Table of Content
- Introduction
- How to Estimate Concrete Strength?
- What is a Rebound Hammer (Schmidt Hammer)?
- The Objective of Hammer Rebound Test
- How to Use the Rebound Hammer?
- How to Calculate Rebound Number?
- Applications of Rebound Hammer
- Estimate Concrete Strength Using Rebound Hammer (In-Place Method)
- What Affects Rebound Hammer Readings?
- Standards for Rebound Test Hammer
- Benefits of Rebound Hammer Test
- Final Word
- Key Takeaways
- FAQs
Introduction
Non Non-destructive Testing (NDT) is the process of detecting quality without destructing the material or changing its actual properties.
The rebound hammer test is also called as Schmidt hammer or Swiss Hammer or Concrete hammer test.
This NDT Method is used to detect the strength of concrete slabs or rocks by identifying their hardness or elastic properties.
How to Estimate Concrete Strength?
Estimating Concrete Strength is a crucial aspect of construction, providing insights into the material's durability and structural integrity.
The rebound hammer test is a Non-destructive Method widely used to estimate concrete strength.
The test involves striking the concrete surface with a rebound hammer and measuring the rebound index.
This index is then correlated with the concrete's compressive strength using conversion charts or tables.
Concrete strength estimation is a complex process that depends on Rebound Hammer Test Factors such as project requirements, accessibility, and precision.
The UPV test measures the speed of ultrasonic pulses through concrete, which indirectly assesses its compressive strength.
The Penetration Resistance Test measures the resistance encountered in concrete, with higher resistance indicating stronger concrete.
Core testing extracts cores from the structure, while the maturity method monitors the curing process's temperature and time history.
The pull-out test involves inserting a steel rod or stud into concrete and determining its compressive strength.
Combined methods, such as rebound hammer and UPV, provide a more accurate evaluation.
Proper curing, sample representativeness, and correlation charts can help in estimating concrete strength.
What is a Rebound Hammer (Schmidt Hammer)?
Schmidt’s Hammer Test is based on the principle that the “rebound of a spring-loaded mass depends on the hardness of the concrete mass on which the mass strikes”.
This rebound distance of the rebounded Plunger mass is noted down in the graduated scale as the Rebound number/rebound index.
The graph in the body of the hammer is used to find out the respective compressive strength.
The concrete with low energy and low stiffness absorbs more energy from the plunger and produces a low rebound value on the scale. This is crucial in Calculating Concrete Strength in the Rebound Hammer Test.
The Objective of Hammer Rebound Test
- To assess the compressive strength of concrete
- To compare the quality of two different concrete pour
- To assess the quality of concrete against the requirements of construction standards
- To detect the locations of weak concrete slabs or concrete with voids.
How to Use the Rebound Hammer?
The rebound hammer is a method used to Measure the Strength of Concrete.
It involves the NDT Inspector and NDT Technician preparing the concrete surface, calibrating the hammer, selecting test locations, holding the hammer, striking the surface, taking multiple readings, recording readings, correlating readings, and interpreting results.
The process involves ensuring the surface is clean and free from loose particles or debris by NDT Professionals.
The rebound hammer is then placed against the concrete surface, releasing the hammer to measure the rebounded energy.
Factors such as NDT Techniques for Surface Flaws, moisture content, and reinforcing bars should be considered when interpreting the results.
Step 1: Calibration
Carry out Rebound Test on a calibrated Anvil, supplied by the manufacturers.
The anvil would be made of Steel with a hardness value of approximately 5000N/mm2.
The rebound hammer should produce reliable results on the anvil.
If yes, proceed with the procedure for inspection; if no, then send the instrument for re-calibration to the manufacturer.
Step 2: Surface Preparation
The test surface should be smooth, clean, and dry. The loosely adhering scales should be ground off using a grinding wheel or stone.
Rough surfaces result from incomplete compaction and loss of grout.
Spalled or tooled surfaces do not give reliable results and should be avoided.
The point of impacting the Rebound Hammer should be at least 20mm away from the edges or corners of the concrete.
Step 3: Testing
Hold the Rebound Hammer at right angles to the test surface with the plunger pin pointing towards the test location.°
Push the plunger against the concrete, and force the cylinder body towards the concrete.
The force causes the latch to release and make an impact on the concrete.
Essentially the rebound of the spring-driven plunger mass is measured in the scale.
Step 4: Interpretation of Results
The result from the test tells us the quality of Concrete or rocks.
How to Calculate Rebound Number?
The measured value is the Rebound Index which is checked in the graph for its corresponding compression strength in N/mm2.
Rebound Hammer Graph
The Graph Contains Three different curves for three Different positions of Testing.
Pos. A – Horizontal Position,
Pos. B- Vertical downwards,
Pos. C – Vertical Upwards,