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.
The rebound hammer test is standardized under several codes including IS:13311 (Part 2)-1992 in India, ASTM C805 in the USA, and EN 12504-2 in Europe. While it's a valuable tool for assessing concrete quality, it should be used in conjunction with other testing methods for critical structural evaluations.
The rebound hammer test is conducted for several important reasons in concrete quality assurance and structural assessment:
To verify the uniformity of concrete quality throughout a structure and ensure it meets the specified strength requirements during construction.
To locate areas of deteriorated or substandard concrete within a structure, helping identify regions that may require repair or further investigation.
To provide a non-destructive estimate of concrete compressive strength in existing structures where core cutting would be impractical or undesirable.
To monitor changes in concrete properties over time, helping evaluate the effects of aging, environmental exposure, or structural damage.
To assist in the overall condition assessment of existing structures, particularly after events like earthquakes, fires, or when planning renovations.
To determine when formwork can be removed safely, when post-tensioning can be applied, or when structures can be put into service.
While the rebound hammer test is valuable, it has limitations: it only assesses the surface hardness to a depth of approximately 30mm, can be affected by surface conditions, and provides relative rather than absolute strength values. For critical structural assessments, it should be used alongside other methods like ultrasonic pulse velocity tests or core testing.
The following equipment is required to conduct a proper rebound hammer test:
The primary testing instrument. Various models are available (Type N for general testing, Type L for low-impact testing, etc.), each with different impact energies.
A hardened steel anvil used to verify the hammer is properly calibrated before each testing session.
Abrasive stone for smoothing rough surfaces and a wire brush for cleaning the test area.
Chalk, marker, or crayon for marking test points and a ruler or template for consistent spacing.
Data sheets or digital device for recording test results, including rebound numbers and location information.
Charts, correlations, or software for converting rebound numbers to estimated compressive strength.
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.
Standard | Country/Region | Key Requirements |
---|---|---|
IS:13311 (Part 2)-1992 | India | Minimum 10 readings per test location, separation of 20-25mm between test points, test surface should be smooth and clean |
ASTM C805/C805M | USA | Minimum 10 readings per test area, all readings outside the median ±6 range should be discarded, impact direction correction required |
EN 12504-2 | Europe | Minimum 9 readings per test area, spacing of 25mm between impacts, discard readings ±20% from median value |
BS 1881-202 | UK | Minimum 12 readings per test area, test points at least 20mm from edges, smoothing of rough surfaces required |
ACI 228.1R | USA (ACI) | Provides guidelines for correlating rebound numbers to compressive strength, recommends calibration curves for specific concrete mixes |
While these standards provide general correlations between rebound number and compressive strength, it's recommended to develop site-specific correlations by comparing rebound numbers with actual core test results for the specific concrete mix under investigation.
Follow these steps to conduct a proper rebound hammer test as per standard guidelines:
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.
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.
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.
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.
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.
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.
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.
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)
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.
Several factors can influence rebound hammer test results, potentially affecting their accuracy and reliability:
Readings vary significantly with the direction of impact (horizontal, vertical upward, vertical downward). Apply appropriate correction factors for non-horizontal orientations.
Older concrete tends to give higher rebound values due to surface carbonation. Young concrete (less than 28 days) might require age correction factors.
Wet surfaces give lower rebound values compared to dry surfaces of the same concrete. Consistent moisture conditions are essential for comparable results.
Surface carbonation increases rebound values without corresponding increase in strength. Carbonation depth should be measured and accounted for in older structures.
Rough surfaces produce inconsistent and generally lower readings. Always test on smooth, ground surfaces for reliable results.
Lightweight concrete gives different rebound values compared to normal-weight concrete of the same strength. Standard correlations may not apply.
Testing over reinforcement, aggregate, or voids can significantly affect readings. Always avoid visible aggregates, reinforcement, and visible defects.
Extreme temperatures can affect hammer mechanism and concrete surface properties. Avoid testing in freezing conditions or on very hot surfaces.
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:
Rebound Number Range | Approximate Strength Range (MPa) | Concrete Quality Assessment |
---|---|---|
< 20 | < 14 | Very Poor |
20-30 | 14-20 | Poor |
30-40 | 20-30 | Fair |
40-50 | 30-42 | Good |
> 50 | > 42 | Excellent |
The above correlations are approximate and based on general relationships. For accurate assessments, it's highly recommended to develop project-specific correlation curves by comparing rebound numbers with actual core test results from the same concrete mix.
