Building Materials: Cement and Concrete, Masonry Units, Timber, Steel, Glass, Finishes, Damp Prevention, Insulation, Repair and Alternative Materials
2. Bricks, blocks and stone
Burnt clay bricks are graded by compressive strength (classes starting at 3.5 N/mm²). Their water absorption after 24 hours of immersion, (wet − dry)/dry × 100, should not exceed 20 per cent for the lower classes and 15 per cent for higher classes. Soluble salts in brick cause efflorescence, the white deposit on the surface of damp walls. Besides clay bricks, walling units now include fly-ash bricks, AAC blocks and concrete blocks, solid or hollow.
Stone is classed by origin and chosen for strength, hardness, toughness, low porosity and ease of dressing. A good stone is free from cracks, soft veins and quarry sap, and is seasoned before use.
| Stone | Class | Characteristics and use |
|---|---|---|
| Granite | Igneous | Hard, dense, takes a high polish; foundations, cladding, flooring, steps |
| Basalt (trap) | Igneous | Dark, tough and durable; road metal, rubble masonry, aggregate |
| Sandstone | Sedimentary | Easily carved, varied strength; traditional facades, jali and carved work |
| Limestone, laterite | Sedimentary | Limestone: cement raw material, flooring such as kota stone. Laterite: soft when quarried, hardens on exposure, cut into blocks in Kerala and Goa |
| Marble | Metamorphic | Calcium carbonate, takes a fine polish, attacked by acids, so poor for polluted outdoor use |
| Slate, quartzite | Metamorphic | Slate splits into thin sheets: roofing, DPC. Quartzite is very hard: aggregate, cladding |
3. Timber, steel, aluminium and glass
Timber. Hardwood (broad-leaved trees such as teak, sal and sheesham) and softwood (conifers such as deodar and pine) are botanical classes, not measures of hardness. Timber is anisotropic: strongest along the grain. Defects are knots, shakes, checks, warping and wane. Seasoning (air or kiln drying) brings the moisture down to a level near its service condition, so that shrinkage and decay are limited, and chemical preservatives guard against fungi and insects. Engineered woods reduce anisotropy and defects: plywood (veneers glued with grain alternately at right angles, always an odd number), block board, particle board, MDF, laminated veneer lumber, glulam and cross-laminated timber.
Steel and other metals. Mild steel has a yield strength of 250 MPa; reinforcing bars are mostly TMT Fe 415, 500 or 550, made by quenching the hot bar's surface and letting the core's heat temper it: a hard outer layer and a ductile core, giving high strength with good ductility and weldability. Steel's elastic modulus is about 200 GPa, and it is equally strong in tension and compression. It corrodes, so it is protected by cover, paint or galvanising, where the zinc coat corrodes in preference to the steel; weathering steel forms a stable rust patina in suitable exposures, and stainless steel carries chromium. Aluminium is about a third as dense as steel and forms a protective oxide film (thickened by anodising), but its modulus is also about a third, so it deflects more, and it expands and conducts heat strongly.
| Glass | Behaviour and use |
|---|---|
| Annealed (float) | Soda-lime glass; breaks into large sharp shards; ordinary windows |
| Toughened (tempered) | Heated and cooled rapidly: several times stronger; breaks into small blunt granules; cannot be cut or drilled afterwards; doors, balustrades |
| Laminated | Two or more panes bonded by a PVB interlayer that holds fragments; overhead glazing, security, sound reduction |
| Insulating glass unit (IGU) | Two panes with a sealed air or argon gap: lowers U-value and noise |
| Low-E, tinted, reflective | Coatings or tints cut solar heat gain and glare while passing daylight |
4. Materials for walls, floors and roofs
The same material is not right everywhere. Choose by function in the element and by climate, fire, acoustic and thermal needs, weight, cost, embodied energy, local availability and maintenance.
