Aggregate crushing value is calculated as a percentage by weight of crushed (or finer) material obtained when test aggregates are subjected to a specified load under standardised conditions, and aggregate strength is expressed numerically.
Aggregates with a lower crushing value have a lower crushed fraction under load, giving the road a longer service life and thus a more economical performance.
If we used a weaker material, it would be crushed under traffic loads, resulting in smaller pieces that were not coated with a binder and would be easily displaced or loosened out, resulting in loss of the surface/layer.
As a result, aggregates used in road construction must be tough enough to withstand crushing from rollers and traffic.
Crushing Test Objectives
- Evaluate the crushing value of aggregate for use in road aggregates
- determine the acceptability of aggregates for use in various types of road pavement
Crushing Test Apparatus
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| Aggregate Crushing Apparatus |
- The test mould consists of a 15.2 cm diameter open-ended steel cylinder with a square base plate, a plunger with a 15 cm piston, and a rod that may be inserted for lifting or inserting the plunger in the cylinder through a hole in the plunger's stem.
- A rounded metal tamping rod with a circular cross-section 16mm in diameter and a length of 45 to 60 cm.
- A 5 kilogramme capacity balance that is readable and accurate to 1 gm.
- Sizes 12.5mm, 10mm, and 2.36mm on an IS sieve.
- A compression testing equipment that can apply loads of up to 40 tonnes per minute at a consistent pace of 4 tonnes per minute.
- A cylinder with a height of 18 cm and an internal diameter of 11.5 cm.
Procedure
- Using 12.5 mm and 10 mm IS sieves, sieve the material. The test material is made up of aggregates that have passed through a 12.5 mm filter and have been retained on a 10.0 mm sieve.
- Take roughly 3.25 kg of this stuff.
- Aggregates should be poured to fill the measuring cylinder to little over 1/3rd depth.
- The material is compacted by using the rounded end of the tamping rod to give 25 moderate blows.
- In a similar fashion, two more layers are applied until the cylinder is completely filled.
- Using a straight edge, cut away the superfluous material. This is the amount of aggregates that will be used to make the test specimen, and it is contained in the measuring cylinder.
- Empty the cylinder and weigh the aggregates to within 1 gramme accuracy.
- Fill the test mould with the entire weighted quantity, three layers thick, in the same manner as the cylindrical measure. The sample's total depth is around 10 cm, and the surface is just below the mold's top.
- Place the plunger over the levelled surface so that it lies horizontally on the aggregates' surface.
- Then, on the compression testing machine's pedestal, place this assembly.
- The load is applied at a rate of 4 tonnes per second.
- Let go of the weight.
- Remove the aggregate from the cylinder and sift it through an IS sieve of 2.36 mm. To an accuracy of 0.1 gramme, weigh the fraction that passes through it. This fraction represents the amount of material lost due to crushing.
- Compute the aggregate crushing value after recording the data in the proforma. The "Aggregate crushing Value" is the average of two observations, rounded to the nearest whole value.
For calculations, specifications for aggregate crushing value, download the below link (IS code for aggregate crushing value)
