Rebound Hammer Test of Concrete – Procedure, Calculation & Interpretation

Introduction

The Rebound Hammer Test of Concrete is a widely used non-destructive test for assessing the surface hardness and uniformity of hardened concrete. It is particularly useful for comparing the quality of different areas of a concrete member and, when an appropriate correlation has been established, estimating the likely compressive strength of concrete.

The test is performed using a spring-driven steel hammer that impacts the concrete surface and produces a rebound number. The rebound number is related to the hardness of the tested surface. However, it should not be treated as a direct measurement of compressive strength because the result is affected by several factors, including surface condition, moisture, orientation, age, carbonation and aggregate characteristics.

For current Indian practice, the test is covered by IS 516 (Part 5/Sec 4):2020, which provides the objective, principle, apparatus, procedure, influence of test conditions and guidance for interpretation.

Quick Answer

The Rebound Hammer Test of Concrete is a non-destructive test used to determine the rebound number of hardened concrete and assess surface hardness and relative uniformity. The current Indian Standard is IS 516 (Part 5/Sec 4):2020. The rebound number should not be treated as a direct compressive-strength value; strength estimation requires an appropriate correlation applicable to the concrete and testing conditions.

What is Rebound Hammer Test of Concrete?

The Rebound Hammer Test is a non-destructive testing method used to determine the rebound number of hardened concrete.

A spring-controlled mass strikes a plunger placed against the concrete surface. After impact, the mass rebounds, and the amount of rebound is indicated as the rebound number.

In general, a harder concrete surface produces a higher rebound number, while a softer surface produces a lower rebound number.

The test can be used for:

• Assessing the uniformity of concrete quality.
• Comparing different areas of the same concrete member.
• Comparing the quality of one concrete element with another.
• Assessing concrete in relation to specified requirements.
• Estimating likely compressive strength when a suitable correlation has been established.

Important: The rebound hammer test is not a direct substitute for compressive strength testing. Strength estimation requires a suitable correlation between rebound number and compressive strength.

Rebound Hammer Test as per IS 516

The current Indian Standard for the rebound hammer test is:

IS 516 (Part 5/Sec 4):2020 – Hardened Concrete — Methods of Test, Part 5 Non-Destructive Testing of Concrete, Section 4 Rebound Hammer Test (First Revision).

This standard covers the objective, principle, apparatus and procedure for determining the rebound number of hardened concrete using a spring-driven steel hammer. It also provides guidance concerning test conditions and interpretation of results.

Earlier Standard

The older standard was:

IS 13311 (Part 2):1992 – Methods of Non-Destructive Testing of Concrete: Part 2 Rebound Hammer.

IS 516 (Part 5/Sec 4):2020 supersedes the earlier rebound-hammer standard.

Therefore, current articles and specifications should preferably refer to IS 516 (Part 5/Sec 4):2020.

Principle of Rebound Hammer Test

The rebound hammer works on the principle that the rebound of a spring-driven mass is related to the hardness of the concrete surface.

When the plunger is pressed against the concrete surface, the spring-controlled hammer mass is released and strikes the plunger. The mass then rebounds.

The rebound distance is converted into a numerical value called the rebound number or rebound index.

The basic sequence is:

Impact → Rebound → Rebound Number → Surface Hardness Assessment → Appropriate Correlation → Estimated Strength

The rebound number generally increases with increasing concrete strength, but the relationship is affected by several test and concrete conditions.

Purpose of Rebound Hammer Test

The rebound hammer test can be used for several purposes.

1. Assessing concrete uniformity

Readings can be taken at different locations to identify variations in surface hardness.

2. Comparing different structural elements

The test can help compare concrete quality between different members when testing conditions are comparable.

3. Preliminary assessment

It can provide useful information during an initial investigation of an existing concrete structure.

4. Estimating likely compressive strength

With an appropriate correlation between rebound number and compressive strength, the test can be used to estimate likely in-situ strength.

5. Identifying areas requiring further investigation

Areas producing unusually low or inconsistent readings may warrant additional investigation using other suitable testing methods.

Apparatus Required for Rebound Hammer Test

The main equipment required includes:

The standard specifically addresses the rebound hammer, abrasive stone and testing anvil as part of the equipment requirements.

Main Parts of a Rebound Hammer

A typical rebound hammer consists of several important components:

• Plunger
• Spring-controlled hammer mass
• Impact spring
• Body
• Scale or display
• Release mechanism
• Locking mechanism

The plunger is placed against the concrete surface during testing. The internal hammer mechanism produces the impact and rebound.

Types of Rebound Hammers

Rebound hammers are available with different impact energies and configurations.

