Rebound Hammer Test of Non-Destructive Testing (NDT) of Hardened Concrete as per Indian Standard

Rebound Hammer Test of Non-Destructive Testing (NDT) of Hardened Concrete as per Indian Standard

Overview:

• The Rebound Hammer Test is a non-destructive method used to estimate the compressive strength of hardened concrete.
• It provides insights into the uniformity of concrete and highlights potential weak areas in the structure.

Objective:

• To determine the surface hardness and indirectly estimate the compressive strength and quality of concrete.

Equipment:

• Rebound Hammer (also known as Schmidt Hammer)

Principle:

• The test measures the hardness of concrete by assessing the rebound of a spring-controlled hammer mass against the surface of concrete.

Procedure:

1. Calibration: Ensure the hammer is properly calibrated using an anvil provided by the manufacturer.
2. Preparation: The surface where testing is to be conducted should be smooth, dry, and clean.
3. Positioning: Hold the hammer firmly against the concrete surface, ensuring it is perpendicular.
4. Impact: Press the hammer gently until it impacts the concrete surface and records the rebound index.
5. Readings: Take a minimum of ten readings for each test area, discarding the outliers.
6. Calculation: Compute the average of the recorded rebound indices.
7. Conversion: Use the conversion charts provided in the relevant Indian Standard to convert the rebound index to compressive strength.

Factors Influencing the Results:

• Surface Texture: Rough surfaces may lead to lower rebound values.
• Moisture Condition: Wet or damp surfaces can affect the measurements.
• Type of Cement: The cement type can influence surface hardness and hence the rebound index.
• Concrete Age: The hardness increases with the age of concrete.
• Carbonation: Affects the surface hardness and may lead to misleading results.

Advantages:

• Non-destructive: Does not damage the structure.
• Quick and Simple: Can be performed rapidly on-site.
• Economic: Cost-effective compared to other methods.

Limitations:

• Provides relative, not absolute, values.
• Surface conditions greatly impact the rebound index.
• Limited to the measurement of surface hardness only.

Application:

• Used to assess the uniformity of properties over a concrete structure.
• Identifies areas that require further testing.
• Preliminary estimation of compressive strength.

Standards and Codes:

• As per Indian Standards (IS: 13311-Part 2), the procedure, calibration, and evaluation must follow specific guidelines to ensure accuracy and reliability.

Formulas:

• Conversion formula: $\text{Compressive strength}=\text{f(rebound index)}$ Specific formulas and charts are typically provided in the standard documents.

Calculations:

1. Mean Rebound Index: $\text{Mean Rebound Index}=\frac{\sum (\text{Rebound values})-\text{Outliers}}{\text{Number of valid tests}}$
2. Conversion to Compressive Strength: Utilize the specific conversion chart to convert mean rebound index to equivalent compressive strength.

Example Calculation:

1. Record the rebound indices: e.g., $30,32,29,28,31,33,35,27,32,34$
2. Discard outliers (e.g., 27)
3. Calculate mean rebound index: $\frac{30+32+29+28+31+33+35+32+34}{9}=31.56$
4. Convert to compressive strength using relevant conversion chart.

Conclusion: The Rebound Hammer Test is a practical tool for evaluating the quality and compressive strength of hardened concrete. Understanding its limitations and ensuring proper calibration and procedure adherence are key to obtaining reliable results.