Resistance to weather - Walls.

Monday, January 24, 2011

Between 1920 and 1940 it became more usual for external walls of small buildings to be constructed as cavity walls with an outer leaf of brick or block, an open cavity and an inner leaf of brick or block. The outer leaf and the cavity serve to resist the penetration of rain to the inside face and the inner leaf to support floors, provide a solid internal wall surface and to some extent act as insulation against transfer of heat.

The idea of forming a vertical cavity in brick walls was first proposed early in the nineteenth century and developed through the century. Various widths of cavity were proposed from the first 6 inch cavity, a later 2 inch cavity followed by proposals for 3, 4 or 5 inch wide cavities. The early cavity walls were first constructed with bonding bricks laid across the cavity at intervals, to tie the two leaves together. Either whole bricks with end closers or bricks specially made to size and shape for the purpose were used. Later on, during the middle of the century, iron ties were used instead of bond bricks and accepted as being adequate to tie the two leaves of cavity walls.

From the middle of the twentieth century it became common practice to construct the external walls of houses as a cavity wall with a 2 inch wide cavity and metal wall ties. It seems that the 2 inch width of cavity was adopted for the convenience of determining the cavity width, by placing a brick on edge inside the cavity so that the course height of a brick, about 65 mm, determined cavity width rather than any consideration of the width required to resist rain penetration. This was adapted to the 2 inch (50 mm) wide cavity for walls that became common until recent years.

In constructing the early open cavity walls it was considered good practice to suspend a batten of wood in the cavity to collect mortar droppings. The batten was removed from time to time, cleaned of mortar, and put back in the cavity as the work progressed. The practice, which was largely ignored by bricklayers as it impeded work, has since been abandoned in favour of care in workmanship to avoid mortar droppings becoming lodged inside the cavity.

With the increase in the price of fuels and expectations of thermal comfort, building regulations have of recent years made requirements for the thermal insulation of external walls that can best be met by the introduction of materials with high thermal resistance. The most convenient position for these lightweight materials in a cavity wall is inside the cavity, which is either fully or partially filled with insulation. A filled or partially filled cavity may well no longer be an efficient barrier to rain penetration so that, with the recent increase in requirement for the thermal insulation of walls it has now been accepted that the width of the cavity may be increased from the traditional 50 to 100mm to accommodate increased thickness of insulation and still maintain a cavity against rain penetration.

Cavity walls.

Sunday, January 23, 2011

Strength and stability.
The practical guidance in Approved Document A to the Building Regulations accepts a cavity of from 50 to 100 mm for cavity walls having leaves at least 90 mm thick, built of coursed brickwork or blockwork with wall ties spaced at 450 mm vertically and from 900 to 750 mm horizontally for cavities of 50 to 100mm wide respectively. As the limiting conditions for the thickness of walls related to height and length are the same for a solid bonded wall 190 mm thick as they are for a cavity wall of two leaves each 90 mm thick, it is accepted that the wall ties give the same strength and stability to two separate leaves of brickwork that the bond in solid walls does.


Chases in Walls.

Saturday, January 22, 2011

To limit the effect of chases cut into walls in reducing strength or stability, vertical chases should not be deeper than one-third of the thickness of solid walls or a leaf of a cavity wall and horizontal chases not deeper than one-sixteenth. A chase is a recess, cut or built in a wall, inside which small service pipes are run and then covered with plaster or walling.

Stability lateral support - Walls.

Friday, January 21, 2011

For stability up the height of a wall lateral support is provided by floors and roofs as set out in Table 3.

Walls that provide support for timber floors are given lateral support by 30 x 5 mm galvanised iron or stainless steel ‘L’ straps fixed to the side of floor joists at not more than 2 m centres for houses up to three storeys and 1.25 m centres for all storeys in all other buildings. The straps are turned down 100 mm on the cavity face of the inner leaf of cavity walls and into solid wallings, as illustrated in Fig. 67. 

Table 3 Lateral support for walls.

Lateral support from timber floors, where the joists run parallel to the wall, is provided by 30 x 5 mm galvanised iron on stainless steel strap anchors secured across at least two joists at not more than 2 m centres for houses up to three storeys and 1.25 m for all storeys in all other buildings.

The ‘L’ straps are turned down a minimum of 100 mm on the cavity side of inner leaf of cavity walls and into solid walling. Solid timber strutting is fixed between joists under the straps as illustrated in Fig. 67.
Solid floors of concrete provide lateral support for walls where the floor bears for a minimum of 90 mm in both solid and cavity walls, as illustrated in Fig. 67.

To provide lateral support to gable end walls to roofs pitched at more than 15° a system of galvanised steel straps is used. Straps 30 x 5 mm are screwed to the underside of timber noggings fixed between three rafters, as illustrated in Fig. 68, with timber packing pieces between the rafter next to the gable and the wall.
The straps should be used at a maximum of 2 m centres and turned down against the cavity face of the inner leaf of a whole building block or down into a solid wall.

Fig. 67 Floors providing lateral restraint to walls.

Fig. 68 Lateral support to gable ends.

Fig. 69 Length of walls.

To provide stability along the length and at the ends of loadbearing walls there should be walls, piers or chimneys bonded to the wall at intervals of not more than 12 m, to buttress and stabilise the wall.
The maximum spacing of buttressing walls, piers and chimneys is measured from the centre line of the supports as illustrated in Fig. 69. The minimum length of a return buttressing wall should be equal to one-sixth of the height of the supported wall.

To be effective as buttresses to walls the return walls, piers and chimneys must be solidly bonded to the supported wall.

Stability thickness of walls.

Thursday, January 20, 2011

The general limitation of wall thickness given for stability is that solid walls of brick or block should be at least as thick as one-sixteenth of the storey height. This is a limiting slenderness ratio relating thickness of wall to height, measured between floors and floor and roof that provide lateral support and give stability up the height of the wall. The minimum thickness of external, compartment and separating walls is given in a table in Approved Document A, relating thickness to height and length of wall as illustrated in Fig. 66. Compartment walls are those that are formed to limit the spread of fire and separating walls (party walls) those that separate adjoining buildings, such as the walls between terraced houses.

Cavity walls should have leaves at least 90 mm thick, cavity at least 50 mm wide and the combined thickness of the two leaves plus 20 mm, should be at least the thickness required for a solid wall of the same height and length.

Internal Ioadbearing walls, except compartment and separating walls, should be half the thickness of external walls illustrated in Fig. 66, minus 5 mm, except for the wall in the lowest storey of a three storey building which should be of the same thickness, or 140 mm, whichever is the greater.

Fig. 66 Minimum thickness of walls.

Walls of brick and block: Walls of brick and block,Height, Witdth, Strength.

Monday, January 10, 2011

Strength and stability.
Up to the middle of the twentieth century the design and construction of small buildings, such as houses, was based on tried, traditional forms of construction. There were generally accepted rule of thumb methods for determining the necessary thickness for the walls of small buildings. By and large, the acceptance of tried and tested methods of construction, allied to the experience of local builders using traditional materials in traditional forms of construction, worked well. The advantage was that from a simple set of drawings an experienced builder could give a reasonable estimate of cost and build and complete small buildings, such as houses, without delay.

