Small Suspended 3" thick Concrete Slab Countertop; Technical Question 6

[steve41337]

My Engineering license and experience is in Mechanical Engineering and HVAC. I'm also a licensed General Contractor working on an unusual project pouring in place two office desks shown in the attachement provided below. The countertop at the top of the page is my bigger concern of the two. Looking at that slab you will notice it is supported on the left side by a built-in cabinet (no problem for that cabinet to support the weight of the concrete). Along the back wall and right right side wall there will be short pony walls that will also adequately support the weight of the concrete pour along these adjacent wall perimeters. By design there are no other supports for the L shaped countertop. The back wall and right side wall are masonry bock so it is possible to anchor rebar into these walls. My instinct is to tie into the back and right wall with #3 rebar on 4" centers. I know the pour will not fall down but I'm not sure it will not crack because of the large unsupported area of concrete countertop where there is no direct support.

I have two questions: 1) is there any advantage to anchoring the 4" x 4" rebar lattice to the adjacent masonry walls behind and to the right of the concrete pour? And 2) Assuming no more than 2 men each weighing 200 pound sit in the edge of the approximate center of this slab how likely is it to crack somewhere; and what psi concrete would be required to make this monolithic pour concrete slab rigid enough transfer its loaded weight to the three perimeter sides without cracking anywhere? Obviously smacking it with a sledge hammer in the right place would easily crack it but I'm not concerned to make is resilient to that kind of abuse.

The bottom of the pour frame will be lined with 1/2" cement board or plywood (supported underneath during pour and curing) with a finished 3.5" front edge (3" concrete depth poured above 1/2" cement board with cement board abandoned in place after the pour. In other words the abandoned cement board will not be visible from the front edge of the concrete pour where a full 3.5" of concrete will be visible (everywhere else the concrete pour is 3" deep on top of 1/2" cement board abandoned in place). My gut instinct is that a 4" x 4" #3 rebar lattice poured with 5000 psi concrete mix 3" deep (plus some glass fiber to further increase concrete strength) should be strong enough to resist any cracking. But I can't say that for sure.

 

[bimr]

There are many online resources for information on concrete countertops.

I would suggest that the support under the countertop should be 3/4 inch plywood. Cement board is not structural.

http://www.finehomebuilding.com/slideshow/9-design-ideas-tips-for-concrete-countertops.aspx

http://training.concreteexchange.com/c/Concrete_Countertops

 

[steve41337]

I have to laugh at your reply. And unfortunately you have not helped. As far as my question goes; 1/2" cement board is more structural than 3/4" plywood because the concrete pour will bond with the cement board. I did not comment but there will be an extensive temporary support system under the 1/2" cheating during the pour and during concrete curing. But seriously, the engineering I am after does not depend on my form lining as far as I know and you have not provided any technical comments. I have done many concrete countertops and I am very familiar with the concrete countertop online forums but I assure you my project details are outside typical countertop designs and these forums. That is why I am here asking my question.

In case you didn't know typically speaking 100% of poured in place concrete countertops are always poured on top of a counter. This is not what I am doing with regard to the question I asked here on this forum.

I appreciate you took time to post a reply . . . but honestly, there was no reason for you to reply the way you did. I'm sorry to say your comments are worthless. Give me a little credit for being a licensed engineer (but lacking in civil engineering needed here) and a licensed general contractor to find my way here to ask my question.

Steve

 

[oldestguy]

Good evening Steve: One of the common faults with original posts here is that the original posts frequently do not give complete information. Had you said you have considerable experience with concrete counter tops, I probably would have responded with "Why are you here?". So, being that you didn't give a complete description of the past experience and since the prime purpose here is to provide tips for those lacking experience, the response you received was proper.

 

[dhengr]

THE END.... We got no more for ya, Stevie, just keep on laughin.

 

[steve41337]

Thanks for your reply oldestguy. I provided a to scale drawing and construction details along specific questions. Let me know what was lacking in the original post.

