Concrete – Complete Chapter-wise Study Notes

Concrete Technology is one of the highest-weightage topics in GATE Civil, ESE (IES) and SSC JE. This page covers every chapter — from constituents and fresh-concrete properties through to mix design, strength, durability, testing, special concretes and IS codes — with all formulae, tables and exam-pattern analysis.

GATE ESE / IES SSC JE State PSC RRB JE

Ch 1 · Constituents Ch 2 · Fresh Concrete & Workability Ch 3 · W/C Ratio & Strength Ch 4 · Mix Design Ch 5 · Properties of Hardened Concrete Ch 6 · Durability Ch 7 · Testing of Concrete Ch 8 · Production & Placing Ch 9 · Special Concretes Ch 10 · IS Codes & Quick Revision

1Constituents of Concrete►

Overview of Ingredients

Constituent	Typical Proportion by Volume	Function
Cement paste (cement + water)	25–40 %	Binds all ingredients; fills voids; provides workability
Fine Aggregate (sand)	25–35 %	Fills voids in CA; reduces cost; improves workability
Coarse Aggregate	35–45 %	Main load-bearing skeleton; reduces shrinkage
Entrapped air	1–3 %	Unavoidable; reduced by good compaction
Admixtures (if used)	< 1 %	Modify fresh or hardened properties

Fine Aggregate (Sand) – IS 383:2016

Passes 4.75 mm IS sieve; retained on 75 µm sieve
Types: natural (river sand, pit sand, sea sand), crushed stone sand, manufactured sand (M-sand)
Sea sand: contains chlorides and shells → not recommended for RCC unless washed
Pit sand: angular particles; good bond; may contain clay / organic matter
River sand: rounded particles; better workability but slightly lower strength

Grading Zones of Fine Aggregate (IS 383)

Zone	Fineness Modulus (approx.)	Nature	Suitability
Zone I	3.5 – 4.5 (coarser range)	Coarse	Harsh mix; use only with low w/c
Zone II	2.9 – 3.5	Medium-coarse	Best for RCC — recommended
Zone III	2.4 – 2.9	Medium-fine	Acceptable; increase sand proportion slightly
Zone IV	1.6 – 2.4	Fine	Avoid in structural concrete; high water demand

Bulking of Sand

Surface tension of moisture film around fine sand particles causes them to push apart, increasing the apparent volume (bulking). Coarse and fine sand bulk less.

Bulking (%) = [(V_bulked − V_dry) / V_dry] × 100
Maximum bulking occurs at ~4–8 % moisture content
Maximum bulking for fine sand: ~25–30 %
Maximum bulking for coarse sand: ~15–20 %

⚠ Bulking is zero for both bone-dry sand AND fully saturated (submerged) sand. Always correct for bulking when measuring sand by volume.

Fig 1 – Bulking of Sand: zero at 0% (bone dry) and ~12%+ (submerged); peak at 4–8% MC. Fine sand bulks more than coarse sand.

Coarse Aggregate – IS 383:2016

Retained on 4.75 mm sieve; maximum size typically 10, 20 or 40 mm
Angular, rough-textured aggregate → better bond with paste → higher strength
Rounded aggregate → better workability but lower bond strength
Specific gravity: 2.6 – 2.8; Water absorption ≤ 2 %

Maximum Size of Aggregate (MSA)

Structural Element	Max Aggregate Size
Mass concrete (unreinforced)	Up to 80 mm
Lightly reinforced slabs, pavements	40 mm
Beams, columns, slabs (normal RCC)	20 mm
Thin slabs, heavily reinforced sections	10 mm
General limit: MSA ≤ 1/4 minimum dimension of member	–
MSA ≤ 3/4 × clear spacing between bars (IS 456)	–
MSA ≤ cover to reinforcement (IS 456)	–

💡 GATE Tip: Larger MSA → less surface area → less cement paste needed → lower w/c possible → higher strength. BUT larger MSA → reduced workability and risk of segregation in thin sections.

Water for Concrete (IS 456:2000, Clause 5.4)

Should be fit for drinking (potable) or tested against IS 456 limits
pH: 6 – 8.5
Sulphates (as SO₃): ≤ 400 mg/L
Chlorides (as Cl⁻): ≤ 500 mg/L (PCC); ≤ 250 mg/L (RCC)
Organic matter: ≤ 200 mg/L
Suspended matter: ≤ 2000 mg/L
Seawater: may be used for PCC in non-aggressive environments; not for RCC

Aggregate Tests Relevant to Concrete Quality

Test	IS Code	Limiting Value	Importance
Aggregate Crushing Value (ACV)	IS 2386 Pt IV	≤ 30 % (pavement); ≤ 45 % (concrete)	Structural strength
Aggregate Impact Value (AIV)	IS 2386 Pt IV	≤ 30 % (pavement); ≤ 45 % (concrete)	Impact toughness
Los Angeles Abrasion Value	IS 2386 Pt IV	≤ 30 % (pavement); ≤ 50 % (concrete)	Wear resistance
Flakiness Index (FI)	IS 2386 Pt I	≤ 15 % preferred (≤ 35 % max)	Workability & strength
Elongation Index (EI)	IS 2386 Pt I	≤ 15 % preferred (≤ 35 % max)	Strength & compaction
Specific Gravity	IS 2386 Pt III	2.6 – 2.8 (normal weight)	Mix design calculations
Water Absorption	IS 2386 Pt III	≤ 2 % (CA); ≤ 3 % (FA)	Effective w/c adjustment
Soundness (Na₂SO₄ / MgSO₄)	IS 2386 Pt V	≤ 10–12 % loss	Weathering resistance
Alkali-Reactivity (ASR)	IS 2386 Pt VII	Non-reactive preferred	Prevents gel expansion

2Fresh Concrete – Workability, Segregation & Bleeding►

Definition of Workability

Workability is the property of freshly mixed concrete that determines the ease and homogeneity with which it can be mixed, transported, placed, compacted and finished with minimum loss of homogeneity. (Defined by Glanville and Collins.)

Factors Affecting Workability

Factor	Effect on Workability
Water content (w/c ratio)	Single most important factor; ↑ water → ↑ workability
Cement content	↑ cement → finer paste → ↑ workability (at fixed w/c)
Aggregate size (MSA)	↑ MSA → less surface area → ↑ workability
Aggregate shape	Rounded > angular for workability (less friction)
Aggregate texture	Smooth > rough for workability
Aggregate grading	Well-graded → fewer voids → less paste needed → ↑ workability
Air entrainment	Air bubbles act as ball bearings → ↑ workability
Admixtures	Plasticizers / superplasticizers → ↑ workability without extra water
Temperature	↑ temperature → faster hydration → ↓ workability (slump loss)
Time after mixing	Workability reduces with time (slump loss)
Cement type / fineness	Finer cement → faster hydration → faster slump loss

