This paper presents a focused comparative case study considering the influence of natural and synthetic fibers on the fresh and mechanical properties of concrete. Locally sourced 19 mm long sisal fibers from sisalana leaves and manufactured polypropylene fibers were incorporated in a normal strength concrete matrix with fiber volumetric contents of 1%. After describing the measured aggregate characteristics, mix designs, and fresh concrete properties, several destructive and non-destructive tests on hardened concrete were undertaken. The former included compression tests on cylinders and flexural tests on prismatic samples, and the latter included ultrasonic pulse velocity and rebound number tests. The workability of sisal-fiber reinforced concrete was generally lower than the nominal concrete and that provided with polypropylene fibers by about 20%, largely due to the hydrophilic nature of the natural fibers. Test results showed that the presence of sisal fibers can improve the compressive strength by about 6%, and the tensile strength by about 4%, compared with the non-reinforced counterpart. This was due to the sisal fibers storing moisture that was released gradually during hydration, helping with the strength development. The concrete with polypropylene had virtually identical properties to the reference concrete. In addition to fresh and mechanical properties, environmental impacts associated with the production of fiber and concrete were also identified and discussed. Based on the assessments from this paper, overall, from the two fibers investigated, the sisal fiber showed more promising results, indicating that natural fibers can be a more sustainable alternative to plastic fibers, providing a good balance between workability and strengths.

Cement is one of the most valuable materials in today’s society, as it is used in most construction developments known to mankind. However, the energy intensive process and significant environmental impacts related to the production of Ordinary Portland Cement have shown the importance of searching for more sustainable materials. Concrete uses different aggregates added to the cement binder to lower, not only cost, but other factors like environmental burden, while maintaining good mechanical properties. This study analyzes the properties of fresh and hardened concrete incorporating recycled rubber to replace fine aggregate. Locally sourced 2 mm diameter rubber was incorporated in a regular strength concrete matrix into three different replacement levels, i.e., 3%, 5%, and 10%. Compression, tensile, flexural, and modulus of elasticity of hardened concrete were carried out in specimens aged 7, 14, and 28 days. In addition, non-destructive ultrasonic pulse velocity and rebound number tests were only performed on specimens aged 28 days. Once the tests were carried out, the fresh and hardened concrete properties were obtained. Similarly, the compressive and flexural strengths had the exact relationship between the values obtained. On the other hand, the modulus of elasticity tends to decrease due to the presence of the rubber. Consequently, it is recommended not to develop mix designs with more than 5% rubber because it is not meaningfully affected. The fine aggregate can be partially replaced by the rubber, keeping almost the same performance compared with sand-only counterparts. In addition, the life cycle assessment showed a reduction of up to 40% in the global warming potential. In fact, the 15% recycled rubber concrete mix has a climate change indicator of approximately 245 kg of CO2 eq.