Rubble walling and random rubble - wall.

Tuesday, February 8, 2011

Rubble walling.
Rubble walling has been extensively used for agricultural buildings in towns and villages in those parts of the country where a local source of stone was readily available. The term rubble describes blocks of stones as they come from the quarry. The rough rubble stones are used in walling with little cutting other than the removal of incon venient corners. The various types of rubble walling depend on the nature of the stone used. 

Those stones that are hard and laborious to cut or shape are used as random rubble and those sedimentary stones that come from the quarry roughly square are used as squared rubble.

The various forms of rubble walling may be classified as random rubble and squared rubble.

Random rubble.

Uncoursed random rubble.

Uncoursed random rubble stones of all shapes and sizes are selected more or less at random and laid in mortar, as illustrated in Fig. 1 16A. No attempt is made to select and lay stones in horizontal courses. There is some degree of selection to avoid excessively wide mortar joints and also to bond stones by laying some longer stones both along the face and into the thickness of the wall, so that there is a bond stone in each square metre of walling. At quoins, angles and around openings selected stones or shaped stones are laid to form roughly square angles. 

Random rubble brought to course.
Random rubble brought to course is similar to random rubble uncoursed except that the stones are selected and laid so that the walling is roughly levelled in horizontal courses at vertical intervals of from 600 to 900 mm, as illustrated in Fig. 11 6B. As with uncoursed rubble, transverse and longitudinal bond stones are used.

 Fig. 116 (A) Random rubble uncoursed (B) Random rubble coursed.

Dowels. Cramps - Walls - Stones.

Dowels.

To maintain parapet stones in their correct position in a wall, slate dowels are used. The stones in a parapet are not kept in position by the weight of walling above and these stones are, therefore, usually fixed with slate dowels. These dowels consist of square pins of slate that are fitted to holes cut in adjacent stones, as illustrated in Fig. 113.

Cramps.
Coping stones are bedded on top of a parapet wall as a protection against water soaking down into the wall below. There is a possibility that the coping (capping) stones may suffer some slight movement and cracks in the joints between theni open up. Rain may then saturate the parapet wall below and frost action may contribute to some movement and eventual damage.
To keep coping stones in place a system of cramps is used. Either slate or non-ferrous metal is used to cramp the stones together.

A short length of slate, shaped with dovetail ends, is set in cement grout (cement and water) in dovetail grooves in the ends of adjacent stones, as illustrated in Fig. 11 5A.

As an alternative a gunmetal cramp is set in a groove and mortice in the end of each stone and bedded in cement mortar, as illustrated in Fig. 115B.

For coping stones cut from limestone or sandstone a sheet metal weathering is sometimes dressed over coping stones. The weathering of lead is welted and tacked in position over the stones.


Fig. 115 (A) Slate cramp. (B) Metal Cramp.

Weathering to cornices, Cement joggle - Stones - Walls

Weathering to cornices.
Because cornices are exposed and liable to saturation by rain and possible damage by frost, it is good practice to cover the exposed top surface of cornice stones cut from limestone or sandstone with sheet metal, The sheet metal covering is particularly useful in urban areas where airborne pollutants may gradually erode stone.

Sheet lead is preferred as a non-ferrous covering because of its ductility, that facilitates shaping, and its impermeability.

Sheets of lead, code No 5, are cut and shaped for the profile of the top of the cornice, and laid with welted (folded) joints at 2 m intervals along the length of the cornice. The purpose of these comparatively closely spaced joints is to accommodate the inevitable thermal expansion and contraction of the lead sheet. The top edge of the lead is dressed up some 75 mm against the parapet as an upstand, and turned into a raglet (groove) cut in the parapet stones and wedged in place with lead wedges. The joint is then pointed with mortar.

The bottom edge of the lead sheets is dressed (shaped) around the outer face of the stones and welted (folded) To prevent the lower edge of the lead sheet weathering being blow up in high winds, 40 mm wide strips of lead are screwed to lead plugs set in holes in the stone at 750 mm intervals, and folded into the welted edge of the lead, as illustrated in Fig. 114.
Where cornice stones are to be protected with sheet lead weathering there is no purpose in cutting saddle joints.

Fig. 114 Lead weathering to cornice.

Cement joggle.
Cornice stones project and one or more stones might in time settle slightly so that the decorative line of the mouldings cut on them would be broken and so ruin the appearance of the cornice. To prevent this possibility shallow V-shaped grooves are cut in the ends of each stone so that when the stones are put together these matching V grooves form a square hole into which cement grout is run. When the cement hardens it forms a joggle which locks the stones in their correct position.

Cornice an parapet walls, Saddle joint - Walls - Stones.

Cornice an parapet walls.
It is common practice to raise masonry walls above the levels of the eaves of a roof, as a parapet. The purpose of the parapet is partly to obscure the roof and also to provide a depth of wall over the top of the upper windows for the sake of appearance in the proportion of the building as a whole.

In order to provide a decorative termination to the wall, a course of projecting moulded stones is formed. This projecting stone course is termed a cornice and it is generally formed some one or more courses of stone below the top of the parapet. Figure 113 is an illustration of a cornice and a parapet wall to an ashlar faced building. An advantage of the projecting cornice is that it affords some protection against rain to the wall below.

The parapet wall usually consists of two or three courses of stones capped with coping stones bedded on a dpc of sheet metal. The parapet is usually at least I B thick or of such thickness that its height above roof is limited by the requirements of the Building Regulations as described in Chapter 4 for parapet walls. The parapet may be built of solid stone or stones bonded to a brick backing.

The cornice is constructed of stones of about the same depth as the stones in the wall below, cut so that they project and are moulded for appearance sake. Because the stones project, their top surface is weathered (slopes out) to throw water off.

Fig.113 Cornice and parapet.

Saddle joint.
The projecting, weathered top surface of coping stones is exposed and rain running off it will in time saturate the mortar in the vertical joints between the stones. To prevent rain soaking into these joints it is usual to cut the stones to form a saddle joint as illustrated in Fig. 113. 

The exposed top surface of the stones has to be cut to slope out (weathering) and when this cutting is executed a projecting quarter circle of stone is left on the ends of each stone. When the stones are laid, the projections on the ends of adjacent stones form a protruding semi-circular saddle joint which causes rain to run off away from the joints. 

Ashlar masonry joints and Tooled finish - Stones.

Ashlar masonry joints.
Ashlar stones may be finished with smooth faces and bedded with thin joints, or the stones may have their exposed edges cut to form a channelled or ‘V’ joint to emphasise the shape of each stone and give the wall a heavier, more permanent appearance. The ashlar stones of the lower floor of large buildings are often finished with channelled or V joints and the wall above with plain ashlar masonry to give the base of the wall an appearance of strength. Ashlar masonry finished with channelled or V joints is said to be rusticated. A channelled joint (rebated joint) is formed by cutting a rebate on the top and one side edge of each stone, so that when the stones are laid, a channel rebate appears around each stone, as illustrated in Fig. lilA. The rebate is cut on the top edge of each stone so that when the stones are laid, rainwater which may run into the horizontal joint will not penetrate the mortar joint.

A V joint (chamfered joint) is formed by cutting all four edges of stones with a chamfer so that when they are laid a V groove appears on face, as illustrated in Fig. 11 lB. Often the edges of stones are cut with both V and channelled joints to give greater emphasis to each stone.


Fig. 111 (A) Channelled joint. (B) V joint.

Tooled finish 
Plain ashlar stones are usually finished with flat faces to form plain ashlar facing. The stones may also be finished with their exposed faces tooled to show the texture of the stone. Some of the tooled finishes
used with masonry are illustrated in Fig. 112. It is the harder stones  such as granite and hard sandstone that are more commonly finished with rock face, pitched face, reticulated or vermiculated faces. The softer, fine grained stones are usually finished as plain ashlar. 

