8 Steps to Mixing Concrete by Hand Without a Cement Mixer
Mixing Cement by Hand
Mixing Concrete Without a Mixer
A cement mixer does a great job at mixing concrete (also referred to as "mixing cement"), thoroughly blending cement, stone and sand to give a consistent result. But is it possible to mix concrete by hand without a mixer? Yes, with a little care and some effort. You might need to mix concrete for setting gate, fence posts or a clothes line in the ground. Sometimes there's a requirement to make a concrete slab to act as a base for something. All you need to mix concrete is the materials, a shovel, a couple of buckets and optionally a sheet of plastic.
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How Do I Mix Concrete?
- Spread a plastic sheet on the ground
- Measure out the materials
- Place the stone and sand into a pile on the sheet
- Place the cement on top of the pile of sand and stone
- Shovel the material in the pile to one side, creating a new pile and repeat three times
- Make a deep crater in the pile and add water
- Fold the mix in from the sides
- Continue to fold inwards and "chop" the pile to distribute water through the mix
See below for lots more details and photos.
What is Cement?
Cement is a binder, used as an ingredient in combination with sand and stone to make a composite material called concrete. The three constituents on their own have no real strength but when bound together, stones interlock like a 3D jigsaw puzzle and sand fills the gaps. Cement just glues the stones together but it's the stone that gives concrete its shear and compressive strength, not the cement.
There are several different types of cement two examples are Portland cement and blast furnace slag cement. Portland cement is made by baking limestone in kilns and grinding the clinker produced with a little gypsum to form a fine powder. Blast furnace slag cement is made from the waste products of the steel industry.
Your Options - Pre-mix Concrete or Make Your Own
If you need to mix concrete by hand, there are two options:
- Buy a bag of pre-mixed concrete (drymix). This is available at all good home improvement stores. It has all the ingredients dry mixed together for making concrete, i.e. the cement, sand and stone. All you have to do is put it into a bucket, wheelbarrow or on a piece of plastic on the ground, add water and spend a few minutes mixing it. Usually the product comes in 2 to 4 stone bags ( 10 kg to 25 kg) with varied setting times. Quick setting concrete is available for fixing posts into the ground so that they don't have to be stayed.
- Premixed products from stores tend to work out more expensive if you're going to need several wheelbarrows of concrete. The alternative is to mix your own. All you need is cement, sand and stone (20 mm). Instead of sand and stone you can use ballast or gravel, which is a mixture containing varying sizes of particles ranging from sand to stone. Gravel contains rounded stone and was deposited in gravel pits as a result of the ice age and other geological activity. Gravel or ballast can have a varying proportion of sand/stone, so you may need to judge whether you need to add additional sand, if any.
Crushed stone is produced in quarries by crushing blasted rock and then the result is graded by being passed through a succession of sieves. In theory, this should make better concrete than rounded stone from gravel pits, because of the sharp angular edges.
Concrete Mixing Ratiios
A general purpose mixture suitable for fixing posts, making concrete pads, door steps, floors for sheds etc. is known as a C20 mix. Materials are mixed by volume not weight.
A C20 mix consists of:
- 1 part cement
- 2 parts sand
- 4 parts stone (aggregate)
Alternatively a mix of sharp sand and stone, sometimes called ballast, can be used, and this is mixed 6 to 1 with the cement.
A stronger, more hard wearing C30 mix, suitable for thinner concrete slabs, e.g. pavements, consists of:
- 1 part cement
- 2 parts sand
- 3 parts stone (aggregate)
Ideally stone should be 15 to 20 mm in size (9/16 to 13/16 inches).
Sharp sand should be used, graded from 0 to 5mm. Don't use fine stuff which is really for mortar/plastering.
This online calculator by DIY Doctor allows you to estimate quantities of materials required.
Note: A C20 mix is not suitable for building foundations
How Much Water Do I Need For Making Concrete?
The max amount of water that should be added is 0.55 times the weight of cement. So for a 25 kg bag:
25 x 0.55 = 13.75 kg or 13.75 litres
This is approximately the volume of a 3 gallon bucket.
