LESSON 2.1 — Building Materials: Properties and Selection
A. Standard Map
| Topic | Governing Source | Exam Focus |
|---|---|---|
| Brick — grades and properties | IS:1077:1992 — Common Burnt Clay Building Bricks | Class designation; compressive strength; water absorption limits |
| Concrete — grades | IS:456:2000 — Plain and Reinforced Concrete | Grade designation M10–M80; min RCC grade; design strength formula |
| Steel — reinforcement | IS:1786:2008 — High Strength Deformed Steel Bars and Wires | Fe415, Fe500 yield strength; elongation |
| Timber | IS:883:2016 — Design of Structural Timber in Building | Strength groups; seasoning methods; defect identification |
| Glass | IS:2553:1990 — Safety Glass; NBC 2016 | Float, tempered, laminated, low-e — identification and use |
| Sustainable materials | NBC 2016 Part 3; BIS guidelines | Fly ash bricks, compressed earth, bamboo — recognition |
| Material selection matrix | Standard design practice | Multi-criteria selection (durability, cost, thermal, acoustic, structural) |
B. Mechanism in Words
- A structural system requires materials; materials must be selected to satisfy simultaneous performance demands — strength, durability, thermal behaviour, fire resistance, cost, and availability.
- Each material category has a governing IS code that specifies characteristic strength, quality tests, and acceptance criteria.
- The designer specifies a minimum grade or class; the contractor procures to that specification; site testing verifies compliance.
- Composite assemblies (brick+mortar, RCC, timber+steel connections) derive their performance from the interaction of component materials — specifying one material without the other is incomplete.
- Sustainable materials substitute for energy-intensive conventional materials where structural, durability, and code requirements can still be met.
C. Core Concept Explanations
C1. Brick — Classification and Properties (IS:1077:1992)
Standard modular brick (IS:1077): Nominal size 200 × 100 × 100 mm (including 10 mm mortar joints). Actual size = 190 × 90 × 90 mm.
Brick classification by designation:
| Designation | Min. Compressive Strength (N/mm²) | Max. Water Absorption (% by weight, 24 hr) | Typical Use |
|---|---|---|---|
| Class A | 35 | 20% | Load-bearing walls; exposed work; pavement |
| Class B | 17.5 | 22% | General construction; protected masonry |
| Class C | 12.5 | 25% | Non-load-bearing partitions |
| Class D | 7.0 | — | Internal partitions only; very limited use |
Source: IS:1077:1992, Clause 6 (compressive strength), Clause 9 (water absorption).
Exam Anchor (GATE/UPSC): Water absorption NOT to exceed 20% by weight for Class A bricks. The standard sets a maximum — ≥ 20% indicates a defective, over-porous brick.
Efflorescence: White powdery deposit on brick surface from soluble salts migrating to the surface during wetting and drying cycles. Tested per IS:3495. Rating: Nil, Slight, Moderate, Heavy, Serious. Class A requires Nil to Slight efflorescence.
Brick testing sequence (IS:3495): Compressive strength → Water absorption → Efflorescence → Dimension tolerance → Warpage.
C2. Concrete — Grades and Properties (IS:456:2000)
Grade designation: The letter M followed by a number denotes the specified characteristic compressive strength (f_ck) of a 150 mm cube at 28 days, in N/mm².
| Grade | Category | Nominal Mix (C:S:A) | f_ck at 28 days (N/mm²) | Application |
|---|---|---|---|---|
| M10 | Ordinary | 1:3:6 | 10 | Blinding concrete; levelling course |
| M15 | Ordinary | 1:2:4 | 15 | Plain concrete foundations; footpaths |
| M20 | Ordinary | 1:1.5:3 | 20 | Minimum grade for RCC per IS:456 |
| M25 | Standard | Design mix | 25 | Multi-storey residential frames |
| M30 | Standard | Design mix | 30 | Commercial buildings; minimum for post-tensioned PT |
| M40 | Standard | Design mix | 40 | Pre-tensioned concrete; high-rise columns |
| M45–M80 | High Performance | Design mix | 45–80 | Bridges; high-rise core walls; prestressed elements |
Source: IS:456:2000, Table 2 (grades); Clause 6.1 (minimum grade for RCC = M20).