Workability Tests – Complete Comparison

Test	IS Code	Range	Best For	Principle
Slump Test	IS 7320; IS 1199 Pt 2	0–175 mm	Medium workability (25–175 mm)	Subsidence of concrete after removing Abrams' cone
Compaction Factor (CF) Test	IS 1199 Pt 3	0.70–0.95	Low to medium workability; more sensitive than slump	Ratio of weight of partially compacted to fully compacted concrete
Vee-Bee Consistometer Test	IS 1199 Pt 4	3–25 seconds	Very low / stiff mixes (pavement, precast)	Time for concrete to re-mould under vibration
Flow Table Test	IS 9103	Spread diameter	Highly workable / SCC / flowing concrete	Spread of concrete under 15 jolts on flow table
Kelly Ball Test	ASTM	0–150 mm penetration	Field test; quick; fresh concrete in forms	Penetration of 30 lb ball under own weight

Slump Test – Full Details (IS 1199 Pt 2)

Apparatus: Abrams' cone: height 300 mm, top dia 100 mm, base dia 200 mm; tamping rod 16 mm dia, 600 mm long
Procedure: Fill in 3 layers, 25 tamps each; lift cone vertically; measure slump
Types of slump: True slump (uniform subsidence — desirable), Shear slump (one side shears off — repeat test), Collapse slump (complete collapse — very high w/c)

Fig 2 – Abrams' Cone (h=300 mm, base ⌀200 mm, top ⌀100 mm) and the three types of slump: True (accept), Shear (repeat), Collapse (reject).

Slump Value	Degree of Workability	Suitable Structures
0–25 mm	Very low	Roads, pavements, mass concrete
25–50 mm	Low	Lightly reinforced foundations, kerbs
50–75 mm	Medium	Normal RCC — slabs, beams (manually placed)
75–100 mm	Medium-high	Columns, walls, heavily reinforced members
100–150 mm	High	Pumped concrete, pile foundations
150–175 mm	Very high	Grouting, tremie concrete, SCC precursor

Compaction Factor Test – Full Details

Compaction Factor (CF) = Weight of partially compacted concrete / Weight of fully compacted concrete
CF = 0.75–0.80 → Very low workability
CF = 0.85 → Low workability
CF = 0.90 → Medium workability
CF = 0.95 → High workability

💡 CF test is more sensitive than the slump test, especially for stiff mixes where slump difference is very small. Preferred for mixes with slump < 25 mm.

Fig 8 – Compaction Factor Test: concrete falls freely through two hoppers into a cylinder. CF = W₂(partial) / W₁(full). Range: 0.70 (very low) to 0.95 (high workability).

Segregation

Segregation is the separation of constituents of concrete such that the mix is no longer homogeneous — coarse aggregate settles to the bottom while mortar rises to the top.

Causes: Excessive w/c, dropping from height, over-vibration, poor grading, too large MSA
Types: (a) Coarse aggregate separates from mortar; (b) Cement paste separates from aggregate (bleeding)
Prevention: Correct w/c ratio, proper grading, limit drop height ≤ 1.5 m, controlled vibration, use of admixtures

Bleeding

Bleeding is a form of segregation where water rises to the surface of freshly placed concrete. It is caused by the inability of the solid particles to hold all the mixing water as they settle.

Effects: (a) Water channel formation → increased permeability; (b) Laitance layer on surface → weak surface; (c) Water pockets under aggregate and rebar → reduced bond; (d) Reduces compressive strength
Factors increasing bleeding: High w/c, high sand content, coarse cement, low cement content, lack of fines, high temperature
Prevention: Reduce w/c, increase cement content, use finer cement, air entrainment, pozzolans
IS 9103 Bleeding test: Bleeding capacity = volume of bleed water / total mixing water × 100 %

⚠ Laitance must be removed before placing the next lift of concrete, otherwise a weak plane (cold joint) forms. Use wire brushing, sandblasting or water jetting.

Stiffening, Setting and Early Strength

Stage	Approx. Time	Description
Fresh / plastic stage	0 – 1 hr	Concrete fully workable; can be compacted
Initial set (IST)	~1.5 – 3 hrs	Concrete begins to stiffen; workability nearly lost; should be placed and compacted by now
Final set (FST)	~5 – 10 hrs	Concrete becomes rigid; demoulding possible (carefully)
Early strength stage	24 hrs – 7 days	Rapid strength gain; curing critical
Long-term strength	7 – 90 days+	Continued hydration; strength increases for months

3Water-Cement Ratio & Strength of Concrete►

Abrams' Water-Cement Ratio Law (1919)

For a given set of materials and conditions of testing, the strength of properly cured and compacted concrete is inversely proportional to the water-cement ratio.

f_c = A / B^w/c
where A and B are empirical constants dependent on cement type, age and testing conditions
A ≈ 100–125 MPa; B ≈ 3.5–4.0 (for OPC at 28 days)

⚠ Abrams' law is valid only for fully compacted, properly cured concrete. It breaks down at very low w/c ratios (insufficient workability) and very high w/c ratios (excessive voids).

Fig 3 – Abrams' Law: compressive strength decreases as w/c increases. Dashed lines show IS 456 max w/c limits for different exposure conditions.

Effective W/C Ratio

Effective w/c = (Total water − Water absorbed by aggregates) / Weight of cement
Free water = Total water − Absorption × Weight of aggregate / 100

Aggregates in Saturated Surface Dry (SSD) condition → no adjustment to mix water needed
Aggregates drier than SSD → absorb water from mix → increase effective water; add extra water
Aggregates wetter than SSD → contribute free water to mix → reduce mix water

Relationship: W/C Ratio and Compressive Strength

w/c Ratio	Approx. 28-day Compressive Strength (OPC 43)
0.30	~65 MPa
0.35	~55 MPa
0.40	~45 MPa
0.45	~38 MPa
0.50	~32 MPa
0.55	~26 MPa
0.60	~21 MPa
0.65	~17 MPa

Minimum w/c Ratio for Complete Hydration

Theoretical minimum: w/c = 0.23 (water chemically combined during hydration)
Practical minimum: w/c = 0.36 (includes gel water required for C–S–H structure)
At w/c < 0.36: unhydrated cement grains remain; self-desiccation occurs
For workability: w/c ≥ 0.40 generally required without superplasticizer

Effect of W/C Ratio on Concrete Properties

Property	Low w/c (<0.40)	High w/c (>0.60)
Compressive strength	High	Low
Tensile/flexural strength	High	Low
Workability	Low (needs SP)	High
Permeability	Very low	High
Durability	Very high	Low
Shrinkage	Lower	Higher
Bleeding	Lower	Higher
Capillary porosity	Low	High

Maximum W/C Ratio Limits (IS 456:2000 Table 5)

Exposure Condition	Max w/c	Min Cement (kg/m³)	Min Grade
Mild	0.55	300	M20
Moderate	0.50	300	M25
Severe	0.45	320	M30
Very Severe	0.45	340	M35
Extreme	0.40	360	M40

Strength at Different Ages (% of 28-day strength) — OPC

Age	OPC 33	OPC 43	OPC 53 / RHPC	PPC / PBFSC
1 day	16 %	20 %	27 %	10 %
3 days	46 %	55 %	65 %	40 %
7 days	70 %	75 %	82 %	65 %
28 days	100 %	100 %	100 %	100 %
3 months	115 %	115 %	110 %	120 %
1 year	135 %	130 %	120 %	150 %

💡 IS 456 Clause 6.2.1: The increase in strength beyond 28 days can be accounted for in design if properly established, but 28-day strength remains the standard design reference.

