Eugene, an avid self-taught DIYer and engineer, has acquired almost 40 years of experience with power/hand tools, plumbing, and woodwork.
Mixing Concrete With a Mixer
In my other guide, 8 Steps to Mixing Concrete by Hand, we discovered how it's feasible to mix concrete with just a shovel. This is fine for small amounts of concrete, however, a mixer thoroughly mixes the materials, lessening the chances of weak spots in the final result. If you're going to be laying floors or pavements or casting gate pillars or walls, a cement mixer is pretty much essential if you want to get a job done reasonably quickly.
What Is Concrete and What Are the Different Strength Mixes?
I go into more detail in my other article, but let's cover the basics here:
- Concrete is a composite material consisting of cement, sand and stone.
- Cement acts as a binder joining the stone and sand together.
- Stone and sand, also known as aggregate, mixed together on their own with no cement added would have no strength. However, cement holds them together and interlocks the particles like a 3D jigsaw puzzle. This is what gives concrete its shear and compressive strength.
Varying the proportion of cement, sand and control controls the strength of concrete and the force it can withstand without failing. "C" types are used to specify the strength of concrete. These recipes specify the proportion of materials (by volume) which will produce concrete that can have a certain strength.
C20 is a general purpose mix for fixing posts, making concrete pads, door steps, bases for garden sheds, floors etc.
This is mixed 1 part cement, 4 parts stone and 2 parts sand.
C30 is a stronger mix suitable for thinner and narrower concrete slabs, e.g. pavements.
This is mixed 1 part cement, 3 parts stone and 2 parts sand.
Quantity of Water Needed When Mixing Concrete
The drier concrete is, the stronger it is. Ideally it must only be wet enough so that it's workable.
By weight of cement:
The quantity of water needed should be 0.4 to 0.6 times the weight of cement. For a 25kg (4 stone approx) bag, if we take a figure of 0.5 this works out at:
25 x 0.50 = 12.5 kg of water
Now since 1 kg of water has a volume of 1 litre, this equates to 12.5 litres of water for 25 kg of cement.
12.5 litres is nearly 2 3/4 imperial (UK) gallons or 3.3 US gallons.
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In terms of buckets:
Loose cement has a density of about 1.5 kg/litre. That's 1.5 times the density of water. So if we have 1 bucket of cement, it takes 1.5 buckets of water to weigh the same amount and again taking a figure of 0.5, we need 0.5 x 1.5 = 0.75 or 3/4 buckets of water per bucket of cement.
Concrete shouldn't be excessively wet after mixing and should have minimum slump. A shovelful deposited on the ground should self-support itself without spreading out (slumping). Concrete that is too dry is unworkable and also won't have sufficient water for the chemical reaction with cement to progress properly and the concrete to cure, therefore reducing its ultimate strength. Too much water is also detrimental to the final strength because as excess water that doesn't combine with cement evaporates, it leaves behind micropores which are tiny air cavities. The concrete is effectively a sponge.
Tools Required for Mixing Concrete in a Mixer
- Cement mixer, electric or gas (petrol).
- 3 buckets, with one for cement, one for sand and gravel and one for water. Aggregate can be wet, so having a separate bucket for cement keeps the bucket dry, otherwise cement will stick to the sides when measuring it out.
- Barrel for holding water.
- Wheelbarrow for transporting concrete.
PPE for Mixing Concrete
- Dust mask to protect your lungs.
- Gloves to protect your hands from abrasion and cement which is an irritant and somewhat caustic.
- Safety glasses to protect your eyes from splashes from the mixer.
- Steel toe cap shoes/boots for foot protection.
Steps on How to Use a Cement Mixer
A cement mixer allows you to mix concrete quickly and you can produce a wheelbarrow of concrete in about 10 minutes. Here are some tips to get the best results.
- A UK builders bucket will typically hold 3 UK gallons or 13.5 litres. These are convenient for measuring out quantities of materials before mixing commences.
