Method of Tests for Strength of Concrete

IS 516: Introduction and Scope — Key Points & References

• Purpose:
  Provides methods of testing concrete strength (compressive, flexural, modulus of elasticity) to ensure quality control in concrete construction.

• Scope:
  Covers technical provisions for concrete testing; does not cover contract provisions.

• Units:
  All dimensions and values are in metric units (SI system).

• Rounding Off:
  Follow IS 2-1949 for rounding numerical test results, maintaining significant figures consistent with the standard.

• Referenced Standards:
  Testing relies on related IS codes, including:

  IS Code	Title
  IS 1199-1959	Methods of Sampling and Analysis of Concrete
  IS 269-1958	Specification for Ordinary, Rapid-Hardening, and Low Heat Portland Cement
  IS 383-1952	Specification for Coarse and Fine Aggregates
  IS 456-1957	Code of Practice for Plain and Reinforced Concrete
  IS 10086-1982	Specification for Beam Moulds and Tamping Bars

• Testing Equipment:

  • Beam moulds per IS 10086:1982
  • Tamping bars per IS 10086:1982 (6.1b)

Summary Table: Key References in IS 516

This standard ensures uniformity in concrete testing methods, facilitating reliable strength assessment and quality control.

IS 516: Scope & Key Specifications

• Scope (Clause 0.7):
  Covers technical provisions for testing concrete only; does not include full contract provisions.

• Units (Clause 0.5):
  All dimensions and values are in metric units to ensure uniformity.

• Rounding Off (Clause 0.6):
  Final test values must be rounded per IS:2-1949 rules, retaining the same significant figures as specified.

• Specimen Size (Clause 9.2):

  • Standard test specimen: Cylinder 15 cm diameter × 30 cm length
  • Alternate: Other cylinders or square prisms allowed if height/diameter or height/width ≥ 2

• Apparatus (Clause 10.3 & 10.3.6):

  • Includes fixed clamps/supports with max width = 1/6th of specimen length
  • Metallic and earthed support if using variable air-gap exciter and pick-up units

Summary Table: Specimen Dimensions

flowchart LR
  A[Concrete Testing Scope] --> B[Specimen Size]
  B --> C[Standard: 15x30 cm Cylinder]
  B --> D[Alternate: Other sizes with H/W ≥ 2]
  A --> E[Apparatus]
  E --> F[Fixed Clamp: Width ≤ 1/6 Specimen Length]
  E --> G[Metallic & Earthed Support for Variable Air-gap Units]

This concise summary aligns with IS 516 clauses for scope and specimen preparation.

IS 516: Materials Preparation and Sampling - Key Points

1. Sampling of Materials (Clauses 2.2 & 7.2)

• Cement Sampling:

  • Take small portions from multiple bags on site.
  • Mix thoroughly dry by hand or mixer for uniformity.
  • Store in dry, airtight metal containers.

• Aggregate Sampling:

  • Take from larger lots by quartering method.
  • Separate into fine and coarse fractions using IS Sieve 480.
  • Air-dry aggregates before testing.
  • Recombine fractions to achieve desired grading.

2. Preparation of Materials (Clause 2.3)

• Bring all materials to room temperature (27°C ± 3°C) before testing.
• Ensure uniform blending to avoid foreign matter intrusion.

3. Mixing & Proportions (Clause 7.2)

• Prepare, weigh, and mix materials exactly as for laboratory compression test specimens.

Summary Table: Sampling & Preparation

flowchart LR
    A[Material Sampling] --> B{Cement}
    A --> C{Aggregates}
    B --> D[Mix dry uniformly]
    D --> E[Store airtight & dry]
    C --> F[Quartering method]
    F --> G[Separate fine & coarse by IS Sieve 480]
    G --> H[Air-dry aggregates]
    H --> I[Recombine for grading]

This ensures consistent, representative samples and reliable test results per IS 516.

IS 516: Making and Curing Compression Test Specimens in the Field

Key Specifications (Clauses 3.3 & 2.12)

• Initial curing (first 24 hrs):

  • Store specimens free from vibration.
  • Cover with damp matting or sacks.
  • Temperature range: 22°C to 32°C.
  • Duration: 24 ± 0.5 hours from water addition.

