Mixing concrete—buying cement and aggregates

Press, 8 March 1984, Page 17

Mixing concrete—buying cement and aggregates

If you decide to mix your own concrete, you will need cement and aggregates. There are several different ways you can buy them. Cement is simple: you will need ordinary Portland cement (0.P.C.) and you can buy it in 40kg bags from builders’ merchants, do-it-yourself (D.LY.) centres, some garden centres and even many hardware shops; many D.I.Y. and similar outlets also sell cement in smaller quantities (2.5 kg upward) for repairs and other small jobs. Brand names do not matter — all ordinary Portland cement made in New Zealand is manufactured to the same New Zealand Standard, 3122. White Portland cement, though about four times the price of O.P.C. is useful for

some kinds of work. Colour apart, it is basically the same as ordinary Portland. A word of warning — masonry cement is specially made for bricklaying and blocklaying mortar, and is not suitable for concrete. Aggregates are available in bulk from builders’ merchants and many D.I.Y. and garden centres; full loads can also usually be purchased direct from the pit or quarry. You will get the best results if you buy coarse and fine aggregates separately and proportion them yourself. When buying sand, make sure you get the right sort. For concrete you want “concreting” or “sharp sand” — not builder’s (bricklayer’s or “soft”) sand. The-alternative to separ-

ately-purchased aggregates is combined or “all-in” aggregate, containing both fine and coarse material.

Generally referred to as “builder’s mix,” it varies widely in quality and needs some care in buying. Have a look at it before you order — it should be clean, without much dust or silt (if it stains your hand when squeezed into a ball, avoid it) and well graded, with about 60 per cent of the particles over smm. If the builder’s mix is too sandy or too gravelly, you can improve it by adding extra coarse aggregate or sand.

Some retailers supply aggregates in heavy-gauge plastic bags. The cost is higher but aggregate purchased this way is much

easier to store and keep clean.

Stone chippings and other special aggregates are available from some garden centres and from stone merchants and monumental masons.

Dry-packed mixes are extremely handy for repairs and smaller jobs. They contain cement and aggregates in carefully measured proportions and are widely available from hardware shops and other retailers as well as builders’ merchants and D.I.Y. or garden centres. A range of different mixes is available, in quantities from 2.5 kg (for some types) upward. A major advantage of dry-packed mixes is that they can bfe taken, in the bag, right to the job and mixed there without making a mess of the garden or the house.

Colouring pigments for concrete and mortar are sold by most builders’ merchants but great care is needed in batching and mixing if unsightly variations in shade and intensity are to be avoided from one batch to the next.

Stains for use on hardened concrete are likely to 'give a patch appearance and are not recommended.

Estimating the volume of concrete you need is easy — simply measure the area to be concreted and multiply by the thickness to be laid. Make sure you use the same units for all calculations: if you are laying a path 1.2 m wide, 10m long and 100 mm thick, the calculation will be 1.2 by o.lm to give you the. number of cubic metres of concrete required. If you are mixing the concrete yourself, order aggregates in plenty of time and store them in separate piles on a hard surface.

Aggregates can be stored indefinitely if they are kept clean so you can order enough for several small jobs at one time, but it is a good idea to keep’ them sheeted over and out-of-bounds to children and animals.

Do not, however, order more cement than you can use in a week’s work — moisture in the air can penetrate the paper bags and cause “air-hardening.” Do not use cement that has gone off in the bag. If you run short, it is not usually very difficult to get another bag or two at short notice.

Stack cement bags flat on a hard dry surface, under cover is possible. If you have to store them in the open, keep them on a raised platform of planks clear of the ground, securely covered with plastic sheeting.

CONCRETE MIXES General purpose concrete mix is suitable for most jobs where there will not be any excessive localised wear or loading. Paving concrete mix is recommended for paving areas where there will be excessive wear and localised loading. The common situation around the home for this mix would be driveways.

It is recommended that ready mixed concrete should be supplied to ensure the consistent quality of concrete. In the summer months it is also recommended that a retarder be specified for inclusion in the mix.

Bedding concrete is more economical for jobs like concreting in fence posts, clothes lines etc.

TOOLS Most of the tools you will need for concrete work are ones you probably already have: hammer, mallet and handsaw, shovels (at least two if you are mixing your own concrete), rakes, linen or steel builder’s tape measure, wheelbarrow and so on.

The rest you should be able to hire, borrow or make up for yourself without great expense or trouble.

You will need a good builder’s spirit level with tubes for both horizontal and vertical levelling, a straightedge, timber for set-ting-out pegs and an improvised tamping beam, buckets for measuring materials (good stout ones — the nine litre heavy-duty type sold for building, and farm use is ideal — but

make sure they are all the same size), ana a large trysquare.

will find invaluable is a timber builder’s square. The square is simply three lengths of strip fastened together in the proportions 3:4:5. If the measurements are accurate this will ensure that the angle between the two shorter sides is 90 degrees. The above diagram shows how to make the square and check it.

(The above information was taken from the 28-page booklet, “Concrete at Home,” which is available from the New Zealand Concrete Research Association, Private Bag, Porirna, priced at $3 a copy, post free.)