Concrete is one of the widely used building materials due to formability and appropriate mechanical properties. One of the widespread utilizations of concrete is in pavement so that it can be used in the construction of roads, airports, industrial zones floors and so on. The binder which is used in ordinary concrete is Portland cement. The production of Portland cement causes serious harms to the environment because of the production of significant amount of carbon dioxide. One of the best measures to be taken in order to reduce these harms is using alkali-activated concrete instead of Portland cement one which can be used in various civil projects such as construction of pavement. On the other hand, by considering the increasing of production of plastic waste and the resulting environmental problems rendered by its disposal, substituting a proportion of natural aggregate in concrete with plastic particles could be a proper way for recycling plastic waste materials. This study investigates the utilization of polyethylene as a partial fine aggregate (5%, 10% and 15% by volume) in alkali-activated slag concrete for pavement construction purposes. Numerous experiments were conducted and then the results were compared with the numbers recommended by two pavement instructions. The fresh concrete experiments were slump and density and the tests that was done on the hardened concrete were some mechanical and durability tests. Mechanical tests were compressive strength, tensile strength, flexural strength and impact and durability tests were ultimate water absorption, water impermeability, carbonation, freezing and thawing resistance and electrical conductivity. It should be noted that in order to inspect the effect of plastic aggregate on the concrete in the microscopic scale, some SEM images were taken of the transitional zones of both natural and plastic aggregates. Generally, the results of this test are representatives of the reduction of both mechanical and durability properties of the concrete by adding plastic particles. The durability reduction happened due to the increasing of penetration of water, carbon dioxide and chloride ion. However, the resistance of the concrete to freezing and thawing cycles increased by adding plastic aggregate. For example, by replacing 10 percent of the fine aggregate with plastic particles the 28-day compressive strength and tensile strength lowered by around 11.3 and 7.5 percent, respectively or by this replacement percentage the total water absorption and 180-day carbon dioxide penetration increased by about 9.3 and 13.7, respectively. After comparing the results of this research with the recommendations of pavement standards, it was concluded that although the mechanical properties of concrete decreased as the plastic replacement increased, higher mechanical strength of alkali-activated concrete compared with Portland cement one compensated this reduction so that mechanical properties of all four mixes met the limitations that were determined by the instructions. In addition to the mentioned studies, the calculations of carbon footprint and embodied energy were also done in order to eva‎luate how much the replacing of this concrete with ordinary one can be environmentally advantageous. The calculations show that by replacing Portland cement concrete with this binder in any of the four mixes, the amount of emitted carbon dioxide and consumed energy decreases significantly. For instance, the CO2 emission and energy consumption of alkali-activated concrete with 10 percent of plastic aggregate are around 16.32 and 29.08 percent less than the equivalent Portland concrete, respectively. These results show that by widespread using of this concrete instead of Portland cement one, a good step is taken toward constructing of environmentally friendly concrete pavements because of both less carbon dioxide emission and less energy consumption.

كياچهر بهفرنيا

محمدرضا افتخار، فرامرز مودي