The selection of a suitable hammer depends on:

• Type of concrete
• Structural element
• Testing purpose
• Required impact energy
• Equipment manufacturer’s specifications
• Applicable testing standard

The hammer used for a particular investigation should be suitable for the concrete and test application.

Different hammer configurations should not be treated as interchangeable without appropriate calibration and correlation.

Preparation of Concrete Surface

Proper surface preparation is one of the most important factors affecting the reliability of rebound hammer readings.

The test surface should be suitably prepared, clean and free from loose material. The moisture condition of the surface should be recorded because moisture can affect rebound readings.

Loosely adhering scale should be removed. Rough surfaces caused by incomplete compaction, loss of grout, spalling or tooling can produce unreliable results and should be avoided.

Why surface preparation matters

The rebound hammer primarily evaluates hardness close to the concrete surface. Therefore, a weak, rough, wet or otherwise abnormal surface can produce a reading that does not represent the concrete adequately.

Selection of Test Locations

Test locations should be selected carefully so that the readings represent the concrete being investigated.

Avoid unsuitable locations such as:

• Very rough surfaces
• Honeycombed concrete
• Spalled concrete
• Obvious cracks
• Areas with loose material
• Locations very close to edges
• Areas containing obvious surface defects

According to the procedure summarised from IS 516 (Part 5/Sec 4):2020, the impact point should be at least 25 mm away from an edge or shape discontinuity.

Rebound Hammer Test Procedure

Step 1 — Select the test area

Select a suitable representative area of the concrete member.

The surface should be suitably prepared, clean and free from loose material.

Step 2 — Prepare the surface

Remove loose material and prepare the surface where necessary using an appropriate abrasive stone.

Do not test directly on unsuitable rough, honeycombed or damaged surfaces.

Step 3 — Check the rebound hammer

The condition and performance of the rebound hammer should be checked using the appropriate testing/calibration procedure.

The testing anvil is used for checking hammer performance.

Step 4 — Position the hammer

Place the rebound hammer firmly against the concrete surface.

The hammer should normally be held at right angles to the test surface. The test may be performed horizontally, vertically upward or downward, or at an intermediate angle depending on the application.

Step 5 — Apply the impact

Press the plunger against the concrete until the impact mechanism operates.

The spring-controlled hammer mass impacts the plunger and rebounds.

Step 6 — Record the rebound number

Record the rebound number indicated by the instrument.

The reading should be associated with its:

• Test location
• Structural element
• Hammer orientation
• Test condition

Step 7 — Repeat the measurements

Multiple readings should be taken within the selected test area according to the applicable procedure.

Do not base an assessment on a single rebound reading.

The readings should be reviewed for consistency and treated according to the applicable standard/test procedure.

Step 8 — Determine the representative result

The readings are processed according to the applicable test procedure to obtain a representative rebound value for the test area.

The result can then be interpreted with the appropriate correlation and test information.

What is Rebound Number?

The rebound number is the numerical value obtained from the rebound hammer after the impact of the spring-driven mass on the concrete surface.

It is an indicator of the surface hardness of the tested concrete.

A higher rebound number generally indicates a harder surface, while a lower rebound number indicates a softer surface.

However:

Rebound number should not be directly converted into compressive strength using a universal formula.

The relationship between rebound number and compressive strength must be established using an appropriate correlation for the concrete and testing conditions.

Rebound Hammer Test Calculation

The rebound hammer test does not use one universal equation for converting rebound number directly into compressive strength.

A fixed formula should therefore not be presented as a universal IS 516 conversion equation.

The general assessment process is:

Rebound Number → Appropriate Correlation → Estimated Compressive Strength

Where:

• Rebound Number (R) = representative rebound index obtained from testing
• Appropriate Correlation = relationship applicable to the concrete and testing conditions
• Estimated Compressive Strength (fᶜ) = strength estimated using that correlation

How to Estimate Concrete Strength Using the Rebound Hammer Graph

The rebound hammer provides a rebound number, while the corresponding compressive strength is estimated using an appropriate correlation graph/curve.

The general process is:

Rebound Number (R) → Applicable Correlation Curve → Estimated Compressive Strength (fᶜ)

Where:

• R = Rebound Number
• fᶜ = Estimated Compressive Strength

How the graph is used

1. Obtain the representative rebound number from the test.
2. Identify the rebound number on the horizontal axis of the applicable correlation graph.
3. Move vertically upward until it intersects the appropriate correlation curve.
4. From the intersection point, move horizontally toward the strength axis.
5. Read the corresponding estimated compressive strength.
6. Apply the appropriate corrections/considerations for the test conditions and correlation used.

