Ribbed Slab Construction: Slab Type – Ribbed and Waffle

GPRS is the nation’s largest private subsurface locating company. GPRS is the company that provides concrete scanning, utility locating, leak detection, and video pipe inspection services. Our dedication to safety has helped us achieve an over 99.8% subsurface damage prevention scanning rate on hundreds of thousands of scanning and location projects. GPRS has unparalleled accuracy, and as a company, we want to raise the industry standard. To help push the industry forward, GPRS is releasing articles about our training methodology. This article will review our training documentation and methods of ribbed and waffle slab scanning.

Ribbed and Waffle Slabs

Ribbed Slabs

Ribbed slabs could also be called joist slabs or beam and joist slabs. These slabs have narrow concrete beams (joists) that span between larger, traditional concrete beams in the column lines. The joists are often 2’ to 3’ on center. The structural support from the joists allow the slab to be as thin as 2”-6”. These slabs are still poured onsite using rebar like a traditional rebar slab but most of the structural support is in the joists. The slab is thinner and could contain minimal reinforcing such
as just wire mesh or rebar that is spaced further than a traditional, flat slab. The rebar in the slab should be marked but the primary goal when scanning these slabs is to identify the joists as no core drilling should take place through a joist. Of course, any conduit in the slab would also
be important to locate.

Ribbed Slabs

The shape and size of the joists can vary as shown here but the concepts are the same.

Ribbed Slabs

Decking, aka pan decking, metal decking, etc. can be used to form slabs, primarily in steel buildings but can also be found in certain areas of concrete buildings. The decking acts as both the form in which to pour the concrete and most of the structural support for the slab.

Ribbed Slabs

Here are two examples of what a typical ribbed slab could look like from below.

Ribbed Slabs
Waffle Slab

A waffle slab is just a two-way joist system. The slabs shown below do not have larger beams at the columns due to the added strength of the waffle system.

How to Diagnosing a Ribbed Slab

The slab being less than 6” thick is usually the first clue. A traditional rebar slab will be at least 6” thick. A slab that appears to be less than 6” could actually be a topping slab with something like foam below and sitting on top of a structural slab. A topping slab on foam can fool the GPR technician into thinking that the bottom of the slab is being seen when, in fact, only the topping slab is being seen and the structural slab is not being seen at all. The general guideline is, if the slab is less than 6” thick then it is probably a topping slab unless a pattern of joists or beams can be confirmed which would allow for a thinner slab.

Diagnosing a Ribbed Slab

Another clue for diagnosing a ribbed slab would be a lack of reinforcing. The slab shown below is elevated based on the strong, flat air reflection at 4” but being less than 6” is a red flag. In addition, the reinforcing appears to be wire mesh. Wire mesh is not structural and will not support an elevated slab. Seeing only wire mesh in an elevated slab or seeing rebar that is spaced greater than 12” on center can be a clue that the slab is only a topping slab unless joists can be confirmed. The joists in this data would have shown up much more clearly with a cross polarized scan but can still be confirmed here. Since joists were confirmed, the bottom of the slab can be confirmed at 4”. If the joists had not been found then this may have been a topping slab with foam below that we are not seeing through.

Diagnosing a Ribbed Slab

Below is another example of a ribbed slab. This slab is only 2” thick and two of the joists can be seen in this image. The bottom of the slab and the joists are outlined in blue.

The image below indicates a cross polarized scan of the same location as the image shown on the previous page. The cross polarized scan causes the wire mesh to disappear and the joists are very easy to identify. Cross polarizing is very useful when scanning ribbed slabs. Cross polarizing is also very useful for any concrete scanning job.

Diagnosing a Ribbed Slab
How Markings done

Markings for a ribbed slab should look similar to this. Rebar in the beams do not need to be
marked because the entire beam or joist will be hashed out as a no drill zone.

How Markings done  for a ribbed slab
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