Saturday, June 8, 2013

Underfloor trunking below structural rebars

Below is a picture of a somewhat "improvised" installation of underfloor uPVC trunking within the structural rebars of a floor slab in a workshop building under construction.

Picture 1: uPVC trunking installed below structural rebars

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About the author:

I have published quite a number of pictures on underfloor trunking systems.

Below is an example which was been posted in May 2010.

Picture 2: An under trunking system at a highrise office building

These underfloor trunking systems, together with their floor draw boxes (i.e. junction boxes) and floor service boxes, have so far been installed inside the concrete screeding of the concrete floor.

In this installation method, the reinforced concrete floor slab is constructed first.

However, the construction of the concrete floor is not yet completed with the completion of the reinforced concrete.

A layer of concrete 50mm to 75mm thick is to be poured and leveled on the finished reinforced concrete to make a finished concrete floor. This layer is called 'concrete screeding'.

It is within this top concrete layer that an underfloor trunking system is installed, or "embedded" as some people call it.

However, in the new picture that I showed above, the underfloor trunking is not installed in the screeding layer (In fact there is no screeding layer here).

The finished floor level here has been designed to be just 50mm above the top of the top of topmost reinforcement steel bars (called "rebar").

With the size of the uPVC trunking of 75mm wide x 25mm height, there would only be "approximately" 25mm thick of concrete above the uPVC ducts.

This concrete thickness is insufficient and it can eventually crack even in a "light duty" environment such as office floor. It will not last very long here ... it's a workshop.

So in this case the contractor had no choice but to install the trunking between the steel rebar mesh. If it had been two layers of reinforcement provided, then the trunking would be installed between the layers.

Picture 3: Close view of a service box opening

As you can see there are also added reinforcement bars around the opening for the underfloor service box.

When I took this picture the structure engineer has not yet inspect the underfloor trunking installation work.

They have inspected them once and had a few major comments.

One of the comments that I wish to share with you is that the uPVC trunking should not be touching the steel reinforcement bars.

The photo below shows how the installation was before it was corrected.

Photo 4: uPVC trunking touching steel reinforcement

At the areas where the trunking touches the rebars, the grip of the concrete material onto the reinforcements would be weaken. This may weaken the strength of the finished reinforced concrete.

How much weaker? Well, I did not really ask. My guess was that even the site structural did not really know "how much weaker".

Photo 5: The corrected trunking installation

In the above picture, the trunking installation was adjusted to leave some gap of around 15mm or more between the trunking and the steel reinforcement.

You can still see the rust marks on the trunking where they touched each other earlier.

Beginners please do not get confused here. The white rectangular boxes connecting the underfloor trunking are not the real underfloor junction boxes or service boxes.

They are just some polystyrene foam materials (the same material some people uses as refrigerator insulators) that are used to create openings for the real junction and service boxes that will be installed at a much later date.

If the contractor installs the boxes now, they will be damaged in a very short time.

OR STOLEN! ... I can almost feel some readers quickly nodding their heads in agreement ...

Okey guys. This is all I can write today. I will post a few more other pictures on this soon.

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