Important technical note

The graph used for strength estimation must correspond to the particular rebound hammer, concrete and correlation established for the investigation.

Example: Estimating Strength from a Correlation Graph

Suppose the representative rebound number obtained from a test area is:

R = 38

Locate 38 on the rebound-number axis of the applicable correlation graph.

Follow the graph’s correlation curve to determine the corresponding estimated compressive strength.

If the applicable correlation graph gives an estimated compressive-strength value for the representative rebound number, that value should be reported as the estimated compressive strength from the applicable correlation, not as a universal conversion for that rebound number.

Important

The numerical value obtained from one project’s graph must not automatically be applied to another project, because the correlation depends on the concrete and testing conditions.

How to Read a Rebound Hammer Correlation Graph

A rebound hammer correlation graph normally relates the measured rebound number to the corresponding estimated compressive strength for the applicable concrete and testing conditions.

To read the graph:

1. Identify the representative rebound number on the horizontal axis.
2. Locate the applicable correlation curve.
3. Project the rebound-number value to the curve.
4. From the curve intersection, project toward the strength axis.
5. Read the corresponding estimated compressive strength.
6. Record the correlation or graph used along with the test result.

Note: The axes, curve and numerical range may differ between correlations. Always use the graph applicable to the concrete and testing conditions being investigated.

How is the Compressive Strength Estimated?

The rebound number is compared with the appropriate correlation curve, or graph developed for the concrete under investigation.

The estimated compressive strength is obtained from the correlation applicable to the particular concrete, rebound hammer and test conditions.

Rebound Number (R) → Appropriate Correlation → Estimated Compressive Strength (fᶜ)

• R = Rebound Number
• fᶜ = Estimated Compressive Strength

Important: There is no single universal rebound-number-to-strength equation applicable to all concrete.

Worked Example of Rebound Hammer Interpretation

Suppose the representative rebound number obtained from a test area is:

R = 38

Assume that an appropriately established correlation applicable to the concrete and testing conditions indicates:

R = 38 → Estimated compressive strength = 28 MPa

Therefore:

Estimated compressive strength ≈ 28 MPa

Important: The value of 28 MPa is illustrative only. It is not a universal IS 516 conversion value for rebound number 38. The actual estimated strength must be obtained from the appropriate correlation applicable to the concrete and testing conditions.

Practical Rebound Hammer Test Examples

Example 1 — Comparing Two Columns

Suppose two comparable columns are tested under similar conditions:

Column C2 has a lower rebound number than C1, indicating a comparatively lower surface rebound response.

Although Column C2 has a lower rebound number, this should not automatically be interpreted as lower compressive strength. The difference should be evaluated considering surface condition, moisture, carbonation, concrete history and other relevant investigation results.

Example 2 — Variation Within One Column

Area D is noticeably different from the other test areas.

This may justify additional investigation rather than immediate rejection of the column.

Example 3 — Effect of Moisture

Suppose comparable locations give:

Dry surface: R = 36

Wet surface: R = 31

The lower rebound number obtained from the wet surface should not automatically be interpreted as lower compressive strength. Surface moisture can influence rebound readings; therefore, moisture condition and other relevant test conditions should be recorded and considered during interpretation.

How Many Rebound Hammer Readings Should Be Taken?

A concrete test area should not be assessed from a single rebound reading. Multiple impact readings should be taken and processed according to the applicable test procedure.

The number and distribution of readings should provide a representative assessment of the selected test area while maintaining the required spacing from edges, discontinuities and unsuitable surface locations.

Important: Follow the current requirements of IS 516 (Part 5/Sec 4):2020 and the project testing procedure when deciding the number, spacing and treatment of readings.

Interpretation of Rebound Hammer Test Results

The rebound number should be interpreted carefully.

Higher rebound number

Generally indicates:

• Higher surface hardness
• Relatively harder concrete surface
• Potentially higher strength, subject to appropriate correlation

Lower rebound number

May indicate:

• Lower surface hardness
• Poor surface condition
• High moisture
• Surface deterioration
• Carbonation differences
• Other factors affecting the rebound response

Therefore, a low rebound number does not automatically prove that the entire concrete section has low compressive strength.

The standard notes that rebound indices are indicative of compressive strength only to a limited depth from the surface.

Effect of Test Direction on Rebound Number

The orientation of the rebound hammer affects the measured rebound number.

The hammer should be held at right angles to the concrete surface.

Testing may be performed:

• Horizontally on vertical surfaces
• Vertically upward
• Vertically downward
• At intermediate angles where required

Different orientations can produce different rebound numbers for the same concrete, so the test direction should always be recorded and the appropriate treatment applied according to the applicable procedure.

