Effect of Silt and Clay Contents in Natural Sand on the Compressive and Flexural Strengths of Standard Grade Concrete

Affiliation:

^1,2,3,4Cagayan State University, Carig Campus, College of Engineering and Architecture, Civil Engineering Department, Tuguegarao City, Philippines

ORCID: ^4*

Licensed under:

1. Introduction

1.1 Background

Concrete, in construction, is a structural material consisting of a hard, chemically inert particulate substance, known as aggregate (usually sand and gravel), that is bonded together by cement and water. Concrete is characterized by the type of aggregate or cement used, by the specific qualities it manifests, or by the methods used to produce it. In ordinary structural concrete, the character of the concrete is determined mainly by a water-to-cement ratio. The lower the water content, all else being equal, the stronger the concrete. The mixture must have just enough water to ensure that the cement paste surrounds each aggregate particle, that the spaces between the aggregate particles are filled, and that the concrete is liquid enough to be poured and spread effectively. Another durability factor is the amount of cement in relation to the aggregate (expressed as a three-part ratio—cement to fine aggregate to coarse aggregate). Where firm concrete is needed, there will be relatively less aggregate (Britannica, 2022).

Quality assurance of building materials is crucial for constructing strong, durable, and cost-effective structures (Savitha, 2012). When construction is planned, building materials should be selected to fulfill the functions for which they are expected. In Kenya, over 14 buildings have been reported to collapse in the last 10 years, leading to deaths and injuries, and various causes of building failure have been suggested. Use of poor-quality construction materials (such as poor-quality sand, aggregates, or water) results in poor-quality structures. It may cause structures to fail, resulting in injuries, fatalities, and financial losses for developers. Impurities in building sands contribute to a reduction in compressive strength. According to Olanitori (2006), the higher the percentage of clay and silt content in sand used in concrete production, the lower the compressive strength of the hardened concrete. Although many studies mentioned above have shown that the use of poor-quality materials is one of the major contributing factors to building collapse, testing these materials has not been carried out to examine the impact of impurities in building sands on the overall performance of concrete. In addition, where tests have been carried out, testing of both clayey soils, silts, and organic impurities has not been conducted to determine their combined effect on concrete strength. To prevent building failures, careful selection of construction materials, including building sands, is paramount to ensure they meet the set construction standards. Impurities in sand negatively impact both compressive strength and bond strength between steel reinforcement and concrete, potentially leading to building failure. The Nigerian Standard Organization specifies the maximum quantity of silt in sand as 8% beyond which sand is regarded as unsuitable for construction work (Olanitori & Olotuah, 2005).

Silt is a granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil (often mixed with sand or clay) or as sediment suspended in water (also known as a suspended load) in a body of water, such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt typically has a dry, floury texture and a slippery texture when wet. Silt can be visually observed with a hand lens (Bashir & Kour, 2019).

In analyzing the influence(s) of silt and clay on the concrete strength, if the silt and clay content is found to increase the concrete strength, it means silt and clay can partially replace fine aggregates. This will reduce the cost of concrete production; however, if not, it will be pivotal information for all concrete designers and users to ensure that fine aggregates are free from silt and clay impurities. Silt and clay is not as strong as typical fine aggregates. They can absorb water, and their properties can change. In fresh concrete, silt and clay will interfere with the bonding of aggregates to cement. In hardened concrete, if the silt and clay come in contact with water in air voids, it can shrink or swell, either building internal pressure (swelling) or leaving larger voids and weakening the concrete (shrinking). Silt and clay are much finer than fine aggregates. Due to its non-cohesive property, which prevents it from reacting with cement, fine aggregates, or water, it starts reacting, such as through shrinking and swelling. Still, it exists in concrete, which can cause unwanted hairline cracks or, in some cases, significant cracks, depending on the percentage of silt and clay, which should not exceed 4%. The failure of concrete structures, resulting in the collapse of buildings, has prompted numerous research studies on the quality of construction materials. This failure has resulted in injuries, loss of life, and significant investments, which have been primarily attributed to the use of substandard concrete ingredients. The fine aggregates of high quality have a high positive effect on the quality of compressive strength of concrete, and the presence of silt and clay above a certain percentage in fine aggregates requires more cement to coat other ingredients of concrete (Hannah et al., 2014, as cited in Ayodele & Ayeni, 2015).

The Philippines does not have similar studies related to the topic; hence, the researchers need to pilot the study. The lack of related studies means that the majority are unaware of the adverse effects of silt and clay in concrete mix if present in aggregates. According to Haileslassie (2014), the presence of dust (silt), loam, and clay, which are finer than sand, used to make concrete or mortar, decreases the bond between the material to be bound together and hence the strength of the mixture. The finer particles not only reduce the strength but also the quality of the mixture produced, resulting in fast deterioration.

Tuguegarao City is home to numerous construction firms that utilize aggregates as one of their primary materials. Some contractors are not washing their aggregates before using them, disregarding the potential effects when mixed into concrete. Demissie (2019) recommended in his study that washing the sand or cement increment should be taken to alleviate the effect of the clayey silt content in the sand. Therefore, conducting this study is relevant to fill the lack of knowledge of those in the construction sector regarding the adverse effects of silt on concrete’s strength and workability.

1.2 Statement of the Problem

Concrete structures can take various shapes and sizes, ranging from simple rectangular columns to slender, curved domes or shells. Olanitori (2006) stated that several factors can adversely affect the strength and durability of concrete structures, including poor design, inadequate supervision, and impurities in aggregates. Olanitori & Olotuah (2005) explained how Civil Engineering structures have experienced various failures during their lifespans due to multiple factors, such as the poor quality of sand, possibly as a result of inadequate studies on sand properties, which constitute the materials on which civil engineering structures are founded. In view of this, an adequate study of sand properties from selected locations cannot be overemphasized, as the failure of many structures can be attributed to inadequate concrete strength that does not meet the design strength. Research on the effect of clay and silt impurities in sand from different sources or locations on the strength of concrete helps determine the suitability of sands for concrete production, considering both strength and economic factors. Concrete strength is based on the quality of aggregates and other materials used in its production, including cement and water (Ezeagu, 2007).

