Method of making, curing and determining compressive strength of accelerated cured concrete test specimens
1978 Edition

Overview of Scope: Accelerated Curing and Compressive Strength Evaluation of Concrete

Objectives:

• To establish a standardized, expedited method for evaluating concrete quality via accelerated curing (warm-water and boiling-water techniques) and compressive strength assessment.
• Enables strength estimation within roughly 24 hours, forecasting 28-day strength to support timely quality control and corrective measures.

Highlights:

• Accelerated curing expedites cement hydration, achieving about half the strength of standard 28-day curing quickly.
• Correlation charts linking accelerated and conventional curing strengths should be developed for local materials (refer Appendix A).
• Suitable for on-site quality control and mixture adjustments.
• Excludes high-pressure steam curing due to practical constraints.

Important Correlation Details (Appendix A)

Summary of Accelerated Curing Methods (Clause 0.5)

References for Sampling and Testing

• Sampling guidelines: IS 1199-1959
• Specimen dimensions, compaction, and capping: IS 516-1959

flowchart LR
    A[Concrete Specimen Casting] --> B[Accelerated Curing]
    B --> C{Select Method}
    C --> D[Warm Water (~60°C)]
    C --> E[Boiling Water (100°C)]
    D & E --> F[Compressive Strength Testing (~24 hours)]
    F --> G[Correlation with 28-day Strength]
    G --> H[Quality Control Actions]

This approach allows early evaluation of concrete strength, facilitating timely interventions and economical mix modifications.

Apparatus and Materials According to IS 9013

1. Equipment (Clauses 2.3.2 & 2.3.3)

• Accelerated Curing Tank:
  • Should hold adequate water volume.
  • Water temperature near specimens must be continuously maintained at the target temperature, except for up to 15 minutes after specimen immersion.
  • Typical tank design is depicted in Fig. 1 (IS 9013).

2. Materials

• Use concrete specimens prepared from locally sourced materials, considering:
  • Cement's chemical composition.
  • Water to cement ratio.
  • Mix proportions.
• These factors influence hydration speed and strength gain.

3. Key Specifications (Clause 10.3 & Appendix A)

• Accelerated curing via warm or boiling water methods achieves roughly 50% of the strength attained by 28-day normal curing in a shorter period.

• Correlation between accelerated and normal strengths varies with mix design.

• Predict strength using regression formulas from Appendix A:

  [ f_{ac} = k \times f_{28} ]

  where:

  • ( f_{ac} ) is strength after accelerated curing,
  • ( f_{28} ) is 28-day normal cure strength,
  • ( k ) typically around 0.5.

4. Application

• Accelerated curing is intended for early quality control, providing results within approximately 24 hours.
• Correlation curves should be established locally due to variability in materials.

Strength Correlation Summary

flowchart LR
A[Concrete Specimen] --> B[Accelerated Curing Tank]
B --> C{Curing Method}
C --> D[Warm Water]
C --> E[Boiling Water]
D --> F[~50% Strength of 28-day Normal]
E --> F
F --> G[Correlate with Normal Curing Strength]

Note: Always verify predictions with local calibration data as per Appendix A.

Test Specimen Preparation Guidelines (IS 9013)

• Sampling and Material Preparation:

  • For correlation testing between accelerated and normal curing, adhere to IS 516:1959 for sampling, mixing, proportioning, workability assessment, specimen dimensions, compaction, and end capping (Clause 3.1).
  • For routine control tests, sampling follows IS 1199:1959; specimen size, compaction, and capping as per IS 516:1959 (Clause 3.2).

• Key Specifications per IS 516:1959:

  • Specimen Dimensions:
    • Cubes: 150 mm × 150 mm × 150 mm (standard size)
    • Cylinders: 150 mm diameter × 300 mm height (optional)
  • Compaction: Employ tamping rods or vibration methods as standardized.
  • End Capping: Use sulfur mortar or neoprene caps to ensure uniform compression load distribution.

• Workability Testing: Conduct slump test according to IS 1199.

• Accelerated Curing Setup: Refer to Fig. 1 (IS 9013) for the curing tank layout ensuring controlled temperature and humidity.