Factors Affecting Rebound Hammer Test Results

Several factors can influence the rebound number.

1. Surface condition

Smooth and hard surfaces generally produce different readings from rough or damaged surfaces.

2. Moisture condition

Wet concrete surfaces can produce lower rebound values than equivalent dry surfaces.

3. Surface carbonation

Carbonation can increase surface hardness and therefore influence rebound readings.

4. Age of concrete

The relationship between hardness and strength changes with age and curing history.

5. Aggregate characteristics

The type and properties of aggregate can affect the rebound response.

6. Test direction

The orientation of the hammer affects the rebound number.

7. Surface defects

Honeycombing, cracks, spalling and poor compaction can produce unreliable readings.

8. Hammer condition

The condition and calibration of the instrument influence the reliability of the result.

9. Concrete compaction

Poorly compacted concrete does not have a unique relationship between strength and rebound number.

10. Surface finish

Trowelled and floated surfaces can be harder than moulded surfaces and may therefore influence the estimated strength.

Rebound Hammer Test on Wet Concrete

Moisture is an important consideration.

A wet concrete surface can produce lower rebound readings compared with an equivalent dry surface calibrated under dry conditions. The standard’s guidance specifically identifies moisture content as an influencing factor.

Therefore, the moisture condition of the concrete should be recorded when interpreting the results.

Rebound Hammer Test for Existing Structures

The rebound hammer is particularly useful during preliminary investigation of existing structures because it allows multiple areas to be tested without removing concrete cores at every location.

It can help identify:

• Variations in concrete quality
• Areas requiring additional investigation
• Relatively weak or strong zones
• Differences between structural members

However, structural decisions should not be based solely on rebound hammer readings.

Advantages of Rebound Hammer Test

The main advantages are:

• Non-destructive testing method
• Rapid testing
• Portable equipment
• Relatively simple operation
• Large numbers of locations can be tested
• Useful for assessing concrete uniformity
• Useful for preliminary investigation
• Limited physical damage to the concrete surface

Limitations of Rebound Hammer Test

The important limitations include:

• Primarily reflects near-surface hardness.
• Results are affected by surface condition.
• Moisture affects readings.
• Carbonation can affect surface hardness.
• Aggregate characteristics affect results.
• Test direction affects the rebound number.
• Strength estimation requires appropriate correlation.
• It cannot independently establish structural capacity.
• Poorly compacted or unsuitable surfaces can give unreliable results.
• A single rebound reading is not sufficient for assessing an entire structural member.

The standard specifically emphasises the influence of test conditions and the need for suitable correlation for strength estimation.

Rebound Hammer Test vs UPV Test

A combination of rebound hammer and ultrasonic pulse velocity testing can provide more useful information than relying on rebound hammer results alone in appropriate investigations. IS 516 (Part 5/Sec 4):2020 specifically emphasizes combined interpretation with UPV in relevant circumstances.

For a detailed explanation of ultrasonic pulse velocity testing, see our [Ultrasonic Pulse Velocity Test of Concrete – UPV Test] guide.

Rebound Hammer Test vs Core Test

The two methods serve different purposes and should not automatically be treated as substitutes.

Rebound Hammer vs UPV vs Core Test

Practical Site Recommendations

For reliable rebound hammer testing:

1. Select representative test locations.
2. Use suitably prepared, clean surfaces and record the moisture condition.
3. Remove loose surface material.
4. Avoid honeycombed and severely damaged surfaces unless they are specifically being investigated.
5. Keep the impact point at least 25 mm from an edge or discontinuity.
6. Hold the hammer at the appropriate orientation.
7. Record the test direction.
8. Take multiple readings.
9. Record the location of every test area.
10. Check the hammer’s condition and calibration.
11. Use an appropriate correlation for strength estimation.
12. Do not interpret rebound number as direct compressive strength.
13. Use additional testing where structural decisions require greater confidence.

What Should Be Included in a Rebound Hammer Test Report?

A proper test report should identify the testing information needed to interpret the results.

Recommended records include:

• Date of testing
• Identification of structure
• Structural element
• Location of test area
• Concrete grade, where known
• Rebound hammer identification
• Test direction/orientation
• Individual readings
• Representative rebound result
• Surface condition
• Moisture condition
• Correlation used for strength estimation
• Estimated strength, where applicable
• Remarks and observations

Recording the test location and hammer orientation is particularly important because rebound readings are affected by testing conditions.

