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  • Sep 5, 2013, 07:47 PM
    Concrete Slab Repair on Garage with No Foundation
    I have a 12'x20' garage with an off-center door. I am in the process of widening the door to 9' and repairing the cracked, uneven floor under the garage door. My plan was to remove part of the slab and repour so the new door would sit on a nice level surface.

    I thought the garage had a foundation, but after I started jackhammer today, I realized I was wrong!

    As of right now, I removed the concrete up to the major crack (photo to be posted in next reply) inside the garage. I also removed about two feet of the concrete skirt outside the garage to provide room for wooden forms.

    I'm trying to pour a new slab and join it to the old slab but I'm not sure how to support the new slab since there is no foundation. My father's car will be stored in the garage but it will only be driven a couple times per week. When parked, the car will not be sitting on the new slab. I live in southern New York. The garage is not heated. The old slab is about 4"-6" thick in the middle and about 12" thick around the perimeter.

    I've read other posts on this site that said drilling rebar approx. 4" into the old concrete, extending the rebar into the new slab, and connecting the rebar with wire mesh will help keep the slabs together.

    But what can I do to prevent the new slab from sinking since there is no foundation?

    Will bagged concrete mix be strong enough or should I bring a small truck in to pour a stronger mix?
  • Sep 5, 2013, 07:53 PM
    1 Attachment(s)
    Here is the photo I mentioned in the question above. I removed all concrete between the entrance and the large crack in the photo. I'm having my driveway repaved in 2 weeks so I'm praying that I can fix this concrete mess before then. In a perfect world I would just rip down the garage and put in a foundation, but that is not in the budget. I am looking to salvage what's there. I appreciate any and all help. Thanks, Logan.
  • Sep 6, 2013, 05:01 AM
    Using a higher strength mix from a producer (if available in a small quantity- maybe not) will help, but mixing on site with a portable mixer using a 7 cement bag (94 lb bag) per cy or approx.
    1-1/2:2:3 ratio (cement:sand;stone) mix will be stronger. The less water used in the mix, the stronger the mix will be. Curing is important, keeping wet, covered with burlap or similar material for approx. 7 days. Insulating to protect from freezing might already be a concern in your area.
    After removing the old concrete, the stone or soil base should be solid enough to walk on without penetration. If moisture or wet clay is encountered it may be the cause of the original failure and that problem should be addressed before continuing with the work.
    Excavating below the planned bottom of slab and replacing with crushed stone may be an option. Going extra deep with concrete on a bad base will only increase your problem. If you encounter free water, you will need to install a drain to get rid of it, maybe just a stone trench to a lower point would work to get the water away from the slab.
    Mesh will help prevent cracking but connection to the rebars is of little value.
    Drilling into the old slab and using an epoxy or similar compound to anchor the rebars into the old slab will help, not just putting bars into drilled holes of course.
    Lastly, if you are using hot mix asphalt for the driveway paving, you need to keep the rollers off the portion of concrete outside the garage until the concrete has set for about 7 days and that assumes a good curing process as described above.
    One good sign is joining into the old slab at an angle as it prevents both wheels from loading the joint simultaneously.
    My experience is in highway and bridge construction (and inspection) and my recommendations may be too idealistic but get as close as you can if you want to do this once and get it right. Good luck.
  • Sep 6, 2013, 05:15 AM
    Thank you for the reply. Since I broke up the concrete last night, I know free water won't be a problem. I don't own a mixer so I will have to weigh the cost of renting vs. buying from a producer. The last mixer I rented from Home Depot was horrible. Looking at the weather, I might luck out with the concrete if I get it poured this weekend.

    Today, I will be removing the concrete debris, compacting the soil, and then compacting item-4 to create a stable base. How many inches of item-4 should I put down and how thick should I pour the new slab where the slab will meet the newly paved driveway?
  • Sep 6, 2013, 06:13 AM
    Not sure what you mean by item-4. But will assume you mean crushed stone. 6 inches or so for both stone and concrete for a total of 12 inches since it was a problem area and had a severe crack previously, and will be a small independent slab pretty much. Rebar both ways (maybe 6 inch centers) about 2 inches up from the bottom of the concrete will provide a lot of strength even if those aren't the bars tied into the existing concrete.
    And bear in mind that the drilling into the existing concrete for bars depending on the condition and thickness of the old concrete could do more harm than good.
    Also, I am assuming that your driveway is hot mix asphalt (HMA) and not concrete. If the driveway is concrete, you would need to create an expansion joint at one end.
    Edit- see by photo it is HMA- at least on final surface.
  • Sep 6, 2013, 06:24 AM
    Yes, item-4 is crushed stone and driveway will be hot asphalt. When you say "6 inch centers" do you mean placing forming squares with the rebar so that the sides of each square are 6 inches apart?
  • Sep 6, 2013, 06:42 AM
    Yes. Or you could say a checkerboard, or a grid ready for a game of x's and o's on 6 inch open squares, with bars tied with wire at intersections and blocked up (absolute minimum number of blocks to hold the wired up grid 2 inches off the stone). Too many blocks will just weaken the slab.
  • Sep 6, 2013, 06:49 AM
    I just finished clearing away more debris and I looks like the slab is only 4 inches thick in the center. Since this is probably too thin to drill into, is there another way to connect the two slabs? Should I dig a few inches under the old slab to create a shelf with the new concrete?
  • Sep 6, 2013, 12:41 PM
    You could try and find a good concrete epoxy bonding component through a good contractor supply store and paint on the face of the old concrete. Materials similar to this link are used to bond old concrete to new.
    Epoxy Bonding Agent for Concrete - Edison Coatings, Inc.

    Build a shelf like you suggested by maybe going an extra inch deep (assuming you are using a 6 inch slab for the patch) where the new meets the old crack edge. This would create a 3 in. ledge under the old slab at the joint and there should be rebars about mid height in the 3 in. ledge and those bars extending well into the new concrete. Go back under about 6 inches if possible.
    You could create a problem with the old concrete "sitting" on the new concrete for that 6 or so inches where they overlap. You could possibly glue (construction adhesive of any type as it has no function after concrete sets) to hold some building paper or old roof shingles to the bottom of the old slab-- to create a bond breaker between old and new concrete. If a bond develops there, expansion and contraction of the two slabs could destroy the "shelf". Every bit more than 3 inch depth of concrete under the exist. Slab you can get, is beneficial.
  • Sep 22, 2013, 05:28 PM
    I decided against building the shelf because it seemed like there was too much that could go wrong. I found three areas that were thick enough to drill into and insert rebar. Everything turned out pretty good and the garage door guy said everything is level enough for the install. Thank you very much for all the advice.
  • Sep 22, 2013, 06:11 PM
    Thanks for the update.

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