Compressive mechanical and durability properties of concrete with polyethylene terephthalate and high-density polyethylene aggregates

In this study, the effects of the water-cement (W-C) ratio and plastic aggregates (PAs) recycled from polyethylene terephthalate (PET) and high-density polyethylene (HDPE) plastic waste on the compressive mechanical and durability properties of concrete are presented. Coarse flake-shaped PET and fine pellet-shaped HDPE aggregates were used as substitutes for natural gravel and river sand with 0-10% and 0-34% volumetric replacements, respectively, in concrete with two different W-C ratios (0.62 and 0.47). Fresh and hardened concrete were evaluated in terms of workability, elastic modulus, Poisson's ratio, compressive strength, drying shrinkage, and resistance to chloride ion penetration. Micro-computed tomography showed that the incorporation of PAs in concrete modified the pore structure by increasing the porosity and volume percentage of large pores. The effect of PAs on the reductions in slump, mechanical properties, drying shrinkage, and chloride ion penetration increased with the increase in content, size, and flaky shape of PAs. For 0.62 W-C ratio concrete, 10% HDPE content lowered the compressive strength, drying shrinkage, and chloride ion penetration by 5.0%, 6.2%, and 12.1%, respectively, compared with those of the control specimen. Meanwhile, for the same amount of PAs (kg m-3 of concrete), PET aggregates increased the percentage reductions to 21.0%, 12.1%, and 17.3%, respectively. For concrete with single and combined types of PAs, the ratios of compressive strength, elastic modulus, and drying shrinkage of PAs to those of control concretes linearly decreased with an increase in the content of each type (in terms of size and shape) of PA. In addition, substitutions of natural aggregates by PAs decreased significantly the stress-strength ratio at the transition between the elastic and inelastic ranges, particularly for concrete with a low W-C ratio and high content of coarse PAs. For 0.47 W-C ratio concrete, the critical stress-strength ratio (0.83-0.89) in the control specimens decreased significantly to 0.57 and 0.75 for 5% PET and 34% HDPE contents, respectively.