| Element | Common materials | Why chosen |
|---|---|---|
| Load-bearing walls | Burnt brick, stone, concrete block, rammed earth | Compressive strength, durability, thermal mass |
| Infill and partitions | AAC, hollow blocks, fly-ash brick, gypsum or fibre-cement board on studs, glass | Light weight, speed, insulation, acoustics, ease of services |
| Cladding | Stone, terracotta, aluminium composite panel, high-pressure laminate, glass curtain wall | Weather skin and appearance, with low weight on the frame |
| Floors | RCC slab, composite deck, precast planks; finishes of stone, tile, terrazzo, timber, resilient sheet | Strength and span; wear, slip resistance, cleaning, comfort |
| Roofs | RCC slab with waterproofing, metal sheeting on trusses, clay tile, membranes, green roof | Weather-tightness, span, weight, solar and rain performance |
5. Finishes and damp prevention
Wall finishes. Plaster levels and protects masonry: cement-sand plaster (commonly 12 mm inside and 15 to 20 mm outside, applied in coats) is standard; gypsum plaster is smooth and fast but for dry interiors only; lime plaster is breathable and soft. Pointing finishes the joints of exposed masonry (flush, recessed, struck, tuck, V-pointing). Texture finishes include stucco, rough-cast and pebble-dash; cladding uses stone, tile or panels. Paints are chosen by site: lime wash and distemper are cheap and breathable but weak in water; plastic (acrylic) emulsion is washable and the usual interior and exterior choice; enamel gives a hard gloss on wood and metal; cement paint serves rough exterior walls. A good coat needs a primer and a sound, dry, clean surface. Ceilings use plaster or false ceilings of gypsum board, mineral fibre or metal on a suspended grid, which also hide services.
Damp reaches a building by capillary rising damp from the ground, penetrating damp through walls and roofs from rain, condensation on cold surfaces, leaking pipes, and moisture left from construction. It shows as stains, peeling paint, efflorescence, rot and mould. Prevention breaks the water's path. A damp-proof course (DPC) is continuous across the full wall thickness, normally at plinth level above the ground, so that capillary rise from the foundation stops.
| Method | How it works |
|---|---|
| Membrane DPC (bitumen felt, polythene, mastic asphalt, lead) | A flexible waterproof layer in the wall at plinth level, or under a floor slab |
| Cement concrete DPC | A rich concrete layer with waterproofing compound: rigid, so it can crack if the wall settles |
| Cavity wall | An air gap breaks the capillary path of rain; ties are sloped or drip-shaped so water does not cross, and the cavity is kept clear of mortar droppings |
| Surface treatment | Water-repellent coatings and waterproof plaster; they reduce penetration but do not stop rising damp from below |
| Chemical injection DPC | For old walls with no DPC: holes are drilled at plinth level and a silicone or siloxane fluid injected to line the pores |
| Tanking and terrace waterproofing | Continuous membrane or crystalline coat around a basement; on terraces, slope to outlets plus a membrane or coba layer turned up at the parapet |
6. Thermal insulation and acoustic materials
Thermal insulation uses materials of low thermal conductivity k (W/m·K): expanded and extruded polystyrene (EPS, XPS), polyurethane or PIR foam, glass wool and rock wool, cork, perlite, vermiculite, AAC and, for thin layers, aerogel. A layer of thickness L has resistance R = L/k (m²K/W); layers in series add, and U = 1/ΣR (W/m²K). Ignoring surface films, a wall of 20 mm plaster (k = 1.0), 230 mm brick (k = 0.8), 40 mm EPS (k = 0.04) and 12 mm plaster (k = 0.6) has ΣR = 0.02 + 0.2875 + 1.0 + 0.02 = 1.3275, so U = 0.75 W/m²K; without the EPS it would be 1/0.3275 = 3.05. Insulation outside the masonry keeps the thermal mass inside to even out temperature swings; wet insulation loses its value, so a vapour barrier goes on the warm side; and a reflective foil works only facing an air gap.
Acoustic materials do two different jobs. Absorbers soak up sound inside a room and cut echo and reverberation: porous materials (mineral wool, foam, carpet, curtains, acoustic plaster, perforated boards with a porous backing) absorb mainly mid and high frequencies; panel (membrane) absorbers and Helmholtz resonators act at low frequencies. The absorption coefficient α runs from 0 to 1, and the room's absorption is A = ΣSα in m² sabins; for 80 m² of α = 0.05 and 20 m² of α = 0.60, A = 4 + 12 = 16 m² sabins. Insulators stop sound passing from one space to another: they need mass, airtightness and decoupling (double leaf walls with a cavity or resilient layer, floating floors for impact noise). Heavier, tighter partitions raise the transmission loss, and gaps, flanking paths and open doors can defeat any wall.
7. Repair and retrofitting
Why concrete deteriorates. The alkaline pore water of concrete (pH about 12.5 to 13) keeps a passive film on the steel. Carbonation, in which atmospheric CO₂ reacts with the lime, lowers the pH, and when the carbonated front reaches the bars the film is lost. Chlorides (marine air, de-icing salts, contaminated mix) attack the film locally. Rust occupies several times the volume of the steel it replaces and splits the cover, which shows as rust-stained cracks along bars and then spalling. Other causes: sulphate attack, alkali-silica reaction, poor compaction and low cover, overload and fire.
