Introduction to Rebound Hammer Test

The rebound hammer test, also known as the Schmidt hammer test, is a non-destructive testing method used in civil engineering to evaluate the surface hardness and compressive strength of concrete structures. It works on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass strikes.

The test provides a quick and economical means of checking the uniformity of concrete, identifying areas of poor quality or deteriorated concrete, and estimating the in-situ strength of concrete without causing damage to the structure. It is widely used for both quality control during construction and for assessment of existing structures.

Why We Conduct Rebound Hammer Test

The rebound hammer test is conducted for several important reasons in concrete quality assurance and structural assessment:

Required Equipment

The following equipment is required to conduct a proper rebound hammer test:

Standard Specifications

The rebound hammer test is standardized under various codes and specifications globally. These standards provide guidelines for conducting the test, interpreting results, and ensuring reliability.

Step-by-Step Procedure

Follow these steps to conduct a proper rebound hammer test as per standard guidelines:

1. Equipment Calibration

   Before testing, verify the rebound hammer's calibration using the calibration anvil. Take at least 10 readings on the anvil, and ensure they fall within ±3 points of the manufacturer's specified value. If not, the hammer needs service.

2. Surface Preparation

   Select a smooth, flat test area free from defects. If the surface is rough, grind it smooth with an abrasive stone. Clean the area thoroughly to remove dust or loose material. The test area should be at least 20mm away from edges or obvious defects.

3. Grid Marking

   Mark a grid pattern for test points with at least 20-25mm spacing between points and from edges. A test area should have at least 10-12 test points. For large structures, select multiple test areas to assess uniformity.

4. Hammer Positioning

   Hold the rebound hammer firmly and perpendicular to the test surface. Note the orientation of the hammer (horizontal, vertical upward, vertical downward, or at an angle), as this affects the readings and will require correction factors.

5. Impact and Reading

   Gradually increase pressure on the plunger until the hammer impacts. Maintain pressure and lock the plunger position. Read and record the rebound number from the scale. Release the locking mechanism after recording.

6. Multiple Readings

   Take at least 10 readings at different points within the test area, maintaining the required spacing. Avoid taking readings on visible voids, reinforcement locations, or areas with obvious defects.

7. Data Analysis

   Calculate the average of all readings for each test area. According to standards, discard readings that differ from the average by more than 6 units or by ±20% (depending on which standard you follow), and recalculate.

8. Apply Correction Factors

   Apply necessary correction factors based on:
   - Hammer orientation (horizontal, vertical upward/downward, or at an angle)
   - Concrete age (if testing young concrete)
   - Carbonation depth (if significant carbonation is present)
   - Member shape and size (if testing thin members)

9. Strength Conversion

   Convert the corrected rebound number to estimated compressive strength using appropriate conversion curves provided by the manufacturer, relevant standards, or site-specific correlations. Document all results.

Factors Affecting Test Results

Several factors can influence rebound hammer test results, potentially affecting their accuracy and reliability:

Correlation Between Rebound Number and Compressive Strength

The relationship between rebound number and compressive strength is not linear and depends on various factors. Different researchers and standards have proposed different correlation curves. Here's how rebound number typically relates to compressive strength:

The graph shows how the same rebound number can indicate different compressive strengths depending on the orientation of the hammer. Some key observations include:

Sample Calculation

Let's walk through a complete sample calculation for rebound hammer testing of a concrete column:

Sample Data

Test location: Reinforced concrete column, 30 years old
Hammer orientation: Horizontal (0°)
Carbonation depth: 5mm (measured using phenolphthalein indicator)

Calculation Steps:

1. Calculate Average Rebound Number

   Average = (36 + 38 + 35 + 34 + 37 + 28 + 39 + 36 + 34 + 37 + 35 + 36) ÷ 12 = 35.4

2. Check for Outliers

   According to ASTM C805, readings outside the range of the median ±6 should be discarded.
   Median = 36
   Range = 36 ±6 = 30 to 42
   Reading #6 (28) is outside this range and should be discarded.

