The concrete hammer test is useful for determining a variety of concrete characteristics in a non-destructive way. With it, a technician can assess the uniformity and quality of the concrete based on standard specifications, compare the quality of concrete elements and establish a relative strength profile for a concrete structure. Each of these results can be determined using the same simple tool: a concrete test hammer.
How to Perform the Concrete Hammer Test
Performing a concrete hammer test on hardened concrete is relatively simple. You simply hold the concrete test hammer (AKA, impact hammer or rebound hammer) of your choice against the concrete structure and release the spring-loaded hammer. Generally, the hammer tool will have a digital window and scale along the side where you can view the rebound value from the hammer’s impact.
Steps for Performing the Rebound Hammer Test
- Use a Schmidt hammer calibration anvil to check the calibration of your test hammer after every 1,000-2,000 impacts. You should also test your hammer before each use.
- Make sure the concrete surface you will be testing is smooth, clean and dry.
- Ensure the surface is not a wet concrete surface, as this can significantly underestimate the compressive strength.
- If the surface is not smooth, use a concrete grinding stone to prepare it.
- Your chosen impact point should be at least 20mm away from the edge of the structure or any shape discontinuities.
- Take six rebound readings at each site and determine the average for a reliable value.
It’s also important to keep in mind that the types of aggregate and cement used to create concrete can influence the results. Therefore, you will need to evaluate the results of the test based on the specifications of the mixture. Age, curing and carbonation of concrete can also affect the rebound number determined by your test hammer.
Pros & Cons of the Non-Destructive Concrete Testing
Like any materials testing procedure, this test has its strengths and weaknesses, and it shouldn’t be taken as an absolute measure of concrete’s compressive strength. The rebound hammer method evaluates the compressive strength of concrete through the correlation between rebound numbers and structural strength, but it has limitations influenced by material types and surface conditions.
Pros
- Use a Schmidt hammer calibration anvil to check the calibration of your test hammer after every 1,000-2,000 impacts. You should also test your hammer prior to each use.
- Make sure the concrete surface you will be testing is smooth, clean and dry.
- If the surface is not smooth, use a concrete grinding stone to prepare it for the hammer test.
- Your chosen impact point should be at least 20mm away from the edge of the structure or any shape discontinuities.
- Take six rebound readings at each site and determine the average for a reliable value.
- Rebound hammer tests are non-destructive and provide immediate feedback on the consistency and quality of concrete.
Cons
- The results are based on a local point and aren’t a complete indicator of concrete’s compressive strength.
- This test alone cannot detect other flaws in the concrete’s structure.
- You must carefully select and prepare the areas for testing.
- The accuracy of rebound hammer test results can be influenced by surface conditions and material types, requiring careful interpretation.
The concrete hammer test is easy, reliable, and non-destructive. It suits any structure within its capabilities.
The concrete hammer test is easy, reliable, and non-destructive. It works well on any concrete structure within its capabilities.
A calibration anvil and a grinding stone are important to have on hand when preparing to perform the concrete hammer test. But, when it comes to the test itself, all you need is the hammer! The rebound hammer reading, represented as a rebound number or index, is obtained from the test and used to correlate the hardness of the concrete with its compressive strength. There are a few main types of concrete test hammers to choose from:
- Schmidt Hammer Type N: Concrete test hammers are sometimes called Swiss Hammers or Schmidt Hammers, and this is where the name comes from. Schmidt Hammers can be used for a wide variety of non-destructive concrete tests. The Type N is ideal for concrete strengths ranging from 1,400-10,000psi.
- Schmidt NR Hammer: The NR Hammer is also ideal for concrete strengths ranging from 1,400-10,000psi. However, the NR has the added capability of automatically recording rebound numbers as a bar chart on a paper strip for quick and easy comparison. Each strip chart can log up to 4,000 impacts.
- Gilson Type N Hammer: The Gilson Type N can test concrete strength within the same range as the Schmidt Type N. You can also use this test hammer to estimate the damage a concrete structure has sustained from freezing or fire. It also comes at a slightly lower price point.
Choose Your Concrete Hammer Test Equipment at Certified MTP
The Concrete Hammer Test, also known as the Schmidt Hammer Test or rebound hammer test, is a non-destructive testing method widely used in the construction industry to assess the compressive strength of concrete structures. Rebound hammers come in various types, each designed for specific applications in testing the quality of hardened concrete. Invented by Ernst Schmidt in 1948, this test offers a quick, economical, and easy-to-use tool for onsite evaluation of concrete strength.
Principle and Equipment:
The test measures how a spring-loaded mass rebounds when it hits the concrete surface. The equipment, known as a Schmidt hammer, has a spring mechanism inside a tube. When released, the spring drives a mass to hit the concrete surface, and the rebound distance of the mass is measured on a scale. This rebound distance, which is a measure of the surface hardness, is then correlated to the concrete’s compressive strength. Using the same concrete materials and mix proportions is crucial for establishing a reliable correlation between compressive strength and rebound index.
Procedure:
To perform the test, first smooth and clean the concrete surface to remove any loose material. Making testing standard specimens with the same concrete significantly enhances the accuracy and reliability of the strength predictions. Hold the hammer perpendicular to the surface and press it against the concrete to carry out the test. A typical test involves taking multiple readings at various points on the surface to achieve a comprehensive assessment. Then, use the average of these readings to estimate the concrete’s compressive strength.
Applications:
The Concrete Hammer Test evaluates the uniformity of concrete, identifies weak spots, and estimates the strength of existing structures. Inspectors often use it for bridge inspections, building evaluations, and quality control in new constructions. However, it provides only an estimated strength, so it should accompany core testing for a comprehensive analysis. Evaluating the structural integrity of older structures or those experiencing environmental stress is crucial to ensure their durability and safety.
Advantages and Limitations: Compressive Strength
This test offers fast, easy, and non-destructive testing. Its limitations include sensitivity to surface conditions and the need for calibration. It serves better as a comparative tool than a precise strength meter. Overall, it’s a valuable construction tool for assessing concrete quality and strength.
