About

A precast prestressed hollow core slab achieves its phenomenal attributes from prestressing and it’s voids.

Prestressing

Prestressing allows for longer spans, shallow depth and the ability to carry heavier loads compared to normal steel reinforced sections of equal cross section.  This also allows for better space planning and a lower floor/floor height. Moreover, lengthening the span is more economical with fewer slabs to make and install. The slab thickness, the location and number of prestress strands dictates the load bearing capacity in a prestressed precast hollow core slab.

Camber

A camber, or upward deflection, in the hollow core slab is the after effects of prestressing. This camber causes the slab to be in tension. By doing so, the camber counteracts the effects of compression in loading, hence reducing deflection. However, it should be noted that the camber present in our prestressed hollow core slabs are of a very minute degree and is unnoticeable.

Slab Voids

The purpose of the continuous voids is to reduce the self weight of the slab. The reduction in self weight increases the ultimate load of the slab, and reduces cost at the same time. Moreover, with proper and adequate coordination and planning, the voids can be used to conceal electrical wiring or mechanical ducts. For example, lighting circuits can be routed through the cores which can allow for fixtures to be mounted in an exposed ceiling slab without unsightly surface mounted conduits.

Load Distribution

Load distribution behaves similarly to a monolithic slab   when the system of individual slabs are installed and grouted together at the keyways. This beneficial behavior allows the slabs to act together and displace loads to adjacent slabs. This is achieved as hollow core slabs are designed to act as one way, individual, simple span slabs.  A major benefit of the slabs acting together is the ability to transfer forces from one slab to another. However, non-uniform loading occurs in the form of line loads, concentrated loads, or load concentrations at openings in most hollow core slab deck applications. Though, the ability for the connected slabs to interact with one another allows these load concentrations to be shared by numerous slabs depending on the system

Bearing Supports for Hollow Core

Many different types of supports designed to handle the dead load and live load from the hollow core slab can be utilized. For example, precast beams, precast walls, poured concrete beams and walls, masonry walls, insulated concrete forming system walls, wood and steel stud walls and structural steel beams are all suitable for use with hollow core as load bearing systems.

Holes and Openings

Holes and openings may be provided in the hollow core system by numerous ways

1) after a deck is installed and grouted – by shoring and saw cutting
2) In the plant – forming or sawing or by installing short slabs with steel headers
It should be noted that in any holes and openings should be cut parallel to a span so that the least number of prestressing strands are cut, and hence reducing the compromise of structural integrity.

Manufacturing

In manufacturing of our prestressed hollow core slab, the drawing out, fixing and prestressing of strands are the first steps in the production. Strands are used because of it is easier to handle. Strands are stressed individually. Casting of our hollow core slab is done by the utilization of an extruder. Extrusion is characterized by high compaction with very low moisture content of mixes. Hollow core slabs casting is carried out on long-line casting beds. The conventional method is to run the machines on steel beds up to 200 m long, equipped with stressing abutments. Travel speed will depend on quality of raw materials and depth of slab. More than single shift operation is possible, if the correct curing conditions are available.

After the slabs have been cured and cut to dimensions, stacks of slabs are removed to stockpiling by an overhead crane.

Normal production width is 900mm. Normal slab depths go from 120 mm to 500 mm.  Angle cuts are possible but cost more. It is desirable to have the plan dimensions fit the slab module, non-module plan dimensions can be accommodated using partial width slabs.

The main basic operations in the manufacturing of prestressed hollow core slab are:

•          Production planning
•          Preparation of casting beds
•          Drawing out and fixing prestressing strands and wires
•          Prestressing
•          Feeding the concrete
•          Casting the total slab length on the bed
•          Plotting and identifying the slabs
•          Removing notches, openings etc in the wet state
•          Covering the slabs, to retain moisture during curing
•          Curing the total slab length
•          Soft detensioning of the strands or wires.
•          Cutting the slabs to the required lengths and angles
•          Sample deflection testing
•          Creating stacks of slabs
•          Removing stacks to the stockyard
•          Delivery to site and erection on the building.

 

 

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