The Brick Directory Blog. Articles mentioning 'bricks' - brick making, Articles and Words taken from news agencies and newspapers, magazines and books about brick and other building materials including reference ('how to') and sometimes amusing 'brick related' stories. The blog is linked with www.brickdirectory.co.uk helping you get in contact with every brick, paver, tile and stone manufacturer in the UK and Ireland.
Wednesday, 19 May 2010
Make sure the Mortar board is at least 600mm x 600mm in size, and does not have any nails in it. Ensure the labourer splashes it with water before putting any mortar on it. This will keep the mortar more workable for longer.
Always cut your mortar down from the top of the pile with the blade of your trowel, and "ROLL IT", before lifting to place on the wall. There is nothing worse than watching a Bricklayer STICK his trowel into a pile of mortar, and lifting a huge lump up onto the wall, most of which falls off and smears the face of the brickwork, or if he is lucky enough to get it onto the wall, he will then have the problem of "overbedding", making more work for himself and putting more pressure on the wet course below, causing bricks below to tip or swim. I completely disagree with those that say it is "too slow" to cut & roll your mortar, It is far slower and more costly trying to correct roughly built brickwork.
When you bed the brick, do so in a relaxed manner, without too much downward pressure on the brick, especially from the base of your hand, as this causes the brick to tip. You must also be aware of the brick laying correctly horizontally as well as vertically, this will prevent any "hatching & grinning". As the brick sinks into the bed caused by its own weight and a little pressure from you, any surplus mortar will spill out, as you run the sharp side of your trowel blade, along the face of the brickwork, keep the back of the trowel as close to the face as possible, before quickly turning it to catch the surplus mortar onto your trowel. If you`ve bedded correctly there will be only enough to use for the perp joint of the next brick you pick up.
When jointing the perp of the next brick, make sure you do so in three quick movements. 1st is the centre of the brick end, then the two edges. Do Not "top & tail" the brick as this leaves a hollow joint, always make sure the joint is full.
After you have finished the length of one course, always run the edge of your trowel along the back of the course collecting any mortar that has spilled out into the cavity face. This is very important, and make sure you catch as much of it as possible, and don't just cut it off, so it falls down the cavity, as this will dirty the ties and build up on any dpc trays etc.
"Remember, Correcting Brickwork is far more expensive, than building correct brickwork"
Brickwork can look good, but may be hiding a full cavity, poorly bedded ties, ties covered in mortar, dpc`s not bedded or split, other obstructions in the cavity etc, or many other defects.
When constructing a corner make sure that when you start on the first couple of courses, you extend your level down below the scaffold boards. This will ensure you carry on the same vertical line of the full lift below. It`s amazing how many times Bricklayers plumb off the top brick only, and when the complete lift is finished it is not on the same vertical line as the entire building. Always read your level correctly, making sure the bubble is not touching any of the lines in the glass vial of your level. Too many Bricklayers "see the bubble go past in the morning and come back in the evening". Take your time and get the bubble right.
When "jointing" or "raking out" the facework timing is critical, keep testing the joints by feeling the moisture content with your fingers every hour or so, until it feels firm but not hard, and damp but not wet.
After jointing, don`t "brush the wall off" immediately, it is best to wait about 30 minutes, then using a nice clean and dry broom head give it a light brush covering the entire face of the wall. Check the mortar joints for any brush marks or holes, then stand back and feel proud as you look at the work YOU have just produced, hopefully it may be there for the next 50-100 years.
Does the lift of Brickwork "Look pleasing to the eye"?
Has the wall been built correctly to the drawings, i.e correct bond, openings in the correct place, any features are correct?
Is the brickwork not only to the correct gauge (4 courses to 300mm) but is it to the correct "engineers value" of the building i.e 54.025m for example? This can be easily checked if the Brickwork Sub-contractor, or the Foreman Bricklayer gave you a "Job specific Gauge Sheet" at the start of the project, this cross references the gauge of each course to the Engineer`s value or the site levels etc.
Is the DPC positioned & bedded correctly? Check the "Spec" because many contracts vary, some like to see a few millimetres of dpc protruding from the brick face, others don`t want to see any at all.
Have the bricks been mixed correctly, to avoid banding?
Has the brickwork been covered correctly each night, down to below the scaffold boards, to avoid "Scaffold Rash"? If there is some rashing, it is wise to get a labourer to at least give it a soaking and light brush before the scaffold is raised, this makes it alot easier when you come to do final clean downs, before handover stage.
Is the cavity clean, and ties free from mortar, make sure any cavity insulation is fixed tightly back to the inner leaf, with no spaces visible?
Remember, when inspecting Brickwork you have to be practical and professional, not over critical, anyone can find faults, but it is a real skill to be able to get over Brickwork problems quickly, without causing too much disruption to the contract program and other following-on trades. If there is something concerning you, think about the threat to the lifespan of the building and whether that will be effected.