Fig. 112 Tooled finishes.

Stone arch - Crossetted arch.

A stone arch consists of stones specially cut to a wedge shape so that the joints between stones radiate from a common centre, the soffit is arched and the stones bond in with the surrounding walling. The individual stones of the arch are tenned ‘voussoirs’, the arched soffit the ‘intrados’ and the upper profile of the arch stones the ‘extrados’.


Figure 109 is an illustration of a stone arch whose soffit is a segment of a circle. The choice of the segment of a circle that is selected is to an extent a matter of taste, which is influenced by the appearance of strength. A shallow rise is often acceptable for small openings and a greater rise for larger, as the structural efficiency of the arch increases the more nearly the segment approaches a full half circle, The voussoirs of the segmental arch illustrated in Fig. 109 are cut with steps that correspond in height with stone courses, to which the stepped extrados is bonded.

The stones of an arch are cut so that there is an uneven number of voussoirs with a centre or key stone. The key stone is the last stone to be put in place as a key to the completion and the stability of the arch, hence the term key stone.

The majority of semi-circular arches are formed with stones cut to bond in with the surrounding stonework in the form of a stepped extrados similar to that shown for a segmental arch in Fig. 109.



Crossetted arch.
The semi-circular arch, illustrated in Fig. 110, is formed with stones that are cut to bond into the surrounding walling to form a stepped extrados and also to bond horizontally into the surrounding stones. The stones, voussoirs, are said to be crossetted, or crossed. This extravagant cutting of stone is carried out purely for appearance sake. This is not a structurally sound idea as a very slight settlement might cause the crossetted end of a stone to crack away from the main body of the stone, whereas with plain voussoirs the slight settlement would be taken up by the joints.

Openings to stone walls - Lintels.

A stone lintel for small openings of up to about a metre wide can be formed of one whole stone with its ends built into jambs and its depth corresponding to one or more stone courses. The poor tensile strength of stone limits the span of single stone lintels unless they are to be disproportionately deep.

Over openings wider than about a metre it is usual to form lintels with three or five stones cut in the form of a flat arch. The stones are cut so that the joints between the ends of stones radiate from a common centre so that the centre, or key stone, is wedge-shaped, as illustrated in Fig. 108. The stones are cut so that the lower face of each stone occupies a third or a fifth of the width of the opening.

To prevent the key stone sinking due to settlement and so breaking the line of the soffit, it is usual to cut half depth joggles in the ends of the key stone to fit to rebates cut in the other stones. The joggles and rebates may be cut the full thickness of each stone and show on the face of the lintel or more usually the joggles and rebates are cut on the inner half of the thickness of stones as secret joggles, which do not show on the face, as illustrated in Fig. 108. The depth of the lintel corresponds to a course height, with the ends of the lintel built in at jambs as end bearing. Stone lintels are used over both ashlar and rubble walling.

The use of lintels is limited to comparatively small openings due to the tendency of the stones to sink out of horizontal alignment. For wider openings some form of arch is used.
Fig. 108 Stone lintel with secret joggle joints.

Functional requirements - Stones used in building.

Strength and stability.

The strength of sound building stone lies in its very considerable compressive strength. The ultimate or failing stress of stone used for walling is about 300 to 100 N/mm3 for granite, 195 to 27 N/mm3 for sandstone and 42 to 16 N/mm3 for limestone. The considerable compressive strength of building stone was employed in the past in the construction of massive stone walls for fortifications and in other large structures. The current use of stone as a facing material makes little use of the inherent compressive strength of the material.

The stability of a stone wall is affected by the same limitations that apply to walls of brick or block. The construction of foundations and the limits of slenderness ratio, the need for buttressing walls, piers and chimneys along the length of walls and the requirements for lateral support from floors and roofs up the height of walls apply to stone walls as they do for brick and block walls.

Resistance to weather an ground moisture.
To prevent moisture rising from the ground through foundation walls it is necessary to form a continuous horizontal dpc some 150 mm above ground level. One way of achieving this is to construct foundation walls of dense stone, such as granite, that does not readily absorb moisture. More usually one of the damp-proof materials described for use with brickwork is used. A sheet lead dpc is commonly used as it is less likely to be squeezed out and forms a comparatively thin and therefore less unsightly joint than a bitumen felt dpc.
The resistance to the penetration of wind driven rain was not generally a consideration in the construction of solid masonry walls. The very considerable thickness of masonry walls of traditional large buildings was such that little, if any, rain penetrated to the inside face.

With the use of stone, largely as a facing material for appearance sake, it is necessary to construct walls faced with stone as cavity walling with a brick or block inner leaf separated by a cavity from the stone faced outer leaf, as illustrated in Fig. 107.

The outer leaf illustrated in Fig. 107 is built with natural stone blocks bonded to a brick backing, with full width stones in every other course and the stones finished on face in ashlar masonry. This is an expensive form of construction because of the considerable labour costs in preparing the ashlared stones. As alternatives the outer leaf of small buildings may be constructed with stone blocks by themselves for the full thickness of the outer leaf, or with larger buildings the outer leaf may be constructed of brick to which a facing of stone slabs is fixed.

The leaves of the cavity are tied with galvanised steel or stainless steel wall ties in the same way that brick and block walls are constructed and the cavity is continued around openings, or dpcs are formed to resist rain penetration at head, jambs and cills of openings.


Fig. 107 Cavity wall faced with ashlared stone and brick backing.




Durability and freedom from.
Sound natural stone is highly durable as a walling material and will maintenance have a useful life of very many years in buildings which are adequately maintained.

Granite is resistant to all usual weathering agents, including highly polluted atmospheres, and will maintain a high natural polished surface for a hundred years or more. The lustrous polish will be enhanced by periodic washing.
Hard sandstones are very durable and inert to weathering agents



but tend to dirt staining in time, due to the coarse grained texture of the material which retains dirt particles. The surface of sandstone may be cleaned from time to time to remove dirt stains by abrasive blasting with grit or chemical processes and thorough washing.
Sound limestone, sensibly selected and carefully laid, is durable for the anticipated life of the majority of buildings. In time the surface weathers by a gradual change of colour over many years, which is commonly held to be an advantage from the point of view of appearance. Limestones are soluble in rainwater that contains carbon dioxide so that the surface of a limestone wall is to an extent self- cleansing when freely washed by rain, while protected parts of the wall will collect and retain dirt. This effect gives the familiar black and white appearance of limestone masonry. The surface of limestone walls may be cleaned by washing with a water spray or by steam and brushing to remove dirt encrustations and the surface brought back to something near its original appearance.
In common with the other natural walling material, brick, a natural stone wall of sound stone sensibly laid will have a useful life of very many years and should require little maintenance other than occasional cleaning.

Fire resitance.
Natural stone is incombustible and will not support or encourage the spread of flame. The requirements of Part 13 of Schedule 1 to the Building Regulations for structural stability and integrity and for concealed spaces apply to walls of stone as they do for walls of block or brick masonry.

Resistance to the passage of heat.
The natural stones used for walling are poor insulators against the transfer of heat and will contribute little to thermal resistance in a wall. It is necessary to use some material with a low U value as cavity insulation in walls faced with stone in the same way that insulation is used in cavity walls of brick or blockwork.

Resistance to the passage of sound.
Because natural building stone is dense it has good resistance to the transmission of airborne sound and will provide a ready path for impact sound.

Ashlar walling.
Ashlar walling is constructed of blocks of stone that have been very accurately cut and finished true square to specified dimensions so that the blocks can be laid, bedded and bonded with comparatively thin mortar joints, as illustrated in Fig. 107. The very considerable labour involved in cutting and finishing individual stones is such that this type of walling is very expensive. Ashlar walling has been used for the larger, more permanent buildings in towns, and on estates where the formal character of the building is pronounced by the finish to the walling. Ashlar walling is now used principally as a facing material.

