Mixing concrete in a Belle minimix 150

Discussion in 'Builders' Talk' started by nomakeover, Jun 1, 2009.

  1. nomakeover

    nomakeover New Member

    I had to look all over to find answers to my question on how to make an approx. C20 concrete mix with the above mixer, and do some basic calculations. I thought I would put the information I collected here to save other people the trouble. NB the 'recipe' I give depends upon how wet the sand is, so use this information at your own risk.

    C20 is a general purpose concrete grade of 20 N/mm^2 or 20 MPa. strength. To make it you mix by volume 1 part cement, 2 parts sharp sand to 4 parts of aggregate, say 20 mm gravel for coarse concrete, or 10mm gravel for fine concrete. First confusion comes when all-in ballast is substituted for the separate sand/aggregate ingredients. Here you will find people recommending anything from a cement :ballast ratio of 1:4 to 1:6, for a C20 mix. The reason why it is not 1:6 by volume is that the finer sand can sit in the some of the holes between the larger gravel/aggregate particles, so the mixture is denser than its constituents. However having looked into the quoted densities for ballast mixtures, it does not appear to get that dense on mixing so that a cement:ballast rato of 1:4 would probably not be correct. That would assume all the sand can be accomodated within the pores between the gravel chunks. Based on the densities quoted, a ratio somewhere between 1:5 - 1:5.5 would seem about right.

    In any case the required all-in ballast for a single load should weigh about (59 + 112 kg, see below) 171 kg, and in theory this should have a volume of about 96 litres, plus or minus ???

    The most confusing bit is that the change in density is much larger when going from the dry mix to the wet mixture, but anyway here are the figures for a single load in a Belle 150 minimix.

    Cement 25 kg ~ 18 litres
    Water 12.5 kg ~ 12.5 litres (use less if sand and ballast is very wet)
    Sharp sand 59.5 kg ~ 35 litres
    Gravel 112 kg ~ 70 litres
    Total 209 kg ~ 135.5 litres

    However when mixed together the volume drops down to the 85-90 litres mark, but obviously the weight stays the same, so you end up with concrete of density around 2.4 tonnes per cubic metre.

    35 litres is reckoned to be 6 No. 2 shovels, but the cautious newcomer is probably best using a bucket, a typical size is 14 litres, but if you make a mark at 12 litres and fill to this mark, then it’s 3 sand plus 6 of aggregate. Use a watering can to ‘calibrate’ the bucket. (works out about 7.5 -8 buckets [12 litres] of all-in ballast to a 25 kg bag of cement)

    VERY IMPORTANT To start the mix put about 7 litres of water into the mixer, then add the 3 buckets of gravel/aggregate, now you can either (1) add all the cement or (2) add some of the cement, then some of the sand, alternating until it is all (cement and sand) added, however after that stage you add the rest of the gravel/aggregate. NB keep your cement away from anything wet before it enters the mixer, i.e. separate bucket etc.. You can not dry mix like you do when doing it by hand on a mixing board, you need the drop in volume for it to fit, that’s the key!

    Adding the rest of the water is the tricky bit, as you don’t really know how wet everything was before you started, so you might not need all the remaining 5.5 litres or so of water. Gradually add some water and give it chance to mix (but not talking 10 minutes here!). Pointers, if the mix goes over the top it is too dry, ideally the paddles should lift it to about ¾ around the drum, if it just runs through the paddles you have added too much water*. Once all the water has been added, 2-4 minutes further mixing is all that is required, do not over mix, it can cause segregation of the aggregate and reduce the strength of the concrete. Main observation here is that at the magic ratio the mix suddenly goes from dry to just right, so you have to be cautious near to this ‘tipping’ point, as you can easily go too wet, but the concrete should be smoothish and flowing when at the correct cement/water ratio.

    *Only thing you can try to rectify this is adding more cement and ballast in the correct 1:5 ratio (the excess will probably slop out of the mixer, just take more care on the next mix).

