Mixing Concrete by Hand Without a Cement Mixer
Mixing Concrete Without a Mixer
Often there is a need in the garden for a small quantity of concrete. For instance you may want to set a rotary clothes line into the ground or erect fence or gate posts. You might need to create a slab of concrete to act as a base for something. Is it possible to mix the concrete by hand without a mixer? Yes, with a little care and some effort.
Your Options - Pre-mix Concrete or Make Your Own
If you wish 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 are going to require 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. Crushed stone is produced in quarries by crushing blasted rock and then the result is graded by being passed through a succession of sieves. This tends to make better concrete as it grips better in the mix because of the sharp angular edges. 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.
Mixing Ratios for Concrete
A general purpose mixture suitable for fixing posts, laying pavements, making concrete pads, door steps, floors for sheds etc. is known as a C20 mix. This consists of 1 part cement, 2 parts sand, and 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. If you are mixing small quantities, you can double up on the cement to increase strength.
This online calculator by DIY Doctor allows you to estimate quantities of materials required.
Note: A C20 mix is not suitable for building foundations
What Does C20 Mix Concrete Mean?
It means the concrete can withstand a compressive force (actually 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 likely crack long before 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 concrete as measured on a test jig.
Tools and Equipment For Mixing Concrete
- Builders shovel (the pointy type one)
- Ideally 3 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
- 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 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.
- 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!
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!
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, cover your concrete with polythene (after it firms up) to prevent moisture loss and cracking.