Post-Tensioned Slab

A post-tensioned slab is a reinforced concrete floor system in which high-strength steel tendons — typically 12.9mm diameter strands — are threaded through ducts or plastic sheaths cast into the slab, then tensioned using hydraulic jacks after the concrete has reached adequate strength. This post-tensioning compresses the slab longitudinally, counteracting the tensile stresses that bending loads would otherwise cause and allowing the slab to span greater distances with less depth than a conventionally reinforced slab of equivalent span.

Post-tensioned slabs are widely used in commercial office buildings, multi-storey car parks, residential towers, and retail developments where maximising floor-to-ceiling height requires minimising structural depth. In the UK, unbonded post-tensioning — where the tendon is coated with grease and enclosed in a plastic sheath, allowing it to move freely within the sheath — is the most common system for building floors.

The inspection and condition assessment of post-tensioned slabs requires specific awareness of the system's characteristics and failure modes. The tendons are under very high stress (typically 1400-1600 MPa) and any corrosion, damage, or section loss is a serious structural concern. Tendon failures — which can occur at the anchorage, at any location along the tendon profile, or at the live end where the jack was applied — are rare but can be catastrophic, sometimes explosive, in nature.

For building surveyors and structural engineers inspecting buildings with post-tensioned slabs, the key inspection points are the tendon pockets at the slab soffit (where the live-end anchorages are typically located and where corrosion can initiate), the slab soffit generally (for signs of tendon-profile cracking, water staining suggesting corrosion of tendons, or differential deflection), and any coring or service penetration locations (where tendons may have been inadvertently damaged).

Documentation of post-tensioned slab condition should record the location and condition of each visible tendon pocket, any signs of tendon distress, and the overall soffit condition. 360° panoramic photography from consistent positions across each floor — capturing the full soffit in each bay — provides the most complete condition record for this purpose.