Expand existing concrete slab

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Old 10-22-15, 07:46 AM
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Expand existing concrete slab

There is an existing 8x8 concrete slab at a house that I just bought. I want to expand it to 12x12, with plans to build a pizza oven & BBQ pit on one end = a lot of weight. Wondering if/how I can do this to prevent the slab from separating at the expansion joint and destroy my grill. All thoughts appreciated.
 
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Old 10-22-15, 08:06 AM
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First thing is, do you have plans for the pizza oven and the fire pit? Also where will they sit on you new plan for the patio extension?
 
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Old 10-22-15, 08:30 AM
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I do have plans, and yes, the grill structure will run the length of one edge of the expanded slab (I assume this is what you are thinking). Since I will expand 2 perpendicular edges of the slab, there is no way to avoid a newly expanded section edge. My thought is to build on one of the 2 edges with 8ft of existing slab. Wondering though, to do my best to prevent separation of the extensions: 1. how to prep the ground, 2. how to tie rebar into the edge of the existing slab, if this I even the route to go. Again, thanks
 
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Old 10-22-15, 09:22 AM
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Your climate looks to be moderate so probably not a lot of frost issues, unless you are in a really cold area of VA.
How thick is the current slab.

What are the soil conditions under the slab and where you will be adding the extensions. I know, difficult to look under the slab, but if they installed a gravel base, that gravel should have extended beyond the existing slab. dig down below the bottom of the existing slab. That may tell you whether they just poured in over the lawn or if they excavated some distance and compacted a gravel base. If your new extensions match the old method, then better chance they will move together.

You will probably be able to tie them together, but the bigger problem will be getting them to move up and down the same amount.

Others (along with landfillwizzard) with more knowledge on this issue will be along.

Bud
 
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Old 10-22-15, 09:27 AM
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First things first--You should evaluate the slab to make sure it's thick and sound enough to support the weight your new grill will be adding. If it's less than 4" thick, and already shows cracking, you can probably expect more cracking to take place. Unless you locate the grill to rest entirely on the new slab, which you should make at least 5" thick. Anchored to the existing slab with a few doweled rebar, to minimize separation.
 
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Old 10-22-15, 09:46 AM
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The slab is 4" thick and no cracks. The soil underneath is a clay loam, pretty solid. Unfortunately, it is on a slope with high side even with or even lower than ground level. On the low side, the slab is on top of the ground. I see no way to make the grill completely on either the new or the existing slabs. Intend to put most of it on the existing slab, with the lighter end (work table with brick pillar corners) on the new part. There is some gravel underneath, but can tell how much, or the extent of reinforcement within the slab.
 
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Old 10-22-15, 10:31 AM
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Although it is only 8x8, the fact that it has not cracked is good. Since it has had time to settle over the years, I would make sure the new addition is resting upon a compacted gravel base. Drill some 1/2" holes horizontally into the edge of the existing slab, at least every 2', 16" would be better. You could go with 3/8" if 1/2 would be too difficult. #3 rebar for 3/8" and #4 rebar for 4/8". The rebar number is the number of 1/8th increments. Some extra rebar for the new addition would be good as well. Corrections and additions from the pros is welcome.

Bud
 
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Old 10-22-15, 11:12 AM
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Drill the holes how deep into the edge of the slab? You wouldn't be concerned with spalling?
 
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Old 10-22-15, 01:45 PM
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IF the weight of this new addition & its static load's significant, i wouldn't use tie bars into any existing conc,,, i'd build it as a ' stand alone ' structure,,, excavate as rqd, place base material, compact, form, & place conc

sidenote - holes for load transfer devices in conc highways aren't drilled 2'0 deep & they're 1 1/4" in diameter as i recall
 
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