- A small Belle mixer such as the Minimix 130 or Minimix 150 will mix about 90 litres of concrete, so if you do a C20 mix, i.e. 1 bucket of cement, 2 buckets of sand and 4 buckets of stone, this will produce about a wheelbarrow full of concrete. You can make the mix stronger by reducing the amount of stone to 3 buckets (C30 mix).
- Measure out the stone, sand and cement into 3 separate piles on the ground. A sheet of plastic or large wooden board such as a sheet of plywood keeps the ground clean and prevents contamination of the materials by soil and dirt.
- It's advisable to use a small barrel or other large container to hold water if you're doing several mixes. You'll need multiple buckets of water and you don't want to have to go off filling a bucket from a tap/faucet or hose if a mix begins to stick to the drum.
- Start the mixer.
- Shovel half of the stone in first and one quarter of a bucket of water. The abrasive nature of stone helps to scour any remaining wet concrete stuck to the drum from an earlier mix session.
- Next add cement, sand and stone alternately, from the piles. As you add cement and sand, watch for a moment to see how the mix is progressing and add water so that the mix is not over sloppy, but at the same time not so dry that it sticks to the drum.
- You can add all the materials to the drum first and do a dry mix. However cement will stick to the inside of the drum and the likelihood is that material at the back of the drum won't mix properly and get wetted as you add water. So it's better to add them a bit at a time.
- Continue to add the remainder of the cement and sand.
- Never put a shovel into a rotating mixer drum. The mixing spoons can catch the shovel and swing it around and hit you in the face. If the mixture seems to be sticking to the back of the drum, stop the mixer, scrape the mix off the back and pull it forward. Repeat this or add a little more water as necessary.
- If the mix starts to overflow out of the drum, place the wheelbarrow underneath to catch any spill.
- Allow to mix on full speed for about 3 minutes.
- Reduce the speed of the engine and commence unloading of the mixer into the barrow.
- When the drum is empty, a new mix can be started. Add some water first to rinse off the concrete stuck to the inside of the drum. This slush can be incorporated into the next mix. If you are working alone, stop the mixer so that you can deal with transporting and unloading the barrow.
- Don't forget to wash all concrete from your tools before it sets and hose out the drum, ideally while the engine is running and the drum is tipped. Try to avoid getting water from the hose onto the engine. If the mixer is electric, unplug it and relocate the extension lead out of range while hosing.
Protecting Concrete in Dry and Cold Weather
Water added to concrete isn't just for making it plastic and workable so it can be formed into slabs and other shapes. Water or H2O combines with cement in a chemical reaction and actually becomes part of the concrete for all eternity (well not quite, but as long as the concrete is in existence!). Any excess water evaporates over a period of weeks. Since water is necessary for this chemical reaction, it's essential that it is retained until concrete cures or hardens over days to weeks. So once you lay concrete, cover it with plastic to stop it drying. This is important in hot weather and/or when windy. Water can be added onto the surface in subsequent days to keep it wet while curing.
Protecting Concrete in Cold Weather
Cold weather is defined as when the average temperature is less than 41 F (5 C) and also less than 50 F (10 F) for 12 hours in a 24 hour period. During cold weather, concrete should be protected with blankets, bubble wrap or other suitable insulation. This traps the heat of the chemical reaction and stops the concrete from freezing.
How Do I Store Cement?
Cement bags should be kept up off the ground so moisture doesn't soak into them. You can use a palette, or place the bags on boards spanned between wood offcuts or bricks. Unused unopened bags of cement can be sealed into plastic bags (use heavy rubble bags instead of trash bags which can puncture) to lengthen it's shelf life, but apparently it still deteriorates if bags are airtight and it's recommended that it should be used within 6 months.
How Long Does Cement Last For? Does it Have a "Use By" Date?
Use cement within 3 months of the manufacture date printed on the bag. After this it often becomes lumpy where it's in contact with the inside of the bag.
For Additional Information
For more detailed information and examples of what concrete can be used for, see my guide: 8 Steps to Mixing Concrete by Hand Without a Cement Mixer
© 2020 Eugene Brennan