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)

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B. Why It’s Used

Building materials questions appear across GATE, UPSC-CPWD, and state PSCs because material specification is a core professional competency — misapplying grades causes structural failures. GATE AR tests both direct recall (Class A brick = 35 N/mm²) and applied reasoning (which glass type for overhead use, and why). Material selection scenarios appear as MCQs requiring the candidate to match a performance requirement to the correct grade or standard.

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C. Mechanism in Words

1. A structural system requires materials; materials must be selected to satisfy simultaneous performance demands — strength, durability, thermal behaviour, fire resistance, cost, and availability.
2. Each material category has a governing IS code that specifies characteristic strength, quality tests, and acceptance criteria.
3. The designer specifies a minimum grade or class; the contractor procures to that specification; site testing verifies compliance.
4. 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.
5. Sustainable materials substitute for energy-intensive conventional materials where structural, durability, and code requirements can still be met.

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D. 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

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.

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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

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

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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

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.

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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

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

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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

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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

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E. 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

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F. Design Criteria

Criterion	Governing Parameter	Code
Structural safety	Characteristic compressive strength (f_ck); yield strength (f_y)	IS:456, IS:1786
Durability	Water absorption (bricks); w/c ratio; cover to reinforcement	IS:1077, IS:456
Fire resistance	Glass type (wired = FR; laminated = overhead safe); timber treatment	IS:2553, IS:3614
Thermal performance	Embodied energy; thermal mass; low-e coatings	NBC 2016 Part 11
Cost and availability	Locally available materials; modular sizes	IS:1077
Sustainability	Embodied energy; recycled content; renewable source	NBC 2016

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Code Comparisons

IS Code	Material	Key Number	Exam Significance
IS:1077:1992	Common burnt clay bricks	Class A = 35 N/mm²; max absorption 20%	Grade/class is the most tested parameter
IS:456:2000	Plain and reinforced concrete	Minimum RCC grade = M20	The single most tested concrete fact
IS:1786:2008	HSD steel reinforcement	Fe415 = 415 N/mm²; Fe500 = 500 N/mm²	Yield strength = grade number
IS:883:2016	Structural timber	SG-1 to SG-4; bending stress 18→5.5 N/mm²	Strength group classification
IS:2553:1990	Safety glass	Tempered = granular fracture; Laminated = PVB interlayer	Overhead glazing = always laminated
IS:12894:2002	Fly ash bricks	Class SP	Low embodied energy alternative to clay brick

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G. Application Zones

Building Zone	Material Specification	Code Basis
Structural frame	M20–M40 concrete + Fe415/Fe500 steel	IS:456
Load-bearing masonry walls	Class A or B bricks	IS:1077; IS:1905
Partitions	Class C bricks; AAC blocks; lightweight panels	IS:1077
Overhead glazing / skylights	Laminated glass	IS:2553; NBC 2016
Ground-floor structural glazing	Tempered (toughened) glass	IS:2553
Fire-rated vision panels	Wired glass (in frames rated ≥ 30 min)	IS:3614
Structural timber (portal frames, trusses)	SG-1 or SG-2 species; kiln-seasoned	IS:883:2016
Low-carbon construction	Fly ash bricks, CSEB, AAC, bamboo	IS:12894, IS:6874

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H. 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.

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I. Compare & Contrast

Pair	Item 1	Item 2	Key Difference
Overhead glass	Tempered	Laminated	Tempered: stronger but fragments fall on breakage; Laminated: fragments adhere to PVB interlayer — only laminated is safe overhead
Steel ductility	Fe415	Fe500	Fe415: 14.5% elongation (more ductile); Fe500: 12% (stronger, less ductile) — Fe415 preferred in seismic zones
Brick class	Class A	Class B	Class A: 35 N/mm² / ≤20% absorption; Class B: 17.5 N/mm² / ≤22% absorption
Concrete minimum	M15	M20	M15: plain concrete only; M20: minimum for RCC — using M15 in RCC is a code violation
Glass safety	Wired glass	Tempered glass	Wired = fire-resistant (NOT safety glass — sharp shards); Tempered = safety glass (NOT fire-rated)
Sustainable material	Fly ash brick	CSEB	Fly ash: industrial byproduct + lime; CSEB: local soil + cement stabiliser — CSEB has lowest embodied energy
Seasoning method	Air seasoning	Kiln seasoning	Air: months–years, MC 15–20%, uncontrolled; Kiln: days–weeks, MC 8–15%, uniform and controlled

