¹Department of Materials Science and Engineering, Kwara State University, Nigeria

²Department of Mining Engineering, Federal University of Technology, Nigeria

³Department of Geology and Mineral Sciences, University of Ilorin, Nigeria

*Corresponding author:  Department of Materials Science and Engineering, Kwara State University, Malete, Kwara State, Nigeria, Tel: +2348033979980, E-mail:  [email protected]

Received Date: April 23, 2022

Published Date: October 12, 2022

Citation: Abdulraman SO. (2022). Investigation of the Effect of Weathering of Selected Charnockite Rocks in Nigeria on Its Engineering Properties. Material Science. 4(2):15.

Copyrights: Abdulraman SO. © (2022). 

ABSTRACT

The effect of alteration in the petrographic characteristics of charnockite on its strength, its engineering properties and transformations of the specific gravity which took place during weathering has been examined in this study. This study characterized selected samples of un-weathered and weathered charnockite rocks of Ijare, Ondo State, Nigeria. The physical properties determined were specific gravity, density, porosity, water content and water absorption while the mechanical properties investigated include the point load strength index. Results from the analysis carried out reveal: the point load index of 5.74Mpa and 3.72Mpa for unweathered and weathered samples; average uniaxial compressive strength of 136.90Mpa and 65.50Mpa for unweathered and weathered samples; slake durability of 98.80% and 97.80% for unweathered and weathered sample; Schmidt rebound value of 49Mpa and 45Mpa for both the unweathered and weathered samples respectively. The rock samples were also subjected to petrographical analysis whose result shows that weathering increases some investigated physical properties like porosity, water content and water absorption of the rock while decreasing the specific gravity and density. The result of the tests shows the bulk and dry densities increase for both the weathered and unweathered samples; increased degree of weathering equally results in a major reduction in the strength of the charnockite rock which is directly associated with changes in mineralogy and associated porosity. The point load strength, uniaxial compressive strength test and rebound hardness result values decreases because of weathering. Weathering processes also affected the physical properties of rock, mineralogy, chemical composition as well as the mechanical properties of the rock thereby reducing the strength and mechanical behavior of the rock materials.

Keywords: Charnockite, Ijare, Ondo State, Weathering, Petrographic, Mineralogical and Engineering Properties.

INTRODUCTION

Charnockite being one of the basement rocks in the earth’s outer surface reputable for esthetics has proven over time to be of a high economic value. Generally, un-weathered charnockite rocks have sufficient strength and are preferred for construction purposes as aggregates, concrete, railway ballast as well as dimension stones in the manufacturing of ornaments and tiles etc. Rock materials depends majorly on the relationship between the particles, inherent cementing materials and rock minerals for its properties especially the physical and mechanical ones. Therefore, the main minerals in charnockite are Quartz and Felspar (Orthoclase and Plagioclase) while some of the other minor ones are biotite, hornblende, pyroxene and microcline etc. In tropical region such as the Southwestern Nigeria, quartz has proved to be very stable that when charnockite is weathered, the quartz in it becomes sand grains. However, the orthoclase feldspar is not as stable. It yields potassium and carbonate ions, silica and flakes of kaolinite when orthoclase reacts with water and carbon dioxide.

Rocks affected by weathering suffers alteration in both its fabric and mineralogy as well as the strength properties. Although, results of several investigations conducted by previous researchers highlights the important role weathering plays in rock formation. Nonetheless, the immediate and resultant effect of weathering has great significance in determining the usage of rocks and its aggregates for both engineering works and design of geotechnical structures. The understanding of the changes of rock strength characteristics due to weathering is important for the discipline of Rock Engineering and Engineering Geology. [1] stated that both the mechanical properties of rocks and the value of strength are dependent on the inherent characteristics of the weathered rocks. The voids in rocks such as micro cracks also affect the rock’s physical and mechanical properties and also accounts for the anisotropy in most granites. Mineral composition, texture voids characteristics are the major factors controlling the physicochemical harm by most rocks introduced to new environments [2­−4].

However, little or no research has been reported on how weathering affects or alter the physico-mechanical properties/characteristics of the charnockite rock in the Southwestern Nigeria. Hence, the key interest of this study is to assess the alteration of the petrographical, physical and mechanical properties of charnockite to further enhance a better understanding of its performances for various engineering applications. This research aims to know the role of weathering has in the engineering properties of selected charnockites in Ondo State. The objectives are to:

1. ascertain the mineralogical composition of the selected rock types;
2. highlight the engineering properties (compressive strength, porosity, PLI, hardness, slake durability, specific gravity, density and water adsorption of both unweathered and weathered rock materials; and
3. evaluate the impact of weathering on the construction/engineering properties of charnockites.

