Post-Tensioned Concrete: When and Why Engineers Specify It

Post-Tensioned Concrete

Post-tensioned concrete is one of the most efficient structural systems available for multi-storey construction. By introducing compressive forces into the concrete through tensioned steel tendons, engineers can achieve longer spans, thinner slabs, and more economical structures.

How Post-Tensioning Works

In a post-tensioned system, steel tendons (strands) are placed within ducts in the concrete slab before casting. After the concrete has gained sufficient strength, the tendons are tensioned using hydraulic jacks and anchored at the slab edges. The tension in the tendons places the concrete in compression, counteracting the tensile stresses that would otherwise cause cracking.

When to Use Post-Tensioning

Post-tensioning is most commonly specified for multi-storey residential and commercial buildings where span lengths exceed approximately 7 metres. It is particularly effective for flat plate construction, where the slab spans directly between columns without beams.

The benefits include reduced slab thickness (typically 200-250mm for spans of 8-10 metres), reduced concrete and reinforcement quantities, reduced building height (which can have planning benefits), and reduced deflections under service loads.

Design Considerations

Post-tensioned design requires specialist expertise. The tendon layout, stressing sequence, and anchor details must be carefully designed to ensure that the structure performs as intended throughout its design life. At ACSES Engineers, we have extensive experience in post-tensioned design for buildings ranging from three storeys to twenty-plus storeys.

Quality Control

Post-tensioning is a specialised construction activity that requires trained operatives and rigorous quality control. The stressing operation must be monitored to verify that the correct tendon force is achieved, and the grouting of tendon ducts must be performed to a high standard to ensure long-term durability.