Specimen Preparation Standards Summary

flowchart TD
    A[Sampling] --> B[Mixing and Proportioning]
    B --> C[Workability Test (Slump)]
    C --> D[Specimen Molding]
    D --> E[Compaction]
    E --> F[Capping]
    F --> G[Curing (Normal or Accelerated)]
    G --> H[Compressive Strength Testing]

This process ensures uniformity and comparability between accelerated and conventional curing test results.

Warm Water Accelerated Curing According to IS 9013

1. Procedure (Clause 4 / 1.2)

• Specimens in molds are submerged in warm water maintained at 55 ± 2°C.
• Immersion time should be at least 19 hours 50 minutes.
• Upon completion, specimens are extracted, demolded, and cooled in water at 27 ± 2°C for no less than 1 hour.
• Total duration from immersion to end of cooling must not exceed 20 hours 10 minutes.

2. Specimen Preparation (Clause 3.3)

• Immediately after molding, protect specimens by covering with a steel plate coated with mold-release oil to prevent sticking.

3. Strength Correlation (Appendix A)

• Accelerated curing yields approximately 50% of the 28-day strength after about 20 hours.
• Strength outcomes depend on cement chemistry, water-cement ratio, and mix proportions.
• Use local correlation data or empirical experience to relate accelerated and normal strengths.

4. Typical Regression Model (Appendix A)

[ R_{20} = a + b \times R_{28} ]

• Where:
  • (R_{20}) is compressive strength after 20 hours accelerated curing (N/mm²),
  • (R_{28}) is compressive strength after 28 days normal curing (N/mm²),
  • (a, b) are empirical constants based on local materials.

Summary Table

flowchart TD
    A[Mold Specimens] --> B[Cover with steel plate and oil]
    B --> C[Immerse in 55±2°C water for ≥19h 50m]
    C --> D[Remove and demold specimens]
    D --> E[Cool in 27±2°C water for ≥1h]
    E --> F[Perform compressive strength test]

This procedure facilitates rapid estimation of concrete strength.

Boiling Water Accelerated Curing Process as per IS 9013

1. Specimen Preparation

• Immediately after molding, cover specimens with a steel plate coated with mold oil to prevent adhesion (Clause 3.3).

2. Curing Procedure

• Specimens inside molds are carefully lowered into boiling water maintained at approximately 100°C at sea level.
• Immersion time: 3 hours ± 5 minutes (Clause 5.2).
• Water temperature should not decline by more than 3°C after specimen placement and must return to boiling within 15 minutes.
• In enclosed spaces, temperature may be kept just below boiling to minimize evaporation.

3. Post-Curing Handling (Warm Water Reference)

• For warm water curing, specimens are held at 55 ± 2°C for roughly 20 hours (Clause 1.2).

Boiling Water Curing Parameters Summary

flowchart TD
    A[Mold Specimens] --> B[Cover with steel plate and mold oil]
    B --> C[Lower into boiling water (100°C)]
    C --> D[Immersion for 3 hours ± 5 minutes]
    D --> E[Maintain water temp: max 3°C drop, recover to boiling within 15 min]
    E --> F[Remove specimens for testing]

This technique accelerates strength development by curing specimens at boiling temperature for a brief period, simulating extended curing duration.

Testing Procedures for Concrete Specimens as per IS 9013

1. Sampling and Specimen Preparation

• Sampling must conform to IS 1199:1959.
• Specimen dimensions, compaction methods, and capping should follow IS 516:1959.
• Preparation steps include:
  • Material sampling, proportioning, weighing, mixing, and workability assessment.
  • Compaction and capping as per IS 516 when correlating normal and accelerated curing test outcomes.

2. Test Report Details (Clause 8.1)

Reports must include:

• Specimen identification marks, type, size, and casting date.
• Testing date and time, specimen age.
• Concrete mix particulars and compaction method.
• Specimen size, mass, and any defects.
• Timing of water addition, specimen fabrication, curing immersion/removal, and cooling.
• Temperature logs of curing water.
• Maximum crushing load and computed compressive strength.
• Description of the fractured surface.

3. Accelerated Curing Tank Setup

• Follow layout specifications as per Fig. 1 (Clause 2.3.3).

