Getting a handle on rebound hammer graphs is far more straightforward than its reputation suggests.
We are talking about a tool that gives you a reliable indication of concrete strength without resorting to destructive measures. In our industry, it’s a cornerstone of quality control, but the real skill is in knowing what a result truly signifies.
Before you can interpret any graph, you must first arrive at a single, reliable number from your series of tests.
First Things First: Mastering the Rebound Hammer Reading Calculation
The foundation of an accurate result is a proper rebound hammer reading calculation. You don’t simply take one reading and use it. This process is the first and most critical step in learning how to calculate rebound hammer test results you can stand behind.
Prepare the Surface and Take Multiple Readings
For any single test area, you should take between 10 to 12 individual readings. It is absolutely essential that the surface is prepared correctly. Use the provided grinding stone to remove any plaster, loose material, or carbonation to get to the clean, dry parent concrete.
Discard Outlying High and Low Readings
Next, review your set of numbers. It is standard practice to discard the highest and lowest 20% of your readings. This step is critical as it prevents an unusually hard aggregate near the surface or a small, localized soft spot from skewing your final result.
Calculate the Final Average
With the outliers removed, calculate the arithmetic mean of the remaining readings. This final number is your average rebound value. This is the manual process you'd use with a classic tool like our Concrete Rebound Hammer QualiCRH-2000A.
For instance, if your 10 readings are 35, 34, 39, 36, 31, 35, 38, 35, 34, 32, you would:
- Discard the two highest (39, 38) and the two lowest (31, 32).
- Average the remaining six numbers (35, 34, 36, 35, 35, 34).
- Your final calculated average rebound number would be 34.83.
How to Read a Rebound Hammer Graph and Test Reading Chart
With your average rebound number in hand, you can now translate it into an estimated compressive strength figure. This is the core skill of how to read rebound hammer graphs and their corresponding charts.
Select the Correct Curve for Your Test Angle
The graph will present a few different curves. These correspond to the orientation of the hammer during the test (horizontal at 0°, vertically upwards at +90°, or vertically downwards at -90°). You must select the correct curve for an accurate rebound hammer test graph reading.
Locate Your Value on the Horizontal Axis
Find your calculated average rebound number along the bottom (X-axis) of the chart.
Trace Vertically to the Curve
From your number on the X-axis, trace a perfectly vertical line up until it intersects with the orientation curve you selected. We find a small, transparent straightedge is invaluable here.
Trace Horizontally to the Vertical Axis
From that exact point of intersection, trace a perfectly horizontal line to the side (Y-axis) of the chart. The value you land on is the estimated compressive strength. This same data is often presented in a rebound hammer test reading table for quick reference.
What Your Readings Reveal: Learning How to Read a Rebound Hammer Test
From our professional standpoint, this is where the rebound hammer demonstrates its full capability. Beyond just finding a single average, the consistency of your readings provides a clear picture of the concrete's uniformity. This is a key part of how to read rebound hammer test results like an expert.
Consistent Readings
If your numbers are all closely grouped, that’s an excellent sign of uniform, well-compacted concrete.
(e.g., A set of readings like 34, 35, 36, 34, 35, 35 suggests uniformity.)
High Variation
If your readings show wide, inconsistent variations, that should be a warning sign that could indicate internal voids or issues from the pour.
(e.g., A scattered set like 28, 39, 31, 42, 33, 37 could indicate underlying issues.)
Localized Low Readings
If one specific area consistently produces a low reading compared to the surrounding area, it suggests a localized problem that warrants direct attention.
(e.g., If most of your readings are in the 35-38 range, but one spot repeatedly gives you readings of 26, 28, 27, that specific location needs a closer look.)
Putting It All Together: A Worked Example
Let's apply the full process. Imagine you take 12 readings on a horizontal concrete wall, yielding these numbers: 34, 36, 35, 38, 32, 35, 37, 39, 31, 35, 36, 34.
- The Calculation: This is how to calculate rebound hammer test results in practice. We discard the highest two (39, 38) and lowest two (31, 32) readings. The average of the remaining eight numbers gives a final average rebound value of 35.25.
- The Graph Interpretation: Now for the rebound hammer test graph reading. We select the curve for a horizontal (0°) test. We find 35.25 on the bottom axis, trace up, and then trace across. The line should meet the axis at approximately 27 MPa (or ~3900 psi).
Related article: A Guide to Concrete Rebound Hammer Calibration
For the Highest Degree of Accuracy: Developing a Custom Curve
While the manufacturer's rebound hammer test reading chart provides a dependable starting point, it's based on a standard, generic concrete mix.
Your project's concrete, with its specific local aggregates, cement type, and admixtures, will have its own unique relationship between surface hardness and strength. For the most precise results, you can develop a custom correlation curve that maps this unique relationship perfectly.
The process is straightforward. When test cylinders or cubes are cast from the same concrete batch used in the main structure, you perform a full rebound test on them before they are sent to the lab for destructive compression testing. For each cylinder, you will have two crucial pieces of data:
- Your calculated average rebound number.
- The actual compressive strength, as determined by the lab's crushing test.
You then plot these pairs of data on a graph—rebound number on the bottom axis and actual strength on the side axis.
After plotting several points from multiple cylinders, you can draw a best-fit curve. This becomes your custom rebound hammer test reading chart, perfectly calibrated to your project's materials.
When you then test the actual structure, you can use this chart for your rebound hammer test graph reading, removing a significant variable and providing a level of confidence in your results that a standard chart simply cannot match.
Trust Qualitest for Rebound Hammer Results
At Qualitest, we operate on the principle that your results must be accurate. That is why we supply a full range of high-quality, cost-effective concrete rebound hammers built to meet key international standards like ASTM C805.
For straightforward and reliable manual testing, our QualiCRH-2000A is a trusted workhorse. For professionals looking for enhanced efficiency with features like automatic calculation, our QualiCRH-2000D is the ideal solution.
We invite you to explore our collection on our product page. Contact us to find out more.
