The effect of grain size on G_max of a demolished structural concrete: A study through energy dispersive spectroscopy analysis and dynamic element testing

Recycled concrete aggregate (RCA) composed of demolition-crushed structural concrete is a promising material in geotechnical engineering applications, for example, as a backfill in retaining walls or as an embankment fill and pavement construction material. Due to the presence of cement mortar component, RCA has a lower unit weight than that of typical soils, thus its use may be considered beneficial in engineering infrastructures with a demand in the reduction of settlements or lateral earth pressures. In this study, a set of torsional resonant column and bender element tests were carried out on uniform fractions of a recycled concrete aggregate with origin from New South Wales, Australia, with varying the mean grain size. The created in the laboratory samples, were prepared in a dry state and tested under isotropic conditions of the confinement varying the effective confining stress from 25 to 800 kPa in a resonant column and in a triaxial apparatus with embedded piezo-element inserts with a particular focus on the elastic stiffness G_max. The results showed that the sensitivity of G_max to pressure increased with decreasing mean grain size. This observed trend was attributed, partly, to the higher cement mortar component for fractions with a smaller grain size. The different composition of the fractions was verified through Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS) analysis in particular quantifying the ratio of Silicon over Calcium contents. The performance of expressions proposed in the literature for the prediction of G_max of sands and gravels, was rigorously evaluated by means of measured against predicted elastic stiffness for all the fractions as well as by means of the state parameter for a particular RCA fraction.