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Article

Study of Various Mechanical Properties of Maize (Zea mays) as Influenced by Moisture Content

by
Manuel Moya-Ignacio
1,*,
David Sánchez
2,
José Ángel Romero
2 and
José Ramón Villar-García
1
1
Forest Research Group, Department of Forestry and Agricultural Medium Engineering, University Center of Plasencia, University of Extremadura, Avda. Virgen del Puerto nº 2, 10600 Plasencia, Cáceres, Spain
2
University Center of Plasencia, University of Extremadura, Avda. Virgen del Puerto nº 2, 10600 Plasencia, Cáceres, Spain
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(8), 1613; https://doi.org/10.3390/agronomy14081613
Submission received: 21 May 2024 / Revised: 18 July 2024 / Accepted: 22 July 2024 / Published: 24 July 2024
(This article belongs to the Special Issue Advances in Agricultural Engineering for a Sustainable Tomorrow)
Browse Figures
Versions Notes

Abstract

:
The mechanical properties of agricultural materials influence not only the loads occurring inside agricultural silos, but also the design of several types of post-harvest machinery. The loads generated by these materials inside silos can be predicted with silo calculation methodologies from their mechanical properties. It has been known for many years that these properties are highly dependent on the moisture content of the material. However, to date, there are not many studies focused on its determination. The goal of this research is the determination of the internal friction angle, apparent cohesion, angle of dilatancy and apparent specific weight of maize when different moisture contents are applied. The equipment used for this study consisted mainly of direct shear and oedometer assay apparatus. The maize samples used were moistened using a climatic chamber. Moisture contents applied to maize samples ranged from 9.3% to 17.4%. Results similar to those provided by other authors were obtained for the internal friction angle, apparent cohesion and apparent specific weight. On the other hand, the values obtained for the dilatancy angle of maize as a function of moisture content could not be compared because nothing has been published so far. The values obtained for this parameter overlap with those published for this material under ambient conditions. In addition, for the samples tested, these results did not allow confirming the existence of a direct relationship between the dilatancy angle and the moisture content. Finally, the increase in moisture content led to an increase in apparent specific weight, which differed from that published in the literature. The values provided here can be used for the optimization of storage and handling structures for granular agricultural materials.

1. Introduction

The mechanical properties used in the calculation of agricultural silos, such as the internal friction angle, coefficient of friction or bulk density, among many others, have been extensively studied for many materials such as wheat, corn, barley, oats, etc., over the years [1,2,3,4,5,6]. The knowledge of these parameters is essential to perform the calculations for structures related to the storage and handling of granular or powdery agricultural materials, either using classical silo theories [7], or resorting to more modern technologies through the application of numerical methods, by performing either finite [8,9,10] or discrete [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25] element analysis, or a combination of both [20,26]. These data refer to samples of the material under normal environmental conditions, i.e., dry samples. However, it has long been known that these material properties depend to a large extent on the moisture content of the material [18,27,28,29,30,31,32,33,34,35,36] and even the variety of the granular material used [37].
In spite of the above, to date not many papers have been published in the literature studying the influence of moisture when the material is subjected to intermediate liquid contents. Most of these studies focus on the analysis of the pendular state, but very few focus on the funicular state [38,39,40,41]. In humid conditions, water molecules are located between the grains of the material thanks to capillary forces, affecting the cohesion and friction of the material [33,42,43]. Depending on the amount of water in a granular medium, capillary bridges may or not be established between the grains, thus determining the state of humidity reached. In addition, as the material moves from a wetter to a drier state, or vice versa, capillary bonds, capillary bridges and/or liquid bridges are created or destroyed. In recent years, some studies have been carried out in order to understand the cohesive behavior of the material under these circumstances [44,45,46]. This behavior affects several factors, such as the tensile strength, among others, which, in turn, determine the behavior of the material when different normal and shear stresses are applied. Therefore, a better understanding of the mechanical behavior of unconsolidated wet granular materials is needed.
The goal of this research was, therefore, the determination of the values of internal friction angle, apparent cohesion, angle of dilatancy and apparent specific weight of maize (Zea mays) at different moisture contents. The results obtained here will be added to those previously published by other authors except in the case of the dilatancy angle, for which no similar studies have been carried out before. These parameters can be used to optimize handling and storage structures.

2. Materials and Methods

The material used in this work was maize (Zea mays) variety P0023, provided by the company Leonesa Astur de Piensos, S.A. (LESA), located in the province of León (Spain).
The following mechanical properties were determined for maize in this research: the internal friction angle (ϕ), the apparent cohesion (C), the dilatancy angle (ψ) and the specific weight (γ).
In previous works [34,47], the different tests carried out here were described in a general manner. Therefore, the particular characteristics of the assays carried out with the maize samples used are described below.

