AGGREGATES
General
The ground on which a structure is to be built has a great influence on the life-span of the structure. If you are building a new house, and are not sure about the ground conditions, it is best to use the services of a professional person to advise you about foundation requirements.
Strip footings are the most common type of foundation, and the requirements will be discussed in some detail.
Except where founded on rock, the minimum founding depth for strip footings should not be shallower than 400mm below the original ground level.
Materials required for good foundation concrete
Once the foundations has been dug, you need to purchase the required materials.
Cement
Quality cement, with the SABS mark on the bag, is required to ensure that the foundations are functional.
Sand for foundation concrete
Sands for use in foundation concrete should comply with all the following:
contain little or no organic material (material produced by animal or plant activities)
not contain any particles which are retained on a sieve or nominal aperture size 5mm
have a clay content such that a “worm” 3mm in diameter cannot be rolled in the palm of the hand by adding a few drops of water to material obtained from sieving a sample of dry sand through a nylon stocking.
when one litre of cement is mixed in a container to three litres of sand and three litres of stone, to a uniform colour, the mixture should not require more than 750 ml of water to be added to reach a wet or “just right” for use consistency.
Stone
Stone for concrete is normally 19mm, although 13mm can also be used, and should always be hard and clean. Stone has a smaller influence on the final characteristics than do the properties of sand.
Water
Water is used for mixing the cement, sand and stone, and should be fit for drinking.
1m3 of concrete requires the purchase of:
# 5.5 pockets of cement
# 0.75m3 of concrete sand
# 0.75m3 of stone
Mixing
Hand mixing of concrete should be undertaken on a surface which is free of contaminants. The sand should be thoroughly mixed with the cement before the addition of the water and stone. The addition of water to the mix should be controlled and should be such that the resulting concrete can be readily compacted into the corners of the formwork and around the reinforcement, without segregation of the materials or excessive bleeding of free water at the surface.
Placing of concrete
Freshly mixed concrete should not be allowed to stand for so long that it stiffens before it is placed. Concrete may be left standing for limited periods, but must be covered with plastic sheets or wet sacks to prevent it drying out. Concrete should not be re-tempered by the addition of water or any other material.
Wet concrete should be remixed before being placed, should the stone particles settle to the bottom of wheelbarrows during transportation.
All excavators and other surfaces of an absorbent nature that are to come into contact with concrete should be dampened with water before concrete is placed.
Wherever possible, the concrete should be deposited vertically into its final position to avoid segregation of aggregates or displacement of reinforcement and other items that are to be embedded. Concrete should be compacted by mechanical means, or by means of spading, rodding or forking, in such a manner that the concrete is thoroughly worked against the formwork and around the reinforcement of other embedded items without displacing them so as to ensure that the concrete is free from honeycombing and planes of weakness.
Wherever practicable, concrete should be placed in a continuous process.
Laying of masonry units
The surface upon which masonry is to be laid should be clean and free of loose aggregate.
Burnt clay units having high initial rates of absorption should be wet 24 hours prior to laying. Units should be surface dry when laid. Immersing of units in water should not be permitted.
A rough but effective field test to determine initial rate of absorption can be made by drawing a 25mm diameter circle (draw around a R2 coin) on the surface of the unit to be mortared. Then, using a medicine dropper, quickly place 20 drops of water within this circle. Note the time it takes for all water to be absorbed. If the time exceeds 11/2 minutes, the unit may not need to be wet prior to laying. If the period is less than 11/2 minutes, the unit should be pre-wet to reduce the water absorption.
Concrete units should not be wet prior to laying and should be laid dry.
Solid units should be laid on a full bed of mortar, with all perpend joints solidly filled with mortar as the work proceeds.
Hollow units should be shell bedded, horizontally and vertically. The face shells of the bed joints should be fully mortared.
Each unit should be laid and adjusted to its final position while the mortar is still plastic. Any unit which is disturbed to the extent that the initial bond is broken after positioning, should be removed and re-laid on fresh mortar.
All perpend and bed joints should have a nominal thickness of 10mm. The bed joint thickness should not be less than 5mm or more than 15mm; perpend joint thickness should not be less than 5mm or greater than 20mm.
Masonry should not be laid when the temperature is less than 5°C. Wet or frozen units should not be laid. In hot or windy weather conditions, the length of mortar runs ahead of units which are to be laid, should be adjusted to ensure that the mortar remains plastic when the units are laid.