 

[bimr]

Someone that does not know the difference between pouring and placing concrete would be better served to purchase the countertop from a professional:

http://www.trueformconcrete.com/

 

[jayrod12]

In all seriousness, you're asking for a 3" thick slab to cantilever 2 1/2 feet from a block wall that is likely not reinforced. There's barely enough room to get rebar in the slab since you'll need it both ways in some areas.

I can only see this working if you somehow created a connection to the existing block walls that could somehow develop full rebar tension.

Maybe you'll get someone here that has made this work before. The problem is the majority of us haven't and wouldn't design something like this, because for actual structural loads it would never work. That goes for most countertops and cabinetry.

 

[steve41337]

I thought this was a forum for engineers by engineers who comment when they have relevant background and experience? I realize the person I'm looking for may not be here; in fact that seems very likely at this point based on comments so far. I'd be happy to pay an experienced engineer here qualified to do a simple analysis to answer a few simple questions.

 

[steve41337]

Hi Jayrod

I've used #3 rebar 4" o.c. in 3" cantilevered slabs before. There's enough room in the form to get a good pour and I was planning to go both directions everywhere in the form. Unfortunately I have nothing on the left side to tie the rebar into (just a large surface areas support under); but can get a good connection to the block wall behind and on the right side. I'm looking for someone to crank some number to see how likely the slab will crack.

 

[jayrod12]

100% it will crack.

The question is whether the cracking will be large enough to be noticeable.

And as I said, providing the rebar with enough anchorage in the existing wall will be the most difficult part. I'm not sure how you would accomplish that without breaking out a portion of the block and hopefully threading in a rod with a reasonably long bend and grouting a lot of cores below. I'm guessing the slab will be about 30" off the floor as well. If it were my job, I would start at providing 30"x30" corner bars into the existing wall and then grouting the entire 4 cores below the slab. And without running any numbers I'm still not comfortable that would work. The existing wall might not have the lateral capacity to deal with the concentrated moment that the counter will subject it to.

No one here is likely going to actually run any numbers, but we might give you our feel on the project (as I have done). It's up to you to actually determine if the wall is adequate and the connections will work. If you aren't comfortable (or are unable) to make those decisions, then you must find and retain an engineer who will. And you are best off finding someone local to you for that for convenience.

 

[cvg]

anchoring the slab to the wall does not make the slab stiffer or prevent the slab from cracking, it only holds it tight to the wall and prevents it from tipping off. if you are able to anchor it well enough, you will then be putting stress on the blocks in the wall which might damage the blocks. in order to prevent the slab from bending and cracking, either make it stiffer or support it on the free / front edge so it is not cantilevered. run your own numbers to determine the allowable bending moment the slab can handle

 

[oldestguy]

OG once more. The actual design of some project is not what this forum is for. The tips are suggestions not design guides.

More of what Steve should have provided is the purpose of this slab. What sort of loading? Will it hold a person standing or sitting on it? Is any special precaution for acid or base activities required? Is some special finish possibly needed? Tints? Air entrainment? Spec for mix? Stainless re-bar?

 

[racookpe1978]

Over 25 years, the poured concrete edge-supported 4 inch thick concrete culvert covers in my neighborhood have sagged more than 1 inch. They are only slightly longer than your countertop.

Can you pour the countertop so it has a heavy lip in back (pointed down perhaps to be out of sight-line) and a lip on the front.

 

[steve41337]

Jared

I really appreciate your comments. Something I could do is build two vertical forms: one next to the cabinet (left end of the slab to the right side of the existing cabinet) and build a 2nd vertical form on the right side of the slab next to the masonry wall. I can anchor rebar to the floor in the bottom of both forms and run rebar vertically up through the form with a 90 bend to tie into the rebar running both directions within the countertop form. To help clarify what I mean the vertical forms would have to completely fill with concrete before the horizontal countertop form could completely fill with concrete. So the wall and countertop get poured together.

