Concrete mixing and casting

Although concrete is considered to be hard, heavy and uninteresting, it is really quite versatile and there are a number of textures and finishes that can be produced to make it more acceptable. Initially it is soft and plastic so that it can be moulded into almost any shape, provided that the correct mix is used, but thin sections may be difficult to reinforce.

The basic materials for making concrete are Portland cement, sharp sand and gravel. Cement is supplied in 50 kg bags, but it can be obtained loose for storage in silos of large static mixing plants. The fine and coarse aggregate, are supplied by the cubic metre. The coarse aggregate is graded for most concrete work, but it is possible to get an ‘all-in’ ballast which is a mixture of sizes from sand to about 19 mm stones. This is suitable for use where the uniformity of the concrete is not important; concreting fence posts into the ground for instance. For quality work the materials must be measured separately.

Bags of ready-mixed dry materials can be obtained and they are useful on sites which are too cramped to provide storage space for both the materials and the mixing machine, and where the quantities needed at any one time would be too small to make deliveries of ready-mixed concrete an economic proposition.

Ordinary Portland cement is grey in colour, but it is possible to get white cement if the job warrants the expense – white cement is much dearer than the ordinary cement. Colouring materials are obtainable either in powder or liquid form and if they are used the maker’s instructions must be followed exactly. This applies especially to quantities as the degree of colour can be maintained only by accurate measurement, because the concrete will vary in colour as it dries out. A newly mixed batch will look quite different from that already laid, which itself will look patchy as the area dries at different rates.

Sand bought at the merchants will have been washed to remove the dirt and impurities. It must therefore be stored on a clean, hard base. On a large site a weak concrete mix is laid a few inches thick over the ground, but on a small site old wooden boardings can be used. Wooden sides are needed if large quantities are to be stored, and also to keep the sand from mixing with the large aggregate. Small quantities can be stored on thick polythene sheeting if care is taken not to cut the plastic with the shovel. The same precautions apply to the coarse aggregate which can be either gravel or crushed rock in sizes between 19 mm and 4 mm.

Mixing machines are obtainable in sizes to suit most jobs, but where hand mixing is to be undertaken it must be done on a clean, hard surface such as a platform of boards. If the concrete is mixed on a drive or road it will cause stains that will not come off.

A gauge box is required to measure the materials accurately. This can be made out of wood and is about 300 mm cube. A bucket is the traditional measure, but as they are all different sizes, it makes calculating the total amount of mix being made rather difficult.

A good general mix suitable for strip foundations, not less than 150 mm thick, for walls or for drives and garage floors not less than 100 mm thick and laid over 150 mm of well compacted hardcore, is mixed at a ratio of 1 : 2, /2 : 4, which is one part cement, two and a half parts sand and four parts coarse aggregate.

When mixed by hand, 2 1/2measures of sand are placed in a flattened heap on the mixing platform. Then, four measures of coarse aggregate are spread evenly over the top of the sand. These two materials are then blended together by turning them over using a shovel and forming them into a new heap. This process is repeated two or three times when one measure of cement is added and mixed in the same way until the mix is of an even colour, before the water is added.

The usual method of adding the water is to hollow out the centre of the heap to form a crater and pour the water into it, gradually pushing the dry mix into it and as the water is soaked up, the wet mix is turned over to ensure thorough wetting. The problem with this method is that the large quantity of water tends to wash the cement off the aggregate, undoing all the careful coating which has taken place during the dry mixing stage. This cement then forms a slurry at the bottom of the crater and if the sides are breached the slurry runs away off the mixing platform and so reduces the strength of the mix.

A better method of adding the water is to sprinkle it on the dry materials using a watering can. Spray just a little at a time and turn the concrete over between spraying, this way the cement coating will stay on the aggregate where it is needed and there will be less chance of the mix being over wetted.

Whatever method is used, either hand or machine, it is important that only enough water is used to make the mix sufficiently workable to be placed into the casting mould or into the foundation without it forming air holes. The mix is tamped into the mould or trench using a stout piece of wood or a mechanical vibrator to ensure that the mould is filled solid without any honeycombing. Over tamping, or vibrating, a mix that is too wet will bring the cement to the top instead of it being evenly spread throughout the mix. Too much water will cause excessive shrinkage as the concrete dries, crazing the surface with tiny cracks.

Once the concrete has been mixed it must be placed in position within one hour. It must not be allowed to dry too quickly; finished work must be covered. Polythene sheeting is used generally as it is impervious and will keep the moisture in the concrete by preventing it evaporating. When the cast, or drive, is hard enough it can be wetted gently and the polythene replaced. At this stage the sides of the casting mould are removed so that the sides of the cast can be wetted and any surface defects made good.

