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Concrete – Chapter 3

parag21pal@icloud.com027 mins
Chapter 3: Concrete — Civil Engineering

Table of Contents

  1. 3.1 Properties of Cement Concrete
  2. 3.2 Classification of Concrete
  3. 3.3 Manufacturing of Concrete
  4. 3.4 Materials Used in RCC Work
  5. 3.5 Methods of Proportioning Concrete
  6. 3.6 IS Code Method of Concrete Mix Design
  7. 3.7 Durability of Concrete
  8. 3.8 Defects in Concrete
  9. 3.10–3.12 Water-Cement Ratio and Workability
  10. 3.13 Tests for Workability
  11. 3.14–3.18 Strength Tests on Concrete
  12. 3.20 Types of Admixtures

3.1 Properties of Cement Concrete

Cement concrete is a mixture of cement, sand, pebbles or crushed rock and water which, when placed in the skeleton forms followed by curing, becomes hard like a stone.

  1. Composite man-made material; most widely used building material in the construction industry
  2. Mixture of binding material (lime or cement), well graded coarse and fine aggregate, water and sometimes admixtures
  3. Basic requirement: good strength in hardened state; remain “fresh” plastic during transportation, placing, compaction
  4. High compressive strength; free from corrosion; hardens with age; more economical than steel
  5. Binds rapidly with steel — steel reinforcement placed at suitable places → Reinforced Cement Concrete (RCC)
  6. Tendency to shrink; forms hard surface capable of resisting abrasion

3.2 Classification of Concrete

BasisTypes
Cementing material(a) Lime concrete   (b) Gypsum concrete   (c) Cement concrete
GradeM10, M15, M20, M25, M30, M35, M40, M45, M50, M55
Bulk densityExtra light weight (<500 kg/m³) | Light weight (500–1800 kg/m³) | Dense weight (1800–2500 kg/m³) | Super heavy weight (>2500 kg/m³)
Place of casting(a) In-situ concrete — placed in position at the site   (b) Precast concrete — used for making prefabricated units in a factory

3.3 Manufacturing of Concrete

Figure 3.1 — Manufacturing of Concrete: Six Steps (A to F)
Six steps of concrete manufacturing: Batching Mixing Transportation Placing Compaction Curing Six sequential steps A through F shown as column headers with detail boxes below: A Batching volume or weight method, B Mixing hand or machine 20 revolutions, C Transportation within initial setting time pans buggies chutes pumps transit mixer, D Placing maximum free fall 1.5m, E Compaction internal surface form vibrators 5 percent voids reduces strength 30 percent, F Curing minimum 7 days at 90 percent humidity IS 456. A Batching B Mixing C Transport D Placing E Compaction F Curing Batching Aggregates, cement, water measured ±3% Methods: • Volume batching • Weight batching Weight batching recommended for important works Admixtures ±5% of batch quantity Mixing Homogeneous, uniform colour IS-456: ~2 min Mixer types: 1. Tilting type 2. Non-tilting 3. Batching plant 20 revolutions = sufficient mixing Hand mixing 2 min max Transportation Must be within initial setting time 30 min OPC Methods: 1. Pans (small jobs) 2. Power buggies (up to 24 km/h) 3. Chutes 4. Concrete pumps (up to 400 m) 5. Transit mixer 6. Belt conveyer Placing Delayed placing can gain strength Max free fall: 1.5 m (IS 456) Dry mix delay: 0.5–1 hour Wet mix delay: several hours Segregation must not take place Compaction Removes entrapped air; uniform dense mass formed Vibrator types: 1. Internal/needle (4000–12000 rpm) 2. Surface vibrators 3. Form vibrators 4. Vibrating tables 5% voids → 30% ↓ strength 10% voids → 60% ↓ Curing (IS 456) Prevents moisture loss; accelerates strength gain Min. duration: 7 days — OPC 10 days — mineral admixtures/blended At 90% RH Temperature: 24–30°C Moist cured 7d = 50% stronger than air-dried concrete
Fig. 3.1 — Six steps in manufacturing of concrete (A→F). Key: max free fall 1.5 m (IS 456); mixing time ~2 min (IS 456); transport within initial setting time (30 min OPC); compaction 5% voids = 30% strength reduction; curing minimum 7 days (OPC) at 90% humidity.

