Tuesday, 7 August 2018

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.