The mix shouldn't be overly sloppy and should be able to self support itself without slumping.
What Does C20 Mix Concrete Mean?
It means the concrete can withstand a compressive force (or more correctly pressure) of 20 newtons per square millimetre (20 MPa) without crushing. So in theory, for a 10 cm x 10 cm square (about 4 inches x 4 inches):
Area = 100mm x 100mm = 10,000 square millimetres
So total compressive force that the concrete can withstand without crushing is:
Force = pressure x area = 20 N/mm2 x 10,000 mm2 = 200,000 newtons.
A newton is equivalent to a weight of about 0.1 kg so:
200,000 newtons = 200,000 x 0.1 = 20,000 kg weight or 20 metric tonnes on a 10 cm by 10 cm square.
In reality a concrete slab would probably crack or sink with much less than this magnitude of load. This is because the underside of the slab would be spanning the individual stones of the sub base foundation and not supported underneath at every point over its full extent. These figures really refer to the compressive strength of a 150 mm concrete cube as measured on a test jig.
Tools and Equipment For Mixing Concrete
- Builders shovel (the pointy type one)
- Ideally three, 3 gallon buckets One for water, one for cement and one for sand/stone. If the sand or stone is wet and you use this bucket afterwards for measuring cement, it will stick to the sides and bottom
- Polythene sheeting A sheet of polythene helps to prevent mess and contamination of the concrete by pebbles, leaves, soil and other debris. It also makes it easier to scrape up all the leftovers with no wastage. I use 1200 gauge polythene (the heavy stuff used as a damp proof membrane under concrete) and this is widely available in hardware stores. You can of course mix concrete in a wheel barrow or bucket, but there's more room to mix on a sheet
- Wheelbarrow or buckets For transporting the concrete to its final destination. Alternatively you can mix in situ
- Steel toe-capped boots Optional. You may end up standing in or close to concrete while mixing/laying. Steel toe-caps protect your toes if you drop e.g. bricks or blocks on them
Cement is generally available in 25kg bags. In the US it is sold in 47 or 94 pound bags
Mixing Concrete: Sand, Cement, Gravel, and Water
When mixing by hand, it is essential to thoroughly mix the concrete to get consistency throughout the mixture.
If you need to mix a small amount, for instance half a bucket, place the stone and sand and finally the cement into the bucket. This needs to be thoroughly dry mixed with a garden or block layers trowel. Water is then added, say half a pint at a time and mixed throughout the bucket. More water can be added until the concrete is at the required consistency. If the mixture becomes too sloppy, add more cement. If you are filling a hole in the ground e.g. around a post, the mixture can be a bit more sloppy so that it flows easier.
Larger quantities of concrete can be mixed on a flat surface on the ground
You can measure the proportions of the mix by counting shovels. However I find it easier to use buckets. If you go for a C20 mix, a single mix using buckets ( which totals 7 buckets of material) is enough to produce a wheelbarrow of concrete.
So remember you need:
- 1 bucket of cement
- 2 buckets of sand
- 4 buckets of stone
- If you're using polythene sheet, spread it on the ground and weigh it down with blocks/bricks or whatever at the edges in windy weather
- Place the stone onto the ground followed by the sand. Alternatively you may have gravel which is a composite mix of large and small stone and sand. If there isn't much sand in the gravel, you can add some more
- Place the cement evenly onto the top of the pile. If you have someone to help you mix, it will be easier. Start at the edge of the pile with the other person facing you. The aim is for both people to keep shovelling the pile to one side to create a new pile adjacent to the old pile. Repeat this three times so that's four mixes in total.
- Now it's time to add the water. Make a crater in the top of the pile about half its diameter so that it looks like a volcano. Again, it's important that the pile doesn't have slopes that are too steep so when you add the water it breaks through the crater and runs down the slope! Add some water, the amount depends on the amount of dry mix you have created.
- Now with the shovel, go around the edges of the crater and keep sliding the mix into the center. Then use a chopping motion with the edge of the shovel to mix the water with the dry mixture. Continue to shovel the dry mixture from the edges of the pile towards the center. Eventually the mixture will become easier to control as the water becomes more distributed. Then keep sliding the shovel under the mixture and turn it over bit by bit and use a chopping motion with the edge of the shovel to thoroughly mix the concrete.