3.3 Batching Methods

  • Volume Batching: Amount of each solid ingredient measured by loose volume (not compacted). Correction for bulking of sand needed if volume batching adopted.
  • Weight Batching: Cement always measured by weight, irrespective of method. Water measured in kg or litres (density of water = 1 gm/cm³). Volume of 1 bag of cement = 0.035 m³ (35 litres). Recommended for important works.

3.4 Materials Used in RCC Work

  • (1) Cement   (2) Aggregates (both coarse and fine)   (3) Steel   (4) Water
  • Water: clean and free from harmful impurities (oil, alkali, acid etc.). Water fit for drinking should be used.

3.5 Different Methods of Proportioning Concrete

1. Nominal Mix

  • No rigid control on strength; widely used for small magnitude works
  • Nominal mix: cement : fine aggregate : coarse aggregate = 1 : n : 2n for normal work
ProportionMax. Agg. SizeNature of Work
1:1:212–20 mmHeavily loaded RCC columns and RCC arches of long span
1:2:212–20 mmSmall precast members, watertight constructions, heavily stressed members
1:1.5:320 mmWater retaining structures, piles, precast products
1:2:3 or 1:2:420 mmWater tanks, concrete deposited under water, bridge construction and sewers
1:2.5:3.525 mmFootpaths and road work
1:2:440 mmAll general RCC works in buildings (stair, beam, column, slab, lintel)
1:3:650 mmMass concrete work in culverts, retaining walls etc.
1:4:8 or 1:5:10 or 1:6:1260 mmMass concrete work for heavy walls, foundation footings
Nominal mixes approximate grades: M5 = 1:5:10 | M7.5 = 1:4:8 | M10 = 1:3:6 | M15 = 1:2:4 | M20 = 1:1½:3 | M25 = 1:1:2

2. Design Mix

  • Proportions of constituents decided by established relationships based on experiments
  • For RCC work: max size of aggregates limited to 20–25 mm
  • Rounded aggregates require least water-cement ratio for given workability
  • Coarse aggregates > 4.75 mm; Fine aggregates < 4.75 mm

3.6 IS Code Method of Concrete Mix Design (IS 10262 : 2009)

Figure 3.2 — IS Code Mix Design: Seven-Step Procedure
IS Code concrete mix design procedure IS 10262 2009 seven steps Seven sequential steps: Step 1 target mean strength fck prime equals fck plus 1.65S, Step 2 selection of water cement ratio from IS 456 table, Step 3 selection of water content from IS 10262 table 2, Step 4 calculation of cementations material content C equals water divided by w/c ratio, Step 5 estimate volume of coarse aggregates from IS 10262 table 3, Step 6 estimation of mass of fine and coarse aggregates using absolute volume method, Step 7 correction for actual site conditions moisture content adjustment. Step 1: Target Mean Strength and Standard Deviation f’ck = fck + k·S where k = 1.65 (IS code) → f’ck = fck + 1.65S  |  S = Standard Deviation Step 2: Selection of Water-Cement Ratio Cl. 4.1 IS 10262:2009 — Refer Table 5 IS 456:2000 for free w/c ratio for different grades and exposure Step 3: Selection of Water Content Cl. 4.2 — Table 2 IS 10262:2009 gives max water content for angular CA, 25–50 mm slump. +3% per 25 mm extra slump. Step 4: Calculation of Cementations Material Content C = Water content used / Water-cement ratio  |  Cl 4.3 IS 10262:2009 — Check vs minimum cement content for durability Step 5: Estimate of Volume of Coarse Aggregates per Unit Volume of Concrete Cl. 4.4 — Table 3 IS 10262:2009 gives volume of CA per unit volume for different zones of fine aggregate (sand) Step 6: Estimation of Fine and Coarse Aggregate Masses (Absolute Volume Method) VA = 1 − [C/Sc + W/1 + F/Sf + P/Sp] × 1/1000 − v  |  Mass of CA = p·VA·Sca×1000  |  Mass of FA = (1−p)·VA·Sfa×1000 Step 7: Correction for Actual Site Conditions If aggregates have free moisture → reduce water by that amount; increase aggregate mass by same amount If aggregates absorb moisture → increase water content; decrease mass of aggregate by same extent
Fig. 3.2 — IS Code method of concrete mix design (IS 10262:2009). f’ck = fck + 1.65S (k = 1.65 per IS code). Steps: target strength → w/c ratio → water content → cement content → coarse aggregate volume → aggregate masses → site corrections.
ExposureMax Free w/c (Plain)Min Grade (Plain)Max Free w/c (RCC)Min Grade (RCC)
Mild0.6—0.55M20
Moderate0.6M150.5M25
Severe0.5M200.45M30
Very Severe0.45M200.45M35
Extreme0.40M250.40M40