With the increasing use of unfamiliar materials, such as steel and concrete, in hitherto unused forms, it became necessary to make calculations to determine the least size of elements of structure for strength and stability in use. The practicability of constructing large multi-storey buildings provoked the need for standards of safety in case of fire and rising expectations of comfort and the need for the control of insulation, ventilation, daylight and hygiene.

During the last 50 years there has been a considerable increase in building control, that initially was the province of local authorities through building bylaws, later replaced by national building regulations. The Building Regulations 1985 set out functional requirements for buildings and health and safety requirements that may be met through the practical guidance given in I I Approved Documents that in turn refer to British Standards and Codes of Practice.

In theory it is only necessary to satisfy the requirements of the Building Regulations, which are short and include no technical details of means of satisfying the requirements. The 11 Approved 

Documents give practical guidance to meeting the requirements, but there is no obligation to adopt any particular solution in the documents if the requirements can be met in some other way.

The stated aim of the current Building Regulations is to allow freedom of choice of building form and construction so long as the stated requirements are satisfied. In practice the likelihood is that the practical guidance given in the Approved Documents will be accepted as if the guidance were statutory as the easier approach to building, rather than proposing some other form of building that would involve calculation and reference to a bewildering array of British Standards and Codes and Agrément Certificates.
In Approved Document A there is practical guidance to meeting the requirements of the Building Regulations for the walls of small buildings of the following three types:

(1) residential buildings of not more than three storeys
(2) small single storey non-residential buildings, and
(3) small buildings forming annexes to residential buildings (including garages and outbuildings).
Limitations as to the size of the building types included in the guidance are given in a disjointed and often confusing manner.

The maximum height of residential buildings is given as 15 m from the lowest ground level to the highest point of any wall or roof, whereas the maximum allowable thickness of wall is limited to walls not more than 12 m. 

Height is separately defined, for example, as from the base of a gable and external wall to half the height of the gable. The height of single storey, non-residential buildings is given as 3 m from the ground to the top of the roof, which limits the guidance to very small buildings. The maximum height of an annexe is similarly given as 3 m, yet there is no definition of what is meant by annexe except that it includes garages and outbuildings.

The least width of residential buildings is limited to not less than half the height. A diagram limits the dimensions of the wing of a residential building without defining the meaning of the term ‘wing’, which in the diagram looks more like an annexe than a wing. Whether the arms of a building which is ‘L’ or ‘U’ shaped on plan are wings or not is entirely a matter of conjecture. How the dimensions apply to semi-detached buildings or terraces of houses is open to speculation.

In seeking to give practical guidance to meeting functional requirements for strength and stability and at the same time impose limiting dimensions, the Approved Document has caused confusion.

One further limitation is that no floor enclosed by structural walls on all sides should exceed 70 m2 and a floor without a structural wall on one side, 30 m2. The floor referred to is presumably a suspended floor, though it does not say so. As the maximum allowable length of wall between buttressing walls, piers or chimneys is given as 12 m and the maximum span for floors as 6 m, the limitation is in effect a floor some 12 x 6m on plan. 

It is difficult to understand the need for the limitation of floor area for certain ‘small’ buildings.

The guidance given in the Approved Document for walls of brick or block is based on compressive strengths of 5 N/mm2 for bricks and 2.8 N/mm2 for blocks for walls up to two storeys in height, where the storey height is not more than 2.7 m and 7 N/mm2 for bricks and blocks of walls of three storey buildings where the storey height is greater than 2.7 m.

Jointing and poiting - Mortar.

Jointing is the word used to describe the finish of the mortar joints between bricks, to provide a neat joint in brickwork that is finished fairface. F’airface describes the finished face of brickwork that will not be subsequently covered with plaster, rendering or other finish.

Most fairface brickwork joints are finished, as the brickwork is raised, in the form of a flush or bucket handle joint. When the mortar has gone off, that is hardened sufficiently, the joint is made. Flush joints are generally made as a ‘bagged’ or a ‘bagged in’ joint. The joint is made by rubbing coarse sacking or a brush across the face of the brickwork to rub away all protruding mortar and leaving a flush joint. This type of joint, illustrated in Fig. 65, can most effectively be used on brickwork where the bricks are uniform in shape and comparatively smooth faced, where the mortar will not spread over the face of the brickwork.

A bucket handle joint is made by running the top face of a metal bucket handle or the handle of a spoon along the joint to form a concave, slightly recessed joint, illustrated in Fig. 65. The advantage of the bucket handle joint is that the operation compacts the mortar into the joint and improves weather resistance to some extent. 

A bucket handle joint may be formed by a jointing tool with or without a wheel attachment to facilitate running the tool along uniformly deep joints.

Flush and bucket handle joints are mainly used for jointing as the brickwork is raised.

The struck and recessed joints shown in Fig. 65 are more laborious to make and therefore considerably more expensive. The struck joint is made with a pointing trowel that is run along the joint either along the edges of uniformly shaped bricks or along a wood straight edge, where the bricks are irregular in shape or coarse textured, to form the splayed back joint. The recessed joint is similarly formed with a tool shaped for the purpose, with such filling of the joint as may be necessary to complete the joint.

Of the joints described the struck joint is mainly used for pointing the joints in old brickwork and the recessed joint to emphasise the profile, colour and textures of bricks for appearance sake to both new and old brickwork.

Fig. 65 Jointing and  poiting 

The words jointing and pointing are commonly loosely used. Jointing is the operation of finishing off a mortar joint as the brickwork is raised, whereas pointing is the operation of filling the joint with a specially selected material for the sake of appearance or as weather protection to old lime mortar.

Pointing is the operation of filling mortar joints with a mortar selected for colour and texture to either new brickwork or to old brickwork. The mortar for pointing is a special mix of lime, cement and sand or stone dust chosen to produce a particular effect of colour and texture. The overall appearance of a fairface brick wall can be dramatically altered by the selection of mortar for pointing. The finished colour of the mortar can be affected through the selection of a particular sand or stone dust, the use of pigmented cement, the addition of a pigment and the proportion of the mix of materials.

The joints in new brickwork are raked out about 20 mm deep when the mortar has gone off sufficiently and before it has set hard and the joints are pointed as scaffolding is struck, that is taken down.

The mortar joints in old brickwork that was laid in lime mortar may in time crumble and be worn away by the action of wind and rain. To protect the lime mortar behind the face of the joints it is good practice to rake out the perished jointing or pointing and point or repoint all joints. The joints are raked out to a depth of about 20mm and pointed with a mortar mix of cement, lime and sand that has roughly the same density as the brickwork. The operation of raking out joints is laborious and messy and the job of filling the joints with mortar for pointing is time consuming so that the cost of pointing old work is expensive.

Pointing or repointing old brickwork is carried out both as protection for the old lime mortar to improve weather resistance and also for appearance sake to improve the look of a wall.
Any one of the joints illustrated in Fig. 66 may be used for pointing.

Aggregate for mortar.

The aggregate or main part of mortar is sand. The sand is dredged from pits or river beds and a good sand should consist of particles ranging up to 5 mm in size, In the ground, sand is usually found mixed with some clay earth which coats the particles of sand. If sand mixed with clay is used for mortar, the clay tends to prevent the cement or lime binding the sand particles together and in time the mortar crumbles. It is therefore important that the sand be thoroughly washed so that there is no more than 5% of clay in the sand delivered to the site. 

