DIY: laying cement

Step 2: buying your materials

Concrete is made by mixing cement, sand, stone aggregate and water. For small jobs, it’s best to buy pre-mixed concrete in 40kg bags. Then all you do is add water. For larger projects, it’s cheaper to buy the materials separately and mix yourself. Cement is normally sold in 40kg bags. Sand and aggregate are both sold by the tonne or fraction of a tonne - the minimum is usually a quarter of a tonne.

Different projects require different thicknesses of concrete and the mix must be adjusted accordingly. Basically, the greater the load on the slab the thicker it must be. Here are the most common mixes:

• Suitable where a minimum thickness of 100mm is needed, such as driveways, garage and workshop floors, - one part cement, 2 1/2 parts sand, four parts aggregate.
• Used for light duty strips and bases up to 75mm thick - garden paths and the like, - one part cement, two parts sand, three parts aggregate.
• Where great strength is not needed, such as garden wall foundations, bedding in slabs, and so on, - one part cement, three parts sand, six parts aggregate. To calculate the volume of cement and aggregates needed, multiply the area’s length by its width by the required thickness of concrete. Then use this handy guide to estimate your quantities, allowing for wastage by rounding up volumes to the nearest half or whole cubic metre.
• Mix, Cement, Sharp Sand, Aggregate To make 40kg

Step 3: preparation

Step 4: fixing the formwork

Finally, to help prevent your concrete cracking by earth movement, tree roots and usage, adding steel reinforcing, this is essential to increase strength. With bolt cutters cut your reinforcing mesh 50mm less than the distance between the formwork to protect it form rusting. Then place it inside the formwork supported off the ground on bar chairs suitable for the thickness of the concrete (refer to "Your Materials Checklist").

Step 5: mixing the concrete

At this stage, you should also consider adding a colouring agent to your mix. Liquid and powder forms are available in a range of earthy colours to highlight the overall design of your path or driveway.

Hand mixing

The cheapest method when the quantity of concrete required is not too large.

Mix on a smooth, clean surface such as an old sheet of board - never on dirt. Using a bucket, measure out the correct amounts of each material into a pile You can double up quantities if you like, but mix only what can be used immediately. With a shovel, turn the pile over several times until it is a uniform colour with no patches of cement or sand. Make a well in the middle of the pile and add a little water. Push in the mix around the edges into the water until the water has been absorbed (Fig. 3). Then turn the mix over until it is evenly moist, form a new well in the middle and add more water. Continue doing this until you have a workable mix.

Power mixer

These can be hired for any reasonably sized job and save a great deal of time and effort. First, with the mixer running add the aggregate into the drum. Then add the sand and cement and let these mix thoroughly. After a minute or so, add some water and let it mix in for two to three minutes. Check the mix and add just enough water to make a workable mix. Once this is obtained, mix for another minute. Be sure to clean the mixer with water after each mix. When you’re finished for the dry or the job, leave the mixer running and add some water and aggregate to clean out the drum.

Ready-mixed

For really large projects. All you have to do is tell the concrete supply company the size and they’ll calculate the amount needed and the cost. Some even have smaller trucks that can drive right into your garden. But remember, be prepared to lay it fast if the concrete can’t be tipped directly into your formwork. Have plenty of helps with heavy duty wheelbarrows (you can hire these, too) standing by.

Step 6: laying & levelling

• Pour the mixed concrete into the formwork, spreading evenly with a rake or shovel (Fig. 4). Make sure the corners and edges are well filled with no air gaps, and that thee reinforcing is fully covered - any exposed steel could rust and deteriorate, weakening your slab over time.
• The concrete should be slightly higher than the sides of the formwork. Concrete slabs shouldn’t be more than 3 metres in any direction without a joint to control cracking.
• The simplest way is to include a 5mm thick piece of softwood or bituminous stripping (plastic and rubber is also available) the same depth as the concrete as you’re laying it (Fig. 5). It acts as an expansion joint and remains embedded in the concrete.
•  To level the surface, lay the tamping board across the concrete at one end and rest on the formwork.
• Move the board to and fro in a sawing action to level the concrete, filling in with more or scraping off excess as necessary (Fig. 6).
• Then use a firm tapping action, moving the tamper forward by half its width each time, to compact it.

Step 7: the finishing touches

• After tamping, the surface is usually suitable for most driveways or paths with a slightly ridged non-slip finish. Or, draw a stiff-bristled broom gently across the concrete to create a non-slip pattern of straight lines (Fig. 7).
• One of the simplest finishes is to run the back of your shovel over the wet concrete, producing a fine swirling pattern.
• If a smoother finish is wanted, use a wooden float. Press the float down firmly and “scrub” with a circular action. Then allow the water in the concrete to rise to the surface. Don’t scoop or brush this water off. Let it disappear and then finish off with the wooded float.
• For a smoother, polished finish, use a steel float at a slight angle and drawing it towards you with a sweeping semi-circular action. Two passes with a float should be enough - any more and you could weaken the concrete.
• Next, slightly round the edges with an edging tool to prevent chipping and to give your job a more professional look (Fig. 8). Finally, allow the concrete to cure.
• Cover it with damp sacking and regularly keep it moist, or use plastic sheeting. Keep it covered seven days - more in cold weather. Now all that’s left is remove the formwork - and admire a job well done.

Useful info

• If hand mixing, mix on a large sheet of plywood or hardwood, or use a heavy layer of polythene sheeting - never mix on dirt.
• Avoid using too much water in mixing - this will weaken the concrete and cause shrinkage as it hardens.
• As a guide to a workable mix, draw the shovel tip over the mix and leave a series of clear-cut ridges. If the mix is too dry, the ridges will crumble - if too wet they will spread out and level.
• If you want curves in the formwork, use hardboard cut into strips as wide as the concrete is deep and support with frequent pegs around the curve.
• If using a power mixer and you want to break off work for a while, add the aggregate and water you’ll need for the next batch and leave the mixer running while away to keep the drum clean.
• When the surface is level, tap the outside edge of the formwork with a hammer to settle the concrete - fill in any resulting hollows and level again.