Fig 4 – Strength gain with age: OPC 53 gains strength faster early; PPC has slower early gain but achieves higher long-term strength due to pozzolanic reaction.

4Concrete Mix Design – IS 10262:2019►

Types of Mixes

Feature	Nominal Mix	Standard Mix	Design Mix
Basis	Fixed volumetric proportions	IS prescriptive proportions	Statistical target mean strength
Applicable grades	M5 to M25 (IS 456)	M10 to M25	M25 and above (mandatory)
IS code	IS 456 Table 9	IS 456	IS 10262:2019
Economy	Conservative (over-designed)	Moderate	Most economical and reliable
Quality control required	Low	Moderate	High (batching, testing)

Nominal Mix Proportions (IS 456:2000 Table 9)

Grade	f_ck (MPa)	Proportion (C : FA : CA) by volume	Max w/c
M5	5	1 : 5 : 10	–
M7.5	7.5	1 : 4 : 8	–
M10	10	1 : 3 : 6	–
M15	15	1 : 2 : 4	–
M20	20	1 : 1.5 : 3	0.55
M25	25	1 : 1 : 2	0.50

⚠ M20 is the minimum grade for RCC in mild exposure as per IS 456:2000. Nominal mix must NOT be used for M30 and above — only design mix.

IS 10262:2019 – Design Mix Procedure (Step by Step)

Determine target mean strength (f'_ck):

f'_ck = f_ck + 1.65 × σ
σ = standard deviation from IS 10262 Table 1:
- M10–M15: σ = 3.5 MPa
- M20–M25: σ = 4.0 MPa
- M30–M50: σ = 5.0 MPa
- M55 and above: σ = 6.0 MPa
Select w/c ratio: From Table 5 of IS 456 (exposure conditions) and from strength relationship
Determine water content: From IS 10262 Table 2 based on MSA and required slump; then adjust for admixtures
Calculate cement content:

Cement content = Water content / w/c ratio
Check against minimum cement content from IS 456 Table 5
Determine aggregate proportions: Volume method using absolute volumes; fine aggregate fraction from IS 10262 Table 3 (based on zone and MSA)
Calculate mix quantities per m³ of concrete:

Volume of concrete = Volume of cement + Volume of water + Volume of FA + Volume of CA + Volume of air
V_cement = C / (S_c × 1000) m³
V_water = W / 1000 m³
V_agg = 1 − V_cement − V_water − V_air
Volume of FA = V_agg × (% of FA / 100)
Mass of FA = V_FA × S_FA × 1000
Trial mixes: Minimum 3 trial mixes at w/c ± 0.05; test workability and 28-day strength; finalise proportions

Fig 5 – IS 10262:2019 Mix Design Procedure: 7-step flow from target mean strength to trial mixes. Feedback loop adjusts proportions if trial results fail.

Water Content from IS 10262:2019 Table 2

Max Agg. Size (mm)	Slump 25–50 mm (kg/m³)	Slump 50–100 mm (kg/m³)	Slump 100–150 mm (kg/m³)
10	208	228	–
20	186	200	215
40	165	178	192

⚠ For each 25 mm increase in slump beyond 50 mm, add ~3 % water. For Zone III sand, add 12 kg/m³; Zone IV sand, add 18 kg/m³. For rounded aggregate, reduce water by 10 kg/m³.

Fine Aggregate Volume Fraction (IS 10262 Table 3)

w/c ratio	FA Zone II, MSA 20 mm	FA Zone II, MSA 40 mm
0.35	0.30	0.27
0.40	0.33	0.29
0.45	0.36	0.32
0.50	0.38	0.34
0.55	0.40	0.36
0.60	0.42	0.38

ACI Method of Mix Design (for reference, ESE)

Recommended water content from ACI 211.1 Table based on slump and MSA
w/c selected from strength and durability requirements
Cement = Water / w/c
Dry-rodded unit weight of coarse aggregate determines CA volume
FA fills remaining volume
ACI uses by weight proportioning; IS 10262 uses absolute volume method

5Properties of Hardened Concrete►

Compressive Strength

Characteristic compressive strength (f_ck): 28-day cube strength below which not more than 5 % of results are expected to fall
Standard cube: 150 mm × 150 mm × 150 mm (IS 516)
Standard cylinder: 150 mm dia × 300 mm height (ASTM)
Cube strength / Cylinder strength ≈ 1.25 (cube strength is ~25 % higher)
Loading rate: 140 kg/cm²/min (IS 516) = 14 N/mm²/min

Target mean strength: f'_ck = f_ck + 1.65σ
For M20: f'_ck = 20 + 1.65 × 4.0 = 26.6 MPa
For M25: f'_ck = 25 + 1.65 × 4.0 = 31.6 MPa
For M30: f'_ck = 30 + 1.65 × 5.0 = 38.25 MPa

Acceptance Criteria for Concrete Strength (IS 456 Clause 16.1)

Fig 16 – Concrete microstructure: C–S–H gel provides strength; Ca(OH)₂ maintains high pH; ITZ (Interfacial Transition Zone) is the weakest link at 20–50 µm around aggregates; capillary pores control durability and permeability.

Criterion	Requirement
Mean of any group of 4 consecutive results	≥ f_ck + 0.825 × s (s = established SD) or ≥ f_ck + 3 MPa, whichever is greater
Any individual test result	≥ f_ck − 3 MPa

Tensile Strength

Concrete is weak in tension; tensile strength ≈ 8–12 % of compressive strength
Split cylinder tensile strength (Brazilian test, IS 5816):

f_t = 2P / (π × L × D) [P = max load; L = length; D = diameter of cylinder]
Flexural strength (Modulus of Rupture, IS 516): Tests 150×150×700 mm beam

If failure within middle third: f_r = PL / (bd²)
IS 456 formula: f_r = 0.7√f_ck MPa (characteristic)

Modulus of Elasticity (IS 456 Clause 6.2.3)

E_c = 5000 √f_ck MPa    (short-term static modulus — IS 456)
E_c = 5700 √f_ck MPa    (ACI 318 — cylindrical strength)
E_c = 9100 (f_ck)^1/3 MPa    (CEB-FIP — more accurate)

Grade	f_ck (MPa)	E_c = 5000√f_ck (GPa)
M20	20	22.4
M25	25	25.0
M30	30	27.4
M35	35	29.6
M40	40	31.6

Poisson's Ratio

Concrete: µ = 0.1 – 0.2 (IS 456 uses 0.2 for elastic analysis)
Steel: µ = 0.3

Creep of Concrete

Creep is the time-dependent increase in strain under a constant sustained stress. It occurs due to viscous flow of cement gel and redistribution of stress to aggregates.

Creep strain = Total strain − Elastic strain − Shrinkage strain
Creep coefficient (θ) = Creep strain / Elastic strain
IS 456 values of θ: Age at loading 7 days → θ = 2.2; 28 days → θ = 1.6; 1 year → θ = 1.1
Effective modulus: E_eff = E_c / (1 + θ)

Creep is higher with: high w/c, high cement content, early loading, low humidity, high temperature, thin members
Creep is lower with: high-strength concrete, humid curing, late loading, thick members, large aggregates
Ultimate creep strain ≈ 2–3 × elastic strain for normal concrete
Effects of creep: Increased deflection, loss of prestress, redistribution of stresses, differential settlement

Fig 6 – Creep of Concrete: on loading, instantaneous elastic strain occurs; creep strain accumulates over time. On unloading: elastic recovery + partial creep recovery; residual strain remains permanently.