Tuesday, 18 May 2010
http://www.directoryofstone.co.uk/ has been put together by Brick and Stone Man, Tim
Bristow. The Directory of Stone (part of the http://www.brickdirectory.co.uk/ Database) is
a comprehensive website of web links which help self
builders/builders/developers/renovators/architects get in contact with stone quarries
as well as stone merchants, importers and factors. According to Tim
Bristow ‘We are not selling anything just helping people choose the right
Cast, Portland, Slate, Granite, Marble, Ashlar, Sandstone, Limestone, Dressed Stone
from the 2,000 or so types available in the UK’. For inclusion in the web site send an
E mail with logo and brief description to email@example.com
For further information, please contact
Tim Bristow, http://www.directoryofstone.co.uk/ Phone 07836 761541. E mail firstname.lastname@example.org or email@example.com
Monday, 17 May 2010
The Brits have a fondness for bricks. Like Yorkshire pud and Sir Trevor McDonald, they have a special place in our hearts. It’s no surprise then that 80 per cent of new homes are clad at least in part using brick, and that it is still the most popular choice in the UK for the exterior of homes.
Think Victorian terrace housing, quaint church rectories and modern cul-de- sacs – bricks make up a large proportion of our built environment. Why then have bricks been frowned upon by recent generations of architects, spurned in favour of concrete or glass? Seen as too conservative by many and too closely associated with the past, perhaps? But as the projects featured in these pages show, bricks can be versatile, they can be daring and they can most definitely be cool.
Whether handmade or mechanically produced, the basic manufacturing process has remained the same for centuries. You dig the clay, mix and roll it, then dry it before it’s stored. On the day of production, water is added and the moist mud is banged into a wooden mould. Once turned on its head, the mould is prized from the clay and the bricks are dried to remove remaining water. Now ready for firing, they are stacked in the kiln and blasted at temperatures ranging from 900-1,200°C, depending on the type of clay. Checked for mistakes, packaged and placed in the stocking yard, the bricks are finally lorried away.
Nobody can call bricks boring. They come in an astounding range of colours and texture – indeed 38 varieties are produced from one Ibstock pit alone. The colour of a brick varies depending on the depth of clay used and the components added. To darken a brick, coal is added; to lighten it, lime can be used. There’s also a plethora of glazed bricks available. For texture, sand is commonly added to the clay to create a rough finish, while the use of oil or water when extracting the brick from the mould ensures a smooth surface. Other methods include dragging rollers indented with various patterns over the bricks and adding glass beads. Using these simple processes, blobs of mud are turned into an attractive and solid construction material. It’s now up to the architect to do the bricks justice and be creative with their designs.
Storage: inspect the bricks on delivery. Unload them directly onto a dry level area or scaffold and protect from the weather.
Mortar: correctly proportion and thoroughly mix the mortar. Add nothing but clean water after this point and never use mortar after it has started to set.
Uniform walls: to avoid patches or bands of different shades, use bricks from at least three packs at the same time and don’t place all the bricks from one pack in one patch of the wall.
Protection: all brick walls must be covered during breaks in construction. Rain on recent brickwork can cause the mortar to change colour, or create lime stains, efflorescence and saturation. In cold, wet weather cover with a water-resistant material; in dry weather use hessian.
BRICK: A BRIEF HISTORY
The mud brick was invented between 10,000 and 8,000 BC making brick one of the oldest building materials known to man. The Mesopotamians developed the moulded brick in around 5000 BC, but the greatest breakthrough came with the invention of fired brick in about 3,500 BC. From this moment on, bricks could be made without the scorching heat of the sun and they soon became popular in cooler climates.
Brick was adopted in the Islamic world and in parts of India, South-East Asia and China – the Great Wall of China was constructed in brick during the Ming Dynasty (1368–1644). The Romans introduced brick building to Europe and it continued to dominate during the medieval and Renaissance period, with the red bricks of the Mediterranean and the austere brickwork of Northern Europe. The Georgians, Edwardians and Victorians all relied heavily on brick for their burgeoning cities.
Bricks crossed the Atlantic with Dutch and British immigrants and many early American skyscrapers are clad in brick or terracotta – an astounding 10 million bricks were used to construct the Empire State Building. It was used by some of the 20th century’s most famous architects including Le Corbusier, Frank Lloyd Wright and Louis Kahn, but towards the end of the century brick was perceived, unfairly, to be a cheap and uninspiring material. Brick lost out to stone, steel and glass, and became synonymous with uninspired developer-fare design.
But Now comes a new attitude: brick is back with a vengeance. Despite competition from prefabrication and other new technologies, brick has regained its place.
