Concreting Skills Explained

Concrete is made by mixing together aggregates (sand, and gravel or stones) and cement with water. For most DIY jobs you can use all-in aggregates, or ballast.

There are different types of cement available but for most DIY jobs ordinary Portland cement will do. This is light grey in colour and is normally supplied in 50kg bags. It is possible to buy smaller bags when only a little is needed for small projects or repairs. Portland cement is a type, not a brand name, so provided that it is manufactured to British Standard BS12, it matters little which brand you use.

For all general purpose jobs a mix of 1 part cement to 4 parts ballast (by volume) is suitable. For footings, foundations and bases for pre-cast paving, use 1 part cement to 5 parts ballast.

Self-mix or ready-mix

The simplest way to order your materials is to tell your supplier the exact dimensions of the area to be concreted, the thickness of the slab, and the concrete mix you will be using

and let him calculate the number of bags of cement and the number of cubic metres of ballast required. As a rule of thumb, a cubic metre is about 25 wheelbarrow loads.

Although the ballast can be stored on a clean, firm area for any length of time, provided that it is protected from the attentions of animals and children, the bags of cement should not be kept more than a week or two in case moisture in the air penetrates the paper bags and causes hardening. Store the bags flat, under cover, on a dry surface. If they have to be kept in the open then place them on strong boards supported on bricks and covered with plastic sheeting.

The alternative to buying separate materials and mixing your own concrete is to use ready-mix. There are several specialist firms who will supply concrete in this form and their addresses can be found under ‘Concrete’ in Yellow pages and in the local press. Specify the amount of concrete required and for what it is to be used, and the supplier will agree a time for delivery.

Ready-mix has certain advantages and disadvantages. The biggest factor in its favour is that it saves a lot of time and effort

in mixing the concrete. If the area you want

to concrete is readily accessible to a large lorry then you could have the whole load poured straight between the formwork for spreading and levelling.

The main disadvantage is that, with certain firms, you will be expected to deal with the load in a restricted amount of time which means having to line up plenty of friends with wheelbarrows — especially if the concrete has to be transported a fair distance from the road. Some firms help here by agreeing to work at your pace but you must remember that a large load can be pretty exhausting to cope with.

The other way to buy concrete ready-mixed is in dry-mixed bags. This method is handy for small jobs or repair work but it is not economical for big jobs. Don’t keep dry ready-mix in store for more than a couple of weeks and always store in the same way as bags of cement.

Equipment

For transporting materials and laying the concrete, you will need: Spade and fork for generally clearing the site and digging foundations.

Garden roller or punner for firming subsoil and hardcore in foundations. A punner is a home-made alternative for a garden roller. Make a square timber mould about 150 x 150 x 100mm in size and fill it with concrete. Insert a broom handle or length of pipe vertically in the mould and keep it supported until the concrete has hardened fully. Remove the timber mould and you have a heavyweight compacting tool.

Hammer, saw, straight-edge, spirit-level, tape-measure, string line and wooden pegs for setting out a level site and formwork to retain the concrete.

Builder’s square for accurate marking out of right-angled corners in formwork when making a rectangular concrete base. Make one from three pieces of wood in the proportions 3:4:5; a useful size would be 450x 600 x 750mm. Join the pieces with L-shaped metal brackets and screws. If you work accurately, the angle between the shorter sides will be 90 degrees.

Use a mixing machine for any sizeable job. For mixing small amounts of concrete by hand you will need:

Two shovels and two same-size buckets, one of each for measuring out the cement, the other for adding ballast and water to the mix and for the actual mixing.

Mixing platform or solid area available for mixing. Make a platform about 1200mm square from timber boards on battens, or from 18 or 25mm thick plywood. Add small side-pieces to keep the mix on the platform. Sturdy wheelbarrow to transfer the materials from the storage or delivery site on to the work area. Also arrange for some strong boards if the barrow has to be pushed over soft ground or steps.

Shovel for transferring the wet, mixed concrete into the formwork (the mixing shovel can be used for this).

Rake for roughly levelling the concrete. Tamping beam for final levelling of the concrete in the formwork. A narrow path can be levelled using a piece of 100 x 50mm timber on edge. A wider path or patio needs a stouter timber of 150 x 50mm to which strong handles have been fixed. In both cases the timber should be about 300mm longer than the width of the formwork. Steel or wood float, or soft or coarse broom for achieving the preferred surface finish. Polythene sheet, straw or sacking to protect fresh concrete from the harmful effects of frost or hot sun while the concrete sets.

