1/4 Inch Per Foot Sloping Slab Standard.....where Did that come from? 2

[bigmig]

I am scouring my ACI code books looking for any reference to this age old industry standard of sloping exterior concrete flat work at least 1/4" per foot
for drainage.....and not finding anything.

Can someone point me in the right direction? Looking for a code reference.....

Thank you.

 

[GeoPaveTraffic]

I've never seen a code reference. Just a constructability issue. If the slab is too flat, you will have lots of bird baths and most owners don't care for those in their slabs.

Mike Lambert

 

[cvg]

exterior landings at doorways may have a requirement. Otherwise, this not a building code issue.

1/4 inch per foot is approximately equal to 2% slope which is an accepted minimum standard for drainage of sheet flow. For example, many roads have a minimum standard 2% cross slope for the same reason. I'm not sure a building code would have a standard for drainage of pavements and walkways.

 

[dhengr]

Bigmig:
Can we do anything these days, or will we do anything any more, if it isn’t codified? I’m sure there will soon be a whole 40 page code section on this matter. 67 pages of calcs., flow charts, and various numeric manipulations will be needed to make the determination, and you’ll be home free, but not blameless when the sh-- hits the fan. In the mean time, I think it’s a biblical thing: either you need to provide proper drainage, or you need an ark.

 

[BAretired]

There was a time when the minimum slope was specified to be 1/8" per foot but it wasn't enough as there were too many bird baths, particularly when the slab was on an expanding clay soil.

BA

 

[jdgengineer]

You likely won't find it in the ACI as the drainage requirements apply to surfaces that are not always concrete.

If you are using 2012 IBC see Section 1804.3 Site Grading. "Impervious surfaces within 10 feet of the building foundation shall be sloped a minimum of 2 percent away from the building".

 

[bigmig]

@jdengineer....I found that IBC reference as well.

My situation is that I am representing a plantiff, who has a slab that was not built sloped enough. The result was water ponding, leading to spalling and dangerous walking conditions as this is a high elevation environment.

Having a codified reference is just cut and dry in terms of litigation (typically). A "best practice" is just a bit grey in the lawyer world.

 

[BridgeEI]

There is no code reference for 2% slope in the transportation world. It's been used since roads were constructed with horses since it was found adequate for road drainage. It's been continued on since it's always been done that way.

 

[cvg]

if you have ponding, than you have no slope and no drainage. should not be hard to prove that is a bad idea. however, if this is a cold climate and there is water draining onto a concrete surface, than no amount of slope will prevent it from icing up.

 

[3DDave]

Why would spalling be related only to ponding? I thought spalling was primarily due to poor concrete finishing that may be aggravated by freeze/thaw, but is otherwise not related to slope or to ponding.

Reading between the lines, the surface was rough from spalling? Then it's the concrete.

If it was a skating rink, then that's a building owner responsibility, to some extent, because ice will form on even vertical surfaces. Even for ice it would be a toss up on liability as ice is very difficult to eliminate.

 

[bigmig]

@3dDave, the low spots hold water, which then freeze and thaw and acts like mini jack hammers, creating "catch pools" that in turn catch and trap more water. You can see the water stains, located right over the areas of spall, which are circular in nature.

 

[bigmig]

So in quick summary, one area that is very specific about how much slope to put into the flatwork is the subsurface drainage specification in the soils report. In this particular case, the recommendation was 2.5% slope.

In regards to there being no advantage over flat vs sloped for ice formation, I would argue that in a situation with an intermittent water source (i.e. the occasional snow storm) snow/ice/water that accumulate on the sloped surface at least have a chance to move away as a result of slope, and dissipate, as compared to a level condition, or a water "pocket" where things can only move by evaporation. Conditions such as shade exposure, maintenance, foot traffic (packing snow down so it is condensed) and the amount of freeze-thaw days will all be factors, but who can argue that a sloped surface does not have a benefit over a flat one?

Will they both hold ice/snow under certain conditions? Sure. Which one would you rather have? I think most would answer sloped....

 

[fel3]

Bigmig…

2.5% would certainly drain better, but if it's on a cross-slope along a traveled way (or something that can be considered a cross-slope on a traveled way), then it violates the 2% maximum per the American's With Disabilities Act.

What we try to do for traveled ways (e.g. sidewalks, in parking lots, etc.) is target about 1.5%-1.6% cross-slope to leave a little safety factor for the ADA maximum, then make up the rest of the slope for drainage in the longitudinal direction. Thus, the vector sum of the slopes meet or exceed the recommended 2% minimum, but we still have some wiggle room on the cross-slope.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[3DDave]

I can't see any water stains. I have seen perfectly level concrete, exposed to water and freezing conditions that did not suffer from the exposure.

Anyway, here's another guy who has given this some thought and never mentions slope:

https://youtu.be/P9OopE3EHFE?t=108

But it looks like you -need- to blame the slope for the results of bad concrete mix or install, so best of luck with that.

 

[DamsInc]

Some support for ponding exacerbating the situation in ACI 201.2:

1.4.1 Exposure to moisture—Because the vulnerability of concrete to cyclic freezing is greatly influenced by the degree of saturation of the concrete, precautions should be taken to minimize water uptake in the initial design of the structure. The geometry of the structure should promote good drainage. Tops of walls and all outer surfaces should be sloped. Low spots conducive to the formation of puddles should be avoided.

Really though, the relative contribution of ponding is pretty small. It may speed up the damage, but the real culprit is the material itself and/or improper finishing.

 

[dhengr]

Bigmig:
Check the specs. and concrete mix design for conc. strength, W/C ratio, air content, slump, aggregate not susceptible to freeze-thaw, etc. These are all important, at the correct values, for the conc. durability. What were the finishing and flatness criteria? A dog-leg for a screed causes low spots which require more slope to drain. I’ve spent a fair amount of time fixing ill drained (iced) walkways, fixing surrounding drainage or discharge from above, slope, etc. The property owner is responsible for keeping his public (all) walkways clear and safe. So, they almost always take a big hit in these cases. Many times, they can go back to the Arch. or contractor, or these people are brought into the case immediately, so there are more deep pockets to go after.

 

[OldBldgGuy]

I think you're picking the wrong reason for the spalling & if the other side has a decent expert, you're toast. Poor drainage is not going to cause spalling. Poor concrete mix or mix design is going to cause spalling during freeze-thaw conditions; especially with deicing chemicals. If you live in Canada, you take that for granted: sidewalks, driveways, parking garage floors, highways, where do you want to stop?. Drainage is done for lots of reasons, but to prevent spalling isn't one of them. If it was, we'd have some pretty serious problems with the hockey rinks in every town up here; slope makes it tough to build ice & people get upset if they are skating on concrete in one end of the rink.