- Investigate first: visual survey; cover meter; rebound hammer and ultrasonic pulse velocity for strength and soundness; half-cell potential for corrosion risk; cores for strength and chloride; and a phenolphthalein spray on a fresh break, which turns pink where concrete is still alkaline and stays colourless where it has carbonated.
- Patch repair: break out the damaged concrete, including behind the corroded bars, clean the steel to bright metal, coat it with a rust-inhibiting primer, apply a bonding coat, fill with polymer-modified mortar or micro-concrete, then seal with a protective coating. Cracks: epoxy injection restores monolithic action in structural cracks; flexible polyurethane suits leaking or moving cracks. Others: guniting (shotcrete), jacketing, cathodic protection.
Repair restores lost condition; strengthening raises capacity; retrofitting upgrades an existing building to a higher performance standard, typically seismic. Global measures add or enlarge the lateral system (new shear walls, steel bracing, base isolation, dampers); local measures fix weak members (column jacketing with concrete, steel or FRP, beam wrapping, foundation enlargement). In masonry: bands, corner and opening reinforcement, ferrocement or mesh overlays, grouting cracks, and anchoring walls to floors and roofs. Carbon or glass FRP wrapping raises a column's confinement and ductility much more than its stiffness.
8. Alternative building materials
Alternative materials aim at lower embodied energy and carbon, use of waste, local sourcing and good thermal performance, while meeting strength and durability. They have limits, so the exam asks both the merit and the caution.
| Material | Merit | Caution |
|---|---|---|
| Fly-ash bricks and blocks | Use an industrial waste, cured rather than burnt, uniform and lighter than clay brick, save topsoil | Need good curing and quality control of the fly ash |
| AAC blocks | Light, insulating, accurate, quick with thin-bed mortar, lower dead load | Absorb water, need special fixings and plaster, lower strength |
| Bamboo | Fast-growing, strong in tension for its weight, renewable; scaffolds, roofs, trusses, mat boards | Must be treated against insects and fungus, kept off wet ground, and joints are the weak point |
| Earth: adobe, rammed earth, stabilised blocks | Low embodied energy, local, high thermal mass, recyclable | Weak when wet: need a raised plinth, wide eaves, and a little cement or lime for stabilised blocks |
| Hempcrete | Hemp hurd bound with lime; light, insulating, moisture-buffering, and the plant stored carbon as it grew | Low compressive strength: non-load-bearing infill within a frame, not a structural wall |
| Geopolymer and blended-binder concretes, ferrocement, recycled aggregate, agro-waste boards | Cut cement use or reuse waste; ferrocement gives thin strong shells and tanks | Need tested mixes, skilled work and code acceptance |
Key takeaways
- OPC grades are 28-day mortar strengths; gypsum controls flash set; PPC gains strength slower with lower heat; lower water-cement ratio gives stronger, more durable concrete; 1.54 is the dry-volume factor in mix arithmetic.
- Brick absorption is (wet − dry)/dry × 100; marble is attacked by acids; AAC gets its cells from aluminium powder; hardwood and softwood are botanical classes; plywood alternates grain with an odd number of plies.
- TMT bars have a hard skin and ductile core; steel's modulus is about 200 GPa and aluminium's about a third; toughened glass crumbles and cannot be cut, laminated glass holds together on breaking.
- Damp prevention breaks the water's path: DPC at plinth, cavity walls, tanking, chemical DPC for old walls; never seal damp in with waterproof plaster before cutting the source.
- Insulators need low k and U = 1/ΣR with R = L/k; absorbers (A = ΣSα) and insulators work differently, so soft foam cuts echo but not transmitted noise.
- Carbonation and chlorides break steel's passive film, and rust splits the cover; investigate, patch-repair or inject, retrofit the whole system, and use alternative materials such as hempcrete, earth and bamboo where they fit their strengths.
