Characterization of recycled materials for use in road construction and determination of the resilient moduli with the cyclic load triaxial test

Project carried out with financial support from the Federal Public Service (FPS) Economy, SMEs, Self-Employed and Energy (Conventions CCN 309 and CCN 359 – 1st March 2006 through 1st September 2008 – Conventions CCN 506 and CCN 556 – 1st September 2008 through 1st September 2010 – Conventions CCN 807 and CCN 857 – 1st September 2011 through 1st September 2013).

In the first two years of the project the resilient moduli of a reference material (limestone) and two secondary materials (recycled concrete aggregate and steel slag) were determined by testing under constant confining stress according to method B (high stress level) in European standard EN 13286-7. The strains of the test samples were measured with three axial sensors and one radial sensor.

The influence of water content and of density after compaction on the resilient modulus was investigated and the effect of compaction method (vibrating hammer in six layers or vibrocompression) was determined. A number of load frequencies (from 0.5 to 3 Hz) were tested. In addition, a number of tests under variable lateral stress were carried out on the reference material (limestone).

In the second two-year period the permanent strain behaviour of the same materials was studied. Under optimum conditions of density and water content, the secondary materials tested (steel slag and recycled concrete aggregate) exhibited smaller strains after 100,000 cycles than the limestone. On the other hand they proved to be more sensitive to water content, especially the steel slag.

The test sample with limestone (reference material) compacted by vibrocompression developed greater permanent strains. At equivalent stress levels this effect was clearly less marked for the steel slag and was virtually inexistent for the recycled concrete aggregate.

A few multi-stage tests (50,000 cycles per stage, or possibly 80,000 cycles) were performed on the three materials. These tests did not produce the same strain levels as single stage tests. However, provided a sufficient number of cycles is performed (generally more than the 10,000 cycles mentioned in standard NBN EN 13286-7) at each stage these tests do reveal the trends in the behaviour of a material (order of magnitude of strains developed after a certain number of cycles, stabilization or increase of permanent strain at a given stress level). Nevertheless, the single stage test remains the reference for thorough research.

In the third two-year period tests under variable confining pressure were made on the limestone and on one recycled concrete aggregate and two types of recycled mixed waste aggregate. The resilient and permanent strains were investigated. The moduli found in these tests under variable confining pressure were smaller than in the tests under constant confining pressure. The highest moduli and the smallest permanent strains were recorded for the recycled concrete aggregate. One of the two types of recycled mixed waste aggregate was subject to severe permanent strain (5 %, or even more). From the tests made and the models developed it appeared that the stress levels suggested by the standard are too high. New procedures have been suggested.