Reconstructed stone - Aggregate.

Reconstructed stone is made from an aggregate of crushed stone, cement and water. The stone is crushed so that the maximum size of the particles is 6 mm and it is mixed with cement in the proportions of 1 part cement to 3 or 4 parts of stone. Either portland cement, white cement or coloured cement may be used to simulate the colour of a natural stone as closely as possible. A comparatively dry mix of cement, crushed stone and water is prepared and cast in moulds. The mix is thoroughly consolidated inside the moulds by vibrating and left to harden in the moulds for at least 24 hours. The stones are then taken out of the moulds and allowed to harden gradually for 28 days.

Well made reconstructed stone has much the same texture and colour as the natural stone from which it is made and can be cut, carved and dressed just like natural stone. It is not stratified, is free from flaws and is sometimes a better material than the natural stone from which it is made. The cost of a plain stone, cast with an aggregate of crushed natural stone, is about the same as that of a similar natural stone. Moulded cast stones can often be produced more cheaply by repetitive casting than similar natural stones that have to be cut and shaped.
A cheaper, inferior, form of cast stone is made with a core of ordinary concrete, faced with an aggregate of crushed natural stone and cement. This material should more properly be called cast concrete.
The core is made from clean gravel, sand and Portland cement and the facing from crushed stone and cement to resemble the texture and colour of a natural stone. The crushed stone, cement and water is first spread in the base of the mould to a thickness of about 25 mm, the core concrete is added and the mix consolidated. If the stone is to be exposed on two or more faces the natural stone mix is spread up the sides and the bottom of the mould, This type of cast stone obviously cannot be carved as it has only a thin surface of natural looking stone.

As an alternative to a facing of reconstructed stone, the facing or facings can be made of cement and sand pigmented to look somewhat like the colour of a natural stone.

Cast stones made with a surface skin of material to resemble stone do not usually weather in the same way that natural stone does, by a gradual change of colour. The material tends to have a lifeless, mechanical appearance and may in time tend to show irregular, unsightly dirt stains at joints, cracks and around projections.

Reconstructed stone is used as an ashlar facing to brick or block backgrounds for both solid and the outer leaf of cavity walls and as facings.

Seasoning natural stone, Bedding stones, Cast stone: used in construction of buildings.

Monday, February 7, 2011

Seasoning natural stone.
Some natural stones are comparatively soft and moist when first quarried but gradually harden. Building stones should be seasoned (allowed to harden) for periods of up to a few years, depending on the size of the stones. Once stone has been seasoned it does not revert to its original soft moist state on exposure to rain, but on the contrary hardens with age. 

Bedding stones.
Natural stones that are stratified, limestone and sandstone, must be used in walling so that they lie on their natural bed to support compressive stress. The bed of a stone is its face parallel to the strata (layer) of the stones in the quarry and the stress that the stone suffers in use should be at right angles to the strata or bed which otherwise might act as a plane of weakness and give way under compressive stress. The stones in an arch are laid with the bed or strata radiating roughly from the centre of the arch so that the bed is at right angles to the compressive stress acting around the curve of the arch. 

Cast stone.
Cast stone is one of the terms used to describe concrete cast in moulds to resemble blocks of natural stone. When the material first came into use some 50 years ago it was called artificial stone. To avoid the use of the pejorative term artificial, the manufacturers now prefer the description reconstructed stone.

Durability of natural stone – used in the construction of buildings.

Natural stone has been used in the construction of buildings because it was thought that any hard, natural stone would resist the action of wind and ram for centuries. Many natural stones have been used in walling and have been durable for a hundred or more years and are likely to have a comparable life if reasonably maintained.

There have been sorne notable failures of natural stone in walling, due in the main to a poor selection of the materia! and poor workmanship. The best known example of decay in stonework occurred in the fabric of the buses of Parliament, the walls of which were built with a magnesian limestone from Ancaster in Yorkshire. 

A Royal Commission reported in 1839 that the magnesian limestone quarried at Bolsover Moor in Yorkshire was considered the most durable stone for the Houses of Parliament. After building work had begun it was discovered that the quarry was unable to supply sufficient large stones for the building and a similar stone from the neighbouring quarry al Anston was chosen as a substitute. The quarrying, cutting and use of the stone was not supervised closely and in consequence rnany inferior stones found their way into the building and many otherwise sound stones were incorrectly laid.

Decay of the fabric has been continuous since the Houses of Parliament were first completed and extensive, costly renewal of stone has been going on for many years. At about the same time that the Houses of Parliarnent were being buili, the Museum of Practical Geology was built in London of Anston stone from the same quarry that supplied the stone for the Houses of Parliament, but the quarrying, cutting and use of the stones was closely supervised for the museum, whose fabric remained sound.

The variability of natural stone that may appear sound and durable, but sorne of which may not weather weIl, is one of the disadvantages of this material which can, when carefully selected and used, be immensely durable and attractive as a walling material.

Portland stone, Bath stone, Sandstone: Used in building.

Portland stone.
Portland stone is quarried in Portland Islands on the coast of Dorset. There were extensive beds of this stone which is creamy white in colour, weathers well and used to be particularly popular for walling for larger huildings in towns. Many large buildings have been built in Portland stone because an adequate supply of large stone was available, the stone is fine grained and delicate mouldings can be cut on it and it weathers well even in industrial atmospheres.

Among the buildings constructed with ihis stone are the great banqueting hall in Whitehall (1639), St Paul’s Cathedral (1676), the British Museum (1753) and Somerset House (1776). More recently, many large buildings have been faced with this stone.

In the Portland stone quarries are three distinct beds of the stone, the base bed, the whit bed and the roach. The base bed is a fine, even grained stone which is used for both external and internal work to be finished with delicate mouldings and enrichment. The whit bed is a hard, fairly fine grained stone which weathers particularly well, even in towns whose atmosphere is heavily polluted with soot and it was extensively used as a facing material for large buildings.

The roach is a tough, coarse grained stone which has principally been used for marine construction such as piers and Iighthouses.

The stones from the different beds of Portland limestone look alike to the layman. It is sometimes difficult for even the trained stonemason to distinguish base bed from whit bed. Roach can be distinguished by its coarse grain and by the remains of fossil shells embedded in it. When taken from the quarry the stone is moist and comparatively soft, but gradually hardens as moisture (quarry sap) dries out.

Bath stone.
Many of the buildings in the town of Bath were built with a limestone quarried around the town. This limestone is one of the great oolites and a similar stone was also quarried in Oxfordshire. Bath stone from the Tayton (Oxfordshire) quarry was extensively used in the construction of the early colleges in Oxford (St Johns, for example) during the twelfth, thirteenth and fourteenth centuries. Many of the permanent buildings in Wíltshire and Oxfordshire were built of this stone, which vanes from fine grained to coarse grained in texture and light cream to buff in colour. Most of the original quarries are no longer being worked.

The durability of Bath stone vanes considerably. Sorne early buildings constructed with this stone are well preserved to this day, but others have so decayed over the years and been so extensively repaired that little of the original stone remains. Extensive repair of the Bath stone fabric of several of the colleges in Oxford has been carried out and continuing repair is necessary.

Sandstone
Sandstone was formed from particles of rock broken down over thousands of years by the action of wind and ram. The particles were washed into and settled to the beds of lakes and seas in combination with clay, lime and magnesia and gradually compressed into strata of sandstone rock. The particles of sandstone are practically indestructible and the hardness and resistance to the weather of this stone depends on the composition of the minerais binding the particles of sand. If the sand particles are bound with lime the stone often does not weather weIl as the soluble lime dissolves and the stone disintegrates. The material binding the sand particles should be insoluble and crystalline. Sandstones are generally coarse grained and cannot be worked to fine mouldings.