2.1. Sample Wetting

The geotechnical laboratory of the University Center of Plasencia (University of Extremadura), where the tests were carried out, is equipped with a climatic chamber, which allows different humidities to be applied to the samples used in the assays. This chamber has a touch screen that allows selecting the relative humidity (expressed as a percentage) and the temperature to be applied to the material deposited inside (in degrees Celsius). In this work, a minimum temperature of 20 °C was selected. At this temperature, four different humidities were programmed in the climatic chamber (35, 45, 65 and 90%). Additionally, in order to obtain a lower moisture content in the material, the maize sample was acclimatized at 55 °C temperature and 25% relative humidity. These temperature and humidity conditions were maintained for a minimum of 72 h to ensure homogeneous moisture in the material prior to the start of the assays.

2.2. Oven Drying

The moisture content of the different samples used in this work was determined using an oven at a temperature ranging between 105 and 110 °C. The tests were carried out in accordance with the standard [48] by weighing the different samples with a 0.01 g electronic balance every 24 h until a constant weight was reached. The moisture content of the samples was obtained as dry basis (d.b.) according to the following expression:
MCdb = mw × 100/md
where:
  • MCdb = dry basis moisture content (%)
  • mw = mass of water (g)
  • md = mass of dry solids (g)

2.3. Direct Shear Assays

Direct shear assays were performed to determine the angle of internal friction, apparent cohesion and dilatancy angle. These assays were carried out in accordance with the standard [49]. The characteristics of the equipment used, as well as the procedure for processing the data collected and obtaining the corresponding results, were described in previous works [34,47]. The normal stresses applied in this work were 10, 20, 50, 100, 200 and 300 kPa, while the test speed selected was 4 mm/min. This speed was used in previous work [34], and it was chosen considering that it was high enough to minimize the loss of moisture from the specimen during the assays, but at the same time to obtain a sufficient amount of data for the subsequent data processing and for the graphical representation of the stress–strain curves. At this speed, the duration of the tests was 5 min because the maximum displacement allowed by the direct shear device is 20 mm. Two replications of each test were carried out for each of the normal stresses applied.

2.4. Determination of Apparent Specific Weight

The apparent specific weight of the material was determined with an oedometer device. The different assays were carried out in accordance with the standard [50]. Since the oedometric equipment was described in previous works [34,47], only the specific characteristics of the assays performed in this work will be discussed below. In order to speed up the tests as much as possible to avoid moisture loss of the material, abbreviated oedometric assays were carried out. In this test mode, each loading or unloading step occurred when, during a minimum interval of three consecutive hours, the maximum deformation was 0.1 mm, assuming that primary consolidation had been reached during that period. A lower value of that maximum deformation could lead to an excessive duration of the test, while the choice of a value higher than this could make it difficult to guarantee that this primary consolidation is reached. The normal stresses applied were 8, 16, 32, 64, 128 and 256 kPa. Three replications were carried out with the maize samples for the different moisture conditions applied to them.
The apparent specific weight could be determined at any time as follows: the volume of the oedometric cell in which the material is deposited is known because it is standardized (see Figure 1). Since the weight of the maize sample was determined immediately before beginning each test with the aid of an electronic balance (0.1 g accuracy), it was possible to determine its initial apparent specific weight. With the help of a displacement transducer, it was possible to determine the height reached by the sample inside the oedometric cell at each moment for each loading or unloading step applied, and thus its apparent specific weight.
Finally, the analysis of the results obtained from the different assays carried out was performed by applying a statistical treatment consisting of the determination of the mean and standard deviation. Excel spreadsheet for Microsoft 365 was used for the statistical analysis and graphs.

3. Results

3.1. Moisture Content of Samples

As mentioned in the previous section, five different combinations of temperature (°C) and humidity (%) were applied to maize samples in the climatic chamber to perform the direct shear and oedometric assays at different humidities. In all cases, immediately before starting the tests, samples were dried in an oven to determine their moisture content. The results for the moisture content of the maize samples after the oven drying process for each of the temperature and humidity combinations applied are shown in Table 1.
The above table shows that the moisture content selected to analyze the mechanical properties of maize ranged from 9.3% to 17.4%. This range is within those used by other researchers [29,32,36,51].

