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PRODUCTION
NOTES ON PRODUCTION TECHNOLOGIES
The production of prestressed hollow core slabs takes place in works on long iron beds (120 H- 150 m) on which prestressing wires are positioned and stretched.
Casting of concrete is continuous and done with machinery designed specifically for the purpose. Generally speaking, two production procedures are employed:
The “slipform” procedure with slide mould machines in which concrete is directed into mobile sectors and vibrated by batteries of vibrators at different frequencies. In slide mould machines casting takes place in three stages: intrados, webs and extrados to arrive at completion of the finished slab.
The “extruder” procedure with the use of extruding machines in which the concrete is forced by special screw-feeders to compact in a single stage to produce the finished slab section.
All production procedures require concrete of the highest quality and homogeneity both in grain size composition and the cement/water ratio. They must assure instantaneous stability in shape so as to form the voids, high initial mechanical strength to allow prestressing and removal from the bed after a short time, and finally optimum adherence of prestression reinforcement and any normal reinforcement included in the casting.
Curing is accelerated by homogeneously heating the concrete until the required degree of strength for release of prestression reinforcement is reached (fck > 30 – 35 MPa). This strength is determined experimentally through the breaking of test pieces that have received the same vibratory and thermal treatment.
At the time of the compression test (28 days from casting), the concrete will have cubic strength above fck cube 55 MPa.
Once the artefact begins to be cast continuously over the entire length of a bed, operating immediately on still fresh concrete, the cut-outs required by the design or the holes for vertical canalization are added manually.
In this phase grooves are made in the slab ends for anchoring tie bars, as well as the transversal holes that may be needed for lifting.
When the concrete is sufficiently hardened and strands are released from their anchorages, slabs are then cut to the required length with abrasive or diamond disks.
This is the moment of concrete prestression in each slab.
On removal from the casting bed, the hollow core slabs present the intrados which is smooth from having been in contact with the metal surface, while the side faces and the extrados are rough. This assures a good bond with in situ castings of joins or structural topping.
