Help me mix concrete for a foundation.

Help me mix concrete for a foundation. I'm wanting to pour a slab about 16x16 and it's cheaper to mix it all by hand since I am 2 hours from the nearest ready mix plant. As I understand I need 1 part cement, 2 parts sand and 3 parts gravel. How important is it that I get these proportions correct? Can I just go dig up my sand pile for the sand and gravel pit for the sand? Both of which I have on the property. Or does it need to be measured out more precisely. What about if there is small amounts of dirt or organic material in the sand?

Thanks lads

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  1. 1 year ago
    Anonymous

    I'm no Mason, but you're gonna have to be more specific for people. Depth, style, framing etc

    • 1 year ago
      Anonymous

      Typical slab for a garden shed. 4 inch thick with deeper footings. Will use rebar and wood forms.

      • 1 year ago
        Anonymous

        Get concrete blend sand/gravel.
        4 blend 1 Cement should do I think, I'm just a labourer.

        4:1 concrete blend and gp Cement respectively should do.

  2. 1 year ago
    Anonymous

    You want washed sand and gravel. No dirt or organics in it. Most concrete batch plants will sell a washed sand/ gravel mix to people for mixing their own. If that is not feasible you could always do the quickcrete bags of premixed stuff. But those cost more.

  3. 1 year ago
    Anonymous

    i don't know where you get 1:2:3 from.
    so first of all 16ft is near enough 5m, i work in metric sorry so a 16ft foundation slab say 150mm thick (6 inch) MINIMUM is about 4m3 of concrete. you will not mix this by hand period.
    but lets say you did it anyway. 16x16 i'm guessing is a shed or something? light? not a 2+ story block house?
    say you want ~C20 which is 20 newtons strength when cured. for max 20mm gravel and s4 slump the prescribed mix is 380kg cement to 1740kg aggregates, the ratio of fine aggregate (sand) to coarse aggregate (gravel) is somewhere between 25% for <=0.5mm and 45% for down to 5% <=0.5mm sand. sand should be sharp for concrete btw, not soft sand.
    now gravel alone is a big topic but suffice to say you want as cubic gravel as possible, not round pebbles not flat shale, if you can get roughly 50% 10mm to 50% 20mm gravel with a wide margin you are doing quite well, you want a bit of a mix of gravel size ideally.
    water is another topic, you want 0.4-0.6 (less is stronger) as much water as cement but the gravel and sand needs to be surface dry, that is not dried out inside, otherwise you need more water. this is critical.
    now, if you want to batch by volume you shouldn't really do anything over C15, which means C20 ratio of roughly 1:2:4 isn't technically allowed by my standard (check your local regs).
    but look at the end of the day its just concrete, they let builders mix it ffs. mix up a test batch and test its strength with a hydraulic press to check your sand and gravel is up to snuff before you mix a big batch.

    ?t=195
    this guy has some really interesting videos.
    you know you don't have to use readymix right they have trucks that mix on site too right, the truck has the ingredients and mixes exactly what you need where you need it.
    finally good luck.

    • 1 year ago
      Anonymous

      1-2-3 is written on most bags of concrete mix.

      • 1 year ago
        Anonymous

        i just looked and you are right
        never noticed that before.

        Typical slab for a garden shed. 4 inch thick with deeper footings. Will use rebar and wood forms.

        alternative for a shed is build two block walls lengthwise and frame a timber base between them, gives you airflow under for damp and you can insulate it too.

    • 1 year ago
      Anonymous

      1-2-3 will work and is easy to remember. This is obviously not the best mix. 4m3 can be mixed by hand (i did it multiple times) but this is not fun or easy and you'll have a hard time getting a good surface finish (because mixing that much will take a lot of time)

      Also add some (real) rebar in the slab itself, you'll need way less concrete that way, so it'll probably be cheaper in total. It's also not that difficult to calculate the required thickness and amount of rebar.

      Round gravel should be fine, but make sure the sand is real sand and the gravel is clean. Any clay will completely ruin the concrete. YOU SHOULD TEST THIS FIRST IF YOU USE YOUR OWN SAND! It will not set if there is clay in the mix.