Shrinkage of Concrete

Type	Cause	When Occurs	Magnitude
Plastic shrinkage	Rapid evaporation from fresh surface before set	0–6 hrs after placing	Large; leads to surface cracks
Autogenous shrinkage	Chemical shrinkage during hydration (self-desiccation)	During hardening	Significant at low w/c (<0.40)
Drying shrinkage	Loss of adsorbed water from C–S–H gel on drying	After hardening, on exposure	300–600 × 10⁻⁶
Carbonation shrinkage	CO₂ reacts with Ca(OH)₂ → CaCO₃	Long-term; surface first	Smaller; may be partially reversible
Thermal shrinkage	Cooling after peak hydration heat	During curing of mass concrete	Significant in large pours

Drying shrinkage (IS 456 Cl. 6.2.4): ε_cs = 0.0003 (for design purposes)
Coefficient of thermal expansion of concrete: α = 10 × 10⁻⁶ /°C (IS 456 uses 11 × 10⁻⁶ /°C)

Fig 10 – Shrinkage types mapped against time. Plastic shrinkage is the earliest and fastest; drying shrinkage is the most significant and longest-lasting; IS 456 design value ε_cs = 0.0003.

Unit Weight and Density

Type of Concrete	Density (kg/m³)	Unit Weight (kN/m³)
Plain Cement Concrete (PCC)	2300–2400	24
Reinforced Cement Concrete (RCC)	2400–2500	25
Lightweight concrete	< 1900	< 19
Heavyweight (radiation shielding)	> 3200	> 32
High Strength Concrete (HSC)	2400–2500	25

Thermal Properties

Coefficient of thermal expansion: α ≈ 10–12 × 10⁻⁶/°C (same as steel → compatible)
Thermal conductivity: k ≈ 1.7 W/m·K (normal weight concrete)
Specific heat: c_p ≈ 840–1170 J/kg·K
Concrete retains strength up to 300 °C; serious loss above 300 °C; spalling above 400 °C

6Durability of Concrete – Mechanisms & Prevention►

Definition (IS 456 Clause 8.1)

A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during the service life for which it was designed. The concrete must resist weathering action, chemical attack, abrasion and other degradation processes.

Permeability – The Master Parameter

Permeability governs transport of water, oxygen, CO₂, chlorides and sulphates → controls all durability mechanisms
Capillary pores (0.01–10 µm) are the main transport pathways
Reduces with: lower w/c, prolonged curing, mineral admixtures (fly ash, silica fume, GGBS)
Water permeability coefficient k ≈ 10⁻¹⁰ – 10⁻¹² m/s for dense concrete
Darcy's Law: v = k × i (flow velocity = permeability × hydraulic gradient)

Chemical Attack Mechanisms

(a) Sulphate Attack

Parameter	Details
Source of sulphates	Soil (pyrite oxidation), groundwater, seawater, industrial effluent
Reaction with C₃A hydrate	Ca(OH)₂ + CaSO₄ + 2H₂O → CaSO₄·2H₂O (gypsum, expansive)
Reaction with C–S–H	CaSO₄ + C–S–H + H₂O → CaCO₃ + silica gel (decalcification)
Secondary ettringite	Gypsum + C₃A hydrate → Ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) — very expansive
Most aggressive form	MgSO₄ — attacks both C₃A and C–S–H gel
Visual symptoms	White deposits, cracking, spalling, softening, loss of section
Prevention	SRC (C₃A < 5 %); low w/c; PPC / GGBS; dense cover; membranes

(b) Carbonation

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
Reduces pH from 12.5 to below 9 → depassivates steel → corrosion initiates
Rate: fastest at 50–70 % RH; slow in very dry and very wet conditions
Carbonation depth ∝ √t (diffusion process)

d = K_c × √t [d = carbonation depth; K_c = carbonation coefficient; t = time in years]

Higher w/c, lower cement content, poor curing → faster carbonation
Prevention: Low w/c, dense concrete, adequate cover, polymer coatings

Fig 7a – Carbonation front moves inward with time (d∝√t). Phenolphthalein shows colourless zone (carbonated) and pink zone (alkaline).

Fig 7b – Chloride ions diffuse through cover → destroy passive film on steel → pitting corrosion → rust expansion → cracking and spalling of cover.

(c) Alkali-Silica Reaction (ASR) / Alkali-Aggregate Reaction (AAR)

Mechanism: Amorphous/reactive silica in aggregate + alkalis (Na⁺, K⁺ from cement) + moisture → expansive silica gel
Three prerequisites (Stanton's Trinity): Reactive aggregate + sufficient alkali (Na₂O_eq > 0.6 %) + moisture
Visual signs: Map cracking (crazing), gel exudation, surface popouts
Reactive aggregates: Opal, chert, flint, volcanic glass, greywacke, certain quartzites
Pessimum proportion: Maximum expansion at ~5–15 % reactive content; reduces at higher amounts
Prevention: Low-alkali cement (Na₂O_eq ≤ 0.60 %); fly ash ≥ 30 % or GGBS ≥ 50 %; silica fume; non-reactive aggregates

Fig 12 – ASR Mechanism: reactive silica + alkalis + moisture → expansive silica gel → swelling → internal pressure → map cracking. Prevention: Na₂O_eq ≤ 0.60%, fly ash ≥ 30%.

(d) Chloride-Induced Corrosion of Steel

Cl⁻ ions migrate through concrete → reach steel → destroy passive Fe₂O₃ layer → pitting corrosion begins
Corrosion threshold: free Cl⁻ > 0.4 % by mass of cement (IS 456)
Corrosion products (rust) volume 2–6 × original steel → expansive cracking and delamination
Chloride transport mechanisms: diffusion (main), absorption, permeation
Prevention: Adequate cover, low w/c (< 0.45), silica fume, epoxy-coated bars, stainless steel, cathodic protection (impressed current / sacrificial anode)

(e) Acid Attack

Any acid at pH < 6.5 attacks concrete; most aggressive at pH < 4.5
Acids dissolve Ca(OH)₂ and C–S–H gel → surface erosion, loss of section
Biogenic sulphuric acid attack (BSAA) in sewers: H₂S → H₂SO₄
Prevention: Use HAC or SSC; GGBS; dense concrete; acid-resistant linings/tiles; epoxy coatings

(f) Freeze-Thaw Attack

Water expands ~9 % on freezing → hydraulic pressure → internal cracking and surface scaling
Osmotic pressure from dissolved salts also contributes
Damage quantified by Durability Factor (DF) from ASTM C666
Prevention: Air entrainment (3–7 % air with 25–250 µm bubble spacing ≤ 200 µm); low w/c; proper curing; de-icing salt resistance

Fig 15a – Freeze-thaw: water in capillary pores freezes, expands 9%, generates hydraulic pressure → micro-cracking → progressive scaling.

