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THE BUILDING PROCESS
FOUNDATIONS & SUPERSTRUCTURE

  • An Introduction to Foundations & Superstructure
  • Mortar
  • Aggregrates
  • Cement
  • Bricks & Blocks

AN INTRODUCTION TO FOUNDATIONS & SUPERSTRUCTURE


This section is the most complex and contains the largest variety of materials. One has only to view the product manuals produced by some manufacturers to appreciate the magnitude of materials that could be used in construction. We have listed the most commonly used materials. Labour in this section forms a very small portion. To relate labour rates to each and every item of material is almost impossible and not practical. The SABS 0400 regulations are clearly and professionally presented and a complete copy should be in the office of all those involved in the industry.


MORTAR

Mortar for Masonry

Mortar binds bricks and blocks together to give strength and stability to a wall.

Freshly mixed mortar must be soft plastic so that it spreads easily and makes good contact with the bricks during laying. It must harden thoroughly without becoming to strong. Too strong a mortar may cause cracking, is wasteful and is more expensive.

Technical information

Materials:
Cement
Use either:
Ordinary Portland cement (CEMI 42.5)
or Portland Cement CEMIIA 32.5

Ordinary Portland cement & lime

Lime
Use building lime with the SABS mark. Do not use quicklime or agricultural lime. Lime is sold in 25kg bags.
Lime should be used if the sand lacks fine material or is single sized as such sands tend to produce mortar with poor workability unless lime is included in the mix.

Lime also helps the fresh mortar to retain water when it is placed against dry cement brick or block and helps to prevent cracking of the hardened mortar.

Sand
The sand should be clean (grass, leaves, roots etc, are harmful) and it should not contain too much clay. It should consist of hard particles which range in size from dust up to about 2mm. Pit sands generally have these characteristics. River, dune and beach sands are often too uniform in size (single sized) to give good results.

Ready-mix mortar (dry)
In some areas mortar can be bought in bags ready for mixing and use. It should be used in accordance with the manufacturer’s instructions.

Batching the materials
A builders wheelbarrow is a convenient measure for large batches; the capacity is 65 litres. Steel drums of 20 or 25 litre capacity and buckets are useful for small batches. Check the capacity of drums and buckets when filled to the brim as this is often more than the nominal capacity. To batch, shovel material into the measure and then strike off, level with the brim.

Mixing
Mixing should be done on a clean hard surface such as a smooth concrete floor or a steel sheet. Small batches may be mixed in a wheelbarrow as the volume of the batch is no more than half the capacity of the barrow.

Sand, cement and lime, if used, should be mixed until the colour of the mix is uniform. Then add water in small quantities, mixing after each increment, until the mix is soft and plastic.

Handling
If mortar is left in the sun before being used, it should be covered with plastic sheeting or a wet sack. Discard mortar that has been stiffened so much that it is impossible to remix it without adding more water.

Note: Concrete bricks and blocks should not be wetted before being laid. Burnt clay bricks should be wetted before being laid.


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.


CEMENT
There are cements formulated primarily for use in concrete, although some may be suitable for sand-cement mixes. Common cements consist of portland cement only, or a blend of portland cement and extender or filler.

From July 1996, when European standard was adopted, the South African standard for common cements became SABS ENV 197-1 “Cement - composition, specifications and conformity criteria. Part 1: Common cements”. The standard specifies a number of properties and performance criteria. Composition and strength are required to be displayed by the manufacturer on the packaging of each cement produced.

The standard specifies composition of cements according to the proportions of constituents, ie portland cement, extenders and fillers.

The standard specifies strengths which are determined in accordance with SABS EN 196-1 “Methods of testing cement. Part 1: Determination of strength”; using a water:cement ratio of 0.5.

The standard permits many different combinations of composition and strength class. In practice, however, the manufacturers will be constrained by what is technically and economically feasible. The number of combinations that are likely to be produced in South Africa will therefore be considerably fewer than the number permitted by the standard.

Masonry cements are formulated primarily to impart good workability to mixes for rendering, plastering and masonry work. Masonry cements are normally a blend of portland cement and finely ground limestone or hydrated lime; some masonry cements include an air-entraining agent.