Thursday, 13 May 2010
13th May 2010
http://www.brickdirectory.co.uk/ has been put together by Brick Man, Tim Bristow.
The Directory of Bricks is a comprehensive website of web links which helps self
builders/builders/developers/renovators/architects get in contact with all brick
manufacturers as well as brick merchants, importers and factors, brick services
companies, builders merchants, roof tile and paver manufacturers. According to Tim
Bristow ‘We are not selling anything just helping people choose the right
reclaimed/rough/smooth textured, metric/imperial Brick from the 1,800 or so types
available in the UK’. For inclusion in the web site send an E mail with
logo and brief description to firstname.lastname@example.org
For further information, please contact
Tim Bristow, http://www.brickdirectory.co.uk/ Phone 07836 761541. E mail email@example.com or firstname.lastname@example.org
Tuesday, 4 May 2010
This is the gist of it… As of 1st October 2008 planning permission is required to lay traditional impermeable driveways that allow uncontrolled runoff of rainwater from front gardens onto roads, because this can contribute to flooding and pollution of water courses. However, householders will not require planning permission if they use permeable paving, or have sufficient drainage or soak away to ensure runoff from non-permeable surfaces does not go onto roads.
Basically, this is a great move. In times of heavy or sustained rainfall our drainage systems comes under extreme pressure. Catastrophic flooding in parts of the UK in recent years has demonstrated the distressing consequences of overloading the system and the importance of water management in our urban areas.
Permeable paving is certainly the best option environmentally, and the easiest choice when it comes to the new legislation. However not everyone has the right soil conditions for permeable paving and for it to work well, it really does need to be installed by a professional.
Whatever your choice – permeable paving or planning permission – there are plenty of products on the market that will suit every project.
There is plenty of evidence that the Romans used clay tiles extensively on their properties. Although the use of clay tiles diminished somewhat during the Saxon period, by the 12th century there are records of clay tile use being encouraged particularly in place
of thatch for fire safety. The size of tile (10½" x 6½" x ½") was standardised in 1477.
In the early years the use of clay tiles, like many other building materials, was limited by cost. Nonetheless, for those who could afford it, clay tile was often the material of choice.
Another limiting factor was transport. Prior to the advent of mass transportation systems it was rare for clay tiles (or any other materials) to be transported any significant distance, typically not more than a day’s cart journey. Exceptions were made for the roofing of churches and the homes of the very rich, who often had access to clay fields and kilns further afield, and employed the labour, which made the costs much cheaper.
As a result the pattern of clay tile usage correlates closely with the areas in which clay and ‘brick earth’ are found, and it is perhaps not surprising to find that the manufacture of clay tile from the later medieval period was closely aligned to that of brick-making.
By the late medieval period a more stable social, economic and political climate resulted in an increase in wealth, generally enabling more people to afford materials such as brick, glass and indeed clay tiles.
From the 17th century clay tile became the ubiquitous roofing material for large parts of the country where the raw material was close at hand – mainly the southeast and east of England and the Midlands.
Greater wealth in the 19th century, improved transportation and the introduction of taxation on fired building products such as tiles and bricks to fund the Napoleonic wars led to a reduction in the use of clay tiles and the increasing use of other roofing materials, particularly slate. However, it was the advent of the railway more than anything else that caused the roof map of England to change from red to grey. During the 19th century slate tended to be cheaper and thus it overtook clay tiles as the roof material of choice for the rapidly developing urban landscape.
During the 20th century mass-production of machine-made clay tiles resulted in a resurgence of clay-tiled roofs, particularly during the inter-war period. However, increase in competition from man-made tiles such as concrete tiles and man-made slate resulted once again in a downturn in the use of natural clay tiles. In more recent years homeowners have rediscovered the beauty of the material and there has been something of a resurgence in the use of handmade clay tiles.
The tile typically found throughout this period is the double-lap tile (one where the overlap between courses of tiles is greater than the length of a tile) but one should not forget the single-lap tile where the tiles interlock at edges only. Although today we are used to seeing the single lap tile in the form of concrete roofing materials, the history of single lap tiling goes back many centuries. The most common form is what we generically refer to as ‘pantiles’. These should not be confused with genuine Roman tiling, which in fact has not reappeared in any significant manner in this country since the 4th century AD.
The use of pantiles is not as widespread as clay tiling generally and it tended to focus on the eastern side of the country. Records indicate that pantiles arrived somewhere around the 17th century, with home-produced pantiles appearing from about 1700. Because the tiles were originally imported, their distribution tends to focus on the ports of the eastern seaboard. The exception is Bridgewater in Somerset, where pantiles were certainly established by the late 1750s and where a prolific pantile-making industry later emerged, supplying tiles throughout Somerset and the neighbouring counties.