Fig 15b – Air entrainment: small voids (25–250 µm, spacing ≤ 200 µm) act as pressure relief chambers. Ice expands into void → no cracking. Trade-off: 1% air ≈ −5% strength.

Cover to Reinforcement (IS 456:2000 Clause 26.4)

Exposure Condition	Nominal Cover (mm)
Mild	20 mm
Moderate	30 mm
Severe	45 mm
Very Severe	50 mm
Extreme	75 mm
Fire resistance (2 hrs)	40 mm (beams); 35 mm (slabs)

Fig 11 – IS 456:2000 Exposure Conditions Staircase: each step up the severity ladder adds 5 MPa to the minimum grade, reduces max w/c, increases min cement content and min cover.

Curing of Concrete

Method	Description	When Used
Water curing (moist curing)	Wet hessian, burlap; ponding; continuous sprinkling	Most common; all RCC
Membrane curing	Impermeable membrane (polyethylene, liquid compound) seals surface	Pavements, slabs, arid conditions
Steam curing (atmospheric)	100 °C steam; 6–18 hrs; 70–80 % 28-day strength in 24 hrs	Precast elements
High-pressure steam (Autoclaving)	180 °C, 1 MPa; extremely fast strength gain; C–S–H → tobermorite	AAC blocks, railway sleepers
Electrical curing	Low-voltage current heats concrete	Cold weather; mass concrete
Infra-red / microwave	Radiant heating	Thin precast panels

Minimum Curing Periods (IS 456:2000 Clause 13.5)

OPC: 7 days
Blended cements (PPC, PBFSC, SRC) in moderate/severe environments: 10 days
Hot and dry conditions (T > 40 °C and RH < 20 %): 14 days minimum
Lean concrete, nominal mix: 7 days
Curing temperature must be ≥ 10 °C; concrete should not be allowed to freeze during curing

7Testing of Concrete – Destructive & Non-Destructive►

Destructive Tests (DT)

1. Compressive Strength Test (IS 516:1959)

Cube: 150 × 150 × 150 mm; cylinder: 150 mm dia × 300 mm (ASTM)
3 specimens per batch; tested at 7, 28 days (and optionally 3, 56, 91 days)
Loading rate: 140 kg/cm²/min = 14 N/mm²/min
Accept average of 3 results if no individual result < f_ck − 3 MPa
Cube vs Cylinder: f_cube = 1.25 × f_cylinder approximately

2. Split Cylinder / Brazilian Test (IS 5816)

Tensile strength f_t = 2P / (π L D)
P = failure load; L = length of cylinder; D = diameter
Typically f_t ≈ 8–12 % of f_ck; IS 456: f_t = 0.7√f_ck (flexural)

3. Flexural Strength Test (IS 516) – Modulus of Rupture

Beam specimen: 150 × 150 × 700 mm; simply supported; third-point loading
Span = 450 mm (3 × depth); loads at 150 mm from each support

If failure in middle third: f_r = PL / (bd²) [P = total load; L = span; b = width; d = depth]
If failure outside middle third (a = distance from support to fracture):
f_r = 3Pa / (bd²)

Non-Destructive Tests (NDT) of Concrete

Test	IS Code / Standard	Principle	Output	Accuracy
Rebound Hammer Test	IS 13311 Pt 2; ASTM C805	Spring-loaded plunger impacts concrete surface; rebound number R measured	Surface hardness → estimated compressive strength	±25 % variation; only surface layer
Ultrasonic Pulse Velocity (UPV)	IS 13311 Pt 1; ASTM C597	Ultrasonic pulse (50–250 kHz) travels through concrete; time measured	Pulse velocity → quality assessment; detect cracks, voids, delamination	Good for uniformity; limited for strength
Combined Method (Hammer + UPV)	RILEM NDT 4	Uses both R and V to estimate strength from empirical charts	Better strength estimate than either test alone	±15–20 %
Core Test	IS 516; IS 1199	Cylindrical cores drilled from structure; tested in compression	In-situ concrete strength	Most accurate in-situ method; invasive but not fully destructive
Pull-out Test (Lok Test)	ASTM C803	Embedded disc pulled out; force measured	In-situ compressive strength	Good for early-age strength monitoring
Penetration Resistance (Windsor Probe)	ASTM C803	Steel probe driven by powder charge; penetration depth measured	Surface hardness / estimated strength	±10–15 %; only near-surface
Radiography / X-ray	ACI 228	X-ray or gamma-ray penetration; film or digital detector	Location of bars, voids, cracks inside concrete	High accuracy; expensive; radiation hazard
Infrared Thermography	ASTM D4788	Thermal camera detects temperature differentials	Delamination, voids, water ingress behind surfaces	Good for large areas; depends on sun and wind conditions
Ground Penetrating Radar (GPR)	ASTM D6087	EM pulses reflect off boundaries (steel, voids, cracks)	Rebar location, cover depth, void detection	Excellent; non-contact; very fast
Half-Cell Potential (HCP)	IS 14977; ASTM C876	Measures electrochemical potential of steel against reference electrode	Probability of corrosion activity of reinforcement	Indicates active/passive; not quantitative
Carbonation Depth Test	BRE Digest 405	Phenolphthalein indicator sprayed on freshly broken concrete surface	Carbonation depth (colourless = carbonated; pink/red = alkaline)	Simple; inexpensive; accurate
Chloride Content Test	IS 14959; ASTM C1152	Core extracted; powdered; chemical or potentiometric titration	Total and free chloride content at various depths	Good; semi-destructive (core required)

Rebound Hammer – Detailed Notes

Rebound Number (R): 10–20 → very poor; 20–30 → fair; 30–40 → good; 40–50 → very good; > 50 → excellent surface hardness
Factors affecting R: carbonation (↑ R), moisture (↓ R), surface texture, age, type of aggregate, direction of impact (calibration chart changes)
At least 12 readings per test location; reject outliers > 6 R from mean
Calibrate on standard anvil; R = 80 ± 2 on IS reference anvil
Not reliable for: Lightweight concrete, highly wet surfaces, surfaces with surface carbonation only

UPV Test – Detailed Notes

Velocity (km/s)	Quality of Concrete
> 4.5	Excellent
3.5 – 4.5	Good
3.0 – 3.5	Medium
2.0 – 3.0	Doubtful
< 2.0	Very poor / completely deteriorated

Transmission modes: Direct (most accurate), Semi-direct, Indirect/surface (least accurate)
Pulse velocity affected by: moisture content, steel reinforcement (increases velocity), aggregate type and content, temperature

Fig 9a – UPV transmission modes: Direct (opposite faces), Semi-direct (adjacent faces), Indirect/Surface (same face). Direct gives highest accuracy.

Fig 9b – Rebound Hammer: spring-loaded plunger impacts surface; rebound number R correlates with surface hardness and estimated strength. Accuracy ±25%.