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J. Memory Hooks

• Brick classes A–D → 35, 17.5, 12.5, 7 N/mm²: “35 is A-grade” — top of the class
• Fe-number = yield strength: Fe415 yields at 415, Fe500 at 500 — the grade IS the strength
• Ductility inverts strength: Higher grade → more brittle; Fe415 more ductile than Fe500
• M20 for RCC: “Twenty for RCC” — M15 is for plain (no steel, no “R”)
• Laminated glass overhead: “Laminated = layers that stick” — broken glass stays in frame
• Wired = fire, NOT safety: “Wire = fire alarm wire” — fire-resistant only, sharp shards remain
• SG-1 = strongest timber: Sal/Teak are dense hardwoods at top of the group ladder

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K. Revision Ladder

Level	Task	Test Yourself
L1 — Recall	State: IS code for each material; Class A brick strength and absorption limit; minimum RCC grade; Fe415 and Fe500 yield strengths	Recite all 5 IS codes with their key numbers without notes
L2 — Apply	Calculate modulus of rupture for M25 (0.7 × √25 = 3.5 N/mm²); select correct glass type for a given application; classify a brick from given test results	Work through the I. Mini-Check questions without referring to notes
L3 — Analyse	Compare CSEB vs fly ash bricks for a school in a seismic zone; justify Fe415 preference over Fe500 for earthquake-prone buildings; explain why M20 is the minimum for RCC and what fails at M15	Explain the structural logic behind each IS code limit in one sentence each

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L. 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.

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M. Answer-Writing Cues

For grade / classification MCQs:

For material selection questions:

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N. PYQ Integration (2007–2026 verified)

Topic	Exam Appearance	Question 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
As per the NBC 2016, materials in increasing order of embodied energy	GATE AR 2025 Q22	MCQ: Ans: (B) Fly-ash Bricks < MDF < Float Glass < Aluminium
The minimum number of masonry units of size 200 mm × 100 mm × 100 mm each, required to…	GATE AR 2026 Q24	NAT: Ans: 600
The graph shows the typical test result of a property of a building material. Identify…	GATE AR 2025 Q53	MCQ: Ans: (D)
____ increases the spreading quality of paints and helps to achieve desired…	GATE AR 2025 Q52 (MTA)	MCQ: cancelled — Marks to All awarded
Seasoning methods	UPSC-CPWD	MCQ: most effective method; moisture content after kiln seasoning
Rebound hammer test is used to measure	GATE AR 2021 Q22	MCQ: Ans: (C) compressive strength of concrete
Statements about surface paint	GATE AR 2021 Q41	MSQ: Ans: (B)(C)
Which of the following processes is used for surface treatment of metals?	GATE AR 2022 Q50	MCQ: Ans: (B) Thermoplating
An Ultrasonic Pulse Velocity (UPV) test was done on a hardened concrete element…	GATE AR 2022 Q52	MCQ: Ans: (C) Doubtful — velocity = 600 ÷ 0.18 ms = 3.33 km/s
If Aluminium : Anodisation :: Glazing : X, which of the following choices represent X?	GATE AR 2022 Q54	MSQ: Ans: (A)(C) Hard coating; Tempering
Match the structural joining systems in Group I with the corresponding materials…	GATE AR 2022 Q59	MCQ: Ans: (D)
Which of the following statements about burnt clay bricks are NOT correct?	GATE AR 2023 Q55	MSQ: Ans: (B)(D) — magnesia ≠ red colour; alkali excess increases efflorescence
Match the items in Group–I with their corresponding items in Group–II.	GATE AR 2024 Q35	MCQ: Ans: (C)
Polytetrafluroethylene (PTFE) coated fiberglass has been used as a roofing membrane in	GATE AR 2024 Q55	MCQ: Ans: (A)
Match the specially shaped bricks in Group–I with their corresponding nomenclature…	GATE AR 2024 Q59	MCQ: Ans: (A)
The following Figure shows the excavation plan of a two room structure…	GATE AR 2024 Q63	NAT: Ans: 13.20
As per the NBC 2016, match the types of glass in Group–I with their applications…	GATE AR 2026 Q59	MCQ: Ans: (C)

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O. 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.

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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.

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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

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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.

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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.

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