The manufacture of clay tiles is relatively straightforward. Traditional handmade tiles are a mixture of clay with aggregates rolled out and cut or moulded to simple rectangles (sometimes shaped) with two holes at one end for fixing. These are then fired in a kiln. Sometimes the ends were extended at right angles to form a nib, but the majority of clay roofing for many centuries was simply a baked clay rectangle.
Due to the firing, flat tiles would come out slightly convex and this added to their character. Uneven temperatures in the kiln and the nature of the hand-making process also contributed to variations in shape and form, and the quality of the clay resulted in rough and therefore textured surfaces. The colour would be determined partly by the clay and the mix of aggregates but also by the temperature and length of firing in the kiln.
Sometimes shaped tiles were produced and occasionally glazed tiles and pantiles can be seen. During the Victorian period there was much experimentation and occasionally one comes across multi-coloured examples. With modern machine-made tiles, dyes are added to bring greater consistency of colour.
Plain clay tiles are laid in regular courses with each tile lapping two others, leaving approximately four inches exposed. The precise method of fixing depends on the nature of the tile itself. In the case of the more basic form of tile, simple tapered wooden pegs were pushed through the two holes at the top of the tile so that the tile could be hung over battens fixed horizontally across the tops of the roof rafters. The tops of the pegs would be trimmed flush to the surface of the tiles so that the next course would lie flat.
Lime mortar, sometimes with straw and other aggregates, would often be applied to the internal face of the tiles to fill the gaps and help improve the general fixing of the tile. This mortar fillet is often referred to as ‘torching’. On many roofs the pegs would be limited to only one per tile. Indeed, roofs can often be seen with no pegs at all, or at least pegs only in occasional courses. Although this can be due to the pegs rotting away, sometimes tiles were laid bedded in lime mortar with no pegs. In such
situations the fixing of the tile relied as much on friction and the weight of tiles above as on any torching or mortar bed.
If a tile had been made with nibs these would be used to hang the tile over the batten, and pegs would not be required. With modern tiling the nibs themselves have holes to enable nail fixing to the battens, although not every course is nailed in place.
As the use of slate increased, the need for nails to fix them also increased. The consequent increase in the production of nails resulted in their increasing use to fix clay tiles as well: nailing was quicker and avoided the need to trim the timber peg before laying the next course.
Today we find a wide variety of tiles available to us, including traditional peg tiles but also handmade tiles with nibs to facilitate fixing.
TYPICAL DEFECTS AND REPAIRS
It is often said that clay tiles have a limited life of up to 60 years or thereabouts. However, walking around the countryside you will often come across peg-tiled roofs that are several hundreds of years old, so this is clearly not a reliable guide.
The failure of the tile itself will depend on many different factors, including the original manufacture, the make-up of the material within the tile and its firing in the kiln.
Because tiles are much thinner in section than brick, they are less susceptible to variations in firing. Nonetheless, there will always be some tiles that are from the cooler parts of the kiln and therefore more vulnerable to early failure, particularly as a result of frost damage. That said, as a rule handmade tiles tend to have great durability and, if well-fired, tend not to be particularly vulnerable to frost damage. Only after many years will the best examples eventually weather, exposing the softer and more porous clay body below to frost damage.
Other factors which can influence the longevity of tiles (and, in fact, any roof covering) will be the orientation of a building, the steepness of the roof and indeed the microclimate around the building. Clay tiles are best used on roof pitches of 40° but some single lap tiles can be used down to 25° pitches.
Other more controllable factors include such matters as tree branches brushing up against the roof covering and dislodging or breaking tiles, climbing plants being allowed to grow over and into tiling to dislodge and damage it, and clumsy workmen treading on the tiles.
Due to the rough texture of a clay tile surface it is likely to harbour lichens and mosses. These plants should not necessarily be regarded as harmful. Although lichens produce acidic secretions and moss can hold moisture and lead to frost damage, they are unlikely to cause much damage. Indeed moss can provide a protective layer and lichens contribute to the characteristic colouring of tiled roofs. However, significant moss growth can increase the weight on the roof structure generally, and when it dies and rolls into the gutter it can cause quite serious gutter blockages.
If moss is to be removed, care should be taken. Simply pulling moss from the roof surface is more likely to cause damage than by letting it die naturally or by appropriate chemical removal means (biocidal treatment). However, care should be taken with chemical removal methods to ensure that the chemicals do not run down to the gutter and into the surface water system.
The defects that most often affect tiled roof coverings are in fact the sort of defects that affect all roof coverings: failure of battens (rot, woodworm etc); failure of the batten fixings (nail corrosion); deterioration of the tile fixings (rotting pegs, corroding nails or crumbling torching); failure of or defects to the roof frame; defects to perimeter details (soakers, flashings, etc); defects to roof details (valleys, verges, eaves, etc); and wind uplift.
