help needed - reinforced slab overlay on an OLD unreinforced slab

[PE_JRM]

First, if anyone can tell me how to contact an admin. for this website I'd appreciate it. I mistakenly signed up as a civil/environmental but I'm actually a structural engineer since 1981. About 10 years into my career I went back to school and focused on aerospace structures. SOOOO, I havent designed a concrete anything since the early 90's. Concrete airplanes don't exist. (yet?)

Anyhow, I bought an old cabin and I need to reinforce and improve the existing 20' x 34' slab on grade foundation. The existing slab is clearly a poorly executed DIY. No vapor barrier, no gravel beneath it, no insulation, apparently no reinforcement. In spite of all of that the slab has no cracks and it seems to be in pretty good shape. The top of the slab is right at grade on some of the edges and those areas get pretty moist but they don't flood. I plan to survey the slab thickness by drilling some holes. Its 4" st the edges, k for the soil is around 130 pci, allowable bearing strength for the soil (silty clay) is 1500-2000 psf. The soil data if from the Georgia DOT preliminary design handbook for thae region in question. I am not permitted to demolish the slab and start over, dig trenches (spread footings) or do anything too invasive since the land is leased from the Army Corps of Engineers.

I want to accomplish the following: 1) Raise the elevation of the slab 6 to 8". 2) Insulate the slab 3) Increase the allowable wall load at the free edge around the perimeter. 4) Provide 15 mil vapor barrier.

I'm near Atlanta so the required roof LL is only 5 psf. I'm in the process of estimating wall loads (considering options) but due to the light roof loading, lack of a second story, slab on grade and light construction the wall loads wont be huge.

I'm trying to see if I can make the following work. Several inches of crusher run or washed crusher run if I can find it on top of the old slab. 2" of rigid eps insulation. 4" (or more) new slab reinforced with #3 or #4 rebar. Thats the unbonded slabs thought. If I go the bonded slab route I would eliminate the crusher run.

I need help with determining the allowable wall load on the renovated and reinforced overlayed slab. I've been using Corps on Engineers document TM-5-809-12 as a refernce but it doesn't really cover my situation since my propsed 4" overlay is off the nomograph provided.

I am retired and have limited analysis resources so things like FEM arent available to me. Whatever I do must be hand calcultion or spreadsheet based.

 

[EdStainless]

Would there be any advantage to covering with crushed except for the outer 8-12"?
You could put an extra layer of steel in edges then.
Effectively thickening the edges of the slab.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[PE_JRM]

Thanks EdStainless.

Crusher run is "crushed" tailings from the local granite quarry. The result is small aggregate that is pretty easy to place by hand with a shovel/wheelbarrow and its easy to level. Washed crusher run has the fines washed out of it. Typically #57 stone is used around here under a slab but its too big and too heavy for me to shovel through open windows and doors.

You make a good suggestion about the thickened slab around the edges. I thought of that also. Yet, a simple analysis tool or method is what I haven't been able to find.

I think its horribly wasteful to completely discount the strength of whatever the exoisting slab has. The best thing I've come up with (not using the thickened edge idea yet) is to calculate the stiffness of the unreinforced slab on grade including the soil (TM-5-809-12 can help me with that) and calculate the stiffness of the reinforced slab as a cantilever of a length back to the location of the maximum moment. Then the load carrying could be apportioned between the two cantilevers based on their respective stiffnesses. I don't want to mix ASD and LRFD techniques though. My analysis of the existing slab is ASD (TM-5-809-12). I havent done ASD for a slab since the first week of concrete class in 1978. Even then its was just a quick intro before going into LRFD.

 

[dik]

I don't think there is an easy remedy. If the slab is 4" and in good condition, this attests to it being constructed properly in the past. It may not be poorly executed. I don't know what your climate is, but can you excavate around it and install vertical or horizontal insulation to use geothermal heat to improve the temperature of the floor. If not, and the slab is only 4" thick, you may be further ahead to scrap it and to redo it the way you want. You may be able to do this in parts and keep the structure in place.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

I think this is how I would do for a simple and conservative approach:

I would ignore the existing concrete slab, use 1500 psf as a uniform resistance, and design the new slab on a per-foot basis for the point load (the wall load per foot) on a 1 foot wide reinforced concrete 'beam'. You can use the applicable (fixed-pinned) beam diagram out of the AISC steel manual (sorry, I don't have my steel manual handy at the moment). You basically just reverse it - use a 1500 lb/ft 'load' and adjust the length of the beam until the reaction at the pinned end equals your wall load.