From July 1996, the stand for masonry cements is SABS ENV 413-1 “Masonry cement. Part 1: Specification’. The standard defines masonry cement as ‘a factory made finely powdered hydraulic binder which relies essentially upon the presence of portland cement clinker to develop strength. When mixed with sand and water only and without the addition of further materials it produces a workable mortar suitable for use in rendering, plastering and masonry work.

The standard specifies composition, strength performance, fineness, setting times, soundness and the properties of fresh mortar.

Fly ash (FA) is collected from the exhaust flow of plant buring finely-ground coal. The finer fractions are used as a cement extender. FA reacts with calcium hydroxide, in the presence of water, to form cementing compounds consisting of calcium silicate hydrate. This reaction is called pozzolanic and FA may be described as a synthetic pozzolan.

The hydration of Portland cement produces significant amounts of calcium hydroxide which does not contribute to the strength of the hardened cement paste. The combination of FA and PC is a practical means of using FA and converting calcium hydroxide to a cementing compound.

FA should not be used on its own as the binder for concrete. The effect of FA on the properties of concrete depends on the FA concrete of the binder. General trends are as follow:

Fresh concrete:
Improves the workability of fresh concrete, ie FA tends to reduce water requirement for a given slump.
Slightly retards the setting of fresh concrete.
Hardened Concrete:
Reduces the rate of hardening and strength gain particularly at low temperatures
Reduces the rate at which heat is generated by the reactions of PC and FA
Improves the sulphate resistance of concrete with adequate FA content. Specialist advice is recommended
Reduces the rate of chloride diffusion through concrete
Can prevent or retard the reaction between alkalis and alkali-reactive aggregates in concrete if used in sufficient quantities, ie > 20%
Results in a finer pore structure and lower permeability if well cured. To achieve good durability all concrete should be well cured.
Condensed silica fume (CSF) is the condensed vapour by-product of the ferro-silicon smelting process. CSF reacts with calcium hydroxide, in the presence of water to form cementing compounds consisting of calcium silicate hydrate. This reaction, as mentioned before, is called pozzolanic and CSF may be described as a synthetic pozzolan. Because the hydration of PC produces calcium hydroxide, the combination of CSF and PC is a practical means of using CSF and improving the cementing efficiency of PC.

In addition to the chemical role of CSF, it is also a ‘fine filler’. The extremely small CSF particles in the mixing water act as nuclei for the formation of calcium silicate hydrate which would otherwise form only on the cement grains. CSF will also change the microstructure of the interfacial zone. The result is a more homogenous microstructure that has greater strength and lower permeability. (To ensure thorough dispersion and effective use of the CSF, the use of plasticising admixtures is recommended).

CSF affects the properties of concrete as follows:

Fresh concrete:
Reduces the workability of fresh concrete, ie CSF tends to increase the water requirement for a given slump.
Increases cohesion
Significantly reduces the bleeding of fresh concrete
Hardened concrete
Marginally retards strength development at one day
Reduces permeability of concrete
Reduces the rate of chloride diffusion through concrete
Increases the strength of concrete.
Finely ground limestone is chemically virtually inert when mixed with portland cement and water (although there are some minor reactions). Depending on its fineness, limestone may however act as a ‘fine filler’ in fresh paste. Limestone may be used as a filler in common cement or as a workability improver in masonry cement. The effect of limestone on the properties of concrete or mortar depends on the specific limestone, whether a grinding aid is used in production, and the fineness of the limestone. General trends are as follow:

Fresh concrete or mortar:
Has no significant effect on water requirement
Prolongs the bleeding period but reduces the amount of bleed water
Limestone may improve the workability of mortar

BRICKS & BLOCKS
Bricks and blocks are the most basic building components utilized in the construction of any structure. Bricks have changed in shape and size over centuries. There are many different types available; not only in size but also colour, texture and strength. Bricks can be divided into two distinct categories, being; Clay and Concrete (concrete bricks are sometimes incorrectly referred to as cement bricks).