    The readymix technicians perform a standard slump test to check this. Basically they fill a metal cone with the mixed concrete, upturn it on a flat surface and see how much the top drops down. Should be about 50mm.

    Slump test

    The freshly mixed concrete is packed into a 300 mm (12 in.) high cone, 200 mm (8 in.) wide at the bottom and 100 mm (4 in.) wide at the top, which is open. The concrete is smoothed off level with the top rim of the cone, and the cone is then carefully lifted, so that the concrete is left unsupported. The slump is the distance that the centre of the cone top settles. In a so-called ‘true’ slump test the base of the concrete does not spread excessively. If the concrete collapses or shears to one side the test results will be unreliable.

    Although the slump test does not directly measure the work needed to compact the concrete, it gives a reasonable indication of the how easily a mix can be placed and is simple to perform. The test is only suitable for reasonably workable, cohesive mixes. Very stiff mixes do not settle enough for useful measurements to be made and uncohesive mixes tend to shear or collapse. A slump of less than 25 mm (1 in.) indicates a stiff concrete and a slump of more than 125 mm (5 in.) indicates a very runny concrete.

    For the normal range the slump is 30-60 mm, best you can do is just get used to the correct degree of self support normal concrete exhibits when you make deep valleys in it using a shovel and a spot board.

    Hope this is helpful to someone.
  2. manassa

    manassa New Member

    I know you mean well nomaker but ***.
    Here is my method.
    Throw half a bucket of water in followed by 1 cement to every six sand and gravel,add water as neccessary until the mixer is full.
    duders likes this.
  3. Mr GrimNasty

    Mr GrimNasty Active Member

    By the time I finished reading, all my cement was past its use by date.

    I prefer method 'B'.
  4. ­

    ­ New Member

    Here's an even better method for a BELLE 150.

    Throw in one and a half buckets of water
    Add about ten shovels of ballast
    Add one whole bag (25kg) of cement
    Allow a minute or so for this to mix to avoid clogging
    Add further ballast until mixer is 3/4 full
    Additional water added to taste.

    No need for all that info from the o/p!

  5. blimey, stick to the paperwork , if your that exact about every part of a job, it will be christmas before you even start a job
  6. Pvc Fitter

    Pvc Fitter New Member

    Take it you take all that fumble with you when on site? and pull it out of ur flask before starting a mix? then measure it out with a tea spoon? then tell everyone that you only pointing round a soil pipe.

    Then charge the customer day rate for the time taken?.

    Just follow a simple mix varying on what you doing its sooo much easier
  7. ecm

    ecm New Member

    Next week's "Trade Tips" - Cleaning your Belle minimix with 2 gallons of water and 4 half-bricks.
  8. manassa

    manassa New Member

    The size and density of brick must be considered relative to drum and water volume.
    The abrasive quality of the bricks can affect cleaning efficiency ,as can the drum rotation speed.
    The drum speed of the belle 150 can be calculated as..etc...etc...etc...etc...:(
    DIY womble likes this.
  9. nomakeover

    nomakeover New Member

    Thanks for all the replies, they really cheered me up. Obviously wrong place to give advice to would be DIYers hiring the Belle 150 for the first time.
    Noticed some poor advice in one reply though, at the risk of another roasting, here goes (try to keep technical stuff out).

    If you start a normal mix off with 1.5 buckets of water & then add extra water to taste, then yes it will pour nicely, like the after effects of bad guts curry, not much tamping required there to get it to fill the formwork! Its probably the worst thing you can do. 4.6 gallons of water to 25kg of cement, do me a favour, in new money thats about 21 litres aka 21kg of water. A free water:cement ratio of at least 0.84. Anybody who doesn't know why that's a bad thing for concrete (whatever the intended use C15-C40) should not be let near a site's mixer, apart from cleaning it, and to keep this short I wont list the reasons.
    Short version of original recipe
    Bung 1/2 bucket of water into mixer
    chuck in 6 shovels of gravel
    add one 25 kg bag of cement
    then add 6 shovels of sharp sand
    then 6 shovels of gravel
    finally up to another 1/2 bucket of water

    4 minutes after adding final water, pour into barrow,
    put next 1/2 load of water and 1/2 gravel into mixer whilst current load is being laid.
    1/2 Housebricks is traditional way to clean drum, but if your the one that has to fork out for new drums, use water and 20mm gravel and plenty of it. It's also way faster. Christ you don't want to know the rotation speeds as a function of the carb settings for the Belle 150, or do you?

