Shape Test on Aggregates

The percentage of flaky and elongated particles in an aggregate particle determines its shape. The form of gravel may be represented in terms of the angularity number.

The presence of flaky and elongated particles in coarse aggregate used for the construction of road pavement base and surface sources is considered undesirable, as these may produce inherent weakness with the possibility of breaking down during compaction and under strong traffic loads.

Due to their increased stability and improved interlocking, angular particle shapes are preferred for use in bituminous mixes and granular base courses. However, when the shape of aggregates deviates more from a spherical shape, as in the case of angular aggregates, the void content in an aggregate of any specified size increases.

So the grain size distribution of graded aggregate has to be suitably altered in order to obtain minimum voids or the highway dry density in the dry mix.

The flakiness index, elongation index, and angularity number are used to evaluate the particle shapes.

Flakiness Index

The aggregate flakiness index (FI) is the proportion by weight of aggregate particles with the smallest size / thickness that is less than three-fifths, or 0.6, of their mean dimension. This test applies to diameters greater than 6.3 mm. In other words, flakiness index (FI) represents the percentage of flaky materials with widths less than 0.6 of the mean dimension.

The thickness or least dimension of aggregate samples is measured using a standard thickness gauge. The flaky aggregates are those that pass through the thickness gauge’s predefined slots, which have elongated slots with the smallest size equal to 0.3 times the mean dimension of each range; these flaky aggregates are separated.

Elongation Index

An aggregate’s Elongation Index (EI) is the percentage by weight of particles whose largest dimension or length is larger than one and four fifth or 1.8 times its mean dimension. The elongation index test does not apply to diameters less than 6.3 mm.

The maximum dimension or length of the aggregate samples is measured using a standard length gauge. Elongated aggregates are those that do not pass through the prescribed slots of the length gauge that are 1.8 times the aggregate’s mean size; these elongated aggregate pieces are separated.

Apparatus

1. Sizes of 63, 50, 40, 31.5, 25, 20, 16, 12.5, 10, and 6.3 mm IS sieves.
2. Metal gauge
3. A 5 kg capacity weighing balance that is readable and accurate to 1 gm.

Preparation of Sample

1. The test sample must be drawn from material that has been well mixed and includes enough moisture to avoid segregation. A typical sample, large enough to give not less than the specified weight of dry material.
2. The test sample shall be dried to constant weight at a temperature of 110°C ± 5°C and weighed to the nearest 0.1 percent.
3. After drying and weighing the test sample, it should be placed in the container with enough water to cover it. The contents of the container must be thoroughly shaken.
4. The agitation must be forceful enough to separate all particles finer than 75 micron from the coarse particles and bring the fine material into suspension.
5. Fading away of the coarse particles of the sample should be avoided as much as possible. The procedure must be continued until the wash water is clear.
6. The wash water containing the suspended and dissolved materials must be put directly over the nested sieves, with the coarser sieve on top.
7. The material collected on the nested sieves must be returned to the washed material. The washed aggregate must be weighed to the closest 0.1 percent and dried to constant weight at a temperature not exceeding 110°C.

The amount of material passing through the 75-micron IS Sieve is determined as follows.

A = {(B-C)/B}*100

where,

A = Percentage of material finer than 75-micron.

B = Original Dry Weight, and

C = Dry weight after washing

Procedure

1. Sieve the sample using IS sieves (as specified in the table).
2. Weigh a minimum of 200 pieces of each fraction to be tested.
3. To separate the flaky materials, use a thickness gauge to measure the thickness of each portion.
4. Weigh the flaky material with the gauge to a precision of at least 0.1% of the test sample.
5. To separate the elongated materials, use a length gauge to measure each fraction for length.
6. Weigh the elongated material kept on the gauge to within 0.1 percent of the test sample.

Observations

Flakiness Index = (X₁+ X₂+…..) / (W₁ + W₂ + ….) X 100

Elongation Index = (Y₁ + Y₂ + …) / (W₁ + W₂ + ….) X 100

The IRC recommends that the FI of aggregates used in bituminous concrete and surface dressing not exceed 25%, and that the flakiness index (FI) of aggregates used in water bound macadam and bituminous macadam not exceed 15%.

Elongated and flaky aggregates are less workable; they are also more likely to break under smaller loads than spherical or cubical aggregates. Flakiness index and elongation index values more than 15% are typically regarded undesirable; however, no recognized limitations for elongation index have been established.

The Government of India and Ministry of Road Transport and Highways (MORT&H) has set the permitted maximum of the combined flakiness and elongation index, or combined index (CI), for coarse aggregates to be used in different types of pavement layers.

MORT&H has proposed that the flakiness index test be performed on a chosen sample of coarse aggregates in accordance with the Bureau of Indian Standards (BIS), and the value of the flakiness index, FI, be determined.

The flaky particles that pass through the thickness gauge slots are removed, and the elongation index test is conducted on the remaining non-flaky particles. Let EI be the value of the determined elongation index. The coarse aggregate sample’s combined index, CI, is thus equal to (FI + EI).

For wet mix macadam base course, dense bituminous macadam binder course, and bituminous concrete surface course, MORT&H has specified a maximum permitted value of the combined index of coarse aggregates of 30%.

Reference

1. IS : 2386 (Part IV) – 1963 – Methods of Test for aggregates for Concrete – Part IV : Mechanical Properties

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