Soft sand and sharp sand.
Sand which is not washed and which contains a deal of clay in it feels soft and smooth when held in the hand, hence the term soft sand. Sand which is clean feels coarse in the hand, hence the term sharp. These are terms used by craftsmen. When soft sand is used, the mortar is very smooth and plastic and it is much easier to spread and to bed the bricks in than a mortar made of sharp or clean sand.

Naturally the bricklayer prefers to use a mortar made with soft or unwashed sand, often called ‘builders’ sand’. A good washed sand for mortar should, if clenched in the hand, leave no trace of yellow clay stains on the palm.

Matrix for mortar.
The material that was used for many centuries before the advent of Portland cement as the matrix (binding agent) for mortar was lime. Lime, which mixes freely with water and sand, produces a material that is smooth, buttery and easily spread as mortar, into which the largely misshapen bricks in use at the time could be bedded.

The particular advantage of lime is that it is a cheap, readily available material that produces a plastic material ideal for bedding bricks, its disadvantages are that it is a messy, laborious material to mix and as it is to an extent soluble in water it will lose its adhesive property in persistently damp situations. Protected from damp, a lime mortar will serve as an effective mortar for the life of most buildings.

Portland cement, which was first manufactured on a large scale in the latter part of the nineteenth century, as a matrix for mortar, produces a hard dense material that has more than adequate strength for use as mortar and is largely unaffected by damp conditions. A mixture of cement, sharp sand and water produces a coarse material that is not plastic and is difficult to spread. In use, cement has commonly been used with ‘builders’ sand’ which is a natural mix of sand and clay. The clay content combines with water to make a reasonably plastic mortar at the expense of loss of strength and considerable drying shrinkage as the clay dries.

‘Compo’ mortar.
During the last 50 years it has been considered good practice to use a mortar in which the advantages of lime and cement are combined. This combination or ‘compo’ mortar is somewhat messy to mix.

Mortar plasticiser.
As an alternative to the use of lime it has become practice to use a mortar plasticiser with cement in the mix of cement mortars. A plasticiser is a liquid which, when combined with water, effervesces to produce minute bubbles of air that surround the coarse grains of sand and so render the mortar plastic, hence the name ‘mortar plasticiser’ mixes.

Ready Mixed mortar.
Of recent years ready mixed mortars have come into use particularly on sites where extensive areas of brickwork are laid. The wet material is delivered to site, ready mixed, to save the waste, labour and cost of mixing on site.

A wide range of lime and sand, lime cement and sand and cement and sand mixes is available. The sand may be selected to provide a chosen colour and texture for appearance sake or the mix may be pigmented for the same reason.

Lime mortar is delivered to site ready to use within the day of delivery. Cement mix and cement lime mortar is delivered to site ready mixed with a retarding admixture.

The retarding admixture is added to cement mix mortars to delay the initial set of cement. The initial set of ordinary Portland cement occurs some 30 minutes after the cement is mixed with water, so that an initial hardening occurs to assist in stiffening the material for use as rendering on vertical surfaces for example.
The advantages of ready mixed mortar are consistency of the mix, the wide range of mixes available and considerable saving in site labour costs and the inevitable waste of material common with site mixing.

Cement mortar.
Cement is made by heating a finely ground mixture of clay and limestone, and water, to a temperature at which the clay and limestone fuse into a clinker. The clinker is ground to a fine powder called cement. The cement most commonly used is ordinary Portland cement which is delivered to site in 50 kg sacks. When the fine cement powder is mixed with water a chemical action between water and cement takes place and at the completion of this reaction the nature of the cement has so changed that it binds itself very firmly to most materials.

The cement is thoroughly mixed with sand and water, the reaction takes place and the excess water evaporates leaving the cement and sand to gradually harden into a solid mass. The hardening of the mortar becomes noticeable some few hours after mixing and is complete in a few days. The usual mix of cement and sand for mortar is from 1 part cement to 3 or 4 parts sand to 1 part of cement to 8 parts of sand by volume, mixed with just sufficient water to render the mixture plastic.

A mortar of cement and sand is very durable and is often used for brickwork below ground level and brickwork exposed to weather above roof level such as parapet walls and chimney stacks.

Cement mortar made with washed sand is not as plastic however as bricklayers would like it to be. Also when used with some types of bricks it can cause an unsightly effect known as efflorescence.

This word describes the appearance of an irregular white coating on the face of bricks, caused by minute crystals of water soluble salts in the brick. The salts go into solution in water inside the bricks and when the water evaporates in dry weather they are left on the face of bricks or plaster. Because cement mortar has greater compressive strength than required for most ordinary brickwork and because it is not very plastic by itself it is sometimes mixed with lime and sand.

Lime mortar.
Lime is manufactured by burning limestone or chalk and the result of this burning is a dirty white, lumpy material known as quicklime. When this quicklime is mixed with water a chemical change occurs during which heat is generated in the lime and water, and the lime expands to about three times its former bulk. This change is gradual and takes some days to complete, and the quicklime afterwards is said to be slaked, that is it has no more thirst for water. More precisely the lime is said to be hydrated, which means much the same thing. Obviously the quicklime must be slaked before it is used in mortar otherwise the mortar would increase in bulk and squeeze out of the joints. Lime for building is delivered to site ready slaked and is termed ‘hydrated lime’.

When mixed with water, lime combines chemically with carbon dioxide in the air and in undergoing this change it gradually hardens into a solid mass which firmly binds the sand.

A lime mortar is usually mixed with 1 part of lime to 3 parts of sand by volume, The mortar is plastic and easy to spread and hardens into a dense mass of good compressive strength. A lime mortar readily absorbs water and in time the effect is to reduce the adhesion of the lime to the sand and the mortar crumbles and falls out of the joints in the brickwork.

Mortar for general brickwork may be made from a mixture of cement, lime and sand in the proportions set out in Table 2. These mixtures combine the strength of cement with the plasticity of lime, have much the same porosity as most bricks and do not cause efflorescence on the face of the brickwork.

The mixes set out in Table 2 are tabulated from rich mixes (1) to weak mixes (2). A rich mix of mortar is one in which there is a high proportion of matrix, that is lime or cement or both, to sand as in the 1:3 mix and a weak mix is one in which there is a low proportion of lime or cement to sand as in the mix 1:3:12. The richer the mix of mortar the greater its compressive strength and the weaker the mix the greater the ability of the mortar to accommodate moisture or temperature movements. 

Table 2 Mortar mixes.

The general uses of the mortar mixes given in Table 2 are as mortar for brickwork or blockwork as follows:

Mix I For cills, copings and retaining walls
Mix 2 Parapets and chimneys
Mix 3 Walls below dpc
Mix 4 Walls above dpc
Mix 5 Internal walls and lightweight block inner leaf of cavity

Hydraulic lime.
Hydraulic lime is made by burning a mixture of chalk or limestone that contains clay. Hydraulic lime is stronger than ordinary lime and will harden in wet conditions, hence the name. Ordinary Portland cement, made from similar materials and burnt at a higher temperature, has largely replaced hydraulic lime which is less used today. 