3.2. Direct Shear Tests

3.2.1. Internal Friction Angle and Apparent Cohesion

With regard to the direct shear assays performed, the stress–strain curves at the different normal stresses applied to the maize samples were plotted at moisture contents of 17.4% (Figure 2) and 9.3% d.b. (Figure 3), respectively. These curves represent the mean value obtained for the shear stress at each value of the horizontal displacement and for each of the normal stresses applied for the two repetitions carried out, while the vertical bars correspond to the standard deviation.
The value obtained for the maximum shear stress when the moisture content was 17.4% (199.08 kPa) was 8.3% higher than that corresponding to a moisture content of 9.3% (183.83 kPa). For a normal stress of 200 kPa, this value was 21.2% higher at 17.4% humidity compared to that obtained at 9.3%, while for a normal stress of 100 kPa, this value was 1% higher at 9.3% humidity compared to that obtained at 17.4%. Finally, when analyzing the differences obtained by applying the lowest normal stresses, higher values of the maximum shear stress of 6.1% and 22.3% were observed at normal stresses of 50 and 10 kPa, respectively, at a humidity of 17.4% with respect to the value obtained at 9.3%. The exception to this trend was at a normal stress of 20 kPa, since the values obtained were 18.5% higher at 9.3% humidity than those obtained at 17.4% humidity. Therefore, it can be stated that the tendency seems to be that the higher the normal stress applied and the higher the moisture content of the sample, the higher the shear stress, although this tendency is not generalized, as there are exceptions when applying normal stresses of 20 and 100 kPa.
Figure 4 shows the regression lines corresponding to the Mohr–Coulomb strength envelopes as a function of the normal stress applied and for the different moisture contents of the maize samples.
The mathematical expressions of the different regression lines obtained are given in the figure above. From them, the mean values and standard deviation of the internal friction angle and apparent cohesion have been determined and are shown in Table 2.
From the above table, it can be deduced that the maximum internal friction angle was reached at the highest applied humidity (17.4%). This value was 13.7% higher than the minimum recorded for this parameter, which was obtained at 12% moisture content. The general trend observed was that the higher the moisture content of the samples, the higher the shear strength. The only exception to this trend was for a moisture content of 12%, for which there was a small reduction of 3% in the value obtained with respect to that obtained with a moisture content of 9.3%. This anomaly could perhaps be due to the fact that the value obtained at the latter moisture content was slightly higher than expected.
With regard to the values obtained for apparent cohesion, it can be observed that very high values were not obtained in any of the cases. Nor was there a general tendency to increase or decrease the values recorded as the moisture content of the samples tested increased. In this sense, it can be observed how the values obtained for this parameter were lower at a humidity of 12% than that reached at 9.3%, or that obtained at 13% humidity lower than that reached at a humidity of 12% (the same occurred at 17.4%) when, a priori, logically, this value should have been higher at the higher humidity.
When considering the range of high (100, 200 and 300 kPa) and low (10, 20 and 50 kPa) normal stresses and observing the regression lines of the Mohr–Coulomb strength envelopes, it was found that in some cases, these lines intersected at a point, as occurred at moisture contents of 9.3 and 14.1% (see Figure 5), while in others, no such intersection occurred, as happened at moisture contents of 12.0 and 13.0% (see Figure 6).
Logically, as in the previous figures, the slopes of the regression lines varied for the same moisture content depending on the range of normal stresses considered (high or low), and the internal friction angles obtained were different. Similarly, this variation also affected the values obtained for the apparent cohesion at high and low normal stresses for each moisture content applied, since the points of intersection of the corresponding regression lines with the ordinate axis were also different in each case. Once the regression lines corresponding to high and low normal stresses for each of the moisture contents applied had been determined, the mean values and standard deviation of internal friction angle and apparent cohesion obtained for the maize samples assayed could be obtained. These values are shown in Table 3.
In general, the highest values for the angle of internal friction were reached in the low normal stress range. When considering the range of low normal stresses, the maximum value of the internal friction angle (37.5°) was achieved at the highest moisture content (17.4%), while the minimum value for this parameter was obtained when applying a moisture content of 12.0% (25.1°). Moreover, in all the moisture ranges analyzed, the value of this parameter exceeded 30°, the only exception being the value recorded at the 12.0% moisture content. In fact, the difference between the highest and the lowest values obtained in the different humidity intervals applied, except for the value obtained at 12.0% humidity, was 15.5%, and this difference increased to 33.1% when considering the value obtained at 12.0% humidity. On the other hand, when analyzing the values obtained in the high normal stress interval, again the highest value (32.2°) was reached at the highest applied humidity (17.4%), with the 12.0% humidity also providing the minimum value of this parameter (25.5°). In this case, 30° was only exceeded at humidities of 13.0 and 17.4%, with a difference of 20.8% between the highest and the lowest values.
Regarding the values for apparent cohesion, these were generally higher in the range of high normal stresses. The only exception was the value obtained at 13.0% moisture content, although the standard deviation obtained could not be disregarded in this case. The difference was 37.4% between the maximum and minimum values in the range of low normal stresses, while it was 81.6% in the range of high normal stresses.

