied, these cables are pulled tight, or tensioned, and permanently anchored to the outer edges of the concrete structure.
In traditional Reinforced Concrete (RCC), structural elements are reinforced to handle tensile forces, resulting in increased self-weight due to the added steel and concrete.
In contrast, the Post-Tensioning (PT) system is far more efficient. PT tendons are specifically profiled (shaped) to directly counteract both tension and compression requirements. The external force applied via the stressing jack significantly reduces the required quantities of both steel reinforcement and concrete.
The current state of prestressed concrete is the result of continuous research spanning the last 90 years.
The concept was first documented in 1886, when Jackson of San Francisco patented a method for pavement construction that introduced prestress by tensioning sleeved reinforcing rods.
The field saw its most significant advancements between 1928 and 1933, thanks to Freyssinet. His contributions included the development of vibration techniques for producing high-strength concrete and the invention of the double-acting jack for stressing high-tensile steel wires.
Is it safest system for structure?
Thanks to over 90 years of continuous research and application, Post-Tensioning (PT) offers greater structural stability than traditional RCC. This superior performance is achieved through:
Thin Slab Elements and Reduced Self-Weight, which lowers overall structural load.
The combined effect of axial forces and the PT hogging effect, which provides significantly better control over the deflection of slabs and beams.
What is the Benefit of Post-tensioning?
Post-Tensioning provides significant advantages, making it the superior choice for modern construction:
Cost Effective & Fast: Delivers a comparatively cost-effective and faster construction process.
Superior Stability: Provides high structural stability while effectively controlling cracks and deflection.
Sleek Design: Achieves larger spans with thinner slabs and a flat plate design, maximizing floor-to-floor clearance.
Space Optimization: Enables clear-spaced car parks and eliminates short-span columns and additional beams.
Foundation Savings: Reduces the building’s self-weight and axial loads, which directly translates to savings on foundation and column elements.
What is the approximate savings in Post-Tensioning?
While the exact cost efficiency of Post-Tensioning (PT) is optimized based on the project's loading and configuration, the material savings are substantial:
Concrete Reduction: PT reduces the necessary concrete volume in both tension and compression zones by 20% to 35%.
Steel Savings: The savings in steel are even more dramatic, typically ranging from 60% to 80%. This massive reduction is possible due to the high permissible stresses allowed in the high-tensile PT wires.
Foundation Savings: The resulting decrease in overall structural weight reduces the design loads, which translates directly into significant cost reductions for foundations and supporting elements.
How to save cost on Foundation with PT?
A Post-Tensioning Raft Foundation streamlines your project by:
Saving Time and Labor: Eliminating the time-consuming processes of deep individual foundation excavation, tie beam installation, and complex reinforcement fixing.
Controlling Costs: Avoiding unexpected and costly de-watering expenses often associated with deep traditional foundations.
Reducing Materials: Utilizing a design based on the reduced self-weight of the PT slab, which significantly cuts the required quantities of concrete and steel in the foundation itself.
Faster Construction: Integrating the foundation and ground floor slab reduces separate concreting processes, saving both time and cost.
What is the form work striking time for PT Slabs?
Slab formwork shall be removed immediately after final stressing is complete (typically by the third day after concreting). Back propping must be installed as required by the structural engineer's recommendations.
What is the Sequence of Post-Tensioning Work?
This sequence details the steps from formwork to final grouting for a Post-Tensioning slab:
Formwork & Decking: Install the slab formwork and decking.
Bottom Steel Reinforcement: Lay the bottom mild steel (rebar) reinforcement.
Edge Shutter Installation: Complete all slab edge shuttering.
Anchor Installation: Fix the Post-Tensioning (PT) anchorages securely into the edge shutters.
Tendon Installation: Install the PT ducts and tendons (cables).
Tendon Profiling: Profile the tendons (adjust their curvature/drape) to the required design specifications.
Top Steel & Punching Reinforcement: Fix the top steel reinforcement and any necessary punching shear hooks or stirrups.
Pre-Pour Inspection: Conduct the Pre-Pour Inspection to verify all steel, ducts, anchors, and profiles are correct.
Concrete Pouring: Pour the concrete into the slab formwork.
Early Formwork Removal: Remove the edge shutters approximately 16 hours after the concrete pour.
Strength Verification: Obtain the third-day cube test report to confirm the concrete has reached the specified transfer strength (fci).
Final Stressing: Perform the Final Stressing of the PT tendons once the required strength is confirmed.
Grouting: Complete the grouting of the slab (filling the tendon ducts) after the back propping has been removed, as per project schedules and structural requirements.
(Note: Step 10 often immediately precedes the strength verification required for stressing, and step 15 was renumbered to follow 12 for better flow.)
What is the maximum design load for Post-Tensioning?
The maximum design load for post-tensioning tendons is strictly limited to 80% of the strand's ultimate tensile strength.
What is the compressive strength requirements for Final Stressing?
While the exact cube compressive strength requirements vary between different codes, the typical range is generally 25 MPa to 28 MPa.
What are the structural application of Post-tensioning ?
Our expertise spans a wide range of building and civil construction projects, including:
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