Don't forget to wash your shovel when finished before the concrete sets on it!
How to Mix Concrete
Steps 1 to 8
Step 1: Spread a Plastic Sheet on the Ground
Step 2: Measure Out the Materials
Step 3: Place the Stone and Sand on the Sheet
Step 4: Place the Cement on Top of the Sand and Stone
Step 5: Shovel to One Side From the Edge of the Pile and Repeat Three Times
Step 6: Make a Deep Crater and Add Water
Step 7: Fold the Mix in From the Sides
Step 8: Continue to Fold Inwards and "Chop" the Pile to Distribute Water Through the Mix
Curing of Concrete
During curing or hardening of concrete, a process called hydration occurs where water chemically bonds to cement. So some of the water you added actually never dries out. It is locked to the cement in a bond forever!
Don't allow concrete to dry out in hot weather or if there are strong drying winds. It needs to cure slowly, so ideally cover with plastic to prevent moisture loss
Protecting Concrete in Cold Weather (or Dry Weather)
The best time to make concrete is when the weather is mild. Freezing weather conditions can weaken concrete and hot dry weather can cause water to evaporate too quickly so that there is insufficient water for it to cure properly, resulting in cracking.
Until concrete cures, it should be protected from the weather and never allowed to freeze for the first 24 hours. Minimum curing temperature should be 40 F (4C). In freezing weather conditions, water in concrete expands as it freezes. As ice crystals grow, they push the concrete outwards, breaking bonds between cement, stone and sand. Then when they melt, they leave millions of micro-cavities, so the concrete ends up porous like a sponge, potentially weakening it.
You can cover concrete slabs after laying with blankets/polystyrene/bubble wrap or whatever to help prevent it freezing. If frost is due to set in at night, lay your concrete early in the day so that it firms up, before covering with insulating material (otherwise it'll get marked by the covering).
In hot, dry weather, wait for a few hours until the concrete firms up. Then cover it with polythene to prevent moisture loss and cracking.
How to Mix Concrete in a Mixer
if you decide to use a mixer to mix concrete faster with greater consistency, the mix ratio is exactly the same. However here are some tips to get the best results. A bucket usually holds 3 UK gallons or about 13.5 litres:
- 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
- Start the mixer
- Empty 4 buckets of stone into the mixer and add some water. The abrasive nature of the stone helps to scour any remaining wet concrete stuck to the drum from an earlier mix session. Take care when emptying stone into the drum that the bucket handle doesn't get caught by the the mixing spoons. If possible, remove bucket handles before mixing. Alternatively you can empty the stone onto the ground in front of the mixer and shovel it into the drum. Never put a shovel into a rotating mixer drum
- Next add cement, sand and water alternately, either directly from buckets or from two measured out piles in front of the mixer. 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
- Continue to add the remainder of the cement and sand
- 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. However if you are working alone, stop the mixer so that you deal with transporting and unloading the barrow
- If you're laying a concrete slab, tip the mix from the barrow within the formwork (the boards which bound the slab). Roughly scrape out the barrow and use a shovel and rake to bring the level of concrete about 1/2 inch above the formwork
- Start a new mix
- If you work fast and alone, you should be able to lay about 3 barrow loads of concrete before a slab needs to be screeded or levelled
- 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
How to Make Concrete Stronger
Concrete is only strong in compression, not tension. This means that it'll withstand being squeezed but not bent. So for instance a concrete pillar in a building can withstand tons of weight pushing down on it, but a concrete beam spanning an opening (e.g. a large doorway in a building) would snap under moderate load. To strenghten it, steel bars called reinforcing bar or rebar is inserted when the concrete is wet. Because steel is strong under tension, the resulting composite material becomes strong both under compression and tension.
You can add rebar rods or grid to concrete when laying floor, pads or anywhere you think that it'll be subject to heavy loads and in danger of cracking. It's also a great way of getting rid of any scrap iron you've accumulated.
Questions & Answers
© 2012 Eugene Brennan