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Old 08-16-2016, 11:12 AM   #1
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Concrete Slab Phantom Front


We are hiring a concrete patio poured in our back yard. The contractor has framed it up and brought in gravel to prepare for the pour, but I don't understand what he's doing. He framed an external structure and an internal structure of wood that he backfilled inside of. That internal structure leaves a 9" wide by 9" deep "footing" to the outside at 3 of the 4 sides of the patio where the yard falls away. He plans on pouring the full patio which will encapsulate the internal wood structure. When I called out that it will rot out, he informed me that was to plan and would rot out in 6-7 years leaving an intentional void. He called it a "phantom front" or something like that.I can't find anything by that name on the web. So when it's finished we will eventually have a 4" thick slab with a roughly 1.75" wide x 9" deep void under it on 3 sides. Is this a real technique? Can someone explain the purpose of this practice? I am attaching a couple of pics. Thanks in advance. It's our first home so we have lots of DIY, lots of hiring, and lots of questions!
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Concrete Slab Phantom Front-20160816_102841.jpg   Concrete Slab Phantom Front-20160816_103008.jpg  
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Old 08-16-2016, 11:29 AM   #2
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Re: Concrete Slab Phantom Front


Not my area of expertise but I certainly understand your hesitation....
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Old 08-16-2016, 03:47 PM   #3
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Re: Concrete Slab Phantom Front


Never seen it done like that. The ones I have seen just piled the stone and sloped it. No wood frame to hold it.
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Old 08-16-2016, 05:04 PM   #4
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Re: Concrete Slab Phantom Front


Normal concrete should be a minimum of 3" thick, and really 4" thick. His idea is OK, but not normal, and voids under concrete are often done intentionally, but I would insist on a minimum edge thickness of 4".
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Old 08-16-2016, 05:06 PM   #5
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Re: Concrete Slab Phantom Front


Ayuh,.... No idea what yer contractor is doin',.....

Never seen such a thing,...

Like Joed says,...

We form up the outside, sculpture the compacted stone inside it, lay the steel, 'n pour the concrete,....

Looks like a future fracture line to me,....
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Old 08-16-2016, 05:23 PM   #6
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Re: Concrete Slab Phantom Front


Also, the way he has it formed, it is creating a fracture plane right at the worst place, the edge. His "phantom front" is likely to crack off as formed.
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Old 08-16-2016, 05:55 PM   #7
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Re: Concrete Slab Phantom Front


I've worked with at least 6 different concrete finishers on at least 100 different slabs and I've never seen it done that way.
Just does not make logical since to me.
I like to see a turned down edge on a slab to make it stronger and less chance of it washing out.
Sort of like this.
https://www.google.com/search?q=turn...RV0aKYtW4eM%3A
No way would I except what he has done.
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Old 08-16-2016, 06:27 PM   #8
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Re: Concrete Slab Phantom Front


I never thought about the cracking but I agree. Any square sharp corner is going to crack.

This is what I would expect to see.

Concrete Slab Phantom Front-uyttebroek2.jpg
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Old 08-16-2016, 08:29 PM   #9
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Re: Concrete Slab Phantom Front


35 years in the ready-mix business, and never seen anything framed like that.
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Old 08-17-2016, 08:09 PM   #10
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Re: Concrete Slab Phantom Front


I also poured a lot of slabs as a mixer driver, don't remember ever seeing that. Not even sure of the logic behind it.
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Old 08-17-2016, 09:25 PM   #11
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Re: Concrete Slab Phantom Front


Like everyone else is that I've never seen anything like that at all in residential or industrial situations. I certainly wouldn't accept it as a normal procedure. Your profile didn't state what part of the country you're in. If you are in an area where frost is a consideration it's even worse!

What joed showed in the picture you posted is what I seen everywhere that I've been around.
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