Effect of waste PET and CR as sand replacement on the durability and acoustical properties of semi dense asphalt (SDA) mixtures

Construction materials research is consistently striving to improve sustainability, in the reduction of virgin materials by use of otherwise landfilled materials of the same purpose. Crumb rubber (CR) from end-of-life tires
and polyethylene terephthalate (PET) from post-consumer liquid containers are two of the most commonly circulating forms of waste in the urban environment. This study investigated the replacement of semi-dense asphalt
(SDA) sand by untreated mechanically shredded CR and PET, at 2.5 and 5.1% respectively by total mass of aggregates. The mixtures were evaluated by compactability, indirect tensile strength (ITS), fracture energy (FE), water
sensitivity by ITS ratio (ITSR%), surface texture and acoustic absorption tests. After compaction, the CR and PET
samples experienced an elastic rebound effect, which resulted in the air voids being higher than expected.
Also, the PET samples required more compaction energy. The ITS, FE and ITSR% were significantly reduced
with CR replacement, while the PET mixture performed similar to the control, especially in FE. The sound absorption was related more to the air voids than the material type, although the absorption coefficients of the SDA was
not found to be significant. The CR reduced the texture level of the pavement significantly in comparison to the
control, while texture level remained the same for the PET mixture, despite a difference in the porosity. Further
studies were performed using a mixture replacing PET by aggregate volume at 5.1%, comparing it to the control
SDA in terms of low temperature cracking and permanent deformation at 50 °C. While the compactability of the
PET mixture was now similar to that of the control, the resistance to cracking and permanent deformation was
lower. Although the PET mixture had some interesting ductility properties, the replacement of sand by CR and
PET is not recommended, and the more common use as asphalt mixture modifiers with fairly low addition contents of around 1% is more sound.