Clay Bricks are manufactured from a mixture of Clay (different clays produce the different colours) and other additives, which are mixed together to make a pliable type mixture almost like plasticine, which is then extruded and cut to size. Concrete bricks and blocks are manufactured from various types of aggregate (sand and other materials like ash mixed with small stone particles) creating the texture and colour, mixed with cement and water, which is then vibrated into a mould creating the shape and size and profile of the brick/block.

Clay Bricks
In the construction of a typical residential house, the cost of clay bricks or clay plaster bricks would generally amount to between 6% and 8% of the total construction cost of the house. In the case of commercial projects, like factories or office blocks, the cost of face bricks and plaster bricks is between 2% and 3% of the total construction cost.

Brickwork being the most visible component of a building often leads to the perception that the vast majority of the cost of the building must therefore be in the brickwork. Fortunately this is not true and should allow one the flexibility of creating different details that can be achieved using Clay bricks in many different ways and applications.

Some traditional advantages of using Clay bricks:
Clay bricks keep interiors comfortable; Heavyweight clay bricks have an inbuilt ability to keep buildings cool in summer and warm in winter. During the day, clay brick slowly absorbs and stores heat thus helping to keep buildings cool. During the night, the stored heat is slowly released and this assists in maintaining the inside temperature at a consistent level thereby minimizing the need for heating and cooling and associated energy costs, which has become increasingly important in a new era.

Clay bricks are fire resistant; because they are fi red at high temperatures during manufacture and are therefore almost incombustible. A clay brickwall is resistant to fi re and collapse. Walls built with quality clay bricks which are properly constructed will produce strong, stable and durable buildings. Clay bricks built in the form of solid or cavity walls offer excellent insulation properties.

Concrete Bricks & Blocks
Concrete masonry has a surprisingly long history and was fi rst seen in Britain in 1840, in the USA it has been in use since 1900. More concrete masonry is used in the USA, Germany and France than any other type of masonry, and in the USA, it accounts for 80% of all masonry used. Although concrete masonry units have been used in South Africa since the early 1900s, their use was initially confi ned to rural areas. Their application increased rapidly after World War 2, due to an increased demand for housing. And this period saw the introduction of mobile block-making machines, where concrete masonry units were produced on site.

The difference between a brick and a block is a matter of size, not material. A block is a masonry unit varying in length from 300mm to 650mm, a brick varies in length from 190mm to 220mm.

Standard specification:
The standard for concrete masonry units is SANS 1215. This standard covers the physical requirements and the sampling of units for testing. Assurance of compliance with the quality requirements of this standard is by obtaining the SABS Certification Mark that the concrete masonry units manufactured comply with the requirements of SANS 1215. This certificate will indicate to purchasers that the concrete masonry units are produced under acceptable controlled conditions with appropriate materials. SABS accredited laboratories are permitted to perform the appropriate testing requirements on behalf of SABS in the awarding of the mark.

Physical conditions and format
Dimensions of concrete masonry units do not appear in SANS 1215, amendment No. 2 but in Appendix F recommended nominal dimensions of concrete masonry units. The use of modular size masonry units is essential if buildings are designed to the 100mm standard module – as stated in SANS 993 modular co-ordination in building. Modular planning is based on a nominal joint thickness of 10mm. Modular wall thicknesses are 90,140 and 190mm.

The permissible thickness of masonry walls in building is 90, 110, 140, 190, and 230 mm and the modular dimensions are 90,140, and 190 mm. In the market place there is a proliferation of different sizes of masonry units. Mainly these are based on the “imperial” brick size of 222 x 106 x 73mm, or multiples of this size up to block size units of 448 x 224 x 224mm. The width of these units exceeds the requirements of SANS 10400, namely 106 and 224mm wall thickness as compared to the “deemed to satisfy” thicknesses of 90 and 190mm.

Thus for commercial reasons, units of reduced width are being made which are non-modular and non-imperial, such as 222 x 90 x 73mm that satisfy the minimum requirements of SANS 10400. Non-modular sizes of units are found in practice not to bond well without considerable cutting of the units. English or Flemish bond and construction of square brick piers is not possible as such units deviate from the basic principle of masonry bonding where the length of a unit should be twice its width plus the thickness of the bedding or perpendicular joint.

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