Durability and performance of South African recycled granulates in unbound (sub)base pavement layers

Abstract

Recycled granular materials such as Recycled Concrete Aggregate (RCA) and Recycled Crushed Masonry (RCM) are widely used in The Netherlands as base layers in asphalt pavements. The lack of natural resources and the growing amounts of demolition waste made that the Dutch industries in the early 1980s started to explore the possibilities to use construction debris in road construction. Recently, the application of recycled materials in pavement structures has also found traction in South Africa. Due to differences in pavement design, however, the mechanical and environmental exposure of these materials will be more severe than in the Netherlands. This results in different challenges with respect to (long term) performance and material durability. Understanding the potential durability issues and the way durability affects pavement performance is crucial to successfully implement these materials in South African pavements.
This research, conducted at Stellenbosch University, South Africa, involves laboratory testing to investigate the performance and durability aspects of recycled aggregates. By means of triaxial testing before and after durability simulation, it is aimed to address the extent of potential material breakdown and the influence this has on performance. Tests are conducted on RCA, RCM, MG65 and MG30. The latter two refer to a mixture of RCA and RCM, with a mass percentage RCA of 65% and 35%, respectively. In addition to the recycled materials, a commonly used crushed rock of G2 quality is tested as well to serve as benchmark. Monotonic triaxial tests, to obtain the shear parameters, are performed on all materials except the pure RCM. Permanent deformation triaxial tests, to gain understanding of the long term response to cyclic loading, are performed on RCA and MG65. Specimens are tested under different confinement and deviator stress levels. For the durability simulation, the South African Durability Mill (DMI) is used. The DMI enables durability testing of the full grading under soaked and dry conditions. After the tests, the milled specimens are sieved out to obtain the change in grading.
The most important findings regarding granulate durability include that the breakdown in recycled materials is significant in comparison with the G2. Mainly the largest particle fractions are affected. Furthermore, for these particular resources of recycled granulates, the RCA suffers more breakdown than the RCM. The breakdown in the blends decreases with increasing masonry content, implying that the RCA is the most prone to mechanical damage. Considering the monotonic triaxial tests, substantial values of shear parameters are measured in all materials. The highest cohesion is measured in the MG30, while the highest internal angles of friction are measured in the pure RCA. The shear parameters in the recycled materials are in all cases higher than those for the G2. Differences in failure type (brittle versus plastic) are observed as well. Durability milling results in a small increase of the internal angle of friction and in a moderate decrease of cohesion. The latter is the most governing for the material’s compressive strength after milling, as this is decreased in all milled specimens. Still, the shear parameters of the milled specimens remain relatively high. In the permanent deformation triaxial tests, a decrease of performance can be observed in both the RCA and MG65. Delayed shear failure is observed in milled specimens tested at a deviator stress ratio (DSR) higher than 30%. Although the number of permanent deformation tests performed in this research is limited, 30% DSR seems the upper limit with respect to cyclic loading. This points out that monotonic triaxial testing alone is not sufficient for an adequate material characterization. A small linear elastic pavement analysis based on the tested materials, however, shows that the occuring DSR levels in a reference pavement caused by standard axles of 80 kN do not exceed 20% DSR, proving the potential of these materials for further studies.