3.2.2. Dilatancy Angle

As mentioned in the previous section, the direct shear assays were also used to determine the dilatancy angle of maize, This parameter can be determined from the measurement of the vertical deformation (εv) of the material generated inside the shear box as the direct shear assay is performed. For this purpose, the deformation curve is plotted, where the abscissa axis is intended for the horizontal deformation (εH) and the ordinate axis for the vertical deformation. The mean vertical deformation values obtained from the two repetitions carried out at each of the different normal stresses and moisture contents applied are shown in Figure 7. Figure 7a,b show the deformation curves obtained at moisture contents of 17.4% and 13.0%, respectively.
The figures above show the different behavior of the material. Thus, at 17.4% humidity, the dilatancy of the direct shear assays was greater the lower the normal stress applied, as would be expected a priori. However, at 13.0% moisture content, the previous trend was not observed. In this case, when a horizontal displacement of 9 mm was reached, the curve obtained at a normal stress of 20 kPa was above that obtained at 10 kPa. Similarly, until 15 mm of horizontal displacement was exceeded, the curve obtained at a normal stress of 100 kPa did not exceed that obtained at 200 kPa. A similar trend, in which the curve at 20 kPa exceeded that at 10 kPa, was observed in the tests carried out applying a moisture content of 14.1%, while at 12.0% moisture content, the curve obtained when applying a normal stress of 200 kPa showed greater dilatancy than that provided when the normal stress was 100 kPa. Finally, in the tests carried out at 9.3% humidity, it was the normal stress of 50 kPa that provided the greatest dilatancy, while the curve corresponding to a normal stress of 20 kPa was exceeded at 11 mm of horizontal displacement by that obtained by applying a normal stress of 100 kPa. These erratic trends observed in some cases can be seen in the data shown in Table 4. In this table, the mean values and standard deviation of the dilatancy angle are provided for the different normal stresses and moisture contents applied:
Tests conducted with a moisture content of 17.4% and a normal stress of 10 kPa yielded the highest value for this parameter (30.7°), while those performed with a moisture content of 9.3% and a normal stress of 300 kPa yielded the lowest value (4.4°). It should also be noted that dilatancy was observed in all the assays performed in this work when the highest normal stress (300 kPa) was applied, regardless of the moisture content applied to the maize samples.
By comparing the maximum and minimum values obtained for the dilatancy angle for each of the moisture contents applied to the samples, the percentage drop in this parameter was obtained. The results showed that the highest percentage drop (81.7%) was recorded at a moisture content of 9.3%, since the maximum value of this parameter at this moisture content was 24.1°, while the minimum value was 4.4°. This was followed by that corresponding to a moisture content of 17.4%, for which the percentage drop was 75.9%. This was followed by percentages of 69.8% and 69.2% at moisture contents of 14.1% and 13.0%, respectively. Finally, the lowest percentage (64.4%) was obtained when applying a moisture content of 12.0%, since at this humidity, the maximum value was 21.5°, while the minimum was 8.1°.

3.3. Apparent Specific Weight

With respect to the oedometric assays performed, by representing the apparent specific weight on the ordinate axis and the normal stress on the abscissa axis, the different resulting curves showing the variation in the apparent specific weight with normal stress can be plotted for each of the applied moisture contents. Figure 8 shows the results obtained for maize. Each curve was obtained from the mean values corresponding to the three replicates carried out for each of the test conditions applied.
The variation in the apparent specific weight with moisture content can be plotted by representing the former on the ordinate axis and the latter on the abscissa axis. Figure 9 shows the curves obtained for each of the applied normal stresses.
As expected, from the above figures, it was generally deduced that the higher the normal stress, the higher the apparent specific weight obtained, as these values were also higher as the moisture content increased. The only exception occurred at a normal stress of 8 kPa, since a lower value was obtained at 14.1% humidity compared to that obtained at 13.0% humidity.
Table 5 lists the mean values and standard deviation of the apparent specific weight for the different moisture contents and normal stresses tested with the oedometric devices:
From the above data, when comparing the initial value of the apparent specific weight determined immediately before starting the test (normal stress equal to 0 kPa) with that corresponding to the maximum normal stress applied (256 kPa), the greatest increase in this parameter (18.8%) was recorded at a moisture content of 17.4%. It was followed by that obtained at a humidity of 14.1% (8.8%) and that corresponding to 13.0% moisture content (7.0%). In addition, the increase was 5.8% at 12.0% moisture content, and the smallest variation (3.6%) was recorded at 9.3%. Finally, when comparing the maximum values obtained for this parameter at the different humidities applied, the greatest difference was recorded, as expected, between the one reached at 17.4% humidity (8.34 kN/m3) and the one corresponding to 9.3% (7.28 kN/m3), reaching a percentage of 14.6%.