Problems that can affect the tile surfaces, apart from those rare occasions when moss or lichen cause damage, are usually brought about by matters such as pollution, the premature failure of poor quality tiles, saturation from leaking pipes or drips from overhanging details such as TV aerials.
Frost damage can occur where moisture is retained on the surface and this sometimes happens at the laps. Sometimes localised frost damage can cause a tile to break at the head lap. Machine-made tiles are particularly prone to frost damage as the surfaces are more even and regular, enabling moisture to be trapped on the underside. Handmade tiles on the other hand have a natural variation which is both less moisture-retentive and more pleasing to the eye.
Other problems can arise due to poor laying in the first instance. Such problems include inappropriate detailing at verges and hips, poor setting and laying of the ridge and poor detailing of abutments such as chimneys and walls. Abutments need particular care. Local vernacular may dictate the use of tile creasings, or else mortar fillets, or lead. Lead flashings are usually the more reliable and mortar fillets the least. Whichever detail is used, lead soakers should always be incorporated between each tile to resist the passage of rainwater horizontally.
Another common failure with clay-tiled roofs is brought about by the failure of the fixings or battens due to rot or rusting. The battens often use sapwood, which is much more vulnerable to decay than heartwood. Pressure-treated battens should always be used for repair and replacement. If care is taken, many of the tiles themselves can usually be salvaged and reused. A word of warning, however: because peg tiles tend not to be pegged every course and therefore rely on friction and/or the torching, there is a risk of mass failure and slippage if a careless roofer steps onto the roof. Before attempting to repair a clay tile roof it is important to check the fixings below in case the attempt at repair itself causes more damage.
When repairing a tiled roof it is important to obtain as close a match as possible to the original in terms of texture and colouring. Non-ferrous fixings should be used to reduce corrosion risks. Any lime torching should be continued across new areas of work, and with the existing torching properly reinstated.
Most roofs can be satisfactorily patch-repaired rather than having to be completely stripped and re-covered. However, if complete re-covering is to take place, every attempt should be made to salvage the tiles and as a rule of thumb one would hope to salvage approximately 70 per cent.
Complete stripping and re-covering requires the new work to comply with building regulations, and this would often mean the use of a lining over the rafters beneath the battens and tiles. Such linings restrict airflow into the roof space, and the roof space then has to be positively ventilated or a modern breathable lining used.
It should be noted that where there is a double-lap roof covering, a lining is not strictly necessary for weathering purposes. Homeowners often attempt to line a roof because they believe it to be appropriate or perhaps to stop unnecessary draughts. However, attempting to line a roof from the underside (within the roof space) can lead to a number of problems. Because the lining is then not laid over the rafters it will direct any penetrating water into the eaves where it will cause rot and damage. Careful thought and installation is needed with regard to retrospective lining and it is best avoided.
In recent years there has been an increase in the use of expanded foam applications to the undersides of tiles. These are often marketed as providing a solution to insulation problems, securing loose tiles in place and reducing draughts. However, the use of such material should be viewed warily and it is suggested that such material should be regarded as a last resort only, particularly for historic buildings. Foam stuck to the underside of the tiles means the tiles cannot be salvaged for re-use at a later date. The practice also makes it very difficult to undertake patch repair in future because of the difficulty in getting individual tiles out. There is also a possibility of reducing the life of the tiles or slates if they are a bit porous, as it reduces the evaporative surface area: water absorbed when it rains will no longer be able to evaporate from the lower surface. The risk of frost damage is therefore greater.
Spray-on foams also perform poorly as a means of insulating roofs. The blocking up of ventilation and the lack of a moisture barrier can lead to condensation problems. Building regulations require a ventilation gap above insulation or a vapour membrane under the insulation, but with spray-on foams neither are provided.
From an aesthetic point of view these foams can also be a problem, as it is often difficult to prevent the foam spilling out between gaps in the tiles (particularly pantiles). Such foams are therefore a short-term form of repair that could increase the long-term cost of later work. If tiles are slipping it is better to undertake a proper repair.
Of course these negatives should always be balanced against the difficulty of access and perhaps the expected future lifespan of the roof. If the building is listed, however, such work would require consent and many conservation officers would probably refuse consent for use of such products.
Traditional clay tiles create beautiful roof coverings that are full of character due to the individual nature of the tiles. Provided they are carefully and properly maintained there is no reason to expect them to perform poorly. Many of the typical problems found can be resolved without loss of the tile itself. Before embarking on any work to a roof seek professional advice on what is required. If altering or extending the roof of a listed building, ensure the appropriate consent has been obtained.
The decision to clean an historic building is not one which should be made lightly, as cleaning can have significant physical and visual results. A period of detailed investigation must be undertaken to determine whether cleaning should be undertaken and, if so, the details of how this should be done. The nature and condition of all substrates must be understood, not forgetting pointing materials, as must the soiling to be removed. The latter may include atmospheric soiling, paint, limewash, metallic staining, anti-pigeon gel and graffiti. Each can require a different cleaning approach or at least modifications to the system selected for use elsewhere.