Concrete fails when it can no longer provide the required strength to support its designed load. The failure of concrete can sometimes be mild, with visible cracks and deflections, or severe, leading to partial or total collapse of the structure, either during construction or the post-construction stage. Incidents of structural failures linked to poor concrete practices are widespread in Nigeria, particularly in major cities such as Lagos, Port Harcourt, and Abuja, among others. Generally, the sand available in the riverbed is very coarse and contains a very high percentage of silt and clay. The silt and clay present in the sand reduce the strength of the concrete and hold moisture.

These findings from various sources emphasize the importance of determining the silt and clay contents present in a fine aggregate before incorporating it into a concrete mix to achieve optimal performance. Therefore, a study on the effects of silt and clay contents in natural sand on the compressive and flexural strengths of standard-grade concrete in Tuguegarao City is needed, as no prior studies have been conducted.

1.3 Objectives of the Study

1.3.1. General Objectives

The primary purpose of this study is to evaluate the effects of silt and clay contents of natural sand on the compressive and flexural strengths of standard-grade concrete.

1.3.2. Specific Objectives

1. To determine the physical properties of natural sand from selected different sources (Site A, B, C, and D) and their contents of silts and clays.

2. To determine the compressive and flexural strengths of M25 grade concrete with natural sand at different curing periods.

3. To analyze the effect of silt and clay contents with varying contents using Simple Linear Regression (SLR) Analysis.

1.4 Conceptual Framework

Fig. 1: Conceptual Framework

1.4 Scope and Limitations of the Study

This study determined the effects of silt content of natural sand on the compressive and flexural strengths of concrete in Tuguegarao City. It tested selected natural sand samples taken from different sources in the place, by doing laboratory tests. The result of this study was analyzed through simple linear regression, and other methods of statistics. It was analyzed to know the relationship of silt content and the mechanical properties of concrete. Likewise, this study was limited to the standard grade of concrete mixes which is made by laboratory tests using only materials available locally. It did not include the effects of other factors such as the quality of coarse aggregates, type of cement, variations in water cement ratio, and the use of admixture. It also did not include the effects of other factors in the local materials and environment.

2. Materials and Methods

2.1 Study Area

This research study area is located in Tuguegarao City, the capital of Cagayan Province. Tuguegarao is home to 62 large and small construction firms, as well as the main offices of the Department of Public Works and Highways and the Department of Agriculture in Region 2. Having this number of firms involved in the construction sector makes this study more relevant, as it will raise awareness not only to contractors but also to all stakeholders of future projects about the possible effects of the amount of silt content present in natural sand. To select the locations for sample collection in the study area, Purposive Sampling was employed. A purposive sample is a non-probability sample that is selected based on the characteristics of a population and the study's objective (Crossman, 2020). This study used three (3) natural sand samples that were collected from the riverbanks of the Cagayan River, at barangays Buntun and Pallua, Tuguegarao City, tagged at Sites A, B, C, and Site D as a control. Figure 2 shows the sample collection.

Fig. 2: Sample collected from the study area

2.2 Research Design

This study adopted an experimental approach. It is also a combination of qualitative and quantitative research. It qualitatively described the physical properties of natural sand from selected different sources and the amount of silts and clays. Moreover, it quantified the flexural and compressive strengths of M25-grade concrete using natural sand. The laboratory test was conducted in accordance with the specific objectives of the research and by reviewing the literature on sand impurities. This design requires that the sample be carefully selected from the population, the experiment be conducted on these samples, and that the results be carefully observed and recorded for further analysis. Attempts are then made, based on the results, to determine the fundamental opinion within the cause-and-effect relationship. Experimental research was deemed appropriate for this study for these reasons:

• To isolate and quantify the effects of changes in the values of independent variables, observations must be adequately taken into account.

• To forecast the correlations between cause and effect, i.e., identify relationships between independent and dependent variables.

Fig. 3: Flowchart of research design process

2.3 Materials

Sand samples were obtained from Sites A, B, and C, and the sand for the control sample was obtained from Site D. From each supply point, 100 kg of sand samples were obtained for physical and mechanical testing. Coarse aggregates from crushed stones 25.4 mm normal size obtained from Site D; Eagle Cement Blended Cement Type 1T ordinary Portland cement from the cement shop, hydrometer, and universal testing machine from the Department of Public Works and Highways.

Fig. 4: Materials and Testing Machine

2.4 Methods

The researchers will prepare data sheets for each test to be conducted, which will serve as a basis for data analysis and be reviewed by the research adviser. After preparing and approving the said data sheets, data collection will proceed through observation and experimentation. The experiment will include the following tests: Sieve Analysis, Slump Test, Silt Test, Compressive Strength Test, and Flexural Strength Test.

2.4.1 Collection and Preparation of Sand Samples

A code ID was assigned to each sand sample to represent the supply point, curing age, and silt content, thereby avoiding any bias. Each local supply point was labeled accordingly, Site D-P14, P28, and Site A-B14-3%, B14-4%, B14-5%, B14-6%, B28-3%, B28-4%, B28-5% and B28-6%. The sampling technique used to reduce the field sample and obtain the necessary test component for each test run was directed by ASTM C702. Visual examination, tests for index qualities, tests for contaminants, tests for workability, and strength tests are all performed. These experiments were conducted to determine if the amount was sufficient to cause harm and withstand forces. On fresh concrete, a slump test was conducted, and 150mm x 150mm x 150mm cubes and 150mm x 150mm x 525mm beam samples were prepared, compacted, demolded after 24 hours of casting, and cured in a water tank for 14 and 28 days. Sand from each source is subjected to a series of laboratory experiments in accordance with ASTM standards.

