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Gradation test

A gradation test, or sieve analysis, is a commonly used procedure in civil engineering that measures the size distribution, or gradation, of an aggregate, such as sand, crushed rock, or clay. A gradation test is performed on a sample of aggregate in a laboratory. The test consists of weighing the sample of the aggregate and passing it through progressively smaller mesh screens known as sieves. Each sieve is then weighed and compared against the total weight of the sample of aggregate. The results of this test are used to describe the properties of the aggregate and to see if it is appropriate for various civil engineering purposes such as selecting the appropriate aggregate for concrete mixes, asphalt mixes, and foundations. The results of this test are provided in graphical form to identify the type of gradation of the aggregate. The complete procedure for this test is outlined in the American Society for Testing and Materials (ASTM) C 136 and the American Association and State Highway and Transportation Officials (AASHTO) T 27

Preparation
In order to perform the test, a sample of the aggregate must be obtained from the source. To prepare the sample, the aggregate should be mixed thoroughly and be reduced to a suitable size for testing. The total weight of the sample is also required.

Procedure
A suitable sieve size for the aggregate should be selected and placed in order of decreasing size, from top to bottom, in a mechanical sieve shaker. A pan should be placed underneath the nest of sieves to collect the aggregate that passes through the smallest. The entire nest is then agitated, and the material whose diameter is smaller than the mesh opening pass through the sieves. After the aggregate reaches the pan, the amount of material retained in each sieve is then weighed.

Results
The results are presented in a graph of percent passing versus the sieve size. On the graph the sieve size scale is logarithmic. To find the percent of aggregate passing through each sieve, first find the percent retained in each sieve. To do so, the following equation is used,

%Retained = $$\frac{W_{Sieve}}{W_{Total}}$$×100%

where WSieve is the weight of aggregate in the sieve and WTotal is the total weight of the aggregate. The next step is to find the cumulative percent of aggregate retained in each sieve. To do so, add up the total amount of aggregate that is retained in each sieve and the amount in the previous sieves. The cumulative percent passing of the aggregate is found by subtracting the percent retained from 100%.

%Cumulative Passing = 100% - %Cumulative Retained.

The values are then plotted on a graph with cumulative percent passing on the y axis and logarithmic sieve size on the x axis.

Types of Gradation

 * Dense gradation – A dense gradation refers to a sample that is approximately of equal amounts of various sizes of aggregate. By having a dense gradation, most of the air voids between the material are filled with particles.  A dense gradation will result in an even curve on the gradation graph.


 * Narrow gradation – Also known as uniform gradation, a narrow gradation is a sample that has aggregate of approximately the same size. The curve on the gradation graph is very steep, and occupies a small range of the aggregate.


 * Gap gradation – A gap gradation refers to an sample with very little aggregate in the medium size range. This results in only coarse and fine aggregate.  The curve is horizontal in the medium size range on the gradation graph.


 * Open gradation – An open gradation refers an aggregate sample with very little fine aggregate particles. This results in many air voids, because there are no fine particles to fill them.  On the gradation graph, it appears as a curve that is horizontal in the small size range.


 * Rich gradation – A rich gradation refers to a sample of aggregate with a high proportion of particles of small sizes.

Properties
Gradation affects many properties of an aggregate. It affects bulk density, physical stability and permeability. With careful selection of the gradation, it is possible to achieve high bulk density, high physical stability, and low permeability. This is important because in pavement design, a workable, stable mix with resistance to water is important. With an open gradation, the bulk density is relatively low, due to the lack of fine particles, the physical stability is moderate, and the permeability is quite high. With a rich gradation, the bulk density will also be low, the physical stability is low, and the permeability is also low. The gradation can be affected to achieve the desired properties for the particular engineering application.

Engineering Applications
Gradation is usually specified for each engineering application it is used for. For example, foundations might only call for coarse aggregates, and therefore an open gradation is needed. Gradation is primarily a concern in pavement mix design. Concrete could call for both coarse and fine particles and a dense graded aggregate would be needed. Asphalt design also calls for a dense graded aggregate. Gradation also applies to subgrades in paving, which is the material that a road is paved on. Gradation, in this case, depends on the type of road (i.e. highway, rural, suburban) that is being paved.