PSI - Issue 64

Francisco Játiva et al. / Procedia Structural Integrity 64 (2024) 1468–1475 Jativa et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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3.4. Dynamic Modulus Results In total, 18 concrete disks were tested for resonant frequency by measuring their acceleration response to an impact force, from which the dynamic modulus, E dyn , was calculated. Fig. 3 shows the dynamic modulus results [GPa] for control and recycled aggregate series with polypropylene, abaca, and coconut fibers. The first observation is that the dynamic modulus, E dyn , of all samples made with control aggregates are between 15% to 30% larger than those made with recycled aggregates. The second observation is that E dyn values for control sets made with abaca and coconut fiber are similar in magnitude as observed by its standard deviation, the same is true for recycled sets made with abaca and coconut fibers. The third observation is that E dyn values are larger in magnitude in both control and recycled series made with abaca and coconut compared to sets made with plastic fiber only. 4. Discussion Combining the results from the material characterization, we found that the recycled aggregates are less dense, have more absorption and abrasion compared to the andesitic control material. However, this reduction is not extreme and concrete mechanical properties can be corrected by an accurate sieve distribution and w/c ratio adjustment. From the mechanical testing point of view, we find that the compressive strength of the cylinders with recycled aggregates is about 65% of the control, and that the peak strength in the flexural test is about 70% of the control. There is a relation between the reduction in material properties and mechanical properties, as also reported in the literature. Replacing the polypropylene fibers with an equal percentage of abaca or coconut fibers does not significantly influence the compressive strength, as expected. The first observation from the flexural tests is that the natural fibers have crack bridging properties and result in a load that can be carried after the peak. However, further research is necessary to study the potential fiber degradation over time in the concrete, as well as finding the optimal percentage of fiber to add to the mixes to obtain the desired toughness. The dynamic modulus results show that concrete reinforced with natural fibers increase its magnitude by enhancing hydration through internal curing (Choi, 2022). This is corroborated by compressive strength results (Figure 2a.) where this effect is also observed. Since these values are only studied for specimens, water cured at standard temperature for a period of 28 days, it is expected for these values to level out over time. Also, since natural fibers degrade in a highly alkaline environment (pH of concrete is around 13) it is expected that E dyn values reduce over time. In future studies, the same disks will be used to determine the rate of degradation using this nondestructive testing method. 5. Summary and conclusion To develop concrete-like materials that form part of the circular economy, and that fulfill requirements for sustainability, the use of recycled aggregates and natural fibers was proposed. The main findings of this study are: • Recycled aggregates as developed in this pilot study in Ecuador are less dense, have more absorption and abrasion compared to the andesitic control material. • Concrete mixes with recycled aggregates have an average compressive strength of 65% of the control mix. • Concrete prisms with recycled aggregates have an average peak strength of 70% of the control mix, and the peak strength is not affected by the fiber type. • The toughness of the prisms with abaca fibers is on average 48% of those with polypropylene fibers and the toughness of mixes with coconut fibers is on average 30% of the control. • Concrete with natural fibers and andesitic aggregates has a dynamic modulus 15% to 30% higher than that of concrete made with recycled aggregates and natural fibers; It is observed that natural fibers cause an internal curing effect in concrete specimens were the dynamic modulus magnitude reduces from 30% to 15% to 20%. The final conclusion is that for application in Ecuador, concrete mixes with recycled aggregates and abaca fibers can be a promising solution, if the adjusted mechanical properties are taken into account. Acknowledgements Financial support for the experimental part of this research is obtained via the program of Poligrants Vinculacion con la Industria 2023 and 2024 from Universidad San Francisco de Quito and Holcim del Ecuador, S.A. Financial support for staff exchanges related to this research is received via Horizon Europe and the project BEST: Biobased Energy-efficient Structures and materials