Mortar platicisers.
Liquids known as mortar plasticisers are manufactured. When these liquids are added to water they effervesce, that is the mixture becomes bubbly like soda water. If very small quantities are added to mortar, when it is mixed, the millions of minute bubbles that form surround the hard sharp particles of sand and so make the mortar plastic and easy to spread. The particular application of these mortar plasticisers is that if they are used with cement mortar they increase its plasticity and there is no need to use lime. It seems that the plasticisers do not adversely affect the hardness and durability of the mortar and they are commonly and successfully used for mortars.

Mortar for brickwork and blockwork.

Clay bricks are rarely exactly rectangular in shape and they vary in size. Some facing bricks are far from uniform in shape and size and if a wall were built of bricks laid without mortar and the bricks were bonded the result might be as shown, exaggerated, in Fig. 64.

Because of the variations in shape and size, the courses of bricks would not lie anywhere near horizontal. One of the functions of brickwork is to support floors and if a floor timber were to bear on the brick marked A it would tend to cause it to slide down the slope on which it would be resting. It is essential, therefore, that brickwork be laid in true horizontal courses, and the only way this can be done with bricks of differing shapes and sizes is to lay them on some material which is sufficiently plastic, while the bricks are being laid, to take up the difference in size, and which must be able to harden to such an extent that it can carry the weight normally carried by brickwork.

The material used is termed mortar. The basic requirements of a mortar are that it will harden to such an extent that it can carry the weight normally carried by bricks, without crushing, and that it be sufficiently plastic when laid to take the varying sizes of bricks, It must have a porosity similar to that of the bricks and it must not deteriorate due to the weathering action of rain or frost.

Sand is a natural material which is reasonably cheap and which, if mixed with water, can be made plastic, yet which has very good strength in resisting crushing. Its grains are also virtually impervious to the action of rain and frost. The material required to bind the grains of sand together into a solid mass is termed the matrix and the two materials used for this purpose are lime or cement.

Fig. 64 Badly shaped Racing Tricks laid without mortar.

Bonding blocks.

Blocks are made in various thicknesses to suit most wall requirements and are laid in stretcher bond.

Thin blocks, used for non4oadbearing partitions, are laid in run- fling stretcher bond with each block centred over and under blocks above and below. At return angles full blocks bond into the return wall in every other course, as illustrated in Fig. 62. So as not to disturb the full width bonding of blocks at angles, for the sake of stability, a short length of cut block is used as closer and infihl block. 

Thicker blocks are laid in off centre running bond with a three quarter length block at stop ends and sides of openings. The off centre bond is acceptable with thicker blocks as it avoids the use of cut blocks to complete the bond at angles, as illustrated in Fig. 62.

Thick blocks, whose length is twice their width, are laid in running (stretcher) bond as illustrated in Fig. 62, and cut blocks are only necessary to complete the bond at stop ends and sides of opening.
At the ‘T’ junctions of loadbearing concrete block walls it is sometimes considered good practice to butt the end face of the intersecting walls with a continuous vertical joint to accommodate shrinkage movements and to minimise cracking of plaster finishes.

Where one intersecting wall serves as a buttress to the other, the butt joint should be reinforced by building in split end wall ties at each horizontal joint across the butt joint to bond the walls. Similarly, nonloadbearing block walls should be butt jointed at intersections and the joint reinforced with strips of expanded metal bedded in horizontal joints across the butt joint.

Concrete block walls of specially produced blocks to be used as a fairface finish are bonded at angles to return walls with specially produced quoin blocks for the sake of appearance, as illustrated in Fig. 63. The ‘L’ shaped quoin blocks are made to continue the stretcher bond around the angle into the return walls. 

Quoin blocks are little used for other than fairface work as they are liable to damage in handling and use and add considerably to the cost of materials and labour.

Fig. 62 Bonding building blocks.

Fig. 63 Bonding block walls.

Clay blocks.

Sunday, January 9, 2011

Hollow clay building blocks are made for use as a wall unit. The blocks are made from selected brick clays that are press moulded and burnt. These hard, dense blocks are hollow to reduce shrinkage during firing and reduce their weight and they are grooved to provide a key for plaster, as illustrated in Fig. 61. The standard block is 290 long x 215mm high and 62.5, 75, 100 and 150mm thick.

Clay blocks are comparatively lightweight, do not suffer moisture movement, have good resistance to damage by fire and poor thermal insulating properties. These blocks are mainly used for non-load- bearing partitions in this country. They are extensively used in southern Europe as infill panel walls to framed buildings where the tradition is to render the external face of buildings on which the blocks provide a substantial mechanical key for rendering and do not suffer moisture movement that would otherwise cause shrinkage cracking.

Fig. 61 Clay blocks.

Moisture movement - Concrete blocks.

As water dries out from precast concrete blocks shrinkage that occurs, particularly with lightweight blocks, may cause serious cracking of plaster and rendering applied to the surface of a wall built with them. Obviously the wetter the blocks the more they will shrink. It is essential that these blocks be protected on building sites from saturation by rain both when they are stacked on site before use and whilst walls are being built. Clay bricks are small and suffer very little drying shrinkage and therefore do not need to be protected from saturation by rain. Only the edges of these blocks should be wetted to increase their adhesion to mortar when the blocks are being laid.

Lightweight aggregate concrete blocks primarily for internal non-loadbearing walls.

The thin blocks are solid and either square edged or with a tongue and groove in the short edges so that there is a mechanical bond between blocks to improve the stability of internal partitions. The
poor structural stability may be improved by the use of storey height door linings which are secured at floor and ceiling level.

Thin block internal partitions afford negligible acoustic insulation and poor support for fittings, such as book shelves secured to them.

The thicker blocks are either hollow or cellular to reduce weight and drying shrinkage.

Lightweight aggregate concrete blocks for general use in building.

The blocks are made of ordinary Portland cement and one of the following lightweight aggregates: granulated blast-furnace slag, foamed blast-furnace slag, expanded clay or shale, or well burned furnace clinker. The usual mix is I part cement to 6 or 8 of aggregate by volume. 

Of the four lightweight aggregates noted, well burned furnace clinker produces the cheapest block which is about two-thirds the weight of a similar dense aggregate concrete block and is a considerably better thermal insulator. Blocks made from foamed blast- furnace slag are about twice the price of those made from furnace clinker, but they are only half the weight of a similar dense aggregate block and have good thermal insulating properties. The furnace clinker blocks are used extensively for walls of houses and the foamed blast-furnace slag blocks for walls of large framed buildings because of their lightness in weight. 

These thin blocks, usually 60 or 75 mm thick, are made with the same lightweight aggregate as those in Class 2. These blocks are more expensive than dense aggregate blocks and are used principally for non-loadbearing partitions. These blocks are manufactured as solid, hollow or cellular depending largely on the thickness of the block.

Dense aggregate blocks for general use.

The blocks are made of Portland cement, natural aggregate or blast- furnace slag. The usual mix is 1 part of cement to 6 or 8 of aggregate by volume. These blocks are as heavy per cubic metre as bricks, they are not good thermal insulators and their strength in resisting crushing is less than that of most well burned bricks. The colour and texture of these blocks is far from attractive and they are usually covered with plaster or a coat of rendering. These blocks are used for internal and external loadbearing walls, including walls below ground.

Buildings Blocks.

Saturday, January 8, 2011

Concrete blocks -Walls.