• ASTM C702, Reducing Samples of Aggregate to Testing Size.

• ASTM C136 is used for particle size distribution analysis.

• ASTM C128 for specific gravity and water absorption.

• ASTM C117 for silt content materials passing 75μm.

• ASTM C 192, making and curing concrete.

• ASTM C109-90 compressive strength testing for the concrete cubes.

• ASTM C78 flexural strength testing for the concrete beams.

2.4.2.1. Particle Size Analysis of Soils (AASHTO DESIGNATION: T 88-00)

The purpose of the grain size analysis test is to estimate the proportion of each size of grain that makes up a soil sample. The test's outcomes can be utilized to create the grain size distribution curve. The soil is categorized, and its behavior is predicted using this information. The two methods generally used to find the grain size distribution are:

• Sieve analysis, which is used for particle sizes larger than 0.075 mm in diameter

• Hydrometer analysis, which is used for particle sizes smaller than 0.075 mm in diameter.

2.4.2.2. Sieve Analysis

Sieve analysis is employed to ascertain the grain size distribution of soils with a diameter greater than 0.075 mm. Although it is frequently used for sand and gravel, it cannot be the sole technique for determining the distribution of grain sizes in finer soils. This technique employs woven wire sieves with square openings. The list of the U.S. standard sieve numbers with their corresponding opening sizes is provided in Table 1.

Table 1. Nominal Dimensions of U.S. Standard Sieves – AASHTO M92

2.4.2.3. Specific Gravity of Coarse Aggregate

The ratio between the substance's specific gravity and that of a given volume of water was used to calculate the specific gravity of the coarse aggregates. According to this definition, the substance must be completely solid. However, they have both permeable and impermeable pores, whose structure influences the specific gravity of the aggregate (size, quantity, and continuity pattern) as well as permeability and water absorption.

Bulk Specific Gravity (Gsb) (also known as Bulk Dry Specific Gravity). The ratio of the weight in air of a unit volume of aggregate at a stated temperature to the weight in air of an equal volume of gas-free distilled water at a stated temperature. This unit volume of aggregates is composed of the solid particles, permeable voids, and impermeable voids.

Gsb = A / (B-C) (Eq.1)

Where: A is the oven-dry weight.

B is the SSD weight.

C is the Weight in water.

Bulk SSD Specific Gravity (Gsb SSD). The ratio of the weight in air of a unit volume of aggregate, including the weight of water within the voids filled to the extent achieved by submerging in water for approximately 15 hours, to the weight in air of an equal volume of gas-free distilled water at a stated temperature.

Gsb SSD = B / (B - C) (Eq. 2)

Where: B is the SSD weight.

C is the weight in water.

Apparent Specific Gravity (Gsa). This ratio of the weight in air of a unit volume of the impermeable portion of aggregate (does not include the permeable pores in aggregate) to the weight in air of an equal volume of gas-free distilled water at a stated temperature.

Gsa = A / (A - C) (Eq. 3)

Where: A = Oven dry weight.

C = weight in water

Fig. 5: Specific gravity and unit weight of coarse aggregate.

2.4.2.4. Specific Gravity of Fine Aggregate

The specific gravity of a soil is defined as the ratio of the weight in air of a given volume of soil particles to the weight in air of an equal volume of distilled water at a stated temperature. According to ASTM D854-92, specific gravity is determined in a lab by adding 500 grams of the sample to the pycnometer. It is filled up with distilled water before being emptied. The pycnometer is turned on its side to release trapped air, and its outside is cleaned before being weighed (W1). The pycnometer’s contents are transferred to a tray, and its weight (W2) is calculated once it has been filled with distilled water to the same level. The aggregates are dried with a cloth to a saturated surface dry condition and weighed (W3).

Fig. 6: Specific gravity of fine aggregate.

2.4.2.5. Hydrometer Analysis

Sieve analysis cannot determine the particle size distribution of soil with a considerable amount of smaller particles (silt and clay). The data are presented on a semi-log plot of percent finer against grain diameters to illustrate the particle size distribution. The hydrometer analysis is a commonly used technique for estimating the distribution of soil particle sizes from the #200 (0.075 mm) sieve to approximately 0.001 mm. To obtain the entire gradation curve of the coarse and fine fractions of many natural soils, both sieve analysis and hydrometer analysis are necessary.

(a). Equipment

• Balance
• Mixer (blender)
• Hydrometer (152H model preferably)
• Sedimentation cylinder (1000 mL cylinder)
• Graduated 1000 mL cylinder for control jar
• Dispersing agent [sodium hexametaphosphate (NaPO3) or sodium silicate (NaSiO3)]
• Control cylinder
• Thermometer
• Beaker
• Timing device

(b). Method

• 50 g of fine soil was placed in a beaker along with 125 mL of the dispersant (sodium hexametaphosphate [40 g/L] solution), and the mixture was stirred until the soil was completely moist. The soil was given at least 10 minutes to soak.

Fig. 7: Sodium hexametaphosphate solution added & meniscus correction reading

• 125 mL of the dispersion agent was added to the control cylinder and filled to the mark with distilled water while the soil is soaking. Record readings less than zero as a negative (-) correction and readings between zero and sixty as a positive (+) correction (the reading at the top of the meniscus created by the hydrometer stem and the control solution is known as the zero connection). The difference between the top of the meniscus and the level of the solution in the control jar—typically about +1—is the meniscus correction. To thoroughly combine the contents, the control cylinder was shaken. The zero correction and temperature were noted, respectively, after inserting the hydrometer and thermometer into the control cylinder.
• If additional distilled water is required, the soil slurry is poured into a mixer until the mixing cup is at least halfway filled. After that, the mixture is stirred for two minutes.