    Back to my real job, or as they tend to say here's that f***g 'ask Elvis' T**T again.
  10. etcdanny

    etcdanny New Member

    would like to do a job for this guy, it's concrete for christ sake, they let labourers use it, three quarter bag cement, as much ballast as it can hold, as much water as you like depending on job get it poured
  11. nomakeover

    nomakeover New Member

    Sorry mate doesn't sound like you have enough civils experience, now a shed base or something unimportatnt hid out of the way :) I'll know who to call.
  12. LadyBoss

    LadyBoss Member

    I have never laughed so much in ..in...hours. I am a girl and I can mix compo by just looking at what I am doing. I have less complex recipes in my kitchen. Go on, tell me its a wind up, go on, go on!
  13. joinerjohn

    joinerjohn New Member

    Thank christ for cut and paste :)
  14. Switcher

    Switcher New Member

    Thought it was Copy and Paste?
  15. Captain Leaky

    Captain Leaky New Member

    Get real mate!

    I know - lets fit cement mixers with recipe book holders! :O
  16. Big Fat Stu

    Big Fat Stu New Member

    MY GOD....has this guy heard of a well known saying....CHUCK THE C**T IN

    Its concrete ***, and tbh if i put what he suggested in my little belle mixer the motor would probably burn out lol. but then maybe i should consider the watt hour amp volt stuff of the motor and add a flux capacitor and a radon gun to make it more powerful pmsl
  17. Euan Lowe

    Euan Lowe New Member

    I know this is an old thread, but I just wanted to post a message applauding nomakeover's article. I'm casting some custom concrete blocks and the info is very enlightening. There is this obsession with the gifted amateur, who are of course awesome. But it's this same belief that leads to so much of the mixed quality of civil engineering and other bodge jobs we come across every day.
  18. sospan

    sospan Well-Known Member

    The whole thing was Spam. Nobody posts an article as specific then gets all those "new members" to add to it
  19. CGN

    CGN Well-Known Member

    The whole thread is comedy gold. At the 'right moment' this eve, I nearly had a heart attack from laughing so hard...there's 'geeks', then there's 'geeks'! Absolutely hilarious. Latest post(not you SP! ;)) is perfectly intune with the original hilarity...I just hope that Armageddon/WW3/ selective cull happens sooner than later otherwise B n' Q will become a holiday resort at this rate!!! :D:confused:
  20. Chris Coles

    Chris Coles New Member

    I found the post really helpful nomakeover.
    Some of these responses all seem a bit ‘finger in the air’... maybe ok for a time done person but for a newcomer not much help for c20 and a 150ltr drum.

    I got from your post:

    3/4x full (maybe slightly more) bucket of water
    1x bag of cement (25kg)
    2.5x buckets of sharp sand
    5x buckets of 20mm gravel

    (Meaning 9 buckets needed for each full drum load - the standard 14ltr 0.99p buckets from B&Q or Wicks). Very easy and quick way to measure out.

    Alternatively... (using ballast)

    3/4x full (maybe slightly more) bucket of water
    1x bag of cement
    7.5-8x buckets of ballast

    Thanks for the volume/weight calcs.
    Up until now I was struggling to find a YouTube video for a C20 mix for long retaining wall footings for either 4:2:1 or ballast (plenty of videos of people using unmeasured amounts - not helpful for a newbie!). I guess this is a good start to then get a feel for the mix when pouring.

    Cheers Chris

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