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Old 03-02-2010, 08:13 PM   #1
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Proper concrete for footing and stem wall


I'm currently remodeling a 5.5' x 12' bathroom. I need to pour a footing and then a stem wall along side of the existing foundation. The loads on the foundation are not that much and I've had a structural engineer take a look at it. I'm just waiting on the final assessment. I'm going to do this properly and to code. My very conservative estimate of the amount of concrete I will need for the project 4 cubic yards. I believe I will be completing this project in 6 sections of 3 feet because I will be digging out underneath the existing foundation as if I were underpinning. Due to the small amount of concrete needed for each section, purchasing ready mix concrete might not be possible.

My question: If I can't order ready mix concrete, how much of a quality difference is mixing my own concrete or using high strength concrete such as Quickrete 5000? Since I will likely be doing this in sections, I know there could be a problem with the consistency of each section. But in an area with very little load and very stable soil, how much difference is there between the three options? Is there anyone that would recommend that I do not use Quickrete 5000 for this project? Thanks in advance.

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Old 03-02-2010, 08:25 PM   #2
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Proper concrete for footing and stem wall


The question is not the material choice - 5,000 far surpasses what you need - but the method of pouring.

You will have 'cold joints', the negatives of which can be largely negated by the use of strong and effective 'keying'. But the problem will come from continuious rebar pieces that will be exposed to moisture, and subject to failure due to eventual rust.

The more logical and secure process might be to be certain all your rebar is contained within the confines of each section of footing replaced. Basically pouring a series of individual 'pad' footings.

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Old 03-02-2010, 10:57 PM   #3
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Proper concrete for footing and stem wall


Thanks for the response Willie. My plan- which is extremely ambitious- is to do a new section each week. Although I haven't taken into account the curing process and if that time frame is possible. But any rate, my plan is to replace each section in the quickest possible time. Do you think that if this is done quickly and the sections of rebar are exposed for the shortest possible amount of time, I will still have problems with moisture negatively effecting the strength?
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Old 03-02-2010, 11:19 PM   #4
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Proper concrete for footing and stem wall


Yes Jay, even if you pour it in lifts a couple hours apart you'll still have cold joints between the pours. The concrete hydrates and begins to cure (and form a crystalline structural bond) immediately when it is placed. Once the water is hydrated out of the concrete, concrete poured against the "dry" or drying concrete will not bond like it should....Cold joint. No way to avoid it other than to pour the footing in one pour and then the wall on top of the footing in another pour. It is commonplace for a wall to be poured on top of a footing as early as a day after placement of the footing. The bond between the footing and the wall isn't critical...You should at least form a keyway in the footing (with a 2x4) to receive part of the bottom of the wall and help with lateral stability of what is effectively a horizontal intentional cold joint. A rubberized waterstop poured into the footing is another option but not as common.

Plan on pouring the footing in one pour and plan on pouring the wall in another pour. Otherwise you're guaranteed a weakened job.

4 yards is normally available from ready-mix plants. They may charge more since it is a short load, but they'll do it. Many batch plants also offer you-cart concrete in a trailer that you pick up and haul to the site. A really great option is site-mixed concrete that is batched on a truck...Many rental companies and some concrete companies offer this service for jobs such as yours. Last time I used it they charged about $100 per yard.

4 cubic yards is 140 bags of concrete at 2/3 cubic foot per bag. That's a heck of a lot of money and it is a heck of a lot of mixing by hand. You'll regret doing bag mix on a job like this.
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Old 03-02-2010, 11:32 PM   #5
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Proper concrete for footing and stem wall


Well it sounds like doing this in sections isn't such a good idea. I think the problem I'm going to run into with with my initial idea of digging out underneath the existing foundation and semi underpin and then pouring a footing and stem wall in one section is that the foundation will certainly cave in if it isn't done in sections at a time. The existing foundation is a 16 inch concrete slab over buried bricks (house was built in 1896).

