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from Table 1, over 3,403,000 tonnes of crushed rock (including Granite) is used on the Rail per year.
Table 1 – Extract from Annual Minerals Raised Enquiry, ONS 2005
The typical arrangement of ballast in relation to the rest of the Permanent Way structure is depicted below in figure 1.1 and 1.2.
Fig 1.1 – Typical layout of permanent way, including ballast formation.
Before its application on the UK Rail Industry, track ballast is required to undergo a number of stringent tests to determine its size and suitability and certification for safe use. These are outlined in the Railtrack Specifications and brought into further detail through the referencing of British Standards for the sampling of Aggregate;
[Ballast Size, Dimensions and Shape]
In accordance with RT/CE/S/006, track ballast is required to be made up of a consistent mixture of sizes ranging from 28mm – 50mm and be tested for this in accordance with the Sieve Test specified in BS812 Section 103.1 (1985) and conform to the limits in the following table;
Square Mesh Sieve (mm)
Cumulative % by Weightpassing BS Sieve
63
100
50
97-100
37.5
35-65
28
0-20
14
0-2
1.18
0.8-0
Table 1.1 – Extract from RT/CE/S/006 Railtrack Specification
[Sieve Testing in Accordance with BS812-103.1 (1985)]
The process of washing and sieving to grade aggregates according to size is necessary to conform to the standards set out by industry. In short, the method described in BS812-103.1 is split into a number of steps as laid out below, culminating in a full percentage breakdown of mass per size by sample.
Preliminary Preparation; requires a test sieve (75 μm size) fitted to a nesting guard sieve and wet on both sides. The aggregate is placed into a suitable sized, clean container, and filled halfway with water, and then agitated by sufficient shaking to separate fine and coarse particles. The sample is then drained into the test sieve and washed with a constant stream of water until the excess runs clear.
Oven Drying; the sample then is oven dried at a temperature of 105 ± 5 oC (95 – 105 oC) until constant mass is achieved. Cooled, weighed and recorded.
Mass of the sample is determined using the weight before sampling less the recorded mass after drying for discarded aggregate.
Sieving Dried Sample; a number of stackable sieves are then placed in a tower (sizes according to required particle sizes wanting to be achieved) and by mechanical means, shaken until the aggregates have been separated into size fractions.
The finished test sieved sample is then weighed in its different size fractions and separated for its different uses.
[Flakiness Index Testing; In accordance with BS 812-105.1 (1989)]
Another test required to determine the suitability of aggregate for track ballast usage is the Flakiness Index test. BS 812-105.1 states “Aggregate particles are classified as flaky when they have a thickness of less than 0.6 of their mean sieve size.”
The flakiness index of an aggregate sample is found by separating the flaky particles and expressing their mass as a percentage of the mass of the sample tested. The specifications in www.51lunwen.org/Materialengineering/ RT/CE/S/006 require track ballast to not exceed 40% in the under mentioned testing procedure.
Before determining the flakiness of an aggregate the sample to b