(a) Describe various defects in clay bricks giving sources/causes of defects. (10 marks) (b) Draw neat sketches, showing complete…

(a) Defects in clay bricks and their causes

• Over-burning: bricks fired too long or too hot partly fuse (vitrify); they become distorted, brittle and glassy. Cause: excess temperature or fusible impurities (alkalis) in the clay.
• Under-burning: bricks not fired enough stay soft, porous and weak, and absorb much water. Cause: low kiln temperature or short firing.
• Black core (black heart): a dark centre in an otherwise red brick. Cause: carbonaceous matter or iron compounds not fully oxidised because firing was too fast.
• Efflorescence: a white, powdery deposit on the surface. Cause: soluble salts (sulphates of sodium, potassium, magnesium, calcium) in the clay, the water or the fuel, dissolved by moisture and crystallised as the brick dries.
• Bloating: a swollen, spongy mass on the brick. Cause: gases from excess carbonaceous matter and sulphur during burning.
• Blisters and lamination: raised patches or thin peeling layers. Cause: air trapped during moulding, or clay not well tempered or pugged.
• Checks and cracks: Cause: stones, pebbles or lime nodules in the clay, rapid or uneven drying, and shrinkage.
• Lime bursting (popping): pieces flake off after the brick is wetted. Cause: lime nodules which, after firing, slake and expand when they meet water.
• Spots: dark patches. Cause: iron sulphide (pyrites) in the clay.
• Chuffs: deformed bricks. Cause: rain falling on hot bricks.
• Warping: twisted or bent bricks. Cause: uneven drying or firing, and poor stacking.

Most defects are prevented by selecting clay free from lime, pyrites, stones and alkalis, thorough tempering and pugging, slow even drying, and controlled firing.

(b) English bond, 1½-brick wall, right-angled corner

• A 1½-brick wall is one header plus one half-brick thick: about 290 mm with modular bricks (190 × 90 × 90 mm plus 10 mm joints) or 13½ in with traditional bricks.
• Every course has a header row and a stretcher row. In one course, the face of wall A shows stretchers (with a header row behind); in the next course, the face of wall A shows headers (with a stretcher row behind). The return wall B always shows the opposite pattern in the same course.
• At the corner the quoin brick is a stretcher of one wall, whose end appears as a header on the face of the other wall. On the header face, a queen closer (a brick split lengthwise, a quarter-brick wide on the face) is placed next to the quoin. This shifts the joints so that the joints in successive courses are staggered by a quarter brick (lap = ¼ brick).
• No vertical joint is continuous through two successive courses, and the quoin ties the two walls together.

(c) Timber preservatives (IS 401)

IS 401 classifies timber preservatives into three types:

• Type 1, oil type: coal-tar or lignite-tar creosote, alone or mixed with coal tar or petroleum oils. It is highly toxic to fungi and termites, permanent, insoluble in water and non-corrosive, so it suits exterior and ground-contact work (poles, sleepers, piles, marine timber), usually applied by pressure or the hot-and-cold process. It is dirty to handle, smells, and treated wood is hard to paint.
• Type 2, organic-solvent type: preservatives dissolved in organic solvents, such as copper and zinc naphthenates, trichloro- or pentachlorophenol, lindane (γ-BHC), chlorpyrifos and synthetic pyrethroids (cypermethrin, deltamethrin). They penetrate well, do not swell the wood and leave it clean and paintable, so they suit joinery, furniture and plywood; most solvents are inflammable, so they are applied cold.
• Type 3, water-soluble type: salts dissolved in water, such as zinc chloride, boric acid–borax, sodium fluoride and sodium pentachlorophenate, and the fixed salts copper–chrome–arsenic (CCA), copper–chrome–boron (CCB) and acid copper chromate (ACC). They are odourless, cheap and allow painting when dry. Unfixed salts leach out in wet conditions, so fixed formulations (CCA, CCB) or a protective coating are used outdoors; boric acid–borax suits plywood, veneers and food-contact packing.

(d) Visibility of the lighthouse

Allowing for curvature and refraction together, the combined correction is c = 0.0673d² (c in metres, d in km). A point at height h can therefore be seen up to d = √(h/0.0673) = 3.855√h km.

• Lighthouse (h = 62 m): d₁ = 3.855√62 = 30.35 km.
• Observer (h = 10 m): d₂ = 3.855√10 = 12.19 km.
• Greatest distance at which the top can be seen = d₁ + d₂ = 42.54 km.

The ship is 55 km away, which is more than 42.54 km, so the lighthouse top is not visible: it lies below the observer's grazing line of sight. The ship must move 55 − 42.54 ≈ 12.5 km towards the lighthouse.

(e) Stopping sight distance

• v = 60 km/h = 16.67 m/s; lag distance = vt = 16.67 × 0.75 = 12.5 m.
• On a descending gradient the grade assists motion, so braking distance = v²/[2g(f − s)] with s = 0.025.
• Dry pavement (f = 0.40): braking distance = 16.67²/(2 × 9.8 × 0.375) = 37.8 m; SSD = 12.5 + 37.8 = 50.3 m. Since 50.3 m < 55 m, the driver can stop in time (about 4.7 m to spare).
• Wet pavement (f = 0.32): braking distance = 16.67²/(2 × 9.8 × 0.295) = 48.0 m; SSD = 12.5 + 48.0 = 60.5 m. Since 60.5 m > 55 m, the driver cannot stop in time (about 5.5 m short).