Upgrade old slab ideas needed

[PE_JRM]

I could use some help from people who design concrete structures every day. I am retired structural engineer and have had an interesting career that was widely varied in nature (marine, oil & gas, research, aerospace, nuclear) but I never did much concrete design work. The current problem I am trying to solve is how to deal with a concrete slab of unknown thickness and unknown reinforcement in an old empty building. I bought a remote cabin (no codes apply) and took on the job of renovating it. It's over 60 years old and built like a rectangular shed (lightweight metal roof, no insulation, no big loads). The slab has lasted all these years with zero cracking. It looks like the slab is just 4" thick at the edges (no thickened, reinforced edge detail). The slab was done by amateurs and the concrete does not appear to be well consolidated or level or flat. The top of the slab is also a few inches below grade (gotta fix that).

My first order of business will be to get an idea of the contour of the slab using a water level and then maybe taking a few cores or drilling some holes to determine the thickness.

Anyhow, my current plan involves "sort of" abandoning the slab and building a wood framed floor above it. A single course of CMUs or a poured curb around the perimeter would get the framing above grade. This plan will result in much larger edge loads (floor + roof) than the slab currently experiences. This will also result in a distributed load down the center of the building (load path for the floor framing). The slab essentially would become the floor in a crawl space. In this scenario it doesn't matter much if the slab cracks. It just needs to transfer the loads to the soil evenly.

Another idea is to pour a new reinforced slab on top of the old one. This could be accomplished fairly easily but it doesn't really solve the improperly done existing slab completely since the edges are still under reinforced (probably). A 4" overlay will take about 8 cubic yards (a truck load)and the site is a little inaccessible so this idea would be difficult to implement. However, its worth considering as it may be the best option.

For what its worth I'm in the deep South and there is no frostline although its generally considered to be 12" or so below the surface.

Any ideas you might have will be appreciated. My budget is small so whatever I do needs to address the problems and be structurally sound but it doesn't have to be perfect.

 

[Greenalleycat]

I'm not really on board with the 'code doesn't apply' idea but it is an interesting project

Does the slab support the wall framing etc at the perimeter?
Often a thickening was poured (usually with a bar down the guts) and then the slab added later with no connection or reinforcement (here anyway)

A concern I have with the existing slab being below grade is moisture ingress
I'm assuming that there is no polythene etc below the slab as it is, let alone the risk of moisture coming in to the top/sides due to being below ground
If you put a subfloor over the top how will you vent it?
Even if you stick a slab over it, it seems there is still a risk of moisture coming through the side of the foundation..?
What are your cladding setdowns like?

I think the structural solution to this problem is probably pretty easy, the challenge is getting the other details right so that you enjoy the result

 

[PE_JRM]

I have designed structures to code for my entire career. UBC, AWWA-D100, API-650, API-750, ASME section VIII-div 1 & 2, AISC, ACI, Federal Airworthiness Regulations, FAA Engineering Orders. So, I'm very familiar with code and meeting code.

I'm not implying that I don't intend to meet code. This structure was originally build by the Army Corps of Engineers 60 years ago. Those guys build to their own specs and often the structures are intended to be cheap and temporary. I bought the structure and it is on property leased from the Corps. What I can do to it in order to renovate it is severely restricted by the Corps. Most everything can be easily upgraded to current standards and I intend to do that. However, I'm stuck with the slab and I need to know how to evaluate it and strengthen it if necessary.

Yes, the slab supports the 8' wall framing at the perimeter. As I mentioned, it is framed like a shed. No insulation, no exterior sheathing, Masonite paneling not backed by drywall as the interior wall covering, aluminum siding on the exterior. Roof loads are carried by steel Z-shaped purlins (on about 3 ft centers) that run the length of the structure. The purlins span about 14 feet and are supported by the interior walls and gable ends of the structure.

The roof is covered by galvanized corrugated steel sheets which spans the purlins. There is no drywall on the ceiling. The entire roof probably weighs 1000 lbs. I estimated the weight once and forget the figure but I was surprised by how light the roof structure is. Actually, the entire existing structure is extremely lightweight.

I'll address some of your concerns. I can mediate the below grade issue in the area where it is a problem (15% of the perimeter) by some grading and drainage. I feel grading and drainage coupled with a perimeter curb of either CMUs or poured concrete will address the ingress of water. The cabin will have to be lifted to allow space for the curb.

I recognize the need to vent the area below the sub-floor if I construct a subfloor. I believe that venting the space in the same manner that crawl spaces are vented in this area would be adequate. I also intend to seal the slab with an epoxy topcoat (or similar) to eliminate moisture coming through the slab.