• Post 24 hrs:

  • Remove specimens from molds.
  • Mark for identification.
  • Store in clean water at 24°C to 30°C until transport.
  • Transport specimens well packed in damp sand or sacks to keep moist.
  • On arrival at lab, store in water at 27°C ± 2°C until testing.

• Water maintenance:

  • Renew curing water every 7 days.
  • Specimens must never dry out before testing.

Important Notes

• Keep daily max/min temperature records during field and lab curing.
• Specimens must be moist and vibration-free throughout curing.

Summary Table of Curing Conditions

flowchart TD
    A[Mixing Concrete] --> B[Place in Moulds]
    B --> C[Store under damp matting, vibration-free]
    C -->|24 ± 0.5 hrs, 22-32°C| D[Remove from moulds]
    D --> E[Mark specimens]
    E --> F[Store in clean water, 24-30°C]
    F --> G[Transport in damp sand/sacks]
    G --> H[Lab storage in water, 27 ± 2°C]
    H --> I[Testing]
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IS 516: Securing and Preparing Hardened Concrete Specimens for Compression Test

Key Specifications from IS 516:

• Curing (Clause 3.3):

  • Store specimens under damp matting/sacks for 24 ± 1 hour after mixing.
  • Temperature range: 22°C to 32°C.
  • After 24 hours, demould and store in clean water at 24°C to 30°C.
  • Transport specimens in damp conditions (damp sand/sacks).
  • At lab, store in water at 27°C ± 2°C until testing.
  • Maintain daily max/min temperature records.

• Securing & Preparing Specimens (Clause 4.1):

  • Specimens must be taken from hardened concrete after placing.
  • Ensure proper identification and handling to avoid damage.

• Test Report Requirements (Clause 5.6.2): Include:

  • Identification mark
  • Date and age of specimen
  • Curing conditions
  • Weight and dimensions
  • Cross-sectional area
  • Maximum load applied
  • Calculated compressive strength
  • Fracture appearance/type

Important Formula:

[ \text{Compressive Strength} (f_c) = \frac{P}{A} ]

• (P) = Maximum load at failure (N)
• (A) = Cross-sectional area of specimen (mm²)

Correction Factor for Height/Diameter Ratio:

• Use Fig 1 in IS 516 to adjust strength if specimen height/diameter ratio ≠ 2:1 (for cylinders).

flowchart TD
    A[Concrete placed & hardened] --> B[Specimen extraction]
    B --> C[Mark & store under damp matting (24±1 hr)]
    C --> D[Demould & cure in water (24-30°C)]
    D --> E[Transport in damp condition]
    E --> F[Store at lab (27±2°C)]
    F --> G[Compression test]
    G --> H[Record & report results]

Summary: Follow IS 516 curing and handling to maintain specimen integrity. Calculate compressive strength as load divided by area, applying correction factors if specimen dimensions differ from standard.

IS 516: Test for Compressive Strength of Concrete Specimen

Key Specifications (Clause 5.6.2)

Report on each specimen must include:

• Identification mark, test date, specimen age
• Curing conditions & manufacture date
• Weight, dimensions, cross-sectional area
• Maximum load at failure
• Compressive strength = (\frac{\text{Maximum Load}}{\text{Cross-sectional Area}})
• Fracture appearance and type if unusual

Correction Factor for Height/Diameter Ratio (Fig 1)

• When specimen height/diameter ≠ 2, apply correction factor (C_f) to strength.
• Corrected strength = Measured strength × (C_f)

Specimen Preparation & Curing (Clause 4.5)

• Core specimens cured in water at 24–30°C for 48 hours before testing.
• Measure specimen height with capping to nearest 1 mm.

