Lab 2: Sieve Analysis

Abstract

A soil’s gradation characteristics are one of the fundamental properties that can describe a material in sufficient detail to classify and utilize the material in a wide variety of applications. Numerous agencies and code bodies (i.e. state highway agencies (SHAs), International Building Code (IBC), American Concrete Institute (ACI), Superpave, etc.) all use gradations to ensure materials such as backfill, structural fill, and pavement mixtures have adequate strength and durability. The gradation itself is a description of the amount of material present at a set or range of specific sizes. For example, part of a gradation analysis may determine that a soil sample has particles that are 65%, by weight, smaller than 0.5 inches.

Required Standards
ASTM C136 – Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates
ASTM D6913 – Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis
ASTM E11 – Specification for Woven Wire Test Sieve Cloth and Test Sieves

Introduction

A sieve¹ analysis is a process in which the soil material is separated into various sizes by using a combination of sieves. A sieve is typically a round or rectangular frame that has a wire cloth inside it. This cloth is a precisely woven material that has a specific opening size as outlined in ASTM E11. Multiple sieves are stacked in descending size to perform the analysis. This sieve stack is then physically or mechanically agitated to ensure all the particles have a chance to fall through the openings in the wire cloth. Finally, the amount retained on each sieve is weighed and the results can be plotted.

Objectives

At the completion of this lab exercise, you will have satisfied the following objectives:

1. Perform sieve analysis on a silty-clayey soil material
2. Perform sieve analysis on a sandy soil material
3. Perform sieve analysis on a coarse aggregate
4. Perform calculations necessary to determine the requisite gradation properties
5. Construct gradation chart of three aforementioned materials down to a #200 (75 \(\mu\)m) sieve size

Learning Objectives

At the completion of this lab exercise, you should be able to:
–understand what a sieve is and how a sieve stack is utilized to obtain gradation information
–perform calculations necessary to characterize the gradation of a sieve analysis
–understand how to interpret a gradation chart and fineness modulus values when provided with specification requirements
–present information on gradations in a useful and professional manner

Procedure

The sieve analysis procedure is divided into three parts: preparation, execution, and analysis. Given the importance placed on accurate sieve analyses, it is critical to keep accurate notes and follow steps carefully and accurately. When an engineer presents their stamped work to a client, a phrase similar to “the values were determined in accordance with ASTM D6913” will be included. This is not a generic phrase! You are legally stating you followed all of the requirements outlined in the specification. If you skipped a step, and the client found out or suspected the numbers were off, they would have legal standing to challenge your results. If you did follow the specification exactly, then you can correctly use that phrase. If you did not, one alternative is “the values were determined using generally accepted procedures such as ASTM D6913 and others”. However, if you were a client, would you want a firm to follow specifications exactly or one that gets most of it?

We are going to run two sieve analysis procedures: ASTM C136 and ASTM D6913. They are fairly similar with a few specific differences. The ASTM C136 procedure is typically used on aggregate sources for concrete and asphalt mixtures. It is a simpler procedure because it assumes there are no significant amount of fines² present in the sample that are plastic³. The ASTM D6913 is better suited for soils with significant amount of fines as the test method specifies procedures to separate the fine particles.

Regardless of which procedure to use, the size of the sieves are governed by ASTM E11. There are three general ways to identify a sieve: number, opening in millimeters, and opening in inches. Each sieve has a nameplate attached to it that identifies its size and that it meets ASTM E11 (Fig. 1).

Preparation

In performing this lab procedure, we can characterize soil and aggregate materials with varying grain sizes including fine-grained soil (silt or clay), coarse-grained soil (sand or gravel), and coarse aggregate. Fine-grained soils will be characterized with ASTM D6913 and the coarse-grained soil or coarse aggregate will be characterized with ASTM C136. The actual sieve sizes we use are minimally specified; that is, the specification outlines the minimum sizes we should use but we could add additional sizes depending on our objective. For this exercise, we will stick with the minimum requirements.