3.7 Durability of Concrete

  • A durable concrete performs satisfactorily under anticipated exposure conditions for its stipulated life

1. Permeability

  • Ingress of water leads concrete susceptible to chemical attack, forest action, rusting of steel
  • Reduce by: (i) High grade concrete (ii) Well-graded dense aggregate (iii) Low w/c ratio with adequate cement and effective curing (iv) Appropriate admixtures (v) Maximum compaction

2. Frost Action

  • Below 0°C, absorbed water expands → ice builds up in large pores → expansion causes disintegration

3. Sulphate Attack

  • Sulphates react with C₃A → form calcium sulphoaluminate (ettringite) → expands → disrupts concrete
  • Magnesium sulphate has the most severe disruptive action
  • Reduce by: blast furnace slag cement, sulphate resisting cement, superphosphated cement; reduce permeability

4. Organic Acids

  • Acetic acid, lactic acid, and butyric acid severely attack concrete

5. Sugar (Retarder)

  • Retarding agent; gradually corrodes concrete if added in excess as admixture

3.8 Defects in Concrete

DefectCauseEffect / Description
CracksExcess water; early loss of water; alkali-aggregate reaction; freeze and thawInherent in concrete; cannot be completely prevented but can be minimized
EfflorescencePoorly washed aggregate; salty water used in mixing; salts leached to surface by rain waterAppearance of fluffy white patches on the surface of concrete members
SegregationExcess water; dropping from heights; badly designed mixes; over-vibration; excess water-pumping; belt conveyor finishing; extra floating and tampingSeparation of coarse from fine aggregate, paste from coarse aggregate, or water from mix. Reduces to small sizes aggregates and uses air-entraining agents, dispersing agents, pozzolana
BleedingExcessive vibrations to achieve full compactionUpflow of mixing water from freshly placed concrete to the surface. Reduce by well-graded aggregates, pozzolana, breaking continuous water channel, air-entraining agents, finer cement, rich mix
CreepConstant sustained loadContinued deformation with time under constant load (plastic flow or time yield). Rate of creep decreases with time; creep strains at 5 years taken as terminal values

3.10 Water-Cement Ratio

  • Water in concrete performs two functions: (1) Chemical action with cement → setting and hardening (2) Lubricates aggregates → makes concrete workable
  • Abram’s law: strength of workable concrete is only dependent on water-cement ratio
  • Higher w/c ratio → lower strength; lower w/c ratio → higher strength
Note (IS 456): Water-cement ratio and degree of compaction are the two major parameters determining strength of concrete. Higher w/c ratio → lower strength. Strength reduced with lower degree of compaction.

3.11 Workability of Concrete

  • Workability = amount of work required to produce full compaction
  • Adding more water → low strength and poor durability
  • Workability affected by: maximum size of coarse aggregate; mainly by water content, w/c ratio and aggregate-cement ratio

3.12 Factors Affecting Workability

  • (a) Water content   (b) Mix proportions   (c) Size of aggregates   (d) Shape of aggregates   (e) Surface texture   (f) Grading of aggregates   (g) Use of admixtures
  • Bigger aggregate size → less surface area → less water needed → less matrix needed → better workability for given w/c ratio
  • Well graded aggregate → less void content → higher workability
  • Rounded aggregates → better workability; angular/flaky/elongated → harsh concrete
  • Strength order of aggregate types: Crushed > Cubical > Rounded > Flaky/Irregular