Exam Anchor: Minimum concrete grade for Reinforced Concrete = M20. Using M15 or below in RCC is a code violation.
Key concrete relationships:
| Property | Formula / Value | Source |
|---|---|---|
| Design strength of concrete in compression | f_d = f_ck / 1.5 = 0.67 f_ck | IS:456, Cl. 38.1 |
| Design strength of steel | f_d = f_y / 1.15 | IS:456 |
| Modulus of rupture (flexural tensile strength) | f_cr = 0.7 √f_ck (N/mm²) | IS:456, Cl. 6.2.2 |
| Water-cement ratio for durability (severe exposure) | ≤ 0.45 | IS:456, Table 5 |
| Minimum cement content (severe exposure) | 320 kg/m³ | IS:456, Table 5 |
Workability — Slump values:
| Workability class | Slump (mm) | Application |
|---|---|---|
| Very low | 0–25 | Road pavement (mechanically compacted) |
| Low | 25–75 | Lightly reinforced foundations; mass concrete |
| Medium | 75–100 | Normal RCC work |
| High | 100–150 | Heavily reinforced sections; pumped concrete |
C3. Steel Reinforcement (IS:1786:2008)
| Grade | Yield Strength f_y (N/mm² min.) | Ultimate Tensile Strength (N/mm²) | Elongation at failure (%) | Application |
|---|---|---|---|---|
| Fe250 (Mild Steel) | 250 | 412 | 23 | Hooks, binders; now rare in new construction |
| Fe415 | 415 | 485 | 14.5 | Standard RCC — slabs, beams, columns |
| Fe500 | 500 | 545 | 12 | High-rise columns; prestressed elements |
| Fe550 | 550 | 585 | 10 | High-performance structures |
| Fe600 | 600 | 660 | 10 | Specialist structural applications |
Source: IS:1786:2008, Table 1 (mechanical properties).
Exam Anchor: Fe415 → yield strength = 415 N/mm²; Fe500 → yield strength = 500 N/mm². The number in the grade IS the yield strength.
Trap: Higher grade steel has LOWER elongation — Fe500 is less ductile than Fe415. For seismic zones III–V, IS:13920 mandates specific ductility provisions (see Lesson 2.8).
Bar identification: HYSD bars (High Yield Strength Deformed) have surface deformations (ribs) that enhance bond with concrete. Mild steel bars are plain round — now rarely used for main reinforcement.
C4. Timber — Classification and Defects (IS:883:2016)
Strength groups (IS:883): Timber is classified into four strength groups (SG-1 to SG-4) based on modulus of rupture, modulus of elasticity, and compressive strength. SG-1 is the strongest; SG-4 is the weakest.
| Group | Permissible Bending Stress (N/mm²) | Example Species |
|---|---|---|
| SG-1 | 18.0 | Teak, Sal, Deodar |
| SG-2 | 12.0 | Mango, Neem, Babul |
| SG-3 | 8.5 | Poplar, Willow |
| SG-4 | 5.5 | Bamboo (treated as timber reference) |
Seasoning methods:
| Method | Mechanism | Result |
|---|---|---|
| Air seasoning (natural) | Stacked timber exposed to circulating air; slow evaporation | Takes months to years; moisture content 15–20%; uncontrolled |
| Kiln seasoning | Timber placed in closed kiln with controlled temperature, humidity, and air circulation | Rapid (days to weeks); moisture content reduced to 8–15%; uniform |
| Water seasoning | Timber immersed in running water to leach out sap and sugars | Reduces warping; common for timber being used in water contact |
| Chemical seasoning | Application of dehydrating agents to accelerate drying | Used for specific species where kiln or air seasoning causes cracking |
Exam Anchor: Kiln seasoning is the most effective and controlled method. Air seasoning is the most economical but slowest. Moisture content after kiln seasoning is specified by end use: structural timber ≤ 12%; flooring ≤ 10%; furniture ≤ 8%.