Fig. 8: Soil slurry preparation using a mixer and pouring the soil sample into the sedimentation cylinder

• The soil slurry was immediately transferred into the empty sedimentation cylinder, and distilled water was added up to the mark.
• The open end of the cylinder is covered with a stopper and secured with the palm. Alternatively, the cylinder was turned upside down and back upright for one minute, inverting it approximately 30 times.
• The time was recorded after setting the cylinder down. To obtain the initial reading, the cylinder's stopper was removed, and the hydrometer was carefully and slowly inserted. (Note: To reduce disruption, the hydrometer is inserted or removed slowly—about ten seconds—and released as close as possible to the reading depth to prevent excessive bobbing.)
• The readings were taken by looking at the meniscus' top, which was created by the hydrometer's stem and the suspension. The hydrometer was repositioned in the control cylinder after it was carefully removed from the original position. To remove any debris that may have adhered to it, gently spin the item in the control cylinder.

Fig. 9: Observing the top of the meniscus and the Hydrometer readings with a time interval

• The hydrometer was read at 15 seconds, 30 seconds, 1 minute, 2 minutes, 4 minutes, 8 minutes, 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours, 16 hours, 24 hours, and 48 hours. These are rough times that will typically result in a pleasing plot spread.
• For each hydrometer reading, the temperature of the soil-water suspension was noted to the nearest 0.5°C.

2.5 Mix Design for Normal Concrete Mixtures

The process of determining the required and specifiable characteristics of a concrete mixture is called mix design. Characteristics can include:

1. Fresh concrete property.

2. Required mechanical properties of hardened concrete, such as strength and durability requirements; and

3. The inclusion, exclusion, or limits on specific ingredients.

Mix design leads to the development of a concrete specification. Mixture proportioning refers to the process of determining the quantities of concrete ingredients, using local materials, to achieve the specified characteristics of the concrete. A properly proportioned concrete mix should possess these qualities:

• Acceptable workability of the freshly mixed concrete

• Durability, strength, and uniform appearance of the hardened concrete

• Economically, understanding the basic principles of mixture design is as important as the actual calculations used to establish mix proportions.

Only with the proper selection of materials and mixture characteristics can the above qualities be obtained in concrete construction (Demissie, 2019).

The batching of concrete was done by weight. One batch consists of two beam moulders (150 x 150 x 525 mm) and two cube moulders (150 x 150 x 150 mm) prepared by a mix ratio of 1:1.6:3 and 0.55 water-cement ratio using a combination of coarse aggregate and fine aggregate with different concentrations of silt and clay as shown in Table 2.

Table 2: Combination of concrete ingredients.

The amounts of coarse aggregate, cement, and water used are kept constant for all batches. Constituent materials were measured and weighed. Cement was mixed with fine aggregate on the platform until the mixture was thoroughly blended. Coarse aggregate was then added and mixed until it was uniformly distributed throughout the batch. Water was finally added and mixed until the concrete appeared homogeneous and of the desired consistency. Molds of 150mm x 150 mm x 150mm cubes and a 150mm x 150mm x 525mm beam moulder were used to mold the concrete cubes. The molds were adequately lubricated with oil to prevent adhesion of the concrete to the surface. After 24 hours in the molds, the specimens were removed and placed in the water tank for 14 and 28 days.

Table 3: Concrete Mix Design Proportions for Beam Samples.

Table 4: Concrete mix design proportion for cube samples

2.6 Workability of Concrete

Workability of Concrete is a broad and subjective term describing how easily freshly mixed concrete can be mixed, placed, consolidated, and finished with minimal loss of homogeneity. Workability is a property that directly impacts strength, quality, appearance, and even the cost of labor for placement and finishing operations. ASTM C 125 workability is termed as "that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity." Thus, Slump measurement is taken immediately after mixing each batch of concrete before casting it into specimen molds. The concrete was cast in three layers into the cube molders, and each layer was tamped 25 times with a 16mm diameter rod to remove entrapped air present in the concrete. While the beam sample was cast in two layers, it was tamped 63 times using the same rod. Excess concrete was removed, and the surface was leveled using a trowel. The slump, as illustrated in Figure 10 below, is measured as the difference between the height of the mold and that of the highest point of the specimen.

Fig. 10: Slump testing of concrete from each batch.

2.7 Casting, Compacting, and Curing of Concrete Cubes

2.7.1. Casting of Concrete

The mold used was made of phenolic board, fastened with a Torx pin, and was built solid enough to contain the concrete during casting. The wet concrete has some considerable weight and requires extensive support arrangements. Before commencing, it was ensured that the molds were clean and contained no debris. The moulds were oiled to allow easy removal of the specimens after 24 hours.

2.7.2. Compacting of Concrete

Having secured the necessary supply, the concrete specimens were tamped. Three layers of concrete were cast into the cube molders, and each layer was tamped 25 times using a rod with a 16 mm diameter to eliminate any trapped air. While utilizing the same rod, the beam sample was cast in two layers and tamped 63 times. These two activities are carried out simultaneously.

2.7.3. Demolding of Concrete Cubes

The molds were left in place until the specimens had hardened sufficiently to hold their own weight, i.e., 24 hours. Care was taken to prevent damage to edges and corners during the demolding process. After 24 hours, the concrete specimens can bear their weight.

Fig. 11: Concrete cube specimens demolding

2.7.4. Curing condition and duration of concrete

Failure to cure concrete will have adverse effects on its durability, as the loss of moisture in fresh concrete will result in poor quality, reduced strength, and increased permeability. Ponding is an effective method for preventing moisture loss from the concrete; it also helps maintain a uniform temperature in the concrete (Parkhe et al., 2016). The curing water was more than about 11°C (20°F) cooler than the concrete to prevent thermal stresses that could result in cracking. The most thorough method of curing with water consists of total immersion of the finished concrete element. This method is commonly used in the laboratory for curing concrete test specimens. Where the appearance of the concrete is important, the water used for curing by ponding or immersion was free of substances that will stain or discolor the concrete.