The stratification of most sandstones is visible as fairly close spaced divisions in the sandy mass of the stone. It is essential that this type of stone be laid on its natural bed in walls. 

Most sandstones have been quarried in the northern counties of England where for centuries this stone has been the material commonly used for the walls of buildings. Sorne of the sandstones that have been used are:

Crosland Hill (Yorkshire). A light brown sandstone of great strength which weathers well and is used for masonry walls as a facing material and for engineering works. It is one of the stones known as hard York stone, a general term used to embrace any hard sandstone not necessarily quarried in Yorkshire.
Blaxter stone (Northumberland). A hard, creamy coloured stone used for wall and as a facing.
Doddington (Northumberland). A hard, pink stone used for walling.

Darley Dale (Derbyshire). A hard, durable stone of great strength much used for erigineering works and as walling. It is hard to work and generally used in plain, unornamented wall. Buff and white varieties of this stone were quarried.

Forest of Dean (Gloucestershire). A hard, durable, grey or blue grey stone which is hard to work but weathers welI as masonry walling.

Natural stones used in building.

The natural stones used in building may be classified by reference to their origin as:

(1) igneous
(2) sedimentary
(3) metamorphic.

Igneous stones
Igneous stones were formed by the cooling of molten magma as the earth’s crust cooled, shrank and folded to form heds of igneous rock. Of the igneous stones that can be used for building such as granite, basalt, diorite and serpentine, granite is most used for walls of buildings.

Granite consists principally of crystais of felspar, which is made up of lime and soda with other minerais in varying proportions and small grains of quartz and mica which give a sparkle to the surface of the stone. The granite that is native to these islands that is most used for walling is sometimes loosely described as Aberdeen granite as it is mined from deep beds of igneous rock near that town in Scotland. The best known Aberdeen granites are Rubislaw which is blue grey, Kemnay which is grey and Peterhead which is pink in colouring. Ah of these granites are fine grained, hard and durable and can be finished to a smooth polished surface. 

Aberdeen granites have been much used for their strength and durability as a walling material for large buildings and are now used as a facing material.

Cornish and Devon granites are coarse grained, light grey in colour with pronounced grains of white and black crystais visible. The stone is very hard and practically indestructible. Because these granites are coarse grained and hard they are laborious to cut and shape and cannot easily be finished with a fine smooth face.

These granites have been principally used in engineering works for bridges, lighthouses and docks and also as a walling material for buildings in the counties of their origin.

Sedimentary stone
Sedimentary stone was formed gradually over thousands of years from the disintegration of older rocks which were broken down by weathering and erosion or from accumulations of organic origin, the resulting fine particles being deposited in water in which they settled in layers, or being spread by wind in layers that eventually consolidated and hardened to form layers of sedimentary rocks and clays. Because sedimentary stone is formed in layers it is said to be stratified. The strata or layers make this type of stone easier to split and cut than hard, igneous stones that are not stratified. The strata also affect the way in which the stone is used, if it is to be durable, as the divisions between the layers or strata are, in effect, planes of weakness. A general subdivision of sedimentary stones is

limestone
sandstone

The limestones used for walling consist mainly of grains of shell or sand surrounded by calcium carbonate, which are cemented together with calcium carbonate. The limestones most used for walling are quarried from beds of stone in the south-west of England, those most used being Portland and Bath stone. Because limestone is a stratified rock, due to the deposit of layers, it must be laid on its natural bed in walls. 



Metamorphic stones.
Metamorphic stones were formed from older stones that were changed by pressure or heat or both. The metamorphic stones used in building are siate and marbie.

Slate.
Slate was formed by immense pressure on beds of clay that were compressed to hard, stratified siate which is used for roofing and as chis and copings in building. Riven, split, Welsh slate has for centuries been one of the traditional roofing materials used in this country. The stone can be split to comparatively thin siates that are hard and very durable.

Marble.
The description marbie is used to include many stones that are not true metamorphic rocks, such as limestones, that can take a fine polish. In the British Isles true marbie is only found in Ireland and Scotland. Marbie is principally used as an internal facing material in this country.

Stone Masonry Walls.

Before the Industrial Revolution, many permanent buildings in hill and mountain districts and many large buildings in Iowland areas in this country were built of natural stone. At that time the supply of stone from local quarries was adequate for the buildings of the small population of this country. The increase in population that followed the Industrial Revolution was so great that the supply of sound stone was quite inadequate for the new buildings being put up. Coal was cheap, the railway spread throughout the country and cheap mass produced bricks largely replaced stone as the principal material for the walls of ah but larger buildings.

Because natural stone is expensive it is principally used today as a facing material bonded or fixed to a backing of brickwork or concrete. Many of the larger civic and commercial buildings are faced with natural stone because of its durability, texture, colour and sense of permanence. Natural stone is also used as the outer leaf of cavity walls for houses in areas where local quarries can supply stone at reasonable cost.

In recent years much of the time consuming and therefore expensive labour of cutting, shaping and finishing building stone has been appreciably reduced by the use of power operated tools, edged or surfaced with diamonds. This facility has improved output in the continuing and extensive work of repair and maintenance to stone buildings and encourages the use of natural stone as a facing material for new buildings.

Because natural stone is an expensive material, cast stone has been used as a cheaper substitute. Cast stone is made from either crushed natural stone or natural aggregate and cement and water which is cast in moulds. 

The cast stone blocks are made to resemble natural stone.

Vapour barrier: Vapour check, External insulation, Resistance to the passage of sound.

Sunday, February 6, 2011

Vapour check.
The moisture vapour pressure from warm moist air inside insulated buildings may find its way through internal linings and condense to water on cold outer faces. Where the condensation moisture is absorbed by the insulation it will reduce the efficiency of the insulation and where condensation saturates battens, they may rot.
With insulation that is permeable to moisture vapour, a vapour check should be fixed on the room side of insulation. A vapour barrier is one that completely stops the movement of vapour through it and a vapour check is one that substantially stops vapour. As it is difficult to make a complete seal across the whole surface of a wall including all overlaps of the barrier and at angles, it is in effect impossible to form a barrier and the term vapour check should more properly be used. Sheets of polythene with edges overlapped are commonly used as a vapour check, providing the edges of panels or boards of these materials can be tightly butted together. 

External insultation.
Insulating materials by themselves do not provide a satisfactory external finish to walls against rain penetration or for appearance sake and have to be covered with a finish of cement rendering, paint or a cladding material such as tile, slate or weatherboarding. For rendered finishes, one of the inorganic insulants, rockwool or cellular glass in the form of rigid boards, is most suited. For cladding, one of the organic insulants such as XPS, PIR or PUR is used because their low U values necessitate least thickness of board.

As a base for applied rendering the insulation boards or slabs are first bedded and fixed in line on dabs of either gap filling organic adhesive or dabs of polymer emulsion mortar and secured with corrosion resistant fixings to the wall. As a key for the render coats, either the insulation boards have a keyed surface or expanded metal lath or glass fibre mesh is applied to the face of the insulation. The weather protective render is applied in two coats by traditional wet render application, by rough casting or by spray application and finished smooth, coarse or textured. Coarse, spatter dash or textured finishes are preferred as they disguise hair cracks that are due to drying shrinkage of the rendering.

Because the rendering is applied over a layer of insulation it will be subject to greater temperature fluctuations than it would be if applied directly to a wall, and so is more liable to crack. To minimise cracking due to temperature change and moisture movements, the rendering should be reinforced with a mesh securely fixed to the wall, and movement joints should be formed at not more than 6 m intervals. The use of a light coloured finish and rendering incorporating a polymer emulsion will reduce cracking.