The minimum requirements for ASTM C136 are in Section 8.2 and simply state that the minimum sieves are those specified by the client or agency. For characterization of our sandy soil sample, the required sieves are: #4, #8, #16, #30, #50, #100, and #200 (Fig. 2). These particular sieves are chosen so that the fineness modulus can be calculated for the sandy soil sample as described in ASTM C136 Section 9.2. As we do not typically calculate a fineness modulus for coarse aggregate, we will use the following sieves when characterizing coarse aggregate: 37.5 mm, 25 mm, 12.5 mm, 9.5 mm, and #4. For all the sieves stacks, do not forget the pan and lid (Fig. 3).

The minimum requirements for ASTM D6913 are in Section 6.1.1 and Table 1 within the standard and apply to the silty-clayey soil. Because this method is designed for soil materials that can have a wide range of particle sizes, there are significantly more sieves. So many, in fact, that we split the test into two “runs”.

The sieve analysis procedure is done on a weight-basis (i.e. all calculations are done using weights, not volumes). We are going to be recording a variety of weights⁴ and it is important to prepare a worksheet prior to lab to aid in collecting the necessary data.

To aid us in creating a worksheet, let’s think about what we want at the end of the test: weight of aggregate sitting on each sieve. Well, we could shake the sieve stack, separate the sieves, then dump the amount in each sieve on the scale and measure it. However, with the smaller sieve sizes, it is really difficult to get all the particles out of the woven cloth. Thus, our measurements are likely to be off, and in some cases significantly off! So, we need to first measure the weight of the empty sieves, run the sieve analysis, then weigh the sieve with the retained material. We can then subtract the weight of the empty sieve and obtain the retained mass! An example data collection worksheet is shown in Appendix A.

As you weigh each sieve to obtain the empty weight, be sure to inspect it for leftover aggregate (Fig. 4) or damaged/torn mesh (Fig. 5). Once you have all the sieves weighed, arrange the stack of sieves so that the largest opening sieve is at the top of the stack and the opening decreases as you go down the stack (Fig. 2).

We will also need the starting weight of the material. It is possible to sieve too much material and get erroneous results. Fortunately, both specifications outline the maximum allowable on the individual sieves and total starting weights. For ASTM C136, this starting sample weight information is found in Section 7.3 and Section 7.4 for fine aggregate and coarse aggregate, respectively. The overload weight for each sieve for ASTM C136 is found in Section 8.3. For ASTM D6913, this starting sample weight information is found in Section 10.2. The overload weight for each sieve for ASTM D6913 is found in Section 11.3.

Checklist
☐ Obtain correct sieves, note any missing sieves
☐ Weigh empty sieves, including the pan
☐ Determine minimum weight of material needed for test
☐ Record starting sample weights

Execution

We are now ready to run the sieve analysis. For both specifications, the end goal is to separate the particles into their individual sizes. Both specifications describe manual and mechanical agitation methods to expedite the sieving process. For this laboratory exercise, we will utilize mechanical shaking methods for both procedures.

Neither specification outlines a precise time to shake the sieves. This is because soil materials do not sieve equally. Both specifications outline trial procedures to use to determine the amount of time to sieve a sample. A typical shaking time for ASTM C136 materials is 5-10 minutes whereas ASTM D6913 materials have shaking times around 15-20 minutes. For this laboratory exercise, the ASTM C136 test will be run with a shaking time of 5 minutes and the ASTM D6913 test will be run with a shaking time of 10 minutes.

Because the sieves are expensive and damage can occur if they are not loaded into the sieve shaker properly, the instructor will assist you with this process. Make note if the correct shaking time is used. An example of a properly loaded sieve stack is shown in Figs. 6, 7, and 8.

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After the shaking period has ended, the sieve stack will be removed from the shaker. The next step is one of the hardest steps for this entire process: separating the sieve stacks without spilling material. The sieves have been pounded together and particles have wedge themselves in-between the sieves. The instructor will demonstrate the technique to use to correctly separate the sieves.

