Building Materials: Chapter 3 - Bricks PDF

University of Halabja Course Name: Building Materials Civil Engineering Department Course Code: First Stage Date / 2021...

University of Halabja Course Name: Building Materials Civil Engineering Department Course Code: First Stage Date / 2021 2020-2021 : Lecturer: Rabar H. Faraj CHAPTER THREE Bricks INTRODUCTION: One of the oldest building material brick continues to be a most popular and leading construction material because of being cheap, durable and easy to handle and work with. Clay bricks are used for building-up exterior and interior walls, partitions, piers, footings and other load bearing structures. A brick is rectangular in shape and of size that can be conveniently handled with one hand. Brick may be made of burnt clay or mixture of sand and lime or of Portland cement concrete. Clay bricks are commonly used since these are economical and easily available. CLASSIFICATION OF BRICKS  On Field Practice Clay bricks are classified as first class, second class, third class and fourth class based on their physical and mechanical properties. 1  A. First class bricks 1. These are thoroughly burnt and are of deep red, cherry or copper colour. 2. The surface should be smooth and rectangular, with parallel, sharp and straight edges and square corners. 3. These should be free from flaws, cracks and stones. 4. These should have uniform texture. 5. No impression should be left on the brick when a scratch is made by a finger nail. 6. The fractured surface of the brick should not show lumps of lime. 7. A metallic or ringing sound should come when two bricks are struck against each other. 8. Water absorption should be 12–15% of its dry weight when immersed in cold water for 24 hours. 9. The crushing strength of the brick should not be less than 10 N/mm2. This limit varies with different Government organizations around the country. Uses: First class bricks are recommended for pointing, exposed face work in masonry structures, flooring and reinforced brick work. B. Second Class Bricks are supposed to have the same requirements as the first class ones except that: 1. Small cracks and distortions are permitted. 2. A little higher water absorption of about 16–20% of its dry weight is allowed. 3. The crushing strength should not be less than 7.0 N/mm2. Uses: Second class bricks are recommended for all important or unimportant hidden masonry works and centering of reinforced brick and reinforced cement concrete (RCC) structures. C. Third Class Bricks are under burnt. They are soft and light-coloured producing a dull sound when struck against each other. Water absorption is about 25 per cent of dry weight. Uses : It is used for building temporary structures. D. Fourth Class Bricks are over burnt and badly distorted in shape and size and are brittle in nature. Uses: The ballast of such bricks is used for foundation and floors in lime concrete and road metal. 2 On Strength and Durability Clay building bricks are graded according to properties related to durability and resistance to weathering, such as compressive strength, water absorption, and saturation coefficient (ASTM C62). (ASTM C62) Classifies three available grades : 1. severe weathering (SW) Grade SW bricks are intended for use in areas subjected to frost action, especially at or below ground level. 2.moderate weathering (MW) Grade MW bricks are recommended for use in areas with no frost action and in dry locations, even where subfreezing temperatures are expected 3.negligible weathering (NW) Grade NW bricks can be used in interior construction, where no freezing occurs Absorption is one of the important properties that determine the durability of bricks. Highly absorptive bricks can cause efflorescence and other problems in the masonry. According to ASTM C67, absorption by 24-hour submersion, absorption by 5-hour boiling, and saturation coefficient are calculated as: 3 The compressive strength of clay bricks is an important mechanical property that controls their load-carrying capacity and durability. The compressive strength of clay bricks is dependent on the composition of the clay, method of brick manufacturing, and the degree of firing. Physical Requirements for Clay Building Bricks (ASTM C62) 4 Sample problem From Table above the maximum allowable absorption by 5-hour boiling= 25 %. Therefore, the brick does not satisfy the ASTM requirements.  On the Basis of Use A. Common Brick is a general multi-purpose unit manufactured economically without special reference to appearance. These may vary greatly in strength and durability and are used for filling, backing and in walls where appearance is of no consequence. B. Facing Bricks are made primarily with a view to have good appearance, either of color or texture or both. These are durable under severe exposure and are used in fronts of building walls for which a pleasing appearance is desired. C. Engineering Bricks are strong, impermeable, smooth, table molded, hard and conform to defined limits of absorption and strength. These are used for all load bearing structures.  On the Basis of Finish A. Sand-Faced Brick has textured surface manufactured by sprinkling sand on the inner surfaces of the mould. 5 B. Rustic Brick has mechanically textured finish, varying in pattern.  On the Basis of Burning A. Pale Bricks are under burnt bricks obtained from outer portion of the kiln. B. Body Bricks are well burnt bricks occupying central portion of the kiln. C. Arch Bricks are over burnt also known as clinker bricks obtained from inner portion of the kiln.  On the Basis of Types A. Solid Small holes not exceeding 25 per cent of the volume of the brick are permitted; alternatively, frogs not exceeding 20 per cent of the total volume are permitted. B. Perforated Small holes may exceed 25 per cent of the total volume of the brick. C. Hollow The total of holes, which need not be small, may exceed 25 per cent of the volume of the brick. D. Cellular Holes closed at one end exceed 20 per cent of the volume. 