As the overall thickness of the external insulation and rendering is too great to be returned into the reveals of existing openings it is usual to return the rendering by itself, or fix some non-ferrous or plastic trim to mask the edge of the insulation and rendering. The reveals of openings will act as thermal bridges to make the inside face of the wall around openings colder than the rest of the wall. Figure 106 is an illustration of insulated rendering applied externally.

Tile and slate hanging, timber weatherboarding and profiled sheets can be fixed over a layer of insulating material behind the battens or sheeting rails to which these cladding materials are fixed.

Slabs of compressed rockwool are cut and shaped with bevel edges to simulate the appearance of masonry blocks. The blocks are secured to the external face of the wall with stainless steel brackets, fixed to the wall to support and restrain the blocks that are arranged with either horizontal, bonded joints or vertical and horizontal continuous joints. An exterior quality paint is then applied to the impregnated surface of the blocks. At openings, non-ferrous or plastic trim is fixed around outer reveals.

Details of insulating materials are given in Table 7. 

Fig. 106 External insulation.

Resistance to the passage of sound.
The requirement of Part E of Schedule 1 to the Building Regulations is that walls which separate a dwelling from another building or from another dwelling shall have reasonable resistance to airborne sound.

Where solid walls of brick or block are used to separate dwellings the reduction of airborne sound between dwellings depends mainly on the weight of the wall and its thickness. A cavity wall with two leaves of brick or block does not afford the same sound reduction as a solid wall of the same equivalent thickness because the stiffness of the two separate leaves is less than that of the solid wail and in consequence is more readily set into vibration.

The joints between bricks or blocks should be solidly filled with mortar and joints between the top of a wall and ceilings should be filled against airborne sound transmission. 

Table 7 Externa insulating materials.

In Approved Document E, giving practical guidance to meeting the requirements of the Building Regulations in relation to walls between dwellings, is a table giving the minimum weight of walls to provide adequate airborne sound reduction. For example, a solid brick wall 215 mm thick, plastered both sides, should weigh at least 300 kg/rn2 including plaster, and a similar cavity wall 255 mm thick, plastered both sides, should weigh at least 415 kg/rn2 including plaster, and a cavity block wall 250 mm thick, plastered both sides, should weigh at least 425 kg/rn2, including plaster.

Solid walls: Mechanical fixing, Internal finish.

Mechanical fixing.
As alternative to adhesive fixing, the insulating lining and the wall finish can be fixed to wood battens that are nailed to the wall with packing pieces as necessary, to form a level surface. The battens should be impregnated against rot and fixed with non-ferrous fixings. The insulating lining is fixed either between the battens or across the battens and an internal lining of plasterboard is then nailed to the battens, through the insulation.

The thermal resistance of wood is less than that of most insulating materials. When the insulating material is fixed between the battens there will be cold bridges through the battens that may cause staining on wall faces.
Details of some insulating materials used for internal lining are given in Table 6.

Table 6 Internal insulting materials.

Internal finish.
An inner lining of plasterboard can be finished by taping and filling the joints or with a thin skim coat of neat plaster. A plaster finish of lightweight plaster and finishing coat is applied to the ready keyed surface of some insulating boards or to expanded metal lathing fixed to battens. 

Laminated panels of insulation, lined on one side with a plasterboard finish are made specifically for the insulation of internal walls. The panels are fixed with adhesive or mechanical fixings to the inside face of the wall. For internal lining the organic insulants such as XPS, P1 R and PU R have the advantage of least thickness of material necessary due to their low U value.

Solid walls: Adhesive fixing.

Adhesive fixing directly to the inside wall face is used for preformed, laminate panels and for rigid insulation boards. Where the inside face of the wall is clean, dry, level and reasonably smooth, as, for example, a sound plaster finish or a smooth and level concrete, brick or block face, the laminate panels or rigid insulation boards are secured with organic based, gap filling adhesive that is applied in dabs and strips to the back of the boards or panels or to both the boards and wall. The panels or boards are then applied and pressed into position against the wall face and their position adjusted with a foot lifter.

Where the surface of the wall to be lined is uneven or rough the laminated panels or insulation boards are fixed with dabs of plaster bonding, applied to both the wall surface and the back of the lining. Dabs are small areas of wet plaster bonding applied at intervals on the surface with a trowel, as a bedding and adhesive. The lining is applied and pressed into position against the wall. The wet dabs of bonding allow for irregularities in the wall surface and also serve as an adhesive. Some of the lining systems use secondary fixing in addition to adhesive. These secondary fixings are non-ferrous or plastic nails or screws driven or screwed through the insulation boards into the wall.

Figure 105 is an illustration of laminated insulation panels fixed to the inside face of a solid wall.
Internal insulation is used where solid walls have sufficient resistance to the penetration of rain, an alteration to the external appearance is not permitted or is unacceptable and the building is not occupied. A disadvantage of internal insulation is that as the insulation is at, or close to, the internal surface, it will prevent the wall behind from acting as a heat store where constant, low temperature heating is used.

The principal difficulty with both external and internal insulation to existing buildings is that it is not usually practical to continue the insulation into the reveals of openings to avoid thermal bridges, because the exposed faces of most window and door frames are not wide enough to take the combined thickness of the insulation and rendering or plaster finish.

 Fig. 105 Internal insulation.

Solid walls: Thermal insulation. Internal insulation.

Thermal insulation.
A requirement of the Building Regulations is that measures be taken, in new buildings, for the conservation of fuel and power. There is no requirement for particular forms of construction to meet the requirement. The practical guidance to the regulation, contained in Approved Document L for dwellings, is based on assumed levels of heating to meet the expectation of indoor comfort of the majority of the largely urban population of this country who are engaged in sedentary occupations.

The advice in the Approved Document is based on an assumption that walls will be of cavity construction with the insulation in the cavity, which is the optimum position for insulation. In consequence it is likely that insulated cavity wall construction will be the first choice for the walls of dwellings for some time to come.

The regulations do make allowance for the use of any form of construction providing the calculated energy use of such buildings is no greater than that of a similar building with recommended insulated construction.
To provide the insulation required to meet the standard for conservation with a solid wall it is necessary to fix a layer of some lightweight insulating material to either the external or the internal face of the wall.

For external insulation it is necessary to cover the insulation material with either rendering, tile, slate or some sheet metal covering as protection against weather. Internal insulation has to be protected with plasterboard or some other solid material to provide an acceptable finish. The cost of the additional materials and the very considerable labour involved is so great that it is an unacceptable alternative to the more straightforward, less expensive and more satisfactory use of cavity wall insulation for new buildings.

Internal insulation.
Internal insulation may be fixed to the solid brick walls of existing buildings where, for example, there is to be a change of use from warehouse to dwelling to enhance the thermal insulation of the external walls. 

Insulating materials are lightweight and do not generally have a smooth hard finish and are not, therefore, suitable as the inside face of the walls of most buildings. It is usual to cover the insulating layer with a lining of plasterboard or plaster so that the combined thickness of the inner lining and the wall have a U value of 0.45 W/m2K, or less.

Internal linings for thermal insulation are either of preformed, laminated panels that combine a wall lining of plasterboard glued to an insulation board or of separate insulation material that is fixed to the wall and then covered with plasterboard or wet plaster. The method of fixing the lining to the inside wall surface depends on the surface to which it is applied.

Brick Arches – Segmental arch, Centering, Flat camber arch, Flat gauged camber arch.

Segmental arch.
The curve of this arch is a segment, that is part of a circle, and the designer of the building can choose any segment of a circle that he thinks suits his design. By trial and error over many years bricklayers have worked out methods of calculating a segment of a circle related to the span of this arch, which gives a pleasant looking shape, and which at the same time is capable of supporting the weight of brickwork over the arch. The recommended segment is such that the rise of the arch is 130 mm for every metre of span of the arch.