It is our hope that this study will serve as a means of understanding the effect and the impact weathering has on the engineering properties of the selected charnockite rocks in Southwestern Nigeria as well as their uses in engineering works.

The area studied is ZIBO FM Quarry in Ijare, Ondo State which lies on latitudes 007° 20ʹ 25ʹʹN and 007° 20ʹ 26ʹʹN and longitude 005° 09ʹ 50ʹʹE and 005° 09ʹ 55ʹʹE (Figure 1). Lithologically, this outcrop is made up of migmatite, intruded by three petrological varieties of older granites. The migmatites consist of melanocratic paleosomes as well as felsic neosomes. The three petrographic varieties of older granite rocks recognized in this group are coarse-porphyritic biotite-hornblende, coarse-medium grained biotite hornblende, and medium-coarse grained charnockite.

Figure 1: Geological Map of Ondo State, Southwestern Nigeria [5].

METHODS OF INVESTIGATION

2.1 Sample Collection and Preparation

This research was conducted through field work and laboratory analyses of samples collected from the charnockite outcrops’ location in Ijare area of Ondo State. GPS machine was employed in locating the study area. Block sizes of weathered and unweathered samples were taken from the outcrops. Samples were prepared (cut and ground) using the acceptable ISRM standard to get the physiomechanical properties of the rock [6]. The quantitative analysis of thin sections was done to the petrographic characteristics using Standard Polarizing Microscope and Scanning Electron Microscope for the analysis of the opaque minerals.

2.2 Rock Classification

Rock mass classification/characterization systems can be of considerable use in the initial stage of a project when little or no detail information is available. Colour being a significant noticeable feature of a rock has arguable being one of the most difficult tool to use for rock’s description. However, in hand specimen, there is a very distinct colour difference between the unweathered and weathered samples. The method adopted for the identification of the samples is the colour indicated by the grinded samples after milling.

2.3 Petrographic Examination of the Rock Samples

2.3.1 Procedure for preparing thin sections of rock samples

Samples from various locations were labelled according to their locations and the chips were cut from each of the samples. The obtained chips were trimmed to obtain smooth edges and grinded to obtain smooth surface with the aid of grinding machine. The grinded chips were then lapped on the successive grade of abrasive 400, 600 and 700. The material chip was placed on hot plate covered with Aluminum foil. The chip and the glass slide were allowed to warm up gently on the hot plate, after few minutes, the mounting medium was applied. The chip was mounted on the glass slide by placing it on chip. It was then placed on the slide bench to cool and set for about 24 hours. The chips were further grinded to reduce the thickness to about 90 microns. Lapping was carried out on grades of abrasives 400, 600 and 700. A paste of the first grade of carborandum 400 was made on the mixture from 400-600 and finally on 700, watching changes in colour from one grade to another under a polarizing microscope. A mineral that is quartz was picked as an index mineral, whose colour changed from purple on carborandum 400, to blue or deep yellow on caborandum 600 and finally to grey on caborandum 700. A standard thin section of about 30 microns was obtained. Razor blade was used to further trim the slide to sharpen the edges of the slides. This was done by carefully scrapping the excess aradite from the slide. The completed thin section was lightly heated on the hot plate with drops of canadal balsams. The cover slip was carefully placed over the slides and pressed down to exclude air bubbles. The slides were allowed to cool and then covered. After covering, the thin sections methylated spirit cotton wool was used to wash them. They were washed with detergent and rinsed with water until they were clean. The slides were air-dried. After drying, the slides were labelled with masking tape and stored in slide boxes.

2.3.2 Study of the Thin Sections

All the slides prepared from rock samples as thin sections were studied using the polarizing microscope while the mineralogical composition of the samples was determined using the modal analysis technique. Three different counts of all different spots on a slide were taken and all the counts were added together for percentage calculation of each mineral based on the total number of all minerals present.

2.4 Rock Density Determination

The essence of this test was to determine the dry and bulk densities of samples (irregular form). The dry density is the density of the rock in dried state (i.e. after oven dried) as density varies with moisture content. The apparatus used for the tests are:

1. oven
2. desiccator
3. balance using 0.01g accuracy and a 100g range
4. container for the sample with air-tight lid.

Irregular form of five specimens were prepared from the representative rock sample. The sizes of the specimen were prepared using the required procedure The M_sub and M_sat of the sample were measured to determine the volume of irregular samples.

The specimen was in a container, but without a cover, and dried in an oven to constant mass (for about 24 hours) at a temperature of about 105°C. After covering it, the specimen was allowed to cool in a desiccator for about 30 minutes. The mass (C) of the container (and cover) with the specimen was determined with an accuracy of 0.1g. The container with the cover was cleaned and dried and its mass (A) determined. This procedure followed acceptable ISRM standard using equations 2.1 to 2.4 [6].

