Cores are typically either circular or elliptical. Slabs may be reinforced or prestressed and are designed as ribbed slabs. The hollow cores afford a reduction in self weight of 30% or more, compared with a solid slab of the same depth. For most applications, no propping is necessary during construction, but crane access is essential. An erection rate of up to 600 m² per day is possible.
Prestressed hollow core slabs are manufactured in units 1200 mm wide with depths of 120,150, 200 and 250 mm. Units are made to order in lengths up to 12 m. The number and disposition of prestressing tendons varies according to span and loading.
Occasionally, structural topping is applied to increase the effective depth for superimposed loads. Propping is not generally specified except when the slab is required to act compositely with the structure above, e.g. where heavy walls are carried on the edge of the slab.
Prestressed hollow core slabs are designed as simply supported pretensioned ribbed slabs, in accordance with the requirements or the appropriate National Code. The prestressing force opposes the tendency to downward deflection and causes an upward camber in the units under no-load conditions. Hence, together with the high-strength concrete employed larger span/depth ratios can be achieved than with reinforced concrete. In lightly loaded roof slabs, for instance, span/ depth ratios around 50 are not uncommon.
Saves time: Time consuming site operations such as propping and shuttering wet concrete are virtually eliminated thus having a positive effect on the construction project.
Immediate platform: Once the Magicrete floor units have been placed in position and grouted, they provide an immediate working platform for all follow-on trades. This reduces site delays to a minimum.
No propping: In most cases propping is not required with Hollowcore flooring, saving time and money. The area below the floor remains free from congestion allowing other trades to progress unhindered.
Design flexibility: Prestressed Hollowcore floors can be designed to suit a very wide range of buildings and meet most project criteria.