Compressive Strength Calculation

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

• (f_c) is compressive strength (N/mm²),
• (P) is maximum load at failure (N),
• (A) is the cross-sectional area of the specimen (mm²).

Specimen Size Summary (IS 516:1959)

flowchart TD
    A[Sampling (IS 1199)] --> B[Specimen Fabrication (IS 516)]
    B --> C[Compaction & Capping]
    C --> D[Curing Process]
    D --> E[Compressive Strength Testing]
    E --> F[Test Report Generation (Clause 8.1)]

This ensures standardized testing and dependable correlation between accelerated and normal curing results.

Compressive Strength Determination of Accelerated-Cured Concrete (IS 9013)

Important References:

• Calculation procedures based on IS 516-1959 (Clause 7.1).
• Specimen preparation as per IS 516-1959 (Clause 3.1).
• Correlation between accelerated and normal curing strength detailed in Clause 10.3 and Appendix A.

Key Points:

• Accelerated curing expedites hydration, yielding strength approximately 50% of the 28-day normal curing strength.
• Correlation factors are influenced by:
  • Cement chemistry.
  • Water-cement ratio.
  • Mix design.
  • Temperature regimen during curing.

Typical Regression Correlation Formula:

[ f_a = k \times f_{28} ] Where:

• (f_a) = compressive strength after accelerated curing (N/mm²),
• (f_{28}) = compressive strength after 28 days normal curing (N/mm²),
• (k) ≈ 0.5 (typical factor from Appendix A).

Specimen Preparation and Testing (per IS 516-1959):

• Sampling, mixing, compaction, curing, and capping follow IS 516.
• Common specimen sizes include 150 mm cubes or 150 mm diameter by 300 mm cylinders.
• Testing age corresponds to the accelerated curing regime applied.

flowchart LR
    A[Material Sampling] --> B[Mixing & Proportioning]
    B --> C[Specimen Molding]
    C --> D[Accelerated Curing]
    D --> E[Compressive Strength Testing]
    E --> F[Calculate \(f_a\)]
    F --> G[Correlate to 28-day Strength \(f_a = 0.5 \times f_{28}\)]

Summary: Employ IS 516 for testing procedures, calculate accelerated strength, and correlate it with 28-day strength using the factor ~0.5 as per IS 9013 Appendix A, adapting for local materials as needed.

Reporting Guidelines under IS 9013

Required Information in Test Reports (Clause 8.1)

• Identification: Specimen mark, type, size, casting date.
• Testing Details: Date and time of testing, specimen age.
• Concrete Mix Data: Composition and compaction method.
• Specimen Details: Size, mass, observed defects.
• Process Timings: Water addition, specimen fabrication, curing/cooling immersion and removal times.
• Temperature Records: Thermographic logs of curing water temperature.
• Test Outcomes: Maximum crushing load, calculated compressive strength, and fractured face description.

Application of Results (Clauses 10.2 & 10.3)

• Enables quick assessment of concrete quality variability and process control.
• Strength values are influenced by materials and curing conditions; local correlations and experience are important.

Correlation of Accelerated and Normal Curing (Appendix A)

• Accelerated curing typically achieves about 50% of normal 28-day strength.
• Correlation depends on cement chemistry, water-cement ratio, and mix proportions.
• Standardized curing procedures help minimize variability.

Typical Regression for Strength Correlation:

[ R_a = k \times R_{28} ] Where:

• (R_a) = Accelerated curing strength (N/mm²)
• (R_{28}) = 28-day normal curing strength (N/mm²)
• (k) ≈ 0.5 for both boiling and warm water methods.

Reporting Summary Table

flowchart TD
    A[Test Specimen] --> B{Report Contents}
    B --> C[Identification]
    B --> D[Testing Details]
    B --> E[Concrete Mix Data]
    B --> F[Specimen Details]
    B --> G[Process Timings]
    B --> H[Temperature Records]
    B --> I[Test Results]

Proper documentation ensures traceability and effective quality control.

Precautionary Guidelines for Accelerated Curing (IS 9013 Clause 9.1)

Tank Maintenance

• Regularly clean the curing tank and replace water to prevent accumulation of impurities.
• Ensure heating and circulation systems remain fully operational.

