DIY Working with Concrete
Home handymen of all levels of experience will find concrete an invaluable material for many outdoor projects. It can be used for making garden paths, floors for greenhouses, sheds and garages, aprons for garages, driveways, garden walls, pergolas, garden pools and rockeries, etc.
Concrete contains three main ingredients; the most important of them being cement. There are many different types of cement; the one in most general use is Portland cement. This is a fine grey powder made from crushed stone. It is supplied in stout paper bags, in quantities of 2 cwt., 1 cwt. and 2 cwt., and this material is very inexpensive in relation to its manifold uses and long life. The second ingredient of concrete is known as aggregate. This may consist of granite chips, crushed stone, shingle, sand or ballast — the latter being a mixture of shingle and sand. The aggregates are used to strengthen the mixture and to save costs — the price of aggregate being much lower than cement powder.
The aggregate varies in size according to the job. For most purposes a 3/4 in. shingle is used. 3/4 in. is a term of measure and is applicable to the size of the screen mesh of the sieve through which the shingle is riddled, and this simply means that no piece of shingle measures more than 3/4 in. either way. The sand used as an additional aggregate with shingle should be coarse and sharp, the grains being anything up to 1/8 in. Both sand and shingle are obtainable from local suppliers and they are supplied as ‘washed’ ready to use.
The third ingredient for making concrete is cold water. The water is used to soften the dry ingredients, and to mix them in a homogeneous mass. The bulk of the ingredients are carefully measured according to the type of concrete being made. The action and size of the various ingredients are such that when they are mixed, the concrete is solid and strong. The fine aggregate–the sand — is well mixed with the coarse aggregate, so that in the finished concrete there are no voids or hollows. The size of the cement powder is such that it fills all the spaces between the coarse and the fine aggregates and bonds them. Well-proportioned and evenly mixed concrete is very strong. Concrete may be further strengthened by reinforcing it. This may be done by using wire chicken netting in the case of small jobs, or expanding netting or iron rods for larger jobs.
Tools and Equipment:
The tools required for mixing and laying concrete are few and simple. They consist of a watering-can fitted with a fine rose, a garden shovel for turning the mixture and for laying it on the site, an ordinary bricklayer’s trowel with a blade of about 7 in. and. Also required is a screed, which is simply a long piece of wood with a straight edge. A small spirit-level is required, a line of stout thin cord, and pegs. The concrete may be mixed on an existing concrete surface, providing this is well washed down after each mixing, or — and it may be more convenient for use close to the site — the material may be mixed on a bunker. The bunker consists of 6-in. by 1-in. planks, firmly nailed to 3-in. by 1-1/2 in. battens on the underside, and three sides of the bunker are edged to make a shallow tray. The advantage of this mixing platform is that it may be easily transported so that the material is mixed near the site where it is to be used.
To mix concrete of maximum strength it is essential that all the materials are clean and that the cement is dry. The cement should be stored by raising it off the floor of a shed or outhouse, and it is a good plan to buy only sufficient cement for the job in hand. The aggregates, both fine and coarse, should be ‘washed’ quality and they should not be contaminated with soil or dust. The water also, which is used for concrete, must be clean and fresh. The best results are obtained when the concrete is used within 30 minutes of mixing it. It is not necessary to mix all the cement for the job in hand, if the bulk cannot be handled and laid within a reasonable time of setting. To ensure that the material is fresh when laid, the handyman should be able to estimate the quantities required for the job. In this way the dry materials can be mixed in bulk, and a smaller part of the dry mixed materials wetted and turned with water, so that an easily manageable amount of concrete is mixed at one time. A simple method of estimating quantities is described below.
The example given below is for the floor of a small garden shed, but the same method of estimating quantities may be applied to any other form of concrete structure. The size of the floor of the example garden shed is 4 ft. wide by 6 ft. long, and the thickness of the floor is 4 in. The square footage of the floor is therefore 24 sq. ft., and if this is multiplied by 4/12 (the 4-in. thickness) the cubic capacity of the floor is 8 cu. ft. Assuming that a reasonable proportion of ingredients for this type of job would be one part cement to two parts sand, with four parts aggregate, the total parts of the ingredients would be seven, and this figure is obtained by adding the one-two-four parts of the total mixture together. Therefore, if the cubic measurement of the floor is 8 cu. ft., and there is a total of seven parts of the ingredients, the cubic footage of each part in their correct proportion is as follows:
Cement (one part)-1-1/7 cu. ft.
Sand (two parts) – 2-2/7 cu. ft.
Aggregate (four parts) – 4-1/7 cu. ft.
Total: (seven parts) 8 cu. ft.
From this simple example it will be seen that it is an easy matter to estimate all the ingredients for a job of any size, even though the ratio of ingredients may vary. There is a point beyond which the addition of more aggregate would weaken the concrete, and the proportion of ingredients will vary according to the type of job being done.
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