These are used extensively for both Ioadbearing and non-loadbearing walls, externally and internally. A concrete block wall can be laid in less time and may cost up to half as much as a similar brick wall. Lightweight aggregate concrete blocks have good insulating properties against transfer of heat and have been much used for the inner leaf of cavity walls with either a brick outer leaf or a concrete block outer leaf.

A disadvantage of some concrete blocks, particularly lightweight aggregate blocks, as a wall unit is that they may suffer moisture movement which causes cracking of applied finishes such as plaster. To minimise cracking due to shrinkage by loss of water, vertical movement joints should be built into long block walls, subject to moisture movement, at intervals of up to twice the height of the wall. These movement joints may be either a continuous vertical joint filled with mastic or they may be formed in the bonding of the blocks.
Because the block units are comparatively large, any settlement movement in a wall will show more pronounced cracking in mortar joints than is the case with the smaller brick wall unit.

For some years it was fashionable to use concrete blocks as a fairface external wall finish. The blocks were accurately moulded to uniform sizes and made from aggregates to provide a variety of colours and textures. Blocks made to give an appearance of natural stone with plain or rugged exposed aggregate finish were used.
These special blocks are less used that they were, particularly because of the fairly rapid deterioration in the appearance of the blocks due to irregular weather staining of smooth faced blocks and the patchy dirt staining of coarse textured blocks.

Concrete blocks are manufactured from cement and either dense or lightweight aggregates as solid, cellular or hollow blocks as illustrated in Fig. 60. A cellular block has one or more holes or cavities that do not pass wholly through the block and a hollow block is one in which the holes pass through the block. The thicker blocks are made with cavities or holes to reduce weight and drying shrinkage.

The most commonly used size of both dense and lightweight concrete blocks is 440 mm long x 215 mm high. 

The height of the block is chosen to coincide with three courses of brick for the convenience of building in wall ties and also bonding to brickwork. The length of the block is chosen for laying in stretcher bond.
For the leaves of cavity walls and internal loadbearing walls 100mm thick blocks are used. For non-loadbearing partition walls 60 or 75 mm thick lightweight aggregate blocks are used. Either 440mm x 215mm or 390 x 190mm blocks may be used.

Concrete blocks may be specified by their minimum average compressive strength for: 
all blocks not less than 75 mm thick and  a maximum average transverse strength for blocks less than 75mm thick, which are used for non-loadbearing partitions.

The usual compressive strengths for blocks are 2.8, 3.5, 5.0, 7.0. 10.0, 15.0, 20.0 and 35.0 N/mm2. The compressive strength of blocks used for the walls of small buildings of up to three storeys, recommended in Approved Document A to the Building Regulations, is 2.8 and 7 N/mm2, depending on the loads carried.

Concrete blocks may also be classified in accordance with the aggregate used in making the block and some common uses.
 Fig.60 Concrete blocks.

Garden wall bonds.

Walls, such as garden walls, that are to be finished fairface both sides and built 1 B thick are often built in one of the garden wall bonds. 

Because of the variations in size and shape of many facing bricks it is difficult to finish a 1 B wall fairface both sides because of the differences in length of bricks that are bonded through the thickness of the wall.
Garden wall bonds are designed specifically to reduce the number of through headers to minimise the labour in selecting bricks of roughly the same length for use as headers.

Usual garden wall bonds are three courses of stretchers to every one course of headers in English garden wall bond and one header to every three stretchers in Flemish garden wall bond, as illustrated in Fig. 59.

The reduction in the number of through headers does to an extent weaken the through bond of the brickwork. This is of little consequence in a freestanding garden wall, Other combinations such as two or four stretchers to one header may be used.

The tops of garden walls are finished with one of the special coping bricks illustrated in Fig. 49 or one of the brick or stone cappings. 

 Fig. 59 Garden wall bonds.

Bonding at angles and jambs - Walls.

At the end of a wall at a stop end, at an angle or quoin and atjambs of openings the bonding of bricks has to be finished up to a vertical angle. To complete the bond a brick ¼ B wide has to be used to close or complete the bond of the ¼ B overlap of face brickwork. 

A brick, cut in half along its length, is used to close the bond at an angle. This cut brick is termed a 6queen closer’, illustrated in Fig. 56. If the narrow width queen closer were laid at the angle, it might be displaced during bricklaying. To avoid this possibility the closer is laid next to a header, as illustrated in Fig. 57. The rule is that a closer is laid next to a quoin (corner) header. 

Fig. 56 Queen closer.

There is often an appreciable difference in the length of facing bricks so that a solid wall 1 B thick may be difficult to finish as a wall fairface both sides. The word fairface describes a brick wall finished with a reasonably flat and level face for the sake of appearance. Where a I B wall is built with bricks of uneven length it may be necessary to select bricks of much the same length as headers and use longer bricks as stretchers. This additional care and labour will add appreciably to costs.

Walls l ½ B thick may be used for substantial walling for larger buildings, such as industrial, storage and civic, for the sake of the appearance of the brickwork and the durability and sense of solidity and pennanence where the walling is finished fairface both sides.

To complete the bond of a solid wall l ½  B thick in double Flemish bond, that is Flemish bond on both faces, it is necessary to use cut half bricks in the thickness of the wall as illustrated in Fig. 57. At angles and stop ends of wall, queen closers are laid next to quoin headers and a three quarter length cut brick is used, as illustrated in Fig. 57.

Cutting the many half length bricks ( bats) and three quarter length bricks and closers is time consuming and wasteful as it is not always possible to cut a brick in half cleanly. This adds considerably to the cost of this walling, which is selected for appearance rather than economy.

A 1 ½ B thick wall, finished fairface both sides and showing English bond both sides, requires considerably less cutting of bricks to complete the bond, as illustrated in Fig. 58. It is only necessary to cut closers and three quarter length bricks to complete the bond at angles and stop ends.

Walls l ½ B thick that are to be finished fairface on one side only may be built with facing bricks for the fairface side and cheaper common bricks for the rest of the thickness of the wall, where the inside face is to be covered with plaster.

Fig. 57 Double Flemish bond.

 Fig.58 English bond.

English and Flemish bond - Walls.

Because brick by itself does not provide adequate resistance to the transfer of heat, to meet the requirements of the Building Regulations for the conservation of fuel and power, it is used in combination with other materials in external cavity walling for most heated buildings. In consequence brick walling 1 B and thicker is less used than it was.

Solid brick walls may be used for heated and unheated buildings for arcades, screen walling and as boundary and earth retaining walling for the benefit of the appearance and durability of the material.
For the same reason that a B wall is bonded along its length a solid wall 1 B and thicker is bonded along its length and through its thickness.

The two basic ways in which a solid brick wall may be bonded are with every brick showing a header face with each header face lying directly over two header faces below or with header faces centrally over a stretcher face in the course below, as illustrated in Fig. 54. 

The bond in which header faces only show is termed ‘heading’ or ‘header bond’. This bond is little used as the great number of vertical joints and header faces is generally considered unattractive.

The bond in which header faces lie directly above and below a stretcher face is termed Flemish bond. This bond is generally considered the most attractive bond for facing brickwork because of the variety of shades of colour between header and stretcher faces dispersed over the whole face of the walling. Figure 54 illustrates brickwork in Flemish bond.

