Theme: bricks, blocks and pavers
Brick manufacture makes a significant contribution to the UK economy with a market size of about ú550 million. There are some 1,200 varieties of brick, ranging from handmade bricks manufactured using traditional techniques to machine-made bricks produced in highly automated factories. Products also include innovative clay cladding systems.
Three basic techniques are used for forming bricks:
l Soft mud process - clay is mixed with 25-30 per cent water to achieve an easily malleable consistency. It is thrown into moulds, either by hand or machine, turned out, dried and then fired. Typically, bricks formed by this method have a soft, irregular outline and an attractive, often rugged, appearance. Bricks made using this process are referred to as 'stock bricks'.
l Extruded wire-cut process - clay is mixed with about 18 per cent water to a stiff plastic consistency. It is forced from a machine through a rectangular die, sized to correspond with the length and width of a brick.
Rollers, scrapers, sand spray or other such methods may be used to texture the surfaces of the extruded strip, which is then cut into individual bricks by a frame of taut parallel wires. Generally, there are rods in the throat of the die to control the flow of the clay and these create the perforations in bricks.
l Semi-dry pressing - semi-dry clay is ground into small particles and pressed into steel moulds in hydraulic presses. In the UK this method is confined to the production of Fletton bricks from Lower Oxford clay.
Firing methods also vary, with four different techniques employed:
l Clamps - in some soft mud bricks, a small amount of crushed fuel is added to the clay and they are fired in clamps. A clamp has no permanent enclosure; a large stack of bricks (between 500,000 and two million) is placed on a bed of solid fuel that is ignited. When the fuel in the base and in the bricks has burned out, the firing is complete. At some works, clamps are gas fired.
l Intermittent kilns - there are various types.
They are permanent enclosures in which bricks are set and fired using solid fuel, oil or gas. When firing is complete, the bricks are allowed to cool and are then removed.
The use of these kilns is not confined to any particular type of brick, but fully enclosed kilns with good temperature control are necessary for the high temperatures needed to produce dense-bodied, high-strength 'engineering' types.
l Continuous burning multi-chamber kilns - these can reduce the waste of time and energy due to cooling and reigniting kilns.
Chambers are arranged in a looped circuit and the fire is conducted from chamber to chamber. Chambers are emptied of fired bricks and refilled with unfired ones in a corresponding sequence. Solid fuel or gas is introduced through the roofs of chambers.
This type of kiln is particularly suited to firing bricks made from Lower Oxford clay (Fletton bricks) because the clay has a significant fuel content. The type was widely used before the introduction of tunnel kilns.
l Continuous burning tunnel kilns - these are about 100m long with a firing zone in the centre. Bricks are set on kiln cars that are pushed through the tunnel, taking three to four days, during which the bricks are heated, fired and cooled. Tunnel kilns are capable of accurate control and high efficiency. The use of kiln cars is consistent with increasing automation of the manufacturing process, and most modern brickworks use tunnel kilns.
A popular image of brick is of mud transformed into a hard, inert, rock-like material by intense fire. The firing is a conspicuous use of energy and is immediately challenged when environmental credentials are considered. However, other characteristics of brick - its status as a plentiful and inexpensive raw material, exceedingly long service life (hundreds of years) and almost negligible demands for maintenance - more than redress the balance.
UK brick manufacturers were quick to respond to the government's Sustainable Development Strategy in 1999 by joining the Pioneers Group, a best-practice group to develop sectoral strategies. A framework to assess the industry's economic, social and environmental performance was published and the industry agreed to measure itself annually against a set of key performance indicators (KPI) and to improve year on year. The Brick Development Association (BDA) recently published a detailed report for 2004 noting good performance in almost all key performance areas. The strategy has the dual advantage of improving industry efficiency and enhancing its claims to be a sustainable supplier of building material.
Research assists manufacture CERAM Building Technology (CBT) at Stoke-on-Trent has completed a two-year research project funded through the Waste and Resources Action Programme (WRAP).
It assessed the cost versus technical benefits of using powdered recycled glass in the manufacture of clay bricks and pavers.
Although the potential of using powdered glass as a fluxing agent in brick production was recognised about 70 years ago, its use was not worthwhile because of low fuel costs. Today, not only does the high cost of fuel change the balance but reduced firing means fewer CO 2 emissions, which also leads to cost saving.
Recycled glass bottles and jars were ground to a fine sand-like powder by a specialist supplier. Three brickmakers joined in the trials so that the effects of the addition could be explored on three different types of clay. Glass was added to replace 5 per cent and 10 per cent of the clay, and quantities of full-sized bricks were made. Test firings were carried out first in the CERAM laboratory and then in manufacturers' kilns.
In all cases, the addition of powdered glass gave economies in firing and improved the physical properties of the bricks. As expected, the effects were not the same for the three clays, due partly to differences in normal firing temperatures and partly to dissimilar mineralogy.
