Compacted or non-compacted fill under ramp?

[WWTEng]

Please see the attached detail/section for an exterior concrete ramp which is somewhat of a standard starting detail I came across. We are in the midwest, so frost & snow are common.

Looking at this detail it appears that the slab is not spanning between the walls on each side since its only doweled into the walls and not bearing on them. So I am guessing that the slab itself sits on grade. Which raises the question of why this detail shows non-compacted granular fill? If the slab is bearing I would want the sub-grade to be compacted. Is it possible that compacted base heaves more in frost then non-compacted fill? That is the only reason I can think of why the fill could be non-compacted.

Amount and location or rebar in the walls is also an issue I have with this detail but my question is focused more on the fill.

Thank you.

 

[hokie66]

Maybe the designer thinks the bars into the walls provide enough support, but I think that is a poor assumption.

 

[JAE]

That is a totally crappy detail. The rebar in the walls is totally wrong (wrong face for one thing).

 

[spats]

No offense, but I would say that an architect designed it. It's a pedestrian ramp because of the width, so no big slab loads. The walls are evidently intended to be deep beams, with longitudinal reinforcing top and bottom. The wall reinforcing at the inside, exposed face is for crack control. Also, architects think that dowels just tie everything together no matter what the size or spacing, so no worries about how the slab is supported, it will simply span... that's why it's reinforced.

Seriously, I have never seen somebody not compact subgrade for frost protection, but I can understand the logic. I wouldn't do it, but it is difficult to frost protect a ramp. In any event, if you want to span the slab with dowels, there's no reason you can't use shear friction principles. It's done all the time. If you check the numbers, #4 @ 18" o.c. probably works just fine, as long as they're developed on both sides of the joint.

 

[hokie66]

I never use shear-friction, so I don't do it all the time. But for this application, even if I did agree with the concept, I wouldn't rely on dowels to carry the load. This is an external ramp, and the bars through the joint are subject to corrosion.

 

[BAretired]

A recess in each wall to receive the slab would be a better detail. Placing the rebar on the tension face would be a definite improvement. Provisions to drain the high fill adjacent to the retaining wall would relieve potential frost pressure.

If frost heave is a concern, there are a variety of voidformers available. It is better to compact the fill, although granular fill may not need much compaction.

BA

 

[RareBugTX]

I think a slab on grade will do just fine with WWF without that wasted steel. Two comments on the detail though. I would use compressible filler on edges. The ramp slope is not that steep.

1) doesnt look like it provides minimum cover for concrete in direct contact with ground per ACI.

2) The embedded post base detail is not good and not allowed in many jurisdictions. Plate with Anchor bolts has been standard for a few years now.

And obviously the retaining wall vertical rebar placement is a fail.

RareBugRA

 

[spats]

Hokie,

I'm shocked that you don't "believe" in shear friction. I guess it's been in ACI 318 all these years without merit. Shear friction is a clamping mechanism due to tension in the reinforcing, not bearing on dowels. I've personally used it in several serious situations where dowel bearing strength alone wouldn't even come close to cutting it. The most serious one has been there for about 20 years now.

 

[spats]

Oops,

Two other quick points... agree with BA, shear friction needs to be on the tension side of the joint.

Regarding hokie's comment about corrosion, my serious use of shear friction was in a cast-in-place parking garage.

 

[spats]

I'm on a roll... I just realized this ties into the same subject as JAE's question about shear friction between rock and concrete.

Sorry, I'll stop. I realize shear friction was not the original question in this thread.

 

[WWTEng]

Thanks all for the feed back. Someone commented that this detail might be from an architect, indeed it is. To me this is SOG but being exterior, I rather use rebar in the slab then WWF (one of those feel good things I guess). I also agree that being exterior slab it will heave, so there is not a lot that can be done in that regard.

I think the thought behind the use of the dowels is to brace the walls so that they are not pure cantilevers.