English bond, illustrated in Fig. 55, avoids the repetition of header faces in each course by using alternate courses of header and stretcher faces with a header face lying directly over the centre of a stretcher face below. The colour of header faces, particularly in facing bricks, is often distinctly different from the colour of stretcher faces. In English bond this difference is shown in successive horizontal courses. In Flemish bond the different colours of header and stretcher faces are dispersed over the whole face of a wall, which by common consent is thought to be a more attractive arrangement.

Fig. 54 Header and Flemish bond.

Fig. 55 English bond.

Stretcher bond - Walls.

Friday, January 7, 2011

The four faces of a brick which may be exposed in fairface brickwork are the two, long, stretcher faces and the two header faces illustrated in Fig. 51. The face on which the brick is laid is the bed. Some bricks have an indent or frog formed in one of the bed faces. The purpose of the frog or indent is to assist in compressing the wet clay during moulding. The frog also serves as a reservoir of mortar on to which bricks in the course above may more easily be bedded.

The thickness of a wall is dictated primarily by the length of a brick. The length of bricks varies appreciably, especially those that are hand moulded and those made from plastic clays that will shrink differentially during firing.

It has been practice for some time to describe the thickness of a wall by reference to the length of a brick as a I B (brick) wall, a l B wall or a 2 B wall, rather than a precise dimension.

The external leaf of a cavity wall is often built of brick for the advantage of the appearance of brickwork. The most straightforward way of laying bricks in a thin outer leaf of a cavity wall is with the stretcher face of each brick showing externally. So that bricks are bonded along the length of the wall they are laid with the vertical joints between bricks lying directly under and over the centre of bricks in the courses under and over. This is described as stretcher bond as illustrated in Fig. 52. This wall is described as a B thick wall.

At the intersection of two half brick walls at corners or angles and at the jambs, sides of openings, the bricks are laid so that a header face shows in every other course to complete the bond, as illustrated in Fig. 52.
The appearance of a wall laid in stretcher bond may look somewhat monotonous because of the mass of stretcher faces showing. To provide some variety the wall may be built with snap headers so that a stretcher face and a header face show alternately in each course with the centre of the header face lying directly under and over the centre of the stretcher faces in courses below and above, as illustrated in Fig.

This form of fake Flemish bond is achieved by the use of half bricks, hence the name ‘snap header’. The combination and variety in colour and shape can add appreciably to the appearance of a wall. Obviously the additional labour and likely wastage of bricks adds somewhat to cost.

Fig. 52 Stretcher bond.

Fig. 53 Flemish bond with snap Readers.

Bonding Bricks - building a wall.

In building a wall it is usual to lay bricks in regular) horizontal courses so that each brick bears on two bricks below. The bricks are said to be bonded as they bind together by being laid across each other along the length of the wall, as illustrated in Fig. 50. 

The advantage of bonding is that the wall acts as a whole so that the load of a beam carried by the topmost brick in Fig. 50 is spread to the two bricks below it, then to the three below that and so on down to the base or foundation course of bricks.

The failure of one poor quality brick such as ‘A in a wall and a slight settlement under part of the foundation such as ‘B’ and ‘C’ in Fig. 50 will not affect the strength and stability of the whole wall as the load carried by the weak brick and the two foundation bricks is transferred to the adjacent bricks.

Because of the bond, window and door openings may be formed in a wall, the load of the wall above the opening being transferred to the brickwork each side of the openings by an arch or lintel.

The effect of bonding is to stiffen a wall along its length and also to some small extent against lateral pressure, such as wind.

Fig. 50 Bricks stacked pyramid fashion.

Properties of bricks - Hardness, Compressive strength, Absorption, Frost resistance, Efflorescence, Efflorescence.

This is a somewhat vague term commonly used in the description of bricks. By general agreement it is recognised that a brick which is to have a moderately good compressive strength, reasonable resistance to saturation by rainwater and sufficient resistance to the disruptive action of frost should be hard burned. Without some experience in the handling, and of the behaviour, of bricks in general it is very difficult to determine whether or not a particular brick is hard burned.

A method of testing for hardness is to hold the brick in one hand and give it a light tap with a hammer. The sound caused by the blow should be a dull ringing tone and not a dull thud. Obviously different types of brick will, when tapped, give off different sorts of sound and a brick that gives off a dull sound when struck may possibly be hard burned.

Compressive strength.
This is a property of bricks which can be determined accurately. The compressive strength of bricks is found by crushing 12 of them individually until they fail or crumble. The pressure required to crush them is noted and the average compressive strength of the brick is stated as newtons per mm of surface area required to ultimately crush the brick. The crushing resistance varies from about 3.5 N/mm2 for soft facing bricks up to 140 N/mm2 for engineering bricks.

The required thickness of an external brick wall is determined primarily by its ability to absorb rainwater to the extent that water does not penetrate to the inside face of the wall. In positions of moderate exposure to wind driven rain a brick wall 215 mm thick may absorb so much water that it penetrates to the inside face.

The bearing strength of a brick wall 215 mm thick is very much greater than the loads a wall will usually carry. 

The current external wall to small buildings such as houses is built as a cavity wall with a 102.5mm external leaf of brick, a cavity and an inner leaf of block. The external leaf is sufficiently thick, with the cavity, to prevent penetration of rain to the inside face and more than thick enough to support the loads it carries.
It is for heavily loaded brick piers and walls that the crushing strength of brick is a prime consideration.

The average compressive strength of some bricks commonly used is:

Scientific work has been done to determine the amount of water absorbed by bricks and the rate of absorption, in an attempt to arrive at some scientific basis for grading bricks according to their resistance to the penetration of rain. This work has to date been of little use to those concerned with general building work. 

A wall built of very hard bricks which absorb little water may well be more readily penetrated by rainwater than one built of bricks which absorb a lot of water. This is because rain will more easily penetrate a small crack in the mortar between bricks if the bricks are dense than if the bricks around the mortar are absorptive.
xperimenta1 soaking in water of bricks gives a far from reliable guide to the amount of water they can absorb as air in the pores and minute holes in the brick may prevent total absorption and to find total absorption the bricks have to be boiled in water or heated. The amount of water a brick will absorb is a guide to its density and therefore its strength in resisting crushing, but is not a reasonable guide to its ability to weather well in a wall. This term ‘weather well’ describes the ability of the bricks in a particular situation to suffer rain, frost and wind without losing strength, without crushing and to keep their colour and texture.

Frost resistance.
A few failures of brickwork due to the disruptive action of frost have been reported during the last 30 years and scientific work has sought to determine a brick’s resistance to frost failure. Most of the failures reported were in exposed parapet walls or chimney stacks where brickwork suffers most rain saturation and there is a likelihood of damage by frost. Few failures of ordinary brick walls below roof level have been reported. Providing sensible precautions are taken in the design of parapets and stacks above roof level and brick walls in general are protected from saturation by damaged rainwater gutters or blocked rainwater pipes there seems little likelihood of frost damage in this country.

Parapet walls, chimney stacks and garden walls should be built of sound, hard burned bricks protected with coping, cappings and damp-proof courses.

Clay bricks contain soluble salts that migrate, in solution in water, to the surface of brickwork as water evaporates to outside air. These salts will collect on the face of brickwork as an efflorescence (flowering) of white crystals that appear in irregular, unsightly patches. This efflorescence of white salts is most pronounced in parapet walls, chimneys and below dpcs where brickwork is most liable to saturation. The concentration of salts depends on the soluble salt content of the bricks and the degree and persistence of saturation of brickwork.