Preparation

A job such as a base for a shed does not involve a great deal of planning. You will have found out its required dimensions and know roughly where you want it to be. However, a path, patio or drive needs more thought. It is best to make a scale plan of the project first to see how it fits with its

A rectangular slab which is to be a base for a shed or garage, for example, has to be level. Here you would first need to set up string lines fixed to pegs, then check with a builder’s square that the corners are true right-angles and with a steel tape that the diagonals are equal. When all is correct, set in the formwork timbers and check that they are level all round.

Surroundings. A curved path or an ornately shaped patio can be ‘drawn’ out on site using a length of string or rope or a sprinkled layer of sand.

Clear the site of all weeds and roots which are likely to cause problems later on. For most jobs the concrete can be laid directly on well-compacted ground with stones being used to reinforce any soft patches. However, on weak ground, clay or peaty soils you will need to lay a 100mm thick base of hardcore — broken bricks and stones — and compact it well with a roller or punner. Dig out the ground bearing in mind where you want the surface of the concrete to be, and take into account the thickness of any hardcore base plus the thickness of the concrete layer.

All concrete has to be laid within a formwork of strong boards which act as a mould to retain the wet concrete until it is set. The boards are left in place for about a week after concreting. Any reasonable straight-edged boards will do provided that they are at least 12 to 18mm thick. If a path needs to curve, bend formwork timbers by sawing through them at several close spacings to about half their thickness.

Setting out

A long path will first have to be defined by stretching two string lines and fixing them to pegs at each end of the site. The formwork boards can then be laid on edge, and supported by stout pegs driven into the ground so that their tops lie below the top edge of the formwork. Butt-join lengths of formwork half-way across a peg and nail through into the peg, ensuring that the nail heads are flush. All paths need a slight crossfall in order to shed rainwater so allow for this by building in a slope of about 12mm in m from one side to the other. To set this slope in the case of a m wide path, therefore, you would place a 12mm thick shim of wood on the lower formwork and use a spirit-level on a straight-edge across the shim and the other formwork. Adjust the height of the formwork to give a level reading.

Hand mixing

Mixing concrete by hand is arduous so only do so if you are well used to tough work, and then only for small batches.

First measure out the required quantity of ballast in a bucket and pour it on to the ground or mixing platform; make a hole in the top of the heap and add the necessary amount of cement. Now turn the heap over and over with the shovel until the ballast and cement become thoroughly mixed and the heap takes on a uniform grey colour. There must be no streaks of ballast in it.

Make a crater in the middle of the heap with solid ‘walls’ round it. Pour some of the water into the crater — don’t overdo it or the water may run out of the sides. Gradually shovel dry material from the edges of the crater on to the water and start turning the mix over and over with the shovel. Use the shovel in a chopping motion occasionally to help the mixing process. Again form a crater in the middle of the partly mixed heap, add more water and continue to mix in dry material. Keep repeating this process until the materials are well mixed together. The concrete should be easily workable but not too crumbly or sloppy. Test this by trowelling the surface with the back of the shovel — it should remain solid but moist.

If you are using a bag of dry-mixed materials then pour all the contents out of the bag before adding water to mix them together. If you want to use only part of the bag still pour the whole lot out and mix it up dry. Then shovel the unwanted material back into the bag before adding water to the remainder which is to be used. This is done because the cement in a dry-mixed bag tends to settle at the bottom so you would not get a true mixture by pouring out part of a bag.

Machine mixing

A mixing machine can be hired locally. For most jobs around the garden a mixer of 100 litres capacity is ideal. Petrol- and electrically-operated types are available. Place the mixer on site and ensure that it is level and that the wheels are chocked to keep it still_ If it can be positioned alongside the formwork then the mixed concrete can be tipped directly into the formwork. The alternative is to discharge it into a wheelbarrow for taking to the formwork.

Put half the ballast and half the water into the revolving drum, let it turn over for a while then add the cement and the remainder of the ballast. This will make the mix crumbly and dryish. Next, slowly add the remainder of the water and allow it to turn over until correctly mixed. A total mixing time of about two minutes is about right. At this point the mix should fall cleanly off the blades of the mixer but shouldn’t be sloppy. At first it is worth testing the consistency of each batch as described for hand-mixing.

You should never leave the mixer for a long time without cleaning it out. If you take a break, pour half the ballast and water for the next batch into the drum and leave it revolving.

