Post-tension slabs require tighter elevation control than standard reinforced slabs because slab thickness directly determines tendon cover — and insufficient cover is a structural deficiency, not just a quality issue. Subgrade elevation errors that would be acceptable on a conventional slab can require remediation or redesign on a post-tension project.
What are the elevation control requirements for post-tension slab construction?
Post-tension slab elevation control requires three sequential verifications: (1) subgrade elevation survey on a 10-ft grid prior to form setting, confirming the subgrade is within ±0.05 ft of design elevation so the minimum slab thickness and tendon cover will be achieved; (2) form elevation check at the slab perimeter before concrete placement, confirming the top of form matches the design slab elevation; and (3) finished slab elevation survey within 72 hours of placement, typically on a 5-ft grid, to confirm the slab surface matches the design and calculate the F-number if specified. Structural engineers typically require the subgrade survey as a condition of pre-pour inspection and sign-off.
In a conventional reinforced concrete slab, minor variations in slab thickness are tolerable within ACI 117 limits. In a post-tension slab, the structural design depends on maintaining the specified slab thickness because tendon eccentricity — the vertical distance from the tendon to the slab centroid — is part of the structural calculation.
If subgrade is high by 0.5 inches, the slab is thin by 0.5 inches. For a 5-inch design slab, this represents a 10% thickness reduction. The structural consequences include:
The structural engineer of record (EOR) defines the acceptable subgrade tolerance in the project specifications. If the specification does not explicitly state a subgrade tolerance, request clarification before grading — the EOR may require a pre-pour inspection that will flag subgrade elevation issues before concrete is placed.
A subgrade survey before a post-tension pour is a pre-pour inspection hold point for most structural engineers. The survey procedure:
Tendon cover is the concrete thickness between the outer face of the tendon sheath and the nearest concrete surface. Minimum cover requirements for post-tension slabs are defined in PTI DC80.3 and ACI 318:
| Exposure Condition | Minimum Bottom Cover | Minimum Top Cover |
|---|---|---|
| Interior slab on grade | 3/4 inch | 1 inch |
| Exterior exposed slab | 1 inch | 1-1/2 inch |
| Slab exposed to deicing salts | 1-1/2 inch | 1-1/2 inch |
| Parking structure (non-exposed) | 3/4 inch | 1 inch |
| Post-tensioned slab per ACI 318-19 Table 20.6.1.3 | 3/4 inch (enclosed) | 1 inch (enclosed) |
Document tendon cover during placement inspection. The pre-pour inspector should verify that tendon supports are correctly placed and that the cover dimensions match the structural drawings at multiple locations across the pour area. Photograph the tendon layout before concrete placement.
After the slab is placed and has reached the minimum specified strength for stressing (typically 2,000-3,000 psi at 72-96 hours), the post-tensioning contractor stresses each tendon. The stressing record is a critical QC document that must be retained for the life of the structure.
Required stressing record elements:
Sitemark organizes post-tension slab documentation — subgrade surveys, tendon placement inspections, and stressing records — into a single QC package ready for structural engineer sign-off. Start free.
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