The efflorescence of white salts on the surface is generally merely unsightly and causes no damage. In time these salts may be washed from surfaces by rain. Heavy concentration of saks can cause spalling and powdering of the surface of bricks, particularly those with smooth faces, such as Flettons. This effect is sometimes described as crypto efflorescence. The salts trapped behind the smooth face of bricks expand when wetted by rain and cause the face of the bricks to crumble and disintegrate.

Efflorescence may also be caused by absorption of soluble salts from a cement rich mortar or from the ground, that appear on the face of brickwork that might not otherwise be subject to effiorescence. Some impermeable coating between concrete and brick can prevent this (see Volume 4). There is no way of preventing the absorption of soluble salts from the ground by brickwork below the horizontal dpc level, although the effect can be reduced considerably by the use of dense bricks below the dpc.

Sulphate attack on mortars and renderigs.
When brickwork is persistently wet, as in foundations, retaining walls, parapets and chimneys, suiphates in bricks and mortar may in time crystallise and expand and cause mortar and renderings to disintegrate. To minimise this effect bricks with a low sulphate content should be used.

Type of brick.


These are bricks which are sufficiently hard to safely carry the loads normally supported by brickwork, but because they have a dull texture or poor colour they are not in demand for use as facing bricks which show on the outside when built and affect the appearance of buildings. These ‘common’ bricks are used for internal walls and for rear walls which are not usually exposed to view. Any brick which is sufficiently hard and of reasonably good shape and of moderate price may be used as a ‘common’ brick. The type of brick most used as a common brick is the Fletton brick.

This is by far the widest range of bricks as it includes any brick which is sufficiently hard burned to carry normal loads, is capable of withstanding the effects of rain, wind, soot and frost without breaking up and which is thought to have a pleasant appearance. As there are
as many different ideas of what is a pleasant looking brick as there are bricks produced, this is a somewhat vague classification.

Engineerig bricks.
These are bricks which have been made from selected clay, which have been carefully prepared by crushing, have been very heavily moulded and carefully burned so that the finished brick is very solid and hard and is capable of safely carrying much heavier loads than other types of brick. These bricks are mainly used for walls carrying exceptionally heavy loads, for brick piers and general engineering works. The two best known engineering bricks are the red Southwater brick and the blue Staffordshire brick. Both are very hard, dense and do not readily absorb water. The ultimate crushing resistance of engineering bricks is greater than 50 N/mm2.

Semi-engineerig bricks.
These are bricks which, whilst harder than most ordinary bricks, are not so hard as engineering bricks. It is a very vague classification without much meaning, more particularly as a so-called semi- engineering brick is not necessarily half the price of an engineering brick.

Composition clays.
Clays suitable for brick making are composed mainly of silica in the form of grains of sand and alumina, which is the soft plastic part of clay which readily absorbs water and makes the clay plastic and which melts when burned. Present in all clays are materials other than the two mentioned above such as lime, iron, manganese, sulphur and phosphates. The proportions of these materials vary widely and the following is a description of the composition, nature and uses of some of the most commonly used bricks classified according to the types of clay from which they are produced.

There are extensive areas of what is known as Oxford clay. The clay is composed of just under half silica, or sand, about one-sixth alumina, one-tenth lime and small measures of other materials such as iron, potash and sulphur. The clay lies in thick beds which are economical to excavate. In the clay, in its natural state, is a small amount of mineral oil which, when the bricks are burned, ignites and assists in the burning.

Because there are extensive thick beds of the clay, which are economical to excavate, and because it contains some oil, the cheapest of all clay bricks can be produced from it. The name Fletton given to these bricks derives from the name of a suburb of Peterborough around which the clay is extensively dug for brickmaking. 

Flettons are cheap and many hundreds of millions of them are used in building every year. The bricks are machine moulded and burned and the finished brick is uniform in shape with sharp square edges or arises. The bricks are dense and hard and have moderately good strength; the average pressure at which these bricks fail, that is crumble, is around 21 N/mm2
The bricks are light creamy pink to dull red in colour and because of the smooth face of the brick what are known as ‘kiss marks’ are quite distinct on the long faces. These ‘kiss marks’ take the form of three different colours, as illustrated in Fig. 46.

The surface is quite hard and smooth and if the brick is to be used for wall surfaces to be plastered, two faces are usually indented with grooves to give the surface a better grip or key for plaster. The bricks are then described as ‘keyed Flettons’. Figure 47 is an illustration of a keyed Fletton.

Fig. 46 Fletton brick showing kiss marks.

Fig. 47 Keyed Fletton.

The term ‘stock brick’ is generally used in the south-east counties of England to describe the London stock brick. This is a brick manufactured in Essex and Kent from clay composed of sand and alumina to which some chalk is added. Some combustible material is added to the clay to assist burning. The London stock is usually predominantly yellow after burning with shades of brown and purple. The manufacturers grade the bricks as 1st Hard, 2nd Hard and Mild, depending on how burned they are. The bricks are usually irregular in shape and have a fine sandy texture. Because of their colour they are sometimes called ‘yellow stocks’. 1st Hard and 2nd Hard London stocks were much used in and around London as facings as they weather well and were of reasonable price. In other parts of England the term stock bricks describes the stock output of any given brick field.

By origin the word marl denotes a clay containing a high proportion of lime (calcium carbonate), but by usage the word marl is taken to denote any sandy clay. This derives from the use of sandy clays, containing some lime, as a top dressing to some soils to increase fertility. In most of the counties of England there are sandy clays, known today as mans, which are suitable for brick making. Most of the marl clays used for brick making contain little or no lime. Many of the popular facing bricks produced in the Midlands are made from this type of clay and they have a good shape, a rough sandy finish and vary in colour from a very light pink to dark mottled red.

The gault clay does in fact contain a high proportion of lime and the burned brick is usually white or pale pink in colour. These bricks are of good shape and texture and make good facing bricks, and are more than averagely strong. The gault clay beds are not extensive in this country and lie around limestone and chalk hills in Sussex and Hampshire.

Clay shale bricks.
Some clay beds have been so heavily compressed over the centuries by the weight of earth above them that the clay in its natural state is quite firm and has a compressed flaky nature. In the coal mining districts of this country a considerable quantity of clay shale has to be dug out to reach coal seams and in those districts the extracted shale is used extensively for brick making. The bricks produced from this shale are usually uniform in shape with smooth faces and the bricks are hard and durable. The colour of the bricks is usually dull buff, grey, brown or red. These bricks are used as facings, commons and semi- engineering, depending on their quality.

Calcium silicate bricks.
Calcium silicate bricks are generally known as sand-lime bricks. The output of these bricks has increased over the past few years, principally because the output of Fletton bricks could not keep pace with the demand for a cheap common brick and sand-lime bricks have been mainly used as commons. The bricks are made from a carefully controlled mixture of clean sand and hydrated lime which is mixed together with water, heavily moulded to brick shape and then the moulded brick is hardened in a steam oven. The resulting bricks are very uniform in shape and colour and are normally a dull white. Coloured sand-lime bricks are made by adding a colouring matter during manufacture. These bricks are somewhat more expensive than Flettons and because of their uniformity in shape and colour they are not generally thought of as being a good facing brick. 