Compressive Strength Formula:

[ f_c = \frac{P}{A} ]

• (f_c) = Compressive strength (MPa)
• (P) = Maximum load (N)
• (A) = Cross-sectional area (mm²)

Typical Cross-Sectional Areas:

flowchart TD
    A[Specimen Preparation] --> B[Curing 24-30°C, 48 hrs]
    B --> C[Measure Dimensions & Weight]
    C --> D[Apply Load till Failure]
    D --> E[Record Max Load]
    E --> F[Calculate Strength: P/A]
    F --> G[Apply Correction Factor if needed]
    G --> H[Report Results]

This summarizes IS 516's key points for compressive strength testing.

IS 516: Determination of Compressive Strength Using Portions of Beams Broken in Flexure

Key Points from Clauses 6.1, 6.3, 8.4, and 5.6.2:

• Specimen Requirements (6.3):

  • Length of broken beam portion ≥ (beam depth + 5 cm).
  • Specimen must be free from cracks, chips, or defects in the test length.

• Test Procedure (6.1):

  • Use broken flexural beam portions as compression test specimens.
  • Compressive strength results are approximately equal or up to 5% higher than standard cube tests.

• Compressive Strength Calculation (Equivalent Cube Method):

  • Use the broken beam portion cross-sectional area and maximum load.
  • Compressive strength, ( f_c = \frac{P}{A} ), where:
    • ( P ) = maximum load on specimen (kg or N),
    • ( A ) = cross-sectional area (cm² or mm²).

• Flexural Strength Calculation (8.4):

  • Modulus of rupture, ( f_t ), is calculated as:

  [ f_t = \frac{P \times a}{b \times d^2} ]

  where:

  • ( P ) = load at fracture (kg),
  • ( a ) = distance from fracture line to nearer support (cm),
  • ( b ) = width of beam (cm),
  • ( d ) = depth of beam (cm).

• Test Report (5.6.2): Include:

  • Specimen ID, test date, age, curing, weight, dimensions, cross-section, max load, compressive strength, and fracture appearance.

Summary Table: Compressive Strength Calculation

flowchart TD
    A

IS 516: Flexural Strength Test (Modulus of Rupture) – Key Points

Formula (Clause 8.4)

The flexural strength, or modulus of rupture ( f_t ), is calculated as:

[ f_t = \frac{P \times l}{b \times d^2} ]

Where:

• ( P ) = Load at fracture (kg)
• ( l ) = Span length between supports (cm)
• ( b ) = Width of the specimen (cm)
• ( d ) = Depth of the specimen (cm)

Alternatively, if ( a ) = distance from fracture line to nearer support (cm), the formula adapts accordingly (see IS 516 Clause 8.4).

Specimen & Test Details

• Specimen size: Usually 500 mm length × 100 mm width × 100 mm depth (or as per IS 516 Clause 7.1)
• Span length: Typically 4 times the depth (e.g., 400 mm for 100 mm depth)
• Curing: Same as compressive strength specimens (Clause 7.5)
• Loading: Two-point loading or center-point loading as per IS 516 recommendations

Notes:

• Flexural strength results are approximately 5% higher than compressive strength test results on broken beam portions (Clause 6.1).
• Use specimens with max aggregate size ≤ 38 mm (Clause 7.1).

Summary Table for Flexural Strength Test Parameters

flowchart LR
    A[Prepare Specimen] --> B[Cure Specimen (28 days)]
    B --> C[Set Specimen on Supports (Span = 4d)]
    C --> D[Apply Load at Fracture Point(s)]
    D --> E[Record Load P]
    E --> F[Calculate Modulus of Rupture \(f_t\)]

This ensures controlled, repeat

IS 516: Flexural Strength Testing Procedure (Clause 8.4)

The flexural strength (modulus of rupture), ( f_t ), is calculated by:

[ f_t = \frac{P \times l}{b \times d^2} ]

where:

• ( P ) = Load at fracture (kg)
• ( l ) = Span length between supports (cm)
• ( b ) = Width of the specimen (cm)
• ( d ) = Depth of the specimen (cm)

If ( a ) is the distance from the nearer support to the fracture line (cm), then:

[ f_t = \frac{3 P a}{b d^2} ]

Key Specifications:

• Specimen size: Usually 100 mm x 100 mm x 500 mm beams.
• Span length ( l ): Typically 4 times the depth (e.g., 400 mm for 100 mm depth).
• Testing age: Specimens cured for 28 days (Clause 7.5).
• Load application: Two-point loading or center-point loading as per test setup.
• Result precision: To the nearest 0.5 kg/cm².