In addition to a reduction in firing temperature and duration, the addition of glass increased the products' compressive strength, reduced their water absorption and improved their resistance to damage by freeze/thaw action. The most marked effect was seen in bricks made from Etruria Marl, a heavy clay normally fired at 1,100 °C. To achieve physical properties at least equivalent to normal production, the addition of glass allowed a 20 per cent fuel saving and reduced firing time by nearly five hours. As an unexpected bonus, hydrogen fluoride emission was reduced by as much as 33 per cent.
This research has identified what seems to be a very beneficial additive to clay for brick manufacture. Not only are fuel savings worthwhile but the quality of the product also seems to be enhanced by the addition of powdered glass. The addition is not likely to result in bricks of different appearance but it looks as if it will save energy, reduce emissions of CO 2 and pollutants, and improve quality - all grist to the mill of sustainability.
Innovative construction for housing Traditional Plus is a new masonry construction system developed by CBT for single-storey and two-storey domestic buildings. External walls consist of a loadbearing 140mm-wide single skin of masonry, lined internally with 100mm-thick waterproof Styrofoam insulation with a pre-bonded 12.7mm plasterboard surface. The U-value is 0.26 W/m 2°K.
The masonry units, specially developed and produced for the system by research partner Ibstock Brick, are 290mm long x 140mm wide x 65mm high and are perforated. This is a rare departure from the brick industry's traditional role of providing standard units for its customers. For Traditional Plus, a width of 140mm was considered as optimum. The UK standard brick used to build a leaf of 102mm presents a problem of poor stability during construction, and if used to build a 215mm leaf it would be unnecessarily thick. The strength requirement for low-rise domestic structures is not great and therefore the unit can be highly perforated to reduce weight and assist manufacture. The length of the new unit is twice the width (plus a 10mm joint width) to permit half-lap bonding. Its height gives an attractive 1:4 face proportion and conforms to a normal brickwork gauge of four courses to 300mm. The perforation pattern incorporates a finger grip in the centre but, in practice, bricklayers have found the unit is comfortable to span single-handed and the units are easy to lay.
Laboratory testing that simulated severe driving rain found the system to be effective in resisting rain penetration. The Styrofoam insulation - waterproof and non-absorbent - is fixed to the inner face of masonry with dabs of waterproof adhesive, creating a 6-8mm gap between the surfaces. Any water that penetrates the masonry will run down in the gap and be diverted out through weepholes in the masonry by a damp-proof course tray at the base of the wall, or in any other appropriate position.
The system has a precast concrete suspended ground floor, a timber-joisted first floor and a trussed-rafter roof structure. Conventional concrete trench fill or raft foundations can be used, but Roger Bullivant has developed a special pile-andground-beam design for the system.
The Department of Trade and Industry's (DTI) 'Partners in Innovation' programme has funded the development of the system, including a demonstration house built on the Bullivant site in Stoke-on-Trent, with the support of the BDA, the National House Building Council and BRC Special Products.
Support is now sought for the next development stage, which ideally would be a pilot scheme to regenerate a city-centre brownfield site to provide affordable housing in the North Staffordshire region.
CBT has published a 38-page design guide for Traditional Plus. It contains extensive details, notes on structural design, specification and workmanship. The system reinstates a basic concept of masonry construction as inherently simple and straightforward. It offers a high standard of construction, superior thermal performance and fast, economic building.
Colourful bricks The variety of clays and firing techniques used by UK brick manufacturers provides a wonderful choice of facing bricks, a choice unrivalled in any other country.
The variety of colours is derived from the reaction of the mineral content of clay to firing at high temperature. Ceramic glazes of almost any colour can be applied to bricks, although some colours present technical difficulties and require expensive pigments.
To look good, glazed bricks must have a virtually blemish-free surface and be free of chips. They must also be accurate and consistent in size because neat, thin joints are generally required and on-site cutting is impractical. These bricks therefore tend to be expensive.
Decorative application of glazed coloured bricks in buildings by architects such as Piers Gough and John Outram has caught the imagination of many designers, and there is now a greater demand for such bricks. Hathernware Ceramics in Leicestershire and Shaws of Darwen in Lancashire are established manufacturers of glazed architectural terracotta and glazed bricks. They have standard ranges but will make other colours to order.
Hanson Brick now includes glazed coloured bricks in its Desimpel range, and Ibstock Brick has introduced its Colour range of six high-gloss glazed coloured standard-format bricks. Ibstock's Fireborn glazed coloured terracotta blocks, made in 13 colours, can be used on their own or incorporated into brickwork. Baggeridge Brick offers glazed bricks imported from the Netherlands, which include a range of colours and metallic glazes in copper, gold, aluminium and nickel.