4. Discussion

4.1. Moisture Content

Regarding moisture content, the range of values selected for the maize samples in this research was within those used in other works [2,29,36,51]. Therefore, the results shown here will be contrasted with those reported by other authors, if available.

4.2. Direct Shear Tests

4.2.1. Internal Friction Angle and Apparent Cohesion

As for the results obtained for the angle of internal friction from the direct shear assays performed, the stress–strain curves obtained were similar to those reported in similar tests with other granular or powdered agricultural materials [4,5,34].
Since the material used was the same as that used in previous works [47], the results obtained can be compared with those published at ambient humidity (12.77%). From this comparison, it was found that the value of the angle of internal friction was similar to that obtained in direct shear assays at ambient conditions (29.0°) at all moisture contents applied to the maize sample, except at 17.4% humidity. Thus, this value ranged from 28.5° (at 12% moisture) to 29.9° (at 14.1% moisture), while at 17.4% moisture, a value of 32.4° was obtained, which is 11.7% higher than that obtained under ambient conditions.
When comparing these values with those obtained in the triaxial assays carried out with the material under ambient conditions, it was observed that the minimum value obtained (28.5°) at 12.0% humidity was 61.0% lower than that obtained at 10% axial strain in the triaxial assay (46.7°), and 59.9% lower than that obtained at 20% axial strain (47.6°). However, the maximum value obtained at 17.4% humidity (32.4°) was 69.4% and 68.1% lower than that obtained at 10% and 20% axial strain, respectively.
On the other hand, the values obtained in this work have also been compared with those published by other authors [2] at different humidities, showing that the value obtained at 9.3% humidity (29.0°) was 10% higher than that determined by them at 10% humidity (26.7°), while the maximum value obtained here (32.4°) was 3% lower than that provided by them at 17.5% humidity (33.6°). Furthermore, when analyzing the values reported in other works [5], referring to ambient conditions, it was observed that the minimum value was, at most, 13.1% higher than the minimum published, while the highest value obtained here was, at most, 8.0% higher than the one provided by those authors, most of the values being in the range of those reported in those conditions.
Analyzing the results obtained for the angle of internal friction when considering the range of values of the applied normal stress, the following was observed: in the interval of low normal stresses (10, 20 and 50 kPa), the differences were similar to those previously mentioned for most of the moisture contents tested. In this case, it should be noted that the maximum value obtained in this interval (37.5°) at 17.4% moisture content was 12.2% higher than that obtained at 17.5% by other authors [2]. In addition, this percentage was increased by approximately 39%, as a maximum, with respect to the maximum values published in other works under ambient conditions [5]. As for the high normal stress range (100, 200 and 300 kPa), the values obtained at the different applied humidities were similar to those published by other authors at different humidities [2] and, at most, 19% higher than those published under ambient conditions [5].
Finally, with all these results, the trend observed was that, in general, the higher the moisture content of the sample, the higher the value of the internal friction angle. The only exception to this trend was the value obtained with a humidity of 12.0%. A similar trend was observed in other works [2,29], although in the latter, the published data referred to the angle of repose, a parameter that is closely related to the internal friction angle.
Possible reasons for the relatively small differences found between all these values may include differences in the moisture content of the samples used, particle arrangement, bulk density, particle orientation within the shear box, shear banding, consolidation time and particle shape [2].
Regarding the apparent cohesion, the range of values obtained here were slightly lower (8%) than those provided for the same material subjected to environmental conditions [47] and overlapped with those reported by other authors [5]. No significant values were obtained in any case, since the maximum value was 11.36 kPa. Similar differences were obtained when analyzing the resulting values at high and low normal stresses. In this case, the highest value obtained at high normal stresses was 26.38 kPa at 12.0% humidity, a value that could not be disregarded but overlapped with those found in the referred literature.
Regarding the lower values obtained at certain moisture contents with respect to those obtained at higher moisture contents for this parameter, probably the least squares adjustment carried out to determine the regression lines corresponding to the Mohr–Coulomb strength envelopes could have caused this circumstance. At very low normal stresses, the Mohr–Coulomb strength envelope follows a curve instead of a straight line, as it has been simplified to obtain that parameter approximately.

