Impact of connection between specimen and load plate on viscoelastic material response of hot mix asphalt

In routine testing of hot mix asphalt (HMA) cyclic compression tests (CCT) are carried out to assess the permanent deformation behavior. However, CCTs can also be employed for determining the material response within the linear viscoelastic domain in terms of stiffness and phase lag, when a frequency and temperature sweep is considered. In the compressive domain two test setups are possible regarding the connection between load plate and specimen: The specimen is either firmly connected (glued) to the load plates, which prevents transverse strain at the end planes, or the specimen is placed between the load plates without firm connection. In this case friction-reducing additives (e.g. silicone grease) applied to the end planes of the specimen help to create a more homogeneous strain distribution over the height of the specimens, since limited transverse strain are also be activated at the end planes of the specimen. This paper investigates whether these two test setups produce comparable results in terms of viscoelastic material reaction of HMA. Therefore, CCTs were run on HMA-specimens with the two test setups. Data evaluation was carried out by means of regression analysis with a standard sine and an advanced function containing the first harmonic oscillation term. It is shown that the parameters of the first harmonic term are able to describe the magnitude and shape of a distorted sine. Data from force and axial deformation signals are analyzed as well as the derived viscoelastic material reaction. From the findings of the study it can be concluded that cyclic material tests in the compressive domain are capable of describing the viscoelastic material behavior of HMA regardless of the connection between specimen and load plates and that they are comparable to results of a well-established, standardized stiffness test, the 4-point-bending test. It is also shown that the regression analysis with the advanced function can be employed to analysis the quality of response of the test machine. Thus, technical limits of test machines and deformation sensors can be detected by using the regression analysis with the advanced approximation function.