2.8 Standard Compression Test

Compressive Strength of concrete is one of the most important and valuable properties of concrete. The primary purpose of designing concrete is to resist compressive strength in structural members. Hence, it is the role of a concrete designer to specify the expected characteristics and strength of the concrete/mix proportion, to enable it to resist external forces. The compressive strength of the concrete is determined using the compressive test, as specified in ASTM C496. The standard acceptance test for measuring the strength of concrete involves short-term compression tests on concrete cubes (150 mm x 150 mm x 150 mm), which are cured and tested in accordance with the building codes. The said cubes were subjected to this test to determine the variation in strength regarding the different percentages of silt and clay present in unwashed samples, and to investigate the effects of silt and clay content on concrete. The standard acceptance test is carried out when the concrete is 28 days old. The water-cement (w/cm) ratio used was 0.55. A maximum water-cement ratio reduces the risk of excessive water being introduced into the mix, resulting in lower-than-required strength gain (Shaikh & Shete, 2022). According to ACI 301-1.6.6 - “The strength of concrete is satisfactory provided that every average of three consecutive strength tests (sets) equals or exceeds the specified compressive strength and no strength test (set) falls below the design strength by more than 500 psi.

Fig. 12: Cubes subjected to compression testing

2.9 Standard Flexural Test

The flexural strength test is an indirect method often used to determine the tensile strength of concrete. It is usually expressed as the concrete modulus of rupture. Specimens were tested to failure using universal testing machines at 14 and 28-day curing ages, respectively. The three-point loading suggested by ASTM was used for the flexural test of the concrete beams. The flexural test was conducted using a universal testing machine with a maximum loading capacity of 1200 KN. The bed of the testing machine was provided with two steel rollers, 38 mm in diameter, on which the specimen is supported, and these rollers were mounted so that the distance from center to center is 60 cm for 15.0 cm specimens or 40 cm for 10.0 cm specimens. Individual beams were fed into the apparatus slowly throughout the test until failure occurred. Two similar rollers, positioned at the third points of the supporting span, are spaced 20 or 13.3 cm center to center. These rollers are used to transfer the load. The force is equally distributed between the two loading rollers, and each roller was placed so that the load is applied axially and without placing any torsional strains or constraints on the specimen. The machine's screen exhibited and recorded the failure force and the accompanying deflection. Equations 3 and 4 were used to calculate the flexural strength and flexural modulus of the concrete beams.

Flexural strength:

S = ${\textstyle {\frac {3WL}{2b{d}^{2}}}}$ (Eq.4)

Flexural modulus:

Fm = ${\textstyle {\frac {{L}^{3}F}{4b{d}^{2}y}}}$ (Eq. 5)

Where: S is the Flexural strength of the concrete beam at the cross-sectional plane of failure (MPa).

Fn is the Flexural modulus of the concrete (MPa or GPa).

W is the Maximum load indicated by the testing machine(N).

L is the Concrete beam Span (mm).

b is the Average width of the concrete beam at the plane of failure (mm).

d is the Average depth of the concrete beam at the plane of failure (mm).

y is the Deflection of the beam corresponding to the load(mm)

Fig. 13: Rectangular beam mobilization and flexural testing.

2.10 Sample Population

The sample population for this study was samples of natural sands collected from selected locations in Tuguegarao City – Site A, B, C, and a control variable obtained from Site D. The sample with the highest silt content – Site A - underwent a series of tests necessary for the study, along with the control variable.

2.11 Sampling Technique and Sample Size

The sample was extracted from river sand, explicitly focusing on sand collected from the riverbanks of Tuguegarao City. Three sand samples from various selected sources underwent a series of tests. The sand sample from Site A, which has the highest silt and clay content, was selected for use in the study.

2.12 Data Processing and Analysis

The analysis involved inferential statistics. Calculation of the weighted mean was performed in the analysis of the compressive strength of four cubes and the flexural strength of four beams for curing ages (14 and 28 days) of sand samples. To measure the strength of a linear association between two variables, Pearson Correlation was used. A simple linear regression model was performed to examine how the dependent variable (Flexural and compressive strength of M25 grade concrete) is influenced by the independent variables.

2.13 Research Tools

1. Pearson Correlation Coefficient

Pearson correlation, also known as the Pearson correlation coefficient, is a measure of the linear relationship between two variables. It quantifies the strength and direction of the relationship between two continuous variables. The coefficient ranges from -1 to +1. A correlation coefficient of +1 indicates a perfect positive linear relationship, meaning that as one variable increases, the other variable increases proportionally. A correlation coefficient of -1 indicates a perfect negative linear relationship, implying that as one variable increases, the other variable decreases proportionally. A correlation coefficient of 0 indicates that there is no linear relationship between the variables.