Laying concrete

One problem when shovelling and levelling concrete in the formwork is that ‘air pockets’ may be left, especially around the edges. These cavities will eventually cause broken edges or hollows to form. So when each batch of concrete has been tipped or shovelled into the site it should be well tamped down — the heel of your boot is often the best method of ensuring that edges are well filled.

When enough concrete has been poured to fill about a metre of the formwork right across its width, use the back of a rake to leave the surface slightly proud of the top edge of the formwork — allow about 10mm for every 100mm of finished thickness. Now use the tamping timber to level off the mix with the top of the formwork; rest the tamper on the formwork and work it backwards and forwards in a sawing movement, then mix and shovel more concrete on to the site and level off this batch.

Finishes

Different surface finishes, smooth or wrinkled, can be achieved. After tamping, the surface will take on a sort of rippled washboard appearance.

There is a variety of brushed finishes which can be used for various effects. The results depend on the type of brush bristles used and at what stage in the setting of the concrete the work is carried out. It is a good idea to practise on a small unimportant area to get the technique right before tackling the whole job. With a narrow path it should be possible to stand on the ground alongside while working. For a wide drive, however, you will have to use a strong plank supported clear of the concrete.

For a smooth finish use a soft broom to gently brush the concrete immediately after it has been compacted. Always use the broom in the same direction. A more pronounced ripple will be formed by a nylon or stiff bass broom used immediately after compacting the concrete. The correct technique is to drag the bristles across the surface, holding the brush at a shallow angle so that the surface is indented but not torn up.

For a textured aggregate effect, spread a thin layer of ballast on the surface after compacting and then firm it into the surface with a float. Allow the concrete to harden further until the aggregate is well gripped and then lightly spray with water and brush the surface to remove any loose material and leave the stones slightly proud of the surface. Use a stiff broom a couple of days later to finish off.

Other finishes are attainable with a float. A wood float used on fresh concrete will give a sandpaper texture. If the mix is allowed to build up on the face of the float a coarser texture will result. If the float is used in overlapping circles then an attractive ‘fish-scale’ appearance will develop. A steel float gives a smooth, fresh finish. As its name implies the tool should literally be skimmed or floated over the surface to avoid leaving marks. Also avoid over-vigorous trowelling of fresh, new concrete, as this will draw water on to the surface, producing a weak layer which will be prone later to dusting or surface damage. If the surface has been allowed to stiffen before steel trowelling, a very tight, hard-wearing finish will be achieved.

An unusual circular combed-finish can be made with a scrubbing brush after the back of the shovel has been ‘floated’ on the surface. It is also possible to create random or fixed patterns in a smooth steel-float finish which has been allowed to harden for a while. Using a pointed implement, a crazy-paving design can be drawn or a regular brick-bonding formation can be set out. The latter entails the use of a taut string-line stretched across the concrete so that an accurate line can be ‘drawn’ at uniform spacings.

The edge of a concrete slab will be sharp when the formwork is removed. Not only will this tend to break off easily but it could also be painful for an ankle to knock against. It is best, therefore, to round it off after the finish has been completed. Make a suitable implement for the job using a piece of sheet metal bent round a piece of rod or dowel. Run the tool along between the concrete and formwork to make the edge neat and uniformly rounded.

Protection

Unless concrete is allowed to dry out in its own time it will be weak and prone to damage. Hot sun will cause rapid drying, leading to shrinkage cracks; frosty conditions can freeze concrete, causing cracks.

As soon as the surface of the concrete is hard enough not to be marked it should be protected from extreme elements. In hot weather cover it with a blanket of polythene sheeting or hessian sacking. Weight down the material at the edges to stop wind blowing underneath it. Tape joints between plastic sheets and sprinkle the sheets with sand to prevent them ballooning in gusty weather. Hessian sacking must be kept damp by sprinkling it with water. Whichever method is used, leave it in place for about three days.

You should only concrete when there is no danger of frost. If it is unavoidable, however, cover the surface with a layer of straw, retained by polythene sheeting or a sprinkling of earth or sand. Are also available. It is best to stick to one manufacturer’s range when choosing your slabs — mixing ranges can cause problems as some are made to metric dimensions and others to imperial sizes. Most ranges, however, will be made to a standard module size, normally 225mm, so that slabs of different sizes can be combined to make patterns.

The slabs may have squared or slanting edges. The latter type automatically provide uniform joint lines after laying which are intended to be filled with mortar. Square-edged slabs are intended to butt up closely so obviating the need to fill joints. They can be laid with a gap of about 8 to 10mm which is later filled with mortar.

Sorry, comments are closed for this post.

Share On Facebook
Share On Twitter
Share On Google Plus