      • 1 year ago
        Anonymous

        Who crossbred that husky with a Black person I thought dogs only fricked white women

      • 1 year ago
        Anonymous

        I have a book on reinforced concrete calculations somewhere. Rebar is around 2-4% of a beams volume, and more concentrated on the bottom and top, mimicking the profile of an H beam. The slab is a sort of beam itself as it has to resist bending

      • 1 year ago
        Anonymous

        >4m3 can be mixed by hand (i did it multiple times)
        i'm not calling you a liar i jut think you have made a mistake
        also rebar ruins concrete and is completely unnecessary 99% of the time its used by diy

        • 1 year ago
          Anonymous

          just buy fiberglass rebar then, its way easier to deal with anyways

          For the people saying its too much concrete, can't do it by hand etc, just do it in sections with individual forms. You won't have a singular smooth surface on the top unless you grind/smooth the top down but it'll be fine enough for a shed floor.

          well, its 5 yards of concrete, big tricky to do yourself solo but you can easily get it all done in a day

          How are you getting materials to the site though, endless runs in the back of your pickup?

          Why not buy premixed bags instead of fricking it up being autistic with mixing gravel you stole from the side of the road

        • 1 year ago
          Anonymous

          Bags of 25kg are pretty cheap here, mixed concrete is even cheaper (100 euro per m3) but i need an expensive pump to get it to where it needs to be.

          Im not sure why you think rebar ruins the concrete. It will not if you put it in the right way, and not all diy is unnecessary heavy foundation work.

          I have a book on reinforced concrete calculations somewhere. Rebar is around 2-4% of a beams volume, and more concentrated on the bottom and top, mimicking the profile of an H beam. The slab is a sort of beam itself as it has to resist bending

          You can calculate the amount of rebar needed, its not always the same percentage. You can also calculate the position of the rebar (from top/bottom). But there are rules about the minimum distance to the edge (depending on the moisture, salt etc) so the rebar doesnt "ruin" the concrete.

          • 1 year ago
            Anonymous

            >Im not sure why you think rebar ruins the concrete.

            Think he's referring to the fact that rebar puts a hard cap on the life of a concrete structure. Iron oxides are less dense than elemental iron, so corroding rebar will slowly force chunks out of the very concrete it's supposed to be reinforcing. There's no practical way to prevent this when using standard rebar, so whatever you build with it starts to literally fall apart after ~50 years. I'm not aware of any way to truly repair the damage as it happens, either. You're basically just fricked at that point and have to rebuild either parts or all of the structure.

            Different types of reinforcement, glass fiber particularly, is preferred where this is a concern. I'm not sure why you'd want to use any in a small slab foundation like this, anyway, unless you absolutely can't have any expansion joints or cracks in the slab for some reason.

          • 1 year ago
            Anonymous

            you put that in a mixer right? not truly by hand?
            either way i'll buy you a beer next time i see you.

            wait until you find out about control joints in large slabs, you fricking moron

            >explaining to building control i didn't use any mortar on my block building because they are 'control joints'
            frick off idiot control joints are placed calculated so the slab can still bear load. discreet chunks of hand mix dont constitute control joints in any form and are a dogshit footing. noce try moving goal posts

            • 1 year ago
              Anonymous

              I did put it in a mixer, still a lot of work because cheap mixers only mix 3 bags at a time.

              >Im not sure why you think rebar ruins the concrete.

              Think he's referring to the fact that rebar puts a hard cap on the life of a concrete structure. Iron oxides are less dense than elemental iron, so corroding rebar will slowly force chunks out of the very concrete it's supposed to be reinforcing. There's no practical way to prevent this when using standard rebar, so whatever you build with it starts to literally fall apart after ~50 years. I'm not aware of any way to truly repair the damage as it happens, either. You're basically just fricked at that point and have to rebuild either parts or all of the structure.

              Different types of reinforcement, glass fiber particularly, is preferred where this is a concern. I'm not sure why you'd want to use any in a small slab foundation like this, anyway, unless you absolutely can't have any expansion joints or cracks in the slab for some reason.

              I have seen the meme threads about this, but this is mostly an issue with the rebar being to close to the surface/additives in the concrete/salt water/too much water in the mix/etc. Normal reinforced concrete should last 100 years+ easy.

              You can do some foundations and floors without rebar, but usually you can make it lighter and cheaper by using rebar. My garage floor is only 5cm thick in most places and still fine after 7 years.

