<![CDATA[Concrete is a widely used construction material due to its high compressive strength and the relative ease of obtaining its constituent materials. With the rising environmental issues caused by plastic waste—specifically Polyethylene Terephthalate (PET)—efforts are needed to repurpose this waste as an alternative construction material. Utilizing PET as a concrete additive offers a way to reduce plastic waste while supporting the development of more eco-friendly construction materials through the use of locally available resources. Additionally, the use of Fly Ash—a byproduct of the North Sulawesi II Steam Power Plant—further supports the utilization of waste materials in concrete mixtures. This study aims to determine the effect of using PET and Fly Ash on the mechanical properties of concrete and to compare the performance of normal concrete with concrete containing 30% Fly Ash and PET at levels of 0.6% and 1%. The research was conducted experimentally at the Materials Testing Laboratory of the Civil Engineering Department, Manado State Polytechnic. Cylindrical specimens with a diameter of 100 mm and a height of 200 mm were used, with a design strength of 20 MPa. Testing included slump tests, compressive strength tests at 7, 14, and 28 days, and splitting tensile strength tests at 7 and 28 days. The test results indicate slump values of 6.0 cm for normal concrete, 3.5 cm for the 30% Fly Ash mix, 4.2 cm for the 30% Fly Ash + 0.6% PET mix, and 4.8 cm for the 30% Fly Ash + 1% PET mix. At 28 days, normal concrete yielded the highest compressive strength at 20.87 MPa, followed by the 30% Fly Ash mix at 19.06 MPa, the 30% Fly Ash + 0.6% PET mix at 19.03 MPa, and the 30% Fly Ash + 1% PET mix at 18.31 MPa. Meanwhile, the splitting tensile strengths at 28 days were 2.51 MPa, 2.40 MPa, 1.62 MPa, and 1.13 MPa, respectively. Based on these results, the use of 30% Fly Ash and 0.6% PET yielded better performance than the use of 1% PET, particularly regarding the 28-day compressive strength. The utilization of PET and Fly Ash in this study demonstrates the potential of using waste materials as local alternatives that can support the implementation of more eco-friendly and sustainable construction practices.]]>