4.2.2. Dilatancy Angle

With respect to the dilatancy angle, as there are no similar works in the literature in which this parameter has been determined for this same material at different moisture contents, the only values with which it has been possible to establish a certain comparison are those published in previous works with the material subjected to environmental conditions [5,47]. In these cases, the environmental humidities determined were 13.71 and 12.77%, respectively. The values obtained here overlap with those reported in those works. In general, the trend observed was that the higher the applied normal stress, the lower the dilatancy angle, coinciding with that corroborated in other works using granular agricultural materials [5,34,52]. This is because as the normal stress applied increases, the materials become softened. This softening can cause the material to compress, even more so in the presence of water, thus reducing the dilatancy. Finally, no direct relationship between the dilatancy angle and the moisture content of the samples could be deduced from the assays performed.

4.3. Apparent Specific Weight

The values obtained for the apparent specific weight from the oedometric assays conducted in this work ranged between 7.01 and 8.34 kN/m3, which were within the values reported by other authors in previous works [2,47]. When analyzing the values of the apparent specific weight obtained just before starting the assays (with zero normal stress), they ranged between 7.01 and 7.04 kN/m3, thus being within the range provided by other authors for this material in a humidity range between 10 and 20% [2].
On the other hand, in the assays carried out, it was determined that the higher the moisture content, the higher the apparent specific weight. Thus, the highest applied moisture content (17.4%) provided the highest values for this parameter, reaching its maximum value (8.34 kN/m3) at the highest applied normal stress (256 kPa). However, this tendency did not correspond to that observed in other studies with other granular agricultural materials in which the value of this parameter decreased with increasing moisture content [8,14,16,30,37]. In addition, in works carried out with corn kernels [11,36], the bulk density increased as the moisture content increased. This trend could not be corroborated in this work in tests on wheat samples [13]. In the same sense, the trend observed in the present work also disagreed with the predictions made with some numerical models for wheat [34,36]. These models considered that this material is more compressible as its moisture content increases, predicting a reduction in bulk density as moisture content increases. However, for maize, some authors have obtained values with compacted material that do not follow this trend [2]. Other authors [35] have even shown that without applying any normal stress, bulk density decreases with increasing moisture content, whereas when increasing normal stress is applied, as occurred in the assays developed here, bulk density increases until it approaches the same maximum value regardless of the moisture content of the sample. The results obtained in this work may have been influenced by the orientation of the grains in the presence of water, as well as by the disposition of the particles of the material inside the oedometric cells used.
Nowadays, society is increasingly aware of the need to optimize the use of natural resources and to minimize the occupational risks, costs and environmental impacts of all construction projects and industrial processes. Therefore, with regard to the design of facilities and infrastructures for the handling and storage of granular and powdered materials, it is essential that they are adequate, efficient and economical [36]. In this sense, numerous studies that have been carried out have demonstrated the fundamental role played by the mechanical properties determined here in the behavior of the material, both when it is stored and when it flows [34,35,36,38]. These properties depend to a large extent on certain factors, such as the humidity of the material when it is stored, the temperature, the relative humidity of the air, etc. Therefore, in order to increase the level of existing knowledge and facilitate the design and management of these facilities, it is necessary to develop similar tests with other granular agricultural materials.

5. Conclusions

The values obtained for the angle of internal friction and apparent cohesion in this research were within the range of values found in the literature for maize.
The values for the internal friction angle were approximately 60% lower than those obtained in triaxial tests under ambient conditions. In addition, the trend of the values obtained for this parameter showed that the higher the moisture content of the samples, the higher the angle of internal friction, which coincides with that found in other published works.
Since there are no similar works in the literature, it was not possible to compare the dilatancy angle values for maize at different moisture contents. However, the results obtained here are similar to those reported for the same material under ambient conditions. It was possible to corroborate that the higher the applied normal stress, the lower the dilatancy angle. Nevertheless, from the assays carried out here, it was not possible to confirm that there is a direct relationship between this parameter and moisture content.
In general, the values obtained for the apparent specific weight were similar to those found in the literature. The trend observed in this work was that the higher the moisture content of the sample, the higher the apparent specific weight, which differed from that published in other works.
The values obtained may be useful for optimizing the design of structures and equipment for handling bulk solids.

Author Contributions

Conceptualization, M.M.-I.; methodology, M.M.-I. and J.R.V.-G.; software, M.M.-I.; validation, M.M.-I., J.R.V.-G., D.S. and J.Á.R.; formal analysis, M.M.-I., D.S. and J.Á.R.; investigation, M.M.-I., J.R.V.-G., D.S. and J.Á.R.; data curation, M.M.-I., D.S. and J.Á.R.; writing—original draft preparation, M.M.-I.; writing—review and editing, M.M.-I., J.R.V.-G., D.S. and J.Á.R.; supervision, M.M.-I., J.R.V.-G., D.S. and J.Á.R.; funding acquisition, M.M.-I. and J.R.V.-G. All authors have read and agreed to the published version of the manuscript.

