PSI - Issue 64

Francisco Játiva et al. / Procedia Structural Integrity 64 (2024) 1468–1475 Jativa et al./ Structural Integrity Procedia

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2.1.2. Polypropylene and Natural Fibers For the control specimens, a polypropylene fiber TUF-Strand-SF made by Euclid® was used with l f = 50 mm, and E = 9.5 GPa. The natural fibers used in this study are abaca and coconut fiber. Abaca fiber is obtained from the pseudo stream of Musa textile, known as Manila hemp and corresponds to a second quality fiber (classification based on color and diameter of the fiber) (Simbaña et al., 2020). The abaca fibers considered in this study come from Santo Domingo de los Tsachilas province in Ecuador and were delivered with a length ranging from 1.5 m to 2 m. Coconut fibers extracted from the outer husk of the coconut are an agricultural byproduct of processing of coconut oil. The coconut fibers for this study were obtained from the coastal area of Ecuador (province of Manabi) and had lengths that varied between 100 – 150 mm. The average mechanical properties of the Santo Domingo de los Tsachilas province abaca fibers, as determined in the laboratories of Universidad San Francisco de Quito are average diameter Ø = 0.2 mm, maximum axial strain of 0.05, elastic modulus of 15.3 GPa. The abaca fibers were cut to fibers with l f = 100 mm for our concrete mixes. The mechanical properties of the Manabi province coconut fibers are average diameter Ø = 0.18 mm, maximum axial strain of 0.15, elastic modulus of 5.3 GPa. No further cutting of the coconut fibers was done for application in the concrete mixes. The aspect ratios (L/Ø) for polypropylene, abaca, and coconut fibers are 74, 500, and 555, respectively. Abaca and coconut fibers are thinner (Ø = 0.2 mm, 0.18 mm) compared to polypropylene fibers (Ø = 0.68 mm) and have double the length ( l f = 100mm Vs. l f = 50mm). A longer natural fiber length was considered ( l f(natural fiber) = 100mm Vs. l f(polypropelene) = 50mm) to maximize strain energy in the sample (Majumder, 2023). 2.2. Mix Design For the concrete mix design, only stone #7 and sand (conforming to ASTM C33-18), were used due to mold size constraints. The mix had a water-to-cement ratio ( w/c ) of 0.45 and aggregates constituted 60% of the total materials weight. Stone #7 represented 60% of the total aggregates weight and sand provided the remaining 40%. A general use blended cement meeting ASTM C1157-17 (ASTM, 2010) was used (Holcim-GU). A high-range plasticizer (polycarboxylate-based) was considered to maintain workability. No extra water was added in the different mixes, rather, workability was compensated for using a high-range water reducer (PCE) based. For all mixes, 1% fiber was added. In total, six mix designs were developed: three mixes with regular andesitic aggregates (polypropylene, abaca, and coconut fibers), and three mixes with the recycled aggregates described in §2.1.1 (polypropylene, abaca, and coconut fibers). Six cylinders (Ø = 101 mm × L = 200 mm) and four prisms (500 mm × 150 mm × 150 mm) were cast per mix. Three cylinders were used for compressive strength tests, one cylinder was sliced into four discs (Ø = 101 mm × L = 25 mm) for dynamic modulus tests. The remaining specimens were kept as duplicates. Mixing process followed ASTM C192 standard. Fibers were added at the end of the mixing cycle. High-range water reducer was added in the mixing water. 2.3. Determination of Mechanical Properties Compressive tests were performed based on ASTM C39-23 (ASTM, 2023a), flexural performance of fiber reinforced concrete strength followed ASTM C1609-12 (ASTM, 2012). For the evaluation of the dynamic modulus, the axisymmetric vibration of a thin disc was measured after applying an impact force by dropping a steel ball in the center of the specimen. A high frequency accelerometer is coupled in the center of the specimen using a commercial quick bonding agent. The accelerometer is connected to an oscilloscope through a signal amplifier and a power supply. The oscilloscope records the vibration response as voltage versus time. Then, a Fast-Fourier-Transform was applied to obtain peak frequency (resonance frequency), f . From this information the dynamic modulus, E dyn , can be obtained (Leming et al., 1998; Hutchinson, 1979). 3. Results and Analysis 3.1. Aggregate Characterization Table 1 presents the characterization of recycled aggregates, including tests for density, absorption, DRUW, materials passing #200 sieve, abrasion, uniformity, and alkali-silica reactivity (ASR). Out of the 67 tons processed for Stone #57, the gradation meets ASTM C33-18 limits. However, for Stone #7, the 25.5 tons produced fall below the lower limit of ASTM C33-18. This finer material results from low abrasion resistance compared to control (34 % vs. 23%) during grinding. The recycled sand produced by grinding coarser recycled aggregates partially meets the