Rebound hammer test: A popular NDT in the evaluation of concrete strength- Part 2

Previously we have discussed about the history and development of rebound hammer, and the operating principle. Here we will talk about the limitations of the use of rebound hammer in the assessment of the strength of concrete. 

In real practice, the number of influencing factors are very large and most of them are uncontrolled. Many factors may affect the test result to different degrees to the overall results. The influencing factors are identified and quantified by many researchers. The main influencing factors are summarised below:

Type of aggregates- Type and grading of aggregates have significant influence on the rebound hammer values. A difference of 7 points in rebound values were reported when concrete are made with limestone and gravel coarse aggregate, harder aggregates results high rebound value.

Age- Upper surface of the concrete reacts with the carbon-di-oxide resulting carbonation making the surface of the concrete harder. Due to the effect of carbonation, error may reach up to 50%.

Moisture content- A difference up to 10 rebound values between dry air concrete and saturated concrete can be observed on site where dry air concrete results in higher rebound value.

Surface regularity and roughness- Increases the variability in the measurements. Concrete just under the hammer crushes when the hammer impacts. For this reason, it is advisable to flatten the surface before conducting the rebound hammer test.

Near surface properties- Factors like type of framework used during the casting, curing conditions etc. that affect near surface properties, influences the measurement of rebound value. The presence of aggregates, air bubble or voids near the surface affect the measurement to a great degree.

Rigidity of the structure: Rebound value is lower if rigidity of the member of the structure is smaller. The rebound value measured on a core is lower than that directly provided on the structure. It is advisable to apply a pressure of 7 MPa to a concrete cube to keep it steady during the rebound hammer testing.

Rebound hammer device- Rebound value depends on the direction of testing (horizontal, upwards, or downwards) and type of hammer used for testing as well. Many types of rebound hammer exists in the market which are used for different cases, N-type, L-type, M-type, and P-type. The N-type rebound hammer is used for normal strength concrete having the impact energy of 2.207 Nm. The L-type rebound hammer is used for testing of small or thin walled (less than 100 mm) concrete members. The impact energy of L-type hammer is 0.735 Nm. The impact energy (29.43 Nm) and size of the M-type hammer is much higher than the N-type hammer and is used for high strength concrete pavement. For the low  strength concrete, such as lightweight or fresh concrete, mortar, P-type hammer is used having the impact energy of 0.833 Nm.

Stress state: Stress state influences the rebound value measurement, although in normal practice this is likely to be small in comparison with many other variables.

Rebound hammer test is considered to be one of the least reliable application in strength assessment of concrete. This is where misuse is most common as, unfortunately, a strength estimate by rebound hammer is frequently addressed by engineers. The accuracy in the estimating concrete strength depends entirely upon the assessing the influencing factors. The accuracy of the rebound hammer test in assessing compressive strength of concrete is within the range of ±15-20% for a properly calibrated hammer and for test specimens cast, cured and tested under laboratory condition, where in practical field, it is unlikely that the accuracy becomes lower, within the range of ±30%. Although it may be possible to address for one or two influencing factors in the relationship which may not be identical on site, the accuracy in predicting strength will decline as a consequence. For this reason, in practice, it is advised to use the Schmidt rebound hammer as a device of assessing relative concrete quality and uniformity, rather than a device for strength estimation.

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.