Concrete is one of the most widely used materials in construction, and assessing its strength is critical for quality assurance and safety. One of the most common non-destructive testing (NDT) methods used on concrete structures is the Rebound Hammer Test. This test is simple, fast, and provides a quick indication of compressive strength without damaging the structure.
In this guide, we’ll explore how the rebound hammer test works, the working principle, testing procedure, apparatus used, and how to interpret the results accurately. Whether you’re a site engineer, quality control manager, or student of civil engineering, this article will help you fully understand this important test.
What is Rebound Hammer Test?
The Rebound Hammer Test, also known as the Schmidt Hammer Test, is a non-destructive testing method used to determine the surface hardness and estimate the compressive strength of concrete.
This test provides a convenient way to assess in-place concrete quality, especially when core cutting or destructive testing is not feasible. The rebound number obtained correlates with the compressive strength of the concrete.
Working Principle of Rebound Hammer Test
The working principle is based on the rebound of a spring-controlled plunger when it strikes the surface of concrete. The degree of rebound depends on the hardness and elasticity of the surface.
A plunger hits the concrete surface with a specific energy. The hammer rebounds, and the extent of rebound (called the rebound number) is measured on a scale. Higher rebound numbers indicate harder surfaces and generally higher compressive strength.
Formula for Estimating Strength
The actual compressive strength is estimated using charts or calibration curves provided by the manufacturer or established through lab calibration.
Apparatus Used in Rebound Hammer Test
The main tool is the Rebound Hammer device itself, which includes:
- Steel plunger
- Spring mechanism
- Rebound scale (graduated dial)
- Locking button and body case
- Optional: Calibration anvil for standardization
Types of Rebound Hammers
| Type | Energy Level | Use Case |
|---|---|---|
| Type N | 2.207 Nm | Standard hammer for general concrete testing |
| Type L | 0.735 Nm | Lightweight hammer for small or thin components |
| Type M | 29.43 Nm | For testing mass concrete structures like dams |
Standard Codes for Rebound Hammer Test
- IS 13311 (Part 2): 1992
- ASTM C805
- BS EN 12504-2
Rebound Hammer Test Procedure Step-by-Step
Step 1: Surface Preparation
Ensure the concrete surface is clean, dry, and smooth. Avoid dusty, flaky, or rough-textured surfaces. Do not test on plastered or coated surfaces.
Step 2: Holding the Hammer
Hold the hammer perpendicular to the test surface. Maintain firm, steady contact.
Step 3: Test Execution
Push the hammer plunger against the concrete until the spring releases. Allow the hammer to rebound automatically. Record the rebound number shown on the scale.
Step 4: Repeat Readings
Take at least 10 readings on each test location. Discard the highest and lowest values. Calculate the average of the remaining values.
Factors Affecting Rebound Hammer Test Results
- Moisture Content: Wet concrete shows lower rebound values
- Age of Concrete: Strength increases with age, affecting rebound
- Surface Texture: Rough or porous surfaces reduce rebound
- Type of Aggregate: Hard aggregates yield higher rebound
- Orientation of Testing: Horizontal vs vertical testing affects results
- Carbonation: Carbonated layers on old concrete can give false high readings
Interpretation of Rebound Hammer Test Results
Typical Rebound Number to Strength Relationship
| Rebound Number | Estimated Compressive Strength (MPa) |
|---|---|
| < 20 | < 15 MPa (Poor concrete) |
| 20–30 | 15–25 MPa (Fair quality) |
| 30–40 | 25–35 MPa (Good quality) |
| > 40 | > 35 MPa (Excellent quality) |
| Note: These are general values. Always use manufacturer calibration or lab correlation for accurate interpretation. |
Advantages of Rebound Hammer Test
- Non-destructive and portable
- Quick results on-site
- No special skills required
- Cost-effective for routine quality checks
- Suitable for comparative strength assessment
Limitations of Rebound Hammer Test
- Surface hardness ≠ internal strength
- Not suitable for lightweight concrete
- Cannot assess concrete below the surface
- Affected by surface conditions, carbonation, moisture, and aggregate
- Should not be used as a standalone method for structural safety assessment
Applications of Rebound Hammer Test
- Assessing uniformity of concrete across different areas
- Estimating in-place compressive strength
- Evaluating old structures before repair or renovation
- Cross-checking quality of ready-mix concrete
- Initial screening of doubtful or damaged areas
Best Practices for Accurate Rebound Testing
- Use calibrated hammer only
- Avoid testing on edges, corners, or near reinforcement
- Take readings at uniform spacing
- Always note surface condition and moisture level
- Perform tests at the same orientation (vertical/horizontal)
- Repeat testing on different areas for better average
Comparison: Rebound Hammer Test vs Other NDT Methods
| Test Method | Parameter Measured | Use Case |
|---|---|---|
| Rebound Hammer | Surface hardness | Rapid quality check, uniformity assessment |
| Ultrasonic Pulse Velocity (UPV) | Pulse speed through concrete | Detects voids, cracks, and overall integrity |
| Core Cutting Test | Direct compressive strength | Destructive but highly accurate |
| Penetration Resistance Test | Resistance to steel pin penetration | Used in concrete pavements and overlays |
FAQs – Rebound Hammer Test
What does the rebound hammer test measure?
It measures the surface hardness of concrete, which correlates with its compressive strength.
Can rebound hammer test be used for all concrete structures?
Yes, but it’s best suited for flat, clean, and cured concrete surfaces. Avoid using it on plastered or painted surfaces.
How many readings should be taken for accurate results?
At least 10 readings per test spot. Remove outliers and average the rest.
Does carbonation affect rebound hammer results?
Yes. Carbonation hardens the surface artificially, giving false high strength readings.
What is the acceptable range for rebound number?
Acceptable values depend on concrete grade. For example, M20 concrete may show a rebound number of 25–30 under ideal conditions.
Is rebound hammer test reliable?
It is reliable for comparative strength evaluation, but not sufficient alone for precise strength determination.
Conclusion: Rebound Hammer Test in Construction
The Rebound Hammer Test remains one of the most useful tools for evaluating the in-situ quality of concrete without damaging the structure. While it has its limitations, when used correctly and interpreted wisely, it offers quick and cost-effective insights into concrete strength and uniformity. For best results, always combine rebound testing with other methods like core tests or ultrasonic pulse velocity, especially when structural integrity is in question.