              • 1 year ago
                Anonymous

                100 years is not long enough i want it to last much longer

              • 1 year ago
                Anonymous

                I believe it should be possible to fill cracks in concrete with lime and stop the rusting. The slight flexibility of cured lime makes the whole mass able to flex a bit without cracking more
                It would last for centuries with minimal maintence, concrete only rots in 50 years if you leave it alone. The cracks are easy to see and you can patch them in time

              • 1 year ago
                Anonymous

                What do scientists and engineers who study concrete for a living have to say about that obviously unique and never previously explored revolutionary idea unprecedented in history?

  4. 1 year ago
    Anonymous

    It's a shed. Forego cement entirely and use the gravel. This may even keep you from getting fricked by building codes and property taxes on it being a permanent structure or not.

    • 1 year ago
      Anonymous

      In the states under 600 ft^2 per IBC doesn't need footings

      • 1 year ago
        Anonymous

        Really?
        Section 1809.5(3) says that the footings don't require frost protection if a light framed structure is less than 600sqft, but it doesn't say that you don't still need footings

        • 1 year ago
          Anonymous

          [...]
          no you don't need footings, the building just floats in the air

          For fricks sake a slab is not a footing

          The footing comes in at frost depth (strip footing, isolated footings, etc)

          My broader point is that the city has no ability to complain about the concrete. If he went down to frost (>600 sq ft) now it's a permanent structure

      • 1 year ago
        Anonymous

        Really?
        Section 1809.5(3) says that the footings don't require frost protection if a light framed structure is less than 600sqft, but it doesn't say that you don't still need footings

        no you don't need footings, the building just floats in the air

  5. 1 year ago
    Anonymous

    cement is the most important. sand the second, gravel the third.

    cement+sand = grout
    if you don't use enough gravel you just end up using more of the others. more cement = stronger bond, more sand = more fill material, never exceed 1:1. Gravel is an elastic fill material, size is more important as if the agrigate is too big then it will not settle correctly and cause... rocks to stick up out of the concrete. most people use 5-7 stone because it is cheaper by volume. if you use unwashed 4-11 stone, know that it contains coarse and fine sand. you will want to increase the amount of fine sand present if you want a smooth finish.

    you typically clean sand of unwanted material by gravity panning it. it is why most sand pits are in creeks and rivers. you drop the sand up stream and the organic material will float and the sand will sink. you can also build a strainer for sand to remove large organic material by using something like chicken wire. it won't work for clay though. (clay is bad). my home state is bad about sandy clay, so you basically have to use a water agitation method to separate it. there are different methods for this some are Lazy others are very technical both require a lot of time to let the soil settle after basically making dirt soup.

    >1:2:3 is a measurement by volume

  6. 1 year ago
    Anonymous

    For the people saying its too much concrete, can't do it by hand etc, just do it in sections with individual forms. You won't have a singular smooth surface on the top unless you grind/smooth the top down but it'll be fine enough for a shed floor.

    • 1 year ago
      Anonymous

      >building a literal foundation
      >nah, doesn't matter if its solid, just make it out of all different non connected bits

      • 1 year ago
        Anonymous

        a slab is not a foundation

        • 1 year ago
          Anonymous

          a slab of concrete, at the bottom of a structure (house/shed etc), is a foundation. thats what its called.

      • 1 year ago
        Anonymous

        wait until you find out about control joints in large slabs, you fricking moron

  7. 1 year ago
    Anonymous

    In my country (balkan) everybody makes their own concrete with those mixers with 1:3 cement:gravel or 1:4 if they're cheap and that has given the best results in the last 50 years. Even tho we are poor we don't make houses out of cardboard
    We also use sifted gravel, dirt and sand are no good for concrete

  8. 1 year ago
    Anonymous

    OP, civil engineer here. I don't really have any constructive advice, but I'll come by with a slump cone once you've got it all mixed up and tell you to do it all over again.

  9. 1 year ago
    Anonymous

    Ok i have a separate question. Sorry to possibly derail your thread OP.

    I am doing a stone patio. My original plan was to excavate the soil from the area, then put down 2-3" of road base, compact it, and then use bagged concrete to set the stones in as I go. It would end up being about 2-3" gravel base layer, then 2" of concrete reinforced with remesh, with the 2" granite stone on top.