Core Test – Correction Factors

Estimated cube strength = Core strength × Correction factor
L/D correction: for L/D = 2.0 → factor = 1.0; L/D = 1.0 → multiply by 1.25
IS 516: If core strength × 1.25 ≥ 0.85 × f_ck → concrete is acceptable

Half-Cell Potential – Corrosion Probability

Potential (mV vs Cu/CuSO₄ electrode)	Probability of Corrosion (ASTM C876)
More positive than −200 mV	< 10 % probability of active corrosion
−200 to −350 mV	Uncertain zone; further investigation needed
More negative than −350 mV	> 90 % probability of active corrosion

8Production, Batching, Mixing, Placing & Compaction►

Batching of Concrete Materials

Method	Description	Accuracy	Preferred For
Weigh batching	All materials weighed individually	±1–3 % cement; ±3 % aggregate; ±1 % water	All concrete above M20; IS 456 requirement
Volume batching	Measured by volume using gauge boxes	Lower; affected by bulking, void ratios	Nominal mix only; M15 and below

⚠ IS 456:2000 recommends weigh batching for all concrete of M20 and above. Volume batching is permitted only for grades below M20 using nominal mix. Always correct for bulking of sand and surface moisture of aggregates.

Mixing of Concrete

Method	Equipment	Minimum Mixing Time	Notes
Machine mixing (drum mixer)	Tilting drum, non-tilting drum, pan mixer	2 minutes (IS 456)	Preferred; most construction sites
Ready-mixed concrete (RMC)	Central plant + transit mixer (agitating drum)	70–100 revolutions at mixing speed; then 4–6 km/h agitation	IS 4926:2003; IS 456 permits use
Hand mixing	Manually on watertight platform	Not recommended for structural concrete	Only M10 and below; add 10 % extra cement

Transportation of Concrete

Maximum time from mixing to placing: 2 hours at ≤ 25 °C; 1.5 hours at > 25 °C (IS 456)
No re-tempering (adding extra water) permitted — reduces strength and durability
Transit mixer: can travel up to 10–15 km if agitating; no segregation/slump loss
Chute: max slope 1 vertical : 3 horizontal; prevent segregation with baffles
Concrete pump: slump ≥ 75 mm; maximum pipe bend radius ≥ 1 m; pressure up to 100 bar
Belt conveyor: can cause segregation; use concrete of low slump; cover to prevent drying
Truck (agitator): maximum 300 revolutions before discharge; maximum 1.5 hours

Placing of Concrete

Maximum free-fall height: 1.5 m (IS 456); beyond this → use tremie or pump
Do not disturb placed concrete after 30 minutes or once initial set begins
Avoid placing against flowing water; pump out water first
Concrete placed in layers: 300–450 mm for vibrated concrete; thinner if hand-compacted
Avoid cold joints: next layer must be placed before initial set of previous layer

Hot Weather Concreting (IS 456 Clause 14.1)

Temperature of concrete at placing: ≤ 40 °C (IS 456 Cl. 14.1)
Precautions: cool water/ice, chill aggregates, use retarder, shade, night pouring, use white cement
For every 5 °C rise in concrete temperature → approximately 1 % increase in water demand → reduce strength by ~2 MPa

Cold Weather Concreting (IS 456 Clause 14.2)

Concrete temperature at placing: ≥ 10 °C
If ambient T < 5 °C: warm water; insulated formwork; accelerators; heated enclosures
Frozen aggregates must NOT be used
Protect from freezing until compressive strength ≥ 3.5 MPa

Compaction of Concrete

Method	Type	Suitable For	Notes
Internal vibration (poker/needle vibrator)	Most common; 25–100 mm dia needle; 50–200 Hz	Most RCC, beams, columns	Best compaction; insertion at 0.5 m intervals; withdraw slowly at 80–100 mm/min
External vibration (form vibrator)	Attached to formwork	Thin members, precast panels	Limited penetration depth; supplement with internal
Vibrating table	Table vibrates at controlled frequency	Precast, small elements	Very uniform compaction
Surface vibration (screed vibrator)	Vibrating screed or plate	Slabs, pavements	Limited to ~150 mm depth
Tamping / rodding	Manual; 25 tamps per 100 cm²	Small elements; test specimens	Very limited effectiveness
Centrifugal compaction	Spinning mould	Pipes, poles, piles	Low w/c possible; very dense concrete

⚠ Over-vibration causes segregation (aggregate sinks, paste rises → laitance). Under-vibration leaves voids → honeycombing → reduced strength and durability. Optimum: insert poker at 0.5–0.75 m centres; vibrate until air bubbles cease to appear at surface.

Fig 13 – Poker vibrator insertion pattern: spaced 0.5–0.75 m centres; penetrates 50 mm into previous layer; withdrawn slowly at 80–100 mm/min. Vibration radius shown as blue circles. Over-vibration → segregation; under-vibration → honeycombing.

Formwork (Shuttering)

Must be watertight, strong, stiff, smooth surface
Apply release agent before placing concrete
Striking/stripping time depends on cement type and temperature

Structural Element	Min. Stripping Time (OPC, T > 15 °C)
Vertical formwork (columns, walls)	16 – 24 hours
Soffit of slabs (props left in)	3 days
Soffit of beams (props left in)	7 days
Props to slabs (slabs spanning ≤ 4.5 m)	14 days
Props to beams and arches (span > 6 m)	28 days

Ready Mix Concrete (RMC) – IS 4926:2003

Designed and batched at central plant; delivered by transit mixer to site
Transit drum: 40–100 RPM (mixing); 2–6 RPM (agitating)
Maximum 70 revolutions at full mixing speed; thereafter agitate
Total revolutions must not exceed 300
Maximum transport time: 2 hours from mixing to discharge
Advantages: Quality control, continuous supply, less labour, reduced site clutter, accurate batching
Disadvantages: Traffic delays cause slump loss, distance limitations, high cost for small quantities

Pumped Concrete

Concrete pumped through steel pipes (100–150 mm dia) at pressure up to 100 bar
Required slump: ≥ 75 mm; use plasticizer / superplasticizer
No segregation in pipe; pump pressure forces bleeding water to surface on exit
Cannot pump concrete with crushed flaky aggregate efficiently
Pumpability: Depends on cement paste content, grading, shape of aggregate, admixtures

Underwater Concreting Methods

Tremie method: 150–300 mm dia pipe; bottom of pipe always submerged in concrete; prevents water entry; most common method
Bottom dump bucket: Concrete placed in bucket and lowered; less control
Prepacked aggregate (grouted concrete): Pre-placed CA; grout injected; excellent bond
Pump method: Flexible hose pumps concrete into water; concrete must be self-compacting or of high consistency
Antiwashout admixtures (AWA) used to prevent cement paste from washing out in turbulent water

9Special Concretes – Types, Properties & Applications►

High Strength Concrete (HSC)

Definition: f_ck > 40 MPa (IS); > 55 MPa (ACI)
Materials: Low w/c (0.25–0.35), silica fume (8–12 %), superplasticizer, high-quality aggregates
Strength up to 100–120 MPa achievable in practice; 150+ MPa in laboratory
More brittle than normal concrete; E_c is higher; less creep
Uses: High-rise buildings, bridges, offshore structures, parking garages

High Performance Concrete (HPC)

Concrete designed to achieve high strength AND high durability simultaneously
w/c ≤ 0.35; silica fume; GGBS; fly ash; superplasticizer; well-graded aggregates
Permeability: k < 10⁻¹² m/s (100–1000 × less permeable than normal concrete)
100-year service life design
Uses: Marine structures, bridges, tunnels, nuclear facilities

