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title: "Post-Tensioned Concrete Slabs Design Construction | AVANORTH"
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**Construction Tips**·April 6, 2026· 4 min read
# **Post-Tensioned Concrete Slabs: Design and Construction**
Post-tensioning allows thinner, longer-spanning concrete slabs with fewer columns. Understanding the principles of prestressing, tendon layout, stressing operations, and grouting is essential for quality construction.
**AVANORTH Team**
AVANORTH Construction
Post-tensioned concrete uses high-strength steel tendons (strands) that are tensioned after the concrete has cured, compressing the concrete and dramatically improving its performance. This allows thinner slabs, longer spans between columns, and reduced deflection compared to conventionally reinforced concrete. In Canadian commercial construction, post-tensioned slabs are standard for office buildings, parking structures, and high-rise residential buildings.
---
## How Post-Tensioning Works
### The Principle
Concrete is strong in compression but weak in tension. When a slab bends under load, the bottom face is in tension and tends to crack. Post-tensioning applies a compressive force to the slab through tensioned steel strands. This pre-compression offsets the tensile stresses from loading, keeping the concrete in compression (or reducing the tension) and reducing cracking and deflection.
1. **Place tendons:** High-strength steel strands (typically 12.7 mm / 0.5" diameter, seven-wire strand, 1860 MPa) are laid inside plastic ducts or sheathing in the formwork before concrete is placed
2. **Pour concrete:** Concrete (typically 35-45 MPa) is placed and cured
3. **Stress tendons:** After the concrete reaches the specified strength (typically 25-30 MPa, 3-7 days), hydraulic jacks tension each strand to the design force (typically 138-207 kN per strand)
4. **Lock off:** Wedge anchors lock the strand at the stressed length
5. **Grout (bonded system):** Cementitious grout is injected into the duct to bond the strand to the concrete and provide corrosion protection
---
## Bonded vs. Unbonded Systems
| Feature | Bonded (grouted) | Unbonded (greased/sheathed) |
| --- | --- | --- |
| Tendon type | Multi-strand in metal duct | Single strand in plastic sheath with grease |
| Grouting | Required (cementitious grout) | Not required |
| Bond to concrete | Full bond after grouting | No bond (strand free to move in sheath) |
| Typical use | Bridges, parking structures, heavy loads | Building slabs, residential, commercial |
| Corrosion protection | Grout + duct | Grease + plastic sheath |
| Repairability | Difficult (strand bonded) | Easier (strand can be de-stressed/replaced) |
| Installation speed | Slower (grouting required) | Faster (no grouting) |
> **Canadian Practice:** In Canada, unbonded post-tensioning dominates building construction (slabs, beams). Bonded systems are common in bridges and parking structures where corrosion protection is critical (de-icing salts). The choice depends on the application, loading, and exposure conditions.
---
## Advantages of Post-Tensioned Slabs
| Advantage | Detail | Impact |
| --- | --- | --- |
| Thinner slabs | Span/thickness ratio of 40-48 (vs. 28-36 for RC) | Reduced floor-to-floor height, material savings |
| Longer spans | Typical spans of 9-12 m (vs. 6-8 m for RC) | Fewer columns, more flexible interior layout |
| Less reinforcing steel | PT strand replaces much of the conventional rebar | Reduced rebar cost and placement time |
| Reduced cracking | Compression counteracts tensile stress | Better durability, fewer repairs |
| Reduced deflection | Upward camber from prestress offsets dead load deflection | Flatter floors, fewer problems with finishes |
| Faster construction | Thinner slabs cure faster, earlier formwork stripping | Faster floor cycle (6-8 days typical) |
---
## Construction Sequence
1. **Formwork:** Shoring and formwork erected to design elevation
2. **Tendon layout:** PT tendons placed per the shop drawings (profile, spacing, anchorage locations). The tendon profile (drape) is critical: high at supports, low at midspan.
3. **Conventional reinforcement:** Mild steel rebar placed as specified (supplementary reinforcement at columns, edges, openings)
4. **Embed placement:** MEP sleeves, inserts, edge forms, blockouts installed
5. **Concrete placement:** Concrete placed and finished. Concrete must reach specified strength before stressing.
6. **Initial stress (if specified):** Some designs require partial stressing at early age (typically 10-15 MPa) to control early shrinkage cracking
7. **Final stressing:** Full stressing when concrete reaches design strength. Elongation measured and compared to calculated values (tolerance +/- 5%)
8. **Tendon pocket patching:** After stressing and lock-off, excess strand is cut, and anchor pockets are patched with non-shrink grout
9. **Form stripping:** Remove forms and shores per the engineer's stripping schedule
### Stressing Quality Control
The key QC check during stressing is the elongation measurement. When a strand is stressed, it stretches. The actual elongation is measured and compared to the calculated (theoretical) elongation. If they do not match within tolerance, there is a problem (friction loss, gauge error, strand damage).
> **AVANORTH Approach:** We work with specialized post-tensioning subcontractors who handle tendon placement, stressing, and grouting. Our site team coordinates the sequence and performs QC verification, particularly elongation checks and concrete strength testing before stressing. Every stressing operation is documented with jack calibration records, elongation measurements, and concrete test results.
#post-tensioning #concrete #structural
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