350mm slab, only top layer mesh UK 2

[DrStrange10]

Hi all, long time lurker first time poster!

Really just wondering if anyone has came across old buildings of similar nature.

An old factory building, disused for the best part of 15 years in the 2000s is being repurposed. It is dated around 1980 for original construction.

There is no H&S file showing foundation drawings or much historic data, so cores were arranged to ascertain slab condition and rebar. A couple of different areas were selected, and no bottom rebar is present despite being 350mm in depth, ground bearing.

I have never came across a slab of this depth only having one layer of mesh in the top and I'm quite confused. There is currently some equipment housed in this area that was installed around 2019, so cannot see the condition or core directly below this equipment. Doesn't appear to have significant cracking around this area.

Problem is, now our client is on possession of this building, new equipment is going to be installed. I will not get the loads to work with a slab of just one layer of mesh in the top.

Has anyone seen this before? I think we're going to need to cast a new slab, else the risk of significant cracking is high for new loads.

Thanks for any views,
L

 

[hetgen]

That is a typical surface bed (slab-on-ground) with top shrinkage rebar.

 

[JLNJ]

Many, many slabs-on-grade (including and especially pavements) are designed and cast completely unreinforced. The load goes through the slab into the base material and imparts minimal flexure to the slab; therefore, no reinforcement is needed.

 

[human909]

Both the above posters have nailed it so I won't drive home that particular point any further. However you say:

I think we're going to need to cast a new slab, else the risk of significant cracking is high for new loads.

How did you come to this conclusion? Try calculating the bending moments. If you calculated it through assuming a suitable value of subbase stiffness you will find that your moments are quite low unless your forces are extremely high. You might find that you don't reach cracking strains of the unreinforced concrete.

 

[DrStrange10]

Thanks all.

Yes just receiving the equipment loads this week - it may be less than anticipated so the slab may be sufficient. In my decade of experience in the UK, at depths greater than 200mm, I've rarely if ever seen a slab have only shrinkage RC in industrial heavy use settings, and we have typically designed for soft spots.

Thanks for experience shared