The advantage of them however is that the material from which they are made can be carefully selected and accurately proportioned to ensure a uniform hardness, shape and durability quite impossible with the clay used for most bricks.

Flint-lime bricks.
Flint-lime bricks are manufactured from hydrated lime and crushed flint and are moulded and hardened as are sand-lime bricks. They are identical with sand-lime bricks in all respects.

Cellular and perforated bricks, illustrated in Fig. 48, are machine press moulded from plastic clays, either pressed from wire cuts or separately formed. The purpose of forming the hollows and perforations is to reduce the volume of moulded, wet clay, the better to control shrinkage and deformation during drying and burning to produce more uniformly shaped bricks. 

Fig. 48 Cellular and perforated bricks.

These bricks, which have good crushing strength and are of semi- engineering quality, are used extensively in the brick enclosures to inspection pits and chambers for underground cable, for foundations and basements where the uniform shape and the density of the brick is an advantage.

They may be used for external walls as the perforations and hollows do not affect the weathering properties of the wall and may provide some little increase in insulation. 

Special brick.
A range of special bricks is made for specific uses in fairface brickwork. These bricks are made from fine clays to control and reduce shrinkage deformation during firing. The finished bricks are dense, to
resist damage by exposure to rain and cold in the exposed positions in which they are most used. Figure 49 is an illustration of some typical specials.

The two coping bricks, the half round and the saddleback, are for use as coping to a I B thick parapet wall. The bricks are some 50 mm or more wider than the thickness of a 1 B wall so that when laid they overhang the wall each side to shed rainwater and the grooves on the underside of the hangover form a drip edge.

The two bulinose specials are made as a capping or coping for I B walls. They are of the same length as the thickness of the wall on which they are laid, to provide protection and a flush finish to the wall.

The plinth bricks are used to provide and cap a thickening to the base of walls, a plinth, for the sake of appearance. A plinth at the base of a wall gives some definition to the base of a wall as compared to the wall being built flush out of the ground. A plinth may be formed by the junction of a solid l B wall built from the foundation up to a cavity wall. 

Fig. 49 Special bricks.

Wire cuts

Press moulded bricks generally have a frog or indent and wire cuts have none. The moulded brick is baked to dry out the water and burned at a high temperature so that part of the clay fuses the whole mass of the brick into a hard durable unit. If the moulded brick is burned at too high a temperature part of the clay fuses into a solid glass-like mass and if it is burned at too low a temperature no part of
the clay fuses and the brick is soft. Neither overburned nor under- burned brick is satisfactory for building purposes.

A brick wall has very good fire resistance, is a poor insulator against transference of heat, does not, if well built, deteriorate structurally and requires very little maintenance over a long period of time. Bricks are cheap because there is an abundance of the natural material from which they are made, that is clay. The clay can easily be dug out of the ground, it can readily be made plastic for moulding into brick shapes and it can be burned into a hard, durable mass at a temperature which can be achieved with quite primitive equipment.

Because there is wide variation in the composition of the clays suitable for brick making and because it is possible to burn bricks over quite a wide range of temperatures sufficient to fuse the material into a durable mass, a large variety of bricks are produced in this country. The bricks produced which are suitable for building vary in colour from almost dead white to practically black and in texture from almost as smooth as glass to open coarse grained. Some are quite light in weight and others dense and heavy and there is a wide selection of colours, textures and densities between the extremes noted.

It is not possible to classify bricks simply as good and bad as some are good for one purpose and not for another. Bricks may be classified in accordance with their uses as commons, facing and engineering bricks or by their quality as internal quality, ordinary quality and special quality. The use and quality classifications roughly coincide, as commons are much used for internal walls, facing or ordinary quality for external walls and engineering or special quality bricks for their density and durability in positions of extreme exposure. In cost, commons are cheaper than facings and facings cheaper than engineering bricks.

Materials from which bricks are made.

Thursday, January 6, 2011

In this country there are very extensive areas of clay soil suitable for brickmaking. Clay differs quite widely in composition from place to place and the clay dug from one part of a field may well be quite different from that dug from another part of the same field. Clay is ground in mills, mixed with water to make it plastic and moulded, either by hand or machine, to the shape and size of a brick.

Bricks that are shaped and pressed by hand in a sanded wood mould and then dried and fired have a sandy texture, are irregular in shape and colour and are used as facing bricks due to the variety of their shape, colour and texture.

Machine made bricks are either hydraulically pressed in steel moulds or extruded as a continuous band of clay. The continuous band of clay, the section of which is the length and width of a brick, is cut into bricks by a wire frame. Bricks made this way are called ‘wire cuts’.

Brick and Block Walls.

The majority of the walls of small buildings in this country are built of brick or block. The external walls of heated buildings, such as houses, are built as a cavity wall with an outer leaf of brick, a cavity and an inner leaf of concrete blocks. Internal walls and partitions are built, in the main, of concrete blocks.

The word brick is used to describe a small block of burned clay of such size that it can be conveniently held in one hand and is slightly longer than twice its width. Blocks made from sand and lime or concrete are manufactured in clay brick size and these are also called bricks. The great majority of bricks in use today are of clay.

The standard brick is 215 x 102.5 x 65 mm, as illustrated in Fig. 45, which with a 10mm mortar joint becomes 225 x 112.5 x 75mm. 

Fig. 45 Standard brick.

Resistance to the passage of sound - Materials construction.

Sound is transmitted as airborne sound and impact sound. Airborne sound is generated as cyclical disturbances of air from, for example, a radio, that radiate from the source of the sound with diminishing intensity. The vibrations in the air caused by the sound source will set up vibrations in enclosing walls and floors which will cause vibrations of air on the opposite side of walls and floors.

Impact sound is caused by contact with a surface, as for example the slamming of a door or footsteps on a floor which set up vibrations in walls and floors that in turn cause vibrations of air around them that are heard as sound.

The most effective insulation against airborne sound is a dense barrier such as a solid wall which absorbs the energy of the airborne sound waves. The heavier and more dense the material of the wall the more effective it is in reducing sound. The Building Regulations require walls and floors to provide reasonable resistance to airborne sound between dwellings and between machine rooms, tank rooms, refuse chutes and habitable rooms. A solid wall, one brick thick, or a solid cavity wall plastered on both sides is generally considered to provide reasonable sound reduction between dwellings at a reasonable cost. The small reduction in sound transmission obtained by doubling the thickness of a wall is considered prohibitive in relation to cost.

For reasonable reduction of airborne sound between dwellings one above the other, a concrete floor is advisable.

The more dense the material the more readily it will transmit impact sound. A knock on a part of a rigid concrete frame may be heard some considerable distance away. Insulation against impact sound will therefore consist of some absorbent material that will act to cushion the impact, such as a carpet on a floor, or serve to interrupt the path of the sound, as for example the absorbent pads under a floating floor.

Noise generated in a room may be reflected from the walls and ceilings and build up to an uncomfortable intensity inside the room, particularly where the wall and ceiling surfaces are hard and smooth. To prevent the build-up of reflected sound some absorbent material should be applied to walls and ceilings, such as acoustic tiles or curtains, to absorb the energy of the sound waves.