The effect of sulfuric acid attack on mechanical properties of steel fiber-reinforced concrete containing waste nylon aggregates: Experiments and RSM-based optimization

Replacing natural aggregate with recycled plastic has been a strategy to reduce pollution associated with conventional concrete production and disposal of plastic materials. Due to the possibility of structures subject to acid attacks, it is of value to check the resistance of concrete against these attacks. The present study evaluates the behavior of concrete containing steel fibers and nylon granules under sulfuric acid attack. 9 batches of concrete containing 0, 10, and 20% (by volume) of nylon granules, replacing natural sand and three volume percentage of steel fibers (0, 0.75, and 1.25%) were prepared. First, concretes were placed in a 5% sulfuric acid solution for 0, 45, and 90 days, and then their ultrasonic pulse velocity (UPV) and compressive capacity were evaluated. The microstructural behavior of concretes was also investigated through scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDXMA). Finally, a model using the response surface method (RSM) was proposed to optimize the content of steel fibers and nylon granules in concrete to achieve the maximum strength of concrete in an acidic environment. The results show that increasing the acid exposure time decreases the compressive capacity and UPV of concrete. Concrete with a higher nylon granule content exhibits a lower strength and UPV decline rate due to acid immersion when compared to that with a lower nylon granule content. After optimization, it was found that the optimal volume percentage of steel fibers and nylon granules in concrete after 90 days of acid exposure is 0.11% and 20% to maximize the compressive strength, respectively. The findings of this study help to minimize the extraction of non-renewable natural resources and the environmental impact of waste nylons by developing cleaner and more sustainable concrete.