Take the max moment from that as the design moment for the slab, choose a depth for the slab and see how much reinforcing you need. It's an interior slab, but I'd still keep 1.5" of cover to the top rebar. Do this for both directions, and remember the layer of rebar in one direction will have a reduced effective depth, since it will be below the other.

 

[PE_JRM]

Thanks dik,

If you read my post carefully you'll see that I can neither excavate around the existing slab or demolish it. The existing slab is only in good condition because it has essentially almost no load on it. I'm renovating what is basically a shed into a living space. Wall load will increase quite a bit.

 

[PE_JRM]

Thanks BridgeSmith,
Conservative is great when someone else is paying for it. I'm ultimately going to be the owner and the budget is sort of tight. Thanks for the ideas though. I'm having to balance spending money on determining what I've got versus just doing a brute force conservative approach. It will probably be a combination of both.

 

[dik]

If you cannot insulate the perimeter, you may have to live with what you have.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

Conservative is great when someone else is paying for it.

It's also great when the cost difference between the simple conservative approach and a more detailed and time consuming approach is minimal, which I believe it will be in this case. You might save a few bucks by using smaller rebar with a more detailed process, but making assumptions about the performance of unreinforced concrete comes with risks, too.

I get trying to save a few bucks wherever you can. I am a dedicated cheapskate myself. However, the foundation of a house is not the place where I would skimp. As I said, that's how I would analyze it, and I would build it accordingly.

 

[PE_JRM]

BridgeSmith,

I didn't intend to dismiss your comments. I thought of a similar solution although my approach involved a beam on elastic foundation and a case from the Roark Manual.

Your comment reminded me of a story. The only concrete structure I ever designed that was actually built was the footings under my 14' x 17' deck. My dad who was also a structural engineer (licensed in 5 states) sat in a chair and critiqued my design as I was building it. He commented that the steel reinforcement was a waste of money (the footings were small). I replied that there was $2500 worth of lumber and $25 worth of rebar and it was a justified expense.... especially since it was extra rebar left over from the dowels used to dowel the footing to the riser. I was about 30 at the time and it seems that 35 years of engineering practice has turned me into my dad. That said, I agree with the steel in my slab renovation as long as the analysis approach I use isn't crazy conservative.

 

[structSU10]

Its unclear to me - is the plan to demolish the existing building and just reuse the slab with a new one placed over? If that is the case I would agree with the ignoring of the existing to work with the new slab in general. Your capacity would likely be limited by the plain concrete capacity of the lower slab so if you want to avoid that problem and treat it as if its just a stone subbase you don't really care if that lower slab gets damaged by the proposed increased load. The only other option I can think of is to share the load based on relative stiffness.

Are you planning to build a new 2 story structure on this thing? What kind of load increase are you thinking?

 

[BridgeSmith]

If you're looking to save money, a heavily reinforced thinner slab beats a lightly reinforced thick slab every day of the week, (at least until shear capacity controls).

I have one of those curved 'roundabout' driveways that I need to make wider, so I'm going to put in a narrow 'sidewalk'. It needs to be able to handle a full-size pickup driving on it though. Instead of 6" of unreinforced concrete, I'm doing 3.5" with 2 layers of the 10ga. 6x6 mesh. Instead of over $200 in concrete mix (and alot of extra mixing), I'll be spending $120 for the concrete and $20 on the mesh.

 

[PE_JRM]

StructSU10,

I didn't give a lot of details because often people start commenting on the details instead of the question asked. But you ask a reasonable question so here is the background info.

