Mixing concrete with 120L Cement Mixer

27 Apr 2015
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United Kingdom
Hi. I have bought a 120L electric cement mixer.

I am wanting to make 0.25 cubic meters of concrete (by weight 1-2-4) using all-in ballast and some bags of cement. I'll need about 3 -ish 25Kg bags of cement.

Okay, but how do I manage the mixing with my new 120L cement mixer?

Of course, I'll be making batches of concrete, so how should I make these batches given that the capacity of the mixer is 120L? I put so many liters of water in first, then so much all -in ballast, then finally the cement. Okay, but does anyone have an idea of what quantities I'd be putting in the barrel for each batch?


P.S. For a C20 equivalent concrete the quantities for 0.25 cu m, came to:

Cement: 80Kg
Sand: 158Kg
Gravel: 293Kg
Water: 75 liters

Note, I am using all-in ballast instead of separate sand and aggregate.
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For me as a DIYer when I did this:
You are making a 1:6 mix (cement:all_in_one_ballast).
Get mixer running first.
Put mixer in good position to pour/tip into the area where you need concrete (hard to move once full of ballast).
Put some water onto Mixer first (washes last mix off the sides and makes next stages quieter).
Add plasticiser if making mortar, not if making concrete.
Use a bucket (I am not good enough to use a shovel) and put in 1.5 buckets of Ballast and then half a bucket of Cement and then 1.5 buckets of Ballast to get 1:6 ratio.
let mix a bit.
Then add more water until mix looks right (I used small bucket so I could count number and be faster next time).
(Note that I found hose pipe was too slow, so I filled a larger builders bucket/dustbin full of water to act as reservoir to fill small bucket).
Leave until mix is even (I used this time to prep by filling buckets) and then pour into hole/barrow.
Put some water water into mixer now to stop leftover bit sticking to sides.

Note that I found with my hired mixer I could JUST get 2.5 bags of ballast and half a bag of cement in giving a 1:5 mix. Saved me time as no need for bucket as simply poured bags in. But a bit messy as some spillage from mixer.
Also in this heat consider in advance if it is going to dry out too fast.

When I did this as a DIYer I think I got all this from:
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I get 20 shovels of ballast and half a bag of cement into my mixer, to fill a barrow. I think mine's about 130L (it's orange - actually mostly grey now).

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On the basis that its not rocket science, put a bit of water in, throw a few shovels of sand in, throw a shovel of cement in, throw a few more shovels of sand in, add a bit of water to taste, throw one more cement in, top up with sand until nearly full, put one more cement in, let it mix, add any more water you like, and then tip it out.
A plastic dustbin full of water is handy. 14 slightly heaped shovelfuls and 1/2 bag of cement for 4:1. Put bag on scrap board and cut with trowel. Or for smaller amounts bag of ballast and 3 1/2 l of cement.
You don't put mortar plasticiser in concrete, you can use a water reducing agent like ready mix use but your mix will be all over the place so there is no need.
You don't use much water in concrete. The sand is normally damp enough, and you just add a bit to help with the mixing. Half a bucket per mixer full is enough.
A higher water content, so a higher slump, makes the concrete much more workable, it also means you don't need to vibrate to remove voids.
The problem is the more water you add the more cement you'd need to add to keep it the same strength. This is where a water reducing agent comes in. When you buy ready mixed concrete you'll see this as an admixture code as WRA/HRWR.
When you mix your own you probably wont have access to a WRA. Don't use mortar plasticiser, although it does contain a water reducing agent, it brakes the surface tension of the water, it also areates the mix reducing it's strength. The optimum amount of water in a concrete mix would only produce a very dry almost damp sand consistency which is hard to work with. And this is why a WRA is used, it reduces costs.

After you have placed your concrete and leveled and tamped it make sure you don't steel float it as you'll lock in the bleed water. Also don't be tempted to towel to early and towel in the bleed water, there should be no water on the surface when you trowel/float.

It's only a tiny amount of concrete to mix and as long as you don't skimp on the cement and don't add too much water you'll be fine what ever you do.
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Okay, what I get from the above is:

* With a mixer of 120L capacity, with a 1:6 mix, the cement portion for a batch needs to be more-or-less half of a 25Kg bag of cement.

* I'll be making then 6 batches as I use up my 3 bags of cement as I make my 0.25 cu m of concrete.

* Can use a bucket for measures if not confident about shoveling in the materials.

* Good to have a container of water to draw upon (something like a dustbin).

* Start with some water, add all-in-one ballast (2+4 part) add cement, add more water to get right consistency
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I have already some all-in-one ballast that came in a one tonne bag a while ago.. Really then what I ought to be going is shoveling in the ballast and then chucking in the half bag of cement. So, I'm minded to buy myself a shovel. Should I be buying a particular size and style of shovel? How do I know I've got the right kind of amount on the shovel? Thanks.
Something like this:- http://www.spear-and-jackson.com/product/fibrelite/fibrelite-taper-mouth-shovel, a large "contractors" shovel for shoveling not digging. My brother once told me that their capacity is measured in pecks, giving " bushels" of mix. I can't find any references in Wiki to back this up, some one a lot older then me will have to help us out. N.B S&J do not give any capacity for their shovels.
Just wondering if this is a massive engineering project or a bit of concrete in the garden?

Guaging buckets, water barrels, retarders, plasticisers, water reducers, vibration, ratios, capacity of shovels? o_O

Get a grip
Stuff we all do because we found out about it 20 years ago are being imparted in a 10 minute burst to DIYalot. Interestingly I have met some old timer type builders, who do not bother to gauge materials. One who used 1/2 a bottle of plasticisier for a one wheelbarrow mix. I noticed this because I was wondering how much longer it would last. At least I found out, about 10 minutes!! That wheelbarrowful cost more in plasticisier then in cement.
As has been said, most of water and small amount of aggregate first to clean the drum .then all the cement and mix to a slurry.then the rest of the aggrigate and finally,the last of the water. If more cement is added at later stages of the mix it ends up sticking to the inside of the drum so more water or longer mix time is required to incorporate it . Been using this method for 50 years so if its wrong,I've been doing it wrong for a long time!

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