Timber defects:
| Defect | Type | Description | Structural Impact |
|---|---|---|---|
| Knots | Natural | Branch stump encased in timber during tree growth | Reduces tensile and bending strength; stress concentration |
| Shakes | Natural | Longitudinal separation along grain — ring shake, heart shake, cup shake | Weakens timber; visible as clefts or cracks in section |
| Checks | Drying | Surface cracks along grain from uneven drying | Reduces strength; cosmetic and structural concern |
| Warping | Drying | Distortion of shape — bow, cup, twist, crook | Dimensional inaccuracy; reduces contact area in connections |
| Dry rot | Fungal | Fungus destroys cellulose; timber becomes brittle and brown | Complete structural failure possible; requires full replacement |
| Wet rot | Fungal | Fungus requires persistently moist conditions; timber becomes soft | Structural failure if untreated; remove source of moisture |
C5. Glass Types and Structural Use
| Glass Type | Production / Treatment | Properties | Application |
|---|---|---|---|
| Float glass | Molten glass floated on molten tin → flat, uniform surface | Flat, optically clear; not strengthened | General glazing (must be secondary-processed for structural use) |
| Tempered (toughened) glass | Float glass reheated and rapidly cooled | 4–5× stronger than float; breaks into small granular cubes — safety glass | Facades, glass doors, shower cubicles, structural flooring |
| Laminated glass | Two or more layers bonded with PVB interlayer | On breakage, fragments adhere to interlayer; best for overhead glazing | Skylights, overhead canopies, bullet-resistant glazing, noise control |
| Low-e glass | Thin metallic coating applied to float | Reflects infrared radiation; reduces heat gain; maintains visible light | High-performance facades, ECBC-compliant envelopes |
| Wired glass | Metal wire mesh embedded during manufacture | Fire-resistant (rated 30 min); not safety glass — breaks with sharp shards | Fire-rated vision panels; industrial skylights |
| Frosted glass | Acid-etched or sandblasted surface | Translucent, not transparent; diffuses light | Privacy screens; bathroom windows |
Source: IS:2553:1990 (safety glass); NBC 2016 Part 6.
Trap: Wired glass is fire-resistant but is NOT safety glass — it breaks into sharp shards. Tempered and laminated glass are safety glasses. Wired glass tested for fire rating per IS:3614.
C6. Sustainable Building Materials
| Material | Description | Advantage over Conventional | Key Standard / Note |
|---|---|---|---|
| Fly ash bricks | Sintered or autoclaved mix of fly ash (thermal plant byproduct) + lime + gypsum | Lower embodied energy than clay brick; reduces industrial waste; lighter | IS:12894:2002; Class SP (Special Purpose) fly ash bricks |
| Autoclaved Aerated Concrete (AAC) blocks | Aerated concrete cured under pressure | Lightweight (~600 kg/m³); good thermal insulation; fast construction | IS:2185 Part 3; BIS; not structural in high seismic zones without reinforcement |
| Compressed Stabilised Earth Blocks (CSEB) | Soil + stabiliser (cement 5–8%) compressed at high pressure | Local material; very low embodied energy; good thermal mass | Auroville Earth Institute guidelines; not under a single IS standard |
| Bamboo (structural) | Treated culms or engineered bamboo products | High tensile strength (~150 N/mm²); fast renewable; lightweight | IS:6874:2008; requires treatment against moisture and insects |
| Recycled aggregate | Concrete demolition waste crushed and graded | Reduces quarrying; waste utilisation | IS:383 permits up to 20–30% replacement in non-structural concrete; structural use requires mix design verification |
D. Design/Parameter Table
| Parameter | Value | Unit | Source |
|---|---|---|---|
| Standard modular brick — nominal | 200 × 100 × 100 | mm | IS:1077:1992 |