Practice questions (20)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Gypsum is ground with Portland cement clinker mainly to
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Answer: B — prevent a flash set by controlling the early reaction of C₃A
Without gypsum, C₃A reacts with water almost at once and the paste stiffens in minutes (flash set); a few per cent of gypsum controls this and gives workable setting times. Higher strength is the tempting answer, but gypsum is not a strength-giving constituent, and sulphate resistance comes from low C₃A or blended cements.A site supervisor adds extra water to a concrete mix to make it flow easily into congested reinforcement. The most likely consequence is
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Answer: D — a higher water-cement ratio, with lower strength and durability
Strength falls as the water-cement ratio rises, because excess water leaves pores when it evaporates; porosity also cuts durability. The belief that cement content alone fixes strength is the tempting error. The proper way to gain flow is a plasticiser, not extra water.A glazed canopy is to be fixed overhead above an entrance. To keep broken pieces from falling on people, the most suitable glass is
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Answer: A — laminated glass with a PVB interlayer
In laminated glass the interlayer holds the fragments in place after breakage, so the pane stays up as a sagging sheet; that is why it is used overhead. Toughened glass is the tempting choice, but it crumbles into granules that all fall at once. Thickness and coatings do not change how annealed or tinted glass fails.Which building stone is generally unsuitable for outdoor use in a polluted city because acidic rain and air attack it?
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Answer: B — Marble, a metamorphic stone of calcium carbonate
Marble is calcium carbonate, which dissolves in acids, so its polish dulls and it etches outdoors in polluted air. Granite, basalt and quartzite are silicate-rich and resist acids; granite is the tempting prestige stone but it is the durable one here.In a cavity wall, rain that penetrates the outer leaf is kept from reaching the inner leaf mainly because
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Answer: C — the air gap breaks the capillary path, and the wall ties are sloped or drip-shaped
The gap is a break in the capillary path, and sloped or drip-shaped ties and clear cavity bottoms stop water bridging across; weep holes drain what runs down. Rigid tying into one mass is the opposite principle, which a solid wall uses. Even rich plaster cracks, so impermeability cannot be relied on.An old brick building with no damp-proof course shows damp and peeling plaster up to about 1 m on its ground-floor walls. Which treatment suits best?
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Answer: A — Injecting a chemical DPC at plinth level, then replacing the salt-laden plaster
Rising damp needs its source cut, and an injected chemical DPC does that in an existing wall; the salt-contaminated plaster is then replaced. A waterproof plaster over the damp wall is the tempting quick fix, but it traps the moisture so it rises higher, and salts soon spoil the finish. Paint and a dado only hide the symptom.Noise from the neighbouring flat passes through a party wall. Which measure is most effective in reducing the transmitted sound?
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Answer: B — Adding mass with a decoupled, sealed second leaf and closing all gaps
Insulation of airborne sound needs mass, airtightness and decoupling, so a sealed, separate second leaf with the gaps closed raises the transmission loss. Foam and curtains are the tempting answer, but they absorb sound within the room and add almost no insulation.A freshly broken face of old concrete is sprayed with phenolphthalein solution. A colourless band near the surface and pink concrete beyond it shows that
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Answer: A — the surface zone has carbonated and lost its alkalinity
Phenolphthalein turns pink above a pH of roughly 9 to 10, so the colourless zone is where carbon dioxide has reacted with the lime and lowered the pH; its depth shows how near the carbonation front is to the bars. Reading pink as the damaged zone is the tempting reversal, and the test says nothing directly about chlorides or sulphates.Autoclaved aerated concrete gets its cellular, lightweight structure because
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Answer: C — aluminium powder reacts to give hydrogen bubbles, and the cake is then steam cured
The aluminium powder reacts with the alkaline lime slurry to release hydrogen, which makes the pores; autoclaving then fixes the structure. Protein foaming makes a different product, cellular lightweight concrete, which is the tempting confusion. Burnt-out sawdust and expanded clay describe other lightweight materials.Hempcrete, a mix of hemp hurd and a lime binder, is best used as
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Answer: C — a non-load-bearing insulating infill within a timber or other frame
Hempcrete is light and insulating with low compressive strength, so the frame carries the loads and the hempcrete fills between. Its fibre is not a structural reinforcement and it is not waterproof, which makes the column and membrane answers wrong. The renewable origin is the tempting reason, but it does not give structural strength.Plywood is made with the grain of adjacent veneers laid at right angles, using an odd number of layers. The main benefit is
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Answer: D — it evens out strength and swelling in both directions of the board
Crossing the grain balances the strong and weak directions of timber, cutting shrinkage and splitting, and the odd count keeps the board symmetrical so it does not warp. Along the grain it is in fact weaker than solid wood, which is the tempting claim, and glue is essential.Which of the following are correct comparisons of Portland pozzolana cement (PPC) with ordinary Portland cement?
