GENERAL DISCUSSION OF AGGREGATES

I. GENERAL DISCUSSION

1. Introduction:

Aggregates of different sizes are being used as a natural filler in concrete and road construction.

Aggregates should be durable, strong, tough, well-graded and free from harmful ingredients. For

these reasons, testing of aggregate is important. As an example, if aggregates are weak, then

whatever the quality of cement/mix-proportion/water-cement ratio/compaction/curing, etc., the

concrete will also be weak. Similarly, for a durable road surface/wearing course in addition to other

factors, the aggregates must have proper resistance against abrasion. Various topics related to

aggregates, specification, testing, etc. have been described in the subsequent articles.

2. General Description of Particle Size:

Aggregates that are larger than ASTM #4 sieve or 4.75 mm are coarse aggregates and size 4.75

mm to 0.075 mm (retained on ASTM #200 sieve) are fine aggregates or Sand as per ASTM (but

according to AASHTO the demarcation line between Sand and Gravel is #10 sieve or 2.00 mm).

Coarse aggregates may contain even particles such as rock fragments. However a good fine or coarse aggregate should contain particles ranging from larger to smaller size to give a satisfactory

gradation.

Note: Pea Gravels are particles ranging from 9.5 mm or 3/8 inch to 2.36 mm or 0.093 inch (ASTM #8 sieve). Gravels are particles passing 75 mm sieve but are retained on 4.75 mm sieve (No. 4 sieve).

Aggregates Discussion,Aggregates,properties of coarse aggregate,aggregate types and sizes,Properties of aggregates,

3. Field Testing of Aggregates:

3.1 Several types of field testing of aggregates are described below, mainly for aggregates used in

concrete. However, field testing only gives a rough idea, and it is not a supplement to detailed

laboratory testing.

3.2 In controlling the quality of aggregates it is important to ensure that the aggregates are clean and do not contain any organic impurities which might retard or prevent the setting of cement.

Cleanness can be assessed visually, though for natural sands the field settling test will give an

approximate guide to the presence of the amount of clay or silt (known as sedimentation test). The

test entails placing about 50 ml of a 1% solution of common salt in water (roughly one slightly

heaped teaspoonful per pint) in a 250 ml measuring cylinder. Sand as received is then added

gradually until the level of the top of the sand is at the 100 ml mark and more solution is added to

bring the liquid level to the 150 ml mark. The cylinder is shaken vigorously and the contents are allowed

to settle for three hours. The thickness of the silt layer which settles above the sand is then measured

and expressed as a percentage of the height of the sand below the layer. The amount of clay and silt

in the sand may be considered acceptable if it does not exceed 8% in some specifications. If a

measuring cylinder is not available, a jam jar or bottle filled to a depth of 50mm with sand and to a

total depth of 75 mm with the salt solution will give comparable results if the contents are allowed to

settle for three hours. The thickness of the silt layer in this case should not be more than 3 mm.

The field settling test gives only an approximate guide. Sands apparently containing more than 8%

of clay or fine silt cannot comply with the specification and further laboratory tests should be done

to assess their suitability.

3.3 There is no suitable site test for the cleanness of coarse aggregates and reliance is usually placed upon the grading analysis to show whether there is an excess of fine dust in the material. Problems

arise, however, because of the tendency for fine dust to adhere to the rough surface of crushed coarse aggregates.

It is important to ensure that aggregate particles are not coated with clay and that lumps of clay are

not mixed in with the aggregate. The presence of clay indicates that the aggregates have not been washed adequately before or the aggregates have subsequently become contaminated after delivery. however, another field test are described below for rough estimation.

3.4 Test for Vegetable Matter:

- Take a straight sided glass jar and give 130cc and 200cc mark on it.

- Fill the jar with sand to be tested up to 130cc mark.

- Add 3% solution of caustic soda until 200cc mark is reached.

Shake the bottle thoroughly, as soon as the solution of caustic soda is added and allow the mixture to stand for 24 hours.

Note the colour of the liquid above the sand as follows:

- If colourless, the sand is free from vegetable matter.

- If straw coloured, this indicates the presence of some vegetable matter, but not enough to seriously

objectionable.

-If dark coloured, the sand contains objectionable quantity of vegetable matter, and should not he

allowed to use without washing.

3.5 Other Tests for Impurities:

Rub a little sand between fingers. Stains left on the fingers will indicate the presence of undesirable

clay impurities. The accurate content of silt & clay may be determined by wet sieving in the laboratory.