Rebound Hammer Testing for Non-Destructive Evaluation of Concrete Strength

Assessing the strength of hardened concrete in existing structures presents a challenge that standard cube compression tests alone cannot fully address. While laboratory testing of concrete cubes remains the conventional method for strength determination, the results from these cubes do not always reflect the actual in-situ strength of concrete within a structure. Differences in compaction, curing conditions, and composition between test specimens and the actual construction can lead to significant discrepancies. This is where non-destructive testing methods become essential. One widely used approach is the rebound hammer test, a surface hardness method that provides a rapid, economical way to evaluate concrete quality on site. For a broader overview of field testing approaches, see Destructive and Non Destructive Field Testing of Concrete.

The rebound hammer, originally developed in 1948 by Swiss engineer Ernst Schmidt, has become the most popular non-destructive testing instrument for in-situ concrete worldwide. Its appeal lies in its simplicity, portability, low cost, and ability to provide fairly accurate results when properly used. The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which it strikes. For concrete, surface hardness correlates with compressive strength, making the rebound hammer a valuable tool for assessing concrete quality, uniformity, and relative strength comparisons within a structure.

Understanding the Rebound Hammer Test Method

Operating Principle and Design

The rebound hammer consists of a spring-controlled mass that slides on a plunger within a tubular housing. When the plunger is pressed against the concrete surface, it retracts against the force of the spring. Once fully retracted, the spring is automatically released, propelling the mass toward the concrete surface. The mass impacts the plunger and rebounds, carrying a rider along a graduated scale. The distance of rebound, measured as the rebound number, is read from the scale after pressing a button to lock the rider in position.

Standardization and Codes

The rebound hammer test has been standardized by various national and international bodies over time. Key standards include:

• IS: 13311 (Part 2) – 1992: Indian Standard for non-destructive testing of concrete by rebound hammer
• BS 4408: Part 4 – 1971: British Standard for surface hardness methods
• BS 1881: Part 202 – 1986: Recommendations for surface hardness testing by rebound hammer
• ASTM C805: Standard test method for rebound number of hardened concrete

The Bureau of Indian Standards published IS: 13311 (Part 2) in 1992, and IS: 456-2000 specifies that non-destructive tests, including the rebound hammer method, may be used to estimate the properties of concrete in a structure. CPWD specifications in India have also recognized the rebound hammer test for payment purposes when cube tests fail.

Objectives of the Test

According to IS: 13311 (Part 2) – 1992, the rebound hammer method serves four primary purposes:

1. Assessing the likely compressive strength of concrete using suitable correlations between rebound index and compressive strength
2. Assessing the uniformity of concrete within a structure
3. Assessing the quality of concrete relative to standard requirements
4. Assessing the quality of one structural element in relation to another

The method is most reliable when used for differentiating between acceptable and questionable parts of a structure, or for making relative comparisons between different structures. For specialized applications such as testing in challenging environments, refer to Non Destructive Testing of Underwater Concrete Structures.

Calibration Procedure for Reliable Results

Proper calibration is the most critical step in obtaining meaningful rebound hammer test results. Each hammer comes with a manufacturer-supplied calibration chart, but this chart is only valid when material and testing conditions closely match those used during the original calibration. Since each hammer varies in performance and different aggregate sources produce different correlations, field calibration is essential.

Step-by-Step Calibration Process

The following procedure outlines the recommended approach for calibrating a rebound hammer for a specific project:

1. Prepare a number of concrete cubes covering the range of strengths expected on the job, using the same cement, aggregates, and mix proportions as the actual construction. Use 150 mm cubes for standard hammers with impact energy of approximately 2.2 Nm.
2. Cast and cure the cubes according to IS: 516-1959 standards.
3. Remove cubes from wet storage 24 hours before testing and allow them to dry in the laboratory atmosphere. Wet-tested cubes typically give strength values 10% lower than dry-tested cubes.
4. Clean the cube faces and grip them in a compression testing machine under a load of 7 N/mm2 (approximately 15.75 tonnes for 150 mm cubes) when using a hammer with impact energy of about 2.2 Nm.
5. Take at least nine hammer readings on each of the two vertical faces accessible in the compression machine. Impact points should be at least 20 mm from edges and 20 mm apart. Never impact the same point more than once.
6. Immediately after taking readings, test the cube to its ultimate compressive load.
7. Repeat the procedure for all cubes and discard extreme values before averaging the readings to obtain the rebound number for each cube.
8. Plot the rebound numbers against cube compressive strengths and fit a curve using the method of least squares to establish the calibration relationship.