3. Recalculate Average

   New Average = (36 + 38 + 35 + 34 + 37 + 39 + 36 + 34 + 37 + 35 + 36) ÷ 11 = 36.1

4. Apply Carbonation Correction

   Carbonation affects rebound readings by increasing surface hardness without proportionally increasing strength.
   Carbonation Correction = 5mm × 0.5 = 2.5
   Corrected Rebound Number = 36.1 - 2.5 = 33.6

5. Convert to Compressive Strength

   Using the standard correlation formula for horizontal orientation:
   Compressive Strength (MPa) = 1.25 × e^{0.073 × Rebound Number}
   Compressive Strength = 1.25 × e^{0.073 × 33.6} = 25.3 MPa

6. Determine Concrete Class

   Based on the estimated compressive strength of 25.3 MPa, the concrete would be classified as M25 grade concrete.

Quality Assessment

Using the Rebound Hammer Calculator

Our Rebound Hammer Calculator simplifies the entire testing process by automating calculations, applying correction factors, and providing instant quality assessments. Here's how the calculator works:

Rebound Hammer Calculator

The calculator features a user-friendly interface that allows you to:

1. Multiple Test Locations

   The calculator supports testing at multiple locations (Location 1, 2, 3), allowing you to assess different areas of a structure and calculate an overall average.

2. Input Parameters

   For each location, you can specify:
   - Hammer orientation (horizontal, upward, downward, or at angles)
   - Concrete age (optional)
   - Carbonation depth (optional)
   - Individual rebound readings (minimum 10 recommended)

3. Automatic Calculations

   The calculator automatically:
   - Filters outlier readings according to standard protocols
   - Calculates average rebound number
   - Applies orientation correction factors
   - Adjusts for concrete age and carbonation
   - Converts to estimated compressive strength
   - Determines concrete strength class (M15, M20, M25, etc.)
   - Provides quality assessment (Poor, Fair, Good, Excellent)

4. Results Presentation

   The results are displayed in a comprehensive table showing:
   - All input parameters
   - Number of valid readings used
   - Average and corrected rebound numbers
   - Estimated compressive strength
   - Concrete strength class
   - Quality assessment with color-coded indicators

5. Documentation Features

   The calculator also offers:
   - PDF report generation for professional documentation
   - Saving of results for future reference
   - Calculation of average strength across multiple test locations

Interpreting Results & Common Issues

Properly interpreting rebound hammer test results is crucial for making informed decisions about concrete quality and structural integrity:

Result Interpretation

Result Variability Assessment

The coefficient of variation (CoV) of rebound readings at a location provides valuable information about concrete uniformity. Low CoV indicates uniform concrete, while high CoV suggests quality control issues or deterioration.

Common Issues & Troubleshooting

Practical Applications of Rebound Hammer Testing

The rebound hammer test finds practical applications across various civil engineering contexts:

Case Study Example

The graph above shows data from a case study of a 30-year-old building where rebound hammer testing was conducted across different floors. The test revealed:

• Consistent concrete quality across middle floors (CoV < 10%)
• Lower strength in ground floor elements, likely due to moisture exposure
• Higher variability in top floor elements, possibly from environmental exposure
• Overall strength sufficient for current use, with localized repairs needed

Combined Testing Strategies

While the rebound hammer test is valuable, combining it with other testing methods provides a more comprehensive assessment of concrete quality and strength:

References & Resources

For further information on rebound hammer testing, refer to the following standards and resources:

• IS:13311 (Part 2)-1992 - Non-Destructive Testing of Concrete - Methods of Test, Part 2: Rebound Hammer
• ASTM C805/C805M - Standard Test Method for Rebound Number of Hardened Concrete
• EN 12504-2 - Testing Concrete in Structures - Part 2: Non-destructive Testing - Determination of Rebound Number
• ACI 228.1R - Report on Methods for Estimating In-Place Concrete Strength
• BS 1881-202 - Testing Concrete. Recommendations for Surface Hardness Testing by Rebound Hammer