Every cleaning system can be used correctly or incorrectly. Poor cleaning should not be blamed on poor application alone as it is often the result of incorrect selection of a process. Glossy trade literature is no guarantee of correct selection. The design of a cleaning regime for an historic building is often deceptively complex, requiring specialist professional input.
The purpose of cleaning is to remove soiling, often a source of long-term deterioration to masonry, while causing little or no disruption of the masonry beneath. This can be difficult to achieve due to the intimate relationship between the stone and its soiling, as the soiling can be embedded deeply in between the surface particles.
Several published sources now exist which outline the basic constituents of various historic masonry materials and the susceptibilities of these to selective cleaning procedures. Previous experience must also come into play in assessing surface conditions and characteristics particular to the job at hand. The basic principles of any cleaning process must be determined if it is to be considered for use. Works should be undertaken by skilled supervisors and operatives from specialist masonry firms experienced in the cleaning of historic masonry.
Air abrasive cleaning systems are usually considered when soiling is not water-soluble and when, for reasons of site logistics or material incompatibility, chemical processes are inappropriate or less preferable.
A wide range of air abrasive techniques is currently available. These include a variety of machines, nozzles and abrasives from Hodge Clemco, Neokleen, Liquabrade, JOS and the suppliers of pencil abrasive techniques. Some larger scale equipment can be used in a very versatile and sensitive manner.
All air abrasive techniques operate by directing particles of abrasive onto the soiled masonry in a stream of compressed air. Cleaning is accomplished by impingement of the particles which dislodge or pulverise the surface layer of the masonry. This may be the layer of soiling or the stonework or brickwork to which it is attached. Most systems also involve the use of water, either additional to the air/abrasive stream or combined as a slurry with the abrasive. The main effect of the introduction of water is to reduce dust (both dry and wet abrasive systems clean in a similar manner), although the mist produced is still a health hazard.
Air abrasive cleaning techniques are most successful on surfaces of even profile and consistent surface texture and hardness. An air abrasive stream cannot on its own differentiate between the removal of soiling and the removal of masonry. Nor can it distinguish portions of masonry which are closer to the nozzle from those further away or areas of masonry which are softer. Damage to the masonry can only be avoided through the skill and ability of the operator to make the necessary adjustments in technique.
Air abrasive cleaning is usually most successful on plain stone surfaces of even hardness. Careful use can enable the technique to be employed on moulded and some carved stone surfaces. However it is difficult to successfully clean brickwork by abrasive means without any damage, due to the many variations in surface texture and hardness that are often present and due to the intolerance of many bricks to its impact. The removal of hard, traditional paints can rarely be achieved successfully from any masonry surface using air abrasives.
In the normal use of abrasive cleaning, two factors are of utmost importance; the velocity and the concentration of the particles which impact on the surfaces. These parameters are controlled by the pressure and volume of the air flow and the concentration of abrasive feed into the line. It is therefore not adequate to specify pressure alone. Important parameters will also include the size of the abrasive particle, its shape and its hardness. Commonly available abrasives for facade cleaning include aluminium silicate, calcium silicate, olivine and calcium carbonate. More specialist materials are also available, particularly for pencil abrasive equipment used by conservators.
Nozzle shape, nozzle size, rate of water flow and working distance must also be established.
It is usually best to determine the many parameters relating to abrasive cleaning on site when all soiling types, the degree of soiling and masonry conditions can be properly assessed. Specific advice such as recommended pressures and abrasive types cannot be given here as they are only a few of the many variables which must be determined, as already described. However, the following general principles can be applied:
i) Smaller particles of the same abrasive type can be less damaging than larger ones, used in the same manner.
ii) Harder abrasives can be more damaging than softer abrasives of the same size, used in the same manner.
iii) A higher concentration of abrasive particles can be more damaging than a lower concentration, all other factors being equal.
iv) Higher air pressure and volume can be more damaging than lower air pressure and volume, all other factors being equal.
v) A closer working distance between the end of the nozzle and the masonry can be more damaging than a greater one, all other factors being equal.
vi) Depending on how they are used, some small scale abrasive systems can be as or more damaging than larger scale systems.
vii) Differences in technique will be required for plain and carved surfaces, sound and deteriorated conditions.
General recommendations cannot be made in relation to air abrasive cleaning, any more than with any other cleaning approach. Pre-contract on-site trials are always recommended for the cleaning of historic masonry. These should be overseen by an experienced professional who can observe and assess the effects of each procedure and produce a detailed specification for the works.
