Flexural strength tests of a composite concrete and coffee husk cobblestone for use in pedestrian and light vehicular traffic surfaces, according to NTC 2017 Standard

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INTRODUCTION
Climate change is currently established as a global emergency that requires coordinated solutions at the national and international levels. With the aim of mitigating the environmental impact generated and its negative consequences, the Paris Agreement Colombia has been increasing its efforts to reduce greenhouse gas emissions generated by large industries, encouraging different companies to create products with natural additives that are more environmentally friendly, allowing them to be environmentally sustainable and encouraging the preservation of resources through the concepts of reduction, reuse, and recycling. To meet this need, it is important to e-ISSN 2357-6014 / Vol. 19, no. 1 / january-april 2023 / Bogotá D.C., Colombia Universidad Cooperativa de Colombia improve existing applications and develop new ecological alternatives based on the use of agroindustry, which generates large quantities of solid waste, rich in organic matter, that due to their biodegradable nature can be safely disposed of and can substitute new value-added products. In Colombia, most of the by-products generated by agroindustry are incinerated or dumped in water streams. Some are included in the production of compost, vermicompost, detoxification, solid biofuel, ethanol, biogas, adsorbents, mushrooms, energy drinks, energy bars and fermentation studies [3].
The final disposal of agro-industrial waste generates an economic loss and significantly alters the environment due to the high consumption for washing and separation, which varies from 0.1 to 0.3 L of water per liter of fruit, while in peeling and/or pulping, the consumption is 3 to 5 L of water per liter of fruit [4]. Consequently, if this water is discharged directly into effluents or not properly treated it can have a great negative impact, with serious consequences such as degrading the oxygen level in the water, which can kill swathes of flora and fauna, clogging waterways, and further contributing to anaerobic conditions. This hostile environment, generated by the discharge of pulping water, favors the growth of health-threatening bacteria, generating a bad odor and dark appearance, caused by toxic chemicals such as tannins, phenols, and alkaloids [4].
The use of organic material for construction is currently being studied with the aim of replacing traditional materials, such as cement and concrete, among others, because their continuous exploitation generates high concentrations of toxic substances that are emitted into the atmosphere. For this reason, the reuse of agro-industrial waste (eggshells, sawdust, coffee husks, rice husks, sugar cane, etc.) is being sought as an alternative. In some research, several types of components have been used as a possible sustainable alternative: Rubber fibers when mixed with concrete provide an effect that is inversely proportional to its resistance to compression [5], sugarcane bagasse ash in the concrete mix, led to the most optimal mechanical properties [6].
Eggshell ash, when substituted in proportions of 1.5% generates an increase in flexural strength of 8.62 % with respect to the mix without ash substitution [7]. Cane bagasse ash with a drying time of 7 days achieves its maximum value with a 40% substitution, but increasing the percentage further decreases the strength of the concrete. [8] Some studies have been carried out on the behavior of concrete with the addition of coffee husks that make it a light compound, with a silicon percentage of 15% in its chemical structure [9]. One of the main sources of income for the agricultural sector in Colombia is the commercialization of coffee with a high contribution to the country's economy [10]. Its properties make it optimal for this research. Coffee husk, as agro-industrial waste, represents 15% of the agricultural GDP; by 2022, coffee Ingeniería Solidaria e-ISSN 2357-6014 / Vol. 19, no. 1 / january-april 2023 / Bogotá D.C., Colombia Universidad Cooperativa de Colombia production will be 13.2 million bags [11]. During the production of coffee powder, the only part that is used is the bean, and all other parts, such as husk, parchment, mucilage, pulp, and skin, known as by-products, are discarded and can contaminate the environment. In other words, more than 50% of the coffee fruit is discarded when it could have a potential use in the pharmaceutical, food, cosmetic, construction, and other industries [12].
The objective of this study is to evaluate the viability of using coffee husks as a partial substitute in the manufacture of a cobblestone, according to the Colombian Technical Standard "NTC 2017, Concrete cobblestones for Paving".

A. Material Used
Materials used for the manufacture of the test specimens were as follows: Estructural Max cement manufactured by Argos (Colombia) and coffee husks from the El Porvenir Specialty Coffee Association Group, located in the south of the municipality of Isnos-Huila (Colombia).

Physical properties of Coffee Husk
The average particle size of the coffee husk was 250 µm, procured from an industrial mill for specialty coffee at the Faherjos factory (located in Isnos-Huila).

Aggregate properties
It was verified that the aggregates from the department of Huila complied with the specifications of Colombian Technical Standard 174 [13] in the manufacture of concrete mixtures.  The calculation for the volume of the mixture of the 5 specimens is presented in equation (1) V=(L*W*H)*N (1) The result of the total volume was 0.006 m 3 . The dosage of cement, sand and gravel per cubic meter was made considering the type of concrete 1:2:3 presented in Table 2.

C. Equipment and Testing
Two molds were manufactured in laminated wood of 0.2 m in length, 0.1 m in width and 0.06 m in thickness for the experimentation of the final mix, substituting 10% coffee husk for the total mix, according to the Colombian Technical Standard 2017. Figure   2 shows the result per test specimen. For the case study, 5 specimens were manufactured with the dimensions shown in Table 4.    Each specimen was placed in the testing machine with the support surface facing down. The load is applied on the section area of the wearing side, and it forms a parallel line with the support points, 10 mm to the inside of the minor sides of the rectangle. The distance between supports is 15 cm above the support side, according to the NTC 2017 Standard [14]. Figure 5 shows the specimens subjected to the maximum load to reach rupture:  Table 5 shows the data obtained from the Flexural Strength test corresponding to a sample of 5 specimens tested using the Colombian technical standard 2017.    It is recommended to study the drying variable at 7, 14, 21 and 56 days to observe if there is a relationship between the drying time and the resistance behavior of the coffee husk, as well as variables of solar radiation and humidity that allow us to verify physical-chemical changes in the concrete [18].
Considering the results obtained with 10% of coffee husk for the total of the mix, it is advisable to carry out test studies with a replacement of 5%, 2%, 1% and 0.5% of coffee husk and to observe if the mechanical behavior of the mixes increases the rupture resistance in the specimens.

CONCLUSIONS
The coffee husks from the municipality of Isnos-Huila meet the requirements for mix design and final finishing, especially in the use of architectural concrete. As a 10% replacement of coffee husk in the total mix, it does not fulfill the resistance criteria for pedestrian and light vehicular use, according to NTC 2017.
From the obtained results, it is feasible to conclude that it is recommended to have low replacement percentages of coffee husk, since the resistance tends to decrease significantly as the percentages of coffee husk increase, with the aim of maintaining the mechanical properties and workability of the mix. In accordance with previous research, it is possible to obtain better resistance results with substitutions of percentages between 0.5 and 1.0% [8].
Although an approximate resistance of 100% was not obtained, it is feasible to make mixes with coffee husk substitution to be used in structures with conventional uses such as architectural concrete; where it is viable to obtain criteria of cohesion, durability, resistance, and homogeneity among the materials, in the same way as with ordinary aggregate concrete, provided the dosage guidelines, mix design and adequate curing time are followed.