2.5 Determination of Specific Gravity of Rock Samples

Specific gravity is the ratio of the weight in air of a given volume of a material at a particular temperature to the weight of the same volume of water at a particular temperature. The test was done using the ASTM (American Society of Testing and Monitoring) standard and procedure. ASTM D854-00 – Standard Test for Specific Gravity (G_S) of Soil Solids by Water Pycnometer is calculated using the equation 2.5 below:

M₁ = Mass of density bottle (pycnometer) + stopper.

M₂ = Mass of density bottle (pycnometer) + stopper + sample.

M₃ = Mass of water + sample + density bottle (pycnometer) + stopper.

M₄ = Mass of water + stopper + density bottle (pycnometer).

2.6 Determining the Water Content of Rock Material

The determination of the water content of the rock material collected from the field is the main objective of this test. To get accurate result of the in-situ water content, the precautions of sampling, handling, transportation and storage should be followed to ensure that the water content is within the (recommended) 1% of the in-situ value.

Equipment used are:

1. oven (24 hours at 105°C +/- 5°C)
2. desiccator
3. balance with accuracy of 0.1g and range of 1000g
4. sample container (non-corrodible) with airtight lid.

Each representative sample of at least 5 lumps was prepared, measuring at least 50g and the minimum lump dimension was at least ten (10) times the maximum grain size of the rock. The lumps were placed in the container, covered by the lid and the mass (B) was determined using accuracy of about 0.1g. The process was repeated but this time keeping the lumps in the container and leaving the lid opened, oven-dried at a constant mass (for about 24 hours) at a temperature of 105°C. Again, the lids were replaced, and the lumps cooled in a desiccator for 30 minutes. The container with the lumps has its mass (C) determined using the accuracy of 0.1g. Thereafter, the container with the lid was cleaned, dried and has its mass (A) determined. This test was done three (3) times and the calculation of average of the result followed. The minimum lump dimension was at least ten times the maximum grain size of the rock. The procedure followed acceptable ISRM standard [6] using the equation 2.6 below:

2.7 Water Absorption Determination (W_A)

Water absorption is considered as one of the most significant parameters for determining the weathering of rocks in the laboratory. This property can be considered as one of the most suitable, among routine tests, to reveal the presence of microcracks connected with weathering increase. Moreover, the direct connection between water absorption and porosity also involves a strict dependence of compression resistance to the capacity of a rock to absorb water. The procedure involves drying the specimen (in the open container) in an oven using a constant mass (for 24 hours) and at about 105°C temperature. After closing the container and cooling it in a desiccator for about 30 minutes, the determination of the mass (C) of the dry sample with the container (and the lid) is done using an accuracy of 0.01g. Thereafter, the specimen was saturated using water immersion technique in a vacuum that was less than 800Pa in 24 hours. Eventually, the calculation of the water absorption as the percentage (%) of the dry mass was done using the equation below:

2.8 Determining the Porosity of the Rock Material

This test was done essentially to determine the porosity of the rock specimens of irregular form. Porosity is defined as the volume of the pores in a rock that is expressed as a percentage (%) of the total volume of the rock. The equipment used are:

1. oven (24 hours at 105°C +/- 5°C)
2. desiccator
3. balance; with a range of about 200g and an accuracy of 0.01g
4. sample container (non-corrodible) with air-tight lid
5. vacuum chamber with pressure less than 800Pa

Irregular form of five specimens from a representative sample of the rocks were prepared. The sizes of the specimen were made with a mass of at least 50g of irregular form and the minimum specimen dimension was at least ten times the maximum grain size of the rock. The bulk volume (V) of the specimen was determined through the measurement of the saturated-submerged mass (M_sub) and the saturated mass (M_sat) of the samples for the dimension of the specimen. The specimen was saturated by water immersion in a vacuum of less than 800 Pa for One (1) hour with periodic agitation to remove trapped air.

The specimen was removed from the water and surface-dried using moisten cloth. Necessary precautions were taken including removing the surface water and ensuring that no fragments were lost. The specimen was located in a container to avoid the loss of the mass during subsequent sample handling. The mass of the specimen plus container (B) was determined with an accuracy of about 0.01g.

The specimen (in the open container) was dried in the oven at constant mass (for 24 hours) and temperature (of 105°C). After closing the container and cooling it in the desiccator for 30 minutes, the mass (C) of the dry sample with the container (and lid) was determined with an accuracy of 0.01g. The container with the lid was cleaned and dried with its mass determined using an accuracy of 0.01g.

The procedure followed acceptable standard [6] using the equation 2.8 to 2.9.