3.13 Tests for Workability

Figure 3.3 — Slump Test: Cone Dimensions, Types of Slump & Compaction Factor Test
Slump test and compaction factor test diagrams with dimensions and recommended slump values table Left shows Abram slump cone with top diameter 10 cm bottom diameter 20 cm height 30 cm and tamping rod 16mm diameter 60cm long. Three types of slump shown: True slump even subsidence, Shear slump one side falls poor cohesion, Collapse slump concrete completely collapses too wet. Right shows compaction factor apparatus with upper hopper 25.4cm x 27.9cm and lower hopper 22.9cm x 20.3cm leading to cylinder 15.2cm x 30.5cm. Compaction factor CF equals weight of partially compacted concrete divided by weight of fully compacted concrete. Slump Test — Abram’s Cone Top ⌀: 10 cm Bottom ⌀: 20 cm Height 30 cm Rod: ⌀16 mm, 600 mm long 25 strokes/layer · 3 layers Types of Slump True Slump Even subsidence Shear Slump One side falls away Collapse Concrete collapses Compaction Factor Test Upper Hopper 25.4 cm × 27.9 cm ← Trap door Lower Hopper 22.9 cm × 20.3 cm Cylinder ⌀15.2 cm H: 30.5 cm CF = Wt. partial / Wt. fully compacted Recommended Slumps (mm) Concrete type Slump Road construction20–40 mm Tops of curbs, parapets, slabs (horizontal)40–50 mm Normal RCC work80–150 mm Mass concrete25–50 mm
Fig. 3.3 — Slump test (left): Abram’s cone — top ⌀10 cm, bottom ⌀20 cm, height 30 cm; rod ⌀16 mm, 600 mm long; 25 strokes/layer, 3 layers. Three types of slump: True (good), Shear (poor cohesion), Collapse (too wet). Not suitable for very wet or very dry mixes. Compaction Factor Test (right): CF = weight partially compacted / weight fully compacted. CF 0.95 = high; 0.92 = medium; 0.85 = low; 0.75 = very low workability.

3.13.4 Vee-Bee Consistometer Test

  • Preferred for stiff concrete mixes with very low workability; this is a good laboratory test to measure indirectly the workability of concrete
  • Consists of a vibrating table, a metal pot and a standard iron rod
  • Time for concrete to change from slump shape to cylindrical shape = Vee Bee Degree
  • Vee-Bee 3–5 sec → stiff plastic, medium workability; 10–15 sec → stiff, low workability; 18–30 sec → very stiff, very low workability

3.13.5 Flow Test

  • Laboratory test; standard mass of concrete subjected to jolting
Flow % = [(Spread diameter in cm − 25) / 25] × 100    (value ranges 0–150%)

3.14–3.18 Strength Tests on Concrete

3.14 Strength Test on Concrete

  • Compressive strength much greater than tensile strength; tensile strength ≈ 15% of compressive strength
  • Grade of concrete designated by letter M and a number (e.g. M20 = 20 MPa characteristic compressive strength of 150 mm cubes at 28 days)
  • Tensile and bending strength = 10 and 15% of compressive strength; Shear strength ≈ 20% of uniaxial compressive strength
  • Characteristic compressive strength = strength below which not more than 5% of test results are expected to fall

3.15 Compressive Strength Test

  • Test specimens: 150 × 150 × 150 mm cubes or cylinders ⌀150 mm × 300 mm height
  • Layers of 50 mm; tamped 35 times with 16 mm bar or vibrated
  • Stored at 27° ± 3°C, 90% humidity for 24 ± 1/2 hour; then immersed in water until testing
  • 7-day strength ≥ 2/3 of 28-day strength
  • Load applied at 0 to 14 N/mm²/min until specimen crushed; average of three values (individual variation ≤ ±15%)
Note: Cube strength = 1.25 × cylinder strength. IS code also recommends 100 mm cubes for aggregate size not exceeding 20 mm.