With the sieves now separated, they can be individually weighed. If material was lost during the separation process, note it in your report but perform the calculations with the values you measure. Do not attempt to guess how much material was lost. You may notice that some sieves appear empty. You should still weigh them. If there was a particle stuck in the mesh prior to the shaking process and it came loose, you will see a decrease in the apparent mass, indicating you were not careful in the preparation of the test.

Checklist
☐ Select correct shaking time for material being sieved
☐ Carefully separate sieve stack after shaking period
☐ Record weights of individual sieves with retained aggregate
☐ Dump retained aggregate and clean each sieve

Analysis

We should now have enough data to calculate and plot the gradation of the three materials characterized in the laboratory exercise. The first thing to do is calculate the mass of material on each sieve. You have likely figured out by now we simply subtract the empty weight of the sieve from the weight of the sieve with the material in it. At this point, the individual masses should be summed to see how close it is to the original starting mass. This is a very important check as any discrepancy would indicate that material was lost during the test. The maximum allowable loss of material is 0.3% for ASTM C136 procedures. Interestingly, ASTM D6913 does not specify a maximum allowable loss.

With individual retained weights and the summed weight, we can calculate the percentage retained on each sieve. With the retained percentages on each sieve, we can then calculate a cumulative, or running total, of percentage retained. Finally, the cumulative passing is simply 100% minus the cumulative retained. An example dataset is shown in Table 1 and can be used to check the spreadsheet you develop for this laboratory exercise. Note that the #200 sieve value for cumulative retained goes to one decimal place. This is required by ASTM C136 Section 10.2. There are a couple of checks one can do to ensure the calculations are correct:

• Cumulative retained must always end at 100% at the pan.
• Cumulative retained values must always increase from sieve to sieve down the stack.
• Cumulative passing must always end at 0% at the pan.
• Cumulative passing values must always decrease from sieve to sieve down the stack.

This fineness modulus is typically used for daily quality control at a quarry or ready-mix plant to monitor the aggregate sources for changes. In some cases, agencies will specify a minimum and maximum fineness modulus value for a project. A lot of times, the fineness modulus is abbreviated FM and is typically only used on fine aggregates, but can be used on coarse aggregates. As previously mentioned, the specifics of the calculation are in ASTM C136 Section 9.2.

Checklist
☐ Calculate cumulative retained and cumulative passing for all three soil material sources
☐ Calculate the FM of the sandy soil sample

Summary

We have successfully planned, executed, and analyzed the results of a sieve analysis on three different materials using two different ASTM specifications. This relatively straightforward process forms the basis of numerous engineering endeavors. All disciplines within civil engineering utilize gradation information for hazardous waste diffusion, concrete mixture design, slope stability, etc.

However, the test methods used to obtain the gradation data are sometimes insufficient. You may have noticed that the silty-clayey soil sample had a significant amount of material that passed the #200 sieve. We need a method to evaluate the gradation to an even smaller particle size. Why? We need to know how much clay and silt we have in our sample as these materials can significantly affect the behavior we encounter in the field.

1. Pronounced “sive” like “give”; not “seeve” like “grieve”. Surest way to look like a rookie in front of your new boss!
2. Recall that fines are usually considered particles smaller than the #200 sieve (75 \(\mu\)m).
3. The topic of plasticity is covered in the companion Atterberg Limits laboratory exercise.
4. For this laboratory exercise, weight and mass are used interchangeably. Since the materials being measured are not moving and we assume the gravitational force exerted on them is constant, we can safely make this assumption. Additionally, we are not calculating the force exerted by the materials. In other parts of this course, we will distinguish between weight and mass using gravity.
5. You may be excited to point out that I am not using consistent decimal places for the Retained and Cuml. Retained columns, but ASTM allows me to report the #200 sieve and smaller to one decimal place provided the value is less than 10%.