The accuracy and reproducibility of the hammer should be verified periodically using a standard calibration anvil, especially before testing an actual structure. It is worth noting that 700 to 1000 individual tests may be needed to reliably plot calibration curves for a rebound hammer. For an overview of testing equipment options, see Equipment for Non Destructive Testing of Concrete.

How to Test a Concrete Structure with a Rebound Hammer

A rebound hammer with impact energy of 2.207 Nm (0.225 kgm) is suitable for testing concrete in ordinary building and bridge construction. The procedure for field testing follows a systematic approach to ensure reliable data collection.

Surface Preparation and Test Point Selection

All members and test points should be clearly marked for identification. Concrete surfaces must be dry before testing. Tests should be conducted on smooth, uniform surfaces, preferably those cast against formwork. Rough, honeycombed, or porous areas must be avoided. If loose scale, plaster, or coatings are present, they should be removed with a grinding wheel or stone. Trowelled or floated surfaces may be used with appropriate corrections or a special calibration curve.

Testing Procedure

1. For concrete sections less than 100 mm thick, support the back side with a heavy mass to prevent elastic deformation from affecting the rebound reading.
2. At each selected test point, take six rebound readings, shifting the hammer 25 mm between impacts. Never impact the same spot twice.
3. Ensure each point of impact is at least 20 mm away from any edge or sharp discontinuity.
4. Record all readings and check consistency. Rebound values are considered reliable when at least six readings do not deviate by more than 2.5 to 3.5 units on the impact scale.
5. Calculate the average rebound number after discarding any outliers.
6. Determine the compressive strength from the calibration curve relating rebound number to strength for the specific materials being used.

Number of Readings Required

Different standards specify varying numbers of readings. The manufacturer of the Schmidt hammer recommends 5 to 10 impact readings per point. BS 4408 specifies at least 9 valid readings and not more than 25. IS: 13311 requires 6 readings per point, while ASTM C805 requires 10 readings. A practical approach is to confine readings to an area not exceeding 300 mm by 300 mm rather than random testing across the entire structure. CPWD specifications require the average of at least 12 readings.

Direction of Testing

While the usual test direction is horizontal or vertically downward, any consistent direction can be used. Calibration corrections for different test directions are typically supplied with the hammer or can be derived from the manufacturer’s data. For additional information on related non-destructive test methods, refer to Non Destructive Tests Concrete.

Factors That Influence Rebound Hammer Test Results

Several factors can significantly affect rebound hammer readings and must be considered when interpreting results. Understanding these influences is essential for obtaining accurate strength estimates.

Type of Cement and Aggregate

Concrete made with high alumina cement can show strengths up to 100% higher than Portland cement concrete when using a calibration based on ordinary Portland cement. Conversely, supersulphated cement concrete can give 50% lower readings. Recalibration is therefore necessary for different cement types. Gravel and most crushed rock aggregates produce similar correlations, but lightweight aggregates and aggregates with unusual properties require special calibration.

Surface and Moisture Conditions

The rebound hammer method is applicable only to close-textured concrete. Open-textured concrete, honeycombed concrete, or no-fines concrete cannot be reliably tested. Trowelled and floated surfaces are harder than moulded surfaces and tend to overestimate strength. Wet surfaces give rebound values approximately 10% lower than dry surfaces, leading to an underestimation of strength when using calibration curves developed from dry specimens.

Age of Concrete and Carbonation

Very old and dry concrete develops a harder surface layer than the interior, producing rebound values higher than the true concrete strength would indicate. New concrete with a moist surface has a relatively softer surface, resulting in lower rebound numbers. Surface carbonation is a particularly significant factor. In old concrete where the carbonated layer can reach up to 20 mm in thickness, the strength may be overestimated by as much as 50%.

Summary of Correction Factors

Accuracy and Limitations

When all influencing factors are properly accounted for, the rebound hammer can estimate concrete strength within an accuracy of plus or minus 15%. This level of accuracy makes the test a valuable screening tool but not a substitute for direct compression testing when precise strength values are required. The rebound hammer is most effective as a comparative tool for assessing uniformity and identifying areas of potential concern within a structure.

The operation of the rebound hammer appears simple, but its effective use requires proper training and experience. Calibration, field testing, analysis, and interpretation must be carried out by trained specialists. In the hands of an expert, the rebound hammer remains an excellent tool for non-destructive evaluation of concrete quality.

In summary, the rebound hammer test offers a practical and economical method for assessing concrete strength and uniformity in the field. Its success depends on proper calibration, careful surface preparation, adherence to standardized procedures, and a thorough understanding of factors that influence results. When used correctly, it provides engineers with valuable information for quality control and condition assessment of concrete structures.