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Old 09-04-2012, 09:57 AM   #1
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Hi guys,
I have been reading this forum for awhile but this is my first post. I have excavated a 3 foot by 18 foot by 8" deep section off the edge of my existing patio to extend it outwards. The patio also butts up next to my house. I have a few questions that I can't seem to find answers to.

1. Everything I read says use gravel but my father seems to think that using red clay, which we have an abundance of here in south Louisiana, as a 4" base for this small addition should be OK. Is this suitable?

2. The existing patio comes up to about even with the existing soil line so my forms will be start at the same level down in the excavation. Should I put the forms in first and then put the base in or visa versa?

3. Having to do with the last question, I understand that you want a level base and I need to create a pitch with the forms but that is confusing to me because it would make my concrete thinner as the patio went out and at a length of 18 feet, almost disappear. Should the sub base be sloped as well?

4. This extends question 2 as well. Do the forms have to sit flush with the base or can there be a little gap. Unless I am sloping the subbase some with the forms, it seems that this is almost impossible. I am assuming that the concrete wont flow out underneath the forms.

Anyway, any help for these questions would be very much appreciated.

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Old 09-04-2012, 08:36 PM   #2
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1. Clay isn't stable when exposed to moisture - a compacted gravel base is the proper way to do it.
2. Gravel base first, then build your forms on top the base. Make sure to extend your base about 6" past the perimeter where the forms will be.
3. With the minimal distances you are talking, I would pitch the forms. A 2% slope over that area won't amount to much to where the base should be adjusted.
4. There can be a small gap at the base of the form. A little concrete will ooze out, but you should be fine.

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Old 09-05-2012, 09:41 PM   #3
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Quote:
Originally Posted by cpusl View Post
Hi guys,
I have been reading this forum for awhile but this is my first post. I have excavated a 3 foot by 18 foot by 8" deep section off the edge of my existing patio to extend it outwards. The patio also butts up next to my house. I have a few questions that I can't seem to find answers to.

1. Everything I read says use gravel but my father seems to think that using red clay, which we have an abundance of here in south Louisiana, as a 4" base for this small addition should be OK. Is this suitable?

This is one of those area's that could qualify as "dollar foolish". You really need to just spend the few dollars on gravel.

2. The existing patio comes up to about even with the existing soil line so my forms will be start at the same level down in the excavation. Should I put the forms in first and then put the base in or visa versa?

We always form first, unless we have a few lifts of gravel that needs to be compacted, but for a few inches, it's easier to use the set height of the forms to establish the gravel height.

3. Having to do with the last question, I understand that you want a level base and I need to create a pitch with the forms but that is confusing to me because it would make my concrete thinner as the patio went out and at a length of 18 feet, almost disappear. Should the sub base be sloped as well?

You pitch the subgrade at the same pitch as the finished concrete, which will give you a uniform thickness regardless of the slab dimensions.

4. This extends question 2 as well. Do the forms have to sit flush with the base or can there be a little gap. Unless I am sloping the subbase some with the forms, it seems that this is almost impossible. I am assuming that the concrete wont flow out underneath the forms.

Anyway, any help for these questions would be very much appreciated.
Small gaps are no problem. We pour most 5" thick flatwork with 3.5" forming material with little problem, even with a vibrating screed at times.........
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Old 09-06-2012, 07:26 PM   #4
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If you have a bunch of concrete oozing out the forms with that sort of gap, you're pouring it waaaaayyyyy too wet.
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