Example Rebound Hammer Test Record

The comparatively lower result at C2-A should not automatically be interpreted as failure of the concrete. The location should be investigated considering surface condition, moisture, carbonation, concrete history, applicable correlation and other available test information.

Frequently Asked Questions

1. What is the rebound hammer test of concrete?

The rebound hammer test is a non-destructive test used to determine the rebound number of hardened concrete and assess surface hardness and uniformity.

2. What is the principle of rebound hammer test?

The test is based on the rebound of a spring-driven mass after impact with the concrete surface. The rebound is related to the hardness of the tested surface.

3. What is the IS code for rebound hammer test?

The current Indian Standard is IS 516 (Part 5/Sec 4):2020 – Rebound Hammer Test. It supersedes IS 13311 (Part 2):1992.

4. Is rebound hammer test destructive?

No. It is generally classified as a non-destructive test because it does not require removal of concrete from the structure.

5. What does rebound number indicate?

Rebound number indicates the response of the concrete surface to the impact of the rebound hammer and is related primarily to surface hardness.

6. Can rebound number directly determine compressive strength?

No. Compressive strength should be estimated only through an appropriate correlation established for the concrete and test conditions.

7. What factors affect rebound hammer readings?

Surface condition, moisture, carbonation, age, aggregate characteristics, test direction, surface defects, compaction and equipment condition can all affect the readings.

8. How far should the impact point be from an edge?

The impact point should be at least 25 mm away from an edge or shape discontinuity according to the procedure in IS 516 (Part 5/Sec 4):2020.

9. Why should the rebound hammer be held perpendicular to the surface?

The test orientation affects the rebound number. Holding the hammer at right angles to the test surface provides the specified test arrangement, while different orientations require appropriate treatment.

10. What is the difference between rebound hammer and UPV test?

Rebound hammer testing measures rebound response related to surface hardness, whereas UPV measures ultrasonic pulse transmission through concrete. Both are non-destructive methods and both require appropriate interpretation.

11. Can rebound hammer test be used for old concrete?

Yes, it can be used for investigating existing concrete, but the effects of age, carbonation, surface condition and other factors must be considered.

12. Is rebound hammer test enough for structural assessment?

Not necessarily. Rebound hammer results should be interpreted within the context of the investigation and, where necessary, supplemented by other appropriate tests such as UPV or core testing.

13. How is compressive strength estimated from a rebound hammer graph?

The representative rebound number is located on the applicable correlation graph and the corresponding estimated compressive strength is obtained from the correlation curve. The graph must be applicable to the concrete, rebound hammer and test conditions being investigated. It should not be treated as a universal IS 516 conversion graph.

14. What is a good rebound hammer number for concrete?

There is no single rebound number that universally establishes good or acceptable concrete. Rebound number is influenced by concrete properties, surface condition, moisture, carbonation, test direction and other factors. Where strength estimation is required, an appropriate correlation applicable to the concrete and testing conditions should be used.

15. What rebound hammer value is required for M25 concrete?

A universal rebound number should not be assigned directly to M25 concrete. Rebound number measures surface rebound response, while M25 denotes a specified concrete compressive-strength grade. An appropriate correlation and the relevant acceptance/testing requirements are necessary for strength assessment.

16. Can rebound hammer detect honeycombing?

Rebound hammer testing may help identify areas with abnormal surface response, but it should not be used alone to confirm the presence or extent of honeycombing. Visual inspection, UPV testing and other appropriate investigations may be required.

17. Which is better: UPV or rebound hammer?

Neither test is universally “better.” Rebound hammer testing primarily evaluates surface rebound response, whereas UPV evaluates ultrasonic pulse transmission through concrete. The appropriate method depends on the purpose of the investigation, and the two tests can provide complementary information.

18. Can rebound hammer replace a core test?

No. Rebound hammer testing is a non-destructive surface assessment method, while core testing involves extracting concrete for direct specimen testing. They serve different purposes and should not automatically be considered interchangeable.

Key Takeaways

• The Rebound Hammer Test is a non-destructive method for assessing the surface hardness and uniformity of hardened concrete.
• The current Indian Standard is IS 516 (Part 5/Sec 4):2020.
• The older IS 13311 (Part 2):1992 has been superseded.
• The test produces a rebound number, not a direct compressive-strength value.
• Compressive strength can be estimated only through an appropriate correlation.
• Surface condition, moisture, carbonation, age, aggregate and test orientation can significantly affect results.
• The impact point should be at least 25 mm from an edge or shape discontinuity.
• Rebound hammer testing is particularly useful for assessing uniformity and identifying areas requiring further investigation.
• For important structural assessments, rebound hammer results should be considered together with other appropriate evidence.

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