Post-Tensioned Slabs & Floors

What is a post-tensioned slab?

A post-tensioned slab is a concrete floor reinforced by high-strength steel cables (1,860 MPa strands) stressed after the concrete has been cast and cured. This prestressing technique creates a permanent compression within the slab, neutralizing the tensile forces that normally cause cracking in conventional reinforced concrete.

Unlike traditional reinforced concrete floors that require beam drops to span distances, a post-tensioned slab functions as a continuous flat structure. The cables follow a computer-optimized parabolic profile, concentrating the prestressing force where bending stresses are greatest.

How is post-tensioning used in floors?

The process begins with laying HDPE (high-density polyethylene) ducts containing the steel strands directly on the formwork. Anchor plates are fixed at the slab perimeter. After concrete placement and once a minimum strength of 25 MPa is reached (typically 3 to 5 days), the strands are stressed using hydraulic jacks.

The prestressing force — typically 150 to 200 kN per cable — is transferred to the concrete via the anchors, creating a biaxial compression state throughout the slab. This compression counteracts the tensile stresses generated by live loads, allowing the slab to perform more efficiently than conventional reinforced concrete.

Advantages over conventional reinforced concrete

The most visible advantage is the reduced thickness: a post-tensioned slab of 18 to 26 cm replaces a reinforced concrete floor of 23 to 33 cm for the same spans. This 20 to 30% thickness reduction triggers a cascade of savings across the entire building: less concrete, less passive reinforcement, lighter foundations, and greater clear height under the ceiling at each storey.

The absence of beam drops completely frees up the space beneath the slab. Architects have full freedom to position partitions, service ducts, and suspended ceilings. For a 10-storey building, the cumulative height saving can amount to an additional storey within the same building envelope — a decisive economic argument for property developers.

Formwork can be stripped in just 3 to 5 days compared with 21 days for conventional reinforced concrete. This accelerated construction cycle reduces labour costs, formwork hire charges, and shortens the overall building delivery schedule. On a large-scale project, total savings reach 20 to 35% of the structural cost.

Typical applications

Post-tensioned slabs are the benchmark solution for residential towers (10 storeys and above), office buildings with open-plan floors, hotels requiring 7 to 9 m spans without intermediate columns, and hospitals where space flexibility is essential. In West Africa, BEPCO has delivered over 1,000,000 m² of post-tensioned slabs for these building types.