Let's walk through a complete sample calculation for rebound hammer testing of a concrete column:
Test location: Reinforced concrete column, 30 years old
Hammer orientation: Horizontal (0°)
Carbonation depth: 5mm (measured using phenolphthalein indicator)
Reading No. | Rebound Value |
---|---|
1 | 36 |
2 | 38 |
3 | 35 |
4 | 34 |
5 | 37 |
6 | 28 |
7 | 39 |
8 | 36 |
9 | 34 |
10 | 37 |
11 | 35 |
12 | 36 |
Average = (36 + 38 + 35 + 34 + 37 + 28 + 39 + 36 + 34 + 37 + 35 + 36) ÷ 12 = 35.4
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.
New Average = (36 + 38 + 35 + 34 + 37 + 39 + 36 + 34 + 37 + 35 + 36) ÷ 11 = 36.1
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
Using the standard correlation formula for horizontal orientation:
Compressive Strength (MPa) = 1.25 × e0.073 × Rebound Number
Compressive Strength = 1.25 × e0.073 × 33.6 = 25.3 MPa
Based on the estimated compressive strength of 25.3 MPa, the concrete would be classified as M25 grade concrete.
The corrected rebound number is 33.6, corresponding to an estimated compressive strength of 25.3 MPa. This indicates "Fair to Good" quality concrete, suitable for most structural applications. The concrete is classified as M25 grade, which meets common requirements for reinforced concrete structures.
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:
The calculator features a user-friendly interface that allows you to:
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.
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)
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)
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
The calculator also offers:
- PDF report generation for professional documentation
- Saving of results for future reference
- Calculation of average strength across multiple test locations
Properly interpreting rebound hammer test results is crucial for making informed decisions about concrete quality and structural integrity:
Rebound Number | Estimated Strength (MPa) | Concrete Quality | Typical Applications |
---|---|---|---|
< 20 | < 14 | Very Poor | Requires immediate attention, possibly unsafe for structural use |
20-30 | 14-20 | Poor | Non-structural applications, may need repair if structural |
30-40 | 20-30 | Fair to Good | Suitable for most structural applications |
40-50 | 30-42 | Very Good | High-strength applications, heavy-duty structures |
> 50 | > 42 | Excellent | Special high-strength applications |
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.
Issue | Potential Cause | Solution |
---|---|---|
Highly variable readings | Poor concrete uniformity, improper testing technique, or surface defects | Increase number of test points, check surface preparation, verify hammer operation |
Unexpectedly low readings | Surface moisture, honeycombing, improper hammer orientation, or concrete deterioration | Ensure dry surface, check for internal voids, verify orientation, inspect for deterioration |
Unexpectedly high readings | Surface carbonation, presence of large aggregate, or over-troweled surface | Measure and correct for carbonation, avoid visible aggregate, select representative areas |
Inconsistent results between tests | Different testing personnel, hammer calibration issues, or varying surface conditions | Standardize testing protocol, verify calibration regularly, document test conditions |
Rebound hammer testing should be considered as one tool in a comprehensive assessment approach. For critical structures or when results are ambiguous, always supplement with other NDT methods or core testing. Professional judgment by qualified engineers is essential when interpreting results.
The rebound hammer test finds practical applications across various civil engineering contexts:
Verifying concrete quality during construction, confirming strength development, determining when formwork can be removed, and ensuring specification compliance.
Evaluating the condition of existing structures, identifying areas of concrete deterioration, and establishing prioritization for maintenance or repairs.
Rapid assessment of concrete structures after earthquakes, fires, floods, or explosions to determine structural integrity and safety for occupancy.
Assessing the strength and quality of concrete in structures planned for renovation, adaptive reuse, or load modifications.
Tracking changes in concrete properties over time to monitor aging, environmental effects, or structural performance.
Studying concrete performance under various conditions, evaluating new mix designs, and developing improved correlation models.
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:
While the rebound hammer test is valuable, combining it with other testing methods provides a more comprehensive assessment of concrete quality and strength:
Combining rebound hammer with UPV testing (known as the SonReb method) significantly improves strength estimation accuracy and helps detect internal defects.
Taking cores from a few selected locations based on rebound hammer results provides actual strength data and allows development of site-specific correlation curves.
Phenolphthalein spray tests to measure carbonation depth help correct rebound readings and assess concrete durability against reinforcement corrosion.
Using a cover meter to locate reinforcement ensures rebound testing avoids rebar locations, which could otherwise produce misleading results.
Thorough visual inspection complements instrumental testing by identifying cracks, spalling, efflorescence, and other visible signs of deterioration.
Advanced statistical treatment of test results from multiple methods provides confidence intervals and reliability metrics for strength estimates.
The most reliable assessment strategy typically involves initial rapid screening with rebound hammer testing across the entire structure, followed by more detailed investigation using other NDT methods in critical areas, and finally, selective core testing to verify findings at key locations.
For further information on rebound hammer testing, refer to the following standards and resources: