Equipment Slab - 6" Compacted AB 1

[jtjc23eng]

Hello. I have always thought that a layer of 6" compacted aggregate base over compacted sub grade is the industry practice for concrete slabs. I would think the reason is for improved sliding resistance + all the benefits of good compaction. Note the slabs I am talking about are heavy equipment slabs on grade say around 10'x20'x8" holding something 15000 lbs +. My questions are:
1. Is it typical practice in your experience?
2. Is it overkill to do compacted AB on top of compacted sub grade?
3. Are there any other benefits to the AB I did not capture?
4. Is it ever acceptable to pour on native?

Thanks.

 

[le99]

I think the need for a compacted aggregate base is mainly controlled by characteristics of the native soil regarding settlement and susceptibility to frost heave.

 

[EireChch]

It sounds reasonable to me, however alot of time the contractor will just place the subbase directly on the exposed subgrade. You'll be lucky if they will do a proof roll.

I dont know what compacted AB is....

 

[GC_Hopi]

1. Is it typical practice in your experience? 4" to 6" of Aggregate base (AB) is typical but could be more if the native soil has expansive potential. My area has soil with expansion potential so often X number of feet below the slab being removed and replace with borrow material or AB.
2. Is it overkill to do compacted AB on top of compacted sub grade? My area does this and its not overkill but other areas do not. For example Florida can have a lot of sandy soils so no AB needed just compact native subgrade material.
3. Are there any other benefits to the AB I did not capture? Is used to allow contractor to fine grade for the slab
4. Is it ever acceptable to pour on native? Yes, or example Florida can have a lot of sandy soils so no AB needed just compact native subgrade material.

 

[jtjc23eng]

Thanks for all the excellent input!

 

[TigerGuy]

It is also utilized to keep the slab separated from ground moisture. If the subgrade soils have moisture wicking upwards, a clean aggregate gives the moisture somewhere to go instead of slowly permeating through the slab. In dry climates, it can give the initial moisture emissions from the slab a second direction to go to reduce curling.