Funding

This research and APC were funded by the Spanish “Agencia Estatal de Investigación” via the research project “Study of the structural behavior of corrugated wall silos using Discrete Element Models (SILODEM)”, grant number PID2019-107051GB-I00/AEI/10.13039/501100011033.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors thank Leonesa Astur de Piensos, S.A. (LESA) company for providing the maize used in this research.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

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Agronomy 14 01613 g001
Figure 1. Oedometric cell used with the maize sample.
Figure 1. Oedometric cell used with the maize sample.
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Figure 2. Stress–strain curves for maize as a function of the normal stress applied at a moisture content of 17.4% d.b.
Figure 2. Stress–strain curves for maize as a function of the normal stress applied at a moisture content of 17.4% d.b.
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Figure 3. Stress–strain curves for maize as a function of the normal stress applied at a moisture content of 9.3% d.b.
Figure 3. Stress–strain curves for maize as a function of the normal stress applied at a moisture content of 9.3% d.b.
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Figure 4. Regression lines corresponding to the Mohr–Coulomb strength envelopes as a function of applied normal stress and for different moisture contents of the maize samples. M.C. = Moisture Content.
Figure 4. Regression lines corresponding to the Mohr–Coulomb strength envelopes as a function of applied normal stress and for different moisture contents of the maize samples. M.C. = Moisture Content.
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Figure 5. Comparison of regression lines corresponding to Mohr–Coulomb strength envelopes obtained at high and low normal stresses in direct shear assays for moisture contents of 9.3% (a) and 14.1% (b), respectively. M.C. = Moisture Content.
Figure 5. Comparison of regression lines corresponding to Mohr–Coulomb strength envelopes obtained at high and low normal stresses in direct shear assays for moisture contents of 9.3% (a) and 14.1% (b), respectively. M.C. = Moisture Content.
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Figure 6. Comparison of regression lines corresponding to Mohr–Coulomb strength envelopes obtained at high and low normal stresses in direct shear assays for moisture contents of 12.0% (a) and 13.0% (b), respectively. M.C. = Moisture Content.
Figure 6. Comparison of regression lines corresponding to Mohr–Coulomb strength envelopes obtained at high and low normal stresses in direct shear assays for moisture contents of 12.0% (a) and 13.0% (b), respectively. M.C. = Moisture Content.
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Figure 7. Deformation curves obtained for maize samples at moisture contents of 17.4% (a) and 13.0% (b).
Figure 7. Deformation curves obtained for maize samples at moisture contents of 17.4% (a) and 13.0% (b).
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Figure 8. Apparent specific weight vs. normal stress at different moisture contents. M.C. = Moisture Content.
Figure 8. Apparent specific weight vs. normal stress at different moisture contents. M.C. = Moisture Content.
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Figure 9. Apparent specific weight vs. moisture content at different normal stresses.
Figure 9. Apparent specific weight vs. moisture content at different normal stresses.
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Table 1. Actual moisture content of maize samples after oven drying (expressed as dry basis, d.b.) with different combinations of temperature (°C) and relative humidity (%).
Table 1. Actual moisture content of maize samples after oven drying (expressed as dry basis, d.b.) with different combinations of temperature (°C) and relative humidity (%).
Climatic Chamber ConditionsMoisture Content (d.b.)
(%)