Centering.
As temporary support for brick arches it is necessary to construct a rough timber framework to the profile of the underside of the arch on which the arch bricks are laid and jointed with mortar.
The timber frame is described as centering. It is fixed and supported in the opening while the bricks of the arch are being built and the coursed brickwork over the arch laid. Once the arch and brickwork above are finished the centering is removed.

A degree of both skill and labour is involved in arch building, in setting out the arch, cutting bricks for the arch and the abutment of coursed brickwork to the curved profile of the arch so that an arched opening is appreciably more expensive than a plain lintel head.

Flat camber arch.
This is not a true arch as it is not curved and might well be more correctly named flat brick lintel with voussoirs radiating from the centre, as illustrated in Fig. 104.

The bricks from which the arch is built may be either axed or gauged to the shape required so that the joints between the bricks radiated from a common centre and the widths of voussoirs measured horizontally along the top of the arch are the same. This width will be 65 mm or slightly less so that there are an odd number of voussoirs, the centre one being a key brick.

The centre from which the joints between the bricks radiate is usually determined either by making the skew or slating surface at the end of the arch 600 to the horizontal or by calculating the top of this skew line as lying 130 mm from the jamb for every metre of span.

If the underside or soffit of this arch were made absolutely level it would appear to be sagging slightly at its centre. This is an optical illusion and it is corrected by forming a slight rise or camber on the soffit of the arch.

This rise is usually calculated at 6 or 10mm for every metre of span and the camber takes the form of a shallow curve.

The camber is allowed for when cutting the bricks to shape. In walls built of hard coarse grained facing bricks this arch is usually built of axed bricks. In walls built of softer, fine grained facing bricks the arch is usually of gauged rubber bricks and is termed a flat gauged camber arch. This flat arch must be of such height on face that it bonds in with the brick course of the main walling. The voussoirs of this arch, particularly those at the extreme ends, are often longer overall than a normal brick and the voussoirs have to be formed with two bricks cut to shape.

 Fig. 104 Flat gauged camber arch.

Flat gauged camber arch.
The bricks in this arch are jointed with lime and water, and the joints are usually 1.5 mm thick. Lime is soluble in water and does not adhere strongly to bricks as does cement. In time the jointing material, that is lime, between the bricks in this arch may perish and the bricks may slip out of position. To prevent this, joggles are formed between the bricks. Thesejoggles take the form of semi-circular grooves cut in both bed faces of each brick, as shown in Fig. 104, into which mortar is run. 

Brick arches: Gauged bricks and Two ring arch.

Gauged bricks.
The word gauge is used in the sense of measurement, as gauged bricks are those that have been so accurately prepared to a wedge shape that they can be put together to form an arch with very thin joints between them. 

This does not improve the strength of the brick arch and is done entirely for reasons of appearance. Hard burned clay facing bricks cannot be cut to the accurate wedge shape required for this work because the bricks are too coarse grained, and bricks which are to be gauged are specially chosen. One type of brick used for gauged brickwork is called a rubber brick because its composition is such that it can be rubbed down to an accurate shape on a flat stone.

Rubber bricks are made from fine grained sandy clays. The bricks are moulded and then baked to harden them, and the temperature at which these bricks are baked is lower than that at which clay bricks are burned, the aim being to avoid fusion of the material of the bricks so that they can easily be cut and accurately rubbed to shape. Rubber bricks have a fine sandy texture and are usually ‘brick red’ in colour, although grey, buff and white rubber bricks are made. These bricks are usually somewhat larger than most clay bricks.

Sheet zinc templates, or patterns, are cut to the exact size of the wedge-shaped brick voussoirs. These templates are placed on the stretcher or header face of the brick to be cut and the brick is sawn to a wedge shape with a brick saw. A brick saw consists of an ‘H’ shaped wooden frame across which is strung a length of twisted steel wires. Because rubber bricks are soft this twisted wire quickly saws through them.
After the bricks have been cut to a wedge shape they are carefully rubbed down by hand on a large flat stone until they are the exact wedge shape required as indicated by the sheet zinc template.

The gauged rubber bricks are built to form the arch with joints between the bricks as thin as 1.5 mm thick. A mortar of coarse sand and lime or cement, is too coarse for narrow joints and the mortar used between the gauged bricks is composed of either fine sand and cement and lime or lime and water, depending on the thickness of joint selected. The finished effect of accurately gauged red bricks with thin white joints between them was considered very attractive. Gauged bricks are used for flat camber arches.

A disadvantage of thin, lime mortar joints with fine grained rubber bricks is that bricks may become saturated with rainwater and crumble due to the effect of frost and the lime mortar joints may break up.

Two ring arch.
Rough and axed bricks are used for both semi-circular and segmental arches and gauged brick for segmental and flat camber arches to avoid the more considerable cutting necessary with semi-circular arches.

Rough, axed or gauged bricks can be laid so that either their stretcher or their header face is exposed. 
Semi-circular arches are often formed with bricks showing header faces to avoid the excessively wedge-shaped bricks or joints that occur with stretcher faces showing. This is illustrated by the comparison of two arches of similar span first with stretcher face showing and then with header face showing, as illustrated in Fig. 103. If the span of the arch is of any considerable width, say 1.8 m or more, it is often practice to build it with what is termed two or more rings of bricks, as illustrated in Fig. 103.

An advantage of two or more rings of bricks showing header faces is that the bricks bond into the thickness of the wall. Where the wall over the arch is more than 1 B thick it is practical to effect more bonding of arch bricks in walls or viaducts by employing alternate snap headers (half bricks) in the face of the arch. 

Fig. 103 Two ring arch.

Brick Arches – Semi-cirular arch – Rough and axed arches.

Saturday, February 5, 2011

Semi-cirular arch.
An arch, which is the most elegant and structurally efficient method of supporting brickwork, has for centuries been the preferred means of support for brickwork over the small openings for doors and windows and for arcades, viaducts and bridges. The adaptability and flexibility of the small units of brick, laid in mortar, is demonstrated in the use of accurately shaped brick in ornamental brickwork and large span, rough archwork for railway bridges.

The traditional skills of bricklaying have for many years been in decline. Of recent years the use of brick arching has, to an extent, come back into fashion in the form of arched heads to openings in loadbearing walls, brick facework to framed buildings and arcading.

The most efficient method of supporting brickwork over an opening is by the use of a semicircular arch which transfers the load of the wall it supports most directly to the sides of the opening through the arch. Figure 100 is an illustration of a semi-circular brick arch with the various terms used noted.

A segmented arch, which takes the form of a segment (part) of a circle is less efficient in that it transmits loads to the jambs by both vertical and outward thrust. 

Fig. 100 Semi-circular brick arch.

Rough and axed arches.
The two ways of constructing a curved brick arch are with bricks laid with wedge shaped mortar joints or with wedge-shaped bricks with mortar joints of uniform thickness, as illustrated in Fig. 101.
An arch formed with uncut bricks and wedge shaped mortar joints is termed a rough brick arch because the mortar joints are irregular and the finished effect is rough. In time the joints, which may be quite thick at the crown of the arch, may tend to crack and emphasise the rough appearance. Rough archwork, which may be used for its rugged appearance with irregularly shaped bricks, is not generally used for fairface work.
Arches in fairface brickwork are usually built with bricks cut to wedge shape with mortar joints of uniform width. The bricks are cut to the required wedge shape by gradually chopping them to shape, hence the name ‘axed bricks’.

Any good facing brick, no matter how hard, can be cut to a wedge shape either on or off the building site. A template, or pattern, is cut from a sheet of zinc to the exact wedge shape to which the bricks are to be cut. 

