Ever found yourself asking, “What is a rebounding hammer?”, “How to use a rebound hammer?” , “Is rebound hammer test accurate”?. If so, you’re in the right place to uncover everything you need to know!
Rebounding hammer? It is a simple, reliable device used to measure the quality of concrete without causing any damage. Whether you’re assessing a high-rise building or a small pathway, this tool transforms the way we understand and evaluate concrete, ensuring safety and reliability with every test.
Rebound hammer plays a vital role in the world of construction. But what makes it so effective? Let’s explore this essential tool and understand it’s a key element in building and civil engineering.
What is a Rebounding Hammer?
Imagine holding a tool that lets you check the strength of a structure with just a few taps. It is possible with a Rebound Hammer.
What is a rebounding hammer? It is a tool widely used for non-destructive testing methods in the field of construction and civil engineering. Rebounding hammers are also known as Concrete Test Hammers, Swiss Hammers, or Schmidt Hammers. It produces more value, beyond estimating the compressive strength.
This tool was invented in 1954 by a civil engineer called Ernst O. Schmidt in Zurich, Switzerland. Later it was developed by Antonio Brandestini through his company Proceq. And now these test hammers have been made available by numerous manufacturers.
According to the Indian standard IS: 13311(2)-1992, the objectives of the rebound hammer test are:
- Establishing the compressive strength of concrete by correlating the rebound index with the compressive strength.
- Evaluating the uniformity of concrete.
- Assessing the quality of the concrete following standard specifications.
- Establishing a comparative quality relationship between different concrete elements.
- Identifying acceptable and weak areas of a structure or comparing the strength of two different structures using the rebound hammer test.
Understanding the Rebound Hammer Test Principle
Testing hardened concrete on-site is essential for assessing structural integrity, especially for existing structures exposed to environmental stresses. And so, nondestructive evaluations during construction help identify material issues early, allowing for timely corrections.
So, how to use a rebound hammer? Here’s a clear step-by-step guide on using it.
- Select Test Locations
Choose test locations on the concrete surface that best represent its overall condition and strength. Ensure these areas are undamaged and free from any visible defects or irregularities.
- Clean the Surface
First, the concrete surface should be clean and free from debris for the accuracy of the rebound hammer readings.
- Prepare the tool
According to the manufacturer’s instructions, the tool should be calibrated to ensure accurate readings. Calibration can be performed using a calibration anvil or a smooth, rigid surface with a known hardness.
- Position and Strike the Concrete Surface
Position the hammer perpendicular to the surface, whether vertical or horizontal, and strike the surface to measure the rebounded energy.
- Take Readingsand Record
Take multiple readings (at least three readings) to ensure accuracy for each test location. If there are significant variations, take additional readings or investigate further. Record them for analysis.
- Correlate with Compressive Strength
Refer to the correlation chart (differ on various concrete mixtures or ages) and convert rebound values into estimated compressive strength.
- Assess Variations
Analyze the results. Higher rebound values typically suggest greater compressive strength, whereas lower values could indicate potential concerns with the quality of the concrete
Factors Influencing Rebound Hammer Test
Aggregate Type
With the type of aggregate used, the relationship between concrete compressive strength and rebound number varies. Standard correlations are available for common aggregates like gravel and crushed aggregates, but testing with lightweight aggregates needs special calibration.
Cement Type
When compared to the Ordinary Portland Cement (OPC), concrete made with high alumina cement generally offers higher compressive strength. While Super Sulphated cement reduces the compressive strength of concrete by up to 50%.
Surface type and moisture condition
The rebound hammer is best on close-textured concrete and is not suitable for high honeycomb or no-fines concrete. The strength tends to be overestimated compared to moulded surfaces when tested on floated or polished surfaces. Additionally, testing on wet concrete surfaces can result in lower rebound values, potentially underestimating the concrete’s strength by up to 20% compared to dry concrete.
Curing and Concrete Age
Concrete strength and hardness evolve over time, with curing conditions and moisture exposure influencing this relationship. Concrete aged between 3 to 90 days is generally unaffected by age-related effects, while older concrete requires specially calibrated curves for accurate assessment.
Carbonation on Concrete Surface
To obtain accurate results, the carbonated layer should be removed, and the test should be conducted on the non-carbonated concrete layer. The rebound hammer tends to estimate a higher strength on concrete surfaces subjected to carbonation, with values potentially 50% higher.
Differences: High-strength rebound hammer vs. standard rebound hammer
The rebound hammer is a popular tool for assessing the surface hardness of concrete, but there are two primary types: the high-strength rebound hammer and the standard rebound hammer.
- Appearance: They look similar, but vary in length.
- Applications: The high-strength rebound hammer tests large concrete components or high-strength concrete, while also detecting aggregate and transition layers.Standard rebound hammers are ideal for testing general building components and bridges, including slabs, beams, and columns, particularly for on-site strength assessments.
- Nominal energy: The impact force of a high-strength rebound hammer is higher than that of a standard rebound hammer. 225-type standard rebound hammer has a nominal energy of 2.207 J, 450-type high-strength rebound hammer has a nominal energy of 4.500 J while 550-type high-strength rebound hammer has a nominal energy of 5.500 J.
- Calibration values: The 225-type standard rebound hammer, when tested on a normal steel anvil with a Rockwell hardness of HRC = 60±2, delivers a rebound value of 80±2. In comparison, the 450-type high-strength rebound hammer, used on a high-strength steel anvil with the same Rockwell hardness, produces a rebound value of 88±2. Meanwhile, the 550-type high-strength rebound hammer, tested on a high-strength steel anvil with Rockwell hardness HRC = 60±2, gives a rebound value of 83±1.
- Measurement Range: Standard rebound hammer is suitable with strengths of 10-60 MPa. On the other hand, a high-strength rebound hammer is suitable for concrete with strengths of 50-100 MPa.
- Impact Area: Unlike the standard rebound hammer, the high-strength rebound hammer’s larger impact energy extends the impact area beyond the cement paste layer, reaching the aggregate and transition layers in high-strength concrete testing. As a result, increasing the impact energy does not necessarily increase the rebound value and may even decrease it due to the influence of these deeper layers.
Pros and Cons of Using Rebound Hammer
Pros
- Non-destructive Testing Method.
- Indicates Concrete Compressive Strength.
- Applicable for In-place Concrete Assessment.
- Requires minimal preparation and is cost-effective.
Cons
- Surface conditions affect the results.
- Limited to Surface Hardness assessment.
- Not suitable for thin or soft concrete layers.
Conclusion
A rebound hammer is a tool that assesses the surface hardness of concrete, offering an indirect measure of its compressive strength. Commonly used in construction, it helps evaluate concrete strength, uniformity, and overall quality in both new and existing structures. When asking what is a rebounding hammer, it’s important to understand that the test provides a quick, non-destructive alternative to more traditional methods. As for is rebound hammer test accuracy, the test’s accuracy can be influenced by factors such as surface texture, moisture, and concrete type, making calibration and careful site selection crucial for reliable results.
For more detailed information on what is a rebounding hammer and its applications, as well as the option to purchase one, you can visit Langry. We offer a wide range of rebound hammers suited for various testing needs in construction and concrete assessments.