Temperature (°C)Relative Humidity (%)
55259.3
203512.0
204513.0
206514.1
209017.4
Table 2. Values obtained from direct shear assays for the angle of internal friction and the apparent cohesion at different moisture contents.
Table 2. Values obtained from direct shear assays for the angle of internal friction and the apparent cohesion at different moisture contents.
Moisture Content
(%) (d.b. 1)		Angle of Internal Friction (ϕ)Apparent Cohesion (C, kPa)
9.329.4° ± 0.511.36 ± 1.80
12.028.5° ± 0.810.46 ± 2.84
13.029.8° ± 0.58.04 ± 1.97
14.129.9° ± 0.410.47 ± 1.42
17.432.4° ± 0.69.56 ± 2.14
1 dry basis.
Table 3. Values obtained from direct shear assays for the angle of internal friction and apparent cohesion when considering the range of applied normal stresses (low or high) for the different moisture contents tested.
Table 3. Values obtained from direct shear assays for the angle of internal friction and apparent cohesion when considering the range of applied normal stresses (low or high) for the different moisture contents tested.
Moisture
Content
(%) (d.b. 1)		Angle of Internal Friction (ϕ)Apparent Cohesion (C, kPa)
Low Normal StressesHigh Normal StressesLow Normal StressesHigh Normal Stresses
9.335.3° ± 1.328.3° ± 1.16.80 ± 1.1116.65 ± 5.57
12.025.1° ± 2.225.5° ± 1.59.74 ± 1.4626.38 ± 6.85
13.032.2° ± 0.830.4° ± 1.57.16 ± 0.614.85 ± 7.55
14.131.7° ± 1.629.6° ± 1.19.05 ± 1.2412.43 ± 5.25
17.437.5° ± 2.132.2° ± 1.46.10 ± 1.8810.16 ± 7.57
1 dry basis.
Table 4. Values of the dilatancy angle for maize at different normal stresses and moisture contents.
Table 4. Values of the dilatancy angle for maize at different normal stresses and moisture contents.
Normal Stress
(kPa)		Dilatancy Angle (Ψ)
Moisture Content (d.b. 1)
9.3%12.0%13.0%14.1%17.4%
1019.3° ± 3.620.8° ± 1.029.9° ± 1.624.8° ± 6.730.7° ± 0.7
2022.1° ± 2.521.5° ± 1.327.0° ± 2.924.8° ± 1.320.3° ± 4.7
5024.1° ± 7.219.4° ± 0.720.4° ± 1.520.8° ± 3.322.4° ± 6.1
10016.7° ± 0.817.5° ± 0.913.9° ± 1.617.1° ± 4.018.9° ± 2.4
20011.1° ± 0.59.9° ± 1.112.7° ± 0.410.2° ± 3.615.3° ± 4.3
3004.4° ± 2.58.1° ± 2.29.2° ± 0.57.5° ± 0.87.4° ± 2.2
1 dry basis.
Table 5. Values for the apparent specific weight of maize at different moisture contents and normal stresses.
Table 5. Values for the apparent specific weight of maize at different moisture contents and normal stresses.
Normal Stress
(kPa)		Apparent Specific Weight (γ) (kN/m3)
Moisture Content (d.b. 1)
9.3%12.0%13.0%14.1%17.4%
07.03 ± 0.017.01 ± 0.017.04 ± 0.077.02 ± 0.017.02 ± 0.01
87.04 ± 0.017.03 ± 0.037.13 ± 0.017.11 ± 0.017.34 ± 0.09
167.06 ± 0.027.07 ± 0.037.17 ± 0.027.18 ± 0.017.41 ± 0.11
327.09 ± 0.027.13 ± 0.027.24 ± 0.037.29 ± 0.037.55 ± 0.12
647.13 ± 0.047.20 ± 0.037.30 ± 0.037.37 ± 0.037.74 ± 0.13
1287.19 ± 0.047.28 ± 0.057.40 ± 0.047.51 ± 0.058.00 ± 0.12
2567.28 ± 0.047.42 ± 0.067.53 ± 0.077.64 ± 0.058.34 ± 0.13
1287.25 ± 0.047.40 ± 0.067.51 ± 0.077.62 ± 0.058.34 ± 0.14
647.23 ± 0.037.38 ± 0.067.49 ± 0.077.60 ± 0.058.32 ± 0.14
327.21 ± 0.037.36 ± 0.067.47 ± 0.067.59 ± 0.048.31 ± 0.13
167.19 ± 0.027.35 ± 0.067.47 ± 0.027.57 ± 0.048.31 ± 0.14
87.18 ± 0.047.33 ± 0.067.46 ± 0.017.56 ± 0.038.32 ± 0.14
1 dry basis.
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Moya-Ignacio, M.; Sánchez, D.; Romero, J.Á.; Villar-García, J.R. Study of Various Mechanical Properties of Maize (Zea mays) as Influenced by Moisture Content. Agronomy 2024, 14, 1613. https://doi.org/10.3390/agronomy14081613

AMA Style

Moya-Ignacio M, Sánchez D, Romero JÁ, Villar-García JR. Study of Various Mechanical Properties of Maize (Zea mays) as Influenced by Moisture Content. Agronomy. 2024; 14(8):1613. https://doi.org/10.3390/agronomy14081613

Chicago/Turabian Style

Moya-Ignacio, Manuel, David Sánchez, José Ángel Romero, and José Ramón Villar-García. 2024. "Study of Various Mechanical Properties of Maize (Zea mays) as Influenced by Moisture Content" Agronomy 14, no. 8: 1613. https://doi.org/10.3390/agronomy14081613

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