The formula for calculating the Pearson correlation coefficient (r) is:

${\textstyle {\mathit {\boldsymbol {r=}}}{\frac {{\mathit {\boldsymbol {n(}}}\sum _{}^{}{\mathit {\boldsymbol {xy)-}}}{\mathit {\boldsymbol {(}}}\sum _{}^{}{\mathit {\boldsymbol {x}}}{\mathit {\boldsymbol {)(}}}\sum _{}^{}{\mathit {\boldsymbol {y)}}}}{\sqrt {{\mathit {\boldsymbol {[n}}}\sum _{}^{}{\mathit {\boldsymbol {x}}}^{\mathit {\boldsymbol {2}}}{\mathit {\boldsymbol {-(}}}{\sum _{}^{}{\mathit {\boldsymbol {x)}}}}^{\mathit {\boldsymbol {2}}}{\mathit {\boldsymbol {][[n}}}\sum _{}^{}{\mathit {\boldsymbol {y}}}^{\mathit {\boldsymbol {2}}}{\mathit {\boldsymbol {-(}}}{\sum _{}^{}{\mathit {\boldsymbol {y)}}}}^{\mathit {\boldsymbol {2}}}{\mathit {\boldsymbol {]}}}}}}}$ (Eq.6)

Where r is the Pearson Coefficient

n is the number of pairs of the stock

∑xy is the sum of products of the paired stocks

∑x is the sum of the x scores (amount of Silt and Clay)

∑y is the sum of the y scores (Workability, Compressive, and Flexural Strengths)

∑x² is the sum of the squared x scores (amount of Silt and Clay)

∑y² is the sum of the squared y scores (Workability, Compressive, and Flexural Strengths)

2. Simple Linear Regression Analysis

Simple linear regression is a statistical technique used to model the relationship between two variables: an independent variable (often denoted as X) and a dependent variable (often denoted as Y). It assumes a linear relationship between the variables and aims to find a linear equation that best fits the data.

The equation for simple linear regression is typically expressed as:

y = β₀ + β₁X₁ + ε (Eq. 7)

Where y is the predicted value of the dependent variable (compressive and Flexural strengths) for any given value of the independent variable (varying percentage of silt and clay contents).

B₀ is the intercept, the predicted value of y when x is 0.

B₁ is the regression coefficient – how much we expect y to change as x increases.

x is the independent variable (varying percentage of silt and clay contents).

e is the error of the estimate, or how much variation there is in our estimate of the regression coefficient.

The goal of simple linear regression is to estimate the values of β₀ and β₁ that minimize the sum of the squared errors (residuals) between the observed values of Y and the predicted values from the regression equation. The estimation of the regression coefficients (β₀ and β₁) is typically done using the method of least squares. This method finds the line that minimizes the sum of the squared differences between the observed Y values and the predicted Y values from the regression equation. Once the regression coefficients are estimated, they can be used to make predictions or draw inferences about the relationship between X and Y. The slope (β₁) represents the change in Y for a one-unit increase in X, while the y-intercept (β₀) represents the predicted value of Y when X is equal to zero.

2.14 Validity and Reliability

Validity is a test or instrument that accurately measures what it is meant to measure, whereas reliability is a measure of the stability or consistency of the test score. A correlation coefficient, which measures the strength of the relationship between two variables, is typically used to describe the reliability of standardized tests. The range of these coefficients is between -1.00 and +1.00, with the former indicating perfect negative reliability and the latter indicating perfect positive reliability. The calibration of the testing equipment was therefore confirmed.

3. Results and Discussion

3.1 Physical Properties

3.1.1. Grain Size Distribution

Sieve analysis was conducted on the sand samples to determine their degree of fineness. Percentages of sand passing and retained were analyzed, and a grading curve was plotted for comparison. Control sand samples were prepared by thoroughly washing river sand with clean water to remove silt, clay, and organic impurities, and then drying. Aggregate grain size distribution or gradation is one of the properties of aggregates that influences the quality of concrete. Therefore, the sand was dried in the air and sieved using a set of sieves with openings ranging from 9.5 mm to 75 μm. The grading curve: percentage (%) passing is plotted against the particle diameter on a standard semi-log graph together with the upper and lower limits of the adopted fine aggregate grading curve envelope. For the determination of Fineness modulus, the Sum of cumulative percentage weight retained on each sieve is divided by 100. Meanwhile, the fineness modulus is calculated by adding the percentage weight of material retained in each of the standard sieves and then dividing the total by 100. The objective of finding the fineness modulus is to grade a given aggregate for the most economical mix and workability with a minimum quantity of cement.

Fig. 14: Particle size distribution curve of Coarse Aggregate of Site D

Fig. 15: Particle size distribution curve of Site A

Fig. 16: Particle size distribution curve of Site B

Fig. 17: Particle size distribution curve of Site C

Fig. 18: Particle size distribution curve of Site D

The figures above show that the grain size analysis test results indicate that local sand samples did not comply with the ASTM C136-05 and DPWH standard requirements, as they were taken directly from their natural locations. However, the control variable, Site D from Figure 18, passed the upper and lower limits of the said requirements. This indicates that local sand samples cannot be used directly in their natural state and must be thoroughly washed and sieved first to separate the various sieve size fractions.

3.1.2. Specific Gravity

Table 5: Specific Gravity of coarse aggregate

Table 6: Specific Gravity of fine aggregate

The specific gravity values indicate that the materials used in this study are aggregates that comply with ASTM C128 standard requirements, ranging from 2.4 to 3.1, which reflects the silica content of the sand. Specifically, the higher the silica content, the higher the specific gravity.

3.2 Hydrometer Analysis

Any sand that is supposed to be used as fine aggregate should be free from silt, clay, and organic impurities. The impurities in the sand affect the bond between the cement paste and the sand surface, thereby decreasing the strength of the concrete. From the result of the silt content test, which is presented in the figures below, it is clearly observed that sand samples from Sites B, C, and D have fulfilled the silt content requirement, which is stated under ASTM C117 and DPWH requirements, which are 0.6%, 0.3%, and 2.3%, respectively. However, the sample from Site A has a 12.2% silt concentration, which did not comply with the said requirements. Site A was then selected as the variable to be studied.

Fig. 19: Particle Distribution Curve of Site A

Fig. 20: Particle Distribution Curve of Site B.

Fig. 21: Particle Distribution Curve of Site C

Fig. 22: Particle Distribution Curve of Site D

3.3 Workability

It has been verified that slump values decrease as the percentage of silt and clay in sand increases, whereas the slump increases as the water-to-cement ratio increases. For the same water-to-cement ratio in the study, the decrease in slump value resulting from an increase in silt and clay percentage in the sand can be attributed to the fact that finer particles have a larger surface area by volume, which absorbs more water in the concrete mix.