Self Compacting Concrete (SCC)

Flows and consolidates under its own weight without vibration; highly flowable, non-segregating
Fresh properties tested by:

Test	Principle	Target Value
Slump Flow Test	Cone removed; measure spread diameter	550–850 mm
T₅₀₀ (Flow time)	Time to reach 500 mm spread	2–5 seconds
J-Ring Test	Flow through closely-spaced rebar ring	Δ height ≤ 10 mm (good passing ability)
V-Funnel Test	Time for concrete to drain from V-shaped funnel	6–12 seconds
L-Box Test	Ratio of heights after flow through rebar gate	H₂/H₁ ≥ 0.80
U-Box Test	Height difference between two compartments	≤ 30 mm difference
Sieve Segregation Resistance	% passing 5 mm sieve after flow	≤ 15 %

Key ingredients: High powder content (> 500 kg/m³), superplasticizer (PCE type), viscosity modifying agent (VMA), low coarse aggregate content
Uses: Congested reinforcement, inaccessible locations, complex formwork, noise-sensitive sites

Fig 14 – SCC workability tests: (a) Slump Flow – spread 550–850 mm, T₅₀₀ = 2–5 s; (b) L-Box – H₂/H₁ ≥ 0.80 measures passing ability through reinforcement; (c) V-Funnel – flow time 6–12 s measures viscosity.

Fibre Reinforced Concrete (FRC)

Fibre Type	Diameter / Length	Aspect Ratio (l/d)	Properties Enhanced	Uses
Steel fibres (hooked end)	0.25–0.75 mm / 25–60 mm	40–80	Toughness, flexural strength, impact resistance, crack control	Pavements, tunnels, industrial floors, crash barriers
Polypropylene (PP) fibres	Monofilament; 6–50 mm	High	Plastic shrinkage crack control; fire resistance (melt at 165 °C → vent pressure)	Slabs, walls, fire-resistant structures
Glass fibres (AR-glass)	10–15 µm dia / variable	High	Tensile strength; lightweight panels	Precast cladding panels (GRC)
Carbon fibres	7–8 µm dia	High	Very high tensile strength; low weight; expensive	Aerospace, structural retrofitting
Natural fibres (jute, sisal)	Variable	–	Low cost; limited strength gain	Low-cost housing, rural construction

Critical fibre volume fraction V_f,crit = f_mu / [f_fu × (1 − V_f)] (approximately)
Fibre aspect ratio = Length (l) / Diameter (d); typical 40–100
Pull-out bond strength governs fibre contribution to toughness

Lightweight Concrete (LWC)

Category	Density (kg/m³)	Method	Uses
Lightweight aggregate concrete	1400–1900	Expanded clay (Leca), expanded shale, pumice, perlite, sintered fly ash (Lytag)	Structural elements; reduces dead load
Foamed (aerated) concrete	300–1600	Stable foam mixed into cement slurry; or aluminium powder generates H₂ (AAC)	Insulation, void filling, AAC blocks
No-fines concrete	1600–2000	Only CA + cement; no sand; high void content	Drainage walls, drainage base courses

Heavyweight Concrete

Density: 3200–5000+ kg/m³
Aggregates: baryte (BaSO₄), magnetite (Fe₃O₄), limonite, iron shot
Uses: Nuclear reactor shielding, medical X-ray rooms, counterweights, offshore ballast

Ready Mix Concrete (RMC) vs Site-Mixed Concrete

Aspect	RMC	Site Mixed
Quality control	Excellent (automated plant)	Moderate (human error)
Cost	Higher per m³	Lower for large projects
Suitability	Urban projects; small-medium quantities	Large projects; remote sites
Water addition at site	Not permitted (IS 4926)	Possible but not recommended

Shotcrete (Gunite)

Concrete pneumatically projected at high velocity onto a surface (no formwork)
Dry mix (Gunite): Dry mix fed through hose; water added at nozzle; high-velocity impact → natural compaction
Wet mix Shotcrete: Premixed concrete pumped and air-projected; better consistency; less rebound (waste)
Rebound: dry-mix ~10–40 %; wet-mix ~5–15 %
Uses: Tunnel linings, slope stabilisation, swimming pools, dam face rehabilitation, structural repair
Steel fibres / mesh reinforcement can be incorporated

Vacuum Dewatered Concrete

Extra water (for workability) removed from fresh concrete surface by vacuum mat
Reduces w/c by 15–25 % → increases surface strength by 40–60 %
Better abrasion resistance, less shrinkage
Uses: Industrial floors, pavements, airfield slabs

Roller Compacted Concrete (RCC)

Zero-slump concrete placed in thin lifts (150–300 mm) and compacted by vibratory rollers
Very low cement content; high aggregate content
Uses: Gravity dams (RCC dams), parking areas, airfields, roads
RCC dams: much faster and cheaper than conventional concrete dams

Pervious (Permeable) Concrete

No fine aggregates; high void content (15–25 %); flow rate: 200–750 L/min/m²
Compressive strength: 3.5–28 MPa
Uses: Stormwater management, parking lots, pedestrian paths, sports courts

Polymer Concrete

Type	Description	Properties	Uses
Polymer Impregnated Concrete (PIC)	Hardened concrete dried, evacuated, then impregnated with monomer (MMA) and polymerised	Very high strength (100–200 MPa), very low permeability	Bridge decks, industrial floors, rehabilitation
Polymer Cement Concrete (PCC)	Latex polymer partially replaces mixing water; co-binds with cement hydrates	Improved flexural strength, bond strength, low permeability	Repair mortars, bridge deck overlays
Polymer Concrete (PC)	Polymer (resin: epoxy, polyester, acrylic) replaces cement entirely as binder	Very high strength, rapid curing, chemical resistance	Chemical plant floors, precision machine bases

Autoclaved Aerated Concrete (AAC)

Mix: cement / lime + finely ground sand/fly ash + aluminium powder + water
Al powder reacts with Ca(OH)₂: 2Al + Ca(OH)₂ + 2H₂O → CaAl₂O₄ + 3H₂↑ (hydrogen bubbles create pores)
Autoclaved at 180 °C, 1 MPa → C–S–H converts to tobermorite → high strength
Density: 400–800 kg/m³; compressive strength: 2–8 MPa
Uses: Non-load-bearing walling, thermal insulation blocks (e.g., Siporex, Ytong)

Sulphur Concrete

Molten sulphur used as binder instead of cement + water
Cools and solidifies quickly; very high early strength
Excellent acid and chemical resistance
Uses: Chemical plants, battery rooms, mining; also used on moon/Mars (abundant sulphur)