The template is laid on the stretcher or header face of the brick as illustrated in Fig. 102. Shallow cuts are made in the face of the brick either side of the template. These cuts are made with a hacksaw blade or file and are to guide the bricklayer in cutting the brick. Then, holding the brick in one hand the bricklayer gradually chops the brick to the required wedge shape. For this he uses a tool called a scutch, illustrated in Fig. 102. 

When the brick has been cut to a wedge shape the rough, cut surfaces are roughly levelled with a coarse rasp, which is a steel file with coarse teeth.

From the description this appears to be a laborious operation but in fact the skilled bricklayer can axe a brick to a wedge shape in a few minutes. The axed wedged-shaped bricks are built to form the arch with uniform 10mm mortar joints between the bricks. 

Fig.101 Rough and axed arches.


Fig. 102 Axed brick.

Brick lintels - walls.

A brick lintel may be formed as bricks on end, bricks on edge or coursed bricks laid horizontally over openings. The small units of brick, laid in mortar, give poor support to the wall above and usually need some form of additional support.

A brick on-end lintel is generally known as a ‘soldier arch’ or ‘brick on end’ arch. The word arch here is wrongly used as the bricks are not arranged in the form of an arch or curve but laid flat. The brick lintel is built with bricks laid on end with stretcher faces showing, as illustrated in Fig. 97. In building a brick lintel, mortar should be packed tightly between bricks.

A brick on end or soldier arch was a conventional method of giving the appearance of some form of support over openings in fairface brickwork.

For openings up to about 900 mm wide it was common to provide some support for soldier arches by building the lintel on the head of timber window and door frames. The wood frame served as temporary support as the bricks were laid, and support against sagging once the wall was built.

A variation was to form skew back bricks at each end of the lintel with cut bricks so that the slanting surface bears on a skew brick in the jambs, as illustrated in Fig. 97. The skew back does give some little extra stability against sagging.

For openings more than 900 mm wide a brick on end lintel may be supported by a 50 x 6 mm iron bearing bar, the ends of which are built into jambs as illustrated in Fig. 98A. The bearing bar provides little effective support and may in time rust. As a more effective alternative a steel 50 or 75 mm angle is built into jambs to give support to the lintel. The 50 mm flange of the angle supports the back edge of the bricks and may be masked by the window or door frame.

Another method of support was to drill a hole in each brick of the lintel. This can only successfully be done with fine grained bricks such as mans or gaults. Through the holes in the bricks a round-section mild steel rod is threaded and the ends of the rod are built into the brickwork either side of the lintel. This method of supporting the lintel is quite satisfactory but is somewhat expensive because of the labour involved.

A more satisfactory method of providing support for brick on edge lintels is by wall ties cast into a concrete lintel. The lintel bricks are laid on a temporary supporting soffit board. As the bricks are laid wall ties are bedded between joints. An in situ reinforced concrete lintel is then cast behind the brick lintel so that when the concrete has set and hardened the ties give support, as illustrated in Fig. 98B.

Bricks laid on edge, showing a header face, were sometimes used as a lintel. Where the soffit of the lintel is in line with a brick course there has to be an untidy split course of bricks, some 37 mm deep above. Alternatively, the top of the lintel may be in line with a course, as illustrated in Fig. 97.

As support for coursed brickwork over openings a galvanised, pressed steel lintel is used. The lintel illustrated in Fig. 99 is for use with cavity walling to provide support for both the brick outer leaf and the block inner.

 
Fig. 97 Brick lintels.


Fig. 98 (A) Bearing bar for lintel. (B) wall tie support for lintel.


Fig. 99 Steel lintel support.

Pressed steel lintels - walls.

Galvanised pressed steel lintels may be used as an alternative to concrete as a means of support to both loadbearing and non-load- bearing internal walls.

Mild steel strip is pressed to shape, welded as necessary and galvanised. The steel lintels for support over door openings in load- bearing internal walls are usually in hollow box form, as illustrated in Fig. 95. A range of lengths and sections is made to suit standard openings, wall thicknesses, course height for brickwork and adequate bearing at ends. For use over openings in loadbearing concrete block internal walls it is usually necessary to cut blocks around the bearing ends of these shallow depth lintels.

Over wide openings it may be necessary to fill the bearing ends of these hollow lintels with concrete to improve their crushing resistance.

The exposed faces of these lintels are perforated to provide a key for plaster.

To support thin, non-loadbearing concrete blocks over narrow door openings in partition walls, a small range of corrugated, pressed steel lintels is made to suit block thickness. These shallow depth, galvanised lintels are made to match the depth of horizontal mortar joints to avoid cutting of blocks.

The corrugations provide adequate key for plaster run over the face of partitions and across the soffit of openings, as illustrated in Fig. 96. 

Fig. 95 Steel lintels in internal walls.



Fig. 96 Corrugated steel lintel in internal wall.

These shallow depth lintels act compositely with the blocks they support. To prevent sagging they should be. given temporary support at mid-span until the blocks above have been laid and the mortar hardened.

Boot lintels - Walls.

When concrete has dried it is a dull, light grey colour. Some think that a concrete lintel exposed for its full depth on the external face of brick walls is not attractive. In the past it was for some years common practice to hide the concrete lintel behind a brick arch or brick lintel built over the opening externally.

A modification of the ordinary rectangular section lintel, known as a boot lintel, was often used to reduce the depth of the lintel exposed externally. Figure 93 is an illustration of a section through the head of an opening showing a boot lintel in position. The lintel is boot-shaped in section with the toe part showing externally. The toe is usually made 65 mm deep. The main body of the lintel is inside the wall where it does not show and it is this part of the lintel which does most of the work of supporting brickwork. Some think that the face of the brickwork looks best if the toe of the lintel finishes just 25 or 40 mm back from the external face of the wall, as in Fig. 94. The brickwork built on the toe of the lintel is usually B thick for openings up to 1.8 m wide. The 65 mm deep toe, if reinforced as shown, is capable of safely carrying the two or three courses of B thick brickwork over it. The brickwork above the top of the main part of the lintel bears mainly on it because the bricks are bonded. If the opening is wider than 1.8 m the main part of the lintel is sometimes made sufficiently thick to support most of the thickness of the wall over, as in Fig. 94.

The brickwork resting on the toe of the lintel is built with bricks cut in half. When the toe of the lintel projects beyond the face of the brickwork it should be weathered to throw rainwater out from the wall face and throated to prevent water running in along soffit or underside, as shown in Fig. 93.

When the external face of brickwork is in direct contact with concrete, as is the brickwork on the toe of these lintels, an efflorescence of salts is liable to appear on the face of the brickwork. This is caused by soluble salts in the concrete being withdrawn when the wall dries out after rain and being left on the face of the brickwork in the form of unsightly white dust. To prevent the salts forming, the faces of the lintel in direct contact with the external brickwork should be painted with bituminous paint as indicated in Fig. 93. The bearing at
ends where the boot lintel is bedded on the brick jambs should be of the same area as for ordinary lintels.

Fig. 93 Boot lintel.


Fig. 94 Boot lintels.

Prestressed concrete lintels and Composite and non-composite lintels- Walls.

Prestressed concrete lintels 

Prestressed, precast concrete lintels are used particularly over internal openings. A prestressed lintel is made by casting concrete around high tensile, stretched wires which are anchored to the concrete so that the concrete is compressed by the stress in the wires. Under load the compression of concrete, due to the stressed wires, has to be overcome before the lintel will bend.
Two types of prestressed concrete lintel are made, composite lintels and non-composite lintels.

