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Post-tensioned concrete slabs span long distances with a thin slab, which suits office blocks, control buildings, multi-storey warehouses and car parks on factory sites. The quality of the slab depends on the strand and anchorage materials, the accuracy of the tendon profile (position and level), the stressing force and the grouting. World Steel controls each stage in the sequence below.
Work sequence
| Step | Work | Control |
|---|---|---|
| 1. Materials | Check the origin and quality of strand, anchors and wedges; sample and test the strand and anchorage system. | Test results before use. |
| 2. Strand and anchor preparation | Cut the strand and thread it into the ducts; prepare the live anchors. Long tendons are cut and threaded on the slab in their final position; short ones are made up in advance and lifted. | A tidy preparation area so strands do not tangle and waste is low. |
| 3. Setting out the tendons | Paint tendon lines and profile levels on the formwork; drill the edge formwork for the live anchors. | Clean the column head areas before fixing the bottom steel. |
| 4. Placing the tendons | After the bottom slab steel: place tendons in both directions, long ones first; tape the duct joints carefully; fix the live anchors firmly to the edge formwork. | Accept each stage; clean formwork before placing. |
| 5. Before the pour | Fix the top slab steel; install grout tubes; adjust the tendon profile to the design. | Accept tendon positions and levels before concreting. |
| 6. Anchors and wedges | After concreting and striking the edge formwork, fit the anchor heads and wedges. | Live anchors free of grout and not blocked by reinforcement. |
| 7. Stressing | Stress when the slab concrete reaches the strength specified by the designer; full stressing platforms in place. | Record the force and elongation of every strand. |
| 8. Grouting | Cut off surplus strand, seal the anchors with mortar and grout the tendons. | Grout mix and pump pressure as designed. |
Common mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Tendon lines and profile not marked on the formwork | Tendons in the wrong position or at the wrong level | Paint tendon lines before fixing steel |
| Reinforcement in the way of live anchors | Anchors cannot be fitted; cutting bars weakens the structure | Check the clearance round anchors at steel acceptance |
| Anchors not sealed | Grout runs in and blocks the anchor | Seal and check before the pour |
| Dead anchors too short | Dead anchor pulls out during stressing | Check anchor length against the drawings |
| Strand cut by mistake when platforms are removed | Tendon cannot be stressed | Mark and protect strand ends waiting for stressing |
| Wrong stressing force or elongation readings | Slab does not reach its design capacity | Calibrate jacks; supervise and record every strand |
| Ducts blocked by concrete grout | Slab has to be drilled to clear the duct | Tape duct joints carefully and accept before the pour |
| Wrong grout mix or pump pressure | Weak grout, tendons not fully grouted | Test the grout and control pump pressure |
Reference standards
The edition in force when the contract is signed applies.
- TCVN 4453:1995 Monolithic concrete and reinforced concrete structures – Code of practice for construction and acceptance
- The approved technical specifications of the designer and of the strand and anchorage supplier