I plan to renovate with standard walls sheathed in OSB, sided with hardi-plank, insulated, and finished with drywall. The roof will be replaced with a ridge beam, rafters, OSB sheathing, standing seam metal, insulation and drywall (or T&G pine) to create a vaulted ceiling. The roof may need special venting details - I need to research that. The wiring will be removed and replaced per code. The plumbing (steel pipe embedded in the slab) will be abandoned and replaced with PEX and PVC per code.

So I have thought through the entire project pretty thoroughly. Yet, ideas to evaluate and structurally reinforce the slab are what I need since concrete isn't my specialty.

 

[Eng16080]

Under normal circumstances, considering that you intend to replace the roof, modify the walls, and potentially replace the slab/foundation, as well as provide new plumbing and I assume electric, it’s usually more economical to demolish and rebuild. It sounds like you’re restricted from doing that though.

Replacing the “1,000 lb” roof with what you describe as well as the modification to the walls will likely add a lot of weight to the structure and it might be necessary to increase the size of the slab/foundation along the perimeter.

If your only concern had been the slab and you weren’t making these other changes, I might take a more pragmatic approach and simply poor a new slab on top to make up the height difference to grade. Maybe also add a layer of crushed stone and vapor barrier between. Then sawcut the new concrete at a controlled spacing to minimize cracking. I wouldn’t add reinforcement nor would I be too concerned with what’s in the existing slab. I don’t really consider a slab on grade like this a structural element. It’s more just a smooth surface on top of the ground.

I wouldn’t construct a new framed floor over the slab. That seems pointless to me, unless you need to have utilities below the floor and the crawl space will be elevated enough to provide access.

Just my two cents. Hope that helps.

 

[PE_JRM]

Eng16080, you are correct that I am not permitted to demolish the building. It is grandfathered and once its gone it can't be rebuilt. I picked the place up for almost nothing and was attracted to the engineering challenge of fixing it. The building is small enough (like a large garage) that re-building it without tearing it down is possible without spending a lot of money or too much effort.

I like your double slab concept. Its actually the first idea I considered months ago before buying the building. Your idea to place some gravel and a moisture barrier between the old and new slab is something I hadn't thought of. Good idea. Finding a crew skilled enough to produce a flat and level slab is an issue that concerns me. Contractors in my area are quick to promise whatever you want and they don't actually deliver. A poorly finished slab isn't easily fixed. That said it may be my best option.

The benefit of a subfloor is that it isn't a slab, Slabs are very unpleasant surfaces in my opinion for a living space since they are too hard. A subfloor also provides A LOT of flexibility with regard to running plumbing and electrical. However, the flexibility is offset by the need to properly vent the space which might prove to be difficult.

I'm actually a structural Engineer. I need to change that designation in my profile if it's possible.

 

[Greenalleycat]

I misinterpreted your 'code doesn't apply' statement - no offense intended.
It would be helpful to know whether any seismic etc provisions apply for this one

I was also favouring the slab option here but I would personally always reinforce the slab
Howver, I have worked on a job once that created a subfloor over a slab
It was a new build and they wanted the depth to run services and have a non-concrete surface to walk on
I can't remember exactly the details but we basically just laid 90x45 joists onto the slab and bolted them down and created a false floor
It was lucky that we were doing that because the slab contractor on that job was one of the worst I have ever encountered.....

Something I'm struggling with is how the edge detailing will look like here
Are you planning to disconnect the existing framing and lift them up onto your new floor?
If not, how are you going to do the perimeter detail
A subfloor may be better than slab to prevent moisture leaching from the new concrete to existing walls if you are retaining the walls onto the existing slab

 

[PE_JRM]

Greenalleycat, I don't really have a plan. I'm trying to optimize some of the ideas I've hadI'm not offended by the code comment, BTW but code limits creativity and I mentioned it for that reason. I need some creative ideas.

I posted this question to see if anyone has better ideas than I've already considered or has better tweaks to the ideas. The goal is to do it cheaply because I'm the one paying the bill. So far I have two possible ways to go. 1) Put in a subfloor. In this case I would put in a perimeter block wall or poured concrete rim 6 to 12 inches tall and raise the subfloor to that level. 2) Pour a new slab on top of the old one. I'm guessing the cost of these alternatives would be a wash although I can do the floor framing myself and control the levelness of the result. Concrete is always a gamble around here (Atlanta, GA). The cabin has a lot of rot in the frame and I intend to replace all of the frame four feet at a time.

The frame replacement plan is to temporarily support the walls with "structural stilts", temporarily attach some CDX sheets all around the perimeter to evenly transfer the shear load and then cut off the bottom foot of the wall framing to make room for the new slab or rim curb. I'll make some 4x8 modular sheathed panels in my garage that can be transported to the site and tipped up into place. Since the roof is supported by purlins that run the length of the building (supported by the gable ends) tearing out wall sections on the long walls will be easy and I can do few of them at a time. I'll have to jack the roof a foot to do this but its not heavy and it won't be difficult. Once the roof is jacked the gable ends can be replaced pretty easily.