6 CHARACTERISTICS OF GOOD BRICK A. Size and Shape The bricks should have uniform size and plane, rectangular surfaces with parallel sides and sharp straight edges. B. Colour The brick should have a uniform deep red or cherry colour as indicative of uniformity in chemical composition and thoroughness in the burning of the brick. C. Texture and Compactness The surfaces should not be too smooth to cause slipping of mortar. The brick should have precompact and uniform texture. A fractured surface should not show fissures, holes grits or lumps of lime. D. Hardness and Soundness The brick should be so hard that when scratched by a finger nail no impression is made. When two bricks are struck together, a metallic sound should be produced. E. Water Absorption should not exceed 20 per cent of its dry weight when kept immersed in water for 24 hours. F. Crushing Strength should not be less than 10 N/mm2. G. Brick Earth should be free from stones, kankars, organic matter, saltpetre, etc. INGREDIENTS OF GOOD BRICK The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulphur, etc. The proportions of various ingredients are as follows: 7  Functions of Various Ingredients  Silica: It enables the brick to retain its shape and imparts durability, prevents shrinkage and warping. Excess of silica makes the brick brittle and weak on burning.  Alumina: absorbs water and renders the clay plastic. If alumina is present in excess of the specified quantity, it produces cracks in brick on drying.  Lime: normally constitutes less than 10 per cent of clay. Lime in brick clay has the following effects: 1. Reduces the shrinkage on drying. 2. Causes silica in clay to melt on burning and thus helps to bind it. 3. In carbonated form, lime lowers the fusion point. 4. Excess of lime causes the brick to melt and the brick loses its shape. 5. Red bricks are obtained on burning at considerably high temperature (more than 800°C) and buff-burning bricks are made by increasing the lime content.  Magnesia: rarely exceeding 1 percent affects the colour and makes the brick yellow, in burning; it causes the clay to soften at slower rate than in most case is lime and reduces warping.  Iron: Iron oxide constituting less than 7 percent of clay, imparts the following properties: 1. Gives red colour on burning when excess of oxygen is available and dark brown or even black colour when oxygen available is in sufficient, however, excess of ferric oxide makes the brick dark blue. 2. Improves impermeability and durability. 3. Tends to lower the fusion point of the clay, especially if present as ferrous oxide. 4. Gives strength and hardness. MANUFACTURING OF BRICKS The fundamentals of brick manufacturing have not changed over time. However, technological advancements have made contemporary brick plants substantially more efficient and have improved the overall quality of the products. A more complete knowledge of raw materials and their properties, better control of firing, improved kiln designs and more advanced mechanization have all contributed to advancing the brick industry.  Raw Materials Clay is one of the most abundant natural mineral materials on earth. For brick manufacturing, clay must possess some specific properties and characteristics. Such clays must have plasticity, which permits them to be shaped or molded when mixed with water; they must have sufficient wet and air-dried strength to maintain their shape after forming. 8 Also, when subjected to appropriate temperatures, the clay particles must fuse together.  Types of Clay Clays occur in three principal forms, all of which have similar chemical compositions but different physical characteristics. 1. Surface Clays. Surface clays may be the up thrusts of older deposits or of more recent sedimentary formations. As the name implies, they are found near the surface of the earth. 2. Shales. Shales are clays that have been subjected to high pressures until they have nearly hardened into slate. 3. Fire Clays. Fire clays are usually mined at deeper levels than other clays and have refractory qualities. Surface and fire clays have a different physical structure from shales but are similar in chemical composition. All three types of clay are composed of silica and alumina with varying amounts of metallic oxides. The manufacturer minimizes variations in chemical composition and physical properties by mixing clays from different sources and different locations in the pit. Chemical composition varies within the pit, and the differences are compensated for by varying manufacturing processes. As a result, brick from the same manufacturer will have slightly different properties in subsequent production runs. Further, brick from different manufacturers that have the same appearance may differ in other properties.  Phases of Manufacturing The manufacturing process has six general phases: 1) mining and storage of raw materials, 2) preparing raw materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products Diagrammatic Representation of Manufacturing Process 9   Mining and Storage Surface clays, shales and some fire clays are mined in open pits with power equipment. Then the clay or shale mixtures are transported to plant storage areas. Continuous brick production regardless of weather conditions is ensured by storing sufficient quantities of raw materials required for many days of plant operation. Normally, several storage areas (one for each source) are used to facilitate blending of the clays. Blending produces more uniform raw materials, helps control color and allows raw material control for manufacturing a certain brick body.  