Performance assessment and ranking of natural and recycled granular materials for road subbase layers by precision cyclic triaxial testing (EN 13286-7)

Abstract

The cyclic load triaxial test is a laboratory test that allows investigating the mechanical behaviour (resilient and permanent strains) of unbound granular materials used in subbase and capping layers of roads. The resilient modulus and permanent strains are required to assess material performance and serve as key input properties in modelling and designing roads. This paper will present the investigation of resilient and permanent strains as specified by European standard EN 13286-7, for a limestone aggregate and recycled materials (one crushed concrete aggregate and two blended crushed waste aggregates) used in subbases and capping layers of road structures in Belgium. Method B (constant confining pressure) was used in most of the tests for resilient strains. The influence of water content was also analysed. Permanent strains were analysed after conditioning by 20,000 cycles of single-stage loading. The paper will also present the results obtained on the limestone aggregate only in analysing resilient strains with both axial and confining cyclic loading – method A. The resilient moduli were deduced from the test results and compared to those obtained with method B. The calibration of the LDT’s in air proved to remain valid when they were used in water (variable confining pressure tests). The natural and recycled materials were ranked starting from the characteristic values of resilient modulus and characteristic permanent axial strain. This ranking based on a mechanistic approach was compared with conventional ranking based on indicative properties determined in empirical tests. This research work produced an excellent set of findings for the mechanical characterization of unbound subbase materials through the cyclic triaxial test, thus contributing to a more widespread and common use of recycled materials in geotechnical engineering.