3.16 Flexural Tensile Strength (Modulus of Rupture Test)

  • Indirect test for tensile strength of concrete; mould 150 × 150 × 700 mm; tamping bar 2 kg, 400 mm long
  • Specimen on two 38 mm rollers at 600 mm c/c; load through two similar rollers at third points (200 mm c/c)
  • Load applied at 0.7 N/mm²/minute until failure
Modulus of Rupture = pl/bd²   (a ≥ 200 mm)
or = 3pa/bd²   (200 mm > a > 170 mm)
where a = distance between fracture line and nearest support, b and d = width and depth, l = span, p = max load

3.17 Split Tensile Strength Test (IS 5816-1970)

  • Standard test cylinder 300 mm × ⌀150 mm placed horizontally; compression load applied diametrically
  • Uniform tensile stress acts over two-third of loaded diameter
σ = 2P / πDL    (P = applied load, D = diameter, L = length of cylinder)

3.18 Non-Destructive Test (NDT)

TestPrincipleResult Interpretation
Ultrasonic Pulse VelocityVelocity of sound in solid = f(√E/ρ). Higher velocity → greater strength. Only dynamic test showing potential in-situ concrete strength.Excellent ≥4.5 km/s | Good 3.5–4.5 | Medium 3.0–3.5 | Doubtful <3 km/s
Rebound Hammer (Schmidt Hammer)Rebound of elastic mass depends on surface hardness. Plunger pressed against concrete; spring-controlled mass rebounds; rebound number read from graduated scale.Suitable for 20–60 MPa concrete; surface layer up to 30 mm depth; compressive strength read from graph on hammer body

3.20 Types of Admixtures

Admixtures categorized by effect: (a) Plasticizers (water-reducing agents)   (b) Superplasticizers (high range water reducers)   (c) Air entrainers   (d) Accelerators   (e) Retarders   (f) Others

Figure 3.4 — Types of Admixtures with Key Properties
Five types of concrete admixtures with key properties Five column cards showing admixture types. Plasticizers water reducers 0.1 to 0.4 percent by cement weight reduce water by 10 percent improve workability. Superplasticizers high range water reducers 20 to 40 percent water reduction enables w/c as low as 0.25 for strength 100 MPa add just prior to placing. Air entrainers 0.1 to 0.3 percent minute bubbles 10-25 microns resist freezing thawing 1 percent air reduces strength 5 percent. Accelerators calcium chloride not for reinforced concrete rapid setting early strength cold weather. Retarders calcium sulphate most common slow setting hot weather concreting long distance transport. Types of Admixtures Plasticizers Water reducers Superplasticizers High range WR Air Entrainers Minute bubbles Accelerators Fast set Retarders Slow set 0.1–0.4% of cement wt Improve workability for given w/c ratio ~10% water reduction Types: ligno- sulphonic acids, hydroxy carboxylic acids, carbohydrates Dose: 200–450 ml per 100 kg cement Improves pump ability of concrete 20–40% water reduction Chemically distinct from plasticizers; more marked action w/c as low as 0.25 → strength 100 MPa Types: SMF, SNF, MLS Add just prior to placing (rapid loss of workability) 0.1–0.3% OPC clinker Minute bubbles 10–25 µm distributed uniformly in concrete 1% air → 5% strength reduction Frost/thaw resistance Reduces segregation and bleeding Types: natural wood resins, animal/veg fats, wetting agents Speed up chemical reaction of cement Main: Calcium Chloride (CaCl₂) NOT for RCC or water-retaining Uses: rapid turn- over of formwork, shaft sinking, cold weather concreting Others: NaCl, Na₂SO₄, NaOH, K₂SO₄, KOH Slow chemical reaction; longer setting time Most common: calcium sulphate Others: hydroxy- lated carboxylic acids, lignins, sugar, cellulose Uses: hot weather concreting; long distance transport cold joints prevention; pumped Sugar 0.05–0.10% = little retardation 0.2% = retarded up to 72+ hours
Fig. 3.4 — Five types of admixtures (from source). Plasticizers: 0.1–0.4% by cement weight, ~10% water reduction. Superplasticizers: 20–40% water reduction, w/c as low as 0.25, add just prior to placing. Air Entrainers: 1% air → 5% strength reduction, frost-thaw resistance. Accelerators: CaCl₂ — NOT for RCC or water-retaining structures. Retarders: calcium sulphate most common, hot weather concreting.
All chloride-based accelerators promote corrosion of reinforcing steel and should NOT be used in: (i) reinforced concrete, (ii) water-retaining structures. Accelerators work more effectively at lower ambient temperatures.

Chapter 3: Concrete — Civil Engineering · Construction Materials
All technical data as per IS 456:2000, IS 10262:2009 and IS 5816:1970

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