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Answer: A — PPC gives lower heat of hydration; C — PPC generally resists sulphates and chlorides better; D — PPC uses less clinker per tonne of cement
The fly ash in PPC reacts slowly with the lime freed by hydration, which lowers the heat, refines the pores, improves resistance to aggressive salts, and replaces part of the clinker. The slower reaction means early strength is lower, not higher, which is the tempting error, so curing must be longer.Which statements about metals used in buildings are correct?
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Answer: A — TMT bars have a hard quenched outer layer and a softer, ductile core; C — A galvanised zinc coat corrodes in preference to the steel beneath it where scratched; D — Weathering steel forms a stable rust layer that slows further corrosion in suitable exposures
Quenching and self-tempering give TMT its hard skin and ductile core, zinc protects sacrificially, and weathering steel's patina is protective. The aluminium statement is reversed: its modulus is about one third of steel's, so it deflects about three times as much for the same section and load, which is the tempting mistake because aluminium is light.Which of the following correctly describe absorption and insulation of sound?
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Answer: A — Porous materials such as mineral wool mainly absorb mid and high frequencies; C — Mass, airtightness and decoupling raise the insulation of a partition; D — Gaps and flanking paths can seriously reduce the insulation of a heavy wall
Porous absorbers work on mid and high frequencies, and insulation depends on mass, tightness and decoupling, which gaps and flanking paths undermine. A thin foam layer is the tempting claim: it absorbs sound in the room but adds almost nothing to the insulation, because it adds little mass.Which statements about concrete repair and retrofitting are correct?
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Answer: A — Epoxy injection can restore monolithic action across a structural crack, while flexible polyurethane suits leaking cracks that still move; B — In patch repair, concrete is broken out behind the corroded bars so they can be cleaned all round and coated; D — Carbon-fibre wrapping of a column mainly raises its confinement and ductility rather than its stiffness
These are the standard repair practices and the known effect of FRP wrapping. The shear-wall statement is wrong because walls at one end make the stiffness eccentric, so the building twists about the stiff side; this is the tempting error of treating retrofit as pure strength addition.Using 1.54 m³ of dry materials per m³ of 1:3:6 concrete and 1 bag of cement = 0.035 m³, the number of cement bags per m³ of concrete is ____ (correct to one decimal place).
Numerical answer — type the value.
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Answer: 4.4
Parts = 1 + 3 + 6 = 10, so cement volume = 1.54 × 1/10 = 0.154 m³. Bags = 0.154/0.035 = 4.4. Check: 4.4 × 0.035 = 0.154.A job needs 20 m³ of 1:2:4 concrete. Taking 1.54 m³ of dry materials per m³ of concrete, the volume of sand needed, in m³, is ____ (correct to one decimal place).
Numerical answer — type the value.
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Answer: 8.8
Dry volume = 20 × 1.54 = 30.8 m³. Parts = 1 + 2 + 4 = 7, and sand is 2 parts, so sand = 30.8 × 2/7 = 8.8 m³. Check: cement 4.4, sand 8.8, aggregate 17.6 sum to 30.8.A dry brick weighs 3.20 kg and after 24 hours of immersion weighs 3.68 kg. Its water absorption, as a percentage of dry weight, is ____.
Numerical answer — type the value.
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Answer: 15
Absorption = (3.68 − 3.20)/3.20 × 100 = 0.48/3.20 × 100 = 15 per cent. Check: 3.20 × 1.15 = 3.68.An AAC block wall has a 200 mm block (k = 0.16 W/m·K) with 15 mm of plaster (k = 0.75 W/m·K) on each face. Ignoring surface films, its U-value in W/m²K, to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 0.78
Block R = 0.200/0.16 = 1.25. Each plaster R = 0.015/0.75 = 0.02, so two give 0.04. ΣR = 1.29 m²K/W and U = 1/1.29 = 0.7752, which is 0.78 W/m²K. Check: 1.29 × 0.7752 = 1.000.A room has 80 m² of wall and floor surfaces with absorption coefficient 0.05 and 20 m² of acoustic ceiling tile with absorption coefficient 0.60. The total absorption A = ΣSα, in m² sabins, is ____.
Numerical answer — type the value.
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Answer: 16
A = 80 × 0.05 + 20 × 0.60 = 4 + 12 = 16 m² sabins. Check: the 20 m² of tile supplies three quarters of the total despite being a fifth of the area.