Material Testing Lab (CE-506) - Unit 2 Short Notes

A. PREREQUISITE & FOUNDATIONAL CONCEPTS

• Review of Key Properties:

  • Strength: Capacity to resist deformation/failure (e.g., compressive, tensile).

  • Stiffness (Modulus): Resistance to elastic deformation (slope of initial stress-strain curve).

  • Ductility: Ability to undergo plastic deformation before fracture (measured by % elongation/reduction in area).

  • Toughness: Energy absorbed before fracture (area under entire stress-strain curve).

  • Durability: Resistance to weathering, chemical attack, abrasion.

• Stress-Strain Curves: Distinguish ductile (yield point, large strain) vs. brittle (no yield, sudden failure) materials.

• Standardization: Tests follow IS (BIS), ASTM, BS codes to ensure uniformity, reliability, and comparability of results.

• Accuracy & Precision:

  • Accuracy: Closeness to true value.

  • Precision: Closeness of repeated measurements.

  • Calibration: Periodic verification of equipment against standards to maintain accuracy.

  • Error Analysis: Identify systematic errors (faulty calibration) vs. random errors (operator variation).

[!TIP]

Exam Focus: Be ready to define all 5 properties and sketch generic stress-strain curves for mild steel (ductile) and concrete (brittle). Always mention the relevant IS code when describing any test.

B. CONCRETE TESTING

1. Fresh Concrete Tests

a) Slump Test (IS 1199)

• Principle: Measures consistency/workability by the subsidence (slump) of a conical mould.

• Apparatus: Slump cone (top dia 10 cm, bottom dia 20 cm, height 30 cm), tamping rod, scale.

• Procedure: Fill cone in 4 layers, each tamped 25 times. Lift cone vertically; measure vertical subsidence.

• Interpretation:

  | Slump (mm) | Workability | Application | | :--- | :--- | :--- | | 0-25 | Very low | Mass concrete, roads | | 25-75 | Low | General RCC | | 75-150 | Medium | Columns, beams | | >150 | High | SCC, heavily reinforced |

  • Zero Slump: Very dry mix.

  • True Slump: Uniform subsidence (most common).

  • Shear Slump: One side slumps (incomplete test).

  • Collapse Slump: Mix collapses (very high workability; use flow table).

• Limitations: Not suitable for very dry or very wet concrete.

b) Compacting Factor Test (IS 1199)

• Principle: Measures the degree of compaction achieved by a standard weight falling from a standard height. Inverse measure of workability.

• Apparatus: Two conical hoppers & cylinder.

• Procedure: Concrete allowed to fall through hoppers into cylinder; top trimmed & weighed. Compacting Factor = (Weight of compacted concrete) / (Weight of fully compacted concrete).

• Significance: More precise for low-workability mixes. Values: 0.7-0.85 (low workability) to 0.85-0.95 (high workability).

c) Vee-Bee Consistometer Test

• Principle: Measures time (in seconds) for a standard mass to consolidate a concrete sample in a cylindrical container. For very dry mixes where slump is zero.

• Procedure: Concrete placed in cylinder; vibrating table operated; time to achieve complete surface consolidation recorded.

d) Flow Table Test (IS 1199)

• Principle: For highly workable/SCC concrete. Measures spread diameter after jolting.

• Procedure: Concrete placed on central disc; table lifted & dropped 25 times; spread diameter measured.

• Result: Flow % = (Spread dia - 150) / 150 × 100. SCC typically has flow > 60%.

e) Air Content (Pressure Method - IS 1199)

• Principle: Boyle's Law. Air in concrete compressed; pressure drop measured indicates air content.

• Apparatus: Air meter (pressure gauge, water-filled bowl).

• Procedure: Fill bowl with concrete; apply pressure; read % air directly from gauge.

• Significance: Entrained air improves freeze-thaw durability but reduces strength (~1% air ≈ 5% strength loss).

f) Setting Time (Vicat Apparatus - IS 4031)

• Initial Setting Time: Time when needle (1 mm dia) penetrates ≤ 25 mm.

• Final Setting Time: Time when needle (5 mm dia) with attachment fails to penetrate ≤ 0.5 mm.

• Standard: Initial: ≥ 30 min (for OPC). Final: ≤ 10 hrs (for OPC).

2. Hardened Concrete Tests

a) Compressive Strength (Cube Test - IS 516)

• Specimen: 150 mm cube (standard). Cast in steel moulds, cure in water (28 days).

• Testing: Load applied centrally on opposite faces at 14 N/mm²/min (≈ 0.25 MPa/s) until failure.

• Calculation:

$$f_{ck} = \frac{P_{max}}{A}$$

Where $$\displaystyle P_{max} $$ = max load (N), $A$ = cross-sectional area (mm²).  