Boiling Water Safety

• Prevent burns by cautiously opening covers to avoid sudden steam release.
• Immerse specimens gently to prevent hot water splashing.

Specimen Handling

• Protect and carefully store specimens during initial curing phases to avoid damage.

Safety Equipment

• Install thermographs and heating control panels.
• Use dedicated electrical switchboards for safety.

Tank Dimensions (Fig. 1)

• Length (a): 1.60 meters
• Width (b): 0.50 meters
• Height (c): 0.65 meters
• Designed to accommodate 12 cubes of 150 mm size.

Additional Notes

• Specimens should rest undisturbed in molds at 27 ± 2°C for at least 1 hour before immersion.
• Time interval between mixing and immersion should be 1 to 3.5 hours.
• Accelerated curing strength generally corresponds to about 50% of 28-day normal strength.

Precaution and Setup Diagram

flowchart TD
    A[Curing Tank] --> B[Water Cleaning & Renewal]
    A --> C[Heating & Circulating Systems]
    C --> D[Safety Devices & Temperature Monitoring]
    E[Specimens] --> F[Initial Protection & Storage]
    E --> G[Immersion in Water (27±2°C)]
    H[Boiling Water Use] --> I[Safety Precautions]
    I --> J[Prevent Steam Burns & Hot Water Splash]

Refer to Appendix A for detailed strength correlation between accelerated and normal curing.

Guidance on Interpreting Results per IS 9013

Use of Results (Clauses 10.2 & 10.3)

• Provides a rapid method for assessing variability and controlling the production process.
• Strength outcomes depend on the combination of materials and curing procedures.
• Results should be interpreted using site-specific correlations and experience (Appendix A).

Correlation Between Accelerated and Normal Curing (Appendix A)

• Accelerated curing methods, such as boiling water curing, typically yield about 50% of the 28-day strength.
• Correlation is influenced by cement chemistry, water-cement ratio, and mix proportions.
• A typical regression model relating 28-day strength ((R_{28})) to accelerated strength ((R_a)) is:

[ R_{28} = 12.65 + 1.30 \times R_a ]

• Calibration for the specific site conditions is recommended for precision.

Test Report Components (Clause 8.1)

Include:

• Specimen identification, size, type, and casting date.
• Test date, time, and specimen age.
• Concrete composition and compaction methods.
• Specimen size, mass, and any defects.
• Times of mixing, casting, curing, and cooling.
• Temperature records during curing.
• Maximum load and calculated compressive strength.
• Description of fracture surface.

Strength Correlation Flow

graph LR
A[Accelerated Curing Strength (R_a)] --> B[Apply Regression Equation]
B --> C[Estimate 28-day Strength (R_{28})]
C --> D[Use in Quality Control and Mix Adjustment]

This procedure ensures dependable interpretation of accelerated curing results aligned with local material and site conditions.

Correlation of Compressive Strengths: Normal vs Accelerated Curing (IS 9013)

Highlights from Clause 10.3 & Appendix A:

• Accelerated curing methods (boiling or warm water) hasten hydration, achieving approximately 50% of the 28-day normal curing strength in a shorter timeframe.
• Strength correlation varies with cement type, water-cement ratio, and mix design.
• Standardized curing methods reduce variability.
• Site-specific correlation curves are advisable for accuracy.

Typical Correlation Parameters:

Regression Equation (from Figure 2):

[ R_{28} = 12.65 + 1.28 \times R_a ]

• This formula enables estimation of 28-day strength based on accelerated test results.
• Accelerated strength is generally about half of the 28-day strength.

Testing Requirements (Clause 3.1):

• Prepare specimens following IS 516-1959.
• Conduct sampling, mixing, curing, compaction, and capping per IS 516.
• Employ standardized accelerated curing procedures.

graph LR
A[28-day Normal Curing] -->|Strength R28| B[Strength Measurement]
C[Accelerated Curing (Boiling Water)] -->|Strength Ra| B
B --> D[Apply Correlation Equation]
D --> E[Estimate R28 from Ra]

Summary: Use the regression (R_{28} = 12.65 + 1.28 R_a) to predict 28-day compressive strength from accelerated curing results, ensuring compliance with IS 9013 and IS 516 standards.