PSI - Issue 68

Asad Zia et al. / Procedia Structural Integrity 68 (2025) 231–237 A. Zia et al. / Structural Integrity Procedia 00 (2025) 000–000

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Recycling and reusing construction waste, particularly aggregates, are therefore essential for sustainable development (Sufian et al., 2021; Zia & Ali, 2018; Zia & Khan, 2021). Aggregates account for 85% of the total C&D waste generated (Tam et al., 2018), making their reuse and recycling crucial for sustainability efforts (Brasileiro et al., 2024; Gebremariam et al., 2024; Santos et al., 2024). Studies have shown that replacing more than 50% of natural aggregates with recycled concrete aggregates (RCA) leads to a notable decline in concrete properties, with particularly significant declines observed when 75% to 100% of natural aggregates are replaced. To address this issue, the use of industrial fibers has been suggested to improve the properties of RCA concrete (RAC). Research indicates that steel fibers, among commercially available fibers, are particularly effective in reinforcing recycled concrete composites. Hook-end industrial steel fibers are the most widely used, appearing in 38% of research studies globally between 2004 and 2021 (Ahmed et al., 2022). Significant improvements in strength, ductility, and water absorption reduction have been reported when using a hybrid of steel fibers (SFs) and polypropylene fibers (PFs) in RAC (Htet et al., 2024). However, steel fibers present challenges due to their high cost and the carbon dioxide emissions associated with their production. In this context, using steel fibers derived from waste tires offers a promising solution for enhancing RAC properties without increasing carbon emissions. Recycled tire steel fibers (RFs) make up 15% to 25% of tire composition, making their recycling or reuse essential for pollution mitigation. Various studies have explored the use of RFs in natural aggregate concrete, observing significant improvements (Awolusi et al., 2021; Zia et al., 2023), with many studies focusing on raw RFs due to their commercial availability. The primary goal of the modern construction industry is to develop durable and sustainable concrete structures that can withstand adverse environmental conditions and meet long-term durability requirements (Nnaemeka & Singh, 2019; Tang & Deng, 2022; Tumba et al., 2018). Water absorption, which affects concrete performance by increasing porosity and reducing strength and durability, is also a critical factor (ASTM C1585-13, 2013; De Schutter & Audenaert, 2004; Dhakal et al., 2007). Its significance in defining the durability of cement composites has been emphasized by other researchers (Christopher et al., 2023; Panesar, 2019). In this context, recycled steel fibers from end-of-life tires represent a viable option for enhancing the performance of RAC, offering a sustainable solution. This study aims to explore the effectiveness of end-of-life tire steel fibers (EFs) in improving RAC performance, particularly by examining the influence of locally accessible EFs on RAC properties. Specifically, concrete with 75% of natural aggregates replaced by RCA was investigated, incorporating 0.60% of hook-end industrial steel fibers (IFs) and EFs. The study evaluates compressive strength, total energy absorption, compression toughness index, split tensile strength, and water absorption following ASTM standards, using plain concrete as a reference. 2. Experimental Program The study investigates concrete with 75% of natural aggregates replaced by recycled concrete aggregates (RCA), incorporating 0.60% of hook-end industrial steel fibers (IFs) and end-of-life tire steel fibers (EFs). The mix design for plain recycled aggregate concrete (RAC) includes the following materials per cubic meter: 280 kg of CEM II cement, 202 kg of water, 780 kg of sand (0/4 size), 1035 kg of coarse aggregates (RCA), 720 kg of 8/16 aggregates, 60 kg of fly ash, and 5.18 kg of plasticizer LZ84. For fiber-reinforced concrete, the same mix design is used, except for the addition of fibers and an increase in plasticizer quantity to 5.71 kg/m³. The raw waste tire steel fibers used in this study comprise multi-scale fibers of various sizes, which differ from the single-size industrial fibers typically employed. The properties of plain concrete (0F7R) serve as a baseline against which the effects of fiber reinforcement on RAC are compared. Locally available concrete materials from Bratislava were utilized. The end-of-life tire steel fibers (EFs) were sourced from Fibercom company in Bratislava and met EN 14889-1:2006 standards. The fibers varied in thickness, with those over 0.80 mm being more curved and resistant to deformation, while those ≤ 0.40 mm were more pliable. The industrial steel fibers used in this study have an average length of 50 mm and a diameter of 0.80 mm. Compression tests were conducted on cylindrical specimens according to ASTM C39/C39M-21 (ASTM C39/C39M-21, 2021). Split tensile tests were performed on specimens that had undergone 21 days of carbonation in a chamber, following ASTM C496/C496M-17 (ASTM C496/C496M-17, 2017). Water absorption tests were carried out in accordance with ASTM C642-13 to evaluate water absorption properties (ASTM C642, 2013). A total of twenty seven specimens were allocated for testing: 9 for water absorption, 9 for compression testing, and 9 for split tensile