Precast concrete ceiling components
• These reinforced concrete components can be precast on the building site without expensive equipment or lifting gear.
• They are designed to provide high strength with a minimum volume of concrete, thus requiring only manual operations in production and installation, and reducing cement consumption.
• The major advantages of using precast concrete components are the avoidance of shuttering for ceiling construction (apart from a few props) and the speed of installation.
• Depending on the costs and availability of cement, these construction methods can be more expensive than non-concrete ceilings, but provide greater strength and durability without special maintenance.
Further information: Bibl. 21.01, 21.04, 21.08.
Channel Units (Bibl. 21.04)
• The units, developed at the Central Building Research Institute, Roorkee, India, are 13 cm high and 30 or 60 cm wide, while the lengths can vary according to the required span, but not more than 4 m, as greater lengths reduce stiffness and load-bearing capacity.
• The moulds can be of timber or steel. The corrugations on the outer sides and the vertical grooves at the ends provide the necessary shear key action.
• The mould is oiled, the reinforcement cage placed with 12 mm spacers and concrete filled and compacted with a plate vibrator. The fresh unit is moist cured for 2 days, after which it is demoulded and cured for 12 days, keeping the trough filled with water. A further 14 days of air-curing is needed before installation in the building.
• Assembly is possible without props by placing the channel units in parallel on top of the walls, and filling the joints with concrete and a reinforcing rod.
Reinforced Concrete Planks (Bibl. 21.01)
• The system, also developed in India, mainly comprises a 3 cm thick reinforced concrete plank measuring 30 x 145 cm, with a 6 cm thick haunch portion in the centre, and 10 cm wide tapering fillets to strengthen the plank during handling. Joists of 15 x 15 cm cross-section, with stirrups projecting out on the top side, are also precast in simple timber or steel moulds.
• The joists are placed at 150 cm centres and propped at mid-span. The planks are placed over the joists side by side. After fixing reinforcements across the joists, screed is cast in-situ. Once it attains its final strength, the props are removed. No structural deck concrete is required over the planks.
Hollow Floor Slabs (Bibl. 21.08)
• This is a simple method for the on-site-prefabrication of reinforced concrete hollow floor slabs, a technology developed and practised in China 20 - 25 slabs of 333 x 60 x 12 cm can be produced during a normal working day.
• The wooden framework is fixed to a cradle-like, (rocking), welded steel substructure. The steel end-pieces with 4 openings define a trapezium-shaped cross-section of the floor slab, so that when finally assembled, the V-shaped gaps be tween slabs can easily be filled with concrete.
• A canvas-like cloth is placed within the formwork to prevent concrete from sticking to it. Reinforcing steel is laid with sufficient distance from the ultimate slab surface. Four steel pipes are pushed lengthwise through the holes in the end-pieces, the concrete is poured and compacted simultaneously, to ensure that no air-pockets develop around the pipes. The concrete is cast very dry so that it will not collapse when the pipes are removed.
• After completing the concreting phase, 3 or 4 men turn the entire cradle-like structure in one continuous movement, such that the freshly made slab lands directly on the ground, covered with loose sand to prevent sticking. The pipes are gently tapped and then pulled out one by one with an electrically-driven winch.
• The formwork is removed and immediately reassembled for the production of the next slab. One complete production cycle takes about 15 minutes with 3 - 4 men.
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