Preparation To break up large clay lumps and stones, the material is processed through size reduction machines before mixing the raw material. Usually the material is processed through inclined vibrating screens to control particle size.  Forming. Tempering, the first step in the forming process, produces a homogeneous, plastic clay mass. Usually, this is achieved by adding water to the clay in a pug mill. a mixing chamber with one or more revolving shafts with blade extensions. After pugging, the plastic clay mass is ready for forming. 10  Pug mill Clay is Thoroughly Mixed with Water in Pug Mill Before Extrusion After pugging, the plastic clay mass is ready for forming. There are three principal processes for forming brick: stiff-mud, soft-mud and dry-press. 1. Stiff-Mud Process - In the stiff-mud or extrusion process, water in the range of 10 to 15 percent is mixed into the clay to produce plasticity. After pugging, the tempered clay goes through a de airing chamber that maintains a vacuum of 15 to 29 in. (375 to 725 mm) of mercury. De-airing removes air holes and bubbles, giving the clay increased workability and plasticity, resulting in greater strength. 11  2. Soft-Mud Process The soft-mud or molded process is particularly suitable for clays containing too much water to be extruded by the stiff-mud process. Clays are mixed to contain 20to 30 percent water and then formed into brick in molds. To prevent clay from sticking, the molds are lubricated with either sand or water to produce “sand-struck” or “water-struck” brick. Brick may be produced in this manner by machine or by hand. 3. Dry-Press Process This process is particularly suited to clays of very low plasticity. Clay is mixed with a minimal amount of water (up to 10 percent), then pressed into steel molds under pressures from 500 to 1500 psi (3.4 to 10.3 MPa) by hydraulic or compressed air rams.  Drying Wet brick from molding or cutting machines contain 7 to 30 percent moisture, depending upon the forming method. Before the firing process begins, most of this water is evaporated in dryer chambers at temperatures ranging from about 100 ºF to 400 ºF (38 ºC to 204 ºC). The extent of drying time, which varies with different clays, usually is between 24 to 48 hours.  Hacking. Hacking is the process of loading a kiln car or kiln with brick. The number of brick on the kiln car is determined by kiln size. The brick are typically placed by robots or mechanical means. 12  Firing (burning)  Brick are fired between 10 and 40 hours, depending upon kiln type and other variables. There are several types of kilns used by manufacturers. The most common type is a tunnel kiln, followed by periodic kilns. Fuel may be natural gas, coal, sawdust, methane gas from landfills or a combination of these fuels. In a tunnel kiln, brick are loaded onto kiln cars, which pass through various temperature zones as they travel through the tunnel. Firing may be divided into five general stages: 1) final drying (evaporating free water); 2) dehydration; 3) oxidation; 4) vitrification; and 5) flashing or reduction firing. All except flashing are associated with rising temperatures in the kiln. Although the actual temperatures will differ with clay or shale, final drying takes place at temperatures up to about 204 ºC, dehydration from about 149 ºC to 982 ºC, oxidation from 538 ºC to 982 ºC)and vitrification from 871 ºC to 1316 ºC.  Cooling After the temperature has peaked and is maintained for a prescribed time, the cooling process begins. Cooling time rarely exceeds 10 hours for tunnel kilns and from 5 to 24 hours in periodic kilns. Cooling is an important stage in brick manufacturing because the rate of cooling has a direct effect on color.  De-hacking De-hacking is the process of unloading a kiln or kiln car after the brick have cooled, a job often performed by robots or machines. 13 HARMFUL SUBSTANCES IN BRICKS  Lime: When a desirable amount of lime is present in the clay, it results in good bricks, but if in excess, it changes the colour of the brick from red to yellow. When lime is present in lumps, it absorbs moisture, swells and causes disintegration of the bricks.  Pebbles, Gravels, Grits : do not allow the clay to be mixed thoroughly and spoil the appearance of the brick. Bricks with pebbles and gravels may crack while working.  Organic matter On burning green bricks, the organic matter gets charred and leave pores making the bricks porous; the water absorption is increased and the strength is reduced.  Alkalis (Alkaline Salts) : forming less than 10 per cent of the raw clay, are of great value as fluxes, especially when combined with silicates of alumina. However, when present in excess, alkali makes the clay unsuitable for bricks. They melt the clay on burning and make the bricks unsymmetrical. When bricks come in contact with moisture, water is absorbed and the alkalis crystallise. On drying, the moisture evaporates, leaving behind grey or white powder deposits on the brick which spoil the appearance. This phenomenon is called efflorescence. Efflorescence should always be dry brushed away before rendering or plastering a wall; wetting it will carry the salts back into the wall to reappear later. 14  Sulphur: is usually found in clay as the sulphate of calcium, magnesium, sodium, potassium or iron, or as iron sulphide. Generally, the proportion is small. If, however, there is carbon in the clay and insufficient time is given during burning for proper oxidation of carbon and sulphur, the latter will cause the formation of a spongy, swollen structure in the brick and the brick will be decoloured by white blotches.  Water: A large proportion of free water generally causes clay to shrink considerably during drying, whereas combined water causes shrinkage during burning. The use of water containing small quantities of magnesium or calcium carbonates, together with a sulphurous fuel often causes similar effects as those by sulphur. 15