2.9 Determining the Point Load Index of Rock Materials

Point load test is a simple test and usually used for the estimation of rock strength. Equipment used for this test consists of a loading frame that can measure the force required to break the sample and a system for measuring the distance between the two plate contact points. The test’s apparatus is: two-point load platens; a rigid frame; a hydraulically activated ram (with pressure gauge); and a device for measuring the loading points’ distance. The procedures conform with acceptable ISRM standard [7]. The point load test ensures the determination of the uncorrected point load strength index (ls). It should be corrected to standard equivalent diameter (De) of 50mm.

But if there is a significant difference, the result is reported as uncorrected strength and it can be multiplied by the correction factor, F; to get the corrected strength [8].

F = (D/50)^0.45                                                                                                                             (2.15)

F = correction factor; D = diameter (mm)

2.10 Determining the Rock Material’s Uniaxial Compressive Strength

This test is usually carried out to determine the unconfined (or uniaxial) compressive strength (UCS) of rock specimens of cylindrical form. It is mostly used for strength classification and the characterization of rocks that are intact. The UCS is adjudged to be the best in measuring the deformation, strength and the fracture characteristics of rocks. Like the above, its procedure is in line with acceptable ISRM standard [6].

Uniaxial compression tests on the unweathered and weathered (altered) charnockite was done on a MTS servo controlled hydraulic testing machine in accordance with ISRM guidelines [6]. In addition, axial and lateral micro-measurement N2A series electric resistance strain gauges were secured to each sample to record strain development on loading. The result of the axial and lateral strain was recorded on the PC attached to the machine in a Microsoft Excel text file. The volumetric strain was calculated from the ratio of axial strain to lateral strain. The following equation gives the strength of the rock according to ISRM standard [6]:

   

                σ_c = unaxial compressive strength (UCS) of the rock (MPa);

                P_max = peak load (kN);                              

A = initial cross sectional area (m²)

2.11 Determination of Deformations in Unconfined Compression

The stress-strain curves are determined through this test. From the curve, the Tangent Modulus E_t50, Modulus of elasticity, and the rock’s Poisson ratio were calculated. The apparatus used include a testing machine with sufficient capacity that is capable of applying axial load at a rate that conforms to the requirements. The steel platens with which the specimen was loaded  consists of hardened steel and were more than double the diameter of the specimen. The upper bearing plate was uquipped with a spherical seat. And the centre of curvature of the spherical seat coincided with the centre of the top face of the specimen. During loading, the determination of the axial and lateral deformation of the specimen was done using dial guages. Meanwhile, the conversion of   load measurement and axial deformation carried out during the different stages of the test to values of stress and strain was carried out with the formula below:

               

                P = axial load (Newton); A₀ = cross-sectional area of the specimen at the start of the test;

                dL = the axial deformation of the specimen (m, and positive for a decrease of length);

                L₀ = the original length of the specimen.

As stated above, it is on the graph of stress versus strain that the compressive stress and axial strain are to be plotted. This is for the curve to give the best description of the behavior of the deformed rock especially from zero stress up to failure; in case the rock shows non-linear stress-strain behavior at low and high stress levels. Strain gauges attached to the rock sample during loading measures the axial and lateral deformation. The volumetric strain is calculated as follows:

                ε_a = Axial strain; ε_r = Lateral strain

The Poisson ratio can be calculated from the formula below:

2.12 Determination of Slake Durability Index of Rock Materials

Rocks are said to be durable when they are resistant to breakdown under weathering conditions over time. When a rock with clay minerals mixes with water, the rock swells and slaking occur. To measure durability in rocks, the slake durability index (ID₂) is used. It works by giving the proportionate information of the behaviour (mechanical) of the rock with respect to the amount of clay and some other minerals that are birthed as a result of the effect of weathering on them [9].

To carry out this test, samples containing 10 rock pieces were selected with each weighing between 40 to 60g; with the whole sample weight range of between 450 to 550g. This sample was then placed in a screen drum with both the sample and the drum oven-dried at a temperature of about 110°C (plus or minus 5°C) using a constant weight. After the sample has been allowed to cool to normal room temperature, the drum was coupled to a motor and rotated then immersed in distilled water at a speed of 20rpm for 10 minutes. The sample was again oven-dried at a temperature of 110°C ± 5°C using a constant weight. Thereafter, the sample was made to undergo a second wetting and drying cycle. It is after the second cycle that the slake durability index is calculated using the formula below:

ID₂ = slake durability index (second cycle), (%);

B = mass of the drum plus oven-dried sample before the first cycle, (g)

WF = mass of the drum + oven-dried sample retained after the second cycle, (g)

C = mass of the drum, (g)

From the test, the visual and index classification was done in accordance with the appearance of the rock pieces remaining and the range of the ID₂. The result is shown in Tables 2.1 and 2.2 below:

Table 1: Visual Description of the Rock Samples Retained in the Test Drum after the Second Cycle (after [10])

Type	Description
I	Pieces remain virtually unchanged
II	Consists of large and small pieces
III	Exclusively small fragments