Additional Notes:

• Flexural strength test specimens are made and cured under controlled conditions (Clause 7.1).
• Portions of beams broken in flexure can be used for compressive strength testing (Clause 6.1).
• Flexural strength is an indicator of tensile strength of concrete.

flowchart LR
    A[Prepare Beam Specimen] --> B[Cure for 28 days]
    B --> C[Set Span Length (l)]
    C --> D[Apply Load (P) at fracture point]
    D --> E[Measure distance a to fracture]
    E --> F[Calculate Flexural Strength \(f_t\)]

Modulus of Elasticity of Concrete in Compression (IS 516)

Key Specifications & Procedures:

• Clause 9.9 (Static Test with Extensometer):

  • Calculate strain for last two load cycles separately.
  • Plot stress vs. strain; draw straight lines for each extensometer.
  • Determine slopes, find average modulus ( E ).
  • If difference between slopes > 15% of average, re-center and repeat.
  • If still > 15%, discard test results.
  • Record ( E ) in kg/cm² rounded to nearest 1000.

• Clause 9.1:

  • Applies for concrete with max aggregate size ≤ 38 mm.
  • Use extensometer-based method on cured specimens.

• Clause 10.6.1 (Dynamic Modulus by Vibration):

  [ E = 4.083 \times 10^6 \times n^2 \times \frac{W^2}{L^3 \times b \times d \times w} ]

  Where:

  • ( E ) = dynamic modulus (kg/cm²)
  • ( n ) = natural frequency (cycles/sec)
  • ( W ) = specimen weight (kg)
  • ( L, b, d ) = length, breadth, depth (cm)
  • ( w ) = density (kg/m³)

• Clause 10.1:

  • Non-destructive electrodynamic method using fundamental vibration frequency.
  • Same specimen can be used for subsequent flexural tests.

Summary Table:

flowchart LR
    A[Test Specimen] --> B[Apply Load (Static)]
    B --> C[Measure Strain (Extensometer)]
    C --> D[Plot Stress vs Strain]
    D --> E[Calculate Slope (Modulus E)]
    E --> F{Slope Difference ≤ 

IS 516 - Equipment & Instrumentation for Testing (Clause 10.3 & 10.8)

Key Apparatus Specifications:

• Exciter Unit: Electro-magnetic type (moving-coil or variable air-gap).
  • Moving-coil mass ≤ 0.2% of specimen mass.
  • Former made of light card/paper, slides loosely between magnet poles.
• Clamp/Support: Fixed, metallic, earthed if variable air-gap exciter used.
  • Max width = 1/6th of specimen length.
• Coil: 50 to 100 turns on former.
• Attachment: Adhesive allowing detachment of paper former after test.

Testing Setup (Fig. 4 Reference):

• Permanent magnet with coil and specimen held by clamp.
• Specimen vibrated longitudinally to find natural frequency.

Report Requirements (Clause 10.8):

• Identification mark, test date, specimen age & curing.
• Average specimen dimensions & wet weight.
• Natural frequency of fundamental longitudinal vibration.
• Dynamic modulus of elasticity.

Dynamic Modulus of Elasticity (E_d) Calculation:

[ E_d = 4 \rho L^2 f^2 ] Where:

• ( \rho ) = density of specimen (kg/m³)
• ( L ) = length of specimen (m)
• ( f ) = natural frequency (Hz)

Summary Table: Apparatus Parameters

flowchart LR
    Specimen -->|Clamp| Support
    Support -->|Holds| Exciter
    Exciter -->|Drives vibration| Specimen
    Specimen -->|Vibration frequency| Pick-up Unit
    Pick-up Unit -->|Signal| Amplifier
    Amplifier -->|Output| Recorder

This setup ensures accurate dynamic modulus measurement per IS 516 guidelines.