Designing a new suspended slab that won't leak

[ajk1]

For a new suspended reinforced concrete slab floor which is a "transfer floor" for municipal residential garbage (this is the municipality's main garbage transfer facility where their garbage trucks unload), is there any way to design the floor not to leak?

I don't want to prestress it (it is an existing garbage transfer floor where the slab will be demolished and a new slab poured) and cannot put a waterproofing membrane on it because of the mechanical damage that would be done by the humungus equipment (6 foot diameter tires) used to push the garbage along the floor to the chutes. I don't think integral crystalline waterproofing will work, because the cracks will be active, under the huge equipment and weight of the garbage.

 

[hokie66]

Then you need to use a lot of reinforcement. What type floor slab? Flat slab, flat plate, banded slab? I would start with 0.6% Ag, and would probably end up with closer to 1.0% Ag. And about 50 MPa concrete. A waste transfer station is not a place for skimping, as I am sure you know. I have been involved with a few of these, but never one with a suspended floor. Usually, the trafficable floor is on ground.

 

[KootK]

Any chance you could drop the new slab and install a topping above it with membrane between the two? I can think of all manner of constraints that might come into play in an exiting structure of course. Even with the active cracking business, I might consider crystalline as a second line of defense.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

I don't think you can prevent the top concrete from being damaged in a heavy industrial environment. So, I think a top wear slab (12" reinforced?) lends itself nicely to this situation. I'd consider a placing a bright red sheet directly under the wear slab to both alert the users when the wear slab needs repair and to provide a slip plane between the wear slab and the structural slab. For the structural slab, I'd think an appropriately deep, reinforced concrete structure could be detailed so that wide cracks are prevented. For waterproofing, a plaza system will need to be repaired whenever the wear slab ever gets eaten through. I like the crystalline option for the structural slab, but I don't have any experience using one.

 

[cliff234]

This slab is going to be severely abused. I suggest you try to find out what the standard practice is for slabs in other trash transfer facilities. (Single slab vs. double slab). If you go with a single slab it should be heavily reinforced with continuous top and bottom reinforcing for crack control and to distribute the wheel loads. Make it thick enough to minimize deflections. The slab should be sloped to provide drainage and prevent puddles of water. What was the thickness of the original slab and how long did that slab last? I am assuming that your new slab will have to be the same thickness as the original one.

 

[hokie66]

Is this the level where the trucks operate, or the level where the waste is dumped to be dealt with by dozers/loaders? I thought you were talking about the level where the garbage trucks dump their loads into the big pit, not the pit itself. The pit is certainly subject to lots of abuse, but that is generally not a suspended slab.

 

[rapt]

Your only chance would be to

- minimize restraints
- use a reduced shrinkage concrete and some good addiditives to increase density, fly ash and Silica Fume might help, 400-450 microstrain should be attainable
- continuous top mat of reinforcement, maximum 200 centres
- keep service stresses really low, so service stress of about 100-130 MPa in the top steel.

 

[Ingenuity]

Type K cement too...but expensive.

 

[ajk1]

Thanks very much to you all.

Floor is the "tipping floor", not the pit. It is 200 feet long. It is one-way slab spanning about 7'-6" between supporting steel beams. Slab will be 10.5" thick and continuously reinforced top and bottom.

My words to the designer were exactly the same as Hokie66's, "Don't skimp on the reinforcement" So glad to see that. We have continuous top and bottom rebar. I told them to check the crack control and they have reduced the spacing of the rebar.

The existing slab is reportedly extensively cracked and gouged. Our designer says that he has had to put 40% more rebar in to satisfy the loading (I believe the equipment weight governs over the uniform loading).

For the new slab, we are using a 2" thick metal reinforced epoxy topping (I think that is too thick and expensive) in 2 layers with the lower layer coloured so that wear is revealed. There is another such facility being currently built but designed by others that uses a 1" thick metal reinforced urethane topping.

With this system, we are specifying 35 MPa (CSA Class C-1) concrete for the structural slab. The epoxy we are told cannot span cracks so cannot be considered a waterproofing membrane from that point of view.

It has been suggested that the cracking is partly due to the brittleness of the 60 MPa in the existing slab, but I am doubtful of that.

I will review our design in light of all excellent comments for all of you. They are really all very good comments. Thank you so very much. But I am still thinking that there is no way to stop leakage. They tell me that the existing slab has little leakage but I do not know why that would be.

I think to put a membrane between the slabs would require a very thick reinforced concrete topping, something like 10" or so, but I have not done any calculation on that and it is rather too late to change now anyway since it will be tendered shortly. It may overtsress the existing beams and timber pile foundations, and that slab would still be subject to mechanical damage.

To Hokie66 - inc you have designed these type facilities, albeit as slab-on-grade, would you have information on the density of the garbage? All the papers I have read pertain to land fill where the garbage is regularly overlaid by a layer of soil.

 

[hokie66]

No, sorry, I don't know about the density, and that is the reason I didn't respond in the other thread. It has never come into play, as the tipping floor has always been essentially a pavement design, designed for the loaded trucks, not loose dumped material. The trucks dump into the pit, not on the floor.

 

[ajk1]

ok, thanks

 

[dicksewerrat]

Can "metal roof" be suspended below to catch any leakage? Steer it away from the lunch room. Make it cleanable.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[ajk1]

Metal roof deck to catch leakage would not do anything to stop leak water running over the steel beams and corroding them, as well as water running along slab soffit and corroding bottom rebar. Putting a metal deck would be hiding a problem not solving it (by the way, the space under the floor is not used). You touched a raw nerve with me, because I think putting a metal pan or deck below is always a really bad practice because it hides the deterioration and eventually leads to very expensive repairs...if you can't easily see the condition of the structural elements, and there is leakage of salt contaminated water, you can eventually get structural collapse (see Algo Mall, 2 people killed)

 

[hokie66]

Amen, ajk1. Concealing a known problem is never the way to go.