Construction World December 2023

PROJECT INFORMATION

• Company entering: Quantibuild • Client: Central Developments • Main Contractor: Quantibuild • Consulting Engineer: Civil Concepts • Subcontractor: Res Spec

that would increase friction during tensioning. Getting the layer under the HDPE slip surface smooth and level with no obstructions was therefore key to achieving design success. The initial alternative design included for 50 mm of blinding followed by 50 mm of topping under the HDPE liner to achieve this. The Contractor was however able to save additional time and cost by eliminating the topping and power-floating a 75 mm thick blinding layer until the surface was smooth. To ensure levels were within tolerance, the blinding was constructed in four-metre wide lanes running north to south. Post-tensioning of round slabs provides a unique challenge in that the jacking force needs to be applied perpendicular to the slab edge whilst the cables still need to span in two directions (north-south and east-west) to achieve compression of the entire slab. Whilst curving the cables does achieve this outcome, significant losses in the stressing force can be experienced due to friction and wobble if the cables are not profiled along long, gradual curves. To achieve these curves

practically, welded reinforcing ladders with a vertical locator for the centre-line between two cables were accurately made up based on dimensions from the AutoCAD drawing. These ladders were then placed around the perimeter in three continuous rings at 1,5 m, 8 m and 14,5 m from the slab edge. Installation of cables then became a matter of simply lining up the anchor points to the locators on the ladders and smoothing out the curves in between. A hybrid concrete mix design between a water retaining mix and a PT slab mix was required for the floor slab which meant striking a balance between increased impermeability, reduced

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22 nd Best Projects Awards 2023