outdoor equipment pad

huangyhg

outdoor equipment pad
those of you practicing in northern climates, do you typically worry about frost around the edges of an equipment pad that will be installed outdoors?  the pad i am designing needs to be at least 8" thick to get all the rebar in, but i am wondering if i should turn down the edges of the concrete to a deeper depth.  the pad is about 8' x 10' in plan size.  normal depth of footing around here to prevent frost heaving is 3'. what is the practice in your office for this type of situation? yours truly is not asked to outdoor design equipment pads very often!   
assuming that there will be no building above this pad (and as such no heating of the pad during winter) there is no reason to thicken (or turn down) the pad. the thought here is to allow the ground around and below the pad to freeze more or less equally. problems can occur whenever limited areas are subject to movement (due to moisture and frost variations)such as the exterior edges of a heated building.
i have to take issue with the last statement.  frost heaving occurs as a result of ice formation around small particles of soil causing an expansion in the mass.  this can occur in any location provided the soil or substrate below does not readily accomodate this expansion due to large void spaces.  therefore, what is most important in the prevention of heaving due to frost is to provide a base material with sufficiently large size particles to accomodate the expansion of the frost susceptible soils. this is seen quite readily in northern climates with frost heaving of roadways due to inadequate base preparation.  discontinuties in types of subgrade material and void ratios will also generate differential movement, not just a heated/unheated conditions.
to prevent heaving beneath utility pads on grade we usually either hire a geotechnical engineer to check the soil to see if it is frost susceptible, or excavate and replace with a granular, non-frost susceptible material of sufficient particle size (gravel, crushed stone)to a depth equal to the depth of frost dictated by local authorities.  turning down the edge of slab will not mitigate the movement unless the pad is designed to span and resist the uplift forces from frost.
thanks for your posts.  
fyi, the pad is completely unprotected from the elements.  subgrade below it will definitely freeze at some point. perhaps turning down the edges is a bad idea- if as wsiladi notes there can be uplift on the pad interior because only part of the supporting subgrade freezes. i am also hesitant to make a prediction about the magnitude of uplift pressure resulting from freezing soil.  
i was planning to require proof rolling of the subgrade and a 6" layer of crushed stone under the pad.  perhaps i may make the stone layer deeper to promote drainage, and provide some resiliance (ie pore space) for absorbing upward forces as noted in the posts above.  
there is not much geotechnical data available at the site, all i have is an allowable subgrade pressure of 2000 psf.  this is a small project (its mostly changes to an existing building), so there is no budget for a geotechnical investigation.   

pay as much attention to the concrete mix design as the slab thickness and steel.