Non destructive testing for concrete
Non Destructive Testing is the process of detecting the quality
without destructing the material or without changing its actual
properties.
Rebound Hammer is also called as Schmidt hammer or Swiss Hammer or Concrete hammer test.
This NDT method is used to detect the strength of Concrete Slab or rock, by identifying the hardness or elastic property.
Principle of Rebound Hammer Test.
Schmidt’s hammer test is based on the principle of “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 Rebound number / rebound index. The graph in the
body of hammer is used to found out the respective compressive
strength.
The concrete with low energy and low stiffness absorbs more energy from plunger and produces a low rebound value on the scale.
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.
Apparatus Required:
Rebound hammer test lab report
Apparatus consists of a spring controlled mass that slides on a
plunger within a tubular housing. The impact energy required for rebound
hammers for different applications is given in table below
How to do Concrete Hammer Test Work:
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 the reliable
results on the anvil. if yes- 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 grinded off using grinding wheel or stone. Rough
surfaces resulting from incomplete compaction, 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 edges or corners of 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.
Repeat the Test for 6 readings, and average the value.
Rebound hammer test procedure
The measured value is 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 down wards,
Pos. C – Vertical Upwards,
Step 4: Interpretation of Results
The result from the test tells us the quality of Concrete or rocks.
Standards for Rebound Test Hammer:
Thus the nondestructive testing methods is a rapid
testing methods which can be handled easily to determine the quality of
civil works.
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