Getting ready to build concrete slabs for 2 - 8x10 plastic storage sheds

Discussion in 'Bricks, Masonry and Concrete' started by voyager, Jan 20, 2019.

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  1. Jan 20, 2019 #1

    voyager

    voyager

    voyager

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    Getting ready to build concrete slabs to erect 2 - 8x10 HD plastic storage sheds on.
    Pano 01 - 600.jpg
    I am now spreading, smoothing, and leveling the base pad to put the forms on.
    I will need to terrace the pad because of the slope of the ground.

    I will need to finish using and/or moving the load of black cinders to be able to use the area under part of it, possibly for an existing 8x8 shed, if I can salvage it.
    RIMG0173sm.jpg

    It will be a while before I finish this part of the project.
    But, I do need to begin working on the concrete plan.

    OK,
    I need to pour 2, hopefully 3, - 4" slabs: 2 - 8x10 and 1 - 8x8.
    At a yd for the 8x10s and a little less for the 8x8, I'm figuring on 3 yds total to mix and pour in 1 yd pours.
    I am too cheap, and too old and slow, to consider renting a mixer to do the job.
    Four days rental will cost as much as buying one.
    It will probably take me 3 days to mix, pour, and work up the 3 slabs, by my plan.
    I can buy, then sell a mixer for less than renting one.
    94# bags of portland cement cost 2x more at HD here, than at an L.A. HD.
    It's a 50 mile RT from town.
    I will not pay to have ready mix delivered.
    I do not want to deal with buying and hauling sand and gravel aggregate.

    After checking around about light weight concrete using porous volcanic aggregates, and self leveling/compacting concretes, I am going to look into making a light weight, Self Compacting Concrete [SCC] for the slabs using the black cinders I have sitting here already.
    All I will need to purchase is form lumber, portland cement, sheep fence rebar, a super plasticizer, and a mixer, as well as the 8x10 sheds.
    I have looked long and hard at this and have drug it by a lot of people looking for reasons not to it this way.
    Everybody objects with reasons that do not apply to this usage.
    If I can get around the real problems, I can make it work.

    This is a light duty application.
    A high compressive strength concrete is not needed.
    The heaviest load it will be carrying is a lawn tractor being parked in one of the sheds on its plastic floor.
    The other will only be used for tool and general storage.

    The real problems to be gotten around are with making a consistent pour of SCC while doing it in small batches, plus calculating and measuring the amount of cinders to substitute for the needed sand and aggregate in the mix.

    My needs are undoubtedly beyond the scope of this forum.
    But, if there is anyone here with knowledge of SCC and light weight concrete mix design, it would make my plan much easier.
    But, I can get it figured out on my own, if I can't find anyone knowlegable to pick their brain.

    The mixers I'm looking at can mix batches of 2.3 or 2.5 cuft.
    That means that to make a single 1yd slab pour, I'll need to figure out how to make 11 or 12 batches all consistantly the same mixture, and get them done within 30 to 45 min of each other with the complete pour finished in one day.
    Does this sound reasonable to do?

    It has put the strike out in.
    I cannot remove it.
     
    Last edited: Jan 20, 2019
  2. Jan 20, 2019 #2

    Sparky617

    Sparky617

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    Is there a question in here? With the palms in the background, I'm assuming freeze/thaw isn't a problem in your area. If you're not going to do a single pour I'd form off your slabs into separate sections that are as big as you can mix and finish at one time. With these sheds could you get by with just setting it on a compacted crusher run base? It looks like it has a floor.
     
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  3. Jan 20, 2019 #3

    nealtw

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    Getting that mix right the first time would be some trick I think.
     
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  4. Jan 21, 2019 #4

    bud16415

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  5. Jan 21, 2019 #5

    voyager

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    @Sparky617
    The question was in the last paragraph under the cross-outs:
    Q:
    The mixers I'm looking at can mix batches of 2.3 or 2.5 cuft.
    That means that to make a single 1 yd slab pour, I'll need to figure out how to make 11 or 12 batches all consistently the same mixture, and get them done within 30 to 45 min of each other with the complete pour finished in one day.
    Does this sound reasonable to do?

    [When I added the bold in the last post, it put the cross-outs in and wouldn't let me remove them.]

