Abstract
One of the most widely spread techniques to estimate the compressive strength of concrete is the rebound hammer test, also known as Schmidt Hammer test. In spite of a large number of scientific works trying to calibrate the test, to identify the parameters affecting its results and to estimate its reliability, the original Schmidt curve is still provided by the producers along with the hammer and is used in Structural Engineering Applications. This paper discussed an extensive research, and application, of this technique to a large number of cubes provided by the Laboratory for Building Materials of the University of Genoa, Italy, showing that several phenomena strongly affect the test: moisture content, maturity, stress state among the others. Strength estimates may differ as much as 70% if these parameters are not taken into account. Besides, several in situ investigations on existing buildings were affected by a large dispersion of data, so that we should conclude that the Rebound Hammer is unable of giving a reliable estimate of the concrete strength. This is probably due to the very limited area of the material on which the test is performed that allows also small local inhomogeneity to affect quite strongly the test. Therefore, the rebound hammer seems to be useless in the estimation of concrete compressive strength, being only a rough tool for estimating material homogeneity inside a specific concrete type.
Highlights
In Civil Engineering practice, the estimation of concrete quality is needed both for quality controls of new buildings and for rapid surveys of existing structures
The most recent results of scientific research show that the Rebound Hammer might provide some information on concrete quality provided that it is calibrated on the specific concrete type it is used on [11 and 12]
7 and 28 days data could be obtained with good precision, while third party specimens are often older than 100 days since the 28 days limit is seldom respected. These circumstances allow to discuss the effect of concrete maturity on rebound tests, which turns out to be quite important: a 30mm rebound index would account for a compressive strength ranging from 32 MPa for 7 days old cubes to 57 MPa for more than 100 days old cubes, which almost 85% more
Summary
In Civil Engineering practice, the estimation of concrete quality is needed both for quality controls of new buildings and for rapid surveys of existing structures. Calibration has been performed on a large number of specimens cured in standard conditions but without separating the contribution of the different factors affecting the test, such as concrete maturity and hardening conditions, moisture, surface finishing, concrete composition, aggregate type and hardness, etc. The most recent results of scientific research show that the Rebound Hammer might provide some information on concrete quality provided that it is calibrated on the specific concrete type it is used on [11 and 12]. These conclusions did not yet enter common Civil Engineering practice. From existing structures of different types and age, are considered in order to allow a rational estimation of the test reliability by comparison of the available data
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