Dynamic properties of crushed concrete used in pavement structures

Abstract

To reduce the environmental impact of transport in expanding cities, mass transport is an effective solution but it needs large amounts of good-quality aggregate. Recycled crushed concrete from demolition sites has the potential of substituting some of the conventionally used materials in railway construction due to its high strength and stiffness. However, knowledge about the dynamic properties of crushed concrete is very limited both in Finland and internationally. This limits the possibilities of assessing the groundborne vibrations, and hence there is a risk that the vibration levels become too large. Given the need for sustainable solutions and the large potential of crushed concrete, the dynamic properties of crushed concrete are studied and compared with conventionally used materials through a finite element simulation. In this thesis, an extensive literature review is conducted on railway dynamics and dynamic and small-strain properties of materials. The railway dynamics are studied through empirical models and conventional engineering dynamics. Based on the information found from the literature review, the small- strain stiffness affects significantly both damping, resonance frequency, and displacements related to the dynamic loading from railways. However, the existing empirical models are simplified to an extend where these properties are not included. Existing methods for estimating the small-strain stiffness and dynamic properties are also studied in the literature review, where the properties are estimated by several different methods. A resulting outcome of the literature review is an estimation of the behaviour of crushed concrete exposed to dynamic loading, where a small-strain shear modulus is found using a method by (He, et al., 2018) to be around 350 MPa at pressure of 400 kPa.

The crushed concrete is tested by resonant column and bender element test to determine the small-strain properties of the material. The small-strain stiffness found from laboratory tests is 656 MPa at 300 kPa, which is around 30% larger than the estimated value. However, by analysing the measured values with the estimated values, a correction of the assumed void ratio and particle density yield a good fit. The conventionally used materials and the crushed concrete is compared through finite element simulations. The compari- son shows no significant difference in the vibration dispersion and propagation resulting from the different materials. The velocities transmitted to the surrounding material are damped nearly equally fast for the compared materials. There is a small difference to the stronger ballast material and similar results between the subgrade and crushed concrete. The surrounding material seem to have a strong effect on the vibration propagation, which calls for an extended study on the geometry’s influence on the wave propagation.