[Badfish740]

I am in the process of finishing a built in entertainment center in the basement consisting of an alcove in the wall where the flat screen TV will hang flanked on either side by built in bookcases. Underneath the TV there will be a built in that will house the satellite boxes, home theater PC, etc... Here is a pic of the unfinished space:

The area underneath of the TV needs something to finish off the top and I was thinking of something kind of like a hearth like you'd see on a large fireplace. I thought about trying to get a piece of stone cut to fit it but it would likely be a lot more than I want to spend. Then I started thinking about concrete. Since this is such a small job (3 bags of concrete plus some reinforcing wire, anti-crack additives, silicone caulk and melamine for the forms would likely total less than $100) I'm willing to give it a shot. One of these days we're going to remodel our kitchen and I'd love to do concrete countertops, so why not practice with this? The big question is how much can you cantilever with concrete? I want the "hearth" to overhang the support structure underneath by 5" with 5" x 4" "ears" on either side (see graphic below) to finish out the look. Would these be prone to breaking? I'm planning on doing a 2" thick slab so I would have some room to add 3/8" rebar in addition to the reinforcing wire if that would help.

 

[SeniorSitizen]

The cantilever won't be a problem but be sure you bend a piece of rebar around the 90° inside corners at the ears. If there is going to be a crack that's where it will happen.

 

[jomama45]

Like Fairview mentioned, the cantilever is fine, but the 90 degree inside corners are begging to crack and break off. I would undoubtedly put a 3/8" rebar in the "ears", to within an inch of the form. I would also consider radiusing that inside corner as large as you deem possible, as that will ease the chance of cracking. Obviously, with the radius, you'll have to remove material from the existing framed corners to make it fit.......

 

[Daniel Holzman]

The rebar you propose will have no structural effect, since you will need to install it at the neutral axis of the 2 inch thick slab (the midpoint). Even at that, you will not have anything close to recommended minimum clearance between the edges of the bar and the edges of the slab (you will have about 3/4 inch clearance, typical minimum required clearance is 1-1/2 inches to resist rebar corrosion).

However, this is an interior installation, so moisture penetration is not an issue, so the bar can be installed with less than normal clearance. It will not have any structural effect, since the neutral axis has no stress, therefore the bar will do no work. It will keep the countertop together if the top cracks, which I suppose is a plus. As an alternative, you may want to get a good book on constructing concrete countertops, and note the techniques that are used to minimize cracking. I have seen some very attractive concrete tops, I like the look, but it is a bit harder than it sounds to achieve a high quality finish. Good idea to practice on this setup.

 

[jomama45]

The rebar you propose will have no structural effect, since you will need to install it at the neutral axis of the 2 inch thick slab (the midpoint). Even at that, you will not have anything close to recommended minimum clearance between the edges of the bar and the edges of the slab (you will have about 3/4 inch clearance, typical minimum required clearance is 1-1/2 inches to resist rebar corrosion).

However, this is an interior installation, so moisture penetration is not an issue, so the bar can be installed with less than normal clearance.

You seriously just typed all this above, and it has absolutely no bearing on what the poster is trying to do?????

It will not have any structural effect, since the neutral axis has no stress, therefore the bar will do no work. It will keep the countertop together if the top cracks, which I suppose is a plus. As an alternative, you may want to get a good book on constructing concrete countertops, and note the techniques that are used to minimize cracking. I have seen some very attractive concrete tops, I like the look, but it is a bit harder than it sounds to achieve a high quality finish. Good idea to practice on this setup.

Of course it's a "plus", what else would the rebar be in the concrete for?????

 

[stadry]

steel / no steel debate continues even amongst artisans who build conc c-tops as their main work :yes: having done more than 30, we picked carbon fiber over steel - have NOT cast 1 less than 2" thick, tho

 

[MJ Force]

Concrete hearth. 3" thick, 10" cantilever. 22" deep.
This also houses 2 10"subwoofers

 

[MJ Force]

 

[MJ Force]

I built this last year and from what I remember I use 3 bags of redimix with a high mpa. Because of the cantilever I used two large L-Brackets hidden at the rear for support which can be easily removed if I need access to the subwoofers. I shaped the top and sides to my desired texture. I later etched then painted then applied a gloss finish. It's a beautiful piece and some believe its real stone. The hearth acts as a cover for the subwoofers. The chimney chase acts as an infinite baffle sound box. There are two voids on the bottom of the hearth to allow for sound pressure from the subs to escape. A 500w sub amplifier drives the two 10" Concepts. Amazing sound really. Better than I expected.

 

[stadry]

nice work, force - typically any reinforcemt's vertically center'd - looks like expanded wire mesh near your c-tops top,,, we would've not used it,,, again, nice work, tho :thumbup:

 

[Badfish740]

I built this last year and from what I remember I use 3 bags of redimix with a high mpa. Because of the cantilever I used two large L-Brackets hidden at the rear for support which can be easily removed if I need access to the subwoofers. I shaped the top and sides to my desired texture. I later etched then painted then applied a gloss finish. It's a beautiful piece and some believe its real stone. The hearth acts as a cover for the subwoofers. The chimney chase acts as an infinite baffle sound box. There are two voids on the bottom of the hearth to allow for sound pressure from the subs to escape. A 500w sub amplifier drives the two 10" Concepts. Amazing sound really. Better than I expected.

NICE!!! I'll be sure to post pics of mine when it's finished. What does high MPA mean? I was planning on using Quikrete's countertop mix. I should be building my form tonight.

 

[MJ Force]

Mpa is the strength. The Quicrete* for countertops is likely better.
Build on a solid surface where you don't have to move the mold during the curing process. Be sure you can easily remove the mold sides if you plan on shaping or texturing. Use screws to fasten the sides and sufficient support to prevent bulging.

When I released the sides for shaping the surface texture one of the ears developed a small 2" crack. The wire mesh or expanded metal prevented it from breaking off. I had to work the butter a bit in that area. No problem after that.

If your happy with the texture from the mold sides, then leave them in place while everything cures. It was risky for me to remove the sides but I wanted to shape a stone look. Just don't move your project until your confident it has cured.

 

[MJ Force]

Speaker fronts

Ready for hearth

 

[Badfish740]

I stopped into Lowes on my lunchbreak and it will take a month to get the countertop mix They have to wait to include it in a large order. What did you use for a release agent?

 

[MJ Force]

On the plywood I just used vegetable oil spread lightly with a brush. The sides were covered with 3mm poly and had wire imbedded to create a stone look. The wire was laid over the top after the pour an worked in with a flat trowel to simulate a stone pattern.

 

[MJ Force]

More pictures.

 

[MJ Force]

Just remember that it was the thickness 3" and the rebar and mesh that allowed me to cantilever this hearth. It is strong and the large L-Brackets at the rear allow me to stand on the edge with no worries.

 

[Badfish740]

Nice technique-I'm definitely going to be upping the thickness of my slab to 3". I was planning on casting mine upside down and I had a thought. It might make for an interesting stone like texture if I spread sand in the form and made it a bit uneven and then put poly over that-I could even incorporate lines using wire like you did as well. Lots of possibilities...

 

[MJ Force]

After the hearth had cured (2-4weeks) I applied an acid wash then stained with a charcoal colour. I finished that with a clear gloss finish. I suppose you could experiment with pigments for concrete but I needed a specific colour which is why I used a stain.

 

[Badfish740]

Made a template from 3/4" plywood and tested the fit:

I'll eventually cut the piece down and use it as an underlayment for extra support.