\boxed{f_{ck} = \frac{P}{150 \times 150} \text{ MPa}}

• Cube vs. Cylinder Strength: Cube strength ≈ 1.25 × Cylinder strength (for same concrete). Conversion factor depends on concrete grade.

• Factors Affecting Strength:

  • Size/Shape: Larger specimens show lower strength (size effect).

  • Curing: Inadequate curing reduces strength significantly.

  • Loading Rate: Too slow/too fast affects measured strength.

  • Capping: Uneven surfaces must be capped with sulfur mortar or cement paste.

b) Tensile Strength - Splitting Tensile Test (IS 516)

• Principle: Indirect tensile test. Load applied along the length of a cylindrical specimen (150 mm dia × 300 mm long) generates tensile stress along the diametral plane.

• Setup: Specimen placed horizontally; load applied via narrow strips.

• Calculation:

$$f_{ct} = \frac{2P}{\pi LD}$$

Where $P$ = load at failure (N), $L$ = length (mm), $D$ = diameter (mm).  

\boxed{f_{ct} = \frac{2P}{\pi \times 300 \times 150} \text{ MPa}}

• Advantage: Simple specimen preparation; no gripping issues like direct tension.

c) Flexural Strength (Modulus of Rupture - IS 516)

• Specimen: 150 mm × 150 mm × 700 mm beam, tested as simply supported with two-point loading (third-point loading).

• Calculation (Two-point loading):

$$f_{r} = \frac{PL}{bd^2}$$

Where $P$ = max load (N), $L$ = span (mm), $b$ = width (mm), $d$ = depth (mm).  

\boxed{f_{r} = \frac{P \times L}{b \times d^2}}

• Note: Flexural strength ≈ 10-15% of compressive strength for normal concrete.

d) Non-Destructive Testing (NDT) Overview

• Rebound Hammer (Schmidt Hammer - IS 13311):

  • Principle: Measures surface hardness via rebound of a spring-driven mass.

  • Reading: Rebound number (R). Higher R → higher surface strength.

  • Use: Rapid comparative assessment, uniformity check.

  • Limitations: Affected by surface condition, moisture, carbonation, orientation. Correlation with compressive strength is empirical and must be established for each mix.

• Ultrasonic Pulse Velocity (UPV - IS 13311):

  • Principle: Measures time taken by an ultrasonic pulse to travel through concrete. Velocity $$\displaystyle v = L/t $$.

  • Interpretation:

    | Velocity (km/s) | Concrete Quality | | :--- | :--- | | >4.5 | Excellent | | 3.5-4.5 | Good | | 3.0-3.5 | Medium | | <3.0 | Poor (cracks, voids) |

  • Use: Detect internal flaws, uniformity, estimate strength (empirical correlation).

• SonReb Method: Combined use of Rebound number and UPV for better strength estimation accuracy.

C. METAL (STEEL) TESTING

1. Tensile Test (IS 1608 / IS 2062)

• Specimen: Prepared from bar, with gauge length (usually 5×dia or 100 mm) and cross-section (circular/rectangular).

• Machine: Universal Testing Machine (UTM) with extensometer/yield detector.

• Procedure: Load applied axially at specified strain rate. Record load & elongation.

• Stress-Strain Curve: Plot $$\displaystyle \sigma = \frac{P}{A_0} $$ vs. $$\displaystyle \epsilon = \frac{\delta}{L_0} $$.

• Key Parameters:

  • Yield Strength ($$\displaystyle f_y $$): For mild steel, clear yield point. For high-yield steel, use 0.2% offset method: draw line parallel to elastic portion from $$\displaystyle \epsilon = 0.002 $$; intersection = yield strength.

  • Ultimate Tensile Strength (UTS): $$\displaystyle \sigma_u = \frac{P_{max}}{A_0} $$.

  • Percentage Elongation: $$\displaystyle \frac{\delta_f - \delta_y}{L_g} \times 100\% $$ (after fracture, $$\displaystyle L_g $$ = gauge length).

  • Percentage Reduction in Area (RA): $$\displaystyle \frac{A_0 - A_f}{A_0} \times 100\% $$ (at fracture zone).

• Failure Mode: Ductile (necking, cup-cone) vs. Brittle (flat fracture, little elongation).

[!TIP]

Common Pitfall: Using original cross-section ($$\displaystyle A_0 $$) for UTS calculation even after necking. Always use original area for all stress calculations. RA uses final area ($$\displaystyle A_f $$).

2. Bend Test / Re-bend Test (IS 1599)

• Purpose: Assess ductility and soundness (absence of internal cracks/lamination) of steel bars, especially for reinforcement.

