Tuesday, 17 July 2018

Rebound hammer test: A popular NDT in the evaluation of concrete strength- Part 1

Good to see you again. This time, I will go through a series explaining some popular non-destructive tests (NDTs) of concrete strength assessment. 

Rebound hammer test can be considered as a method of measuring surface hardness of the concrete. The increase in the hardness of concrete with age and strength encouraged the development of the test method to measure the hardness of concrete. The use of hardness testing method in concrete can be traced back to the early twentieth century after the introduction of the Brinell indentation method for metals. The test method is based on the rebound principle consisting of measuring the rebound of a spring driven hammer mass after its impact. Ernst Schimdt in 1948 introduced spring impact device of concrete by rebound principle. To date, it is the most popular in-situ testing method for concrete due to being inexpensive and its relatively simple use and fast operation. With the development of this device, the hardness measurement became much easier. The measurement can be read directly on the scale of the device. 

The technique provides an easy measurement of rebound of impact hammer and the operation is quite simple. During the operation, a hammer impacts the concrete at a fixed energy with the help of a spring and rebounds from the surface of the concrete after the impact. Energy of the hammer is applied by tensioning of spring and it is independent of the operator. The test method consists of measuring the rebound of the hammer after its impact with concrete and correlating the rebound value with compressive strength of concrete.

When the hammer impinges on plunger, a compressive wave propagates into the concrete through the plunger. The plunger deforms elastically during the stress wave propagation. When the compression stress wave reaches the other end of the plunger (i.e. the concrete), part of the energy is absorbed in the concrete and the rest of the stress wave is reflected back in the plunger. The reflected compression wave returns to the free end of the plunger and causes the hammer to rebound. From the theoretical point of view, the rebound of the hammer is dependent on the energy absorbed during the impact. The energy absorbed in concrete results from both elastic deformation and plastic deformation (local crushing) of the concrete. When the acceleration of the plunger is brought to rest, the elastic deformation of the concrete recovers. A residual set is formed in the concrete under the tip of the plunger. The absorbed energy is dependent on the properties of the concrete at the vicinity of the tip of the plunger. The relationship between rebound value and concrete strength depends on the strength, stiffness and damping capacity of concrete in the vicinity of the tip of the plunger of the rebound hammer. Concrete with high strength, high-stiffness will absorb less energy than the concrete with low strength and low stiffness. Therefore, for two concrete mixes with same strength but different stiffness, the rebound values can be different even if the strengths are equal and vice versa.

Rebound hammer is the most popular NDT in this field. The test is affected by high number of factors, most of them are uncontrolled, thus affecting the overall assessment of the strength of concrete. We will talk about that in the next episode. 


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