European Standards New European Standards (EN) for masonry materials were published last year. To avoid the upheaval of an instantaneous change, they presently coexist with the current British Standards (BS) for masonry materials, which will be withdrawn in spring 2005. However, there are other British Standards that specify design methods, selection of materials and workmanship for masonry construction and they will continue for some considerable time. Therefore, BS 5628 (Parts 1 and 2) and BS 8103, which deal with structural design, and BS 5628:3, which deals with non-structural aspects, are in process of revision to incorporate references to European product standards.
Nomenclature in the European Standards differs from that in the British Standards, but it is important to understand that masonry products and their applications will essentially remain the same. For example, in both BS 3921, the British Standard for clay bricks, and BS EN 771-1, the European Standard for clay masonry units that will supersede it, bricks are classified with regard to durability by reference to frost resistance and solublesalts content. Although the terms and test methods used to classify these properties are not identical, there are practical equivalents.
Reference to them will be included in the updated version of BS 5628:3. Compressive strength and water absorption are measured in both BS and EN, but different test methods are used and they give different values.
Guidance on the application of these values for structural design will be included in the updated versions of design codes BS 5628:1 and 2 and BS 8103.
BS EN 771-1 is a standard for clay masonry units and distinguishes between high-density (HD) and low-density (LD) units. All bricks produced and traded in the UK are of the HD class. LD units, highly perforated blocks larger than brick format, are not made in the UK. The EN includes a National Foreword and a National Annex that give informative commentary on the differences between the EN and BS 3921. A significant difference is the method specified for measuring bricks to determine permitted limits of deviation from specified size. The EN involves measurement of a sample of 10 individual bricks using special precision instruments; it is not practical as a routine site test. A simplified method for site verification is given in BS PAS 70, HD clay bricks - A guide to appearance and site measured dimensions and tolerance, a new BS publication sponsored by the BDA.
BS EN 771-2 to 5 specify masonry units of calcium silicate, aggregate concrete, autoclaved aerated concrete and manufactured stone. The position with pavers is more advanced.
British Standards for concrete block pavers and clay pavers have been withdrawn and replaced by BS EN 1338 and 1344 respectively.
Manufacturers and trade associations have already published guidance to assist designers, specifiers and other construction professionals in interpreting their requirements with reference to the new standards.
This is likely to continue as the process of changeover proceeds.
Simplifying mortar Mortar is an essential component of masonry but it is poorly understood by many specifiers.
Perhaps that is because of the variety of mixes that produce broadly equivalent performance, of additives and admixtures that modify properties and the vagaries of site production that may or may not produce good results.
Faults can have expensive consequences.
To minimise failure, there has been a marked change in dealing with mortar in recent years. A survey by the Mortar Industry Association indicates that less than 25 per cent of mortar is now made on site. Ready-to-use wet mortars (with set retarder) account for about 35 per cent, semi-dry lime/sand mortars (gauged on site with cement) about 30 per cent, and dry mortar in silos is rapidly increasing its share of the market.
The new BS EN 998-2 'Specification for masonry mortar' has now been published to promote factory-produced quality-controlled mortars. Although a mortar can be specified by a type and mix recipe, this is deprecated and it is preferable to specify that mortar of guaranteed strength is required. A mortar supplier can then adjust precise mix proportions to suit the constituent materials being used.
Of course, mortar colour and texture may well be a factor in design and this aspect of specification should be clarified and agreed with the supplier.
Mortar strength is generally correlated with durability, but when masonry is subject to severe exposure to weather or soluble sulphates, specific durability requirements should be confirmed.
Prefabrication benefits Large-scale prefabrication of brickwork - reinforced concrete cladding panels faced with brick - is often used for large commercial and public buildings where quick erection is a financial benefit. Brickmakers report an increase in orders for supply of brick slips or specially prepared bricks for this purpose.
Brick slips are also used in proprietary lightweight cladding systems. In Hanson Brick's Wonderwall and EuroBrick ranges, slips are bonded to rigid plastic thermal insulation, carrier-boards that are fixed to the backing structure. In Baggeridge Brick's Corium, slips are clipped into specially profiled steel carrier strips. In all cases the joints between the slips are pointed with mortar to replicate normal brickwork.
Prefabrication of discrete brickwork elements is a means of incorporating special features into work that would otherwise require extra time and craft skill. Gauged brick arches of various types and dimensions can be supplied for fixing on site as preassembled units. Ibstock Brick provides several such items bonded to lightly reinforced aerated-concrete backings. Hanson Brick produces gauged brick arches in two forms: FabrikArch, on a lightweight concrete backing, and CombiArch, bonded to a structural steel lintel.
Michael Hammett was senior architect at the Brick Development Association (BDA) until retirement in 2001. He represents the BDA on various BSI committees and is chairman of the one responsible for BS 5628:3