The amount of making good required will depend on how well the mould has been made and how carefully the concrete has been placed in it. Concrete that has been properly mixed and placed into moulds that have been designed so that they can be taken apart easily, will produce casts that need little or no making good. The sides of casts can usually be removed after 24 hours, but the cast will not be strong enough to be lifted for another three or four days. In the case of lintels or beams cast in situ, the bottom or soffit must remain supported for at least a week after the sides have been removed.

Honeycombed surfaces, caused by poor compaction, or using a mix that did not contain enough fine aggregate or which is the result of insufficient mixing, should be made good with a mortar mix of the same proportions as the concrete mix, but without the coarse aggregate. That is, in the case of a 1 : 2 ½: 4 mix, the mortar should be made of 1 part cement to 2 ½ parts sand, but patching should be avoided if at all possible because the patch generally dries darker than the original concrete.

Cement mortar and concrete shrink slightly as they dry and harden, so use the mix as dry as possible, because the amount of shrinkage depends largely on the amount of water present in the mix. Before filling the holes, all loose and thin, fragile material must be removed from the area and if a large patch has to be filled it must first be primed with neat cement grout which is simply cement and water mixed to a creamy consistency and brushed into the surface. Cement mortar is applied immediately and smoothed off with a wooden float. Patches should not be smoothed with a steel float because this would darken the surface making the patch show even more. Patches will need the same curing treatment as the main concrete.

Concrete walling blocks and paving slabs are easy items to cast as they need no reinforcing and can be made in simple square boxes. Planed timber should be used as it will give a better finish and will not stick to the concrete as much as rough timber would.

The most important point to remember when making any kind of concrete mould is that it must be easy to take apart after the concrete has started to harden. For this reason it is better to use bolts to hold the mould together than to use nails or screws. Nails get bent and soon loose their grip and screw heads soon get badly damaged and cannot be reused. These fixings are suitable only if the mould is to be used for just one cast, as when casting a concrete lintel in situ.

Mould boxes can be made separately or in multiple units, depending on the number of casts needed and the type of casting. Paving slabs for instance are made flat with the sides of the mould not more than 50 mm high. If the mix is not made sloppy there will not be much outward pressure and 25 mm thick timber will do the job. If there is sufficient space a number of slabs can be cast in a long mould.

The dividing battens are housed into the long sides about 6 mm, not for strength, but to ensure that they are held in the correct position. The long sides can be held at the ends by long threaded rods with nuts and washers or they can be held by wedges driven between them and blocks of wood screwed to the baseboard. If this wedge method is used, stripping the sides when the concrete has set is simply a matter of knocking the wedges out and lifting the side away. Avoid making casts too heavy, a 600 mm square slab 50 mm thick is the maximum that one person can be expected to move for laying. A thickness of 38 mm is sufficient for slabs for paths around dwelling houses. Slabs 600 mm X 450 mm X 38 mm are better for handling and, used with 300 mm square slabs, can be laid to various patterns.

Building blocks can be made either standing on edge in the laying position or they can be cast lying flat. Blocks are usually 450 mm X 230 mm and in thicknesses from 50 mm to 100 mm. They can be cast singly or in multiple boxes. As the mould will have to be used many times it is worthwhile taking care with its construction. Timber at least 25 mm thick is used and the sides are held firmly by bolts, one at the top and one at the bottom at each end. The ends are housed into the sides to ensure that they are fixed in the same position each time the box is used.

The mould is fixed to the baseboard to keep it firm while it is being filled and the inside of the mould is given a coating of a releasing agent to ensure that the wooden mould will leave the concrete block cleanly, without any sticking. For this reason it is essential that the moulds are cleaned thoroughly between castings.

Moulds should be filled with concrete in layers about 50 mm deep, each one being tamped well before the next layer is laid. If a smooth-faced block is required a trowel is used with a chopping action at the sides of the mould to compact the concrete and bring the cement forward so that honeycombing is avoided. A bolted box can be removed as soon as the concrete has set as this can be done without shaking and cracking the casting. The blocks must be kept damp and are not moved for a few days after the mould box has been removed.

Apart from making blocks and slabs, concrete can be used for making other building units such as lintels and copings. Lintels need only simple boxes like the building-block boxes, but longer and with centre supports to prevent the sides bowing under the pressure of the wet concrete. This support can be given by wire passed through holes drilled in the sides of the box. The wire is simply cut when the box is removed and the ends snipped off close to the cement.

Moulds for copings are slightly more complicated because the copings must have sloping tops to drain the water off and they must have a groove or throat at each side on the underside to make the water drip clear and run down the wall. They can be cast either the right way up or upside-down. If the coping is to be cast the right way up then the mould must have one side higher than the other and sloping ends so that they can be used as a guide for levelling off the top. Small beading strips must be nailed to the base board to form the groove or throat. Small headings frequently become so tightly embedded in the concrete that they pull out the nails when the cast is removed, and have to be chiselled out of the concrete.

