Apron Slab Deterioration 1

[ansys54]

We have a 100' long heated(in winter) scale for weighing pulp trucks. At each end is a 10' long x12" thick approach slab which is deteriorating immediately adjacent the scale where the tires run over. The potholes are 30"x30" and 4" deep and appeared slowly over the past 5 years since installation. Is there something other than freeze/thaw or deicing salt damage going on here. Could it be the transition from the scale to a different surface. Has anyone seen a similar type situation before? One more thing, one end of each apron slab is free and one end is tied with rebar into the end wall supporting the scale. We've checked the axle loads vs thickness and we have a F.S. over 4 so I don't think stress at the support is an issue.

 

[dik]

Some additional information is needed:
Do you have a spec for the concrete that was placed (strength, air entrainment,etc.)? An idea of the loading? the type of fill? gradation? fines? and degree of consolidation?

Potholing is often an indication of frost heave susceptible soil with the slab collapsing into the void caused by the ice lens.

 

[ansys54]

Thank you dik. The native soil in the area is a stiff clay and I wouldn't be surprised if clay got mixed in with some of the subbase backfill. I'm going to consider some of your points as I seek to unravel this mystery.

 

[dik]

Stiff clay shouldn't normally be a problem; it's normally not susceptible to frost heave in particular if it's stiff. Often ordinary people and contractors cannot distinguish between clay and silt (generally frost susceptible).

Stiff clay is more likely to deform (compared to well compacted granular material) and if your slab is underdesigned (need wheel loads) then this could cause failure, but it would likely require some 'big chunks' of clay.

 

[ansys54]

I agree dik. However I've done a lot of work around this mill and many times when we're breaking out old deteriorated slabs, we've found big and many chunks of clay mixed in with the granular base. Your comment this morning reminded me of that fact and that's why I wouldn't be surprised to find clay chunks in this situation as well.
I did check the wheel loads and they are well within a reasonable limit, actually we have a factor of safety of 4+.
We'll find out soon enough when we break the old apron slabs out. I'll report back as to what we found.

 

[dik]

If the 'clay' is mixed in with the granular material, are you sure the 'clay' isn't silt? In most environs where I've been involved with projects, granular material is more likely to have silt than clay.