I’m wondering if I poured these two vertical forms (walls) with #3 rebar 4” o.c. opposing each other on the right and left end of the countertop if I could do a much less comprehensive job supporting and anchoring the rebar in the back masonry wall. I cannot pour a vertical wall there because the existing pony walls provide access and covers that hide all the typical electric wiring and cables found under office desks. If 30” x 30” corner bars are still required in the back wall what would the minimal spacing be?

I should have clarified I am worried only about cracking large enough to visually look bad. Bottom of the horizontal pour is 26” above the floor so this is how much grouting would have to be done below the slab to anchor the rebar in the wall. Just my instinct that because this is more like a “span” than a cantilever and there is really no load on the countertop like there would be in a building structure it seems this thing should pretty much hang together even if I didn’t anchor the rebar to the back wall (assuming I poured the two vertical walls mentioned).

Agreed actual number crunching calcs are not necessary. I’m so far out of my league with this kind of engineering I was just hoping someone here is familiar enough to help me work this out and prevent me from making a big mistake (by big mistake I mean ending up with a large visible crack somewhere other than next to the back wall).



cvs

I appreciate your effort to educate me. Thank you for your reply along with Jared’s. Both you guys are great. Your explanation about “tipping off” the wall makes sense. If the final horizontal concrete slab was stiff and strong enough then it could easily be supported on three sides and make the required spans without developing a visible crack. Is it your opinion (and Jared’s) that 3” concrete loaded with rebar as I have proposed will immediately develop a visible major crack when supported all along the back, right, and left sides when not tied to reinforced block walls?

Is there a way of determining how far off the mark I am when starting with a 3” pour ( plus 1/2” bonded cement board on the bottom of the pour) using 4” x 4” lattice #3 rebar and 5000 psi concrete with glass fiber? I mean am I a long way away from something strong and rigid enough to not develop a big visible crack if I was simply supporting the slab on back, right and left sides?

I don’t know how to run my own numbers to determine the allowable bending moment the slab can handle which is why I am here. Perhaps if there is a very simple tutorial online somewhere you can point me to or someone can help me through one calculation the first time. Assuming a simplified geometry of 2.5’ x 8’ rectangular slab supported on 3 sides would be a good place to start?

Steve

 

[steve41337]

hi racookpe1978

Yes I could open up the countertop form and pour a lip along the back edge over the entire length. But I already have a 2" X 4" pony wall supporting the back edge of the concrete form that will always remain in place. Are you thinking I need to pour a thick back lip in addition to this pony wall support?

appreciate your input.

Steve

 

[steve41337]

I wish there was a way to go back and edit pervious posts . . .

cvg I was obviously referring to you in my previous reply above; see (cvs) typo

racookpe1978 I can see how adding a deeper ridge in the bottom of the countertop form near the front edge (or even in the middle) would strengthen the final slab and make it more rigid. Unfortunately adding anything lower here (closer to the floor) will interfere with anyone sitting and working at the desk (by bumping their knees into it).

 

[bimr]

If you took out the non-structural cement board, you would be able to add thicker #4 bars and concrete. What is the purpose of the cement board? Are you thinking that you would not have to sand the bottom?

There is no method to model the combined thickness of the cement board bonded to the concrete.

 

[steve41337]

The cement board (or plywood is another option) was intended to support the concrete pour and then be abandoned in place. I can pour the slab thicker than the proposed 3" (1/2" thicker on the front edge to hide plywood or cement board would make a 3.5" front edge). If analysis says a thicker slab is required that is possible to do.

 

[steve41337]

What would the minimum thickness of the pour need to be be to accommodate 1/2" rebar on 6" centers (in both directions)? How likely would it be to have a crack show up doing that with 5000 psi concrete delivered with glass fiber added? I don't know how much the glass fiber increases 5000 psi concrete. Can anyone help answer this question? If no one can help without charging me analysis time please private message me.