    Everything before that was an attempt to keep from digressing into options already considered and discarded.
    I live in a very wet climate, ~150 in/yr rainfall.
    The greatest complaint in this area is these plastic sheds leaking and not holding up when placed on an unstable base.

    A wood base is not an option.
    The existing 8x8 shed pictured was placed on 4x4 beams.
    They have rotted out, the floor is collapsing, and the walls are sagging.
    It is now leaking.
    If I can, I will salvage it and place it on a slab in its new location.

    Erecting them directly on the red cinder pad is also not an option.
    It is not stable enough to insure water tightness through time.
    Plus, they cannot be fastened down to hold then in high winds.

    Sectioning it up into 1/2 yd pours is not a bad idea.
    I will keep that option in mind.
    But, I would rather complete a full 1 yd pour in a day, if possible.
    Which is what my question was asking.


    By using a SCC type of mix it will require little working or handling once in the forms, other than leveling and floating or brooming.
    Compacting will not be required.
    But, it will require that all the batch pours be consistently the same, as if they were one single pour.
    That can be achieved by measuring each component exactly the same for each batch.

    The superplasticizer loses its effect in 1/2 to 3/4 of an hour, or 1 hour depending on who is telling it.
    That is the reason for the time limit on the batches being added to the pour.
    With luck and efficiency the full 1 yd pour could be done in 6 to 8 hours, or less [I hope?].

    @neiltw
    Exactly.
    I might need to do some slump tests to get the water portion right.
    But, I would think that once I got it right, I could crank the batches out for each pour.
    That may be a bit too confident , though.
    That's why I was hoping to find someone here familiar with concrete mix design.

    @bud16415
    The black cinders are a crushed rock screened to 1-1/2" minus, but are actually closer to being 3/4" minus.
    Very few pieces larger than 3/4".
    There is a very high percentage of sand like fines in it.
    The red cinders are also 1-1/2"minus but are much coarser and compact better.
    That's why they make a better base for the pad.
    The soil concrete, while different, makes the black cinders sound even better than before.
     
    Last edited: Jan 21, 2019
  6. Jan 21, 2019 #6

    Steve123

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    I have poured a shed slab and can offer a few suggestions:

    -I also was too cheap and self confident to rent a mixer. That was a mistake. Hand mixing is darn hard work. Did not help that I ended up doing it on the hottest day of the year (at least it felt that way). I was rushing for fear of having a cold joint between pours (not positive that was or was not a valid concern, but takes a while to hand mix a couple of bags).
    -Take time to level your forms and make sure you don't have outside stakes rising above top of forms. Easiest/best way (at least for regular concrete) to finish is to simply use a 2x4 as a screed over the top of the forms.
    -Have a sheet of plastic handy to put over the slab. Every day for a week, lift the plastic and spray some water onto the surface to keep it hydrated.
     
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  7. Jan 21, 2019 #7

    voyager

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    @Steve123
    I am not going to attempt to hand mix 3 yds of concrete, not even 1 yd.
    I will purchase a mixer for the job, then either keep it or sell it when finished.

    Your advise on leveling is pretty much my plan.
    Screed is the term I was looking for when I said float.

    With the high rainfall and heavy morning dew keeping the new concrete moist is not a great concern except for hot sunny days.
    But, I do have several rolls of visqueen handy for just in case.
    Because of the porosity of the cinders and their softness compared to a regular crushed rock, I will probably protect the slabs for about a month before placing any sheds on them.

    EDIT:
    One thing I'm considering is to pour about 1/2 [2"] of the depth of the concrete, then lay the sheep fence rebar on that, and finally finish the final 2" of the pour over the rebar.

    That would make 2 batch sessions of 3 to 4 hours where the 1/2 to 1 hour time limit for 6 batches instead of 12 would need to be followed.
    Plus, the sheep fence would be in the middle of the slab, not on the bottom. There would only be a partial setting up of the lower pour before the final layer was laid over it, probably little more than the superplasticizer losing its effectiveness.
     
    Last edited: Jan 21, 2019
  8. Jan 21, 2019 #8

    nealtw

    nealtw

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    If you do 2 lifts with rebar in between it will not mix together, you will end up with 2 layers. steel needs to be buried at least 2" into concrete so air and moisture can not get to it, rust will take concrete apart.
     
