When it comes to mixing concrete, there are three options to choose from:
- Mix by hand - where less than ¼m³ of concrete is needed, it can be mixed by hand by a reasonably fit person.
- Use a cement mixer - where the quantity is between about ¼m³ and 2m³, a powered concrete mixer is useful
- Buying in Ready Mix - where more than 2m³ of concrete is needed, it is easier and probably cost effective to go for a delivery of ready mixed concrete.
These suggestions are broad guidelines and the quantities are those needed at one time. If you have a number of sections to concrete which you could do over a number of days, then you need to think about the volume of concrete required per day. Mixing by hand or using a cement mixer gives you flexibility (you can do how much, or how little, you want, when you want) - while a delivery of ready mix must be dealt with when it arrives on site.
- Avoid breathing-in cement dust - wear a face mask.
- Wet concrete releases an alkali which can damage the skin, if it comes into contact with the skin, immediately wash it off with plenty of water.
- If concrete splashes get into the eyes, flush out with water and seek medical attention immediately.
Mixing concrete by hand.
Generally in the UK, we hand mix concrete 'on the deck', either using a slab of existing concrete (which can be hosed off and cleaned down afterwards) or on a large mixing board. In other countries they seem to prefer to mix in a barrow or tub - (seems awkward and harder work to me!).
When measuring out the cement and ballast (or sharp sand/gravel), it is best to actually use a bucket rather than measuring by 'shovel loads', you will get more ballast on a shovel than cement. You may not want to use a large bucket; when you add up all the ingredients, you could end up with an enormous pile. It is far easier to mix two small piles than one large one.
- Clean off the working surface, use a brush and hose it down.
- Carefully measure out onto the mixing surface about half the ballast (or sand and gravel - do one of each in turn so that they start to mix) needed into a rough cone shape.
- With a shovel, make a crater in the centre of the heap then measure and add all the cement required.
- Measure out the rest of the ballast (or sand and gravel) and add to the heap forming a cone shape.
- Mix all the ingredients together using a shovel, work around the heap turning the mixture over three or four times to achieve an evenly coloured (grey) mixture.
- With a shovel, form another crater in the top of the heap and add some water.
- Shovel the ballast from the sides of the heap into the central crater and turn part of the heap to distribute the water throughout the mixture.
- Repeat forming the heap, making the crater, adding water, turning the heap until the whole mixture is wetted.
As the dry material becomes absorbed with water, flatten out the heap and start 'chopping' across the top with the shovel, move around the heap to evenly mix in the water.
A good mix should be smooth and plastic, not wet and runny or dry and crumbly. As a guide, keep an eye on the ridges as the top of the heap is 'chopped' - if between the ridges fills with cement slurry, the mixture is OK; if they fill with a watery mix or the ridges disappear, the mix is too wet; if between the ridges stays dry, the mix is too dry.
Note: One of the most common mistakes made when mixing concrete by hand is adding too much water. Add it in small quantities and the heap will suddenly be the right consistence. If you keep part of the dry mix to one side, you could add this into the heap if it does become too runny.
When you have finished mixing, wash and brush down the surface to remove any remains.
Mixing concrete using a cement mixer
- Make sure that you understand the operating instructions for the mixer; especially where the 'OFF' switch is.
- If using an electric mixer ensure the supply is routed through an RCD.
- Make sure that the mixer is positioned on a firm and level surface.
- Never put your hand, or any tool into a rotating drum - pour the ingredients in from above.
- Always wear goggles as concrete can splash out of the drum as it rotates.
Various shapes and sizes of cement mixer are available either petrol or electrically powered. Both are readily available for purchase or hire; don't forget that you will need to plug an electric one into the mains (via a RCD), you need it near the heap of ballast and the work, you won't want to keep pushing heavy barrow loads around more than necessary.
Always stand the cement mixer on a slab of existing concrete (which can be hosed and cleaned down afterwards) or on a large mixing board; however hard you try, odd pieces of ballast, sand, gravel and mixed concrete will find their way onto the ground under the mixer.
When measuring out the cement and ballast (or sharp sand/gravel), it is best to actually use a bucket rather than measuring by 'shovel loads', you will get more ballast on a shovel than cement. You may not want to use a large bucket, when you add up all the ingredients, you could end up with more ingredients than the mixer can take.
Take care that the amount of concrete which you mix in one go is right for the size of drum, too small an amount won't mix correctly, too large an amount could overload the machine.
- Start the machine running with the drum at about 45°. Keep the mixer running until the mixer has been emptied.
- Add about half the ballast (or half the sand and half the gravel) required.
- Add a small amount of water and allow it to wash around with the ballast for a minute or two.
- Add all the required cement.
- Add the remaining ballast/sand/gravel and allow it to mix for a minute or two.
- Gradually add more water giving it a chance to mix in and keeping an eye on the consistency. A good mix should be smooth and plastic, not wet and runny or dry and crumbly. If parts of the mix stick to the side and 'goes over the top', the mix is too dry; if the mix on the inside goes about ¾ of the way up to the top and then falls back into the mix, it is about right; if the mix is not taken up the side, the mixture is too wet.
Note: One of the most common mistakes made when mixing concrete is adding too much water. Add it in small quantities and the contents of the drum will suddenly be the right consistency. If the mixture does get too wet, sprinkle in handfuls of ballast and cement in the rough proportions until it firms up.
- When the mixing is complete, place a wheelbarrow under the drum and empty it into a wheelbarrow. It helps to have two people, one to hold the wheelbarrow steady while the other tips the drum.
One can then wheel the barrow to the work, while the other person can start the next load of concrete.
- When the job is complete, or you wish to take a break of half an hour or more, the cement mixer must be cleaned; never leave the cleaning up until later - the same applies to wheelbarrow, buckets and other tools which have come into contact with the ingredients or final mixture.
Always clean inside the drum; put a shovel load of ballast and some water in the drum and run it for 5 minutes, tip the whole load and then hose the inside of the drum and, with the mixer switched off, use an old brush to clean out any awkward corners (i.e. behind the mixing blades etc).
Using ready mixed concrete
When you order Ready Mixed Concrete for a simple diy job, you will need to tell the supplier four things:
- The quantity (i.e. volume in cubic metres) required. Order 4% to 10% more concrete than you estimate by calculation of the plan dimensions. This will allow for spillage, wastage, over-excavation, spreading of forms etc.
- The formulation of concrete required - it is often easier to tell them what you want it for - i.e. footings, garden path, driveway etc - and the supplier will use their knowledge to choose the right mix.
- When you want it - you may not get be able to tie them down to the half hour, you may have to accept AM or PM of a day.
- The address.
- One thing you need to know from your supplier is the free cancellation time, how late can you leave it to cancel should you need to.
Ordering ready mixed concrete is the easy part, the hard part is when it arrives - the quantity you ordered will arrive, hopefully, more or less at the time you expect it. So you need to be ready to handle it.
Things to consider for the day of delivery include:
- Where is the lorry going to park ? Will it obstruct a main road - do you need some 'No Parking' signs ? Will it fit between the gate posts ? Will it sink into the mud on the site ?
- Are you ready for the concrete ? Are all the excavations dug and forms in place?
- 'Many hands make light work' - the lorry won't want to wait around, so, within reason, the more help, wheelbarrows etc you have, the better.