Table 7: Slump result of sand samples

3.3 Effect of Sand Impurities on Workability and Strength Reduction of Concrete

In this study, simple linear regression was employed because there is only one independent variable that can affect the dependent variable being forecasted. The following variables are used in the linear regressions presented in Table 8 below.

Table 8: Organized Raw Data for Linear Regression Analysis

Where: X1 is the Percentage of Silt content.

Y1 is the Compressive Strength

Y2 is the Flexural Strength

3.4 Compressive Strength Test

To observe the performance changes due to impurities present in river sand in concrete production, standard-grade concrete M-25 was prepared and tested for compressive strength at 14 and 28 days. The test results for all samples. However, for discussion purposes, the summarized test results are presented in Figure 23 below, organized by the curing time of the specimens.

Fig. 23: Compressive strength vs. Curing time graph

Generally, a concrete is required to provide a specified strength. The most common measure of concrete strength is its compressive strength, which is determined in either a cube test or a cylinder test. For each sample, a total of five cubes were cast, cured under water at room temperature, and tested for compressive strength. Five concrete cubes were made from each sample and tested at ages of 14 days and 28 days to obtain the average value. The results are shown in the Figure above.

• From the above results, based on ACI 301-1.6.6, three samples (B-3%, B-4%, B-5% and B-6%) failed to meet the minimum strength expected at day 14 and at day 28.

• The range of the maximum silt content recommended by the Standard Specifications of the Department of Public Works and Highways is 3%. Sand samples with a silt content greater than 3% were rejected and excluded from further investigthe sampling processring sampling.

• The compressive strength of concrete cubes varies when samples are subjected to similar casting and curing conditions; this failure is primarily attributed to the presence of a high amount of impurities in the sand, which left insufficient water in the mix for the complete hydration of cement, and to some extent to particle shapes, sizes, and texture.

3.4.1. The effect of sand impurities on compressive

The regression analysis was used to derive the relationship between compressive strength of concrete with varying silt content, as illustrated below.

Table 9: Regression Summary Output for Workability on Compressive Strength

Therefore, the equation of the regression line can be written as:

Y= -2.0697X + 26.575 (Eq. 7)

Fig. 23: Compressive strength vs. Percentage of Silt Content graph

From the Figure above, an increase in silt and clay content significantly reduces the compressive strength of concrete. The 95% contribution represents a contribution of 25 MPa for concrete. Therefore, the presence of silt and clay cannot be ignored during the concrete production process, as they may lead to the failure and collapse of the structure; therefore, care must be taken during design and supervision.

Table 10: Compressive Strength Correlation Summary Output

Where: X1 is the Percentage of Silt content.

Y1 is the Compressive Strength

Y2 is the Flexural Strength

From Table 10, it is deduced that the contribution of silt and clay content toward the compressive strength of concrete is a significant 97%. It is observed that the contribution of silt and clay content towards the strength of concrete is significant.

3.5 Flexural Strength Test

To observe the performance changes due to impurities present in river sand in concrete production, standard-grade concrete M-25 was prepared and tested for flexural strength at 14 and 28 days. The test results for all samples are presented. However, for discussion purposes, the summarized test results are presented in Figure 24 below, organized by the curing time of the specimens.

Fig. 25: Flexural strength vs. Curing time graph

3.5.1. The effect of sand impurities on flexural reduction

The regression analysis was used to derive the relationship between the flexural strength of concrete with varying silt content, as illustrated below.

Table 11: Regression Summary Output for Workability on Flexural Strength

Therefore, the equation of the regression line can be written as: Y= -0.4833X + 4.5358.

Fig. 26: Flexural strength vs. Percentage of Silt Content graph

From the Figure above, an increase in silt and clay content significantly reduces the flexural strength of concrete. The 98% contribution represents a contribution of 4.5 mPa for concrete. Therefore, the presence of silt and clay cannot be ignored during the concrete production process, as they may lead to the failure and collapse of the structure; therefore, care must be taken during design and supervision.

Table 12: Flexural Strength Correlation Summary Output

Where: X1 is the Percentage of Silt content.

Y1 is the Compressive Strength

Y2 is the Flexural Strength

From Table 12, it is deduced that the contribution of silt and clay content toward the compressive strength of concrete is a significant 98%. It is observed that the contribution of silt and clay content has a significant impact on the strength of the concrete sample.

4. Summary, Conclusion, and Recommendation

4.1 Summary

The general objective of this study is to evaluate the effects of silt and clay contents of natural sand on the compressive and flexural strengths of standard-grade concrete. This study specifically aimed to determine the physical properties of natural sand from selected different sources and their amount of silt and clay, to determine the compressive and flexural strengths of M25 grade concrete with natural sand at different curing periods, and to analyze the effect of silt and clay with varying contents using Linear Regression Analysis. To discuss the results of the specific objectives, the following summary was established:

1. Aggregates occupy about 80 percent of the volume of typical concrete mixtures, and their characteristics have a definitive impact on the performance of fresh and hardened concrete. This study primarily focused on the physical characteristics of the amount of organic impurities in sand, including silt and clay content, as well as their impact on the workability (assessed by the slump) of fresh concrete and the compressive and flexural strength of hardened concrete.

• Based on grain size analysis, Sites A, B, and C did not comply with the standard requirement set by ASTM C136-05 and DPWH, while Site D passed the said requirement.
• The level of silt and clay content found in Sites A, B, C, and D was 12.2%, 2.3%, 0.3%, and 0.6%, respectively. The specific gravity was 2.64, 2.62, 2.72, and 2.71, respectively.
• According to ASTM C136, test aggregate should have a 2 to 3.5 fineness modulus to give a good workability under economical conditions. Site A has a fineness modulus of 1.24, Site B has 2.43, Site C has 0.867, and Site D has 3.2.