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  9. Jan 22, 2019 #9

    voyager

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    Well, so much for that plan.
    I guess I'll just have to lift it off the bottom to the middle of the slab as the batches are added.
     
  10. Jan 22, 2019 #10

    nealtw

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    hold it up with stones or you can buy plastic chairs for that.
     
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  11. Jan 22, 2019 #11

    voyager

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    I have seen the plastic chairs.
    The pricing I saw was extortionary, meant for commercial operations where high labor costs are a major concern.

    After a lot more thought and a day's work on the retaining wall for the terracing, I'm beginning to drop the SCC line of thought.
    I do not see me finishing a complete slab in one day, a cuyd of materials, 94# bags of cement, 12 pours in a day, etc.
    I am Superman, but even Superman would have trouble with that.

    RIMG0174sm.jpg
    I stacked most of the cut concrete blocks into the wall yesterday.
    They are heavy!
    I'm getting old, and I hadn't taken my anti-inflammatory.
    I have trouble with thinking that I still have the strength and energy of a 20-something.
    Yesterday and this morning were a comeuppance.
    Gotta get my anti-inflammatory Rx refilled.

    The red cinder base pad will not hold water, not even in torrential rains.
    The plastic shed should protect the slab from rainfall.
    The edges of the slab can be sealed.
    The sheep fence will be more effective if placed in the upper third of the slab, not on the ground or just above it.
    Placing it on the bottom is little better than having none at all, which is an option.

    I am back to thinking of a two part pour of a standard type mix, the bottom 1/2, then the top 1/2 with the sheep fence in the middle, or none at all.
    A horizontal cold joint in a slab of this size is much better than a vertical one.
    Flexure strength and other characteristics of the slab could actually be enhanced.
    I'm going to look further into a 2 day - 2 part pour per slab.
    It will require compacting.
    Cold joints seem to be very common in DIY multi batch pours of any type, even for professionally done residential pours because of contractor/ready mix expediencies.
    I'd be more concerned if it were a wall, foundation, beam, or something similar.
     
    Last edited: Jan 22, 2019
  12. Jan 23, 2019 #12

    bud16415

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    If it were mine I would go buy two bags of Portland and do two experiments. You might only need one bag depending on the size. I would mix one batch in your little cart and pour a 2x2 pad as thick as you want the real job. Then I would loose mix on the ground dry like a rototiller would do, then wet it down a little mix a bit more and tamp it flat and sprinkle it a little more. Keep both damp for a few days and see how they look.


    I know lots of people pour dry ready mix in a posthole and then dump a bucket of water in and that’s it. It cures good and hard. I know if I open a bag and use half and sit it in the garage when I come back a month later it is a rock.


    If nothing else the experiment will tell you how much you need to add to your lava stuff to get it to work.
     
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  13. Jan 24, 2019 #13

    voyager

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    Hi bud16415,

    Using the cinders as aggregate can be compared to a standard mix using sand and gravel.
    Engineering studies have been made for the use of pumice and other volcanic materials.

    I do live on a volcano.
    Soil here is either lava rock, lava cinders, or pig mud, the rotted organic material laying over and mixed into the cinder base.
    The subtropical very wet climate here creates a lot of vegetative growth [jungle].
    I do not think soil cement is an option.
    Plus, I already have a load of both red and black cinders sitting here.
    The cinders should give a much better compressive strength than soil.
    They did mention in that video that the resulting concrete was somewhat soft.
    I would expect that.

    In my searching around on this subject, I have found the accepted weights, #/ft³, and volumes, yd³, of the components, sand, gravel, and cement in a standard 1 yd mix.
    What I will be doing is weighing 1 ft³ of the black cinders, then substituting that weight for the total of the standard gravel/sand weight/volume.
    It will be less weight because the cinders are porous and lighter.

    I need to decide whether I want a 5, 6 or 7 cement bag/yd mix.
    The more cement, the more tensile strength the setup concrete will have.
    Flexural strength is a form of tensile strength.
    A two part pour might make a 6 bag mix just as viable as a 7 bag mix.
    A 7 bag mix would only cost another $20/yd.
    The actual change in tensile strength would probably not be enough to be worth doing a 7 bag mix.
    Decisions, decisions.