• Procedure (Bend Test): Bar bent around a mandrel of specified diameter (related to bar dia) at a specified angle (usually 180° or 90°). No cracks should appear on tension side.

• Re-bend Test: Bar bent to 45° after initial 180° bend; held for 1 min, then straightened. Bent again to 180° in opposite direction. No cracks should appear.

• Acceptance: Based on bar grade and diameter (refer IS 1599 table).

3. Hardness Tests (Brief)

• Brinell: Large indenter (10 mm ball), high load. Measures HB. For coarse structures, castings.

• Rockwell: Minor then major load; depth of indentation gives HR scale (A, B, C etc.). Quick, direct reading.

• Vickers: Diamond pyramid indenter, loads from 10g to 100kg. Measures HV. For thin materials, surface layers.

• Relation: Hardness ≈ 1/3 of UTS (MPa) for steels.

D. BITUMINOUS MATERIALS TESTING

E. SOIL MECHANICS TESTS (If Included)

1. Classification & Index Properties

• Grain Size Analysis (IS 2720):

  • Sieve Analysis: For coarse-grained soils (>75 µm). Dry/wet sieving. Plot % finer vs. size (log scale) → Gradation Curve.

  • Hydrometer Analysis: For fine-grained soils (<75 µm). Uses Stokes' Law. Plot % finer vs. time → ** sedimentation curve**.

• Atterberg's Limits (IS 2720):

  • Liquid Limit (LL): Water content at which soil changes from plastic to liquid state (Casagrande cup, 25 blows).

  • Plastic Limit (PL): Water content at which soil just ceases to be plastic (thread method).

  • Plasticity Index (PI): \boxed{PI = LL - PL}. Indicates clay content & behavior.

2. Compaction & Strength

• Standard/Modified Proctor (IS 2720):

  • Principle: Determine Optimum Moisture Content (OMC) & Maximum Dry Density (MDD) for a given compaction effort.

  • Procedure: Soil mixed with varying water contents; compacted in mould (3/5 layers, 25/56 blows per layer); dry density calculated.

  • Result: Plot Dry Density vs. Moisture Content; peak = MDD at OMC.

• California Bearing Ratio (CBR - IS 2720):

  • Principle: Penetration resistance of soil relative to standard crushed stone. Measured as %.

  • Procedure: Soaked/unsoaked specimen; piston (50 mm dia) penetrates at 1.25 mm/min; load recorded at 2.5 mm & 5 mm penetration.

  • Calculation:

$$CBR (\%) = \frac{\text{Soil load at 2.5/5 mm}}{\text{Standard load at 2.5/5 mm}} \times 100$$

    Standard load at 2.5 mm = 1370 kg; at 5 mm = 2055 kg.

*   **Use:** Pavement design (flexible). Higher CBR → better subgrade.

F. CROSS-CUTTING THEMES & ADVANCED TOPICS

• Quality Control (QC) & Assurance (QA):

  • Sampling: Representative sampling (IS 2430 for aggregates/concrete).

  • Specimen Preparation: Strict adherence to mould dimensions, compaction, curing (temp, humidity).

  • Test Report: Must include: material source, test method (IS code), specimen details, results, observations, signature.

• Calibration:

  • UTM: Using calibrated proving ring/load cell.

  • Measuring Devices: Gauges, scales, thermometers against standards.

  • Frequency: As per lab manual/IS code (often monthly/quarterly).

• Data Representation & Analysis:

  • Graphs: Stress-strain, load-deflection, moisture-density.

  • Statistics: For multiple specimens:

    Mean $$\displaystyle \bar{x} = \frac{\sum x_i}{n} $$

    Standard Deviation $$\displaystyle \sigma = \sqrt{\frac{\sum (x_i - \bar{x})^2}{n-1}} $$

    Coefficient of Variation (COV) = $$\displaystyle \frac{\sigma}{\bar{x}} \times 100\% $$ (measure of dispersion).

• Comparison with Standards: Results must satisfy IS code limits for the material grade (e.g., M20 concrete: $$\displaystyle f_{ck} \geq 20 $$ MPa at 28 days).

• Common Sources of Error & Troubleshooting:

  • Concrete Cube: Improper capping, uneven loading, side friction, incorrect loading rate.

  • Steel Tensile: Misalignment, slippage in grips, incorrect extensometer mounting.

  • Slump Test: Non-uniform filling, improper lifting, wet cone walls.

  • General: Temperature variations, equipment not calibrated, poor specimen handling.

[!TIP]

Exam-Winning Strategy: In long answers, always structure as: 1. Objective/IS Code, 2. Apparatus, 3. Procedure (stepwise), 4. Calculation Formula, 5. Result Interpretation, 6. Precautions/Errors. For NDT, emphasize empirical nature and limitations.