· C Andrew et al, Stone Cleaning: A Guide for Practitioners, Historic Scotland, 1994
· N Ashurst, Cleaning Historic Buildings, Volumes 1 and 2, Donhead, London, 1994
· British Standards Institution BS 6270: Code of practice for cleaning and surface repair of buildings, Part 1, BSI, London, 1982
· Technical Pamphlet 4: Cleaning Brick and Stone, SPAB (Society for the Protection of Ancient Buildings), London, 1994
· ME Weaver and FG Matero, Conserving Buildings: A Guide to Techniques and Materials, John Wiley, New York, 1993
· RGM Webster, Stone Cleaning and the Nature, Soiling and Decay Mechanisms of Stone, Donhead, London, 1992
Previously considered to be an inferior material to stone, brick construction was rarely used in Britain until the close of the Middle Ages. Gerard Lynch looks at its historical development over the last 600 years and its conservation and repair.
TUDOR BRICKWORK 1485-1603
The popularity of the material can be traced to the revival of brick-making in eastern England in the late 13th and early 14th centuries. This was a direct result of lack of local stone, an increasing shortage of good timber, and the influence of Europe where brickwork was used extensively.
By the Tudor period the brick-makers and bricklayers had emerged as separate craftsmen well able to rival the masons. From unsophisticated early work, brick building entered its heyday, rivalling stone in its popularity as a structural material.
Bricks were generally made on site in wood, heather or turf fired clamps by itinerant workers. Not only were standard bricks produced but also many in extravagant and elaborate shapes, epitomised by those that formed the spiral twisted chimney stacks for which the period is renown.
The Tudors further patterned their brickwork by inserting headers of over burnt or vitrified bricks into the walling. These dark surfaces ranging from deep purple to slate in colour, were laid carefully in quarter brick offsets in mainly English bond or English cross-bond, to form a diaper or chequered pattern within the predominantly red brickwork.
Tudor bricks were irregular in size and shape and therefore thick (15-25mm) mortar joints were necessary to even these out. The slow setting mortar was of matured non-hydraulic lime (often containing particles of the fuel used in its production), and coarse sand in a ratio varying from 1:2-1:5, the joints being finished flush from the laying trowel.
With the building of Hampton Court Palace, we have not only the seal of royal approval, but a monument to the achievement of brick in this period.
THE GEORGIAN PERIOD 1714-1830
The late 17th and early 18th centuries were a high point in the use of brick. Their manufacture was much improved, with blended clay, better moulding and more even firing which lead to greater consistency in shape and size. The colours of bricks changed in popularity from red, purple or grey bricks fashionable in the late 17th century until 1730, when brownish or pinkish grey stocks replaced the hot colours. These were followed in the mid 18th century by grey stocks, and, by 1800, the production of yellow marl or malm London stocks, which were closer to the stone colour desired for a classical facade.
Brickwork was generally of a very high standard, in mainly Flemish bond although header bond was also popular in the early 18th century.
Pointing was executed to a similar standard. As well as giving more protection to the weaker bedding mortar, fine detailing also helped to minimise the visual impact of the joints so that the classical details could be displayed more clearly. 'Tuck' pointing was the ultimate development in this quest.
A more expensive solution was to use 'gauged' brickwork popularised by Wren using a facade of fine, colour-matched bricks cut and rubbed to exact units, and laid in thin lime putty joints. However after 1730 this was considered too expensive and was reserved for window arches, aprons and other ornamentation only.
VICTORIAN BRICKWORK 1830-1914
This was a period of revivalism in domestic architecture and industrial building. The former seeking a return to 'medievalism' and other exotic building forms as a relief from the unspirituality of the Machine Age. The latter, for the infrastructure of factories, warehouses, railway bridges and so on, all largely met through the cheap use of bricks.
During this period, a greater number of bricks were made and laid than during all the previous periods. Brick manufacturing methods had improved in all respects including quality accuracy, regularity and in range of colours available. From the mid 18th century onwards the manufacturing process, like many others, was becoming mechanised. This enabled deeper clays to be used for pressing into dense bricks for use on civil engineering works.
With improvements in travel and communications, bricks could be transported over wide areas which removed the traditional local variations.
Improvements in the production of mortar also occurred during the late 18th century through the use of washed and graded aggregates, often with colouring. Also, the development of natural cements including Roman cement and other hydraulic limes, which set quicker and stronger, were vital to the speed of construction that the Industrial Age demanded. Portland Cement appeared in the mid 19th century.
Joints reduced to 0.3 inches (8mm) due to the accuracy of the machine pressed bricks and continued to be finished in various profiles. These were popular from the 17th century although the new 'weather-struck' and 'cut' style of joint became particularly popular.
A variety of face bonds were now used although, in the main, Flemish bond predominated domestically, whilst English bond was favoured industrially.