    Sodium Silicate, water glass, could seal the two pours to stop water infiltration into the joint.
    Might even be able to paint just the outer edge area of the 1st pour just prior to the 2nd pour being added.
    Still need to look further into this.

    Quote:
    I know lots of people pour dry ready mix in a posthole and then dump a bucket of water in and that’s it. It cures good and hard. I know if I open a bag and use half and sit it in the garage when I come back a month later it is a rock.

    Yeah, Quikrete sold by HD does say to just dump it into the hole then add water.
    Quikrete is too expensive for this large of a job.

    I did use Quickrete and a tube form for our new mailbox.
    But, I did mix it before putting it into the tube.
    Did a nice job, taking a whole 80# bag for it.
     
    Last edited: Jan 24, 2019
  14. Jan 26, 2019 #14

    voyager

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    Relocated the retaining wall to allow the slabs for the 8x10 sheds to be built in front of the Samoan Palm.
    When I began this [ordered the cinders] it was before the eruption, evacuation, etc, about 10 months ago.
    I had planned to allow placing a beehive or two behind the sheds.
    I have gone back to that line of thought.
    The area is almost finished smoothing and leveling.
    It's within a inch or 2 in 8'.
    I think it is getting close enough to begin to assemble one of the forms.
    I should be able to square and level the form, then fine tune the leveling of the surface to match.

    I'll go into town tomorrow to pick up the form wood and a hand tamper.

    I found something to justify the use of cinders as aggregate.
    On HD's web site, the Quikrete 94 lb. Portland Cement page, in the questions section.

    Question:
    Can I mix this with Cinder and Lava Rock/ gravel
    Asked by
    John
    October 14, 2015
    I am on the Big Island of HI and have plenty of cinder/ lava rock

    Answer:
    BrowneBearLLC
    Detroit, MI

    Top 500 contributor
    Pro

    September 18, 2017
    You can mix any type of stone that you want its just the straighten that will be effected.


    I believe the use of "straighten" is a misspelling of "strengthen".
    I can find nothing about a BrowneBear LLC in Detroit.
    None the less, that is in agreement with everything I have found about using a porous light weight aggregate mix.
    A softer aggregate will make a mix with a lower compressive strength, but adequate for this application.
     
  15. Jan 26, 2019 #15

    bud16415

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    I was more concerned about the porous nature of your stone and the reason I suggested doing samples. In a regular mix sand gravel and stone are mixed in a ratio that allows for tha smaller bits to fill in the space between the larger. Thus minimizing the amount of Portland required. I have seen concrete made from just sand and Portland and it is very strong but requires much more Portland.


    The Portland will work into the pores in your stone and get used up is my thought. If it is widely used there then you should find more information on mix rates.

    I guess one way to get a clue would be to take a five gallon bucket of your lava stone mix and carefully add water to it and see how many gallons of water it will hold and compare that with regular stone and sand mixed.
     
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  16. Jan 27, 2019 #16

    voyager

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    I made a breakthrough today for info about using cinders as an aggregate for concrete.
    I appears that the correct term for cinders is scoria.
    putting "scoria concrete aggregate" into a Google search bring out engineering papers like coins from a nickel machine jackpot.

    I have only scanned some of the papers so far.
    But, I did see something about the porosity of the scoria and the moisture it may hold, being a good thing, helping with the curing process adding strength as a result, while not necessarily affecting the water for the mix.
    I do need to go back and do a more thorough reading of some of the papers.
    A lot of weight and volume figures are there for mix designs.
    Although, I'm pretty sure "close enough will be good enough" for my use.

    The only real concern I have right now is the sizing of the cinders.
    There are a lot of fines in the material.
    That is the reason I was drawn to the SCC type of mix.
    The particle sizing looked to be close to what is used in those types of mixes.
    But, I think I saw something somewhere in today's find about scoria fines not being a bad thing.
    Again, I need to confirm that.

    Also, from some more reading, it sounds as if I might be able turn the batches out fast enough that cold joints may not be much of a problem, if I get one of those low to the ground wheelbarrow type mixers, relatively little lifting to be done.
    And if the cold joints become a problem, the sodium silicate treatment of the older pour before adding the new one over it, should be a very beneficial treatment, bonding the surfaces together and adding strength in the areas treated.
     
    Last edited: Jan 27, 2019
  17. Jan 27, 2019 #17

    FatGans

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