    Well I fricked up and ordered about double the amount of road base that I needed.

    So my current plan is to use the road base to make concrete by mixing it with portland cement. That way I won't have to get rid of a yard of road base and I won't have to spend any more money on bagged concrete. The road base I got has a lot of sand in it and the gravel is all crushed concrete or stone. It looks like it would be good for making concrete. I'll probably mix it with the portland at a 1:5 ratio.

    any thoughts?

    • 1 year ago
      Anonymous

      what you are trying to do is called soil cementing. it is method of Harding poor soil but in your case you are making a very weak concrete. the end result is you will have a very strong compacted patio with a layer of granite bricked. for a patio you won't need rebar with your method and are basically making a brick walkway and not a solid pour concrete slab.

      • 1 year ago
        Anonymous

        here is an example of soil-cementing road base.

      • 1 year ago
        Anonymous

        Well the road base actually doesn't have much dirt in it so it's a bit more like actual concrete than soil cement. The road base i got is just crushed concrete so it probably has a bit of dirt in it but only like 10% or less

        • 1 year ago
          Anonymous

          >the coarse sand.
          it will be stronger than a soil cement but weaker than true concrete. if you wash the stones, you can remove the excess sand as it will naturally fall to the bottom. the rock aggregate can be directly added to the concrete mix as additional aggregate. most bagged concrete uses 8-9 stone aggregate. road base is a unwashed 4-11 with extra sand. you don't want the sand or else you will need to get some pure bags of cement-grout to pour with it to get it to act like an aggregate.

          your options are to
          >wash the road base to remove the extra sand and use the rock as aggregate.
          >soil cement the road base and have an extra thick sub-base.
          >add grout (cement +fine sand) to the road base to create concrete.

          the problem is that the ready mix concrete already has aggregate in the form of some 9-11 stone. you can add a larger stone to the concrete to fill space, but not smaller because it will weaken the mixture. road base is classified as an unwashed 4-11 stone. repurposed asphalt is not ideal for this but will work because asphalt is rock+tar. the tar will weaken the structure because it is basically rubber "chips". this will prevent the cement from working. however in road base this is fine because the properties of the rock are all you are looking for. in addition the asphalt that is normally placed on top would only benefit from more tar/oil to bond to.

          if it is crushed concrete that is a much better option because the cement in the concrete will continue to react with water and help fuse the base together, otherwise it is just rock.

          • 1 year ago
            Anonymous

            Thanks for the info.

            I'm not so much worried about how strong it will be since it's going on a bed of compacted roadbase and i'll be using tile adhesive on the back of the stones to make sure they stick to the concrete, so just as long as the 'concrete' stays solid for a few years it's fine

            • 1 year ago
              Anonymous

              I wouldn't be worried either because you are basically making a brick-tile walk way, I don't even know if you need the steel reinforcement, mostly sounds like overkill but adding that isn't a problem. if you do that and the cement doesn't set right you are basically doing this:

              the only part that might matter is the perimeter, but what you can do is to make a high quality concrete "curb"/footer as a facade or you can tile the perimeter to hide the imperfections. if you make that perimeter ring it is like the iron band on a barrel.

  10. 1 year ago
    Anonymous

    for your 4" slab (64 cft*1.1=70.4 cft)

    400L of water
    1080kg of cement
    1770kg of 1/2" crushed stone
    1570kg of sand

  11. 1 year ago
    Anonymous

    >I'm wanting to pour a slab about 16x16 and it's cheaper to mix it all by hand
    Lmao you REALLY don't want to do this, anon

    • 1 year ago
      Anonymous

      Everyone who says this has never even lifted a bag of sackcrete

  12. 1 year ago
    Anonymous

    Get concrete blend sand/gravel.
    4 blend 1 Cement should do I think, I'm just a labourer.

  13. 1 year ago
    Anonymous

    Size 4 rebar at a foot is plenty for your shed foundation. T rebar guy. Size 5 at 8 inches will withstand a strongman's gym.

  14. 1 year ago
    Anonymous

    Look up how the Chinese do it by laying all the cement dry and running a rototiller through it with water and then floating it.

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