You might be interested to know that most commercial service concrete runways and aprons are not reinforced. They are pretty thick and the have steel dowels that transfer the shear shear at the construction joints. I was shocked by this when I went to work for the FAA - its a different way of thinking.

If you know how to get my designation changed from civil/environmental to structural please let me know. I signed up late at night and chose the wrong designation to describe me. I get easily bored and have designed a pretty big variety of things over the years. Among them are municipal water storage vessels, oil storage vessels, refinery process equipment, pressure vessels, vacuum vessels, penstocks, smokestacks, a rocket nose, aircraft wing and fuselage parts, crack growth prediction in aircraft, aircraft landing gear parts, airport infrastructure projects, etc. Civil Engineering prepared me to do all of those and never get bored. I feel lucky that I got to do it.

 

[Greenalleycat]

I can't see how to change the discipline either sorry

This project is starting to make a bit more sense to me now.
Block could be a goer if you back yourself to lay them
You could setup a perimeter wall 1-2 courses high and epoxy starters into the slab
Internally you could do the same thing to support bearers - pseudo piles onto your slab
Use some sort of galvanised bracket to bolt into the top of the pile then screw into the bearer (don't forget your DPC between timber and concrete!!)

I'm surprised to learn that about runways - I always assumed they were heavily reinforced
My reluctance comes from my practice area - I do a lot of work with earthquake damaged houses
I've seen the performance difference between reinforced and unreinforced slabs both in terms of damage to house + superstructure, and also the insurance complications with unreinforced slabs
However, there are definitely region-specific concerns there that may not apply to you

 

[PE_JRM]

What you describe for the subfloor support is what I've been thinking also. I don't like the fact that there will be hundreds of what you describe as pseudo-piles. What I do like is that they could potentially even be pressure treated two by fours since they would be short with galvanized simpson strong tie brackets to tie to the floor joists and slab. A support system like this approximates a distributed load yet allows for ventilation below the subfloor.

Additionally, I could level up the floor joist and cut each 2x4 to match the existing elevation of the slab....each one would be quickly custom fit. This would be an easy DIY to construct. It seems a bit second rate but I think it would do the job.

Regarding seismic...Atlanta, Georgia is in seismic zone 2 (I think) and seismic just isn't a governing design issue here (usually).

 

[Greenalleycat]

Standard practice here is for timber piles to be cast into unreinforced concrete - you wouldn't really be deviating from that
I don't see the concern with blocks - you'd use 200x200 or 400x200 blocks so that has a much larger bearing area than a standard 125sq pile
Obviously none of this is ideal but it's a quirky project so you end up with some funky solutions
Mortar joints give you some means to level the floor at the end too which is helpful

I wouldn't have thought you'd need too many - I'd expect to have 2/ 140x45 bearers spanning between piles at 1.65m crs, spaced ~1.2m apart with 90x45 joists spanning across min. two bearer lines

There are some code challenges to work through but maybe your jurisdiction doesn't care too much
For instance, subfloor bracing - now that you have an elevated floor you need to brace it for EQ loading I assume
I'm not sure of your dimensions and whether it's feasible to argue that the floor diaphragm spans it between end walls
If not, you'll need to allow for some sort of internal bracing - this is where epoxying a starter in for your block piles would allow you to put a number to each one as a mini cantilever
It'd be a bit of a funky solution but I imagine the loads will be low enough that it works

Another challenge is to consider geotechnical conditions
If you're on solid ground it's probably fine, but if you're on soft stuff then you're changing from low distributed loads to concentrated point loads under whatever pile support you're using
You'll need to assume some sort of soil properties to satisfy yourself that bearing works - you also have some risk of punching locally around your slab over time if the soil is soft
I don't know whether your local authority will require you to demonstrate any geotechnical testing but be aware that this could be an issue

 

[PE_JRM]

Greenalleycat, what you describe is slightly different than what I thought you meant originally. When I get time I'll sketch something up, post it and see if we are talking about the same thing. Thanks for your comments. You have helped me focus my thoughts enough to make a first stab at a plan.

 

[Eng16080]

PE_JRM, If getting contractors to pour a quality slab isn’t possible, then that’s certainly a good reason to construct an elevated floor. I can understand the desire to do something yourself as you know you won’t cut corners!

Using a self-leveling cement is perhaps another option which could be used to try to correct a less than perfect poured slab. I’ve never used it myself but have heard good things about the product.

Good luck with your project. It seems you have a good handle on things.