10IS Codes, Key Formulae, Values & Exam Quick Revision►

Complete IS Code Reference for Concrete

IS Code	Year	Subject
IS 456	2000	Plain and Reinforced Concrete – Code of Practice (primary design code)
IS 10262	2019	Concrete Mix Proportioning – Guidelines
IS 516	1959	Methods of Tests for Strength of Concrete
IS 1199	Various Pts	Sampling and Analysis of Concrete (fresh concrete tests)
IS 383	2016	Coarse and Fine Aggregates from Natural Sources
IS 2386	Pts I–VIII	Methods of Test for Aggregates for Concrete
IS 4926	2003	Ready-Mixed Concrete – Code of Practice
IS 7320	1974	Specifications for Concrete Slump Test Apparatus
IS 5816	1999	Split Cylinder Tensile Strength of Concrete
IS 9013	1978	Method of Making, Curing and Determining Compressive Strength of Accelerated Cured Concrete Cubes
IS 9103	1999	Admixtures for Concrete
IS 13311 Pt 1	1992	Non-Destructive Testing – Ultrasonic Pulse Velocity Method
IS 13311 Pt 2	1992	Non-Destructive Testing – Rebound Hammer Method
IS 14977	2001	Assessment of Corrosion in Steel Reinforcement by Half-Cell Potential Method
IS 4991	1968	Criteria for Blast Resistant Design of Structures for Explosions Above Ground
IS 2974	Various Pts	Design and Construction of Machine Foundations
IS 1343	2012	Prestressed Concrete – Code of Practice
IS 3812	2013	Pulverised Fuel Ash (Fly Ash)
IS 12089	2013	Granulated Slag for Portland Slag Cement
IS 15388	2003	Silica Fume

All Key Formulae – Concrete Technology

── STRENGTH ──
Target mean strength: f'_ck = f_ck + 1.65σ
Cube vs cylinder: f_ck,cube ≈ 1.25 × f_ck,cylinder
Modulus of elasticity (IS 456): E_c = 5000√f_ck MPa
Modulus of rupture (IS 456): f_r = 0.7√f_ck MPa
Split tensile strength: f_t = 2P / (πLD)
── CREEP ──
Creep coefficient (IS 456): θ = 2.2 (7 days load); 1.6 (28 days); 1.1 (1 year)
Effective modulus: E_eff = E_c / (1 + θ)
── SHRINKAGE ──
Design drying shrinkage (IS 456): ε_cs = 0.0003
Thermal expansion: α = 10–11 × 10⁻⁶/°C
── MIX DESIGN ──
Cement content = Water / (w/c)
Volume of CA + FA = 1 − V_cement − V_water − V_air
── WORKABILITY ──
Compaction Factor = W_partial / W_{full compaction}
── BULKING ──
Bulking = [(V_bulked − V_dry) / V_dry] × 100
── NDT ──
Core acceptance: core strength × 1.25 ≥ 0.85 f_ck
Carbonation: d = K_c × √t

Critical Numerical Values – Master List

Parameter	Value
Min. grade for RCC (mild exposure)	M20 (IS 456)
Max. water fall height (placing)	1.5 m
Max. time from mix to placing (T ≤ 25 °C)	2 hours
Max. time from mix to placing (T > 25 °C)	1.5 hours
Min. concrete temp. at placing	10 °C
Max. concrete temp. at placing	40 °C
Min. curing period (OPC)	7 days
Min. curing period (PPC / blended)	10 days
Standard cube size (IS 516)	150 × 150 × 150 mm
Cube loading rate (IS 516)	14 N/mm²/min
Cube strength / Cylinder strength	≈ 1.25
Slump test cone height	300 mm
Slump test base dia / top dia	200 mm / 100 mm
Slump test tamping rod dia	16 mm, 600 mm long; 25 blows per layer, 3 layers
MSA ≤ (minimum member dimension)	1/4 of min. dimension
MSA ≤ (bar spacing)	3/4 × clear spacing between bars
Mixing time (machine mixer)	Min. 2 minutes (IS 456)
Transit mixer max. revolutions	300 total; 70–100 at mixing speed
Min. w/c for complete hydration (theory)	0.23
Min. w/c for complete hydration (practical)	0.36
Unit weight – PCC	24 kN/m³
Unit weight – RCC	25 kN/m³
Poisson's ratio (concrete)	0.1 – 0.2 (IS 456 uses 0.2)
Thermal coefficient (concrete = steel)	10–12 × 10⁻⁶/°C
Creep coefficient θ (age 28 days)	1.6
Design shrinkage strain (IS 456)	0.0003
UPV — Excellent concrete	> 4.5 km/s
Corrosion threshold (half-cell)	< −350 mV (90 % probability)
Carbonation indicator	Phenolphthalein (pink = alkaline; colourless = carbonated)
SCC slump flow target	550–850 mm
SCC L-box ratio	H₂/H₁ ≥ 0.80
Chloride limit (free, IS 456) for steel corrosion	> 0.4 % by mass of cement
Phenolphthalein — turning pH	~pH 9.5 (pink above; colourless below)
Hand mixing — extra cement	Add 10 % over design
Core acceptance criteria (IS 456)	Core × 1.25 ≥ 0.85 × f_ck
Standard deviation σ (M20–M25)	4.0 MPa
Standard deviation σ (M30–M50)	5.0 MPa

Mnemonics & Memory Aids

Exposure → Minimum Grade (steps of 5 MPa):
Mild=M20, Moderate=M25, Severe=M30, Very Severe=M35, Extreme=M40
"Men Must Suffer Very Extreme"

Workability Tests (increasing sensitivity for stiff mixes):
Slump → Compaction Factor → Vee-Bee → Flow Table
"Slippery Concrete Vibrates Fast"

Cover (mm) by exposure:
Mild=20, Moderate=30, Severe=45, Very Severe=50, Extreme=75
20–30–45–50–75 (not uniform steps; severe jumps to 45)

Standard deviation σ for target mean strength:
M10–M15: 3.5 | M20–M25: 4.0 | M30–M50: 5.0 | M55+: 6.0

Which special concrete for which problem:
No vibration access → SCC
High strength + durability → HPC + silica fume
Slope / tunnel lining → Shotcrete
Pavement / industrial floor → Vacuum dewatered / FRC
Thermal insulation → AAC / LWC
Radiation shielding → Heavyweight concrete (baryte)
Dam (fast construction) → Roller Compacted Concrete (RCC)

GATE / ESE Previous Year Question Pattern

Topic	Exam Frequency	Question Type
Target mean strength formula: f'_ck = f_ck + 1.65σ	Every year (GATE)	NAT
Modulus of elasticity: E_c = 5000√f_ck	Very high	NAT / MCQ
Mix design water content / cement content calculation	High (GATE)	NAT
Workability test identification and ranges	High (SSC JE / ESE)	MCQ
Slump test types (true / shear / collapse)	Moderate	MCQ
Cover requirements by exposure	High (ESE)	MCQ
Durability mechanisms (ASR, carbonation, sulphate)	High (GATE / ESE)	MCQ / Descriptive
Creep coefficient and effective modulus	Moderate (GATE)	NAT
NDT — UPV quality table and rebound hammer	High (SSC JE / ESE)	MCQ
Core test acceptance criteria	Moderate	NAT / MCQ
Special concretes — SCC, FRC, RCC identification	Moderate (ESE)	MCQ
W/C ratio vs strength relationship	High (all exams)	MCQ / NAT
Formwork stripping times	Moderate (SSC JE)	MCQ
Bulking of sand percentage and mechanism	Moderate	MCQ
Half-cell potential corrosion threshold	Moderate (GATE)	MCQ / NAT