| Standard brick — actual | 190 × 90 × 90 | mm | IS:1077:1992 |
| Class A brick min. compressive strength | 35 | N/mm² | IS:1077 |
| Class A brick max. water absorption | 20 | % | IS:1077 |
| Min. concrete grade for RCC | M20 | — | IS:456:2000 |
| Min. concrete grade for post-tensioned concrete | M30 | — | IS:456 |
| Min. concrete grade for pre-tensioned concrete | M40 | — | IS:456 |
| Modulus of rupture | 0.7 √f_ck | N/mm² | IS:456 Cl. 6.2.2 |
| Concrete design compressive strength | 0.67 f_ck | N/mm² | IS:456 Cl. 38.1 |
| Fe415 yield strength | 415 | N/mm² | IS:1786:2008 |
| Fe500 yield strength | 500 | N/mm² | IS:1786:2008 |
| Fe415 elongation (min.) | 14.5 | % | IS:1786:2008 |
| Fe500 elongation (min.) | 12.0 | % | IS:1786:2008 |
| Kiln-seasoned structural timber moisture | ≤ 12 | % | IS:883:2016 |
| W/C ratio max. — severe exposure | 0.45 | — | IS:456 Table 5 |
| Min. cement content — severe exposure | 320 | kg/m³ | IS:456 Table 5 |
E. Common Confusions
| Confusion | Correct Distinction |
|---|---|
| M20 vs minimum grade for RCC | M20 is the minimum. M15 is used for PLAIN concrete (mass concrete, levelling). M15 in RCC is a code violation. |
| Fe415 vs Fe500 ductility | Higher grade = higher strength but LOWER elongation (less ductile). Fe415 is more ductile; Fe500 is stronger. For seismic zones III–V, IS:13920 specifies minimum ductility requirements. |
| Water absorption limit for bricks | IS:1077 sets a MAXIMUM (not minimum) of 20% for Class A. A brick absorbing MORE than 20% fails the test. |
| Tempered glass vs laminated glass | Tempered: strongest; breaks into granular pieces (safe at ground level). Laminated: fragments held by interlayer (safest OVERHEAD). For overhead glazing, always specify laminated. |
| Wired glass = safety glass | Wired glass is fire-resistant but NOT safety glass — it breaks with sharp shards. Safety glass = tempered or laminated. |
| Float glass for structural use | Float glass alone is not used for structural glazing — it must be secondary-processed (tempered, laminated, or heat-strengthened) for any structural application. |
| Bamboo as a structural material | Untreated bamboo is not a structural material. It requires treatment against moisture, insects, and fire. IS:6874 governs structural bamboo; specific species and culm age matter. |
F. Exam Traps
| Trap | Incorrect Assumption | Correct Answer |
|---|---|---|
| T01 | “The minimum grade for RCC is M15” | IS:456:2000 mandates M20 as minimum for RCC. M15 is for plain concrete only. |
| T02 | “Class A brick can absorb up to 25% water” | Class A maximum = 20%; Class B = 22%; Class C = 25%. The class determines the limit. |
| T03 | “Fe500 is more ductile than Fe415 because it is stronger” | Strength and ductility trade off. Fe500 has lower elongation (12%) vs Fe415 (14.5%). Fe415 is more ductile. |
| T04 | “Tempered glass is best for overhead skylights” | Laminated glass is prescribed for overhead glazing — fragments adhere to the interlayer, preventing injury from falling shards. |
| T05 | “CSEB and AAC blocks have the same embodied energy” | CSEB has the lowest embodied energy (local soil + small cement content). AAC requires autoclaving but is still lower than clay brick. Not the same. |
| T06 | “Wired glass is a safety glass” | Wired glass is fire-resistant, not safety glass. On breakage, it produces sharp shards. Safety glass categories = tempered (toughened) and laminated. |
G. Answer-Writing Cues
For grade / classification MCQs:
“Per IS:456:2000, the minimum grade of concrete for reinforced concrete construction is M20. Grades below M20 are permitted only for plain (unreinforced) concrete elements such as levelling courses and blinding. The grade designation ‘MXX’ denotes the characteristic cube compressive strength at 28 days in N/mm².”
