Hollow core concrete slabs construction - step by step construction project

Hollow core concrete slabs also called voided slab or hollow core blank, it's a prestressed concrete elements and the reason for calling it hollow core because of the voids or cores which run through the units which are manufactured using high tensile strength prestressed strands.

Since these slabs are prestressed units, according to the level of prestress and the span the hollow core concrete slab units will have an upward camber because of the applied tensile force on each unit of the slab. However, after applying other dead load and toppings this camber will be reduced

These hollow core concrete slab units are manufactured and cured in the manufacture with many dimensions, then supplied to the construction site according to the request of the site engineer from the manufacturer, where there are many dimensions of these slabs depends on the manufacture.

So, what are the standard dimensions which widely used ? What the manufacturer should know ?

The members are manufactured with standard width of 120 cm and special widths can be manufactured as well. The thickness of these slabs are manufactured with 15, 20, 25, 32, 40, 50 cm depending on the span. However, the site engineer should give the service openings dimensions by shop drawings so that the sanitary system could be suspended pipes. Also, there are units which manufactured with thermal insulation.

Advantages and Structural use:

  1. Using hollow core slabs speeds up the construction process.
  2. The units are lighter than the traditional slabs because it has cores or voids which covers 65% - 75% from the slab thickness. 
  3. These voids in the members give larger capacity for these slabs than the traditional slabs, where the units can capable up to 17 meters span. So, this type of slabs is used when long spans are needed such as factories and parking.

Some frequently asked questions:

➧ Do these hollow core concrete slabs have firefighting resistance characteristics ?
Yes, these slabs have firefighting resistance from 60 - 120 mins and that depends on the manufacturer.

➧ How to prepare for the hollow core slabs in the construction site ? These slabs are supported on girders or ledge which is a short cantilever from the girder (you can see them from the pictures of the project below)

➧ How to transfer the members to the site ? The slabs are supplied from the factory by a trucks

➧ Is there a structural topping on the slab ? Hollow core concrete slabs are normally used without topping, but it's better to use a topping 5 to 7 cm reinforced concrete with minimum reinforcement to fill the voids between the units, and to make the structure as one unit for seismic purpose

➧ Are these slabs used in multi story building ? Yes, there's no problem to use the hollow core slabs in multi story building, but the reinforcement between beams and columns must be taken in consideration to resist the lateral forces from seismic loads.

➧ Are the girders and columns which support the hollow core slabs should be precast ?
No, columns and girders can be cast on site (as in the case in the structural project below)

Project which constructed by hollow core concrete slabs step by step:

First: Excavation and casting foundations works

This stage is like any project, which contains the excavations works and then reinforcement of the foundations as per construction details.




Second: Building columns and retaining walls and girders

The stage of building the columns and girders must be at a high level of accuracy, because the dimensions the site engineer gives the manufacturer are as per plans. So, as built must me the same as per plan with the amount of error allowed by manufacturer. Otherwise, any fault could cause a big amount of money.



















Third: Using crane setting the hollow core concrete slab members which supported on the girders.

Every unit has four hooks so it can be hanged by the crane to set up on the girders, according to the site area one crane needed, other sites with bigger area two or more cranes could be required.







The casting topping 7 cm reinforced concrete to fill the voids between to members, also for the seismic load because this topping makes the slab as one unit.




Don't hesitate to ask us about anything. 


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