How to Measure and Mix Concrete
In order to mix the ingredients in their correct proportions, when estimating the cubic footage of a job, it will be necessary to use a measuring device. A suggestion is to make a bottomless box with square sides, measuring 12 in. each way inside. A pair of metal handles is screwed to the top edges of the box at opposite sides. If the box is placed on the mixing site, and filled with either of the solid ingredients to the level of the top edges, the box may be lifted by the handles, leaving 1 cu. ft. of the ingredient being measured on the mixing site.
There are two ways of using concrete for garden projects. The concrete may be laid in situ — this of course means that the unset mixture is placed in the site position of the structure — or the concrete may be shaped into preformed sections away from the site, and in this case it is made in the form of bricks or slabs.
Whatever the application of the material and the proportion of the ingredients, there is a correct method of mixing concrete to obtain maximum strength. It has been explained that the concrete may be mixed on an existing concrete surface, providing it is swilled down with water immediately after the material has been used, or that it can be made on a bunker, which of course may be placed near the site where the concrete is to be used. The first part of mixing consists of spreading the sand in a flat layer on the mixing site. The measured quantity of cement should then be spread evenly on the layer of sand. The mixture of sand and cement should then be turned over with a shovel, until the mixed ingredients are of even colour throughout — that is without streaks of yellow sand or grey cement. It may be necessary to turn the dry ingredients three or four times before they are properly mixed. The mixture of sand and cement is then spread over the mixing site and the measured aggregate is spread evenly over the previous mixture. The heap should then be turned over again — at least three or four times — until the aggregate has been distributed evenly. Thoroughness in this part of the work is essential — the more thorough the mix, the stronger the concrete.
The dry ingredients are then sprinkled with water. To do this properly the ingredients should again be spread in an even layer. Only sufficient water is added to make the mass plastic. A good mixture, when squeezed in the hand, should retain its shape and be slightly moist on the surface, without dripping water. The water is best added with a watering-can fitted with a fine rose, and only a small amount of water should be added at a time, the heap being thoroughly t it riled after each addition of water. It is again emphasized that only the minimum amount of water necessary to make concrete workable should be added to the dry ingredients. The concrete should be used within 30 minutes of mixing, and after laying the concrete should be matured. This is done to prevent too rapid drying which will weaken the material.
Maturing consists of keeping the concrete dampened for four or five days after it has been laid, and this is especially necessary in hot summer weather. The best method of maturing concrete is to cover it with wet sacks, and to re-dampen the sacks by sprinkling them with water from a rose-fitted watering-can as the sacks dry out. In addition, to protect the concrete against the heat of the sun, the covering of sacks will prevent drying by strong winds.
Concrete should not be mixed or laid in frosty weather if this can be avoided, but if this cannot be avoided an anti-frost solution or accelerating solution should be added to the water with which the concrete is mixed to speed up the time of drying. The treated concrete is hard enough to walk on within 24 hours of mixing However, in normal circumstances, it should not be necessary to accelerate setting, and the concrete should not be walked on for at least seven days after laying. In addition to the use of additives for anti-frost and accelerating, ordinary concrete mixed with Portland cement may be rendered waterproof by the addition of waterproofing powder — several proprietary brands are available — to the cement powder. Also for special jobs, such as swimming-pools, a waterproof brand cement may be obtained.
Cement work in the garden may be brightened- up by using coloured concrete, and concrete made from ordinary Portland cement may be tinted by the addition of a colouring powder. These powders are obtainable from local builders’ merchants and the proportion of powders to use with different mixtures is printed on the colour-powder containers. Container instructions should be carefully followed to obtain good results. In this way, the concrete used in the same job may be differently coloured. For instance a path can be made of smallwhich can be constructed of batches of slabs, each one made in a different colour. In the case of a garden wall, colour may be added by colouring the mortar, or by using coloured cement bricks.