The structure is on property leased from the Army Corps of Engineers. I am not permitted to tear it down and rebuilt it. It is on a lake and the government once encouraged privately owned recreational property around the lake. Now they are tired of dealing with those properties and want to see them gone (my opinion). If torn down the building is no longer grandfathered and can't be rebuilt. However, if renovated an element at a time that is permitted.

Because the structure was originally built by amatuers 60 years ago there is a lot of rot in the building frame in the bottom foot. My plan is to put temporary stilts around the perimiter and cut off the bottom 2 feet of the wall framing to gain free access to the slab from the outside. I'll run a temporary rim joist (1/2 sheet of plywood) right above the cut line to shear the stilt loads into the frame. I'll go back and sister on new 2x4's (about 100 of them) and replace the sill plate when the new slab is in place. Ultimately, I will strip off the aluminum siding, sheath the building and probably side it in hardi-plank.

The existing building is basically a 700 sf shack. 2x4 construction, no exterior sheathing, 1/8" 60's paneling inside, no insulation, aluminum siding outside, aluminum shed windows. It has electric service (one outlet) and septic. I intend to replace the plumbing and upgrade the electric. The roof is galvanized corrugated steel supported by Z section steel purlins in the long direction. The roof has no insulation and no drywall finish. As I said, like a shack. There is nothing about it currently that meets any code. I intend to fix that. The current wall loads are light because there is nothing substantial there. The new loads will be much higher because I intend to meet code with whatever I do. That means adding exterior sheathing, regular roof framing, insulation and designing for code required loads (live, dead, wind, EQ, etc.).

I haven't settled on a plan yet so I don't know the wall loads. A shed roof dumps all of the roof load into the two long exterior walls. A gable roof dumps half of the roof load into a structural ridge beam. So there will be a significant difference based on what option I choose. The existing 4" unreinforced slab on grade is good for about 250 plf if it was done properly which it was not. I might give it credit for half of that or nothing if being conservative.

Right now I'm trying to determine what strength I can get out of the exist slab by overlaying it. Since I have a lot of work ($$$) to do on every aspect of the structure and I'm the final owner I'm focused on keeping the cost as low as possible. With some idea of what the slab is capable of through a reinforced overlay I can then move on to the building frame.

 

[PE_JRM]

BridgeSmith,

I forgot that I designed my own driveway about 15 years ago. According to the concrete contractor who built it I have the finest driveway in the area. He thought I overdid it. Driveway contractors around here do shoddy work knowing that it will take a few years before the results of cutting corners show up. My old driveway was 2" thick in some places.

I went with recycled concrete rubble instead of gravel, 4-5 inches of 4000 psf concrete, ACI recommended minimum temperature/shrinkage steel. The contractor didn't understand the concept of temp/shrinkage steel and he insisted on hand tooling and then saw cutting control joints. I still hate all of the joints but accepted them because the contractor insisted that was the only way he would take the job. I had three bids and the other two contractors didn't seem like they would do a good job.

Anyhow, 15 years later there are no cracks and no spalling ... still looks like the day it was placed. The day it was placed it was 98 F. I spent a lot of time watering it while it cured.

 

[BridgeSmith]

It sounds like you have a pretty good handle on how to build this. I'd encourage you to make an estimate of the wall loads, try the analysis I suggested, and see what the slab thickness and reinforcing comes out to be. I think you're gong to find it's not excessive. Then, try it assuming support from the existing slab, maybe double bearing capacity, and see what you need. I think you'll find the end result for the new slab doesn't change much.

 

[PE_JRM]

BrdgeSmith,

I a pretty good handle on it but its been a long time (decades) since I wasn't working on an airplane structure which is an entirely different thing. If you knew how aircraft were designed you would think twice about getting on one...A margin of zero for the worst case load event.

Back when I worked for Chicago Bridge & Iron company (CB&I) decades ago I was pretty good at using the beam loading diagrams in the back of the AISC manual (the old red one) to get quick, approximate results instead of waiting until the next day for computer output from the mainframe. I understand what you suggested and I'll see what that results in. The beam on elastic foundation analysis I mentioned resulted in a very doable 4" slab without that much reinforcement using very conservative soil modulus conditions. I was surprised by the result.