For material selection questions:
“The selection of structural steel grade depends on the required yield strength and ductility. Fe415 (IS:1786) offers a yield strength of 415 N/mm² with 14.5% elongation — preferred for general RCC and seismic applications. Fe500 provides higher strength (500 N/mm²) with reduced ductility (12% elongation) — suited for high-load columns and prestressed elements where seismic ductility demands are lower.”
H. PYQ Linkage Note
| Topic | Exam Appearance | Pattern |
|---|---|---|
| Minimum concrete grade for RCC | GATE multiple years | MCQ: “Per IS:456, minimum concrete grade for RCC is…” |
| Fe415 / Fe500 yield strength | GATE, UPSC-CPWD | MCQ: direct recall; NAT possible (design strength calculation) |
| Brick compressive strength | UPSC-CPWD, state PSC | MCQ: identify Class A strength; distinguish from Class B |
| Timber defects | GATE, UPSC-CPWD | MCQ: name of defect from description |
| Glass type for overhead use | GATE, UPSC-CPWD | MCQ: which glass type for skylight / overhead canopy |
| Seasoning methods | UPSC-CPWD | MCQ: most effective method; moisture content after kiln seasoning |
I. Mini-Check — Lesson 2.1 (5 Questions)
Q1 (MCQ): Per IS:456:2000, what is the minimum grade of concrete permitted for reinforced concrete work?
(A) M10 (B) M15 (C) M20 (D) M25
A1: (C) M20. IS:456:2000 Clause 6.1 explicitly mandates M20 as the minimum grade for all RCC. M15 and below are for plain (unreinforced) concrete only.
Q2 (MCQ): A Class A brick per IS:1077:1992 must have a minimum compressive strength of:
(A) 17.5 N/mm² (B) 25.0 N/mm² (C) 35.0 N/mm² (D) 7.0 N/mm²
A2: (C) 35.0 N/mm². IS:1077 Table 2: Class A = 35 N/mm² minimum; Class B = 17.5 N/mm²; Class C = 12.5 N/mm²; Class D = 7.0 N/mm².
Q3 (NAT): Compute the modulus of rupture (flexural tensile strength) for M30 concrete per IS:456. (Answer in N/mm², correct to 2 decimal places.)
A3:
– Formula: f_cr = 0.7 × √f_ck
– f_ck = 30 N/mm²
– f_cr = 0.7 × √30 = 0.7 × 5.477 = 3.83 N/mm²
Q4 (MCQ): Which type of glass is most appropriate for an overhead glass skylight in a public building?
(A) Float glass (B) Wired glass (C) Tempered glass (D) Laminated glass
A4: (D) Laminated glass. On breakage, fragments adhere to the PVB interlayer — no dangerous falling shards. Tempered glass (C) breaks into granular pieces that can fall; float glass (A) produces sharp shards; wired glass (B) is fire-resistant but not safe overhead.
Q5 (MSQ): Which of the following are correct statements about Fe415 and Fe500 steel per IS:1786:2008? Select all that apply.
(A) Fe415 yield strength = 415 N/mm²
(B) Fe500 is more ductile than Fe415
(C) Fe500 yield strength = 500 N/mm²
(D) Fe415 minimum elongation = 14.5%
A5: (A), (C), and (D). Fe415 yield strength = 415 N/mm² ✓; Fe500 yield = 500 N/mm² ✓; Fe415 elongation 14.5% ✓. Statement (B) is wrong — Fe500 has LOWER elongation (12%) than Fe415 (14.5%), making Fe415 more ductile.