The rate of new construction should be limited so as to eliminate any possibility of joint deformation, slumping or instability which may reduce bond strength.
Cutting of units should be kept to a minimum.
Joints in face masonry should be finished and compacted to the required profile with a jointing tool in the period between initial and final set. Joints in faces of walls constructed of hollow units should not be raked.
Temporary supports should be provided to support masonry in arches and above openings. Such supports should only be removed once masonry has developed adequate strength.
Masonry walling should not overhang concrete foundation slabs by more than 20mm.
Plastering successfully
The masonry surface to which plaster is to be applied should be free from oil, dirt and other substances that may affect the bond with the plaster.
Plaster should be mixed on a surface free of contaminants, or by mechanical mixer, for a period of time that ensures all the ingredients are properly mixed.
Before any plastering commences, all chases should be complete and all electrical, plumbing conduit boxes and the like be fixed in position.
Plastering should be carried out in one operation. Joints in plasterwork should only be provided at intersections between surfaces. Plaster should be firmly troweled onto the walls.
Plaster should not be allowed to dry too quickly and should be dampened by means of a light spray for a period of not less than 2 days.
Materials required for good Mortar &-Plaster
Cement:
Quality cement, with the sabs mark on the bag, should be useful to ensure that your brickwork or plasterwork is successful.
Sand
Sand is the most critical selection when doing bricklaying or plastering, and is generally the cause of most plaster or mortar problems.
To avoid most of the problems, sand used in mortar and plaster should comply with the following, and it will be worth checking the sand to avoid major problems later:
contain little or no organic material (material produced by animal or plant activities)
not contain any particles which are retained on a sieve or nominal aperture size 5mm
have a clay content such that a “worm” 3mm in diameter cannot be rolled in the palm of the hand by adding a few drops of water to material obtained from sieving a sample of dry sand through a nylon stocking
when 2.5kg of cement is mixed to 12.5kg of air dry sand the mixture does not require more than 3.75 litres of water to be added to reach consistency suitable for plastering or mortaring
have adequate plasticity
Mortars are best when coarse and medium sand fractions are predominant. These sizes can be viewed through a transparent plastic ruler using a hard lens. (Place graduals on ruler over sand)–
Very coarse 2 – 1mm
Coarse sand 1 – 0.5mm
Medium sand 0.5 – 0.25mm
Fine sand 0.25 – 0.125mm
The visual examination should reveal a high proportion of coarse and medium sand fractions, but also some very coarse sand.
If the visual measurement of sand indicates that it is too coarse or too fine, a complementary sand should be sought and blended with the original sand to improve performance.
Sand for plaster should be predominantly coarse to medium (1.0 – 0.25mm). If the available sands are predominantly fine sand (<0.25mm), they should be blended with a suitable coarser sand.
Concrete Floors, Paths and Driveways
Concrete uses are endless, and whether you are building your own home or tackling a project around your home, a little knowledge will help you make a success of each job.
Concrete is simply a mixture of PPC cement, sand, stone and water, and strength depends on three things:
the amount of each material in the mix, including water
how well it is compacted or packed
keeping the finished concrete damp for as long as possible
Sand for concrete
Sands for use in concrete should comply with all of the following
contain little or no organic material (material produced by animal or plant activities)
not contain any particles which are retained on a sieve or nominal aperture size 5mm
have a clay content such that a “worm” 3mm in diameter cannot be rolled in the palm of the hand by adding a few drops of water to material obtained from sieving a sample of dry sand through a nylon stocking
when one litre of cement is mixed in a container to three litres of sand and three litres of stone to a uniform colour, the mixture does not require more than 75 ml of water to be added to reach a wet or “just right” for use consistency.
the mixture prepared to check on the amount of water required to reach an acceptable consistency should be left in the mixing container, in the shade, for a period of 10 minutes. If a layer of water more than 1mm deep appears on the surface, it is likely that the sand lacks fine material and should be blended with plaster sand.
Stone
Stone for concrete is normally 19mm, although 13mm can also be used, and should always be hard and clean. Stone has a much smaller influence on the characteristics than the properties of the sand do.
Water
Water is used for mixing the PPC cement, sand and stone, and should be water that is fit for drinking.
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