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Structures33 (Structural) (OP)
22 Jul 09 11:07
hey there - I have two scenarios regarding the sloping of concrete slabs for which I would like some opinions.
 
1. typical concrete slab on grade (6" or 8") used for maintenance facility type application; has multiple floor drains to which we need to slope the slab.  The building dimensions vary but let's say 75'x50' for this example.  I'm looking for opinions on methods for sloping the slab to the drains - slope entire floor maintaining thickness or slope entire floor with varying thicknesses... or feather at the drain?  There will not be regular "traffic" on the slab but it will see an occasional truck or forklift.

2. bottom slab (approx 2'-10" thick) of concrete basin containing water or wastewater; multiple "trains" divided by 2'-0" walls - each one has it's own drain.  Approximate layout is four 40' wide trains x 100' long (side by side so they share one base).  Right now we specify grout fill to acheive the slopes.  What are opinions on this?  

Thanks for sharing any opinions and/or experiences
Delchi (Structural)
22 Jul 09 11:21
For the maint floor, I almost always specify constant-thickness slab with slope (if any) to drain. A flat slab is a misnomer--there are always dips, so a min slope (1/8" per ft, often) helps keep water from ponding. On the other hand, if the space usage requires a level slab, then you might have to live with some ponding.

No experience with bottom slabs for wastewater.
mtu1972 (Structural)
22 Jul 09 11:30
Always more than one way to do anything.  For your first case I would slope the subgrade and place constant thickness slabs with the slope as required.  For the second case, I would prepare a level subgrade and place sloping concrete as required.  With the 40' width there is plenty of room to accommodate the slopes.  This would avoid the added labor required to place the sloping grout.
RHTPE (Structural)
22 Jul 09 13:10

From someone who has participated in the construction of many parking structures and water treatment facilities ...

Floor slopes for areas that will see vehicular traffic, particularly in areas using chlorides in the winter, should slope at least 1/8" per foot - 1/4" per foot is preferable.  1/4" per foot is becoming more common as it drains water faster and helps to reduce penetration by chlorides into the slab.  If this were an elevated slab, I would advocate for a level soffit (bottom) first, and a one-way bottom slope second.  For a slab on grade it all depends on the contractor's ability to accurately trim the base material.  Certainly you can save a bit of concrete by maintaining a constant slab thickness.

For your "train" - gnuguy is correct about the cost associated with installing fill concrete or grout to provide the slopes, BUT he has not considered the additional cost to construct the walls.  Setting today's large form panels would require considerable built-in-place wall forms to provide a "starter" form that is level to set the wall form panels on.  Setting these forms directly on a sloped slab surface simply does not work.  

Ralph
Structures Consulting
Northeast USA

BENDOG (Structural)
22 Jul 09 16:35
Always more than one way..

My experience is that the field does not like to slope the subgrade and would rather slope the concrete floor.  But that also may depend on where the drains are.  If they're in the middle of the floor then it might be eassier to slope the floor.  If the drains are at one end then maybe it is easier to slope the subgrade in just one direction.

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