PT slabs are not just regular slabs
A post-tensioned slab takeoff that follows the same pattern as a conventionally-reinforced slab takeoff will be wrong by 8-15%. The geometry is the same; the cost driver is different. Tendons, anchors, stressing, and grouting account for the bulk of the in-place cost difference, and most estimators new to PT understate them.
This is the breakdown we run on every PT slab engagement at Vortex.
What gets under-counted
1. Tendon weight by strand size, force, and length
Post-tensioned tendons are specified by their force capacity (1/2”, 0.6”, 0.7” diameter strand most common in US PT slabs), the number of strands per tendon, and the path geometry. Total tendon weight is the cost driver for both material and labor.
A typical 8” PT slab uses 0.8-1.2 lb/SF of tendon. The actual number depends on:
- Slab thickness
- Span between supports
- Live load and dead load assumptions
- Number of strands per tendon
- Tendon profile (straight, parabolic, harped)
The mistake we see most: estimators counting tendons by linear foot of slab edge, missing the actual path length of the parabolic tendon (which is 8-15% longer than the slab span).
2. Anchorages
Every tendon has a stressing anchorage at one end and a dead-end anchorage at the other. Both are individual hardware items with material cost ($45-$95 each depending on tendon size) and labor cost (jig setup, alignment, attachment).
Anchorage count is one of the easiest line items to miss when estimating from a tendon plan that shows tendon lines without explicitly counting anchorages. Always: anchorage count = 2 × tendon count.
3. Stressing labor
Post-tensioning stressing is its own crew operation, distinct from normal concrete placement. The stressing crew shows up after concrete cures (typically 3-7 days), runs the hydraulic jack on each tendon, records elongation measurements, cuts and grouts the dead-end anchorages, and installs the corrosion-protection caps.
Productivity is roughly 60-100 tendons stressed per crew-day depending on tendon size and access. For a 30,000 SF PT slab with 0.9 lb/SF of tendon and 4-strand tendons averaging 80 ft long, total tendons = ~85. That’s 1.5 crew-days of stressing — typically a 2-person specialty crew at union or quasi-union rates.
4. Grouting and corrosion protection
Bonded PT (the most common system in US commercial slabs) requires grouting the tendons after stressing. Grout pump cost, grout material, and labor are separate line items. Unbonded PT systems use grease-protected tendons with plastic sheathing — different cost model, no grouting.
The mistake: estimators using bonded vs unbonded interchangeably. They cost differently (~$0.40/SF apart). Read the spec.
5. Formwork SFCA realities for PT
PT slab formwork costs MORE per SFCA than conventional slab formwork. Why:
- Edge forms must accommodate stressing anchorages — chamfer cuts, blockouts, drop-out boxes
- Forms must remain in place longer (concrete cures, then stressing happens 3-7 days post-pour) — extending the reuse cycle
- Tendons run through formwork at specified profiles — formwork must include supporting chairs that don’t conflict with tendon path
- Drop-out panels at stressing anchorages add field labor
We typically apply a +25-40% labor premium to PT slab formwork compared to conventional slab.
The real cost breakdown
For a typical 8” PT structural slab in a Class A office tower, here’s the in-place cost we’d estimate (mid-2026, mid-tier US market):
| Component | Cost per SF | % of total |
|---|---|---|
| Concrete material (8”, 5000 psi PT mix) | $5.80 | 22% |
| Pumping | $0.95 | 4% |
| Placement labor | $1.10 | 4% |
| Finishing labor (steel-troweled) | $1.25 | 5% |
| PT tendons (1.0 lb/SF average) | $4.20 | 16% |
| Tendon installation labor | $1.85 | 7% |
| Anchorages (material + install) | $0.95 | 4% |
| Stressing labor + equipment | $1.65 | 6% |
| Grouting | $0.45 | 2% |
| Mild reinforcement (where required by code) | $0.90 | 3% |
| Embeds, accessories | $0.50 | 2% |
| Formwork (shoring + deck + edge) | $7.20 | 27% |
| Total in-place | $26.80/SF | 100% |
Note formwork is 27% of total cost — the single biggest item. The premium over conventional slab formwork is the longer reuse cycle and accommodation of stressing operations.
QA checklist for PT slab estimates
Before any Vortex PT slab takeoff ships, we verify:
- Tendon weight matches calculated weight from spec (lb/SF × area, cross-checked against tendon length × weight/LF for the spec’d strand size)
- Anchorage count is 2× tendon count, broken out stressing vs dead-end
- Stressing labor crew-days computed from total tendon count and crew productivity
- Bonded vs unbonded system identified, grouting line included or excluded correctly
- Formwork productivity premium applied (typically +25-40% over conventional)
- Mild reinforcement included where local code or spec requires
- Stressing schedule consistent with overall construction sequence
- Anchorage drop-out blockouts in formwork included
- Tendon profile geometry accounted for in length calculation
- Temperature / curing requirements noted in spec are reflected in placement sequence
The bottom line
PT slabs are 25-40% more expensive in-place than conventionally-reinforced slabs of equivalent capacity — and most of that difference is in line items that conventional-slab estimators don’t habitually count. If your takeoff treats a PT slab like a regular slab, you’re 8-15% low on cost.
Vortex Estimating handles PT slab work as a recognized specialty within concrete estimating. Send us your PT specs and tendon plans — we deliver a complete PT takeoff in 48-96 hours with all the line items most takeoffs miss.