I thought about cart concrete being the best option. I just haven't looked into the availibility of it in my area.
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Old 03-03-2010, 06:59 AM   #6
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Proper concrete for footing and stem wall


I did around 20 80 lbs bags 2x - hot tub pad & shed "footing"
It was a PIA both times & back breaking work
I can't imagine doing 140 bags
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Old 03-03-2010, 07:03 AM   #7
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Proper concrete for footing and stem wall


people underpin large old structures all the time with rebar exposed and cold joints. use some bonding agents at cold joints
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Old 03-03-2010, 10:08 AM   #8
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Proper concrete for footing and stem wall


I'm imagining 140 bags would not be fun. My thought was bags would be easier if I were to do this project in sections. All of your responses have been extremely helpful and have enabled me to do addtional searches and learn more about what I'm trying to accomplish. The idea of a series of pad footings seems to be a viable option. Is it fair to see that a series of pad footings would create a more stable footing and stem well than joining cold joints? Given the great soil stability in my area, I would think this could be the better option. Thanks again for the information. This will be very helpful for me when my structural engineer gives me his final assessment.
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Old 03-03-2010, 01:48 PM   #9
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Proper concrete for footing and stem wall


At my concrete company, we have a 1 yard minimum. You'll get a short load charge, but its worth every penny if the alternative is mixing 140 bags of concrete. I've been to jobs before where they pour the footing, wait about 45 minutes to an hour, then wet the rest of the load up and pour a wall. Doesn't happen too often, but it can be done. Only reason to not do it all in one shot is that the concrete will ooze out of the top of the footing form. If you take steps to eliminate that as a factor, you can do it all at once.

Just make sure your mix has air entrainment. Its not necessary for the footing, but it is for the wall, and won't hurt anything. A 3000 psi mix should be sufficient.
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Old 03-03-2010, 08:41 PM   #10
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Proper concrete for footing and stem wall


If you decide to mix it yourself based on what your engineer tell you, I'd suggest getting a mixer and sell it afterwards on CL. I can do about 20x80# bags in 2 hours using my Kushlan. Make sure you're not placing it in freezing weather. I don't mind it so much as I consider it a gym membership.

I would definitely go the trailer route if I had a truck big enough to bring it home.
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Old 03-03-2010, 11:52 PM   #11
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Proper concrete for footing and stem wall


Here's an idea of how you would set up each of the sections... following the numbers, so they will 'KEY' into one another. Of course these are shown separated by10" or so. In reality, they will fit (pour) right into one another.

The 'OPEN-SIDED' sections show where the rebar would run (chairs and cross ties not shown)

As you can see, after the 'odd' numbered sections are poured and stripped, only the sides of the 'even' numbered sections require forms because the hardened parts of the 'odd' sections will make the ends of the forms.

BTW....... These CAN be formed a little differently so that you can just keep them going, one right after the other. But this way allows you to leave the original support walls in place for every other section till you get the new walls built.
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Proper concrete for footing and stem wall-foundation-sections-1.jpg   Proper concrete for footing and stem wall-foundation-sections-2.jpg   Proper concrete for footing and stem wall-foundation-sections-3.jpg  
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Old 03-04-2010, 12:48 PM   #12
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Proper concrete for footing and stem wall


Wow. Thanks for the visual WillieT. This definitely makes much more sense in my head now. One follow up. What you are describing is for the footing and stem wall together? I'm looking at probably a 50 inch stem wall.
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Old 03-04-2010, 02:05 PM   #13
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Proper concrete for footing and stem wall


perfect willie do you have a program that lets you do those or is it from a library of material?
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Old 03-04-2010, 02:05 PM   #14
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Proper concrete for footing and stem wall


This will give you an idea of how you leave alternating sections of old supporting block in place while you lay up the new wall in between.

You just make sure of two things.

One: You make your footing lengths appropiately spaced multiples of 16" so that they will fully support the new block walls.

Two: You space your new blockwork accurately so you will be able to fill in the missing areas perfectly. (Allow for the mortar joints on each end of the blocks.)

It may take some study to grasp all this.

Also, the alternate way of forming the footing sections (I mentioned it above) is shown in the foreground.
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Old 03-04-2010, 02:07 PM   #15
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Proper concrete for footing and stem wall


Quote:
Originally Posted by tpolk View Post
perfect willie do you have a program that lets you do those or is it from a library of material?
I just use SketchUp (the free version). I drew the foundations, but imported the block wall from a library.

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