2. With lower silt and clay contents, the use of river sand would affect the quality control of the concrete production.

• The compressive strengths of sand having 3%, 4%, 5% and 6% at 14^th day curing are 11.63 MPa, 9.47 MPa, 9.21 MPa, and 8.91 MPa, respectively. The control variable, with a strength of 14.89 MPa, obtained a value of 0.6%.
• The compressive strengths of sand with 3%, 4%, 5%, and 6% content at 28^th day curing are 21.63 MPa, 17.176 MPa, 17.07 MPa, and 13.613 MPa, respectively. The control variable, with a strength of 24.89 MPa, obtained a value of 0.6%.
• The flexural strengths of sand with 3%, 4%, 5%, and 6% content at 14^th day curing are 1.37 MPa, 0.9 MPa, 0.83 MPa, and 0.79 MPa, respectively. The control variable, with a strength of 3.21 MPa, obtained a value of 0.6%.
• The flexural strengths of sand with 3%, 4%, 5%, and 6% content at 28^th day curing are 3.31 MPa, 2.62 MPa, 1.93 MPa, and 1.68 MPa, respectively. The control variable, with a strength of 4.15 MPa, obtained a value of 0.6%.

3. Silt and clay content on concrete in terms of workability and strength reduction of concrete under regression analysis.

• Referring to Table 9, the value of R Square represents an excellent fit as it is 0.947. This means that a 95% variation in the amount of silt and clay content can have a significant impact on the compressive strength of concrete. The coefficient of silt and clay content is approximately -2.07.
• Referring to Table 11, the value of R Square represents an excellent fit as it is 0.976. This means that a 98% variation in the amount of silt and clay content can have a significant impact on the flexural strength of concrete. The coefficient of silt and clay content is approximately -0.48.

4.2 Conclusion

The use of aggregates in a project is one of the most necessary considerations in construction. Aggregates are used in a significant portion of the structure, and it is essential to investigate their quality and strength before using them to ensure the quality and strength of the structure to be built.

1. The gradation results indicated that only the control variable passed the standard requirements set by ASTM C 33-03, while the sands collected at Sites A, B, and C did not pass. Results obtained from hydrometer tests indicated that Site A has the highest percentage of silt among the other sands. This also indicates that sand in its natural state should not be used for construction, especially for high-rise and heavy structures.

2. Based on the results obtained for compressive strength tests, the sample with 3% silt and clay content and the control variable passed the standard requirement set by the ACI 301-1.6.6, and the rest failed. The results of the flexural strength test indicated that only the control variable met the standard requirements.

• Based on upper and lower limit requirements, three samples (B-3%, B-4%, B-5% and B-6%) failed to meet the minimum compressive strength expected at day 14, and only one sample (B-6%) failed to meet the compressive strength expected at day 14. While in flexural strength, only the control variable (P-0.6%) meets the minimum strength expected at day 14.
• Based on upper and lower limit requirements, three samples (B-4%, B-5% and B-6%) failed to meet the minimum compressive strength expected at day 28, and two samples (P-0.6% and B-3%) met the compressive strength expected at day 28. While in flexural strength, only the control variable (P-0.6%) meets the minimum strength expected at day 28.

3. The compressive and flexural strength of the concrete is indirectly proportional to the amount of silt and clay in the fine aggregate. The above data show that any increment of this finer material – silt and clay – decreases the strength and workability of the concrete. Gradation analysis also shows that riverbank sands in their natural state, or in situ, are not suitable for use in construction, as they do not meet the standard requirements provided by ASTM C136-05. Therefore, the presence of silt and clay cannot be ignored during the concrete production process, as they may lead to the failure and collapse of the structure; therefore, care must be taken during design and supervision.

4.3 Recommendation

In this study, sand in its natural state is used to investigate its index properties and impurities. Much better correlations between experimental and theoretical values were obtained in this manner, rather than blending and washing the sand, to determine the level of impurities and their effect on workability, compressive, and flexural strength. The reason behind this was that all sand used – except for the control variable – in the concrete production was directly sourced from the source to attain the specific objectives stated. It is observed that sand in its natural state does not meet the gradation requirement, and grading plays a significant role in concrete workability. Since significant variations in grading are expected across different samples of fine aggregates, it is recommended to sieve and recombine the aggregates for optimal concrete workability.

The result of the compressive strength test shows that, with respect to ASTM C 33–86, ASTM Destination D-2419, and the standard set by the DPWH, only the sample with a 3% silt and clay content passed the requirement. With this, the user has the option to use different concentrations, following the standard requirements they prefer. However, for safety purposes, we recommend that fine aggregates with a silt and clay content of 3%, 4%, and 5% can only be used for low-rise residential structures and other minor projects.

If the percentage of silt and the content of sand exceed the limits set by the standards, then the following remedial measures must be taken.

• Washing of sand increment should be taken to alleviate the effect of the clayey silt content of the sand.
• Before concrete mix preparation, attention should be given to critically assessing the physical quality, impurities, and gradation of fine aggregates.
• Sand is one of the most important construction materials, but availability is limited, and there is a need for proper management, awareness, and understanding of construction sand by suppliers, quarry operators, washing plant operators, builders, and the respective govlevant government agenciesocal area officials and related authorities need to support the legal aspect and action to control the illegal operation, like obtaining sand from riverbanks without consulting quarry checkers for economic gain, and need to maintain the supply chain management.

Acknowledgment

The researchers would like to acknowledge the faculty of the Civil Engineering Department, College of Engineering and Architecture, Carig Campus, Cagayan State University, Philippines, for their unwavering support in finishing this research.

Funding

The authors received no direct funding for this research.

Conflict of Interest

The authors declare that there is no conflict of interest regarding the publication of this article.

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