When copings are cast upside-down in the mould boxes, these little beads are at the top and can be removed while the concrete is still fairly green, so that any nibs that are holding the beads will break off easily and at the same time, any damage can be repaired with a better chance of keying into the concrete than if the concrete were fully cured.

This method of casting means that tapered wooden slips have to be fitted into the bottom of the mould box to form the slope of the coping top. The angle need not be steep – 25 mm is sufficient to make the water run off. Saddle-back coping which slopes to each side need be only about 75 mm at the centre and 50 mm at each side. The coping must over hang the wall by 25 mm to 38 mm with the throating about central in the overhang.

In order to get a dense and smooth finish, the concrete is mixed with pea-shingle as the coarse aggregate, or a one-part cement to three-parts sand mix is used. Fine material is spread over the bevel slips and well compacted to make a smooth surface for the finished coating.

A mould box for an upside-down casting needs to be fixed to a flat wooden base so that the sides can be held in place by wedges. Then, when the concrete has hardened sufficiently, the whole box can be turned over and the wedges knocked out and the base and sides removed. This will expose the face of the concrete so that any blemishes can be made good and the surface trowelled to a uniform smoothness. When the cast is made the right way up this operation can be carried out without disturbing the casting at all.

Castings are kept damp while the concrete cures, in the same way as the drives and paths. Spray the surface with water gently so that it does not get pitted, then cover it with polythene.

Reinforcing is not needed in copings and small castings, but it is needed in lintels and gate posts. Lintels for dwellings rarely need more than two reinforcing bars at the bottom edge, and the ends of the bars are bent round to form a hook so that they will be anchored in the cement. Reinforcing is needed because the concrete is not strong under tension, although it has a very high compressive strength.

When a beam supported at each end is loaded, there is a bending action imposed on it. This puts the upper part of the beam in compression and the lower part of the beam, which is stretched, is under tension. Therefore, the reinforcing is placed in the bottom part of the lintel or beam. It needs 50 mm of concrete around it to protect it from the weather which would make it rust. However, the reinforcing bars do not have to be bright metal or even galvanised metal, the patina of rust that they normally carry helps to give a grip on the concrete. No oil or grease must be allowed to get on the bars. The ends of the bars are bent back to form a hook which will anchor them and hold them in place against the tension which is set up.

Where beams are supported in the middle as well as at both ends, the reinforcing has to be bent to rise to the top of the beam where it passes over the central support because the beam will tend to bend over the support and the tension will be at the top. It can be seen that except for simple lintels, the designing of reinforcing must be done by a skilled engineer.

When posts are cast, four reinforcing bars are better than one in the middle of the post. Place a reinforcing bar at each corner to resist any tendency to bend in any direction. Give the bars as much concrete cover as possible. The bars themselves need to be 6 mm diameter for most posts. Bars 12.5 mm diameter are only needed when load bearing columns are being cast, and in this case the reinforcing must be designed by an engineer.

Reinforcing is not usually needed for domestic drives or for the bases of garages and sheds, but where it is needed it is again placed at the bottom of the concrete. The reinforcement for concrete slabs of this nature is in the form of a light metal mesh made of about 4 mm rods welded at the cross joints to form 75 mm squares. The mesh is held about 50 mm off the ground or hardcore, by tying 50 mm concrete cubes to the underside of it with wire for it to rest on. These blocks can then stay in place when the concrete has been poured.

Only properly cut and shaped reinforcing bars or mesh are effective, it is no use just putting into the concrete any old lengths of iron bar or mesh that happen to be lying around. For domestic work it is easier and perhaps better to use the ready-made pressed steel lintels which are made to the size and shape necessary for the size of opening which they are long enough to span. These lintels are light in weight and can be positioned by one man.

Provision for fixings in the castings must be made before the concrete hardens. One way of providing fixing points is to set dovetail-shaped blocks in the concrete. They can be tacked to the inside of the mould just sufficiently to hold them while the concrete is poured. When the sides are removed later, they will pull off the points of the nails which can be bent over out of the way.

Bolt holes can be made by putting the bolt in place through the sides of the mould and fitting a cardboard sleeve over it so that the bolt can be removed when the sides are taken off the mould. A well greased, tapered wooden peg can be used for forming the bolt hole, but there is always the danger than the wood will swell and become tightly wedged in the casting in spite of its being covered with grease.

One of the best ways to provide holes in concrete castings is to use pieces of expanded polystyrene. They can be held in place by wire passed through small holes drilled in the sides of the mould. When the sides are removed the wire can be cut and then the polystyrene melted out of the concrete, using a blow-torch.

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