Composite and non-composite lintels.
Composite lintels are stressed by a wire or wires at the centre of their depth and are designed to be used with the brickwork they support which acts as a composite part of the lintel in supporting loads. These comparatively thin precast lintels are built in over openings and brickwork is built over them. Prestressed lintels over openings more than 1200 mm wide should be supported to avoid deflection, until the mortar in the brickwork has set. When used to support blockwork the composite strength of these lintels is considerably less than when used with brickwork.

Non-composite prestressed lintels are made for use where there is insufficient brickwork over to act compositely with the lintel and also where there are heavy loads.

These lintels are made to Suit brick and block wall thicknesses, as illustrated in Fig. 92. They are mostly used for internal openings, the inner skin of cavity walls and the outer skin where it is covered externally.

Precast, or prestressed lintels may be used over openings in both internal and external solid walls. In external walls prestressed lintels are used where the wall is to be covered with rendering externally and for the inner leaf of cavity walls where the lintel will be covered with plaster.

Precast reinforced concrete lintels may be exposed on the external face of both solid and cavity walling where the appearance of a concrete surface is acceptable.

Fig. 92 Prestressed lintels.

Reinforcing rods and Casting lintels - Walls.

Friday, February 4, 2011

Reinforcing rods.
Reinforcing rods are usually of round section mild steel 10 or 12 mm diameter for lintels up to 1.8 m span. The ends of the rods should be bent up at 900 or hooked as illustrated in Fig. 91.

The purpose of bending up the ends is to ensure that when the lintel does bend the rods do not lose their adhesion to the concrete around them. After being bent or hooked at the ends the rods should be some 50 or 75 mm shorter than the lintel at either end. An empirical rule for determining the number of 12mm rods required for lintels of up to, say, 1.8 m span is to allow one 12mm rod for each half brick thickness of wall which the lintel supports. 

 Fig. 91 Ends of reinforcing rods.

Casting lintels.
The word ‘precast’ indicates that a concrete lintel has been cast inside a mould, and has been allowed time to set and harden before it is built into the wall.

The words ‘situ-cast’ indicate that a lintel is cast in position inside a timber mould fixed over the opening in walls. Whether the lintel is precast or situ-cast will not affect the finished result and which method is used will depend on which is most convenient.

It is common practice to precast lintels for most normal door and window openings, the advantage being that immediately the lintel is placed in position over the opening, brickwork can be raised on it, whereas the concrete in a situ-cast lintel requires a timber mould or formwork and must be allowed to harden before brickwork can be raised on it.

Lintels are cast in situ, that is in position over openings, if a precast lintel would have been too heavy or cumbersome to have been easily hoisted and bedded in position.

Precast lintels must be clearly marked to make certain that they are bedded with the steel reinforcement in its correct place, at the bottom of the lintel. Usually the letter ‘T’ or the word ‘Top’ is cut into the top of the concrete lintel whilst it is still wet.

Concrete lintels.

Since Portland cement was first mass produced towards the end of the nineteenth century it has been practical and economic to cast and use concrete lintels to support brickwork over openings.

Concrete is made from reasonably cheap materials, it can easily be moulded or cast when wet and when it hardens it has very good strength in resisting crushing and does not lose strength or otherwise deteriorate when exposed to the weather. The one desirable quality that concrete lacks, if it is to be used as a lintel, is tensile strength, that is strength to resist being pulled apart. To provide the necessary tensile strength to concrete steel reinforcement is east into concrete.

For a simple explanation for the need and placing of reinforcement in concrete lintels suppose that a piece of india rubber were used as a lintel. Under load any material supported at its ends will deflect, bend, under its own weight and loads that it supports. India rubber has very poor compressive and tensile strength so that under load it will bend very noticeably, as illustrated in Fig. 90. The top surface of the rubber becomes squeezed, indicating compression, and the lower surface stretched, indicating tension. A close examination of the india rubber shows that it is most squeezed at its top surface and progressively less to the centre, and conversely most stretched and progressively less up from its bottom surface to the centre of depth.

A concrete lintel will not bend so obviously as india rubber, but it will bend and its top surface will be compressed and its bottom surface stretched or in tension under load. Concrete is strong in resisting compression but weak in resisting tension, and to give the concrete lintel the strength required to resist the tension which is maximum at its lower surface, steel is added, because steel is strong in resisting tension, This is the reason why rods of steel are cast into the bottom of a concrete lintel when it is being moulded in its wet state.

Lengths of steel rod are cast into the bottom of concrete lintels to give them strength in resisting tensile or stretching forces. As the tension is greatest at the underside of the lintel it would seem sensible to cast the steel rods in the lowest surface. In fact the steel rods are cast in some 15mm or more above the bottom surface. 

The reason for this is that steel very soon rusts when exposed to air and if the steel rods were in the lower surface of the lintel they would rust, expand and rupture the concrete around them, and in time give way and the lintel might collapse. Also if a fire occurs in the building the steel rods would, if cast in the surface, expand and come away from the concrete and the lintel collapse. The rods are cast at least 15 mm up from the bottom of the lintel and 15 mm or more of concrete below them is called the concrete cover.

Fig. 90 Bending under load.

Head of openings in solid walls and Timber lintels.

Head of openings in solid walls.
Solid brickwork over the head of openings has to be supported by either a lintel or an arch. The brickwork which the lintel or arch has to support is an isosceles triangle with 600 angles, formed by the bonding of bricks, as illustrated in Fig. 89. The triangle is formed by the vertical joints between bricks which overlap 1/4 B. In a bonded wall if the solid brickwork inside the triangle were taken out the load of the wall above the triangle would be transferred to the bricks of each side of the opening in what is termed ‘the arching effect’.

Lintel is the name given to any single solid length of timber, stone, steel or concrete built in over an opening to support the wall over it, as shown in Fig. 89. The ends of the lintel must be built into the brick or blockwork over the jambs to convey the weight carried by the lintel to the jambs. The area of wall on which the end of a lintel bears is termed its bearing at ends. The wider the opening the more weight the lintel has to support and the greater its bearing at ends must be to transmit the load it carries to an area capable of supporting it. For convenience its depth is usually made a multiple of brick course height, that is about 75 mm, and the lintels are not usually less than 150 mm deep. 

 Fig. 89 Head of openings.

Timber lintels.
Up to the beginning of the twentieth century it was common practice to support the brickwork over openings on a timber lintel. Wood lintels are less used today because wood may be damaged during a fire and because timber is liable to rot in conditions of persistent damp.

Bonding of bricks at rebated jambs - Walls.

 Just as at an angle or quoin in brickwork, bricks specially cut have to be used to complete, or close, the 1/4 B overlap caused by bonding, so at jambs special closer bricks B wide on face have to be used.

Provided that the outer reveal is 1/4 B wide, the following basic rules will apply irrespective of the sort of bond used or the thickness of the wall. If the rebate is B deep the bonding at one jamb will be arranged as illustrated in Fig. 88. In every other course of bricks a header face and then a closer of 1/4 B wide face must appear at the jamb or angle of the opening. To do this and at the same time to form the 1/4 B deep rebate and to avoid vertical joints continuously up the wall, two cut bricks have to be used.

These are a bevelled bat (a ‘bat’ is any cut part of a brick), which is shaped as shown in Fig. 88, and a king closer, which is illustrated in Fig. 88. Neither of these bricks is made specially to the shape and size shown, but is cut from whole bricks on the site.

In the course above and below, two other cut bricks, called bevelled closers, should be used behind the stretcher brick. These two bricks are used so as to avoid a vertical joint. Figure 88 shows a view of a bevelled closer. 

Fig. 88 Bonding at rebated jambs.


Where the rebate is 1/2 B deep the bonding is less complicated. An arrangement of half bats as quoin header and two bevelled closers in alternate courses for English bond and half bats and king closers in alternate courses for Flemish bond is used.

 
 
 

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