In all matters of brickwork, the Victorian desire for enrichment was readily achieved by the use of often garish polychromatic work, and the lavishing of ornamentation by detailing mass produced purpose moulded 'specials' or by gauged brickwork.
DECAY, CONSERVATION AND REPAIR
Before considering the most appropriate method of repairing brickwork, correct diagnosis of the cause of failure is vital.
Manufacturing defects in bricks can be the result of under firing or impurities in the clay used. These bricks decay more rapidly than better burnt bricks, especially with frost action. They can also act as a point of entry for moisture, which in turn will affect the whole wall, leaving it open to damage from frost and chemical action. Free standing walls, parapets and retaining walls are particularly vulnerable, and some judicious replacement may be necessary.
Poor Detailing can also contribute to failure through construction defects such as:
Decayed bond timbers, joists, timber lintels, plates or bearers which have been embedded or built in to the masonry.
The expansion of rust on corroding iron and steel structural members, wall ties or reinforcement embedded in the brickwork.
Failure of arches and lintels from inadequate bearings, or abutments.
Poor bonding and inadequate or even non-existent tying-in of brickwork. This can be due to a habit in the 18th and 19th centuries of 'snapping' headers leading to a wall of two skins, instead of one mass. Alternatively failure can occur at the junctions between walls, particularly where front and rear walls are insufficiently tied to the cross walls.
'Corbelling' (over projecting brickwork) and oversailing are especially prone to being insufficiently tailed-in to the main walling. They are also susceptible to water penetration due to inadequate, or non-existent protective weathering.
Sulphate attack occurs when water is present with cement based mortars, producing slow steady expansion of sulphate crystals within the mortar or the bricks as the water evaporates. It can result in damage and even failure of the masonry. This is particularly common in unlined chimney stacks, where sulphates have been introduced by the burning of sulphur-rich fossil fuels. Where chimneys have been designed without bends, allowing rain straight down the flue, damp may appear on the chimney breast with a possible resultant salt problem. This can especially occur when the air is humid, or where the fireplace has been sealed without proper ventilation.
Poorly designed parapet copings without damp proof courses, inadequate overhangs, and poor jointing techniques, which encourage damp penetration.
Remedial work to historic brickwork must be carefully selected after expert analysis and should always be kept to an absolute minimum.
Consolidants should only be applied to decaying brickwork as a last resort. Although predictably effective on soft porous bricks their use is still in its infancy, and the long term affects of new techniques is less certain. The consolidated brick face may behave in a different manner from the base through thermal movement, resulting in eventual separation.
Sealants may induce similar problems. By sealing the surface they may inhibit or reduce surface evaporation leading to a build up of moisture. This can result in concentrating evaporation in other areas where crystallisation and frost damage may be exacerbated. Sealants should only be used in localised areas to prevent problems such as the staining which occurs from water run-off from limestone dressings, where it may be used after removal of the deposits.
The introduction of hard mortars is one of the most common causes of failure in historic brickwork, leading to a failure of the mortar and of the brickwork itself.
Inappropriate cleaning methods may cause substantial damage by removing not only the dirt but also the fireskin, leaving a pitted face. Rotary carborundum heads again, destroy the surface as well as dishing and scouring the 'arrises'. Such methods may actually accelerate re-soiling and rate of decay by producing a more textured surface.
Vegetation although often attractive, is generally harmful to older brick walls of traditional construction. Many types of ivy can cause serious damage to brickwork particularly if it is in poor repair, or constructed of soft, possibly spalling, bricks bedded in soft lime mortar where the pointing is defective.
If not carefully removed, ivy should at the least, be heavily controlled and never allowed to reach eaves level where it might block gutters and downpipes. In a strong wind, vegetation can also transfer additional wind-load, pulling out guttering, parapets, and even a chimney-stack.
Pigeons can also present problems, especially in city centres. Not only can they force up loose roof coverings, but they will block up gutters and downpipes with feathers, detritus and excrement, causing water penetration and consequent decay. The faeces rapidly deface the external (as well as internal) fabric, and may damage porous brickwork. Removal is difficult and expensive.
Control is therefore imperative, and can involve bird nets, repellent gels, poisons, traps and even shooting.
J Ashurst and N Ashurst, Practical Building Conservation, Volume 2: Brick, Terracotta and Earth, Gower Technical Press, Aldershot, 1988
TG Bidwell, The Conservation of Brick Buildings, Brick Development Association, Windsor, 1977
RW Brunskill, Brick Building In Britain, Victor Gollancz Ltd, London, 1990
N Lloyd, A History of English Brickwork, H Greville Montgomery, London, 1925
GCJ Lynch, Gauged Brickwork: A Technical Handbook, Gower Technical Press, Aldershot, 1990
J Woodforde, Bricks To Build a Brick House, Routledge & Kegan Paul, London, 1976